Organic thermal stabilizers for rigid poly(vinyl chloride) V. Benzimidazolylacetonitrile and some of its derivatives

Abstract

2-Benzimidazolylacetonitrile (BAN) and some of its conjugated unsaturated derivatives namely: 2-benzimidazolylbenzylidineacetonitrile (BBeAN), 2-benzimidazolyl-ω-phenylpropenylidineacetonitrile (BPAN) and 2-benzimidazolylbut-2-enylidineacetonitrile (BBuAN) have been investigated as organic thermal stabilizers for rigid PVC at 180 °C in air. Their stabilizing efficiencies are based on measuring the length of the induction period ( T s), the period during which no detectable amounts of HCl gas could be observed, and also from the rate of dehydrochlorination as measured by the continuous potentiometric titration on the one hand, and the extent of discoloration of the degraded polymer samples on the other. The stabilizing efficiencies of the investigated stabilizers were compared with that of dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate) and dibutyltin maleate (DBTM) which are commonly used industrial stabilizers. The results reveal the higher stabilizing efficiency of the investigated materials relative to the reference stabilizers concerning the T s values. However, the rate of dehydrochlorination at the later stages of degradation for PVC stabilized with the investigated stabilizers is found to be relatively higher than that in the presence of reference stabilizers. These experimental findings indicate that almost all the stabilizers actions are exhausted during the induction period. The stabilizing efficiencies of the investigated stabilizers can be attributed to the replacement of the labile chlorine atoms on the polymer chains by a relatively more thermally stable groups derived from the stabilizer. This substitution reaction proceeds most probably through a radical mechanism. Moreover, the results also reveal the effect of the extent of conjugated unsaturates in the organic stabilizer molecule. The stabilizing potency of the investigated materials increases with the extent of their conjugated unsaturation sites. On the other hand, the results also reveal that while the extent of discoloration of thermally degraded PVC in the presence of BPAN stabilizer is comparable with most of the investigated reference stabilizers, the other derivatives showed a relatively higher extent of discoloration.