CA1121230A

CA1121230A - Method of bonding silicone rubber to a substrate surface

Info

Publication number: CA1121230A
Application number: CA000332651A
Authority: CA
Inventors: Milton C. Murray
Original assignee: Dow Corning Corp
Current assignee: Dow Silicones Corp
Priority date: 1978-09-01
Filing date: 1979-07-27
Publication date: 1982-04-06
Also published as: SE434060B; DE2934203B2; SE7907273L; GB2030999A; FR2434701A1; JPS6224457B2; FR2434701B1; BR7905626A; IT1193496B; GB2030999B; JPS5534993A; DE2934203A1; BE878521A; AU5044079A; IT7925377A0; AU525384B2

Abstract

ABSTRACT OF THE DISCLOSURE

A simplified method of bonding organic peroxide vulcanized silicone rubber compositions to substrate surfaces has been developed. By adding specified amounts of acryloxyalkyl-silane to the unvulcanized silicone rubber composition, a composition is obtained which bonds to substrates when vulcanized in contact with the surface of the substrate. This method can be used to produce fabric reinforced rubber articles such as tubes, hoses, tapes and diaphragms.

Description

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This invention relates to a method of bonding heat activated organic peroxide vulcani~ed silicone rubber to the surface of substrates during the process of vulcanizing the rubber.
Certain uses for silicone rubber such as shock mounts and metal-enclosed shaft seal~ require that the rubber be firmly bonded to the surface of the substrate.
Two general methods are used for bonding silicone rubber to surfaces. The silicone rubber can be formed to shape and vulcanized, as in a mold, then bonded to a substrate surface with an adhesive. Alternatively, the unvulcanized silicone rubber stock can be applied to the subs~rate surface and then vulcanized.
In either case, most types of substrate surfaces must be carefully cleaned and then treated with special primers in order to obtain satisfactory adhesion of the vulcanized silicone rubber to the substrate surface. ~he priming of the substrate surface before the bonding step is a costly and time consuming operation that is not necessary in the method of this invention.
A majority of the commercial primers presently available are activated when applied to a substrate surface by the moisture in the air. The variability of the drying and hydrolyzing conditions due to day to day differences in the humidity in the air can lead to variability of results.
A different method of obtaining adhesion to a substrate surface is throu~h the addition of adhesion additives to the unvulcanized silicone rubber stock. U.S. Patent 4,033,924 to Mine et al. discloses a heat curable organopolysiloxane composition containing an organosilicon compound having at least one A~R~o)2si group and at least one alkyl, low molecular weight alkenyl group, or hydrogen atom bound to silicon, A is a monovalent epoxy . ~ . . . . .. . ... . . . .

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containing hydrocarbon group and R' is a low molecular weight alkyl group.
An improved method of obtaining bonding of a heat activated organic peroxide vulcanized silicone rubber to substrate surfaces has been developed. A silicone rubber composition of the type commonly known as "high viscosity" that is vulcanized through the use of organic peroxides is used as the basic material. The first step of the method is mixing into the silicone rubber composition an acryloxyalkylsilane. ThiS modified composition is then formed into the desired shape in contact with the cleaned substrate surface to which it is to be bonded. The modified composition is then heated to vulcanize it while it is in contact with the substrate surface. Bonding of the modified composition to the substrate surface takes place during the vulcanization ~ step.
This invention relates to a method for improving the bonding of a vulcanized silicone rubber to a substrate surface comprising (a) mixing 100 parts by weight of silicone rubber base consisting essentially of polydiorganosiloxane ;
containing organic radicals selected from the group consisting of methyl, vinyl, phenyl and 3,3,3-trifluoropropyl, reinforcing silica filler, and anticrepe-hardening agent; from 0 to 150 parts by weight of siliceous extending filler with an average particle size of less than 25 micrometres and a surface area of less than 50m2/g; from 0.1 to 5 parts by weight of organic peroxide vulcanizing agent suitable for vulcanizing the silicone rubber base; and from greater than 0.1 to 1.5 parts by ~ 2 .Z;3~

