EPA-R2-72-046 Environmental Protection
August 1972
An Investigation
of the Biodegradability
~ *
of Packaging Plastics
Office of Reiejfch and Monitoring
U.S. Envifjiiine-itj! Protection Agency
Washington. D C :04?J
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EPA-R2-72-046
August 1972
AN INVESTIGATION OF THE
BIODEGRADABILITY OF
PACKAGING PLASTICS
By
James E. Potts, Robert A. Clendinning, and Watson B. Ackart
Union Carbide Corporation
River Road, Bound Brook, New Jersey 08805
Contract No. CPE-70-124
Project Element 1D2064
Project Officer
Charles Rogers
Solid Waste Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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REVIEW NOTICE
The Solid Waste Research Laboratory of the
National Environmental Research Center, Cincinnati,
U.S. Environmental Protection Agency, has reviewed
this report and approved its publication. Approval
does not signify that the contents necessarily re-
flect the views and policies of this laboratory or
of the U.S. Environmental Protection Agency, nor
does mention of trade names or commercial products
constitute endorsement or recommendation for use.
The text of this report is reproduced by the
National Environmental Research Center, Cincinnati,
in the form received from the Grantee; new prelim-
inary pages and cover have been supplied.
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FOREWORD
To find, through research, the means to protect,
preserve, and improve our environment, we need a focus
that accents the interplay among the components of our
physical environment - the air, water, and land. The
missions of the National Environmental Research Centers
— in Cincinnati, Ohio; Research Triangle Park, North
Carolina; Corvallis, Oregon; and Las Vegas, Nevada —
provide this focus. The research and monitoring activ-
ities at these centers reflect multidisciplinary
approaches to environmental problems; they provide for
the study of the effects of environmental contamination
on man and the ecological cycle and the search for
systems that prevent contamination and recover valuable
resources.
Man and his surrounding air, water, and land must
be protected from the multiple adverse effects of pesti-
cides, radiation, noise, and other forms of pollution
as well as poor management of solid waste. These
separate pollution problems can receive interrelated
solutions through the framework of our research programs
— programs directed to one goal, a clean livable environ-
ment.
This publication/ issued by the National Environ-
mental Research Center, Cincinnati, reports on a 14-month
research contract study of one of today's more pressing
solid waste disposal problems—disposing of the plastics
that inexpensively and attractively clad and protect
so much of what we buy. That these plastics will con-
tinue to be used is indisputable; and it follows that
solutions must be found for their disposal. The find-
ings here will contribute greatly to solving this
problem.
ANDREW W. BREIDENBACH, Ph.D.
Director, National Environmental
Research Center, Cincinnati
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CONTENTS
ABSTRACT 1
SCOPE OF INVESTIGATION 2
SUMMARY 5
INTRODUCTION H
Plastics in Solid Waste 11
Definition of Biodegradability 13
Biodegradability as a Means of Recycling 14
PREVIOUS STUDIES OF THE BIODEGRADATION OF PLASTICS ... 18
MICROBIOLOGICAL PROCEDURES 20
BIODEGRADABILITY OF COMMERCIAL PLASTICS 22
Packaging Films 22
Other Commercial Plastics 23
EFFECT OF POLYMER MOLECULAR WEIGHT AND BRANCHING .... 25
Polyethylene 25
Pyrolyzed Polyethylene 35
Polyethylene Containing Terminal Functional Groups. . 37
Polystyrene 38
Pyrolyzed Polystyrene 38
BIODEGRADABILITY OF RANDOM COPOLYMERS 41
Copolymers of Ethylene 41
Copolymers of Styrene 44
BIODEGRADABILITY OF POLYESTERS 46
SOIL BURIAL TESTS ON EPSILON-CAPROLACTONE POLYESTER . . 49
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BIODEGRADABILITY OF BLOCK AND GRAFT COPOLYMERS 57
BIODEGRADABILITY OF ADDITIVES USED IN PLASTICS 61
PLASTICIZED POLYVINYL CHLORIDE 66
EFFECT OF ENERGY TREATMENT ON PACKAGING FILMS 70
RECOMMENDATIONS FOR ACTION 74
ACKNOWLEDGEMENT 77
REFERENCES 78
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ABSTRACT
An Investigation Of The Biodegradability Of Packaging Plastics
This study investigates the effects of various struc-
tural parameters on the biodegradability of plastics. It in-
cludes a determination of the effect of molecular weight and
polymer end group composition on the biodegradability of poly-
ethylene and polystyrene, and an evaluation of the biodegrad-
ability of various block, graft and random copolymers containing
polyethylene or polystyrene chain segments. Also included is a
study of the biodegradability of organic chemicals used commer-
cially as additives to plastics.
The study verifies the popular belief that the current
high volume, high molecular weight packaging plastics are not
biodegradable at practical rates. Aliphatic polyesters and de-
rivatives were the only synthetic, high molecular weight poly-
mers found to be biodegradable. The report also establishes
that the structural modification of polyethylene and polystyrene
by random copolymerization with other monomers will not lead to
•
biodegradability.
This report was submitted in partial fulfillment of
Contract No. CPE-70-124 under the sponsorship of the Solid Waste
Research Division of the National Environmental Research Center,
Cincinnati, U.S. Environmental Protection Agency.
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AN INVESTIGATION OF THE BIODEGRADABILITY OF PACKAGING PLASTICS
James E. Potts, Robert A. Clendinning, and Watson B. Ackart
SCOPE OF INVESTIGATION
This is the final report of a 14 month investigation
of the biodegradability of packaging plastics performed by the
Research and Development Department of the Union Carbide Cor-
poration, Chemicals and Plastics, under a research contract
with the Solid Waste Research Division of the National Envi-
ronmental Research Center, Cincinnati, U.S. Environmental
Protection Agency.
The major objectives of this investigation, as
described in the work statement of the contract are:
1) to screen the major commercial packaging
plastics for biodegradability
2) to determine the biodegradability of other
plastics that might be useful as packaging
plastics
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3) to determine the effect of polymer molecular
weight on the biodegradability of polyethylene
and polystyrene
4) to determine the effect of placing metaboli-
cally active polymer end groups on the bio-
degradability of polystyrene and polyethylene
5) to determine the biodegradability of additives
that are customarily found in packaging plastics
(antioxidants, slip agents, and plasticizers,
for example)
6) to examine the biodegradability of random co-
polymers of ethylene and of styrene
7) to study the effect of block and graft polymer
i
structure on biodegradability, and
8) to examine the effects of thermal, ultra-
violet, and ionizing radiation on polymer
biodegradability.
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The commercial polymer samples used in this investi-
gation were either obtained in-house or purchased from other
plastic manufacturers. The experimental polymers and copoly-
mers involved in this program were synthesized in the labora-
tories of the Union Carbide Corporation and contributed with-
out cost to the investigation.
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SUMMARY
In a study embracing the large volume thermoplastic
packaging plastics and several hundred experimental polymers,
aliphatic polyesters and derivatives were the only synthetic,
high molecular weight polymers found to be biodegradable.
Although high molecular weight polyethylene is not biodegrad-
able, pure linear paraffin molecules below about 500 mol wt
were found to be utilized by microorganisms. Lower molecular
weight polyethylenes, prepared by either direct polymerization
or by pyrolysis of high molecular weight polymers, also sup-
ported the growth of microorganisms. Evidently, the polymer
fractions being utilized were those below 500 mol wt. Low
molecular weight polystyrene, down to 5,000 mol wt , did
not support the growth of microorganisms. The placement of
metabolically active organic functional groups at the end of
polyethylene and polystyrene chains did not observably en-
hance biodegradability. Copolymers of ethylene with comono-
mers such as vinyl acetate, ethyl acrylate, lauryl acrylate,
acrylic acid, carbon monoxide, aconitic acid, itaconic acid,
and vegetable oils such as safflower, linseed, and soy bean
oil, did not support fungal growth. Copolymers of styrene
with acrylic acid, dimethyl itaconate, 2-ethyl-hexyl fuma-
rate, and dodecyl acrylate were also inactive.
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An explanation of these observations from the view-
point of molecular biology was outside the scope of this in-
vestigation. When the insolubility in water and the inert-
ness of most plastics to biological attack is considered, it
is not surprising that most published fundamental studies of
microbiological decomposition involving polymers were carried
out with naturally occurring polymeric materials such as cel-
lulose, starch, protein, lignin, etc. Prom these studies we
can elicit general principles that may well be relevant to
the systems examined in this investigation.
