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Biodegradable Plastics - Developments and Environmental Impacts

Nolan-ITU Pty Ltd
Prepared in association with ExcelPlas Australia
October, 2002

Standards and Test Methods

Australia at present has no standards, or test methods, that specifically apply to biodegradable plastics. There are however a range of international standards, and test methods, developed specifically for biodegradability, product safety, and also for compost derived products.

The main international organisations that have established standards or testing methods are:

These standards and testing methods are summarised below.

6.1 Biodegradation Standards and Tests

6.1.1 American Society for Testing and Materials

A family of ASTM standards addresses physical property deterioration in a variety of specific environmental conditions including simulated composting (D5509, D5512), simulated landfill (D5525), aerobic microbial activity (D5247) and marine floating conditions (D5437).

A second group of ASTM standards addresses CO2 generation in aerobic environments including sewage sludge (D5209), activated sewage sludge (D5271), and controlled composting (D5338).

A third group of ASTM standards addresses CH4/CO2 evolution in anaerobic environments such as anaerobic sewage sludge (D5210), anaerobic biodegradation (D5511), and accelerated landfill (D5526). D6400 differentiates between biodegradable and degradable plastics, and D5152-addresses environmental fate.

The test methods for the key ASTM standards are summarised below,

ASTM D5338-93 (Composting)

For biodegradable plastic materials to be accepted in composting plants, both biodegradability and disintegration are important. Disintegration is the physical falling apart of the biodegradable plastic material, or more precisely of the product that has been made from it, into fine visually indistinguishable fragments at the end of a typical composting cycle.

A compostable material is understood to be a material in which:

Those materials having a degree of biodegradation equivalent to that of cellulose (maximum permissible tolerance of 5%) are considered to meet the compostability criteria under these standards.

To measure compostability, the ASTM in the United States has developed the key standard ASTM D5338-93. This is a standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions. In this method the plastic is mixed with stabilised and mature compost derived from the organic fraction of municipal solid waste. The net production of CO2 is recorded relative to a control containing only mature compost. After determining the carbon content of the test substance, the percentage biodegradation can be calculated as the percentage of solid carbon of the test substance which has been converted to gaseous carbon in the form of CO2. In addition to carbon conversion, disintegration and weight loss can be evaluated.

To meet the ASTM D5338-93 standard, 60% of single polymer materials must mineralise in six months, and 90% must do so in blends. Materials should give way to intense microbial activity and be converted from carbon to carbon dioxide, biomass and water. Materials also should begin to fragment, at which point disintegration begins. In this phase, the material must completely physically and visually disintegrate. Ninety percent of the disintegrated material must not adversely affect the quality of the compost. Finally, even after land application, remaining materials should be safely converted into carbon dioxide by microorganisms. The resultant compost should not be toxic and should not deter plant growth.

ISO CD 14855 and the CEN test procedures are similar to the ASTM D 5338-92. The only difference is the temperature profile which is continuously at 58C in both ISO and CEN procedures while it follows a temperature profile of 35-58-50-35C in the ASTM test.

ASTM D5209-91 (Aerobic, Sewer Sludge)

The Sturm test (ASTM D5209-91) was developed to measure the biodegradability of both watersoluble and insoluble compounds in an aquatic environment. As with ASTM D5338-93 these tests are based on the measurement of CO2 produced during biodegradation of the plastic test material. The percentage biodegradation is calculated from the CO2 produced by the plastic sample compared to the total theoretical amount. Continuous aeration ensures that there is sufficient oxygen in the bioreactor at all times. The measurement of carbon dioxide evolved during degradation gives direct information on the bioconversion of the carbon backbone of the polymer to metabolic end products.

A 10 mg plastic sample is used in the test. For this reason it gives an artificially high breakdown rate. While the measurement of evolved CO2 is very accurate the rate of biodegradation can have an error of up to 80%.

The Technical Research Centre of Finland (VTT) have developed a 'headspace test', which like the Sturm test, is also based on the measurement of carbon dioxide under aerobic conditions. The VTT test is performed in 'headspace' bottles (volume 125 mL) containing 50 mL of a mineral nutrient medium and sewage sludge. The benefits of the VTT test are its simplicity and the ability to conduct an extensive number of samples simultaneously and thereby perform statistical evaluation. The carbon dioxide evolved during biodegradation is determined from the gas and liquid phases at weekly intervals.

ASTM D5210-92 (Anaerobic, Sewage Sludge)

Anaerobic degradation is biodegradation in the absence of oxygen. A mixed population of microorganisms is needed for complete degradation of the polymer. In the first stage, acidogenic bacteria convert organic substances into lower molecular weight components such as alcohols and short chain fatty acids. Subsequently acetogenic bacteria further degrade these substances to acetone, carbon dioxide and hydrogen. In the final phase, methane and carbon dioxide are the end products of anaerobic degradation.

The standard test method for determining the anaerobic degradation of plastic materials is ASTM D5210-92. This test measures the amount of biogas released during polymer biodigestion by microorganisms. The biodegradation percentage is the ratio of biogas produced by the test sample compared with the theoretical amount produced in the case of complete mineralisation.