weight of an acrylo~yalkylsilane of the formula R O Ra C~2=C-C-O-R'~Six(3-a) in which R is selected from the group consisting of hydrogen and methyl radicals, R' is an alkylene radical of from 1 to 4 inclusive carbon atoms, X is a radical selected from the group consisting of alkoxy radicals of from 1 to 3 inclusive carbon atoms and acetoxy radicals, and a is from 0 to 2 }0 inclusive, to yield a curable silicone rubber composition, (b) forming a composition wherein the curable silicone rubber composition contacts a surface of a substrate, and thereafter (c) heating the combination to a temperature sufficiently high to vulcanize the composition, thereby yielding a vulcanized silicone rubber bonded to the substrate surface.
The silicone rubber base used in the present invention can be any mixture of polydiorganosiloxane and reinforcing silica filler including types commercially available. The polydiorgano-siloxane of this invention contains organic radicals selected from the group consisting of methyl, vinyl, phenyl and 3,3,3-trifluoro-propyl, said radicals being bonded to the silicon atoms of the polydiorganosiloxane. The polydiorganosiloxanes are commonly of a viscosity of from 1000 Pa-s up to and including non-flowing gums.
These polydiorganosiloxanes are well known in the art and are commercially available.
A silicone rubber base contains a reinforcing silica 30~ filler to improve the physical strength of the polymer.

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Reinforcing silica fillers have surface areas of from 1~0 to greater than 400 m2/g. These reinforcing silica fillers are well known in the art and can be obtained commercially. The reinforciny filler can be untreated, treated, or treated in situ during the manufacture of the silicone rubber base. The treated reinforcing silica fillers can be treated by any of the conventional methods described in the prior art, wherein the treating agents include organosilanes, organosiloxanes and silazanes. The amount of reinforcing filler can vary from 10 to as much as 100 parts by weight and the usual amount varying between 15 to 75 parts by weight per 100 parts by weight of the polydiorganosiloxane.
A silicone rubber base can also contain anti-crepe hardening agents. These anti-crepe hardening agents are used to reduce the reaction between the polydiorganosiloxane and the reinforcing silica that causes the base to become harder or pseudo-vulcanized. Such a reaction can cause the base to become too "nervy" to be of further use.
Suitable anti-crepe hardening agents are well known in the art. They can be such additives as hydroxyl endblocked short chain polydimethylsiloxane fluids. If the reinforcing filler is treated as discussed above, the silicone rubber base may not need an additional anti-crepe hardening agentO
The silicone rubber base may also contain minor amounts of additives to improve, among other things, the heat stability, handling, compression set and oil resistance. A single silicone rubber base may be used or a mixture of bases may be used to obtain the desired range of physical properties for the cured silicone rubber.

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In use, a silicone rubber base may be extended with an extending filler to increase the bulk of the composition. This helps to lower the cost of the finished part as the extending fillers are much lower in cost than the silicone rubber base.
When a silicone rubber base i5 extended with an extending filler such as ground quartz, the tensile strength of the cured composition is lower than that of the beginning base. The amount of tensile strength lost is dependent upon the relative amounts of base and extending filler used as well as ~he exact nature of both I0 ingredients.
The addition of an extending filler may also lower the bond strength of a composition intended to be bonded to a substrate surface. The method of this invention provides a means whereby compositions containing large amounts of extending filler can still be successfully bonded to substrate surfaces. As higher levels of extending filler are added, it becomes more difficult to achieve a satisfactory bond to a substrate surface. The maximum amount of extending filler that can be used and still obtain satisfactory bonding to a substrate surface will depend upon the nature of the silicone rubber base used and the extending filler used. The maximum is about 150 parts by weight of extending filler per 100 parts by weight of silicone rubber base. ;~
The siliceous extending fillers used with silicone rubber bases are finely ground particles of heat stable inorganic materials with an average particle size of under 25 micrometres.
The finest extending fillers approach a particle size and configuration such that they have a surface area of as high as 50 m2/g. Examples of siliceous extending fillers include ground quartz, diatomaceous earth and glass.

