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MAKING
POLYETHYLENE
MORE
DISPOSABLE
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MAKING
POLYETHYLENE
MORE DISPOSABLE
This condensation (SW-14C.1) of
Feasibility Study of the Disposal of Polyethylene Plastic Waste
was prepared for the Federal solid waste management program
by IRENE KIEFER
U.S. ENVIRONMENTAL PROTECTION AGENCY
1973
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Ever-increasing amounts of plas-
tics are being used to fill the
needs of our affluent society. At the
same time, ever-increasing amounts
are being discarded, becoming part
of municipal and industrial wastes.
The most common type of plastic
material in the United States is poly-
ethylene; in 1969, it accounted for
about 30 percent of total production
of plastics. Large concentrations of
polyethylene present unique problems
in some of the common methods of
processing and disposing of solid
wastes:
Sanitary landfill. When buried in a
sanitary landfill, polyethylene can
survive intact for many years. It is
resistant to chemical attack; no bac-
teria are known that can attack it
fast enough to promote its effective
disposal. Also, rigid wastes resist be-
ing compacted, thereby reducing the
amounts that can be buried. Whether
or not these reactions represent prob-
lems in sanitary landfills is not under-
stood.
Composting. Since polyethylene re-
sists bacterial action, it cannot be
converted into compost. It is difficult
to pulverize or remove from mixed
municipal solid waste.
Incineration. Conventional munici-
pal incinerators are designed primari- Environmental Protsctior
ly to bum materials that release much
less heat than polyethylene. When
heavier plastics are burned, they
sometimes block the air supply by
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depositing on the furnace grate in a
molten mass. Present municipal
wastes contain only a few percent of
plastics, but the increasing use of
disposable plastic products and pack-
aging materials could push the level
in wastes to the point where they will
begin to cause problems in conven-
tional municipal incinerators.
How can polyethylene be disposed
of, then, in an efficient, safe, and
economical manner? To find new ap-
proaches to this problem, Kurt Gut-
freund of IIT Research Institute
investigated ways of degrading poly-
ethylene to make it more amenable
to disposal. The study, supported by
the Office of Solid Waste Manage-
ment Programs of the U.S. Environ-
mental Protection Agency, focused on
how degradation affects the mechani-
cal, thermal, and biological properties
of the plastic.
HOW
POLYETHYLENE
DEGRADES
Polyethylene consists of molecules
of ethylene, CH2 = CH2, joined into
long polymer chains. If the chains are
straight, the polymer molecule is
symmetrical and hence crystalline. If
there are branches off the main
chain, however, the molecule cannot
be packed into as tight a structure
and so is less dense and less crystal-
iTROWMENTAL PROTECTION AGENCY
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line. The overall crystallinity of poly-
ethylene can vary from 55 to 99 per-
cent, with the noncrystalline regions
being most susceptible to chemical
attack. The number of double bonds
in polyethylene also influences its
susceptibility to attack, since oxygen
can add to the bond and break the
polymer chain. Even more important,
the hydrogen atoms on the double-
bonded carbons are very reactive.
Polyethylene, like most polymers,
degrades through a chain reaction.
As energy—from heat, radiation, or
chemical reactions, for example—is
added to the polymer, a hydrogen
atom is removed and a free radical
formed. The reaction is then self-
propagating through the free radical
mechanism. Eventually, the chemical
bonds between atoms are broken. In
polyethylene, the products of degra-
dation are not predominantly the
ethylene molecules that joined to
form the polymer Rather, they are
fragments intermediate in size or
molecular weight between ethylene
and polyethylene. At the same time
that the bonds are being cut, the op-
posite process, cross'inking, can also
occur. This increases the size of the
polymer molecule and makes it more
brittle
Burning is one way of degrading
polyethylene Under ideal conditions.
with combustion complete, only car-
bon dioxide and water are produced
when polyethylene burns. In practice,
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the combustion is not complete, and
chain fragments are produced. Com-
bustion is not, however, the only deg-
radation process; so the IITRI study
began by surveying the literature to
assess the numerous approaches to
promoting degradation and ultimate
disposal of polyethylene.
OXIDATIVE
DEGRADATION
The oxygen in air does not degrade
polyethylene at a significant rate.
