Miniemulsion Polymerization And Application Of Cyclic Siloxane

Polydimethylsiloxane is the main product in the silicone industry, because of its 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.In the textile industry, polydimethylsiloxane is used as lubricant. But polydimethylsiloxane is hard to be washed off on the textile because of its high viscosity. Polydimethylsiloxane emulsion as lubricant provides a way to improve the scouring property of polydimethylsiloxane, but direct emulsification of polydimethylsiloxane proved difficult because of its high viscosity. What's more, the polydimethylsiloxane emulsion from direct emulsification has poor stability.This thesis used miniemulsion polymerization technology to prepare polydimethylsiloxane emulsions. Octamethylcyclotertrasiloxane(D₄) was used as monomer, sulfuric acid or hydrochloric acid as catalyst, sodium dodecylbenzene sulfonate(SDBS) as surfactant, hexadecane(HD) as costabilizer, and ultrasonication as pre-emulsification method. A polydimethylsiloxane emulsion whose particle size was about 200nm provides a way to improve the scouring property and stability of polydimethylsiloxane emulsion.The effects of ultrasonication, surfactant concentration, monomer concentration, costabilizer concentration and catalyst concentration on particle size and miniemulsion stability were investigated. The technological conditions of stable miniemulsion polymerization were obtained.The surface tension measurement showed that the surface tension of miniemulsion after ultrasonication was higher than that of SDBS solution when reached CMC. The particle size of miniemulsion during polymerization remained stable. The two aspects confirmed that the system investigated was droplet nucleation dominated miniemulsion polymerization.The macro kinetics of D₄ cationic miniemulsion polymerization was investigated by measuring the monomer conversion with time. The effects of ultrasonication, surfactantconcentration, and catalyst concentration on kinetics were studied. The kinetics orders of these factors were obtained.A general scheme of D₄ cationic miniemulsion polymerization was proposed after the kinetics study. The location of D₄ miniemulsion polymerization was the oil-water interface. The dodecylbenzene sulfonate acid (DBSA) formed by the reaction of SDBS and hydrogen ion was the real catalyst. D₄ diffuses from inner droplet to oil-water interface, participating ring-opening polymerization. The area of oil-water interface has great influence on polymerization rate.The change of molecular weight of polydimethylsiloxane with time and conversion were obtained by GPC. The change of polymer chains with time was obtained from the definition of number molecular weight and monomer conversion. The polymerization consisted of four major reactions: ring opening reaction of D₄ to form active center, polyaddition of D₄ between active center, termination between active center and water to form silanol-ended polydimethylsiloxane, condensation between two silanol-ended polydimethylsiloxane. All of them took place at the surface of the droplets, and the monomer diffused from inner to the surface of droplets. The effects of temperature and catalyst on molecular weight were also investigated. A simplified mechanism of early stageof polymerization was investigated, the relationship of [D₄]₀-[D₄]_t wasfound.The friction coefficient between polyurethane film and stainless steel was used to characterize the lubricating property of polydimethylsiloxane miniemulsion. The change of contact angle of water on the polyurethane film during washing was used to characterize the scouring property of polydimethylsiloxane miniemulsion. The effects of molecular weight of polydimethylsiloxane and technological conditions on the lubricating property and scouring property were investigated.