Thermal ageing of plasticized PVC: effect of loss kinetics on plasticizer depth distribution

Initial weight loss kinetics of industrial extruded samples of poly(vinyl chloride) (PVC) plasticized respectively by 15, 20, 25 and 30 weight % of di(2-ethyl hexyl)phthalate, dinonylphthalate, didecylphthalate and diundecylphthalate were studied at 85, 95, 105 and 120/spl deg/C. Principally two distinct kinetic regimes of weight loss were observed: Regime 1 characterized by weight loss proportional to time and corresponding to a process controlled by evaporation. This regime predominates for high molecular weight plasticizers especially at high plasticizer concentrations (/spl ges/20 weight %). Regime 2, in which the weight loss is proportional to a square root of time, is typical for kinetic control by diffusion according to Fickian law. It was observed for low molecular weight plasticizers at low concentrations (/spl les/20 weight %). The depth distribution of plasticizer in the bulk sample depends also of the principal kinetic regime. In all samples in which evaporation controlled weight loss was observed, the plasticizer concentration was rather homogeneously distributed into the sample depth. On the contrary, the important decrease of plasticizer concentration in the superficial layers was observed in all cases of diffusion controlled of plasticizer loss.<
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