Particle Formation in Emulsion Polymerization: Particle Number at Steady State

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Abstract
The number of particles formed in batch emulsion polymerization over wide ranges of emulsifier and initiator concentration has been investigated by computer simulation with a mathematical model developed in a previous paper. The influence of particle coagulation is also considered. The results show that, at low emulsifier concentration, the steady-state particle number N s is governed by homogeneous nucleation so that N s increases slowly with increasing emulsifier concentration [S]. In this range, N s increases with increasing monomer polarity. The steep rise in N s with emulsifier concentration after [S] exceeds a critical value suggests a transition from homogeneous nucleation domination to micelle nucleation. The slope of the N s vs [S] relationship increases as the particle coagulation rate constant Kf increases. The power x in the empirical relationship N s ≊ [S]x decreases with increasing polarity of monomer in this region. At very high micelle concentration, insufficient radical generation and the increasing tendency for particle coagulation cause N 2 to be less dependent on emulsifier concentration. These phenomena have been reviewed by Vanderhoff and confirmed by the experimental data presented by Sutterlin. The particle number increases with increasing initiator concentration [I] when [S] is above the CMC. As [I] continues to increase, however, N s becomes relatively constant. Experimental data for styrene, butyl acrylate, and methyl acrylate from the literature are compared with the model predictions. Agreement between the theoretical predictions and the experimental data is evident over a wide range of emulsifier and initiator concentrations.