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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Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time.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

Other Degradable Polymers

4.1 Water Soluble Polymers

There are two main water soluble polymer types commercially available - polyvinyl alcohol (PVOH) and ethylene vinyl alcohol (EVOH).

4.1.1 Polyvinyl Alcohol (PVOH)

PVOH is a readily biodegradable, water soluble polymer. Conventional PVOH however, cannot be processed by traditional extrusion technologies since it decomposes close to its melting point of 230C. Even partially hydrolysed grades, which melt at 180-190C, undergo some decomposition during melt processing.

Developments

Until recently PVOH film was prepared by casting films from an aqueous solution. Recently however, new grades of PVOH have been commercialised which incorporate an internal plasticiser that are thereby extrudable and retain their water solubility.

Table 4.1 shows some of the PVOH biodegradable plastics which are commercially available.

Table 4.1 - PVOH Polymers (Commercially Available)
Trade-name Supplier Origin
Vinex Air Products USA
Elvanol DuPont USA
Poval Kuraray Co. Japan
Exceval Kuraray Co. Japan
Hi-Selon Nippon Gohsei Japan

Note: VinexTM is a copolymer of polyvinyl acetate with poly(alkyleneoxy) acrylate.

Applications

PVOH can generally be utilised in a range of film applications.

Degradation Mechanisms and Properties

The degradation of PVOH is influenced by its crystallinity and molecular weight. Testing carried out by Swinburne University have shown that PVOH does not biodegrade, but simply dissolves in water. Literature from PVOH manufacturers such as Kuraray Co. Ltd. indicated that PVOH can be biodegraded by activated sludge treatment. Biodegradation of PVOH in soil is expected be very slow.

4.1.2 Ethylene Vinyl Alcohol (EVOH)

EVOH is another water-soluble synthetic plastic, and is used as an oxygen barrier layer in multilayer film packaging. The high cost of EVOH is a significant barrier to its widespread use in other biodegradable plastics applications.

4.2 Photo-biodegradable Plastics

Photodegradable plastics are thermoplastic synthetic polymers into which have been incorporated light-sensitive chemical additives or copolymers for the purposes of weakening the bonds of the polymer in the presence of ultraviolet radiation. Photodegradable plastics are designed to become weak and brittle when exposed to sunlight for prolonged periods. Photosensitisers used include diketones, ferrocene derivatives (aminoalkyferrocene) and carbonyl-containing species. These plastics degrade in a two-stage process, with UV light initially breaking some bonds leaving more brittle lower molecular weight compounds that can further degrade from physical stresses such as wave action or scarification on rocks.

Developments

Photodegradable products can have a positive impact on plastic litter in both land and marine situations. The effectiveness is dependent on exposure intensity and will vary with factors such as the season, geography, dirt or water cover, and shading. A new approach to making photodegradable plastics involves adding catalytic metal salts or chelates to initiate the breakdown process.

Applications

Photodegradable plastics may be useful in applications where littering is an issue and in those that pose a threat to animal and marine life (i.e. six-pack plastic beverage rings).

Degradation Mechanisms and Properties

In photodegradable systems, biodegradation occurs only after an initial photo-degradation stage. Degradation of the polymer is triggered by UV light, and assisted by the presence of UV sensitisers in the polymer. The polymer is initially converted to low molecular weight material (i.e. waxes), and then converted to carbon dioxide and water by bacterial action.

4.3 Controlled Degradation Additive Masterbatches

Additives that impart controlled degradation behaviour to conventional thermoplastics, as well as to inherently biodegradable plastics, are becoming a popular strategy due to price competition. Such additives are known as prodegradant concentrates, and are generally based on catalytic transition metal compounds such as cobalt stearate or manganese stearate. The additive is typically used at levels of 1-3% and leads to additional costs of between 10-35% over that of polyethylene.

Developments

The principal company that has developed these prodegradant additives is EPI Environmental Technologies (Conroe, TX, USA) and their products are trademarked TDPATM - an acronym for Totally Degradable Plastic Additives. Plastic products manufactured with EPI's TDPA technology progressively degrade to lower and lower molecular weights. They become brittle, disintegrate and are ultimately digested by microorganisms back to the basic elements of carbon dioxide (CO2), water (H2O) and biomass with no harmful residues. TDPAs have been shown not to affect bacteria, fungi or earthworms and they leave no hazardous residues. TDPAs can control the degradation rates of plastics in various degrees, from as short as a few weeks to months or years, at a competitive cost.

The prodegradants developed by EPI are also known as degradable and compostable polymer (DCP) additives. Compostable bags and bin liners that utilise polyethylene modified by DCP additives are claimed to totally degrade within 90 days in commercial composting facilities. Such prodegradant additives are also being marketed by CIBA under the trade name EnvirocareTM.

Prodegradant containing films represent a significant development over earlier biodegradable films based on starch-filled polyethylene. The starch-based bags had significantly inferior physical and mechanical properties compared with polyethylene; the prodegradant containing films on the other hand possess the same mechanical properties as polyethylene. A biodegradable additive marketed under the tradename BioefectTM is used in polyolefins such as polypropylene.

In Australia a number of companies are using such pro-degradant additives. Enviro Covers Australia (Nerang, Qld) is distributing degradable polyethylene landfill covers based on the EPI technology. Valpak (Cromer, NSW) are producing degradable plastic bags called BioBagTM also based on the EPI technology.

AMCOR Flexibles have licensed this technology from EPI which allows them to produce plastic film and bags that will degrade in a dry landfill. The films are made by adding a prodegradant masterbatch to standard polyethylene. The additive, a metal chelate, initiates chain scission processes that cause the polymer chain to be reduced to molecular weights 20% that of the original, at which point bacterial degradation takes over. The additive is used at relatively low levels and contributes little add-on cost. The process begins essentially as soon as the film is extruded.

Such prodegradant containing films possess the same mechanical properties as polyethylene which represents a significant development over earlier starch-filled polyethylene films which had inferior physical and mechanical properties.

Applications

Potential applications include:

Degradation Mechanisms and Properties

The addition of a 'masterbatch' (i.e. concentrate) to a plastic base resin allows for degradation behaviour to be controlled. Polyethylene containing 3% of the additive is claimed to degrade to a 95% weight loss after 4 weeks at 60C. Such environmentally degradable plastic compositions are not strictly biodegradable but rather 'bioerodable'.

DCP-containing polymers do not initially biodegrade, but rather chemically oxidise to lower and lower molecular weights, become brittle and fragment. The fragments are then ingested slowly by microorganisms, ultimately leaving carbon dioxide, water and biomass. As a consequence, DCP-containing plastics do not meet the ASTM D6400-99 standard, due to their degradation through chemical oxidation before the onset of biodegradation and because the mineralisation occurs at a slower rate than is acceptable.

EnvirocareTM additives promote oxidative degradation of plastics such as standard polyethylene. Although the additives promote an oxidation process of the polymer, neither light nor heat is essential, and the additives can work at ambient temperature. When the molecular weight of the plastic drops sufficiently, the plastic is effectively converted to wax and at this point it will naturally biodegrade.