Patent Description:
Release sheets, e.g. release paper and release film which are releasable from sticky or pressure-sensitive adhesive (PSA) materials are manufactured by coating an organopolysiloxane composition onto the surface of various substrates such as paper, laminated paper, synthetic film, transparent resins and metal foil, and inducing crosslinking reaction to form a cured film. There are known various means for curing the organopolysiloxane composition, for example, condensation reaction with the aid of organometallic compounds, vulcanization with the aid of organic peroxides, and hydrosilylation reaction with the aid of platinum group metal catalysts. While these curing methods need heating, low or room temperature cure is required for productivity increases or energy savings. Additionally, radiation curing methods are used as a curing method other than heating.

The radiation curing method encompasses cure modes through radical polymerization of acrylic modified polysiloxane, cation polymerization based on ring-opening of epoxy groups on epoxy-modified polysiloxane, and thiol-ene reaction of mercapto-modified polyorganosiloxane.

When a cured film is formed by radical polymerization of acrylic modified polysiloxane using a photo-radical initiator, the film is tightly adhesive to substrates and effectively curable. However, a device for reducing oxygen concentration is necessary because cure is inhibited by the presence of oxygen.

When a cured film is formed by cation polymerization based on ring-opening of epoxy groups on epoxy-modified polysiloxane using a photoacid generator, cure in air is possible because cure inhibition by oxygen is avoided. However, cure inhibition can occur where a substrate contains a compound capable of inhibiting acid generation, or under the influence of air-borne humidity.

When a cured film is formed by thiol-ene reaction of mercapto-modified polyorganosiloxane using a photo-radical initiator, cure proceeds via radical polymerization as in the case of acrylic modified polysiloxane. Advantageously, the system is curable in air without the influence of oxygen concentration, and is less susceptible to the influence of cure inhibitory substances unlike the cation polymerization. By virtue of these advantages, the composition is applicable as a backing treating agent of PSA tape, release agent of PSA label and tape based on a substrate containing various additives. The future expansion of the market is expected. For this reason, many inventions relating to release agents utilizing thiol-ene reaction of mercapto-modified polyorganosiloxane have been proposed.

The radical polymerization by thiol-ene reaction allows cure to proceed without the influence of oxygen concentration in air unlike the radical polymerization of acrylic groups, but is slower in reaction rate than the radical polymerization of acrylic groups, leading to a tendency to lower the adhesion to substrates.

<CIT> (Patent Document <NUM>) proposes a UV-curable silicone release composition comprising:.

<CIT> (Patent Document <NUM>) proposes a releasable UV-curable silicone composition comprising:.

Patent Document <NUM> discloses a release agent composition comprising a mercapto-functional polysiloxane, a vinyl-functional polysiloxane and a compound comprising multiple acrylate groups. Its Example <NUM> discloses a composition based on a mercapto-functional polysiloxane, a vinyl-functional polysiloxane and an acrylic silicone resin.

An object of the invention, which has been made under the above-mentioned circumstances, is to provide a releasable radiation-curable silicone composition which is effectively radiation-curable to form a cured product having improved adhesion to substrates, and a release sheet.

Making extensive investigations to attain the above object, the inventors have found that the outstanding problem is solved by adding a small amount of a compound having a plurality of acrylic groups per molecule as an adhesion improver to a releasable radiation-curable silicone composition for release sheets comprising an alkenyl-containing organopolysiloxane, a mercapto-containing organopolysiloxane and a photo-initiator, because a release sheet which is significantly improved in adhesion to a substrate while maintaining the release properties unchanged can be manufactured therefrom. It has been confirmed that the releasable radiation-curable silicone composition of the invention forms a cured product capable of maintaining the release properties despite excellent adhesion, as compared with prior art radiation-curable silicone compositions. The invention is predicated on this finding.

Accordingly, the invention provides a releasable radiation-curable silicone composition comprising the following components (A), (B), (C), and (D):.

The invention also provides a release sheet prepared by coating the radiation-curable silicone release composition onto a substrate and irradiating radiation onto the coating for curing the coating.

The dependent claims further define other embodiments of the silicone composition.

According to the invention, there are provided radiation-curable silicone release compositions which are effectively radiation-curable to form cured products having improved adhesion to substrates, and release sheets.

