Several aromatic 1,3,4-oxadiazole derivatives have been investigated a
s thermal stabilizers for rigid poly(vinyl chloride) (PVC) at 180 degr
ees C in air. Their stabilizing efficiencies are evaluated by measurin
g the rate of dehydrochlorination and the extent of discoloration of t
he degraded polymer. The results obtained show the greater stabilizing
efficiency of the investigated compounds as compared with dibasic lea
d carbonate (DBLC), barium-cadmium-zinc stearate (Ba-Cd-Zn stearate) a
nd n-octyltin mercaptide (n-OTM), which are commonly used industrial s
tabilizers. Their higher efficiencies are indicated not only by their
longer induction periods during which no detectable amounts of hydroge
n chloride gas are liberated, but also by their lower dehydrochlorinat
ion rates during the subsequent stages of degradation. The stabilizing
efficiency of these compounds is influenced by the number of the 1,3,
4-oxadiazole rings as well as by the introduction of electron donating
substituents in the phenyl ring of the stabilizer molecule. This can
be attributed to the replacement of the labile chlorine atoms on the p
olymer chains by more stable groups derived from the stabilizer. This
substitution reaction proceeds most probably through an ionic mechanis
m. Moreover, stabilization with the investigated compounds results in
an improvement in the extent of discoloration of the degraded PVC rela
tive to the reference stabilizers. This may be attributed to the abili
ty of these compounds to disrupt the formation of the conjugated doubl
e bonds which are responsible for discoloration. Combination of the in
vestigated compounds with industrial stabilizers greatly improves the
stabilizing efficiency which reaches its maximum at a molar ratio of o
xadiazole stabilizer/reference stabilizer of 3:1. This synergism occur
s due to the interaction between the residual investigated stabilizers
with the reference metallic stabilizers' by-products, metal chlorides
, thus protecting the polymer from the deleterious effect of these chl
orides that accumulate when the metallic stabilizers are used alone. (
C) 1998 Elsevier Science Ltd. All rights reserved.