GB1569681A

GB1569681A - Photocurable organopolysiloxane compositions

Info

Publication number: GB1569681A
Application number: GB44172/77A
Authority: GB
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1976-10-29
Filing date: 1977-10-24
Publication date: 1980-06-18
Also published as: JPS5443017B2; JPS5355362A; DE2748527A1

Description

(54) PHOTOCURABLE ORGANOPOLYSILOXANE COMPOSITIONS (71) We, SHIN-ETSU CHEMICAL CO., LTD., a corporation organised and existing under the laws of Japan, of 6-1 Otemachi 2-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and bythe following statement: The present invention relates to organopolysiloxane compositions curable b ultraviolet irradiation. More particularly the invention relates to organopolysiloxane compositions capable of affording releasing properties to various substrates, such as paper, laminated paper, plastics films, metal foils, and the like.

It is well known in the prior art to treat a substrate such as paper or plastic film with a particular organopolysiloxane composition in order to prevent the substrate adhering or bonding to other surfaces that are sticky, or to give water repellency to the substrate. Paper or plastic films thus treated are widely used as protective liners for adhesive tapes, adhesive labels, and the like.

The organopolysiloxane compositions known for the purpose are those capable of forming release coatings by a condensation reaction in the presence of an organotin compound as the catalyst (see, for example, U.S. Patents 2,803,613, 2,985,545 and 2,985,546) and those capable of forming release coatings by an addition reaction in the presence of a platinum compound as the catalyst (see, for example British Patent Specification 1,240,520). These compositions are generally used either in the form of a solution in an organic solvent or in the form of an aqueous emulsion.

The above-mentioned prior art organopolysiloxane composition in the form of an organic solution may contain various auxiliary additives, such as co-catalysts or various other organopolysiloxanes, in order to attain improvements in their releasing properties, wearing resistance, curability, and anti-blocking properties; nevertheless, the major component of the composition is always a high molecular weight diorganopolysiloxane of gum-like consistency, and because of this, large volumes of an organic solvent should be contained in the composition when it is applied. Such a high content of the organic solvent will, disadvantageously, give rise to the possibility of air pollution or increases in expenditure arising from recovery of the solvent.

Another prior art aqueous emulsion type organopolysiloxane composition may involve less of an environmental pollution problem, and yet have the possibility of secondary pollution inherent in the use of emulsifying agents. Besides, the aqueous emulsion type composition is disadvantageous in having to be cured by heating at higher temperatures for long period of time to form cured films, as well as in the difficulty in obtaining cured films with superior physical properties, since it is difficult to use the high molecular weight diorganopolysiloxane which has a poor emulsion-forming property, to form the major component of the composition.

In order to overcome the above described disadvantages, the so called non diluent type organopolysiloxane compositions which contain no organic solvent or water have been proposed. The compositions comprise (a) a vinyl-containing diorganopolysiloxane having a viscosity of 10 to 10,000 centistokes at 250C, (b) an organohydrogenpolysiloxane having at least three hydrogen atoms bonded directly to the silicon atoms in a molecule, (c) a platinum catalyst, and (d) a curing retarder (see, for example, British Specification No. 1,469,808). These compositions, despite the content of the curing retarder therein, still have a low and insufficient pot life.

Further, these compositions require heat application to form cured films, necessitating large scale curing facilities as well as enormous heat energy for the purpose of commercial production of, for example, release papers.

Furthermore, organopolysiloxane compositions that are curable by ultraviolet irradiation have been proposed, comprising a vinyl-containing organopolysiloxane, an organohydrogenpolysiloxane and a photosensitizer, or alternatively a mercaptocontaining organopolysiloxane, a vinyl-containing organopolysiloxane and a photosensitizer (see British Patent Specification No. 1,433,461).

The above ultraviolet light curable organopolysiloxane compositions have a drawback encountered in the manufacture of release papers in that their curing velocity is relatively low. This drawback may be overcome only by introducing large amounts of photocurable functional groups into their structure, resulting disadvantageously in poorer compatibility between the organopolysiloxane components as well as decreases in the releasing property of the release papers prepared therefrom, such decreases being remarkably great particularly after prolonged storage.

