TWI856033B - Photocurable adhesive composition - Google Patents

Abstract

A photocurable adhesive composition comprising a group 8 transition metal metallocene compound and a 2-cyanoacrylate compound, and do not comprising a compound having a benzoquinone structure or comprising a compound having a benzoquinone structure and wherein the content of a compound having a benzoquinone structure is more than 0 ppm and less than 4 ppm.

Description

Light-curing adhesive composition

The present invention relates to a photocurable adhesive composition.

Adhesive compositions containing 2-cyanoacrylate compounds can rapidly start polymerization by using trace amounts of water present near the surface of the material to be adhered, and can adhere almost all of the materials to be adhered made of various materials within a very short time of about a few seconds to a few minutes. Moreover, the adhesive force is strong, so it is used as the main component of instant adhesives in a wide range of fields such as electricity, electronics, mechanical parts, precision machinery, household products, and medical care.

In addition, a photocurable adhesive composition is known that can be cured simply by light without damaging the excellent characteristics of an instant adhesive such as a one-liquid adhesive at room temperature. Known photocurable adhesive compositions include compositions described in patent documents 1 to 4. Patent document 1 describes a photocurable composition characterized in that it is composed of (A) α-cyanoacrylate and (B) a transition metal metallocene compound of group VIII (group 8) of the periodic table containing an aromatic electron system ligand as essential components.

In addition, Patent Document 2 describes a photocurable composition characterized by comprising the following components as essential components: (A) α-cyanoacrylate, (B) a ferrocene compound substituted with a lower alkyl group, and (C) a photocuring accelerator.

In addition, Patent Document 3 describes a photocurable composition characterized in that it is composed of (A) α-cyanoacrylate, (B) a transition metal metallocene compound of Group VIII of the periodic table containing an aromatic electron-based ligand, and (C) a photocuring accelerator having a structure represented by the following general formula (1) as essential components.

In formula (1), R is a hydrogen atom, or an alkyl group having 1 to 8 carbon atoms, a halogen group, or an alkylthio group having 1 to 8 carbon atoms, and each R may be the same or different.

Furthermore, Patent Document 4 describes a cyanoacrylate adhesive composition which is capable of moisture curing and light curing and contains the following components as essential components: (A) α-cyanoacrylate, (B) a multifunctional compound having a (meth)acryloyl group, (C) a peroxide, (D) a metallocene compound of a transition metal of Group 8 of the periodic table containing an aromatic electron-based ligand, and (E) an intramolecular cleavage-type photoradical initiator.

Patent document 1: Japanese Patent Publication No. 9-249708 Patent document 2: Japanese Patent Publication No. 11-166006 Patent document 3: Japanese Patent Publication No. 2003-277422 Patent document 4: Japanese Patent Publication No. 2007-217484

[Problem to be solved by the invention] The problem to be solved by the present invention is to provide a photocurable adhesive composition having excellent storage stability. [Technical means to solve the problem]

The technical means for solving the above-mentioned problems include the following aspects. >1> A photocurable adhesive composition, which contains: a Group 8 transition metal metallocene compound and a 2-cyanoacrylate compound; and does not contain a compound having a benzoquinone structure, or the content of the compound having a benzoquinone structure is more than 0 ppm and less than 4 ppm. >2> The photocurable adhesive composition as described in >1>, wherein the Group 8 transition metal metallocene compound is at least one compound selected from the group consisting of ferrocene and ruthenocene. >3> The photocurable adhesive composition as described in >1> or >2>, wherein the Group 8 transition metal metallocene compound is ferrocene. >4> The photocurable adhesive composition as described in any one of >1> to >3>, further comprising a photoradical generator. >5> The photocurable adhesive composition as described in >4>, wherein the photoradical generator is at least one compound selected from the group consisting of acylgeranyl compounds, acylphosphine oxide compounds and alkylphenone compounds. >6> The photocurable adhesive composition as described in any one of >1> to >5>, further comprising a polymerization inhibitor. >7> The photocurable adhesive composition as described in >6>, wherein the polymerization inhibitor comprises a phenolic radical polymerization inhibitor. >8> The photocurable adhesive composition as described in >6> or >7>, wherein the polymerization inhibitor comprises a radical polymerization inhibitor having a hydroquinone structure. >9> The photocurable adhesive composition as described in >7> or >8>, wherein the content of the polymerization inhibitor is greater than 10 ppm and less than 1,000 ppm. [Efficacy]

According to the present invention, a photocurable adhesive composition having excellent storage stability can be provided.