The first principle is that a living cell must take
food materials inside itself to obtain energy and organic
buildup blocks for growth. Soluble organic substances diffuse
into the cell or are carried in by various transport mechan-
isms involving enzymes called permeases. Inside the cell,
the food substances are attacked by digestive enzymes and the
fragments used for growth or energy.
For insoluble materials such as many natural poly-
mers, the organism prepares one or more enzymes that leave
the cell and break down the insoluble material in the vici-
nity of the cell to a soluble substance that can be assimi-
lated by the cell and utilized.
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A second general principle involving the metabolism
of living cells is that as a result of evolutionary processes,
microorganisms utilize many metabolic processes in common with
other more complex organisms such as man. These metabolic
pathways involve the action of many highly specific enzymes.
As mentioned above, low molecular weight linear
polyethylene molecules are biodegradable, even though they
are of synthetic origin. These products are closely related
structurally to the paraffin based crude oils which are of
natural origin. It would appear that the oxidase enzymes de-
veloped by nature to facilitate the biooxidation of naturally
occurring linear paraffinic hydrocarbons are also specific for
low molecular weight linear polyethylene fractions.
The mechanism of biodegradation of paraffinic hydro-
carbons involves the action of oxidase enzymes, which catalyze
the beta oxidation of the molecular chain by removing two
carbon atoms at a time from the end of the chain. Why are
the enzymes that must catalyze the decomposition of low mole-
cular weight polyethylene seemingly unable to bring about the
decomposition of high molecular weight polyethylene used for
packaging applications? Rodriguez > suggested that this may
be due to the fact that polyethylene crystallizes in a folded
chain configuration in which chain ends are unlikely to be
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found near the surface of the polymer. In order for the en-
zyme to catalyze the oxidation of the polymer, it must be
able to complex with the ends of the polymer chains that are
both low in concentration and not very accessible. This
steric hindrance may play an important role in inhibiting the
biodegradability of high molecular weight polyethylene.
Other factors that must be considered in addition
to enzyme specificity and steric hindrance are the effect of
pH and the tendency of the polymer to absorb water. The
water repellant nature of polyethylene certainly does not
facilitate the attainment of the aqueous conditions (pH and
swelling) favorable to the optimum functioning of enzymes,
which are proteins.
Although this investigation did not establish which
enzymes are responsible for the biodegradability of the ali-
phatic polyesters examined, it is reasonable to assume that
initial attack is by esterase enzymes, which catalyze the hy-
drolysis of ester linkages along the chain.
f1'
In summary, this study has verified the popular
belief that the current high volume, high molecular weight
packaging plastics are not biodegradable at practical rates.
It has also established that structural modification of
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polyethylene and polystyrene by random copolymerization with
other monomers will not lead to biodegradability. The econo-
mics of the plastic packaging industry indicate that poly-
ethylene, polystyrene, and polyvinyl chloride will continue
to be used in large volume for packaging applications because
of their low cost, light weight, eye appeal, and functional
utility. For these materials to be made biodegradable, the
molecular weight of the polymer will have to be reduced sub-
stantially. This deliberate degradation of molecular weight
must be carried out after the plastic package is discarded,
because degradation results in a severe reduction in the
strength properties of the package.
Two methods of achieving this degradation require
no prior modification of the plastic packaging material. The
discarded plastic is either pyrolyzed (thermally degraded) or
treated with oxidizing chemicals in an autoclave. The third
method involves modification of the plastic formulation during
manufacture so that the plastic package is rendered much more
susceptible to oxidative attack by environmental oxygen. The
plastic formulation can be modified either by adding substances
that will enhance the rate of oxidation in a controllable
manner, or by modifying the structure of the polymer suffi-
ciently to make it more susceptible to oxidative degradation.
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The development of packaging plastics that will undergo con-
trolled auto-oxidation will help alleviate the plastic litter
problem and will insure that such products will be recycled
back into the natural carbon cycle of the planet.
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INTRODUCTION
Plastics in Solid Waste
It has been estimated2 that by 1976, the U.S. will
be generating about 450 billion Ib of solid waste per year,
compared with 375 billion Ib estimated for 1970. By 1976,
total plastic wastes are expected to be about 9.5 billion Ib
per year, with packaging plastics accounting for about 6.6
billion Ib. Thus plastics represent less than 5% by weight
of the total solid waste generated annually in this country.
Table 1 lists the types of plastics found in muni-
2
cipal refuse as of 1966. It can be seen that 90 percent of
the total plastic waste is made up of polyethylene (38%),
polyvinyl chloride (31%), and polystyrene (21%).
TABLE 1
TYPES OF PLASTICS POUND IN MUNICIPAL WASTE
Type of plastic
Nylon
Phenolics
Polyacetals
Po ly carbonate
Polyethylene
Po ly pr opy lene
Polyvinyl chloride
Polyesters
Polystyrenes
Urea and melamine
Urethane foam
Cellulosics
Weight
(in millions of pounds)
7.9
22.5
3.5
3.4
1446
71
1204
51
823
79
89
60
% of total
0.2
0.6
0.1
0.1
37.6
1.8
31.2
1.3
21.3
2.0
2.3
1.5
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Because these three plastics account for the bulk
of packaging plastics, it is felt that they should be the
focus of attention of research and development pertaining to
the disposability or recycling of plastic waste.
A recent report prepared for the Solid Waste
Management Office of the Environmental Protection Agency by
Battelle Memorial Institute presented data on the expected
ranges in municipal refuse composition (see Table 2).
TABLE 2
EXPECTED RANGES IN MIXED MUNICIPAL REFUSE COMPOSITIONS
Component
Paper
Newsprint
Cardboard
Other
Metallics
Ferrous
Nonferrous
Food
Yard wastes
Wood
Glass
Plastic
Miscellaneous
Percent composition as received
(dry weight basis)
Anticipated range
37-60
7-15
4-18
26-37
7-10
6-8
1-2
12-18
4-10
1-4
6-12
1-3
<5
Nominalt
55
12
11
32
9
7.5
1.5
14
5
4
9
1
3
tBattelle estimate.
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The anticipated range for plastics is 1 to 3 per-
cent, a figure that is quite low compared to paper and paper
board (37-60%) and other solid wastes such as glass and metals.
Definition of Biodegradability
Strictly speaking, biodegradable materials are
those whose chemical structures make them susceptible to assi-
milation by microorganisms such as molds, fungi, and bacteria
when buried in the ground or otherwise contacted with the or-
ganisms under conditions conducive to their growth. Some non-
biodegradable plastics are erroneously believed to be bio-
degradable because they often contain biodegradable additives
that will support the growth of microorganisms without causing
the plastic itself to become assimilated.
The term "biodegradable" is often used indiscrimi-
nately to refer to various types of environmental degradation,
including photodegradation. Because a polymeric material is
degraded by sunlight and oxygen does not necessarily mean that
the material will also be assimilated by microorganisms. The
term "biodegradable" should be reserved for that type of de-
gradability that is brought about by living organisms, usually
microorganisms.
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Biodegradability as a Means of Recycling
The reclamation and recycling of plastic wastes are
already being practiced to some extent in those applications
where the plastic waste can be kept homogeneous and clean, or
in applications where substandard plastics are completely ac-
ceptable. In almost all large volume plastic applications,
however, very rigid standards of product quality, including
cleanliness, are absolutely necessary for several reasons:
1. In many packaging applications, the plastics
fabrication must meet very demanding standards set by the
Food and Drug Administration with regard to the cleanliness,
absence of toxic or undesirable additives, or presence of
material which may be extracted by the food being packaged.
The vendor must guarantee the compositional uniformity of
his product.
2. The plastic material must be suitable in melt
flow properties and in heat stability for the particular fa-
brication process used and must emerge from fabrication with-
out any loss of mechanical strength or appearance properties.
3. The packaging material must exhibit the proper
balance of physical properties for the specific application
for which it was designed — modulus, impact strength, stress
crack resistance, softening temperature, abrasion resistance,
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and permeability to gases and liquids to name a few. Proper-
ties required for a plastic milk bottle are quite different
from those of a meat wrap, for example. Many of these physi-
cal properties requirements are subject to regulation by
State and National agencies, and by organizations such as the
National Sanitation Foundation.
