True stabilization: the behaviour of lead compounds against the thermal decomposition of polyvinyl chloride

Abstract

Investigations which have been conducted within the last two decades into the behaviour of basic lead compounds as stabilizers against the thermal dehydrochlorination of polyvinyl chloride are summarized. It is shown that the results can be explained satisfactorily in terms of a free radical dehydrochlorination mechanism and a regenerative exchange process which interferes with the dehydrochlorination. This may be regarded as “true stabilization” in contrast to “hydrogen chloride scavenging”. In true stabilization it is proposed that aliphatic carboxylate groups (such as the stearate ion) react with reservoirs of basic lead compounds (such as white lead) to form relatively mobile salts (such as stearates of lead). Also, it is proposed that these salts take part in exchange reactions with chlorine atoms released by the PVC during free radical decomposition, to give innocuous chlorides of lead and the corresponding aliphatic carboxylate free radicals. Hence the chlorine atoms are trapped and no longer able to propagate dehydrochlorination of the polymer. Further, it is proposed that the aliphatic carboxylate free radicals can esterify PVC chains at sites where prior attack by chlorine atoms has abstracted methylenic hydrogen atoms leaving unpaired electrons. This step eliminates the stimulus for loss of chlorine atoms from the PVC, thereby also interfering with the free radical propagation mechanism. Subsequently, the pendant aliphatic carboxylate groups dissociate from the polymer chains with neighbouring chloromethylenic hydrogen atoms to form the corresponding acids, and leaving the chlorine atoms adjacent to carbon-carbon double bonds in relatively stable vinyl-type positions. The aliphatic carboxylic acids so formed can react with the basic lead compounds reservoir to regenerate mobile salts so that the stabilizing process is therefore continuous and cyclic. It is demonstrated that ionic and unimolecular mechanisms which normally are put forward to explain the behaviour of primary stabilizers in PVC are not satisfactory because they do not account for all observations made with basic lead stabilizing regimes. Other evidence which favours the free radical decomposition and stabilization mechanisms is cited, and a suggestion is made for further work.