DEGRADATION AND STABILIZATION OF POLY(VINYL CHLORIDE) .1. KINETICS OFTHE THERMAL-DEGRADATION OF POLY(VINYL CHLORIDE)

The kinetics of the thermal degradation of solid powdered poly(vinyl c hloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution and rate of polyene sequence formation. M onoalkenes are present in commercial PVC and their concentration incre ases during thermal degradation. Sequences of 2 to 25-30 conjugated do uble bonds are formed in parallel reactions and their concentrations i ncrease linearly versus time and extrapolate back through the origin. Their rate constants decay exponentially with the number of double bon ds. These results can be accommodated by a chain mechanism involving i nitiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugate d polyene sequence (Scheme 1). Structural irregularities such as allyl ic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation.