Improved method for the manufacture of silanol diorganopolysiloxanes having low molecular weight and high silanol content

摘要

1533158 Diorganopolysiloxanes GENERAL ELECTRIC CO 11 Feb 1976 [11 March 1975] 05332/76 Heading C3T A silanol end-stopped diorganopolysiloxane having a viscosity of less than 1,000,000 centipoises at 25‹ C. and having a silanol content of from 6À5 to 14% wt. is prepared by a process comprising (a) contacting a cyclic trimer of the formula (R 2 SiO) 3 , wherein R is a monovalent hydrocarbon radical, a halogenated monovalent hydrocarbon radical or a mixture thereof, with an acid-activated hydrated aluminium silicate clay as catalyst in the presence of water and a polar solvent, and (b) separating the desired product, wherein the amount of water used is from 0À12 to 0À20 p.b.w. per part by weight of said cyclic trimer. The cyclic trimer, water and polar solvent are preferably present in a uniform homogeneous phase. Suitable R radicals are C 1-8 alkyl, haloalkyl, in particular fluorinated alkyl, and cycloalkyl radicals, vinyl and phenyl. Preferred polar solvents are those having a boiling point of 50-80‹ C. such as acetone, dioxane and tetrahydrofuran. The clay catalyst is preferably a montmorillonite clay activated with either sulphuric or hydrochloric acid. Preferably 0À1 to 10 p.b.w. of the said clay catalyst is used per 100 p.b.w. of the cyclic trimer, and 0À5 to 10 p.b.w. of the polar solvent per part by weight of the cyclic trimer. The reaction is preferably carried out at a temperature of 50-80‹ C. for 2-12 hours. The product of the process may be separated in step (b) either (1) by adding 0À1 to 5 p.b.w. of magnesium oxide per part by weight of the clay catalyst, stripping the polar solvent at a temperature of 20-60‹ C. and a pressure of 100-300 mmHg, optionally adding 0À1 to 5 p.b.w. of a diatomaceous earth filtering aid, per part by weight of the clay catalyst, then filtering out the MgO and clay catalyst and stripping off the low boiling siloxanes and water at 100-140‹ C. and 10-100 mm Hg pressure; or (2) by optionally adding 0À1 to 5 p.b.w. of diatomaceous earth per p.b.w. of clay catalyst as filtering aid, then filtering out the clay catalyst, stripping off the polar solvent at 20-60‹ C. and 100-300 mm Hg pressure, stripping out the low boiling siloxanes and water at 100-140‹ C. and 10-100 mm Hg pressure. The product is generally obtained in a 70-95% yield. The products can be used as processing aids for the mixing of fillers, e.g. SiO 2 fillers, into linear diorganopolysiloxanes of viscosities ranging from 1 Î 10SP6/SP to 200 Î 10SP6/SP c.p. at 25‹ C. for the manufacture of heat vulcanizable silicone rubber compositions; or the products may be reacted with each other or with other silanol-terminated diorganopolysiloxanes to form linear diorganopolysiloxanes having viscosities ranging from 2000 to 200 Î 10SP6/SP c.p. at 25‹ C., which can be used as base polymers for RTV or heat vulcanizable silicone rubber compositions. In the examples silanolterminated diorganopolysiloxanes are prepared as above using as the cyclic trimer either hexamethylcyclotrisiloxane, or 1,3,5-trimethyl, tris- (3,3,3 - trifluoropropyl)cyclotrisiloxane or 1,3,5 - trimethyl - 1,3,5 - trivinylcyclotrisiloxane, and as the polar solvent, acetone, and as the catalyst, acid-activated montmorillonite clay.