More severe conditions are needed if
polyethylene is to be decomposed by
an oxidation process. Ozone is known
to accelerate the oxidative degrada-
tion of many polymers, including rub-
ber and plastics. Nitrogen tetroxide
and nitric acid are also reactive
agents that can oxidize otherwise
resistant polymers. Red fuming nitric
acid, which is a mixture of nitrogen
tetroxide and concentrated nitric acid,
is even more effective. In the initial
stages of oxidation of polyethylene
by these nitrogen-containing com-
pounds, nitrates are formed; this al-
lows the possibility of using the
treated material in fertilizers and other
applications requiring nitrogen. In ad-
dition, the treated material is more
readily attacked by bacteria, facilitat-
ing disposal.
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THERMAL
DEGRADATION
When heated, polyethylene does not
decompose into ethylene but, instead,
is randomly broken and reformed into
fragments. The proportion of larger
fragments in the products of pyrolysis
decreases as temperature increases.
At lower temperatures, fewer frag-
ments are formed, but they are larger
in size.
These relationships are relevant in
combustion since it is both a thermal
and an oxidation process. The larger
fragments are harder to burn com-
pletely and are also more likely to
pollute the air. The smaller fragments
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are easier to burn and so release
more heat, a disadvantage in con-
ventional incinerators not designed
for so much heat. Thus there is a
tradeoff between, on the one hand,
the heat needed to break more bonds
and create smaller fragments and, on
the other hand, the heat given off by
thorough oxidation of the smaller
fragments. The heat generated by
oxidation outweighs the heat required
to break the bonds and so determines
the heat balance.
The problems related to excessive
heat liberation during combustion of
polyethylene could be minimized by
partially oxidizing the waste before
it is incinerated. Adding chlorine or
one of the other halogens would have
a similar effect, but their deliberate
introduction must be carefully evalu-
ated because corrosion and pollution
problems may also be introduced at
the same time.
MECHANICAL
MEANS OF
DEGRADATION
Studies on masticated rubber estab-
lished long ago that polymers can be
degraded by mechanical means. The
shearing forces that the molecules
are subjected to during mastication
cut the bonds, leading to formation
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of free radicals, which then react
with the oxygen in air. The bonds are
cut at certain sites in the polymer
chain; hence the fragments fall into
a narrow range of sizes or molecular
weights, rather than a broad range.
Stress cracking is another form of
mechanically induced degradation.
When a high stress is applied in only
one direction, the polyethylene mole-
cule will deform; but if stresses are
applied along two axes the molecule
stores the energy. In the presence of
a crack-inducing sensitizer in the en-
vironment, the polyethlyene rapidly
becomes brittle and fractures. En-
vironmental stress cracking thus can
induce failure at stresses much lower
than the molecule's ultimate strength.
The susceptibility of polyethylene to
stress cracking depends on the stress
and the nature of the sensitizer; also,
polymers having a narrow molecular
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weight distribution have superior re-
sistance to environmental stress
cracking.
ULTRASONIC
DEGRADATION
Ultrasonic energy creates stresses
within the polyethylene molecule,
causing it to disintegrate. As with the
shearing forces involved in mastica-
tion, bonds are cut at selected sites,
producing fragments having a narrow
distribution of molecular weights.
Usually there is a minimum molecular
weight that cannot be decreased
further by prolonged exposure to
ultrasonic energy. Although ultrasonic
degradation has attractive features—
for example, it can be more selective
and thorough—the cost of equipment
and power may preclude its use in
waste disposal.
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DEGRADATION
INDUCED BY
RADIATION
Solar radiation, particularly in the
ultraviolet region, degrades many
synthetic polymers, frequently chang-
'ng their color. Radiation ruptures the
oonds in the backbone of the poly-
mer, this action is followed by forma-
tion of free radicals. If oxygen is
present, oxidative degradation also
occurs. The use of ultraviolet sensi-
tizers to promote degradation is
limited by the fact that some forms
of radiation energy cannot penetrate
the polymer molecule Films thicker
than 0004 inch require extended ex-
posure before they show signs of
degradation
High-energy radiation, however, can
penetrate deeply into the polymer
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and induce rapid deterioration. Ioniz-
ing radiation from cobalt-60, for ex-
ample, ruptures the bonds and also
brings on cross-linking. In radiation-
induced cross-linking, the polymer
can form a rigid, three-dimensional
network, which in extreme cases is
so brittle that a small mechanical
force can fracture it. Thus, degrada-
tion of polyethylene using high-energy
radiation merits study, although de-
veloping a practical method may be
extremely difficult.