ASTM D5511-94 (High-solids Anaerobic Digestion)

ASTM D5511-94 is the standard test method for determining anaerobic biodegradation of plastic materials under high-solids anaerobic digestion conditions. This method determines the inherent biodegradability of plastic in an anaerobic solid waste digester or a sanitary landfill under optimal conditions. In this case the total volume of biogas produced per unit weight of sample is measured. Knowing the carbon content of the test material, the percentage of biodegradation can be calculated as the percentage of solid carbon in the sample which has been converted to gaseous carbon in the form of CH4 and CO2.

ASTM Tests for Specific Disposal Environments

Tests to evaluate the generation and quality of humic substances in landfills, composting or terrestrial or aquatic environments are beginning to appear. Aerobic composting with activated vermiculite provides the opportunity to recover and quantify the polymeric residues, allowing more complete carbon balances as well as assessments of toxic compound generation and humus quality.

The property of disintegration of biodegradable plastics can be measured in a pilot-scale or full scale composting test. The test substance is subjected to a spontaneous composting process for a duration of 12 weeks. At the end, a sieving over 2 mm followed by a precise sorting analysis is executed. It may be that a bioplastic passes the test at a specific thickness but fails the test at a higher thickness. It must also be demonstrated that the bioplastic materials have no negative effect on compost quality. This involves chemical analyses (e.g. heavy metals), required to evaluate conventional compost quality. In addition, ecotoxicity tests are advised, including plant germination tests as well as animal toxicity tests. The purpose of these tests is to make sure that small additives (e.g. from the plastics) do not have an adverse effect on compost quality.

6.1.2 International Standards Research

The performance of biodegradable plastics in composting facilities and under laboratory conditions has been studied by International Standards Research (ISR). ISR has determined that plastics need to meet the following three criteria in order to be compostable:

6.1.3 International Standards Organisation

Three International Standards Organisation (ISO) standards have set the criteria by which European biodegradable plastics are currently assessed. These are:

ISO 14855 is a controlled aerobic composting test, and ISO 14851 and ISO 14852 are biodegradability tests specifically 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 disintegration tests, the tests cannot differentiate between biodegradation and abiotic disintegration, but instead demonstrates that sufficient disintegration of the test materials has been achieved within the specified testing time.

6.1.4 European Committee for Normalisation

The European Committee for Normalisation (CEN) 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 materials based on four main areas:

Importantly, the packaging material that is intended for entering the bio-waste stream must be 'recognisable' 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:

For a material to pass the standard, it must not persist for longer than 6 months under any of the conditions stipulated in the above tests and have a pass level of 90%. In addition, the material must not exceed a heavy metals content above 50% of that for 'normal' compost, as follows:

'OK Compost' Certification and Logo

The 'OK Compost' 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, and is based on the CEN - 13432 standard.

6.2 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 commercial degradable plastics using cell culture testing has been reported by Dang et al. (1997). A number of polyester types were tested including a plasticized cellulose acetate, an aliphatic polyester (Bionolle), polyhydroxybutyrate-co-hydroxyvalerate (BiopolTM), and polycaprolactone (TONETM 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 (Dang et. al; 1997).

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.

Phytotoxicity testing can be conducted on two classes of flowering plants. These are monocots (plants with one seed leaf) and dicots (plants with two seed leafs). Representatives from both of these classes are typically used in toxicity 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.

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 feeds 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) in which earthworms are exposed to soil and compost in varying amounts. Following 14 days of exposure, the number of surviving earthworms is counted and weighed and the percent survival rate is calculated. The earthworms are exposed to several mixture ratios of compost and soil mixtures.

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 degradation products present in liquids pose any problem to surface water bodies. In the test, Daphnia are placed in test solutions for 24 hours. After exposure the number of surviving organisms is counted and the percent mortality is calculated.

6.3 Difference Between Standards for Biodegradation

The main point of differentiation between the various international standards is the percentage of biodegradation required for compliance. This is an important issue that is under discussion at ISO level. The compliance requirements for the key standards are shown in Table 6.1.

Table 6.1 - Standards Compliance Requirements
Standard Biodegradation Requirement
DIN 60% 6 months
ASTM 60% 6 months
CEN 90% Nil
OECD 60% (for chemicals) 28 days

A 60% biodegradation of a chemical (that is, small molecules) will generally represent a strong modification of its original chemical structure. On the other hand, if a copolymer is formed by two monomers statistically distributed, say 60% A and 40% B, where A is biodegradable and B is recalcitrant, a 60% biodegradation may mean a 100% biodegradation of one component but a 0% biodegradation of the other.

6.4 Development of Australian Standards

There is a need for the adoption of Australian Standards for biodegradation which match the potential application areas and disposal environments in Australia. These Standards may be developed with reference to the excisting international standards.

A number of products presently being imported into Australia are purported to be biodegradable plastics, but are simply hydrocarbon polymers such as polyethylene. It is currently difficult to verify whether polymers comply with relevant overseas standards other than by conducting expensive testing. For example, the knives and forks from the Sydney Olympics have been tested in laboratory compost vessels and were found not to breakdown after 6 months. The development of Australian Standards that address local requirements and circumstances may alleviate these problems.