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Publications archive - Waste and recycling

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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Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time.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 assocation with ExcelPlas Australia
October, 2002


Plastics Sorting and Reprocessing

The plastics recycling collection, sorting and reprocessing industry is well established in Australia, from pre-consumer industrial scrap right through to post-consumer domestic packaging materials. This situation has led to the development of a viable plastics reprocessing industry in Australia, and a high level of plastics waste export to Asian reprocessing markets. The National Plastics Recycling Survey undertaken by Nolan-ITU for PACIA (2002) showed a total of 164 000 tonnes of plastics recycled in 2001.

Most plastics are sorted and reprocessed as specific polymer types. Over the past decade, a polymer identification code has been added to many products to assist consumers and reprocessors to identify which plastics can be recycled. Advice to consumers on the scope of recycling is often a combination of reference to the polymer identification code and the product or packaging application (eg. beverage bottles marked with a 1, 2 or 3).

8.1 Key Issues

The plastics reprocessing industry and policy makers are concerned about the potential impact of biodegradable plastics on the current mechanical recycling industry and its continued expansion. One of the challenges faced by the plastics recycling sector over the past decade has been to build confidence in the technical integrity of the reprocessed material and to demonstrate its ability to perform as a viable alternative to virgin plastics.

Of greatest concern is the possibility that a proportion of reprocessed plastic will contain biodegradable material. This could result in changing the characteristics of the material (flow rates, strength etc). Most importantly it may lead to a failure of products as degradation occurs. Even a minor increase in failure would be significant in applications such as pipe fittings or liquid packaging. The growing confidence in recycled plastics will be eroded if this technical integrity is comprised.

8.2 Recyclable Plastics Sorting Considerations

Plastic sorting and separation is required to produce quality end products and maximise economic return. Most plastics are mutually incompatible with each other. If incompatible plastics are melt blended (by extrusion) then on cooling the mixture phase separates and the resultant products usually have low mechanical properties since cracks develop through the plane of weakness between the immiscible phases. Plastic enrichment and purification to a level to enable marketability of the recyclate is therefore necessary.

A range of measures are used to sort different plastics. Manual sorting is the most basic form of separating plastic categories. This is then backed up at some plastics reprocessors with sorting technologies based on specific gravity or optical sorting equipment.

Perhaps the best current technologies to sort biodegradable plastics from non-biodegradable plastics is near infra-red (NIR) detection. In recent years a number of such systems have been commercialised including one wholly developed in Australia by Rofin Pty. Ltd. The basis of this plastics sorting system is the use of a near infra-red light beam that impinges on the plastic whilst it is moving rapidly (2 m/s) on a conveyor belt. The reflected light beam is collected and sent to a detector which can then positively identify the plastic type. An air jet then deflects the plastic article into a collection bin. This sorting technology is usually used for positive sorting - that is the plastics of interest are positively removed from the mixed stream and the unwanted plastic types remain on the conveyor and fall off the line into a waste bin that is generally landfilled.

NIR sorting systems are almost all used for rigid plastic packaging (bottles, jars trays, etc.). Since biodegradable plastics have a characteristically different NIR signature to commodity polymers such as PE, PET, PP they will not cross-contaminate the recycling processes for these polymers. In addition, the NIR system can be tuned, for example to positively sort starch-based polymers and in this way a pure biodegradable stream could be segregated for composting. Where cross-contamination is more likely, is in plastic film recycling. Presently plastic film is being collected and recycled (such as commercial pallet wrap, stretch and shrink wrap). Due to the nature of film it does not lend itself well to automated sorting techniques.

8.3 Reprocessing Considerations

Almost all biodegradable plastics are based on polar polymers such as polyesters, polyketones and polyalcohols. In each case, oxygen-containing functional groups are present in the polymer to provide sites for microbial attack. Common conventional packaging plastics on the other hand, such as polyethylene, polypropylene and polystyrene, are non-polar and are thus not compatible with biodegradable plastics.

Another consequence of the polar nature of biodegradable plastics is that they absorb moisture from the atmosphere and can have equilibrium water contents as high as 6% by weight. In thermoplastic starch biodegradable plastics, water is often deliberately added as a plasticiser to induce flexibility. During reprocessing in an extruder, recycled biodegradable plastics will liberate this water as steam and this will cause problems such as bubbles, blisters and broken strands during pelletisation and downstream fabrication. A key issue for consideration in this context is the likely variance in the quantities of biodegradable plastics within the recyclate, which would decrease the ability to produce an end product with consistent characteristics and quality.

Polyolefin Reprocessing

The likelihood of cross-contamination of polyolefins such as polyethylene and polypropylene by biodegradable plastics is high, given the fact that biodegradable plastics for film and flexible packaging applications have been designed to match, as far as possible, the appearance and characteristics of these polymers.

During recycling and extrusion of polyolefins both starch and PCL-based biodegradable plastics will tend to degrade and caramelise causing discoloration of the resultant recyclate. This is due to the limited thermal stability of biodegradable plastics - for example PCL begins to degrade at just 200C. Starch materials begin to caramelise at slightly higher temperatures.

Given that polyethylenes and polypropylenes are reprocessed at temperatures of 220 - 240C respectively, it is clear that melt degradation of biodegradable plastics is an issue. This is exacerbated by the fact that biodegradable plastics usually contain no thermal stabilisers or antioxidants.

Polyethylene Reprocessing

Biodegradable plastics containing prodegradant additives (such as the EPI films) could have a major impact on existing plastic recycling operations. Since the prodegradant-containing plastics are indistinguishable from conventional polyethylene they will almost certainly enter the polyolefin recycling stream. The prodegradant catalysts are so effective at sensitising polyolefins towards oxidative degradation that even low levels of such cross-contamination has the potential to destabilise large volumes of polyethylene recyclate.

Polyethylene recyclate is commonly used in strength critical applications such as builders film, dam liners, garbage bags, etc. In this way biodegradable plastic contamination in the conventional plastic recycling stream can undermine the integrity and mechanical properties of polyethylene recyclate. Since there is a time delay before the onset of accelerated degradation and concomitant loss of mechanical properties, failures would most probably be detected not during fabrication, but rather in the field where the loss potential and consequential damage would be highest.