GB2039231A

GB2039231A - Coating crosslinked organopolysiloxanes

Info

Publication number: GB2039231A
Application number: GB7941492A
Authority: GB
Original assignee: Wacker Chemie AG
Current assignee: Wacker Chemie AG
Priority date: 1978-12-01
Filing date: 1979-11-30
Publication date: 1980-08-06
Also published as: NL7908083A; BE880329A; JPS5575430A; IT7950946A0; CH641191A5; DE2852090C2; FR2442668A1; NO793902L; ES486473A1; FR2442668B1; SE438610B; AU5339679A; CA1142818A; SE7909907L; DE2852090A1

Abstract

The adhesion of the coatings to crosslinked organopolysiloxanes, is improved by applying to the surface, before coating or adding to the coating composition a silane containing per molecule (a) at least one amino group that is bonded through one or more carbon atoms via an SiC-linkage and optionally one or more other atoms, to a silicon atom, and (b) at least one silicon-bonded monovalent hydrocarbonoxy radical that is unsubstituted or is substituted by an amino or alkoxy group, and/or a partial hydrolysate thereof. Preferred silanes have the formula [R<2>2N(CH2)mYa(CH2)n]b Si(OR<3>)cR4-b-c in which R denotes optionally substituted hydrocarbon, R<2> denotes hydrogen or optionally substituted hydrocarbon, R<3> denotes unsubstituted or (amino or alkoxy) - substituted hydrocarbon, Y denotes -O- or -N(R<2>)-, a denotes 0 or 1, b and c denote 1 to 3, and m and n denote 1 to 10; for example the silane (CH3O)3Si(CH2)3NH(CH2)2NH2.

Description

SPECIFICATION Coating crosslinked organopolysiloxanes The present invention relates to a process for coating a crosslinked organopolysiloxane with a coating agent.

The coating of the crosslinked organopolysiloxane with a coating agent has for some time presented a problem because the coating agent often will not adhere readily to the crosslinked organopolysiloxane. Various solutions have been proposed to this problem, for example, including various additives in the organopolysiloxane prior to crosslinking (see, for example, German Offenlegungschrift No.2407290); by applying certain silanes (including those of the type used according to the invention as discussed below) to the coating agent shortly after applying the coating agent to the crosslinked organopolysiloxane and then allowing the silanes to penetrate into the coating agent (see U.S. Patent Specification No. 3 561 996), and by using, as coating agents, dyes containing organosiloxane units (see, for example, U.S.

Patent Specification No. 4038293).

The present invention provides a process for coating a crosslinked organopolysiloxane with a coating agent, wherein (i) a silane that contains, per molecule, (a) at least one amino group that is bonded through one or more carbon atoms, and optionally one or more other atoms, to a silicon atom via an SiC-linkage, and (b) at least one silicon-bonded monovalent hydrocarbonoxy radical that is unsubstituted or is substituted by an amino or alkoxy group, and/or (ii) a partial hydrolysate of such a silane, is applied, prior to coating, to at least that part of the surface of the crosslinked organopolysiloxane that is to be coated, and/or is mixed with the coating agent prior to applying the coating agent to the surface of the crosslinked organopolysiloxane.

The process according to the invention has the advantages over the mentioned known solutions to the problem of coating a crosslinked organopolysiloxane with a coating agent that, for example, it is suitable for coating a greater number of types of crosslinked organopolysiloxane and/or that it is suitable for use with a greater number of coating agents and/or that the organosilicon compounds used in this process, namely the said silane or partial hydrolysate thereof, are more readily available.

