Material of the month: Biodegradable plastics

Materials World magazine
24 Sep 2013

In UK households, 60% of plastic waste comes from packaging and more than 80% of that plastic is used just once and then sent to landfill where it remains, consumers are often told, because plastics do not break down. Recently, however, a new generation of plastics have begun to appear on the market and in shopping bags. 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.

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.

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. So has science found a solution to the Great Pacific Garbage Patch? Not quite.

When biodegradable plastics are buried in landfill, there is a limited supply of oxygen and water so they break down anaerobically, releasing methane. In fact, the EU’s Landfill Directive limits the amount of biodegradable waste that can be disposed of in landfill in an attempt to control greenhouse gas production. It requires member states to reduce the amount of biodegradable municipal waste being sent to landfill to 50% by 2009 and to 35% by 2016, compared with 1995 levels.

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. Bioplastics are an attractive option because they are often compostable and are made from renewable sources. However, if the raw material required for the production of the plastic grows in place of a food crop, the potential impact on a growing global population is undeniable.

Nevertheless, all the plastic types mentioned can be considered more sustainable than conventional plastics. But they don’t hold the key to the world’s problem of plastic waste, in part because of their mentioned drawbacks. So what can we do? The trusty Waste Hierarchy can help us answer this question. It presents a list of waste management options, often in a pyramid, with the least favourable at the bottom.

Short of littering, the worst thing we can do with waste is dispose of it in a landfill. Even if biodegradable plastic waste breaks down in a completely harmless way, the material and energy resources used to produce it are lost. When consumers see the words biodegradable or compostable, they might be put at ease, but the word disposable should put users off.

Next up in the hierarchy, we find energy recovery and recycling, which are often not as sustainable as they sound, especially for plastics, which emit harmful gases when burned and degrade when recycled. In addition, the cost of cleaning and sorting plastic waste – including the removal of biodegradable plastics from the mix – is one factor that is hindering the uptake of plastic recycling. An interesting suggestion is that compostable plastics might be advantageous when it comes to the problem of plastic waste that is contaminated with food. This is because these plastics can be composted together with food waste and materials such as paper packaging, meaning establishments such as cafes can put everything in one bin with an eased conscience. Next up in the Waste Hierarchy, after recycling, is reusing. This can be done easily with items such as plastic water bottles.

The best thing to do with regards to waste is prevent it, or at least minimise it, in the first place. In many cases, if this decision is made then an alternative product will be required and it is important to choose wisely. For example, cotton bags might seem an attractive alternative to conventional plastic carrier bags. However, they require more resources to manufacture and transport so must be used more than 100 times to compensate for this. Incidentally, the future of biodegradable plastic bags – at one time a very popular option – is uncertain, because of the disadvantages mentioned above.

In conclusion, the solution when it comes to disposable and throw-away plastic goods seems to be to drastically reduce the use of these goods, because of the material they are made from and because of the practice of using them only once. Alternative products and habits must be selected carefully. With regards to longer life plastic goods, compostable bioplastics might offer the best possible solution. Biome Bioplastics in Hampshire is one company that is developing bioplastics for the more durable plastics market, in collaboration with the University of Warwick. The joint team is looking at lignin, which is found in plant cells and provides structural support to plants, as a source of chemicals that will impart properties such as toughness to the bioplastics. Lignin is not only renewable but it is a waste product from the pulp and paper industry. In the future we might find this type of plastic under our car bonnets – our electric car bonnets of course.