Patent Description:
As represented by mobile electronic instruments such as smart phones and tablet terminals, touch panel instruments including both a display unit and an input means become rapidly widespread nowadays. The touch panel instruments are constructed using optical members, which include liquid crystal display panels and complex polarizers such as polarizing plates and phase difference plates. These optical members are bonded together via an optical clear pressure-sensitive adhesive (PSA).

The optical clear PSAs are generally divided into OCA (optical clear adhesive) or carrier-less PSA tape and OCR (optical clear resin) or liquid reactive resin, each of which mainly includes heat cure types and UV-cure types. Among these, the UV-cure type PSAs are nowadays widely used in the manufacture of optical products because of containing no solvent media such as solvents and water, possible formation of a PSA layer without removal of solvent media, and no need for heating.

As OCA and OCR materials of UV-cure type, a number of acrylic PSAs were proposed (Patent Document <NUM>: <CIT> and Patent Document <NUM>: <CIT>).

However, since these acrylic PSAs undergo noticeable shrinkage upon curing, liquid crystal display panels, for example, can be deformed by the internal stresses associated therewith. This is an outstanding problem in the modern age when panels have advanced to the stage of high luminance, high definition and large size.

As compared with silicone PSAs, acrylic PSAs undergo noticeable transparency loss and yellowing at high temperatures and a substantial variation of modulus with temperature changes. There remains the problem that the acrylic PSAs tend to peel from liquid crystal display panels, polarizers and the like.

As for the silicone PSAs, especially of solventless type, most silicone PSAs proposed thus far are of heat-cure type as described in Patent Document <NUM>: <CIT> and Patent Document <NUM>: <CIT> whereas only a few are of UV-cure type. Patent Document <NUM>: <CIT> proposes a UV-cure type silicone resin composition comprising a combination of linear and branched organopolysiloxanes having specific (meth)acryloxy groups while its adhesion to plastic films used in displays, typically polycarbonate (PC) and polymethyl methacrylate (PMMA) is evaluated nowhere.

Patent Document <NUM> discloses an ultraviolet-curable pressure-sensitive silicone adhesive composition comprising <NUM> parts of an organopolysiloxane end-capped by acrylate-functional group, <NUM>-<NUM> parts of a monofunctional (meth)acrylate compound containing no siloxane structure and/or a polyfunctional (meth)acrylate compound containing no siloxane structure, <NUM>-<NUM>,<NUM> parts of a silicone resin comprising M and Q units, and <NUM>-<NUM> parts of a photopolymerization initiator.

Patent document <CIT> describes a UV-curable silicone compositions based on functional organopolysiloxanes used for the preparation of pressure-sensitive adhesives. The compositions comprise an organopolysiloxane comprising M and Q units, an organopolysiloxane end-capped by acrylate groups and a photoinitiator.

Patent document <CIT> describes a UV-curable silicone composition based on silicone components bearing an acrylate functional group and used for the preparation of a pressure sensor comprising the cured product. The UV-curable silicone composition comprises <NUM> parts by weight of a linear organopolysiloxane end-capped at both terminals by an acrylate-functional group and having a viscosity at <NUM> of <NUM>,<NUM> mPa. s, <NUM> parts by weight of an organodisilloxane comprising an acrylate-functional group, <NUM> parts by weight of a photo-curing agent and <NUM> parts by weight of an organopolysiloxane end-capped by vinyl groups.

Patent document <CIT> describes a pressure-senstitive adhesive composition comprising a vinyl-functionalised polysiloxane and a silicone resin MQ in a weight ratio of <NUM>/<NUM> exhibiting improved adhesion strength on a polyethylene terephthalate substrate when compared to a silicone composition free of MQ silicone resin.

Finally, patent document <CIT> describes pressure-sensitive adhesive compositions comprising an organopolysiloxane end-capped by acrylate group, an unreacted MQ silicone resin having a ratio M/Q = <NUM> together with <NUM>,<NUM>-bis(acryloxypropyl)-tetramethyldisiloxane. The compositions are cured in present of a UV-initiator.

An object of the invention, which has been made under the above-mentioned circumstances, is to provide a UV-curable silicone PSA composition which quickly cures upon UV exposure and has satisfactory adhesion to sheet-like substrates, typically plastic films, and a silicone PSA film.