The silanes used according to the invention and defined at (i) above have previously been used as a component of diorganopolysiloxane compositions that are storable in the absence of water and that will crosslink at room temperature in the presence of water to form elastomers (see, for example, German Offenlegungschrift No. 19 64502) in order to improve the adhesion of elastomers manufactured from these compositions to substrates on which they are manufactured. When used as components of such compositions, however, these silanes do not improve the adhesion of coating agents applied to such elastomers.The silane used in the process according to the invention is preferably a compound of the general formula R22N(CH2)rnYa(CH2)rj bSi(OR3)CR4-b--C in which R denotes an unsubstituted or substituted monovalent hydrocarbon radical, R2 denotes a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon radical, R3 denotes a monovalent hydrocarbon radical that is unsubstituted or is substituted by an amino or alkoxy group, Y denotes -0-or N(R2)-, a denotes 0 or 1, each ofb and c denotes 1, 2 or 3, with the proviso thatthesum!;+c does not exceed 4, and each of m andn denotes an integerfrom 1 to 10.

In the above general formula and throughout this specification, any symbol representing a chemical atom, group or radical that appears more than once in the same formula may have the same meaning or different meanings on each occurrence.

Unsubstituted monovalent hydrocarbon radicals denoted by R in the above formula are, for example, alkyl radicals, for example methyl, ethyl, n-propyl, isopropyl and octadecyl radicals; alkenyl radicals, for example vinyl and allyl radicals; cycloaliphatic hydrocarbon radicals, for example cyclopentyl, cyclohexyl, methylcyclohexyl and cyclohexenyl radicals; aryl radicals, for example phenyl and xenyl radicals; aralkyl radicals, for example benzyl, p-phenylethyl and p-phenylpropyl radicals; and alkaryl radicals, for example tolyl radicals. Substituted monovalent hydrocarbon radicals denoted by R are, for example, haloaryl radicals, for example chlorophenyl and bromophenyl radicals; andcyanoalkyl radicals, for example p-cyanoethyl radicals. Preferably, R denotes a methyl radical because of its ready availability.

Examples of unsubstituted and substituted monovalent hydrocarbon radicals denoted by R2 in the above formula are all those radicals listed above as examples of radicals denoted by R, with the exception of vinyl radicals, and also n-butyl, tert-butyl, 3, 5, 5 - trimethylcyclohexyl and 2,3,4 - triethylcyclohexyl radicals. Preferably, at least one symbol B2 in the above general formula denotes a hydrogen atom.

Examples of monovalent hydrocarbon radicals that are unsubstituted or are substituted by an amino or alkoxy group, denoted by R3 in the above formula, are alkyl radicals having up to 6 carbon atoms, for example methyl, ethyl, n-butyl, sec-butyl and and tert-butyl radicals; aminoalkyl radicals, for examplep-aminoethyl radicals; alkoxyalkyl radicals, for example p-methoxyethyl radicals; cycloalkyl radicals, for example cyclohexyl radicals; aryl radicals, for example phenyl radicals; and aralkyl radicals, for example benzyl radicals.

Examples of silanes of the above general for mula suitable for use in the process according to the invention are the compounds of the following for mulae: CH3Si(OCH2CH2NH2)2it(CH2)30(CH2)2NH2], (CH3O)3Si(CH2)3NH(CH2)2NH2, H2N(CH2)2o(CH2)3Si(oCH2CH2NH2)3 H2N(CH2)20(CH2)3Si(OC2Hs)3, and also the following compounds:: N - ss - aminoethyl - y - aminopropyltriethoxysilane of the formula H2N(CH2)2NH(CH2)3Si(OC2Hs)3, ss-,aminoethyltriethoxysilane of the formula H2N(CH2)2Si(OC2Hs)3, N - '3 - aminoethyl - 8 - aminobutyltriethoxysilane of the formula H2N(CH2)2NH(CH2)4Si(OC2Hsht y - aminopropyltriethoxysilane of the formula H2N(CH2)3Si(OC2Hs)3, aminomethyltrimethoxysilane of the formula H2NCH2Si(OCH3)3, N - ss - amino - y - aminopropyltris (methox yethyleneoxy) silane of the formula H2N(CH2)2NH(CH2)3Si(OCH2CH20CH3)3, 8-aminobutyltriethoxysilane of the formula H2N(CH2)4Si(OC2H5)3.

