High‐conversion emulsion polymerization

Abstract

SUMMARY The four important factors that determine the rate of emulsion polymerization are the propagation rate coefficient, the latex-particle concentration, the monomer concentration in the latex particles, and the free-radical concentration in the latex particles. Both theoretical considerations and experimental evidence suggest that the important factors that may reduce the rate of emulsion polymerization at high conversion are the propagation rate coefficient and the monomer concentration in the latex particles, and not the free-radical concentration in the latex particle, if the initiator is not depleted. Various approaches for increasing the rate of emulsion polymerization at high conversion are suggested. One of the important considerations in any industrial application of the emulsion polymerization process is residual monomer in the final emulsion latex product. Often it is very difficult to remove the last few percent of monomer during the polymerization reaction, since the rate of an emulsion polymerization can become relatively slow at high weight fractions of polymer in the latex particles. It should be noted that the problem of slow rates of emulsion poiymerization at high conversions apply for both continuous and batch processes, since in continuous and semi-continuous reactors residual monomer is normally removed by reaction in a post (batch) reactor. In this paper theories of emulsion polymerization are considered in an attempt to show the causes of the often slow rate of emulsion polymerization at high conversions of monomer. Case studies are presented for the emulsion polymerizations of methyl methacrylate and butadiene. The aim of this paper is to distinguish the effect of high conversion upon those factors that determine the rate of an emulsion polymerization. Specifically, we wish to determine why the rate of emulsion polymerization of a specific system may become (uneconomically) slow at high conversion. Having isolated some of the major difficulties, some techniques for circumventing the slow rates of emulsion polymerization at high conversion are suggested. 2. Theory In the following discussions we consider, for simplicity, emulsion homo-polymerization, i.e. the emulsion polymerization of one monomer. However, most of the theoretical description can be easily extended to account for two or more monomers. The rate of any free-radical polymerization can be written as: