bio degradation of agricultural plastic films- a critical review
TRANSCRIPT
ORIGINAL PAPER
Biodegradation of Agricultural Plastic Films: A Critical Review
Ioanna Kyrikou Æ Demetres Briassoulis
Published online: 12 April 2007
� Springer Science+Business Media, LLC 2007
Abstract The growing use of plastics in agriculture has
enabled farmers to increase their crop production. One
major drawback of most polymers used in agriculture is the
problem with their disposal, following their useful life-
time. Non-degradable polymers, being resistive to degra-
dation (depending on the polymer, additives, conditions
etc) tend to accumulate as plastic waste, creating a serious
problem of plastic waste management. In cases such plastic
waste ends-up in landfills or it is buried in soil, questions
are raised about their possible effects on the environment,
whether they biodegrade at all, and if they do, what is the
rate of (bio?)degradation and what effect the products of
(bio?)degradation have on the environment, including the
effects of the additives used. Possible degradation of
agricultural plastic waste should not result in contamina-
tion of the soil and pollution of the environment (including
aesthetic pollution or problems with the agricultural prod-
ucts safety). Ideally, a degradable polymer should be fully
biodegradable leaving no harmful substances in the envi-
ronment. Most experts and acceptable standards define a
fully biodegradable polymer as a polymer that is com-
pletely converted by microorganisms to carbon dioxide,
water, mineral and biomass, with no negative environ-
mental impact or ecotoxicity. However, part of the ongoing
debate concerns the question of what is an acceptable
period of time for the biodegradation to occur and how this
is measured. Many polymers that are claimed to be ‘bio-
degradable’ are in fact ‘bioerodable’, ‘hydrobiodegrad-
able’, ‘photodegradable’, controlled degradable or just
partially biodegradable. This review paper attempts to
delineate the definition of degradability of polymers used
in agriculture. Emphasis is placed on the controversial is-
sues regarding biodegradability of some of these polymers.
Keywords Degradation � Biodegradation � Mulching
films � Agriculture � Polymers
Introduction
Polymers are synthetic and natural macromolecules com-
posed of smaller units called monomers that are bonded
together. Examples of natural polymers include proteins,
polysaccharides, and nucleic acids [1]. Synthetic polymers
have been developed for durability and resistance to all
forms of degradation [2]. These characteristics and others,
such as rigidity, permeability and transparency can be
controlled by changing the polymer synthesis, molecular
weight and/or by the use of specific additives. The resulting
plastics’ versatility allows them to be used in a very wide
range of applications, including agriculture.
Because of their wide use, the problems with the dis-
posal of agricultural plastic wastes become more and more
severe. Since 1990 the plastics industry, as individual
companies and through organizations such as APC
(American Plastics Council), has invested more than
$1 billion to support increased recycling and educate
communities [3]. Despite the continuing growth of recy-
cling, source reduction and energy recovery, some pro-
portion of the waste will always require disposal. The most
common method for disposing of municipal solid waste is
landfilling [4]. A more significant combination of source
reduction, recycling, incineration and composting is being
developed in Western Europe, the United States and else-
where as an alternative to solid waste disposal in landfills.
I. Kyrikou � D. Briassoulis (&)
Department of Agricultural Engineering, Agricultural University
of Athens, Iera Odos 75, 11855 Athens, Greece
e-mail: [email protected]
123
J Polym Environ (2007) 15:125–150
DOI 10.1007/s10924-007-0053-8
Many synthetic polymers are produced and utilized
because they are resistant to chemical and physical deg-
radation. On the other hand, polymers resistant to degra-
dation present disposal problems when their usefulness
ceases [5]. The degradation of used plastics is not a simple
process, when referring to environmental degradation. The
detailed mechanisms of how some of the plastics degrade
after disposal in specific environments are not thoroughly
understood and are the subject of intensive research during
the last decades. Degradation under extreme conditions,
such as incineration (which is one of various disposal op-
tions) is not a physical process, thus is not considered when
referring to environmental degradation.
Specifically for the case of agricultural plastic wastes,
one of the alternative ways of disposal is biodegradation.
Biodegradation concerns specially designed polymers, the
so-called biodegradable polymers. Most experts and
acceptable standards [6, 7] define a fully biodegradable
polymer as a polymer that is completely converted by
microorganisms to carbon dioxide, water, minerals and
biomass (or in the case of anaerobic biodegradation, carbon
dioxide, methane and humic material1) without leaving any
potentially harmful substances. However, part of the
ongoing debate concerns the question of what is an
acceptable period of time for the biodegradation to occur
and how it is measured. Given enough time (that may be
even thousands of years), nearly all carbon-based materials
may eventually biodegrade [4]. This does not mean that all
carbon-based materials are considered to be biodegradable
materials. Only those materials that biodegrade within a
limited period of time, as this is defined by international
standards (cited later on) and also satisfying a set of
additional criteria (e.g. no ecotoxicity or negative envi-
ronmental impact), shall be considered to be biodegrad-
able.
The most acceptable disposal method for biodegradable
polymers is composting. However, composting requires an
infrastructure, including collection systems and compo-
sting facilities [4], while it does not represent a practical
solution for most cases of agricultural plastics wastes as
compared to biodegradation in soil.
Use of Plastics in Agriculture
The Importance of Plasticulture
Polymers have been used in agriculture and horticulture
since the middle of the last century [8]. The growing use of
plastics in agriculture has enabled farmers to increase their
crop production. Today’s plasticulture (use of plastics in
agriculture) [9] results in increased yields, earlier harvests,
less reliance on herbicides and pesticides, better protection
of food products and more efficient water conservation.
Plastic films are used as coverings of greenhouses or tun-
nels over crop rows, as silage covers, as bale-wrap films,
and as mulching films to cover soil [9]. Plastic films can
improve product quality and yield by mitigating extreme
weather changes, optimizing growth conditions, extending
the growing season and reducing plant diseases. Green-
houses are mainly concentrated in two geographical areas:
the Far East (especially China, Japan, and Korea) with
almost 80% and the Mediterranean basin with about 15%
of the worlds’ greenhouse covered area. The area covered
by greenhouses has been steadily increasing at a rate of
20% per year during the last decade. Development in
Europe is very weak but Africa and the Middle East are
growing at 15–20% annually. Of special interest is the case
of China, which has grown from 4200 ha in 1981 to
1,250,000 ha in 2002 (30% per year). The volume of
plastic films used for this application would thus be about
1,000,000 t/year [10, 11].
In general, an estimated 2–3 million tons of plastics are
used each year in agricultural applications [7, 9, 12]. Al-
most half of this amount is used in protected cultivation
(greenhouses, mulching, small tunnels, temporary cover-
ings of structures for fruit trees, etc.). Of extreme impor-
tance is the financial impact of products originating from
protected cultivation to the agricultural income in many
less favoured regions of southern European Union (EU)
countries. The vast majority of the protected cultivations
area covered by plastic materials is dominated by the use of
plastic made out of polyethylene (PE) [13]. In particular,
low-density polyethylene (LDPE) is the most widely used
polyethylene grade, due to its relatively good mechanical
and optical properties, combined with a competitive market
price.
Disposal of Agricultural Plastics
As the concentration of the plastics used in certain agri-
cultural regions (rather than the overall quantities at na-
tional level) is very large and each year the usage is
increased, the collection and clearing problems and the
final disposal problems of the accumulated in specific
locations agricultural plastic waste is a fact. Therefore, the
1 Both humate and humic refer back to organic compounds found in
the soil. Humate generally refers to compounds that are generated by
the breakdown of plants and animals. Humic generally refers to either
one of the organic acids found in soil resulting from the degradation
of organic material, or it can refer to the organic layer itself found in
many soils. The humic layer in a soil generally appears as a rich, dark
earthy layer that is usually found in the upper portions of a soil profile
126 J Polym Environ (2007) 15:125–150
123
degradation and disposal options, or possibly the biodeg-
radation of the used polymers, represent a very important
subject with both economical and environmental aspects.
Degradation of Agricultural Plastics
Degradation During the Useful Life-time
Degradation of a plastic in general, is defined as a detri-
mental change in its appearance, mechanical, physical
properties and chemical structure [14–18] (refer to the
definitions in Appendix). It is important to make a dis-
tinction between the initiation of the degradation process
(commencing in the extruder, at temperatures of around
200�C, but controlled) and its manifestation during their
useful life-time. The degradation process is delayed in
actually damaging the plastic by a special balance of
inhibitors engineered to the specific application and to the
anticipated life expectancy of the plastic. Heat, ultra-violet
radiation and stress can accelerate the degradation process
of the material [19]. Degradation of agricultural plastics
during their useful lifetime is due to a combination of
factors (mainly UV radiation) [20] and may be controlled,
to some extent, through the use of appropriate additives.
Degradation after the Useful Life-time
Further degradation of the aged agricultural plastic (i.e.
agricultural plastic waste) following their useful lifetime is
directly related to the various disposal options. In any case,
degradation of agricultural plastic waste should not result
in contamination of the soil and pollution of the environ-
ment (including aesthetic pollution) and the agricultural
products safety. Describing plastics degradation, measuring
it, and controlling it are all complicated by three major
factors.
(i) Mechanisms: Plastics can and do degrade by many
routes, consecutively or simultaneously. The plastics can
be fragmented through physical forces. Fragmentation of-
ten plays an important role in the early stages of degra-
dation and can be brought about by physical forces of
mechanical nature. Chemical changes within the plastic
can occur and may begin with abiotic degradation.
Chemical degradation occurs through reactions of the
plastic with agrochemicals or other chemicals. Degradation
brought by chemical reactions generally involves chain
scission—fragmentation of the polymer chains. Surface
erosion can be the result of chain scission resulting from
chemical hydrolysis. At some point, some specific plastics
may be attacked effectively by microorganisms—the onset
of biodegradation. Biodegradation is generally considered
as consisting of both enzyme-catalyzed hydrolysis and non-
enzymatic hydrolysis [21]. Enzymatic degradation can be
carried out either by extracellular enzymes present in the
microorganisms’ environment or by intracellular enzymes
[22]. Both result in chain scission whereby the polymer
chains are cleaved into smaller segments. The enzymes
may be either endoenzymes, which cleave internal linkages
within the chain or exoenzymes, which cleave terminal
monomer units sequentially. Endoenzymes cleave the
internal chain linkages randomly which results in a rapid
decrease in molecular weight; the sequential cleavage of
terminal segments leads to less dramatic immediate chan-
ges in molecular weight. Under some conditions microor-
ganisms contribute to degradation of polymers through
ingestion, mastication and excretion. All of these pathways
are potential routes for polymer degradation [23].
(ii) Environmental conditions: How polymers’ degra-
dation proceeds in a specific case depends on the envi-
ronment the plastics are exposed to, during their useful
lifetime and the environment the polymer wastes are dis-
posed to, afterwards. The kinetics of polymer degradation
depend on whether the environment is dry air, humid air,
soil, a landfill, a composting environment, sewage, fresh-
water or a marine environment. Each environment has its
own characteristic concentration profile of important fac-
tors: oxygen, water, other chemicals, daylight and
degrading microorganisms [7, 24–27]. According to the
nature of the environment there may be a relatively more
efficient or less efficient mechanism by which degradation
can occur. In one environment a very efficient degradation
mechanism may be available, whereas in another envi-
ronment the same mechanism might not be available at all
for lack of appropriate conditions. Also according to the
nature of the environment, there may be a larger or a
smaller concentration of chemicals that react with the
plastic during the degradation process. More specifically,
the environmental factors affecting the rate of degradation
that is due to microorganisms—that is the value of bio-
degradation—include temperature, moisture level, atmo-
spheric pressure, and pressure of oxygen, concentrations of
acids and metals, and the degree of exposure to light.
Factors relating to microorganisms include their concen-
tration, whether or not they have enzymes for which the
polymer is substrate, the concentration of enzymes, the
presence of trace nutrients for the microorganisms and the
presence of inhibitors or predators. If any of the required
elements is absent, or if it is present at a level that falls
below a critical threshold, biodegradation may not only
slow down but may stop altogether until proper conditions
are once again present [28–31].
