Organic thermal stabilizers for rigid poly(vinyl chloride) II. Benzal thiobarbituric acid and some of its derivatives

Benzal thiobarbituric acid (BTBA) and two of its derivatives: para-methoxy benzal thiobarbituric acid (p-MeO-BTBA) and para-chloro benzal thiobarbitur ic acid (p-Cl-BTBA) have been investigated as thermal stabilizers for rigid PVC at 180 degreesC, in air. Their stabilizing efficiency is based on meas uring the rate of dehydrochlorination and the extent of discoloration of th e degraded polymer. The results reveal the higher stabilizing efficiency of the investigated materials as thermal stabilizers for rigid PVC as compare d with the industrially used thermal stabilizers: dibasic lead carbonate (D BLC), dibasic lead stearate (DBLS), n-octyl tin mercaptide (n-OTM), dibulyl tin maleate (DBTM) and barium-cadmium-zinc stearate (Ba-Cd-Zn stearate). T his is well illustrated by the longer induction period (T-s) values and by the lower rate of dehydrochlorination. The stabilization efficiency is affe cted by the nature of the substituent found in the phenyl ring of the BTBA derivatives. Moreover, it was found that these materials lower the extent o f discoloration of the polymer during degradation as compared with the refe rence stabilizers. A radical mechanism for the stabilizing effect of BTBA a nd its derivatives is proposed. The effect of blending p-Cl-BTBA with eithe r of the reference stabilizers on the stabilizing efficiency of the thermal ly degraded rigid PVC has been also investigated. The results reveal that m ixing of the stabilizers improves both the T-s values, the rate of dehydroc hloration and the extent of discoloration. This improvement attains its max imum when both the investigated and the reference stabilizers are taken in equivalent weight ratio. (C) 2000 Elsevier Science Ltd. All rights reserved .