DESIGN OF A WATER-DISPOSABLE
                          research grants
                           UI-00651
                           EC-00033
                           Clemson
                           University

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This report has been reviewed by the U.S. Environmental
Protection Agency and approved for publication.  Approval
does not signify that the contents necessarily reflect
the views and policies of the U.S. Environmental Pro-
tection Agency, nor does mention of commercial products
constitute endorsement or recommendation for use by the
U.S. Government.

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    DESIGN OF A WATER-DISPOSABLE GLASS PACKAGING CONTAINER


    Part I:  Protective Oxide Coatings for Glasses by Chemical Vapor Deposition
      Part II: Mechanical Properties of Water-Soluble Sodium Silicate Glasses
  Part III: The Rate of Dissolution of Sodium Silicate Glasses in Aqueous Solutions
              These interim reports (SW-llrg) on work performed under
  Research Grant No. UI-00651 (Parts I and II) and under Grant No. EC-00033 (Part III)
                           to the Clemson University
were prepared by SAMUEL F. HULBERT, C. CLIFFORD FAIN, and MICHAEL J. EITEL
               and have been reproduced as received from the grantee.
               U.S. ENVIRONMENTAL PROTECTION AGENCY
                                   1971

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An  environmental  protection publication in  the solid waste
                                 (

management series (SW-llrg).
   For sale by the Superintendent of Documents, U.S. Government Printing Office

                    Washington, D.C. 20402 - Price $1.75

                         Stock Number 5502-0046

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                        FOREWORD

PACKAGING WASTES constitute a significant and growing fraction of
the total solid waste load, and a major component of this total
is the glass container.
     In a study done for this agency, the Midwest Research Insti-
tute found that shipments of glass containers have grown from 20.2
billion units in 1958 to 29.4 billion units in 1966.]  They pro-
jected a consumption rate by 1970 of 34.9 billion units and by
1976 of 45.7 billion units.  During the period from 1966 to 1976
they expect the shipments of glass beverage containers to more
than double, as nonreturnable containers account for a larger and
larger share of the market.
     We applaud the nationwide program by which glass container
manufacturers are supporting recycling activities.  Clearly,
though, other avenues must be explored, including the concept of
reducing solid wastes at the source—reducing the quantities of
waste that will ultimately require storage, collection, transport,
processing, and disposal.
     Under the authority of the Solid Waste Disposal Act of 1965,
amended by the Resource Recovery Act of 1970, research has been
undertaken for the study and solution of problems caused by
packaging wastes.  The research work discussed in this report
illustrates the innovative and imaginative approach being brought
to bear on the problem.  The development of a water-soluble glass
container, presented in this three-part work, has high potential
for promoting source reduction, and thereby improving the quality
of our environment.
      Darnay, A., and W. E. Franklin.  The role of packaging in solid
waste management, 1966--1976.  Public Health Service Publication
No. 1855.  Washington, U.S. Government Printing Office, 1969.  205 p.
                               m

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                         PREFACE
THIS REPORT was prepared by the Division of Interdisciplinary
Studies, Clemson University, Clemson, South Carolina, under
U.S. Public Health Service Research Grants  UI-00651  and
EC-00033.
     The work reported in Part I was performed in fulfillment
of the thesis requirements for the degree of Master  of Science
in Ceramic Engineering by Martin M. Cooper under the direction
of Dr. C. Clifford Fain.  The work reported in Part  II was
performed in fulfillment of the requirements for the degree of
Master of Science in Materials Engineering by Charles Wesley
Jennings under the direction of Dr. Samuel  F. Hulbert.  That
reported in Part III was performed in fulfillment of the re-
quirements for the degree of Master of Science in Ceramic
Engineering by David Taylor Ballenger, also under the direction
of Dr. Hulbert.
                               IV

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                             PARTI




PROTECTIVE OXIDE COATINGS FOR GLASSES BY CHEMICAL VAPOR DEPOSITION

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                                  ABSTRACT



        A method of coating water soluble silicate glasses with oxide  films by

chemical vapor deposition was studied.  The work represents the initial phase of a
                                                                           •
three year investigation to determine a feasible method of processing  glass con-

tainers after use in order to relieve a source of solid waste currently reaching

gigantic proportions nationally.

        A detailed  description of the system was provided to include coating equip-

ment design features, input material requirements, and techniques of coating.

Particular attention was given to key process control parameters such as temperature

control, evaporation rates, and vapor transport procedures.

        Coatings of silica, titania, and other oxides were readily obtained by

pyrolytic decomposition of organic ester vapors.   The reaction kinetics of selected

materials were analyzed in terms of the process conditions used to derive them.

For example, the effects of deposition temperature, carrier gas flow rate, reactant

vapor concentration in  the carrier gas stream, and substrate orientation were  corre-

lated with deposition rates and coating efficiencies.

        High quality coatings of both silica and titania were obtained  on soluble

glass substrates by exercising proper judgment in the choice of process conditions

to be used.  Oftentimes the best coatings were obtained at the expense of longer
                                      •
coating times and lower efficiencies.  The films produced, however, were found to

be continuous, highly resistant to various forms of corroding media, and adherent

to the surface of the substrate.

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        The titania films were most readily applied as uniform thin films at a growth


rate of approximately 6000 A per hour between temperatures of 500   F - 800  F.


It was determined that the film structures were readily controlled by proper atten-
                  \

tion to process parameters.  The silica films were generally applied under more


restricted conditions, typically between 1000° F - 1050° F at about 4000 A per


hour.


        Both types of oxide coatings were found to  produce undesirably high


residual stress levels, primarily because  of a mismatch of thermal  expansion


coefficients between the coating and the substrate.  In addition,  many coatings


exhibited microstructural  variability primarily caused by uncontrollable coating


conditions such as poor control on vaporization and gas delivery processes,


impurities, substrate surface variations,  localized temperature fluctuations, and


others.  The variable coating  microstructures  in conjunction with  high residual


stress levels are believed  to promote a generally  higher defect level  such as


microcracks and pinholes, thus contributing to a  premature  breakdown of the


continuous barrier under water attack.  The fact  that tentative investigations


revealed a wide scatter in the times required  for  water to penetrate and dissolve


a coated specimen tends to amplify this observation.
                                     Vi i

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                                 PARTI



                           TABLE OF CONTENTS


Chapter                                                              Page



  I.   INTRODUCTION	     1



  II.   REVIEW OF LITERATURE	     3



        A.  Solid Waste Control	     3



        B.  Soluble Silicate Glasses	     4



        C.  Coating Considerations	     6



 III.   OBJECTIVES OF RESEARCH	    14



 IV.   EXPERIMENTAL  PROCEDURE	    15



        A.  Coating Apparatus Design	



        B.  Other Design Considerations	    25



        C,  Input Material Considerations	    26



        D.  Deposition Techniques	    31



        E.  Analytical Procedures	    34



                1. Microscopy	    34



               2. Phase Analysis	    34



               3. Coating Thickness and Uniformity	    35



               4. Coating Efficiency	    33



               5. Coating Stresses  • •	    40



 V.   RESULTS AND DISCUSSION OF RESULTS	    42



        A.  Effect of  Process Variables Upon Reaction Kinetics	    «~



                1. Substrate Temperature	    43



               2. Coating Time at Different Temperatures	    43



                                   • * •
                                  vm

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Chapter                                                           Page

              3.  Carrier Gas Flow Rate and Substrate Orientation. .  .    46

              4.  Reactant Gas Concentration .  .  . ,	    46

        B.  Coating Stresses	    48

        C. Coating Uniformity	    53

        D. Substrate Considerations	    54

        E.  Characterization Of SiO2 Coatings	    58

        F.  Characterization Of TIO2 Coatings	    62

        G. Other Coatings	    72

        H. Initial Corrosion Studies	    76

 VI. CONCLUSIONS	    78

VII. RECOMMENDATIONS FOR FUTURE WORK	    80

LITERATURE CITED	    84

APPENDICES	

    APPENDIX I.  DETERMINATION OF PLATING VAPOR
                 CONCENTRATION IN THE CARRIER GAS STREAM.  .

    APPENDIX II.  DETERMINATION OF COATING EFFICIENCY. ...   90

    APPENDIX III.  DETERMINATION OF RESIDUAL DEPOSIT STRESS  .   92
                             IX

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                                 PARTI






                            LIST OF TABLES






Table                                                                 Page




   I.   Chemical  Properties of Plating Compounds	    30




  It.   Optical Properties of Coatings and Substrates	    39




  III.   Mechanical Properties of Coatings and Substrates	    41




  IV.   X-Ray Diffraction Data for Coatings Obtained	    63




  V.   Chemical  Durability of Coatings Obtained	    66

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                                    PARTI

                               LIST OF FIGURES


Figure                                                                    Page

 1.  Schematic of the chemical vapor deposition apparatus   	      16

 2.  Photograph of the chemical vapor deposition apparatus	      17

 3.  Schematic of the temperature control circuit	      20

 4.  Plot of evaporation rates of tetraisopropyl titanate under
       various conditions	      23

 5.  Photograph of the dual furnace assembly	      27

 6.  Schematic of the two substrate positions used	      33

 7.  Analysis of film thickness and uniformity shown by interference
       colors	      36

 8.  Plot of deposition rates at different temperatures	      44

 9.  Plot of deposition rates at two different temperatures for varying
       coating time periods	      45

10.  Plot of deposition rates at different substrate orientations and
       gas flow rates	      47

11.  Plot of deposition rates at different reactant vapor concentrations
       at an overall constant flow rate	      49

12.  Photograph of residual stresses in a SiO2 deposit	      50

13.  Photomicrographs of residual stresses in a Ti03 deposit	      52

14.  Photograph of a representative deposition pattern  for SiO2 deposits  .      55

15.  Photomicrographs of atmospheric attack upon a soluble silicate
       surface at three  time periods	      57

16.  Plot of deposition rates at different temperatures for SiOo deposits  .     60

17.  Photomicrograph of surface topography  of a typical S«O2 deposit  .  .     61

                                      Xi

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Figure                                                                  Page

18.  Photomicrograph of Cristobalite crystals	       64

19.  Photomicrograph of columnar growth habit of SiO2 Deposits  ....       65

20.  Plot of TiO2 structure at different temperatures and reactant              69
       vapor mass flow rates	

21.  Photomicrographs of typical anatase microstructures	       71

22.  Photomicrographs of secondary growth features in TiO2 deposits . .       73

23.  Photomicrograph of surface topography of amorphous TiO2
       deposit	       74

24.  Photomicrograph of a TiO2~SiO2"TiO2 duplex coating	       75

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                                   CHAPTER I







                                INTRODUCTION






        The great natural resources of our nation have been ingeneously utilized by




a resourceful people to create a vast industrial capability which today provides an




expanding population with the highest standard of living on earth.  This epic growth




has been accompanied, however,  by a constantly expanding quantity of material




wastes emanating from our nation's communities.  By virtue of the current magni-




tude of the problem we are now  urgently compelled to find new methods to process




or neutralize these wastes rather than simply digest or absorb them as in the past.




Our people can no longer afford to ignore the effects of belching factory smoke,




gaseous emissions from vehicles, foul liquid contaminants dumped into our streams,




and solid  wastes piled  up at refuse dumps or discarded along our roadways, for to




do so will  subject  much of our citizenry to potentially severe health hazards as well




as contaminate the beauty of our national landscape.




        Recent reports  (1) indicate that the nation has already taken conclusive




steps  to control gaseous and liquid wastes.   In contrast, however, it is pointed out




that we are just beginning to focus our attention on the  problems related to solid




wastes. The Federal Government is  now providing funds and leadership for research




and planning programs to curb the undesirable effects of inadequate solid waste




control.   Essential to this effort  is a  method of processing incombustible,  unreactive




glass  containers after use.  The potential impact of such a development upon solid




waste control may  be readily visualized by considering  that approximately  135 glass

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bottles and jars are produced each year for every person in the United  States.  This

amounts to about 27 billion glass containers per year (2).

       This paper represents the initial segment of a projected three year study to

formulate a method of reducing  the source of glass  container waste by utilizing a

water soluble glass encapsulated in a thin,  impervious coating which resists corro-

sion by the environments normally encountered by the glass packaging industry.

After the  consumer empties the container, the continuous physical barrier may be

broken and the water soluble glass dissolved.  It  is anticipated that the decompo-

sition could be made to occur gradually over several  months' duration,  as at a

municipal refuse  dump, or relatively rapidly, perhaps within several  minutes,  by

adjustment of the chemical and  physical  parameters of the base glass  and  its over-

lay coating.

       This paper describes the use  of chemical vapor deposition processes for
               *
applying chemically inert  metallic oxide coatings to soluble glass substrates.   A

detailed analysis of the method  is presented and its potential feasibility for vitreous

container waste control is  discussed.

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                                  CHAPTER II







                            REVIEW OF LITERATURE







A.  Solid Waste Control




       The problem of solid waste control in our country is rapidly approaching




critical proportions.  This statement surprises many people, for generally speaking,




the public is not aware that there is a problem.  A vivid illustration of public apathy




can be seen in  the attitude toward the throw-away glass bottle which relieves the




producer, distributor, and consumer of handling costs and inconvenience, but cre-




ates more burdens for refuse collectors.  Unfortunately, most people are not even




aware of the additional  problems that have been created by this  convenience item.




The refuse collector is painfully aware of his increased burdens,  however.  The use




of non-returnable glass bottles is increasing (3), and no immediate solutions are in




sight, although some states have begun to introduce legislation (4) in an effort to




curb their production.




        It is estimated (5) that glass comprises five to ten percent by weight of all




refuse ending up on municipal dumps.  Of course, scrap glass represents a major




solid  waste problem because, unlike much refuse, it is non-degradable.  In addi-




tion to being aesthetically objectionable, discarded glass containers may harbor




rodents and catch other vermin.  They may catch and  hold water in which mosqui-




toes may breed and subsequently transmit malaria, yellow fever, dengue,  mosquito-




borne encephalitis, and filariasis (6)-

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        At present, non-combustible glass containers can best be disposed of by




sanitary landfill techniques.  Obviously, this does not represent the complete




answer, for convenient sites are not always available.




        In a major effort to generate fresh approaches directed toward solution of




the problem, the Federal Government passed the Solid  Waste Disposal Act, which




became law on October 20, 1965 (1).  The Congress assigned responsibility for




administering the Act to the Secretary of Health,  Education and Welfare and to




the Secretary of the Interior.  It is interesting to note that similar legislation to




combat water and air pollution has been in effect for twenty, and twelve years,




respectively (1).




        This work represents one facet of the total program which has been imple-




mented in selected  universities and industrial research facilities throughout the




country.







B.  Soluble Silicate Glasses




        Glass containers are carefully formulated today to yield high chemical dura-




bility.  The resistance which glass offers to various corrosive agents is a property of




great practical  significance, according to Morey (7).  This eminent glass authority




further points out that the chemical durability of glass is oftentimes the chief reason




for its preference over competing materials.  As an example, he cites the use of




glass  containers which are used in enormous numbers for the distribution of commod-




ities ranging from milk to medicine to acids.  In this respect, he concludes that the




superiority of glass  leaves it without a competitor.

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        Of course, the hypothesis of this paper requires that a vitreous container




must be soluble  in order to be readily eliminated after use.  Thus,  a water soluble




glass, with satisfactory optical, mechanical and  chemical properties for container




use is required.   Fortunately, a vast market exists today which makes use of water




soluble  glasses.   Vail  (8) discusses the diverse uses for these alkaline silicate




materials, which may  serve such purposes as adhesives, cleaners, cements, defloc-




culants, and protective coatings.




        Vail  (9) points out that  the largest quantity of soluble glass produced for the




soluble  silicate  industry is the eutectic composition between sodium disilicate and




quartz, which  is  readily dissolved by water in less than one hour.   Potassium sili-




cate glasses are also known to be readily soluble.




        The properties of the alkali silicate glasses have been investigated, but




generally the reasons were for scientific curiosity rather than to achieve any prac-




tical objective.  Their properties tend to change  continuously as a  function of com-




position rather than show the abrupt type of discontinuity characteristic of hetero-




geneous crystalline systems.  The solution rate depends primarily upon the  ratio  of




alkali to silica in the  glass (9).




        The alkali silicate glass system would offer some  inherent advantages if




adopted for glass container production.  As reviewed  by  Vail (8), sodium silicate




solutions are now added to polluted waters,  as a flocculant in the settling  and fil-




tering steps of water purification.  At the same time,  these solutions tend to form




a thin film on the inside of water pipes which would protect them against water

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corrosion.  In addition,  it is noteworthy to add that no adverse effects from potable

waters which have been  sodium silicate treated have been detected.

        The production of alkali silicate glasses is simple and cheap.  Most of the

material made in this country is easily produced by firing mixtures of sand and

sodium  carbonate in open-hearth type furnaces of regenerative or recuperative

design with gas or oil.  The clear glass reaction product is then drawn directly  into

water to produce silicate solutions (9). The objective of this study is to use a

similar  procedure with the exception that the vitreous silicate will be made into a

container and used prior to putting it into solution.


C.  Coatmg  Considerations

        The choice of a coating mechanism for this work entailed the critical eval-

uation of several methods.  These methods can best be categorized in terms of the

size of  the particles which are being used to produce the coating structure (10).

They are  (a) atomic species; (b)  particulate material; and, (c) sheet material.  On

this basis,  the following categories provide a reasonable framework for discussing

coating methods,

        1.  Atomic species processes

           a.  Vacuum evaporation
           b.  Cathode  sputtering
           c.  Electrode position
           d.  Chemical vapor deposition

        2.  Particulate material processes

           a.  Flame and plasma spraying
           b.  Enamelling
           c.  Painting
           d.  Trowelling

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        3.  Sheet material processes

           a.  Laminating
           b.  Cladding

        Chemical vapor deposition was used as the coating mechanism in this study

because it is believed to be the best method for producing thin, impervious oxide

coatings of desired quality.  Lesser considerations governed ttie choice to some

3xtent, such as potential  adaptability to other materials systems, construction,

•naintenance, and production costs, and scale-up considerations.

        Chemical vapor deposition may  be defined (11) as a method of plating in

/vhich deposits are  produced  by heterogeneous gas-solid chemical reactions at the

surface of a heated substrate.  A volatile compound of the element or substance to

DC deposited is vaporized and the vapor thermally decomposed, or reacted with

Dther gases or vapors at the substrate  to yield non-volatile reaction products which

deposit on the substrate, as a  coating.

        Blocher (12) refers to chemical vapor deposition,  vacuum evaporation,

cathode sputtering, and electrodeposition as "molecular forming" processes, that

is,  they tend to build up coatings or deposits atom-by-atom upon a substrate. As

a result, these processes have in  common  the potential of yielding dense deposits

of controlled thickness and orientation.  Such properties are essential for a sound

protective coating.

        Each of the molecular forming processes has its advantages and limitations.

However, when these are analyzed, chemical vapor deposition comes out quite well

by Blocher's comparisons.  To illustrate, chemical vapor deposition is capable of

yielding the greatest variety  of products at rates of deposition equivalent to or

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exceeding those of the other techniques.   It has the greatest throwing power, that




is, the ability to deposit uniformly on relatively complex shapes.   Blocher regards




the major drawback of chemical vapor deposition as the fact that the substrate




must usually be heated to relatively high  temperatures, which can  lead to residual




stresses in the deposit and subsequent mechanical failure provided the differential




thermal expansion coefficients between the substrate and the coating are sufficiently




high.  The other  techniques have the advantage of maintaining the substrate at




ambient temperatures.




        Accountius (13) regards chemical  vapor deposition as still largely a labo-




ratory operation, except in a relatively few instances.  He states that the deposi-




tion of each material is unique, and in nearly every case,  further study of the ther-




modynamics, kinetics, heat transfer, chemistry, and solid state phenomena are




required to elevate chemical vapor deposition from the status of an  art to that of a




science.




        In spite of its many unexplored facets at present, the process has evolved




quite rapidly from a technological standpoint in the last fifteen years.  One  may




gain an accurate idea of its recent growth by comparing the 1955 state-of-the-art




survey entitled Vapor Plating (14) with its 1966 revision, Vapor Deposition (]5\




both prepared under the auspices of Battelle Memorial Institute, an organization




which has spearheaded the bulk of fundamental and applied research that has gone




into the field.




        The use of chemical vapor deposition is already well entrenched in protec-




tive  coatings technology.  It is interesting to note, in fact, that  the first significant





                                     8

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historical application of the technique grew out of a need for a protective coating




material in the incandescent lamp industry in the last two decades of the nine-




teenth century (11).  Sawyer and Man (16) deposited pyrolytic carbon, and Ayls-




worth (17) and deLodyguine (18) deposited metals onto carbon filaments.  Unfor-




tunately, this particular application was never very satisfactory because of the




brittle carbon core, and interest in chemical vapor deposition as a coating tool




soon subsided for several years.




        Since 1935, however, several interesting applications of chemical  vapor




deposition have been  devised to protectively coat metals, alloys, and non-metallic




materials against erosion and abrasion, general corrosion, and high temperature




oxidation. These have been reviewed by Krier (19).  Countless materials are cur-




rently protected by siliconizing, chromizing, nitriding, and other like processes




which make up a vast commercial endeavor.  There are also a large number of




research and  development programs active today.  Emanating from these programs




are such developments as new silicide and aluminide coatings for the aerospace




industry, pyrolytic  carbon coatings for rocket nozzles and re-entry vehicle surfaces,




and others.   These coatings are usually chemically vapor deposited by pack cemen-




tation or fluidized bed methods, two sub-processes of chemical vapor deposition




which hold high promise for future applications.




        Another use of chemical vapor deposition has been recently discovered in




the atomic energy field. Small spherical fuel  particles such as uranium dioxide  are




coated with oxides or carbides by fluidized bed techniques to produce a dense,




impervious barrier to fission  products or other reaction products (20).  The fission

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product retention of these coatings has been found to be significantly  low.  Char-

                                                    1 ^o
acteristically only a few parts per million (ppm) of Xe Ov5 fission gas generated in


neutron activation is released on post irradiation heating.  As a primary result of


this caliber of performance particulate fissionable material encapsulated in a vapor


deposited coating is believed to have the potential for solving many problems in


reactor technology.


        Curiously, as mentioned previously, the molecular forming processes such


as chemical vapor deposition have the ability  to control  the structure of a coating.


For example, a coating may  be made to exist  in either a crystalline or amorphous


form, or a combination of both, depending upon the  proper selection of deposition


parameters. Walton (21) explained the phenomena by pointing out that no orien-


tation  is produced when a single bond (i.e., an atom joined to a cluster by a single


bond) is stable.  Single bonds rather than multiple bonds occur provided the sub-


strate temperature is low enough or the reactant gas  incidence rate high enough.


If, on  the other hand, the supersaturation is low enough  for multiple bonds to be


stable, but not single bonds, an oriented deposit can result.  Interpreted another


way,  if one is obtaining crystalline films on a given  substrate, the process may be


adjusted to yield amorphous or glassy films by  increasing  the deposition rate and/or


decreasing the substrate temperature.


        Several investigators have discussed whether  amorphous films should be


termed glasses.  Secrist and Mackenzie (22) have proposed that there is no dis-


tinction, based upon their work.  They compared the critical structure indicators
                                      10

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of amorphous silica prepared by conventionally cooling from a melt and from var-




ious deposition techniques and found no significant changes.




        This exciting discovery means that glassy materials may be prepared by such




processes as sputtering, vacuum evaporation, and chemical vapor deposition as




well as by  the classical method of cooling from a melt.




        The concept has radically altered ideas on what substances are  capable of




forming glasses.  Many oxides and non-oxides are capable of being  made into




glasses by the molecular forming techniques although most of them cannot be




obtained as glasses in bulk form by cooling from a melt.   Sinclair (23) states that




essentially any material which can be deposited by the molecular forming processes




can be made into amorphous or glassy films.




        An amorphous film has inherent  advantages for a coating application such




as the one  proposed in this paper. Sinclair  (23) points out, for example,  that




glass  films  have no grain boundaries  and are isotropic.  For this reason, they are




currently .used extensively in the field of  thin-film microelectronics as diffusion




barriers to  keep water and ionic contaminants from junctions.




        On the other hand,  Sinclair  (23) points out that the properties of glass films




are greatly effected by methods of preparation, techniques, and conditions.  He




illustrates the effect of impurities by stating that a monolayer of water absorbed




onto a substrate surface, if incorporated into a 1000 Angstrom (A) silica film,




would represent 0.5 mole percent of the composition.  Additional water soluble




surface contaminants such as sodium  ions would have the dual  effect of  being




impurity sources in their own right and also causes of increased water absorption
                                 11

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on the surface.  He concludes the the predeposition cleaning procedure is very




important.




        Holland (24) generally takes a dim view of the feasibility of protecting




glass surfaces by vapor depositing oxide films onto the substrate.  Citing results




from sputtering and vacuum evaporation techniques, he states that it is difficult to




produce films completely free from pinholes.  He  also observes  that only a limited




range of film materials have suitable optical properties to allow their use.




        Overall, the writer believes that selected oxide coatings, chemically inert




to water and other corroding media, offer a splendid source of  potential  for




research in the field of coating soluble glasses by chemical vapor deposition.  The




literature contains several references to work which has been recently accomplished




in this area on relatively durable compositions.  For example,  thin titania films are




now commonly applied to glass bottles after annealing primarily to impart a more




effective scratch-resistant finish (25).  The method involves the use of a volatile




organic titanate which is easily pyrolyzed to effect the reaction.  The resulting




film is extremely thin  and transparent and is said  to impart a substantial degree of




additional durability.  The method  is also applicable  to the oxides of aluminum and




Tirconium (26).




        Badger (27) has discovered that silica deposited on glass surfaces by treat-




ment with ethyl orthosilicate or other alkoxysilanes,  at a  temperature between the




melting point and the  softening point of the glass, increases the chemical durability




of the glass.  The same system has been  devised for aluminum oxide, deposited from

-------
aluminum isopropoxide, and  is said to work for any oxide which forms constituents




of conventionally prepared durable glasses.




        Mixed oxides may be deposited by using single oxide deposition  processes




and mixing the metal compounds in the plating atmosphere (28).  Such procedures




are readily adaptable to producing borosilicate glass coatings produced by the




mixed vapors from ethyl orthosilicate and triethyl  borate.




        Several oxides have been coated sequentially onto a substrate to form




composite layers (28).  Such materials as  silica, alumina, titania,  and zirconia




have  been mentioned.  This latter method would possibly offer advantages in mini-




mizing  the effects of pinholes and other growth defects.
                                 13

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                                 CHAPTER  ill







                         OBJECTIVES OF RESEARCH







        The objective of this research was to define the capabilities of chemical




vapor deposition as a tool for coating soluble glass substrates with metallic oxide




films.   The work represents a segment of a projected three year study which will




deal with methods of processing glass packaging containers after use.   In order to




accomplish the objective of this investigation, the research was divided into three




phases, as discussed below.




