Na. Mohamed, AROMATIC HYDRAZIDES AS STABILIZERS FOR RIGID PVC AGAINST THERMOOXIDATIVE DEGRADATION, Polymer degradation and stability, 56(3), 1997, pp. 317-329
Aromatic hydrazides and their derivatives have been investigated as th
ermal stabilizers for rigid polyvinyl chloride (PVC) at 180 degrees C
in air by measuring the rate of dehydrochlorination and the extent of
discoloration of the degraded polymer. The results reveal the greater
stabilizing efficiency of the investigated compounds as shown by their
longer induction periods (Ts) and lower dehydrochlorination rates in
relation to n-octyltin mercaptide, dibasic lead carbonate and cadmium-
barium-zinc stearate stabilizers commonly used in industry. The stabil
izing induction periods at the early stages of the degradation process
increase with increasing number of hydrazide linkages as well as with
the introduction of electron donating substituents in the phenyl ring
of the stabilizer molecule. The stabilizer efficiency is attributed t
o the replacement of the labile chlorine on the PVC chains by a relati
vely more thermally stable aromatic hydrazide moiety. An ionic mechani
sm far the stabilizing action of the investigated aromatic hydrazide d
erivatives is offered. Moreover, the investigated stabilizers impart b
etter color stability for the degraded samples as compared with the re
ference stabilizers. This may be attributed to the ability of these co
mpounds to disrupt the formation of conjugated double bonds which are
responsible for discoloration. A synergistic effect is achieved when t
he materials under investigation were blended in various molar ratios
with either cadmium-barium-zinc stearate or dibasic lead carbonate, re
aching its maximum at a 3:1 molar ratio of hydrazide compound to refer
ence stabilizer. This synergism may be due to the ability of the hydra
zide linkages to form stable complexes with the metal chlorides formed
as a reference stabilizer by-product, thus protecting the polymer fro
m their auto catalytic deleterious accelerating degradative effect. (C
) 1997 Elsevier Science Limited.