Mechanism of polyvinyl chloride degradation and stabilization

Many articles have been written attempting to explain the degradation processes which take place when polyvinyl chloride is aged. Everyone agrees that hydrogen chloride is evolved, color develops, and the polymer eventually becomes stiff and insoluble as a result of crosslinking. Early workers believed that hydrogen chloride catalyzed the decomposition and that the role of stabilizers was to react with it to render it ineffective. More recent work has shown that hydrogen chloride is not a catalyst for the decomposition in the absence of oxygen and that the function of stabilizers may be quite different from that earlier assumed. In this paper an attempt is made to weigh the results of many workers and to explain the observable changes brought about during degradation by a consistent free radical mechanism. Under milling conditions, temperatures are attained at which even saturated hydrocarbons will oxidize; hence it is not unreasonable to assume the presence of free radicals in polyvinyl chloride. Free radicals may also result from the ejection of a chlorine atom from a polymer chain. At room temperatures chlorine atoms may be formed as the result of the absorption of light energy. Once radicals are produced, degradation in the presence of oxygen may take many routes. A few of these are discussed. Many stabilizers are believed to act by disrupting radical chain processes.