Ethylene-vinyl acetate semi-batch emulsion polymerization: Kinetics, reactor design and modelling.

摘要

Ethylene-vinyl acetate (EVA) emulsion copolymers are useful materials for paint, adhesive and coating applications. The kinetics of their production remain largely unstudied, probably due to the inherent difficulties associated with polymerizations at higher pressures. These types of polymerizations are in general more difficult to understand and control, and relatively more expensive since one has to consider the added cost of increased safety precautions.;Through a series of carefully designed factorial experiments a copolymer of about 33 weight percent ethylene was produced. This ethylene content is higher than what can be achieved industrially under similar operating conditions. In addition to the copolymer composition, the molecular weight averages of the copolymer, particle size and number, gel content and rates of polymerization were examined as well. Thus, as the experimental work progressed an improved understanding of the EVA emulsion process was achieved.;Examination of the kinetic results revealed a copolymer composition drift, under certain experimental conditions, which suggested a mass transfer limitation of the gaseous ethylene from the headspace of the reactor to the locus of polymerization (i.e., the polymer particles). Through a study of the reactor design itself, several ways of overcoming the gas-liquid mass transfer limitations which exist during EVA emulsion polymerization were identified. Thus, the factorial experiments, coupled with a study of various reactor design configurations, identified possible modifications to either the recipe or the reactor configuration, which may lead to the production of homogeneous copolymer.;A limiting conversion was observed during all of the semi-batch polymerizations. This limiting conversion was thought to be due to a partitioning phenomenon. The vinyl acetate comonomer was found to partition in a manner dependent upon the cumulative copolymer composition of the polymer produced. This partitioning behaviour was further clarified through the collection of high pressure solubility data, which in turn led to the development of several partitioning models.;This thesis describes the results of an extensive experimental kinetic study in which the effects of twelve variables on the EVA process were studied. These variables include pressure, temperature, emulsifier type and concentration, initiator type and concentration, the addition of stabilizer, the addition of co-solvent, agitation, buffer, vinyl acetate feed rate and reactor configuration. The primary objectives of the research were to increase the amount of ethylene which could be incorporated into the copolymer at reduced temperatures and pressures (our target was a copolymer with an ethylene content of 30 percent by weight at 500 psig and 20;The monomer partitioning information was eventually incorporated into a mechanistic EVA emulsion polymerization mathematical model. All mechanistic features identified earlier during the experimental portion of the thesis were integrated into this model. The model, although at a preliminary stage, was shown to provide reasonable predictions of the rate of polymerization, copolymer composition, molecular weight averages, and particle size and number. The model will be a valuable tool in the future for sensitivity analysis of the process, parameter estimation and sequential experimental designs, thus leading eventually to an improved process understanding and optimization.