Aromatic hydrazides as stabilizers for rigid PVC against thermo-oxidative degradation

Abstract

Aromatic hydrazides and their derivatives have been investigated as thermal stabilizers for rigid polyvinyl chloride (PVC) at 180 °C in air by measuring the rate of dehydrochlorination and the extent of discoloration of the degraded polymer. The results reveal the greater stabilizing efficiency of the investigated compounds as shown by their longer induction periods ( Ts) and lower dehydrochlorination rates in relation to n-octyltin mercaptide, dibasic lead carbonate and cadmium-barium-zinc stearate stabilizers commonly used in industry. The stabilizing induction periods at the early stages of the degradation process increase with increasing number of hydrazide linkages as well as with the introduction of electron donating substituents in the phenyl ring of the stabilizer molecule. The stabilizer efficiency is attributed to the replacement of the labile chlorine on the PVC chains by a relatively more thermally stable aromatic hydrazide moiety. An ionic mechanism for the stabilizing action of the investigated aromatic hydrazide derivatives is offered. Moreover, the investigated stabilizers impart better color stability for the degraded samples as compared with the reference stabilizers. This may be attributed to the ability of these compounds to disrupt the formation of conjugated double bonds which are responsible for discoloration. A synergistic effect is achieved when the materials under investigation were blended in various molar ratios with either cadmium-barium-zinc stearate or dibasic lead carbonate, reaching its maximum at a 3:1 molar ratio of hydrazide compound to reference stabilizer. This synergism may be due to the ability of the hydrazide linkages to form stable complexes with the metal chlorides formed as a reference stabilizer by-product, thus protecting the polymer from their auto catalytic deleterious accelerating degradative effect.