Copolymer composition control during the seeded emulsion copolymerization of vinyl acetate and methyl acrylate

Abstract

AbstractThe effect of various monomer addition policies on the copolymer composition during the seeded copolymerization of methyl acrylate and vinyl acetate in a semicontinuous reactor was investigated. Four policies were considered. In the first, which is called optimal policy, the reactor was initially charged with all of the less reactive monomer (vinyl acetate) plus the amount of the more reactive monomer (methyl acrylate) needed to initially form a copolymer of the desired composition. Then, the addition of the remaining methyl acrylate was made at a flow rate that ensured the formation of a copolymer of the desired composition. In the second, the initial charge is the same as in the previous case but the time dependent addition rate of methyl acrylate was decomposed into three periods of constant feed rate. In the third, the initial charge is the same as in the previous policies but the remaining methyl acrylate was added at a constant feed rate. In the forth, both monomers were added under starved conditions. In order to calculate the optimal addition policy, the time evolution of the average number of radicals per particle, ñ, is required. The calculation of ñ involves rate parameters which values are usually unknown. Therefore, a semi‐empirical method for the calculation of the time evolution of ñ was developed. It was found that ñ was always greater than 2. For the experimental conditions used in this work, this result suggested that termination in the polymer particles was affected by gel effect. A gel effect function was estimated and then the optimal addition policy was calculated. Comparison between the different processes showed that, for a fixed process time, the copolymer obtained through the optimal addition policy was more homogeneous than that formed when the methyl acrylate was fed in three periods of constant feed rate and even better than the one obtained using a constant feed rate throughout the process. The copolymer quality of the later process is similar to that of the copolymer formed by means of the starved process. Nevertheless, a more homogeneous copolymer composition can be obtained through the starved process when longer process times are used.