Na. Mohamed et al., Organic thermal stabilizers for rigid poly(vinyl chloride) I. Barbituric and thiobarbituric acids, POLYM DEGR, 70(1), 2000, pp. 5-10
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.
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