        The first phase involved the design and construction of a  mechanism cap-




able of depositing metallic oxide films from the vapor phase  onto soluble glass




substrates.  The proper selection of equipment components and materials necessarily




involved an extensive research effort and numerous calculations to assure that satis-




factory coatings could be attained over a wide range of controlled conditions.




        The second phase of the research effort was directed  toward characterizing




selected coatings in terms of various process parameters, as well  as accumulating




basic data designed to reveal  the overall  feasibility of the system's capability of




performing its ultimate task by considering various pertinent  properties of the com-




posite produced.




        The third phase of this work involved a critical assessment of the system




investigated.  Its basic limitations were discussed,  and suggestions were given for




possible improvements to be incorporated  into the current facility.
                                     14

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                                CHAPTER IV







                        EXPERIMENTAL PROCEDURE







A.  Coating Apparatus Design




       The vapor deposition apparatus designed and constructed for this study is




shown schematically in Figure 1 and photographically  in Figure 2.  The choice of




components was carefully screened to  provide a system capable of depositing a




wide spectrum of materials over a range of controlled conditions.  At the same




time, it was deemed advisable to maintain a simplified and straightforward design




concept which can be ultimately tailored to scaled up production operations with




minimum difficulty.




       A mullite-lined tube furnace served as the reaction chamber for this inves-




tigation.  The combustion tube, purchased from McDanel Refractory Porcelain




Company, measured thirty-six inches in length.  Its cross sectional dimensions were




2 1/4 inches  inside diameter with a 1/8 inch wall thickness.   The  ends of the tube




were readily  closed off with rubber-gasketed aluminum doors supplied by the same




company. O-ring compression seals were mounted on the doors to provide  1/4 inch




gas tight lead-throughs for the entrance and exit of gases. Thermocouples and




other control  components were also introduced into the interior of the reaction




chamber via the same type lead-throughs.   The thermocouple wires extending




through the double bore porcelain  insulator tubing were sealed with epoxy cement




to prevent gas leakage.  The  sealed door arrangement provided a gas tight system




capable of being dynamically monitored or controlled during a coating run.
                                 15

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        PRESSURE GAUGE
        FLOWMETER
   ORGANIC LIQUID
        COMPRESSED GAS
FURNACE

  COMBUSTION TUBE
                                        SUBSTRATE
                                            EXHAUST-
                                                         •SCRUBBER
                                         THERMOCOUPLE  TUBE
                                         AND POTENTIOMETER
                             •CONTROLLER
Figure 1. Schematic of the chemical vapor deposition apparatus

-------
Figure 2.  Photograph of the chemical vapor deposition apparatus

-------
        A review of other designs (11) suggested that the cross-sectional dimen-

 sions of the chamber should be as small as practically possible for improved control

 effects.  This consideration is thermodynamically related to gas mixing uniformity

 and heat transfer effects within the tube (29).  It was recognized at the outset of

 this investigation that two primary control parameters would be temperature uni-

 formity of the substrate on all external faces and a  uniform supply of the reactant

 gas and carrier gas mixture to the substrate.  A compact reaction chamber with  a

 small internal volume enhances the control of such  variables.

        The tube was fired with a 230 volt,  3200 watt Lindberg "Hevi-Duty" labo-

 ratory combustion  furnace.  The furnace measured 25 inches along the axial length

 of the tube.  The unit  incorporated eight 1/2 inch by 16 inch series connected

 silicon carbide heating elements which were arranged transverse to the axis of the

 tube, four above and four below, on four inch horizontal and vertical  centers,  to

 form a twelve inch firing zone.

        Excellent temperature control of the unit was readily achieved through the

 use of a Barber Coleman Model 477 Capacitrol electronic indicating controller  to

 drive a Model 621A silicon controlled rectifier for  the proportional control of load

 power.  The solid state circuitry of the Model 477 makes it possible to eliminate

 temperature overshoot and  undershoot by properly adjusting the proportional control

action and reset features of the instrument.

       The control thermocouple was composed of platinum and platinum  alloyed
                                                                          ,>
with 13 percent  rhodium wires.  The junction was maintained in contact with the

external face of the combustion tube at the center of the firing zone.  The final



                               18

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location of the controlling sensing element was determined after many trial and




error experimental runs which consisted of monitoring the response and recovery




characteristics of the above control system to arbitrary changes  in temperature and




gas flow with  the thermocouple junction located inside the firing zone of the com-




bustion  tube as well  as against the outside face.




        For the purpose of the above trials, a monitoring thermocouple was con-




nected to a potentiometer with its junction positioned inside  the firing zone of the




combustion tube.  This thermocouple was also subsequently used during actual




coating runs to record the substrate temperature.




        In general, it was found that the control characteristics  for an externally




heated system under  stable conditions were excellent for both thermocouple loca-




tions.  However, process lag and cycling were less for the external thermocouple




location when an arbitrary process change was introduced.  Mullite heat reflectors




were placed inside the tube at the ends of the firing zone to  help balance the tem-




perature profile, and with this arrangement, it was possible to maintain plus or




minus 2 degrees Fahrenheit (° F) at a given spot and plus or minus  15° F at different




locations within the  firing chamber under stabilized conditions.  A schematic




representation of the control circuit is shown in Figure 3.




        This system as described will operate successfully to 2700  F.  However,




longer life of  the heating elements and thermocouple wires is expected if the tem-




perature does  not exceed 2500° F.  The unit is readily responsive to input power




changes.  Heating and cooling rates above 350° F per hour are considered dele-




terious to  extended component life.






                                     19

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ro
o
                                                             MODEL
                                                              477P
                                                           INDICATING
                                                           CONTROLLER
                        THERMOCOUPLE
                   LOAD
   MODEL
    62IA
   POWER
CONTROLLER
              Figure 3. Schematic of the temperature control circuit

-------
       Control of the plating vapor concentration in the carrier gas stream over a




range of flow rates is important in chemical vapor deposition processes, for such




variables partially govern deposit thickness, uniformity, orientation, and purity.




For the purposes of this investigation, cylinders of prepurified grade compressed




gases were purchased from Matheson  Company to serve as the transport vehicle.




These gases were pressure regulated on Matheson equipment in two stages to yield




a resultant gas force which could be  controlled in integral  units of water pressure




up to 35 inches.  This controlled pressure was fed into  the system through a flow-




meter calibrated  to read between 0-10 cubic feet of air per hour (cfh) in increments




of 0.25 cfh up to 2 cfh and in  increments of 1.0 cfh at higher flow rates.  A cali-




bration chart supplied by the vendor  allowed different  gases to be used with the




same flowmeter by applying correction factors to the settings used.  The resultant




system yielded a  highly reproducible, control-sensitive gas supply to  the vaporizer.




       The liquid plating agents used in this study were vaporized into the carrier




gas from a  simple 125 milliliter (ml) Pyrex flask.  The flask was sealed at the top




with a rubber stopper containing two sections of  1/4 inch Pyrex tubing, one to allow




for the entrance of the carrier  gas and the other for the exit of the resultant gas




mixture.  A constant temperature water bath was utilized to elevate the vapor




pressure  of the liquid for those runs that required a higher concentration of plating




vapor in the gas stream mixture entering the reaction chamber. Higher temperatures




(between 200°F  - 500° F) were achieved  by using an electrical blanket around  the




flask,  connected through an autotransformer for temperature regulation. The liquid







                                      21

-------
 temperature  Inside the vaporizer was read with a mercury thermometer,  vertically





 suspended by the rubber stopper at the top of the flask.




        The carrier gas was admitted to the vaporizer by bubbling through the





 liquid, or alternately, by simply passing  it over the liquid surface.   The type  pro-




 cedure used was found to influence the evaporation rate significantly.   In the first




 case, the carrier gas becomes saturated with plating vapor, whereas in the second




 case  saturation does not necessarily occur.  Thus, the bubble-type vaporizer was




 found to give a constant concentration of plating vapor in the carrier gas over a




 range of flow rates whereas the non-bubbling type was largely uninfluenced by




 flow  rate except at higher flows and at temperatures near the boiling point of the




 liquid.  Figure 4 depicts the relationship  for tetraisopropyl titanate, one of the




 plating compounds used.




        Both  methods generally worked satisfactorily for the range of conditions used




 in this study.  However, high vaporization temperatures and high flow rates leading




 to excessive  plating vapor saturation in the gas stream oftentimes resulted in spray




 formation or misting conditions and subsequent coating defects on the substrate with




 either method. This situation was partially remedied by wrapping the downstream




 surfaces of the vaporizer and gas lines with electrical  heating tape and packing the




 interior of the flask with loosely fitting binder free glass cloth to lessen  the  possi-




bility of the vapor condensing and subsequently reevaporating on downstream sur-




faces. Such  measures however, were only marginally  successful  and as a result it




was concluded that more sophisticated vaporizer design concepts would be required
                                22

-------
  10.0
  5.0
X
UJ

1  1.0

5
V.

-------
 under such conditions.  Li (30) and Powell (11) have reviewed some advanced





 design concepts used in this field.




        The vaporizer equipment described thus allowed a given plating compound




 to be vaporized at a  given rate and mixed under controlled conditions with a




 selected carrier gas.  The ability to regulate the concentration of the plating




 vapor in the resultant gas stream is highly important, as previously mentioned.




 Appendix 1 contains  a sample calculation illustrating the method used to derive




 plating vapor concentration in the gas stream under a given set of conditions.




        After mixing, the effluent gas stream exited the vaporizer by forced con-




 vection and passed into a 1/4 inch stainless steel tube leading into the interior of




 the reaction chamber via  an O-ring  compression seal mounted on the entrance door.




 The stainless steel tube was water cooled along its length by a copper coil.  The




 entrance and exit water leads of the coil were  also extended through O-ring seals




and connected to a nearby water source and drain system.  The purpose of the water




 cooling was to protect the various vapor species from premature decomposition,




primarily at higher temperatures and lower flow rates.   In many cases, water cooling




was not required, and a 1/4 inch Pyrex glass inlet tube was substituted under those




conditions to promote cleanliness and lessen the possibility of side reactions.  The




length of the inlet tube was sufficiently long to allow its terminal end inside the




reaction chamber to be pre-adjusted laterally with  respect to the substrate position




in the firing zone.  This arrangement allowed a range of distances between the gas




outlet and the substrate to be selected for orientational studies.
                                 24

-------
        After reaction at the substrate, the spent gases were exited through the




rear end of the sealed combustion tube, bubbled  through 1/2 inch of water level




in a 1000 ml Pyrex flask and exhausted through a fume hood collection system




erected over the deposition apparatus.







B.  Other Design Considerations




        As an adjunct to the method of vaporization described above, a method of




vaporizing solid materials or liquids with very low vapor pressures was derived.




This procedure  involved the use of higher temperatures (500° F - 1800° F) supplied




by a 120 volt,  1650 watt nichrome wire-wound combustion tube furnace located




upstream from the  deposition furnace previously described and supplied with a




common mullite tube 57 inches in length extending through both furnaces.  The




smaller furnace was independently controlled with a 120 volt General  Radio auto-




transformer supplying power to the system as dictated by a Barber Coleman Model




472 Capacitrol  indicating time proportioning controller.  A chromel-alumel thermo-




couple vertically located in contact with the external surface of the combustion




tube at the center of the 12 inch  firing zone served as the sensing  element.




        For this system,  evaporation was carried out inside a 1 inch by 1 inch




mullite crucible which contained the substance to be evaporated.  The crucible was




placed into  the center of the firing zone, and a carrier gas passed over its surface.




The actual vaporization temperature was read with a chromel-alumel thermocouple




located 1/2 inch from the evaporating  surface.




        Intimate mixing of the gaseous  species was difficult to attain, and the




method was generally unsatisfactory.   However, the system has merit for high




                                     25

-------
temperatures and should qualify as a satisfactory method if procedures to better




direct the gas  flow for improved  mixing effects are installed.  The external arrange-





ment of the complete system is shown in Figure 5.




        Efforts to meter liquids dropwise into the gas tight chamber for evaporation





proved fruitless.  A titrimeter charged with the plating substance was installed




above the evaporation furnace and the liquid head produced drops of liquid which




entered the chamber by gravity,  being collected in a crucible under the influence




of controlled heat as described in the preceding paragraphs.  The rate of metering




did not  lend itself to close control by this method since  the head pressure of the




titrimeter constantly changed.  No coatings of value were obtained in this way.







C .  Input Material Considerations




        The compressed gases used in this study were prepared by Matheson Com-




pany.  Nitrogen was the  primary gas employed, with argon and helium used to a




lesser extent.  As in all chemical vapor deposition processes, the relative purity




level of the carrier gases largely  influences coating characterizations and reaction




kinetics.  The presence of water vapor in these gases was considered especially




deleterious because of the pronounced tendency of tetraisopropyl titanate to hydro-




lyze.  As can  be seen by a comparison of the molecular  weights, 284.2 for  tetra-




isopropyl titanate and  18.0 for water, only a small amount of water is required to




reduce a relatively large amount of the plating compound. All compressed gases




used were prepurified grade or higher with a purity level of greater than 99.99




percent.
                                 26

-------
I
                   Figure 5.   Photograph of dual furnace assembly

-------
        The plating compounds used in this study were selected for their ability to



 yield controlled coatings of desired composition  through the medium of chemical




 vapor deposition.  The physical mechanism consists of causing the vaporized plating




 compound to decompose or  pyrolyze at the heated substrate, leaving the nonvolatile




 components of the compound behind as a chemically bonded coating.  Obviously,




 a coating compound must possess certain characteristics to enable  it to be processed




 in this way.  Ideally, it should be a liquid at room temperature with a sufficiently




 high vapor pressure to obviate  the necessity for elevated vaporizer temperatures.




 Its purity content must be high to minimize the possibility of undesirable contami-




 nants collecting on the substrate and also to prevent the  possibility of side reactions




 neutralizing the plating vapor.  Lastly, the source material  must be chemically




 stable even though it is volatile, that is, it must not prematurely decompose, and




 it should contain only one nonvolatile component.  Two organometallic compounds




 which readily fit these criteria were selected for this  investigation.  They are




 tetraisopropyl titanate, Ti(C3H7O)4, known as TPT, and ethyl orthosilicate,




 Si(OC2H5)4, known as ETOS.   In addition, several runs were made using aluminum




 isopropoxide, Al (C3H7O)3, and ethyltriethoxysilane, (C2H5)Si(OC2H5)3.  These




 materials are all relatively  stable liquids with boiling points ranging from 322° F




 to 478° F.  All  of these  materials yield oxide coatings with  the associated metallic




 cation.  Their proposed reaction mechanisms are listed below, in the form of



chemical equations.




             Ti(C3H70)4        —» Ti02 +  2C3H6 + 2C3H7OH




             Si(OC2H5)4        	,>  SiO2 +  hydrocarbon gases




             (C2H5)Si(OC2H5)3	»  SiQ2 +  hydrocarbon gases




             2AI(C3H70)3      — * AI203 + 3C3H6 +  3C3H7OH




                                 28

-------
        The purity levels of the plating compounds used were the highest obtain-




able for stock items from the major chemical supply houses.  Generally, this is




specified to be greater than 90.0 percent pure.  The writer believes that future




work in this field should incorporate even higher purity levels if possible to further




preclude the possibility of side reactions and impurity deposits.  The chemical




complexity of these materials requires that they be stored under cool, dry conditions,




No evidence of deterioration with time was noticed.  Table I may be consulted for




chemical information pertaining to the plating compounds.




        Glass microscope slides were  employed in this study to serve as substrates




for all process evaluations.  The  1  inch by 1 3/4 inch microslides were composed of




Pyrex glass manufactured by W.H.  Curtin and Company.  These slides were care-




fully individually washed with warm, soapy water, rinsed, and dried prior to use.




        The soluble glass compositions were produced as 1 1/4 inch by 1/4 inch




discs.   Raw materials consisted of commercial grade sodium carbonate and silicon




dioxide weighed and  mixed in the proper proportions to yield the desired oxide ratio




after melting.  The range of compositions studied varied between 43-57 weight




percent SiC^.  Powders were intimately mixed with a V-blender mixer, poured  into




a mullite crucible and fired to 2500° F. The resultant fused mixture was removed




from the furnace after 30 minutes and with the aid of hand tongs was poured into a




steel die and pressed  into the desired shape. The pressed pellet was rapidly removed




from the die and transported to an annealing furnace at 800  F. After stabilization




at that temperature for thirty minutes the furnace was deenergized and slowly cooled
                                      29

-------
                                                  TABLE I
                                CHEMICAL PROPERTIES OF PLATING COMPOUNDS
co
o
     MATERIAL (REFERENCE)
FORMULA
MOLECULAR
  WEIGHT
SPECIFIC   MELTING
GRAVITY   POINT, °F
   BOILING POINT, UF
AT A VAPOR PRESSURE OF
60 mm.  400 mm.  760 mm.
Terra isopropyl titanate (25) TUCqHjO)^.*
Ethyl orthosilicate (37)
Ethyltriethoxysilane (37)
Aluminum isopropoxide (38)
Si(OC2H5)4**
(C2H5)Si(OC2H5)3*
A,(C2H70)3*
284.2
208.3
192.3
204.3
0.955 68
0.935 -121
0.889
1.058 223
300 410 450
333
188 282 322
478
     'Supplied by K & K Laboratories,  Inc.
     ^Supplied by Fisher Scientific Company

-------
The resultant stress-free product was subsequently removed from the furnace and


ground and lapped on conventional metallographic processing equipment using 80,


240, 400, and 600 grit silicon carbide particles.  Mechanical polishing was


accomplished with 1 micron alumina by Linde.  The  finished discs were visually


inspected at  80x under vertical illumination.  Samples exhibiting scratches, pits,


or other defects were repolished.  The finished product was then stored in a desic-


cator prior to use.



D. Deposition Techniques


       The reaction chamber was charged with glass substrates at room temperature


to prevent  stress cracking the samples.  Immediately after sealing the system, a 1.0


cfh flow of nitrogen sweeping gas was initiated to purge the system.  After a five


minute purge, the furnace power was turned on,  When the substrate temperature


reached the specified value as read on the monitoring thermocouple located 1/2 inch


away from  its surface, the compressed gas flow was adjusted to a specified level and


rerouted through the plating  compound vaporizer, thus introducing a reactant gas


mixture into  the deposition zone.  The gas mixture was allowed to flow through the


furnace for a  specified length of time determined by the thickness of the  coating


required.  The furnace was then turned off and purging was resumed until  room tem-


perature was reached and the sample  removed.

                                                                           ,>
       Two methods of holding the substrates in the  deposition chamber were devised


Those substrate?, subjected to the upper temperature limits  for glass were simply laid


on one face onto a 2 inch by 6 inch sheet of corrugated alumina.  The alumina


holder tended to protect the  substrate from a tendency to slump at the higher


                                     31

-------
 temperatures.  The major drawback to this method was that it required an additiona




 coating cycle for the other side of the substrate.




        Lower temperature coating operations were alternately carried out by sus-




 pending the substrate vertically with  the front face perpendicular to the gas stream




 The substrate was held by the hooked ends of a short length of wire which was spiral




 wrapped in the middle portion around the outside of the monitoring thermocouple




 insulator tubing for anchoring purposes.  In this manner, minimum surface contact




 with the substrate was maintained. This mode of orientation allowed some coating




 to be deposited on the back side of the sample at higher gas flow rates.  In  addi-




 tion, Hi is method of suspension allowed the substrate to be rotated during the coatinc




 operation, thereby improving  the overall coating uniformity to a high degree.  Rota-




 tion of the sample during a run was accomplished by exerting a torque on the moni-




 toring thermocouple insulator  tubing extending through the exit end of the reaction




 chamber.   The insulator tubing was linked to the heat reflector and substrate, and




 twisting it  through a given angle transformed the same rotation to these component




 inside the chamber.  The arrangement of the two substrate positions is shown sche-




 matically in Figure 6.




        It was found that several successive runs tended to produce such contami-




 nation products as non-adherent white powder particles on the interior surfaces of




 the chamber.  These were analyzed and found to be either TiO2 or SiC^, depending




 upon the plating material used.  Black soot (presumably carbon) and oily spots




(partially polymerized silane from ethyltriethoxysilane) were also observed.   These




substances were easily removed with a warm detergent solution, with the exception






                               32

-------
                   •ALUMINUM DOOR
                    MULLITE COMBUSTION TUBE
        TO POTENTIOMETER
           TO  EXHAUST	
           0-RING COMPRESSION SEAL
           GAS INLET  TUBE
                   HEAT REFLECTOR
THERMOCOUPLE  TIP
SUBSTRATE
CO
CO



•
B



ft
\
\
\
\
\
1
s
s
s
\
s
>
1 /^
' S/SSSSSS/SS/S S/SSSSS/S S S S S S / / / / / SAT S S
/
. ./.
^jj^2^SEB^n_
4
' / / SS / S ///////S////// //////// SIS /// //s
1
&\j\j*j i r\j"i i i_
s sssssssss


•

///////// V





B

yy xy y yy v^y yyv y y<

0
^
fe
3
s
s
s

J
1

In
%
CM



                                                ALUMINA SUBSTRATE HOLDER
     Figure 6.  Schematic of the substrate positions used.  The top view shows the substrate in a vertical position.
              The bottom view shows the substrate in a horizontal position.

-------
of the carbon, which was removed only under high temperature oxidizing condi-




tions.  Silica glass was also found inside the  tube after a series of runs.  It was




removed  periodically by  scrubbing with a weak solution of hydrofluoric acid.




These contaminants did not noticeably alter any results, although their effect on




coating defects was not definitely established.   Jordan (31) reported no deleterious




effect from silica contamination using a similar process for coating semiconductors,




where purity level is critical.







E.  Analytical Procedures




                                 1.  Microscopy




       The  coatings obtained were viewed under a vertically illuminated Leitz




Panphot me tall ©graphic microscope, and photographic work was accomplished on




this equipment with the aid of a Nikon photomicrographic attachment coupled to




a 35 millimeter (mm) Leica camera.  The photomicrographs yielded information pri-




marily related to the defect structure and orientation of the deposits.







                               2. Phase Analysis




       Chemical phase analyses of the TiO2  coatings were determined  with the aid




of X-Ray diffractometry and spectroscopy,  using Norelco instrumentation.  The




structure of  the deposit was determined by observing the presence or absence of




diffraction lines for a given material, using copper K-alpha radiation,  30 kilovolts




and 10 mil I lamps.  TiO2  deposits which failed to diffract X-Rays were declared to




be amorphous;  those deposits which exhibited characteristic diffraction  peaks were




crystalline.





                                 34

-------
       Crystalline SiO2 was determined by the same method outlined previously.




The silica glass was detected by X-Ray diffraction and infra-red spectroscopy.  The




broad, diffuse pattern revealed by the diffractometer was not considered to be very




definitive, so the  spectroscopy technique was primarily relied upon.  A Perkin-




Elmer 621 Grating  Infrared Spectrophotometer was utilized for the production of




these data.   It was necessary to use a substrate which was transparent to infrared




energy, so glass was replaced by a 10 mil wafer of semiconductor grade silicon,




which served as the base for the deposited coating.







                      3.  Coating Thickness and Uniformity




       The TiOo coatings provided an additional dimension to the analytical tech-




niques used because they were found to produce optical interference colors when




deposited on glass. The presence of these colors is related to the differential refrac




tive index between the substrate and the coating, as well as the coating thickness.




An excellent review of the subject is provided  by A. Vasicek (32).




       Generally, the color fringes may  be  interpreted in the same way one reads




a topographic map, except in this case the differential thickness between bands of




the same color equals one-half of the wavelength of light transmitted through the




film producing that color.  Figure 7 illustrates  the concept.  The target effect in




photograph (A) resulted from the plating vapor  contacting the substrate  in a concen-




trated stream rather than diffusing out to produce a more uniform color, as seen in




(B).  Both of these samples contained  roughly the same amount of coating  by weight.




However, the coating on the left varied progressively  in thickness across its surface,






                                     35

-------
to
a
                             (A)               (B)                           (C)                  (D)
     Figure 7.  Analysis of TiOo film thickness and uniformity shown by interference colors.  All films were derived by bubbling
               nitrogen gas through TPT at 212° F.  Exhibit (A):  Carrier gas flow 4 cfh; deposition temperature 575° F; substrate
               orientation 4 inches; time  15 minutes.  Exhibit (B):  Carrier gas flow 10 cfh; deposition temperature 765° F; sub-
               trate orientation  10 inches; time 30 minutes.  Exhibit (C):  Carrier gas flow 10 cfh; deposition temperature 765° F;
               substrate orientation 10 inches; time 5 minutes. Exhibit (D):  blank sample.

-------
whereas (B) was coated relatively evenly, resulting  in an average thickness of
approximately 3000 A. Photograph (C) depicts an extremely thin, uniform coating,
approximately 500 A  in thickness, which  may be contrasted with the uncoated
sample (D).  Whereas the thin films (less than 1000 A) were highly reflective, as
a silvered mirror, if crystalline,  or merely tinted if  amorphous, the thick films (over
50,000 A) were opaque and reflected little light.
       Thus, the TiO2 films were easily interpreted visually for uniformity and
thickness.  Such was  not the case for SiC>2 films which yielded no colors because
of a close similarity of refractive indices  between the coating and the substrate.
       Alternately, film thickness was measured by weighing the specimen before
and after coating on a Mettler grammatic balance.  The weight gain represented
the amount of coating deposited on the substrate, accurate to 0.0001 grams.   The
average coating thickness was computed from the following  formula:
                 w
        t =
           \d) (A) (10-8)
       where
       t  = coating thickness in Angstrom units
       w = weight gain of substrate in grams
       d  = density of coating in grams per cubic centimeter
       A = area of coating surface in square centimeters
       Use of the foregoing relationship yielded an average thickness for all exposed
surfaces coming  in contact with the plating vapor.  Thus, to be used with validity,
steps were taken to insure that all surfaces of the substrate received equal coating
                                   37

-------
 treatment by rotating the sample and reversing its orientation as required, or alter-




 nately to mask out all surfaces except the one being coated.  This last procedure




 was accomplished on microslides by pressing two faces together as in a stacking




 sequence, coating the composite, and separating the  two microslides after coating




 to reveal one uncoated surface for each  slide, as well as one coated surface.   Of




 course,  the weighing procedure is unable to distinguish between a diffusion coating




 and an overlay coating.  For this reason, its use is restricted to lower temperature




 coating  operations (below 1500  F).




        Table II lists the refractive indices and densities of all coatings and sub-




 strates used in this investigation.







