Steric StabilizationIn emulsion polymerization, free radicals are generated by an initiator to begin the polymerization. These free radicals react with insoluble monomers suspended in water to begin the polymerization reaction. When hydroxyethyl cellulose (HEC) is present in the reaction solution, free radicals can also be generated on the cellulose backbone of hydroxyethylcellulose. These reactive radical sites can participate in the emulsion polymerization as well causing the hydroxyethylcellulose to be chemically bonded to the monomers in the reaction to become an integral part of the polymer that is being formed. When this grafting occurs it provides optimal protective colloid performance and allows for drastically improved emulsion stability. The figure below illustrates the reaction mechanism described.
This grafting of hydroxyethyl cellulose to the polymer is not the only reaction occurring during polymerization. The backbone of the hydroxyethyl cellulose is also being cleaved to some extent resulting in smaller hydroxyethyl cellulose fragments. The two possible reaction routes will normally occur simultaneously with one route being favored based on the reactivity of the monomer and chemical structure of the hydroxyethyl cellulose. Hydroxyethylcellulose that is not grafted to the polymer does not provide optimal stabilization to the emulsion. For this reason it is important to understand which CELLOSIZE™ grade is best suited for your system. The competitive reactions are illustrated below.