Preparation of polymethylsiloxanes and polymethylsiloxane varnishes

Contemporary polymethylsiloxanes differ in the structure of the chains formed, viscosity and gelatinisation time. Depending on the degree of cross-linkings, these characteristics are naturally different. 

Gelatinisation time is the period within which a polymer transfers from a meltable and fluid state (stage A) into a nonfluid (stage B) and non-meltable (stage C) states. Gelatinisation time is usually determined with the help of a polymerisation stove at a definite temperature (180, 200 or 250 °C). 

The properties of polymethylsiloxanes are also greatly determined by the conditions of the reaction and the type of the solvent used. Thus, the hydrolytic condensation of methyltrichlorosilane (the main raw stock in the production of polymethylsiloxanes) with iced water or water vapour in a medium of nonpolar solvents forms a nonmeltable and nonsoluble amorphous substance. If the condensation is carried out by pouring methyltrichlorosilane into the emulsion of water and butyl alcohol gradually (to avoid gelation), at intensive agitation and reduced temperature (about 0 °C), it forms a viscous substance, which is soluble in organic solvents and briefly heated to 150 °C loses its meltability and solubility. 

This difference in the product properties can be explained by the fact that the introduction of methyltrichlorosilane into the aqueous-alcoholic emulsion in the system CH3SiCl3+H20+C4H90H causes three competing reactions at the same time  

These reactions greatly differ in speed the speeds of the first two are commensurable however, the third one occurs much slower. As a result, the hydrolytic condensation of methyltrichlorosilane in butil alcohol can be presented by the following overall scheme  

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