X‐Ray Powder Diffraction Studies of Hydrocerussite (Basic Lead Carbonate) as a Stabiliser in Plasticised Polyvinyl Chloride

Abstract

AbstractStrips from a plasticised PVC blend containing pure hydrocerussite, 2PbCO3.Pb(OH)2, as stabiliser were heated under nitrogen at 180°C for various periods up to the time when hydrogen chloride (HC1) was liberated and could be detected externally by Congo‐red indicator paper. Evolved carbon dioxide (CO2) was measured and crystalline phases in the heated samples were studied by X‐ray powder diffraction (X.R.P.D.).Qualitative X.R.P.D. showed that the hydrocerussite reacted to form the uncharacterised basic lead chloride X (B.L.C.X.) and normal lead chloride, PbCl2, in sequence. Quantitative X.R.P.D. showed that the hydrocerussite reacted according to first order kinetics, and the rate constant and half‐life were confirmed by data from the CO2 measurements. These results accord with previous studies using the other basic lead carbonate ‘plumbonacrite’, 6PbCO3.3Pb(OH)2.PbO, as stabiliser. However, the rates of stabiliser reaction differed and, surprisingly, efficient stabilisation continued long after the hydrocerussite had all reacted in the present experiments, whereas stabilisation failed soon after the plumbonacrite had all reacted previously.These findings are interpreted on the basis of a free‐radical exchange and temporary esterification mechanism in which mobile lead salts of organic acids are regenerated as the active stabilising intermediates, and which is believed to retard the PVC thermal decomposition (preventing the formation of HCl). This is called ‘true stabilisation’. The popular idea that basic lead stabilisers merely behave as scavengers for HCl from the hot polymer is rejected because this cannot be reconciled with the fact that stabilisation continued in the absence of hydrocerussite, or with the observed first order kinetics of stabiliser reaction. These studies are briefly related to commercial PVC processing.