GB2185749A - Opaque silicone rubber compositions - Google Patents

Abstract

A silicone rubber composition comprising an opacifier such as carbon black, titanium dioxide or calcium carbonate is particularly useful as an opacifying coating for translucent materials (e.g. glass).

Description

SPECIFICATION Silicone rubber compositions The present invention relates two a room temperature vulcanizable silicone rubber composition and in particular the present invention relates to a self-bonding two-package room temperature vulcanizable silicon rubber opacifying composition.

Modern buildings are often designed with a complete glassfrontage. The "all glass" buildings usethe same glass, often solar reflecting glass, for constant visual effect across the aspects ofthe building. However, the building is made up ofvision glass (i.e. see through glass) and spandrel glass (i.e. glass which covers the building structure). The problem with spandrel glass is that it must lookthe same as vision glass during daylight hours to ensure the architect'svisual design requirements for the building are met. The spandrel glass provides a panelling, for example, between floors to hide the structure of the building.As a consequence, the spandrel glass is subjected to harsh environmental conditions including direct exposure to UV light and, because of the lack of ventilation, glass temperatures of 1 000C and more during exposure bythe sun. During the night hours the glass temperature mayfall to - 1 0,C or lower, depending on ambient atmospheric conditions.

Various types of spandrel glasses have been used. For example, it may be a single sheet or a hermetically sealed insulating glass unit with an enamel coating on the inner side and an insulating glass panel behind.

The disadvantages of insulating glass units are high cost, weight and appearance. Changes in temperature or pressure flex the glass and visible distortion in reflective images can occur.

Single glass panels can be enamelled particularly for solid colours. However, the architectural trend is towards all glass solar reflecting walls. This mirror type glass cannot be enamelled. Polyethylene film can be heat bonded to solar reflecting glass. However, although it can be effective, it is very difficu It to applythe polyethylenewithout air bubbles. Other way of overcoming the problem include the use of insulating foam or mineral wool closely adjacent to the glass. The main disadvantage ofthe latter system is that under certain lighting conditions light passage can occur giving a non-pleasing effect to the outside of the building. In addition, the insulation can be displaced, volatile deposits or condensation staining can occur and damage themirrorsurfacespoilingthevisual effectofthebuilding.

GB1394206 (corresponding to US3888815) describes a two package room temperaturevulcanizable composition which is useful as a roof coating material. During such use the two parts of the composition are mixed and applied to the roof by means of a spray gun. On mixing and application to the roofthematerial successfully bonds to materials such as aluminium concrete, wood and stainless steel. In use, the thickness or bulk of composition also provides protection not onlytothe roof but also to the bond between the roof and the bulk of the material.

US381 3364 describes a self-bonding heatvulcanizable silicone rubber composition which is self-bonding to various substrates such as plastics, metals or glass.

Gel 278798 and Go1381933 describes silicone rubber compositions which are curable at moderate temperatures to produce a silicone rubber which bonds strongly to surfaces with which it is in contactwith during curing.

The present invention attempts to provide a coating which can be attached or bonded to a translucent material and in particularto solar reflecting glass on the inside of the glass when in use. The coating will need to meet extreme bonding requirements as the bond ofthe coating to the substrate will be exposed to extremes of, for example, temperature and UV light without the protection of the bulk of the coating. The purpose of the bulk of the coating is to provide consistent opacity to the translucent material.

According to the present invention, there is provided a curable silicone rubber composition and an opacifier present in 0.1 to 35% by weight of the organopolysiloxane contained in the silicone rubber composition.

The present invention also provides a curable silicone rubber composition and an opacifier present in 0.1% to 35% by weight ofthe organopolysiloxane contained in the silicone rubber composition, the silicone rubber being coated onto a translucent sheet material and cured.

In a further aspect, the present invention provides a method of coating a surface of a translucent material to stop lighttransmission by the translucent material, which method comprises applying the opaque curable silicone rubber composition to the translucent material and curing the silicone composition.

The silicone rubber composition may be a room temperaturevulcanizable silicone rubber composition or may be a heat-curable silicone rubber composition.

The important aspect ofthe present invention is the opacifierfurther described below. In addition, fil ler may also be added to the composition.

In one embodiment, the silicone rubber may be a room temperature vulcanizable silicone rubber composition comprising (a) a linear, organopolysiloxane containing terminal silicone-bonded hydroxy groups and having a viscosity of 500 to 10,000,000 centipoises when measured at 25"C, the organic groups of the aforesaid organopolysiloxane being substituted or unsubstituted monovalent hydrocarbon radicals, (b)from 0.1 to 15% by weight, based on organopolysiloxane, of(1) an organoxysilane orsilicate corresponding to the general formula, (RO)3Si - R1 (1) where R is a monovalent hydrocarbon or halogenated hydrocarbon radical and Rr is an alkyl, haloalkyl, aryl, haloaryl, alkenyl, cycloalkyl, cycloalkenyl, cyanoalkyl, alkoxy or acyloxy radical, or (2) a liquid partial hydrolysis product ofthe aforementioned organoxy silane or silicate compounds, (c) from 0.1 to 5% by weight, based on the organopolysiloxane, of a catalyst which is metal salt of an organic monocarboxylic or dicarboxylic acid in which the metal ion is lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth or manganese, and (d) from 0.1 to 10% by weight, based on the organopolysiloxane, of a nitrogen-containing silane ofthe formula::

where R is a monovalent hydrocarbon or halogenated hydrocarbon radical, Q is an alkoxy, phenoxy, halo, amino ordíarkylamino group, and Q' is a saturated, unsaturated or aromatic hydrocarbon residue substituted by at least one amino hydrazone, azirane or, cyano group, and optionally one or morethio, sulphone, oxa, oxo, diorganosilicon and/or ester grops, and is 0,1 1 or 2.

These compositions are self-bonding, i.e. they do not require the use of a primer. The presence ofthe nitrogen-containing silane in an amount of 0.1 to 10% by weight, based on the linear organopolysiloxane (a) imparts the desired self-bonding properties to the room temperature vulcanizable silicone composition. The nitrogen-containing silane (d) acts both as a self-bonding agentand as a catalytic agent in the composition.

