Aromatic 1,3,4-oxadiazoles as thermal stabilizers for rigid poly(vinyl chloride)

Several aromatic 1,3,4-oxadiazole derivatives have been investigated as thermal stabilizers for rigid poly(vinyl chloride) (PVC) at 180 degrees C in air. Their stabilizing efficiencies are evaluated by measuring the rate of dehydrochlorination and the extent of discoloration of the degraded polymer. The results obtained show the greater stabilizing efficiency of the investigated compounds as compared with dibasic lead carbonate (DBLC), barium-cadmium-zinc stearate (Ba-Cd-Zn stearate) and n-octyltin mercaptide (n-OTM), which are commonly used industrial stabilizers. Their higher efficiencies are indicated not only by their longer induction periods during which no detectable amounts of hydrogen chloride gas are liberated, but also by their lower dehydrochlorination 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 polymer chains by more stable groups derived from the stabilizer. This substitution reaction proceeds most probably through an ionic mechanism. Moreover, stabilization with the investigated compounds results in an improvement in the extent of discoloration of the degraded PVC relative to the reference stabilizers. This may be attributed to the ability of these compounds to disrupt the formation of the conjugated double bonds which are responsible for discoloration. Combination of the investigated compounds with industrial stabilizers greatly improves the stabilizing efficiency which reaches its maximum at a molar ratio of oxadiazole stabilizer/reference stabilizer of 3:1. This synergism occurs 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 chlorides that accumulate when the metallic stabilizers are used alone. (C) 1998 Elsevier Science Ltd. All rights reserved.