Mechanism of Thiol Based PVC Stabilisers

Abstract

The effect of the thiols 3,5-di-tertiary-butyl-—4-hydroxybenzyl
mercaptan (BHBM), 3,5-di-tertiary—butyl-4-hydroxybenzy1thio-—
glycollate (BHBT), 3,5-di-tertiary—butyl—4-hydroxybenzylthiopropionate
(BHBP), 2-hydroxy—4-ethoxymercaptoacetatobenzophenone
(EBHPT) and 2-hydroxy—4-ethoxymercaptopropionatobenzophenone
(EBHPP) on PVC during processing, thermal ageing and uv irradiation
was examined by measuring unsaturation, colour, peroxides, retained
thiol, gel and carbonyl formation. BHBM, EBHPT and EBHPP were
found to cause adduct formation, destroy peroxides and improve
processing and thermal-oxidative stability. The hindered phenolic
thiols were subject to radical and hydrogen chloride attack leading to
stilbenequinone formation during processing. The latter caused photopro-
oxidation while EBHPT and EBHPP caused photostability but were
destroyed by hydrogen chloride. Dioctyl tin bis(isooctyl thioglycollate)
(DOTG) and dibutyltin bis(4-ethoxy-aceto-mercaptide-2-hydroxybenzo-—
phenone) (UVST) neutralised hydrogen chloride stoichiometrically and
destroyed hydroperoxides and their derived thiols added to developing
mono-unsaturation. Photo-pro-oxidation of DOTG was attributed to
photolysis of the derived sulphenate esters. Dibutyltin maleate
(DBTM) was particularly concentration dependent in its activity with
a high critical concentration and functioned mainly by hydrogen chloride
neutralisation and Diels Alder reaction with conjugated unsaturation in
PVC. The activity of DBTM in PVC was synergistically improved under
processing and thermal oxidative conditions by the addition of an
optimum concentration of BHBM, BHBT or BHBP. Substantial substitution
of DSTM by mixtures of both BHBM and EBHPT and BHBM and UV531 resulted
in optimisation of the overall stability of PVC. The mechanism of the
rapid interaction between peroxides and hydrogen chloride was found to
be fundamental to PVC degradation. Efficient PVC stabilisers
invariably favoured ionic interaction between peroxides and hydrogen
chloride by neutralising the latter, increasing the polarity of the
medium and/or active peroxidolysis.

Date of Award	1980
Original language	English