Scientific background to oxo-biodegradable polymers
A1 Introduction
This annex describes some of the more important observations on oxo-bio-degradable polymers, most
of which have been published in the scientific literature over the past 15 years. These have led to the
understanding that polymeric materials, both natural and synthetic, undergo biodegradation in the
environment by two quite distinct and readily distinguishable mechanisms. Nevertheless, nature uses
both of these mechanisms to return carbon-based materials to the biological cycle.
A2 Natural Polymers
Natural rubber (NR) is a carbon-chain (hydrocarbon} polymer, identical to synthetic cis-polyisoprene (IR) and
closely related to the polyolefins (PO), of which polyethylene (PE) and polypropylene (PP) are the most
important. Unlike cellulose, starch, the polyesters and polyamides, NR does not hydrolyse either biotically or
abiotically yet, in its native state, it biodegrades completely. Nevertheless, NR tyres used in heavy goods
vehicles are among the most environmentally durable of all automotive components. They almost always fail by
abrasion or tread separation and never by biodegradation [1]. This does not mean that the NR chemical
structure is not biodegradable but rather that the antioxidants, antiozonants and antifatigue agents that are
added to rubber to inhibit oxidative degradation [1] also inhibit oxo-biodegradation [2]. Synthetic IR behaves in
exactly the same way as NR on exposure to microbial environments [3,4,20] and there is therefore no significant
difference between the biodegradability of bio-based and fossil-based polymers with the same chemical
structure.
Wood, straw and twigs are composed largely of lignocellulose. The lignin component of these abundant natural
products is a carbon-chain polymer containing aromatic rings, resembling the synthetic phenol-formaldehyde
resins, that is relatively resistant to biodegradation compared with pure cellulose [20]. However lignin, like
rubber, oxo-biodegrades slowly. A great deal of research has been carried out in recent years by the papermaking industry directed toward the selective removal of lignin by microorganisms during the manufacture of
high quality paper. Interestingly some fungi have been identified that remove lignin leaving the cellulose intact
[5-7,20]. Straw is a particularly appropriate model for the biodegradation of synthetic polymers since it causes
no problem when left on the fields or is included in compost in spite of its slow rate of biodegradation. It may
take up to ten years for straw to be fully bioassimilated in biologically active soil [8].
The most important conclusion from these studies is that nature does not depend on just one biodegradation
mechanism. Oxygen is equally as important as a bio-reagent as is water and microorganisms. The following
definition of oxo-biodegradation agreed by CEN TC 249/WG9 [9]. Is as follows.
“Oxo-biodegradation is degradation identified as resulting from oxidation and cell mediated phenomena, either
simultaneously or successively”
This definition embraces the ISO definition [10], which defines the external influences on the polymer as
“chemical, physical and biological interactions”
A3. Synthetic Polymers
High molar mass saturated polymers with physical properties similar to polyethylene are not produced in
biological systems, although many closely related lower molar mass products such as hydrocarbon waxes are
natural products. The polyolefins, like NR degrade to closely analogous low molecular weight biodegradable
products by the same free radical peroxidation mechanism [11,20]. The only difference is the rate at which this
process occurs. The low molecular weight oxidation products of PE mineralise in biometric tests as rapidly as
pure cellulose and much more rapidly than the original hydrocarbons from which they were derived [12,13,14].
Polyolefins are normally protected (stabilised) from environmental peroxidation and biodegradation [2] and
cannot support microbial growth due to the presence of antioxidants [2,17,18,20,22,23]. However, Pandey and
Singe have shown that commercial polyolefins that have been extracted to remove antioxidants, on exposure to
light, lose between 20% and 90% of their mass in compost in five months [16]. Polypropylene (PP), even
without prior peroxidation loses 30% of its mass in this time. Interestingly, PP biodegrades more rapidly than PE
an observation that runs counter to the conventional wisdom that branched-chain hydrocarbons biodegrade
1
much more slowly than straight-chain analogues. This indicates that the initiating mechanism of polyolefin biodegradation is essentially abiotic free radical attack, initiated by transition metal ions in combination with
hydroperoxides [20] and biotically by oxygenase enzymes. It is consistent with experience from extensive
agricultural practice that, if thermo- or photo-labile antioxidants are used to give the polymers durability during
service [9,10] then, following depletion, biodegradation occurs rapidly from the polymer surface in a biotic
environment [17]. This bio-erosion process has been to involve the rapid formation of a biofilm, which rapidly
covers the whole of the surface of the plastic. Loss of mass of the polymer begins in the surface of the polymer
and rapidly progresses into the polymer bulk leading to further fragmentation and increase in area of the
polymer surface [17,18,23].
