GB1588510A - Bisilicon elastomer composite articles - Google Patents

Description

(54) BI-SILICONE ELASTOMER COMPOSITE ARTICLES (71) We, DOW CORNING LIM ITED, a British Company of 12 Whitehall, London, SW1A 2DZ do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method for bonding a silicone elastomer to a silicone elastomer.

It is soften required to bond a silicone elastomer to a substrate. Although some types of silicone elastomer will adhere directly to the surfaces of some substrates it is usually necessary to prime the substrate to obtain a satisfactory bond. The choice of a suitable primer for any particular set of circumstances, is however, not easy. It has been found that a primer which provides an acceptable bond with one type of silicone elastomer/substrate gives only a weak bond or no bond at all with a different silicone elastomer and/or substrate.

Elastomers prepared by reacting together a vinyl containing polydiorganosiloxane and a siloxane containing -SiB groups are known. The reaction whereby such elastomers are obtained is generally catalysed by a compound or complex of a platinum metal and may occur at room temperature or elevated temperatures. In the production of certain electrical structures, for example in high volt-, age cable terminations, it has been found' necessary to bond such elastomers to themselves or to other silicone elastomers, for example those of the peroxide-cured type. Attempts to procure such a bond with known primers or without a primer have, however, been unsuccessful.

We have now found that the bonding relationship between said silicone elastomers can be improved if there is employed as a primer substance an organosiloxane containing silicon-bonded hydrogen atoms.

Accordingly this invention provides a process for forming a composite article in which a silicone elastomer is bonded to a silicone elastomer which comprises (1) applying to the surface of a silicone elastomer a coating of an organosiloxane wherein at least 50 molar per cent of the units are

units, any remaining units being dimethylsiloxy units, triorganosiloxy units or diorgano(hydrogeno)siloxy units, (2) heating the applied coating to effect at least partial crosslinking of the organosiloxane, (3) applying to at least a part of the coating a silicone elastomer-forming composition comprising a polydiorganosiloxane having silicon-bonded vinyl radicals in the molecule, an organosiloxane having in the molecule at least two silicon-bonded hydrogen atoms and a catalyst which promotes the reaction of silicon-bonded vinyl radicals with silicon-bonded hydrogen atoms, and (4) curing the silicone elastomerforming composition.

In the process of this invention any silicone elastomer may form the substrate to which bonding is desired. The silicone elastomer may be of the heat cured type or of the so-called room temperature curing type. Thus, the base organopolysiloxane may be cured, for example by means of an organoperoxide or per-ester, by high energy radiation or by the addition of a silane crosslinking agent, with or without a curing catalyst. Such elastomers are well-known in the art and are described, for example, in U.K. Patent Specification Nos. 760,451, 764,246, 802,355, 862,576, 1,097,379, 1,118,016, 1,110,429, 1,178,055, 1,272,705 and 1,296,302. The silicone elastomer may contain any desired fillers and other additives, for example, fume silica, precipitated silica, diatomaceous earth, aluminium hydrate, magnesium oxide and zinc oxide.The process of this invention is however, especially useful for effecting bonding to silicone elastomers containing carbon black.

Bonding to such elastomers when they contain relatively high loadings of carbon black is normally particularly difficult.

According to Step (1) of the process of this invention there is applied to the silicone elastomer substrate an organosiloxane in which at least 50 molar per cent of the structural units are

units. Any remaining structural units in the organosiloxane may be one or more of dimethylsiloxy [(CH,)2SiO j units, triorgano- siloxy (R,3SiC)2iD units, in which R is a monovalent hydrocarbon radical free of aliphatic unsaturation, e.g. methyl, ethyl or phenyl, or diorgano(hydrogeno)siloxy (R2HSiO) units.

Preferred are the trimethylsilyl end-stopped polymethylhydrogensiloxanes having a molecular weight within the range from about 1500 to about 3000.

The organosiloxane employed in Step (1) may be applied to the silicone elastomer substrate as such or from a volatile diluent, for example xylene, toluene or benzene. As satisfactory bonding may be obtained employing only a very thin coating of the organosiloxane it is preferably applied to the silicone rubber substrate as a solution in a volatile organic solvent.

After application, the applied coating is heated to effect at least partial crosslinking of the organosiloxane. If desired, crosslinking may be expedited by the incorporation in the organosiloxane of a catalyst, for example a basic substance or an organic metal compound.

Usually exposure of the coating to a temperature of from 120 to 1500,C for up to about 10 minutes is sufficient to effect a sufficient degree bf crosslinking. Lower temperatures and shorter times can, however, be employed when a crosslinking catalyst is present. A sufficient degree of crosslinking is indicated by the coating being dry or substantially nonztacky to the touch.

The elastomer-forming composition which is bonded to the silicone elastomer substrate according to this invention comprises a polydiorganosiloxane having silicon-bonded vinyl radicals, an organosiloxane having siliconbonded hydrogen atoms and a catalyst which promotes the addition of silicon-bonded hydrogen atoms to silicon-bonded vinyl radi- cals. Examples of suitable catalysts for such compositions are chloroplatinic acid and complexes of platinum halides with compounds containing aliphatic unsaturation, for example cyclohexene and vinyl-containing organo siloxanes. Elastomer-forming compositions of this type are known in the art and are described in, for example, U.K. Patent Specifications 1,141,168, 1,055,777, 1,272,705 and 1,281,343.