                             4.  Coating Efficiency




        Chemical vapor deposition processes inherently produce low deposition




 efficiencies which  are consequently a major cost control factor, especially since the




 plating substances involved are oftentimes very expensive (13). Therefore,  the




 efficiencies yielded by this process were carefully studied.   The coating efficiency




 for a given run was derived by  comparing the weight gain of the substrate  during a




 coating run to the weight loss of the coating material  in the substance being vapor-




 ized and expressing the result as a percentage.  The amount of plating vapor in the




substance being vaporized is derived from a balanced chemical equation of the




reaction. Gravimetric measurements were obtained  on the Mettler balance des-




cribed previously.  The method of computing coating efficiency is illustrated in




Appendix II.




                                     38

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                              TABLE II
         OPTqCAL PROPERTIES OF COATINGS AND SUBSTRATES
                                     INDEX OF
                                     REFRACTION
                                                         DENSITY,
                                                         g/cm3
 I .  Coatings

   A.  Si O2 glass
B.
C.
            anatase
            amorphous
II. Substrates
   A.  Pyrex Microslides

   B.  !.ONa2O-1.3SiO2

   C.  1.3Na2O-l.OSiO2
1.43(22)

2.54(39)
                                   1.46 (7)

                                   1.51 (7)

                                   1.52(7)
2.20(22)

3.84(39)
                      2.23(7)

                      2.42 (7)

                      2.43(7)
                              39

-------
                              5.  Coating  Stresses

        As mentioned earlier, the fact that vapor deposits are applied at elevated


temperatures usually leads to residual stresses, primarily caused by thermal expan-


sion coefficient differences between the deposit and the substrate.  Powell (33) has


pointed out that stress analysis in chemical vapor deposition systems is complex

because thermal expansion stresses tend to mask out smaller stresses which may arise
                                                                                r
from other sources.  In addition,  elevated coating  temperatures sometimes allow
                                                                               /
stress relief to take place by diffusional and  other relaxational processes. The  use


of glassy materials introduces an additional dimension of complexity into any stress

analysis calculation,  because  their mechanical properties generally present anomalies

which are well known (7).  The strength  properties  of glass are largely controlled by

surface  conditions.


        Table  III contains the mechanical property constants of the various materials


used in  this study.  They may be used in  conjunction with the calculation of residual

stresses  between a coating and substrate.  Appendix III contains an explanation of

the procedure used to derive the residual stress.
                                   40

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                              TARE III
       MECHANICAL PROPERTIES Of COATINGS AND SUBSTRATES
MODULUS Of
ELASTICITY, E,
psi
                                            COEFFICIENT OF
                                            THERMAL EXPAN
                                            SION,*, /°F
                                                           TENSILE
                                                           STRENGTH,
                                                           psi
I. Coatings
A.
            Glass
   B.  TiOo Amorphous

   C.  TiO  Anatase
   D.  TiO2 Rutile

II. Substrates

   A.  Pyrex Microslides

   B.  1.ONa2O-1.3SiO2

   C.  1.3Na2O-l.OSiO2
 10 x 10* (40)      0.55 x 10~6 (40)     7250 (40)
                          41 x 1^(41)      4.f  x 10"6 (41)    7500 (41)
                          f x 106(7)

                          10 x 10^(7)

                          10 x 106(7)
                 3.2  x 10"°(7)      8900(7)

                 14.0  x 10"6(7)

                 14.0  x 10"6(7)
                              41
                                           U.S EPA Headquarters Library
                                           1200 Pennsylvaniavenue
                                                    ton, »X

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                                 CHAPTER V


                  RESULTS AND DISCUSSION OF RESULTS


A.  Effect of Process Variables Upon Reaction Kinetics

        The  coating process variables associated with a given vapor deposition

system are interrelated in a complex manner, and are uniquely dependent upon the

geometry of the unit, as  well as upon  the chemical characteristics of the coating,

materials and the substrate used.  It is therefore important to understand that

although the general relationships among the  process variables of a given system
                                                                         j
are valid for all systems, the translation of absolute data from one  system to another

is not.
                                                                       i
                                                                       I
        For this work, a series of TiO2 coating  runs were made over a range of con-

trolled conditions in order to study the effects of important variables upon deposi-

tion rates and coating efficiencies. These variables and their limits of investiga-

tion were as follows:

                    VARIABLE                             LIMITS

        1.  Substrate temperature, ° F                    450-1085° F
       2.  Coating time, minutes                        5-75 minutes
       3.  Carrier gas flow rate/ cfh                     2-10 cfh
       4.  Substrate orientation, inches*                 2-12 inches
       5.  Reactant gas  concentration in
           carrier gas (ppm) at a given flow
           rate                                         19-2300 ppm
*Substrate orientation may be defined as the lateral distance in inches between the
effluent end of the input gas tube and the front face of the substrate.
                                     42

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             coatings from tetraisopropyl  titanate were selected for this study




because of their ease of application to glass substrates over a wide range of con-




ditions. By contrast,  SiO2 coatings from  ethyl orthosilicate are energetically




feasible only within a narrow range of temperatures immediately below the softening




point of the substrate.







                            1.  Substrate  Temperature




        Figure  8 illustrates the strong  influence that the temperature of the substrate




exerts over the deposition rate and coating efficiency.  A linear relationship was




found  to exist for the range of conditions investigated.  It is especially interesting




to observe that for  the temperature range shown the efficiency can be increased by




a factor of four merely by operating at the upper temperature extreme rather than




the lower.  It is anticipated that the curve shown would begin to deviate from




linearity at higher  temperatures with an accompanying decrease in plating efficiency




because of premature decomposition of the vapor species inside the  reaction chamber.







                 2. Coating Time at Different Temperatures




        Figure 9 depicts  the interesting effect exhibited by coating  time upon depo-




sition rates at different temperatures.  The coating grown at the lower temperature




was characterized by an initial period of slow growth, known as an induction period,




and a subsequently  increased rate of steady-state growth thereafter.  This dual growth




rate effect was absent  in the higher temperature coating which grew at a faster  rate,




as seen by the slopes of the two curves.  The overall coating efficiency for the  low




temperature coating was  severely damaged by the  presence of the induction period.
                                  43

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  12
 X
 LU
 I-
LU
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5

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 CM
O
                                                                        7-
                                                                                                  6-
                                                                     >  5
                                                                     o
                                                                     2
                                                                     UJ

                                                                     O
                                                                     iZ  4.
                                                                     u_
                                                                     LU
                                                                     2  3-
                                                                                              o
                                                                                              o
                                                                                                  2-
                                                                        I-
   400
600                    800
      DEPOSITION  TEMPERATURE (°F)
1000
1200
    Figure 8.  Plot of TiC^ deposition rates and coating efficiencies at different deposition temperatures.  The data

              was derived by bubbling nitrogen gas through TPT at 10 cfh; vaporization temperature 70° F; substrate

              orientation 4 inches; time 60 minutes.

-------
tn
                                              30
                                             COATING
                                            45
                                          (IVItNUTES)
60
75
          Figur© 9.
Plot of TiO2 deposition rates at two different deposition fr©mperatur©s for varying coating time p©nods.
The data was derived by bubbling nitrogen gas through TPT at 10 cfn; vaporization temperature 70° F;
substrate orientation 4 inches.

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              3.  Carrier Gas Flow Rate and Substrate Orientation




        The carrier gas flow rate plays an important role in coating reaction kinetics,




as shown by Figure  10.  The relationship is especially important when considered as




a function  of the  substrate orientation.  The plot shown illustrates the effect of two




different flow rates upon the coating efficiency at different substrate orientations.




Coating reactions at a given gas flow rate were generally enhanced by decreasing




the mean average time of residence of the reactant vapors in the deposition zone




prior to contacting the substrate.  This of course, was readily accomplished by




decreasing  the substrate orientation.  Curiously, however, this relation did not




hold at extremely short distances, presumably because the increased cooling effect




of the  impinging gases on the substrate was sufficient to retard the reaction.




       A change in gas flow promoted marked changes in the reaction rate—sub-




strate orientation profile, as shown.   Reaction rates under low gas flow conditions




were highly sensitive to the substrate orientation.  To illustrate,  for the data shown,




the coating deposition rate and efficiency at two cfh varied from a maximum at two




inches to no reaction at all at six inches, whereas these parameters did not  change




appreciably in this range at the higher flow rate, although the maximum attainable




reaction rate at the higher flow was less than one-half that which could be  developed




by the lower gas flow condition.







                       4. Reactant Gas Concentration




       TiC>2 coatings were readily obtained over a range of reactant gas concen-




trations in the carrier gas stream at total  flow rates ranging from two to ten  cfh







                                     46

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                            SUBSTRATE ORIENTATION  (INCHES)
Figure 10.  Plot of TiO2 coating efficiencies at different substrate orientation and carrier
           data was derived by bubbling nitrogen gas through TPT; vaporization
           temperature 620° F; time 5 minutes.
—•« flow rates

  212° F;

-------
 The  deposition rate was found to vary linearly with concentration at a given total




 fVow rate, as shown by Figure 11.  The coating  efficiency for these runs was gen-




 ertilly the same.  The quality ef the coatings  produced at the different concentra-




 tion $ was significantly affected, however.  At high concentrations, the adherence




 was i x>or and the coatings examined shewed split and cracked areas.  These samples




 were t requently covered with a  loosely -adherent brownish colored powder which




wai" fou  nd to be a mixture of TIO2 and carbon.  Many ©f the coatings produced




exhibitec  ^ a dirty cast, indicative of carbon inclusions in the deposit.  None of




these advt *rse conditions were found in etepesits grown under similar conditions at




lower reaci ;°nt 9Qs concentrations.
B.  Coating . Stresses




       The cc Bating stresses developed by SiC>2 coatings applied to either Pyrex




microslides or &  oluble glass substrates were compressive in nature,  and very pro-




nounced, as wou 'd generally be expected from an inspection of the physical con-




stants involved  in  the stress calculation as well as the  high deposition temperatures




employed.  An exeu "nple of a highly stressed SiO2 coating is shown in Figure 12.




In this case,  the coating was applied t© a Pyrex microslide at 1220° F.  During the




initial stages of  the  cooling cycle sufficient differential stress developed to warp




the four corners  of t he substrate while #ie glass was plastic enough to flow.  Further




cooling with the gla: ss in a warped, rigid condiHon subjected the coating to a com-




bined compressive shisar stress which caused  it to craze.




        Depending u pon conditions, several  i>iO'2 coated soluble glass substrates




cracked in tension u nder the  influence of the residual  stress developed during the





                                     48

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10
                                 350
                                    1100
1655"
           Figure 11
                REACTANT  GAS  CONCENTRATION (PPM)
Plot ef TiC>2 deposition rotes at different reactant vapor concentrations.  The data was derived by
mixing the carrier gas with TPT under both bubbling and non-bubbling conditions and the vaporization
temperatures regulated between 70° F - 212° F; carrier gps flow rate 8 cfh; deposition temperature
780° F; substrate orientation 4 inches; time 15 minutes.

-------
rn
o
           Figure 12.  Photograph of high residual stress in SiO2 deposit.  The coating was derived from ethyl orthosilicate

                       vaporized into nitrogen gas at 1  cfh; vaporization temperature 70° F; deposition temperature 1220° F;

                       substrate orientation 3 inches; time 14 hours.

-------
cooling cycle, and others were sufficiently weakened to allow manual breakage by


application of a torque to the stressed sample.


        Despite these obvious drawbacks, satisfactory coatings were developed by


resorting to low-temperature slow growth techniques.  In this case, temperatures

                   o
did not exceed 1050  F, and the resultant product demonstrated adequate mechan-


ical properties.


        TiO2 coatings were found to develop smaller compressive stresses on soluble


glass substrates, because the temperature of deposition was generally much  lower


and the thermal expansion coefficients were  more closely matched.  Coatings


applied to borosilicate  microslides actually developed a small  tensile stress. Yet


even under these much  improved conditions, TiC>2 coatings could be made to pro-


duce a definite crack pattern, provided  the coating conditions were not properly


controlled. Figure 13 illustrates two TiO2 coatings produced at 780°  F and a


reactant gas concentration of 2150 ppm  in the gas stream flowing at a  rate of eight


cfh.  In this case, the concentration of  the  reactant vapor distributed within the


carrier gas was very high  and non-uniform.   In fact,  small droplets of  tetraisopropyl


titanate actually contacted the substrate prior to decomposing, and it  is believed


that this factor was primarily the reason  for the failure of the coating.  Figure  13(B)


shows the mode of failure to be a tensile stress in the coating.


        In general, edges along the  substrate surfaces developed stress concentra-


tions, and coating could be made to flake off along these points under relatively


low abrasive shear stresses.   This was especially noticeable for the  thicker films of


SiO2 (over 75,000 A) and did not appear prevalent on the thinner coated specimens,



                                  51

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 '
' I
                                        (A)                                                       (B)
              Figure 13.  Photomicrographs of residual stresses in a TiC^ deposit.  Exhibit (A):  thin coating,  Exhibit (B): thick
                         coating.  These coatings were derived by bubbling nitrogen gas through TPT at 8 cfh; vaporization
                         temperature 212° F; deposition temperature 780° F; substrate orientation 4 inches; time  15 minutes.
                         One inch on photomicrographs equals 88.9 microns.

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C.  Coating  Uniformity




        Coating uniformity was found to depend chiefly upon the ability of the




system to deliver a constant supply of plating vapor to all surfaces of the substrate




under uniform temperature conditions. Generally, the substrate temperature was




well balanced for all conditions investigated, with the design previously described.




        A uniform dispersal of the plating vapor to all  surfaces of the substrate was




more difficult to attain, especially for the TiO2 coatings.  The  deposit tended to




build up primarily on the front face of the vertically-positioned substrate with a




characteristic pattern, most notably the concentric circle effect referred to earlier,




which  was a  direct consequence of the non-turbulent flow characteristics of the




system.  Geometric limitations precluded Hie possibility of practically achieving




turbulent flow within the tube.  As measured by the Reynolds number, NRC/ the




criteria for turbulent gas flow within the system would  have required a cost prohi-




bitive  carrier gas flow rate of almost 200  cfh.  In addition, the  cooling effect from




this amount of gas flow would probably have rendered  temperature control inside the




tube more difficult to attain.




        For the above reasons, the control of coating uniformity was achieved in




other ways.  Periodic rotation of the substrate within the coating chamber during a




run was found to even out the deposition pattern of TiO2 coatings considerably.  In




addition, the act of increasing the substrate orientational distance relative to the




effluent end  of the input gas tube while simultaneously increasing the carrier gas




flow rate improved the  TiOj coating uniformity to a high  degree by allowing the




gases to disperse and more intimately mix prior to contacting the substrate. Although






                                  53

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this procedure had a detrimental effect on coating efficiency (see Figure  10) the




uniformity for thin coatings was greatly enhanced (see Figure 7).




        The influence of generally higher temperatures for the SiC>2 coatings




tended  to more effectively disperse the plating vapor prior to contacting the sub-




strate,  through the medium of thermal energy. As a consequence, relatively uni-




form SiO2 overlay coatings were developed without resorting to the physical and




mechanical dispersion methods described for TiC^-  Figure  14 illustrates the depo-




sition pattern for a thick film of SiO2 deposited onto a thin wafer of semiconductor




grade silicon.  The coating is approximately 40,000 A in thickness and is interpreted




through the medium of interference colors to be very uniform in the central portions




of the wafer with a gradual  tapering off in thickness around the edges.  Thinner




coatings would probably be even more uniform.







D.  Substrate Considerations
        The important role played by the substrate in vapor deposition operations




has been well documented in the literature.  In addition to its already discussed




influence on residual stresses, the  physical and chemical properties of a given sub-




strate are capable of influencing the nucleation kinetics of a given system (34),




the structure and properties of the  deposit formed (35 and 36) and the  bonding




strength of the deposit (11).




       These  important effects are believed to be operative  in the system discussed




in this paper.  Furthermore, the use of soluble silicate substrates for vapor deposited




coatings represented the addition of another major variable  to be considered.  These






                                     54

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en
en

                                                                                                             • .
                                          (A)    ^          ^                          (B)
           Figure 14.  Photograph of a representative deposition pattern for SiC>2 deposits.  Exhibit  (A):  coated silicon wafer
                      Exhibit (B):  blank sample.  The deposit was derived by bubbling nitrogen through ethyl orthosilicate;
                      carrier gas flow rate 2 cfh; vaporization  temperature 70° F; deposition temperature  1350° F; substrate
                       orientation 3 inches; time  12 hours.

-------
materials, which have a profound affinity for moisture, must be handled and pro~




cessed under special conditions if controlled coatings are to be obtained on their




surfaces.  No reports were found of previous vapor deposition work dealing with




this class of materials.




        One may readily gain an insight into the dynamic solution kinetics of a




typical composition used in this study by referring to the sequential photomicro-




graphs of Figure 15, which illustrate three stages of corrosion of a sodium  silicate




composition containing 43 percent SiC>2 by weight which was attacked by atmos-




pheric moisture under controlled room conditions of 70 percent relative humidity




and 70° F.  The samples shown from left to right were photographed (a) immediately




after removal from the desiccator, (b) 30 minutes later, and (c) 1 1/2 hours later.




        The identification of the mechanism of corrosion, and the correlation of




corrosion rates with induced physical and chemical changes are currently being




studied and will be reported in a  separate paper.  For the purposes of this work,




the accumulated residue shown on the exposed glass surface is believed to be a




precipitated alkaline compound which formed on the surface as a non-adherent solid




after reaction with either the water or carbon dioxide contained in the atmosphere.




        The presence of this residue during  a coating run generally resulted in




poorly bonded deposits which cracked or spoiled during the  cooling cycle, pre-




sumably because the crystalline contaminant effectively blocked the ability of the




plating vapor to form a continuous, chemically bonded film along  the glass surface.




        Furthermore, the residue usually promoted coating structures which micro-




scopically appeared to exhibit more variability or defect areas than those produced







                                  56

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                                                                                                 m


                   (A)                                     (B)                                       (C)
Figure 15.  Dissolution kinetics of sodium silicate glass by atmospheric attack with time.  Exhibit (A): fresh surface.
            Exhibit (B):  30 minutes later.  Exhibit (C): 90 minutes later.  One inch on photographs equals 107
            microns.

-------
on clean surfaces.  It is uncertain whether these effects are related to heterogeneous




nucleation sites or other effects.




        It was determined  that improved coatings were attained on ground and




polished substrates.  Those substrates which were not ground and polished generally




exhibited some warpage and the surfaces were microscopically undulating.  These




conditions generally produced non-continuous, variable coatings.  Removal of all




microscopic contours from the glass surface by grinding to produce a generally flat




surface was  found to be more important than polishing  the surface to a high reflec-




tance level, in terms of tfie coating quality achieved.  These effects may be




explained by considering that the residual shear stress  components of the film and




the glass were decreased by a flat surface. Furthermore,  it may be assumed that




all  mechanically worked surfaces, regardless of the degree of polish, contain some




roughness; slightly microscopically  roughened surfaces would probably promote




coating adhesion.






E.  Characterization of SiC>2 Coatings




        The  highest quality SiC>2 glass films were obtained at deposition temperatures




between 1000° F and 1050° F, yielding an average coating growth  rate of approxi-




mately 4000 A per hour.  Higher deposition temperatures created higher residual




stresses in the deposits, and lower temperatures were found to result in excessively




slow reaction rates and films which were  more clouded in appearance. The films




were grown  by bubbling nitrogen gas at two cfh through ethyl  orthosilicate con-




tained in a flask at room temperature.  The substrate was positioned upon one face




in the center of the deposition zone approximately three inches from the effluent





                                 58

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end of the input gas tube. The concentration of the reactant gases in the vapor




mixture approximated 800 ppm under these conditions, and the efficiency averaged




2 percent.




        Figure  16 shows the relationship between deposition temperature and the




rate of reaction for SiO2 coatings.  It is noted that the reaction rate reached a




maximum at 1150  F and decreased at higher temperatures because of premature




decomposition of the plating vapor prior to contacting the substrate.




        Various analytical schemes were employed to classify the films obtained.




Visual inspection revealed the appearance of a coating which was clear and glassy




in nature. This observation was reinforced by  microscopic examination, which also




brought out the topographical details as seen in Figure 17.




        The composition of the film was checked  by infrared absorption.  The spectra




showed a strong absorption band at 9.2 microns and lesser bands at 12.5 and 21.9




microns, thus corresponding with the known bands for silicon dioxide glass (42).




No evidence of non-stoichiometric silicon oxide could be  found.




        X-Ray diffractometer scans showed a broad, diffuse band at 4.3 A, thus




indicating the presence of SiO2 glass (43).  Some of the higher temperature coatings




were found to have a slightly  crystalline nature.   This was generally more prevalent




on the bottom face of the substrate where slower deposition rates, higher temperatures,




and impurity nucleating sites probably occurred at the interface between the substrate




and the substrate holder.  Under these conditions, the presence of cristobalite was




confirmed by X-Ray  analysis.   The crystalline material was deposited with  a  high




degree of preferred orientation, as observed  by the fact that diffraction occurred for





                                      59

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      o

      X
      UJ
      I-

      z

      2
en
o
     o
     Q.
     LJ
     O


     o1

     
         900                   1000                   iioo                     saoo                1300

                                      DEPOSITION TEMPERATURE  (*F)

         Figure 16. Plot of SiC>2 deposition rates at different deposition temperatures.  The data was derived by bubbling

                   nitrogen gas through ethyl orthosiiicate at 1 cfh; vaporization temperature 70° F; substrate orientation

                   3 inches; time 14 hours.

-------
Figure 17.   Photomicrograph of surface topography of a typical SiO2 deposit.
            The coating was derived by bubbling nitrogen gas through ethyl
            orthosilicate at 2 cfh; vaporization temperature 70° F; deposition
            temperature 1135° F; substrate orientation 3 inches; time 17 hours.
            One inch on photomicrograph equals 53.3 microns.
                               61

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 only one of the major intensity peaks for that material—at an interplanar spacing




 of 4.15 A.  Table IV lists the important diffraction peaks and their intensities for




 this substance.  Optical examination showed the crystals  to be large in size,




 approximately  100 microns in diameter.  Figure  18 depicts several cristobalite




 crystals which have grown and coalesced together.  No crystals of comparable size




 were found on the top face of the substrate -




        Figure  19 reveals the curious appearance of a columnar growth habit which




 was found to typify fractured samples, as well as the presence of an interlocking




 (mechanical) bond at the interface on roughened substrate surfaces.  Based on the




 excellent adherence of  SiO2 coatings on smooth as well as rough surfaces, it is




 believed that the basic  source of bond strength is derived from a chemical bond.




        The  SiO2 as shown by Table V, exhibited a high degree of chemical inert-




 ness.   In order  to measure this property, films were microscopically observed before




 and after 30 minute soaks in 50 percent concentrated sulfuric, nitric, hydrochloric,




 and acetic acids, as well as in sodium hydroxide.  No chemical deterioration was




 observed in these media. Only hydrofluoric acid was found to degrade the film,




 and it  could be removed very rapidly by exposure to this material.







 F.  Characterization of  TiO2 Coatings




       Whereas optimized S?O2 coatings were obtained under a somewhat limited




 range of conditions, such was not the case for TiO2 coatings, as previously discussed.




 Furthermore, it was determined that TiO2 could  generally be deposited in either




 crystalline or amorphous form at  faster rates and  with lower residual stresses developed




at the substrate interface than SiO2-  Consequently, it was determined at the outset





                                 62

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                                                 TABLE IV
                             X-RAY DIFFRACTION DATA FOR COATINGS OBTAINED (44)
CO
     CRYSTALLINE
       MATERIAL
MINERALOGICAL
     NAME
INTERPLANAR SPACING AND RELATIVE
INTENSITIES OF THREE STRONGEST LINES
SPACING, (A)      RELATIVE INTENSITY
H ANNA WALT
CARD FILE
INDEX NUMBER
SiO2


TiO2


Ti203


Cristobal ite


Anatase


Titanium
Sesquioxide

4.15
2.53
1.64
3.51
1.89
2.38
1.68
0.90
1.85
100
80
60
100
33
22
100
100
70
4-0359


4-0477


2-1359



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Figure 18.   Photomicrograph of cristobalite crystals.  The coating was derived
            by bubbling nitrogen gas through ethyl orthosilicate at 2 cfh;
            vaporization temperature 70° F; deposition temperature 1220° F;
            substrate orientation  3 inches; time  11  hours.  One inch on
            photomicrograph equals 53.3 microns.
                               64

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                                                                      \
Figure 19.   Photomicrograph of SiC>2 coating—substrate interface showing
            columnar growth habit of SiC^-  The substrate is oriented below
            the coating.  The coating was derived by bubbling nitrogen gas
            through ethyl orthosilicate at 1 cfh; vaporization temperature
            70  F; deposition temperature  1050  F; substrate orientation 3
            inches; time 48 hours.  One inch on photomicrograph equals
            53.3 microns.
                         65

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                 TABLE V
CHEMICAL DURABILITY OF COATINGS OBTAINED
COATING TESTED CORRODING AGENT
SiO2 glass H2SO4
HNO3
HC1
HC2H3O2
HF
NaOH
TiO2 amorphous H2SO4
HNO3
HC1
HC2H3O2
HF
NaOH
TiO2 anatase H2SO4
HNO3
HC1
HC2H302
HF
NaOH
EFFECT
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
                66

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of this work that TiO2 should make a good candidate for coating soluble glass,




if not alone then certainly as a prime coat for a composite coating composed of,




for example, TiC>2 and SiO2.  In the latter case, it is believed that the TiO2




would buffer the stress distribution over two interfaces rather than one, and would




also probably enhance the overall protective characteristics of the overlay barrier.




Corollary studies in these areas are currently in progress and will be discussed in




a subsequent paper.




        For applications as either single or composite coatings, the use of extremely




thin, uniform,  amorphous TiO2 films would be desired because they would be least




objectionable from an optical standpoint.  Because of the wide divergence in




refractive indices between substrate and coating,  interference colors become




prominent as the thickness increases over 1000 A. An amorphous, uniform film




in this thickness range would reduce the overall light transmission through the




piece, which would appear tinted to the eye.  Crystalline films, however, would




tend to reflect the light by scattering, thus decreasing the light transmission to an




even greater extent.  Unfortunately, no method was devised to retain the amorphous




structure of TiO2 prime coats during the subsequent SiO2 deposition at higher




temperatures, and the amorphous phase was ruled out except for external coating




applications.



        The structure of the deposits obtained over a range of conditions was studied




by X-Ray diffraction. The structure of the TiO2 deposits was found to depend pri-




marily upon deposition temperature and to a lesser extent upon the mass flow rate of







                                      67

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the reactant vapor.  Figure 20 illustrates the relationship between these two




structure determining variables as derived from a systematic investigation of




approximately one hundred samples.  The boundary line,  however, is shown




hatched because its accuracy is believed to be limited to plus or minus 25° F.