The description of the constituent elements described below is sometimes carried out in accordance with the representative implementation form of the present invention, but the present invention is not limited to such implementation form. Furthermore, in the specification of this case, the so-called "～" is used to mean the lower limit value and the upper limit value including the numerical values described before and after it. In the numerical range described in stages in this specification, the upper limit value or lower limit value described in one numerical range can be replaced by the upper limit value or lower limit value of the numerical range described in other stages. In addition, in the numerical range described in this specification, the upper limit value or lower limit value of the numerical range can be replaced by the value disclosed in the embodiment. In the present invention, when there are multiple substances equivalent to each component in the composition, the amount of each component in the composition means the total amount of the above multiple substances present in the composition unless otherwise specified. In the present invention, the term "step" includes not only independent steps, but also steps that cannot be clearly distinguished from other steps as long as the desired purpose of the step can be achieved. In the present invention, "mass %" and "weight %" have the same meaning, and "mass parts" and "weight parts" have the same meaning. In addition, in the present invention, a combination of two or more preferred aspects is a more preferred aspect. In addition, in this specification, "(meth)acryloyl" means both or either of an acrylyl group and a methacryloyl group, and "(meth)acryloyloxy" means both or either of an acryloxy group and a methacryloyloxy group. In addition, in some of the compounds in this specification, the hydrocarbon chain is sometimes described in a simplified structural formula in which the symbols of carbon (C) and hydrogen (H) are omitted. The content of the present invention is described in detail below.

(Photocurable adhesive composition) The photocurable adhesive composition of the present invention contains: a Group 8 transition metal metallocene compound and a 2-cyanoacrylate compound; and does not contain a compound having a benzoquinone structure, or the content of the compound having a benzoquinone structure is more than 0 ppm and less than 4 ppm.

In the known method for producing 2-cyanoacrylate compounds, a compound having a hydroquinone structure such as hydroquinone is often used as a polymerization inhibitor during depolymerization. However, the inventors have discovered the following facts: when a compound having a hydroquinone structure is used in the production of 2-cyanoacrylate compounds, the compound having a hydroquinone structure is oxidized by the action of impurities and decomposition products, and a compound having a benzoquinone structure is generated, albeit in a trace amount; even if distillation is performed, the compound having a benzoquinone structure in the 2-cyanoacrylate compound cannot be completely removed; and the presence of the compound having a benzoquinone structure greatly contributes to the storage stability of the photocurable adhesive composition. The inventors of the present invention have made intensive research and found that a photocurable adhesive composition having excellent storage stability can be provided by preparing the following composition, wherein the composition does not contain a compound having a benzoquinone structure in a 2-cyanoacrylate-based photocurable adhesive composition, or the content of the compound having a benzoquinone structure is more than 0 ppm and less than 4 ppm.

The present invention is described in detail below.

>Compounds with benzoquinone structure> The photocurable adhesive composition of the present invention does not contain compounds with benzoquinone structure, or the content of compounds with benzoquinone structure is more than 0 ppm and less than 4 ppm. In addition, from the viewpoint of storage stability, the photocurable adhesive composition of the present invention preferably does not contain compounds with benzoquinone structure, or the content of compounds with benzoquinone structure is more than 0 ppm and less than 3 ppm, more preferably does not contain compounds with benzoquinone structure, or the content of compounds with benzoquinone structure is more than 0 ppm and less than 2 ppm, and further preferably does not contain compounds with benzoquinone structure, or the content of compounds with benzoquinone structure is more than 0 ppm and less than 1.5 ppm. Furthermore, when the photocurable adhesive composition of the present invention contains two or more compounds with benzoquinone structure, the above content is the total content of the two or more compounds with benzoquinone structure.

The compound having a benzoquinone structure in the present invention may be a compound having a 1,4-benzoquinone structure or a compound having a 1,2-benzoquinone structure. Examples of the compound having a benzoquinone structure include 1,4-benzoquinone, 1,2-benzoquinone, 1,4-naphthoquinone, 1,2-naphthoquinone, 9,10-anthraquinone, and the like.

There is no particular limitation on the method for reducing the content of the compound having a benzoquinone structure in the photocurable adhesive composition of the present invention. A preferred method is, for example, to use a 2-cyanoacrylate compound that does not contain a compound having a benzoquinone structure or has a low content of a compound having a benzoquinone structure as the 2-cyanoacrylate compound used. As a method for producing a 2-cyanoacrylate compound, there is known a method such as the following: depolymerizing a condensate of a cyanoacetate compound and a formaldehyde compound, i.e., a 2-cyanoacrylate condensate, and further distilling the obtained crude 2-cyanoacrylate compound to obtain a purified 2-cyanoacrylate compound. In the past, at least a compound having a hydroquinone structure was often used as a polymerization inhibitor during the above-mentioned depolymerization, and commercially available 2-cyanoacrylate compounds contain a relatively large amount of compounds having a benzoquinone structure. As a method for reducing the content of the compound having a benzoquinone structure in the 2-cyanoacrylate compound, a method of using a polymerization inhibitor without a hydroquinone structure during the above-mentioned depolymerization is preferred; and a method of using a polymerization inhibitor without a hydroquinone structure during the above-mentioned depolymerization and during the above-mentioned distillation is more preferred.