In the process according to the invention, a mixture of two or more silanes as defined at (i) above, for example a mixture of two or more silanes of the above general formula, may be used. Moreover, in addition to or instead of using a silane as defined at (i) above in the process according to the invention, it is possible to use a partial hydrolysate of such a silane but, in this case, the partial hydrolysate should preferably contain not more than 10 silicon atoms per molecule. In general, however, it is preferable to use the silane rather than the partial hydrolysate.

Any desired crosslinked organopolysiloxane may be coated according to the process of the invention.

The process is, however, particularly suitable forthe coating of crosslinked organopolysiloxanes that have been produced from diorganopolysiloxane compositions that are storable in the absence of water and that will crosslink at room temperature in the presence of water. Elastomers (or crosslinked organopolysiloxanes) produced from such compositions are, for example, often used for sealing joints, such as joints around wooden windows and joints on ships decks. It is often particularly desirable to coat elastomers used in such situations.

Diorganopolysiloxane compositions that are storable in the absence of water and that will crosslink at room temperature in the presence of water used in the manufacture of crosslinked organopolysiloxanes to be coated according to the process of the invention may be any such known diorganopolysiloxane compositions. The compositions may be prepared using components that have previously been used, or that could be used, in the manufacture of compositi6ns of this type and such components may be used in the same quantities as previously. Compositions of this type generally comprise a silicon compound having at least three silicon-bonded hydrolysable groups per molecule, a diorganopolysiloxane having condensable terminal groups and, optionally, a filler and/or a condensation catalyst and/or a plasticiser.

Suitable silicon compounds having at least three silicon-bonded hydrolysable groups per molecule are silanes of the general formula RaSiZ4 a in which R and a are defined as above, and Z denotes a hydrolysable group, and partial hydrolysates of such silanes having not more than 10 silicon atoms per molecule.

Examples of unsubstituted and substituted monovalent hydrocarbon radicals denoted by R in the above formula are all those radicals previously mentioned as examples of radicals R in the earlier formula.

Examples of hydrolysable groups denoted by Z are (i) acyloxy groups, for example those of the formula -OOCRI, for example formyloxy and acetoxy groups; (ii) unsubstituted or substituted hydrocarbonoxy groups, namely groups of the formula -OR', for example methoxy, ethoxy, isopropenyloxy and methoxyethoxy groups; (iii) aminoxy groups, for example groups of the formula -ONR'2, for example dimethylaminoxy and diethylaminoxy groups; (iv) amino groups, for example groups of the formula -NR'2, for examplen-butylamino, secbutylamino and cyclohexylamino groups; (v) acylamino groups, for example groups of the formula -N(B1)C(O)R1, for example the benzoylamino groups;; (vi) oxime groups, for example groups of the formula -ON=CR1,, for example acetaldoxime, acetoneoximeand 2-butanoneoxime groups; and (vii) phosphate groups, for example groups of the formula

for example the di - n - butylphosphate group; in all of which formulae R' denotes an unsubstituted or substituted monovalent hydrocarbon radical and, in at least some of the above formulae as will be known by those skilled in the art, at least one of the symbols R' canalterna- tively denote a hydrogen atom.

Suitable fillers are, for example, fume silica and calcium carbonate. An example of a suitable condensation catalyst is dibutyltin dilaurate. Suitable plasticisers are, for example, trimethylsiloxy - terminated dimethylpolysiloxanes that are liquid at room temperature and polyoxyalkylene glycols.

Crosslinked organopolysiloxanes that can be coated by the process according to the invention are also, for example, elastomers or resin-like products produced from so-called "2-component diorganopolysiloxane compositions" for example systems consisting of one pack containing a filler and dimethylpolysiloxane having a silicon-bonded hydroxy group in each terminal unit and a second pack containing a mixture of polyethyl silicate and dibutyltin dilaurate.