Making extensive investigations to attain the above object, the inventors have found that a UV-curable silicone PSA composition is obtained by using an organopolysiloxane resin comprising essentially R<NUM><NUM>SiO<NUM>/<NUM> units (wherein R<NUM> is a substituted or unsubstituted C<NUM>-C<NUM> monovalent hydrocarbon group) and SiO<NUM>/<NUM> units, in admixture with organopolysiloxane containing on the average <NUM> to <NUM> specific (meth)acryloxy groups per molecule and having a specific viscosity, and a reactive diluent in the form of a low molecular weight or low viscosity organopolysiloxane containing one acryloxyalkyl group having the general formula (<NUM>), defined below, per molecule, in a specific blend ratio, the resulting silicone PSA composition being quickly curable upon UV exposure and having satisfactory adhesion to sheet-like substrates, typically plastic films. The invention is predicated on this finding.

The invention provides a UV-curable silicone PSA composition and a silicone PSA film as defined in the claims.

The UV-curable silicone PSA composition of the invention is effectively curable even with a small dose of UV exposure and is fully bondable to plastic films such as polyester films. Since the UV-curable silicone PSA composition not only has excellent resistance to oxygen inhibition upon UV exposure, but is also a low viscosity composition despite its solventless type, the composition is significantly improved in workability over prior art silicone base PSA compositions.

The invention provides a UV-curable silicone PSA composition comprising.

Component (A), which serves to provide a bonding force to sheet-like substrates, is an organopolysiloxane resin or silicone resin of three-dimensional network structure comprising essentially (a) R<NUM><NUM>SiO<NUM>/<NUM> units wherein R<NUM> is a substituted or unsubstituted C<NUM>-C<NUM> monovalent hydrocarbon group and (b) SiO<NUM>/<NUM> units, wherein a molar ratio of units (a) to units (b) is in the range between <NUM>:<NUM> and <NUM>:<NUM>.

Examples of the C<NUM>-C<NUM> monovalent hydrocarbon group R<NUM> include alkyl groups, preferably of <NUM> to <NUM> carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, and tert-butyl, alkenyl groups, preferably of <NUM> to <NUM> carbon atoms, such as vinyl, allyl, propenyl, isopropenyl, and butenyl, aryl groups, preferably of <NUM> to <NUM> carbon atoms, such as phenyl and tolyl, aralkyl groups, preferably of <NUM> to <NUM> carbon atoms, such as benzyl, and halo- or cyano-substituted monovalent hydrocarbon groups in which some or all of the carbon-bonded hydrogen atoms are substituted by halogen atoms, such as chloromethyl, bromoethyl and trifluoropropyl, or by cyano groups, such as cyanoethyl.

In component (A), the molar ratio of (a) R<NUM><NUM>SiO<NUM>/<NUM> units (M units) to (b) SiO<NUM>/<NUM> units (Q units), i.e. the molar ratio (a)/(b) is in the range between <NUM>/<NUM> and <NUM>/<NUM>, preferably between <NUM>/<NUM> and <NUM>/<NUM>. If the molar ratio is less than <NUM>, the cured composition (silicone resin) may lose bonding force and tackiness. If the molar ratio exceeds <NUM>, there may be losses of bonding force and holding force.

Notably, the organopolysiloxane resin may further contain R<NUM>SiO<NUM>/<NUM> units (T units) and/or R<NUM><NUM>SiO<NUM>/<NUM> units (D units) in a total amount of <NUM> to <NUM> mol%, especially <NUM> to <NUM> mol%, based on the total of all siloxane units (i.e. total of M, D, T and Q units) in the molecule as long as the benefits of the organopolysiloxane resin are not impaired.

The organopolysiloxane resin has a hydroxysilyl (or silanol) group content which is preferably <NUM> to <NUM> mol/<NUM>.

The organopolysiloxane resin defined above is a well-known material in the art, which is obtainable from cohydrolytic condensation of a hydrolysable triorganosilane (e.g. triorganoalkoxysilane or triorganochlorosilane), a R<NUM>-free hydrolysable silane or siloxane (e.g. tetraalkoxysilane, tetrachlorosilane, ethyl silicate or methyl silicate), and optionally a hydrolysable diorganosilane and/or monoorganosilane (e.g. diorganodialkoxysilane, diorganodichlorosilane, organotrialkoxysilane or organotrichlorosilane).

It is noted that component (A) may be used in dilute form in a solvent such as toluene, xylene or heptane when the inventive composition is prepared by uniformly mixing component (A) with components (B) and (C) and optionally (D). After the composition is prepared, the solvent is eventually removed by such means as stripping and the composition is used in solventless form.

