Production of polydiorganosiloxane fluids with low silanol content

Claim 1

A continuous process for producing a triorganosilyl end-blocked polydiorganosiloxane fluid with reduced silanol content and reduced branching content, wherein said triorganosilyl end-blocked polydiorganosiloxane fluid has a viscosity in a range from about 3 to 5,000 centipoise at 25° C., said process comprising (A) feeding to a first packed column, a liquid siloxane feed mixture, containing as a major portion HO(R.sub.2 SiO).sub.y H, separately, or as a mixture with (R.sub.2 SiO).sub.x, and as a minor portion (R.sub.3 Si).sub.2 O, R.sub.3 SiO(R.sub.2 SiO).sub.z SiR.sub.3, or mixtures thereof, wherein each R is independently selected and is from a group consisting of hydrogen, alkyl groups containing 1 to 6 carbon atoms, alkenyl groups containing 2 to 6 carbon atoms, and aryl groups; wherein x has a value of 3 to 9; wherein y has a value of 1 to 100; and wherein z has a value of 1 to 10, and further wherein at least one SiR 3 group is triorganosilyl, said first packed column being filled with an acid-treated clay which has an acid equivalent in a range from about 5 to 20 milligrams of KOH per gram of acid-treated clay; and wherein the acid-treated clay is a mixture of solid particles ranging in size from about 300 to 10,000 microns; (B) contacting said liquid siloxane feed mixture with the acid-treated clay in the first packed column to effect reaction of silanol groups to water and to effect formation of the triorganosilyl end-blocked polydiorganosiloxane fluid, wherein said first packed column is maintained at a temperature in a range from about 100° to 150° C.; wherein within said first packed column of the acid-treated clay there is maintained a pressure in a range from about 5 to 1520 mm Hg; and wherein the liquid siloxane feed mixture is in contact with the acid-treated clay for less than about 15 minutes; (C) taking from said first packed column an intermediate mixture comprising the triorganosilyl end-blocked polydiorganosiloxane fluid, and a mixture of (R 2 SiO) e and water; wherein (R 2 SiO) e is a by-product of producing the triorganosilyl endblocked polydiorganosiloxane fluid; wherein R is defined above, and e is 3 to 9, inclusive; and wherein the water is in the form of water vapor in equilibrium with water soluble in the triorganosilyl end-blocked fluid and the (R 2 SiO) e ; (D) separating said water vapor from the intermediate mixture; (E) after separating the water vapor, feeding the intermediate mixture to a second packed column filled with acid-treated clay having an acid equivalent from about 5 to 20 milligrams of KOH per gram of acid-treated clay; and wherein the acid-treated clay is a mixture of solid particles ranging in size from about 300 to 10,000 microns; (F) contacting the intermediate mixture with the acid-treated clay in the second packed column to effect further reaction of remaining silanol groups; wherein said second packed column is maintained at a temperature in a range from about 100° to 150° C.; wherein within said packed column of the acid-treated clay there is maintained a pressure in a range from about 5 to 1520 mm Hg; and wherein the intermediate mixture is in contact with the acid-treated clay for less than about 15 minutes; and (G) collecting from said second packed column a product mixture which contains a triorganosilyl end-blocked polydiorganosiloxane fluid with silanol content substantially reduced from the intermediate mixture.