Mechanism of poly(vinyl chloride) stabilisation

Poly(vinyl chloride) (PVC) thermal degradation is the result of a 'zipper dehydrochlorination' process that involves allylic chlorides as intermediates and is catalysed by hydrochloric acid. The degradation process can be stopped by PVC stabilisers, which either react with allylic chlorine atoms (primary stabilisers) or scavenge hydrochloric acid (secondary stabilisers). Scavenging HCl cannot stop the degradation process completely as it is diffusion controlled. However, HCl scavenging considerably reduces the rate of degradation and avoids the very fast process that eventually causes PVC blackening (catastrophic degradation). Stabilisers that scavenge HCl improve long term stability but do not give very good early colour. Compounds that substitute the allylic chlorine atoms can stop the degradation in its early stages and also maintain good early colour. They must be able to associate with PVC at the surface of primary particles where the mobility of PVC chains allows dehydrochlorination to occur. The propagation step in chain dehydrochlorination is a very fast process; it can be stopped by nucleophilic substitution only if the stabiliser is already associated with the chlorine atom that becomes allylic when HCl is eliminated. Different types of stabilisers are discussed concerning their mechanism of PVC stabilisation based on the authors' own research and also literature data. PRC/1553 (C) 1999 IoM Communications Ltd.