Degradable plastic

In recent years there has rightly been a marked increase in concern for the environment. The continual global population explosion together with the increase in purchasing power has led to a vast increase in the amount of pollution and rubbish. Because of their visibility and inability to degrade at a reasonable rate, plastics materials have been particularly criticised. 

For more than 20 years, polymer scientists and plastics technologists have been working to develop plastics materials that would be more acceptable environmentally, and in the third edition of this book, published in 1975, the author devoted a section to photo- and biodegradation of polymers. In spite of such effort, an article in 1992 stated that Degradable plastics are still in the early 

Whereas cellulose films are biodegradable, that is they are readily attacked by bacteria, films and packaging from synthetic polymers are normally attacked at a very low rate. This has led to methods of degrading polymers to a sufficiently low molecular mass (typically about 10000) which are then accessible to biodegradation. 

Several approaches are used, either individually or collectively, to degrade polymers in this way. Of these the most important are  

Photodegradation may involve use of inherently photo-unstable polymers or the use of photodegradant additives. An example of the former are ethylene-carbon monoxide polymers in which absorption of light by the ketone group leads to chain scission. The polymer becomes brittle and forms a powder. Such materials are marketed by Dow and by Du Pont. Other examples are the copolymers of divinyl ketone with ethylene, propylene or styrene marketed by Eco Atlantic. 

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Degradable plastic is a plastic designed to undergo a significant change in its chemical stmcture under specific environmental conditions, resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic and the appHcation in a particular period of time that determines its classification. 

Hydrolytically degradable plastic is a degradable plastic in which the degradation results from hydrolysis. 

ASTM Standards on Environmentally Degradable Plastics, ASTM Pubhcation Code Number (PCN) 03-420093-19, American Society for Testing and Materials, Philadelphia, Pa., 1993. 

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

The effects of UV radiation on degradable plastics are usually confined to the exposed surface layers. Tlie general effect is one of embrittlement. Tensile strength may either increase or decrease, but the elongation upon breaking is always reduced. A loss of impact... 

Degrade plastics (solid waste) by one of four processes biodegradation, photodegradation, chemical degradation, and hydrodegradation. 

These are polymers that break down over time when exposed to environmental conditions, such as light and bacteria. The Chemistry Bulletin on the next page gives more information about degradable plastics. 

Did you know that the first plastics were considered too valuable to be thrown out Today, many plastics are considered disposable. Plastics now take up nearly one third of all landfill space, and society s use of plastics is on the rise. Recycling initiatives are helping to reduce plastic waste. Another solution to this problem may involve the technology of degradable plastics. 

The ability of a degradable plastic to decay depends on the structure of its polymer chain. Biodegradable plastics are often manufactured from natural polymers, such as cornstarch and wheat gluten. Micro-organisms in the soil can break down these natural polymers. Ideally, a biodegradable plastic would break down completely into carbon dioxide, water, and biomass within six months, just like a natural material. 

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