(iii) Polymer composition: Regardless of the environ-
ment, the mechanism and rate of degradation also depends
on the chemical composition of the polymer. The rate of
possible biodegradation in particular, depends on the
polymers characteristics because the polymer is the
J Polym Environ (2007) 15:125–150 127
123
substrate for the enzymes. One factor that determines the
degradability or biodegradability of a polymer is the nature
of the chemical bonds that are present. The chemical
structure of the polyolefins contains only carbon–carbon
single bond in their backbones. That feature makes them
particularly resistant to degradation [7, 17, 32]. Also the
carbon-carbon single bonds of polyolefins make them
hydrophobic. Thus they are not susceptible to hydrolytic
degradation. They can be degraded through oxidative
mechanisms but not very readily while processing in-
creases their resistance [2, 14, 33]. Also, it is well known
that antioxidants are added to increase their stability
against various degradation mechanisms. Besides the nat-
ure of the chemical bonds that are present the details of
chain branching and even stereochemistry (the detailed
spatial arrangement of atoms and bonds) are also impor-
tant, because enzymes are often specific to attacking one
particular type of chain branching and one particular ste-
reochemistry. The polymer’s molecular weight and the
degree of chain flexibility can also be important. The
morphology of the polymer is important as well, including
the extent of surface and the degree of crystallinity. The
degree of crystallinity is important in the case of polyole-
fins because oxygen does not easily enter the crystalline
regions; they are impermeable to oxygen. Oxidation of
polyolefins occurs mainly in the amorphous regions.
Plastics’ Categories
The main causes of degradation of agricultural plastics
during the useful lifetime are photodegradation and oxi-
dation [34]. The plastics may be categorized according to
how readily they degrade during their exposure in a spe-
cific environment and the nature of degradation to which
they are subjected to, into the following categories [14, 16,
17, 32, 35–37]:
• Non-degradable plastics
• Readily Degradable Plastic
• Plastics of controlled degradation (Programmed
Degradable Plastics)
• Environmentally Degradable Plastics (plastics of this
category may also fall under the two previous catego-
ries)
Non-Degradable Plastics
Commodity plastics are typically stable for a specific
useful life-time, depending on the application and the
environment, and they degrade thereafter to some degree
(e.g. in terms of retaining their initial mechanical proper-
ties). In some environments, objects made from them, even
if degraded, remain intact for many years [29, 30, 38–41].
Their persistence, for most part, originates in three of their
properties that make them so useful for many applications:
they are generally strong mechanically, water resistant and
micro-organisms do not readily attack them [32] (and if
they do, the extent and rate of attack are not practically
significant).
Readily Degradable Plastic (gradually degradated)
A readily degradable plastic, usually after its useful life has
ended simply ‘self-destructs’. The degradation of these
materials is gradual and cannot be really controlled. The
timing of degradation can be predefined however empiri-
cally to some extent based on the selection of the type and
amount of stabilizing additives. Such a material, after the
time required for a useful service is over, during which
retains more or less all the properties that it was formulated
and processed to have, it simply falls apart and it may, or
may not be assimilated by the pervasive microorganisms
found in nature. If not, it simply becomes very brittle and
its fragments pollute the environment. If yes, it returns to
the ecosystem in an environmentally harmless manner,
provided that this is not associated with any ecotoxicity
effects or negative environmental impact [32] (to be veri-
fied by performing testing for ecotoxicity following rele-
vant international standards). This brings forward the
question on the biodegradability of several polymers, as-
sumed or claimed to be biodegradable.
Plastics of Controlled Degradation (Programmed
degradable plastics; step degradated)
Programmed degradation, or controlled degradation, is an
idea that has started 20–30 years ago. The goal of pro-
grammed degradation is to program plastics to degrade in a
predetermined time under specific conditions according to
the needs of particular applications. The difference be-
tween programmed degradable plastics and the materials of
the previous category lies with the degree of the control of
degradation in terms of timing and also the shape of the
degradation curve. Programmed degradable plastics ex-
hibit, at least theoretically, a step-wise degradation, with
the onset of a rather abrupt and so much more severe
degradation occurring at a timing that is better predefined
than that of the gradually degraded polymers.
The programmed degradable materials approach aims at
the elimination of the litter problem; one such approach is
based on exposure to natural sunlight that is common in
agricultural applications. The objective in this case is to
modify the resin so as to promote photodegradation (deg-
radation that results from the UV radiation) [42–45]. The
strategy is to attach a photosensitizing group to the polymer
128 J Polym Environ (2007) 15:125–150
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chains by chemical means. When this photosensitive group
is exposed to natural light, it absorbs radiation, which
causes the chain to break and form smaller segments, a
process called scission. As photodegradation proceeds, the
chains are broken in more and more places, and the plastic
litter is destabilized through embrittlement. Eventually it
becomes fragile and is fragmented. Erosion by wind and
rain completes the breakdown of the embrittled plastic into
friable powder [46].
In more general terms, oxidative programmed degrada-
tion is a programmed degradation that results from oxida-
tion. Some polyolefin additives allow oxidative
degradation to be initiated at a pre-defined time (to some
extent), either by natural daylight, or by heat or both or
even by mechanical stress [19, 47–49]. The applications
from such programmed resins are plastic products for
which disposal might include earth burial, as with compost
bags and agricultural mulch covers. Moreover the frag-
ments formed by oxidative degradation may be wettable,
leading to increased interactions with water and promoting
hydrolysis [50, 51]. Through oxidative and hydrolytic
degradation the polyolefin undergoes a progressive chain
scission. In time, severe embrittlement leads to extensive
fragmentation.
Fragmentation is not the same as total degradation. A
plastic may fragment after being discarded, but may not
degrade readily so as to be decomposed for example by
microorganisms [32, 52–54]. Through embrittlement a
large piece of plastic becomes fragile and the many small
pieces of plastic eventually turn into a friable powder
sometimes even invisible to the naked eye.
In theory, fragmentation of the polymer chains in the
plastic makes them more susceptible to other modes of
degradation, possibly including some kind of biodegrada-
tion activity that should be defined however, in terms of
rate and the specific conditions under which it is measured,
including the measurement methodologies.
What is desirable practically for a programmed
degradable plastic is adequate performance properties ini-
tially and no significant decrease in performance properties
during the planned useful lifetime. On the other hand after
a pre-determined (to some extent) period of use degrada-
tion is to begin upon disposal, starting with fragmentation
or surface erosion. In any case, it should be ensured that the
fragmentation will continue with 100% biodegradation
without any ecotoxicity effects and that will leave no re-
mains, whether invisible or not, in the soil within a rea-
sonable time period to avoid accumulation in the soil. Such
characteristics have not been confirmed yet beyond any
doubt with the available programmed degradable plastics.
In practice, questions related to the environmental fate
and ecological effects of polyolefin fragments have not
been conclusively resolved. For example, if agricultural
covers made from modified polyethylene resins (e.g. PE
with pro-oxidants) are ploughed into the soil at the end of a
growing season, it is not yet known what might be the fate
of these remains and what degree of accumulation may be
experienced with time (note that the remains are in the
form of intermediated-length polyethylene chains due to
degradation). Most important is the long-term effect of any
accumulating residues of such materials in the soil envi-
ronment, the agricultural product safety for the consumer
and the agricultural productivity for the agricultural soil. It
has not been verified systematically with scientific justifi-
cation so far if there are any, or there are no harmful effects
or negative environmental impact. The answers to these
questions need long term systematic multidisciplinary re-
search work.
Environmentally Degradable Plastics
Many polymers that are claimed to be ‘biodegradable’ are
in fact ‘bioerodable’, ‘hydrobiodegradable’ or ‘photo-
degradable’ [6], or just partially biodegradable. These
different polymer classes are grouped by some authors
under the broader category of ‘environmentally degradable
polymers’. Of course the use of the label ‘environmentally’
may be unjustified and misleading in these cases. Even
though this term is widely used in literature [52–57 etc],
and while it can also be found in the title of ASTM D6002-
96 (Standard Guide for Assessing the Compostability of
Environmentally Degradable Plastics) [6], it was not pos-
sible to find an official definition for it.
Environmentally degradable plastics (EDP), based on
the use of the term rather than on a specific definition,
can be considered to include a wide group of natural and
synthetic polymeric materials that undergo chemical
change under the influence of environmental factors. The
chemical change must be followed by complete microbial
assimilation of degradation products resulting in carbon
dioxide and water. In particular, according to literature
[54, 58] the process of EDP degradation comprises two
phases, disintegration and mineralization. During the
initial phase, disintegration is significantly associated
with the deterioration in physical properties, such as
discoloration, embrittlement and fragmentation. The sec-
ond phase is assumed to be the ultimate conversion of
plastic fragments, after being broken down to molecular
sizes, to CO2, water, cell biomass (aerobic conditions),
and CH4, CO2 and cell biomass in the case of anaerobic
conditions. The EDP degradation and assimilation must
be complete and occur at a sufficiently rapid rate so as to
avoid accumulation of materials in the environment [42,
56]. There are questions however concerning the validity
and the conditions of the assumptions about the second
phase [7].
J Polym Environ (2007) 15:125–150 129
123
EDPs can be synthesized on renewable or non-renew-
able feedstocks. Examples of EDP from renewable feed-
stocks are cellulose, starch, starch esters, collagen, viscose,
cellulose acetate (DS < 2), polyhydroxy alkanoates,
polylactic acid etc, and from non-renewable feedstocks are
polyvinyl alcohol, polycaprolactone, aliphatic-aromatic
copolyesters, blends of starch and biodegradable polyesters
etc. Renewable feedstocks used for EDP production can be
simple natural compounds (such as amino acids, sugar,
sesources of vegetal, aquatic, and animal origins) or can be
derivatives of natural compounds that have undergone
chemical transformation to give an appropriate building
block for EDP. EDP can also be produced from non-
renewable feedstocks, most commonly from natural oil and
gas. EDP are often used as blends or composites in which
two or more biodegradable materials are combined to
provide optimal performance while maintaining or
enhancing complete biodegradability [54, 56, 58].
Regardless of feedstock source according to the ICS
EDP guideline—2003 (International Centre for Science
and High Technology of the United Nations Industrial
Development Organization (ICS-UNIDO)) [56], an EDP
material must have the following properties:
• Rapid degradation and/or biodegradation
• Bioassimable degradation products (leading to CO2 and
water via biological pathways)
• Ease of processing
• High versatility
• Acceptable performance
• An acceptable price for intended use.
According the Australian Department of Environment
and Heritage2 the classes of Environmentally Degradable
Plastics are [55]:
• Biodegradable polyesters
• Biodegradable starch—based polymers
• Water—soluble polymers
• Photo—degradable polymers
• Controlled degradation masterbatches
Biodegradation
Biodegradation or biotic degradation is chemical degrada-
tion of materials (e.g. polymers) brought about by the action
of naturally occurring microorganisms such as bacteria,
fungi and algae (chemical degradation that does not involve
biological activity is defined as abiotic degradation) [32, 59,
60]. As biodegradation proceeds it produces carbon dioxide
and/or methane and water. If oxygen is present the biotic
degradation that occurs is aerobic degradation and carbon
dioxide is produced. If there is no oxygen available, the
biotic degradation is anaerobic degradation, and methane is
produced instead of carbon dioxide. Under some circum-
stances both gases are produced.
Mineralization is defined as the conversion of biode-
gradable materials or biomass to gases (like carbon diox-
ide, methane, and nitrogen compounds), water, salts and
minerals, and residual biomass. Mineralization is complete
when all the biodegradable materials or biomass is con-
sumed and all the carbon in it is converted to carbon
dioxide. Complete mineralization represents the rendering
of all chemical elements into natural biogeochemical cy-
cles [32, 61].
Usually, there are two steps involved in the biodegra-
dation of the polymer [61]:
• Mechanical (grinding), chemical (irradiations by ultra-
violet rays; e.g. photodegradation), or thermic degra-
dation. During this stage, microscopic fungi and
bacteria, or other biological agents (earthworms,
insects, roots of plants, even rodents), can also fragment
the product (biofragmentation). This first phase is very
useful, because it leads to the increase of the surface of
the material exposed to the microbodies occurring in
the second phase.
• The second phase corresponds to the biodegradation
Sensu Stricto. Microbodies attack and digest the
product, which is transformed in by-products which
are assimilated by the microbodies, the final result
being CO2 or CH4, water and biomass production. This
second phase is often concomitant of the first one.
Factors Affecting Biodegradation
Biodegradation is fundamentally an electron transfer pro-
cess [32]. Biological energy is obtained through the oxi-
dation of reduced materials. Microbial enzymes catalyze
the electron transfer. Electrons are removed from organic
substrates to capture the energy that is available through
the oxidation process. The electrons are moved through
respiratory or electron transfer chains (metabolic path-
ways) composed of a series of compounds to terminal
electron acceptors [32]. A large proportion of the microbial
population in soil depends upon oxygen as the terminal
electron acceptor for metabolism. Loss of oxygen induces a
change in the activity and composition of the soil microbial
population. Facultative anaerobic organisms (which can
use oxygen when it is present or can switch to alternative
electron acceptors, such as nitrate and sulphate, in the ab-
sence of oxygen) and obligate anaerobic organisms become
dominant when oxygen is not available [32], but aerobic
biodegradation is typically more efficient.