It was therefore desirable to provide an organopolysiloxane composition free from the above-described disadvantages encountered in the prior art compositions, that is to say, diluent-free and curable by irradiation with ultraviolet light in a very short period of time to produce cured films having superior properties.

The organopolysiloxane composition of the present invention comprises (a) an organopolysiloxane having in a molecule at least one mercaptoalkylcontaining organosiloxane unit represented by the general formula HSR Si (R')aO3~a where R is an alkylene group, R1 is a substituted or unsubstituted monovalent hydrocarbon group other than an alkenyl group and a is 0, 1 or 2, (b) an organopolysiloxane having in a molecule at least one allyl-containing organosiloxane unit represented by the general formula CH2=CH-CH2 Si(R2)bO3~b where R2 is a monovalent hydrocarbon group unsubstituted or substituted with a substituent group other than a mercapto group and b is 0, 1 or 2, and (c) a photosensitizer.

The present invention is based on a discovery resulting from extensive investigations directed to organopolysiloxane compositions capable of producing a cured film having superior properties by a brief irradiation with ultraviolet light, viz. that an organopolysiloxane composition including the above-mentioned components (a), (b) and (c) can afford a very satisfactory and reliable cure when irradiated with ultraviolet light for a very short time, that components (a) and (b) are sufficiently compatible with each other even when the content of functional groups in one of them widely differ from that of the other, and further that cured films having releasing properties versatile for various applications can be readily formed.

The organopolysiloxane used as component (a) in the organopolysiloxane composition of the present invention contains in a molecule at least one mercaptoalkyl-containing organosiloxane unit as represented by formula (I) above, in which the group denoted by R is an alkylene group, and the group denoted by R1 is a substituted or unsubstituted monovalent hydrocarbon group other than an alkenyl group. Illustrative of the R groups are methylene, ethylene, propylene. and butylene groups, while illustrative of the R1 groups are alkyl groups such as methyl, ethyl, propyl'and butyl groups; aryl groups, such as a phenyl group; and those groups which are substituted by a substituent or substituents, such as halogen atoms, e.g. chlorine atoms, in place or part or all of the hydrogen atoms in the above-named hydrocarbon groups.

It is preferred that the molar fraction of the organosiloxane units of formula (I) is at least 0.05 mole Ó or, preferably, at least 0.1 mole % of all of the organosiloxane units of which the organopolysiloxane used as component (a) is composed.

The organopolysiloxane used as component (a) in the present invention may be one which is composed substantially of the organosiloxane units of formula (I) alone or a copolymeric one with one or more different organosiloxane units. The organosiloxane units useful for forming the above copolymeric organopolysiloxanes may be those represented by the general formula R ' SiO,, where R' has the same meaning as defined in formula (I) and m is a number of 0, 1, 2 or 3. Examples are dimethylsiloxane units, methylphenylsiloxane units, methylsiloxane units, and phenylsiloxane units.

In order to impart a good releasing property to a paper or other substrate, it is preferred that most of the organic groups bonded to the silicon atoms other than the mercaptoalkyl groups in component (a) are methyl groups. The molecular configuration of the organopolysiloxane used as component (a) is not limitative; it may be linear, branched, or cyclic.

The viscosity of the organopolysiloxane used as component (a) is not limitative also. However, it is preferred not to exceed 10,000 centistokes at 250C if no diluent or organic solvent is included in the composition of the present invention when applied to a substrate in order to prevent atmospheric pollution, or if the composition is intended to have a good workability. Component (a) may, of course, have a higher viscosity insofar as its mixture with the second component (b) has a viscosity which is not excessively high.

The organopolysiloxane used as component (a) can be readily prepared by a known method using one or more organochlorosilanes or alkoxy-containing organosilanes including at least one mercaptoalkyl-containing organosilane, e.g.

(HS CH2CH2CH2)(CH3)Si Cl2, as combined in accordance with the molar ratio of the siloxane units of the desired organopolysiloxane, in which the silane or the silane mixture is subjected to hydrolysis or co-hydrolysis and condensation reaction, followed, if necessary, by an equilibration reaction.

A typical example of the organopolysiloxane used as component (a) is represented by the general formula

where p and q are each zero or positive integers and r is a positive integer.