Further examples of crosslinked organopolysiloxanes that can be coated according to the process of the invention are elastomers or resin-like products produced from organopolysiloxanes at high temperatures, usually temperatures of at least 100"C, by condensation or by free-radical formation by means of, for example dicumyl peroxide.

Still further examples of crosslinked organopolysiloxanes that can be coated according to the process of the invention are elastomers or resinlike products produced by reacting organopolysilox anes containing silicon-bonded hydrogen atoms with organopolysiloxanes containing aliphatic unsaturation in the presence of a catalyst promoting the addition of silicon-bonded hydrogen atoms to aliphatic multiple bonds, for example platinum compounds.

In the process according to the invention the silane or partial hydrolysate thereof is preferably applied, prior to coating with the coating agent, to at least that part of the surface of the crosslinked organopolysiloxane to be coated. This method has the advantage that smaller quantities of the silane or partial hydrolysate thereof are generally required than are required if the silane or partial hydrolysate is mixed with the coating agent When applying the silane or partial hydrolysate to the surface of the crosslinked organopolysiloxane in this manner, it is generally advantageous to apply it in admixture with or in solution in a liquid diluent. In this case, the amount of silane or partial hydrolysate is advantageously within the range of from 1 to 90% by weight, based on the total weight of the liquid diluent and the silane or partial hydrolysate.Suitable liquids for diluting the silane or partial hydrolysate are, for example, aliphatic hydrocarbons, for exam ple alkanes having a boiling point of from 120 to 1800C measured under a pressure of 1 bar; aromatic hydrocarbons, for exampletoluene and xylene; ketones, for example methyl ethyl ketone; alcohols, for example methanol and isopropanol; and chlorohydrocarbons, for example trichloroethylene.

The application of the silane or partial hydrolysate, optionally diluted with a liquid diluent, to the surface of the crosslinked organopolysiloxane to be coated may be effected in any known manner, for example by spraying, by brush-application, by roller application or by immersion.

In many cases, it is advantageous to apply the condensation or hydrolysis catalyst to at least that part of the surface of the crosslinked organopolysiloxane to be coated, prior to or subse quest to the application of the silane or partial hyd rolysate, or during the said application and in admix ture with or separately from the silane or partial hyd rolysate.Examples of condensation and hydrolysis catalysts suitable for this purpose are metal and organometal carboxylic acid salts in which the metal is one from lead to manganese inclusive in the elec tromotive series (as given in "Handbook of Chemis 'try and Physics, 31st Edition (1949), Cleveland, Ohio, U.S.A., page 1465), especially tin and organotin car boxylic acid salts, for example dibutyltin dilaurate, dibutyltin diacetate, dibutyltin diVersatates ("Versa tic" is a Trade Mark; Versatic acid is a mixture of carboxylic acids having 9 to 11 carbon atoms per molecule in which in at least 90% of the acids the carboxylic group is bonded to a tertiary carbon atom), dibutyltin dioctoate, and distannoxanes, (for example diacetoxytetrabutyl distannoxane and dioleoyltetramethyl distannoxane).Examples of other such carboxylic acid salts include iron octoate, lead octoate, lead laurate, cobalt napthenate, titanium esters (for example tetrabutyl titanate), amines (for example n-hexylamine), and amine salts (for example n-hexylamine hydrochloride and n-butylaminoacetate).

When, in the process according to the invention, the silane of partial hydrolysate thereof is mixed with the coating agent prior to applying the coating agent to the surface of the crosslinked organopolysiloxane, the silane or partial hydrolysate is preferably used in an amount within the range of from 0.1 to 10% by weight, based on the total weight of the silane or partial hydrolysate thereof and the coating agent.

Coating agents that may be used in the process according to the invention include, for example, varnishes, glazes, and aqueous dispersions. The coating agent may, for example, comprise a solution in an organic solvent of an alkyd resin, a polyurethane, reactants to produce a polyurethane, nitrocellulose, an epoxide, a polyester, or a homopolymer or copolymer of vinyl chloride. Alternatively, the coating agent may, for example, comprise an aqueous dispersion of an acrylic resin, polyvinyl acetate, our a styrene/vinyl acetate copolymer, for example polystyrene acrylate.