~ 5 ~`` 3~ ;23~3 About 25 parts by weight of extending filler per 100 parts by weight of silicone rubber base is necessary to signficantly lower the cost of the composition. The preferred siliceous extending fillers for use with the present invention are ground quartz and diatomaceous earth with the most preferred filler being ground quartz with an average particle size of about 5 micrometres.
- The composition of this invention contains an organic peroxide vulcanizing agent suitable for vulcanizing the polydi-organosiloxane in the silicone rubber base. When the polydiorgano-siloxane does not contain any vinyl radicals, it must be vulcanized with organic peroxides that are efficient in causing reactions in such polydiorganosiloxanes. Such organic peroxides are labeled "non-vinyl specific" and are represented by such well known organic peroxides as benzoylperoxide, dicumylperoxide and 2,4-dichlorobenzoylperoxide. When the polydiorganosiloxane contains vinyl radicals, it can be vulcanized with either "non-vinyl specific" or "vinyl specific" organic peroxides.
Representative of the vinyl specific organic peroxides are ditertiary-butyl peroxide and 2,5-bis-~tert-butylperoxy)-2,5-dimethylhexane. All these organic peroxide vulcanizing agents and their properties are well known in the art. The properties of the vulcanized silicone rubber can be altered by the type and amount of vulcanizing agent used to vulcanize the composition. Typical changes due to such choices are well recognized in the art. The vulcanizing agent can be present in amounts of from 0.1 to 5 parts by weight 2er 100 parts by weight of silicone rubber base, preferably from 0.5 to 2O0 parts by weight.
The critical component of the composition used in the

3~ method of this invention is an acryloxyalkylsilane of the formula : ,. .: : : , :
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R O Ra CH2=C-C-O-R'-Six~3-a) in which R is selected from the group consisting of hydrogen and methyl radicals, R' is an alkylene radical of from 1 to 4 inclusive carbon atoms, X is a radical selected from the group consisting of alkoxy radicals of from 1 to 3 inclusive carbon a~oms and acetoxy radicals, and a is from 0 to 2 inclusive. The silane is preferred where R is a methyl radical, a is 0, and X is a methoxy radical or acetoxy radical. The most preferred silane is gamma-methacryloxypropyltrimethoxysilane because of its efficiency in causing the vulcanized silicone rubber to bond to a substrate surface against which the silicone rubber has been vulcanized.
The acryloxyalkylsilanes used in this invention are known in the art. They are disclosed in U.S. Patent No. 3,567,497 by Plueddemann and Clark which describes the silanes and their method of manufacture. The preferred gamma-methacryloxypropyltrimethoxy-silane is commercially available.
The compositions of this invention bond to a substrate surface when the composition is cured while in contact with the subtrate surface during the vulcanization of the composition. In order to obtain bonding, it is necessary to use at least about 0.1 part by weight of silane per 100 parts by weight of silicone rubber base. The exact amount of silane necessary to obtain bonding along with the optimum property profile of the cured silicone rubber composition can be easily determined by simple experimentation. The results will depend upon the silicone rubber base selected, the kind and amount of extending filler used, the kind and amount of vulcanizing agent used, and the nature of the substrate surface to be adhered to. The preferred amount of silane is from greater than 0.1 to 1.0 parts by weight per 100 ~ ,~ ~

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parts by weight of silicone rubber base. The addition of more than about 1.5 parts by weight of silane will not improve the adhesion and will begin to adversely effect the physical properties of the cured silicone rubber composition.
The mixing step of this invention used to prepare the composition can be any suitable means that will lead to a homogeneous mix~ure of the several components. Methods of mixing that are common in the silicone rubber art and which are suitable for this invention include mixing with a dough mixer, a rubber 1~ compounding mill or with a Banbury mixer. The order of mixing is not critical. Ordinarily, the silicone rubber base is placed in the mixer, the extending filler and silane are added and mixed until homogeneous, then the vulcanizing agent is added and mixing continued until homogeneous. Any additional additives such as heat stability additives, antioxidants, processing aids, pigments, etc. would ordinarily be added before the vulcanizing agent.
The compositions can be formed to the desired configuration by any of the well known methods of forming elastomeric curable compositions such as press molding, injection molding, calendering and extruding, both supported and unsupported. Since the compositions bond without primers, special precautions must be taken during vulcanizing operations to insure that the vulcanized composition adheres only to surfaces where adhesion is desirable. The surfaces of press plates or molds for instance must be well coated with a suitable release agent. ~ `~
Suitable release agents for the method of this invention are heavy coats of a 2 to 5 percent by weight solution of detergent in water, or more preferably, a coating of fluorocarbon mold release agent. For flat surfaces, a sheet of polytetrafluoro-ethylene is satisfactory.