The transition between amorphous and crystalline deposits was found to occur at




620° F plus or minus 25° F for the range of tetraisopropyl titanate mass flow rates




investigated.  Coatings formed below the transition line were amorphous and those




above were crystalline.  Curiously, the transition temperature was found to decrease




at extremely low mass flow rates, and several samples analyzed  in that range were




found to contain various oxygen deficient modifications of TiO2/ principally




      *  Crystalline deposits derived from higher mass flow rates were all stoichio-




metric.  Table IV contains the essential X-Ray diffraction data  for the crystalline




forms of titanium oxide  found in this study.




       The stoichiometric crystalline deposits consisted of preferentially oriented




anatase.  This fact was  established by examining the  location and intensities of




the three major diffraction peaks for that material and comparing the data to that




produced during an actual X-Ray scan. Two of the three lines, at interplanar




spacings  of 3.51 A and  1.89 A could always be identified; the line at 2.38 A was




always characteristically absent, thus indicating preferred orientation of the




deposit.




       High quality, uniform thin films of TiCX> were readily produced by bubbling




nitrogen  gas at ten cfh through tetraisopropyl titanate maintained in a temperature-




controlled  flask at 212  F.  The substrate was vertically positioned inside the center





                                  68

-------
  lOOOr
  800-
u_
o

uj600
cr
or
UJ
a.
5
UJ
  400
  200
                                      CRYSTALLINE
                                      AMORPHOUS
                           I                    2                    3

                           TPT  FLOW  RATE (MOLES / MINUTE X IO"4)
       Figure 20. Plot of TJO2 structures at different temperature and reactant vapor mass flow rates

-------
of the deposition zone with its front face perpendicular to the impinging gas stream




issuing from the end of the input tube ten inches away.  Although the coating




efficiencies were low (less than 3 percent), the uniformity of the coatings was




excellent and the associated film properties such as adherence and chemical




inertness were quite good between 500° F - 800° F. The conditions described allowed




amorphous coatings to be formed at the lower temperatures at growth rates of approxi-




mately 4000 A per hour, and crystalline films were produced at the higher temperatures




at growth rates of approximately 6000 A per hour.




       The overall microstructural uniformity of the crystalline deposits was very




sensitive to the coating conditions used.  Many of the crystalline samples produced




were found to contain obvious  variations when viewed under the microscope.




Several of the most extreme conditions  have previously been mentioned in con-




junction  with coatings that cracked or spoiled when subjected to a  concentrated,




poorly mixed vapor.  More typically, the variations observed were much more




subtle, as shown in Figure 21,  which depicts two regions of the same sample.




Exhibit (A) was taken from an area which received the direct impact of a poorly




diffused, reactant  vapor-rich gas stream resulting in many small grains.  Exhibit




(B) was located on the periphery of this impinging stream and consequently received




less cooling effect as well as a more dilute dosage of reactant vapor.  It is charac-




terized by a lesser number of larger grains.   The effects to be expected from a




consideration of the principles of nucleation and grain growth are exactly as




represented by the two microstructures shown.
                                    70

-------
      cz
  o
       m
                    ••••/ ,    • -:•'.'•'  . SiSjC?^^ i  1
                       •'--.  ;: /.  9   :  c^O^M-!^S
                                 •;•.• •• ',.;„',;  r •  :   - '. '• ,  •?'•

                     •' v.  * i •.     i' • K#<  •• :-,  . " •.'•-'*.'> '}•', {. -. •
                    '*^- ^. ', vl- '- *'"'" W* '^ i\ * -'''•"•' V -'- ' ••'>%' "'V*'. •'•-''' ••"'"'
  <  O ™
0^09^
13 ro  O- CX
                   V^tS  ^« * s A^a^rS *—*•' * *  v - ** ^- 4
^^;V\''*^.l3^^  S   ^  .>.'.. "^-Yw
**?*   - * »'''.' ft•J*W*l'a?%4": ',i,.,'3.':-i«-'%.ivJi I  '  • •        " '~f..."'_.
                                                                                             .,.
                                                                                             '-   -
                                                                                                        »«^i:-lwi*l', i^ef j

                                                                                                                   •*,*< p -.


                i^'^l'jTT'*/1.**-,*1.S->V i-> • *( *.•
                *^iiS£vw 2*K ^ *£*&£%
                   •*4&/at «fc ^%" v /  ' • ^i'1^"
o?
       cr
        -
                                           (A)
             Figure 21 .   typical anatase microstructures taken from different areas of same sample.  Exhibit (A)  received a more
                         concentrated dosage  of reactant vapor than Exhibit (B),  but was also cooled more by the impinging gas
                         stream.  The photomicrographs were derived by bubbling nitrogen gas through TPT at 10 cfh; vaporization
                         temperature 70° F; deposition temperature 690° F; substrate orientation 4 inches; time 60 minutes.  One
                         inch on photomicrograph equals 88.9 microns.

-------
       Crystalline coatings which were slowly grown were sometimes found to




contain one and two dimensional growth features incorporated into the deposit.




Figure 22 shows two examples of dendritic growth.




       A general characteristic of the  TiC>2 coatings was their smooth, continuous




appearance.  This was especially true for the amorphous films, as shown by Figure




23.  This photomicrograph shows a cracked film with some portions stripped away




to reveal the substrate.  It was selected to portray the effects denoted above, which




are readily brought out because of the visual contrast.




       Figure 24 shows an interesting view of a duplex coating.   In this instance,




two 15,000 A layers of TiO2 were deposited around a 45,000 A layer of SiC>2.




The photomicrograph indicates that both coatings were applied evenly and




continuously.




       The chemical durability of the TiC>2  films was observed and generally found




to be  excellent. The same reagents employed for measuring the chemical inertness




of the SiC>2 coatings were used for both amorphous and crystalline TiO2«  The results




are shown in  Table V.







G.  Other Coatings




       Several runs were made with ethyltriethoxysilane as the source material




for the SiC>2  coatings.  The coatings were applied in a similar manner to the SiC>2




films derived from ethyl orthosilicate, except that the optimum temperature of




deposition was slightly higher, around 1200° F - 1250° F.  Oily traces of partially
                                  72

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                                                                                                  • •*, f





                                                                                                '"•':;V t'  #'''* "^
                                                                                                    "
                                                                                             .


                                (A)                      ^                               (B)
Figure 22.  Photomicrographs of secondary growth features in  TiC>2 deposits.  Exhibit  (A): dendrites.  Exhibit (B):  crosses
            These coatings were derived by bubbling nitrogen gas through TPT at 10 cfh; vaporization temperature 70° F;
            deposition temperature  976° F; substrate orientation 4 inches; time 60 minutes. One inch on photomicrograph
            equals 88.9 microns.

-------
Figure 23.   Photomicrograph of surface topography of amorphous TiC>2 film.
            Coating was derived from nitrogen gas bubbled through TPT at
            10 cfh; vaporization temperature 212° F; deposition temperature
            620° F; substrate orientation 4 inches; time 5 minutes.  One inch
            on photomicrograph equals  53.3 microns.
                             74

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 I
en
             TiOo  —
             SiO2 —
TiO2 —
          Substrate	
      Figure 24.  Photomicrograph of a duplex coating.  Structure is composed of two 15,000 A layers of TiOo with 45,000 A
                  layer of SiO2 sandwiched between.  The TiC>2 coatings were derived by bubbling helium gas through TPT at
                  9 cfh; vaporization temperature 70° F; deposition temperature 700° F; substrate orientation 4 inches; Hme 3
                  hours.   The SiO2  coating was  derived by bubbling helium gas through ethyl orthosilicate at 4 cfh; vapori-
                  zation temperature 70° F; deposition temperature 1100° F; substrate orientation 3 inches; time  12 hours.  One
                  inch on photomicrograph equals 53.3 microns.

-------
decomposed si lane could be found inside the combustion tube for lower deposition




temperatures.  No differences were observed in the SiC>2 films produced from the




two different materials.  Because of this factor and the significantly higher unit




cost of the silane, it was not explored further.




       Aluminum oxide was deposited from aluminum isopropoxide vapors at depo-




sition temperatures between 550° F -  700° F. The deposits obtained were appar-




ently amorphous A1203.  The isopropoxide was not easily vaporized in the  flask




that was used for the other plating agents, primarily because of the fact that the




material  required heating to 223° F in order to melt it.  The coating process used




resembled the technique employed for coating substrates with TiC^. The deposi-




tion rates were slower, however. A^Og low deposition temperature coatings




offer some potential, and should be investigated further.







H.  Initial Corrosion Studies
        Since the various films produced on Pyrex micros! ides were found to be




highly resistant to boiling water as well as other corrosive media, the major




remaining question concerning the utility  of the films revolves around their ability




to form a continuous, impenetrable  barrier between the soluble glass and the




corrodent.  A thorough analysis of this important subject is currently in progress




and will be reported on at a later date.  Nevertheless, several tentative obser-




vations may be made at this point, based on experimental runs made during this study.




        Uncoated soluble glass discs were  readily dissolved in tap water between




room temperature and the boiling point of water. The rate of dissolution was found
                                 76

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to be highly sensitive to the amount of alkali in the glass, the temperature of the




water, and the amount of agitation.  Typically, 57 percent  sodium oxide and




43 percent silicon dioxide compositions were completely dissolved in less than 15




minutes by agitated cold tap water, whereas the most resistant composition con-




taining 43 percent Na2O typically required over 90 minutes to accomplish the




same task.  However, water at 140  F was found to completely dissolve the two




compositions in less than five minutes, and 30 minutes,  respectively.




        The application of both SiO2 and  TiC>2 coatings to these materials was




found to increase the durability substantially.  Generally, however, there appeared




to be a very wide variation in the  protective quality of  both types of films.  Some,




in fact deteriorated almost as rapidly as if they were uncoated while others demon-




strated an ability to withstand the  ravages of 140° F water for several  hours without




deteriorating.




        It is thought that any break or discontinuity in the film structure allows




moisture to attack the interfacial bond between the substrate and  the coating at a




localized point.  Once  initiated,  the process spreads rapidly across the entire




surface, destroying the protective  film.
                                      77

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                                CHAPTER VI







                              CONCLUSIONS







1.   The coating apparatus designed and constructed for this investigation is a




     sound laboratory tool, capable of producing many types of chemically depos-




     ited coatings over a range of controlled conditions.  Its utility is not restricted




     to this particular system; rather,  it has a broad capability for research work in




     many areas of this technology, such as diffusion masking for semiconductors,




     and others.




2.   Soluble silicate glass compositions are readily coated by chemical vapor depo-




     sition processes. The surface condition of the glass must be closely controlled




     in order for satisfactory coatings  to result.




3.   Metallic oxides derived from selected organic esters  provide excellent coating




     materials  for soluble glass substrates.  The high vapor pressure and chemical




     stability of the mother liquid readily allows coatings to be produced by pyrol-




     ysis reactions at the heated substrate surface.




4.   Titanium oxide coatings are readily  produced over a  wide range of controlled




     conditions such  as deposition temperature, time, carrier gas flow  rate,




     reactant vapor concentration, substrate orientation,  and others.  The coatings




     produced  can be amorphous or crystalline, depending upon conditions.  These




     coatings are chemically stable, and form c chemical  bond with the glass




     surface.
                                78

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5.   Silicon oxide glass coatings are produced under somewhat more restricted


     conditions; however, the coatings formed likewise possess excellent properties.


     The problem of residual stress in the coated sample is a greater hazard with


     SiC>2  coatings because of generally higher deposition temperatures  and a


     greater mismatch of  thermal expansion coefficients.


6.   The high residual stress level  contributes to microscopic defects in the finished


     coatings. Generally uncontrollable coating conditions also promote defects.


     These are typified by variable vaporization rates, non-uniform dispersion of


     the vapor species, uneven substrate temperatures, variable substrate surfaces,


     and impurities.
                                                    U 8 IPA Headquarters Library
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                                       79           -J200 Pennsylvania Avenue NW
                                                       Washington, DC 20460
                                                           202-566-0556

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                                CHAPTER VII







                 RECOMMENDATIONS FOR FUTURE WORK







       The feasibility of ultimately applying the technology derived from this




investigation to the solution of glass container waste control  problems is bright;




however, many basic problems  have been found which must be solved in the  labo-




ratory prior to significant scaled-up operations becoming a reality.




       The significance of this work was that it demonstrated that oxide coatings




may be readily applied to soluble silicate glass substrates by  chemical vapor depo-




sition processes with some flexibility in the selection of process parameters used.




This represents in itself an important early finding,  fundamental  to much of the




work which will follow. Corollary studies are presently being pursued in the areas




of glass corrosion kinetics, mechanical properties, and other types of coating media




As rapidly as possible, new information must be disseminated to all  interested par-




ties, to assure that the central objective of the overall project remains  in focus




and that wasted motion is minimized.




       The major product deficiencies denoted in this work were coated specimens




characterized by high residual stresses and variable microstructures.   The author




believes that these problems must be resolved prior to further significant advances




in this area.  The solutions may involve new materials systems, equipment modifi-




cations,  new procedures of coating, or a combination of all.  In all  likelihood,




each of the corollary study areas will be correspondingly affected by any changes




that take place.







                                 80

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        In terms of the specific problems just recounted, a high residual  stress level




obviously damages the mechanical and chemical durability properties of the com-




posite structures.  A need exists to more closely match the thermal expansion




coefficients of the materials used in order to decrease these effects.  As a general




class of materials, the oxides do not match up satisfactorily with the glass compo-




sitions used;  in fact, the residual stresses for several coatings produced in this study




were calculated and found to exceed  the theoretical  mechanical stress limits of the




substances involved.  The mechanism  of stress relief which allowed the coating and




substrate to remain intact is unknown  at the present time, but the example clearly




illustrates the point.




        Alternately, methods may be devised to coat  substrates at much lower tem-




peratures.  Klerer (45) has reported that amorphous SiO2 films may be readily




formed  from the vapor phase at deposition temperatures as low as 150  F - 250° F by




going through a radical-coupling step.  This technique would represent an inter-




esting solution to the problem and should be  investigated.




        It is surmised that the variability that currently exists in the coating micro-




structures is at least partially responsible for the erratic  tentative results which




have been obtained from an early study of the corrosion  rates of the coated samples.




Defects have been microscopically observed  in many  coatings at relatively  low




magnification; this suggests that the defects which are likely present on a truly




microscopic scale as well are producing pinholes and other faults which are the




early failure points  during a corrosion evaluation.







                                      81

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        The question remaining to be answered is how to effectively minimize or




even erase these variations.  The author believes that the problem of structural




defects can be largely defeated by devising an improved  vaporizer and vapor




delivery system. A basic requirement in this process is that the substrate must




receive a constant, well-mixed and diffused vapor supply to all surfaces.  It is




well  known that local  concentrations of reactant vapor in the total vapor stream




under the influence of elevated temperatures will upset the thermodynamic balance




of the system and possibly cause growth defects to occur during the growth  process.




        Another area which should be upgraded is that of input materials.  Higher




purity coating reagents than the ones employed in this work should be obtained and




used  for future studies  if possible.  Special precautions should be  installed  to mini-




mize contamination at all levels and from all potential sources.  The soluble sili-




cate  substrates must be prepared with particular emphasis placed on mixing uni-




formity, annealing, and surface condition.  It is believed that each of these items




plays some part  in determining  the relative number  of defects in a coating.




        Solution of these problem areas would provide a giant step in the right




direction.  Still remaining, however,  is the matter of basic cost.  The process is




obviously inherently expensive, and the investigator is limited in  the possible




significant approaches that might defray costs. During the course of  this study,




few conditions were explored that yielded a coating efficiency above 10 percent.




The best coatings that were produced were less than 5 percent, in fact.   Reclqim




systems would elevate this figure somewhat, but the basic cost factors such as




expensive plating agents, sophisticated control equipment, highly trained






                                  82

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operators, and the  like probably point to a basic problem which can only be




improved upon by diligent effort—not solved completely.  To illustrate this last




point, compressed dry air should suffice as a substitute for the carrier gases




employed, thus resulting in some savings.




        The author  believes that a logical long range approach to the program at




this stage of its development would  be to design a glass composition which might




be dissolved by the elements, as at  a  refuse dump, over a period of several  months'




duration.  This composition would likely possess a lower expansion  coefficient




than the ones used  in this work; consequently,  it is likely that a stress-compatible




coating system could be worked out easier for this material, and the resultant




coated product would thereby possess  improved mechanical properties.  Since the




durability of the  glassy material would also likely be upgraded to a significant




degree, it is possible that the coating microstructural sensitivity factor would be




correspondingly reduced, growth defects might be less critical, and thinner




(cheaper) coatings  would thereby suffice.  This approach seems to make more prac-




tical  sense at the present time than  any other.
                                      83

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                             LITERATURE CITED
 1.  Breidenbach, A.W., "Research Activities Of The Solid Wastes Program Of
        The  Public Health Service—A Status Report," U.S. Department of Health,
        Education and Welfare, Washington,  D.C.,  1-2, (1968).

 2.  President's Science Advisory Committee's Report  Of The Environmental Pollu-
        tion Panel, "Restoring  The Quality Of Our Environment/1 Washington,
        D.C., HI, (November, 1965).

 3.  Glass Industry, 92 (1):  40, (1968).

 4.  Alexander, T., "Where Will We Put All That Garbage?"  Fortune Mag.
        126(5):  151, (1967).

 5.  Anonymous,  "Municipal Refuse Disposal," American Publ ic Works Association,
        47-52, (1966).

 6.  Anonymous,  "Proceedings Of The National Conference On Solid  Waste Research,"
        American Public Works Association, (1963).

 7.  Morey,  G.W., The Properties Of Glass, Reinhold Publishing Corp.,  New York,
        (1954).

 8.  Vail, J.G., Soluble Silicates:  Their Properties  And Uses, Volume  II, Rein-
        hold Publishing Corp., New York, (1952).

 9.  Vail, J.G., Soluble Silicates:  Their Properties  And Uses, Volume  I, Reinhold
        Publishing Corp., New York, (1952).

10.  Plunkett, J.D.,  "NASA Contribution to the Technology of Inorganic  Coatings,"
        NASA SP-5014 Technology Survey, Washington, D.C., (1964).

11.  Powell, C.F., "Chemical  Vapor Deposition," Vapor Deposition, Edited by
        C.F. Powell, J.H. Oxley, and J.M. Blocher, Jr., John Wiley and Sons,
        Inc., New York, (1966).

12.  Blocher, J.M., Jr., "The Promise And Problems Of Chemical Vapor Deposi-
        tion," Chemical Vapor Deposition, Defense Metals Information Center
        Report 170, Battelle Memorial  Institute, Columbus, Ohio,  (1962).

13.  Accountius,  O.E.,  "Vapor Deposition,"  Critical Evaluation Of Ceramic
        Processing At Subconventional  Temperatures, Edited by S.D.  Brown, Air
        Force Materials Laboratory Report TR-67-194, Wright-Patterson Air Force
        Base, Ohio,  (1967).

                                 84

-------
14.  Powell, C.F., Campbell, I.E., and B.W. Gonser, Vapor Plating, John
       Wiley And Sons,  Inc., New York, (1955).

15.  Powell, C.F., Oxley,  J.H., and J.M. Blocher, Jr., Vapor Deposition,
       John Wiley And Sons, Inc., New York, (1966).     "

16.  Sawyer, W.E., and A. Man, U.S. Patent 229, 335 (June 29,  1880).

17.  Aylsworth, J.W., U.S. Patent 553,296 (January 21, 1896).

18.  deLodyguine, A., U.S. Patent 575,002 (January 12, 1897).

19.  Krier, C.A., "Protective Coatings," Vapor Deposition, Edited by C.F.
       Powell, J.H. Oxley and J.M.  Blocher, Jr., John Wiley And Sons, Inc.,
       New York, (1966).

20.  Blocher, J.M., and J.H. Oxley, "Chemical Vapor Deposition Opens New
       Horizons In Ceramic Technology," Bull. Am. Ceram. Soc.,  41 (2):  81-84,
       (1962).

21.  Walton, D., "The Orientation Of Vapor Deposits," Phil.  Mag., 82 (7):
       1671-1679, (1962).

22.  Secrist,  D.R., and J.D. Mackenzie, "Identification Of Uncommon Non-
       crystalline Solids As Glasses," J. Am. Ceram. Soc., 48 (9):  487-91,
       (1965).

23.  Sinclair, R.W., "Inorganic Glass Films—Their Preparation, Properties, And
       Uses," The Glass Industry, 92 (1):  22-28, (1968).

24.  Holland, L., The Properties Of Glass Surfaces, John Wiley And  Sons, Inc.,
       New York, (1964).

25.  Anonymous, "Tyzor Organic Titanates," E.I. Dupont deNemours And Co.,
       Wilmington, Del.

26.  Deyrup, A.J., U.S.  Patent 2,881,780 (April  22,  1958).

27.  Badger,  A.E.,  U.S. Patent 2,881,566 (April  14, 1959).

28.  Powell,  C0F.,  "Chemically  Deposited  Nonmetals," Vapor Deposition,
       Edited by C.F. Powell, J.H0 Oxley, and J.M.  Blocher, Jr., John Wiley
       and Sons,  Inc., New York, (1966).
                                   85

-------
29.  Oxley, J.H., "Transport Processes," Vapor Deposition, Edited by C.F.
       Powell,  J.H. Oxley, and J.M. Blocher, Jr., John Wiley And Sons, Inc.,
       New York, (1966).

30.  Li, C.H.,  "Epitaxial Growth Of Silicon," J. Electrochem. Soc.,  109(10):
       952-57, (1962).

31.  Jordan, E.L., "A Diffusion Mask For Germanium," J. Eiectrochem.  Soc.,
       108(5):  478-81, (1961).

32.  Vasicek, A., Optics Of Thin Films, Interscience Publishers,  Inc., New York,
       (1960).

33.  Powell, C.F.,  "Stresses In Deposits," Vapor Deposition, Edited by C.F.
       Powell,  J.H. Oxley, and J.M. Blocher, Jr., John Wiley and Sons, Inc.,
       New York, (1966).

34.  Hirth, J.P., "Condensation Processes," Vapor Deposition, Edited by C.F.
       Powell,  J.H. Oxley, and J.M. Blocher, Jr., John Wiley and Sons, Inc.,
       New York, (1966).

35.  Coffin, L.F., "Structure-Property Relations For Pyrolytic Graphite," J. Am.
       Ceram. Soc., 47 (10): 473-78, (1964).

36.  Gretz, R.D., "Structure Of Deposits," Vapor Deposition,  Edited by  C.F.
       Powell,  J.H. Oxley, and J.M. Blocher, Jr., John Wiley and Sons, Inc.,
       New York, (1966).

37.  Handbook Of Organometallic Compounds, Edited by H.C.  Kaufmann, D.
       Van Nostrand Company,  Inc., New York, (1961).

38.  Handbook Of Chemistry and Physics, Edited by R.C. Weast, The Chemical
       Rubber Company, Cleveland, O., (1967).

39.  Turnbull, R.C., and W.G. Lawrence, "The Role Of Titania In Silica Glasses,"
       J. Am. Ceram. Soc., 35 (2):  48-53, (1952).

40.  Sosman,  R.B.,  The Properties Of Silica, Reinhold Publishing Corp.,  New
       York, (1927).

41.  Engineering Properties Of Selected Ceramic Materials, Edited by J.F. Lynch,
       C.G. Ruderer, and W.H. Duckworth, The American Ceramic Society,
       Columbus, O., (1966).

42.  Pliskin, WeA., "The Evaluation Of Thin Film Insulators," Thin Solid Films
       2 (1):  1-26, (1968).                                        "      '

                               86

-------
43. Law, H.B0, "The Formation Of Insulating Layers By The Thermal Decompo-
       sition of Ethyl Silicate," The Review Of Scientific Instruments, 20 (12):
       958, (1949).                                     ~~~

44.  Index To The X-Ray Powder Data  File, Edited by Joseph V. Smith, ASTM
       Special Technical Publication 48-H, American Society For Testing
       Materials,  Philadelphia, Pa.,  (1959).

45.  Klerer, J., "A Method For The Decomposition Of SiO2 At Low Temperatures,"
       J. Electrochem. Soc., 108(11):  1070-71, (1961).
                                    87

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                                APPENDIX I
         DETERMINATION OF PLATING VAPOR CONCENTRATION
                       IN THE CARRIER GAS  STREAM
For the purposes of this illustration, assume that it is desired to obtain the concen-

tration of tetraisopropyl titanate (TPT) in nitrogen gas flowing through the system

at a rate of two cubic feet per hour.

       1.  Calculate the number of moles of carrier gas utilized per unit

           of time, using the following relationship from the ideal  gas law:
                                    _
                                  ng'RT

                where n  = the number of moles of carrier gas
                       Pa = pressure in  atmospheres
                       V = volume in liters per unit of time
                       R  = universal gas constant
                       T  = temperature in degrees Kelvin

            It may be assumed that the carrier gas is exhausted to the atmosphere

            for purposes of calculation.  The universal gas constant expressed in

            the choice of units used is 0.082 I iter-atmospheres .  The volume is
                                          mole -degree K.

            given in cubic feet per hour and must be converted to liters per hour

            by a conversion factor of 28 liters per cubic feet.  Substituting in the

            equation yields

            n  - (DP* 28)     - , ,    ,    t -,
            n  - M Q32\ (300) — ~     moles ot nitrogen gas per hour

       2.   The next step is to determine the amount of TPT used during that time

            interval.  This is determined by weighing the flask before and after

            the specified time interval.  Assuming that 3.8206 grams of TPT were

-------
    used, one may divide this number by the molecular weight of TPT


    to determine the number of moles of TPT used.


                  - we'9n* l°ss 'n grams  _ 3.8206

             nTPT " molecular weight    ~ 284.2  " °'0134



3.  The resultant concentration is expressed as the  ration of the  number


    of moles of plating compound to the number of moles of carrier gas.



                     ^   nTPT     0.0134   A „,,
                     C=	  =  0-55	 =0.0061
                          ng       2.2



    The resultant gas stream is composed of 6.1  parts of TPT to 1000 parts


    of carrier gas or 6100 parts per million (ppm).
                             89

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                               APPENDIX II







               DETERMINATION OF COATING EFFICIENCY







For the purposes of this illustration, assume that the coating efficiency of TPT is




required during a run.




       1.   Determine the loss in weight of the vaporizer containing the liquid




           plating substance.  Assume that this loss amounts to 4.7394 grams of




           TPT over a given period of time.




       2.   Determine the gain in weight of the substrate during the same period




           of time.  Assume that this gain is 0.0731 grams of TiO2.




       3.   Write a balanced chemical equation for the reaction taking place to




           compute the yield of the reaction, that is, the number of moles of




           TiO2 formed by the pyrolysis of  1 mole of TPT.




