Organic thermal stabilizers for rigid poly(vinyl chloride) - III. Crotonaland cinnamal thiobarbituric acids

Crotonal thiobarbituric acid (CTBA) and cinnamal thiobarbituric acid (CiTBA ) have been investigated as thermal stabilizers for rigid poly (vinyl chlor ide) (PVC) at 180 degreesC in air. Their stabilizing efficiency is evaluate d by measuring the length of the induction period (T-s), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potent iometric titration on one hand and the extent of discoloration of the degra ded polymer on the other. The results reveal the higher stabilizing efficie ncy of the investigated materials as compared with industrially used therma l stabilizers such as dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), n-octyl tin mercaptide (n-OTM), dibutyl tin maleate (DBTM) and bari um-cadmium-zinc stearate (Ba-Cd-Zn stearate). This is well illustrated by t he longer T-s values, and by the lower rates both of dehydrochlorination an d discoloration of the polymer during degradation relative to those of the reference stabilizers. A radical mechanism for the stabilizing effect of th e investigated materials is suggested. The results indicate the important r ole played by the extent of conjugation in thiobarbituric acid derivatives on the stabilizing efficiency of the stabilizers. The stabilizing potency o f the stabilizer increases with the extent of conjugation. The effect of bl ending CiTBA with either of the reference stabilizers on the stabilizing ef ficiency of the thermally degraded rigid PVC has been also investigated. Th e results reveal that mixing of the stabilizers improve both the T-s values , the rate of dehydrochlorination and the extent of discoloration. This imp rovement attains its maximum when both the investigated and the reference s tabilizers are taken in equivalent weight ratios. (C) 2001 Elsevier Science Ltd. All rights reserved.