The esterification of metal soaps, in the combination of zinc and calcium soaps, on poly(vinyl chloride) (PVC) has been carried out both in liquid and solid phase to determine why the reactivity of zinc soap decreases on addition of calcium soap, improving the long‐run heat stability of PVC and causing the well‐known synergism. It is suggested as the mechanism of synergism that metal soaps having low ionization‐potential values, such as calcium soap, act as ester‐exchangers of metal chlorides, such as zinc chloride, which is generated after the stabilization of PVC, and also decrease the reactivity of the zinc soap by forming a complex, as that of zinc soap—calcium soap. If a third material, such as organic ester silane, reacting as ester‐exchanger on zinc chloride and preventing the formation of a zinc—calcium soap complex, is added to the system, these compounds have an excellent synergistic effect.
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Mitsuo Onozuka (1967) studied this question.
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