FEASIBILITY STUDY OF
The Disposal of Polyethylene
        Plastic Waste

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        FEASIBILITY STUDY OF
The  Disposal  of Polyethylene
            Plastic Waste
            This report (SW-14c) was prepared
       for the Federal solid waste management program
              by KURT GUTFREUND
               IIT Research Institute
            under Contract No. PH 86-67-274
       U.S. ENVIRONMENTAL PROTECTION AGENCY

                    1971

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                     An Environmental Protection Publication
This publication  is also  in  the Public  Health  Service numbered series  as  Public Health Service
Publication No.  2010; its entry in  two government publication series is  the result of a publishing
interface reflecting the transfer of the Federal solid waste management program from the U.S. Public
Health Service to the U.S. Environmental Protection Agency.
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 55 cents
                                     Stock Number 5502-0036

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                         FOREWORD

THE PLASTICS INDUSTRY has created an impressive record in developing
many kinds of plastics for industrial and domestic use. The versatility of
shape, size, color, thickness, strength, and density, as well as resistance to
attack by other substances, has  catapulted plastic containers into wide use
for beverages, foods, cosmetics, and toiletries. In addition to being formed
into  containers per se, plastics appear as coatings and laminants—in combi-
nation with metal, glass, and paper—thereby permeating the entire packag-
ing field. As  a result, today's packaging  waste reflects not only a rapid
growth of plastics but also a decline in wood and textiles.
  Packaging wastes thus constitute a significant and growing fraction of the
total  solid waste load. Of the  350 million tons of residential, commercial,
and industrial solid wastes generated in 1966, for example, 13 percent was
discarded packaging materials.  In a typical year, Americans will throw away
48 billion cans, 26 billion bottles, 4 million tons of plastic, and 30 million
tons of paper. Per capita generation of packaging wastes in rising steeply. In
1958, 505 pounds of packaging materials were discarded on an annual per
capita basis; this figure had risen to 525 pounds by 1966. If present projec-
tions are accurate, it will reach 661  pounds per capita by 1976, unless the
means are found to modify this trend.
  A national research and development program was initiated by Congress'
with passage of the 1965 Solid Waste Disposal Act in an effort to deal more
effectively with the Nation's volumes of solid wastes. As part of this effort,
IIT Research Institute was asked—through the contract mechanism—to ex-
plore several  approaches to facilitate the disposal of polyethylene wastes.
Special emphasis  was to be placed upon  methods that  would (1) enhance
the brittleness of this polymer, (2) reduce its resistance to combustion, and
(3) promote its biodegradability. Throughout the term of the contract, the
Federal solid waste management program was represented by E. P. Floyd as
Project Officer.
  We trust that the resultant volume, which reports IITRI's study of the
plastic waste disposal problem, the degradation of hydrocarbon polymers,
and  the  results of  experimental investigations,  will provide some much
needed basic information. Perhaps this report will also stimulate and en-
courage other workers to continue  the  search for imaginative and varied
solutions to match  the array  of disposal  problems posed by the versatile
plastics.

                                     -RICHARD D. VAUGHAN
                                       Deputy Assistant Administrator
                                            for Solid Waste Management
                                 111

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                             PREFACE
  This is Report No. IITRI-C6120-11 (Final Report) on IITRI Project No.
C6120 entitled,  "Feasibility Study of the Disposal of Polyethylene Plastic
Waste." The  work  reported herein was  conducted for the Public  Health
Service, Solid Wastes Program  under Contract No. PS-86-67-274 during the
period from June 29, 1967 through September 28, 1968.*
  Kurt Gutfreund,  Senior Chemist,  Polymer  Research,  was the Project
Leader. The  assistance of J.  O'Neill in interpreting infrared data, of I.
Lesevicius,  who performed DTA measurements,  of C. Hagen and P. Barbera,
who conducted biodegradation studies on polyethylene, and of R. Glass, V.
Adamaitis,  and T. Meyers, who assisted in various experimental measures, is
gratefully acknowledged. Thanks are also due  to E.  P. Floyd, the Project
Officer of this project for helpful suggestions, and to T.H. Meltzer and G.A.
Zerlaut, who provided technical and  administrative guidance for this work.


                                     -K. GUTFREUND, Project Leader
                                           Polymer Research Chemistry
     *The Solid Wastes Program is now the Office of Solid Waste Management Programs
 of the U.S. Environmental Protection Agency.

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                                 CONTENTS
                                                                             Page
INTRODUCTION  	      1
GENERAL CONSIDERATIONS OF WASTE DISPOSAL PROBLEMS  	      2

    Sanitary Landfill Practices .  .      .       	       	      2
    Composting       	                 	     ....       .       3
    Incineration .          .  .                   ...           ....       .3

DEGRADATION OF HYDROCARBON POLYMERS 	      4
    Structural and Chemical Factors in the Degradation of Polyethylene	       5
    Oxidative Degradation of Polyethylene                                          6
         Mechanism
         Temperature Effects
         Pressure
         Degree of Subdivision
    Selected Methods of Polyolefin Oxidation                               .        1
         Ozonization
         Oxidation by Nitrogen Tetroxide
         Oxidation by Nitric Acid
    Thermal Degradation         .                         ...              9
         Physical Implications of Chain Scission
         Thermo chemical Relationships
    Other Methods of Polymer Degradation                     ...       ...     11
         Mechanical Effects
         Ultrasonic Degradation
         Stress Cracking
         Radiation Effects
         Biodegradation

EXPERIMENTAL INVESTIGATIONS	     15

    Materials. .                      .         • •         ....      .15
         Poly ole fins
         Reagents
    Methods	      .            	      .  .     16
         Differential Thermal Analysis
         Calorimetry
         Mechanical Properties
         Infrared and Viscometric Studies

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         Analytical Measurements
         Biological Studies
    Experimental Results  	     18
         Differential Thermal Analysis Studies
         Calorimetric Measurements
         Infrared and Viscometric Studies
         Mechanical Properties
         Biochemical and Analytical Investigations

CONCLUSION	     41

REFERENCES	     42
FIGURES


                                                                                Page

   1      Environmental Stress-Cracking Behavior for Some Olefin Polymers	      13

   2      Differential Thermograms of Different Polyethylenes  	      19

   3      Differential Thermograms of Polyethylene After Exposure to N2O4 and
         RFNA for 5 hr	      21

   4      Differential Thermograms of RFNA-Treated Polyethylene 	      22

   5      Differential Thermograms of Polyethylene After Treatment  	      24

   6      Differential Thermograms of Polyethylene Before and After PC13 Treatment      27

   7      Infrared  Spectrograms  of Polyethylene  Sensitized with  CoMoO4 and
         Co(NO3 )2-Mn(NO3 )2 Before and After Oxidation 	      30

   8      Infrared  Spectrograms  of Polyethylene  Sensitized with  Co(NO3 )2 and
         CoCrO4 Before and After Oxidation 	      31

   9      Infrared Spectrograms of Benzoyl-Peroxide-Treated Polyethylene Before and
         After Oxidation	      32

  10      Infrared Spectrograms of  FeCl3-Treated  Polyethylene Before and  After
         Exposure to UV Radiation	      33

  11      Infrared Spectrograms of Unexposed and Gamma-Irradiated Polyethylene  . .      34

                                          viii

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 12     Infrared Spectrograms of Untreated and RFNA-Exposed Polyethylene	     35




 13     Oscilloscope Tracings of Compression-Loaded Polyethylene 	     37
TABLES

1
2
3

4
5
6
7
8
9
10
11
12

Use and Production Figures for Selected Plastics 	
Gaseous Constituents in Flame Produced by Burning Polyethylene (450C) . .
Heats of Combustion of Paraffins, Oxygenated Hydrocarbons, and
Halogenated Hydrocarbons 	
Suspension Medium 	
Thermoanalytical Data for Treated and Untreated Polyethylene 	
Thermal Data for Chemically Treated Polyethylene 	
DTA Data for Treated Polyethylene 	
Heats of Combustion of Treated Polyethylene 	
Mechanical Behavior of Treated Polyethylene in High-Speed Compressive Tests
Mechanical Behavior of Polyethylene in Compression 	
Nitrogen Content in Treated Polyethylene 	
Composition of Broth Medium for Bacterial Growth Studies 	
Page
2
10

11
18
20
23
25
28
38
38
40
41
                                         IX

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                      FEASIBILITY  STUDY OF
          The Disposal  of Polyethylene
                           Plastic Waste
THE TIME IN WHICH WE LIVE has often
been characterized as an age of plastics. This
is  in  recognition  of the  ever-increasing
amounts  of polymeric materials  that  are
fabricated and employed to fill the numerous
needs of  our  civilization. Many kinds of
polymers are manufactured today. The major
plastic  types  in tonnage  production  are
polyethylene,  polystyrene,  and polyvinyl
chloride.  The  overall  production  data  for
these materials  and their  applications in
products  used  by  consumers are increasing
(Table 1).
  Although  the  home market  for  plastic
products is small in proportion to the total
volume  of  synthetics  manufactured  in this
country, this market deserves consideration in
municipal  waste  disposal  problems, because
consumer  goods  are  increasingly  reaching
their final destination in public disposal sites.
  The  production  of polyolefins,  which
include polyethylene as a  major constituent,
has  steadily increased during the past decade
and is expected to reach a volume of 6 billion
Ib by 1969. Polyethylene is therefore entering
communal  and industrial wastes in substantial
amounts. About  40  percent of polyethylene
waste derived  from  commercial sources is
burned in incinerators.  Disposing of
polyethylene by  burning  it  in combustion
units  is not  entirely  satisfactory,  because
these  units  are   primarily  designed  for
cellulosic materials (paper and wood) and the
low-calorific constituents of municipal refuse.
In general, oxygen and heat suffice to dispose
completely of cellulosic derivatives by proper
incineration. Polyethylene is a material that is
chemically different  from cellulose, and its
high molecular weight  and its hydrocarbon
nature  prevent the efficient combustion of
this plastic in conventional incinerators.
   Disposal of communal and industrial wastes
in landfills has been attempted in this country
with some success. However, this  approach is
restricted by economic considerations relating
to  the  availability   of  landsites   near
metropolitan centers. The practice of burying
decomposable  waste  (garbage,  cellulosic
refuse,  and metals) has an advantage in that
the disposal  site  can  be reused after the
materials completely decompose  or corrode.
However, synthetic plastics, with very few
exceptions,  do not  undergo  significant
decomposition when deposited in  landfills.
These materials survive intact for many years,
and thus delay reuse of the site. There are no
known  microorganisms  that  attack
polyolefins  at a rate  sufficiently  rapid to
promote effective disposal.
   It is  apparent  from  the foregoing
considerations  that an  efficient, safe, and
economical   method for the  disposal of
polyethylene  waste is needed. A feasibility
study directed toward  these  objectives and

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                                    FEASIBILITY STUDY

                                      TABLE 1
            USE AND PRODUCTION FIGURES FOR SELECTED PLASTICS*

                                                                      Overall
                                                              ;   U.S. production
                                                                    (million Ib)
                                                                      Year
           Plastic
     Home market application
1966
1967
Polyethylene
Polystyrene and
  copolymers

Polyvinyl chloride
  and copolymers
Packaging (baking goods, produce,
meat, garments)                      3,448

Packing material (food containers),
housewares, combs, and brushes        2,417

Clothing (outerwear, baby pants),
footwear, home furnishings
(upholstery, shower curtains), and
garden hose                          2,126
                3,632


                2,400
                                                                               2,127
  *From the plastic industry in 1967. Modern Plastics 45 (5): 84, 1968.
 designed  to determine the applicability  of
 selected  approaches  to  the  problem  of
 polyethylene disposal has been conducted at
 IIT Research Institute.  Particular attention
 was given to chemical methods of modifying
 the  polyolefin and  the effects  of  the
 treatments  on the mechanical, thermal, and
 biological  properties of  the  polymer. The
 background and  various aspects of this work
 are discussed in this report.