The invention provides a releasable radiation-curable silicone composition comprising components (A), (B), (C), and (D).

Component (A) is an alkenyl-containing organopolysiloxane having the average compositional formula (<NUM>). <CHM>
Herein R<NUM> which may be the same or different is a C<NUM>-C<NUM> monovalent hydrocarbon group, a ≥ <NUM>, b ≥ <NUM>, c ≥ <NUM>, d ≥ <NUM>, and <NUM> ≤ a+b+c+d ≤ <NUM>,<NUM>, R<NUM>, a, b, c, and d are selected such that at least two silicon-bonded alkenyl groups are present per molecule. The organopolysiloxane may be used alone or in a combination of two or more.

In formula (<NUM>), R<NUM> which may be the same or different is a C<NUM>-C<NUM> monovalent hydrocarbon group, and is selected such that at least two silicon-bonded C<NUM>-C<NUM> alkenyl groups are present per molecule. The C<NUM>-C<NUM> monovalent hydrocarbon groups are substituted or unsubstituted C<NUM>-C<NUM> monovalent hydrocarbon groups which include alkyl groups such as methyl, ethyl, propyl and butyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl, allyl, butenyl, hexenyl, and octenyl, aryl groups such as phenyl and tolyl, and substituted forms of the foregoing in which some or all of the carbon-bonded hydrogen atoms are substituted by halogen atoms such as fluorine and chlorine, for example, <NUM>,<NUM>,<NUM>-trifluoropropyl, perfluorobutylethyl, perfluorooctylethyl, or by (meth)acrylic functional groups such as acryloxy and methacryloxy, for example, <NUM>-methacryloxypropyl and <NUM>-acryloxypropyl, or by polyoxyalkylene groups such as polyoxyethylene or polyoxypropylene, or by alkoxy groups, for example, methoxypropyl and ethoxypropyl. While component (A) contains at least two silicon-bonded alkenyl groups per molecule, the alkenyl groups are preferably vinyl, allyl or hexenyl.

Preferably in view of release properties, groups other than the alkenyl group include phenyl groups in an amount of <NUM> to <NUM> mol% of overall groups R<NUM> in the molecule, and the remaining groups are methyl groups. A phenyl content of less than <NUM> mol% leads to less UV cure whereas a phenyl content in excess of <NUM> mol% leads to tight release and is unsuitable for release sheets.

R<NUM>, a, b, c, and d are selected such that at least two, preferably at least three silicon-bonded alkenyl groups are present per molecule and may also be selected such that the alkenyl and phenyl contents fall in the above ranges. The preferred range is a ≥ <NUM>, b ≥ <NUM>, c ≥ <NUM>, d ≥ <NUM>, more preferably a ≥ <NUM>, b ≥ <NUM>, c ≥ <NUM>, d=<NUM>. The sum is <NUM> ≤ a+b+c+d ≤ <NUM>,<NUM>, preferably <NUM> ≤ a+b+c+d ≤ <NUM>,<NUM>.

Component (B) is a mercaptoalkyl-containing organopolysiloxane having the average compositional formula (<NUM>). <CHM>
Herein R<NUM> which may be the same or different is a C<NUM>-C<NUM> monovalent hydrocarbon group or mercaptoalkyl group, e ≥ <NUM>, f ≥ <NUM>, g ≥ <NUM>, h ≥ <NUM>, and <NUM> ≤ e+f+g+h ≤ <NUM>, R<NUM>, e, f, g, and h are selected such that at least two silicon-bonded mercaptoalkyl groups are present per molecule. The organopolysiloxane may be used alone or in a combination of two or more.

R<NUM> which may be the same or different is a C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon group or mercaptoalkyl group. Examples of the C<NUM>-C<NUM> monovalent hydrocarbon groups other than the mercaptoalkyl group include alkyl groups such as methyl, ethyl and propyl, aryl groups such as phenyl and tolyl, and substituted forms of the foregoing in which some or all of the carbon-bonded hydrogen atoms are substituted by halogen atoms such as fluorine and chlorine, for example, <NUM>,<NUM>,<NUM>-trifluoropropyl, perfluorobutylethyl, perfluorooctylethyl, or by alkoxy groups, for example, methoxypropyl and ethoxypropyl. While component (B) contains at least two silicon-bonded mercaptoalkyl groups per molecule, it is preferred in view of release properties and compatibility with component (A) that the silicon-bonded group other than mercaptoalkyl be methyl which accounts for at least <NUM> mol% of overall groups R<NUM> in the molecule.

