THERMAL DEHYDROCHLORINATION AND STABILIZATION OF POLY(VINYLCHLORIDE) IN SOLUTION .6. DIHYDROPYRIDINE AS ORGANIC COSTABILIZER WITH ZN-CA STEARATES

1,4-Dihydropyridine diester derivative (DHP) has been used in the stabilisation of PVC carried out in trichlorobenzene (TCB) at 187°C in the absence and in the presence of zinc and calcium stearates. The kinetics of HCl evolution as well as of chloride ion formation on consumption of the metal soaps were studied by coulometry. DHP, used alone, can slow down the rate of degradation of PVC, but it does give a synergistic effect in combination with Zn and Ca stearates. Studies carried out by FTIR, UV and NMR have shown clearly that DHP, as secondary stabiliser, is highly sensitive to a reduction reaction by either heat or by chemical agents, especially by Zn stearate and Zn chloride. This property explains the improvement of both initial colour and long term stabilisation: the first effect is due to the partial hydrogenation of the polyene sequences, the second to the ability of its oxidised derivative (the pyridinic form) to complex Zn chloride strongly, avoiding the catastrophic blackening after full consumption of the metal soaps. Finally, the possibility of substitution of DHP on allylic chlorine in PVC was also discussed. © 1990.