The composition, however, also contains an additional catalyst (c) constituted by from 0.1 to 5% byweight, based on organopolysiloxane, of a catalyst which is a metallic salt of an organic monocarboxylicor dicarboxylic acid in which the metal ion is lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth or manganese. Preferred nitrogen containing silanes (d) have the formula,

where R2 is a monovalent hydrocarbon or halogenated hydrocarbon radical of up to 10 carbon atoms; most preferably an alkyl radical of 1 to 5 carbon atoms; a has the meaning given above and preferably has avalue ofO.

The present composition may additionally include a branched or straight polymer compound of (R3)2 SiO units, (R3)SiO112 units and R3SiO3/2units having a 0.05 to 8% by weight, preferably 0.1 to 8% by weight of hydroxyt radicals (the viscosity ofthe polymer being preferably between 500 to 1.0 1 05 centipoise at25 C).

The ratio of the organosiloxy units to the diorganolsiloxy units is from 0.11 to 1.4 and the ratio ofthe trioganosiloxy units to the diorganosiloxy units is from 0.02 to 1, inclusive.

The preferred linearfluid organopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of 500to 10,000,000 centipoises when measured at25 C, has preferably the formula,

where R3 is a monovalent hydrocarbon or halogenated hydrocarbon radical and r is a whole numberfrom 250 to 7,275. The radicals R, R2, and R3 are preferably alkyl radicals, such as methyl, ethyl, propyl, butyl or hexyl; aryl radicals such as phenyl, ordiphenyl; alkaryl radicals such astolyl, xylyl, or ethylphenyl; aralkyl radicals such as benzyl, or phenylethyl; haloaryl and haloalkyl such as chlorophenyl, tetrachlorophenyl, or difluorophenyl; and alkenyl radicals such as vinyl or allyl. Further, R3 may also represent cyanoalkyl, cycloalkyl orcycloalkenyl radicals. The R3 groups attached to a single silicon radical may be the same groups or different groups. It has been found that at least 50% and preferably 70 to 100% of the R3 groups in the diorganopolysiloxane molecule should be methyl, Further, the diorganopolysiloxane can be a holopolymer, ora copolymer having different types of units in the chain such as dimethyl, diphenyl, or methyl-phenyl.

The organopolysiloxanes offormula (4) may also be represented by the average unitformula, R3,Si04.m 2 (5) where R3 is defined above andthevalue of m mayvaryfrom 1 .99two to 2. The above average unitformulaonly represents organopolysiloxanes having monofunctional terminal groups and optional trifunctional units.

However, in the present case, it is preferred that the terminal groups be hydroxy and the monofunctional and trifunctional groups be kept to a minimum.

In orderforthe diorganopolysiloxane fluids to cure there must be present in the composition the cross-linking agent offormula (I). In that formula, R groups may be alkyl radicals such as methyl, ethyl, propyl, isopropyl, butyl, amyl, isoamyl, octyl, isooctyl, decyl, or dodecyl; haloalkyl such as the chlorinated, brominated, orfluorinated alkyl radicals. In addition, R may represent aryl, aralkyl and alkenyl radicals such as vinyl, allyl, phenyl, tolyl, xylyl, benzyl, phenylethyl, naphthyl, anthracyl, or biphenyl, as well as the halogen-substituted derivatives of the above radicals. In addition, R may represent cycloalkenyl, cycloalkyl and cyanoalkyl radicals. The radical R1 represents the same radicals as R and, in addition, preferably represents alkoxy or aryloxy radicals such as methoxy, ethoxy, butoxy and phenoxy.

Alternatively to the monomeric compounds of formula (1), liquid partially hydrolyzed products thereof can also be used as cross-linking agents. Such hydrolysis products are obtained by effecting partial hydrolysis in waterofthe particular monomeric compound in the presence of small amounts of acid to a pointwhere itis still water-insoluble and still possible to isolate a liquid partially hydrolyzed organosilicon compound.Thus, the ethyl silicate having the formula (C2H50)4Si may be partially hydrolyzed by adding acids oracid-forming metal salts, such as FeCI3, CuCI2, AICI3, or SnC4to the liquid monomeric organosilicate, and thereafter effecting suitable hydrolysis ofthis mixture of ingredients in water to obtain thetwo-phasecompositions, from which the water-insoluble, partially hydrolyzed organosilicate can readily be separated from the aqueous phase and catalyst. A partially hydrolyzed ethyl silicate is sold underthe tradename Ethyl Silicate-40, by Union Carbide Corporation.

There is added from 0.1 to 15.0% by weight of the cross-linking agent offormula (I) (or its hydrolysis product) and preferably 0.1 to 10% by weight, based on the weight ofthe diorganopolysiloxane offormula (4) and (5). If more than 15.0% byweight of cross-linking agent were to be used, the excess would not function as a cross-linking agent since the initial hydroxy positions on the organopolysiloxane would already have reacted with the cross-linking agent and the excess would act as a filler, reducing the elasticity ofthe cured silicone rubber composition. If less than 0. 1% by weight of cross-linking agent were to be used, there would not be sufficient cross-linking agent to reactwith the organopolysiloxane to form the cured silicone rubber.

Although the above mentioned cross-linking agents must be used, there may additionally be used as cross-linking agents, organopolysiloxane resins having a functionality greater than 2 and preferably greater than 2.5. The organopolysiloxane resins are methylsiloxanes, or resins which contain both monomethyl and dimethyl or monophenyl units. There may also be used ethylsiloxane resins, in which the ratio R"Si is 1 .4to 1 and which contains 15 mol % of butoxy groups, orthere may be used resins in which the ratio R"Si is 1.1 to 1 and which contain 10 mol % of methoxy groups or there may be used methylphenylsiloxane resins containing 50 mol % of monomethyl units, 25 mol % of dimethyl units and 25 mol % of monophenyl units.

Other suitable additional cross-linking agents are organohydrogenpolysiloxanes of the formula, R37aHSio3 a 2 (6) in which R37 iS an alkyl oraryl radical and a is a number less than 2, but is not zero. The organohydrogenpolysiloxane cross-linking agents have the disadvantage that during curing there is evolved hydrogen gas which can result in bubbles being trapped in the silicone rubber composition. Although the above cross-linking agents can be used in the compositions, the organosilicates offormula (1 fortheir partial hydrolysis products must be present sincethe processability of the composition is facilitated and the cured silicone rubber composition has better physical properties.A more detailed description of these other cross-linking agents is to be found in U.S. patent 3,127,363.