It is important to recognise that the process of bioerosion depends on the ratio of surface area to mass of the
polymer sample and that this ratio governs the rate of removal of low molar mass products from the polymer.
Therefore, the thicker the polymer artefact, the slower will be the abiotic and biotic processes leading to ultimate
mineralisation. However, fragmentation increases the surface area of the plastic, which to some extent
counterbalances the effect of polymer thickness. It follows then that oxo-biodegradation is most effective in
polymer film products (notably packaging materials) where the effect of thickness is relatively unimportant in
affecting biodegradability.
A good deal is now known about the stabilisation of carbon-chain polymers against the effects of oxygen and
microorganisms in the environment. Peroxidation is the rate-determining step in biodegradation and the kinetics
of peroxidation can be measured from laboratory experiments [13,18]. As already indicated, the key determinant
of the time to oxidation is the effectiveness of the antioxidants used and the rate of diffusion of oxygen and
oxidation products in polymer artefacts. The service life is controllable by balancing the concentrations of
prooxidant and antioxidant. The antioxidant is destroyed during the induction period and the final rate of
biodegradation depends primarily on the effect of the prooxidant on the rate of peroxidation after discard in a
biotic environment. [13,14,18-25,].
In view of the above experimental findings, it is important to recognise that polyolefins that have been made
biodegradable by formulation with prooxidant transition metal ions, when subjected to environmental influences
(e.g temperatures up to 70oC during commercial composting) are no longer inert “wastes”. In practice they
behave very much like natures lignocellulosic wastes (e.g. leaves and twigs) and act as soil conditioners by
providing plant nutrients. There is now very good evidence to show that in biotic environments, thermally aged
oxo-biodegradable polyethylenes are substantially converted to carbon dioxide and cell biomass within two
years [13,14]. The polyolefins are in fact more rapidly mineralised than straw (10 years) [8]. It has also been
shown that polyethylene is biodegraded by the same microorganisms that attack lignin [15,20] and that the latter
produce extra-cellular oxygenase enzymes similar to those that attack rubber. A proportion of “green waste”
(wood, twigs, etc.) is not completely reduced to fine particulate materials during composting and in order to be
acceptable in commercial compost, they are normally screened and returned to the composting process. Any
incompletely fragmented plastic would in practice be similarly “recycled” with the naturally occurring materials.
A4 Composite materials
A4.1 Multi-component devices
A major problem with any of the three major recycling procedures (mechanical, energy and biological) is that
many packaging items are made up of two or more parts that involve the use of different materials [26]. The
best-known example is a PE bottle with a PP cap. When these are mechanically recycled, the two components
have to be segregated either at the collection stage or subsequently, often at considerable additional cost. In the
case of biocycling, however, provided that both components are hydrocarbons and are ultimately biodegradable, there is no need to segregate them before composting. In the example cited, both PE and PP can
be made independently oxo-biodegradable [16]. It is the responsibility of the manufacturer of the packaging to
ensure that all components of multi-component packaging satisfy the Standard for these materials.
