RECENT ADVANCES IN THE PREPARATION OF LATEXES

The industrial production of latexes has grown from the first poly(butadiene-co-styrene) produced in the U.S. for synthetic rubber and the first poly(vinyl acetate) produced in Germany for adhesives to many different polymer families produced for a wide variety of applications. Both conventional (oil-in-water) and inverse (water-in-oil) emulsion polymerization comprise the emulsification of an immiscible monomer in a continuous medium, followed by polymerization with a free radical initiator, to give a colloidal sol of submicroscopic polymer particles smaller by an order of magnitude than the original emulsion droplets. Both processes give ''emulsion polymerization kinetics,'' i.e., a proportionality of both polymerization rate and number-average degree of polymerization to the number of particles, instead of the inverse relationship between these two parameters observed for mass, solution, and suspension polymerization. The emulsion polymerization process can be divided into particle nucleation and particle growth stages, and it can be carried out using batch, semi-continuous, or continuous processes. Seeded emulsion polymerization can be used to obviate the particle nucleation stage in all three processes. The mechanisms proposed for initiation of emulsion polymerization can be divided into four categories, according to the locus of particle initiation: 1) monomer-swollen emulsifier micelles; 2) adsorbed emulsifier layer; 3) aqueous phase; 4) monomer emulsion droplets. Three generations of latex development are defined, according to the emulsifiers used: 1) conventional emulsifiers; 2) functional monomers; 3) polymeric emulsifiers. The formation of coagulum (polymer recovered from the latex in a form other than stable latex) during emulsion polymerization is attributed to two causes: 1) a failure of the colloidal stability of the latex; 2) polymerization by a different mechanism, e.g., mass (monomer layer), surface (walls, roof, agitator shaft), vapor (atmosphere). The failure of the colloidal stability is attributed to diffusion-controlled, agitation-induced, or surface flocculation of the particles, according to the agitation rate (low Reynolds numbers) and the power consumption (high Reynolds numbers).