Preparation of polysiloxane block polymers and their use as foam inhibitors

A method for preparing a foam-inhibiting composition comprising the steps of:

• (1) mixing (a) 100 parts by weight of polydimethylsiloxane that is liquid at 20.degree. C., and (b) from about 2 to 20 parts by weight of a spatially cross-linked polysiloxane resin with a molecular weight of from about 1000 to 10,000, which is comprised of structural elements of the general formula ##STR4## wherein R consists of from about 40 to 100 mol percent of a phenyl, phenylethyl, or alkyl radical having from 2 to 8 carbon atoms and of from 0 to about 40 mol percent of a methyl group, and X and Y, which can be the same or different, have the same meaning as R or can be OH, O.sub.1/2, methyl, or --OC.sub.n H.sub.2n-1, wherein n is a number of from 1 to 4, with the proviso that the content of OH groups is from about 0.1 to 10 percent by weight, based on the total weight of the polysiloxane resin, to form a mixture thereof;
• (2) heating said mixture to temperatures of at least 80.degree. C. in the presence of a catalytically effective amount of a catalyst comprising an acid substance selected from the group consisting of Lewis acids and acid-activated bleaching earths or a basic substance selected from the group consisting of quaternary ammonium bases with alkyl groups and/or hydroxyalkyl groups having from 1 to 4 carbon atoms and alkali metal hydroxides, and optionally, an organic solvent that does not react with the reaction components, until the viscosity increases steadily after an induction period and has reached a value of at least 200 mPa.s, as measured at 20.degree. C., to form a block polymer;
• (3) mixing said block polymer in a known manner with (c) from about 1 to 15 parts by weight of a silicic acid aerogel to form a mixture thereof;
• and
• (4) heating said mixture from step (3).