2 Classification based on work performed for the Australian Depart-
ment of Environment and Heritage by Nolan-ITU.
130 J Polym Environ (2007) 15:125–150
123
For microflora (fungi, bacteria and the like) to convert
and assimilate the carbon in any substrate, a number of
criteria must be met. The substrate must be water–wettable,
and the constituent molecules must be sufficiently small
that a very large number of their chain ends are accessible
at the surface of the material [27]. Hydrocarbon thermo-
plastics are bioinert because they are hydrophobic, and
because their good mechanical properties require very high
molecular weights, leading to very few accessible chain
ends. Also are resistant to hydrolysis (and for this reason
cannot hydrobiodegrade) and to oxidation and biodegra-
dation due to the presence of anti-oxidants, and stabilizers
additives [27].
A wide variety of organic materials are easily degraded
under aerobic conditions. In aerobic metabolism, O2 is the
terminal electron acceptor. When biodegradation follows
this pattern, microbial populations quickly adapt and reach
high densities. As a result, the rate of biodegradation quickly
becomes limited by rate of supply of oxygen or some
nutrient, not the inherent microbial capacity to degrade the
polymer or other contaminant [32]. Some organic com-
pounds can also be degraded under anaerobic conditions.
When oxygen is absent, nitrate (NO�3 ), sulphate (SO�4 ),
ferric iron (Fe3+), manganese (Mn3+, Mn4+), and bicarbonate
(HCO3–) can serve as terminal electron acceptors, if the mi-
crobes have the appropriate enzyme systems [32].
Under anaerobic conditions, the rate of degradation is
usually limited by the inherent reaction rate of the active
microorganisms; adaptation is slow, requiring months or
years, and metabolic activity results in the formation of
incompletely oxidized, simple organic substances, such as
organic acids, and by-products such as methane or hydro-
gen gas. Microorganisms help decompose organic matter in
marine environments as well [52].
Numerous factors affect the potential force and the rate
of naturally occurring biodegradation at a given site, such
as [32, 52]: soil moisture content, porosity, soil tempera-
ture, soil pH, O2 availability, presence of suitable mi-
crobes, presence of contaminants and their concentration,
availability of nutrients, presence of other electron accep-
tors, redox potential etc.
Especially for biodegradable in soil polymers, the rate at
which biodegradation occurs depends on soil conditions
such as temperature, water content (a measure of the
concentration of water), degree of aeration (a measure of
the concentration of oxygen), acidity (a measure of the
concentration of acids) and the concentration of the
microorganisms themselves. Under extremely unfavour-
able conditions degradation rates can be reduced to nearly
zero [32, 61, 62].
Low temperatures strongly inhibit degradation in soil.
Water content of the soil is also important; it supports
hydrolytic degradation. Aeration supports oxidative
degradation and the degree of aeration determines whether
aerobic or anaerobic biotic degradation or both—takes
place. Although there are many bacteria that thrive on an
oxygen-free environment, there are many more that use
oxygen. Biotic degradation also requires that the soil may
be microbial active. Biotic degradation rates can be re-
duced to nearly zero in a sterile environment, or when the
concentration of microorganisms is very low or even if the
material is not really biodegradable.
Many harmful metabolites may be generated microbio-
logically in a variety of environments. These products may
represent substantive threats to the health, growth, or vig-
our of humans and a variety of animals and plants, thus
determining the environmental impact of the biodegrada-
tion. What microorganisms do to that chemical may be of a
great importance to human health, agricultural productivity
or populations in natural ecosystems. The biological active
metabolite formed from a toxicant may not always be
toxic. Sometimes, it may be stimulatory [36].
Biodegradable Polymers
Materials
It is beyond the scope of this work to present an analytical
overview of the various biodegradable materials. Only
some general information is offered in the present section
in support of the main objectives of this work.
Biodegradability represents a complex phenomenon
difficult to measure. As stated above, a material may be
considered to be ‘‘biodegradable‘‘ if it can be shown be-
yond any doubt that it is fully and environmental safely
degraded by microbodies under special conditions. The
result is the formation of water, CO2 and/or of CH4, and of
minerals and a new biomass, leaving no toxic elements for
the environment and any remains or fragments.
Biodegradable polymers may be naturally occurring or
may be synthesized by chemical means [1, 19]. Biode-
gradable polymers can be divided in general into three
groups [8]:
1. natural polymers such as starch, cellulose, proteins,
poly-b-hydroxybutyrate
2. natural polymers biologically or chemically modified
(e.g. cellulose acetate, lignocellulose esters, polyalk-
anoate copolymers...)
3. readily biodegradable synthetic polymers modified
(complexed or blended etc.) with added natural bio-
degradable components (starch, reclaimed cellulose,
natural rubber, etc.) [63, 64, 65] (note that blends of
non-biodegradable polymers with natural biodegrad-
able materials are not and should not be considered to
be biodegradable materials [66]).
J Polym Environ (2007) 15:125–150 131
123
Synthesized polymers may come from the processing of
crops grown for this purpose or the by products of other
crops (renewable resources) or may come from petro-
chemical feedstocks (non-renewable resources).
Biodegradable polymers form a unique class of mate-
rials that created an entirely new concept when originally
proposed as biomaterials. That is, for the first time, a
material performing a structural application was designed
to be completely absorbed and to become weaker over time
[67]. This concept was first applied successfully with cat-
gut sutures and later with more arguable results, on bone
fixation plates and pins [68].
Later on, biodegradable materials were also introduced to
agricultural applications. Systematic reviews of biodegrad-
able materials and examples of really biodegradable mate-
rials used in agriculture, such as mulch films, flowerpots and
controlled-release fertilisers, can be found in literature [13,
69–73]. One of the prototype biodegradable plastics used in
agriculture is Mater-Bi� [13, 74, 75]. This is a biodegrad-
able and water-soluble thermoplastic material, based on
starch, complexed with biodegradable polyesters. The
material satisfies the biodegradability and compostability
requirements of European norm EN 13432, and the national
norms UNI 10785 and DIN54900. Furthermore, Mater-Bi�does not contain any dangerous substances, as defined by the
Community Directive 67/548/CEE, as modified by the
Commission Directive 97/69/CE and subsequent modifica-
tions [73]. Other biodegradable polymers used for produc-
ing biodegradable agricultural films include copolyesters
[76], poly(vinyl alcohol) [77] and poly(vinyl chloride) [78],
acylated starch-plastic [79], modified starch, vegetable oil-
based resins and others [69, 73].
Environmental Impact of Biodegradable Polymers
It is reported in literature [67] that polymers that degrade
by peroxidation followed by bioassimilation (or by a deg-
radation that is assumed to be bioassimilation) of the oxi-
dation products (oxo-biodegradable polymers) are in
general more environmental acceptable (‘green’) than
some biologically produced hydro-biodegradable poly-
mers. The claim that the use of renewable sources of
feedstocks is better than that of non-renewable ones, is a
judgment that cannot be made without carrying out a
complete environmental impact analysis on each source.
According to an analytical approach [80], in the manu-
facture of hydrocarbon polymers, carbon is taken from one
carbon sink3 (e.g. an oil deposit) to another carbon sink
(plastic) with no net production of atmospheric carbon
other than that generated during energy production for the
conversion process. The environmental impact of natural
products like biopolymers produced by growing bacteria on
an appropriate feedstock or starch based or other biode-
gradable materials has been analyzed by several research-
ers [81, 82]. According to other analyses on biopolymers,
the production based on fermentation is not a sustainable
process [83]. In fact, when considering the energy and
material requirements for corn farming and wet milling,
fermentation, and polymer recovery, a rather discouraging
picture emerges. All major environmental indicators such
as carbon emissions, air acidification, eutrophication, and
depletion of natural resources show that fermentative pro-
duction has considerably more negative environmental
impact than conventional plastic production [83]. This
analytical approach suggests that while the major emis-
sions of greenhouse gases occur during the production and
use phases of products, the end-of-life phase should not be
neglected. Whether to prevent greenhouse emissions from
biodegradable materials or to recover resources as material
or energy, it is now accepted that diversion of untreated
waste from landfills is an important factor in reducing re-
source use and associated climate change effects [84, 85].
Along the same line, supporters of the ‘programmed
degradable polymers’ claim [16] that the growing of agri-
cultural crops may involve the application of fertilizers,
herbicides and pesticides which may leave a deep envi-
ronmental footprint. Unless appropriate soil management
practices are in place the soil risk severe depletion of
nutrients, microorganisms etc. In addition chemical or
biochemical processes usually are required to extract and
purify the polymer. These processes may require water,
energy and chemical or biological additives. They also
produce wastes which require treatment and disposal.
Such claims however, have to be compared against the
corresponding footprint of the programmed degradable
polymers that includes the fossil-oil based polymers pro-
duction and the corresponding plastics wastes problem
(heavy environmental impact from oil extraction and oil
refinery industries, processing with energy, waste man-
agement impact etc). On the other hand, another important
factor to be taken into account is the fact that care is (or can
be) taken recently for promoting sustainable low-input
agriculture.
Additional environmental considerations for the use of
biodegradable over conventional polymers are the declin-
ing petrochemical sources (the fact that is also responsible
for major international geo-strategic conflicts), and also the
fact that the biodegradable plastics offer a safe and cheap
alternative option to recycling, in terms of waste manage-
ment (especially for mulching films) [72]. It is important to
take into account the fact that the difficult, in any case,
3 A sink is a reservoir that uptakes a chemical element or compound
from another part of its cycle. For example, soil and trees tend to act
as natural sinks for carbon—each year hundreds of billions of tons of
carbon in the form of CO2 are absorbed by oceans, soils, and trees.
132 J Polym Environ (2007) 15:125–150
123
waste management of non-biodegradable agricultural films
and other plastic materials used in agriculture is also
associated with a significant negative environmental im-
pact and with the corresponding waste management cost.
Concerning the environmental impact of various bio-
degradable materials, one of the most important issues is
the degree of biodegradation of the material. The degree
and rate of biodegradation is dependent on the chemical
composition of the polymer and its working environment,
and so there is no single optimal method for determining
biodegradation. When comparing the degree to which
different polymers biodegrade, several factors must be ta-
ken into consideration. The first of these factors is the
environment. Polymers may be tested for biodegradability
in a natural or simulated environment [16] (refer to the
section on the standard testing methods later on).
Finally, the environmental effects of the polymer must
be considered. A polymer that biodegrades is of little value
if the products that forms are found to contaminate water
supplies or be toxic to living organisms in the environment
or to be accumulated in the soil over the years because of
very slow degradation rate.
Claims and Controversies on Labelling Biodegradable
Polymers
Various studies have shown that several naturally-occur-
ring polymers biodegrade, and chemically modified natural
polymers may biodegrade depending on the extent of
modification. It has been reported in literature that syn-
thetic addition polymers, with carbon as the only atom in
the backbone do not biodegrade at molecular weights
above 500 Daltons [62, 86]. According to other authors it
has been reported that when the molecular weight is re-
duced to around 100,000 Daltons the plastic embrittles and
flakes. According to them, below 40,000 Daltons the
molecular structure becomes bio-degradable [87] (possibly
under specific conditions). The structure becomes water
wettable and microbes and fungi can attach to it, to convert
carbon to cell wall structure. If an addition polymer con-
tains atoms other than carbon in the backbone, it may de-
grade depending on the attached functionality groups.
Synthetic condensation polymers may biodegrade to dif-
ferent (more) extent depending on chain coupling
(ester > ether > amide > urethane), morphology (amor-
phous > crystalline), molecular weight (lower > higher),
while biodegradation of hydrophilic materials is faster than
hydrophobic. However, if a polymer is water soluble, that does
not necessarily mean that it is biodegradable [32].
Another major issue is the environmental conditions
under which materials can biodegrade. For example, some
materials are biodegradable under composting conditions
but not in the soil.
In several cases, readily biodegradable polymers are
mixed-up with non-biodegradable polymers or polymers
with additives. Blends of non-biodegradable polymers with
starch have been used [88], among others. In other cases,
degradable mulching films have been developed that break
down into small brittle pieces (disintegrate), which pass
through harvesting machinery without difficulty, but do not
actually biodegrade [89]. Of course, real biodegradable
polymers have been developed as well [88].