Preferably component (A) has a viscosity at <NUM> of <NUM> to <NUM> mPa s, more preferably <NUM> to <NUM> mPa s, as measured in <NUM> wt% toluene solution. As used herein, the viscosity is measured by a rotational viscometer, e.g. BL, BH, BS, cone plate type or rheometer (the same holds true, hereinafter).

The amount (calculated as solids) of component (A) blended is <NUM> to <NUM>% by weight, preferably <NUM> to <NUM>% by weight, more preferably <NUM> to <NUM>% by weight based on the total weight of components (A) to (C) (inclusive of components (B) and (C) to be described later), specifically <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 components (A) to (C) combined. If the amount of component (A) is too much, losses of bonding force and tack may occur due to agglomeration of the organopolysiloxane resin. If the amount of component (A) is too small, bonding force or tack may not be developed.

Component (B) used herein is a base polymer constituting the main skeleton of a siloxane matrix which is formed as a result of crosslinking or curing of the inventive composition, or a UV-curable component which crosslinks upon UV exposure, and specifically, a linear or branched organopolysiloxane having a backbone composed substantially of repeating diorganosiloxane units and containing <NUM> to <NUM> groups, preferably <NUM> to <NUM> groups, more preferably <NUM> groups on the average, the groups having the general formula (<NUM>) and/or (<NUM>):
<CHM>
wherein R<NUM> is hydrogen or methyl, a is an integer of <NUM> to <NUM>, and the broken line designates a valence bond bonded to silicon atom,
<CHM>
wherein R<NUM> and a are as defined above, and the broken line designates a valence bond bonded to silicon atom, in the molecule (preferably at both molecular chain ends, more preferably only at both molecular chain ends), as the silicon-bonded monovalent substituent group in the molecule (preferably as the monovalent substituent group bonded to the silicon atom at the molecular chain end, more preferably as the monovalent substituent group bonded to only the silicon atom at the molecular chain end), the organopolysiloxane having a viscosity at <NUM> of more than <NUM> mPa·s to <NUM>,<NUM> mPa·s, preferably <NUM> to <NUM>,<NUM> mPa s, more preferably <NUM> to <NUM>,<NUM> mPa s.

In formula (<NUM>) or (<NUM>), a is <NUM>, <NUM> or <NUM>, preferably <NUM> or <NUM>.

The position in the organopolysiloxane molecule of component (B) at which the group of formula (<NUM>) or (<NUM>) is attached may be the end of the molecular chain or a non-terminal position of the molecular chain (i.e. a midway position or side chain of the molecular chain), or both. Preferably the group of formula (<NUM>) or (<NUM>) is attached at both ends of the molecular chain, more preferably only at both ends of the molecular chain.

In particular, groups of the general formula (<NUM>) and/or (<NUM>) are preferably attached at both ends of the molecular chain. <CHM>
Herein R<NUM> and a are as defined above. R<NUM> is independently a substituted or unsubstituted C<NUM>-C<NUM> monovalent hydrocarbon group. X is a C<NUM>-C<NUM> alkylene group, and b is <NUM> or <NUM>. The broken line designates a valence bond bonded to silicon atom.

In formulae (<NUM>) and (<NUM>), R<NUM> is independently a substituted or unsubstituted C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon group, preferably exclusive of an aliphatic unsaturated group such as alkenyl group. Examples of the unsubstituted monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, cyclohexyl, octyl, <NUM>-ethylhexyl and decyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, and butenyl; aryl groups such as phenyl, tolyl, xylyl, and naphthyl; and aralkyl groups such as benzyl, phenylethyl and phenylpropyl. Examples of the substituted monovalent hydrocarbon group include halo or cyano-substituted monovalent hydrocarbon groups in which some or all carbon-bonded hydrogen atoms are substituted by halogen, such as chloromethyl, bromoethyl, and trifluoropropyl, or by cyano moiety, such as cyanoethyl. Inter alia, R<NUM> is preferably an alkyl group, aryl group or haloalkyl group, more preferably methyl, phenyl or trifluoropropyl.

X is a C<NUM>-C<NUM> alkylene group, for example, ethylene, propylene (trimethylene or methylethylene), butylene (tetramethylene or methylpropylene), or hexamethylene.