The other essential component in this silicone rubber composition is a catalyst. It has been found that only certain metallic salts of organic carboxylic acids and dicarboxylic acids, in addition to the nitrogen-containing silanes offormula (2), may be employed with the organopolysiloxanes offormula (4) and (5) as a curing catalyst. Suitable acid radicals are the resinate, lineoleate, stearate, and oleate, as well as the lower radicals such as acetate, butyrate, and octoate. Metallic salts of lauric acid have been found to be especially effective. The metal ion of the metal salt is lead,tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth or manganese.Thus, examples of suitable metallic salt catalysts aretin naphthenate, lead octoate, tin octoate, iron stearate, tin oleate, antimony octoate,tinbutyrate, basic dibutyl tin laurate and dibutyl tin dilurate. The tin and lead salts are preferred since they are usually soluble in the diorganopolysiloxanes offormulae (4) and (5) and since they have enhanced catalytic activity in combination with the alkyl silicate. It is important to note that other compounds which would be expected to exercise good catalytic activity in the mixture of diorganopolysiloxane, filler and alkyl silicate. It is important to note that other compounds which would be expected to exercise good catalytic activity in the mixture of diorganopolysiloxane, filler and alkyl silicate exercise no catalytic activity whatsoever. This class of compounds are zinc salts of organic acids, cobait oleate, cobalt naphthenate, manganese naphthenate, nickel naphthenate and calcium stearate. From 0.1 to 5% by weight ofthe metallic salt is used, based on the weightofthe diorganopolysiloxane.

Various heat curable silicone rubber compositions may also be used. These compositions may comprise, by weight (1)100 parts of a liquid vinyl chain-stopped polysiloxane having the formula

where R10 and R12 are each an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5 to 7 ring carbon atoms or a mononuclear aralkyl radical of which the alkyl radical(s) contain(s) from 1 to 8 carbon atoms with at least 50 mole per cent of R' radicals being methyl and where n has a value sufficient to provide a viscosity of 1,000 to 750,000 centistokes at 25"C, preferablyfrom 50,000to 150,000 inclusive.

(2) from 0 to 50, preferably from 20 to 50 parts of an organopolysiloxane copolymer comprising (Fl")3SiO0.5 units, (R11)2SiO units and SiO2 units, where each R" is a vinyl radical, an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5 to 7 ring carbon atoms ora mononuclear aralkyl radical of which the alkyl radical(s) contain(s) from 1 to 8 carbon atoms, where the ratio of (Fl")3SiO0.5 units to SiO2 units is from 0.5:1 to 1::1 and where from 2.5 to 10 mole per cent of the silicon atoms contain silicon-bonded vinyl groups, (3) a catalyst comprising platinum and/or a platinum compound in an amount sufficientto providefrom 10-3to 10-6 gram atoms of platinum per mole of silicon-bonded vinyl radicals in the composition, (4)an amountofa liquid organohydrogenpolysiloxane having theformula:: (R)a(H)bSiO4-a-b 2 sufficientto provide from 0.5 to 1.2 and preferably 1.0 silicon-bonded hydrogen atom per silicon-bonded vinyl group in the composition described in (1), where R is as previously defined, a has a value of from 1 .00to 2.00, b has avalueoffrom 0.1 to 1.2, preferably 0.1 to 1.0, and the sum of a plus b is from 2.00 to 2.67,there being at least two silicon-bonded hydrogen atoms per molecule, (5) from 0.1 to 1 parts a liquid vinyl siloxane hydrolyzate of the formula:: [(HO), (Fl"'O)y (CH2=) SiO3zJx 2 preferably prepared bythe hydrolysis of a mixture of vinyl trichlorosilane and a vinyl trialkoxysilane, such as vinyl triethoxysilane, where Fl"' is an alkyl radical having one to 8 carbon atoms, xis a number greaterthan 3, y has avalue offrom 0.01 to 0.4, and preferably has a value offrom 0.05 to 0.1 and z has avalue of 0.1 to 0.4, preferably from 0.2 to 0.4, (6) from 0 per cent to 85 per cent, based upon the total weight ofthe above described mixture, ofa halocarbon catalyst inhibitor which is a halocarbon having 2 carbon atoms and at least3 halogen substituents, said halogen substituents having an atomic weight of less than 126 and being positioned anywhere on the molecule.

When a completely transparent laminate is desired, the fluid vinyl siloxane hydrolyzate of (5) is not mixed into the compositions but can be used to prime the transparent surfaces to be joined. The hydrolyzate is usually applied to the surfaces in an ethyl alcohol solution containing from 3to 6 per cent by weight ofthe hydrolyzate.

The compositions are prepared by mixing in a suitable fashion all of the components described above plus any additional components such as will be described subsequently and maintaining the mixture ata temperature at which it is to be cured The compositions cure attemperatureswhich can vary from about 50"C or lower to temperatures of the order of 11 0"C or higher depending upon the particular amount of platinum compound catalyst present in composition and depending upon the time which is allowed for cure.

Likewise, the compositions can be prevented from curing by maintaining them at a reduced temperature such as a temperature of 0 C, in which case all of the components can be kepttogetherfor extended periods oftime without cu ring. The compositions can also be prevented from curing by the utilization of the above described halocarbon catalyst inhibitor.

The compositions can vary from readilyflowable liquids to slowly flowing liquids depending upon the viscosity ofthevarious components employed in the reaction mixture and depending upon the amountof filler included in the reaction mixture. Regardless of the flow characteristics of the compositions and the proportions of the various ingredients, the compositions cure to a hard, tough silicone elastomer upon maintaining the compositions atthe curing temperatureforthe required amount of time. The compositions aretranslucentor opaque and the colour of the cured product is a function of any added filler and the opacifying agents added to the compositions.When a halocarbon inhibitor, as above described, is used in the compositions ofthe present invention, the viscosity of the vinyl containing fluid can be increased up to 3,000,000 centistokes and still have a readily workable material.

