Tham khảo tài liệu 'fundamentals of polymer engineering part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7 Emulsion Polymerization INTRODUCTION Emulsion polymerization is a technique of polymerization where polymer formation occurs in an inert medium in which the monomer is sparingly soluble not completely insoluble . Traditionally water is the inert medium and the initiator is chosen such that it is water soluble. Monomers undergoing stepgrowth reaction do not require any initiation and are not polymerized by this method. Emulsion polymerization is commonly used for vinyl monomers undergoing addition polymerization and even among these those that polymerize by the radical mechanism are preferably polymerized by this method. Water-based emulsions for ionic polymerizations are uncommon because of high-purity requirements. This discussion is therefore restricted to the polymerization of monomers following the radical mechanism only. The water-soluble initiator commonly used is potassium or sodium persulfate and the usual recipe for emulsion polymerization is 200 parts by weight of water 100 parts by weight of the monomer and 2-5 parts by weight of a suitable emulsifier 1 2 . The monomer should be neither totally soluble nor totally insoluble in the water medium and must form a separate phase. The emulsifier is necessary to ensure that the monomer is dispersed uniformly as in a true emulsion 3-8 . The polymer that is formed from emulsion polymerization is in the form of small particles having an average diameter around 5 mm. The particles form a stable emulsion in water. Their separation can be effected only through the 299 Copyright 2003 Marcel Dekker Inc. 300 Chapter 7 evaporation of water and once the water is evaporated these particles coalesce to a solid mass. The rate of emulsion polymerization rp is found experimentally to be much greater than that for the corresponding bulk polymerization and the average molecular weight of the polymer formed is simultaneously very high a property that is not achieved in bulk polymerization. AQUEOUS EMULSIFIER SOLUTIONS .