The organopolysiloxane molecule of component (B) contains a silicon-bonded organic group other than the group of the general formula (<NUM>) or (<NUM>) (or the general formula (<NUM>) or (<NUM>)), examples of which include substituted or unsubstituted C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon groups, preferably exclusive of aliphatic unsaturated groups such as alkenyl groups, like the above-exemplified R<NUM>. Examples of the substituted or unsubstituted monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, <NUM>-ethylhexyl and decyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, and butenyl; aryl groups such as phenyl, tolyl, xylyl, and naphthyl; aralkyl groups such as benzyl, phenethyl and phenylpropyl; and substituted forms of these monovalent hydrocarbon groups in which some or all hydrogen atoms are substituted by halogen (e.g. chloro, fluoro or bromo), typically haloalkyl groups such as chloromethyl, bromoethyl, <NUM>-chloropropyl, and <NUM>,<NUM>,<NUM>-trifluoropropyl, or by cyano moiety, such as cyanoethyl. For ease of synthesis, the silicon-bonded organic group is preferably an alkyl group, aryl group or haloalkyl group, more preferably methyl, phenyl or trifluoropropyl.

Also, the molecular structure of component (B) is basically a linear or branched backbone (inclusive of generally linear backbone which is partially branched) composed of repeating diorganosiloxane units. Preferably component (B) is a linear diorganopolysiloxane which is blocked with groups of the general formula (<NUM>) or (<NUM>) at both ends of the molecular chain. Component (B) may be a single polymer of such molecular structure, a copolymer having such molecular structures, or a mixture of two or more polymers.

Component (B) has a viscosity at <NUM> of from more than <NUM> mPa·s to <NUM>,<NUM> mPa·s, preferably from <NUM> to <NUM>,<NUM> mPa·s, more preferably from <NUM> to <NUM>,<NUM> mPa·s, because the composition is effectively workable and the cured composition has improved dynamic properties.

The above range of viscosity generally corresponds to a number average degree of polymerization of about <NUM> to <NUM>,<NUM>, preferably about <NUM> to <NUM>,<NUM> in the case of a linear organopolysiloxane. In the disclosure, the degree of polymerization or molecular weight may be measured as number average degree of polymerization or molecular weight, for example, by gel permeation chromatography (GPC) using toluene as developing solvent versus polystyrene standards (the same holds true, hereinafter).

Examples of component (B) defined above include compounds having the general formula (<NUM>). <CHM>
Herein R<NUM>, X and a are as defined above. R<NUM> and R<NUM> are each independently a substituted or unsubstituted C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon groups, preferably exclusive of an aliphatic unsaturated group, and c is such a number that the organopolysiloxane may have a viscosity in the above range, preferably an integer of <NUM> to <NUM>,<NUM>, more preferably <NUM> to <NUM>,<NUM>, even more preferably <NUM> to <NUM>.

In formula (<NUM>), examples of R<NUM> and R<NUM> are as exemplified above for the silicon-bonded organic group. Inter alia, R<NUM> and R<NUM> are preferably alkyl, cycloalkyl, aryl, aralkyl or haloalkyl groups, more preferably methyl, phenyl or trifluoropropyl.

Also in formula (<NUM>), -(SiR<NUM>R<NUM>-O)c- preferably contains a diphenylsiloxane unit having the formula (<NUM>). <CHM>
Herein Me stands for methyl, Ph stands for phenyl, c1 and c2 are integers in the ranges: c1 ≥ <NUM>, c2 ≥ <NUM> and c1+c2=c, preferably c2/c is <NUM> to <NUM>, more preferably <NUM> to <NUM>, and the broken line designates a valence bond.

Exemplary of component (B) are compounds having the following formula. <CHM>
<CHM>
Herein Me stands for methyl, Ph stands for phenyl, c1' and c2' are the same as c1 and c2, respectively, the repetition numbers of dimethylsiloxane units and diphenylsiloxane units in the formula are arbitrary integers selected in the range from <NUM> to a positive value such that the organopolysiloxane may have a viscosity at <NUM> of <NUM> to <NUM>,<NUM> mPa·s, especially <NUM> to <NUM>,<NUM> mPa s. The arrangement of repeating units in the backbone is random.

