Vinyl Chloride Polymerization

Abstract

<p>Relevant mechanisms involved in the heterogeneous free radical polymerization of vinyl chloride have been identified including elementary chemical reactions. physical phenomena of polyvinylchloride particle formation and reactant species distributions in phases during polymerization. A comprehensive reactor model for batch and semi-batch processes has been developed on the basis of these mechanisms. The present model accounts for comprehensive elementary chemical reactions, for the effect of diffusion-controlled reactions, for the monomer and initiator distributions among phases and for radical migration between monomer and polymer phases during polymerization. Wide ranging kinetic data covering commercially significant conversion and reaction temperature ranges were measured in the present investigation to better understand the mechanisms involved and to estimate model parameters. The present model predictions are in excellent agreement with experimental data obtained in the present work and independently in other laboratories. The model allows one to predict reactor pressure development; monomer conversion histories; polymerization rates; the critical conversion at the end of two phase polymerization; the limiting conversion for polymerization at temperatures below the polyvinylchloride glassy-state transition temperature; instantaneous and accumulated molecular weight averages and distributions, and other kinetic features of vinyl chloride polymerization over temperature and conversion ranges of commercial interest.</p> <p>The effect of polymerization conditions on polymer properties, especially the thermal stability of polyvinylchloride, has been elucidated. Deterioration of the thermal stability of polymer at high conversions is attributed to a decrease in monomer concentration. The secondary reactions which form the defect structures in polyvinylchloride are favoured at high conversions because of the increase in radical and polymer concentrations and the decrease in monomer concentration. The defect structures which are responsible for the low thermal stability of polyvinylchloride can be minimized significantly by using a semi-batch process at high conversions. Significant improvement in thermal stability of polyvinylchloride at high productivity by semi-batch processing at the monomer vapour pressure was demonstrated in the present investigation.</p> <p>A novel method for monitoring monomer conversion online during the suspension polymerization of vinyl chloride was developed in the present study. This method provides an effective tool not only for vinyl chloride polymerization kinetic studies but also can be adapted for use with other monomer and comonomer systems.</p>