Exemplary mercaptoalkyl groups include mercaptomethyl, mercaptoethyl, mercaptopropyl, and mercaptohexyl, with mercaptopropyl being preferred for availability of reactants and ease of synthesis. It is preferred in view of crosslinking efficiency that the content of mercaptoalkyl groups based on the overall groups R<NUM> in the molecule (i.e. the content of silicon-bonded mercaptoalkyl based on the overall silicon-bonded organic groups) be <NUM> to <NUM> mol%.

R<NUM>, e, f, g, and h are selected such that at least two silicon-bonded mercaptoalkyl groups are present per molecule. The subscripts e, f, g, and h are preferably, in case of e=<NUM>, f ≥ <NUM>, g ≥ <NUM>, and h=<NUM>, or in case of e≠<NUM>, e ≥ <NUM>, f ≥ <NUM>, g ≥ <NUM>, and h ≥ <NUM>; more preferably in case of e=<NUM>, f ≥ <NUM>, g ≥ <NUM>, and h=<NUM>, or in case of e≠<NUM>, e ≥ <NUM>, f ≥ <NUM>, g ≥ <NUM>, and h=<NUM>. The sum is <NUM> ≤ e+f+g+h ≤ <NUM>, preferably <NUM> ≤ e+f+g+h ≤ <NUM>.

An amount of component (B) blended is <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, more preferably <NUM> to <NUM> parts by weight per <NUM> parts by weight of component (A). Less than <NUM> part by weight of component (B) leads to a lowering of cure whereas an amount in excess of <NUM> parts by weight achieves no improvement in cure and reduces release properties.

Component (C) is a silane compound having at least three acrylic groups per molecule, or a hydrolytic condensate thereof. It is a component for enhancing the adhesion of the releasable radiation-curable silicone composition. The compound may be used alone or in a combination of two or more as component (C). When a compound having at least <NUM> acrylic groups per molecule is added, a significant improvement in adhesion is achievable.

Suitable silane compounds having at least <NUM> acrylic groups per molecule include those having the general formula (<NUM>). <CHM>
Herein n is an integer of <NUM> to <NUM>, m is an integer of <NUM> to <NUM>, X is a C<NUM>-C<NUM> alkoxy group, R<NUM> is a C<NUM>-C<NUM> alkyl group, and R<NUM> is a structural group having the formula (<NUM>) or (<NUM>'). <CHM>
Herein A is an organic linking group in the form of a C<NUM>-C<NUM> straight, branched or cyclic divalent hydrocarbon group which may be separated by an oxygen or nitrogen atom, but does not contain another heteroatom, R<NUM> is hydrogen, and p is an integer of <NUM> to <NUM>. <CHM>
Herein B is an organic linking group in the form of a C<NUM>-C<NUM> straight, branched or cyclic trivalent hydrocarbon group which may be separated by an oxygen or nitrogen atom, but does not contain another heteroatom, R<NUM> is hydrogen, and p' and p" each are an integer of at least <NUM>, and p'+p" is an integer of <NUM> to <NUM>.

Of the compounds having formula (<NUM>), silane compounds represented by the general formulae (<NUM>) and (<NUM>) are preferred. Herein X and n are as defined above.

The silane compound having at least <NUM> acrylic groups per molecule, as represented by formulae (<NUM>) and (<NUM>), may be obtained by reacting a compound having acrylic and hydroxyl groups in a common molecule, as described in <CIT>, with an organosilicon compound having an isocyanate group.

Examples of the organosilicon compound having an isocyanate group include <NUM>-isocyanatopropyltrimethoxysilane, <NUM>-isocyanatopropylmethyldimethoxysilane, <NUM>-isocyanatopropyltriethoxysilane, and <NUM>-isocyanatopropylmethyldiethoxysilane. Of these, <NUM>-isocyanatopropyltrimethoxysilane and <NUM>-isocyanatopropyltriethoxysilane are preferred for availability of reactants.

