Process strategies in emulsion copolymerisation

The emulsion polymerisation process strategy, can have a considerable effect on molecular structure and particle morphology. The intrinsic factors as well as the process conditions determine the colloidal aspects of the copolymer latex (particle diameter, surface charge density, colloidal stability etc.), the characteristics of the polymeric material in the particles and the structure of the particles (copolymer composition as a function of particle radius etc.). In turn, these factors determine the properties of the latex and the copolymer product. 

The ultimate goal of most of the investigations on emulsion copolymerisation is to be able to control the process in such a way as to produce a copolymer product (latex or coagulate) with desired properties. For this purpose the semi-continuous (sometimes called semi-batch) emulsion copolymerisation process is widely used in industry. The main advantages of this process as compared with conventional emulsion batch processes include a convenient control of emulsion polymerisation rate in relation with heat removal and control of chemical composition of the copolymer and particle morphology. These are important features in the preparation of speciality or high performance polymer latexes. 

Semi-continuous emulsion copolymerisation processes can be performed by applying various monomer addition strategies. 

Constant addition strategy. The most widely investigated and described procedure is the addition of a given mixture of the monomers (sometimes pre-emulsified monomers) at a constant rate. 

For instance, this procedure is followed in manypapers dealing with the semi-continuous emulsion copolymerisation of vinyl acetate and butyl acrylate (e.g. El-Aasser et al, 1983). Two main situations can be distinguished with respect to the monomer addition rate, (a) Flooded conditions the addition rate is higher than the polymerisation rate, (b) Starved conditions the monomers are added at a rate lower than the maximum attainable polymerisation rate (if more monomers were to be present). The latter process (starved conditions) is often applied in the preparation of homogeneous copolymers/latex particles. In this case after some time during the reaction, because of the low addition rates, a steady state is attained in which the polymerisation rate of each monomer is equal to its addition rate and a copolymer is made with a chemical composition identical to that of the monomer 

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