Methacrylate monomers for Polymeric Additives
Emulsion polymerization is a heterogeneous, free-radical polymerization process that has wide industrial application in the methacrylate-based production of polymer colloids or latexes.
Emulsifiers play a major role in the emulsion polymerization process as well as in the formation and application of latexes. In emulsion polymerization, emulsifiers are key to solubilizing highly water-insoluble monomers and determining the mechanism of particle nucleation. Furthermore, they determine the number of nucleated particles, and thus the rate of polymerization. They also help maintain colloidal stability during the particle growth stage as well as control average particle size and size distribution of the final latex system. Emulsifiers are also essential in stabilizing latexes during post polymerization processes, such as stripping to remove residual monomers, formulation, storage, shipping and onward transformation.
Emulsifier molecules comprise a hydrophobic ‘tail’ and a hydrophilic ‘head.’ These emulsifiers can be classified into four categories according to the charge of the head group: anionic emulsifiers, cationic emulsifiers, nonionic emulsifiers and ampholytic emulsifiers. Another classification is based on reactive emulsifiers and non-reactive emulsifiers according to their respective chemical properties.
VISIOMER® MPEG methacrylates are often used as nonionic reactive emulsifiers. Like non-reactive emulsifiers, reactive emulsifiers are molecules that typically have a hydrophobic segment and an ionizable and/or polar group. The hydrophobic segment tends to adsorb onto the surface of the latex particle during – and following – particle polymerization. The hydrophilic group, on the other hand, extends into the aqueous solution phase, providing a steric barrier or charge repulsion against particle agglomeration and coagulation.
Unlike their non-reactive counterparts, reactive emulsifiers also contain a reactive group on the hydrophobic segment that is capable of covalent bonding to the latex surface. Usually this is a moiety such as a methacrylate ester or a terminal olefin that can participate in free-radical emulsion polymerization reactions. When used in emulsion polymerization, a large fraction of the emulsifier molecules become irreversibly bound to the emulsion polymer chains and droplets. This can both improve latex stability and reduce foaming.