Patent GB-608956-A (GB)
Title: Improvements in organo-silicon compositions and methods of making them
IPC: C08K5/14
Filed: 1945-07-17
Abstract: An organo-polysiloxane containing 1.75-2.25 of alkyl, chloroalkyl, aryl or chloroaryl radicals per silicon atom and containing at least 40 mol. per cent of units of the formula RR1SiO where R+R1 are alkyl groups is heated with a diacyl peroxide containing at least one aromatic acyl radical at a temperature of at least 100 DEG  C. until a coherent rubbery solid is obtained.  The process may comprise three steps-first a low molecular weight organosiloxane is further polymerized, e.g. by use of a dehydrating agent, by treatment with strong acid or alkali, by blowing air through the siloxane at elevated temperature or by treatment with an aromatic aryl peroxide at raised temperature.  The product of this step is a very viscous liquid or gel.  In the second step, this product is mixed with 2-6 per cent of an aromatic acyl peroxide and, if desired, a filler to yield a workable and mouldable composition which in the third step is heated in a mould, preferably in absence of oxygen, e.g. in an atmosphere of carbon dioxide.  The filler may be added in the first step if desired.  Suitable fillers are asbestos, clay, hydrated calcium silicate, zinc sulphide, silica aerogel, barium titanate, bentonite, lithopone, magnesia, graphite, ground slate, ground mica, diatomaceous earth, lead oxide, blue lead. Tables are given showing the physical properties of products with different fillers; glass, alumina, titania, calcium carbonate, iron oxide, or zinc oxide. Suitable peroxides are benzoyl, benzoyl acetyl, dinaphthoyl, and benzoyl lauroyl.  Benzoyl peroxide is used in a form precipitated on calcium sulphate.  In example 1 liquid dimethyl silicone is heated with benzoyl peroxide the resulting gel milled with asbestos and benzoyl peroxide poured into a mould and heated in an oven at 150 DEG C.  By adjusting the amounts of benzoyl peroxide used in the different stages so that a liquid or tacky product is rapidly reacted in a mould which permits of expansion but from which air is excluded a cellular moulded product can be obtained.  In example 2, diethyl silicone and benzoyl acetyl peroxide were used with fibre glass flock as the filler.  In example 3 dimethyl silicone fluid and benzoyl peroxide were mixed in a Baker-Perkins mixer to a soft oily rubbery gel to which calcium carbonate and more benzoyl peroxide were added and mixing continued.  The product was malaxated on a two-roll rubber mill to yield a "crepe" material from which flat sheets or cable insulation was moulded.  Figures are given for moulding conditions and physical properties for various fillers and amounts of peroxide. In Example 4 a gasket is prepared from laminations of fibre glass cloth and siloxane compositions.  The products are suitable for washers, gaskets, flexible tubes, cable insulation, potting compounds for high-temperature condensers, flexible bags for high-temperature mouldings, or paints, or applied to glass tape for sticky electrical insulation.  Samples have been furnished under Sect. 2 (5) of (1) a product prepared by further condensing together by heating with potassium hydroxide dimethylsiloxane and phenyl methyl siloxane, milling the gelled copolymers with titania and benzoyl peroxide and heating the compounded mixture, and (2) a similar product obtained from a copolymer of dimethyl siloxane and tolylmethyl siloxane. Specifications 592,018 and 608,955 are referred to.