The amount of component (B) blended is <NUM> to <NUM>% by weight, preferably <NUM> to <NUM>% by weight, more preferably <NUM> to <NUM>% by weight based on the total weight of components (A) to (C) (inclusive of component (C) to be described later), specifically <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 components (A) to (C) combined. When the amount of component (B) is not more than <NUM>% by weight, the desired properties, typically flexibility, of a cured product resulting from curing of the inventive composition are readily achieved or adjusted. If the amount of component (B) is too small, the cured product may not be endowed with sufficient elasticity and hence, with desired bonding force or tack.

Component (C) used herein serves as a reactive diluent in the inventive composition and is an organo(poly)siloxane containing one group (acryloxyalkyl group) having the general formula (<NUM>) at an end or on a side chain of the molecular chain (preferably only one end of the molecular chain) as the silicon-bonded monovalent substituent group in the molecule (preferably as the monovalent substituent group bonded to only the silicon atom at one end of the molecular chain), the organo(poly)siloxane having a relatively low molecular weight (low degree of polymerization or low viscosity) as compared with component (B) and specifically a viscosity at <NUM> of <NUM> to <NUM> mPa s. When the composition contains a specific proportion of component (C), component (C) functions as a curing sensitizer capable of sensitizing the curing reaction utilizing radical polymerization for thereby improving oxygen inhibition resistance in the step of curing the composition by UV exposure. Component (C) also enables to reduce the viscosity of the composition and to control the cured storage modulus to a relatively low value. <CHM>
Herein a is as defined above and the broken line designates a valence bond.

The molecular structure of component (C) is basically a linear structure having a backbone composed of repeating diorganosiloxane units. Component (C) is preferably a linear diorgano(poly)siloxane which is blocked at an end of the molecular chain or side chain with a group having the general formula (<NUM>) (preferably blocked at one end of the molecular chain with a group having the general formula (<NUM>) and at the other end of the molecular chain with a triorganosilyl group). Component (C) may be a single polymer of such molecular structure or a mixture of two or more polymers.

The group having the general formula (<NUM>) is typically contained in the molecule in either of the bonds: the direct bond to silicon atom at an end or non-terminal position in the diorganopolysiloxane unit of which the backbone is composed (i.e. CH<NUM>=CH-C(=O)-O-(CH<NUM>)a-Si-), and the bond to said silicon atom via an ethereal oxygen atom (i.e. CH<NUM>=CH-C(=O)-O-(CH<NUM>)a-O-Si-). In particular, the group having the general formula (<NUM>) is preferably in direct bond to the silicon atom. Also preferably, the diorgano(poly)siloxane as component (C) is free of a hydrolysable functional group such as alkoxy group in the molecule.

Component (C) has a viscosity at <NUM> of <NUM> mPa·s to <NUM> mPa·s, preferably <NUM> to <NUM> mPa s, more preferably <NUM> to <NUM> mPa s, because the function of a cure sensitizer becomes stronger at a lower viscosity or a lower molecular weight. The organo(poly)siloxane as component (C) has a number average degree of polymerization (or number of silicon atoms per molecule), which is as low as desirably about <NUM> to about <NUM>, more desirably about <NUM> to about <NUM>.

The organo(poly)siloxane molecule of component (C) contains a silicon-bonded organic group other than the group of the general formula (<NUM>), examples of which include substituted or unsubstituted C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon groups, preferably exclusive of aliphatic unsaturated groups such as alkenyl groups. Examples of the substituted or unsubstituted monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, heptyl, octyl, <NUM>-ethylhexyl and decyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, and butenyl; aryl groups such as phenyl, tolyl, xylyl, and naphthyl; aralkyl groups such as benzyl, phenethyl and phenylpropyl; and substituted forms of these monovalent hydrocarbon groups in which some or all hydrogen atoms are substituted by halogen (e.g. chloro, fluoro or bromo), typically haloalkyl groups such as chloromethyl, bromoethyl, <NUM>-chloropropyl, and <NUM>,<NUM>,<NUM>-trifluoropropyl, or by cyano moiety, such as cyanoethyl. For ease of synthesis, the silicon-bonded organic group is preferably an alkyl group, aryl group or haloalkyl group, more preferably methyl, phenyl or trifluoropropyl. Especially an organic group providing high compatibility with component (B) is preferably used.

Exemplary of component (C) are compounds having the general formulae (<NUM>) and (<NUM>). <CHM>
<CHM>
Herein R<NUM> to R<NUM> each are a substituted or unsubstituted C<NUM>-C<NUM>, preferably C<NUM>-C<NUM> monovalent hydrocarbon group, preferably exclusive of an aliphatic unsaturated group, a is as defined above, d is an integer of <NUM> to <NUM>, e and f each are <NUM> or <NUM>, d to f are selected such that the organo(poly)siloxane may have a viscosity in the above range.