All of the components of the composition are well known in the art. The vinyl chain-stopped organopolysiloxane component (1 ) is typified by various compositions within the scope offormula (1 ) where the monovalent hydrocarbon radicals represented by R and R' include alkyl radicals containing from one to 8 carbon atoms, e.g., methyl, ethyl, propyl, butyl and octyl radicals; mononuclear aryl radicals, e.g., phenyl, tolyl and xylyl radicals; cycloalkyl radicals containing 5 to 7 ring carbon atoms, e.g., cyclohexyl and cycloheptyl radicals, mononuclear aryl C1 -C8 alkyl radicals, e.g., benzyl and phenylethyl radicals.

Further examples of heat curable compositions include a self-bonding heat-vulcanizable silicone rubber composition comprising an organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25 Coftheformula, (7) (R2 )aSiO4 a 2 a curing catalyst and an additive selected from the class consisting of an alkenylisocyanurate oftheformula,

and a cyanurate of the formula,

and mixtures thereof, where R20 is selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, R21 is selected from unsaturated monovalent hydrocarbon radicals and unsaturated halogenated monovalent hydrocarbon radicals, R22 and R23 are selected from R' radicals, saturated monovalent hydrocarbon radicals and saturated halogenated monovalent hydrocarbon radicals and a varies from 1.95 to 2.01, inclusive. In the above composition, there is preferably 82% to 99.65% by weight of the organopolysiloxane, 0.1% to 8% by weight of the curing catalyst and 0.25% to 10.0% byweight of the isocyanurate, based on the weight of the composition. There may further preferably be included in the composition a filler such as silica filler, which comprises 20%to 60% by weight of the organopolysiloxane and there may also be present a process aid which comprises 1% to 25% by weight ofthe organopolysiloxane. The curing catalyst is preferablyt-butyl perbenzoate or dick my peroxide.

The self-bonding, curable silicone rubber components are mixed and heated to a temperature in the range of 80"C to 650"C, so as to cure the resulting mixture to a silicone rubber mass.

In the above composition, a critical ingredient is the isocyanurate and cyanurate. The non-silicone isocyanurate or cyanurate is preferred since it has very good shelf-aging properties. If shelf-aging is not an important factor, then there may be used in place of the isocyanurate of Formula (8) or the cyanurate of Formula (9) above, an additive selected from the class consisting of a silylisocyanurate oftheformula,

and a silylcyanurate of the formula,

in the above formulas, R21 is as defined previously, G is selected from R21 radicals and radicals oftheformula, E(3 b) Rb25SjR24 where E is selected from R25O- radicals and R26C00- radicals, where R25 and R26 are selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, R24 is selected from divalent hydrocarbon radicals and halogenated divalent hydrocarbon radicals and b is a whole number equal to Oto 3, inclusive. The silylisocyanurate and silylcyanurate of Formulas (10) and (11) may have one silyl or two silyl substituent groups thereon on the isocyanurate moiety orcyanurate moiety, but preferably has only one silyl groupthereon. Further, in the silylisocyanurates and silylcyanurates, preferably, G is represented by an R' radical,that is, an unsaturated monovalent hydrocarbon radical.

The curing ofthe silicone rubber composition can be effected by chemical vulcanizing agents or by high energy electron radiation. More often, chemical vulcanizing agents are employed for the curing operation and any of the conventional curing agents can be employed. The preferred curing agents are organic peroxides conventionally used to cure silicone elastomers.

Especially suitable are the peroxides which may have the structural formulae,

wherein R20 represents the same alkyl group throughout or alkyl groups of two or more different types and n is zero ora positive integer.

Among the specific peroxidic curing catalysts that are preferred are di-tertiary-butyl peroxide, tertiary-butyltriethylmethyl peroxide, tertiary-butyl triphenyl methyl peroxide,t-butyl perbenzoate and a ditertiary alkyl peroxide such as dicumyl peroxide. Other suitable peroxidic catalysts which effect curing through saturated as well as unsaturated hydrocarbon groups on the silicon chain are aryl peroxideswhich include benzoyl peroxides, mixed alkyl-aryl peroxidic compounds which include tertiary-butyl perbenzoate, chloroaryl peroxides such as 1,4-dichlorobenzoyl peroxide; 2, 4-dichlorobenzoyi peroxide and monochlorobenzoyl peroxide.From 0.1-8 per cent of said peroxidic compound byweight ofthe composition is used to cure the silicone rubber composition and preferably 0.5-3.0 per cent by weight of the above curing catalyst,t-butyl perbenzoate, is preferred.

Other examples of heat curable compositions include a self-bonding, heat curable silicone rubber composition which comprises: (a) from 82 to 99.65% byweight of a linear organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25"C, and having the average unitformula: (Fl30)aSiO4.a 2 (12) (b) from 0.1 to 8% by weight of a curing catalyst, and (c) from 0.25 to 10% by weight of a self-bonding additive oftheformula::

in which the formulae ahasavalueoffrom 1.95to2.01 inclusive, R30 is a monovalent hydrocarbon or halohydrocarbon radical, R36 is alkyl or hydrogen, Z is phenylene or a group ofthe formula -CO-O-, -CO-, -CO-NH- or-CO-NR32- in which R32 is a monovalent hydrocarbon or halohydrocarbon radical, G is hydrogen, a saturated monovalent hydrocarbon or halohydrocarbon radical, or has the same meaning as R35, R35 is an unsaturated monovalent hydrocarbon or halohydrocarbon radical, or a group oftheformula:: -R34-SiRn32(M)3 n (14) in which R34 is a divalent hydrocarbon or halohydrocarbon radical, R32 has the meaning given above, M is a group ofthe formula R330- or Fl33-CO-O- in which R33 is a monovalent hydrocarbon or halohydrocarbon radical, and n is O or a whole numberfrom 1 to 3.

The above composition preferably comprises 1% to 25% by weight, based on the organopolysiloxane of a process aid. There may also be present from 10 to 100% by weight, preferably 20to 60% by weight, based on the organopolysiloxane, of a filler, preferably silica. In addition, there of course can be any ofthe other ingredients and additives normally to be found in heat-curable silicone rubber compositions.