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The formed compositions of this invention can bevulcanized by any suitable means that will cause decomposition of the organic peroxide vulcanizing agent. Heating is the preferred method. The time and temperature necessary to cause vulcanization of the composition is dependent upon the organic peroxide vulcanizing agent chosen, the method of heating, the method of shaping the composition to the desired configuration, and the thickness of the part. The temperature that is appropriate for a given set of conditions is well known in the silicone rubber art.
Typical temperatures are from 110C. to 175C. for molding operations, to as high as 300C. for the ovens used in continuous hot air vulcanization operations.
The method of this invention is useful for making silicone rubber articles that are bonded to a subtrate surface.
Examples of such articles are metal enclosed shaft seals, shock mounts, rolls, and various types of fabric reinorced articles such as tubing, tapes and diaphragms.
The following examples are included for illustrative purposes only and should not be construed as limiting the invention which is properIy delineated by the appended claims.
All parts are parts by weight.
Example 1 A series of samples were made to evaluate the bonding characteristics of stock containing an acryloxyalkylsilane as compared to the same stock without the silane.
A stock was compounded consisting of:
(a) 100 parts of commercial silicone rubber base described as a vinyl-containing silicone rubber designed for compounding general purpose silicone rubber stock. The base was translucent with a ;~
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specific gravity after curing of 1.09. The base consisted of a vinyl containing polydimethyl-siloxane, a reinforcing fume silica, and a hydroxyl endblocked polydimethylsiloxane fluid to prevent crepe-hardening of the base, (b) 50 parts of ground quartz with an average particle size of 5 micrometres, (c) ~ parts of iron oxide paste, (d) 1 part of organic peroxide vulcanizing agent consisting of 50 weight percent 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane dispersed on an inert carrier powder.
To a portion of this stock was added 0.5 parts of gamma-methacryloxypropyltrimethoxysilane based on 100 parts of the silicone rubber base.
, Each stock was then calendered onto a piece of glass fiber fabric to a total thickness of 0.5 mm. The silicone rubber surface of the calendered fabric was then press molded against the ;
cleaned surface of metal panels as shown in Table I. The surface of the metal panels was cleaned by wiping thoroughly with chlo~othene, then with acetone. Two pieces o~ the calendered fabric were also molded against each other with the rubber `
sur~aces in contact. The moldings were for 10 minutes at 171C.
After molding, each sample was cut into 2504 mm wide strips. The calendered fabric strips were then pulled from the various substrate surfaces using a standard tensile testing machine with a rate of 50.8 mm per minute. The strips were ~ulled `
from the substrate surface at an angle of 180. The glass fiber fabric samples were pulled from one another At a total angle of 180 or at 90 each at the point of peeling apart.
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The method of failure was noted for each sample. If there was no adhesion, it was recorded as ~ero percent cohesive failure. If the sample failed by tensile failure within the rubber itself, it was recorded as 100 percent cohesive failure.
The results are shown in Table I. The addition of the silane to the silicone rubber stock greatly improved the adhesion to all of the substrate surfaces tested.
TABLE I

Amount of Subs~rateAdhesion Failure lOSilane kN/m TyPe none Aluminum 0.05 0%
cohesion none C. Ro Steel 0.13 0 none Stainless0.13 0 Steel none Glassfiber0.56 5 Fabric 0.5 part Aluminum 2.45 100 0.5 part C. R. Steel 2.62 100 0.5 part Stainless1.75 50 Steel 0.5 part Glassfiber2.80 80 Fabric Example 2 A different commercial silicone rubber base was used to prepare samples in the same manner as in Example 1.
A stock was compounded consisting of (a) lO0 parts of a commercial silicone rubber base similar to that of Example 1 but with a higher loading of reinforcing silica. The specific gravity of the translucent base was l.10, ~ZL1%~23~