One of the requirements of the ASTM definition of
Compostable Plastic [6] is that it leaves ‘‘no visible, dis-
tinguishable or toxic residue’’ after degradation. Thus to be
considered as environmentally degradable a plastic must
become brittle rapidly enough to disappear visually, and
the degraded material must be susceptible to biological
attack giving complete conversion to biomass without re-
lease of toxic products within an acceptable time period
(defined by relevant Standards). The ASTM Standard
specifies certain tests to determine conformance.
The classes of plastics promoted and labelled in the
market, in terms of the claimed degradation and/or bio-
degradation mechanisms, are summarized as follows [35]:
Oxo-degradable Polymers
The oxo-degradable materials are claimed to be inherently
degradable that will degrade through photo, thermic deg-
radation and molecular scission in a time controllable
manner [35].
Photodegradable Polymers
The breakdown of photodegradable polymers depends on
irregularities in them. These irregularities cause them to
slowly degrade when exposed to ultraviolet (UV) radiation,
typically sunlight. In photodegradable plastics, the rate of
degradation is increased by adding photosensitive sub-
stances, called promoters, to the plastic material [90].
Two common promoters are carbonyl groups (carbon
double bonded to oxygen) and metal complexes (metals
blended with many ingredients) [69]. The exposure of a
mulching film to ultra violet radiation, for example, is
variable and non-existent when the material is buried or
even just covered by the cultivation. Thus there is insuf-
ficient ultraviolet radiation, leading to insufficient photo
degradation of parts of the material. The fate of these
materials in the soil, even if photodegradated to very small
fragments is strongly disputed [27, 44].
Carbonyl Group: Ketone Carbonyl Copolymers
This type of photodegradable plastic is produced by adding
a carbonyl group, vinyl ketone comonomer, to the
J Polym Environ (2007) 15:125–150 133
123
polymers such as polyethylene (PE) and polystyrene (PS)
[90]. The resulting copolymer degrades when the carbonyl
group absorbs sunlight. Because these products require
direct sunlight to degrade, this material is ideal for mulch
film (assuming that the film is exposed to sun-light and not
covered by the plants; part of the film is buried in the soil
along the two longitudinal sides of the film anyway and so
it is not exposed to sunlight). In order for the material to be
completely consumed by microorganisms a biodegradation
process should be activated and completed following the
end of the useful lifetime of the material. Such a biodeg-
radation process has not been confirmed for these materi-
als, at least not confirmed beyond any doubt.
Carbonyl Group: Carbon Monoxide Copolymers
In this category of materials, the carbonyl group carbon
monoxide is added to produce a degradable copolymer,
called carbon monoxide copolymers. Manufacturers of this
material—including Dow Chemical, DuPont, and Union
Carbide—claim that carbon monoxide copolymers are able
to degrade into benign by-products. However, questions
remain about the extent of degradation. More research is
needed to determine whether carbon monoxide products
completely degrade into non-plastic products or whether
they simply disintegrate into smaller pieces of plastic [90].
Metal Complexes
A relatively new approach to photodegradable plastics is
adding metal salts. Degradation of these polymers is
invariably activated by a transition metal. The main dif-
ference between plastics containing metal salts and other
photodegradable materials is its ability to break down also
in the absence of solar radiation. In fact, if they receive
enough UV radiation before burial, these products may even
be able to degrade in landfills. Products being made from
polymers with metal complexes include mulching films and
tree shelters. Currently, one of the main concerns with these
materials is the heavy toxic metal residues remaining after
degradation takes place [90]. More details about the metal
complexes used are needed to evaluate the possible appli-
cability and limitations of these materials. In any case,
strong concerns on the possible biodegradation of these
materials have been expressed in the literature and the rel-
evant scientific questions remain open [7, 47, 48, 68].
Thermodegradable Polymers
Degradation of these polymers initially begins when the
product is exposed to heat (refer to the definitions in the
Appendix). Usually these polymers contain azo-groups in
the main polymer chain. Thermolytic cleavage is known to
proceed by b-H elimination or syn-elimination. This pro-
cess, known as internal elimination (Ei), is generally be-
lieved to be a single step mechanism with a cyclic
transition structure, as first proposed by Hurd and Blunck
[89]. Questions are raised however with respect to the
possible biodegradation of these materials as well [91].
Compostable Polymers
Compostable biodegradable plastics must be demonstrated
to biodegrade and disintegrate in a compost system during
the composting process (typically within around 12 weeks
at temperatures over 50�C) [7, 27, 70] (refer to the defini-
tions in the Appendix). The EU Directive on Packaging and
Packaging Waste (94/62/EC) [92] defines requirements for
packaging to be considered recoverable. The compost must
meet quality criteria such as heavy metal content, ecotox-
icity, and leave no obvious distinguishable residues caused
by the breakdown of the polymers. Compostable biode-
gradable plastics are a subset of biodegradable plastics.
Hydro-biodegradable Polymers
Some biodegradable polymer materials experience a rapid
dissolution when exposed to particular (chemically based)
aqueous solutions [93]. These polymers, the hydro-biode-
gradable polymers, are claimed to be broken down in a
two-step process—an initial hydrolysis stage, followed by
further biodegradation [87, 94]. Single degradation phase
‘water-soluble’ polymers also exist.
Bioerodable Polymers
Many polymers that are claimed to be ‘biodegradable’ are
in fact ‘bioerodable’ (i.e. biodegradation is limited at the
surface level) and may degrade further (i.e. in the bulk
material) without the action of micro-organisms. This is
also known as abiotic disintegration, and may include
processes such as dissolution in water, ‘oxidative embrit-
tlement’ (heat ageing) or ‘photolytic embrittlement’ (UV
ageing) [2, 46, 47, 69, 87].
Systematic research work is needed to clarify the ter-
minology and the conditions under which the claimed
degradation behaviour and rate is ensured and can in fact
be measured by standard testing methods (see below).
The Question on Possible Biodegradation of Polyolefins
The Polyethylene Case
There is a very long controversial discussion going on for
years now over the possible biodegradation of polyethylene
in soil. Many sources clearly indicate that polyethylene
134 J Polym Environ (2007) 15:125–150
123
(PE) is perfectly stable and thus is not directly oxidisable
[70], or that PE is inert [8] and not biodegradable [67]. Two
preliminary treatments (heat and ultra violet radiation) are
essential to modify its chemical structure. These are found
to oxidize (introduce oxygen in the form of hydroxyls,
carbonyls, peroxides), degrade (reduce the molecular
weight or increase it by crosslinking reactions) and de-
structure (modify the crystalline structure) the PE. Karlsson
et al. [34] have estimated PE oxidization to be very slow,
namely, in the order of 1 mg per 100 g of the product per
week or about 0.001% of the product per week. It has been
reported that the molecular weight of degraded polyelyth-
ene varies between 200,000 and 600 Daltons [26, 34, 59],
and that apparition of the double bonds has been observed
at a rate of 0.0035% [70], that is 1 in 30,000. Other con-
tradictory results show notable increases of molar mass
during the oxidization of the PE [33, 59].
(Bio?)degradation of low-density polyethylene (LDPE)
has been reported, but the rate is very slow. Albertsson
[95] found 0.2% weight loss for LDPE films buried in soil
for 10 years. Of course, such a rate cannot justify the use
of the term ‘biodegradation’, as biodegradation is also
related to the rate. In this survey, Albertsson et al. [95]
used LDPE films labelled with 14C, cut them into small
pieces, and buried them under soil, which was kept in
controlled conditions. The degree of biodegradation was
estimated by the yield of 14CO2, which is the possible
final product of the metabolic cycle of the degradation of
the LDPE. These phenomena are very slow (300 years to
break down a thickness of 60 lm) and affect the outer
surface of the material [30], they are limited to a thick-
ness of the order of 1 lm. The UV light accelerates the
breaking down [29, 30], as much as doubling the break-
down speed process.
In a report published in 2005, Hadad et al. [96] claimed
that that polyethylene—considered to be inert—can be
biodegraded if the right microbial strain is isolated. They
used enrichment culture methods which were found to be
effective for isolating a thermophilic bacterium (brevi-
baccillus borstelensis) capable of utilizing polyethylene as
the sole carbon and energy source. Incubation of polyeth-
ylene with B. borstelensis (30 days, at 50�C) reduced its
gravimetric and molecular weights by 11 and 30%
respectively. Brevibaccillus borstelensis also degraded
polyethylene in the presence of mannitol Maximal bio-
degradation was obtained in combination with photo-oxi-
dation, which showed that carbonyl residues formed by
photo-oxidation play a role in biodegradation. They re-
ported that Brevibaccillus borstelensis also degraded the
CH2 backbone of nonirradiated polyethylene. Of course
such a selective microbial treatment under laboratory
conditions has only theoretical interest as it is not possible
under real agricultural soil conditions.
Fragmentation of the films can result from the presence
of microorganisms [59]. Rain, temperature, light etc. are
factors that affect the degradation, which may be acceler-
ated by their synergist activities or inhibited by their ad-
verse effects. As a result of degradation (fragmentation),
the condition of the degraded LDPE may encourage attack
by microbes (i.e. an affinity of LDPE to microbes is as-
sumed to become so strong that the biodegradation—along
with a complex degradation mechanism composed of
several factors such as normal oxidization, effects of
moisture and metal ions in soil—is promoted [30]). The
degradation in which microbes may participate, is devel-
oped from the surface to the inside of the film; the rate of
degradation progressing to the inside is determined by the
diffusion rate of metabolites from microbes, not by that of
oxygen, water and metal ions [30].
According to other authors, when the level of oxides
formation is increased (and that can be accomplished with
the use of special additives), the result could be biodeg-
radation. The assimilation of the order of 60%, of the total
carbon after 180 days in an artificial soil maintained at
60�C has been reported [70]. However this procedure does
not simulate real soil conditions and the conclusions
reached may be misleading with regard to possible bio-
degradation of agricultural films in soil. In another work
published by Karlssson et al., it has been reported that after
426 days, 27.8% mineralization is obtained in compost.
Some theory has been established to demonstrate the role
of additives in the PE degradation mechanism [20].
Ohtake et al. [30, 97] tested LDPE bottles exposed in
aerobic soil for over 30 years and observed some evidence
of biodegradation using SEM of the degraded parts. These
bottles were found buried under bioactive soil. It was esti-
mated that the period that these plastics were buried under
the soil was 32–37 years, as this was specified by the time at
which these plastics were discarded under soil. In another
study [29], sampling for buried LDPE films was carried out
in garden soil located in Japan. The samples were collected
at about 10 and 50 cm depth from the surface respectively.
It has been shown that LDPE films are gradually degraded,
accompanied by a molecular weight reduction caused by
microbes in the soil, as opposed to other common plastics
such as urea-formaldehyde resin (UF), polyvinyl chloride
resin (PVC) and polystyrene resin (PS).
Based on the studies of [29], it is estimated that it takes
about 300 years to entirely degrade LDPE films with
thickness of 60lm (the estimation of 300 years comes from
the rate of weight loss estimated from computed ratios
between parts in contact with soil and parts-not-in-contact
with soil in weight per unit area of the LDPE films con-
cealed in soil for more than 30 years [29, 30]). The rate of
the weight loss then predicts complete degradation at a
period of 300 years provided the rate of weight loss is
J Polym Environ (2007) 15:125–150 135
123
constant. However, this estimation implies an unrealisti-
cally high rate of biodegradation for LDPE in contrast to
the conventional supposition, justified with many published
reports [67, 90, 98, 99 etc.] that it takes several thousand
years for the LDPE (a non-degradable polymer) to be
completely degraded [29]. But even with this overesti-
mated rate (300 years), if the accumulation rate in the soil
is considered, the end result is that the LDPE remains will
continue to increase with time, instead of decreasing, with
irreversible contamination of the soil.
For many years there has been an approach to creating
polymers that could eventually function as biodegradable
polymers, by the addition of organic starch materials into a
petrochemical product (mainly PE). Unfortunately the re-
sults obtained, suggest a number of flaws to this system:
a. The two materials (i.e. starch and polyethylene) are
entirely incompatible. The properties of what was once
a highly efficient and minimalist material are highly
compromised [88, 100].
b. The starch has to be present in large amounts to be
accessible to microorganisms. This in turn, requires the
use of thicker films and thus more PE to ascertain good
mechanical properties to the composite material [101].
c. Starch-based technologies and aliphatic polyesters, are
hydro-degradable and rely on microbial active envi-
ronments to degrade. This is not true for polyethylene.
So finally only a part of the starch of such polymers,
the part that is accessible by the microorganisms can
biodegrade. These materials cannot therefore be con-
sidered and labelled as biodegradable but only partially
biodegradable [88]. These materials accumulate in the
environment with time as pollutants and so cannot be
characterized as environmental friendly ones either.