The following Examples 1 and 2 illustrate the process according to the invention and Examples 3 and 4 illustrate, for comparison purposes, similar processes in which the silane or partial hydrolysate is not used as in the process of the invention. All parts and percentages specified in the examples and in the following description are calculated by weight unless otherwise stated.

Two crosslinked organopolysiloxanes, namely organopolysiloxane elastomers, were used in the examples and these were prepared as described below.

ElastomerA A mixture of 120 parts of a dimethylpolysiloxane having a silicon-bonded hydroxy group in each terminal unit and having a viscosity of 80,000 mPa.s at 250C, 80 parts of a trimethylsiloxy - terminated dimethylpolysiloxane having a viscosity of 35 mPa.s at 25"C, and 8 parts of a polyoxyalkylene glycol (40% ethylene oxide and 60% propylene oxide) having an average molecular weight of 3400, was mixed with 28 parts of a mixture of 2 moles of methyltris (butanone - 2 - oxime) silane, and 1 mole of methyltris (cyclohexylamino) silane, and with 2.4 parts of the silane of the formula (CH2O)2Si(CH2)2NH(CH2)2NH2.

To this mixture, the following components were added in the given sequence: 180 parts of calcium carbonate, the particles of which had been coated with 3% of stearic acid, 16 parts of fume silica having a BET specific surface area of 150 m2/g, and 0.4 partofdibutyltin dilaurate.

The resulting composition was storable in the absence of water but curable at room temperature in the presence of water. It was spread on a flat base to form a layer approximately 5 mm thick, and was then exposed to atmospheric moisture at room temperature. The composition thus crosslinked to form an elastomer, and this was used 7 days after spreading the composition onto the base.

Elastomer B 100 parts of a trimethylsiloxy - terminated diorganopolysiloxane (99.93 mole% of dimethylsiloxane units and 0.07 mole % of vinylmethylsiloxane units) having a viscosity of approximately 5 x 106 mPa.s at 25 C were mixed first with: 7 parts of a dimethylpolysiloxane having a silicon-bonded hydroxy group in each terminal unit and having a viscosity of approximately 40 mPa.s at 25JC and 40 parts of fume silica having a BET specific surface area of 200 m2/g, and then with: 1 part of 95% strength dicumyl peroxide.

The resulting composition was heated in the form of a plate for 15 minutes at 165"C and then for 4 hours at 200go, thus causing it to crosslink to form an elastomer.

The following coating agents 1 to 4 were used in the examples: Coating Agent 1 - a commercially available varnish base on an alkyd resin -- "Ducolux" (Trade Mark) gloss colour grey DG 704-Herman Wiederhold Lackfabrik, 4010 Hilden, Federal Republic of Germany, Coating Agent 2 - a commercially available wood glaze based on an alkyd resin -- "Tixoton" wood glaze, nut brown, "Meisterpreis" ("Tixoton" and "Meisterpreis" are Trade Marks) Spangenberg, 4010 Hilden, Federal Republic of Germany, Coating Agent 3 - a commercially available twocomponent polyurethane varnish -- "Efdedur KRO-423" and "Efdedur HU 7" in a ratio of 4::1 ("Efdedur" is a Trade Mark) - Emil Frei, 7715 Braunlingen-Doggingen, Federal Republic of Ger many, Coating Agent 4 - a commercially available white coating agent based on an aqueous dispersion of a mixed styrene/vinyl acetate polymer -- Wacker- Chemie GmbH., 8000 Munched, Federal Republjc of Germany.

Examples 1 and2 A solution of 50 parts of the silane of the formula (CH3O)3Si(CH2)3NH(CH2)2NH2 in 50 parts of toluene and 5 parts of tetra-n-butyl titanate was brushed onto parts of the surface of each of Elastomers A and B, and then allowed to dry for 1 hour at room temperature. The parts of the surface of the elastomers to which silane had been applied were then coated, by brush-application, respectively, with Coating Agents 1 to 4 iisted above.

