Thermal decomposition of chlorinated poly(vinyl chloride) (CPVC)

Chlorinated poly(vinyl chloride) (CPVC) shows reductions in flammability and smoke production over PVC. The thermal decomposition of pure CPVC (without stabilizer or lubricant) was studied by dynamic thermogravimetric analysis (TGA) at heating rates from 5 to 100degreesC/min in atmospheres of nitrogen, air, and oxygen. In each case, a two-step decomposition was observed similar to that for PVC where dehydrochlorination is followed by pyrolysis/oxidation of the carbonaceous residue. The rate of dehydrochlorination was dependent on atmosphere, occurring slightly slower in nitrogen than in air, and slightly more quickly in oxygen than in air. The decomposition of the residual char was clearly dependent on the conditions in which it was formed. Under dynamic conditions, chars formed at high heating rates appeared more resilient to oxidative degradation than those formed more slowly. However, when chars were formed by heating at different rates and then held at 500degreesC, the char formed at the slowest heating rate was the slowest to be oxidized. The uptake of oxygen by the char appears to be rate-limiting. At low heating rates char oxidation is similar in both air and oxygen. As the heating rate is raised, the rate of mass loss of char in air becomes progressively closer to that in nitrogen until at 100degreesC/min they are almost identical. This work is important to the understanding of the decomposition and flammability of CPVC and flame-retarded CPVC, where the char formation is one of the flame-retardant mechanisms.