Conditional Polymers (bio?)Degradation
Concerning the question of possible (bio?)degradation of
polyethylene, many reports have been published claiming
that PE is biodegradable. In most cases the samples used in
the corresponding research projects are pre-treated or
special conditions and inoculums have been used for the
experiments. Several characteristic cases are summarized
in Table 1 for illustrative purposes.
In general it can be pointed out that:
1. In all but two cases, polyethylene was reported as
‘biodegaradable’, following however thermal pre-
treatment to induce thermal oxidation; this pre-treat-
ment does not simulate real soil conditions though.
2. In one case, biodegradation of PAH (polyaromatic
hydrocarbons) is reported under extremely acidic
environment.
3. In another case biodegradation of a LDPE/starch blend
is reported under controlled biologically active soil.
4. Biodegradation was measured through the percent
conversion of the carbon content from the designed
biodegradable plastic to CO2 in aerobic environment.
The condition under which polyethylene is tested for
possible biodegradation constitutes the key factor to the
interpretation of the results obtained. Thus, in order to
estimate the material life of polymers which undergo oxi-
dative degradation, the accelerated test is used [60]. This
test is based on the assumption that the oxidative degra-
dation is an activated process (i.e. needs some external
factor in order to get started). In general, it requires long
extrapolation from high-temperature data to obtain the
room temperature based life needed for oxidation. As far as
the biodegradation is concerned however, the oxidation
proceeds via an enzymatic process with very low activation
energy which is the only possibility for mild conditions in
the range of room temperature. Thus, it is practically
impossible to apply the accelerated test to study biodeg-
radation, especially in soil. It is necessary to carry out long-
term degradation tests under model conditions which are
similar to those of a real (e.g. soil) system in order to get
reliable results concerning biodegradation [60].
Additives
Functions of Additives
Most conventional plastics contain additives to facilitate
their processing and to enhance the physical characteristics
of the products manufactured from them. This is also true
for the degradable materials appearing in the market [46,
48, 49]. Some of the most common types of additives are
process additives, stabilizers, performance additives and
plasticizers [18, 32]. Additives are useful to modify poly-
mers for three basic features [101]:
• They are chemically active and they react with the
polymer to lead to a new chemical structure or they
slow down the degradation of the polymer.
• They are physically active and modify the rheology,
mechanical properties, optical or electrical characteristics.
• They may be inexpensive and simple, reduce costs (or
sophisticated and expensive; depending on the applica-
tion), and at the same time alter the polymer properties.
Theoretically, each additive is added into a compound
to enhance a single property of the polymer to allow a
precise engineering application [46, 48, 49]. They modify:
• Aesthetics, mechanical, thermal, electrical, optical
performances.
136 J Polym Environ (2007) 15:125–150
123
Ta
ble
1R
epo
rted
(bio
?)d
egra
dat
ion
of
po
lym
ers
inli
tera
ture
Mat
eria
lR
epo
rted
resu
ltC
on
dit
ion
sR
ate
or
du
rati
on
of
deg
rad
atio
nR
epo
rted
deg
rad
atio
nR
efer
ence
LD
PE
/sta
rch
Par
tial
ly
bio
deg
rad
able
Usa
ge
of
acti
vat
edsl
ud
ge
1m
on
thin
ino
culu
mN
ot
men
tio
ned
Nak
amu
raet
al.
[65
]
LD
PE
Par
tial
ly
bio
deg
rad
able
Bio
acti
ve
soil
(rat
her
shal
low
)3
2–
37
yea
rsin
soil
•A
bo
ut
2/3
dec
reas
eo
fth
ick
nes
s
•S
low
rate
of
ox
idat
ive
deg
rad
atio
n
Oh
tak
eet
al.
[29]
LD
PE
con
tain
ing
tota
lly
deg
rad
able
pla
stic
add
itiv
es(T
DP
A)
and
pro
-ox
idan
ts
Bio
deg
rad
able
•P
re-t
her
mal
ly-o
xid
ized
at5
5�C
•F
rag
men
ted
80
0w
eek
s(6
00
day
s)
inin
ocu
lum
•C
um
ula
tiv
eC
O2
emis
sio
ns
~17
00
mg
CO
2(7
0m
g/g
soil
)
•4
4%
min
eral
izat
ion
Ch
iell
ini
etal
.[2
]
LD
PE
wit
hp
ro-o
xid
ants
LL
DP
E
En
vir
on
men
tall
y
Deg
rad
able
Th
erm
ally
-ox
idiz
edat
10
0�C
14
day
s•
Dro
pin
mo
lecu
lar
wei
gh
t
•C
arb
on
yl
form
atio
n
Kh
abb
azet
al.
[40
]
LD
PE
,L
LD
PE
,H
DP
E,
UH
MW
PE
Th
erm
ally
deg
rad
able
Ad
dit
ion
of
met
als
Acc
eler
ated
Ag
ein
g.
Th
erm
al
deg
rad
atio
nca
use
db
yco
nta
ct
wit
hm
etal
s
•D
ecre
ase
inch
emil
um
inen
sen
ce
inte
nsi
ty•
Incr
ease
ino
xid
atio
n
rate
s
Go
rgh
ium
etal
.
[10
2]
PE
Bio
deg
rad
able
Pre
-hea
ted
at6
0�C
inan
air
ov
en
tosi
mu
late
dth
eef
fect
of
the
com
po
sten
vir
on
men
t.
Incu
bat
edin
the
pre
sen
ceo
f
sele
cted
mic
roo
rgan
ism
s
•S
teri
lize
db
yU
V/i
no
cula
ted
30
min
]
•In
cub
ated
for
6m
on
ths
at2
7�C
inso
ilco
nta
inin
g8
5%
of
wat
er
•M
icro
bia
lg
row
th
•E
rosi
on
of
the
film
surf
ace
Bo
nh
om
me
etal
.
[61]
PA
HB
iod
egra
dab
leB
uri
edin
extr
emel
yac
idic
env
iro
nm
ent
(co
alru
no
ffb
asin
)
28
day
s•
60
%m
iner
aliz
atio
n
•C
O2
pro
du
ctio
nfr
om
0–
10
%
dep
end
ing
on
the
hy
dro
carb
on
Sta
ple
ton
etal
.[9
8]
LD
PE
,H
DP
E,
LL
DP
ED
egra
dab
leA
rtifi
cial
acce
lera
ted
wea
ther
ing
(UV
-an
dX
eno
nar
cra
dia
tio
n)
16
00
hu
sin
ga
Xen
on
lam
p
of
65
00
0W
and
80
0h
usi
ng
UV
Bla
mp
0.6
0W
/m2
irra
dia
nce
at3
13
nm
•R
epo
rted
den
sity
~0.9
6g
/cm
3
•3
5%
wei
gh
tlo
ssfo
rH
DP
E,
5%
for
LD
PE
,>
5%
for
NP
Gu
lmin
eet
al.
[99]
LD
PE
/sta
rch
(12
%)
Bio
deg
rad
able
Co
ntr
oll
edb
iolo
gic
ally
acti
ve
soil
7m
on
ths
Pro
du
ced
bio
mas
s~3
00
lg
/l
~7g
CO
2/5
0m
l
Orh
anet
al.
[10
3]
LD
PE
LL
DP
EB
iod
egra
dab
leP
re-t
her
mal
lyo
xid
ized
inan
ov
en
40
–7
0�
and
then
inco
mp
ost
14
0d
ays
17
–2
7%
O2
con
sum
pti
on
Wei
lan
det
al.
[59]
LD
PE
/sta
rch
wit
h
pro
-ox
idan
tsH
DP
EN
P
Bio
deg
rad
able
So
ilm
ixed
wit
h5
0%
(w/w
)
mat
ure
mu
nic
ipal
soli
dw
aste
com
po
st
15
mo
nth
sat
roo
mte
mp
erat
ure
and
40
%h
um
idit
y
•D
ecre
ase
inte
nsi
lest
ren
gth
•>
10
%C
O2
pro
du
ctio
n
Orh
anet
al.
[10
4]
PP
/st
arch
(ble
nd
edw
ith
Mat
er-B
i
Par
tial
ly
Bio
deg
rad
able
Ph
oto
ox
idiz
edw
ith
ult
rav
iole
t
rad
iati
on
atso
lar
wei
gh
t
len
gth
sfr
om
Xen
on
lam
p
1,3
80
,00
0k
J/m
2fo
r9
8.7
h
6m
on
ths
Wei
gh
tlo
ssM
ora
nch
oet
al.
[10
0].
J Polym Environ (2007) 15:125–150 137
123
Ta
ble
1co
nti
nu
ed
Mat
eria
lR
epo
rted
resu
ltC
on
dit
ion
sR
ate
or
du
rati
on
of
deg
rad
atio
nR
epo
rted
deg
rad
atio
nR
efer
ence
PE
con
tain
ing
pro
-ox
idan
t
Po
ten
tial
assi
mil
atio
nb
y
mic
roo
rgan
ism
s.
Pre
-ag
edth
erm
ally
Th
erm
ally
ox
idiz
edin
air
inan
ov
enat
two
dif
fere
nt
tem
per
atu
res,
55
and
70�C
,
un
der
dry
(un
con
tro
lled
hu
mid
ity
)an
dap
pro
xim
atel
y
75
%re
lati
ve
hu
mid
ity
(RH
,sa
tura
ted
NaC
lso
luti
on
)
con
dit
ion
s.
Ch
ang
esin
mo
lecu
lar
wei
gh
t.
Ox
idat
ion
acco
mp
anie
db
y
the
pro
du
ctio
no
flo
wm
ola
r
mas
s,o
xid
ized
frac
tio
ns,
wh
ich
are
du
eto
thei
r
wet
tab
ilit
yan
dfu
nct
ion
alit
y,
May
bec
om
ev
uln
erab
leto
mic
roo
rgan
ism
s
Ch
iell
ini
etal
.
[10
5]
Eth
yle
ne-
pro
py
len
e
cop
oly
mer
s
(E-P
cop
oly
mer
),Is
ota
ctic
po
lyp
rop
yle
ne
(i-P
P),
LD
PE
Bio
assi
mil
atio
nth
at
can
be
foll
ow
ed
by
mic
rob
ial
atta
ck
UV
-irr
adia
ted
(ph
oto
irra
dia
tio
n
of
the
film
sw
asca
rrie
do
ut
ina
acce
lera
ted
wea
ther
ing
cham
ber
(SE
PA
P1
2/2
4)
at6
0�C
).
10
0h
irra
dia
tio
nB
uri
ed
4–
6w
eek
sin
com
po
st
and
cult
ure
env
iro
nm
ent
Det
ecte
d
•su
rfac
eer
osi
on
•v
aria
tio
ns
inin
trin
sic
vis
cosi
ty
•w
eig
ht
loss
per
surf
ace
area
,
•co
lon
izat
ion
of
fun
gu
s
•ch
ain
scis
sio
n,
evo
luti
on
of
hy
dro
xy
l
and
carb
on
yl
gro
up
s
Pan
dey
etal
.[1
06].
LD
PE
Bio
deg
rad
able
Ph
ysi
coch
emic
altr
eatm
ents
ther
mal
trea
tmen
tat
10
5an
d
15
0�C
or
acce
lera
ted
agin
g
trea
tmen
t
Su
bje
cted
tob
iod
egra
dat
ion
by
aco
nso
rtiu
mo
ffo
ur
fun
gi
du
rin
g9
mo
nth
s
Mo
rph
olo
gic
al,
stru
ctu
ral,
surf
ace
chan
ges
and
min
eral
izat
ion
.
Man
zur
etal
.[1
07
]
LD
PE
Bio
deg
rad
able
Po
lyet
hy
len
e-d
egra
din
gm
icro
-
org
anis
mB
revi
ba
ccil
lus
bo
rste
len
sis
stra
in7
07
(iso
late
dfr
om
soil
)
Incu
bat
ion
of
po
lyet
hy
len
e
wit
hB
.b
ors
tele
nsi
s(3
0d
ays,
50
�C)
Deg
rad
atio
no
fp
oly
eth
yle
ne
inth
ep
rese
nce
of
man
nit
ol.
Max
imal
bio
deg
rad
atio
n
was
ob
tain
edin
com
bin
atio
n
wit
hp
ho
to-o
xid
atio
n.
Had
adet
al.
[96]
138 J Polym Environ (2007) 15:125–150
123
• The processing: moulding, extrusion etc, accuracy of
shaping.
• The long-term behaviour: ageing (heat, sunlight, weath-
ering, wet environment), creep, relaxation, fatigue.
• The cost.
