Process for producing a low viscosity silicone resin

A process for providing a molding composition having a low coefficient of thermal expansion during cure, comprising the steps of:

• I. first providing a silicone resin having a viscosity which does not exceed 100 centipoise at 140.degree. by
• A. adding with continuous agitation a mixture of acetone and organohalosilanes over a period of at least 40 minutes to a heterogeneous hydrolysis mixture having a water phase and an organic phase wherein said organic phase is composed of water, acetone and a water-immiscible solvent where in the final heterogeneous hydrolysis mixture, per part by weight of organohalosilanes there is present:
• 1. from at least 1.7 parts to about 10 parts of water;
• 2. from about 0.2 to about 5 parts of acetone;
• 3. from about 0.3 to about 5 parts of a water-immiscible organic solvent, and
• 4. from 0 to about one mole of an aliphatic monohydric alcohol having from 1 to 8 carbon atoms per mole of halogen attached to the silicone atom of said organohalosilane;
• B. maintaining the temperature of said heterogeneous hydrolysis mixture below 45.degree. C. during Step (A);
• C. removing the water from said heterogeneous hydrolysis mixture and reducing the acid content of the organic phase containing a formed silicone resin to below 5 parts per million;
• and
• D. stripping off the solvent at a temperature not to exceed about 120.degree. C. to yield a polyorganosiloxane having an average ratio from about 1 to 1.8 organo radicals per silicon atom;
• said organohalosilanes in Step (A) are selected from the class consisting of
• a. a mixture of organotrihalosilane and diorganodihalosilane,
• b. a reaction product of an aliphatic monohydric alcohol having from 1 to 8 carbon atoms and a mixture of organotrihalosilane and diorganodihalosilane which reaction product may have up to one alkoxy radical substituted for each halogen radical in said organotrihalosilane and diorganodihalosilane;
• and
• c. a mixture of the reaction product of (b) and an organohalosilane selected from organotrihalosilane and diorganodihalosilane, and where the organo radicals of said organohalosilanes are selected from the class consisting of monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals;
• II. mixing said silicone resin with a filler and a catalyst effective for providing a curable silicone molding composition having a low coefficient of thermal expansion during such cure, wherein said silicone resin comprises 15 to 20 percent by weight of the total molding composition.