      GENERAL CONSIDERATIONS OF
        WASTE DISPOSAL PROBLEMS
                   year to 260 million tons per year predicted
                   for the period  1963 to 1980.2  These figures
                   and  the realization that new materials (for
                   which  effective disposal methods have not yet
                   been  developed)  are  contributing  to the
                   accumulation   of  refuse  indicate  that an
                   imaginative  and  sophisticated  approach  is
                   necessary  for  our  future  handling of
                   environmental  control   problems.  The
                   practices of the past are not adequate to meet
                   the challenges of today. Their limitations are
                   indicated below in  a brief review of current
                   disposal methods.
    The steady population  growth,  changing
 technology,  expanding economy, and higher
 living standards are all contributing factors to
 the problems of solid waste disposal. The per
 capita production of rubbish has increased in
 the past 50  yr from  2.75  Ib per  day per
 person in 1920 to 5.3 Ib per day per person in
 1968.1 An even sharper increase in the rate of
 refuse production is  expected in  the  near
 future, with a rise from 150 million  tons per
                            Sanitary Landfill Practices

                      Open dumping of solid waste into isolated
                   areas—a  method  that  disregards  both
                   long-term  effects  and  aesthetic
                   considerations—is no longer practiced in most
                   metropolitan areas because of strict municipal
                   regulations.  However, sanitary  landfilling, a
                   more sophisticated version of this approach, is

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
regarded by the Federal solid waste manage-
ment  program  as  a  currently  acceptable
method for waste disposal.3  In this method,
waste  is  deposited in  topographically de-
pressed areas  (quarries,  gullies, etc.)4 or in
trenches  excavated  by  specially  designed
equipment that is sometimes capable of per-
forming other operations, such  as compacting
and extruding. The deposit  is then covered
with a layer of soil that makes the site sani-
tary. There are several disadvantages in using
landfill methods:  (1) There  are unfavorable
economic factors associated with the acquisi-
tion of land in municipal areas.5'6 (2) There
is  the  necessity of  reducing the  volume of
landfill-deposited  waste.7  (3)  There  are
groundwater pollution  problems that arise
from leaching of refuse constituents in landfill
sites.8  (4) There are combustion hazards due
to waste-generated gaseous effluents.9 Diffi-
culties are also experienced in volume reduc-
tion  of some  polymers  because of their in-
herent flexibility  and resistance to permanent
deformation by compression. These character-
istics  of polymers decrease the effectiveness
of land utilization and  reduce  the quality of
landfill.10

               Composting

   The conversion of refuse  into useful soil
conditioners presents an attractive approach
to the problem  of waste disposal, because
utilization of refuse  in the terrestrial cycle of
biologically transformed  compounds has dual
effectiveness in that it combines disposal with
reclamation.  The feasibility  of  refuse
reclamation  through  composting and  the
biological  digestion of  organic  matter  was
proved in the operation of a composting plant
in Houston, Texas.  This plant  converts one-
sixth of the city's refuse into soil conditioner
by treating waste that is free of metal objects
(50,000  tin cans are removed  daily) with
thickened sewage sludge, grinding the residue,
and processing.11  A similar plant in Johnson
City,  Tennessee,  has  also  successfully
operated in this way.12
   There  are,  however,  disadvantages in
composting  municipal waste.  Some  of these
disadvantages relate to the economics of the
process (segregating,  grinding, and  treating
refuse components) and  to  the   market
possibilities  of  the  product,  while  others
relate to sanitary considerations.12
   The application of composting techniques
to plastics, particularly polyolefins, is rather
ineffective  because of the  pronounced
biological  inertness  of  these synthetic
materials.  However,  the  possibility  of
subjecting  chemically treated  polymers to
microorganisms  that   could  utilize  the
modified  material makes  the composting
approach  for  the  disposal  of   plastics
theoretically feasible. The effectiveness of this
approach  would depend  on the selection of
appropriate  treatments and microorganisms.
Some  attention  has been  given   to  the
microbiological  treatment  of modified
polyethylene in the course of our work. The
results  are described  in  the  section  on the
Degradation of Hydrocarbon Polymers.
               Incineration

  The  disposal  of  municipal  refuse  by
incineration offers a  number  of  advantages,
among  which  is volume  reduction  of the
waste.  Benefits are  also  derived  from  this
method by the  destruction  of  pathogenic
microorganisms, although  secondary sanitary
considerations  relating   to  the  potential
discharge  of air pollutants may  reduce the
overall usefulness of incineration processes in
municipal  areas.
  A voluminous literature is available on the
subject  of incineration, and from this only a
few   representative   articles   are    cited
here.13'18  The   unresolved   problems  of
incineration have  been discussed in recent
technical  meetings  and   engineering
conferences.  The  need  for  improving the
performance  of existing  and  even  recently
designed  incinerator facilities  has  been
emphasized.19  The desirability of upgrading

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                                     FEASIBILITY STUDY
instrumentation  in  municipal  incinerator
plants20  and controlling emissions has also
been considered.21
  Effective  combustion  of  waste requires
maintenance  of proper  temperature,  time,
oxygen  admission,  and  fuel  distribution.
While low temperatures (below 1,400 F) may
result in excessive smoking of burned organic
waste, high temperatures (above 2,400 F) are
damaging to conventional refractory furnace
linings and can  also promote reactions that
lead to the formation  of  pollution-causing
emissions.
  Complete combustion also depends on the
maintenance  of  flame  turbulence.  This is
achieved  by the use of stationary or moving
grates and rotary kilns. Grates can  easily be
rendered  ineffective when  thermoplastic
polymers, such as polyethylene, block the air
supply by depositing on the grate in a molten
mass. The high  calorific  value  of plastics is
also  responsible  for  the  excessive   heat
generated  during their combustion and  the
damage incurred by the gratings.
  Although incineration of plastic  waste, as
found in municipal refuse, does not present
unusual difficulties at the present  level of
synthetics in  waste   (1-2%),  engineers  in
sanitation departments of large cities, such as
Chicago,  are  concerned with  the  growing
amount  of plastic  materials  in garbage,
because it is believed that  a  3- to 4-percent
waste  content  causes problems  in  city
incinerators. This level may soon be reached
because of the  increasing use of disposable
plastic products and packaging materials.
  The potential  problems  of plastic  waste
incineration  in  heterogeneous  municipal
refuse  are  closely  related  to  the  large
differences in  the combustion  properties of
plastic and other  waste  constituents and to
the  pollution  danger associated  with  the
discharge  of  hydrocarbon  combustion
products.  The  excess oxygen  required for
adequate combustion  of  organic polymers is
substantially  greater  than  that needed for
low-calorific   refuse.  Consequently,  the
thermal  output (20,000  Btu)  exceeds the
combustion ratings of municipal refuse (5,000
Btu) by a factor of 3. It  is therefore not
surprising to find unsatisfactory  performance
of refuse incinerators in those applications in
which  a substantial  amount of polymers is
present in  the  waste. The manufacturers of
incinerators   generally   claim  that  the
difficulties  encountered could be alleviated by
redesigning  conventional  combustion
furnaces. They  believe  that changes in the
construction  of the furnace,  proper  stack
design,  and  maintenance  of  optimum
gas-pressure relationships  in the incinerator
will improve the combustibility of plastics.22
  A modern incinerator  facility operating in a
small  community  in Derby, England,  was
designed to cope with current waste disposal
problems.23 The  plant has a  1,000-cu  yd
capacity; it operates on two 8-hr shifts and
disposes of waste at  a rate of 15 tons per hr.
Special oxygen supply nozzles were designed
to facilitate the combustion  of hydrocarbons.
Waste  gases are passed through mechanical
dust collectors  before being discharged into
the  atmosphere   through  a  130-ft-high
chimney. Thus the emission of the particulate
materials is limited  to the specified level of
0.35 gr per cu ft.
  At the present time, the disposal of plastics
by incineration does not seriously affect the
overall  solid   waste disposal   operation.24
Continued  use  of  incineration  methods in
large-scale  disposal  operations,  however, is
predicated  on the  effectiveness of the
combustion process and  the  control  of
environmental contamination.25
   DEGRADATION OF HYDROCARBON
                POLYMERS

   The combustion of organic polymers and
their  substituted  compounds  to  carbon
dioxide  (CO2),  water,  and  nonoxidizable
products essentially is the ultimate stage in
the degradation of these compounds.26  The
simple oxidation products just listed can be

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                           POLYETHYLENE PLASTIC WASTE DISPOSAL
obtained only under ideal conditions.2 6 Even
incineration  results  in  the  formation of
measurable  amounts  of  partially  oxidized
products,  which  in the  combustion of
polyethylene  may  contain  more  than  six
carbon atoms in the fragmented chain.
  Chain  fragments  are also  obtained in  the
degradation  of  polymers  by  other  than
combustion  methods.  For  instance,  the
degradation  of  high-molecular-weight
materials  by  oxidation,  thermal treatment,
and  radiation  can  lead  to a significant
reduction of the  molecular size of polymers,
thereby making them more accessible for final
oxidation in  combustion. The  changes in
mechanical and chemical properties can also
promote  physical disintegration  of plastics
and  enhance  their utilization  by
microorganisms,  thereby  facilitating  the
disposal  of  high-molecular-weight  organic
materials. For these reasons,  a discussion of
the  degradation  processes of polymers
deserves attention.
      Structural and Chemical Factors
     in the Degradation of Polyethylene

  The heterogeneous nature of polyethylene,
particularly its  morphological  differences
(degree of crystallinity), is largely responsible
for localized degradation effects observed in
the polymer Polyethylene  consists of  long
hydrocarbon  molecules  that  are  tightly
packed  in a relatively regular array of chains
held together by van  der Waals forces. The
regularity of this array depends on the extent
of chain branching, i.e., the presence of side
chains  (ethyl,  butyl)  along the  main
carbon-carbon backbone. Because the  chain
branches interfere with packing the polymer
into a  tight structure, the lessened molecular
order  of branched polyethylene causes lower
crystallinity   and   density.   However,
branch-separated  chains  overlap at  some
points, and the  enhanced chain interaction at
these  points results  in the development  of
micro crystalline  (spherulitic) regions  that
constrain  the  polymer in a locally ordered
network.  The  overall  crystallinity  of
polyethylene  may  thus vary from 55 to 99
percent. These morphological  differences in
polyethylenes  influence  their  responses  to
chemical reagents.
   Although  the  polymer's  hydrocarbon
nature protects it  from corrosive chemicals,
the amorphous regions are  not immune to
chemical attack. The  susceptibility  of these
regions to oxidation is not only a result of the
greater  accessibility  of  low-density
polyethylene  to the  oxidizing reagent, but
also a result of the greater affinity for oxygen
that  the  branched   polyethylene  has  in
comparison  to  its linear  counterpart.
Therefore, the chemical effects relating to the
presence of substituents on methylene (CH2-)
groups in  the  main chain of the polymer are
equally  important  in the  degradation  of
polyethylene, because  the substituted tertiary
carbon atoms  present sensitive sites for the
formation of free radicals. The free radicals,
in turn, are instrumental in propagating the
degradation  of  high-molecular-weight
materials, as will be shown later.
   The oxidative behavior of polyethylene  is
also affected by its residual unsaturation, i.e.,
by the  presence  of double bonds. Because
oxygen  (O2)  is capable of adding to the
double bond, it may break the polymer chain
by the following reaction:
 -CH = CH- + o2
-CH
 o—
 CH	-CH.-CH
•O      00
However, the formation of hydroperoxides is
more  important than  the addition of O2 to
the  double  bonds.  The  hydrogen  atoms
situated on carbon atoms that juxtapose the
double bond are very reactive. These allylic
hydrogens   are  most  easily  engaged  in
hydroperoxide formation.
    H H H H
    i  i   i  i
   •c-c-c=c-
    I  I
    H H
           H H  H H
            i  i  i
          -c-c-oc -
            I  I
           H 0-0-H
                 (2)

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                                   FEASIBILITY STUDY
Because  the  formation  of hydroperoxides
governs the chain of events that lead to the
oxidative degradation of polymers, the extent
of  unsaturation  in  polyethylene  has  a
pronounced influence on  the stability of the
polyolefin.
  The  foregoing  considerations  of
morphology,  chain  branching,  and  the
number  of  double bonds  in  polyethylene
indicate  the  different  responses  of
polyethylenes  to  degradation-promoting
influences.
   Oxidative Degradation of Polyethylene2 7

  Mechanism.  The  degradation of polyeth-
ylene  is  a  result  of  the  simultaneous
operation of several types of reactions, each
leading to  different structural  and chemical
changes  of the polymer.  These changes, in
turn, produce inordinately large effects on the
chemical and  physical  properties  of  the
substance.  In  most  polymer degradation
processes-whether  induced  by  the
expenditure  of  mechanical  energy,  heat,
radiation, or chemical oxidation—degradation
takes  place through a chain mechanism. This
mechanism,  which  characterizes  free-radical
processes, provides  the  unifying  concept in
the deterioration  of  high-molecular-weight
compounds.   In   the  presence  of  O2,
polyethylene  decomposes by the free-radical
mechanism  according  to  the  following
scheme:
- CH2
           CH3
                        -C  -CH2 /wv\  - 1
                       H20
                    CH,
           I            1 IT *-»      '     —•*
 «— /WVA CH2 -C  -CH2 -vw + "OH ,.	CH2 -C  -Otf