In order to meet these demanding and varied require-
ments, the plastics industry has developed thousands of formu-
lations containing various plastics in combination with many
different additives that are necessary for different appli-
cations. Even if techniques for separating mixtures of waste
plastics from other municipal waste are eventually developed,
such mixtures may not be suitable for most plastic applica-
tions because of their nonuniformity with respect to such
characteristics as color, composition, additives, molecular
weight, molecular weight distribution, softening point, and
heat resistance. The resulting inability to predict the per-
formance of mixed waste plastics discourages their use in most
plastics applications.
As stated above, another way to recycle organic sub-
stances is by way of the carbon cycle of the earth — nature's
way. A simplified diagram of the carbon cycle is given in
Figure 1. Green plants convert carbon dioxide gas from the
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atmosphere into organic carbon with the aid of sunlight by
the process known as photosynthesis. Animals and plants
ultimately revert to dead organic matter, which yields C02
when decomposed by microorganisms. Animals and plants also
produce CO^ by respiration. Note that animals are true para-
sites in the carbon cycle - they can only utilize it for
growth and energy. It should also be observed that the CO2
used by plants in photosynthesis comes from many sources,
including the burning of gasoline, oil, coal, wood and the
incineration of solid waste, as well as from the respiratory
processes of plants, animals, and microbes.
In principle, biodegradable plastics would decom-
pose in sanitary landfills and backyard compost heaps, and
would disappear more rapidly when littered. Biodegradable
plastics would not be expected to cause any unusual incinera-
tion problems.
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PHOTOSYNTHESIS
C02
CARBON DIOXIDE
DECOMPOSITION BY
MICRO-ORGANISMS
RESPIRATION
DEAD ORGANIC
MATTER
Figure I. A simplified diagram of the carbon cycle.
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PREVIOUS STUDIES OF THE BIODEGRADATION OP PLASTICS
The literature in this field is almost entirely con-
cerned with the problem of preventing or retarding attack on
4
plastics by microorganisms. H. J. Hueck discussed the bio-
logical deterioration of plastics by microorganisms, insects,
and rodents in a short review article written from the point
of view of a biologist. An excellent, more comprehensive
review of the effects on plastics of attack by fungi, bac-
teria, and other larger organisms was published by C. J. WesseL5
In a similar vein, but restricted to plasticized polyvinyl-
chloride, is an article by G. Tirpak. Darby and Kaplan
discussed the fungal susceptibility of 100 experimental poly-
urethanes, using a mixed-culture petri dish method. Poly-
ether polyurethanes were moderately to highly resistant to
fungal attack, whereas all polyester polyurethanes tested
Q
were highly susceptible. P. K. Barua and co-workers demon-
strated that Trichosporon species utilized only normal paraf-
fins from a mixture of n- and iso-paraffins. Utilization of
individual n-alkanes ranging from n-decane to n-eicosane was
generally good. In mixtures, however, the shorter chains
(decane, undecane, and dodecane) were consumed faster and at
a more uniform rate than longer chain paraffins.
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Rodriguez reviewed the published information per-
taining to the biodegradation of rubber, cellulose, paraf-
finic hydrocarbons, polyester plasticizers, and polyurethanes
in a recent article, which gives 72 references. He suggests
that the resistance of high molecular weight polyethylene to
attack by microorganisms may be due to the fact that poly-
ethylene crystallizes in a folded chain configuration in which
chain ends are unlikely to be found near the surface of the
polymer. Accessibility of chain ends is undoubtedly a rate-
determining factor in beta-oxidative degradation catalyzed by
oxidase enzymes.
g
Jen-Hao and Schwartz studied the rate of increase
of bacteria in contact with a series of polyethylenes as the
sole source of carbon. The bacteria count increased for a
week and dropped slowly toward the value for the carbon-free
control. The data suggested that only the low molecular
weight fractions were being assimilated.
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MICROBIOLOGICAL PROCEDURES
Samples of polymers of high molecular weight were
pressed or molded into plaques from which test specimens
were cut. These specimens were tested for degradation by
fungi, using ASTM Method D-1924-63. This procedure re-
quires the placement of test specimens in or on a solid agar
growth medium that is deficient only in carbon. The medium
and specimens are inoculated with the test microorganisms and
incubated for 3 weeks. Any growth that may occur is depen-
dent on the utilization of the specimen as a carbon source
by the test organism. The test fungi consisted of a mixture
of Aspergillus niger, Aspergillus flavus, Chaetomium globo-
sum, and Penicillium funiculosum.
Because of the possibility that growth might occur
as a result of additives in the polymer, it was necessary
that the polymers tested be free from stabilizers, plastici-
zers, lubricants, and other extraneous organic substances,
or that the presence of such additives be recognized. If a
pure polymer sample showed heavy growth and concurrent loss
of weight and mechanical properties, this was considered
good evidence of its degradability.
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Method D-1924-63 was also used to examine seraisolid
waxes and greases through the use of biologically inert fiber-
glass cloth that was impregnated with the test substance and
then examined as above.
In a few instances, ASTM-D-2676T was used in ad-
dition to D-1924. This test uses Pseudomonas aeruginosa
bacteria in place of fungi. Because the fungi are more active
than the bacteria, D-1924 was the test of choice in this work.
After various exposure times (up to 3 weeks),
samples were examined and assigned growth ratings as shown
below:
Growth ratings:
0 = No growth
1 = Traces (less than 10%
covered)
2 = Light growth (10 - 30%
covered)
3 = Medium growth (30 - 60%
, covered)
4 = Heavy growth (60 - 100%
covered)
In addition to the agar plate methods described
here, some plastic samples were buried in a mixture of equal
parts by volume of garden soil, builders sand, and peat moss,
which was placed in flower pots and watered daily.
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BIODEGRADABILITY OF COMMERCIAL PLASTICS
Packaging Films
Table 3 gives the growth rating (GR) for several of
the most widely used packaging films. The GR of 2 exhibited
by the polyethylene film (sample 1J is evidently due to
the presence of an additive that is readily removed by extrac-
tion with cold toluene. The extracted polyethylene is not
attacked by fungi. Two samples of polyvinyl chloride film,
both containing plasticizer, were also found to be biodegrad-
able. Removal of the epoxidized soy bean oil plasticizer by
toluene extraction reduced the GR from 3 to 1. The plasti-
cizer in the Resinite film, though not identified, is almost
certainly responsible for the observed growth. Samples of
vinylidene chloride-vinyl chloride copolymer, polypropylene,
polystyrene, and polyethylene terephthalate in film form
were all resistant to attack by fungi.
TABLE 3
BIODEGRADABILITY OF PACKAGING FILMS
Characterisation
1 Polyethylene household wrap 2
2 Sample 1 extracted with toluene 1
3 PVC-epoxidized soy bean oil plasticizer 3
4 Sample 3 extracted with toluene 1
5 Polypropylene 1
6 Polystyrene 1
7 Polyethylene terephthalate 1
8 Saran Wrap copolymer 1
9 PVC-plasticized Resinite 3
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The susceptibility of various additives customarily
found in plastics is treated in detail in a later section of
this report.
Other Commercial Plastics
Table 4 lists 31 commercially available plastics;
almost all are resistant to attack by microorganisms. Ther-
molastic gave a GR of 3, which dropped to 1 on extraction,
thus indicating the presence of a degradable additive. Kydene
also gave a GR of 3f but neither polyvinyl chloride nor poly-
methyl methacrylate are susceptible. This suggests the pre-
sence of a biodegradable plasticizer in the blend.
In contrast to the above are the results obtained
with Estane polyurethane (sample 22) and caprolactone poly-
ester (sample 24). The biodegradability observed for
samples 23 and 24 is in agreement with the results of Darby
•j
and Kaplan , who demonstrated the susceptibility of aliphatic
polyesters and urethanes derived from such polyesters to
microbiological attack. Since Darby and Kaplan also found
polyether polyurethanes much more resistant than polyester
urethanes, it is likely that Estane polyurethane is made
from an aliphatic ester diol.
It should be noted that Barex 210 and LOPAC plastics,
both of which are used for soft drink bottles, are not bio-
degradable .