The organopolysiloxane used as component (b) in the organopolysiloxane composition of the invention contains in a molecule at least one allyl-containing organosiloxane unit represented by formula (II) above, in which the group denoted by R2 is a substituted or unsubstituted monovalent hydrocarbon group, the substituent groups excluding a mercapto group. Illustrative of the R2 groups are alkyl groups, such as methyl, ethyl, propyl and butyl groups; aryl groups, such as phenyl group; and those groups which are substituted by a substituent or substituents, such as halogen atoms, e.g. chlorine atoms, in place of part or all of the hydrogen atoms in the above-named hydrocarbon groups.

It is preferred that the molar fraction of the allyl-containing organosiloxane units of formula (II) is at least 0.05 mole Ó or, preferably, at least 0. 1 mole % of all of the organosiloxane units of which the organopolysiloxane used as component (b) is composed.

The organopolysiloxane used as component (b) in the present invention may be one which is composed substantially of the organosiloxane units of formula (II) alone or a copolymeric one with one or more of different organosiloxane units. The organosiloxane units useful for forming the above copolymeric organopolysiloxanes may be those represented by the general formula Rn sio4-n where R2 has the same meaning as defined in formula (II) and n is a number of 0, 1, 2 or 3.

Most of the organic groups bonded to the silicon atoms in component (b) are preferred to be methyl groups for the same purpose as in component (a). The viscosity of this component is preferably 10,000 centistokes or below at 25"C. Of course, higher viscosities are operable when combined with component (a) having a sufficiently low viscosity insofar as the resultant composition will have a viscosity not exceeding 10,000 centistokes.

The molecular configuration of component (b) is not limitative; it may be linear, branched, or cyclic.

The organopolysiloxane used as component (b) can be prepared readily by a known method using one or more organochlorosilanes or alkoxy-containing organosilanes including at least one allyl-containing silanes, e.g.

(CH2=CH-CH2)(CH2)SiCl2 and (CH2=CII-CH2)SiCl3 as combined in accordance with the molar ratio of the siloxane units of the desired organopolysiloxane, in which the silane or silanes are subjected to hydrolysis or co hydrolysis and condensation reaction, followed, if necessary, by equilibration reaction.

Typical examples of the organopolysiloxane used as component (b) are those which are represented by the general formula

where s is zero or a positive integer and t is a positive integer, and others which are represented by the average unit formula

where u, v and ware each zero or positive numbers and x is a positive number, each expressing the molar fraction of the organosiloxane unit.

The photosensitizers used as component (c) in the organopolysiloxane composition of the present invention include various conventional ones, which may be exemplified by amino, nitro and phenolic compounds, such as 4hydroxydiphenyl, 4-nitroaniline, picramide, 2,4-dinitrophenol and 2,6-dichloro-4nitroaniline; carbonyl compounds, such as benzaldehyde, acetophenone, benzophenone, 4,4'-diaminobenzophenone and N,N,N',N'-tetramethyl-4,4'diaminobenzophenone, i.e. Michler's ketone; quinone compounds, such as benzoquinone, anthraquinone and 1,2-naphthoquinone; anthrone compounds, such as 3-methyl-1,3-diazo- 1,9-benzanthrone; benzoin compounds, such as benzoin and methyl ether of benzoin; organic dyes, such as malachite green, methylene blue, Chrome Green, Rhodamine Blue and Azo Green TEG; and pyrylium salts, such as 2,4,6-triphenylpyrylium perchlorate, 2,4,6-triphenylthiapyrylium perchlorate, 2,4,6-triphenylpyrylium fluoborate and 2,4,6-triphenylthiapyrylium fluoborate.

In addition to the above-named conventional photosensitizers, certain organosilicon group-containing benzophenone derivatives including 4trimethylsilylbenzophenone and 4-dimethylamino-4'-dimethylvinylsilylbenzophenone as disclosed in U.S. Patent 4,042,613 may be recommended when it is essential that their miscibility with components (a) and (b) is higher. The organosilicon group-containing benzophenone derivatives can work to eliminate any drawbacks that may arise from low miscibility between any photosensitizers and the organopolysiloxanes, i.e., decreases in the photocurability of the composition caused by the precipitated or separated photosensitizer, local variations in curing, low storability of the composition, etc.

