Effect of metal stearate stabilizers on the thermal degradation of PVC in solution: The reversible blocking mechanism of stabilization: Effect of Metal Stearate Stabilizers

Systematic thermal degradation studies of PVC solutions have been carried out in the presence of different metal carboxylates (Pb-, Cd-, Ba-, Ca- and Zn- stearates). ZnSt2 markedly accelerates degradation of the polymer. Significant induction periods (ti) for the appearance of free HCI are obtained in the presence of the other salts. CaSt2 acts mainly as an HCI-scavenger and has no direct influence on the elimination process. Mainly short polyenes are formed during the induction period when PbSt2, CdSt2, and BaSt2 are used. After consumption of stabilizers rapid HCI loss occurs, and suddenly longer polyenes are formed, i.e., the amount of double bonds and the average length of polyenes sharply increases. At the same time, the rate of initiation (i.e., the rate of the polyene sequence formation) does not change at all around ti. None of the investigated stabilizers retard effectively the random initiation of HCI loss. In the presence of PbSt2, BaSt2, and CaSt2, and CaSt2, the initiation rate is nearly identical to that of the unstabilized PVC. The concentration of labile sites (h0) in PVC at the end of the induction period has been estimated by kinetic analysis. It has been found that in some cases h0 is higher than the concentration of labile structures in the virgin PVC. These results indicate that the main role of metal soap stabilizers is the blocking of the fast zip-elimination of HCI from the PVC chain. It is likely that blocking occurs by attachment of a carboxylate group at the end of a propagating zip. This is a reversible process: the blocked structures become active again mainly after the consumption of stabilizers presumably by HCI-catalyzed hydrolysis. In contrast to other stabilization mechanisms by Frye and Horst, Minsker and coworkers, and Michell the reversible blocking mechanism of PVC stabilization is able to explain the experimental findings presented in this study. It also explains such facts of practical importance as color stability of the resin during the induction period and fast blackening after the consumption of stabilizers.

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