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TABLE 4
BIODEGRADABILITY OF COMMERCIAL PLASTICS
Sample
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Characterization
Acrylonitrile-butadiene-styrene
copolymer (ABS) (Kralastic
ABS-PVC blend (Cycovin)
ABS-polycarbonate blend (Cycoloy)
Butadiene- aery lonitrile rubber (Hycar)
Styrene-acrylonitrile copolymer (C-ll)
Rubber modified polystyrene
Styrene-butadiene block copolymer
(Thermolastic )
Thermolastic extracted
Polymethyl methacrylate-PVC (Kydene)
Polymethyl methacrylate (Lucite)
Rubber modified polymethyl methacrylate
Polyethylene terephthalate (Arnite)
Polyethylene terephthalate-isophthalate
(Vitel)
Polycyclohexanedimethanol terephthalate
(Kodel)
Bisphenol A polycarbonate (Lexan)
Poly(4-methyl-l-Pentene) (TPX)
Polyisobutylene
Chlorsulfonated polyethylene (Hypalon)
Cellulose acetate
Cellulose butyrate
Nylon-6, nylon-66 (Zytel) , nylon-12
Polyurethane (Estane)
Caprolactone polyester urethane
Caprolactone polyester
Polyvinyl butyral
Polyformaldehyde (Celcon)
Barex 210 soft drink bottle
Lopac soft drink bottle
Polyvinyl ethyl ether
Polyvinyl acetate
Polyvinyl acetate 50 percent hydrolyzed
to alcohol
Growth
rating
0
0
0
0
0
0
3
1
3
0
0
0
0
0
0
0
0
0
0
0
0
4
4
4
0
0
0
0
0
1
1
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EFFECT OF POLYMER MOLECULAR WEIGHT AND BRANCHING
Polyethylene
The preceding section has demonstrated the re-
markable resistance of almost all types of high molecular
weight polymers to attack by microorganisms. These results
are generally in agreement with other published data. ' ' '
It has also been observed by a number of investigators that
low molecular weight normal paraffins are readily utilized
by microorganisms, ' but their branched isomers are very
p
poorly utilized. Barua reported utilization of n-paraffins
up to n-eicosane (C2QH42f mo^ wt 282.5). Miller and Johnson
cultivated a mixture of Candida lipolytica and C. intermedia
on gas oil samples and paraffin wax and found that the organ-
isms utilized n-paraffins ranging from dodecane through dotri-
acotane (C-^H-.,, mol wt 450.9), with the maximum efficiency
34 DO
of alkane removal in the nonadecane (cigH4Q' m°l wt 268.5) to
tetracosane (C24H5Qf mol wt 338.7) range.
The biodegradability of several pure linear hydro-
carbon samples in the molecular weight range 170-620 was
measured by the screening test described earlier. The growth
rating results are shown in Table 5.
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I
to
TABLE 5
BIODEGRADABILITY OP STRAIGHT CHAIN HYDROCARBONS
Compound
Dodecane
Hexadecane
Octadecane
Docosane
Tetracosane
Octacosane
Dotriacontane
Hexatriacontane
Tetracontane
Tetratetracontane
Formula
C12H26
C16H34
C18H38
C22H46
C24H50
C28H58
C32H66
C36H74
C40H82
C44H90
Mol Wt
170
226
255
311
339
395
451
507
563
620
Growth
Ratincj
4
4
4
4
4
4
4
0
0
0
-------
These data show clearly that biodegradability dimi-
shes sharply above 450 mol wt for the test organisms and is
negligible at 619 mol wt.
The effect of chain branching in alkanes has been
studied by Barua and others who have found that isoparaffins
are utilized poorly or not at all. The biodegradability of
several branched hydrocarbons were measured and compared to
their corresponding straight chain analogs. The data are
shown in Table 6. Here, the first, third, fifth, and seventh
samples are the straight chain hydrocarbons, as reported in
Table 5. The second, fourth, and sixth samples are branched
chain hydrocarbons. The chain length of each of these samples
is the same as the preceding hydrocarbon, but methyl groups
have been introduced along the chain. In each case, the in-
troduction of branching points decreases the susceptibility
of the hydrocarbon to fungal attack, and a growth rating of
0 is obtained. Note that the molecular weight of all of these
compounds is below the 450 cutoff point found in Table 5.
The above data can be used to explain the results
we have found for low and high density polyethylene of varying
molecular weights. Before presenting these results, however,
it will be helpful to discuss the concept of molecular weight
distribution as it applies to polymers. Each of the above
- 27 -
-------
pure organic compounds (Tables 5 and 6) consists of a collec-
tion of molecules having the same chain length and molecular
weight. In a polymer sample there are molecules of different
chain lengths, and the measured molecular weight of the sample
is an average value. Because of kinetic considerations during
polymerization, there is an approximately gaussian distribu-
tion of molecular weights about the average value. This is
illustrated by Figure 2, in which the probability of a given
molecular weight molecule being present in a polymer sample
is plotted versus the average molecular weight of the same
sample.
As the average molecular weight of the polymer is
decreased (by varying polymerization conditions or by post-
polymerization treatment to bring about random chain scis-
sion) , the proportion of low molecular weight polymer in the
sample increases.
In addition to the complication of molecular weight
distribution, polymer molecules may fall into three main
structural shape classification. They may be linear,
branched, or crosslinked. Linear molecules are those in
which the monomer units are linked together to form one
long continuous molecule. Branched molecules are those in
which side branches of monomer units of varying length extend
- 28 -
-------
vo
TABLE 6
EFFECT OF BRANCHING ON HYDROCARBON BIODEGRADABILITY
Compound and structure
Dedecane ^io^26
(CH3CH2CH2CH2CH2CH2)2
2 , 6 , 11-Trimethyldodecane cicH32
CH0 CH- CH.
| 3 | 3 | 3
CH3GHCH2CH2CH2CHCH2CH2CH2CH2CHCH3
Hexadecane ^13^34
(CH3CH2CH2CH2CH2CH2CH2CH2) 2
2 , 6 , 11 , 15-Tetramethylhexadecane C2QH42
CH- CH-
i 3 i 3
Mol wt Branched Growth
170 no 4
212 yes 0
226 no 4
(CH3CHCH2CH2CH2CHCH2CH2)2 283 yes
Tetracosane C24H50
(CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2)2 339 no
Sgualane ^30^62
^**i*« ^•jni'^ N^n*^
i 3 ,3 j 3
(CH3CHCH2CH2CH2CHCH2CH2CH2CHCH2CH2)2 423 yes
Dotriacontane C32Hg6
CH CH CH CH CH ^ CH 451 HO
-------
u>
o
ffi
§
o
tr
o.
A=AVERAGE MOLECULAR
WEIGHT
MOLECULAR WEIGHT
Figure 2. Polymer molecular weight distribution.
-------
from a main polymer chain. Cross-linked molecules are those
in which main polymer chains are connected to each other at
points other than the ends by "bridges" of varying length.
These types of structures can be illustrated as shown in
Figure 3.
Based on the data in Table 6, it can be seen that
branching could seriously interfere with the ability of a
microbe to utilize a polymer chain, since a branch point on
a polymer molecule is like a methyl group in the compounds
of Table 6. Branching is also related to crystallinity and
density. Thus as the amount of branching increases, the
density decreases and the crystallinity decreases because
the polymer chains can no longer fit easily into a crystal
lattice. High density polyethylene, for example, is a more
crystalline, linear polymer than low density polyethylene,
which is more branched.
Table 7 shows the effect on biodegradability of
varying molecular weights in low and high density poly-
ethylenes. Since the high density polyethylene is a linear,
nonbranched molecule, the molecular weight distribution in
- 31 -
-------
LINEAR
BRANCHED
CROSS-LINKED
Figure 3. Linear, branched, and cross-linked
polymer molecules.
- 32 -
-------
samples 1 and 2 is evidently such, that the low molecular
weight, biodegradable molecules are present in sufficient
concentration to give a positive reading in the test. As
the average molecular weight increases into the commercial
range, the molecular weight distribution is shifted so that
there is not a sufficient quantity of these low molecular
weight species present to be detected by the test.
The same conclusion can also be applied to the
low density polyethylene, except that samples 6 and 7
appear to be anomalous. The discussion of branching offers
an explanation here. Sample 6 is a grease, which is a very
low density, highly branched sample that contains very few
linear molecules. It received a low GR. Sample 7 is a
crystalline wax with a high density and a higher crystal-
linity than sample 6. It therefore contains more straight
chain molecules below about 500 mol wt than sample 6 and
hence receives a higher rating. Sample 6 represents a
borderline case since it showed no growth in one test and
slight growth in another.
- 33 -
-------
TABLE 7
EFFECT OF POLYETHYLENE MOLECULAR WEIGHT
ON BIODEGRADABILITY
Sample
no.
