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Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.

Biodegradable Plastics - Developments and Environmental Impacts

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

Introduction to Biodegradable Plastics

The 'biodegradability' of plastics is dependent on the chemical structure of the material and on the constitution of the final product, not just on the raw materials used for its production. Therefore, biodegradable plastics can be based on natural or synthetic resins. Natural biodegradable plastics are based primarily on renewable resources (such as starch) and can be either naturally produced or synthesised from renewable resources. Non-renewable synthetic biodegradable plastics are petroleum-based. As any marketable plastic product must meet the performance requirements of its intended function, many natural biodegradable plastics are blended with synthetic polymers to produce plastics which meet these functional requirements.

Many polymers that are claimed to be 'biodegradable' are in fact 'bioerodable', 'hydrobiodegradable' or 'photo-biodegradable'. These different polymer classes all come under the broader category of 'environmentally degradable polymers'. For the purpose of this document the term 'biodegradable plastics' shall imply 'environmentally degradable plastics'. The classes of biodegradable plastics considered, in terms of the degradation mechanism, are:

  1. Biodegradable
  2. Compostable
  3. Hydro-biodegradable
  4. Photo-biodegradable
  5. Bioerodable

These definitions of degradation are used throughout the report to describe the degradation processes of the 'biodegradable plastics' currently available or under development. Definitions of these degradant mechanisms for different materials are provided below.

Biodegradable

The failure of early 'biodegradable' plastics to properly degrade led to the American Society of Testing and Materials (ASTM) creating definitions on what constitutes 'biodegradability'. The ASTM definition, updated in 1994 (ASTM Standard D-5488-84d), has led to the establishment of labeling terminology for packaging materials.

The ASTM defines 'biodegradable' as:

"capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms, that can be measured by standardized tests, in a specified period of time, reflecting available disposal condition."

Biodegradation is degradation caused by biological activity, particularly by enzyme action leading to significant changes in the materials chemical structure. In essence, biodegradable plastics should break down cleanly, in a defined time period, to simple molecules found in the environment such as carbon dioxide and water.

Biodegradation rates are highly dependent on the thickness and geometry of the fabricated articles. While rapid breakdown rates are often quoted these generally apply to thin films. Thick-walled articles such as plates, food trays and cutlery can take up to a year to biologically degrade.

Compostable

Compostable biodegradable plastics must be demonstrated to biodegrade and disintegrate in a compost system during the composting process (typically around 12 weeks at temperatures over 50C). The compost must meet quality criteria such as heavy metal content, ecotoxicity, and no obvious distinguishable residues caused by the breakdown of the polymers. Compostable plastics are a subset of biodegradable plastics.

'Compostable' is defined by the ASTM as:

"capable of undergoing biological decomposition in a compost site as part of an available program, such that the plastic is not visually distinguishable and breaks down to carbon dioxide, water, inorganic compounds, and biomass, at a rate consistent with known compostable materials (e.g. cellulose)."

Hydro-biodegradable and Photo-biodegradable

Hydro-biodegradable and photo-biodegradabe polymers are broken down in a two-step process - an initial hydrolysis or photo-degradation stage, followed by further biodegradation. Single degradation phase 'water-soluble' and 'photodegradable' polymers also exist.

Bio-erodable

Many polymers that are claimed to be 'biodegradable' are in fact 'bioerodable' and degrade without the action of micro-organisms - at least initially. This is also known as abiotic disintegration, and may include processes such as dissolution in water, 'oxidative embrittlement' (heat ageing) or 'photolytic embrittlement' (UV ageing).

Sections 3, 4 and 5 outline the current biodegradable polymer types, technologies and developments. Many blends of these materials, particularly starch and polyester blends, are also considered. Biodegradable plastics that are intended to be 'degradable in vivo' (in the body, i.e. implants), are considered to be beyond the scope of this study.