Latex particles formation mechanisms

Lewis J.A. Colloidal processing of ceramics. J. Am. Ceram. Soc. 2000 83(10) 2341-2359 Lichti G., Gilbert R.G., Napper D.H. The mechanisms of latex particle formation and growth in the emulsion polymerization of styrene using the surfactant sodium dodecy 1-sulfate. J. Polymer Sci. 1983 21 269-291... 

A number of mechanisms and models have been proposed for latex particle formation in emulsion polymerization systems [18-20]. These include particle formation by entry of a free radical into a micelle [6,7] or by homogeneous nucleation of oligomeric free radicals in the aqueous phase [18,21-23] or within microdroplets of the monomeric emulsion [24]. After their formation, these primary particles may simply grow by conversion of monomer into polymer within these particles, or undergo coagulation [22,25]. 

We begin the discussion of EPM by elaborating on this physical picture. Figure 1 shows a typical emulsion CSTR reactor and polymerization recipe. The magnified portion of the latex shows the various phases and the major species involved. The latex consists of monomers, water, surfactant, initiator, chain transfer agent, and added electrolyte. We used the mechanism for particle formation as described in Feeney et al. (8-9) and Hansen and Ugelstad (2). We have not found it necessary to invoke the micellar entry theory 2, 2/ 6./ 11/ 12/ 14. 

The particle generation rate was calculated by a step mechanism, namely formation of primary precursor particles by homogeneous nucleation (JLQ.) followed by coagulation to latex particles (8-9). This homogeneous nucleation mechanism is often referred to as the HUFT mechanism for its originators Hansen, Ugelstad, Fitch, and Tsai. 

PVC latex particles consisting of two size populations can be generated in a miniemulsion polymerization. The mechanism for the formation of two discrete particle families relies upon polymerization of two distinct kinds of droplets [74]. 

Although emulsion polymerization has been carried out for at least 50 years and has enormous economic importance, the detailed quantitative behavior of these reactors is still not well understood. For example, there are many more mechanisms and phenomena reported experimentally than have been incorporated in the existing theories. Considerations such as non-micellar particle formation, non-uniform particle morphologies, polymer chain end stabilization of latex particles, particle coalescence, etc. have been discussed qualitatively, but not quantitatively included in existing reactor models. 

It was established by Yeliseyeva and Bakaeva (1968) that in the polymerization of polar monomers (MA) the decrease of emulsifier adsorption depends on tbe structure of the latter and for some types of emulsifiers may reach limiting values. This was observed in tbe polymerization ofMA in the presence of a mixed type of emulsifier, partially sulfurated with sulfuric acid oxyethylated alkylphenol (emulsifier C-I0 ). Its adsorption on the particle surface increases with the initial concentration and reaches > 100% filling of the adsorption layer, conditionally corresponding to 0.3S nm per molecule. Stable, concentrated latexes with small particles are formed. Therefore, emulsifier adsorption and the mechanism ctf particle formation associated with it depends not only on monomer polarity but also on the chemical structure of the emulsifier. 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

The final properties of latex coatings are dependent on the mechanism of film formation and how the film forms. For example, the development of mechanical strength is a direct consequence of polymer chain inter-diffusion. Prediction of this strength is only possible from an understanding of the transformations occurring on the particle length scale. Here, the three major transformations in latex film formation are briefly outlined. 

This article has reviewed latex processing. The polymers used, synthesis of particles, major uses, and reasons for loss of dispersion stability have been outlined. The mechanism of latex film formation has been described, and the different properties resulting from different film forming conditions in latex explored. 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

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