Abstract
Benzal thiobarbituric acid (BTBA) and two of its derivatives:
para-methoxy benzal thiobarbituric acid (
p-MeO-BTBA) and
para-chloro benzal thiobarbituric acid (
p-Cl-BTBA) have been investigated as thermal stabilizers for rigid PVC at 180°C, in air. Their stabilizing efficiency is based on measuring the rate of dehydrochlorination and the extent of discoloration of the degraded polymer. The results reveal the higher stabilizing efficiency of the investigated materials as thermal stabilizers for rigid PVC as compared with the industrially used thermal stabilizers: dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS),
n-octyl tin mercaptide (
n-OTM), dibulyl tin maleate (DBTM) and barium–cadmium–zinc stearate (Ba–Cd–Zn stearate). This is well illustrated by the longer induction period (
T
s) values and by the lower rate of dehydrochlorination. The stabilization efficiency is affected 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 of discoloration of the polymer during degradation as compared with the reference stabilizers. A radical mechanism for the stabilizing effect of BTBA and its derivatives is proposed. The effect of blending
p-Cl-BTBA 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 improves both the
T
s values, the rate of dehydrochloration and the extent of discoloration. This improvement attains its maximum when both the investigated and the reference stabilizers are taken in equivalent weight ratio.