BIODEGRADATION
Like most hydrocarbon polymers,
polyethylene is resiSiant to bacteria.
It may be possible, however, to re-
duce this resistance by chemically
modifying polyethylene and exposing
it to bacteria known to be able to at-
tack hydrocarbon polymers.
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EXPERIMENTAL STUDIES
On the basis of this literature survey,
the IITRI study concentrated on how
gaseous and liquid oxidants affect the
thermal properties of polyethylene,
how ultraviolet and high-energy
gamma radiation affect thermal and
mechanical properties, and how
chemical treatment affects biological
response. Alathon 20, a polyethylene
with a molecular weight of 40,000 to
60,000, was used in most of the
studies. Manufactured by E. I. duPont
deNemours & Company, it is widely
used in extrusion and blow-molding
processes.
THERMAL
PROPERTIES
Alathon was exposed to nitrogen
tetroxide (N204), red fuming nitric
acid (RFNA), ozone (03), and chlorine
(CI2). Thermal behavior was deter-
mined by Differential Thermal Anal-
ysis (DTA), a useful method for study-
ing and comparing reaction rates and
energy changes of high-temperature
processes under dynamic conditions.
Results indicate that treatment with
chlorine, nitrogen tetroxide, and
RFNA significantly decreases the total
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heat given off, while treatment with
ozone has a lesser influence. Intro-
duction of chlorine into polyethylene
could cause corrosion and air pollu-
tion problems in incinerators. Nitrogen
tetroxide reacts explosively when
temperatures reach 80 C. RFNA,
therefore, is the preferred treatment.
A number of other treatments were
also tested:
• Catalysts and sensitizing agents,
followed by exposure to pure
oxygen or ultraviolet radiation.
Both organic and inorganic
chemicals were tested.
• Phosphorus trichloride.
• High-energy radiation from co-
balt-60.
All these treatments markedly re-
duce the amount of heat measured
by the DTA studies.
CALORIMETRY
To obtain information on how treated
polyethylene would behave during
incineration, the heats of combustion
were also measured. Most of the
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treatments reduce the heats of com-
bustion by about 10 percent (Table
1). Treatment with RFNA, however, is
considerably more effective—poly-
ethylene exposed for 40 hours at 80 C
releases 30 percent less heat during
combustion. To determine if the heat
output could be reduced still further,
polyethylene was exposed to mixed
acids according to the procedures for
nitrating synthetic fibers. Two solu-
tions were used:
• A nitric acid-sulfunc acid mix-
ture consisting of 56 parts of
nitric acid, 26 parts of sulfuric
acid, and 18 parts of water.
• A nitric acid-phosphoric acid
mixture consisting of 100 parts
of nitric acid and 40.4 parts of
phosphorus pentoxide
Treatment with the nitric-phosphor-
ic mixture is quite effective in de-
creasing the heat of combustion, but
the RFNA treatment proved to be the
most damaging of all those used in
the calorimetric studies.
INFRARED
AND VISCOSITY
MEASUREMENTS
The chemical changes caused by
some of the treatments were studied
using infrared spectrophotometry and
viscosity measurements. The infrared
spectrograms of samples treated with
catalysts containing cobalt, manga-
nese, or chromium indicate that some
degradation occurs; but it is not
15
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Table 1: Treating Alathon 20 Polyethylene
Reduces the Heat of Combustion
Duration
Treatment (hr)
None
RFNA
RFNA
Nitric acid-phosphoric acid
Nitric acid-phosphonc acid
Nitric acid-phosphonc acid
Nitric acid-sulfunc acid
Nitric acid-suifunc acid
Ozone
Chlorine
NaCIO
NaCIO,
Benzoyl peroxide 4- 02
Iron chloride J- 02
CO(NO3)2 - 0;
Co(N(X)2 - Mn(NO3)2 x 0;
CoMoO^ - 02
CoCrO.4 -»- 02
Benzoin + UV
2,4-dimethyl pentanone + UV
2.2'-azobis (methyl) — propionitnle + UV
Cobalt-60 100 megarads
200 megarads
400 megarads
Phosphorus trichloride
Phosphorus trichloride
20
30
1
5
10
10
20
46
20
20
20
20
20
20
20
20
20
72
96
20
—
—
—
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
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enough to induce major changes in
the heat or mechanical properties of
the treated polyethylene. Similar re-
sults were obtained with benzoyl
peroxide, iron chloride, and gamma
radiation. The RFNA treatment in-
duces more obvious degradation. The
modified polyethylene was so brittle
that it was difficult to prepare samples
thin enough for infrared analysis.
