Biodegradable plastics can be plant- or oil-based. The plant-based variety are known as bioplastics and are derived from raw materials such as corn and potato starch, so manufacturers claim they are sustainable as well as biodegradable. Oil-based plastics are typically derived from non-renewable sources such as crude oil, and are processed using energy-intensive and environmentally hazardous techniques. Degradable plastics break down relatively quickly under specific environmental conditions – photodegradable plastics degrade when exposed to light and biodegradable plastics can be decomposed by bacteria or other living organisms.
Plastic materials are made up of long chains of molecules and the molecular weight of a plastic gives an indication of the length of its chains. Plastic is a potential food source for microbes because it is organic (contains carbon atoms) and the shorter the chains, the more easily digestible the plastic is. For example, polythene is biodegradable as long as it has a molecular weight of less than 500. In some cases, additives are used to enhance the biodegradation of a plastic, and some types work by breaking up the plastic’s chains. Additives can be introduced in different amounts so that biodegradation begins after the required shelf life and at a controlled rate.
When biodegradable plastics are buried in landfill, there is a limited supply of oxygen and water so they break down anaerobically, releasing methane. Oxo-biodegradable plastics seem to offer an advantage in this regard because they break down without releasing methane. However, as their name suggests, their requirement for oxygen to enable the degradation process to occur means it will not break down if buried. The degradation consists of two steps – first an oxidisation process takes place under the action of heat or light, which reduces the molecular weight of the plastic. Then microbes break down the plastic further. A plastic material, such as polythene, can be made oxo-biodegradable by the addition of salts of transition elements such as cobalt or iron, which are referred to as the pro-oxidants.
Two problems with the breakdown of biodegradable plastics are that the process can take a long time and the remaining solid products, while existing in very small and often invisible fragments, are sometimes toxic. This is where compostable plastics (sometimes confused with biodegradable plastics) differ, because for plastics to be termed compostable they must break down in a timely manner and leave no toxic residue. The resulting compost supports plant growth, but it can contain inorganic materials, so differs from garden compost. The time compostable plastics take to break down must be similar to that taken by other compostable materials, such as plant waste, but the process normally requires an industrial composting facility due to the need for much higher temperatures than those in a domestic composter.