The Need for Polymers to be Protected Against Oxidation

Most synthetie polymers are fairly stable for long periods provided that they are (a) not heated and (b) kept away from light. But they can be attacked very slowly by oxygen, and the process of oxidation is aceelerated by either heat or hght. The topics of antioxidants and light stabilisers are related and tend to overlap, although in practice the additives are usually classified under different headings. 

Opportunities for oxidation also arise in long-term storage, outdoor weathering, exposure to heat and air, or recycling. Heat, ozone, oxides of nitrogen and sulphur, and certain metal ions accelerate the process. Polyolefins are susceptible, and their usefulness would be severely restricted without protective additives. 

Thin films are oxidised more quickly than thick ones because oxygen diffuses more rapidly into most of the film 

The tendeney of polyolefins to undergo oxidative degradation decreases in the order PP LDPE HDPE. The polyamides, polyurethanes, the saturated polyesters (PET, PBT), certain other engineering thermoplastics and many rubbers are also susceptible to various extents. 

PVC is a special case because it rapidly loses hydrogen chloride on heating, leaving unsaturated carbon-carbon double bond sequences (polyenes) that are sites of instability. As a result PVC always needs a heat stabiliser. 

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