In formulae (<NUM>) and (<NUM>), examples of R<NUM> to R<NUM> are as exemplified above for the silicon-bonded organic group. Among these, alkyl, cycloalkyl, aryl, aralkyl and haloalkyl groups are preferred, with methyl, phenyl and trifluoropropyl being more preferred.

As component (C), those compounds of formulae (<NUM>) and (<NUM>) wherein d, e and f each are <NUM> are preferred. Especially, (acryloxyalkyl)-pentaorganodisiloxanes corresponding to formula (<NUM>) wherein d=<NUM> are preferred.

The amount of component (C) blended is <NUM> to <NUM>% by weight, preferably <NUM> to <NUM>% by weight, more preferably <NUM> to <NUM>% by weight based on the total weight of components (A) to (C), specifically <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 components (A) to (C) combined. If the amount of component (C) is too small, not only the sensitizer function may become weak, but also the composition may undergo a noticeable increase of viscosity, detracting from workability. If the amount of component (C) is too much, the cured product may not be endowed with sufficient elasticity and hence, with desired bonding force or tack.

The amount of component (C) blended is <NUM> to <NUM> parts by weight, more desirably <NUM> to <NUM> parts by weight per <NUM> parts by weight of components (A) and (B) combined.

Component (D) is a photo-initiator which is optional or a component to be blended if necessary. It may be selected from those initiators which are used in prior art UV-curable organopolysiloxane compositions. Examples include acetophenone, propiophenone, benzophenone, fluorene, benzaldehyde, anthraquinone, triphenylamine, carbazole, <NUM>-methylacetophenone, <NUM>-methylacetophenone, <NUM>-pentylacetophenone, <NUM>-methoxyacetophenone, <NUM>-bromoacetophenone, <NUM>-allylacetophenone, p-diacetylbenzene, <NUM>-methoxybenzophenone, <NUM>-methylbenzophenone, <NUM>-chlorobenzophenone, <NUM>,<NUM>'-dimethoxybenzophenone, <NUM>,<NUM>'-diethoxyacetophenone, <NUM>-chloro-<NUM>'-benzylbenzophenone, <NUM>-chloroxanthone, <NUM>,<NUM>-dichloroxanthone, <NUM>-chloro-<NUM>-nonylxanthone, benzoin, benzoin methyl ether, benzoin butyl ether, bis(<NUM>-dimethylaminophenyl)ketone, benzyl methoxyketal, <NUM>-chlorothioxanthone, diethylacetophenone, <NUM>-hydroxycyclohexyl phenyl ketone, <NUM>-methyl-<NUM>-[<NUM>-(methylthio)phenyl]-<NUM>-morpholin-<NUM>-one, <NUM>-benzyl-<NUM>-dimethylamino-<NUM>-(<NUM>-morpholinophenyl)butanone, <NUM>-[<NUM>-(<NUM>-hydroxyethoxy)phenyl]-<NUM>-methyl-<NUM>-propan-<NUM>-one, <NUM>,<NUM>-dimethoxy-<NUM>-phenylacetophenone, <NUM>-hydroxy-<NUM>-methyl-<NUM>-phenylpropan-<NUM>-one, and cyclohexyl phenyl ketone.

When used, the amount of component (D) blended is about <NUM> to <NUM>% by weight, preferably about <NUM> to <NUM>% by weight, more preferably about <NUM> to <NUM>% by weight based on the total weight of the composition, specifically the total weight of components (A) to (D). If the amount of component (D) is too small, its addition effect may not be exerted. If the amount of component (D) is too much, the influence of decomposition residues of component (D) becomes stronger whereby the physical properties of the cured product may be degraded. Accordingly, the 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 components (A) to (C) combined.

Besides the foregoing components (A) to (D), other optional components may be added to the inventive composition as long as the benefits of the invention are not compromised. Suitable components include silica-base fillers (not inhibiting UV-cure reaction) such as fumed silica, extenders such as silicone rubber powder and calcium carbonate, tackifiers (contributing to an improvement in adhesion or pressure-sensitive adhesion) such as alkoxyorganosilanes, heat resistant additives, and flame retardants.