                 Ti(C3H7O)4	> TiO2  + 2C3H6 +  2C3H7OH




           In this case, one mole of TiO2 is formed  from one mole of TPT, or




           expressing it in terms of molecular weights, 1 mole of TiO« (79.9




           grams) is created every time  1 mole of TPT (284.2 grams)  is decom-




           posed.   Expressed as a fraction,  the TiO2 yield from the reaction is




           simply 1^ =0.281





       4.   Multiply the TiO2 yield fraction by the number of grams of TPT




           vaporized to obtain the number of grams of TiO2 contained  in the




           TPT that vaporized.  For this example,




                            (4.7394) (0.281) =  1.332






                               90

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5.  The coating efficiency is expressed as a fraction of the TiOo con-



    tained in the vaporizing TPT divided into the TiO2 deposited on the


    substrate.  For this run,

                                   0.0731
               Coating Efficiency = ,  330  = 0.548 = 5.48%
                              91

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                               APPENDIX ill


             DETERMINATION OF RESIDUAL COATING STRESS


       If a substrate and coating are under isothermal conditions during applica-

tion, and the thermal expansion coefficients of both vary in approximately the

same manner, the coating stress  Sj, induced by thermal variations  in a coating

applied to a flat plate, assuming no plastic flow occurs, can be estimated (33):


           =
       ST  -
              EB+2Ec(tc/tB)

       where

       Sy       - coating stress in pounds per square inch

       En       - elastic moduli of base in pounds per square inch

       E£      - elastic moduli of coating  in pounds per square inch

       AT       = temperature change from deposition temperature, in degrees
                  Fahrenheit

       °*B' °^C  = l'near thermal expansion coefficients of the base and coating
                  respectively,  in reciprocal degrees Fahrenheit.

       *C' *B   = thickness of the coating and base respectively, in inches

       If AT  («*B - «<£) is  positive,  a tensile stress will be induced in the coating

and a compressive stress in the substrate.  If  the term is negative, tfie  reverse will
occur.
                                92

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                          PART 11




MECHANICAL PROPERTIES OF WATER-SOLUBLE SODIUM SILICATE GLASSES

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                           ABSTRACT






     A method for mechanically testing sodium silicate glass by the




bending test was presented.   Five glass compositions between 1.ONa^O*




l.OSiO  and 1.0-Na^O'1.6$iCL are investigated.   The strength was ex-




amined as a function of four variables: composition, re-anneal  time,




atmospheric exposure,  and coatings.




     This work represents the second  segment  of a two year investiga-




tion to determine a feasible method of processing glass containers




after use.




     The description of fabricating and testing rectangular glass  bars




3/8" x 3/8" x V was described,  with  the author's interpretation of




the results.




     Sodium silicate glasses between  the composition range l.ONa^O-




1.3Si02 and 1 .ONa20-1.65iO-2  possess adequate  strength to be utilized




as a container material.  However,  only after the completion of  con-




current studies on dissolution kinetics and coatings can a just  ap-




praisal of the feasibility of this  project be made.

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                            PART II






                       TABLE OF CONTENTS






Chapter                                                          Page




   I.   INTRODUCTION 	     ]




  II.   REVIEW OF LITERATURE 	     2




             Solid Waste Disposal 	     2




             Soluble Silicate Glasses 	     ^




             Physical  Properties	     5




             Strength of Glass	    12




             Measurement of Strength	    13




 III.   OBJECTIVE OF RESEARCH	    16




  IV.   EXPERIMENTAL PROCEDURES	,    17




             Melt Preparation	    17




             Specimen Fabrication 	    17




   V.   PRESENTATION AND DISCUSSION OF RESULTS 	    27




             Composition Analysis 	    27




             Effect of Re-annealing 	    31




             Atmospheric Exposure 	    31




             Coatings	>	    36




  VI.   CONCLUSIONS	    40




 VII.   CONTINUANCE OF RESEARCH	    41




 LITERATURE CITED  	    43




 APPENDIX ....'.'..,	    45

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                           PART 11
                        LIST OF TABLES
Table                                                            Page
   I.   Composition  by Weight of Glasses  Investigated	18

  II.   Annealing  Temperature for Glass Compositions
          under  Investigation	26
                              iv

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                            PART II


                        LIST OF FIGURES


Figure                                                            Page

   1.   Refractive Index Versus Composition 	    8

   2.   Density Versus Composition	    7

   3.   Linear Thermal Expansion Below the Transformation
          Temperature	    q
   k.  Linear Thermal Expansion Above the Transformation
          Temperature
   5.  Transformation Temperature ................   11

   6.  Photograph of Cylindrical Disk Mold ...........   20

   7.  Photograph of Disk Specimens being Prepared ,  ......   21

   8.  Photograph of Rectangular Specimens being
          Prepared .......................   22

   9.  Photograph of Bubble Defects in Rectangular
          Specimens .......................   23

  10.  Photograph Showing the Effect of the Addition  of
          As?0_, as a Fining Agent, on the Incidence  of
          Bubbles in the Test Specimens .............   25

  11.  Plot of Bending Strength Versus Composition .......   28

  12.  Photograph of the Fracture Patterns for the
          Specimens from the Two Types of Molds .........   30

  13.  Plot of Percent Change in Strength Versus Re-Anneal
          Time for a 1 .ONa20 ' 1 . 3Si02 Composition ........   32

  14.  Plot of Percent Change in Strength Versus the  Time
          of Atmospheric Exposure ................
  15.  Illustration of How Surface Flaws are Eliminated
          after Exposure to the Atmosphere ...........   35

  16.  Series of Photographs Showing the Absorption of
          Moisture by the Surface of a 1 .ONa^O- 1 .2S50
          Composition .....................     37

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                            CHAPTER 1






                          INTRODUCTION






     The use of glass has so saturated the market of commodities distri-




bution, that glass has become one of the largest and most troublesome




constituents of solid waste disposal.  The high chemical durability that




glass possesses makes it indispensable in the containment of corrosive




systems, but renders it a formidable opponent on the trash heap.  The




formulation and design of a glass container whose durability and sus-




ceptability to environments can be controlled would greatly assist in




the degradation of such containers.




     The solubility of sodium silicate glasses has been observed as a




natural oddity in the field of glass technology.  Morey (l) points out




that special attention should be called to the Na^O-ZSiO^ quartz eutec-




tic.  There is a remarkable melting point lowering of about 1000°F after




the addition of 25% Na«0.  The low temperature of this eutectic is of




vital  importance to glass technology.  More importantly, Morey suggests




that glass of this composition would be excellent "were it not easily




attacked by water."



     If a suitable coating could be developed that provided an impene-




trable barrier for water vapor, certain sodium silicate glass could be




utilized as container material.



     This paper is the second in a series of papers that investigates




the feasibility of using sodium silicate glass as a method of improving




the degradement of solid waste by rendering glass easily disposable.

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                           CHAPTER 11






                      REVIEW OF LITERATURE






     Glass making is older than recorded history.   No one can say when




or where or who first discovered how to make glass.   Perhaps the dis-




covery resulted from an experiment,  but more likely  the discovery was




accidental.  At some time in history the conditions  existed where some-




one heated sand and soda over a hot  fire and upon  cooling discovered




this vitreous substance - glass.






Sol id Waste Disposal




     The United States is facing a staggering problem in trying to dis-




pose of the  immense amounts of solid wastes that are the unwanted resi-




due of our nation's achievements in  production and consumption.  Our




cities and their surrounding urbanized areas are already bearing the




brunt of our explosive growth and increased industrial activity.  This




country has  to deal with 3-5 billion tons of solid waste each year.




This consists of 48 billion cans, 26 billion bottles, more than 30




million tons of paper, 4 million tons of plastics, and 100 million




rubber tires weighing a million tons (2, 3).  With this soaring problem




in solid waste disposal becoming more critical, the general public is




not aware of the problem.  A vivid illustration of public apathy can




be seen in the attitude toward the non-returnable glass bottles which




relieves the producer, distributor,  and consumer of handling cost and




inconvenience, but  creates more burden for refuse collectors.   It  is




unfortunate  that most people are not aware of the additional problems




that have been created by this convenience item.  The refuse collector



                               2

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is aware of his increased burden, however,  and  the  use of non-returnable

bottles is increasing at an alarming  rate  (^).


     Initial action was  instigated by  passage of  the Solid Waste Dis-

posal Act of 1965, Title 11 of Public  Law  89^272, which President John-

son signed on October 20 of that year.  This  new  legislation directs

the Secretary of Health, Education, and Welfare to  initiate, encourage,

and support a national program aimed  at discovering and evaluating

better methods of coping with the solid waste problem  (5, 6).

     Containers, with respect to solid waste  disposal procedures, can

be divided into two classifications:   combustible and non-combustible.

Containers made from paper, wood, or  plastic  are  the major portion of

the organic compounds of refuse.  Combustible containers have heat value

and when dry will burn freely in incinerators without forced draft and

without fuel.

     A non-combustible container is any container that is unburnable at

ordinary incinerator temperatures  (1300° to 2000°F).  Although metal

containers undergo slow  disintegration by  oxidation, this process is

far too slow to be employed  in connection  with  incinerator procedures.

Carelessly stored, used  containers are aesthetically objectionable and

may harbor rodents and other vermin.   Open cans and bottles catch and

hold water  in which mosquitoes can breed,  so  that many individual
                                                         •
citizens unknowingly but actively encourage the proliferation of these


disease carrying pests  (7).

     Funds and  leadership for research and planning programs to curb the

undesirable effects of  inadequate solid waste control are starting to

be provided (8).* Essential to this effort is a method of processing

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incombustible, unreactive glass containers after use.  The potential




impact to such a development upon solid waste control may be readily




visualized by considering that approximately one hundred thirty-five




glass bottles and jars alone are produced each year for every person




in the United States.   This amounts to twenty-six billion glass con-




tainers each year (3).  The control of the disposal of glass would be




a tremendous step toward the control  of all  solid waste.






Soluble Silicate Glasses
     With the development of glass  blowing techniques,  the formulation




of glass containers was directed to yield high chemical  durability.




The resistance which glass offers to various  corrosive  agents is of




great practical significance.   Morey (1)  points out that the cKemical




durability of glass is often times  the chief  reason for its preference




over competing materials.




     There are six important types  of glass used today:   soda-lime,




lead alkali, borosi1icates,  aluminosi1icates,  36% silica, and fused




silica.  The most common kind  of glass  is soda-1ime-si1ica glass com-




posed of silica sand, soda,  ash, and limestone.  The addition of in-




creasing amounts of soda to  silica  while  decreasing other constituents




significantly lowers the melting and softening temperatures.   However,




the chemical resistance is also impaired, and  if enough soda is  used



the glass is readily attacked  by many environments (1).




     In the past the properties of  alkali  silicate glasses, like sodium




silicate glass, have been investigated  more from scientific curiosity




than from practical scientific investigation.   Soluble  silicates have




been desired more for other  industrial  uses than for glasses.  They

-------
have been used for such applications  as  adhesives,  cleansers,  cements,



deflocculants, and protective  coatings  (9).




     The alkali silicate glass system would  offer  some  inherent  advan-




tages if adopted for glass  container  production.   Vail  (9)  points out




that sodium silicate gels are  now added  to polluted water  as a




flocculant in  the settling  and filtration steps  of  water purification.




At the same time these gels form solutions that  tend  to form a thin




film on the inside of water pipes which  protect  them  against water




corrosion.  These are  important by-products  of soluble silicates.




     Morey (1) has the most complete  record  of the  investigation of




sodium silicate glasses.  However, the  range of  his investigation is




limited between  100% Si02 - 0% Na20 and  70%  Si02 -  30% Na20, and this




did not extend into the  range  of compositions under investigation.




Morey points out that with  the high SiO^ composition, a sodium disili-




cate  (Na^O'ZSiO^)  >s formed that is easily made  into  a glass, while as



the soda content  increases, a  second  compound is formed:   sodium meta-




silicate  (Na~0*SiO?) .   It has  been shown by  a number  of workers  (10,  11,




12) that the range of glass formation in the Na20  - Si02 system  is con-




tinuous from SiO? to a  limiting composition  close  to  that  of the meta-




silicate Na70-Si09.  The  limiting composition depends on the experimen-




tal conditions.  As  is  to be expected,  the smaller  the melt  and  the



greater the rate of cooling, the higher  the  Na20 content at which the




material can still be made  as  glass free from any  crystalline material.




The work of Moore and Carey (11), working with 6 gram melts, established




the limiting composition as kl mole percent  Na20.   Imaoba  and Yamazaki




(12), in a general survey of glass forming regions  in simple silicate

-------
systems, found it possible to make glasses containing up to 58.8 weight

percent Na?0 (57.8 mole percent)  on a scale of 1-2 grams.  The different

results for the Na?0-SiO? system clearly show the effect of the scale

of melting, and illustrate the importance of specifying as clearly as

possible the melting and cooling conditions when reporting the region

glass formation in a system.   It may be significant that the limit in

the Na-0-SiCL system coincides almost exactly with the composition at

which Na?0 becomes the primary crystalline phase, that is, the first

phase to crystallize from the melt.  At this composition a change will

occur in the nature of the crystallization process which will  be re-

flected in the kinetics of the process.  Scholes (13) substantiates

the fact that sodium metasilicate can be obtained as a glass,  but only

in small quantities.  However, to meet the specifications necessary for

use  in  the container industry, it is only necessary for these glasses

to meet the qualifications of a water soluble container and not those

that make  it a glass.


Physical Properties

     The knowledge of the physical properties of sodium silicate glass

is somewhat limited.  There have only been a few researchers that have

done extensive work with these glasses because its solubility has

greatly hindered any thorough investigation.  Not only have the inves-

tigations  been limited, but they have produced significant discrepancies

in these researchers' results.  Some properties that are easier to ob-

tain than  others have been sufficiently substantiated.  These  include

changes in density,  refractive index, and linear thermal expansion
                                                       »
below and  above the  transformation temperature.  These properties are

-------
   2.6 O -
    2.20
100
0
90
10
80
20
70
30
60
40
50
50
40
60
% Si
% N
                            COMPOSITION
FIGURE  I.
DENSITY  VS   COMPOSITION

-------
           1.52  -
00
        X
        UJ
        o
        O
        <
        
-------
X
H
yj
     26


     24


     22
CO
z
CL
X
UJ
-I
2
ce
• - I
20
18

16

14
12


1O
1 	
100
0
I
90
10
s
so
20
I
70
30
i
60
40
1
50
50
I
4O
6O

% Si
    FIGURE 3.
                       COMPOSITION


            LINEAR  THERMAL  EXPANSION  BELOW  THE
            TRANSFORMATION  TEMPERATlfRE.

-------
o
X
o
CO
CL
X
UJ
IT
UJ
80

70

60

50

40

30

20
             1
                 I
1
I
             o BUREAU OF STANDARDS
               SAMSOEN
1
1
     100
      0


  FIGURE 4.
         90      80     70     60     50
         10      20     30     40     50
                   COMPOSITION
        LINEAR THERMAL EXPANSION  ABOVE THE
        TRANSFORMATION  TEMPERATURE
              40
              60
                            SI0

-------
o
e
111

OC
CC

UJ
a.
   450
Ul
   400
O
    350
 o
 b.
 CO
    300
a  BUREAU OF  STANDARDS


O  SAMSOENS
100
0
90
10
80
20
70
30
60
40
50
50
40
60
%C i
O 1
FIGURE 5.
               COMPOSITION

TRANSFORMATION  TEMPERATURE
     VERSUS  COMPOSITION

-------
shown in Figures 1, 2, 3, and 4 respectively  and are taken from




Morey (1).   An example of the discrepancies in results is seen in




Figure 5 which shows the change in the transformation temperature with




changes in  composition.  The results from Samsoens (14)  and the Bureau




of Standards (1) are used.






Strength of Glass



     "Even  when conducted with utmost care, studies of the mechanical




properties  of glass are plagued by what seems  to be an inherent and un-




avoidable scatter, a condition stemming from the susceptibility of the




glass surface to flawing by either mechanical  or chemical action (15)."




     it is  practically impossible to predict the strength of a particu-




lar specimen of glass, except within very broad limits.   Only the




average obtained from a large number of strength measurements has real




significance.  An average strength figure alone does  not  characterize




the property measured since it is also necessary to know  the extent to




which individual measurements are likely to depart from  the mean.  This




fact makes  it necessary to approach the measurement of the strength of




glass with  a statistical procedure.




     It is  widely accepted that glass fails only under tensile stres-




ses.  The strength of glass is conditioned by  the existence of flaws or




weak points in the material.  Several observations regarding the



strength of glass are summarized below.




     (1)  The strength of a massive glass specimen measured by




          bending or tension experiments is much less than pre-




          dicted by any theory which assumes that strong  inter-




          molecular forces co-operate in a regular way.






                              12

-------
     (2)   The strength of similar pieces of glass broken in




          tension may vary widely and only average figures




          hold any significance.




     (3)   The strength of glass  is easily affected by the




          treatment of the material, and in particular the




          factors affecting the  conditions of the surface,




          such as environments,  and annealing times and




          temperatures.




The Griffith Flaw Theory  (2k) provides an explanation for the difference




between the theoretical strength and the experimentally observed results




from the presence of small cracks or flaws around which a strong stress




concentration arises when the solid is stressed.  The first and third




statements above can be used to  substantiate the Griffith Flaw Theory.




The strength of massive glass specimens may also vary as a function of




the geometrical shape and size.  Holland and Turner (16) have shown




that for specimens tested in bending, the flexural strength decreases




as the thickness is increased.




     In summary, any attempt to  find the strength of glass as a func-




tion of any other parameter is a very difficult task.  Some (17) feel




that there is no correlation among glass parameters, such as strength




versus composition, while there  are others (1,  19, 20, 21, 23) that




disagree.






Measurement of Strength



     Two methods can be used to  measure strength of brittle material.




The most direct way to measure strength is to take a rod or fiber with




enlarged ends and load it with a gradually increasing weight until the





                                    13

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specimen breaks.  If the cross sectional  area of the specimen is A, and


the load is W, then the nominal breaking  stress is W/A.  The value of


W/A obtained by this method of testing will  usually be much greater


for fine fibers than for large rods due to the greater probability of


encountering a flaw in a larger cross sectional area than a smaller


one.  The direct measurement of tensile strength by rod or fiber


method is not so convenient as the indirect  method using a bending


test.  For the direct test it is necessary to work with specimens with


enlarged ends while with the bending test it is not.


     A rod can be easily tested by bending it under a load and simply


measuring the load required to break the  specimen.  If the load  W


pounds is applied at the center of a span L  inches in length, the


maximum bending moment developed is WI_A  inch-pounds.  This maximum


bending moment occurs in the cross section of the beam directly under


the load, that is, at the center of the span.  If the section modulus


of the cross section is A, then the maximum  tension developed Is WL/4A.


This parameter is called the "modulus of  rupture."  Assuming the beam


to be symmetrical above and below the central axis, the value of A wi 11


be 21/d, where 1 is the flexural moment of Inertia, and d is the depth


of the beam.  For different geometrical configuration the value of A


will change.  In the rectangular specimen used, the value of A is

  2
bd /6 where b and d are the specimen dimensions respectively, and the

                                                                r\
value of the modulus of rupture for rectangular beams is 3WL/2bd .


     As Scholes (13) emphasized in his work  on the strength of glass,


the beam may not break directly under the load, where the stress is


greatest, but at some other point along the  span.  This happens because
                               14

-------
in glass the weakness of its surface is being tested, and not the


strength of the hulk samples.  This fact holds more importance when a
                                                           >

glass, like the sodium silicate glass, is attacked by the constituents


of the atmosphere.

     Rudnick, Hunter, and Holden  (21) used a third method for deter-


mining the strength of brittle materials, that of the diametral-compres-

sion test.  However, this is limited to a specific fracture pattern.
                                     15

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                           CHAPTER 111






                     OBJECTIVES OF RESEARCH






     The basic objective of this research was to expand the knowledge




of the mechanical properties of sodium silicate glass.  This work




represents the second segment of a projected two year study which will




deal with the feasibility of, and methods for, the degradement of glass




packaging containers.  The research was composed of three phases.




     The initial  phase concerned a method of fabricating suitable test




specimens that when tested would yield representative mechanical data.




This included the selection of methods for testing.




     The second phase concerned physical  testing of specimens and the




interpretation of results.  This was  achieved by relating strength as




a function of four variables:  composition, anneal  time, exposure time,




and coatings.




     The final phase involved a critical  assessment of the present




procedure and recommendations for certain changes in both the method




and procedure.  The basic limitations of  present methods are discussed




with ideas for future research presented.
                              16

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                           CHAPTER  IV






                     EXPERIMENTAL PROCEDURES






Melt Preparation




     The preparation of melts consisted of mixing the desired quantity




of Baker Analyzed Reagent Grade Na^CO- and Pennsylvania Pulvarizing Com-




pany's highest quality silica.  The  ratio necessary to yield the desired




composition was calculated from the  basic equation:
                   + b Si02 + c C0£ + + d Na20- e Si02




where a, b, c, d, and e are the molar ratio determined by the desired




composition.  The percent by weight for the five compositions investi-




gated are given  in Table  1.




     The preparation was  accomplished by weighing out the desired quan-




tities of material and thoroughly mixing them  in a mortar and pestle.




The mixture was  initially fired in common fireclay crucibles.  However,




erosion of the crucible at the glass-air interface necessitated the use




of another crucible material.  The selection was a high quality alumina




crucible.  The desired composition and the alumina crucibles were heated




slowly in a silicon carbide resistance furnace to reduce the problem of



thermal fracture to the crucible.  After the melting temperature of




1250°C was reached, this  temperature was maintained for a minimum of




sixty minutes to allow for adequate soaking.






Specimen Fabrication



     The fabrication of adequate test specimens proved to be the most




formidable obstacle encountered.   Initially, cylindrical^ shaped disks




one and one-half inches in diameter and approximately three-eighths of






                                    17

-------
        TABLE 1
 COMPOSITION BY WEIGHT
OF GLASSES INVESTIGATED
GLASS COMPOSITION
1 .0 Na20 •
1 .0 Na20 •
1 .0 Na20 •
1.0 Na 0 •
1 .0 Na 0 '
1.0 Si02
1.1 Si02
1.2 Si02
1.3 Si02
1.6 Si02
WT % Si02
49.2
51.6
53.7
55.6
60.7
WT % Na20
50.8
48.4
46.3
44.4
39.3
         18

-------
an inch thick were cast  in  the  steel mold  shown  in  Figure  6.  These




disks were made to fulfill  a  three-fold  purpose.   It was hoped  that




these disks could be used as  a  suitable  sample to determine strength




through diametral testing,  solution  rates,  through  rate of dissolution,




and to, determine the degree of  workability  of the glass.  These disks




proved unsatisfactory for the diametral  testing  technique because they




broke with the pattern that was not  typical  for  tensile failure.  The




diametral compression test  is limited  to only tensile  failures so




another testing method had  to be utilized.   Other test methods were




investigated for strength determination  even though the disks continued




to be used for solubility studies.   ASTM,industry,  and other researchers




(17,  18,  19, 20) provided several  testing methods for  the modulus of




rupture of glass but these  tests were  for  sheets of glass and were not




applicable.  The lack of correlation between the strength of bulk sam-




ples and  the strength of fibers tested in  tension prevented the use of




tensilely testing fibers.




     The,bending test was then  chosen  to yield strength in the form of



modulus of ruptures  (MOR).  This was accomplished by molding rectangu-




larly shaped bars and breaking  these bars  in three  point loading.  The




initial material used for the mold was graphite  because it was readily




available and glass  does not  wet it.   Another mold  of mild steel was




later made.  Several molds  were made in  an  attempt  to  find a practical




size.   It was decided that  bars three-eighths inch  by  three-eighths




inch by four inches  could both  be adequately fabricated and tested.




Figures 7 and 8 show specimens  being molded in both types of molds.




      Initially the bars were  of poor quality as  seen  in Figure 9.  How-
                                     19

-------
Figure 6.  Photograph of initial  disk mold
                                20

-------
Figure 7-  Photograph of a disk specimen being poured.

-------
ro
PO

       Figure 8.  Photograph of bar specimen being poured

-------
i O
    Figure 9. Photograph of the bubble defects encountered in initial test specimens

-------
ever, as the proper technique was developed the quality improved.  No




fining agent was needed in the earlier melts because the disk specimens




were pressed with sufficient force to squeeze the bubbles from the




glass.  No prevision was made to press the rectangular specimens so 2%




by weight of As?0~ was added to help sweep the bubbles from the melt.




Figure 10 shows the effect of various amounts of As^O, on the incidence




of bubbles.




     During the initial attempts to mold the disk specimens, the mold




was heated by a gas or electric heater to raise the temperature of the




mold close to that of the annealing temperature.  However, this had no




effect on the quality of the test specimens so the heater was removed.




After the glass specimens were cast, they were immediately placed in an




annealing furnace and held at the annealing temperature suggested by




Morey and listed in Table 11.  The annealing temperature was maintained




for thirty minutes and then the furnace was shut down and allowed to




cool with the specimen inside.  Upon reaching atmospheric temperature,




the furnace was opened and the samples were removed and placed in a




desiccator for storage and transportation to the testing area.




          The three point bending method was used for the bending test.




This was performed on a Baldwin-Tate-Emery testing machine.   A span of




two inches, and a loading rate of .05 inches/minute was used.  The data




was collected on the basis of readings to the nearest pound.




     The same procedure was used for the preparation, fabrica'tion, an-




nealing, and storage of all specimens tested.
                             24

-------
l\3
c n
        Figure  10. Photograph  showing  the effect of  the  addition of As9CL,  as  a  fining  agent,  on  the incidence
                                                                       £-  j

                   of  bubbles  in  the test specimens  (  From  top  to  bottom respectively,  0%,  i%,  1%,  2%  As?0  ).

-------
TABLE 11
COMPOS
1 .0 Na?0 •
1,0
1.0
1.0
1 .0
noN
1.0 Si02
1.1
1,2
1.3
1.6
ANNEAL ING 'TEMPERATURE °C
415
440
450
450
420
  26

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                            CHAPTER V






             PRESENTATION AND DISCUSSION OF RESULTS






     Since the strength of glass  is not understood  in the same detail




as the strength of other brittle materials, it  is very difficult to




explain exactly what happens under every condition.  The strength of




glass is primarily controlled by flaws on the surface and secondarily




those in the interior.  It is this author's intent  to first present




his results and then give the explanation he feels  the results show.






Composition Analysis




     The first systematic investigation of the effect of composition on




the strength of glass was that of Gehlhoff and Thomas (19) in 1926.




Some (17) felt that it was almost impossible to draw convincing conclu-




sions from their results, where others (1, 19, 20, 21, 23) have con-




sidered the effect of composition on strength.




     The results of the present work on the effect of composition on




the strength of glass is shown in Figure 11.  It should be noted, that




the results from Morey  (l), and Gehlhoff and Thomas (23)  are also




depicted and the results from Morey's work with low Na20 content is




used.  The points represented by the triangle are the averages, with the




range given as a bar above and below the average.  The results for the




strength of the specimens from both the graphite mold and the steel




mold are shown.