Degradation and Biodegradation Related Additives
General
The most common types of additives used in
(bio?)degradable films are cobalt acetylacetonate, nickel or
ferrous dithiocarbamate, magnesium stearate or carboxyl-
ate, styrene-butadiene copolymer, starch [70] where the
incorporated amount is up to 20%, of which 7% is generally
starch, which is often associated with peroxides [20, 26, 29,
53, 67, 70]. In the case of polyethylene, the degradation rate
of the material in soil is independent of the nature of these
additives [30]. The additives accelerate the breakdown and
increase the production of oxide derivatives [53, 70].
The photodegradation can be enhanced by the addition of
small levels of UV degradation promoters or photo-initia-
tors. For example, Fe, Ni or Co chlorides or dithiocarba-
mates, organometallic additives, cerium based additives
(Rhone-Poulenc) [108]. More or less high levels of como-
nomers can also be used such as Ethylene-carbon monoxide
copolymer (Dow), Ecolyte masterbatches of PE, PP, PS
(Enviromer Enterprise, Dow and DuPont) [101, 102, 109].
In an effort to promote possible biodegradation condi-
tions, for well-known non-biodegradable polymers, high
levels of biodegradable additives have been used. In such
case, sources of nutrients for micro-organisms were used
but the conventional polymer, polyethylene particularly,
has not been biodegraded [101]. Only the biodegradable
additives are completely biodegraded. In particular, in
these cases it has been shown that:
• The skeleton of the conventional polymer is weak and
brittle and can disappear visually (but not necessarily
physically) more easily.
• The surface area is highly increased and promotes
chemical and bacterial attacks (at unknown rate of
possible (bio?)degradation, if any; refer to the contro-
versial data described earlier).
• One of the most industrialized ways is the addition of
starch-based materials but other biodegradable addi-
tives have also been used.
Pro-oxidants
Pro-oxidants such as manganese soaps can be used to speed
up the thermo-oxidation of the polymers. This solution is
difficult to control because of the high decrease of the
thermal stability of the system for a little variation of the
temperature [2, 19]. The most active pro-oxidants are those
based on metal combinations capable of yielding two metal
ions of similar stability and with oxidation number differ-
ing by one unit only e.g. Mn2+/Mn3+ [19]. Thus material
degrades by a free radical chain reaction involving oxygen
from the atmosphere. The primary products are hydroper-
oxides which can either thermolyse or photolyse under the
catalytic action of a pro-oxidant, leading to chain scission
and the production of low molecular mass oxidation
products such as carboxylic acids, alcohols, ketones and
low molecular mass hydrocarbon waxes [19]. The rate of
degradation depends on the type of polymer, type and
amount of the additives, temperature and other conditions.
As the fate of the remains of degraded polymers containing
such additives in the soil is unknown, systematic research
is needed in this direction.
Specialties such as Addiflex, Ciba EnvirocareTM and
AGPTM, ECM Masterbatch Pellets by ECM Biofilms,
EcoSafe Biodegradable Compost, TDPA� are examples of
some marketed specialties (the information given below,
including the suggested applications, is provided by the
corresponding industries):
Addiflex for PE, PP and PS. AddiFlex additives are
degradable, biodegradable, and/or photodegradable [110].
They are sold in pellets of masterbatches. The addition
level depends on the degradation mode. Biodegradable
ones are normally added to PE or PP, and eventually PS at
15% to 30%. Photodegradable ones are added at 3% level.
The pellets can be fed directly into the extruders or pre-
blended.
EcoSafe Biodegradable Compost, TDPA�. These addi-
tives lead to two-steps degradation by oxo-degradation in
12–18 months in a landfill [110].
Ciba Envirocare TM and AGPTM for PE, PP. These
additives are based on thermal and/or photo-oxidation. The
films after a season of outdoor exposure are sufficiently
brittle to be easily mixed with the soil during ploughing
[110].
ECM Masterbatch Pellets by ECM Biofilms. The pellets
are added at 2% to PE [110].
Limitations
One major drawback of most polymers is the problem with
their disposal. Since they may be resistive to degradation
(depending on the polymer, additives, conditions etc), non-
degradable polymers tend to accumulate in what is today’s
most popular disposal system, the landfill. This brings
questions about what effects polymers have on the envi-
ronment, whether they biodegrade at all, and if they do,
what is the rate of (bio?)degradation, what effect the
J Polym Environ (2007) 15:125–150 139
123
products of (bio?)degradation have on the environment,
including the effects of the additives used.
Regarding the additives, according to the National
Industry Chemical Notification and Assessment Scheme or
the EU Directive 67/548/EEC [7] a polymer in order to be
(bio?)degradable:
• Must not contain additives that are assigned or may be
assigned any of the following risk phrases (or combi-
nations thereof):
• greater than 0.1% of substances classified as carcino-
genic according to the approved criteria for the
classification of hazardous substances by the National
Industry Chemical Notification and Assessment
Scheme;
• greater than 0.2% of any ingredient that is classified or
may be classified as mutagenic or tetragenic according
to the approved criteria for the classification of
hazardous substances;
• Must have a minimum natural material or starch
content of 30%.
• Plastic additives must not include more than 40%
aromatic polyesters or other degradable plastics by
weight.
• The hazardous material content shall be limited to one
of the following internationally determined benchmarks
for compostable biopolymers depicted in Table 2.
• Ecotoxicity: The germination rate and the plant
biomass of the sample composts of plant species
should be more than 90% of those from the corre-
sponding black compost.
No benchmarks have been established so far concerning
biodegradation of polymers in soil.
Tests and Standards on Biodegradability
Testing Biodegradability According to Standard
Testing Methods
In the United States an acknowledged authority for estab-
lishing definitions, test methods and standards, is the
American Society for Testing and Materials (ASTM),
through its Institute for Standards Research. The European
counterpart is the Comitee Europeen de Normalisation
(CEN)—the European Committee for Standardization.
Individual European Countries have their own organiza-
tions. The International Standards Organization (ISO) aims
to reconcile differences. The development of testing
methods is presented in the table below (Table 3) [111].
Some of the methods used to assess biodegradability
include the measurement of carbon dioxide production, as
with the Sturm test and soil test. Other methods involve
measurements of molecular weight and molecular weight
distribution; tensile properties, weight loss; extent of
fragmentation; enzyme assays; biochemical oxygen de-
mand (BOD); and ecotoxicity, as with cress read test and
earth worm test (ASTM D5338-98, ASTM 6340-98) [6].
Multiple test procedures are necessary in evaluating the
biodegradability of a material because some tests are sub-
ject to false-positive interpretations, that are concluding
incorrectly that degradation or biodegradation has occurred
[6, 7]. For example, observed weight loss may result not
from polymer degradation, but from the leaching of addi-
tives, including plasticizers. However, carbon dioxide
production might result from the degradation of low-
molecular weight fraction of the polymer, with no degra-
dation of longer chains. In another case, a large loss of
material strength might come from a very small change in
its chemical makeup. Strength is often disproportionately
affected by the loss of additives and 90% decrease of
strength can result from as little as 5% mineralization [14].
Because of its dependence on many environmental
factors, the biodegradability of a plastic, or polymer in
laboratory evaluations will not be relevant to all disposal
environments. Some tests might show it to be potentially or
Table 2 Internationally determined benchmarks for compostable
biopolymers concerning hazardous material
Chemical DIN V 54900-1
(German Standard)
EN 13432
(EU Standard)
Green Plastics
(Japanese Standard)
Limit values
(mg/kg)
Limit values
(mg/kg)
Limit values
(mg/kg)
Zn 100 150 150
Cu 23 50 37.5
Ni 15 25 25
Cd 0.3 0.5 0.5
Pb 30 50 50
Hg 0.3 0.5 0.5
Cr 30 50 50
Mo – 1 1
Se – 0.75 0.75
As – 5 3.5
F – 100 100
Table 3 Development of degradable methods
Year Test Method Standard
Before
1995
Photo degradable test ISO 4892
1995~ Fungus erosion testing
composting testing
ISO 846, ASTM G 21, ASTM
D 5338
2001~ Biodegradable testing ISO 14851, ISO 14852, ISO
14855
2003~ Disintegration or
biodegradation
ISO 16929, ISO 17556, ISO
20200, ISO 14853
140 J Polym Environ (2007) 15:125–150
123
inherently biodegradable without showing it to be actually
biodegradable in a specific disposal environment (ASTM
6400-99, ASTM 5338-98, ASTM 6340-980 [6].
Criteria used in the Evaluation of Biodegradable
Polymers
A large number of intrinsic properties of a polymer can be
measured. Some properties whilst producing absolute
indicators that the polymer has undergone scission e.g.
molecular weight distribution or intrinsic viscosity chan-
ges, require specialist analytical equipment but tell little of
the ultimate mineralization or biodegradability of the
polymer, Weight loss has limited use particularly where the
polymer fragments and the integrity of the specimens are
lost during the test. Mechanical property changes are dif-
ficult to interpret in relation to structural alterations al-
though they may be sensitive to small changes in molecular
weight [52] and they are applicable only before fragmen-
tation of the samples. The test methods used for the eval-
uation of biodegradability are depicted in Fig. 1.
Plastic bags and other products, e.g. agricultural
mulching films, made with polyethylene (PE) are appearing
on the market with the claim of being ‘‘degradable’’, or
‘‘bio-, UV- or oxo-degradable‘‘, and sometimes even
‘‘compostable’’. The underlying technology is based on
special additives (master batch) which, if incorporated into
standard PE resins, are purported to accelerate the degra-
dation of the film products (refer to earlier sections).
However this technology and the products are not new, and
since their first appearance on the market in the 80s many
doubts have been expressed as to whether these products
provide what they promise. Such doubts are still valid.
IBAW (the international industry organisation for bio-
plastics and biodegradable polymers), has published a po-
sition, which outlines the questions raised by
‘‘degradable‘‘ PE products [7].
Compliance with EN 13432—The Underlying Test Scheme
for Evaluation
The EU Directive on Packaging and Packaging Waste (94/
62/EC) [92] defines requirements for packaging to be
considered recoverable. The harmonized standard EN
13432 [92] amplifies these requirements with respect to
organic recovery and biodegradable packaging. The EN
13432 lays down laboratory test procedures for biode-
gradability and compostability and for the determination of
potential harmful material constituents in packaging and
packaging materials. Whenever a packaging product is
placed on the market as ‘‘degradable’’, conformance with
the requirements of 94/62/EC is to be assessed through the
Determine disposal route in environment
Is environmental compartments part of
biosphere?
Select test system most appropriate to disposal route
Determine solubility and toxicity of chemical
substance
Incorporate or disposal system and CO2 measuring system Is solubility >10
ppm?
Apply any measurement
system
Are anaerobic conditions required? Modify test system to
accommodate anaerobic conditions
Apply test system suitably modified
Is level of biodegradation
acceptable?
Is more data required?
Apply modifications to test system to encourage biodegradation (second
tier of testing)
Consider long term simulation (third tier of
testing)
Fig. 1 Test methods for
evaluating biodegradability [36]
J Polym Environ (2007) 15:125–150 141
123
use of EN 13432. No PE additive or PE with special
additives has yet been shown to comply with EN 13432
[92].
Certification and Labelling Required
Product certification based on EN 13432 [92] and labelling
through an accredited conformity assessment body are to be
applied to all plastic products that are claimed to be
‘‘degradable‘‘, ‘‘biodegradable’’ or ‘‘compostable‘‘ [92].
A responsible industry has developed an environmental
self-commitment on product certification to achieve the
highest possible product safety and lowest possible envi-
ronmental impact. The commitment was officially
acknowledged by the EU Commission in February 2005 [7].
Presence of Oxo-biodegradable Additives
Product Safety and Ecotoxicity: The so called ‘‘oxo-bio-
degradable’’ additives pose several concerns regarding
safety and ecotoxicity. These additives are based on ionic
metals that trigger PE fragmentation. Some metal com-
pounds used in these products are classified and labelled
under the EU Directive 67/548/EEC [92] on Dangerous
Substances as causing adverse effects on humans and the
environment. For instance, cobalt Co(II), has been found in
concentrations higher than 4,000 mg/kg in some ‘‘oxo-
biodegradable‘‘ additives [IBAW position dated 6th June
2005 on degradable PE shopping bags—7]. At such high
concentrations these materials are considered harmful if
released into the environment, and are regulated at the
workplace of plastic manufacturers and converters, since
metal fumes might be released through dust or under
heating [7]. During the fragmentation process however,
regulated metals may be liberated into the environment
with the consequence of adding (eco)toxic persistent and
bio accumulative CMR substances (Carcinogenic, Muta-
genic, toxic to Reproduction) [7].