A4.2 Blends and copolymers
2
One of the earliest attempts to make synthetic commodity polymers biodegradable was to blend biopolymers
with polyolefins. Starch-filled polyethylene was introduced to the market as “environmentally friendly” packaging
in the 1970s. Griffin subsequently realized that starch in such composites is encapsulated in polyethylene so
that microbial attack is possible only in the surface [20,27]. This ‘solution’ to the problem of packaging waste
resulted in considerable criticism from environmental groups, leading to a report by Greenpeace [28] that
pilloried all degradable synthetic plastics on the grounds that only biopolymers can be biodegradable. In the
1980s, Griffin demonstrated that by applying the prooxidant transition metal ion technology of Scott and coworkers [29] to the PE matrix, starch-filled polyethylene could indeed be made biodegradable. In other words,
by making the polyethylene oxo-biodegradable, the hydro-biodegradable starch was released to biodegrade
normally; a situation very similar to that of lignocellulose. By itself starch played no part in inducing
biodegradation. It is also possible to have two different mechanisms (e.g. hydro- and oxo-biodegradation)
operating in the same polymer molecule. For example, polyesters containing carbonyl groups in the polymer
backbone, photolyse rapidly in sunlight and hydro-biodegrade in compost. The rate-determining process
therefore depends crucially on the test environment (see Section 4) and it is not possible to predict without
simulated compost or soil tests what will be the rate of eventual biodegradation [30].
A5 Time to biodegradation in different environments
EN 13432: 2000 requires that the plastic must be 90% converted to the theoretically obtainable carbon dioxide
in six months. This is entirely consistent with the requirement that plastics that are intended to substantially
biodegrade in aqueous media such as rivers and sewage systems [23,31] should mineralise rapidly. However, it
is not a satisfactory process for converting plastics wastes to compost or when spread in land, since rapid
mineralisation is not a recovery process and does not therefore comply with the EU Waste Framework Directive
of March 1991, which defines ‘recovery’ as:
“Recycling/reclamation of organic substances, use as fuel to generate energy and spreading on land resulting in
benefit to agriculture or ecological improvement including compost and other biological transformation
processes.”
Similar reasoning applies to plastic films that must have a well-defined lifetime when used for agricultural
purposes and which may be exposed to the outdoor environment for some time before disintegrating [20,21,2225]. For example, in the case of agricultural packaging animal feedbags, fertiliser sacks, etc., a predetermined
lifetime is necessary to satisfy the user requirements. The time scale for many applications may be up to one
year before degradation commences and quite clearly EN 13432 is not the correct standard in this case. For the
ecological reasons given in the EU Waste Framework Directive carbonaceous biomass should be retained in
the soil for the benefit of growing plants in the same way as natural lignocellulosic materials are retained [8].
A6 “Heavy metals” and essential trace elements
Concern has recently been expressed by environmentalists about the effect of transition metal compounds
(often in-appropriately called “heavy metals”) present in oxo-biodegradable plastics. Extensive research has
been carried out on potential eco-toxicity effects of particulate and extensively degraded plastics when mixed
with soil. Such effects include seed germination and plant growth rates, compared with the same soil without
degraded plastics [32,33], the effects on macroorganisms (worms, daphnia, etc.) in the soil [33] and on the
accumulation of transition metal ions in the stems, leaves and fruit of plants during the growing season [25,34].
So far with the present range of degradable plastics used in agriculture, which incorporate fractions of a percent
of transition metal ions, have no negative effects in any of the above tests. It is also recognised in horticultural
practice that fragmented plastics films play a positive role as soil conditioners [35].
The commonly used transition metal compounds in commercial oxo-biodegradable plastics are manganese,
iron, cobalt and nickel. None of these have been shown to be toxic and until recently have not been in national
lists of dangerous substances. All the above transition metal ions, which are required in human nutrition, are
absorbed from foodstuffs and water. They are therefore considered to be “essential” minerals required in
oxygen transport systems. The non-toxicities of iron, which is present in haemoglobin, catalase and peroxidases
and of manganese, required for manganese peroxidase, have never been questioned [36].
An Expert Group on Vitamins and Minerals of the UK Food Standards Agency has carried out a risk assessment
[37] on trace elements and the following is a summary of their findings on the risk to humans of cobalt and
nickel.