(b) 2 parts of iron oxide paste, and (c) 1 part of the organic peroxide vulcaniæing agent of Example 1.
To a portion of the stock, 0.5 parts of the silane used in Example 1 was added based on 100 parts of stock.
Test samples were prepared and tested in the same manner as described in Example 1. The results are recorded in Table II.
The addition of the silane to a silicone rubber stock not containing extending filler greatly improved the adhesion to all the substrate surfaces tested.
A sample of each type of substrate surface was primed with a commercial primer. The primer is used with organic peroxide catalyzed, heat cured, silicone rubber stock to bond without oven post curing. A sample was prepared using the above stock of this Example without the silane added. The samples were molded and tested as described in Example 1. The results are recorded in Table II. The stock bonded with the primer did not give as high a bond strength as that of the s ock bonded by adding the silane to the stock. The adhesion shown by the glassfiber fabric samples is partially due to mechanical trapping of the rubber into the rough surface of the fabric. The amount of cohesive failure is also more difficult to judge due to the very uneven nature of the surface.

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TABLE II
-Amount of Substrate Adhesion Failure Silane _ _ _ _ _kN/m ___Type_ none Aluminum 0.12 0~
cohesion C. R. Steel 0.35 0 Stainless 0O53 0 Steel Glassfiber 0.88 0 Fabric 0.5 Aluminum 3.7 100 C. R. Steel 3.3 100 Stainless 3.2 95 Steel Glassfiber 3.8 80 ~abric 1.O Aluminum 4.3 100 C. R. Steel 4.0 100 . .
Stainless 3.4 100 Steel Glassfiber 4.2 .20 Fabric primer Aluminum 0.88 40 C. R. Steel 1.1 20 Stainless O.g6 50 Steel Glassfiber 4.1 15 Fabric Example 3 Different types of additives were mixed with a commercial silicone rubber base to compare their usefulness in improving bonding to glassfiber fabric.

A stock was compounded consisting essentially of (a) 100 parts of a commercial silicone rubber base designed to give a 70 durometer, high tensile strength product. The specific gravity of the base was 1.~1, (b) 50 parts of ground quartz with an average particle size of 5 micrometres, and (c) 1 part of the organic peroxide vulcanizing agent of Example lo Portions of the stock were mixed with 1 part of the additives, detailed below, on a 2 roll mill. Each sample was then molded under low pressure in a press against a coarse weave untreated glassfiber fabric for 10 minutes at 171C. The samples were then evaluated by pulling the cured silicone rubber stock and the glassfiber fabric apart. The results are recorded in Table III.
Sample 1 had no additive.
Sample 2 was a mixture of a trimethylsiloxy endblocked polymethylhydrogensiloxane with a silicon-bonded hydrogen atom content of about 1.6 weight percent and ethylpolysilicate. This mixture is known to give bonding.
Sample 3 was gamma-glycidoxypropyltrimethoxysilane. This epoxy functional silane is used to aid bonding with many di~ferent types of polymer.
Sample 4 was gamma-methacryloxypropyltrimethoxysilane.
The results show that the silane of this invention gave superior bonds as compared to the other additives tested.

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TABLE III
Sample Result 1 Blank weak mechanical bond, 0% cohesive failure 2 Comparative Example some bond, 0% cohesive failure 3 Comparative Example stronger than 2, o% cohesive failure

4 This invention strong bond, I0 100% cohesive failure Example 4 A series of samples were made to evaluate the effect of adding acryloxyalkylsilane on the physical properties of the cured silicone rubber composition. -A stock was compounded consisting of 100 parts of the commercial silicone rubber base of Example 1, 100 parts of the ground quartz extending filler of Example 1, and 1 part of the organic peroxide vulcanizing agent of Example 1.
Portions of the above stock were then mixed with gamma-methacryloxypropyltrimethoxysilane in the amounts shown in Table IV for 100 parts of base.
Each portion was molded into test slabs in a press, using ;
aluminum plates treated with a commercial soap-type release agent specified for use with heat cured silicone rubber. The molding was for 10 minutes at 171C. The samples containing 0.5 and 1.0 part of the silane were very difficult to remove from the aluminum plates, even though the plates were coated with a release agent.
The physical properties of the slab were determined in accordance with the procedures described by ASTM-412 for tensile strength and elongation, by ASTM-D625, die B for tear strength, :