The organopolysiloxane composition of the present invention can be prepared by uniformly blending components (a), (b) and (c) by a suitable means. The proportions of components (a) and (b) may vary in a wide range depending on which is a base component or a crosslinking agent and, accordingly, the contents of the mercapto groups and the allyl groups in mole % as the functional groups, the molecular structure, the viscosity, and the like.A generally acceptable formulation is such that the molar content of the functional groups in the main chain which contribute to the formation of crosslinkings is 20 mole % or less of all of the organic groups, while the ratio of the mercapto groups in component (a) to the allyl groups in component (b) is in the range from 1:20 to 20:1 or, preferably, from 1:10 to 10:1, in order to obtain satisfactory compositions having sufficient curability by ultraviolet irradiation.

The amount of the photosensitizer used as component (c) to be added to the composition is determined depending on several factors including the kinds of components (a) and (b), the blending ratio of these components, the desired curing velocity and the like, but not limited specifically. In general, however, the amount is in the range from 0.05 to 5% by weight based on the total amount of components (a) and (b). When the photosensitizer is the afore-mentioned organo-silicon group containing benzophene derivative, even larger amounts can be used causing no precipitation.

The organopolysiloxane compositions of the present invention prepared preferably have a viscosity not exceeding approximately 10,000 centistokes at 250C to give efficiency in coating operations. It is naturally desired that the compositions have a viscosity as high as possible in order to form cured films having superior mechanical properties, but the upper limit of the viscosity depends on several factors, such as coating amount, method of coating, substrate, and the like.For example, a composition having a relatively low viscosity, say, about 400 centistokes or below, is recommended when applied to polyethylene laminated paper, plastic films or metal foils by a known coating method using a roll coater, gravure coater, air coater or curtain flow coater, the coating amount practically ranging from 0.5 to 5 g/m2. Or, a composition having a higher viscosity is recommended for application to glassine paper, kraft paper or clay-undercoat paper, since these substrates are more tended to soak the coating liquid thereinto.

It may be added that recent remarkable advances in coating techniques, for example, by utilizing offset printing techniques, have made it possible to form coatings in an amount of 1.0 g/m2 or less, using a liquid having a viscosity of about 100,000 centistokes or higher. However, it is recommended that the viscosity of the organopolysiloxane composition of the present invention is about 10,000 centistokes at 25"C at the highest if the practicability of the generally applicable coating techniques is taken into consideration.

Cured films having satisfactory properties can be obtained by irradiating the coating layer on a substrate with ultraviolet light. The light source suitable for the purpose is desirably a lamp which emits light rich in the ultraviolet region, such as xenon lamps and low or high pressure mercury lamps.

It is optional that various conventional additives may, if necessary, be included in the composition in an amount not too excessive to reduce the effects of the composition. Such additives include diorganopolysiloxanes free of functional groups, fillers, coloring agents, and the like.

The following examples are given to further illustrate the present invention but are not be be regarded as limitative. In the examples, parts are all parts by weight and all viscosities are measured at 250C.

Example 1.

A coating liquid was prepared by blending 93 parts of a mercaptopropylcontaining organopolysiloxane having a viscosity of 610 centistokes, composed of 95 mole ?/o of dimethylsiloxane units and 5 mole % of mercaptopropylmethyl siloxane units, and terminated at both chain ends with trimethylsilyl groups, 6 parts of an allyl-containing organopolysiloxane having a viscosity of 45 centistokes, composed of allylmethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups, and 1.0 part of benzophenone. The resultant liquid had a viscosity of 520 centistokes and a molar ratio of the mercaptopropyl to allyl groups of about 1.0.

The coating liquid thus prepared was applied uniformly onto polyethylene laminated paper in a coating amount of 0.7 g/m2 by means of an offset-printing coater, followed by exposure to ultraviolet light emitted from an 80-watt high pressure mercury lamp 15 cm apart for 2 seconds. As a result, a glossy and non sticky cured film having strong wear resistance, anti-blocking property and high speed peeling property was formed. The releasing property of the thus coated laminated paper was tested against several kinds of adhesives by the transfer process at the peeling velocity of 300 mm/minute in the same manner as specified in TAPPI, Volume 43, No. 8, August 1960 except that the release paper used was 5 cm wide instead if 1 inch wide.The results of the test recorded the peeling forces of 25 g/5 cm against a gum adhesive and 35 g/5 cm against an acrylic adhesive, the residual adhesiveness being 95%, indicating no inferiority compared to commercially available conventional releasing agents of the solution or non-solvent type. The releasing property of the test specimen remained almost unchanged even after one month of storage.

