Biodegradable Plastics

When the "solid waste crisis" hit the United States in the 1980s, plastics were often attacked as particular problems because they are nonbiodegradable. There was a perception that biodegradation resulted in recovery of valuable landfill space. Some states even passed laws requiring that certain types of plastics (usually merchandise bags) be degradable.

Degradability therefore became a marketing asset, and a number of manufacturers sold merchandise and grocery bags that claimed to be degradable. Some of these structures were photodegradable, rather than biodegradable, capitalizing on the lack of public understanding of the difference between these characteristics. Others were made from a blend of starch and low-density polyethylene, sometimes with photo-oxidants added. The manufacturers claimed these starch-plastic blends were biodegradable. They claimed that microorganisms, after removing the roughly 6% starch contained in the blend, would be able to attack and consume the LDPE that remained since it would now have greatly increased surface area. However, no convincing evidence in support was produced, and some independent researchers found no evidence of significant biodegradation of the LDPE component.

However, within a few years, evidence mounted that biodegradation is very slow in a well-constructed landfill, designed to protect against groundwater pollution due to generation of contaminated leachate. The combination of lack of oxygen and relatively dry conditions leads to very slow microbiological activity. Even such highly biodegradable substances as food have been found to be still recognizable after 10 years or more in a landfill. Some state attorneys general took legal action against manufacturers of photodegradable trash bags, accusing them of misleading consumers. Several environmental groups called for boycotts of degradable plastics on the same grounds. Degradability was, by many, regarded as a scam.

Since that time, there have been two important developments. First, new plastics have been developed that are truly biodegradable. Second, the use of composting for waste disposal has increased dramatically. Composting is designed to increase microbiological activity and the rate of biodegradation. In most composting operations, the process is managed to assure ample supplies of oxygen and moisture, and temperature may also be maintained at optimum levels. There are also some anaerobic composting operations designed to yield methane as a product. The output from composting is humus, which can be used as mulch or applied to fields. The presence of nonbiodegradable plastics (or other non-biodegradable materials) is undesirable and can render the compost unusable if contamination levels are too high. Therefore, for waste streams that will be composted, use of biodegradable plastics is an asset. Many municipal composting programs accept yard waste only. However, composting of source-separated residential organics is growing, as is composting of food service waste streams of various types, including restaurant waste. For example, the Brookfield Zoo in the Chicago area recently began composting the wastes generated from its food service operations. The zoo has begun using only biodegradable materials for disposable plates, cups, and utensils in its food service operations to facilitate composting. San Francisco is moving toward citywide collection of source-separated residential organics, which will be composted. In some parts of Europe, there is a long tradition of composting, but it is still quite rare in the United States, except for yard waste.

One of the first high-profile biodegradable plastics to be used in packaging was polyhydroxybutyrate-valerate (PHBV), a natural polyester copolymer produced by certain types of bacteria. Imperial Chemical Industries, Ltd. (ICI) developed a process for controlling the relative amounts of butyrate and valerate units in the polymer by modifying the diet fed to the bacteria and then harvesting the polymer, and sold it under the name Biopol. Properties of the plastic are generally similar to polypropylene. PHBV was used for a relatively short time in Europe by Wella for shampoo bottles. However, PHBV was much more expensive than the synthetic plastics it was competing with; its price ranged around $20/kg. ICI spun off the Biopol business, and it was eventually bought by Monsanto, which withdrew it from the market in 1998. In May, 2001, Metabolix purchased

Biopol and licensed the production technology from Monsanto, with the intent to launch commercial products in late 2001. Other researchers and companies have developed similar bacteria-based polyesters, and there have also been successful efforts to transfer the plastics manufacturing genes to plants. However, so far none of these processes have achieved commercial success.

The starch-polyethylene blends that claimed to be biodegradable have been succeeded by materials containing nearly 100% starch that are truly biodegradable. These have had limited success, primarily in foam form as cushioning materials and in film form for compost bags. Water is used as a plasticizer to make the starch thermoplastic so it can be melt processed. In some cases, relatively small amounts of other biodegradable substances, including plastics, are blended with the starch to improve properties and processability. Many of these materials are water soluble in addition to being biodegradable. The foam cushioning material manufacturers point to the water solubility and lack of static cling as highly desirable in preventing litter problems associated with loosefill polystyrene cushioning.

One of the most promising of the new generation of biodegradable plastics is based on polylactic acid (PLA). Cargill Dow, a joint venture of Dow Chemical and Cargill, has begun selling these materials under the name NatureWorks PLA, with applications targeted at films, thermoformed containers, coating for paper and paperboard, and bottles. Full-scale production is scheduled to start in 2002. In addition to waste disposal benefits, Cargill Dow claims PLA uses 20-50% less fossil fuel resources than conventional plastics and emits less net carbon dioxide. In addition to Cargill Dow, a few other companies manufacture PLA in relatively small amounts, many targeting the high-value medical market rather than packaging.

Several companies are manufacturing synthetic biodegradable polyesters. DuPont's Biomax can be used in film or containers. Showa Highpolymer makes Bionolle for film and containers. Several other companies make similar materials. Prices are generally higher than for competitive nonbiodegradable plastics.

Polyvinyl alcohol (PVOH) is a water-soluble biodegradable plastic that has been available for many years. Its high cost and water solubility limit its suitability for packaging, but it does have important niche markets. For example, it is used to package some agricultural chemicals, enabling the still-packaged product to be placed in a mixing tank, thus avoiding human exposure to hazardous chemicals. Modified grades of PVOH have limited solubility.

In the waste disposal area, the environmental benefits of biodegradable plastics are limited to waste streams that will be composted, items that are associated with litter problems, and items that are apt to get into sewage treatment systems. If wastes will be disposed by landfill or incineration, biodegradability offers no real advantage.

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