3
High concentrations of cobalt are found in fish (0.01 mg/kg), nuts (0.09 mg/kg), green leafy vegetables (0.009
mg/kg) and fresh cereals (0.01 mg/kg). Most of the cobalt ingested is inorganic. Fresh water concentrations of
Co range from 0.001 to 0.01 mg/L. The mean population intake of Co is 0.012 mg/day. Cobalt is also included in
some multi-constituent licensed medicines, at a maximum daily dose of 0.25 mg. Although cobalt is an essential
trace element, Co deficiency has not been reported in humans (presumably because of its widespread
availability from food and water). Gastrointestinal absorption of cobalt depends on the dose. Very low doses are
almost completely absorbed, whereas larger doses are less well absorbed. Most excess cobalt is excreted in
urine. The only toxicity data for cobalt reported in the literature was in 1960, when heavy beer drinkers suffered
cardiomyopathy as a result of the use by the brewing industry of cobalt chloride as a “foam stabiliser” at 1.0-1.5
ppm. Ethanol and cobalt have a synergistic effect in reducing blood flow causing damage to the heart. Massive
doses of cobalt salts (30 mg/day), evaluated as a treatment for anaemias led to skin rashes and hot flushes.
Prolonged use of cobalt “therapy” led to depression in iodine uptake.
Nickel is present in a number of enzymes in plants and microorganisms and in humans it influences iron
absorption and metabolism. It is found in a variety of foods as ionic Ni, particularly in pulses and oats (0.18
mg/kg in miscellaneous cereals), and in nuts (1.77 mg/kg). Lower levels are found in water. Total intake of
nickel by humans from all sources is up to 0.26 mg/day and no potential high intake groups have been
identified. The average intake from food and drinking water is 0.16 mg/day. Nickel is excreted in urine and in
sweat. Acute nickel exposure is associated with nausea, vomiting abdominal discomfort and diarrhoea. The
lowest reported oral dose associated with acute effects of nickel in humans was 1.2 mg in a 60 kg adult. Chronic
inhalation of nickel and its compounds is associated with lung cancer in humans and in animals but orally
administered nickel was found not to be carcinogenic. It was the exposure of humans to nickel during mining
that led to the believe that nickel is carcinogenic however it is imbibed but administration of nickel compounds
orally has shown that the main effects in humans is in skin sensitisation but only over 5.6 mg.
From the above, it is now understood how and why the common transition metals are obtained by humans as
essential nutrients. It will also be useful when discussing “dangerous substances” in the environment to see how
they are absorbed into the food chain from the soil. In fact, the amounts of transition metal ions available to
plants from common soils is very much greater than could be produced from degradable plastics in the soil are
much higher than can be absorbed by plants [25]. Particular attention has been paid to cobalt and nickel for the
reasons discussed above. Volcanic soils contain very high concentrations of cobalt oxide (up to 100 ppm) and
nickel oxide (up to 750 ppm). Sandstone and limestone contain 90 ppm and 10-20 ppm of nickel respectively
[25]. However, the amount of nickel taken up by the plant appears to have little to do with its concentration in the
soil. Table 1 shows the effect on plant uptake of nickel sulphate applied to the soil to simulate the deposition of
nickel from degradable polyethylene mulching films up to 180 years of application to the same soil [25]. It is
Table 1. The accumulation of nickel in melons (ppm, measured by atomic absorption) grown in soils containing
increasing amounts of nickel sulphate [25].
Control
60 years*
120 years*
180 years*
Leaves
17.3
15.2
13.5
13.7
Stems
5.0
4.5
5.2
5.0
Flesh
2.7
2.0
3.0
3.2
Skin
3.0
3.5
3.2
3.0
*The soil was sprayed with NiSO4 to give nickel concentrations in the
topsoil equivalent to the accumulation from S-G mulching films used
for the number of years indicated.
clear that the accumulation of nickel in various parts of the plant remains constant within experimental limits,
whatever the concentration of nickel in the soil. Furthermore, It can be calculated that in the ‘worst case
4
scenario’, it would take 500 years to increase the nickel content of soil using typical nickel contents of
degradable polyethylene mulching films by 1 ppm [24].