           OOH                 0-0.
       CH3              CH3
OH +-CH2 -C  -CH2 /WNA —. CH2 -C  + -CH2
       Q               O"
                             (3)
  The initial step of removing a hydrogen and
forming  a  free  radical requires  the
expenditure  of a  certain amount of energy
(heat, radiation,  etc.).  Then  the  process
becomes self-propagating  through  the
free-radical mechanism. Eventually,  scission
of the chemical bonds ensues and ketone or
aldehyde-terminated molecular fragments are
formed. The free  radicals  generated during
degradation  are capable of sustaining the
decomposition of the polymer by molecular
collision and the  removal of hydrogen until
the reacting  chain is eventually terminated by
combination  of   the   free  radicals,
disproportionation of  the  radical  chain, or
another mechanism.  In polyethylene,  the
products  of ultimate  degradation  are  not
predominantly  monomer units,  i.e.,  basic
chemical entities  (ethylene) that constitute
the  polymer chain,  but  are  organic
compounds  that  present   a  spectrum of
intermediate-size molecules.
  The process of bond scission may also be
accompanied  by  the opposite  process,
cross  linking, which leads  to an increase in
molecular  weight  and   results  in  the
embrittlement of the polymer. The conditions
of   polyethylene   degradation   such  as
chemical  environment, temperature,  and
structural relationships in the polymer largely
determine the predominance of either effect.
Extensive investigations have been conducted
on  the  oxidative  behavior  of polyethylene.
These investigations include studies of the
kinetics  of peroxide-induced  reactions,28
effects  of  branching  on   the  degradation
process,29   and  physical  changes during
degradation.3 °
  Temperature Effects.  The  oxidation of
polyethylene is greatly enhanced when the
polymer is heated. Above 170 C, the enthalpy
of the oxidation process decreases3 * from 33
kcal per mol to a much lower value of 15 kcal
per mol at 200 C. This decrease is apparently
a result of a change  in  the rate-controlling
step  in the  oxidation of   polyethylene at
elevated temperatures.  The  chemical reaction

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                           POLYETHYLENE PLASTIC WASTE DISPOSAL
between polymer and O2 occurs so readily at
high temperatures that diffusion  of  the  gas
into the polymer cannot keep pace with  the
process;  diffusion  no  longer controls   the
reaction.
  Pressure.  The   rates  of  oxidation  of
polyethylene are proportional to  the square
root of O2 concentration for a pressure range
of 76 to 760 mm Hg.32 If this relationship
should hold over an extended pressure scale,
an  increase in  pressure by a factor  of  100
would increase the reaction rate by a factor of
10.  This  increase   combined with  proper
temperature  control should permit variation
of the rate  of polyethylene  oxidation.
  Degree of Subdivision. Because  the rate of
degradation of  polyethylene  resulting from
exposure of the polymer to a gaseous reagent
is to some extent  diffusion controlled,  the
state  of polymer subdivision is important in
the oxidation process. Studies of  the effects
of  film thickness on oxygen absorption  by
Ziegler polyethylene indicate a rate thickness
proportionality for  specimens investigated in
the  0.12-  to  3.0-mm-thickness range.33
Although  the  absorption  rate  loses  its
proportionality  beyond the 3-mm-thickness
limit,  a valid relationship  is reestablished
when the extent of O2 absorption is followed
as   a  function of surface  area rather  than
thickness.
  During  combustion, in  which  oxidative
processes are superposed on thermal  effects,
the rate  of reaction of polyethylene is
expected to depend  on the size and state of
subdivision  of pyrolyzed products.  The
foregoing considerations should  make clear
the  importance  of physical relationships in
the  oxidation of polyethylene.

 Selected Methods of Poly olefin Oxidation

  Although  the  main  prerequisite for   the
oxidative degradation of polyethylene is  the
availability of O2, the oxidation reaction does
not  proceed at  a significant  rate when   the
polymer  is  exposed  to  air  at  normal
temperatures. Therefore, from a viewpoint of
polyethylene disposal, more severe oxidation
conditions are required to produce the desired
effects of polymer degradation. Some of these
conditions  are  discussed in  the  following
paragraphs.
   Ozonization.  The  degradative  effects  of
ozone   (O3)  on  elastomers and flexible
polymers—particularly  the  crack-promoting
action of  this strongly-oxidizing agent—have
received attention in recent years.34'35  The
primary step in the ozonization of polymers is
believed to  involve  chemisorption. Surface
attack  is enhanced  by  the  thermal  energy
liberated  in  the  decomposition of  O3,
according to
-O
                      O +  35kcal.
(4)
   The  oxidation  of  the  polymeric bulk,
unlike the  surface  reaction,  is  diffusion
controlled, and the rate of this process greatly
depends on concentration.  The propagation
of a cut in rubber in  an O3 environment is
found  to require a characteristic energy at the
tip of  the crack. When this stress is exceeded,
the  growth  of  surface  cracks  occurs
spontaneously.  Thus  O3  may  also  be
considered a stress-sensitizing agent in stress
cracking.
   The behavior  of polyolefin films  on
exposure  to  O3 has also been a  subject of
extensive  investigations.36'37   A  peroxide
mechanism was proposed for the degradation
of  these polymers.  Changes  in  physical
properties, such as melting  point, solubility,
and  viscosity,  indicated preferential bond
scission for the exposed films. The rate of O3
attack  on polyethylene was  greatly enhanced
when the reaction was performed at elevated
temperatures.38  However, other  workers
found  that even  at room temperature small
amounts of O3, such as  those generated in a
discharge  tube, sufficed to accelerate greatly
the oxidative degradation of polyethylene.39
  Oxidation by Nitrogen Tetroxide. Nitrogen
tetroxide (N2 O4), a gas at temperatures above

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                                    FEASIBILITY STUDY
21.3 C and a yellow-brown liquid below this
temperature, exists in equilibrium with nitro-
gen dioxide (NO2):
(N02):
             N204-
NO,
(5)
In a manner similar to O2  and O3, N2O4
oxidatively attacks organic materials through
a  free-radical  mechanism.  At temperatures
above  100  C,  N2O4,  which  under  these
conditions  is largely  dissociated into  NO2,
reacts with unsaturated groups. The reaction
is initiated  by  hydrogen  removal  from
polyethylene, and  the free radicals that  are
produced combine with NO2. The  activation
energy for  hydrogen removal is estimated to
be 14 to 17 kcal per mol. In its nonionized as
well  as  ionized form,40  N2O4  represents a
very reactive agent that can oxidize otherwise
resistant polymers.
   Crystalline  polymers,  including
high-density polyethylene,  are not  easily
destroyed by oxidation processes; e.g., N2O4
degrades  high-density  polyethylene  to a
greater extent  than  low-density and
cross-linked  polyethylene.41   This suggests
that the dense crystalline areas are penetrated
by N2O4  and are disrupted  by  its  strong
solvating   action,  or  chemical  attack.
Polyethylene,  polypropylene, and  especially
unsaturated hydrocarbons become embrittled
and  degrade in the presence of N2 O4, while
silicone rubbers  and  aromatic  polyesters
disintegrate upon contact with the oxidizing
agent.  Even materials such  as  Teflon  are
attacked at elevated temperatures by N2O4.
   The reactivity of N2 O4 with polyethylene
has  been  studied  by  infrared  absorption
spectroscopy.42  The results show that in  the
initial stages of oxidation N2O4  reacts to
form nitration compounds. These compounds
consist  of nitro groups, nitrate  and nitro
esters, and  carbonyl and hydroxyl groups. At
room  temperature,  NO2  adds to  double
bonds,  producing dinitro  or  nitro-nitrite
compounds.
   Excessively  high  temperatures  are  not
required  for  these  reactions.  Experience
acquired at HTRI  with  N2O4 permeation of
polyethylene  has  shown  that  even  at
temperatures as low as 75  C, the hydrogen
removal  occurs  quite  readily,  and  chain
scission is so extensive that  the polyethylene
samples  lose  their coherence.  The  nitrate
esters,  as  well  as carbonyl  and hydroxyl
groups, are produced by the decomposition of
nitrite groups.
   The  formation  of  nitrates  by  the
decomposition  of  polyethylene  admits  the
possibility of  utilizing  N2O4  =  oxidized
polyethylene  in fertilizers  and  other
applications.  Decomposition of nitrite esters
into carbonyl groups assures the possibility of
using this decomposition product in the usual
areas in which  carboxylic acids are required.
   Oxidation  by  Nitric  Acid.  Nitric  acid
(HNO3), a strong mineral acid that forms an
azeotrope with water at a  concentration of
68.8  percent, is  a very effective oxidizing
agent for  organic compounds. The formation
of nitrates in its reactions with polyols, such
as in the manufacture of nitrogylcerine  and
nitrocellulose, indicate the  reactivity of the
acid with organic compounds. Nitration  can
also occur at the carbon  atom of a paraffin, in
accordance with the following reaction

      RH + HONO2    RNO2+H2O.   (6)

The oxidizing capability of  HNO3 is utilized
in rocket  systems in which it functions as an
oxidizer for rocket fuels. The effectiveness of
this  oxidizer  was  found  to  increase with
increasing N2O4 content. The fuming HNO3
thus  formed  causes rapid  oxidative
degradation of polyhydrocarbons through a
free-radical  mechanism, as  discussed
previously.  Pronounced  changes  in  the
physical properties of polyethylene film were
found  after  exposure  to  fuming  HNO3.
Containers made  from this polymer were
considered unsuitable for storage of  dilute
acid.43

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
   In  addition  to  its degradative effects on
polyolefins,  nitric  acid  treatment  of the
polymer presents  an  additional  advantage
from  the viewpoint of plastic-waste disposal.
This  advantage is  the potential use of the
acid-treated material as a fertilizer, because
partial nitration could provide the nitrogen
for assimilation by  plants.   The  modified
hydrocarbon  would  also  be  more  easily
attacked  by  microorganisms, thereby
facilitating the  ultimate  disposal  of the
plastic. These   factors  have  received
consideration in our study.
           Thermal Degradation

   Physical  Implications of Chain
Scission.  The  thermal  degradation of
polyolefins  has  been a  subject  of many
investigations.44-47 The relevance of thermal
relationships  to incineration or combustion
processes  makes consideration  of  these
relationships  particularly  desirable  in  a
discussion of polyolefin waste disposal.
   The reaction mechanism of the  thermal
degradation of polyethylene closely follows
the free-radical  scheme  previously described
for the oxidative degradation of the polymer.
This  mechanism  applies   most strictly to
thermal  processes  that  take place in the
presence of air, because in the case of air the
thermal effects are superposed on  oxidation
processes.
   Polyethylene  produces   degradation
products  in  a  continuous  spectrum of
intermediate-size  molecules.  This  polymer
does not act like the acrylic plastics or even,
to some extent, the styrene derivatives-which
upon  heating  are  almost  quantitatively
converted to monomer (the simple  structural
units that make up the high-molecular-weight
material).  The  polyethylene chain  does not
unzip  in   the  depropagation  step  of the
degradation process, but rather is  randomly
broken and reformed into fragments of larger
molecular sizes. Thus the monomeric yield for
the degradation of polyethylene is very  small
(0.1%  below  200  C),  and  the  rate  of
volatilization  in  pyrolysis  experiments
conducted above 300 C is  low (0.4%/min).48
  These  relationships  undoubtedly  have
implications  for  the  combustion  of
polyethylene,  which  may  relate  to  the
incomplete burning  of  polyethylene waste
and  the discharge  of  low-molecular-weight
hydrocarbons along with emitted combustion
products  and  smoke.  The  proportion  of
heavier fragments  in  the  products  of
polyethylene  pyrolysis  decreases  with
increasing temperature. Thus the percentage
of residual hydrocarbons with chain  lengths
exceeding an 8-carbon  backbone diminishes
from  94 percent at 500 C to 41 percent at
1,200 C. This decrease  is explained  by the
competing  reactions that  take  place by
random   scission  in  the  decomposition  of
polyethylene.
  At  lower pyrolysis temperatures formation
of large fragments  from  polymer  chains is
favored  according to the following scheme:
    HHHHHHHHH

AA/VN C-C-C-C-C-C-C-C-C-
    i  I I  I I I  I i  I
    HHHHHHHHH
 HHHH    HHHH

—C-C-C-C-H +OC-C-C/vw\
 i  i i  i    i i  i i     C~l\
 HHHH    HHHH    \l)
At higher temperatures fragmentation is more
extensive, and the amount of monomer in the
products of pyrolysis increases:
    HHHHHHHH
   •> C-C-C-C-C-C-C-C-v
    I I  I I I I  I I
    HHHHHHHH
    + OC
     i  i
     H H
H H  H
i i    i
C=C + C /wv\
i i    i
H H  H
  Several  problems  result  from  the
fragmentation  of polyolefins during thermal
degradation  and combustion. The danger of
incomplete  combustion  and  discharge  of
pollutants is greater for large fragments than
for  small fragments.  Yet the formation of

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10
FEASIBILITY STUDY
small  molecular fragments would  favor
complete oxidation of the polymer and result
in  the  dissipation  of  more  heat during
incineration.  Actually,  the  effects  of
carbon-hydrogen  and carbon-carbon  bond
rupture, which  are  endothermic  (heat
consuming),  counteract  the  exothermic
(heat-dissipating)  process of oxidation.
Nevertheless, the efficiency  of the oxidation
reaction  determines the  heat balance of the
process, and the thermal energy  expended in
bond breaking  is  outweighed by the heat
generated by  the  thorough  oxidation  of
smaller chain fragments.

   Thermo chemical  Relationships.  In
considerations  of  the  applicability  of
incineration to the disposal of plastic waste, it
is important  to define  the thermochemical
and  thermophysical  properties  of the
materials investigated. Polyethylene packaging
film melts into a small bead and burns like a
candle when  subjected to incineration.49  A
study  of the  combustible   behavior  of
branched polyethylene (27.7 CH3 groups per
1,000 carbon atoms)  disclosed that the flame
maintained by burning a 1-in-diameter rod of
this polymer had temperatures  of  200 and
700 C at the respective  distances of 1 and 2
cm from the  surface of  the polymer.50 The
combustion  products  analyzed  by  mass
spectrometry  and withdrawn from  different
sections  of the  flame indicated that  thermal
oxidation occurred extensively at 400 to 450
C,  although  a  significant  amount  of
unoxidized hydrocarbons was found among
the degradation products at  450  C (Table 2).