     The rectangular specimens made in the steel mold had a smoother




surface than those produced in the graphite mold.  Thus, it seemed




logical  that those specimens would be stronger.  However, upon testing,






                                    27

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oo
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    CO
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    O
    O
    O
    O
    LJ
    a:
    CD
    Z
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    Z
    111
    ffl
32


28


24


20


(6


12


 8


 4
• GEHLHOFF  AND

  THOMAS

Q MOREY

^ GRAPHITE MOLD


O STEEL  MOLD

I RANGE  OF  POINTS


100
0
1
90
10
1
80
20
t
1
70
30
«s\ tji e*t\& i-
I
60
40
n f\ k.i
I
50
50
1
40 (
60

% Si02
% Na20
    FIGURE  If.  BENDING STRENGTH (MOR)   VERSUS  COMPOSITION

-------
the specimen from the steel mold revealed much  lower strengths than




those from the graphite mold.  This difference  is also seen  in Figure




11.  Also, there was a distinct difference  in the fracture patterns




between the two types of specimens.   Figure  12  shows the different




fracture patterns.  The difference  in  thermal conductivity between the




steel mold and the graphite mold may  have produced the differences in




the strengths by altering  the  cooling  rates  for the specimens.  This




change  in the cooling rate also led to the difference in fracture




patterns.  The leaching of iron or other substances from the steel mold




may be  another factor that influenced  the surface of the specimens and




reduced their strengths.   These impurities would form complex glasses




on the  surface that could  lead to a heterogenity between the surface




and interior glass.




     The  rapid decline of  the  strength of the specimen from  the graphite




mold with an additional  increase  in the Na«0 content past bk% is due to




the saturation of the SiCL structure's interstitices by sodium ions.




Coupled with this fact,  the Na20 phase is becoming the primary crystal-




line phase, that  is, the first phase  to crystallize from the melt.




These processes would produce  large clusters of Na  ions or  crystalline




Na20 that would disrupt  the Si02 structure and  reduce the strength.  The




compositions of higher silica  contents fit the  projected curve of the




work by Morey.



     The  change in color with  the change  in  composition was  a noted




visable change.  The specimens of higher silica content, 1.0 Na20.1.4 Si02





and above, possessed a clear and very  transparent color.  However, as




the N»?0  content was increased, the presence of an aqua-bluish tint






                                    29

-------
0
 :•
      Figure 12.  Photograph of the fracture patterns  for  the specimens  from the  two  types  of  molds.  The pattern
                 representative of the steel  mold is on the left,  and  the pattern  representative of the graphite


                 mold is on the right.

-------
appeared and increased until the  1.0Na20-  1.0Si02  specimen  became very




blue-green.  The change  in the color  could  be  from the  presence of  the




sodium ion, other ions leached from the  crucible by the higher Na+




concentration, or a change in the valence state of the  iron  impurities.






Effect of Re-annealing




     Since the specimens from the steel  mold displayed  significantly



lower strength than specimens from the graphite mold, they were re-



annealed to determine what effect this would have  on  their strengths.



Figure 13 shows the results from  reannealing over  a 2k  hour  period.



The composition used for this experiment was 1 .ONa-0- 1 ^SiO- because
concurrent studies  indicate  that  this composition exhibits the dissolu-




tion kinetics required for adequate  hydrolyzing.  The  relief of resi-




dual strains left from the first  annealing period resulted with an in-




crease in strength.  However, after  a period of nucleation, there is




sufficient grain growth of Na^O crystals  to reduce  the strength by




disrupting the SiO« structure.  Eventual  complete devitrification




would follow.  This crystal  growth would  then decrease the strength as




is shown after a re-annealing period of seven hours.  This does not indi-




cate that these specimens would require that length of time for anneal-




ing but indicates that more  research should be done on mold materials




and annealing schedules.






Prolonged Atmospheric Exposure



     The 1 .ONa-0-1 .3SiCL composition was  also used  in this experiment.




The specimens were periodically removed from a desiccator and exposed




to the atmospheric conditions dictated by a dry bulb temperature of 77°F
                              31

-------
CO
ro
      h-
      O
      z
      LU
      IT

      CO

      O
       LU
       CD
       LU
       0
       Z
       O
      111
      O
      flC
      LU
      Q.
-25 -
          O AVERAGE

          I  RANGE  OF POINTS
                              8
                       10  12

                       TIME
 14   16

HOURS
18   20  22   24
        FIGURE  13.  EFFECT OF RE-ANNEALING  A I.ONa20-l.3Si02  COMPOSITION
                   AT  45O°C   ON  ITS  BENDING  STRENGTH.

-------
and a wet bulb temperature of 71°F  (relative  humidity  of  6]%).   Figure

]k shows the percent change  in  strength  for the  times  indicated.   Ini-

tial examination of this figure indicates  a peculiar situation,  a

decrease and then an increase in  strength.

     These changes  in strength  can  be  explained  through the examination

of Figure 15, which shows  the cross section of a specimen at  four

important time periods.  The strength  of glass  is primarily surface

dependent so the  introduction of  surface flaws decreases  the  strength

by  increasing the probability of  propagating  a crack through  the sample,

Figure  ISA shows  the unflawed surface  of a molded sample.  Figure  15B

shows the flawed  surface caused by  abrasion from atmospheric  dust  and

water attack.  These additional flaws  disrupt the surface and cause a

decrease  in the strength of  the sample.  As the  silicate  glass surface

continues to absorb moisture and  other atmospheric gases,  it  erodes

these areas of higher energy, or flaws with small radii of curvature.

This erosion, as  seen  in  Figure 15C, forms a  layer across the surface

of  the  sample.  The removal  of  these highly stressed flaws results  in

an  increase in strength.   Applying  the Griffith  Flaw Theory to the con-

ditions  present  in  Figures 15B  and  15C,  the actual stress concentration

at  a crack  tip can  be expressed by:
      am =
               P

 and

      am =  actual  stress for failure

       a =  theoretical  stress for failure
                             33

-------
   O
   Z
   UJ
   a:
CO
   CO
   UJ
   CD
   O
         100
50
         -50
         -100

0
1
1
1
2
1
3
1
4
I
5
i
6
1
7
          FIGURE 14.
          TIME ,  HOURS

PERCENT  CHANGE IN STRENGTH

OF  ATMOSPHERIC  EXPOSURE
                                           VS THE TIME

-------
      ATMOSPHERE
           GLASS
           ~


    A. SURFACE  AS  FORMED
           GLASS
     B. SURFACE  AFTER  BEING  FLAWED
           SILICA-CARBONATE LAYER
    C. SURFACE AFTER SHORT  EXPOSURE
            SILICA-CARBONATE   LAYER
     D. SURFACE  AFTER  PROLONGED EXPOSURE



FIGURE 15.  ILLUSTRATION  OF HOW  SURFACE FLAWS

          ARE  ELIMINATED  AFTER  PROLONGED


          EXPOSURE TO THE  ATMOSPHERE
                    35

-------
     c = one half the axis length of the crack




     p = radius of curvature of the crack tip



it can be shown that as the radius of curvature of the crack tip in-




creases the strength increases by decreasing the probability of propo-



gating the crack through the specimen.   When there is sufficient erosion



of the glass to produce a reduction in  the cross sectional area as il-



lustrated in Figure 15D, there is a reduction in the overall strength.



     The surfaces shown in Figure 16 support the author's theory on the



change in strength.  As seen in this series of pictures, the layer



grows across the surface until it completely covers the surface.  This




completion occurs between four to five  hours and coincides with the




leveling off and decline of the curve shown in Figure 14.



     The author is not in a position to say of what this surface layer




is composed.  However, Phillips (21) says that sodium silicate is



leached from the glass and undergoes hydrolysis so that the layer con-




tains sodium hydroxide and colloidal silicic acid.  In the presence of




carbon dioxide a further reaction occurs between this gas and the al-



kali, resulting in the formation of sodium carbonate and a surface



layer of finely divided silica.





Coating Evaluation



     There are three types of coatings  under investigation for use as



a moisture barrier.  Organic epoxies, ion exchanged surfaces, and a



chemical deposited surface are under simultaneous study.  It is impos-



sible to conclusively evaluate their effect on strength at this time.




     The only coating available for testing was the organic epoxy.  This




was applied to a heated specimen  (200°C) by passing the specimen-through
                                  36

-------
CO
                                             .......      ••
                                                         :•,.
                                              '
    A. 30 Minutes                           B.  60 Minutes                            C.  90  Minutes




    Figure 16. This  series of photographs  shows the absorption of  moisture  by  the  surface of  a  1.ONa20•1.2Si




               glass and  the growth of  the resulting  layer  across  the  surface   (2000X).

-------
CO
oo
    0.  120 Minutes
                                           E.  180 Minutes
F.  2MD Minutes
    Figure 16.  Con't

-------
a fluidized bed of the powdered  resin.   Upon  cooling,  the specimens




were placed in a desiccator and  stored until  testing.  As in the other




tests, the 1.ONa20*1-3Si02 composition was used.  The  results of




testing yielded a decrease in strength by approximately  ]2%.  This is




a very incomplete experiment at  this time; however, continuation of




present work should yield more pertinent information concerning coatings
                              39

-------
                         CHAPTER VI






                         CONCLUSIONS






1.   The glass compositions from the  1 .ONa20-1.3Si02 to the l.




    1.6SiO  range seem to possess sufficient  strength  and ease of




    workability to be used as  a container  material. However,  no




    decision for the selection of a  definite  composition  can be made




    until  all system parameters are  evaluated.




2.   The technique of molding glass test specimens  is not  the optimum




    procedure for producing samples  because it  is  too  time consuming.




    Another method, like pulling glass  rods,  should be developed to




    mass produce specimens.




3-   The formation of the silica-carbonate  layer on the surface of




    the glass after prolonged  exposure  indicates that  some type of




    atmospheric control will be necessary  during the production




    process.




k.   The change in strength between the  specimens from  the two types




    of mold indicates that more research into mold materials is




    needed.




5.   The change in strength of  the specimen formed  in the  steel mold,




    after a period of rean.neal i nq, indicates  that  an optimum period




    of annealing exists'and continued  research  in  this area is needed.




6.   The. addition of 2% by weight of  As^O-  is  needed as a  bubble re-




    lease agent to sweep the melts of  bubbles prior to molding.
                                   40

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                          CHAPTER VII






                   CONTINUATION OF  RESEARCH






     At present there are corollary  studies being pursued  in the areas




of glass dissolution kinetics, organic  coatings,  ion exchanged sur-




faces, and a continuation of  the work on chemical vapor deposition




(25).  All these areas will  influence the  composition selection and




determine the feasibility of  this project.




     This work has answered  but a few of the questions concerning the




feasibility of using a sodium silicate  glass as a water soluble con-




tainer.  However,  it has been a guide in determining what paths will




be followed in future research.  The molding of glass specimens is




not the optimum method for producing test  samples.  This author sug-




gests that the mass production of uniform  samples by a pulling tech-




nique, similar to  the one used by Watanbe  and associates (26), be




given serious consideration  as a faster and more efficient method.




     The work on dissolution  kinetics is vitally important in select-




ing the desired composition.  This  is very time consuming work that




encounters many obstacles.   But, after  the results are analyzed, that




work will combine  with this  work on  mechanical strength to yield the




selection of a composition that possesses  sufficient mechanical




strength and adequate solubility.   Also important is the coating that




acts as a vapor barrier  to protect  the glass surface.  This coating




must resist chipping or  scratching  from handling, and yet be feasibly




applied and reasonable in cost.  These  components must all function




ss an integral system before  success is achieved.
                              41

-------
     This work has justified the continuation of research on this




project, while indicating that additional  work is needed on mechanical




properties and new research should be investigated on viscosity and




other properties that will be of vital  importance to the glass manu-




facturing industry.
                                     42

-------
1.  Morey, G. W. , The  Properties  of Glass.  Reinhold  Publishing
         New York,  (195*0.

2.  Anonymous, "Why  the  U.S.  Is  In  Danger of Being  Engulfed  by  its
         Trash," U.S.  News  and World Report, Washington   D   C
          (September  8,  1969).'   ''

3.  President's  Science  Advisory  Committee's Report  of the Environ-
         mental  Pollution Panel,  "Restoring the  Quality  of Our  En-
         vironment ," Washington,  D. C.,  141, (November,  1965).

4.  Glass  Industry,  92(1):40,  (1968).

5.  Anonymous, "Solid  Waste Handling in  Metropolitan Areas," U. S.
          Department  of Health,  Education, and Welfare, Washington,
          D. C.,  (February,  1964).

6.  Anonymous, "Proceedings--The  Surgeon General's  Conference on
          Solid Waste Management," U.S.  Department of Health, Educa-
          tion, and  Welfare, Washington,  D.  C., (July, 196?).

7.  Anonymous, "Proceedings of  the National Conference on Solid Waste
          Research," American Public Works Association,  (1963).

8.  "Research  and  Training  Grants," U.S. Department  of Health,  Educa-
          tion, and  Welfare, Washington,  D.  C., (1968).

9-  Vail,  J.  G., Soluble Silicates:  Their Properties and Uses, Vol.
          _H_,  Reinhold  Publishing  Corp.,  New York, (1952).

10.  Rawson, H.,  Inorganic Glass  Forming  Systems, Academic Press, New
          York,  (T9&7T

11.  Moore, H., and Carey, M. ,  "Limiting  Composition  of Binary Glasses
          of  the  Type xR20'SiO?  in Relation to Glass  Structure,"
          Journal of the Society of Glass Technology, 35:43-57,  (1950-

12.   Imaoka, M.,  and Yamazaki ,  T., "Survey of Glass-Forming Regions  in
          Simple  Silicates," Journal of the Ceramic Association  of
          Japan,  61:13-14,   (1963).

13.  Scholes,  S.  R., Modern  Glass  Practices, Industrial Publications,
          Inc.,  Chi cago, (1951).

14.  Samseons,  M. 0., Journal of the Society of Glass Technology, 182,
          517  (1926).

15.  Pranatis,  A. L.  , "Coaxing  Effect Furing the  Dynamic  Fatigue of
          Glass," Journal of the American Ceramic Society, Vol.  52  (6),
          340-341 (1968).

16.  Holland,  A.  J.  and Turner,  W. E. S., Journal of the  Society of
          Glass Technology,  20,  72  (1936).
                                             U 8  EPA Headquarters Library
                               43             '   Mail code 3404T
                                             1200 Pennsylvania AvenijeNW
                                                Washington. DC  204oi>
                                                   202-566-0556

-------
17.   1969 Book of ASTM Standards,  Part  13,  Refractories,  Glass  and^
          Other Ceramic Materials,  Published  by  the  American  Ceramic
          Society for Testing and  Materials,  Philadelphia,  Pa.,  (1969)

18.   Anonymous, "Glass Strength  Test,"  Glass  Industry,  45(2),  80-81,
          (1964).
19.   Guyer,
          Gl
          (1930).
E. M., "Mechanical  Properties  of  Some  Rolled  and  Polished
ass," Journal  of the American  Ceramic  Society,  J13:624,
20.  Williams, A.  E.,  "The Mechanical  Strength  of Glazing  Glass,"
          Journal  of the American  Ceramic  Society,  J6:980,  (1923).

21.  Phillips, C.  J.,  Glass:   The  Miracle  Maker, Pitman  Publishing
          Corp., New York, (1941),

22.  Rudnick, A.,  Hinter, A.  R., and  Holden,  F.  C.,  "An  Analysis of
          the Diametral-Compression Test," Materials  Research  and
          Standards, Vol. 3:4, 283-289,  (April ,  1963) .

23.  Gehlhoff, G., and Thomas, M., "An Investigation  of  the  Effect of
          Composition  on the  Strength  of Sodium Silicate Glass,"
          Zeitschrift  Fver. Physik.,  7:105,  (1926).

2k.  Kingery, W. D., introduction  to  Ceramics,  Wiley  Publishing Co.,
          New York, (I960).~

25.  Cooper, M. M., "Protective Oxide  Coatings  for  Glasses by  Chemical
          Vapor Deposition,"  Progress  Report  No.  1  submitted to United
          States Public Health Service under  Research Grant  No.
          UI00651,  (April, 1961).

26.  .Watanabe, M., Caporali,  R.  V., and Mould,  R. E., "The Effect of
          Chemical Composition on  the  Strength  and  Static  Fatigue of
          Soda-lirne Glass," Physics and Chemistry of  Glasses,  Vol. 2,
          Mo.  1, (February, 1961).
                                    44

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                           APPENDIX






               TABULAR RESULTS OF VARIOUS TESTS






     Although the graphical results are displayed during the text of




the thesis, this section was added to clarify any questions concerning




the number of specimens tested yielding usable data, numerical averages,




and standard deviations for each experiment.






                                       NO. SPECIMENS NUMERICAL STANDARD
TEST
Bending Strength
Versus Composi-
tion using
graphi te mold.

Bending Strength
Versus Composi-
tion using steel
mold .

Effect of Re-
anneal ing on
Bending
Strength .



COMPOSITION
1 .0 Na20 • 1 .6 Si02
l.P Na20 • 1.3 Si02
1 .0 Na20 • 1 .2 SiO
1.0 Na20 ' 1.1 Si02
1 .0 NaJD • 1 .0 Si02
1.0 Na20 • 1.6 Si02
1.0 Na20 • 1.3 Si02
1.0 Na20 • 1.2 Si02
1.0 Na20 "1.1 Si02
1 .0 Na20 ' 1 .0 Si02
1.0 Na20 • 1.3 Si02






TESTED
11
18
9
10
7
12
23
19
11
8
23 at 0 hr
6 at 2 hr
5 at 4 hr
6 at 7 hr
4 at 12 hr
5 at 18 hr
4 at 23 hr
AVERAGE
29,320
30,850
22,010
17,870
13,990
6,880
8,750
10,960
12,030
12,790
13,980
11,930
13,2/0
1 13,980
13,650
11,800
10,910
DEVIATION
2,390
1,810
2,520
2,670
1,900
1,470
970
1,620
1,270
1,080
760
1,860
2,010
870
1,170
1,170
1,840
                               45

-------
                                      NO. SPECIMENS NUMERICAL STANDARD
      TEST            COMPOSITION         TESTED      AVERAGE  DEVIATION

o- 1 .0 Na00 ' 1.3 Si00 23 at
2 2
ure ^ at
chere 6 at 1
0 hr

1 hr
3/4 hr
8,750

5,510
6,810
970

690
560
longed  Exposure

ic the  Atmosphere

                                      5 at 2 1/2 hr   9,180

                                      5 at 3 1/2 hr  11,370      860

                                      k at k 1/2 hr  12,680    1,370

                                      3 at 5 1/2 hr  10,500    2,3^0

                                      5 at 7 hr      12,650    1,750
                                    46

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                              PART III




THE RATE OF DISSOLUTION OF SODIUM SILICATE GLASSES IN AQUEOUS SOLUTION

-------
                          ABSTRACT






     The rate of reaction of soluble silicate glasses with aqueous




solutions was studied.  The work reported is part of an investigation




into the feasibility of processing glass containers after use so that




one large source of solid waste can be relieved and possibly elimin-




ated.



     The apparati used in studying the rate of reaction of the glasses




with water is described in detail.  Two basic methods of rate deter-




mination are discussed.  These two methods are measurement of the




sodium ion concentration in solution, and continuous measurement of




the total bulk weight loss.  These two parameters are plotted versus




time and related to the rate of reaction of the glasses with the water




     The glasses studied were simple sodium silicate glasses in the




range 1.0 Na20. 1.0 Si02 to 1.0 Na20 .  21. SiO-.   The glass batches




were melted at 2050°F and annealed at 800°F.  Surface characteristics




of samples were difficult to control, but were generally uniform.




These samples were dissolved, and the rates of reactions with water




under various experimental  conditions were plotted.  Particular




emphasis was placed on the variables affecting the rates of reaction.




The variables observed were the effects of time,  temperature, volume




of solvent, and turbulence.




     The rates of dissolution were found to increase with temperature




and turbulence.  The effect of the volume of solvent to which the




glass  is exposed was less than expected.

-------
     It  was  found that glasses of a rapid dissolving sodium silicate



composition  dissolved as a simple solution process in the same man-




ner as a sugar crystal.  In more durable glasses, there is the forma-




tion of a boundary layer through which sodium  ions and water mole-



cules must diffuse for solution to occur.
                                  ill

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                           PART III

                       LIST OF FIGURES


Figure                                                            Page

 1.   Photograph of the mold used  to form the glass disks	18

 2.   Photograph of the soluble glass before and after
       KOLDMOUNT has been applied	20

 3.   Photograph of the experimental apparatus used to
       measure water attack under static conditions 	  22

 k.   Photograph of the Beckman Sodium Ion Analyzer  	  23

 5.   Photograph of the experimental apparatus used to
       measure water attack under turbulent conditions	26

 6.   Plot of sodium ion concentration versus time at  72  C
       under static conditions	30

 7.   Photograph of the boundary layer forming on the
    *   surface of more durable glasses	32

 8.   Photographs of the breakdown of the boundary layer
       of more durable glasses	33'

 9.   Plot of sodium ion concentration versus time at  90  C
       under static conditions	35
10.   Plot of sodium ion concentration  versus  time  at  65  C
       under turbulent conditions
37
11.  Plot of sodium ion concentration  versus  time  at  70  C
       under turbulent conditions  ................  38

12.  Plot of sodium ion concentration  versus  time  at  75  C
       under turbulent conditions  ................  39

13.  Plot of bulk weight loss  versus time at  2^°C  in  300
       mill iliters of water ...................  42

14.  Plot of bulk weight loss  versus time at  72°C  in  300
       mi 11 iliters of water
15.   Plot of bulk weight loss versus time at 24°C  in ]k liters
       of water
16.  Plot of bulk weight loss versus time at 72°C  in 300
       mi 11 iliters of water for a more durable 68% silica
       glasses
                               vi

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                           PART III
                       LIST OF TABLES


Table                                                            Page

 I.  Analysis of the Silica Raw Material Used	1&

 II.  Analysis of the Sodium Carbonate Raw Material
       Used	   17

-------
                            PART III
                      TABLE OF CONTENTS






Chapter                                                           Page




  I.  INTRODUCTION ........................   1
 II.  LITERATURE SURVEY
III.  EXPERIMENTAL PROCEDURES ..................  15






 IV.  PRESENTATION AND DISCUSSION OF RESULTS ...........  28






  V.  CONCLUSIONS ........................  51






 VI.  RECOMMENDATIONS FOR FUTURE WORK ..............  53






LITERATURE CITED .........................  55






APPENDICES ............................  57




      APPENDIX I.   Solutions ...................  57




      APPENDIX II.  The Solution Process .............  55
                                IV

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FI9ure                                                           Page

17.  Photograph showing the similarity of concentration
       gradients flowing from the surface of sugar  and
       rapidly soluble glass	
                               vn

-------
                           CHAPTER  I


                         INTRODUCTION


     In a society which  has  its  share  of  problems  such as air and

water pollution, conservation  of natural  resources,  and deterioration

of the cities, there now is  another problem which  must be met and

solved if Americans are  to go  on enjoying their  present standard of

living.  The problem is  called "solid  waste management."

     Solid waste management  is not a new  problem,  but  it is odd that

many people do not realize that  it even  is  a  problem today.  If a

person happens to live near  a  city dump,  he may  be quite aware of the

problem, for it strikes  home on  two sides.  He finds that his property

value is not what it might otherwise be and also the environment may
                                 t
be poor due to air pollution or  the unsightly dump.

     However, most people  go about their  daily lives never giving

thought to what the "garbage man" does with their  solid waste.   All

they ask is that it be carted  off regularly.  In general, the public

has been apathetic until their trash cans stay full  for an extra day.

     The amount of solid waste produced in  a  country is closely cor-

related to the standard  of living of the  people.   All prognosticators

are pointing to a boom in  the  1970's for  the  United  States economy.

As bad as our solid waste  problems are today, everv  indication  is that

they are going to become much  greater, unless some positive steps are

taken immediately.  Government has already  recognized the problems and

started to move to aid in  solving them in numerous ways.   It still

-------
remains for the public and industry to become as concerned as they




should be about the magnitude of the problem.  Municipalities and




industry need a public mandate to get together and try to alleviate




the problem.




     The importance of convenience to the consumer has now become




firmly established as a guiding principle in the design of a packag-




ing material.  With this in mind, beverage container manufacturers




are going more and more to the disposable bottle.  The Midwest Research




Institute has projected that between 1966 and 1976 the number of



beverage containers in use in the United States will double to an es-




timated 62.9 billion units (1). In this period the use of cans and non-




returnable bottles will also double.  The problem is:  what do we do




with these wastes?!



     It should be realized that it is going to be necessary to deal




with this problem on a long term basis.   At the local level, sanitary




landfills are the primary method of disposal.  However, this is only




a temporary means of disposal which holds a remarkable similarity to




the housewife sweeping the dust under the carpet.  It may be put out




of sight temporarily, but it is still there.  This all points to one




of the basic problems concerning sanitary landfills.  This problem is




the settling rate which occurs when raw refuse is used.  "Compaction




of the various packaging and other materials generally is not complete,




creating undesirable voids which trap liquids and gases.   Along with




decaying organic matter, this commonly results in noticeable settling




of the landfill for perhaps 15 to 25 years even in a properly operated



landfill"  (2).

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     Perhaps the answer to the problem  lies  in  taking a different ap-



proach.  Instead of trying to devise  cheap methods of disposal as is




the general practice today,  it may  prove  beneficial  to work on means




of attaining long-range results  even  if they  prove more costly than



the short-range operations.




     It is this approach  that  is being  taken  at  Clemson University.



Work is being done to perfect a  one-way container, made of water-



soluble glass, which when discarded can be broken and dissolved.



While  in use the container must  be  covered by a  protective coating



both inside and out to prevent any  degradation.  Later this coating



can be broken and the bottle can be dissolved.



     This  research, supported by a  Solid  Wastes  Program grant, could




aid in the waste disposal problems  resulting  from the increasing use



of nonreturnable glass containers.   It  is estimated  that 26 billion



of these containers are discarded each year  in  the United States (2).



The objective of this research was  to give insight into the factors



affecting  the dissolution of sodium silicate  glasses.  The variables



which were studied were the effects of composition,  temperature, ex-



posure time to solvent, turbulent conditions  in  the  system, and



sodium concentration in the solution.  The data  collected was used to




determine  the mechanisms  which control the solution  process.



     These results give criteria as to what type of  glass is more



suitable for implementation on a practical basis in  the glass in-



dustry as a water-soluble glass  container.   It also  serves as a guide



for the avenues of approach which would prove most beneficial to




future researchers in this area.

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                         CHAPTER I I






                      LITERATURE SURVEY






     There has been a substantial  amount of work done in the area of




the reaction of glass with aqueous  solutions, or the chemical dura-




bility of glasses.



     Silvermann (3) lists some general factors to consider in the chem-




ical durability of any glass.  These factors are:



     (1)  State of division:   The amount of surface exposed by a




body determines how fast it reacts.