1
2
3
4
5
6
7
8
9
10
Product type
High density polyethylene
High density polyethylene
High density polyethylene
High density polyethylene
High density polyethylene
Low density polyethylene
Low density polyethylene
Low density polyethylene
Low density polyethylene
Low density polyethylene
Mol wt
10,970
13,800
31,600
52,500
97,300
1,350
2,600
12,000
21,000
28,000
Growth
rating
2
2
0
0
1
1
3
1-2
1
0
- 34 -
-------
Pyrolyzed Polyethylene
Samples of high and low density polyethylene have
been thermally degraded using a unique, continuous process
which was developed by the Union Carbide Corporation and re-
cently described by Potts. ' ' This process converts
high molecular weight polymers into lower molecular weight
polymers by thermal treatment in the absence of air in a con-
tinuous, hydraulically filled system. Because the polymer is
thermally degraded under isothermal conditions, it is possible
to obtain lower molecular weight polymer without the gassing
and charring associated with conventional thermal decomposition.
Samples of polyethylene pyrolyzed at temperatures
between 400 C and 535 C were examined for biodegradability
(Tables 8 and 9).
High density polyethylene, with an initial molecular
weight of 123,000, exhibits biodegradability when pyrolyzed to
a molecular weight of 3,200 or below (Table 8). Low density,
pyrolyzed polyethylene shows a lesser GR at 2,100 mol wt than
high density polyethylene at 3,200 mol wt. This can be ac-
counted for by the difference in chain branching in the two
types of polyethylene, as mentioned earlier. Perhaps the
most significant fact is that the pyrolysis process yields
biodegradable polyethylene derivatives and thus increases the
usefulness of this process as a waste plastic recycling tool.
- 35 -
-------
TABLE 8
BIODEGRADABILITY OF PYROLYZED,
HIGH DENSITY POLYETHYLENE
Pyrolysis
tempera ture (C)
Control
400
450
500
535
Viscosity average
molecular weight
123,000
16,000
8,000
3,200
1,000
Growth
rating
0
1
1
3
3
TABLE 9
BIODEGRADABILITY OF PYROLYZED,
LOW DENSITY POLYETHYLENE
Pyrolysis Viscosity average Growth
temperature (C) molecular weight rating
Control 56,000 0
400 19,000 1
450 12,000 1
500 2,100 2
535 1,000 3
- 36 -
-------
Polyethylene Containing Terminal Functional Groups
Low molecular weight polyethylene was synthesized
by the high pressure, free radical process using acetic acid
in one experiment and acetone in another as chain transfer
agents. Because of the chemistry of the chain transfer pro-
cess, a carboxyl group is placed at the end of the chain in
the acetic acid experiment, and a methyl ketone group in the
case of the acetone experiment. The purpose of these experi-
ments was to determine the effect on biodegradability of pla-
cing nominally biologically active functional groups on the
end of the polymer chain.
The presence of carboxyl groups and carbonyl groups
in the appropriate polymers was shown by infrared spectro-
scopy. The products were low molecular weight, brittle waxes.
The molecular weight of the carboxyl terminated sample was
determined by titration and found to be about 3,000, assuming
monofunctionality. For the acetone transfer product, the
molecular weight was calculated from the intensity of the
carbonyl peak in the infrared spectrum and found to be about
1,500, again assuming monqfunctionality. Both of these pro-
ducts gave a GR of 1, indicating no susceptibility to attack
by fungi. These results, along with those obtained for low
mol wt polyethylene samples described earlier, suggest that
these terminal functional groups in some way interfere with
biodegradability. Another possibility is the presence of un-
detected branching in these samples.
- 37 -
-------
Polystyrene
Table 10 shows the effect of molecular weight
variation on the biodegradability of polystyrene. Samples
1 through 5 represent a series of polystyrenes prepared with
an initiator that places carboethoxy groups at the end of the
polymer molecule. Samples 6 through 11 are polystyrene samples
having a carboxylic acid group at one end of the molecule.
Samples 12 and 13 have -CN groups on the end of the molecules.
All of the samples of polystyrene showed a zero GR over the
molecular weight range 5,900 to 214,000. The presence of
metabolically active ester and carboxylic acid end groups did
not increase the susceptibility of polystyrene to biodegradation.
Pyrolyzed Polystyrene
Polystyrene was pyrolyzed using the Union Carbide
continuous plastics pyrolysis process, which was described
earlier under Pyrolyzed Polyethylene. '17'18 pyrolysis tem-
peratures were the same as those used in the polyethylene py-
rolysis study. The products were characterized by determining
their viscosity average molecular weights and subjecting them
to the agar screening procedure for biodegradability. As the
data in Table 11 show, the susceptibility of polystyrene to
attack by fungi is not enhanced by the lowering of molecular
weight.
- 38 -
-------
TABLE 10
EFFECT OF POLYSTYRENE MOLECULAR WEIGHT
ON BIODEGRADABILITY
Sample
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
Molecular weight
*v
89,000
43,600
18,600
11,700
5,900
214,000
62,400
43,600
18,600
13,800
5,900
47,300
11,700
* -COOC2H5
t -COOH '
f -CN
End group composition
Carboethoxy (Ester) *
Carboethoxy (Ester)
Carboethoxy (Ester)
Carboethoxy (Ester)
Carboethoxy (Ester)
Carboxylic acidt
Carboxylic acid
Carboxylic acid
Carboxylic acid
Carboxylic acid
Carboxylic acid
Cyanidef
Cyanide
Growth
rating
0
0
0
0
0
0
0
0
0
0
0
0
0
- 39 -
-------
TABLE 11
BIODEGRADABILITY OF PYROLYZED POLYSTYRENE
Temperature of
pyro lysis (C)
Control
400
450
500
535
Viscosity average
molecular weight
220,000
93,000
67,000
26,000
4,000
Growth
rating
1
1
0
0
0
In terms of chain length, the molecular
weights of pyrolyzed polystyrene are comparable to
those generated in the pyrolysis of polyethylene. A
polyethylene chain with a molecular weight of 1,000 has
about 35 monomer units, and a polystyrene chain with a
molecular weight of 4,000 has about 40 monomer units.
It is evident that this study will have to be extended
to even lower molecular weights in order to determine
if the problem is due to the lack of a specific enzyme
for metabolizing polystyrene chain fragments. Because
of the branched structure of polystyrene, the latter
explanation is probably correct.
- 40 -
-------
BIODEGRADABILITY OF RANDOM COPOLYMERS
OF ETHYLENE OR STYRENE
Copolymers of Ethylene
Several dozen ethylene copolymers were tested for
biodegradability during this investigation. A description
of the copolymers and their GR are given in Tables 12 and 13.
All of the samples were made by free radical, high pressure
copolymerization. Several of the samples were also tested in
the form of derivatives, such as the vinyl alcohol (samples
4 and 5) or the partial sodium or ammonium salt (samples 7, 8,
9, 11). The composition (parts by weight) is listed after the
name.
With the exception of sample 1 in Table 12, all of
the ethylene copolymers tested gave GR of 0 or 1. The rating
of 2 shown by sample 1 is out of line with the results for
samples 2 and 3 which are of similar structure.
Table 13 gives the results of the copolymerization
of ethylene with several vegetable oils. The copolymers were
carefully purified before analysis and testing to remove un-
reacted vegetable oil. None of the copolymers containing
vegetable oil were susceptible to fungus attack.
- 41 -
-------
TABLE 12
BIODEGRADABILITY OF ETHYLENE COPOLYMERS
Sample
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Characterization
Ethylene-vinyl acetate 82-18
Ethylene-vinyl acetate 67-33
Ethylene-vinyl acetate 55-45
Ethylene-vinyl alcohol 30-70
Ethylene-vinyl alcohol 70-30
Ethylene-acrylic acid 85-15
Sample 6, 35% converted to NH. salt
Sample 6, 100% converted to NH. salt
Ethylene-acrylic acid 80-20, 100%
converted to sodium salt
Ethylene-ethyl aery late 82-18
Sample 10, 35% converted to Na salt
Ethylene- carbon monoxide 52-48
Ethylene- carbon monoxide 94-6
Ethylene-aconitic acid 82-18
Ethylene-itaconic acid 79-21
Ethylene- lauryl acrylate 75-25
Growth
rating
2
1
1
0
0
0
0
0
0
0
0
1
0
0
1
1
- 42 -
-------
TABLE 13
COPOLYMERS OF ETHYLENE WITH VEGETABLE OILS
Sample
no.
