Emulsion polymerization composition control

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

Artificial control of the monomer concentrations is possible by changing the monomer feed methods, which includes multishot, stage feed (19), and continuous feed. A multishot emulsion polymerization is expected to form multilayered particles if the monomers are chosen properly. When the layers are sufficiently thin, the particles exhibit unique thermal and mechanical properties. The stage feed system is shown in Figure 11.1.6. It makes it possible to produce particles having gradient composition of different monomer units. 

Wenz and colleagues at Bayer Polymers Inc. describe the use of Raman spectroscopy to monitor the progress of a graft emulsion polymerization process, specifically the manufacture of ABS graft copolymer, in order to select the appropriate reaction termination point.40 Early termination reduces product yield and results in an extra product purification step termination too late reduces product quality. As Figure 5.5 illustrates, the reaction composition over time is not smooth and predictable, making it unlikely that off-line analysis would ever be fast enough to ensure correct control decisions. 

Gugliotta, L.M., Leiza, ).R Arot arena, M., Armitage, P.D. and Asua, ).M. (1995) Copolymer composition control in unseeded emulsion polymerization using calorimetric data. Industrial Engineering Chemistry Research, 34, 3899-906. 

Usually or most widely applied, polymer latexes are made by emulsion polymerization [ 1 ]. Without any doubt, emulsion polymerization has created a wide field of applications, but in the present context one has to be aware that an inconceivable restricted set of polymer reactions can be performed in this way. Emulsion polymerization is good for the radical homopolymerization of a set of barely water-soluble monomers. Already heavily restricted in radical copolymerization, other polymer reactions cannot be performed. The reason for this is the polymerization mechanism where the polymer particles are the product of kinetically controlled growth and are built from the center to the surface, where all the monomer has to be transported by diffusion through the water phase. Because of the dictates of kinetics, even for radical copolymerization, serious disadvantages such as lack of homogeneity and restrictions in the accessible composition range have to be accepted. 

In emulsion polymerizations semibatch operation provides better control of the particle size of the product. The properties of the product polymers can be modified, also, by continuous or intermittent changes in the composition of the monomer feed in emulsion copolymerizations, where a given monomer can be preferentially concentrated in the interior or on the surface of the final particles, as described in Chapter 8. 

The ability of micellized surfactants to catalyze, or inhibit, reactions and to control stereochemistry and product composition, suggests that these agents could have a useful role in organic synthesis. A micelle can speed a desired reaction and inhibit an undesired one, and, for example, cationic micelles can control the ratio of unimolec-ular, Sfjl, substitution to bimolecular, E2, ehmination [54,55]. Micellization is of great importance in emulsion polymerization, but little use has been made of aqueous micelles in synthesis. 

A series of latex copolymers were prepared using a typical emulsion polymerization recipe and procedure only the monomer composition was varied. The control composition (80/20 vinyl acetate/butyl acrylate) is similar to that used for interior latex paint. Table V lists the compositions and properties of the latexes. Percent solids, pH, and particle size are similar for all the latexes. Viscosity varies somewhat, but is within limits for this type of latex. The only unreacted monomer detected was the vinyl acetate. Thus, the incorporation of VEC into the emulsion polymerization via the monomer mixture did not affect the latex synthesis. The Tg and minimum film formation temperature (MFFT) of the latexes increase with increasing VEC content, which is expected based on the previous results. 

Let us first ignore contributions from monomer partitioning and examine the effects of conversion upon copolymer composition. In most cases, thCTe will be a difference in monomer reactivities (Le. reactivity ratios 1) and a consequent drift in copolymer composition with conversion as the more reactive monomer is consumed preferentially (see Section 1.6.3). Since the total quantity of each monomer is added at the beginning of a batch emulsion polymerization, there is no control over this drift in copolymer composition. Hence, copolymors formed using batch processes can have quite broad composition distributions, the breadth of the distribution for each particular system depending upon the monomer reactivity ratios, the initial comonom composition and monomer partitioning (which is dealt with in Section 7.3.2.2). 

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