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.