Examples of the compound having acrylic and hydroxyl groups in a common molecule include difunctional acrylic alcohols such as <NUM>-hydroxy-<NUM>-acryloyloxypropyl methacrylate, and polyfunctional acrylic alcohols such as pentaerythritol triacrylate and dipentaerythritol pentaacrylate.

In preparing the silane compound having a plurality of acrylic groups per molecule, a catalyst may be used if necessary. It may be any of catalysts commonly used in isocyanate reaction, preferably a tin compound. The catalyst is used in an amount of <NUM> to <NUM> mole, more preferably <NUM> to <NUM> mole per mole of the organosilicon compound having an isocyanate group. A catalyst amount in excess of <NUM> mole is uneconomical because the effect is saturated. If the catalyst amount is less than <NUM> mole, the catalytic effect may be short, resulting in a low reaction rate and low productivity.

In preparing the silane compound having at least three acrylic groups per molecule, the reaction is exothermic, with the risk that side reactions can occur at extremely high temperature. For this reason, the reaction temperature is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, even more preferably <NUM> to <NUM>. A temperature below <NUM> leads to a low reaction rate and low productivity. If the temperature exceeds <NUM>, there is the risk that side reactions, for example, polymerization reaction of the organosilicon compound having an isocyanate group and polymerization of acrylic groups can occur.

A hydrolytic condensate of the silane compound having at least three acrylic groups per molecule may also be used as the adhesion-improving component. For hydrolytic condensation, prior art well-known methods are applicable.

Suitable hydrolytic condensation catalysts include acids such as hydrochloric acid, nitric acid, acetic acid, and maleic acid, alkali metal hydroxides such as NaOH and KOH, amine compounds such as ammonia, triethylamine, dibutylamine, hexylamine and octylamine, salts of amine compounds, bases including quaternary ammonium salts such as benzyltriethylammonium chloride, tetramethylammonium hydroxide, tetrabutylammonium hydroxide, fluoride salts such as potassium fluoride and sodium fluoride; solid acidic catalysts or solid basic catalysts (e.g. ion exchange resin catalysts); organometallic compounds, for example, metal salts of organic carboxylic acids such as iron-<NUM>-ethylhexoate, titanium naphthate, zinc stearate and dibutyltin diacetate, organotitanium esters such as tetrabutoxytitanium and dibutoxy-(bis-<NUM>,<NUM>-pentanedionate)titanium, organozirconium esters such as tetrabutoxyzirconium and zirconium dibutoxide(bis-<NUM>,<NUM>-pentanedionate), alkoxyaluminum compounds such as aluminum triisopropoxide, aluminum chelate compounds such as aluminum acetylacetonate complex; aminoalkyl-substituted alkoxysilanes such as <NUM>-aminopropyltrimethoxysilane and N-(β-aminoethyl)-<NUM>-aminopropyltrimethoxysilane, which may be used alone or in admixture.

The desired condensate is obtained by subjecting the silane compound having at least three acrylic groups per molecule to hydrolytic condensation in the presence of the hydrolytic condensation catalyst, water and optionally, an organic solvent.

An amount of component (C) blended is <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, more preferably <NUM> to <NUM> parts by weight per <NUM> parts by weight of component (A). Less than <NUM> part by weight of component (C) is ineffective for adhesion improvement whereas an amount in excess of <NUM> parts by weight achieves no improvement in adhesion and reduces release properties.

Component (D) is a radical polymerization initiator capable of generating a radical upon irradiation of radiation.

