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Biodegradable Plastics - Developments and Environmental Impacts

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

Potential Negative Environmental Impacts

10.1 Pollution of Aquatic Environments

Increased Aquatic BOD

Pollution from high nutrient levels in waterways, determined by high biological oxygen demand (BOD) and chemical oxygen demand (COD), lead to the degradation of aquatic ecosystems and algal blooms. The breakdown of starch-based biodegradable plastic materials can result in increased BOD if the plastics make their way into water ways.

Water Transportable Degradation Products

Plastic degradation by-products, such as dyes, plasticisers or catalyst residues, in landfills or compost can potentially migrate to groundwater and surface water bodies via run-off and leachate. Organisms inhabiting these water bodies could thereby be exposed to degradation products, some of which may be toxic.

Groundwater contamination from pigments, catalyst residues and isocyanate coupling agents may also occur. Liberated plastic additives and polymer degradation products in landfills or compost heaps can potentially migrate to nearby bodies of water through liquids percolating in the ground (leaching) or via rainfall run-off. If current metal-based pigments continue to be used in biodegradable plastics, then the potential for release and accumulation in soil and water is high.

Risk to Marine Species

Plastic pollution in marine environments can result in the death of marine species who ingest it in the belief that it is a jellyfish, squid or other translucent, amorphous organism. In the animals gut, biodegradable plastics will not degrade rapidly and injury to the animal is likely to remain an outcome. Turtles can die of starvation as plastic bags block the alimentary canal.

10.2 Litter

The visual impact of littering is unlikely to decrease with the use of biodegradable plastics since windblown plastic litter and plastic films/bags snagged on branches and bushes will not be exposed to sufficient level of microbes for proper biodegradation to take place. Consequently biodegradation of such litter may take many years. This problem may potentially be combined with the possibility that conspicuous littering by plastic may actually increase due to the belief by consumers that biodegradable plastics will disappear quickly in the environment.

10.3 Compost Toxicity

Composted biodegradable plastics will expose plants, soil dwelling organisms (such as worms) and aquatic organisms to polymer degradation products such as manufacturing residues or additives used in their formulation. Due to the complex nature of polymer breakdown, it is not possible to identify all the compounds present in a mix of degradation products, some of which may be toxic. Hence, the possible toxicity of biopolymer degradation products is assessed using toxicity tests (see Section 6.2).

10.4 Recalcitrant Residues

There is currently little evidence to show that recalcitrant polymer residues in the soil are harmful. Some results suggest that pure polymeric fragments may function like the long-lived components in humus and may provide useful properties as a soil additive.

Grass growing studies using municipal waste derived compost in combination with chopped plastic fibres demonstrated improved growing rate and root structure development to accelerate sod production (Gallagher, 2001). German studies indicate that crushed polystyrene foam improves the aeration of soil (Styropor Technical Information Bulletin, 1994), and is widely used in German orchards, vineyards and potting mixes as a soil conditioner.

However, fragments from partially biodegraded plastics will accumulate in cultivated soils and fragments such as polyethylene (which has a specific gravity less than one) could float and potentially block drains.

Further work is required to understand the fate and consequence of recalcitrant residues in the environment.

Aromatic Compounds

While the aliphatic portion of AAC polymers are biodegradable, the aromatic segments will form small molecules such as terephthalic acid (TPA) whose biodegradation is less certain. In other cases the residual compounds will be bishydroxyethylene terephthalate (BHET). Some new AAC are being made from bishydroxyethylene terephthalate (BHET), capralactone (CL) and tetra-nbutyl titanate (Ti(OBu)4 as a catalyst. Accordingly their breakdown can result in the production of BHET which is the aromatic segment. Both TPA and BHET are also formed from the glycolysis of PET - the difference being PET does not undergo glycolysis or hydrolysis in the natural environment at appreciable rates.

10.5 Additives and Modifiers

Biodegradable polymers are rarely used on their own to make biodegradable plastics. A range of additives and modifiers (e.g. coupling agents, plasticisers, fillers, catalysts, dyes and pigments) are generally added to obtain useful performance properties that approach those of conventional plastics. Once the biodegradable polymer matrix degrades, the additives and modifiers become liberated into the environment. These compounds and their potential negative impacts are outlined below.

Isocyanate Coupling Agents

Starch and PLA do not have readily reactive functional groups, so their mutual compatibility is poor. To overcome this shortcoming a chemical with isocyanate functional groups such as methylene-diisocyanate, (MDI) is reactively blended with the starch and PLA. MDI is recognised as a toxic substance, however further studies are required to assess the problems that could occur if MDI entered the environment.

Plasticisers

Plasticisers are often added to increase biodegradable plastic flexibility. Typical plasticisers used in biodegradable plastics include:

Most of these are organic and readily fully break down in the environment.

Ethylene glycol, however, is a recognised environmental pollutant. Direct exposure to the compound can cause skin and eye damage in humans, with a lethal dose if ingested of 100mL. The lethal concentration for fish has been found to be 100mg/L. (Material Safety Data Sheet - Ethelyne Glycol, 2001).

Fillers

Fillers are often added to biodegradable plastics to reduce cost. Since fillers are generally inorganic, they have the potential to accumulate over time in soil or other disposal environments. Fillers are, however, often inert and of mineral origin, thus posing no toxicity concerns at the levels found in biodegradable plastics (Hohenberger, 2000, p.901). Typical fillers include:

Catalyst Residues

Monomers are generally polymerised in the presence of certain catalytic metals in order to achieve sufficient commercial productivity. Remnants of the catalysts remain in the final polymers. In non-biodegradable plastics these catalyst residues remain encapsulated with the polymer matrix and are not mobile or leachable. In biodegradable plastics these catalyst residues are liberated and can enter the disposal environment. Table 10.1 shows the typical catalytic metals present in biodegradable plastics.

Table 10.1: Typical Metals in Biodegradable Plastics
Metal type Polymer type
Tin PLA, PCL
Antimony Modified PET
Cobalt Modified PET
Chromium PE-blends
Cobalt Manganese Prodegradant Polyethylenes
Titanium Copolyesters

10.6 Prodegradants and Other Additives

Biodegradable plastics based on conventional thermoplastics that contain prodegradant additives (refer to Section 4.3) may cause negative environmental impacts because they:

10.7 Source of Raw Materials

The source materials for biodegradable plastics include both petroleum raw materials and starch produced by agricultural methods. In the Australian context, LCA studies are needed to determine the environmental impacts of biodegradable polymers including their production from raw materials.