All four Coating Agents covered well and did not run.

After 1 month, a portion of each coated surface (that is each elastomer coated with each coating agent) measuring 2.54 cm square was cut, with a razor blade, into 100 squares of equal size, each square having a surface area of 6.45 mm2. A pressure-sensitive adhesive tape was pressed onto the resulting grid of squares and was then slowly removed at an angle of about 30". The number of small squares of coating agent remaining on the elastomer was taken as a measure of the precentage retention of the coating agent. Retention values of 95% or more show excellent adhesion and retention values of 65% or less show poor adhesion. The average results are given in the table below.

Examples 3 and 4 The procedure described above was repeated with the variation that the surface of the elastomers was not treated with the silane solution Drior to application of the coating agents. Running of the coating agents was observed in all cases. The test for retention of the coating agent was also carried out as described above and the average results are given in the table below.

Table Example Elastomer Silane Retention of Coating Agent 1%) Coating Coating Coating Coating Coating Agent Agent Agent Agent 1 2 3 4 1 A Yes 95 100 99 99 2 B Yes 100 100 100 100 3* A No 2 2 0 0 4* B No 4 0 5 0 NOTE: An asterisk (* ) denotes a comparison example.

Claims

1. A process for coating a crosslinked organopolysiloxane with a coating agent, wherein (i) a silane that contains, per molecule, (a) at least one amino group that is bonded through one or more carbon atoms, and optionally one or more other atoms, to a sili con atom, via an SiC-linkage, and (b) at least one silicon-bonded monovalent hyd rocarbonoxy radical that is unsubstituted or is substituted by an amino or alkoxy group, and/or (ii) a partial hydrolysate of such a silane, is applied, prior to coating, to at least that part of the surface ofthe crosslinked organopolysiloxane that is to be coated, and/or is mixed with the coating agent prior to applying the coating to the surface of the crosslinked organopolysiloxane.

2. A process as claimed in claim 1, wherein the silane is a compound of the general formula [R%N(CH2)rnYa(CH2) hSI(OR3)CR4-b - in which R denotes an unsubstituted or substituted monovalent hydrocarbon radical, R2 denotes a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon radi cal, R3 denotes a monovalent hydrocarbon radical that is unsubstituted or is substituted by an amino or alkoxy group, Y denotes -O- or-N(R2)-, a denotes0orl, each of b and c denotes 1,2 or 3, with the proviso that the sum b +c does not exceed 4, and each of m andn denotes an integerfrom 1 to 10.

3. A process as claimed in claim 2, wherein, in the general formula given in claim 2, R denotes an alkyl, alkenyl, cycloaliphatic hydrocarbon, aryl, aralkyl, alkaryl, haloaryl or cyanoalkyl radical.

4. A process as claimed in claim 2, wherein, in the general formula given in claim 2, R denotes a methyl radical.

5. A process as claimed in any one of claims 2 to 4, wherein, in the general formula given in claim 2, R2 denotes a hydrogen atom or an alkyl, alkenyl, cycloaliphatic hydrocarbon, aryl, aralkyl, alkaryl, haloaryl or cyanoalkyl radical.

6. A process as claimed in any one of claims 2 to 5, wherein, in the general formula given in claim 2, at least one of the symbols R2 denotes a hydrogen atom.

7. A process as claimed in any one of claims 2 to 6, wherein, in the general formula given in claim 2, R3 denotes an alkyl radical having up to 6 carbon atoms, an aminoalkyl radical, an alkoxyalkyl radical, a cycloalkyl radical, an aryl radical, or an aralkyl radical.