1
2
3
4
5
6
7
8
9
10
Vegetable oil P
Castor oil
Linseed oil
Safflower oil
Soybean oil
Neat's foot oil
Peanut oil
Rapeseed oil
Olive oil
Corn oil
Oleic acid
erco£l?ierin
26.1
28.3
26.6
27.1
40.8
20.0
19.2
15.7
18.0
8.6
Growth
rating
0
0
0
0
0
0
0
0
0
0
- 43 -
-------
Copolymers of Styrene
Since polystyrene was shown not to biodegrade at
a measurable rate, it was- thought that copolymers of sty-
rene containing metabolically active groups along the
chain might be more susceptible to attack. Accordingly,
copolymers of styrene with one or more monomers such as
acrylic acid, sodium aerylate, ethyl acrylate, dibutyl
maleate, and dimethyl itaconate were synthesized and
tested for susceptibility to fungus attack. As seen in
Table 14, the presence of such groups did not alter the
resistance of the styrene polymer to attack.
- 44 -
-------
TABLE 14
BIODEGRADABILITY OP STYRENE COPOLYMERS
**%]* Char.cteri.atio.
1 Styrene-acrylic acid 84-16 0
2 Styrene-sodium aerylate 84-16 0
3 Styrene-acrylic acid-dibutyl
maleate 85-10-5 0
4 Styrene-acrylic acid-dimethyl
itaconate 85-10-5 0
5 Styrene-dimethyl itaconate 70-30 0
6 Styrene-ethyl acrylate 50-50 0
7 Styrene-2-ethyl hexyl fumarate
85-15 0
8 Styrene-methacrylonitrile 13-87 0
9 Styrene-dodecyl acrylate 85-15 0
10 Styrene-ethyl acrylate-dodecyl
acrylate 85-10-5 0
- 45 -
-------
BIODEGRADABILITY OF POLYESTERS
Thus far in this investigation, the only synthetic
high molecular weight polymers that have been found to be
biodegradable are those having aliphatic ester linkages in
the main chain, as for example samples 22, 23, and 24 of
Table 4. Those polymers having aliphatic ester linkages in
a pendant position on the main chain (polyvinyl acetate, for
example) are not utilized by microorganisms,. Previous in-
vestigators have also observed the susceptibility of ali-
phatic polyesters to attack by microorganisms, as cited
above. This situation is being re-examined in the hope of
better understanding the structural factors in a polymer
that favor biodegradability. Table 15 lists polyesters of
varying structure and molecular weight (as measured by re-
duced viscosity) and the GR observed for each. Sample
numbers in the following discussion refer to Table 15.
Sample 1, an epsilon caprolactone polyester that
has a molecular weight of about 40,000 and no branching is
quite readily utilized by fungi and bacteria. Sample 2, a
branched polyester derived from pivalolactone and of much
lower molecular weight, was not utilized at all.
- 46 -
-------
TABLE 15
BIODEGRADABILITY OF POLYESTERS
Sample
no.
1
2
3
4
5
6
7
8
9
10
11
12
Character! z ation
Caprolactone polyester
Pivalolactone polyester
Polyethylene succinate
Polytetramethylene succinate
Polytetramethylene succinate
Polyhexamethylene succinate
Polyhexamethylene fumarate
Polyhexamethylene fumarate
Polyethylene adipate
Polyethylene terephthalate
Polycyclohexanedimethanol
terephthalate
Polybisphenol A carbonate
r?±si?y
0.7
0.1
0.24
0.59
0.08
0.91
0.25
0.78
0.13
high
high
high
Growth
rating
4
0
4
1
4
4
2
2
4
0
0
0
- 47 -
-------
Polyesters based on fumaric acid, which is an un-
saturated, dibasic acid, appear to be utilized more poorly
than those based on saturated dibasic acids such as succinic
and adipic acid.
A marked dependence of biodegradability on molecular
weight was observed for .polytetramethylene succinate (samples
4 and 5). Aromatic structures as exemplified by samples 10,
11, and 12 render the polyester unassimilable.
Soil burial tests have been carried out on epsilon-
caprolactone polyester to determine the rate of weight loss
as a function of length of burial. The results are discussed
in the following section of this report.
- 48 -
-------
SOIL BURIAL TESTS ON EPSILON-CAPROLACTONE POLYESTER
The polyester derived from the ring opening poly-
merization of epsilon-caprolactone was chosen for further
biodegradation testing by the soil burial technique. A
polymer sample of about 40,000 mol wt was molded into tensile
test bars, which were found to have an ultimate tensile
strength of 2,610 psi, and an ultimate elongation of 369 per-
cent, measured at room temperature. Test bars of this ma-
terial were buried in a mixture of equal parts of New Jersey
garden soil, Michigan peat moss and builders sand. At inter-
vals of 1.25, 2.0, 4.0, 6.0 and 12 months, samples were re-
moved, tested for tensile strength and elongation, and measured
for weight loss. With increasing length of soil burial, the
test bars became more pitted and eroded (Figure 4) and were
much weaker (see Table 16). At the end of 12 months, the
samples were too weak to measure strength properties and had
lost 42 percent of their original weight. The break in the
2-month sample in Figure 4 is the result of our physical
testing, not of attack by the fungi.
-------
Figure 4. Soil-buried caprolactone polyester (about 1.5x)
- 50 -
-------
TABLE 16
EFFECT OF SOIL BURIAL ON CAPROLACTONE POLYESTER
Months of
burial
0
1.25
2.0
4.0
6.0
12.0
Tensile
strength
psi
2,610 ± 103
1,890 ± 215
1,610 ± 180
520 ± 220
100
Negligible
Percent
elongation
369 ± 59
9 ± 1.4
7 ± 2.0
2.6 ± 1.1
Negligible
Negligible
Percent
weight loss
0
8
16
25
42
Scanning electron micrographs of the surface of the
2-month, soil-buried sample reveal the extent of the attack
(Figures 5, 6, and 7).
Figure 5 is a photo of the surface magnified about
980 times before any degradation. The streaks and straight
lines are indicative of the surface of the mold in which the
tensile bar was made. Figure 6 shows the surface of a ten-
sile bar at about the same magnification (950x) that has
been soil-buried for 2 months. Figure 7 is a similar photo
at a slightly lower magnification (about 600x). The deep
pitting, channelling, and cavernous appearance resulting
from the degradation process is readily apparent in these
pictures.
- 51 -
-------
Figure 5. Scanning electron micrograph (about 98Ox) of
caprolactone polyester. Not soil-buried.
- 52 -
-------
Figure 6. Scanning electron micrograph (about 950x) of
caprolactone polyester soil-buried for 2 months
- 53 -
-------
Figure 7. Scanning electron micrograph (about 600x) of
caprolactone polyester soil-buried for 2 months,
- 54 -
-------
In addition to tensile bars we have also soil
buried 1.5 ounce injection molded containers. Weight loss
measurements on these samples are given in Table 17 and
photographs of the samples are shown in Figure 8. The
greater percentage weight loss observed for these samples
relative to the tensile bars is due, of course, to their
larger surface areas.
TABLE 17
WEIGHT LOSS ON CAPROLACTONE POLYESTER CONTAINERS
Percent
Months of burial weight loss
0 0
2 12
4 29
6 48
12 95
- 55 -
-------
Figure 8. Soil-buried caprolactone polyester.
- 56 -
-------
BIODEGRAPABTLITY OF -BLOCK AND GRAFT COPOLYMERS
One synthetic approach to biodegradable polymers
is to construct a polymer molecule containing chain seg-
ments or blocks of nonbiodegradable polymer connected to
chain segments of a biodegradable polymer. Such products
can be made either by block polymer synthesis or graft
polymer synthesis. Several of these block and graft
copolymers have been synthesized and subjected to our
screening program. The nonbiodegradable polymer, or A
segment, of the graft copolymers. can be a polyethylene
type chain or a polystyrene type chain. The biodegradable,
or B segment, is based on an aliphatic polyester. The
data obtained on the ethylene based polymer is presented
in Table 18, and data for the styrene based polymer is
presented in Table 19.
TABLE 18
BIODEGRADABILITY OF GRAFT COPOLYMERS BASED ON AN
ETHYLENE POLYMER AND A POLYESTER
Sample no.
1
2
3
4
Percent of
ethylene polymer
24
48
60
88
Percent of
polyester
76
52
40
12
Growth
rating
4
4
2
0
- 57 -
-------
TABLE 19
BIODEGRADABILITY OF GRAFT COPOLYMERS BASED ON A
STYRENE POLYMER AND A POLYESTER
Sample no.
