| Résumé | Key degradative processes for various polymers are itemised, together with photostabilisation mechanisms which can minimise these processes. Polymers can be conveniently classified as inherent absorbers, which absorb large amounts of solar uv, and as ‘non-absorbers’, that is polymers transparent in the near uv. The former group which includes aramids, polycarbonates and polyesters is predominantly photodegraded by primary processes causing direct bond scission and stabilisation is best effected by the use of uv absorbing additives or opaque pigments. The ‘non-absorbers’ include polyolefins and poly(vinylchloride) which are degraded as the result of oxidative chain processes initiated by chromophoric impurities. For polypropylene the dominant photooxidation product is hydroperoxide which photocleaves to initiate further oxidation. Various uv. stabilisers for polyolefins appear to owe their effectiveness to their ability to decompose hydroperoxide groups and/or to their ability to scavenge free radical intermediates in the oxidative process. |
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