In addition to household waste, there is also the issue of plastic litter (mainly packaging) which is
discarded in the environment. On land it is ugly but in the sea it can be deadly – it can cause
injury and death to birds, fish and other sea creatures. Given all this, it is not surprising that
people are concerned about plastic waste and are increasingly looking to scientists to produce a
degradable plastic to help solve the problem. However, there is much debate among scientists
and environmentalists as to whether degradable or non-degradable plastic is better for the
environment. The conclusions drawn by researchers studying this topic are not always what you
might expect. To understand fully the effect a product has on the environment it is necessary to
carry out a ‘life cycle analysis,’ which means looking at the impacts of production, use and
disposal of the product.
There are two kinds of degradable plastic: photodegradable and biodegradable.
Photodegradable plastic is usually made of oil-based polymers, just like ordinary plastic. It either
has bonds in its structure that can be weakened and broken by sunlight, or it contains a chemical
additive which absorbs light and then attacks the polymer and breaks some of the bonds. Once a
photodegradable plastic is exposed to light it begins to break down – whether you want it to or
not. This can be disastrous if it is mixed in with other plastics during recycling. Photodegradable
plastics tend to break down into small particles of plastic rather than decomposing completely.
The idea is that these small pieces will then biodegrade. Unfortunately, they are often not
biodegradable and so remain in the environment. The effect that a build-up of small pieces of
plastic in the soil might have on the environment has not been investigated. At present, most
plastic waste ends up in a landfill site where it is buried in a dark hole in the ground. Under these
conditions, photodegradation cannot take place.
Biodegradable plastics can be made from oil or from plant-based products. They are attacked by
bacteria, fungi or other micro-organisms which use them as food. The most commonly used
plastics have been tested to find out whether they are biodegradable. Polythene is biodegradable
as long as its chains have a molecular weight of less than 500; most other polymers are not.
Polyesters are susceptible to biodegradation but they are used very little in packaging materials.
As the majority of plastic waste is from packaging, using polyesters will not really help solve either
the problem of the large quantity of waste produced or that of litter in the environment.
There is a lot of interest in the idea of making a plastic that can biodegrade in a landfill site.
Unfortunately, once a landfill is covered with soil, there is limited oxygen and water available (the
conditions are said to be ‘anaerobic’) and the rate of degradation of all materials (even those that
biodegrade rapidly on the surface) becomes extremely slow. Discarded food has been found in
landfill sites several years after it was thrown away.