   The  relatively  high  percentage  of
incompletely  oxidized degradation products
of  polyethylene  emphasizes  the potential
pollution problem that may  be caused by the
combustion  of  polyolefins. The  hazards
associated with  the generation of substantial
amounts of CO also deserve attention. These
conditions  are  responsible  for  the special
precautions that fire  departments take  when
combating fires in polyethylene storage areas.
                          TABLE 2
             GASEOUS CONSTITUENTS IN FLAME
               PRODUCED BY BURNING (450 C)
                     POLYETHYLENE5 °
Gaseous species
N2
02
CO2
CO
CH4
C2H4
nC6H10
Concentration
mole (%)
75.0
1.4
10.3
3.7
0.8
1.6
0.8
            The  problems related to the liberation of
          excessive  heat  during combustion of
          polyethylene waste could be  minimized by
          oxidizing the polymer prior to incineration.
          The extent to which the heat of combustion
          of hydrocarbons can be reduced by oxidation
          or  halogenation  is  indicated by the
          thermochemical data (Table 3) compiled from
          published information.5 * ~5 3

            The  oxidation  of  propane  (C3H8) to
          acetone (CH3COCH)  reduces  the heat of
          combustion  of  the  hydrocarbon  by  16
          percent. Halogenation  has a similar  effect,
          because it results in an 18-percent reduction
          of  the  thermal output  of completely
          chlorinated methane.  Volatile  bromine
          compounds actually inhibit  the combustion
          of polyethylene, as has been demonstrated in
          studies  with  methyl  ethyl  and
          isopropybromide  combustion-retarding
          additives.54  The deliberate  introduction of
          chlorine into hydrocarbons as a  means of
          controlling the  thermal relationships of

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL

                                      TABLES

                              HEATS OF COMBUSTION
                  OF PARAFFINS, OXYGENATED HYDROCARBONS,
                       AND HALOGENATED HYDROCARBONS
                                 (Gaseous state, 25 C)
                                                                                  n
      Chloromethane
 Oxygenated
hydrocarbon
Paraffin
  Heat of
combustion
 (kcal/mole)
        CH3C1
        CH2 C12

        CC14
                          CH3CH2CH2OH
                          CH3COCH3
                                                CH
                                                C3H8
                                                n-C4H10
                                                iso-C4 Hj |
                                                n-C5H12
                                                n-C6H14
                                                 183.2

                                                 164.5
                                                 156.8
                                                 452.4
                                                 403.7
                                                 191.7
                                                 341.3
                                                 488.5
                                                 635.4
                                                 633.7
                                                 782.0
                                                 928.9
polymers must be critically evaluated in view
of possible corrosion and pollution problems
related to the discharge of hydrogen halides.
  Differences in thermal response  were also
noted   in  studies   on  high-  and
low-molecular-weight  polyethylene.55  The
rates of polymer degradation at  376 C were
0.078 and 0.011 percent  for polyethylene
with respective molecular  weights of 16,000
and 23,000. Linear  polyethylene also had a
higher  activation  energy for thermal
degradation (74 kcal/mole) than the branched
polymer  (63   kcal/mole). This  energy
difference  substantiates  the  previous
observations  on the greater  stability  of
unbranched polyethylene and the  degradation
of low-molecular-weight chain  fragments.
                       Other Methods of Polymer Degradation
                       Mechanical  Effects. The  degradation  of
                    polymers by mechanical means has long been
                    established in studies on masticated rubber.56
                    In this degradation, formation of free radicals
                    in the process of mechanically induced bond
                    scission leads  to interaction  of free-radical
                    chains with  the oxygen  that is present in a
                    normal  environment,  thereby  causing
                    permanent  disruption of  the  polymer
                    structure. Shear degradation  of polymers
                    takes place  in a nonrandom  fashion,  as  is
                    indicated  by  a  sharp  rather than  broad
                    molecular-weight  distribution  of  the
                    degradation products.57  The  preferential
                    fracture of polymer chains at selected sites
                    along the chain has been attributed to the

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12
                                    FEASIBILITY STUDY
localized  stress  conditions  in the polymer
when subjected to tensile loads. Consideration
of  stress  conditions  in  a  chain-entangled
polymer system demonstrates the existence of
maximum  stress  at  the center  of polymer
chains.5 8 This condition explains the narrow
molecular-weight distribution of mechanically
degraded polymers.
   The  degradation  of  polymers  by
mechanical  methods has  been  generally
criticized for  its  undesirable although often
unavoidable effects, and several  approaches
have been utilized to inhibit these undesirable
effects.  However,  when these   effects  are
beneficial,  it is also  possible to enhance the
process  by  the  use   of   selected,
degradation-promoting  catalysts.  The
applicability of  this  approach will be
discussed later.
   Ultrasonic  Degradation. The  degradation
of polymers by ultrasonic methods involves a
physical phenomenon  slightly different from
that   which  is  operative in  the  shear
degradation of plastics.  In this degradation,
the  stresses  that  are created  within  the
macromolecule by  the collapse of a cavity
produce a  shock wave that radiates from the
cavity.59-60 Pressures  of the order of 1,600
atm  may  develop,  thus   leading to  the
disintegration  of  polymer.  The  process
involves bond scission at preferential sites and
results  in a relatively narrow distribution of
the  molecular weights  of  the  degradation
products,  analogous  to  the  distribution
obtained in polymers  subjected to shearing
forces during mastication.  Usually there is
obtained a  critical average molecular weight
that   cannot be  further decreased by
prolonged exposure to ultrasonic energy. For
polystyrene,  this  molecular-weight limit is
30,000,  regardless  of  the initial  molecular
weight of the parent polymer.
   Although ultrasonic  methods of polymer
degradation have many attractive features, the
cost   involved  (power requirements and
equipment cost)  may  be too prohibitive to
justify the use of this approach in the disposal
of waste.
  Stress  Cracking.  A  variant  of  the
mechanically  induced  deterioration  of
plastics,  particularly  of polyethylene,  is
presented  in  stress  cracking.  This
phenomenon is defined as the failure in the
surface-initiated  brittle  fracture  of  a
polyolefin  part under polyaxial  stress, when
the part is in contact with a stress-sensitizing
gaseous, liquid, semisolid, or solid medium.61
  The stress state has an  important part in
environmental stress cracking. High uniaxial
stresses  cause  most  polyolefins  to  flow
excessively  and thus change  their  physical
characteristics. However, biaxial stresses allow
high-energy  storage  without  excessive
deformation, and,  under  these  conditions,
environmental  attack  rapidly  leads  to
embrittlement  and  fracture. Environmental
stress cracking  may thus  induce failure in
polyethylene at stresses much lower than its
ultimate  strength.  The  susceptibility  of
polyolefins to stress cracking depends on load
conditions, the  nature of the stress-sensitizing
medium,  and  the  molecular,  weight
distribution of the  polymer.  Polyethylenes
with a narrow molecular weight distribution
were found to  exhibit  superior resistance to
environmental stress cracking.6 2
  An effective  crack-inducing sensitizer  was
found  for  polyethylene  in  a surface
energy-reducing  organic compound, Igepal
(General  Aniline and  Fiber  Co.).  The
time-failure stress relationship for polyolefins
subjected to Igepal solutions  is shown in
Figure 1. The catastrophic onset of fracture in
polyethylene after a 10-hr exposure to the
crack-promoting  environment  indicates  the
potential applicability of this method for the
enhancement of polymer degradation.
  Radiation Effects.  Photolysis. The degrada-
tive influence of  solar radiation on synthetic
polymers, often manifested in visual (color)
changes, is well known. Photolytic processes
involving bond  rupture within the backbone
of the polymer chain require a minimum en-
ergy expenditure equivalent to that of the C-C
bond strength. Since this energy is of the

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                 POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                           13
10,000

 8,000

 6,000

 5,000

 4,000


 3,000



 2,000
 1,000

  800


  600

  500

  400

  300



  200
  100
      0.01
         POLYPROPYLENES A,B,C, IN AIR
                         TYPE II POLYETHYLENE IN AIR
                          TYPE I POLYETHYLENE IN AIR
                   O
                     O

    TYPE I POLYETHYLENE
    IN 3% IGEPAL CA-630
    WATER SOLUTION
0.1
  1             10

TIME TO FAILURE (hi)
100
1,000
            Figure 1. Environmental stress-cracking behavior for some olefin polymers.

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14
                                     FEASIBILITY STUDY
order of 80 to 100 kcal/mol, it is not surpris-
ing to find a radiation threshold from 3,000
to 4,000 A (ultraviolet range) beyond which
photo degradation of organic polymers  does
not occur  (visible range). Thus  radiation in
the ultraviolet range is most  injurious (highly
degradative) to high-molecular-weight organic
compounds.
   Photolytic  bond scission is the initial step
of polymer degradation  which leads to  the
formation of  free radicals that react with the
polymer  chains  to   produce  further
fragmentation  through  the  free-radical
mechanism  of   a  chain reaction.  In  the
presence  of  oxygen,  the  photoinitiated
degradation process  continues by  the
hydro per oxide  scheme of  the  oxidative
degradation process.
   The principles  and operative considerations
in  photochemical processes,  including
photodegradation, were  recently the subject
of  a  symposium.   New,  powerful,
ultraviolet-light  sensitizers for  the  cross
linking  of  polyethylene  plastic  were
discussed.63  The  old function  of  the
sensitizer, long performed by uranyl salts and
benzophenone, has been  augmented by other
agents  such  as  anthrone.  Because  of  its
aromatic substituents, anthrone does not take
part in coupling reactions and therefore gives
a higher quantum yield  than benzophenone.
However, the use of ultraviolet sensitizers to
promote the  degradative  interaction of plastic
and oxygen is sharply limited by the relative
impenetrability  of the  polymer  by  some
forms of radiant energy. Film sections ranging
from  0.001  to  0.004   in.  are  affected  by
ultraviolet radiation; but  films  of greater
thicknesses  require extended  periods  of
exposure before they exhibit noticeable  signs
of degradation.
   High-Energy  Radiation.  The  great
advantage  of high-energy  radiation  is  its
ability  to penetrate deeply  into polymer
sections. This penetration allows chemically
disruptive  influences to  manifest themselves
within coarse plastics.
  In  radiolytic  methods,  higher radiation
energies than those obtained from ultraviolet
sources  can  induce  rapid  deterioration  of
polymers. Thus ionizing radiation provided by
Coi60  predominantly  produces chain scission
in  some  polymers,  such  as  alpha-methyl
styrene, while in others, such as polyethylene,
it causes bond scission and cross linking. The
relationship  between  the  molecular
parameters  of polymers and their response to
ionizing  radiation applies  to scission and
cross-linking processes.
  The  degree   of  crystallization,  chain
branching,  and  the  nature  of substituents
influence  the  extent  of  degradation.  In
radiation-induced cross-linking, the polymer
can form a rigid, three-dimensional network,
which  in  the extreme  case  may  form  an
embrittled  material that could fracture with
the expenditure of relatively little mechanical
energy.  The effectiveness of radiation-induced
changes in  polymers  depends  on  thermal
conditions and the nature of the environment
in  which  the  material  is  subjected  to
radiation.  This  thermal  dependence  is  not
surprising.  An  abrupt  increase  in
radiochemical energy at the  glass  transition
point  could  be  expected, because  the
interaction  of free-radical chains above and
below  this point  could  occur by different
processes,  such  as  disproportionation and
recombination.
  Degradation  of  polymers  by  radiolytic
methods merits attention for  facilitating the
disposal of polyolefin waste. However, the
practical implementation  of  this approach
requires thorough investigation.
  Biodegradation.  The   microbiological
degradation of organic compounds, including
polymers, has  been  investigated  for  many
years,  particularly  the  adverse  effects  of
biological   action  on  the  durability  of
synthetic   fabrics   and  insulating
materials.64-65  More recently, attention has
been  given  to  the  positive  effects  of
microbiological degradation of high polymers,
specifically in studies of waste disposal.66'68

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                                     15
  Transformations  of paraffins, petroleum,
and  other  aliphatic  hydrocarbons are
significant in the terrestrial cycle of carbon,
because they are constituents of plant  tissue.
Microflora decompose the naturally occurring
aliphatic  hydrocarbons,  but  the
higher-molecular-weight homologs  are  not so
easily attacked  by microorganisms. However,
acid-fast   mycobacteria,  Nocardia,
Pseudomonas, Streptomyces, Desulfovibrio,
Corynebacterium, and some cocci and fungi
have  been  found  to attack  hydrocarbon
polymers.6 9

  Vinyl  plastics  appear  to  be particularly
sensitive   to  Alternaria,  Aspergillus,
Penicillium,  Rhodotorula, Streptomyces, and
Trichoderma.  However,  because  vinyl
polymers are generally used  in a plasticized
state, the plasticizer rather  than the polymer
might  provide  the nutrient   for  the
microorganisms. Plasticizers are susceptible to
fungal attack—glycol  derivatives being more
readily  utilized  by  microorganisms than
derivatives  of phthalic acid. Generally, the
growth rate of fungi on plasticizers varies with
the  test organism  used. Thus, conclusions
concerning   fungal  resistances  of  aromatic
plasticizers  based on the  response of a single
microorganism are grossly misleading.