Résumé
Synthetic hydrocarbon polymers (e.g. polyolefins, polystyrene and synthetic rubbers) biodegrade in the
environment by the same abiotic and biotic processes as naturally occurring polymers (e.g. natural rubber,
resins and lignin). In the case of conventional commercial plastics the rates of formation of oxidation products
depend on the presence of prooxidant transition metal ions (Mn, Fe, Co, Ni) and commercial antioxidants. The
lifetimes and hence biodegradation times of commercial biodegradable polyolefins may vary by orders of
magnitude depending on the application. The time-scale from the end of the user life to final conversion in the
environment to carbon dioxide, water and biomass lies within the range of many natural product wastes such as
straw and related lignocellulosic materials. The oxidation products of both natural and synthetic hydrocarbon
polymers biodegrade rapidly and are absorbed by microbial cells. Consequently, abiotic or biotic oxidation is
normally rate controlling and there is no accumulation of low molar mass products in the environment.
The rates of abiotic peroxidation of carbon-chain polymers in the environment can be predicted from laboratory
tests and it has been shown that the biodegradation rates of the oxidation products correlate with mass loss of
the polymer. The purpose of eco-toxicological tests is to ensure that neither the fragmented polymers nor their
oxidative breakdown products have an adverse effect on plants or to humans and animals that may imbibe the
crops.
It is clear from published work that the four popular prooxidant transition metal ions (Mn, Fe, Co and Ni) are no
more toxic in the environment than the abundant naturally occurring minerals and that both are the source of
essential elements for human nutrition. However, it is possible that other toxic metal ions could be used in the
future and it is therefore necessary for the manufacturer of such products to consult EU Directive 67/548/EEC to
ensure that his product does not contain named “dangerous substances”.
References
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2. G.Scott in Degradability, Renewability and Recycling, 5th International Scientific Workshop on biodegradable
Plastics and Polymers, Macromolecular Symposia 144, Eds., A-C. Albertsson, E. Chiellini, J. Feijen, G.
Scott and M.Vert, Wiley-VCH, 1999,113-125.
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8. L. Janssen in The use of isotopes in soil organic matter studies, Report of the FEO/IAEA Technical Meeting,
Sept, Pergamon Press (1963).
9. CEN TC 249/WG9 N120 Plastics – Guide for vocabulary in the field of degradable and biodegradable
polymers and plastic items (ISO/PDTR: 2004)
10. ISO TC I-94.
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Polym. Deg. Stab., 1994, 46, 211-224.
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(2003)
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Chapman & Hall (Kluwer), 1995, Chapters 1,9.
20 G.Scott in Degradable Polymers: Principles and Applications, 2nd Edition, ed. G.Scott, Kluwer, 2002,
Chapter 3.
5
21 G.Scott and D.M.Wiles in Degradable Polymers: Principles and Applications, 2nd Edition, ed. G.Scott,
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6
Method of measuring mineralisation rate of oxo-biodegradable polymers
This annex describes the preferred biometer for the measurement of carbon dioxide evolution from
particulate oxo-biodegradable plastics.
1. Principles
Since the presence of atmospheric oxygen is a prime requirement during oxo-biodegradation, it is essential that
there is a continuous access of air. This is achieved by providing a continuous flow of CO 2-free air evenly
through the compost. A biometer developed by the Swedish National and Research Institute and used in
published studies if degradable polyethylene mineralisation (I.Jakubowicz, Polym. Deg. Stab., 80, 39-43 (2003))
is shown in the Figure. This ideally satisfies this requirement for an adequate air supply and is relatively simple
and cheap.
Figure A simple biometer for monitoring the production of CO2 from compost (I. Jacubowicz, reproduced with
permission)
The salient features of this biometer are
(1) The perforated porcelain plate on which the compost rests, which allows an even distribution of inlet air
(2) The presence of vermiculite as a diluent for compost or soil, which increases the sensitivity of CO 2
measurement by reducing the amount formed from the mature compost.
(3) Removal of CO2 in the inlet air and in the exit gases may be achieved by passing through NaOH solution
7