Almost all of the world's plastic is made from petroleum. The plastic is not biodegradable. Even in sunlight, it degrades very slowly; in landfill it may persist for very much longer.
Biodegradability is an advantage in certain situations, and some plastics can be altered chemically so that they have a limited lifetime and spontaneously decompose after a few weeks or months.
TYPES OF BIODEGRADABLE PLASTIC
(1)MODIFIED PETROLEUM-BASED PLASTICS
These are becoming popular in agriculture. Covering the ground with black polythene sheet in spring raises the soil temperature and allows earlier planting. After the harvest, however, large amounts of used plastic has to be disposed of; a univeristy study from Oklahoma says that about 8 hours of labour is needed to remove an acre of plastic mulch, which then has to be incinerated.
If a suitable additive is included at the manufacturing stage, the plastic is rendered biodegradable and will decompose in the field, typically beginning to break up and disappear around the end of July.
These are based on material from plants, such as maize or sugar cane. Bacterial fermentation forms lactic acid, which is then formed into poly-lactic acid (PLA), the structure of which is shown below:
This plastic is digested by bacteria, so does not persist for long in the environment; typically a few months to a couple of years. It is currently more expensive than petroleum-based plastics.
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