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and by ASTM-D2240 for durometer, type A. The measured physical properties were as shown in Table IV in which the tensile strength is recorded in megapascals tMPa) and the tear streng~h is recorded -in kilonewtons per meter ~kN/M).
The addition of the silane to the compounded stock containing a siliceous extending filler did not harm the physical properties. It caused a significant improvement in the tensile strength.
TABLE IV

Tensile Amount of Silane Durometer StrengthElongation parts/100 p_rts base _ _ _ MPa _ %
none 60 3.1 400 0.1 62 6.8 200 0.5 62 6.9 180 1.0 63 ~ 6.0 150 Example 5 A series of samples were made to evaluate the level of acryloxyalkylsilane needed in a vulcanized silicone rubber stock to bond to glassfiber fabric~

A stock was compounded consisting of 100 parts of the comme,rcial silicone rubber base of Example 3, 25 parts of the ground guartz of Example 3, 1 part of a commercial heat stability additive, and 1 part of the organic peroxide vulcanizin~ agent of Example 1.
Portions of the stock were then mixed with the amounts of gamma-methacryloxypropyltrimethoxysilane shown in Table V based on 100 parts of the silicone rubber base~
Each portion was then calendered onto style 1528 glassfiber fabric to a total thickness of 0.5 mm. Test samples -were prepared by placing pieces of each calendered sample together such that the sample was 4 plies thick. The two center plies were rubber face to rubber face, while the outer plies were rubber face to glassfiber fabric face. Each sample was molded for 10 minutes at 171C. in a press under light pressure to vulcanize the stock and bond the pieces together.
The samples were then evaluated by pulling the pairs of plies apart in a standard test machine at a rate of 50.8 mm per minute causing the plies to separate at the center interface where two rubber layers were together. The plies were pulled from one another at a total angle of 180 or at go each at the point of peeling apart.
The method of failure was noted for each sample. The results are shown in Tablè V.
The addition of the silane to the stock used in this method of bonding improved the bond over that obtained with no silane. The failure at the 0.1 part level appeared to be an adhesive failure, but the higher peel strength shows that some adhesion must have been taking place. The lower peel strengths for the samples with 0.75 part silane and 1.0 part silane are probably due to the higher modulus of those stocks and its effec~
on the geometry of the failure point as the pieces are pulled apart.

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Amount of Adhesion Failure S ilane _ kN/m _Type 21one 0. 35 0%
cohes ion Ool . 0~78 10 O~ 25 1~3 100 0~5 1~1 100 O ~ 75 0 ~ 91 100 1~0 0~96 100 ,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for improving the bonding of a vulcanized silicone rubber to a substrate surface comprising (a) mixing 100 parts by weight of silicone rubber base consisting essentially of polydiorganosiloxane containing organic radicals selected from the group consisting of methyl, vinyl, phenyl and 3,3,3-trifluoropropyl, reinforcing silica filler, and anticrepe-hardening agent; with from 0 to 150 parts by weight of siliceous extending filler with an average particle size of less than 25 micrometres and a surface area of less than 50 m2/g; from 0.1 to 5 parts by weight of organic peroxide vulcanizing agent suitable for vulcanizing the silicone rubber base, and from greater than 0.1 to 1.5 parts by weight of an acryloxyalkylsilane of the formula in which R is selected from the group consisting of hydrogen and methyl radical, R' is an alkylene radical of from 1 to 4 inclusive carbon atoms, X is a radical selected from the group consisting of alkoxy radicals of from 1 to 3 inclusive carbon atoms and acetoxy radical, and a is from 0 to 2 inclusive, to yield a curable silicone rubber composition, (b) forming a combination wherein the curable silicone rubber composition contacts a surface of a substrate, and thereafter (c) heating the combination to a temperature sufficiently high to vulcanize the composition, thereby producing a vulcanized silicone rubber bonded to the substrate surface.

2. The method of claim 1 wherein the siliceous extending filler is selected from the group consisting of ground quartz and diatomaceous earth, the organic peroxide vulcanizing agent is present in an amount of from 0.5 to 2.0 parts by weight, and the acryloxyalkylsilane is gamma-methacryloxypropyltrimethoxysilane.

3. The method of claim 1 or claim 2 wherein the substrate surface is metal or glass.

4. A vulcanized silicone rubber bonded to a substrate surface by the method of claim 1 or claim 2.