     (2)  Condition of the glass surface:   If the glass surface is




continuous, the effect will be different than if the surface has




been chipped, scratched, or abraded.




     (3)  Time of exposure to the solvent:   Exposure time may deter-




mine the extent of external or internal influence on stability.




     (4)  Temperature:  Conditions  during firing and after fabrica-




tion affect durability.



     (5)  Pressure:  External pressure during treatment or use and




internal pressure of contents or strain may affect stability.




     (6)  Concentration of extraneous reactants




     (7)  Radiation



     (8)  Mold and Bacteria




     (9)  Composition




     F. R. v Bichowsky (k) has presented six mechanisms or steps which




take place theoretically at some time during the "weathering" of

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glass in an aqueous environment.   These mechanisms  are:




     (1)  Adsorption of water  on  the surface




     (2)  Diffusion of adsorbed water molecules  into  the  body of  the



glass




     (3)  Reaction of  the  dissolved and adsorbed  water with  the glass,




forming a surface film of  a  character depending on  the glass composi-



tion




     (4)  Soaking up of water  by  this film thereby  getting more



reaction




     (5)  Extraction of soluble salts from this surface film




     (6)  Solution of  the  silica  skeleton film in the strongly alka-



line solution formed.




     It has been shown  that  the extent to which any glass may be




attacked or partially  dissolved by water is  greatly dependent upon




its composition.  Sodium disilicate has long been melted  for the manu-




facture of water glass.  This  composition is used because it is water




soluble.  Sodium metasilicate  glasses dissolve even more  rapidly.




Whenever metallic oxides are introduced,  insoluble  silicates are formed




which decrease  the rate of solution.   This rate  is  also affected by




other factors such as water  impurities, striations  in the glass in



which the ratio of alkali  to silica varies from the main  composition




of the glass, and strain which is  set up by  cooling through an un-




controlled temperature cycle.  Of  course such physical things as




amount of exposed surface  area and the degree of  circulation of the




liquid will affect the rate  of solution (5).

-------
     Morey (6) and Scholes (7) are of the opinoin that ft fs Incorrect




to speak of the solubility of glass as one would discuss the solubility



of salt.  The action according to them is one of diffusion and disin-



tegration and not one of true solution.  All that can be measured and



compared is the rate of attack of the solution on the glass, not the



solubility of the glass in that medium.  The  problem arises because



the glass changes its internal composition.  "The mechanism of the



attack of water upon an ordinary glass cannot be accurately set forth




in the present state of the knowledge.  But it is highly selective" (7).



     When silicate glass is exposed to water, there is the transport



of water constituents into the glass.  The reaction which follows is




dependent upon the composition of the glass and the capacity of the



water to act as a sink for  metal hydroxides (8).  There are two types



of general attack to be considered:  attack on non-bridging oxygen




atoms by reagents with an electron deficit and attack on the network



silicon atoms by reagents with electron excess.  Acids and neutral



solutions are called electrophi1ic reagents.  They attack positions



with excess electrons.  This type of attack is typified by a form



of ion exchange between the glass and attacking solution (9).   When in



contact with a liquid aqueous phases, glasses which contain alkali



cations will  undergo an ion exchange mechanism in which the liquid




water acts as a sink for alkali hydroxide by dissolving them (8).




     Alkalines are nucleophilic reagents.  They attack positions with



a deficit of electrons.  Alkaline attack is usually typified by de-



gradation of the glass network.

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     Water is a very good solvent of  glass  because  it can attack the



glass both electrophi1ically and nucleophi1ically.  Water ionizes to




form H  ions which are electrophi lie  and OH~  ions which are nucleo-




phi lie.  Even a neutral water molecule  can  be weakly nucleophi He (9).



     In an ordinary commercial  glass, Scholes  (6) says that sodium sili



cate is leached from the glass  and  undergoes hydrolysis.  The solution




which  is formed contains sodium hydroxide and colloidal silicic acid.



The sodium hydroxide which  is produced  on  the surface reacts with car-



bon dioxide  in the air while the silica separates out.  The surface



film then becomes a deposit of  crystals of  sodium carbonate together



with finely  divided silica.   If the attack  has  not  proceeded too far,



the film can be washed off  bv a dilute  acid or  even by water, but if



attack  is severe, the  surface  is permanently ruined.  The relative



insolubility of silica causes  it to exert a protective action which



causes  the attack to proceed more slowly as time goes on.



     Most literature concerns  itself  with solving the problem of



making  a glass which  is  less susceptible to attack  by aqueous solu-



tions.  Therefore,  it  is necessary  to use the  information obtainable



in reverse to get a soluble glass.  The literature  is also of some




assistance in specimen preparation  in the  laboratory.



     It has  been shown that the range of glass  formation in the



Na 0-SiO  system is continuous  from Si02 to a  limiting composition



close  to that of the 'metasiIIcate Na20-Sl02.  The limiting composition



depends on the experimental conditions.  The Na20 content of the glass




can be  increased as the melt becomes  smaller and the cooling rate be-

-------
comes greater.  Imaoka and Yamazaki found It possible to make glasses




containing up to 57.8 mole per cent Na«0 on a scale of 1-2 grams.  It




is essential that in all work the melting and cooling conditions be




reported when talking about the region of glass formation in a system.




     The limit to glass forming compositions in the Na20-Si02 system




coincides almost exactly with the composition at which Na20 becomes




the primary crystalline phase or the first phase to crystallize from




the melt.  At this composition, sodium begins to crystallize before the




glass crystallization can be stabilized.  This change will be reflected




in the kinetics of the process (10).



     The compos it ion wi11 have a pronounced effect on the rate of




devitrification.   Dietzel and Wickert (11) studied this effect by




making measurements on twenty compositions in the Na-O-SIO^ system




containing 0-42 mole per cent Na20.  "The stability decreases markedly




in the range 0 to 0.5 mole per cent Na20.  Then in the range above




10 mole per cent Na20, it increases to a maximum at 23 mole per cent




Na20, a composition close to the eutectic between silica and sodium




disilicate, Na^SiJDJ'Oo).  The explanation for this type of curve




is that the stability will decrease as more Na-0 is introduced because




the extra non-bridging oxygens will break down the continuity of the




silica network.  Low stability at those compositions corresponding to




the compounds Na^iO,. and Na2$iO. occurs because there is a close




similarity between the structure of the melt and that of the correspond-




ing crystal.  This condition enables crvstal1ization to occur readily



(10).
                              8

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an-
     Bowen (12) acknowledges  the  problems  associated with obtaining




the proper annealing temperature  for  a  glass.   It  is necessary to




neal the glass to free  it from excessive strain.   This strain can have



a marked effect on the  internal properties of  the  glass and  its chemi-




cal susceptibility to hygroscopic attack.   The choice of temperature




is quite important.  To be able to choose  the  proper annealing temper-



ature, it is necessary  to know the crystallizing power for various



temperatures of the glass to  be annealed,  in order to choose a temper-



ature that will not induce devitrification.  Both  annealing and



devitrification will take place faster  at  higher temperatures.  In the



same manner, lowering the temperature may  not  help  because there is




no definite lower limit of devitrification temperatures and the rate



of annealing may be too slow  if the temperature is  too low.  Composi-



tional changes, of course, can be made  which will  affect the anneal-




ing rate or decrease the rate of  devitrification.   Cox (13) points



out that those glasses  higher in  soda,  low in  barium, and high in



silica, tended most readily to devitrification.  Little devitrification



was noticed in glass high in  potash content, if arsenic was not present.



     There are many literature sources  which relate methods of in-



creasing durabi1ity of a glass.   It is  known that  solubility and the



tendency to weather are increased  directly as  the  lime to soda ratio



in a glass (14).  Baillie (15) and  Vail (5) both point out that a



glass becomes less durable  as the  simple  silicate content is in-



creased.   It has been shown that  there  is  better chemical  durability



for soda glasses than for potash  glasses made  up to the same molecular

-------
formula,  However, a glass composed of both alkalies is more durable




than one containing  either NaJ) or ICO alone (6).   Sen and Tooley (16)




state that the maximum durability of a glass composed of simple 1^0




and Na20 is at a K20/Na20 ratio of 2.6/KO by weight.



     It is widely known that the presence of alumina in the glass will



retard the weathering of the glass.  The addition  of 1.5 to 2.5% alum-



ina to the glass composition will improve the chemical  durability



significantly.  The greatest benefit will occur when the alumina is



one-eighth the soda content (17).



     The reaction product of the proper quantities  of sodium carbonate



and silica will be a clear glass of two types.  One type is neutral



glass, Na20 to SiO- ratio of 1/3.3.  It is pale bluish  or greenish in



color.  The other type is alkaline glass, ratio of  1/2.1, and its




color is usually yellowish (5).



     Niggli (18) studied the reaction of sodium carbonate and silica



at 898°-956  Centigrade and one atmosphere carbon  dioxide.   All  CO-



was expelled and equilibrium could be approached from both directions.



It was also found that the amount of C02 displaced  per  mole of silica



was greater the smaller the amount of silica in the melt.



     Howarth,  ej^ aj_ (19), studied the reaction from equimolar to a



composition of 4 silica:! soda.  The equimolar mixture  took 40 hours



for complete expulsion of the C02 at 700 C.  At temperatures above



900°C, the speed of reaction in all the mixtures increased rapidly.




Commercial soluble silicate glasses are for reasons of  economy made




at temperatures in the range of 1200 to 1^00 C.  Preliminary heating
                              10

-------
at about 700 C allows most  of  the carbon dioxide to escape.   Niggli




found that it takes a ratio more siliceous  than 1  alkaline oxide  to



2 Si02 to expel all of  the  C02 from glasses at 900 to 1000°C  (5).




     There have been many attempts made at  measuring the  durability



or weathering or  rate of attack of a glass  by its  environment.  Morey



is quite skeptical of any such results  which may be obtained  by the



numerous methods  now  is use and probably justifiably so.  The re-



quired precise control  of experimental  conditions  is one  reason for



such skepticism.  The tests used in the past range from visual clas-



sification of "not affected" and "very  affected" to some  very precise




and elaborate titration measurements recommended by the Committee on



the Chemical Durability of  Glass of the American Ceramic  Society.



All have some common  points of agreement and some  differences.  All



recognize the  importance of controlling surface area; of  attaining



homogeneity  in the glass; and  of controlling external factors such



as temperature, exposure time, and concentration of solution.  The



basic differences lie  in the method of  determining how much deteriora-



tion has taken place.   One  school of thought maintains that the best



method is the direct method of weighing both before and after exposure,



The other way  is  to measure the amount  of attack by measuring the



quantity of  alkali,.yielded  to  the solvent under definite  conditions.



This has been done by the use  of flame  photometry,  by measuring



electrical conductivities,  and by using elaborate  titration techniques




Morey is quick to bring attention to the fact that any comparison of



the results  of one observer with those  of another  Is rarely possible,
                              11

-------
and in any case the validity of any such comparison requires proof.



     Morey dealt with systems of wide ranges of temperatures and



pressures and decided that true phase equilibria do not exist in



systems involving water and complex silicate minerals.  When per-



forming an experiment on the reaction of a glass with aqueous solutions,




several mechanisms may be involved in any experiment.  The rate-con-



trolling processes may vary during the experiment as well as with time




and temperature, with the ratio of glass surface area to the volume



of solution, and particularly to the way the pH of the solution is



controlled.  The basic objective of an experiment in this area is




to control the time and temperature of the experiment adequately



and exercise control over the compositional  change of the attacking




medium (20).



     Douglas and Isard (21)  suggest that the process of dissolving



a soda-1ime-si1ica glass is diffusion controlled.  A model  has been



developed in which the rate controlling process is the diffusion of




Na  through the glass to the surface in order to react.  This work



found in general a linear relation of the quantity of alkali  extracted



from the glass to the square root of time in the early stages of the




reaction.   In later stages it was found that the quantity of alkali



was linearly dependent on time.



     In later work, Rana and Douglas (22) found also that the rate




of reaction varies to a close approximation  / t   in the early stages,




but later the rate of reaction becomes constant.  It was also found




that in binary silicate glasses the ratio of silica to alkali removed

-------
during the reaction  is  much  less than the ratio of  the  constituents




in the glass.   It was shown  that the change in dependence  of  the  rate




of reaction from  /T~  to linearity takes place in  a  short time  inter-




val.  This "time  of changeover"  varies from glass to  glass  and  from




temperature to  temperature.   The amount of alkali  removed  is  approxi-



mately constant up  to the changeover.




     Haller  (8) proposes that "water uptake follows initially a para-



bolic law and  is  accompanied by a volume expansion.  The rate of  up-




take is affected  by  the previous hydration history  of the  samples,




suggesting that an  irreversible change of the glass structure is



associated with the  diffusion of the water into the glass."   The




model proposed  by Douglas and El Shamy (20)  suggested that a  leached




layer is formed on  the  surface of the glass.   Silica  is then  dis-




solved at the  leached layer-water interface while sodium ions  dif-




fuse across the leached layer from the glass into the water.   The




change in rate  from  root time dependence to linear  reflects that as




time passes diffusion will  slow as the leached layer  becomes  thicker.




     The rate of  extraction  of alkali from the glass  is independent




of pH+ when  it  lies  between  1 and 9.   If the pH  Is greater than 9,




the rate of extraction  decreases with increasing pH .   The effect on




silica extraction is opposite.  The rate of extraction  increases with




increasing pH+.   It  was found that pure water below 100°C  does little




compared to alkaline solutions.  There is a strong  influence  of al-




kali level on  the rate  of silica attack.  This work served to con-




firm Morey's deduction  that  the action of water on  glass Is a com-
                                 13

-------
plicated decomposition reaction and not a normal  solution process.



This is shown by the occurrence in solution of species chemically



different from the original  glass which impl ies some rearrangement




of chemical  bond occurs.   It was also shown that  aklali  ions are,



always, extracted in excess  of the silica.   This  leaves  an alkali-



deficient leached layer which is not appreciably  different in volume



from the original glass.   At the boundary between the leached layer



and the solution, there are  sites available to cations.   It was



surmised that the diffusion  process must be a  flow of hydrogen ions



from the solution and an  equivalent flow of sodium ions  from the



glass into the solution.   It is believed that  a glass of low chemical



durability would have a large leached layer-solution zone and a



smaller leached layer-glass  zone (20).
                               14

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                          CHAPTER  III






                   EXPERIMENTAL PROCEDURE






     The glasses studied  were  simple  sodium  silicate glasses with




Na20/Si02 ratios in  the  range  1.0/1.0 to  1.0/2.1.  These glasses were




chosen because of their  susceptibility to water attack and the ease



with which they could be  processed.




     The raw materials used were  of reagent  grade.  Their analyses are




shown in Tables 1 and 2.  These materials were hand mixed in a porce-




lain mortar pestle.  The  batches  were fired  in both alumina and high




temperature porcelain crucibles.  The firing temperature was 2050°F




with a soak time of  approximately thirty minutes.  All firing was



done in a small globar heated  furnace.




     After soaking for 30 minutes, the glass was formed into small




disks.  The mold used to  form  the disks was  made of steel (See



Figure 1).  The glass was poured, pressed, allowed to cool  slightly




in the mold,, and annealed at 800°F for thirty minutes.  The furnace




power was turned off, and the  furnace was allowed to cool at approxi-




mately 158°F per hour.  After  cooling,  the glass disks were removed




and placed in a desiccator until  such time as the tests were made.




     The surface area of  the sample exposed  to the water in experiments




had to be constant,  or some geometric time factor had to be considered




to account for changes in the  surface area.  It was decided to try to




isolate one surface  of a  piece of glass and work with that surface




only.  In this way several things would be accomplished:  the surface
                                15

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                 TABLE I
                 SILICA
Silica



Chloride



Sulfate



Calcium



Heavy metals (as lead)



I ron



Magnesium



Potassium
0.0005%



0.0030%



0.0100%



0.0010%



0.0050%



0.0005%



0.0050%
                      16

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    TABLE 11
SODIUM CARBONATE
Sodium Carbonate




Insoluble matter



Lost on heating at 285°C



Chlorine



Nitrogen compounds




Phosphate



Sulphate



Ammonium hydroxide precipitate




Arsenic precipitate



Calcium and magnesium precipitate




Heavy metals (as  lead)




 I ron



Potassium




 Silica
                            99.5000%



                             0.0100%



                             0.5000%



                             0.0010%



                             0.0010%




                             0.0005%



                             0.0020%



                             0.0100%



                             0.0001%



                             0.0100%




                             0.0005%



                             0.0005%



                             0.0050%



                             0.0050%
           17

-------
Figure 1.   Photograph  of  the  mold  used  to  form  the glass  disks
                               18

-------
area exposed to the water  at  any time would be known,  and  the  labora-




tory fabrication problem of producing samples with identical total




volume  (or total surface area)  each time could be circumvented.




     A  disk shape was  selected  because it had the advantage of being




a regular shape which  was  easily fabricated.   All  that was needed




was a cylindrical glass  mold  in which a sample of any  desired  thick-




ness could be  formed.  Having chosen to work with the  disk shape,  it




soon became apparent  that, if the sides and bottom of  the disk were




covered,  it would  leave  only  one surface of known area exposed to  the




aqueous solution.   It, therefore, became standard procedure to cover




the side and bottom of the disk with an insoluble epoxy resin  (trade-




name KOLDMOUNT) before each test in order to isolate one surface




area (See Figure 2).




     There were three  experimental  methods used  to measure the rate




of reaction of the glass with water.  The first  two involved the




measurement of the sodium  ion concentration in the aqueous solution.




The third method employed  the measurement of  the total  bulk weight




lost by the sample during  exposure to water.



     To analyze the  rate of reaction of the sodium silicate glass with




water,  the sodium  ion  concentration of the water both  before and after




exposure  to the glass  surface was measured.   This  sodium concentration




was plotted versus time  to give some idea of  the rate  of reaction of




the glass when exposed to  water.



     In these  experiments, the  water to which the glasses were ex-




posed contained only  about 18 parts per billion  sodium ion concentra-
                                19

-------
Figure 2.   Photograph of the soluble glass  before and after KOLDMOUNT
           has been applled.

-------
tion.  Therefore, eighteen  ppb  was  used  as  the  zero  point or  starting



point for sodium concentration  in all  readings  taken.




     A coated glass disk was  placed in a flask  to which  200 mi 111-




liters of distilled water had been  added.   This flask was placed  in




a controlled temperature bath and  left for  a  certain time interval




(See Figure 3).  When  the appropriate  time  interval  had  elasped,  the




sample solution was extracted in approximately  the same  manner each




time.  The solution was analyzed for the sodium ion  content present




by the Beckman Sodium  Son Analyzer.  In  this  manner  it was possible




to study the rate of dissolution at different temperatures and at




different times.




     The Beckman Sodium  Son Analyzer has the  capability  to measure




from 1 to 10,000 parts per  billion  sodium ion concentration.  This




analyzer is a process  instrument that  can make  a continuous measure-




ment of the concentration of  sodium ions in a solution by determining




the difference in potential developed  between two electrodes, a silver/




silver chloride electrode and a sodium ion  electrode, immersed in




the solution being analyzed (See Figure  4).



     The operating principle  of the analyzer  is  that the sodium ion




measurement is made with a  glass electrode-reference electrode pair.




The glass electrode  is of a special  glass that  is very responsive to




changes In sodium ion  concentration.   This  response comes in the form




of a potential which is proportional  to  the log  of the sodium  ion




concentration present  In the  sample.   A  reference electrode is also




used to complete the circuit  and provide a  constant  reference potential
                                21

-------
Figure 3-
Photograph of the experimental  apparatus used to measure
water attack under static conditions.
                               22

-------
                             i
Figure 4.  Photograph of the Beckman Sodium  Ion Analyzer
                               23

-------
     ihe sodium Ion electrode is also sensitive to hydrogen ions




(pH ).   In order to permit effective sodium ion measurement regardless




of the pH  of the sample, it is necessary to control  the pH  of the




sample.  In the Sodium Ion Analyzer, a simple system provides for ad-




justment of pH  by the addition of anhydrous ammonia.  The electrode




is not sensitive to the ammonia ion or other cations, with the excep-




tion of silver, which was not present in the sample.



     The system operates in the following manner.   The sample passes




through the flow chamber which contains the sodium ion electrode and




the reference electrode.   Sample flow is controlled and the pH  of the




system is raised by bleeding ammonia into the stream through a capil-




lary tube.  As the sample passes through the chamber, the sodium




electrode senses changes in sodium ion concentration.  The difference




between the fixed potential of the reference electrode and the vary-




ing potential of the sodium electrode results in a signal which is




sent to a high impedance amplifier.  The amplifier gives a direct




meter scale readout.  It is a system similar to many standard pH




systems currently in use (23).




     The concentration of sodium in solution after a given time was




thus determined for a reasonably static situation.  It then was neces-




sary to determine what effect turbulent conditions would have on the




rate of attack of the glass by water.




     In this part of the study it was desirable to control the tur-




bulent  conditions as much as possible.   A system was developed which




could pump the water at a constant rate into the system at a given
                               24

-------
point.  The system  in  this  instance was a water vat  of approximately




10 gallons volume.   It was  found that an aquarium pump could  be  set




up to work very  adequately.   A tube from the pump was  attached so that



water flowed  into the  system 1/8 inch above the glass  sample, which




had been coated  as  explained before and mounted on a stainless steel



pedestal.  Samples  of  the solution were withdrawn from the same



position in the  system each time.   This withdrawal was  from a point



1/4 inch above the  sample.   The water vat was heated by a resistance



element.   In  addition, the  temperature was controlled  to ±1°C.




     By using this  apparatus (See Figure 5),  the rate  of water attack



upon the soluble glass at given times and temperatures  could be ob-



served under  turbulent conditions.   The sample was exposed to the



water at a constant temperature, under constant turbulent conditions,



for varying times.   Sample  solutions were extracted  and then analyzed



for sodium ion concentration by the Sodium Ion Analyzer, and plots



were made of  sodium ion  concentration versus  time.



     The rate of water attack upon the bulk glass composition was



tested.  This procedure  entailed suspending a glass  sample in water



under varying conditions.   While being exoosed to water attack, the



weight of the sample suspended in  the solution could be monitored



and the overall  weight loss of the bulk composition  could be deter-



mined at variable times.  There was data collected under the follow-



ing conditions:   (1) the sample was placed in a small  amount of water



(300 ml) at 24°C;  (2)  the sample was placed in a large  amount of water



    liters) at 24°C;  (3)  the sample was placed in a  small amount of
                                25

-------
                                         ':«»	
Figure 5-
Photograph of the experimental apparatus used  to measure
water attack under turbulent conditions.
                               26

-------
water  (300 ml)  at  a  high temperature (72°C);  (k)  the sample was placed




in turbulent water (14 liters)  at 2^°C; and (5)  a different glass com-



position was placed  in 300 ml  of water at 24°C and compared with the



more soluble composition.




     The samples were in each  case weighed continuously  by an Ainsworth



analytical balance that was connected to a strip chart recorder which



plotted  the weight loss versus time.  This work was done so that the



results  of the decomposition of the total bulk could be  compared to



the  loss of sodium ions from the sample.



     Spectrographic analyses were run on solutions which had been ex-



posed  to different glass compositions.  The glass samples were ex-



posed  to water for various times, and sample solutions were withdrawn.



A drop of  this solution was then analyzed by use of a Bausch and Lomb



dual grating  spectrograph.  The resulting photographic plates were



analyzed for  the relative intensities and were qualitatively related



 to the amount of the element present in the solution. These compari-



 sons coupled  with the other data enabled an understanding of how the




 attack of  water upon soluble glass proceeds.
                                 27

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                         CHAPTER IV






           PRESENTATION AND DISCUSSION OF RESULTS






     The samples used in this experimental  research were sodium sili-




cate glasses of varying composition.  In the discussion text these




samples will be referred to as being of a rapid dissolving, a moderate-




ly soluble, or a more durable composition.   The rapid dissolving




glasses are those having a Na20/Si02 molar ratio of between 1.0/1.0




and 1.0/1.3.  The moderately soluble glasses are those which have a




ratio of 1.0/1.3 to 1.0/1.6, and the more durable compositions have




molar ratios greater than 1.0/1.6.   Any of these glass compositions



will become more durable, less soluble, if there are impurities present




in the glass.




     The first experimental  measurements were made on a moderately




soluble glass  disk exposed to distilled water at a temperature of




72 C.   The only current flow or turbulence in the system was that set




up by a thermal gradient in the system.  This gradient was minimized




by the immersion of the flask in the water bath to give more isotherm-




al conditions.




     In these  measurements,  the disks were oriented horizontally on




the bottom of  the flask.  Although  strictly speaking it cannot be said




that the surface remained exactly constant, the change in surface




area was so slight as to not warrant consideration.  This statement




can be made because it was observed that the glass dissolved rather




evenly across  the exposed face.  This characteristic helps to minimize
                                28

-------
any additional concentration caused by an increase in surface area




due to a curved  surface  being exposed rather than a planar surface.




It was noted  that  after  a  certain period of exposure to the water a




surface film  or  coating  was being deposited on the glass disk.   This



surface deposit  remained for the entire duration of the experiment.



If the glass  was allowed to completely dissolve, the film would  re-



main  in the bottom of the epoxy shell.  After dehydration,  the resi-



due became a  brittle, white, flaky substance.  Other workers have



indicated  in  the literature that the substance is probably a complex



sodium carbonate-fine grain silica residue.



      Samples  of  the solution in the flask were withdrawn at various



times and analyzed for sodium ion concentration.  These concentrations



were  then plotted  versus time to yield the curve seen in Figure  6.



The ordinate  in  this figure is sodium ion concentration in  parts per



million as measured by the Beckman Sodium Ion Analyzer.   The abscissa



 is time of exposure to water in hours.  The time at which the sample



 is placed  into the water in the flask is considered the zero point.



 Initial sodium ion concentration is taken as 18 ppb,  the initial con-



centration of sodium in  the distilled water.  If the concentration



of sodium  is  considered  C, and the time is considered T,  then the slope



of the curve  dC/dT will  yield the rate at which sodium ions are  coming



 into  solution from the glass.  This may be the rate of solubility of



the glass, or it may be  the rate of water attack on the glass surface.



 Its exact  correlation with the solubility of the glass is not known.



All that can  be  said is  that the rate observed is the rate at which
                                29

-------
               soor
               400 -
               3OO -
CO
o
z
UJ
o
z
o
o
      o
      z
               200 -
               100 -
                                           34

                                           TIME, hours
      FIGURE 6.  PLOT OF SODIUM  ION CONCENTRATION VERSUS TIME AT  72°C UNDER

               STATIC CONDITIONS.