  The deterioration  of  polyvinyl  chloride
films subjected  to  the  influence of  soil
microorganisms for 5 years in  underground
exposure studies disclosed marked  biological
action.  The  embrittlement and  loss  of
elongation  of  these  films  indicate  partial
removal  of  the  plasticizer.  However,  the
observed  increase  in  the  concentration of
hydroxyl and carboxyl groups  suggests that
bacterial and fungal organisms directly  attack
the  polymer.

  For waste disposal, the resistance of some
polymers to biodegradation could be greatly
reduced  by  chemical modification of the
polymer   prior to  its  exposure  to
microorganisms.   This  modification
particularly  applies to polyethylene  whose
biological inertness could  be  overcome  by
appropriate chemical sensitization.
   EXPERIMENTAL INVESTIGATIONS

  The possibility of facilitating the disposal
of polyolefins  by the previously  discussed
methods  of polymer  degradation  suggested
the   desirability  of  determining  the
applicability of  some of  these methods to
polyethylene waste. Accordingly, concurrent
with the  literature survey on the practices of
plastic-waste handling, an experimental study
was  initiated  to investigate  the  merits of
selected approaches to promote degradation
and   ultimate  disposal  of hydrocarbon
polymers. Particular consideration was given
to the effects of gaseous and liquid oxidants
on thermal  properties of polyethylene, the
influence of  ultraviolet  and high-energy
(gamma)  radiation  on  the  thermal  and
mechanical behavior of the polyolefin,  and
the  effects  of chemical  treatment on the
biological response of the  modified material.
These studies are described in  the  following
paragraphs.
                Materials

  Polyolefins.  In most studies, commercial,
low-pressure polyethylene with a molecular
weight of 40,000 to 60,000 (Alathon20, E. I.
du Pont  de Nemours & Co.) was  employed.
This material is widely used in  extrusion and
blow-molding  processes and  was  therefore
selected for our investigations. In preliminary
characterization  studies,   several  other
polyethylenes  were  used.  These  polymers
differed  in molecular weight  and structure.
They  included  a low-molecular-weight
material  (AC-6, molecular  weight  2,000,
Allied  Chemical  Co.), an extrusion-grade,
low-melt-index  polymer  (Alathon  10,
molecular  weight  30,000),  and  two
high-molecular-weight  experimental

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16
FEASIBILITY STUDY
compounds (molecular weight 200,000)—one
having a linear structure (No. 46892, Phillips
Petroleum  Co.),  and  the other  having  a
branched-chain configuration with seven side
chains per 1,000  carbon atoms (No. 46894,
Phillips Petroleum  Co.).  Polyethylene was
used  when possible  in  pellet  form (1/8-in.
diameter discs) or as  powder sieved through a
20-mesh screen.  For studies  on larger-size
specimens, samples 1/2-in. diameter x  10 in.
were prepared by extrusion.
  Reagents. The  chemical compounds used
as reagents, or promoters, for the degradation
of polyethylene were reagent-grade chemicals
purchased from  commercial sources.  These
chemicals   included  the  following sensiti-
zing   compounds:    benzoin,   2,2'-azobis
(2-methylpropionitrile),  cobalt nitrate
(Co(NO3)2),  cobalt  molybdate  (CoMoO4),
manganese  nitrate  (Mn(NO3)2), cobalt
chromate (CoCrO4),   and  iron  chloride
(FeCl3). Among the  oxidizing  reagents used
in this study  were the following: nitric acid
(HNO3),  red  fuming nitric acid  (RFNA),
nitrogen tetroxide (N2 O4), benzoyl peroxide
(BzO2), chlorine (C12), sodium hypochlorite
(NaOCl), and  sodium  chlorate (NaClO3). Pure
oxygen (O2) and a mixture of O2 and ozone
(O3) (5.5%) were used in thermal oxidation
studies on  polyethylene.
                Methods

  Experimental investigations of the effects
of selected treatments and reaction conditions
on the properties of polyethylene were based
on  thermal,  mechanical,  and  analytical
measurements. These measurements included
differential  thermal  analysis   (DTA),
calorimetry,  determination  of
stress-relaxation and stress-strain behavior at
high  and  low rates  of loading,  infrared
spectrophotometry,  and viscometry.
Analytical and bichemical methods were used
to determine  chemical changes  in  treated
polymers  and  changes in  their biological
responses.
           Differential Thermal Analysis. Differential
         thermal  analysis is a  useful  method  for
         investigating and comparing the reaction rates
         and energetics of high temperature processes
         under  dynamic  conditions.  Reversible
         decomposition-oxidation  reactions  can  be
         followed  during the cycles of heating  at
         elevated temperatures and subsequent cooling
         at constant heating  or  cooling rates. In  the
         DTA  experiments,   the  temperature
         difference, AT,  between  the  sample under
         investigation and an inert reference, A12 O3, is
         measured  as a function of temperature during
         heating  or  cooling.   Reactions  such  as
         decomposition and phase transition, which
         involve absorption of heat by the sample, are
         indicated by   endothermal bands  in  the
         downward  direction  on  the differential
         thermogram,   and reactions such  as  in
         oxidation,  condensation, and crystallization,
         in which  heat is liberated,  are indicated by
         exothermal bands in the upward direction of
         the differential thermogram.*
           The temperature range of 25 to 700 C was
         scanned at a rate of 10 C per  min. Primary
         consideration was given  to the changes in the
         thermal   response  of   polyethylene  after
         exposure  to different degradation-promoting
         treatments.  To  assess  more  critically  the
         changes in the thermal  behavior of modified
         polymer,  areas under the DTA curves were
         integrated  for  the 200-to-450 C  range by
         using a planimeter, and the  thermal  outputs
         were  compared  for specimens  that received
         different treatments.
           Calorimetry.  The  heats of combustion of
         polyethylene before and after treatment were
         determined  by conventional calorimetric
         methods as a corollary of DTA measurements.
         The  measurements  were  performed  with a
         Parr bomb calorimeter at a pressure of 25 atm
         O2  on samples  that weighed approximately
         0.5 g each. The thermal  changes after ignition
         were followed for 15 min. The heat dissipated
            *A model KA-2H DTA apparatus, manufactured by
         the R.L. Stone Co. was used.

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                                     17
during  combustion  of the  specimen  was
determined  from  the maximum  thermal
increase of the system, and its heat capacity
was  determined after  the usual corrections
were applied for  the formation  of reaction
products other than CO2  and  water.  The
specimens  were  compressed  into a  pallet
whenever   the  polyethylene disintegrated
during  exposure to  degradation-promoting
conditions,  in order  to  permit  a   better
correlation  for the data obtained on treated
polyethylene.
  Mechanical  Properties. The effects of
chemical treatments on mechanical properties
of  Alathon   20  were  investigated in
compression tests designed to determine the
crushability of modified polymer. In these
tests, polymer specimens  (l/8-in.-diameter
disks) were subjected to the crushing force of
a steel plunger descending on the sample at a
rate of 0.5 in. per  min. Lateral displacement of
the  disk was  eliminated  by the use of a
concave  hemispherical  holder.
Load-deformation diagrams were  obtained on
the Instron testing machine. The decay of the
force of the  impressed load (at constant
strain) was also followed for  specimens  that
were compression loaded to 30 Ib at a rate of
0.5 in.  per min.  The  decay of  the initially
impressed   force  reflected  the  extent of
internal damage of the crushed specimens.
   The changes in the structural integrity of
polyethylene  after  exposure  to
degradation-promoting conditions were  also
investigated by  mechanical  test methods
involving the rapid compression of cylindrical
specimens,  1/2-in. in  diameter and 1/2-in. in
length.  Measurements  were performed on a
Plastechon  high-speed  testing  machine in
which the samples were compressed at a rate
of 4,000 in. per min.
   Infrared and Viscometric Studies. Treated
samples  were  subjected  to  infrared
spectroscopic  measurements and viscometric
studies to determine the extent of changes in
chemical   properties  that  resulted  from
specific treatments of polyethylene. Whenever
possible,  infrared  measurements  were
performed  on  films  prepared  at  elevated
temperatures from polyethylene  samples that
were  powdered by  compression  molding.
Mulls with Nujol oil  or Kel-F fluorocarbon
were  prepared  when the  treated  polymer
could not be compressed or cast into a film.
The  main emphasis  in infrared spectroscopic
investigations was  placed  on .the  chemical
changes resulting from oxidation or nitration
of the treated polyethylene.
  Corollary   studies  on  changes  in
polyethylene  after treatment were conducted
by viscometric  measurements. The  viscosity
of polyethylene in xylene was determined at
80 C for solute concentrations ranging from
0.1  to 5 percent. The exact concentration of
the solutions was determined by gravimetric
methods.  The  effects  of  selected,
degradation-promoting  treatments on
viscosity relationships were  assessed  from
plots  of  relative   viscosity  against  solute
concentration. An  Ostwald viscosity pipette
was used for the measurements.
  Analytical Measurements.   The
effectiveness   of  HNO3  treatments for
polyethylene was  determined by analytical
measurements  of  the amount of  residual
nitrogen in treated samples. The procedure,
based on micro-Kjeldahl techniques,  involved
heating a 30-mg sample for 12 hr in a 30-ml
Kjeldahl digestion  flask containing  1.3 g of
potassium  sulfate  (K2SO4),  40   mg  of
mercuric  oxide   (HgO), and  2  ml  of
concentrated  sulfuric  acid  (H2SO4). A 2-mil
aliquot  of  the digested sample was  removed
from the flask  after dilution to 50  ml. This
aliquot  was further  diluted with 3  ml of 2 N
sodium  hydroxide   (NaOH) and 2  ml of a
color reagent containing 4  g  of potassium
iodide (KI), 4 g of mercuric iodide (HgI2), and
1.75 g  of  ghatti gum. The intensity  of the
developed color was determined  after 15 min
by a photometer at a  wavelength of  490 m^.
A calibration curve  obtained  with known
concentrations of ammonium   sulfate
((NH4)2SO4)   permitted  the  quantitative

-------
18
FEASIBILITY STUDY
determination  of  the  residual amount  of
nitrogen in the samples.
  Biological Studies. The  susceptibility  of
chemically  treated  polyethylene   tb
biodegradation was investigated by exposure
of test specimens to selected microorganisms
in an  inoculated agar  system containing  1
percent (by weight) of dispersed material. The
organisms  used were Aspergillus  niger,
Aspergillus  flavus,  Aspergillus  versicolor,
Penicillium funiculosum, Trichoderma, and
Pullularia  sullans.   Studies  were later
conducted  on  strains  of  Pseudomonas
aeruginosa. *  The  composition  of  the
suspension medium is given in Table 4.
  The agar system was sterilized and the pH
of the system  was adjusted  to  6.2 by  an
addition of 0.01  N NaOH. Tests conducted at
room-temperature  incubation  (25  C) at  a
relative humidity of 95  percent comprised a
modified  ASTM  D1924-63   procedure  in
fungal utilization studies.

           Experimental Results

  Differential   Thermal  Analysis
Studies.  Preliminary  investigations of  the
thermal response of polyethylenes differing in
molecular weights  and  structures were
performed  by  DTA  methods  on  a
low-molecular-weight  polymer  (2,000)  and
high-molecular-weight polyethylenes (200,000)
with linear- and  branched-chain
configurations. The thermograms obtained for
these materials at high instrument sensitivity
are   shown   in  Figure  2.  The
low-molecular-weight  polymer  is  the least
thermally stable, with its melting endotherm
having a peak at 106 C, while the branched
and  linear polymer specimens have  peaks at
 117  and  137 C,  respectively. The thermal
degradation  of  the  low-molecular-weight
hydrocarbon also occurs earlier than that of
                          TABLE 4

                   SUSPENSION MEDIUM
Ingredient
KH2PO4
K2HPO4
MgSO4.7H2O
NH4NO3
NaCl
Agar
Concentration (g/1)
0.7
0.7
0.7
1.0
0.005
15.0
   *Strains ATCC 10145 and  QMB  1468 were
obtained from American Type Culture Collection and
the U.S. Army Natick Laboratories, respectively.
          the  high-molecular-weight  material.  The
          oxidation  exotherm is  much  more
          pronounced for the linear polymer than for
          its branched counterpart. The smaller amount
          of  heat  evolved  during  heating  of  the
          branched polyethylene is apparently a result
          of the concurrently occurring degradation and
          oxidation processes. Because of the absence
          of tertiary carbon atoms, the linear polymer
          does not undergo substantial  degradation at
          lower temperatures. The  thermal plateaus for
          all  polymers  tested  above  570  C indicate
          complete combustion of the polyolefins at
          this  temperature.  These data confirm  the
          dependence of thermal relationships  on the
          intrinsic properties of ethylene polymers.
            Alathon 20 was exposed to N2 O4, RFNA,
          O3  and C12 to study the effects of chemical
          treatment  on  the  thermal behavior  of
          polyethylene. The  treatment with N2O4  was
          initially conducted at -10 and +20 C,  with
          samples exposed to the reagent for 5 and 20
          hr. The greater tendency of  the polyolefin to
          disintegrate when treated with N2O4 at the
          higher  temperatures suggested a need for a
          further increase in  temperature to ensure the
          completeness of the reaction within a shorter
          time. Thus Alathon 20 was exposed at 80 C
          to  the reagent  in a pressure-withstanding

-------
                                   350
                     423-


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                                                                     500
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Figure 2. Differential thermograms of different polyethylenes.

