Patent GB-1489637-A (GB)
Title: Organopolysiloxane compositions curable to transparent elastomers
IPC: C08L83/00
Filed: 1975-05-08
Abstract: 1489637 Organopolysiloxane compositions WACKER-CHEMIE GmbH 8 May 1975 [15 May 1974] 19464/75 Heading C3T A process for the manufacture of an organopolysiloxane composition curable to a transparent elastomer comprises mixing a diorganopolysiloxane having a viscosity of at least 500,000 cSt. at 25 C. with at least 0À02% by weight of a nitrogen-silicon compound having at least one nitrogen atom bonded either directly or through an oxygen atom to a silicon atom, and having at least one triorganosilyl group, and having not more than one triorganosilyl group bonded directly to any one nitrogen atom, and having not more than one condensable group bonded directly to any one silicon atom, and with from 4-40% by weight of a hexaorganodisilazane, and subsequently with from 10-150% wt. of silica having a BET specific surface area of at least 50 m.<SP>2</SP>/g., and removing any volatile components from the resulting mixture preferably by heating under reduced pressure, all percentages being based on the weight of the diorganopolysiloxane; and the volatile components being those having a B.Pt. not exceeding 250 C. at 760 mm. Hg. The diorganopolysiloxane may be represented by the formula where R represents an optionally substituted monovalent hydrocarbon group, Z is OH or a C 1 -C 18  alkoxy group and n = 0 or 1. Preferably the majority of the R groups are methyl, any remaining R groups preferably being vinyl and/or phenyl groups. The nitrogen-silicon compound may be an aminosilane, an acylaminosilane, an N-triorganosilylcarbamic acid ester, an N-triorganosilylurea, an O-triorganosilyloxyamine, an O-triorganosilylketoxime, an aminoorganosiloxane or an N-triorganosilyloxy- N - triorganosilylamine, e.g. those of the formulµ where R<SP>1</SP> represents hydrogen or a C 1 -C 10  monovalent hydrocarbon group, R<SP>2</SP> is the same as R<SP>1</SP> with the proviso that not more than one R<SP>2</SP> represents hydrogen, or both R<SP>2</SP> groups together represent a bivalent hydrocarbon group, m = 2-6 and p = 1-20. Preferably the nitrogen-silicon compound and the low M.Wt. reaction products each have a B.Pt. not exceeding 250 C. at 760 mm. Hg. The hexaorganodisilazane may be represented by the formula R 3 SiN(R<SP>1</SP>)-SiR 3 , e.g. hexamethyldisilazane or divinyltetramethyldisilazane. Preferred silicas are fume silica and silica hydrogels and xerogels. Preferably 1-40% wt. of water, and optionally up to 20% wt. of a compound of the formula HOSi(R) 2 [OSi(R) 2 ] y -OH, where y=0-50 and R is preferably a C 1 -C 3  alkyl group, may be added to the other components prior to, during or after the addition of the silicon dioxide, the percentages being based on the weight of the diorganopolysiloxane. The process may be carried out in a discontinuous, semi-continuous or continuous manner. The composition can be cured by means of organopolysiloxanes containing at least 3 Si-H groups per molecule, e.g. methylhydrogenosiloxanes, in the presence of a platinum catalyst, in which case the diorganopolysiloxane contains at least 2 alkenyl groups per molecule, or by free radicals using a free radical initiator, e.g. a peroxy or azo compound, or high energy rays. Uses.-Medicinal devices; contact lenses; gas masks; and encapsulating electrical and electronic devices.