1
2
3
4
Percent of
styrene copolymer
23
65
80
92
Percent of
polyester
77
35
20
8
Growth
rating
4
4
2
1
Note that in both of these tables, the biode-
gradability of the system increases as the degradable
polyester content increases. Thus, having small amounts
of a biodegradable polymer chemically bound to a nonbio-
degradable polymer is not sufficient to impart rapid dis-
integration to the overall system. It appears in the
above case that it is necessary to have about 40 percent
of the degradable polymer present in order to get reason-
ably rapid degradation rates; but more data is needed to
clearly define the lower limit, since in Table 19 a growth
rating of 2 is reported for a system containing 20 percent
of the biodegradable segment.
In the area of block copolymers, some preliminary
experiments have been done in which a commercially available
- 58 -
-------
nondegradable polyester, polyethylene terephthalate, has
been modified with a degradable polyester to yield products
that support microbiological growth. The data obtained
are reported in Table 20.
TABLE 20
BIODEGRADABILITY OF BLOCK COPOLYMERS OF DEGRADABLE
POLYESTER (DP) WITH POLYETHYLENE TEREPHTHALATE (PET)
Sample no.
1
2
3
4
DP
(% by weight)
73
43
27
17
PET
(% by weight)
27
57
73
83
Growth
rating
4
2
0
0
As might be expected, the biodegradability of
the block copolymers is directly related to the biodegrad-
able polyester content, which was also true of the graft
copolymers reported above.
No property evaluation of these systems has been
done with regard either to their utility or to their loss
of properties as biodegradation proceeds. Both of these
are beyond the scope of the current investigation. What
has been demonstrated is that biodegradability of block
- 59 -
-------
and graft copolymers is dependent on the content of the
biodegradable segment of the copolymer, and that appreci-
abe contents are required to confer biodegradability on
these particular systems.
- 60 -
-------
BIODEGRADABILITY OF ADDITIVES USED IN PLASTICS
Additives are used in plastic formulations for
a variety of reasons — to inhibit oxidative degradation,
and to act as plasticizers, molding lubricants, ultra-
violet absorbers, and heat stabilizers, to mention a few.
Accurate determination of the biodegradability of a poly-
mer cannot be accomplished without knowing what additives
are present and whether they are biodegradable, inert, or
perhaps growth inhibiting. Some paints contain fungicides,
such as phenyl mercuric acetate.
Table 21 lists commonly used additives, their
function, and the GR observed for each. In some instances
where the chemical name of the substance was not available,
the trade name was used. Inclusion of an additive in the
list does not imply that it is in any way preferable to
other additives not included in this study. The additives
are classified according to the polymer with which they
are most commonly associated.
Generally, hindered phenol antioxidants are not
biodegradable, and thio esters are readily utilized by
microorganisms. Nonyl phenyl phosphite was found to be a
growth inhibitor. Plasticizers also vary widely in their
biodegradability-aliphatic polyesters are readily utilized
- 61 -
-------
by microorganisms, while those based on aromatic acids
are inert. A more complete discussion of plasticizer
utilization can be found under Plasticized Polyvinyl
Chloride.
- 62 -
-------
TABLE 21
BIODEGRADABILITY OF ADDITIVES
Common trade name Chemical name or Growth rating
chemical type
POLYETHYLENE AND OTHER POLYOLEFINS
Antioxidants:
Butylated hydroxy Hindered phenol 0
toluene (BHT)
Santonox R Hindered phenol; also 0
thioether
Topanol CA Hindered phenol 0
Irganox 1010 Hindered phenol 0
DLTDP, dilauryl Thioether ester 4
thiodipropionate
DSTDP, distearyl Thioether ester 4
thiodipropionate
Polygard NonyIphenylphosphite Z.I.*
Slip or anti-block agents:
Erucamide C22 unsaturated primary amide 4
Oleamide C,0 unsaturated primary amide 4
J.O
Stearamide C,g saturated primary amide 4
Behenamide C22 saturated amide 2
HTSA-1 Olealyl palmitamide (a 2
secondary amide)
UV absorbers:
Eastman DOBP 2-hydroxy-4-dodecyloxy- 0
benzeophenone
Eastman OPS p-o.ctylphen.ylsalicy.late 0
*Zone of inhibition. These compounds are essentially fungicides.
_ 63 _
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TABLE 21
BIODEGRADABILITY OF ADDITIVES -- Continued
Common trade name
Chemical name or
chemical type
Growth rating
POLYVINYL CHLORIDE
Plasticizers :
Phthalates
Flexol DOP
Phosphates
Flexol TCP
Epoxies
Flexol EPO
Trimellitates
Rucoflex 2, 5TM
Polyesters
Plastolein 9765
Santicizer 409
Adipates
Di-2-ethylhexyl phthalate
Tricresyl phosphate
Epoxidized soy bean oil
Tris-2-ethylhexyl trimellitate
Aliphatic polyester
Aliphatic polyester
Di-2-ethylhexyl adipate
0
1
4
0
4
4
0
Flexol A 26
Heat stabilizers:
Vanstay HTA
Vanstay SD
Dibutyltin
dilaurate
Lubricants:
Zinc stearate
Hoechst wax E
Phosphite
Tin compound
Metal salt
Hydrocarbon
4
0
4
4
2
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TABLE 21
BIODEGRADABILITY OF ADDITIVES — Continued
Common trade name
Chemical name or
chemical type
Growth rating
POLYVINYL CHLORIDE — Continued
Antioxidants:
Butylated hydroxy
toluene (BHT)
Processing aids:
Acryloid K 120 N
Hindered phenol
Acrylic polymer
POLYSTYRENE AND RELATED POLYMERS
Plasticizers:
Mekon white wax
Lubricants:
Zinc stearate
Antioxidants:
Butylated hydroxy
toluene (BHT)
Polygard
Rubber:
Diene 35
Microcrystalline wax
Metal salt
Hindered phenol
Nonylphenyl phosphite
Polybutadiene
0
Z.I.*
*Z.I. = Zone of inhibition. These compounds are essentially fungicides.
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PLASTICIZED POLYVINYL CHLORIDE
Of the approximately 3 billion Ib of polyvinyl
chloride (PVC) and copolymers sold in the United States
in 1970, about 75 percent, or 2.3 billion Ib, falls into
the category of plasticized or nonrigid PVC. The sus-
ceptibility of many types of PVC plasticizers to micro-
biological attack is well documented in the literature
and in an earlier section of this report. There are no
instances, either in our work or in the literature, where
biological assimilation of the PVC molecule has been
proven. In addition to plasticizers, PVC contains other
additives such as heat stabilizers that prevent thermal
decomposition and hydrogen chloride evolution. Some of
these stabilizers such as dibutyltin dilaurate are bio-
degradable. Loss of these stabilizers could result in a
system more susceptible to dehydrohalogenation and sub-
sequent oxidative attack. Certain types of tin stabilizers
are the preferred stabilizers for food contact applications
such as wraps for luncheon meats, bottles, etc.
The major additive present in PVC formulations
however, is the plasticizer. Work by Tirpak (6) and by
the Union Carbide Corporation has shown that as the
plasticizer is utilized by microorganisms, the vinyl
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becomes stiff, brittle, and increases in tensile strength
and modulus. We have examined three samples of PVC
film containing different plasticizers to show the vari-
ability of the attack and to demonstrate that not all
plasticizers are biodegradable. The data are presented
in Table 22.
TABLE 22
EFFECT OF PLASTICIZER ON PHYSICAL PROPERTIES
OF POLYVINYL CHLORIDE AFTER EXPOSURE TO BACTERIA
Plasticizer Tensile modulus of elasticity (psi) Percent of
Code
R2H
H707
DOP
Type
Polyester
Hercules
proprietary
Dioctyl
phthalate
Unexposed
2,373
1,372
1,102
Exposed
2,887
4,895
1,166
change
22
258
6
The amount of plasticizer consumed in these
experiments was not determined, but the surface of the
film was severely pitted. The increase in modulus on
exposure to the bacteria indicates that the sample was
becoming embrittled. Additional environmental degrada-
tion would be required before a product is obtained that
will crumble to a dust. Plasticizer utilization by micro-
organisms can, however, leave a material with a much
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larger surface area, thus enabling environmental degrada-
tion to proceed at a faster than normal rate.
