REEVALUATION OF THE STABILIZATION MECHANISMS OF VARIOUS LIGHT STABILIZER CLASSES

The mechanisms of stabilization of polymers by UV absorbers, Ni stabilizers and hydroxybenzoates are re-examined. This is done in the light of recent findings concerning hindered amine light stabilizers (HALS) stabilization mechanisms as well as photooxidation of polyethylene (PE) and polypropylene (PP). In polyethylene, photoinitiation is thought to originate from excited polymer-oxygen complexes. In polypropylene, after a short initial period, during which such complexes may also be important, photoinitiation is caused mainly by photolysis of tertiary hydroperoxides. Furthermore, in polypropylene, oxidation chains are usually rather long whereas, in polyethylene, oxidation chains are very short or even non-existent. UV absorption by various UV absorber classes and a Ni stabilizer is demonstrated for polypropylene and polyethylene. It is also shown that typical light stabilizers such as UV absorbers, Ni stabilizers, hydroxybenzoates and HALS can protect polyethylene and polypropylene from thermal oxidation in circulating air ovens. However, it is also found that only part of these light stabilizers is efficiently inhibiting formation of trans-vinylene groups thought to be formed in the initiation reaction in polyethylene. The same stabilizers that are efficient in reducing this initiation reaction show good performance in protecting polyethylene from photooxidation. Light stabilizers protecting mainly or exclusively by free radical scavenging are efficient with PP only. Hydroxybenzoates and some Ni stabilizers fall into this category. The work on stabilization mechanisms is illustrated with data from practical stabilization.