The Risk of Persistency and Bio-Accumulation: It is well
established that standard PE is not biodegradable [7]. It has
been demonstrated in case studies that the so-called ‘‘oxo-
biodegradable’’ PE products may fragment into very small
particles after exposure to UV light or dry heat [7]. How-
ever after fragmentation, PE is still to a large extent
resistant to biodegradation and, therefore, due to the slow
process, the potential of persistency in the environment and
bioaccumulation of liberated regulated metals and PE
fragments in organisms is high [7]. Therefore, is has been
suggested that the presence of ‘‘oxo-biodegradable‘‘
additives in polyethylene, does not justify ‘‘labelling’’ the
material as biodegradable material [70].
Littering: ‘‘Oxo-biodegradable’’ PE products have been
described as a solution to littering problems, as after
trashing they supposedly decompose in the natural envi-
ronment [7]. De facto such a concept promotes littering and
endangers organic recovery schemes which are built up to
promote sustainability [7].
Plastic Recycling Schemes: ‘‘Oxo-biodegradable‘‘
products endanger not only organic recovery but also
recycling processes of plastics. The additives destabilise
plastic recyclates of mixed origin, which may lead to a
reduced value of recycled plastics. Plastic recovery and
recycling schemes may not be prepared to accept products
that contain additives that promote degradation [7].
Parameters Affecting the Performance of Standard
Tests
Studies and investigations aimed at improving the feasi-
bility and the reproducibility of laboratory methods to as-
sess the biodegradation of EDPs are continuously in
progress. This is due to the fact that some operative diffi-
culties can arise during the performance of the tests, thus
affecting the accuracy of the measurements as based on the
monitoring of suited parameters of choice. and the
increasing number of new and structurally different EDP
based materials [54].
A relatively large number of specific problems might be
encountered during the performance of tests designed to
assess the extent of biodegradation as CO2 release or O2
uptake especially under solid-state conditions and in the
presence of organic rich incubation media such as mature
compost. On the contrary, the tests carried out in aqueous
medium are considered easier to set up and generally more
reproducible. Therefore, the response of a standard test
could be significantly affected by some external parameters
that often are intrinsically variable (e.g. microbial inocu-
lum) or relatively poorly addressed in the standard speci-
fications. In particular, the biodegradation kinetics of a test
material under solid state conditions can be influenced by
the material’s concentration in the solid medium, as well as
by the nature of the microbial populations; whereas the test
results might vary significantly depending upon the test
duration and the reference (positive) material designed in
the standard test specifications [54].
Standards for Testing Biodegradable Plastics
The main international organizations that have established
standards or testing methods are:
• American Society for Testing and Materials (ASTM) [6]
• European Standardisation Committee (CEN) [92]
• International Standards Organisation (ISO) [112]
• National Institute for Standards Research (ISR) (USA)
[113]
142 J Polym Environ (2007) 15:125–150
123
• German Institute for Standardisation (DIN) [114]
• Organic Reclamation and Composting Association
(ORCA) (Belgium) [115].
• Association Francaise de Normalization (AFNOR,
France) [116].
International Standards Organization Criteria
Three International Standards Organization (ISO) [112]
standards have set the criteria by which European biode-
gradable plastics are currently assessed (additional EN
standards have also been developed or are under develop-
ment as discussed later on). These are:
• ISO 14855 (aerobic biodegradation under controlled
conditions);
• ISO 14852 (aerobic biodegradation in aqueous envi-
ronments); and
• ISO 15985 (anaerobic biodegradation in a high solids
sewerage environment).
ISO 14855 is a controlled aerobic composting test, and
ISO 14851 and ISO 14852 are biodegradability tests spe-
cifically designed for polymeric materials.
An important part of assessing biodegradable plastics is
testing for disintegration in the form in which it will be
ultimately used. Either a controlled pilot-scale test or a test
in a full-scale aerobic composting treatment facility can be
used. Due to the nature and conditions of such disintegra-
tion tests, the tests cannot differentiate between biodegra-
dation and abiotic disintegration, but instead demonstrates
that sufficient disintegration of the test materials has been
achieved within the specified testing time.
ASTM Standards
The ASTM standards have test methods that measure the
intrinsic biodegradability of plastic materials designed for
biodegradability, and are full-fledged. These test methods
measure the percent conversion of the carbon from the
designed biodegradable plastic to CO2 in aerobic envi-
ronment and CH4 (plus some CO2) in a anaerobic envi-
ronment. The test material is the sole carbon source for the
microorganism in the experiment. The two bio-test meth-
ods that apply in the presence of municipal sewer sludge,
and so they do not simulate soil conditions, are:
• Standard Test Method for Determining the Aerobic
Biodegradation of Plastic Materials in the Presence of
Municipal Sewer Sludge (D5209-91) [6].
• Standard Test Method for Determining the Anaerobic
Biodegradation of Plastic Materials in the Presence of
Municipal Sewer Sludge (D5210-91) [6].
Three other bio-test methods have become ASTM
Standards. They are:
• Standard test method for assessing the aerobic biodeg-
radation of plastic materials in an activated sludge-
waste water treatment system (D5271-92) [6].
• Standard test method for determining the aerobic
biodegradation of plastic materials under controlled
composting conditions (D5338-93) [6].
• Standard test method for determining the aerobic
biodegradability of degradable plastics by specific
microorganisms (D5247-92) [6].
The first two simulate environments that are representa-
tive of waste management infrastructures such as compo-
sting and waste-water treatment system. The test methods
permit the quantification of biodegradability in specific
waste management infrastructures. While these test methods
give a quantitative measure of biodegradability in such
environments, parallel tests in ‘‘real world systems’’ need to
be run to confirm and establish biodegradability. ASTM is
currently developing standard practices for exposing
degradable plastics to such ‘‘real systems‘‘ environments
and reporting the resulting data [6]. The specific microor-
ganisms test method does not represent any real world waste
management infrastructure but provides a standard test
method to quantify biodegradability using well-defined
microbial cultures commonly present in the environment [6].
Aquatic Biodegradability: Mitigating the hazards to
marine life by designing bio and photodegradable plastics
that would degrade in a marine environment is one of the
targets for industry. Thus, to evaluate the biodegradability
potential in an aquatic environment, a Standard Practices for
Exposing Plastics to a Simulated Marine and Fresh-Water
Environments were developed and are now at the Society
balloting stage [6]. A Standard Test Method to quantify the
amount of degradation in such environments is currently
being developed and will build on the two aquatic test
practices discussed: A Standard Practice for Weathering of
Plastics under Marine Floating Exposure [D5437-93] [6].
Composting Environment: Composting is fast becoming
an important waste management strategy. Biodegradable
plastics that will be compostable in an appropriate com-
posting infrastructure are being designed. As discussed
earlier, a Standard Test Method for Determining the Aer-
obic Biodegradation of Plastic Materials under Controlled
Composting Conditions has been developed. Two Standard
Practices for exposing plastics to a simulated compost
environment with and without an externally heated reactor
have also been developed [6].
Others: A number of other specific test methods are
under various stages of development for example, a high
solids anaerobic digester system, and accelerated (biolog-
ically active) landfill conditions.
J Polym Environ (2007) 15:125–150 143
123
EN Standards (European Committee for Normalisation)
The European Committee for Normalisation (CEN) [92]
established the norm standard (CEN prEN 13432) in 1999.
The norm provides the European Commission’s European
Directive on Packaging and Packaging Waste with
appropriate technical regulations and standards. This norm
is a reference point for all European producers, authorities,
facility managers and consumers.
The standard specifies requirements and procedures to
determine the compostability of plastic packaging materi-
als based on four main areas, biodegradability; disinte-
gration during biological treatment; effect on the biological
treatment process; and effect on the quality of the resulting
compost.
Importantly, the packaging material that is intended for
entering the bio-waste stream must be ‘recognizable’ as
biodegradable or compostable, by the end user.
The strictest European standard for biodegradability is
CEN 13432. This standard can apply to other packaging
materials in addition to polymers, and incorporates the
following tests and standards, ISO 14855; ISO 14855
(respirometric); ISO 14852; ASTM D5338-92; ASTM
D5511-94; ASTM D5152-92; ASTM E1440-91; Modified
OECD 207; and CEN TC 261/SC4/WG2.
‘OK Compost’ Certification and Logo: The ‘OK Com-
post’ logo can be used on the labelling of biodegradable
plastics and other materials to signify that the material is
100% compostable and biodegradable. The logo is owned
and managed by AVI (AIB Vincotte Inter, Brussels, Bel-
gium), [117], and is based on the CEN – 13432 standard [92].
Compost Toxicity Tests
For a comprehensive assessment of toxicity associated with
compost applications, plastics can be tested on both plant
and animal species. Toxicity screening of some commer-
cial degradable plastics using cell culture testing has been
reported in literature [98]. A number of polyester types
were tested including a plasticized cellulose acetate, an
aliphatic polyester (Bionolle), polyhydroxybutyrate-co-
hydroxyvalerate (BiopolTM), and polycaprolactone (TO-
NETM polymer). Cell culture medium with serum was used
as the extraction medium. The relative MTT activity of
cells cultured in fresh extracts indicate that TONETM
polymer (all shapes) and Bionolle (test bars and films) are
comparable to materials currently used in food with no
toxic effects on cells.
Plant Phytotoxicity Testing
While a product may not negatively impact plant growth in
the short term, over time it could become phytotoxic due to
the build-up of inorganic materials, which could potentially
lead to a reduction in soil productivity. For this reason
some manufacturers use plant phytotoxicity testing on the
finished compost that contains degraded polymers. Phyto-
toxicity testing can be conducted on two classes of flow-
ering plants. These are monocots (plants with one seed
leaf) and dicots (plants with two seed leafs). Representa-
tives from both of these classes are typically used in tox-
icity testing—summer barley to represent monocots and
cress to represent dicots. Tests involve measuring the yield
of both of these plants obtained from the test compost and
from control compost [98].
Animal Toxicity Test
Animal testing is generally carried out using earthworms
(as representative soil dwelling organisms) and Daphnia (as
representative aquatic organisms). Earthworms are very
sensitive to toxicants. Since earthworms feed on soil, they
are suitable for testing the toxicity of compost.
In the acute toxicity test, earthworms are exposed to
high concentrations of the test material for short periods of
time. The toxicity test is a European test (OECD guideline
#207) [7, 92] in which earthworms are exposed to soil and
compost in varying amounts. Following 14 days of expo-
sure, the number of surviving earthworms is counted and
weighed and the percent survival rate is calculated.
Compost worms (Eisenia fetida) are used for testing the
toxicity of biodegradable plastic residues. These worms are
very sensitive to metals such as tin, zinc, heavy metals and
high acidity. For this test worms are cleaned and accurately
weighed at intervals over 28 days. The compost worm
toxicity test is considered to be an accurate method.
The Daphnia toxicity test can establish whether degra-
dation products present in liquids pose any problem to
surface water bodies. In the test, Daphnia are placed in test
solutions for 24 h. After exposure the number of surviving
organisms is counted and the percent mortality is calcu-
lated.
Difference between Standards for Biodegradation and
Compostability
ASTM standard establishes criteria (specifications) for
plastics and products made from plastics to be labelled
degradable, compostable and biodegradable. It establishes
whether plastics and products made from plastics will
compost satisfactorily, including biodegrading at a rate
compared to known compostable materials. These stan-
dards are comparable to those that have been developed
(since 2002) by the European Committee for Standariza-
tion (CEN) and in harmony with the Deutsches Institut f}ur
Normung (DIN) standards. The main point of differentiation
144 J Polym Environ (2007) 15:125–150
123
between the various international standards is the percent-
age of biodegradation (under physical conditions in the
landfill) required for compliance but also the conditions and
the time. This is an important issue that is under discussion
at ISO level [46]. Some of the standards that apply for
degradation, biodegradation and composting of polymers
are summarized in Table 4 [119].
Appendix
Definitions of Degradation Processes
Ageing: the process of growing old or developing the
appearance and characteristics of old age; the change of
properties that occurs in a material as a result of degradation
Table 4 List of standards on degradation, biodegradation and compostability
No. of
standard
Date of
publication
Title Conditions Nature/Objective
of the test
Evaluation
parameter
Application
98/710671 DC 31/07/1998 Test scheme and evaluation
criteria for the final
acceptance of packaging.
New European Standard
Packaging. Requirements for
packaging recoverable
through composting and
biodegradation
Composting,
biodegradation
Packaging
ASTM D 5210 01/12/1998 Test method for determining
the anaerobic biodegradation
of plastic materials in the
presence of municipal
sewage sludge
Aerobic Biodegradation CO2/CH4 Plastic
ASTM D 5247 01/07/1992 Test method for determining
the aerobic biodegradability
of degradable plastic by
specific microorganisms.