In the above formulae, that is, formulae (12) to (14), the radicals R30, R32 and R33 may be aryl radicals and halogenated aryl radicals such as phenyl, chlorophenyl, xylyl ortolyl, aralkyl radicals, such as phenethyl, or benzyl; aliphatic, haloaliphatic and cycloaliphatic radicals such as alkyl, alkenyl, cycloalkyl, haloalkyl, including methyl, ethyl, propyl, chlorobutyl, orcyclohexyl. Preferably, the R30 radical is represented by methyl and phenyl radicals, where at least 50% of the R30 radicals are methyl. Further, in the organopolysiloxane polymer represented by formula (1 2),there is preferably 0.1 to 0.6 weight per cent ofthe polymer of vinyl radicals.Further, preferably the R32 and R33 radicals are alkyl radicals of not more than 8 carbon atoms and are preferably methyl or ethyl. The R35 radical is selected from hydrogen and alkyl radicals of preferably up to 10 carbon atoms. Preferably, the R36 radical is hydrogen. Radicals represented by R35 are alkenyl radicals, cycloalkenyl radicals and arylalkenyl radicals, such as vinyl, allyl, cyclohexyl, and phenyl-2-propenyl. In addition, the R35 radicals may be represented by alkynyl radicals, such as propargyl. It is preferred that R5 be eithervinyl or allyl or an alkenyl radical of less than 8 carbon atoms.The R32 radical (when R35 is a group ofthe formula R34-SiRn32( M)3 n) may be saturated monovalent hydrocarbon radical or an unsaturated monovalent hydrocarbon radical and is preferably represented by the radicals recited in the exemplification of the R30, R32and R33 radicals, However, more preferably, the R32 radical is selected from unsaturated monovalent hydrocarbon radicals and halogenated unsaturated monovalent hydrocarbon radicals such as alkenyl radicals of up to 8 carbon atoms. It is preferred that G be an unsaturated monovalent hydrocarbon radical.When R35 represents a group of the formula -R34SiR,32(M)3.n, it is preferable that G bean unsaturated monovalent hydrocarbon radical e.g. an alkenyl radical of up to 8 carbon atoms orarylene radical. It is preferable that Z be a carboxyl radical, since when Z has the other meanings enumerated above, these compounds are more difficult to synthesize. In formulae (13), both the cis and trans isomers have been shown and are intended to be covered. Any of the isomers of the maleates, and fumarates and the silylmaleates and silylfumarates disclosed or mixtures of the isomers may be used. In addition, single compounds may be used or a mixture of any of the self-bonding additives.Radicals included by R34are divalent saturated and unsaturated hydrocarbon radicals such as alkenyl, alkenylene, alkynylene and arylene radicals, which are exemplified by ethylene, trimethylene, tetramethylene, phenylene, and ethylene-phenylene. The radical R34 may have 2 to 20 carbon atoms, and is preferably ethylene.

Maleatescoming within the scope offormula (13) are diallyimaleate, dipropenylmaleate, and dibutenylmaleate. The preferred silylmaleates coming within the scope of these formulae are bis-trimethoxysilylpropylmaleate and bis-trimethoxysilylbutylmaleate. The preferred compounds within the scope offormulae (13) are asfollows: bis-trimethoxysilyl propylmaleate diallyl fumarate allyl hydrogen maleate bis-(3-chloropropenyl) maleate ethyl allyl fumarate diisopropenyl fumarate bis-trimethoxysilylpropyl fumarate bis-dimethoxymethylsilylpropyl maleate trimethoxysilyipropyl allyl fumarate bis-ethoxydimethylsilyl propenyl maleate.

There are also within the scope of formula (12) polydiorganosiloxanes which can be copolymers containing two or more differentdiorganosiloxane units therein and copolymers of dimethylsiloxane units and methyiphenylsiloxane units; or copolymers of methylphenylsiloxane units, diphenylsiloxane units, dimethylsiloxane units and methylvinylsiloxane units as well as copolymers of dimethylsiloxane units, methylvinylsiloxane units and diphenylsiloxane units.

The curing of the silicone rubber composition invention can be effected by any of the conventional curing agents. The preferred curing agents are organic peroxidic compounds conventionally used to cure silicone elastomers as described above.

There may be incorporated into the organopolysiloxane a filler which may be of the reinforcing fillertype or ofthe semi-reinforcing type. Generally, the reinforcing fillers having 100-300 square meter surface areas per gram whilethe semi-reinforcing fillers having a surface area of 1-20 square meters per gram.

The reinforcing fillers may be added when it is desired to have a high strength silicone rubbercomposition, that is, a composition with high values fortensile strength and percent elongation. Illustrative ofthe many fillers which can be employed are lithopone, zine oxide, zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, fumed silica, precipitated silica, glass fibers, magnesium oxide, chromium oxide, zirconium oxide, aluminum oxide, crushed quartz, calcined clay, asbestos, cork, cotton and synthetic fibers.

There can also be used silica filler treated with an organosiloxane cyclic trimer ortetramersuch thatthefiller is hydrophobic. Generally, there may be added to said organopolysiloxane, 5 to 300% by weight offillerand preferably 10-200% by weight.

An essential feature of the composition is the opacifier. Any opacifying agents can be used although the preferred opacifying agents are one or more of titanium dioxide, carbon black and calcium carbonate. The opacifier is present in an amount of 0.1% to 35% by weight ofthe organopolysiloxane, preferably the amount ofopacifierwill vary according to the shade of glass required. For example, if a black opaque glass is required, it is preferred that from 0.1 to 3% by weight based on the weight of the polysiloxane of carbon black is used. If a grey opacifier is required a mixture of carbon black and titanium dioxide may be used in a ratio of between 1:10 and 1:100 byweight of carbon black:titanium dioxide and preferably in an amount of 1 to 25% by weight.Titanium dioxide can be used in an amount of 1 to 25% by weight all based on the weight ofthe organopolysiloxane.

The present invention provides a method of coating surface of a translucent material to stop light transmission by the translucent material which method comprises applying a composition comprising an organopolysiloxane and an opacifier and curing the composition.

Any ofthe organopolysiloxanes described may be used for the opacifying coating ofthe present invention.

The method of the present invention relates to the coating oftranslucent materials. Many types of translucent material can be coated such as polymethyl methacrylate, polystyrene, polycarbonate, and glass, particularly solar reflecting glass. Glass is a particular material which causes difficulty because of problems of bonding any form of coatings with the glass. As glass is an inorganic material, the opacifying coating is thought to bond physically with the material. i.e. adhere to its surface. This type of adherence is subjected to the ravages of UV light when the glass is to be the exposed material and the opacifying coating is then on the inside of the glass. The opacifying coating has good bonding properties to glass.