The radical polymerization initiator is not particularly limited as long as it is capable of generating a radical upon irradiation of radiation. Examples include diethoxyacetophenone, <NUM>-hydroxy-<NUM>-methyl-<NUM>-phenylpropan-<NUM>-one, <NUM>'-isopropyl-<NUM>-hydroxy-<NUM>-methylpropiophenone, <NUM>-hydroxymethyl-<NUM>-methylpropiophenone, <NUM>,<NUM>-dimethoxy-<NUM>,<NUM>-diphenylethan-<NUM>-one, p-dimethylaminoacetophenone, p-tert-butyldichloroacetophenone, p-tert-butyltrichloroacetophenone, p-azidobenzaracetophenone, <NUM>-hydroxycyclohexyl phenyl ketone, <NUM>-methyl-<NUM>-[<NUM>-(methylthio)phenyl]-<NUM>-morpholinopropanone-<NUM>, <NUM>-benzyl-<NUM>-dimethylamino-<NUM>-(<NUM>-morpholinophenyl)-butanone-<NUM>, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin n-butyl ether, benzoin isobutyl ether, benzyl, anisyl, benzophenone, methyl o-benzoylbenzoate, <NUM>,<NUM>'-bis(dimethylamino)benzophenone, <NUM>,<NUM>'-bis(diethylamino)benzophenone, <NUM>,<NUM>'-dichlorobenzophenone, <NUM>-benzoyl-<NUM>'-methyldiphenyl sulfide, thioxanthone, <NUM>-methylthioxanthone, <NUM>-ethylthioxanthone, <NUM>-chlorothioxanthone, <NUM>-isopropylthioxanthone, <NUM>,<NUM>-diethylthioxanthone, bis(cyclopentadienyl)-bis(<NUM>,<NUM>-difluoro-<NUM>-(hydropyrrol-<NUM>-yl)titanium, and <NUM>-(<NUM>-(<NUM>-hydroxyethoxy)phenyl)-<NUM>-hydroxy-<NUM>-methyl-<NUM>-propan-<NUM>-one. These radical polymerization initiators may be used alone or in admixture, depending on the desired performance.

An amount of component (D) blended is <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, more preferably <NUM> to <NUM> parts by weight per <NUM> parts by weight of component (A). Less than <NUM> part by weight of component (D) leads to a lowering of cure whereas an amount in excess of <NUM> parts by weight achieves no improvement in cure and reduces release properties.

The releasable radiation-curable silicone composition is obtained by metering predetermined amounts of the foregoing components and mixing them. Besides, additives such as a silicone resin, polydimethylsiloxane, filler, antistatic agent, flame retardant, defoamer, flow control agent, photo-stabilizer, solvent, non-reactive resin, and radical polymerizable compound may be used as optional components. The amounts of optional components added may be conventional amounts as long as the benefits of the invention are not compromised.

The term "releasable" in the releasable radiation-curable silicone composition means that a cured product of the silicone composition is releasable. The term "releasable" preferably corresponds to a release force of <NUM> to <NUM> N/<NUM>, more preferably <NUM> to <NUM> N/<NUM> as measured under the conditions of "Release force test <NUM> of releasable radiation-curable silicone composition" in Example <NUM> below.

The radiation-curable silicone release composition prepared above is coated on various substrates and cured with radiation, yielding release sheets, for example. The composition is coated on a substrate and cured with radiation. The substrate is not particularly limited and any of various substrates commonly used in the art may be used. Examples include glassine paper, clay-coated paper, wood-free paper, polyethylene-laminated paper, plastic films such as polyester film, polystyrene film, polyethylene film, and polypropylene film, transparent resins such as polycarbonate, and metal foils such as aluminum foil. The coating weight of the silicone composition is typically about <NUM> to about <NUM>/m<NUM> though not particularly limited.

For the radiation cure, energy rays in the ultraviolet to visible region (-<NUM> to -<NUM>) emitted from high or ultra-high pressure mercury lamps, metal halide lamps, xenon lamps, carbon arc lamps, fluorescent lamps, semiconductor solid laser, argon laser, He-Cd laser, KrF excimer laser, ArF excimer laser, and F<NUM> laser, are preferably used as the radiation energy source. A radiation source having a high luminous intensity in the region of <NUM> to <NUM> is preferred. Further, high-energy radiation such as electron beam or X-ray may also be used. The irradiation time of radiation energy is typically about <NUM> to about <NUM> seconds at normal temperature. Where the energy ray is less penetrative or a coating of the curable composition is thick, it is sometimes preferred to take a longer time. If necessary, after irradiation of energy ray, the coating may be post-cured by heating at room temperature to <NUM> for several seconds to several hours.

The releasable radiation-curable silicone composition is curable by radiation irradiation even in air, and still curable even when the oxygen concentration is reduced for more efficient cure. Since the cure becomes more efficient as the oxygen concentration is reduced, a lower oxygen concentration is preferred. Specifically the oxygen concentration is reduced below <NUM>% by volume, preferably below <NUM>% by volume, more preferably below <NUM>% by volume. As the case may be, the inventive composition may be diluted with an organic solvent prior to use.

