Organic thermal stabilizers for rigid poly(vinyl chloride) I. Barbituric and thiobarbituric acids

Barbituric acid (BA) and thiobarbituric acid (TBA) have been investigated a s thermal stabilizers for rigid poly(vinyl chloride) (PVC) at 180 degrees C , in air. The efficiency of these compounds as thermal stabilizers was eval uated by measuring the length of the induction period (T-s), during which n o detectable amount of hydrogen chloride gas could be observed, and also fr om the rate of dehydrochlorination as measured by continuous potentiometric titration on one hand, and the extent of discoloration of the degraded pol ymer on the other. The stabilizing efficiencies were compared with that of dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium-cadmium -zinc stearate (Ba-Cd-Zn stearate), n-octyl tin mercaptide (n-OTM) and dib utyl tin maleate (DBTM). With the exception of DBTM reference stabilizer wh ich shows a comparable stabilizing efficiency with BA and TEA, the investig ated organic stabilizers exhibit a greater efficiency than those of all oth er reference stabilizers. Moreover, they impart better color stability to t he degraded samples. A radical mechanism for the stabilizing action is sugg ested. The stabilizing efficiency is attributed to the replacement of the l abile chlorine atoms on the PVC chains by a relatively more stable moiety o f the stabilizer. This disrupts the formation of conjugated double bonds, w hich are responsible for the discoloration. (C) 2000 Elsevier Science Ltd. All rights reserved.