Biodegradable plastics are plastics that are decomposed by the action of living organisms, usually bacteria.
Two basic classes of biodegradable plastics exist: Bioplastics, whose components are derived from renewable raw materials, and plastics made from petrochemicals containing biodegradable additives which enhance biodegradation.
Under proper conditions, some biodegradable plastics can degrade to the point where microorganisms can completely metabolise them to carbon dioxide (and water). For example, starch-based bioplastics produced from sustainable farming methods could be almost carbon neutral.
There are allegations that Biodegradable plastic bags may release metals, and may require a great deal of time to degrade in certain circumstances and that OBD plastics may produce tiny fragments of plastic that do not continue to degrade at any appreciable rate regardless of the environment. The response of the Oxo-biodegradable Plastics Association (www.biodeg.org) is that OBD plastics do not contain metals. They contain salts of metals, which are not prohibited by legislation and are in fact necessary as trace-elements in the human diet. Oxo-biodegradation of polymer material has been studied in depth at the Technical Research Institute of Sweden and the Swedish University of Agricultural Sciences. A peer-reviewed report of the work shows 91% biodegradation in a soil environment within 24 months, when tested in accordance with ISO 17556.
There is much debate about the total carbon, fossil fuel and water usage in manufacturing bioplastics from natural materials and whether they are a negative impact to human food supply.
To make 1 kg (2.2 lb) of polylactic acid, the most common commercially available compostable plastic, 2.65 kg (5.8 lb) of corn is required. Since 270 million tonnes of plastic are made every year, replacing conventional plastic with corn-derived polylactic acid would remove 715.5 million tonnes from the world’s food supply, at a time when global warming is reducing tropical farm productivity. “Although U.S. corn is a highly productive crop, with typical yields between 140 and 160 bushels per acre, the resulting delivery of food by the corn system is far lower. Today’s corn crop is mainly used for biofuels (roughly 40 percent of U.S. corn is used for ethanol) and as animal feed (roughly 36 percent of U.S. corn, plus distillers grains left over from ethanol production, is fed to cattle, pigs and chickens). Much of the rest is exported. Only a tiny fraction of the national corn crop is directly used for food for Americans, much of that for high fructose corn syrup.”