In some cases, depending for example on the type of catalyst and presence or absence of a cure inhibitor, the applied composition can be cured to a firm rubber in a few hours by exposure to normal ambient temperatures, that is about 20 to 250C. Other, less active, formulations, especially those containing a low temperature cure inhibitor such as an oxime or an unsaturated alcohol, may require exposure to temperatures above about 600C if cure is to be effected in a reasonablysshort period.

The process of this invention can be employed in the formation of any composite structure comprising the specified silicone elastomers in a bonding relationship. For example the process can be employed for the manufacture of terminations in electrical cables wherein at least a part of the insulation is a cured silicone elastomer. It can also be employed in applications requiring the formation of a layer of silicone elastomer of one type or hardness over a layer of different type or hardness.

The following example in which the parts are expressed by weight, illustrate the invention.

Example.

A silicone elastomer substrate was prepared by mixing: Gum copolymer of dimethyl siloxy and vinylmethylsiloxy groups (mw ~ 0.75 X 106, 0.25 mol % vinyl) 100 parts Shawinigan carbon black 55 Trimethylsilyl end-stopped polymethylhydrogensiloxane (mw -- 2400) 3 Complex of chloroplatinic acid and vinylmethylsiloxane 0.5 The mixture was pressed into a sheet and cured in the press at 140 degrees C. After removal of the sheet from the press it was allowed to cool and its surface coated by brushing with a 50% by weight solution in a chlorinated solvent of a trimethylsiloxy- terminated polymethylhydrogensiloxane having a molecular weight of approximately 2400. The sheet was then placed in an oven at 1500C for 10 minutes to provide a primed substrate.

An elastomer-forming composition was prepared by mixing together: Polydimethylsiloxane with phenylmethylvinylsiloxy end groups (M.Wt approximately 40,000) 100 parts Polymethylhydrogensiloxane (M.Wt approx. 2400) 2 Reinforcing silica filler 40 parts Platinum catalyst 0.15 The mixture thus obtained was flowable in consistency and was spread over the primed substrate obtained as described above. The sheet was placed in an oven at 1500C for 30 minutes to effect cure of the elastomer-forming c6mposition and provide a composite in which the elastomers were strongly bonded together.

WHAT WE CLAIM IS: 1. A process for forming a composite article in which a silicone elastomer is bonded to a silicone elastomer which comprises (1) applying to the surface of a silicone elastomer a coating of an organosiloxane wherein at least 50 molar per cent of the units are

units, any remaining units being dimethylsiloxy units, triorganosiloxy units or diorgano(hydrogeno)siloxy units, (2) heating the applied coating to effect at least partial crosslinking of the organosiloxane, (3) applying to at least a part of the coating a silicones elastomer-forming composition comprising a polydiorganosiloxane having silicon-bonded vinyl radicals in the molecule, an organosiloxane having in the molecule at least two silicon-bonded hydrogen atoms and a catalyst which promotes the reaction of silicon-bonded vinyl radicals with silicon-bonded hydrogen atoms, and (4) curing the silicone elastomerforming composition.

2. A process as claimed in Claim 1 wherein the organosiloxane employed according to Step (1) is a trimethylsiloxy end-stopped polymethylhydrogensiloxane having a molecular weight within the range from 1,500 to 3,000.

3. A process as claimed in Claim 1 or Claim 2 wherein the organosiloxane employed according to Step (1) is applied to the silicone elastomer as a solution in a volatile organic solvent.

4. A process as claimed in any one of the preceding claims wherein the substrate silicone elastomer contains carbon black.

5. A process as claimed in any one of the preceding claims substantially as described with reference to the Example herein.

6. A composite article in which a silicone elastomer is bonded to a silicone elastomer according to the process claimed in any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. Reinforcing silica filler 40 parts Platinum catalyst 0.15 The mixture thus obtained was flowable in consistency and was spread over the primed substrate obtained as described above. The sheet was placed in an oven at 1500C for 30 minutes to effect cure of the elastomer-forming c6mposition and provide a composite in which the elastomers were strongly bonded together. WHAT WE CLAIM IS:

1. A process for forming a composite article in which a silicone elastomer is bonded to a silicone elastomer which comprises (1) applying to the surface of a silicone elastomer a coating of an organosiloxane wherein at least 50 molar per cent of the units are

units, any remaining units being dimethylsiloxy units, triorganosiloxy units or diorgano(hydrogeno)siloxy units, (2) heating the applied coating to effect at least partial crosslinking of the organosiloxane, (3) applying to at least a part of the coating a silicones elastomer-forming composition comprising a polydiorganosiloxane having silicon-bonded vinyl radicals in the molecule, an organosiloxane having in the molecule at least two silicon-bonded hydrogen atoms and a catalyst which promotes the reaction of silicon-bonded vinyl radicals with silicon-bonded hydrogen atoms, and (4) curing the silicone elastomerforming composition.

2. A process as claimed in Claim 1 wherein the organosiloxane employed according to Step (1) is a trimethylsiloxy end-stopped polymethylhydrogensiloxane having a molecular weight within the range from 1,500 to 3,000.

3. A process as claimed in Claim 1 or Claim 2 wherein the organosiloxane employed according to Step (1) is applied to the silicone elastomer as a solution in a volatile organic solvent.

4. A process as claimed in any one of the preceding claims wherein the substrate silicone elastomer contains carbon black.

5. A process as claimed in any one of the preceding claims substantially as described with reference to the Example herein.

6. A composite article in which a silicone elastomer is bonded to a silicone elastomer according to the process claimed in any one of the preceding claims.