Synthesis Examples, Reference Examples, Examples and Comparative Examples are given below for illustrating the invention. In the Examples, physical properties are measured by the following methods. Viscosity is a measurement at <NUM> by a BN type rotational viscometer.

The releasable radiation-curable silicone compositions of the invention were evaluated by the following methods.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM>, to form a cured film. The cured film was held at <NUM> for <NUM> hours, after which an acrylic pressure-sensitive adhesive tape (trade name TESA <NUM>) of <NUM> wide was rested on the surface of the cured film and press-bonded thereto by moving a roller of <NUM> back and forth, obtaining a sample for release force measurement. With a load of <NUM>/cm<NUM> applied, the sample was aged at <NUM> for <NUM> to <NUM> hours. Using a tensile tester, the tape was peeled at an angle of <NUM>° and a peel rate of <NUM>/min, the force (N/<NUM>) required for peeling was measured. The results are shown in Table <NUM>.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM>, to form a cured film. The cured film was held at <NUM> for <NUM> hours, after which an acrylic PSA tape (trade name TESA <NUM>) of <NUM> wide was rested on the surface of the cured film and press-bonded thereto by moving a roller of <NUM> back and forth, obtaining a sample for release force measurement. With a load of <NUM>/cm<NUM> applied, the sample was aged at <NUM> for <NUM> to <NUM> hours. Using a tensile tester, the tape was peeled at an angle of <NUM>° and a peel rate of <NUM>/min. The tape was rested on a stainless steel plate and press-bonded thereto by moving a roller of <NUM> back and forth. After holding at <NUM> for <NUM> minutes, a force (Y) required to peel the tape was measured. Similarly, a force (Z) required to peel a tape TESA <NUM>, which had not been bonded to the cured film, from a stainless steel plate was measured. A percent residual adhesion was computed by dividing Y by Z. The results are shown in Table <NUM>.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM>, to form a cured film. The cured film was rubbed with the finger <NUM> strokes, after which it was visually observed for smear and rub-off and evaluated according to the following criterion. The results are shown in Table <NUM>.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM> in a nitrogen atmosphere having an oxygen concentration of <NUM> ppm, to form a cured film. The cured film was held at <NUM> for <NUM> hours, after which an acrylic PSA tape (trade name TESA <NUM>) of <NUM> wide was rested on the surface of the cured film and press-bonded thereto by moving a roller of <NUM> back and forth, obtaining a sample for release force measurement. With a load of <NUM>/cm<NUM> applied, the sample was aged at <NUM> for <NUM> to <NUM> hours. Using a tensile tester, the tape was peeled at an angle of <NUM>° and a peel rate of <NUM>/min, the force (N/<NUM>) required for peeling was measured. The results are shown in Table <NUM>.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM> in a nitrogen atmosphere having an oxygen concentration of <NUM> ppm, to form a cured film. The cured film was held at <NUM> for <NUM> hours, after which an acrylic PSA tape (trade name TESA <NUM>) of <NUM> wide was rested on the surface of the cured film and press-bonded thereto by moving a roller of <NUM> back and forth, obtaining a sample for release force measurement. With a load of <NUM>/cm<NUM> applied, the sample was aged at <NUM> for <NUM> to <NUM> hours. Using a tensile tester, the tape was peeled at an angle of <NUM>° and a peel rate of <NUM>/min. The tape was rested on a stainless steel plate and press-bonded thereto by moving a roller of <NUM> back and forth. After holding at <NUM> for <NUM> minutes, a force (Y) required to peel the tape was measured. Similarly, a force (Z) required to peel a tape TESA <NUM>, which had not been bonded to the cured film, from a stainless steel plate was measured. A percent residual adhesion was computed by dividing Y by Z. The results are shown in Table <NUM>.

After a releasable radiation-curable silicone composition was prepared, it was roll coated on a polyethylene-laminated paper sheet of <NUM> thick so as to give a coating weight of ~<NUM>/m<NUM>, and irradiated with UV from two high-pressure mercury lamps of <NUM> W/cm in a dose of <NUM> mJ/cm<NUM> in a nitrogen atmosphere having an oxygen concentration of <NUM> ppm, to form a cured film. The cured film was rubbed with the finger <NUM> strokes, after which it was visually observed for smear and rub-off and evaluated according to the following criteria. The results are shown in Table <NUM>.

