Continuous emulsion polymerization of vinyl chloride

Abstract

For batch emulsion polymerizations of vinyl chloride, the reaction rate can be well represented by the empirical expression dx/dt = k (1 + ax), where x is the polymer concentration, and k and a are constants dependent upon initiator concentration and particle number. A simple mathematical model for continuous stirred-reactor polymerizations has been derived from this expression. The model predicts that at short residence times, a steady state of low PVC concentration is reached quickly; with longer residence time, the system more slowly approaches a higher steady-state polymer concentration. Above a critical residence time, equal to 1/ak, the rate of monomer supply becomes the limiting factor. Experimental data on continuous polymerizations are in good qualitative accord with the predictions of the model using constants fitted to batch-charge data. Quantitative agreement is best when the particle number is large; discrepancies may be attributed to a broader particle size distribution in continuous runs. Particle size measurements show evidence of intermittent particle formation in unseeded continuous runs; continuous seeding narrows the particle size distribution, but it is still broader than in batch charges. The model seems sufficiently valid to yield a number of useful implications about practical aspects of continuous emulsion PVC polymerization.