FEASIBILITY STUDY OF
The Disposal of Polyethylene
Plastic Waste
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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-
High-Molecular-Weight, linear
High-Molecular-Weight, branched
Low-Molecular-Weight
!l
512^
i !
!i
I '
^-x ii
"*T*I
\ '$
v"' " ill
i • »
I l
II
i •
i
/ >f
il
i:
\
i i
! H
J
Til
i«l!
]l\
|! ;
1 II
i'
1 1 1 1
'111
'!
1 1 ' '
1 1 i i
II
\\ >
l\
\i\ /
\^
-531
ri
525
s
t-1
i
£
3
fc
H
O
g
E9
w
O
3
CO
500
518-
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
(-L
I
w
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
-------
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.
-------
I
CHLORINATED
RFNA
I
100
200 300 400
TEMPERATURE, C
Figure 5. Differential thermograms of polyethylene after treatment.
500
600
-------
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
-------
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.
-------
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
-------
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
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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
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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
-------
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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