-------
20
FEASIBILITY STUDY
reaction vessel. Unfortunately, under  these
conditions  the  reaction occurred violently
after 1 hr,  causing an explosion that injured
one  of  the  workers.   Treatment  of
polyethylene  with RFNA  at  80 C did not
present  the reaction problems encountered
with N2O4, and this method was substituted
for  the  more  sensitive and hazardous
treatment with N2 O4.
  Chlorination  involved a  5-hr exposure of
100  g of  polymer in a sample-holding glass
tube to a stream of C12 that entered the tube
at a rate of 1 liter per min. To enhance the
reaction, the  tube was heated in a furnace at
80 C.  Difficulties in maintaining a  constant
reaction temperature below the melting point
of  the  polymer  were  eliminated  by  a
thermocouple control unit that was used with
an auxiliary heater.
  The possibility  of degrading polyethylene
by  exposure  to  O3  was  investigated in  a
parallel experiment in which the polymer was
subjected to  oxygen containing 5.5 percent
03 at a flow rate of 0.5 liter per min. The
O3 -enriched environment flushed through the
test  specimens  was  obtained  from  a
commercial O3 generator.  To enhance the
reaction,  a  temperature of  40  C  was
maintained during the  exposure of Alathon
20 to the oxidizing environment.
            The thermograms obtained for N2O4- and
          RFNA-treated  polyethylene  after  a  5-hr
          exposure are shown in Figure 3. As indicated,
          N2O4  appears to enhance the degradation of
          the polymer to a greater extent than RFNA
          during the  5-hr  exposure.  A shift  toward
          lower  melting  temperatures  is noticed for
          RFNA- and  N2O4-treated  polyethylene in
          comparison  with the  parent material. Also,
          the chemically treated polyolefin has terminal
          endothermal peaks  (associated  with
          decomposition)  shifted  toward  lower
          temperatures than the peaks of the unexposed
          standard. Early thermal decomposition as well
          as oxidation occurs at lower temperatures for
          N2O4- exposed polyethylene than  for its
          RFNA-treated  counterpart. These  data
          emphasize  the effectiveness  of the  N2O4
          treatment in inducing changes in the polymer
          in short-duration exposure tests (Table 5).
            The  changes  in   the  differential
          thermograms of polyethylene after  5-  and
          10-hr  exposures  to  RFNA at 80   C are
          presented in Figure 4. The longer duration of
          exposure  results  in a reduction] of  the
          amplitude of  the exothermal peak  in the
          450-to-500 C  temperature range. The  most
          significant change in the thermal response of
          treated polyethylene in  comparison with the
          untreated material occurs in  the initial stages
                                      TABLE 5

                     THERMO ANALYTICAL DATA FOR TREATED
                         AND UNTREATED POLYETHYLENE


Property
Melting point
Exotherm
Early decomposition
Terminal endotherm A
Terminal endotherm B


None
109
205-375
375-475
515
530
Temperature, C
Treatment
RFNA*
103
190-360
350-450
500
525


N204*
103
190-325
325-425
490
512
   *Five-hr exposure at 20 C.

-------
OS
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                             Figure 3. Differential thermograms of polyethylene after exposure to N204 and RFNA for 5 hr.

-------
w
s
w
                                                                                                          .UNTREATED
                100
200                300                400
                    TEMPERATURE, C
  Figure 4. Differential thermograms of RFNA-treated polyethylene.
500
                                                                                                              600

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                                    23
of oxidation, when the early exotherm in the
200-to^50 C  range is greatly reduced after
RFNA exposure. Comparing the effectiveness
of  different  treatments  shows  that
chlorination and  RFNA  treatment decrease
the total  exotherm of the  polyolefin to a
significant  extent,  while  ozonization
influences the thermal   properties  of
polyethylene  to a lesser  degree (Figure  5).
Integration of the areas under the DTA curves
indicates  a  23-percent  reduction  in  the
thermal response of ozonized polyethylene in
comparison  with  43  percent for  the
RFNA-treated  polyethylene  (Table  6). The
increasing ratios of the total/initial areas with
increasing exposure to the oxidizing media
(with  the exception   of the  5-hr  RFNA
treatment)  suggest  that   polyethylene
undergoes substantial oxidation when  treated
with RFNA, and that the oxidation products
obtained  require  exposure to   higher
temperatures to be fully oxidized by thermal
methods.
  In  additional  studies  of  the  thermal
behavior of chemically treated  or sensitized
polyethylene,  Alathon 20  was exposed  to
different  reagents.  Benzoyl  peroxide,
2,2'-azobis(methyl)propionitrile, and benzoin
were used  in • a  10 percent toluene  solution
(10%  solute)  in which  polyethylene was
heated at 80 C for 20 hr to acquire a desired
degree of sensitization to further oxidative or
photolytic  degradation.  The  preference for
toluene as  solvent  was suggested by its
swelling  action on polyolefms  and the
resulting increase in polymer  absorptivity  of
the  sensitizers. After the 20-hr treatment, the
polymer specimens were removed from the
treating solution  and were placed in one layer
on  aluminum   dishes.  These  dishes   were
introduced into an ultraviolet (UV) irradiation
facility in which  they were kept for 70 to 96
hr under an AH-6 lamp with an irradiance of
110 mv  per cm.2  The irradiated samples were
subsequently  tested  for changes in  thermal
properties.
                                       TABLE 6
            THERMAL DATA FOR CHEMICALLY TREATED POLYETHYLENE
Integrated area
under DTA curve* Relative
Specimen
Alathon 20
Alathon 20
Alathon 20
Alathon 20
Alathon 20
Alathon 20
Treatment
None
03
C12
5-hr RFNA
10-hr RFNA
20-hr RFNA
200-
450 C
range
0.425
0.394

0.163
0.177
0.148
Total
0.605
0.525
0.351
0.356
0.347
0.380
Control
1.0
0.867
0.580
0.588
0.573
0.628
exotherm
Total:
200-450 C
range
1.4
2.2

3.6
3.2
4.2
     * Arbitrary units.

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I
                                                                                                    CHLORINATED
                                                                                                              RFNA
                                                      I
               100
200                300                400
                      TEMPERATURE, C
Figure 5. Differential thermograms of polyethylene after treatment.
500
600

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                           POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                                     25
   Concurrently  with the  photodegradation,
studies,  polyethylene  samples  were  also
exposed  to  pure O2  at 80 C. Prior to this
exposure the   polymer was  treated  with
benzoyl peroxide,  or  FeCl3  catalyst, in a 10-
percent methanol solution (10% solute). The
oxidation apparatus  essentially consisted of
five Pyrex glass  tubes connected in series to
an oxygen tank. Each tube, containing 10 g of
polymer, was flushed with O2 at a rate of 0.5
1/min. The  tubes  with the  samples  were
partially immersed  in a heating bath that was
maintained at 80±2C.  After a 20-hr exposure
to O2, the specimens were subjected to DTA.
                                      lene
  In  corollary experiments,  polyethylene
pellets were exposed to high-energy radiation
in a  Co60  facility.  The specimens received
increasing doses of gamma radiation  (100,
200, 400 megarads) and their chemical and
mechanical properties were investigated. The
thermal responses of the treated samples are
summarized in Table 7. Closer analysis of data
suggested  greater,  although  not  unusual,
thermal effects in samples that were subjected
to oxidation and UV irradiation. The ratio of
the  total  to  initial  heat  output of
benzoyl-peroxide-sensitized polyethylene  is
greater  after O2  treatment than  after UV
irradiation. This increase  could  suggest the
possibility of preferential bond scission of the
UV-treated  polymer,  because its
fragmentation, followed  by oxidation,  would
tend to decrease the exothermal ratio.  In the
absence of degradation-promoting catalysts,
gamma  radiation at the 200-megarad level
                                       TABLE?
                     DTA DATA FOR TREATED POLYETHYLENE
                                           Integrated area
                                          under DTA curve
Treatment
Benzoyl peroxide + O2
Benzoyl peroxide + UV (96 hr)
Benzoyl peroxide
FeCl3 , + UV
FeCl3 , + O2
Benzoin + UV (70 hr)
2,2'-azobis(methyl)propionitrile + UV
Co60 (200 megarads)
Co60 (400 megarads)
(arbitrary
200-450 C
range
0.160
0.178
0.204
0.182
0.224
0.180
0.186
0.200
0.188
units)
Total
0.220
0.184
0.258
0.222
0.316
0.190
0.240
0.269
0.218
Relative
Specimen
control
0.743
0.622
0.872
0.750
1.067
0.642
0.811
0.902
0.736
exotherm
Total
initial range
1.375
1.030
1.260
1.220
1.411
1.060
1.290
1.345
1.160

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26
FEASIBILITY STUDY
appears to favor cross linking of the polymer.
The cross linking was suggested not only by
the higher exothermal ratio of the irradiated
polyolefin,  but  also  by  the apparent
interactability of the modified  polymer  at
temperatures exceeding 150 C.
   The differential thermograms obtained for
polyethylene  subjected  to oxidation after
treatment  with  sensitizing compounds  of
selected transition elements (Co, Mn, Cr, Mo)
generally  showed an increase  in the areas
under  the  DTA  curves   for  the
high-temperature/total-temperature range  in
comparison  with unsensitized  polymer,  thus
indicating the relative  effectiveness  of  the
treatments.  However,  these treatments  did
not significantly  change the overall thermal
response  of polyethylene,   as was  also
substantiated  by calorimetric measurements.
Greater effects were observed for  Alathon 20
after a  3-hr   exposure to   phosphorus
trichloride (PC13) at 60 C. The results are
shown  in  Figure  6.  In  order   to  obtain
information  about  the effects of treatments
on  the  behavior  of polyethylene  during
incineration, the  thermoanalytical   studies
•were supplemented  by heat-of-combustion
measurements. The  data derived  from these
studies  are  discussed  in  the following
paragraphs.
   Calorimetric  Measurements.  The  initial
indications of a  reduced  thermal output of
 RFNA-treated polyethylene suggested  a need
to determine the heat of combustion of the
modified  polymer.  The data  obtained
indicated a  progressively decreasing thermal
output  with  increasing  temperatures   and
durations of treatment. Polyethylene,  treated
with RFNA  at  80 C for 40 hr,  released 30
percent less   thermal  energy  during
combustion than unmodified polymer. These
and  other  data obtained for  chemically
treated polyethylene are summarized in Table
8.
   In subsequent studies, attempts were made
to decrease  the thermal output by exposing
the polymer  to mixed acids according to the
          nitration procedures used for synthetic fibers
          in  the  textile industry.  Two  solutions were
          prepared containing:  (1) 56 parts HNO3, 26
          parts H2SO4, and 18 parts H2O; and (2) 100
          parts  HNO3   and  40.4   parts  P2O5.
          Polyethylene samples placed in each of these
          solutions were treated at 80 C for 5, 10, and
          20 hr.  A significant decrease  in the heat of
          combustion  was  observed  in
          HNO3.P2O5-treated polymer.  The low heat
          of combustion  (8,912  cal/g) obtained  for
          polyethylene treated  with the mixed system
          in comparison with HNO3 alone (9,523 cal/g)
          indicated  the  greater  effectiveness  of the
          anhydride-containing  acid.  However,  in all
          comparative  tests, RFNA appeared to be the
          most damaging reagent for polyethylene.
            The  10-percent  reduction  in the heat of
          combustion  of  chlorinated polyethylene
          indicated  a relatively low degree  of halogen
          substitution  in  the  polyhydrocarbon
          following the chlorine treatment. To obtain
          quantitative information about the extent of
          halogen substitution in the laboratory-treated
          polyethylene, the heat of combustion of a
          commercial, unplasticized polyvinyl chloride*
          was also determined. The value of 4,580 cal
          per g obtained for this material was substan-
          tially lower  than the 10,015 cal per  g  ob-
          tained  for the chlorinated polyethylene. The
          hydrochloric acid (HC1) generated during com-
          bustion was  equivalent to a chlorine content
          of 49 percent for commercial  polyvinyl chlo-
          ride  as compared with  8  percent for  the
          laboratory-treated polyolefin.  It is therefore
          evident that  a degree of halogenation greater
          than that realized in  our   experiments  is
          necessary to  maintain a low caloric output of
          the hydrocarbon.
            Ozonization  under relatively   mild
          conditions appeared to have a small effect on
          the heat  of combustion of polyethylene.
          Exposure to  O2 at elevated temperatures also
          did not perceptibly  change  the  amount of
          heat dissipated  by the  sensitized polymer
          *Pliovic WO-2, Goodyear Tire and Rubber Co.