The present invention also provides a translucent material and in particular glass when coated by a method as described above. In particular, the present invention provides a cladding material comprising a translucent material as the portion exposed to the elements and the opacifying coating as the inner portion of the cladding.

In carrying outthe method of the present invention, the translucent material, particularly glass, is preferably cleaned in a washing machine using demineralised water. Because ofthe problems relatingto bonding ofthe opacifying coating, i.e. the bond being exposed to UV light and large temperature variations, the glass should be thoroughly cleaned without the use ofalkali. Following cleaning, the glass is preferably wiped with a solvent such as methyl ethyl ketone or isopropanol. If a plastics translucent material is to be cleaned priorto application ofthe opacifying coating then care must be taken in the slection of cleaning solvents to prevent damage to the translucent material.

The application ofthe opacifying coating can be performed in a number ofdifferentways. The essential feature is that the composition is thoroughly mixed so that the curing agent, the opacifying agent and the catalyst are uniformly dispersed with the other components of the silicone composition. Preferablythe curing agent and the catalyst are separate from the remaining components of the silicone composition. In a two part form, the components are mixed in a spray gun and are sprayed onto the glass, the glass being generally in sheet or panel form.

The opacifying coating of the present invention can be applied to a number of differenttypes of glass including clear colourless glass, solar reflecting glass, mirror glass and glass for fire walls. The opacifying coating of the present invention can provide a consistent, even, homogenous coating on glass and can therefore provide suitable coatings for external mirror glass.

The present invention will bye further described with reference to the following examples in which all parts are byweight unless otherwise stated.

Examples 1-3 Organopolysiloxane compositions are prepared as follows:- Components Parts by weight Example 1 Example2 Example3 Base Material Dimethyl silanol terminatedpolysiloxane 100 100 100 (blended to 9000 + 1000 cps) Ground silica 137 137 137 Hydrogenated castor oil 1.60 1.60 1.60 Hydrocarbon solvent 65.4 64.0 64.0 Titanium dioxide 22.6 15.0 Carbon black ---- 1.0 1.0 Curing Agent Catalyst- alkylsilicate 1:10 1:10 1::10 (by volume) (by volume) (by volume) Properties (Examples 1-3) Temperature resistance -50 C Tensile strength 3.5-40 N/mm2 (ASTM-412) Heat Agency after 26 weeks at 82"C, no change, within the limits of test error, is tensile strength or elongation UV resistance No cracking, checking or pinholes (ASTM-526-70, 5"C) Water absorption 7 days at 240C < 0.5% byweight (ASTM-D-570) Each ofthe above compositions, Examples 1 to 3, were coated onto a glass surface as follows. Atwo component airless spray gun, a King 45:1, was loaded with the base material and a pressure pot was loaded with the curing agent. Cool light glass was washed in demineralised waterfree of alkalis in an insulating glass washing machine. The glass was wiped with isopropanol. The glass was placed in a vertical position and sprayed to a thickness of 250 microns with the base material and curing agent in a volume ratio of 10:1.

The coverage of the material was 0.72 Kg/m2. The coatings cured to a tack free condition in 10 to 15 minutes and could be handled within3to4hours.

The composition of example 1 was coated onto solar reflecting glass as indicated above and sampleswere exposed to daylight for 20 months. No peeling, fading or degradation of the coating was noted.

Claims (29)

1. A curable silicone composition and an opacifier present in an amount of O.l%to 35% by weight ofthe organopolysiloxane contained within the silicone rubber composition.

2. A composition as claimed in claimed in claim 1 when coated onto a translucent material.

3. A composition as claimed in claim 2 wherein the translucent material is glass.

4. A composition as claimed in any one of claims 1 to 3 when cured.

5. A composition as ciaimed in any one of claims 1 to 4 wherein the silicone rubber composition comprises: (a) a linear, organopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of 500 to 10,000,000 centipoises when measured at 25 C, the organic groups ofthe aforesaid organopolysiloxane being substituted or unsubstituted monovalent hydrocarbon radicals.

(b)from 0.1 to 15% byweight, based on organopolysiloxane,of (1) an organoxysilane or silicate corresponding to the general formula, (FlO)3Si - R1 (1) where R is a monovalent hydrocarbon or halogenated hydrocarbon radical and Ra is an alkyl, haloalkyl, aryl, haloaryl, alkenyl, cycloalkyl, cycloalkenyl, cyanoalkyl, alkoxy or acyloxy radical, or (2) a liquid partial hydrolysis product of the aforementioned organoxy silane or silicate compounds, (c) from 0.1 to 5% by weight, based on the organopolysiloxane, of a catalyst which is metal salt of an organic monocarboxylicor dicarboxylic acid in which the metal ion is lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth or manganese, and (d) from 0.1 to 10% by weight, based on the organopolysiloxane, of a nitrogen-containing silane ofthe formula:

where R is a monovalent hydrocarbon or halogenated hydrocarbon radical, Q is an alkoxy, phenoxy, halo, amino ordialkylamino group, and Q' is a saturated, unsaturated or aromatic hydrocarbon residue substituted by at least one amino hydrazone, azirane or, cyano group, and optionally one or morethio, sulphone, oxa, oxo, diorganosilicon and/or ester groups, and a is 1 or 2.