The inventive composition may be prepared by metering components (A) to (D) and other optional components and mixing them uniformly.

The inventive composition is cured by resorting to UV exposure. Effective UV is of wavelength <NUM> to <NUM>, especially <NUM> to <NUM>, and the exposure dose is preferably <NUM>,<NUM> to <NUM>,<NUM> mJ/cm<NUM>, especially <NUM>,<NUM> to <NUM>,<NUM> mJ/cm<NUM>. The curing temperature may be room temperature, typically <NUM>±<NUM>.

A cured product (i.e. cured silicone having a pressure-sensitive adhesive surface, or silicone PSA) obtained from curing of the inventive composition by UV exposure is characterized by softness and a low rubber hardness as compared with prior art PSA compositions. That is, the cured product of the inventive UV curable silicone PSA composition preferably has a storage elastic modulus of up to <NUM> MPa, especially up to <NUM> MPa as measured at <NUM> by a viscoelasticity meter such as rheometer. Notably, the lower limit of storage elastic modulus at <NUM> is preferably at least <NUM> MPa, more preferably at least <NUM> MPa, though not critical.

Also, a PSA article may be obtained by coating the inventive UV curable silicone PSA composition to any of various substrates and curing the coating with UV.

The substrate is selected from plastic films and glass. Suitable plastic films include polyethylene film, polypropylene film, polyester film, polyimide film, polyvinyl chloride film, polyvinylidene chloride film, polyvinyl alcohol film, polycarbonate film, polystyrene film, ethylene-vinyl acetate copolymer film, ethylene-vinyl alcohol copolymer film, and triacetylcellulose film. The glass is not particularly limited with respect to thickness and type, and even chemically strengthened glass is acceptable. The substrate is not limited to the foregoing examples.

Also useful are those substrates which have been subjected to primer treatment or plasma treatment for improving the adhesion between the substrate and the PSA layer.

The coating means or method may be selected as appropriate from well-known coating means or methods such as a comma coater, lip coater, roll coater, die coater, knife coater, blade coater, rod coater, kiss coater, gravure coater, screen printing, dipping and casting methods.

Since the UV curable silicone PSA composition is of solventless type, a cured product thereof can also be prepared by potting. In case bubbles are entrapped in the potting step of pouring the composition into a container, the bubbles can be removed under reduced pressure. If it is desired to take out the cured product from the container after curing, preferably the container is treated with a parting agent before potting of the composition. For example, fluorine and silicone base parting agents may be used.

The UV curable silicone PSA composition is generally used as such. Where the composition must be improved in use conditions because of awkward handling or other inconvenience, it is acceptable to add an organic solvent to the composition prior to use as long as the desired properties are not compromised.

Examples and Comparative Examples are given below for further illustrating the invention although the invention is not limited thereto. In Examples, parts are by weight. Me stands for methyl, Ph for phenyl, and tert-Bu for tert-butyl. The viscosity is as measured at <NUM> by a rotational viscometer.

(A-<NUM>) A <NUM> wt% toluene solution of an organopolysiloxane resin of three-dimensional network structure consisting of Me<NUM>SiO<NUM>/<NUM> units and SiO<NUM> units in a molar ratio (Me<NUM>SiO<NUM>/<NUM> units)/(SiO<NUM> units) of <NUM>, viscosity <NUM> mPa·s.

(B-<NUM>) A linear diphenylsiloxane-dimethylsiloxane copolymer having a backbone consisting of repeating diphenylsiloxane units and dimethylsiloxane units wherein the diphenylsiloxane units and dimethylsiloxane units are randomly arranged in the backbone, and containing four acryloxyethyloxy groups in the molecule at both ends of the molecular chain (i.e. two groups at each molecular chain end), as represented by the following formula (<NUM>), viscosity <NUM>,<NUM> mPa·s.

(B-<NUM>) A linear dimethylpolysiloxane having a backbone consisting of repeating dimethylsiloxane units and containing two acryloxymethyl groups in the molecule at both ends of the molecular chain (i.e. one group at each molecular chain end), as represented by the following formula (<NUM>), viscosity <NUM> mPa·s.

(B-<NUM>) A linear dimethylpolysiloxane having a backbone consisting of repeating dimethylsiloxane units and containing per molecule one acryloxyethyloxy-dimethylsilylethyl group and one tert-butyl group at both ends of the molecular chain, as represented by the following formula (<NUM>), viscosity <NUM> mPa s.

