AROMATIC 1,3,4-OXADIAZOLES AS THERMAL STABILIZERS FOR RIGID POLY(VINYL CHLORIDE)

Several aromatic 1,3,4-oxadiazole derivatives have been investigated a s thermal stabilizers for rigid poly(vinyl chloride) (PVC) at 180 degr ees C in air. Their stabilizing efficiencies are evaluated by measurin g the rate of dehydrochlorination and the extent of discoloration of t he degraded polymer. The results obtained show the greater stabilizing efficiency of the investigated compounds as compared with dibasic lea d carbonate (DBLC), barium-cadmium-zinc stearate (Ba-Cd-Zn stearate) a nd n-octyltin mercaptide (n-OTM), which are commonly used industrial s tabilizers. Their higher efficiencies are indicated not only by their longer induction periods during which no detectable amounts of hydroge n chloride gas are liberated, but also by their lower dehydrochlorinat ion rates during the subsequent stages of degradation. The stabilizing efficiency of these compounds is influenced by the number of the 1,3, 4-oxadiazole rings as well as by the introduction of electron donating substituents in the phenyl ring of the stabilizer molecule. This can be attributed to the replacement of the labile chlorine atoms on the p olymer chains by more stable groups derived from the stabilizer. This substitution reaction proceeds most probably through an ionic mechanis m. Moreover, stabilization with the investigated compounds results in an improvement in the extent of discoloration of the degraded PVC rela tive to the reference stabilizers. This may be attributed to the abili ty of these compounds to disrupt the formation of the conjugated doubl e bonds which are responsible for discoloration. Combination of the in vestigated compounds with industrial stabilizers greatly improves the stabilizing efficiency which reaches its maximum at a molar ratio of o xadiazole stabilizer/reference stabilizer of 3:1. This synergism occur s due to the interaction between the residual investigated stabilizers with the reference metallic stabilizers' by-products, metal chlorides , thus protecting the polymer from the deleterious effect of these chl orides that accumulate when the metallic stabilizers are used alone. ( C) 1998 Elsevier Science Ltd. All rights reserved.