6. Acomposition as claimed in anyone of claims 1 to4whereinthe silicone rubbercomposition comprises by weight (1)100 parts of a liquid vinyl chain-stopped polysiloxane having theformula

where R10 and R12 are each an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5 to 7 ring carbon atoms or a mononuclear aralkyl radical of which the alkyl radical(s) contain(s) from 1 to 8 carbon atoms with at least 50 mole per cent of the R' radicals being methyl and where n has a value sufficientto provide a viscosity of 1,000 to 750,000 centistokes at 25"C, preferably from 50,000 to 150,000 inclusive, (2) from 0 to 50, preferably from 20 to 50 parts of an organopolysiloxane copolymer comprising (R")3SiO0.5 units, (R")2SiO units and SiO2 units, where each R" is a vinyl radical, an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5 to 7 ring carbon atoms ora mononuclear aralkyl radical of which the alkyl radical(s) contain(s)from 1 to 8 carbon atoms, where the ratio of (R8)3SiOo.5 units to SiO2 units is from 0.5:1 to 1::1, and where from 2.5 to 10 mole per cent ofthe silicon atoms contain silicon-bondedvinyl groups, (3) a catalystcomprising platinum and/or a platinum compound in an amount sufficientto providefrom 10 3to 10~6gram atoms of platinum per mole of silicon-bonded vinyl radicals in the composition, (4) an amount of a liquid organohydrogenpolysiloxane having the formula:: (R)a (H)b SiO4 a.b 2 sufficientto provide from 0.5 to 1.2 and preferably 1.0 silicon-bonded hydrogen atom per silicon-bonded vinyl group in the composition described in (1), where R is as previously defined, a has a value of from 1.00 to 2.00, b has a value of from 0.1 to 1.2, preferably 0.1 to 1.0, and the sum of a plus b is from 2.00 to 2.67,there being at leasttwo silicon-bonded hydrogen atoms per molecule, (5) from 0.1 to 1 partofa liquid vinyl siloxane hydrolyzate oftheformula:: [(HO)z(RO)y(CH2=CH)SiO3+zlx 2 preferably prepared by the hydrolysis of a mixture ofvinyl trichiorosilane and a vinyl trialkoxysilane, such as vinyul triethoxysilane, where R"' is an alkyl radical having one to 8 carbon atoms, is a number greater than 3, y has a value offrom 0.01 to 0.4, and preferably has a value of from 0.05 to 0.1 and z has a value of 0.1 to 0.4, preferablyfrom 0.2 to 0.4, (6) from 0 per centto 85 per cent, based upon the total weight of the above described mixture, of a halocarbon catalyst inhibitor which is a halocarbon having 2 carbon atoms and at least 3 halogen substituents, said halogen substituents having a atomicweightof less than 126 and being positioned anywhere on the molecule.

7. Acomposition as claimed in any one of claims 1 to 4, wherein the silicone rubber composition comprises an organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25"C ofthe formula, (R2 )aSiO4va 2 (7) a curing catalyst and an additive selected from the class consisting of an alkenylisocyanurate oftheformula,

andacyanurateoftheformula,

and mixturesthereof, where R20 is selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, R21 is selected from unsaturated monovalent hydrocarbon radicals and unsaturated halogenated monovalent hydrocarbon radicals, Fl22 and Fl23 are selected from R' radicals, saturated monovalent hydrocarbon radicals and saturated halogenated monovalent hydrocarbon radicals and a varies from 1.95 to 2.01, inclusive.

8. A composition as claimed in any one of claims 1 to 4 wherein the silicone rubber compositions comprises (a) from 82 to 99.65% by weight of a linear organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25"C, and having the average unitformula: (R30),Si04-a 2 (12) (b) from 0.1 to 8% by weight of a curing catalyst, and (c) from 0.25to 10% by weight of a self-bonding additive oftheformula::

in which theformulae a has a value from 1.95to2.01 inclusive, R30 is a monovalent hydrocarbon or halohydrocarbon radical, R35 is alkyl or hydrogen, Z is phenylene or a group ofthe formula -CO-O-, -CO-, -CO-NH- or-CO-NR32- in which R32 is a monovalent hydrocarbon orhalohydrocarbon radical, G is hydrogen, a saturated monovalent hydrocarbon or halohydrocarbon radical, or has the same meaning as R35, R35 is an unsaturated monovalent hydrocarbon or halohydrocarbon radical, or a group oftheformula:: -R34-SiRn32(M)3-n (14) in which R34 is a divalent hydrocarbon or halohydrocarbon radical, R32 has the meaning given above, Mis a group oftheformula R330- or R33-CO-0- in which R33 is a monovalent hydrocarbon or halohydrocarbon radical, and n is O or a whole numberfrom 1 to 3.

9. A composition as claimed in any one of the preceding claims wherein the opacifier is one or more of titanium dioxide, carbon black or calcium carbonate.

10. A composition as claimed in claim 9 wherein the opacifier is carbon black present in the amount of 0.1 to 3% by weight ofthe organopolysiloxane.

11. A composition as claimed in claim 9 wherein the opacifier is titanium dioxide present in an amount of 1 to 25% by weight of the organopolysiloxane.

12. A composition as claimed in claim 9 wherein the opacifier is a mixture of carbon black and titanium dioxide present in the amount of 1 to 25% byweightand is a ratio of between 1:10 and 1:100 byweight carbon black:titanium dioxide.

13. A method method of coating a surface of a translucent material to stop lighttransmission by the translucent material which method comprises applying a composition comprising an organopolysiloxane and an opacifier and curing the composition.

14. A method as claimed in claim 13 wherein the silicone rubber composition comprises (a) a linear, organopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of 500to 10,000,000 centipoises when measured at 25"C,the organic groups of the aforesaid organopolysiloxane being substituted orunsubstituted monovalent hydrocarbon radicals.

(b) from 0.1 to 15% by weight, based on organopolysiloxane, of (1) an organoxysilane or silicate corresponding to the general formula, (RO)3Si-Ra (1) where R is a monovalent hydrocarbon or halogenated hydrocarbon radical and R1 is an alkyl, haloalkyl, aryl, haloaryl, alkenyl, cycloalkyl, cycloalkenyl, cyanoalkyl, alkoxy or acyloxy radical,or (2) a liquid partial hydrolysis product of the aforementioned organoxy silane or silicate compounds, (c) from 0.1 to 5% by weight, based on the organopolysiloxane, of a catalyst which is metal salt of an organic monocarboxylic or dicarboxylic acid in which the metal ion is lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth or manganese, and (d) from 0.1 to 10% byweight, based on the organopolysiloxane, of a nitrogen-containing silane ofthe formula:

where R is a monovalent hydrocarbon or halogenated hydrocarbon radical, Q is an alkoxy, phenoxy, halo, amino or dialkylamino group, and Q' is a saturated, unsaturated oraromatic hydrocarbon residue substituted by at least one amino hydrazone, azirane or, cyano group, and optionally one or morethio, sulphone, oxa, oxo, diorganosilicon and/or ester groups, and a is 1 or2.