Example 2.

A coating liquid was prepared by uniformly blending 88 parts of a mercaptocontaining organopolysiloxane having a viscosity of 280 centistokes, containing 3 mole % of mercaptopropylmethylsiloxane units, the rest being dimethylsiloxane units and trimethylsiloxane units, 4.5 parts of an allyl-containing organopolysiloxane having a viscosity of 130 centistokes containing 10 mole % of allylsiloxane units (CH2=CH-CH2-SiO15), the rest being dimethylsiloxane units and trimethylsiloxane units, and 1.0 part of benzophenone. The viscosity of the resultant coating liquid was 265 centistokes and the molar ratio of the mercaptopropyl to allyl groups was about 0.5.

Glassine paper was coated with the coating liquid thus prepared by an offsetprinting coater in the same manner as in Example 1 in a coating amount of 1.0 g/m2, followed by exposure to ultraviolet light from an 80-watt high pressure mercury lamp 15 cm apart for 3 seconds, to form an excellently non-sticky cured film without rub-off. The releasing property of the thus prepared release paper was determined in the same manner as in Example 1, resulting in the peeling force of 30 g/5 cm against the gum adhesive or 28 g/5 cm against the acrylic adhesive, proving a satisfactory release paper. The releasing property of this specimen remained almost unchanged even after one month of storage.

Example 3.

A mixture of organopolysiloxanes having a viscosity of 820 centistokes was prepared by blending a mercaptopropyl-containing diorganopolysiloxane having a viscosity of 750 centistokes, composed of 7 mole % of mercaptopropylmethylsiloxane units and 93 mole % of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups and an organopolysiloxane obtained by the hydrolysis of a silane mixture of 1 mole of trimethylchlorosilane and 3 moles of allyltrichlorosilane in a ratio such that the molar ratio of the mercaptopropyl groups to the allyl groups was 1/1.5. This organopolysiloxane mixture was admixed with 0.5"/, by weight of 4-trimethylsilylbenzophenone as the photosensitizer to produce a uniform liquid. The liquid was applied onto polyethylene-laminated paper by a gravure offset coater in a coating amount of 0.8 g/m2, followed by exposure to ultraviolet light emitted from a 3-kW high pressure mercury lamp 25 cm apart at a line speed of 70 m/minute corresponding to the irradiation time of 0.6 second. As a result, a glossy release paper with an organopolysiloxane film cured to non-stickiness and no rub-off state was obtained.

The releasing property of the release paper thus obtained was determined in the same manner as in Example 1 to indicate the peeling force of 35 g/5 against the gum adhesive or 30 g/5 cm against the acrylic adhesive. The releasing property of this specimen remained almost unchanged even after one month of storage.

Example 4.

An allyl-containing diorganopolysiloxane having a viscosity of 30 centistokes, composed of 4 mole V of allylmethylsiloxane units and 96 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups and a mercapto-propyl-containing diorganopolysiloxane having a viscosity of 750 centistokes, composed of 30 mole V of mercaptopropylmethylsiloxane units and 70 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups were blended in a ratio such that the molar ratio of the mercaptopropyl groups to the allyl groups was 1.3 to give an organopolysiloxane mixture with a viscosity of 630 centistokes.The organopolysiloxane mixture was admixed with 1.0% by weight of benzophenone, and the resultant liquid was applied onto Kraft paper. followed by exposure to ultraviolet light as in Example 1 for 1.5 seconds. As a result a release paper with an organopolysiloxane film cured to no rub-off state was obtained.

This release paper was evaluated for its releasing property in the same manner as in Example 1, to find that the peeling force was 95 g/5 against the gum adhesive or 70 g/5 cm against the acrylic adhesive. The releasing property of the specimen remained almost unchanged even after one month of storage.

Example 5.