Aerobic Biodegradability Mw, MP
(molecular
weight,
mechanical
properties)
Plastic
ASTM D 5271 01/09/1993 Test method for determining
the aerobic biodegradation
of plastic materials an
activated sludge wastewater
treatment system
Aerobic Biodegradation O2 Plastic
ASTM D 5338 01/08/1999 Test method for determining
aerobic biodegradation of
plastic materials under
controlled composting
conditions
Aerobic Composting CO2 Plastic
ASTM D 5509 01/08/1996 Practice for exposing plastics
to a simulated compost
environment.
Composting Plastic
ASTM D 5511 01/04/1994 Test method for determining
anaerobic biodegradation of
plastic materials under high-
solids anaerobic-digestion
conditions
Anaerobic Biodegradation Plastic
ASTM D 5512 01/08/1996 Practice for exposing plastics
to a simulated compost
environment using an
externally heather reactor
Composting Plastic
ASTM D 5526 01/11/1994 Test method for determining
anaerobic biodegradation of
plastic materials under
accelerated landfill
conditions
Anaerobic Biodegradation Plastic
J Polym Environ (2007) 15:125–150 145
123
Table 4 continued
No. of
standard
Date of
publication
Title Conditions Nature/Objective
of the test
Evaluation
parameter
Application
ASTM D 5988 01/09/1996 Test method for determining
aerobic biodegradation in
soil of plastic materials or
residual plastic materials
after composting
Aerobic Biodegradation in
the soil
Plastic
ASTM D 6003 01/02/1997 Test method for determining
weight loss from plastic
materials exposed to
simulated municipal solid
waste (MSW) aerobic
compost environment
Composting Plastic
ASTM D5209 01/09/1992 Standard Test Method for
Determining the Aerobic
Biodegradation of Plastic
Materials in the Presence of
Municipal Sewage Sludge
Aerobic Biodegradation Plastic
ASTM D5210 1992 Standard Test Method for
Determining the Anaerobic
Biodegradation of Plastic
Materials in the Presence of
Municipal Sewage Sludge
Anaerobic Biodegradation Plastic
ASTM E 1196 01/12/1992 Test method for determining
the anaerobic biodegradation
potential of organic
chemicals
Anaerobic Chemical and
organic
products
ASTM D 5929 01/05/1996 Standard test method for
determining biodegradability
of materials exposed to
municipal solid waste
composting conditions by
compost respirometry
Composting
ASTM D 5975 01/10/1996 Test method for determining
the stability of compost by
measuring oxygen
consumption
Composting O2
ASTM E 1279 1989 Test method for biodegradation
by a shake-flask die-away
method
CEN ENV 12920 11/1997 Characterization of waste—Methodology for the determination of the leaching behaviour of waste under
specified conditions
CSA Z 218-0
(Canada)
1993 Test method for determining
the anaerobic
biodegradability of plastic
materials
Anaerobic Biodegradability Plastic
EN 13193 01/05/2000 Packaging and the environment—Terminology Packaging
EN 13427 01/09/2000 Packaging Requirements for the use of European Standards in the field of packaging
and packaging waste
Packaging
EN 13432 01/09/2000 Packaging. Requirements for packaging
recoverable through composting and
biodegradation. Test scheme and
evaluation criteria for the final acceptance
of packaging
Composting Packaging
EN ISO 846 01/06/1997 Plastics. Evaluation of the action of microorganisms Plastic
FD ISO/TR
15462
1997 Qualite de l’eau. Selection des
essais de biodegradabilite
Biodegradability
146 J Polym Environ (2007) 15:125–150
123
(whether degradation is due to one factor or is due to the
combined action of several factors) [118].
Biodegradation: degradation that is caused by biological
activity, especially by enzymatic action, (ISO/CD 16929).
Biodegradation phase: the time in days from the end of
the lag phase of a test until about 90% of the maximum
level of biodegradation has been reached (ISO/DIS 17556).
Degradation: an irreversible process leading to a sig-
nificant change of the structure of a material, typically
characterized by a loss of properties (e.g. integrity,
molecular weight, structure or mechanical strength) and/or
fragmentation. Degradation is affected by environmental
conditions and proceeds over a period of time comprising
one or more steps (ASTM D-6400.99) [6].
Disintegration: The falling apart into very small frag-
ments caused by degradation mechanisms (ASTM D-
6400.99) [6].
Lag phase: the time required in days for adaptation and
selection of the degrading micro-organisms to be achieved
and the biodegradation degree of a chemical compound or
Table 4 continued
No. of
standard
Date of
publication
Title Conditions Nature/Objective
of the test
Evaluation
parameter
Application
ISO 10707 15/01/1994 Water quality—Evaluation in
an aqueous medium of the
ultimate aerobic
biodegradability of organic
compounds—Method by
analysis of biochemical
oxygen demand (closed
bottle test)
Aerobic
aquatic
media
Total
biodegradability
DBO Organic
composition
ISO 10708:1997 01/01/1997 Water quality—Evaluation in
an aqueous medium of the
ultimate aerobic
biodegradability of organic
compounds—Determination
of biochemical oxygen
demand in a two-phase
closed bottle test
Aerobic
aquatic
media
Total biodegradability DBO Organic
composition
ISO 11733 2004 Water quality—Determination
of the elimination and
biodegradability of organic
compounds in an aqueous
medium—Activated sludge
simulation test
Aquatic
media
Biodegradability Organic
composition
ISO 11734 14/12/1995 Water quality—Evaluation of
the ultimate anaerobic
biodegradability of organic
compounds in digested
sludge—Method by
measurement of the biogas
production
Anaerobic Total biodegradability Free biogas Organic
composition
ISO 14593:1999 15/03/1999 Water quality—Evaluation of
ultimate aerobic
biodegradability of organic
compounds in aqueous
medium—Method by
analysis of inorganic carbon
in sealed vessels (CO2
headspace test)
Aerobic
aquatic
media
Total biodegradability Analyses of
inorganic
carbon
Organic
composition
AFNOR NF U
52-001
Biodegradable Mulching Film:
Test Methods And Criteria
soil or
aqueous
media
material characteristics,
ecotoxicity,
biodegradation
hazardous
substances
heavy
metals,
organic
substances,
earthworms,
algae, CO2
mulching
films
J Polym Environ (2007) 15:125–150 147
123
organic matter has reached 105 of the theoretical maximum
biodegradation derived form the theoretical amount of
evolved carbon dioxide and theoretical oxygen demand
(ISO/DIS 17556).
Maximum level of biodegradation: the maximum bio-
degradation in percent a chemical compound or organic
matter achieves in a test, above which no further biodeg-
radation takes place (ISO/DIS 17556).
Natural ageing: a standardized artificial process for
imparting the characteristics and properties of age [118].
Plateau phase: The times form the end of the biodeg-
radation phase (maximum level of biodegradation) until the
end of the test (ISO/DIS 17556).
Primary Biodegradation is the alteration in the chemical
structure of a substance, brought about by biological ac-
tion, resulting in the loss of a specific property of that
substance (EPA OPPTS 835.3110).
Primary Biodegradation: Minimal transformation that
alters the physical characteristics of a compound while
leaving the molecule largely intact. Partial biodegradation
is not necessarily a desirable property, since the interme-
diary metabolites formed can be more toxic than the ori-
ginal substrate. Therefore, mineralization is the preferred
aim (EPA OPPTS 835.3110).
Theoretical amount of evolved carbon dioxide: the
maximum theoretical amount of carbon dioxide evolve
after completely oxidizing a chemical compound calcu-
lated from the molecular formula; expressed as mg carbon
dioxide evolved per mg or g test compound (ISO/DIS
17556).
Theoretical oxygen demand: the maximum theoretical
amount of oxygen required to oxidize a chemical com-
pound completely calculated from the molecular formula;
expressed as mg oxygen required per mg or g test com-
pound (ISO/DIS 17556).
Ultimate biodegradation (aerobic) is the level of
degradation achieved when the test compound is totally
utilized by microorganisms resulting in the production
of carbon dioxide, water, mineral salts, and new micro-
bial cellular constituents (biomass) (EPA OPPTS
835.3110).
Ultimate Biodegradation (Complete biodegradation):
Molecular cleavage must be sufficiently extensive to re-
move biological, toxicological, chemical and physical
properties associated with the use of the original product,
eventually forming carbon dioxide and water (EPA OPPTS
835.3110).
Ultimate biodegradation: degradation achieved when a
material is totally utilized by microorganisms resulting in
the production of carbon dioxide (and possibly methane in
the case of anaerobic biodegradation), water, inorganic
compounds, and new microbial cellular constituents (bio-
mass or secretions or both) (ASTM D-6046.02) [6].
Weathering: the natural process under real conditions
imparting the characteristics and properties of age [118].
Definitions of Materials Undergoing Various
Degradation Processes
Biodegradable material: a material that has the proven
capability to decompose in the most common environment
where the material is disposed of within 3 years through
natural biological processes into non-toxic carbonaceous
soil, water, carbon dioxide or methane [120]. Biodegra-
dation is measured according to the ASTM defined stan-
dards [6].
Biodegradable material: a material for which the bio-
degradation process is sufficient to mineralise organic
matter into carbon dioxide or methane respectively, water
and biomass (ISO/CD 16929).
Biodegradable plastic: a degradable plastic in which the
degradation results from the action of naturally occurring
microorganisms such as bacteria, fungi, and algae (ASTM
D-6400.99), (ASTM D-2096.01) [6].
Biopolymer: a material that is partially comprised of
natural starch additives with the characteristics of a plastic
product (ASTM D-6400.99) [6].
Compostable material: a material that is biodegradable
under composting conditions (ISO/CD 16929).
Compostable plastic: a plastic that undergoes degrada-
tion by biological processes during composting to yield
CO2, water, inorganic compounds, and biomass at a rate
consistent with other known compostable materials and
leave no visible, distinguishable or toxic residue (ASTM
D-6400.99), (ASTM D-2096.04) [6].
Compostable plastic: plastic capable of undergoing
biological decomposition in a compost site as part of an
available program, such that the material is not visually
distinguishable and breaks down into carbon dioxide, wa-
ter, inorganic compounds, and biomass, at a rate consistent
with known compostable materials (ASTM D-6002) [6].
Degradable plastic: a plastic designed to undergo a
significant change in its chemical structure under specific
environmental conditions, resulting in a loss of some
properties that may vary as measured by standard test
methods appropriate to it (ASTM D-6400.99) [6].
Degradable plastic: a plastic designed to undergo a
significant change in it is chemical structure under specific
environmental conditions resulting in a loss of some
properties that may vary as measured by standard test
methods appropriate to the plastic and the application in a
period of time that determines its classification (ASTM D-
2096.01) [6].
Degradable: A material is called degradable with re-
spect to specific environmental conditions if it undergoes
degradation to a specific extent within a given time
148 J Polym Environ (2007) 15:125–150
123
measured by specific standard test methods (ASTM
D-6400.99) [6].
Hydrolytically degradable plastic: a degradable plastic
in which the degradation results from hydrolysis (ASTM
D-2096.03) [6].
Inherently biodegradable: is a classification of chemi-
cals for which there is unequivocal evidence of biodegra-
dation (primary or ultimate) in a standard test of
biodegradability. Requires ‘‘worst possible case’’ esti-
mates of likely environmental concentrations and therefore
further simulation tests may be required (EPA OPPTS
835.3110).
Non-biodegradable: Negligible (as compared to inher-
ently biodegradable) biotic removal of material under
standard test conditions (EPA OPPTS 835.3110)
Oxidatively degradable plastic: a degradable plastic in
which degradation results from oxidation (ASTM D-
2096.03) [6].
Readily biodegradable is an arbitrary classification of
chemicals which have passed certain specified screening
tests for ultimate biodegradability; these tests are so strin-
gent that it is assumed that such compounds will rapidly
and completely biodegrade in aquatic environments under
aerobic conditions (EPA OPPTS 835.3110).
Readily biodegradable: Rapid and complete minerali-
zation (EPA OPPTS 835.3110)
Photodegradable plastic: a degradable plastic in which
degradation results from the action of natural daylight UV
radiation (solar weight lengths). (ASTM D-2096.02) [6].
Partially biodegradable: Blends of non-biodegradable
polymers with biodegradable (usually starch) material.
Biodegradation of these materials is limited to the acces-
sible by the micro-organisms part of the biodegradable
compound [6].
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