15. A method as ciaimed in claim 13 wherein the silicone rubber composition comprises by weight (1)100 parts of a liquid vinyl chain-stopped polysiloxane having theformula

where R10 and R12are each an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5 to 7 ring carbon atoms or a mononuclear aralkyl radical of which the alkyl radical(a) contain(s) from 1 to 8 carbon atoms with at least 50 mole per cent of the R' radicals being methyl and where n has a value sufficient to provide a viscosity of 1,000 to 750,000 centistokes at 25"C, preferably from 50,000 to 150,000 inclusive, (2)from Oto 50, preferably from 20 to 50 parts of an organopolysiloxane copolymercomprising (R")3SiOo5 units, (R")2SiO units and SiO2 units, where each R" is a vinyl radical, an alkyl radical containing from 1 to 8 carbon atoms, a mononuclear aryl radical, a cycloalkyl radical having from 5to 7 ring carbon atoms ora mononuclear aralkyl radical of which the alkyl radical(s) contain(s) from 1 to 8 carbon atoms, where the ratio of (Fl")3SiO0.5 units to SiO2 units is from 0.5:1 to 1:1, and where from 2.5 to 10 mole per cent of the silicon atoms contain silicon-bonded vinyl groups.

(3) a catalyst comprising platinum and/ora platinum compound in an amount sufficient to providefrom 10-3 to 10.8 gram atoms of platinum per mole of silicon-bonded vinyl radicals in the composition, (4) an amountofa liquid organohydrogenpolysiloxane having the formula:: (R)a(H)bSiO4 a-b 2 sufficientto provide from 0.5 to 1.2 and preferably 1.0 silicon-bonded hydrogen atom per silicon-bonded vinyl group in the composition described in (1), where R is as previously defined, a has a value of from 1 .00to 2.00, b has a value of from 0.1 to 1.2, preferably 0.1 to 1.0, and the sum of a plus b is from 2.00 to 2.67,there being at least two silicon-bonded hydrogen atoms per molecule, (5)from 0.1 to 1 part of a liquid vinyl siloxane hydrnlyzate oftheformula:: [(HO)z (Fl"'O)y (CH2=CH) SiO3 y z]x 2 preferably prepared by the hydrolysis of a mixture ofvinyl trichlorosilane and a vinyl trialkoxysilane, such as vinyl triethoxysilane, where R'D is an alkyl radical having one to 8 carbon atoms,x is a number greater than 3, yhas avalue offrom 0.01 to 0.4, and preferably hasa valueoffrom 0.05to 0.1 and z has a value of 0.1 to 0.4, preferably from 0.2 to 0.4, (6) from 0 per centto 85 per cent, based upon the total weight of the above described mixture, of a halocarbon catalyst inhibitorwhich is a halocarbon having 2 carbon atoms and at least 3 halogen substituents,said halogen substituents having an atomic weight of less than 126and being positioned anywhere on the molecule.

16. Amethod as claimed in claim 13 wherein the silicone rubber composition comprises an organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25"C oftheformula, (R20)aSiO4.s 2 (7) a curing catalyst and an additive selected from the class consisting of an alkenylisocyanurate of the formula,

and a cyanurate of the formula,

and mixtures thereof, where R20 is selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, R21 is selected from unsaturated monovalent hydrocarbon radicals and unsaturated halogenated monovalent hydrocarbon radicals, R22 and R23 are selected from R' radicals, saturated monovalent hydrocarbon radicals and saturated halogenated monovalent hydrocarbon radicals and a varies from 1.95 to 2.01, inclusive.

17. A method as claimed in claim 13 wherein the silicone rubber composition comprises (a) from 82 to 99.65% by weight of a linear organopolysiloxane polymer having a viscosity of at least 100,000 centipoise at 25"C, and having the average unitformula: (R30),Si04, 2 (12) (b) from 0.1 to 8% byweight of a curing catalyst, and (c) from 0.25 to 10% by weight of a self-bonding additive of the formula: :

in which the formulae a hasavalueoffrom 1.95to2.01 inclusive, R30 is a monovalent hydrocarbon or halohydrocarbon radical, R36 is alkyl or hydrogen, Z is phenylene or a group ofthe formula -CO-O-, -CO-, -CO-NH- or-CO-NR32- in which R32 is a monovalent hydrocarbon or halohydrocarbon radical, G is hydrogen, a saturated monovaient hydrocarbon or halohydrocarbon radical, or has the same meaning as R35, R35 is an unsaturated monovalent hydrocarbon or halohydrocarbon radical, or a group oftheformula:: R34~sjRn32(M)3-n (14) in which R34 is a divalent hydrocarbon or halohydrocarbon radical, R34 has the meaning given above Mis a group ofthe formula R330-orR33-CO-O- in which R33 is a monovalent hydrocarbon or halohydrocarbon radical, and n is O or a whole numberfrom 1 to 3.

18. A method as claimed in any one of claims 13 to 17 wherein the translucent material is polymethyl methacrylate, polystyrene, polycarbonate or glass.

19. A method as claimed in claim 18 wherein the glass is solar reflecting glass.

20. A method as claimed in any one of claims 13 to 19 wherein the opacifier is one or more oftitanium dioxide, carbon black or calcium carbonate.

21. A method as claimed in claim 20 wherein the opacifier is carbon black present in an amount of 0.1 to 3% byweightofthe organopolysiloxane.

22. A method as claimed in claim 20 wherein the opacifier is titanium dioxide present in an amount of 1 to 25% by weight of the organopolysiloxane.

23. A method as claimed in any one of claims 1 to 19 wherein the opacifier is a mixture of carbon black and titanium dioxide present in the amount of 1 to 25% by weight and is a ratio of between 1:10 and 1:100 by weight carbon black:titanium dioxide.

24. An opacifying coating as claimed in claim 1 substantially as hereinbefore described in any one ofthe examples.

25. A method of opacifying translucent material as claimed in claim 13, substantially as hereinbefore described in any one ofthe examples.

26. An opacifying coating when produced by a method as claimed in any one of claims 13 to 23 and 25.

27. Atranslucent material when opacified by a method as claimed in any of claims 13 to 23 and 25.

28. An opacifying coating as claimed in any one of claims 1 to 12,24 and 26 togetherwith translucent material.

29. An opacifying coating as claimed in claim 2817 wherein the translucent material is glass.