The mechanism of polyvinyl chloride stabilization by barium, cadmium, and zinc carboxylates. II. Radioactive tracer studies

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Abstract
Polyvinyl chloride deteriorates primarily by a zipperlike elimination of hydrogen chloride from the polymer and the concomitant formation of a colored and easily ozidizable polyene chain. Baum and Wartman have shown that the elimination is initiated by the rupture of certain especially labile carbon‐chlorine bonds. In Part I of this series, we presented infrared evidence to show that the barium, cadmium, and zinc salts of 2‐ethylhexanoic acid, when milled into polyvinyl chloride, react with the resin by an esterifying displacement of chlorine, and proposed that their stabilizing action arises, at least in part, from substitution of certain labile chlorine atoms with more stable carboxylate groups. In the present work we have prepared the barium, cadmium, and zinc salts of radioactive 2‐ethylhexanoic acid and have shown that when these salts are milled into polyvinyl chloride, and the films heat‐treated and then subjected to repeated dissolution in tetrahydrofuran and precipitation with methanol, the precipitated resin shows an effectively constant retention of radioactivity. Hence, the proposed esterifying displacement is unequivocally proved. We have also shown that the amount of radioactivity which is retained by the precipitated resin varies with the particular salt, the temperature, and duration of heat treatment, and that it qualitatively parallels the intensity of the 5.75‐μ band observed in Part I. Finally, we have shown that one function of the organic phosphites (frequently components in commercial stabilizers) is to render innocuous the chlorides of barium and cadmium, which otherwise exert a very deleterious action upon the resin, but which are inevitably generated by the interaction of eliminated hydrogen chloride upon the metal carboxylates.