Organic thermal stabilizers for rigid poly(vinyl chloride) III. Crotonal and cinnamal thiobarbituric acids

摘要

Crotonal thiobarbituric acid (CTBA) and cinnamal thiobarbituric acid (CiTBA) have been investigated as thermal stabilizers for rigid poly (vinyl chloride) (PVC) at 180℃ in air. Their stabilizing efficiency is evaluated 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 potentiometric titration on one hand and the extent of discoloration of the degraded polymer on the other. The results reveal the higher stabilizing efficiency of the investigated materials as compared with industrially used thermal stabilizers such as dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), n-octyl tin mercaptide (n- 0TM), dibutyl tin maleate (DBTM) and barium-cadmium zinc stearate (Ba-Cd-Zn stearate). This is well illustrated by the longer Ts values, and by the lower rates both of dehydrochlorination and discoloration of the polymer during degradation relative to those of the reference stabilizers. A radical mechanism for the stabilizing effect of the investigated materials is suggested. The results indicate the important role played by the extent of conjugation in thiobarbituric acid derivatives on the stabilizing efficiency of the stabilizers. The stabilizing potency of the stabilizer increases with the extent of conjugation. The effect of blending CiTBA with either of the reference stabilizers on the stabilizing efficiency of the thermally degraded rigid PVC has been also investigated. The results reveal that mixing of the stabilizers improve both the Ts values, the rate of dehydrochlorination and the extent of dis- coloration. This improvement attains its maximum when both the investigated and the reference stabilizers are taken in equivalent weight ratios.