An organopolysiloxane mixture having a viscosity of 1,410 centistokes was prepared by blending equal amounts of an allyl-containing organopolysiloxane having a viscosity of 220 centistokes, composed of 5 mole V of allylsiloxane units and 95 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups and a mercapto-containing diorganopolysiloxane having a viscosity of 1,800 centistokes, composed of 5 mole V of mercaptopropylmethylsiloxane units and 95 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups. The molar ratio of the mercaptopropyl groups to the allyl groups in this organopolysiloxane mixture was about 1.0.

Polyethylene laminated paper was coated by an offset-printing coater, using the liquid that was prepared by adding 1.5% by weight of a methyl ether of benzoin to the above organopolysiloxane mixture in a coating amount of 0.8 g/m2, followed by exposure to ultraviolet light as in Example 3 for 0.5 second. As a result, an excellent cured film in no rub-off state was obtained. For comparison, the exposure time was shortened by half by increasing the line speed in the cure box, to find that the resulting cure was in a somewhat insufficient state so that the surface of the cured film became cloudy when rubbed by a finger tip.

The releasing property of the above prepared release paper was evaluated in the same manner as in Example 1 to conclude that the peeling force was 41 g/5 cm against the gum adhesive or 35 g/5 cm against the acrylic adhesive. A similar test was carried out after one month of storage to show the peeling value of 43 g/5 cm against the gum adhesive or 33 g/5 cm against the acrylic adhesive. This can be interpreted that the release paper has a releasing property which is very little variable with lapse of time.

Example 6.

An organopolysiloxane mixture having a viscosity of 2,100 centistokes was prepared by blending a mercapto-containing organopolysiloxane having a viscosity of 1,900 centistokes, composed of 3 mole V of mercaptopropylmethylsiloxane units and 10 mole V of methylphenylsiloxane units, the rest being dimethylsiloxane units and trimethylsiloxane units, and an organopolysiloxane obtained by hydrolysis of an organochlorosilane mixture composed of 1 mole of trimethylchlorosilane, 3 moles of allyltrichlorosilane and 0.5 mole of methylphenyldichlorosilane in ratio such that the molar ratio of the mercaptopropyl groups to the allyl groups was about 0.5 This organopolysiloxane mixture was admixed with 1.00ó by weight of 4trimethylsilylbenzophenone, and the resulting liquid was applied onto glassine paper by an offset-printing coater in a coating amount of 1 g/m2, followed by exposure to ultraviolet light emitted from an 80-watt high pressure mercury lamp 15 cm apart for 3 seconds, to give a non-sticky cured film in no rub-off state.

Example 7.

This is an example for comparison specifically with Example 4.

For the purpose of comparison, an organopolysiloxane mixture was prepared by blending the two ingredients, one being the same mercaptopropyl-containing diorganopolysiloxane as component (a) in accordance with the present invention but the other being a vinyl-containing organopolysiloxane which had a viscosity of 670 centistokes and was composed of 4 mole V of vinylmethylsiloxane units and 96 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups, instead of the allyl-containing one as component (b), and the blending ratio being such that the mercaptopropyl to the vinyl groups was 1.3. The mixture had a viscosity of 630 centistokes.To this mixture was added the photosensitizer in the same manner to produce a coating liquid, which was then applied onto Kraft paper, and exposed to ultraviolet light in the same manner. As a result it was found that it took about one minute before the film formed on the paper substrate was cured to no rub-off state.

WHAT WE CLAIM IS: 1. An organopolysiloxane composition comprising (a) an organopolysiloxane having in a molecule at least one mercaptoalkylcontaining organosiloxane unit represented by the general formula HSR Si (R'),O,~, 2 where R is an alkylene group, R1 is a substituted or unsubstituted monovalent hydrocarbon group other than an alkenyl group and a is 0, 1 or 2, (b) an organopolysiloxane having in a molecule at least one allyl-containing organosiloxane unit represented by the general formula CH2=CH-CH2 Si (R2)bO3~b 2 where R2 is a monovalent hydrocarbon group unsubstituted or substituted with a substituent group other than a mercapto group and b is 0, 1 or 2, and (c) a photosensitizer.

2. The organopolysiloxane composition as claimed in claim 1 wherein the alkylene group denoted by symbol R is selected from methylene, ethylene, and butylene groups.