-------
sodium ions are going  into solution under these specific  conditions.
It is quite possible  that  slight changes in the experimental condi-
tions will yield  changes  in this rate.
     In Figure 6,  it  can  be seen that from the time  the sample was
placed into the water  until after about 2 hours of exposure, the rate
of increase of the sodium ion concentration was constant.  The initial
increase was  caused by the removal of sodium ions from the glass by
the aqueous solution.   The removal of these sodium  ions from the sur-
face causes the glass  to  develop a glass-solution  interface which is
depleted of alkali ions and is rich in  silica.   The  macroscopic ef-
fect of this  leaching  is  the formation  of a boundary layer through
which sodium  ions and  water molecules must diffuse for the dissolu-
tion to proceed  (See Figure 7).   As the boundary layer is formed,
the  rate  slows  to an almost negligible   increase in  sodium ion con-
centration  because diffusion is  occurring.
     After  three  hours exposure  time, the rate begins to  increase
again.  This  rapid increase in rate can be attributed to  two factors.
One  factor  is that as more sodium enters the solution the more basic
the  solution  becomes.   This basic solution is much more corrosive to
the  actual  silica framework than water alone.  As  the framework  is
corroded  at a faster rate, more  sodium is released  into solution at
a  faster  rate.
     The  increase in rate at this point is also caused by the break-
down of  the boundary layer (See  Figure 8).  After  the boundary layer
has  been  exposed  to the water for a time, it begins  to deteriorate
                                 31

-------
                  (A)
                                          (B)
                 (C)
                                        (D)
Figure 7•
Photographs of the boundary layer forming on  the  surface
of  a more durable glass (A) originally,  (B)  after  1  hour,
(C) after 2 hours, (D) after k hours.
                               32

-------
                   (E)
                                           (F)
                  (G)
                                          (H)
Figure 8.
Photographs showing breakdown of the boundary layer on a
more  durable glass (E) after 5 hours,  (F) after 6 hours,
(G) after 7 hours,  (H) after 8 hours.
                               33

-------
and allow water to diffuse through it at a faster rate.  This deterio-




ration then allows the glass matrix to be attacked at a faster rate




and yields the higher sodium concentration in solution.




     This theory can be supported by spectrographic analysis for the




silica content of a solution.  For times coinciding with the initial




leaching of sodium from the glass and for times coinciding with the




formation of a boundary layer,  it was observed that there is very




little increase in the amount of silica in solution.  However, for




times coinciding with the breakup of the boundary layer and alkaline



attack of the silica framework,  there is a very substantial  increase




in the amount of silica in solution.   This indicates that there is an



extended period of exposure during which very little silica goes into



solution followed by a period during which silica is rapidly dissolved.




     After about 5 hours, the rate gradually slows as there is degrada-




tion of the surface of the sample.  At this time the sample has been




corroded to the extent that it no longer presents a continuous surface




to the solution.  Water is able to go completely through the sample




and attack the sample on more surfaces.  Also the sample has been




almost completely eroded away.   Therefore, at this point the test




ce" as to be valid.




     Figure 9 shows the effect obtained by increasing the temperature.




The immediately noticeable effect is that the reaction takes place



much faster at higher temperatures.  For example, after one hour of




exposure at 72 C there was approximately 50 ppm sodium concentration




in solution.  After one hour at 90 C there was an average 115 Ppm
                               34

-------
                 600 r
GO
in
           O
           O
                 500 -
                 400 -
                  300 -
                  200 -
                  100 -
                                                               O
                              I        23456

                                          TIME,  minutes


           FIGURE  9.  PLOT OF SODIUM  ION CONCENTRATION VERSUS TIME AT

                     9O°C UNDER STATIC CONDITIONS.

-------
sodium concentration In solution.   This represents a very marked ef-




fect by temperature increase of just 18°C.   This increase in rate




is reflected in the greater slope  (dC/dT)  of the curve.   There is no




change in the rate of sodium uptake over the duration of the experi-




ment.  The sodium concentration increases  at a linear rate.   This in-




dicates that at higher temperatures there  is not an induction period




involved during which diffusion through a  boundary layer slows the



reaction.  This effect could be due to several reasons.   It  could be




that the boundary layer in fact never forms  at this temperature.   The




author considers this very unlikely from the standpoint  of the sur-




faces which have been studied and  photographed (See Figure 7).   It is




possible that at this temperature  a boundary layer does  form, but it




is broken down by water turbulence so that  it has no effect  on the




reaction rate.  Finally, at this temperature viscosity of the water




may have been reduced sufficiently for it  to diffuse through any




boundary layer that is present at  such a fast rate that  the  boundary




layer diffusion is no longer a rate controlling step.  The pronounced




effect of temperature is demonstrated by the results shown in Figures




6 and 9-  With the exception of temperature, these two experiments




were performed under the same static conditions.




     The effect of turbulence on the rate  of reaction of the glass




with water was observed next (See  Figures  10, 11, and 12).  The most




evident result of the turbulence is a dramatic increase  in the rate  of




sodium uptake.  From Figure 6, it  can be seen that it takes  one hour




under static conditions for 50 ppm sodium ion concentration  to show
                               36

-------
CO
--J
Hi
O
            200 -
            160 -
            120 -
                                 8
                                   12       16

                                   TIME, minutes
20
24
28
      FIGURE IO.  PLOT OF SODIUM ION CONCENTRATION VERSUS  TIME AT 65°C UNDER

                 TURBULENT CONDITIONS.

-------
             200 r
CO

CD
or

z
UJ
o

o
o
             160 -
             120 -
1 1
3
1 ,
4
1 ,
8
1
12
1 i
16
1
20
1
24
i 1
28
                                      TIME, minutes


       FIGURE  II. PLOT OF SODIUM ION CONCENTRATION VERSUS TIME AT  70°C

                 UNDER  TURBULENT  CONDITIONS.

-------
                 2OOr
00
cc
I-
z
UJ
O
z
o
o
                 I60h
                  I20h
                                      8        12

                                      TIME, minutes
                                            16
20
24
            FIGURE 12.  PLOT OF SODIUM ION CONCENTRATION VERSUS  TIME  AT 75°C

                      UNDER TURBULENT CONDITIONS.

-------
up in solution.  In Figure 10, it is found that under turbulent




conditions there is 50 ppm sodium ion concentration in solution after




only eight minutes.  This is an Increase of 7.5 times as fast under




turbulent conditions.  At this point, once again the buildup of a




boundary layer is sufficient to have a leveling effect on the curve.




The rate of increase of sodium ion concentration becomes quite small




until twenty minutes of exposure time has passed.   At this time, there




is once again a very rapid increase in the slope of the curve.  This




increase is probably due to the breakdown of the  barrier layer.  This




deterioration occurs rapidly because of the turbulent conditions around




the surface.  The constant flow of water against the surface causes




enough abrasion to deteriorate the boundary layer  and expose the




glass to attack as exhibited by the very steep slope after 20 minutes




exposure in Figure 10.




     Figures 10 and 11 show again in a clear manner the influence on




the attack process of even very minor temperature  changes.  If the




point at which the slope begins to increase again  after the induction



period is used as a reference, it can be seen that an increase in




temperature serves to shift the reaction to a shorter time along the




abscissa.  For example, in Figure 10 (run at 65 C) that change in slope




occurs at 20 minutes exposure time.   In Figure 11  (run at 70 C)  that




slope change occurs'after only 16 minutes exposure time.   In Figure 12,




the time has been reduced to only 14 minutes of exposure to water at-




tack.  So it can be seen that the effect of turbulence and temperature




in the system is very pronounced.  These results indicate that tur-
                              40

-------
bulent conditions  promote the dissolution of the glass.   All  of  the




results discussed  thus  far were obtained by analyzing the solution  in




contact with moderately soluble glass after certain exposure  times.




This analysis was  made  with the Beckman Sodium Ion Analyzer.




     The  remainder of  the results were concerned with changes  in  the




total bulk composition  of the glass.   In particular,  the  weight  loss




of the glass was monitored by an Ainsworth analytical  balance which




weighed the sample continuously.  This weight loss was plotted versus




time of exposure to water.  Typical  results of this analytical tech-




nique can be seen  in Figure 13.




      In Figure  13, a sample of rapidly dissolving glass was placed in




300 ml of distilled water at 2k C.  The sample was weighed continuous-




ly.  The  resulting curve is linear until  120 minutes  of exposure  has




occurred.  At this time, the sample had decomposed to the extent




that  there was  no  longer a continuous surface presented to the cor-



roding medium.   By this it is meant that the surface  crumbled and




depleted  to the extent  that the test  could not be considered valid




for  longer times.



      The  effect of turbulence was attempted to be shown,  but the  ex-




perimental apparatus could not be operated continuously due to the




swaying effect  caused  by the water flow.   Two weight  measurements




were made after 20 and  50 minutes of  exposure under turbulent condi-




tions.  These measurements are superimposed on Figure  13-  There  is




not enough data to make any conclusions except that,  as in the pre-




vious experiments, turbulence seems to have increased  the rate of
                               41

-------
ro
            to
            QC
            O
            CO
            CO
            O
            -J
O
UJ
                    8.0 -
                                                         0°
                                                            O
                                                  O w
                               4.0
                           8.0
   120      160

TIME, minutes
2OO
240
            FIGURE 13. PLOT OF BULK WEIGHT LOSS VERSUS TIME AT 24°C IN 300
                      MILLILITERS OF  WATER.

-------
attack on the glass.




     Next,  the effect  of  an  increase in  temperature was observed




using the weight  loss  measurement method.   The  water was preheated




to 72°C and  then  brought  into contact with  the  glass sample.  The




effect was  as expected.   The glass lost  weight  at  a much faster rate




at 72 C than at 2^°C  (See Figure H). An example  of this  is that at




2^°C the glass sample  had lost 1.7 grams after  50  minutes of exposure.




However,  if  exposed  to 72°C  water for only  5  minutes,  the same glass




lost 1.8 grams.   The  rate of bulk weight loss at this  temperature




once again  displays  linear characteristics.




     The effect of the volume of solvent (or  corroding medium) to




which the sample  is  exposed  was also investigated.   It was of interest




to observe  if  the amount  of  water to which  the  glass was exposed had




a great effect on the  rate of attack. Of particular interest was




whether or  not the process was self-limiting  in small  volumes of




water.   If  so,  it would indicate that the water was becoming saturated




with the  solute and  causing  the reaction to change, probably to slow




or stop completely.



      It was found that, if the sample were  placed  in \k liters of




water  instead  of  in  300. ml of water, there  was  very  little change in




the  rate of weight loss observed.  If the curves  in  Figures 15 and 13




are  compared,  it  is  found that they could almost be superimposed over




each other  without any change.  The only effect that was observed was




that with the  larger  volume  of water (14 liters) the loss  took place




at only a minutely faster rate; the increase  was to be expected.  The
                               43

-------
cr
o
CO
(0
o
o
UJ
5t
                                   12       16

                                   TIME, minutes
24
28
FIGURE 14. PLOT OF BULK WEIGHT LOSS VERSUS TIME AT  72«C IN 300 MILLI LITERS

          OF WATER.

-------
               10.0
en
        CO
         <0
         CO
         o
         g
         UJ
                                                          O
                                                            O
                                                              O
                                                                 O
                                                                   r* O
r • '
> 40
. i i
80
i I i
120
1
160
i I
200
i 1
240
                                      TIME, minutes


         FIGURE  15.  PLOT OF BULK  WEIGHT LOSS VERSUS TIME AT 24«C IN

                    14 LITERS OF WATER.

-------
slightness of this increase is surprising.   The lack of effect of




solute volume is shown in the following example:  After 40 minutes,




1.7 grams weight loss is observed in 300 ml  of 24°C water.  If the




sample is exposed to 14 liters of 2k C water, the weight loss in-




creases only 0.2 gram to 1.9 grams weight loss.  After 140 minutes




in 300 ml of water, the weight loss is 6.4 grams; in 14 liters,  it




is 6.5 grams.




     This very small  difference in weight loss between the two




volumes leads to the conclusion that the amount of solvent present




has very little effect on the rate of attack on the glass sample.




At least this appears to be true for the amounts of water used in




these experiments.  It is possible that at even smaller volumes




of water there is a self-limiting effect.  However, these experiments




indicate that, as long as there is enough solvent present to dissolve




the glass,  it does not matter how much excess solvent is present in




the system.




     The weight loss measurement was also plotted versus time for a




less soluble glass.  This glass was 68 per cent silica and 32 per




cent soda.  The results of this measurement showed an initial  loss,




followed by an extended period of very little or no weight loss  and




a final rapid weight loss (See Figure 16).   The various sections of




the curve can be explained by applying the boundary layer diffusion




theory outlined previously.  In fact, this curve is remarkably similar




to those obtained by sodium ion analysis.  It appears to follow the




same sequence as the rate of sodium ion  increase shown in Figure 6.
                               46

-------
      I.O
X
<
K
o
o
tal
     O.8
                                              -CX
C i
* IO
1
20
I
30
1
4O
i
SO
1
6O
i
7O
1
SO
                                     TIME, hours
 FIGURE  16.  PLOT OF BULK WEIGHT LOSS VERSUS  TIME  AT 72»C IN 3OO  MILLILITERS

            OF WATER FOR A MORE DURABLE 66H SILICA OLASS.

-------
There is an initial weight loss, a leveling-off effect  (a boundary
layer induction period), and a final  sharp increase in  the rate of
weight loss.
      If the results of the weight loss measurements of  rapidly dis-
solving glass  (Figure 1^) and more durable glass (Figure 16) are com-
pared, it is observed that there is a difference in the way that the
two types of glass dissolve.  The rapid dissolving glasses lose weight
at a constant  linear rate.  More durable glasses go through a change
in surface characteristics which causes it to dissolve  in steps.
     The constant linear rate of weight loss in rapidly dissolving
glasses is indicative of a true solution process (See Appendix I).
By simple solution it is being implied that there is no breakdown
of the glass structure into its individual components as far as the
solvent is concerned.  Both soda and silica complexes are being at-
tacked as one  single phase much as a single sugar crystal is dis-
solved in water (See Figure 17 and Appendix II).  This  theory is
verified by spectrographic analysis which shows that in rapidly dis-
solving glasses the silica concentration in solution increases at a
relatively constant rate.  In more durable glasses the  silica con-
centration remains low until such time as the silica framework is
attacked by solution.  This attack is exhibited by a dramatic increase
in silica concentration  in solution.
     The conclusion reached from this comparison is that sodium
silicate glasses which dissolve rapidly without the formation of a
boundary layer dissolve by a mechanism quite similar to true solution.
                                48

-------
Figure 17•
            soluble glass  (B) .
                               49

-------
In more durable glasses there is the formation of a boundary layer




which causes the dissolution of the glass to be more complex.  This




process can be described as the preferential leaching of sodium ions,




followed by the formation of a boundary layer, and ultimately al-




kaline attack on the remaining silica network.
                                50

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                          CHAPTER V

                         CONCLUSIONS

1.   The attack of water on a  rapidly soluble silicate glass released
    sodium ions  into  the solution at a  linear rate.  Weight loss also
    followed a linear  relationship versus time.  This linearity re-
    presents a simple  solution process  and not a complex decomposi-
    tion process as observed  by experimenters working with more com-
    plex glasses.
2.   In more durable but yet soluble glasses, there  is the formation
    of a boundary layer after a certain time of exposure to water.
    It then becomes necessary for the water molecules to diffuse
    through the  boundary layer before the solution  process can con-
    tinue.
3.  The effect of temperature is  that the rate of  reaction of the
    glass with the water is greatly  increased as the temperature
    is increased.  This  result  is to be expected if the dissolution
    of the glass  is an endothermic  reaction.   It was also found that
    temperature  can affect  the  boundary layer and  water viscosity to
    the extent that water can diffuse through  to the glass surface
    at a fast enough  rate to  maintain a constant rate of reaction with
    the glass.
*».  The effect of turbulence  was  demonstrated  to be a substantial
     increase  in  the rate of attack  of  the water on the glass  surface
    under turbulent conditions.   The explanation of this  is that
    turbulence produces more  abrasion on the  surface.  Subsequently,

-------
    the water molecules are able to cluster around the sodium sili-




    cate molecules more easily and carry them into the solution.




5.   Volume dependency was shown to not be as great as was originally



    thought.   The solution process did not seem to be self-limiting




    in small  volumes of water.  In fact, the amount of solvent pre-



    sent appears to have very little effect at all  on the rate of



    attack on the glass.  Larger volumes of water did show a slight



    increase in the amount of glass dissolved, but the increase is



    not very significant.
                                52

-------
                          CHAPTER  VI

               RECOMMENDATIONS  FOR FUTURE WORK

     The principles observed  in this  investigation  should  ultimately
aid in the utilization of soluble sodium silicate glass  in  the  glass
industry as a partial answer  to the solid waste  dilemma.
     The significance of  this work is  that it  demonstrated  the  feasi-
bility of producing a rapidly dissolving glass which  could  be easily
decomposed without undue  pollution of  water  resources.
     Having established  this  fundamental  ability to produce such a
glass, correlated study  can be  accomplished  to develop more suitable
glasses and study the solution  characteristics of those glasses.
Corresponding studies are being conducted in the area of coating
materials and procedures  for  various soluble glass  compositions.
     The major deficiencies of  the rapid dissolving glass appeared
in  two areas, fabrication and the ability to be  coated.  The solu-
bility of the glass  is such that  it will  easily  meet  any requirements
as  to speed of dissolution.   In fabrication, however, the glass
proved to be quite unstable requiring  close  attention to prevent
devitrification.  The forming of  nucleation  sites and subsequent
devitrification was almost immediately evident if fabrication proce-
dures were not closely controlled. Problems in  the area of-coatings
developed because of  the  extreme  hygroscopic!ty  of  the glass surface.
     The solution of  these problems will  likely  involve  the develop-
ment of more overall  suitable glass compositions, equipment modifica-
tions, better coating procedures  and materials,  and probably a  com-
                                53

-------
bination of all of these.
     Better quality control over the firing conditions and forming
conditions of the glass is a definite requirement at this stage.  A
furnace is required which will  not allow contamination of the batch
and has better control over the heating and soaking periods.   With
more sophisticated forming equipment, sample quality control  could
be upgraded to eliminate sample variations in uniformity, annealing
stresses,  and surface condition.  The elimination of these problems
would greatly enhance the potential  for success of the project.
     The author is of the opinion that, for the optimum development
of the project, new glass compositions which would dissolve more
slowly should be investigated.   A small pilot plant setup would  be
extremely helpful.  This would  be the next logical step if progress
is to take place in the fabrication  of a soluble glass container.
                              54

-------
                      LITERATURE  CITED
1.  Abrahams,  J.  H,   "Packaging Industry Looks at Waste Utilization."
         Glass  Container Manufacturers  Institute, Inc., Washington,
         D.  C.

2.  Vaughan,  R.  D.   "Packaging and Solid Waste Management."  U.  S.
         Department  of Health, Education, and Welfare,  Consumer
         Protection  and Environmental  Health Service.

3.  Silvermann,  A.   "Chemical  Durability of Glass:  An  Outline."
         Bui 1.  Am.  Ceram. Soc.  30:399.   1951.

4.  Bichowsky,  F. R.  v.  "Note on the  Mechanics of the  'Weathering1
         of Glass."   J^. Am.  Ceram. Soc.   3:309-  1920.

5.  Vai1,  J.  G.   Soluble Si 1icates:   Thei r Properties and Uses,
          Volume  I.   Reinhold  Publishing  Corp., New York.   1952.

6.  Morey, G.  W.  The Properties of Glass.  Reinhold Publishing
          Corp., New York.  1954-

7.   Scholes, S.  R.   Modern Glass Practice.  Industrial  Publications,
          Chicago.  1952.

8.   Haller, W.  "Kinetics of  the Transport of Water Through Sili-
          cate Glasses at Ambient Temperatures."  Physics and
          Chemistry of Glasses.  1(2):46-51.  I960.

9.   Budd,  S. M.  "The Mechanisms of Chemical Reaction  Between Silica
          Glass and Attacking  Agents."  Physics and Chemistry of
          Glasses.  2(4):111-114.  1961.

10.   Rawson, H.   Inorganic Glass-Forming Systems.  Academic Press,
          New York.   196?.

11.   Dietzel, A. and H. Wickert.   Glastech. Ber.  29:1-4.  1956.

12.   Bowen, N.  L.  "Devitrification of Glass."  J_. Amer. Ceram.  Soc.
          2:262.  1919-

13.   Cox,  S. F.   "Devitrification  of Glass."  J_. Amer.  Ceram. Soc.
          2:576.  1919-

14.   Ford,  K. L.  "The Weathering  of Glass Containers."  £. Amer.
          Ceram.  Soc.  5:837.    1922.

15.   Baillie, W.  L.   "The Effect of Additives on Chemical Durability
         of Glasses."  J. Soc. Glass Tech.  6:68-101.  1922.
                              55

-------
16.  Sen, S. and F. V. Tooley.  "Effect of Na-O/K-0 Ratio on Chemical
          Durability of Alkal i-Lime-SM ica Glasses."  £. Amer. Ceram.
          Soc.  38:175.  1955.

17.  Lyle, A. K. ,  W. Horak, and D. E. Sharp.  "The Effect of Alumina
          Upon the Chemical Durability of Sand-Soda-Lime Glasses."
          J_. Amer. Ceram. Soc.  19: 1^*2.  1936.

18.  Niggli, P.  "The Reaction of Sodium Carbonate and Silica."
          J_. Amer. Ceram. Soc.  35:1693.   1913.

19-  Howarth, J. T. , W. Maskill, and W. E. S. Turner.  "The Rate of
          Reaction Between Silica and Sodium Carbonate at Different
          Temperatures and the Process of Vitrification."  J_. Soc.
          Glass Tech.  17:25.  1933.

20.  Douglas, R. W. and T. M. M. El-Shamy.  "Reactions of Glasses
          with Aqueous Solutions."  J_. Amer. Ceram. Soc.  50:1-7.
          1967.

21.  Douglas, R. W. and J. 0. Isard.   "Action of Water and of Sulfur
          Dioxide on Glass Surfaces."  J. Soc. Glass Tech.  33(15*0:
          289-335.
22.  Rana, M. A. and R. W. Douglas.  "The Reaction Between Glass
          and Water."  Physics and Chemistry of Glasses,  2(6):
          179-205.  1961.

23.  Beckman Instruction Manual:  Trace Sodium Ion Analyzer.  Beck-
          man Instruments, Inc.  Fullerton, California.

2k.  Maron, S.  H. and C. F. Prutton.  Principles of Physical Chemistry
          The MacMillan Company.  New York.  1958.

25.  Sienko, M.  J. and R. A.  Plane.  Chemistry.  McGraw-Hill Book
          Co.,   Inc.  New York.  1957.
                              56

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                         APPENDIX  I



                           SOLUTIONS



     When several nonreacting  substances  are  mixed,  three possible


types of mixtures may  be obtained:   (a) a coarse  mixture,  (b)  a  col-

loidal dispersion, or  (c)  true solution.   In  a  coarse  mixture, the

individual particles are  readily discernible  and  may be  separated


from each other  by mechanical  means.   In  a colloidal dispersion, the

particles are much finer and  the heterogeneity  is not  as  apparent.

It is, however,  not a  homogeneous  dispersion.   The true  solution


possesses the characteristic  that  the  constituents cannot  be

separated by mechanical means.  A  true solution is a physically

homogeneous mixture of two or  more substances.

     The extent  to which a substance will  dissolve in  another  varies

greatly with different substances  and  depends on  the nature of the

solute and solvent, the temperature, and  the  pressure.   The effect

of pressure is usually small  unless  gases are being  used.  However,

temperature usually has a  marked effect.   The direction  in which the

solubility of a  substance  in  a solvent changes  with  temperature

depends on the heat of solution.   If the  substance dissolves exo-

thermal ly, the solubility  at  higher temperatures  will  decrease.

If, however, the dissolution  is an endothermic  reaction,  the solu-


bility will increase as the temperature is raised.

     A general rule is that likes  dissolve likes. Which means that


compounds similar  in chemical  character are more  readily soluble in
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each other than those of entirely different chemical character.




     The extent to which solids dissolve in liquids varies greatly




with the nature of the solid and liquid, the temperature, and to




some extent the pressure on the system.




     The influence of temperature on the solubility of a solute




in some solvent is usually quite pronounced.  Because most solutes




tend to absorb heat on solution, they are more soluble at higher




temperatures.  The opposite is true of exothermic reactions, but




all behavior is not regular in this respect.




     Whenever the solubility of any substance is plotted versus




temperature, the curve is continuous until  there is a change in the




nature of the solid phase.  As soon as the solid changes, a break




will appear in the solubility curve and a new solubility curve will




originate from this point.  These breaks are characteristic of the




substance involved.




     The particular change in the nature of the solid phase involved




could be a number of things.   It could be a transformation from one




crystalline form to another;  it could be a  change from a hydrate




to an anhydrous salt; or it could be a change from one hydrate to



another (24).
                               58

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                         APPENDIX  I I






                    THE SOLUTION PROCESS






     When sugar is placed  in a beaker of water, the sugar dissolves.




In this process of solution, the sugar  lattice  is ripped apart, and




the individual sugar molecules are  distributed  throughout the body




of the solution.  The entire process can be theorized to occur in




the following way:  First, a sugar  molecule must be pulled from the




surrounding molecules.  Since there is  molecular attraction between




molecules  in  the crystal  lattice, work  must be  done to overcome this




intermolecular attraction  (solute-solute attraction).  Second, water




molecules  have to be displaced to accommodate the sugar molecule.




This displacement also  requires the expenditure of energy, since




there  is molecular attraction between water molecules (solvent-




solvent attraction).   If  both of these  processes require energy, they



cannot be  the driving  force  for the solution to occur.  There has to




be a third step which  provides the  energy  that  is needed.  This step




is called  solvation.   Solvation arises  from the attraction between




the sugar  molecules and the  water  molecules.  This sugar-water




attraction provides almost enough  energy to overcome  the sugar-sugar




attraction and the water-water attraction.  The last  bit of energy




necessary  is  supplied  by  a reduction in the average kinetic energy




of all the molecules  in the  system.



     The actual state  of  the particles  once they get  into the solu-




tion is one of the really  challenging problems  facing chemists
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today.  Progress  is very slow.  Present  theory  assumes  that solution



occurs by some such mechanism as  the  following:   In  their random




thermal motion, solvent molecules  continually bounce against the




surface of the solute.  Solution  occurs  when solvent molecules  carry




off solute molecules  by clustering about them.  A  sugar  molecule




with  its associated cluster of water  molecules  is  called a  solvated




molecule and  is believed to move  off  into solution as a  unit.   Sol-




vent  molecules are continually exchanging places with other solvent




molecules  in  the  surrounding area  of  solvent even  though a  strong




attraction for the solute particle is present.  The  character of




the solvated  particles  is constantly  changing and  this tends to




make  any study of the system difficult.
                                      * U. S. GOVERNMENT PRINTING OFFICE : 1971 O - 446-907
                               60

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