From the data in Table 22, we conclude that the
polyvinyl chloride containing DOP (the most common, work-
horse plasticizer) is essentially unaffected by the
bacteria. The sample containing R2H is attacked to a
slight degree, and the sample containing H707 is severely
attacked.
At present, biodegradable plasticizers are used
in PVC in one-time use applications. A good example is
PVC film for packaging meats. In the packaging-marketing
area in 1970, film and sheet markets accounted for the use
of 175 million Ib of PVC resin.
Although this study did not make detailed studies
of the progressive embrittlement of PVC film as the plasti-
cizer is removed by microorganisms, there is definite
evidence, as indicated in Table 22, that the sample becomes
more friable, as one would expect. In addition to the
samples examined in Table 22, PVC meat wrap film contain-
ing epoxidized soy bean oil plasticizer was buried in the
soil for 3 months and then removed for examination.
Whereas the original meat wrap film was limp
and flexible, the film that had been buried for 3 months
- 68 -
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had the appearance of parchment paper and cracked when
flexed more than once. Although the test was not continued
it was judged that soil burial for a year or more would
result in eventual removal of all of the plasticizer from
the film, leaving a brittle film of greatly increased
surface area.
Simultaneous removal of the biodegradable
stabilizer (dibutyltin dilaurate) further enhances the
susceptibility of the film to dehydrohalogenation, which
results in an unsaturated polymer of greatly increased
oxidative and microbial susceptibility.
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EFFECT OF ENERGY TREATMENT ON PACKAGING FILMS
The treatment of plastics with ionizing radia-
tion, corona discharge, ozone, etc. has been proposed on
several occasions as a means of increasing the suscepti-
bility of plastics to attack by microorganisms. In an
attempt to demonstrate the feasibility of these approaches,
we have treated several of the more prominent packaging
materials with (1) high energy electrons from a
van de Graaff electron accelerator, (2) corona discharge,
and (3) ozone.
The irradiation with van de Graaff electrons was
carried out at three different dose levels: 5 megareps,
10 megareps, and 20 megareps. The higher the dose, the
more intense the radiation and the more pronounced the
effect should be. Radiation doses of 20 megareps are
considerably higher than those normally used in commercial
radiation processes.
The film exposed to the corona discharge was
exposed on one side for 15 seconds at 120 volts and
2.3 amps. This is approximately what is used to surface
treat film to improve the acceptance of printing inks
by the film.
- 70. -
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For treatment with ozone, the films were exposed
in a flow system at 50 C to an atmosphere containing
2 percent ozone. The ozone was generated at a rate of
9 g per hr over a 6-hr period, thus exposing the film
to a total of 54 g of ozone.
After exposure to these treatments, the samples
were submitted to our standard screening procedure. The
results are presented in Table 23.
It is obvious that none of the treatments used
in this experiment has enhanced the biodegradability of
any of these film samples enough to be measurable by the
standard screening test. The vinyl films contained
plasticizers that are biodegradable.
In addition to the above physical methods,
some preliminary experiments were done on outdoor-weather-
ed polyethylene in which a GR of 2 was observed. The
deterioration of physical properties upon outdoor weather-
ing or aging of a polymeric material is due to sunlight
activated oxidative attack on the polymer structure.
This oxidative attack generally leads to a decrease in
the molecular weight of the polymer. Although the average
molecular weight of the polymer at the stage where it
crumbles to a powder is still relatively high, continued
- 71 -
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oxidative attack over a longer period of time could lead
to sufficient molecular weight reduction so that, depend-
ing on the structure of the polymer, assimilation by
soil microorganisms might take place. For hydrocarbon
polymers such as polyethylene, evidence was presented
earlier that a reduction in molecular weight to about
500 would be sufficient for assimilation by fungi in the
soil. It should be recognized that microbiological growth
on the degraded polyethylene would be in evidence before
all of the molecules in a.given plastic object were
reduced to the 500 mol wt level because of the distribu-
tion of molecular sizes that exists in polyethylene even
after oxidative degradation.
There has been a great deal of publicity recently
devoted to the degradation of polymer systems by the action
of ultraviolet light. Work in England, Canada, and the
United States has pointed out several ways that the
ultraviolet degradation of plastics can be accelerated.
Most of this work is pointed towards alleviating the
litter problem. In some cases this approach has been
claimed to result in a biodegradable product. Because
of the profound effect that chain branching has on inhibit-
ing biodegradation of synthetic polymers (an effect that
persists even to low molecular weights), it is essential
that such claims be substantiated by adequate experimental
data.
- 72 -
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TABLE 23
EFFECT OF PHYSICAL METHODS OF TREATMENT ON PACKAGING FILMS
Film
sample
Polyethylene
Polystyrene
Resinite
vinyl film
EPO plasticized
vinyl film
Saran wrap
polyvinylidene chloride
Mylar polyethylene
terephthalate
Polypropylene
Growth rating
Control Corona
discharge
0]
0
3
3
0
0
0
0
0
3
3
0
0
0
of
treatment
method
Ozone Radiation(megareps)
0
0
3
3
0
0
0
5
0
0
3
3
0
0
0
10
0
0
3
3
0
0
0
20
0
0
3
3
0
0
0
- 73 -
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RECOMMENDATIONS FOR ACTION
1. Additional studies are needed to determine the effects
that the use of biodegradable additives, such as
plasticizers and stabilizers, will have on the dis-
integration of plastic packaging, particularly poly-
vinyl chloride. Investigations would include data
on the optimal use of additives with regard to both
the end use and eventual disintegration of the plas-
tic packaging when buried. The ultimate purpose of
such studies would be to assess the use of such
additives as a standard practice in packaging plas-
tics such as PVC.
2. Further studies of biodegradable, aliphatic polyesters
are needed to assess their usefulness as packaging
materials. These studies would include fabrication
of selected polyesters into various packaging containers
and evaluation of their physical properties with regard
to various packaging applications. The objective of
these studies is to provide the basic technical data
needed to determine the commercial feasibility of this
approach to biodegradable packaging plastics. These
studies will also include an evaluation of blends of
the biodegradable polyesters with polyethylene, poly-
- 74 -
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styrene, and polyvinyl chloride.
3. The studies of block and graft copolymers containing
blocks that were found to be attacked by microorganisms
should be continued to determine which of these types
of block copolymers show promise in packaging applica-
tions. This would involve (a) synthesis of larger
quantities of specific products carefully screened
for promising physical properties, (b) fabrication
into various useful packages, and finally (c) measure-
ment of their rate of assimilation by microorganisms.
The purpose of these studies would be to demonstrate
the feasibility of this approach to biodegradable
packaging materials.
4. Preliminary studies reported in a previous section
of this report demonstrated that pyrolyzed polyethylene
becomes more biodegradable as the average molecular
weight decreases. These pyrolysis studies should be
extended to other plastics to determine the effect
of thermal degradation on their biodegradability.
The Union Carbide continuous pyrolysis process will
be used in these studies because of its freedom from
tar formation and its greater degree of control over
the molecular weight of the pyrolyzed polymers.
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5. Perhaps the most practical route to biodegradability
for polyethylene involves the development of poly-
ethylene packaging containing additives that will
protect the package during its useful life but that
will promote its disintegration and ultimate degrada-
tion to a low molecular weight polymer that will be
biodegradable. It is recommended that sufficient
work be done to demonstrate the feasibility of this
approach.
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ACKNOWLEDGMENT
The authors wish to acknowledge the advice and
encouragement offered by Mr. Clarence demons and Mr.
Charles Rogers of the U. S. EPA, who served as contract
monitors. They also wish to acknowledge the excellent
laboratory support provided by Mr. Kerrait Craig and Mr.
Jesse Doll, both of this laboratory.
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REFERENCES
1. Rodriguez, F. The prospects for biodegradable plastics.
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2. Fulmer, M. E., R. F. Testin. Role of plastics in
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3. Drobny, N. L., H. E. Hull, and R. F. Testin. Recovery
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4. Hueck, H. J. The biological deterioration of plastics.
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5. Wessel, C. J. Biodeterioration of plastics.SPE Trans-
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6. Tirpak, G. Microbial degradation-of plasticized PVC.
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- 78 -
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9. Jen-Hao, L., and A. Schwartz. On the behavior of bacte-
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16. Potts, J. E. Continuous pyrolysis of plastic waste.
In Proceedings; Solid Waste Symp, University of
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~ 80 ~ * US.SOVEMMEXTPSIKTIMOFFICE 1972- 759-548/1021
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