Viscosity measurements were used
to determine how much the polymer
chains were being broken. The vis-
cosity of polyethlyene exposed to
RFNA for 40 hours at 80 C was re-
duced by a factor of 3, indicating
that many chains were being broken.
The main damage occurs in the first
few hours of exposure.
MECHANICAL
PROPERTIES
The changes in structural integrity
of treated polyethylene were studied
by mechanical tests involving rapid
compression of cylinders 1/2 inch in
diameter and Vz inch long. Measure-
ments were made on a high-speed
testing machine at a compression
rate of 4,000 inches per minute. In
some tests, the cylinders themselves
were exposed; in others, pellets were
exposed. The cylinders were then
formed by heating the material and
extruding it.
In one test, rods were formed from
pellets exposed to both RFNA and
nitrogen tetroxide for 5 hours at 20 C
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and were compressed to half of their
original height. The maximum com-
pressive stress is reached rapidly;
then it levels off on a plateau. Under
mild exposure conditions, the nitrogen
tetroxide is more effective than RFNA
in reducing the strength of poly-
ethylene. It reduces the initial stress
to 73 percent that of the untreated
material and the plateau strength to
72 percent. RFNA reduces the initial
stress to 85 percent and plateau
strength to 93 percent.
With both RFNA and nitrogen te-
troxide treatment, the extruded cyl-
inders take on a pronounced dis-
coloration. With nitrogen tetroxide,
the discoloration appears when the
pellets are treated for 5 hours at 20
C. With RFNA, it appears after 10
hours at 80 C; after 20 hours, the
washed and dried pellets were com-
pletely fluidized when heated in the
extruder, and the cylinders had to be
prepared by molding them at a lower
temperature. These molded cylinders
were very brittle, as indicated by their
tendency to shatter when dropped
from a height of 8 feet and to pulver-
ize when hit with a hammer.
Crushability of some treated poly-
ethylenes was also determined. Disks
Va inch in diameter were subjected
to a steel plunger descending at 0.5
inch per minute. All the treatments
reduced the load-carrying capability
of polyethylene (Table 2).
In an effort to enhance the brittle-
ness of polyethylene, rods and sheets
were exposed to Igepal AC-630, a
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Table 2: Treated Alathon 20 Polyethylene
Loses Its Ability To Carry Loads
Treatment
None
Cobalt-60, 400 megarads
Phosphorus trichloride
20 hours at 25 C
Nitric acid-suifuric acid
10 hours at 80 C
Nitric acid-phosphoric acid
10 hours at 80 C
Initial
load
(Ib)
30.3
29.8
29.9
29.5
29.7
Load after
5 minutes
llbL
27.4
15.1
19.4
9.3
5.8
Percent
change
9^T
49.3
35.1
68.5
80.5
Loading rate: 0.5 inch/minute
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liquid that reduces surface energy,
to determine if it would promote
cracking. Under stress, both rods and
sheets developed craze marks clearly
visible under the microscope. These
imperfections did not, however, affect
the mechanical properties enough to
suggest that the stress-crack ap-
proach could enhance the disposal of
bulk polyethylene. The RFNA treat-
ment is much more effective for pro-
moting brittleness because it converts
the inherently flexible plastic into a
rigid and rather fragile material.
BIOCHEMICAL
INVESTIGATIONS
The changes in the mechanical and
thermal properties of polyethylene
20
exposed to RFNA suggested that it
would be desirable to investigate the
changes in chemical behavior of the
treated polymers. The amount of ni-
tration is of particular interest since
more residual nitrogen could increase
biodegradation by nitrogen-utilizing
microorganisms.