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                                                                                                    UNTREATED
U.
a



><
u.
                                                                       \
PC13-TREATED
                                   Figure 6. Differential thermograms of polyethylene before and after PC13 treatment.

-------
28
FEASIBILITY STUDY

    TABLE 8
                HEATS OF COMBUSTION OF TREATED POLYETHYLENE

Oxidative
None
RFNA
RFNA
HN03-P2O5
HNO3-P2O5
HN03-P205
HNO3-H2SO4
HNO3-H2S04
Ozone
Q2
NaCIO
NaClO3
















Treatment
Degradation-promoting












Benzoyl peroxide + O2
FeCl3 + O2
Co(NO3)2 +O2
Co(NO3)2-Mn(NO3)2 + O2
CoMoO4 + O2
CoCrO4 + O2
Benzoin + UV
2,4-dimethyl pentanone +
UV
2,2'-azobis(methyl)-
propionitrile + UV
Co60,


PC13
PC13
Duration
Other (hr)

20
40
1
5
10
10
20
46
20
20
20
20
20
20
20
20
20
72
96

20

1 00 megarads
200 megarads
400 megarads
23
20
Temper-
ature
(C)

80
80
80
80
80
80
80
40
80
80
80
80
80
80
80
80
80
20
20

80




60
25
Heat of
combustion
(cal/g)
1 1 ,064
9,963
7,991
10,782
9,847
8,912
10,971
10,927
10,922
10,015
10,986
11,045
10,688
10,974
10,901
10,922
10,895
10,977
10,913
10,811

10,007

10,948
10,727
10,605
10,455
9,744
during combustion. Ultraviolet radiation and
exposure of polyethylene to gamma radiation
at doses ranging from 100 to 400  megarads
did not significantly affect the thermal output
of Alathon  20,  although   the  degradative
influence  of  the radiation  treatments was
noticed in mechanical tests.
  Infrared and  Viscometric Studies.  In  an
effort to determine the nature  and  extent of
          changes that occurred in the polymer as a
          result  of  chemical  treatment,  infrared
          measurements were performed  parallel with
          the calorimetric studies on a series of selected
          specimens.  Because the  metal  salts of the
          transition  elements  are  known to  affect
          polyolefins7 ° >7 *  and  catalyze the oxidation
          of hydrocarbons,72 an attempt was made to
          utilize these salts as possible catalysts for the

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                          POLYETHYLENE PLASTIC WASTE DISPOSAL
                                       29
thermal  oxidation  and  degradation  of
polyethylene and  determine their  influence
on  the chemical  properties  of  modified
polymer by  infrared spectrophotometric
measurements.  Polyethylene samples  in
separate tests were treated for 4 hr at 100 C
in  three  aqueous solutions containing  by
weight:  (1)  11  percent AgNo3,  (2)  12.8
percent  CoMoO4, and  (3)  11.2  percent
CoCrO4.  After  drying,  the polymer  was
subjected  to an oxidizing environment (O2
admitted at a rate of 0.5 1/min)  at 80 C.

   Although  the  calorimetric measurements
did  not  indicate  significant changes in the
heats  of   combustion  of sensitized
polyethylene,  distinct  differences  were
observed in the infrared spectrograms of these
samples.  The sensitization of polymer  with
CoMoO4 and a mixture of Co(NO3)2 and
Mn(NO3 )2  (Figure 7) was more effective in
promoting  oxidative  degradation  than
treatment with Co(NO3)2 or CoCrO4 (Figure
8). The absorption bands in the 8.7- to 9.4-M
region, representing C-O stretching  modes of
aliphatic ethers,  are more pronounced for
CoMoO4   and   Co(NO3 )2-treated
polyethylene than for the untreated material.
However, the degradation-promoting activity
of these catalysts was by no means  sufficient
to  induce  major  thermal or  mechanical
changes  in  the oxidized  polymer. Similar
results  were   also  obtained  with
benzoyl-peroxide-treated  polyethylene
(Figure 9). The appearance of the absorption
band  at 5.9n,  characteristic  of a  C-O.
stretching mode, is indicative of the chemical
change in the polymer. The FeCl3-catalyzed
polyethylene  subjected  to  UV  radiation
(Figure  10)   and   gamma-irradiated
polyethylene  (Figure  11) also  exhibited
carbonyl  (C=O)  bands,  because  partial
destruction of bonds during irradiation caused
the saturation of these bonds by available O2
(air). In neither case did oxidation result in
mechanical failure of the polymer.

  More  obvious effects  of  polymer
degradation were  found  in  polyethylene
subjected  to  RFNA  treatment.  The
pronounced  brittleness  of this material
necessitated the preparation of O.OlO-in.-thick
films  to  prevent  fracture of  the
compression-molded polymer  during
mounting  of  infrared  specimens.  This
thickness  reduced  the  transparency of the
specimen and  made resolution of the major
absorption  peaks  difficult.   Figure  12
represents  the  infrared  spectrograms of
untreated  and  RFNA-exposed polyethylene.
Both  specimens  were  prepared by
compression  molding. The peaks  at 6 and 6.2
M, representing NO2-stretching  modes, are
missing in the control specimen. Symmetric
NO2-stretching modes at 6.7 to  8.0 (j. are
obscured by intense absorption bands in this
region. The two absorption peaks at 10.9 and
12.0 ju represent C-N vibrations  of attached
NO2 groups. Their absence in the untreated
polymer  indicates  the  effectiveness of the
nitration  treatment.  Although  the
spectroscopic  method was qualitatively
helpful in identifying the NO3 functionality
of RFNA-treated Alathon 20, the technique
could  not  be  used  for  quantitative
measurements because of the fragility of thin,
nitrated films.  Therefore, analytical methods
for nitrogen  analysis  had  to be  used to
determine  the  extent  of polyethylene
nitration  in relation  to the duration  of
treatment.

  The oxidative chain scission resulting from
exposure  of  polyethylene  to  RFNA  was
assessed  from  viscometric  measurements
performed on treated polyethylene in xylene
solution.  Problems  arising   from  the
precipitation of  polymer from  solution at
higher  concentrations  made  the  analysis of
data unreliable in terms of intrinsic viscosities.
Therefore, the  viscosity-exposure  duration
relationship  of RFNA-treated polyethylene
was determined at   a  relatively  low
concentration,  1  X 10'3 g per ml, for which
the solute remained completely  in solution.
Under these conditions, the relative viscosities
(normalized  with  respect  to the solution

-------
                                                                                                         W
4
1
12
13
14
                                             8        9       10
                                              WAVELENGTH (ju)
Figure 7. Infrared spectrograms of polyethylene sensitized with CoMoC>4 andCo(N03)2-Mn(N03)2 before and after oxidation.

-------
  Co(N03)2


BEFORE
i i..   -"•


AFTER
                 12
13
14
                                                 9      10


                                             WAVELENGTH Qi)

Figure 8.  Infrared spectrograms of polyethylene sensitized with Co(NO3)2 and CoCr04 before and after oxidation.
                                                         f

                                                         3
                                                          r
                                                          w
                                                          fc
                                                          d
                                                          n
                                                          w

                                                          o
                                                          I— I
                                                          1/3
                                                          "d

                                                          8

-------
                                                                                         UNOXIDIZED
w
u
z
—

s
                                                       8
10
11
12
13
14
15
                                                         WAVELENGTH ftz)
                       Figure 9. Infrared spectrograms of benzoyl-peroxide-treated polyethylene before and after oxidation.

-------
                                             9         10
                                     WAVELENGTH GU)
11
12
13
                               14
15
Figure 10. Infrared spectrograms of Fed-}-treated polyethylene before and after exposure to UV radiation.

-------
CJ
S
00
Z
                            200-MEGARAD

                            TREATED
                                                          TJ

                                                          W
                                                                                                                            3

                                                                                                                            I
                                                                                                                            CO
                             I
I
I
I
I
                                                                           10
                  11
                  12
                  13
                  14
15
                                                       WAVELENGTH (ju)




                             Figure 11. Infrared spectrograms of unexposed and gamma-irradiated polyethylene.

-------
                                                                                                     UNTREATED
tu
u
z
                                  i
_L
I
                                                       I
          i
              '4
                            11
12
                                    13
6        7        8         9        10

                         WAVELENGTH (M)


 Figure 12. Infrared spectrograms of untreated and RFNA-exposed polyethylene.
                                                       14
                            15

-------
36
FEASIBILITY STUDY
density) remained  in  the  proportion
3.5:1.3:1.1:1.0 for untreated polyethylene
and polymer that was treated with RFNA for
10, 20, and 40 hr, respectively. The threefold
reduction  in  viscosity  of polyethylene
subjected to  RFNA  treatment  for 40 hr
indicates  the   appreciable  decrease in
molecular weight  of the  treated  material.
Bond scission  induced by extended exposure
of  polyethylene  to  RFNA is not as
pronounced  as changes in  other properties,
such  as  mechanical behavior and degree of
nitration, would  suggest. As far as viscosity
relationships are  concerned, the main damage
appears to be incurred by the polymer in the
first 10 hr of RFNA exposure.
  Mechanical Properties.  The changes in the
structural integrity  of  polyethylene  after
exposure   to  degradation-promoting
conditions were  investigated by mechanical
test methods involving rapid compression of
cylindrical  test  specimens.  The  rod-like
specimens were extruded on a Killian 1:24-in.
extruder at  400   F.  In  one  series of
experiments, the  mechanical properties were
determined for rods  exposed in bulk to the
degradation-promoting conditions,  while in
other experiments polyethylene pellets were
subjected to  chemical  treatment  and the
treated  specimens  were subsequently
prepared. Measurements were performed on a
high-speed testing machine  at a compression
rate of 4,000 in. per min.
  The   oscilloscope tracings  for samples
compressed  to  50 percent of their  initial
height are shown in Figure 13. As indicated,
the maximum compressive strengths of the
polymer after  short-duration RFNA  and
N2O4  treatment are  respectively  93.4 and
7.17  percent  of the  initial value.  Similar
relationships are also observed for the plateau
stress values  of compressed  specimens. Under
the relatively mild exposure conditions  used
in preliminary tests,  N2O4 appeared  more
effective  in  reducing  the strength of
polyethylene than RFNA.  The temperature
used  in  the  treatment  and the degree of
         subdivision of the polymer during treatment
         affected the mechanical properties as shown
         in Table 9.
            It should be  emphasized that appreciable
         changes in appearance  were noticed in  the
         rods extruded from the polyethylene pellets
         after  the  pellets  received  the  selected
         treatments.  A  pronounced  brown
         discoloration was found in cylinders that were
         extruded from pellets treated for 5 hr at 20 C
         with N2 O4. Similar effects were observed for
         polyethylene subjected to RFNA at 80 C for
         10 hr.  After  treatment with this oxidizing
         acid  for  20  hr,  the  washed  and  dried
         polyethylene  pellets  were completely
         fluidized when  subjected to  elevated
         temperatures in the  extruder. Under these
         conditions the  polymer could  no longer  be
         extruded  into  rods.  Therefore,  it  became
         necessary  to mold cylindrical specimens from
         the fluidized plastic at a lower temperature
         (150 C). The  solidified specimens that were
         obtained,  unlike polyethylene,  exhibited  the
         properties of melt viscosity and the brittleness
         of a low-molecular-weight paraffin.  Brittleness
         was indicated  by the tendency of specimens
         to  shatter into  small pieces when dropped
         from a height of 8 ft and to pulverize when
         hit with a  hammer.
            The effects of chemical treatment on  the
         mechanical properties of polyethylene were
         also investigated at low rates of loading (0.5
         in./min) in a test designed to determine  the
         crushability  of modified polymer.  Tests
         performed on disklike  specimens  did  not
         permit determination of the ultimate crushing
         force,  because  an  abrupt .change  in  the
         impressed force could not be discerned under
         the experimental test conditions.  Therefore,
         the specimens were compression loaded to 30
         Ib  at a  rate of 0.5  in./min, and the decay in
         impressed  force  was  followed for  5 min.
         Additional information about the compressive
         strength  of the material was  also obtained
         from the initial slope of the load-deformation
         curve in the early  strain cycle. The results,
         summarized in Table  10, indicate  10, 35, 68,

-------
800
600
400
200
                      UNTREATED
                                                                                                  N204,20C,5hr
d
o
$
1
o
53

I
                                     Figure 13.  Oscilloscope tracings of compression-loaded polyethylene.