3. The organopolysiloxane composition as claimed in claim I wherein the alkylene group is a propylene group.

4. The organopolysiloxane composition as claimed in any preceding claim wherein the group denoted by symbol R1 is one selected from alkyl groups, aryl groups and the halogen-substituted derivatives thereof.

5. The organopolysiloxane composition as claimed in claim 4 wherein the group denoted by symbol R1 is a methyl group.

6. The organopolysiloxane composition as claimed in any preceding claim wherein the group denoted by symbol R2 is one selected from alkyl groups, aryl groups and the halogen-substituted derivatives thereof.

7. The organopolysiloxane composition as claimed in claim 6 wherein the group denoted by symbol R2 is a methyl group.

8. The organopolysiloxane composition as claimed in any preceding claim wherein the molar fraction of the mercaptoalkyl-containing organosiloxane units in component (a) is at least 0.05 mole 0',.

9. The organopolysiloxane composition as claimed in any preceding claim wherein the viscosity of component (a) is 10,000 centistokes or below at 250C.

10. The organopolysiloxane composition as claimed in any preceding claim wherein the molar fraction of the allyl-containing organosiloxane units in component (b) is at least 0.05 mole , Ó.

Il. The organopolysiloxane composition as claimed in any preceding claim wherein the viscosity of component (b) is 10,000 centistokes or below at 25"C.

12. The organopolysiloxane composition as claimed in any preceding claim wherein the photosensitizer is benzophenone.

13. The organopolysiloxane composition as claimed in any one of claims 1 to 11 wherein the photosensitizer is 4-trimethylsilylbenzophenone or 4-dimethylamino-4'-dimethylvinvlsilvlbenzophenone.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. but the other being a vinyl-containing organopolysiloxane which had a viscosity of 670 centistokes and was composed of 4 mole V of vinylmethylsiloxane units and 96 mole V of dimethylsiloxane units, and terminated at both chain ends with trimethylsilyl groups, instead of the allyl-containing one as component (b), and the blending ratio being such that the mercaptopropyl to the vinyl groups was 1.3. The mixture had a viscosity of 630 centistokes. To this mixture was added the photosensitizer in the same manner to produce a coating liquid, which was then applied onto Kraft paper, and exposed to ultraviolet light in the same manner.As a result it was found that it took about one minute before the film formed on the paper substrate was cured to no rub-off state. WHAT WE CLAIM IS:

1. An organopolysiloxane composition comprising (a) an organopolysiloxane having in a molecule at least one mercaptoalkylcontaining organosiloxane unit represented by the general formula HSR Si (R'),O,~, 2 where R is an alkylene group, R1 is a substituted or unsubstituted monovalent hydrocarbon group other than an alkenyl group and a is 0, 1 or 2, (b) an organopolysiloxane having in a molecule at least one allyl-containing organosiloxane unit represented by the general formula CH2=CH-CH2 Si (R2)bO3~b 2 where R2 is a monovalent hydrocarbon group unsubstituted or substituted with a substituent group other than a mercapto group and b is 0, 1 or 2, and (c) a photosensitizer.

14. The organopolysiloxane composition as claimed in any preceding claim

wherein the molar ratio of the mercaptoalkyl groups present in component (a) plus the allyl groups present in component (b) to all of the organic groups bonded to the silicon atoms of components (a) and (b) is 20 mole V or less.

15. The organopolysiloxane composition as claimed in any preceding claim wherein the molar ratio of the mercaptoalkyl groups present in component (a) to the allyl groups present in component (b) is in the range from 1:20 to 20:1.

16. The organopolysiloxane composition as claimed in any preceding claim wherein the content of the photosensitizer in the composition is in the range from 0.05 Ó to 5% by weight.

17. The organopolysiloxane composition as claimed in any preceding claim of which the viscosity is 10,000 centistokes or below at 250C.

18. A release paper substrate provided with a coating layer of the organopolysiloxane composition as defined in any preceding claim.

19. A process of preparing a release paper comprising coating a substrate by the organopolysiloxane composition as defined in any one of claims 1 to 17 and exposing the coated substrate to ultraviolet light to cure the siloxane compound.

20. The organopolysiloxane composition as claimed in claim 1, substantially as described in any of Examples 1 to 6.

21. A process as claimed in claim 19, substantially as described in any of Examples 1 to 6.

22. A release paper prepared by a process according to Claim 19 or 21.