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The percentage of residual nitrogen
after treatment at room temperature
is low. but prolonged exposure at
elevated temperatures is more effec-
tive. The amount increased to 3.8
percent after 40 hours at 80 C. Even
better results were obtained with
treatments involving two acids (Table
3).
Specimens treated with ozone,
chlorine, nitrogen tetroxide, and RFNA
were exposed to several fungi, in-
cluding some belonging to the Peni-
cillium and Aspergillus genera. None
of the treated polymers showed
fungal growth after 28 days of ex-
posure. The studies were then ex-
tended to bacteria since they grow
faster. After exposure to Pseudo-
monas aeruginosa for two days, the
treated polymers showed little change
After three days, bacterial growth was
noticeable in the polyethylene treated
with RFNA for 40 hours. Samples
treated with ozone and chlorine, as
well as those with shorter exposure
to RFNA, showed no visible changes
over 10 days, at which time the test
was ended.
The polyethylene treated with RFNA
for 40 hours contained 3.8 percent
nitrogen. At the time the biochemical
studies were conducted, the two-acid
systems had not been studied. Since
these treatments produce even higher
amounts of residual nitrogen in the
polymer, it is possible that the vul-
nerability of treated polyethylene to
bacterial action can be increased still
more.
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Table 3: Treating Alathon 20 Polyethylene
Increases the Nitrogen Content
Treatment
Nitrogen
Duration Temperature content
(hr) (C) (%)
RFNA
RFNA
RFNA
RFNA
RFNA
Nitric acid-sulfunc acid
Nitric acid-sulfunc acid
Nitric acid-sulfunc acid
Nitric acid-phosphoric acid
Nitric acid-phosphoric acid
5
5
10
20
40
5
10
20
1
5
20
80
80
80
80
80
80
80
80
80
0.21
1.02
1.52
1.58
3.82
4.40
4.60
4.82
1.42
3.14
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AN ASSESSMENT
The IITRI study concludes that the
properties of polyethylene can be
modified so as to make it easier to
dispose of. Using RFNA (or two-
component systems containing nitric
acid) to oxidize and nitrate the poly-
mer is especially attractive. The
RFNA method scored well in the
thermal, mechanical, and biochemical
testing. The possible use of poly-
ethylenes containing nitrogen (or
phosphorus) as soil conditioners or
fertilizers is another advantage to this
treatment.
The treatments using ozone, chlo-
rine, ultraviolet radiation, and gamma
radiation do not produce effects sig-
nificant enough to warrant their use.
Some of the difficulties in promoting
the degradation of polyethylene might
result from ineffective methods of
applying the sensitizing agents and
catalysts. They were essentially
applied to the surfaces of the poly-
mer. Adding the chemicals after deg-
radation has been started by thermal
oxidation or irradiation might permit
degradation to occur throughout the
bulk of the polymer.
The IITRI study leaves some other
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unresolved problems. Following sani-
tary landfill of treated polymers,
groundwater should be analyzed to
determine if it is being polluted by
leaching from the fill. The air pollu-
tion from the combustion products of
polyethylene, both treated and un-
treated, deserves attention; special
consideration should be given to
treating polyethylene with chemicals
that promote combustion but reduce
air pollution. And, finally, the selected
treatments should be studied further
to find out if they might also facilitate
the disposal of other commercially
important plastics such as polyvinyl
chloride and polystyrene.
24
Agency
US GOVERNMENT PRINTING OFFICE 19730-494-075
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his summary report is based on Feasibility Study of the Disposal of
. Polyethylene Plastic Waste (SW-14c) by Kurt Gutfreund of III Research
Institute. The full report is also numbered as Public Health Service Publica-
tion No. 2010 (Library of Congress Catalog Card No. 70-614032). The full
report is available from the Superintendent of Documents. U.S. Government
Printing Office. Washington. D.C. 20402. Price is 55 cents in paper cover.
The 45-page publication includes 12 tables and 13 figures. It consists of
these major sections:
General Considerations of Waste Disposal Problems
Degradation of Hydrocarbon Polymers
Experimental Investigations
Conclusion
References
Mention of commercial products does not constitute endorsement
or recommendation for use by the U.S. Government
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