-------
38
                                 FEASIBILITY STUDY

                                    TABLE 9

              MECHANICAL BEHAVIOR OF TREATED POLYETHYLENE
                       IN HIGH-SPEED COMPRESSIVE TESTS
Treatment conditions
Rod, untreated
Rod, 20 C, 5 hr
Rod,- IOC, 5hr
Pellet, 20 C, 5 hr
Pellet, 20 C, 20 hr
Pellet,- IOC, 5 hr
Compressive
stress (psi)
Untreated RFNA
Initial Plateau Initial Plateau
7,030 5,316
6,145
(87)*
5,985
(85)



5,166
(97)
4,968
(93)



N2
Initial

6,549
(93)
5,160
(73)
6,133
(88)
5,679
(81)

04
Plateau

5,266
(99)
3,815
(72)
4,928
(93)
5,108
(96)
   *The figures in parentheses indicate the relative compressive strengths of treated polyethylene, in percent,
 in comparison with the parent material.
                                    TABLE 10
           MECHANICAL BEHAVIOR OF POLYETHYLENE IN COMPRESSION
Treatment
None
Co60, 400 megarads
PC13,25C, 20 hr
HN03-H2S04,80C, lOhr
HNO3-P2OS,80C, lOhr
Initial load-
deformation slope
(Ib/in.)
64.2
83.0
66.6
58.8
15.0

Initial
load
(Ib)
30.3
29.8
29.9
29.5
29.7
Relaxation
Load after
5 min (Ib)
17.4
15.1
19.4
9.3
5.8

Change
(%)
9.8
49.2
35.4
68.2
80.7

-------
                           POLYETHYLENE PLASTIC WASTE DISPOSAL
                                                                                     39
and 81 percent reductions in the load-carrying
capability  of  untreated   polymer  and
polyethylene exposed toPCl3,HNO3-H2SO4,
and HNO3 -P2 O5, respectively.
  The exposure of polyethylene to chlorine
resulted in a very dark discoloration  of the
modified  polymer  during  extrusion.  This
enhanced decomposition could be attributed
to the  presence  of bound chlorine on the
polyolefin  and  the  thermolytic cleavage of
hydrogen  halide  during   extrusion,  which
accelerated the  degradation of polyethylene.
The  temperature  for  incipient  thermal
degradation would then be  shifted toward
lower  values,  thus  accounting  for the
instability of the chlorine-treated polymer at
normal extrusion temperatures.
  Ozonization appeared to have the opposite
effect  from  chlorination   on  the  extrusion
behavior of oxidized polymer. As a result of
partial  cross-linking, the  ozonized  plastic
resisted  extrusion. The lower  heat of
combustion of  O3-treated polyethylene in
comparison with  the parent material suggests
that bond scission occurred concurrently with
cross-linking. The inherently  flexible  plastic
becomes relatively brittle  if the cross-linking
effects  outweigh  the bond-scission process
during prolonged exposure of polyethylene to
O3. In our experiments, the O3 doses did not
produce these effects, and the polyethylene
retained a fair degree of flexibility.
  In an effort  to enhance the brittleness of
polyethylene, two parallel  experiments were
performed  on  rod and  sheet specimens
exposed to a stress-crack-sensitizing  medium.
A  0.004-in.-thick polyethylene  sheet
subjected to  biaxial tension equivalent to 25
percent  elongation  was  treated with  a
surface-energy-reducing,  crack-promoting
fluid*. After 2 weeks, the  film, mounted in a
multiclamp  biaxial stretch  device, developed
small cracks in the direction of film extrusion.
A similar effect was also observed in rods of
0.5-in. diameter when these rods, subjected to
"Igepal CA-630, General Aniline and Film Co.
multiaxial stress by 0.3-in.-tapered aluminum
insertions forced into them, were immersed in
the  surface-energy-reducing  fluid.  Craze
marks, clearly visible under the microscope,
developed at the  periphery of the stressed
specimens. However,  the  imperfections did
not affect the  mechanical  properties  of the
rods or sheets significantly enough to suggest
the applicability of the stress-crack approach
as a method for enhancing the disposal of
bulk   polyethylene  by  promoting  its
embrittlement.  In  this  regard,  the  RFNA
treatment was much  more  effective  for
promoting  embrittlement,  because  it
converted the inherently flexible plastic into a
rigid  and  rather fragile  material.  Other
potential advantages  derived  from  the
nitrating procedure  and  relating to  the
conversion  of  polyethylene  into  a
biodegradable material were investigated in
subsequent studies.

  Biochemical  and Analytical
Investigations. The changes in the mechanical
and thermal  properties  of  polyethylene
exposed  to RFNA suggested the desirability
of investigating the changes in the chemical
behavior  of the treated polymer, particularly
with regard  to  the  extent of  polyolefin
nitration, as residual nitrogen could enhance
the biodegradability  of the polymer by the
nitrogen-utilizing   m icroorganisms.
Consequently,  the  residual  amount  of
nitrogen in  polyethylene  subjected  to
different  nitration treatments was determined
by  the  previously  discussed micro-Keldahl
method.  The  results are summarized in Table
11.

  The  percentage  of residual nitrogen after
treatment at room temperature was relatively
low.   Prolonged  exposure  at  elevated
temperatures   increased  the amount  of
nitrogen to  3.8  percent after  a  40-hr
treatment. Even better results were obtained
from  two-component  systems,  particularly
with HNO3   and P2O5,  in which the final
amount of nitrogen after 5 hr at 80 C was 3.1
percent.

-------
40
                  FEASIBILITY STUDY

                    TABLE 11

NITROGEN CONTENT IN TREATED POLYETHYLENE
Treatment
Compound
RFNA
RFNA
RFNA
RFNA
RFNA
HNO3 + H2SO4
HNO3 + H2S04
HNO3 + H2SO4
HN03 + P205
HN03 + P2O5
Duration (hr)
5
5
10
20
40
5
10
20
1
5
Temperature (C)
20
80
80
80
80
80
80
80
80
80
Nitrogen
content
(%)
0.21
1.02
1.52
1.58
3.82
4.40
4.60
4.82
1.42
3.14
  In an effort to determine the susceptibility
of  chemically  treated polyethylene  to
biodegradation,  six specimens  that  had
received different treatments were exposed to
selected  organisms  in  an  inoculated agar
system under the conditions described in the
previous  section on  methods.  The samples
investigated included untreated plastic, as well
as  ozonized,  chlorinated,  N2O4-  and
RFNA-treated polyethylene.  The  attack  by
fungal  organisms  of these  samples  was
determined in  a   nitrogen-  and
dextrose-depleted, minimal-salt agar medium
to  obtain information about  the selective
utilization of carbon and nitrogen from the
plastic source by the organisms. The positive
control  for  these experiments provided the
minimal-salt  agar medium  to which 1 percent
dextrose  was added. None  of the  tested
polymer  specimens  revealed  fungal growth
after  a  28-day  exposure  to  the
microorganisms,  but   the
carbohydrate-containing control was heavily
invaded by fungi.
  The  negative  results  obtained  from
exposing  treated   polyethylene to  fungi
suggested the need to extend the biological
                           investigations to  bacterial  microorganisms.
                           The use of bacteria  appeared  desirable
                           because  of their  more rapid growth rate in
                           comparison with  fungi.  Two  strains of
                           Pseudomonas  aeruginosa were  used in
                           investigations  conducted  with  polymeric
                           specimens dispersed  in  minimal-salt broth
                           media buffered at pH 7.  The composition of
                           the medium is shown in Table 12.
                             Tests  were performed  in media that  had
                           glucose  and  NH4NO3   selectively  omitted
                           from the broth  system  to determine  the
                           extent of polymer utilization as a source of
                           carbon   or   nitrogen  for the  organisms
                           considered, and a complete medium was used
                           for control studies.  Visual determination of
                           bacterial  growth disclosed  little  change in
                           inoculated polymer systems  during the first 2
                           days of observation. However, after 72 hr, the
                           polyethylene  that  received the RFNA
                           treatment at 80 C for  40  hr exhibited
                           noticeable bacterial growth. Ozonized  and
                           chlorinated polyethylene, as well  as polymer
                           exposed to RFNA for less than 40 hr, showed
                           no visible changes over a  10-day period, after
                           which the test was terminated. It should be
                           emphasized that the 40-hr-exposure condition

-------
                           POLYETHYLENE PLASTIC WASTE DISPOSAL
                                       41
 does not represent the optimum treatment for
 obtaining a  maximum  amount  of residual
 nitrogen.  However,  at  the  time when the
 biochemical  tests  were  conducted,  the
 information  on  the  efficiency  of acid
 treatment in mixed systems was not available.
 Therefore, it is  quite possible that the extent
 of bacterial attack on modified polyethylene
 could be further increased by prior exposure
 of polymer to HNO3-H2SO4  or HNO3-P2O5.
 The  selective  removal  of  nitrogen  from
 nitrated polyethylene should greatly weaken
 the  polymer  structure  and  effect its
 degradation. Thus the use of more effective
 treatments  deserves  consideration  in future
 studies of the degradation of  polyethylene by
 biological methods.
              TABLE 12

    COMPOSITION OF BROTH MEDIUM
   FOR BACTERIAL GROWTH STUDIES
Constituent
KH2PO4
Na2HPO4.H2O
MgS04
Glucose
NH4NO3
Concentration
(g/D
1.34
4.08
0.02
10.00
2.00
              CONCLUSION

  The chemical and biological inertness of
polyethylene, which  is primarily due  to  its
hydrocarbon nature and its ordered structure,
makes the disposal of polyolefin waste  by
chemical and  biological  methods  difficult.
However, despite   this  difficulty  the
experimental studies conducted in the course
of this  program have shown that  chemical
treatment  of  plastic  can  modify  the
mechanical,  thermal,  and  biochemical
properties of the material in such a way as to
facilitate the ultimate disposal of plastic. The
approach  that  has  appeared  particularly
attractive involves the oxidative degradation
and concomitant nitration of polyethylene by
exposure  to  RFNA,  or binary  systems
including  HNO3, as  a   constituent.
Considerations for this approach follow.
  •  Thermal treatment  of polyethylene,
    following exposure to HNO3, resulted in
    pronounced  embrittlement  of  this
    inherently flexible plastic to the  extent
    that it  could  be shattered by impact
    force. This behavior is important in the
    disposal  of  polyethylene,  because
    effective  compaction reduces the volume
    of  solid waste  and  minimizes  space
    requirements in landfill and  incineration
    processes.
  •  Thermal  response of  acid-treated
    polyethylene  changed   noticeably   in
    comparison  with untreated material,  as
    indicated  by  DTA  and  calorimetric
    measurements.  A 30-percent reduction
    in the heat of combustion was observed
    for the oxidized polymer. This reduction
    represents the  additional benefit that
    could be derived from the oxidizing-acid
    treatment,  because  a   lowered heat
    output should extend  the life of gratings
    and other  furnace  components. The
    exposure of polyethylene  to  chlorine
    had  a  similar, although much  less
    pronounced  (10% reduction in  the heat
    of combustion) effect.
  • Although polyethylene resists attack by
    fungal  and bacterial microorganisms,
    nitration  modified the polyhydrocarbon
    to  an extent  that  its  utilization  by
    bacteria  (Pseudomonas)  became
    apparent  after  a 72-hr  exposure. The
    residual  amount (3.8%)  of nitrogen  in
    modified polyethylene  used  in these

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42
                                     FEASIBILITY STUDY
     experiments does  not  represent  the
     maximum nitrogen content. The attack
     of polymer  by  selected microorganisms
     could probably be enhanced by treating
     the hydrocarbon with binary acids to
     produce higher nitrogen  yields.  In this
     regard,  the   possible  utilization  of
     nitrogen-(or  phosphorus-)  containing
     polyolefins as soil  conditioners  or
     fertilizers deserves attention.

   • Efforts  to  enhance the degradation of
     polyethylene  by ozonization  or  by
     exposure   to  ultraviolet  light  and
     gammairradiation did not  produce large
     enough  effects  to warrant  the  use of
     these  methods  for facilitating  the
     disposal of  polyolefin waste. Some of
     the  difficulties  encountered  in
     promoting  the  disintegration  of
     polymers might relate to the ineffective
     methods of applying sensitizing agents
     and  catalysts  to  the plastic.  These
     compounds  were essentially  applied to
     the surfaces  of the high-molecular-weight
     materials on which they  exhibited only
     limited effectiveness. Adding the agents
     to  the  polymer after  initiation of the
     degradation  process   by  thermal
    oxidation or irradiation should influence
    the mechanical  and physical properties
    of  polyethylene,  because  degradation
    would occur throughout the bulk of the
    material.

  The foregoing considerations indicate the
desirability of continuing the studies on the
disposal of polyethylene  waste by chemical
treatment. Attention should be given to the
degradation  of  the  polymer  in
multicomponent  oxidizing  systems,  the
effects of the latter  on biodegradability, and
the  use  of  selected  catalysts.  Following
sanitary landfilling, special studies should be
made  to  determine if  there could  be
groundwater pollution  due to leaching of the
treated  polyethylene.   The  aspects  of  air
pollution by  combustion products of parent
and chemically modified polyethylene deserve
attention, and particular consideration should
be given to the use of  combustion-promoting
and pollutant-reducing  compounds.  These
studies  should  be   extended to  other
commercially important vinyl polymers, such
as polyvinyl   chloride   and  polystyrene,  to
determine the applicability  of the  selected
methods in  a more  comprehensive study of
polymer waste treatment.
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     20

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