8. A process as claimed in claim 1, wherein the silane is selected from the compounds of the formulae CH3Si(OCH2CH2NH2)2[(CH2)3O(CH2)2NH2J (CH3O)2Si(CH2)3NH(CH2)2NH2 H2N(CH2)20(CH2)3Si(OCH2CH2NH2)3 H2N(CH2)2O(CH2)2Si(OC2H5)3 H2N(CH2)2NH(CH2)3Si(OC2Hs)3 H2N(CH2)2Si(OC2Hs)3 H2N(CH2)2NH(CH2)4Si(OC2Hs)3 H2H(CH2)3Si(OC2Hs)3 H2NCH2Si(OC2H3)3 H2N(CH2)2NH(CH2)3Si(OCH2CH2OCH3)2 H2N(CH2)4Si(OC2Hs)3.

9. A process as claimed in any one of claims 1 to 8, wherein a partial hydrolysate of the silane is used and wherein the said partial hydrolysate contains not more than 10 silicon atoms per molecule.

10. A process as claimed in any one of claims 1 to 9, wherein the crosslinked organopolysiloxane to be coated is one that has been produced from a dior ganopolysiloxane composition that is storable in the absence of water and that will crosslink at room temperature in the presence of water.

11. A process as claimed in claim 10, wherein the diorganopolysiloxane composition comprises a silicon compound having at least three silicon-bonded hydrolysable groups per molecule, a diorganopolysiloxane having condensable terminal groups and, optionally, a filler and/or a condensation catalyst and/or a plasticiser.

12. A process as claimed in claim 11, wherein the silicon compound having at leastthree hydrolysable groups per molecule is a silane of the general formula RaSiZ4-a in which R and a are defined as in claim 2, and Z denotes a hydrolysable group, or is a partial hydrolysate of such a silane having not more than 10 silicon atoms per molecule.

13. A process as claimed in any one of claims 1 to 12, wherein the silane or partial hydrolysate thereof is applied, in admixture with or in solution in a liquid diluent, prior to coating, to at least that part of the surface of the crosslinked organopolysiloxane to be coated.

14. A process as cld;nrzd in ciaim 13, wherein the liquid diluent is an aliphatic hydrocarbon, an aromatic hydrocarbon, a ketone, an alcohol or a chlorohydrocarbon.

15. A process as claimed in claim 13 or claim 14, wherein the silane or partial hydrolysate thereof is used in an amount within the range of from 1 to 90% by weight, based on the total weight of the liquid diluent and the silane or partial hydrolysate thereof.

16. A process as claimed in any one of claims 13 to 15, wherein the silane or partial hydrolysate thereof is applied, prior to coating, to at least that part of the surface of the crosslinked organopolysiloxane to be coated, and wherein a condensation or hydrolysis catalyst is applied to the said surface or part surface, prior to or subsequent to the application of the said silane or hydrolysate, or during the said application and in admixture with or separately from the said silane or hydrolysate.

17. A process as claimed in claim 16, wherein the said catalyst is a metal or organometal carboxylic acid salt in which the metal is one from lead to manganese inclusive in the electromotive series.

18. A process as claimed in claim 17, wherein the said catalyst is a tin or organotin carboxylic acid salt.

19. A process as claimed in any one of claims 1 to 12, wherein the silane or partial hydrolysate thereof is mixed with the coating agent, in an amount within the range of from 0.1 to 10% by weight, based on the total weight of the silane or partial hydrosylate thereof and the coating agent, prior to applying the coating agent to the surface of the crosslinked organopolysiloxane.

20. A process as claimed in any one of claims 1 to 19, wherein the coating agent is a varnish, a glaze or an aqueous dispersion.

21. A process as claimed in any one of claims 1 to 19, wherein the coating agent comprises a solution in an organic solvent of an alkyd resin, a polyurethane, reactants to produce a polyurethane, nitrocellulose, an epoxide, a polyester, or a homopolymer or copolymer of vinyl chloride.

22. A process as claimed in any one of claims 1 to 19, wherein the coating agent comprises an aqueous dispersion of an acrylic resin, polyvinyl acetate, our a styrene/vinyl acetate copolymer.

23. A process as claimed in claim 1, carried out substantially as described in the examples herein.