l Polydimethylsiloxane emulsions are widely used in coatings, personal care products, because of their low toxicity, high chain flexibility and low surface energy. It has various good properties, such as thermal stability, low temperature performance and lubricity. It has been applied to different areas of industry. A polydimethylsiloxane miniemulsion was prepared with sodium dodecylbenzene sulfonate (SDBS) as emulsifier, sulfuric acid or hydrochloric acid as catalyst, octamethylcyclotertrasiloxane (D-4) as monomer, hexadecane (HD) as costabilizer and ultrasonication as pre-emulsification method. By reducing the monomer droplet size using pre-emulsifying method, and using hydrophobic substance as costabilizer to retard monomer transfer in aqueous phase, the monomer droplets become independent mini-reactors in miniemulsions. The effects of ultrasonic time, amount of catalyst and emulsifier, and temperature on kinetics of miniemulsion polymerization were investigated. Short ultrasonication time is not sufficient for obtaining the stable miniemulsion and high conversion of D-4. When or after a suitable ultrasonication time, a constant reaction rate period during polymerization was found. Unlike conventional emulsion polymerization, the interfacial polymerization of D-4 occurs on the oil-water interface during miniemulsion polymerization. It also showed that in E: certain range of catalyst concentration the polymerization rate increased with increasing, the concentration of catalyst, and the apparent reaction order for sulfuric acid and hydrochloric acid was 0.81 and 1.02, respectively. However a slight decrease of polymerization rate was observed at high catalyst concentrations. The apparent reaction order of emulsifier was -0.66 which indicated partial participation of emulsifier in the polymerization. The overall apparent polymerization activation energy was 40.56 kJ/mol for D-4 miniemulsion polymerization.
