THE MODE OF ACTION OF METAL STEARATE STABILIZERS IN POLY(VINYL CHLORIDE) .3. INFLUENCE OF PREHEATING ON POLYENE FORMATION AND SECONDARY REACTIONS

Spectroscopic evaluations of thermally degraded samples of PVC stabili sed with CaSt(2) and ZnSt(2) were carried out in order to explain the anomalous behaviour of the metal stearates when pre-heated. At pre-hea ting temperatures of 100, 130, 150 and 180 degrees C, samples at 5, 10 , 15 and 20 min of degradation were analysed by visible reflectance sp ectroscopy. Samples preheated at 180 degrees C were used to determine polyene formation by UV spectroscopy. Visible reflectance traces showe d the development of the long conjugated double bond sequences, the ap pearance of pale coloured compounds and dark colouration of the polyme r throughout the degradation time. UV absorptions were used to analyse the formation of short, long and total polyenes, as well as the ratio of the short/long polyenes. The trends observed here consolidate our previous findings and suggest that the preheated metal soaps direct th e stabilisation mechanism through different routes.