(C-<NUM>) (acryloxypropyl)-pentamethyldisiloxane represented by the following formula (<NUM>), viscosity <NUM> mPa·s.

(C-<NUM>) <NUM>-(acryloxypropyl)-<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>-heptamethyltrisiloxane represented by the following formula (<NUM>), viscosity <NUM> mPa s.

(C-<NUM>) <NUM>-acryloxypropyl-methyldimethoxysilane (KBM-<NUM> by Shin-Etsu Chemical Co.

(C-<NUM>) <NUM>-acryloxypropyl-trimethoxysilane (KBM-<NUM> by Shin-Etsu Chemical Co.

Silicone PSA compositions S1 to S6 were prepared by blending components (A) to (D) in accordance with the formulation in Table <NUM> and distilling off toluene at <NUM> in vacuum. The silicone PSA compositions were cured by exposure to UV of wavelength <NUM> from an Eye UV electronic control instrument (model: UBX0601-<NUM>, Eye Graphics Co. ) at room temperature (<NUM>) in air atmosphere such that the exposure dose of UV light was <NUM>,<NUM> mJ/cm<NUM>.

Notably, the amount of component (A) shown in Table <NUM> is the net amount (pbw) of solids with the solvent removed.

The silicone PSA compositions and cured products thus obtained were evaluated with respect to the following items.

The cured product of the silicone PSA composition was measured for storage elastic modulus by a viscoelasticity analyzer ARES-G2 (TA Instruments) under measurement conditions: frequency <NUM>, oscillation angle γ <NUM>%, and temperature <NUM>±<NUM>. The results are shown in Table <NUM>.

Each of the silicone PSA compositions S1 to S6 was coated onto a support (sheet-like substrate) in the form of polyethylene terephthalate (PET) resin film so as to give a dry thickness of around <NUM> after curing, and cured by irradiating UV light of wavelength <NUM> in a dose of <NUM>,<NUM> mJ/cm<NUM>, whereby a PSA sheet was obtained. The PSA sheet was attached to a PET resin film of <NUM> wide, pressed by moving a pressing roller of <NUM> once back and forth, and allowed to stand for <NUM> hours in a <NUM> atmosphere, after which a <NUM>° peeling force (bonding force) was measured at a pulling rate of <NUM>/min. The test piece was also evaluated for breaking mode (CF: cohesive failure, AF: adhesive failure (interfacial peeling)). The results are also shown in Table <NUM>.

Claim 1:
UV-curable silicone pressure-sensitive adhesive composition comprising
(A) <NUM> to <NUM> parts by weight of organopolysiloxane resin comprising essentially (a) R<NUM><NUM>SiO<NUM>/<NUM> units wherein R<NUM> is substituted or unsubstituted C<NUM>-C<NUM> monovalent hydrocarbon group, and (b) SiO<NUM>/<NUM> units, wherein a molar ratio of units (a) to units (b) is from <NUM>:<NUM> to <NUM>:<NUM>,
(B) <NUM> to <NUM> parts by weight of linear or branched organopolysiloxane having a backbone composed of repeating diorganosiloxane units and containing per molecule on average <NUM> to <NUM> groups having the general formula (<NUM>) and/or (<NUM>):
<CHM>
wherein R<NUM> is hydrogen or methyl, a is an integer of <NUM> to <NUM>, and the broken line designates a valence bond bonded to silicon atom,
<CHM>
wherein R<NUM> and a are as defined above, and the broken line designates a valence bond bonded to silicon atom, as the silicon-bonded monovalent substituent group in the molecule, the organopolysiloxane having a viscosity as measured at <NUM> by a rotational viscometer of from more than <NUM> mPa·s to <NUM>,<NUM> mPa·s, and
(C) <NUM> to <NUM> parts by weight of reactive diluent in the form of an organo(poly)-siloxane containing one group having the general formula (<NUM>):
<CHM>
wherein a is as defined above and the broken line designates a valence bond, at an end or on a side chain of the molecular chain as the silicon-bonded monovalent substituent group in the molecule, the organo(poly)siloxane having a viscosity as measured at <NUM> by a rotational viscometer of <NUM> to <NUM> mPa·s,
with the proviso that the total of components (A) to (C) is <NUM> parts by weight, wherein the amount of component (C) blended is <NUM> to <NUM> parts by weight per <NUM> parts by weight of components (A) and (B) combined.