X‐ray powder diffraction studies of basic lead carbonate as a stabiliser in plasticised polyvinyl chloride

Strips from a polyvinyl chloride (PVC) sheet plasticised with di‐2‐ethyl hexyl phthalate (dioctyl phthalate, DOP), containing hydrocerussite 2PbCO3‐Pb(OH)2 stabiliser and milled with stearic acid lubricant, and which had been heated for various periods up to the Congo‐red test‐time at 180°C under N2, have been examined in reflection by X‐ray powder diffractometry. The penetration of X‐rays into these strips was rather low and so examination was confined to surface layers. Studies of the inorganic crystalline phases accord with results obtained during recent examinations of sections cut normal to the strip lengths (which represented the bulk materials) showing that the hydrocerussite reacted homogeneously throughout the hot samples and was completely used long before stabilisation failed at the Congo‐red test‐time. Surprisingly, crystalline salts of both orthophthalic and stearic acids were found in the surface regions, but not in the bulk. These results are interpreted in terms of a free‐radical chain mechanism for PVC decomposition and true stabilisation involving exchanges (which trapped chlorine free‐radicals) and temporary polymer esterifications (which inhibited release of chlorine free‐radicals from the PVC), with lead carboxylates being regenerated as the active intermediates. The inhomogeneous distributions for the lead salts of organic acids are then attributed to migration of intermediates towards the surfaces, combating potentially faster degradation than in the bulk (caused by oxidation of polymer in the surface regions when the newly milled PVC sheet was cooling to room temperature in air). It is concluded that true stabilisation involving lead compounds benefits PVC in commercial processes by retarding thermal decomposition and preventing the formation of HCl.