DEVELOPMENT OF VISCOUS COUPLING OIL (PART 5) - EFFECTS OF CACO3 AND SOME METAL-OXIDES ON THERMAL-STABILITY OF DIMETHYLSILICONE OIL

High-viscosity dimethylsilicone oils, which are used as a fluid for vi scous couplings, are susceptible to decrease in viscosity by decomposi tion in the absence of oxygen at temperatures above 170 degrees C. For the purpose of preventing the decrease in viscosity of dimethylsilico ne oils, effects of addition of CaCO3 and metal oxide powders were stu died by closed system heat test at 200 degrees C. Further, influence o f water contents on the decrease in viscosity was investigated to expl ore the mechanism of decomposition of dimethylsilicone oils. CaCO3 and metal oxides, such as CaO, MgO, ZnO, CuO, Al2O3, and Fe3O4, were effe ctive to prevent decreases in viscosity. CaO was especially effective. On the other hand, it was experimentally confirmed that the decrease in viscosity is due to decomposition caused by presence of dissolved w ater and that the degree of decrease in viscosity was influenced great ly by water contents in a range below about 100 ppm. From analyses of molecular weight distribution and molecular structure unit of oils aft er the heat test. it was demonstrated that the main reaction process o f decomposition of dimethylsilicone oil is the hydrolysis of siloxane bonds, and the action of CaCO3 and some metal oxide powders for inhibi ting the decomposition is mainly capture of water.