A silicone composition #<NUM> was prepared by intimately mixing.

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (A-<NUM>) <NUM> parts by weight of a vinyl-containing polydimethylsiloxane represented by the following average compositional formula and having a viscosity of <NUM> mPa·s at <NUM> as component (A),
<CHM>
wherein Me stands for methyl, Ph for phenyl, and Vi for vinyl.

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (B-<NUM>) <NUM> parts by weight of a mercapto-containing polydimethylsiloxane represented by the following average compositional formula and having a viscosity of <NUM> mPa·s at <NUM> as component (B),
<CHM>
wherein Me stands for methyl and S for mercaptopropyl.

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (C-<NUM>) <NUM> part by weight of trimethylolpropane triacrylate as component (C).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (A-<NUM>) <NUM> parts by weight of the vinyl-containing polydimethylsiloxane as component (A).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (B-<NUM>) <NUM> parts by weight of the mercapto-containing polydimethylsiloxane as component (B).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (C-<NUM>) <NUM> part by weight of dipentaerythritol hexaacrylate as component (C).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (C-<NUM>) <NUM> part by weight of a silane compound having the general formula as component (C):
<CHM>
wherein Me stands for methyl, as component (C).

A silicone composition #<NUM> was obtained by the same procedure as in Example <NUM> aside from using (A-<NUM>) <NUM> parts by weight of the vinyl-containing polydimethylsiloxane as component (A).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (C-<NUM>) <NUM> part by weight of a silane compound having the general formula as component (C):
<CHM>
<CHM>
wherein Me stands for methyl, as component (C).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using (C-<NUM>) <NUM> parts by weight of dipentaerythritol hexaacrylate as component (C).

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from omitting (C-<NUM>) from Reference Example <NUM>.

A silicone composition #<NUM> was obtained by the same procedure as in Reference Example <NUM> aside from using <NUM> part by weight of <NUM>-hydroxybutyl acrylate (X) instead of (C-<NUM>) in Reference Example <NUM>.

A silicone composition #<NUM> was obtained by the same procedure as in Comparative Example <NUM> aside from using <NUM> parts by weight of <NUM>-hydroxybutyl acrylate (X) in Comparative Example <NUM>.

A silicone composition #<NUM> was obtained by the same procedure as in Comparative Example <NUM> aside from using <NUM> parts by weight of <NUM>-hydroxybutyl acrylate (X) in Comparative Example <NUM>. Comparative Example <NUM> was evaluated by release force test <NUM>, residual adhesion test <NUM>, and bond test <NUM>.

Claim 1:
A releasable radiation-curable silicone composition comprising the following components (A), (B), (C) and (D):
(A) <NUM> parts by weight of alkenyl-containing organopolysiloxane having the average compositional formula (<NUM>):
<CHM>
wherein R<NUM> which may be the same or different are C<NUM>-C<NUM> monovalent hydrocarbon groups, a ≥ <NUM>, b ≥ <NUM>, c ≥ <NUM>, d ≥ <NUM>, and <NUM> ≤ a+b+c+d ≤ <NUM>,<NUM>, and R<NUM>, a, b, c and d are selected such that at least two silicon-bonded alkenyl groups are present per molecule,
(B) <NUM> to <NUM> parts by weight of mercaptoalkyl-containing organopolysiloxane having the average compositional formula (<NUM>):
<CHM>
wherein R<NUM> which may be the same or different are C<NUM>-C<NUM> monovalent hydrocarbon group or mercaptoalkyl group, e ≥ <NUM>, f ≥ <NUM>, g ≥ <NUM>, h ≥ <NUM> and <NUM> ≤ e+f+g+h ≤ <NUM>, and R<NUM>, e, f, g and h are selected such that at least two silicon-bonded mercaptoalkyl groups are present per molecule,
(C) <NUM> to <NUM> parts by weight of a silane compound having at least three acrylic groups per molecule or a hydrolytic condensate thereof, and
(D) <NUM> to <NUM> parts by weight of radical polymerization initiator.