From the viewpoint of storage stability, the polymerization inhibitor having no hydroquinone structure preferably contains a compound having a phenolic hydroxyl group, more preferably a compound represented by the following formula (1), and particularly preferably a compound represented by the following formula (2).

In formula (1) and formula (2), _R1 to _R5 each independently represent a hydrogen atom or a substituent other than a hydroxyl group (however, excluding a phenolic hydroxyl group) that can bond to each other to form a ring, _R6 represents a hydrogen atom or an alkyl group, _R7 to _R10 each independently represent an alkyl group, a cycloalkyl group, or an alkenyl group, and _R11 represents a hydrogen atom or a (meth)acryloyl group.

In formula (1), from the viewpoint of storage stability, it is preferred that at least one of R ₁ to R ₅ is the aforementioned substituent, more preferably R ₁ and R ₅ are at least the aforementioned substituent, and particularly preferably R ₁ , R ₃ and R ₅ are at least the aforementioned substituent. In formula (1), from the viewpoint of storage stability, it is preferred that R ₁ and R ₅ are each independently a linear or branched alkyl group, a cycloalkyl group, an alkyl group containing a structure having a phenolic hydroxyl group, or an alkyl group having a (meth)acryloxyphenyl structure. It is more preferred that R ₁ is a linear or branched alkyl group and R ₅ is an alkyl group containing a structure having a phenolic hydroxyl group or an alkyl group having a (meth)acryloxyphenyl structure. It is particularly preferred that R ₁ is a linear or branched alkyl group and R ₅ is an alkyl group having a (meth)acryloxyphenyl structure. In formula (1), from the viewpoint of storage stability, _R3 is preferably a hydrogen atom, an alkyl group, or an alkoxy group, preferably a linear or branched alkyl group, a cycloalkyl group, or an alkoxy group, and more preferably a linear or branched alkyl group, or an alkoxy group. The alkyl group in the aforementioned _R1 , _R3 , and _R5 is preferably an alkyl group having 1 to 8 carbon atoms, preferably an alkyl group having 1 to 6 carbon atoms, more preferably a linear or branched alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 1 to 6 carbon atoms, a tertiary butyl group, or a 2-methyl-2-butyl group, and particularly preferably a methyl group, a tertiary butyl group, or a 2-methyl-2-butyl group. The aforementioned alkyl group may be linear, branched, or have a ring structure, and may also have a substituent. As a substituent, any group that does not lose the polymerization inhibitory ability may be used, and examples thereof include halogen atoms, alkoxy groups, aryl groups, etc. In addition, the aforementioned substituents may be further substituted by at least one group selected from the group consisting of the aforementioned substituents and alkyl groups. In formula (1), _R2 and _R4 are each independently preferably a hydrogen atom or an alkyl group, and preferably a hydrogen atom.

In formula (2), from the viewpoint of storage stability, _R6 is preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and more preferably a hydrogen atom or a methyl group. In formula (2), from the viewpoint of storage stability, _R7 and _R10 are preferably a tertiary alkyl group, and more preferably a tertiary alkyl group having 4 to 8 carbon atoms, and particularly preferably a tertiary butyl group or a 2-methyl-2-butyl group. In formula (2), from the viewpoint of storage stability, _R8 and _R9 are preferably an alkyl group or an alkoxy group having 1 to 8 carbon atoms, and more preferably a methyl group, a tertiary butyl group, a 2-methyl-2-butyl group, a methoxy group, an ethoxy group, a propoxy group, or a butoxy group. In formula (2), from the viewpoint of storage stability, _R11 is preferably a hydrogen atom or a (meth)acryloyl group.

In addition, it is preferred that the photocurable adhesive composition of the present invention contains a compound wherein the 5% weight reduction temperature of the polymerization inhibitor having no hydroquinone structure in a nitrogen environment is within a range of -150°C to +50°C relative to the boiling point of the 2-cyanoacrylate compound contained in the photocurable adhesive composition of the present invention.

Among them, from the viewpoint of storage stability, the polymerization inhibitor not having a hydroquinone structure is preferably at least one compound selected from the group consisting of 2,2'-methylenebis(6-tert-butyl-p-cresol) (221°C), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol) (229°C), 2,2'-methylenebis(4-methyl-6-tert-butylphenol) monoacrylate (234°C), 2,2'-ethylenebis(4,6-di(t-pentyl)phenol) monoacrylate (253°C) and 2,2'-methylenebis(6-(1-methylcyclohexyl)-p-cresol) (285°C). In addition, the temperatures in parentheses are all 5% weight reduction temperatures.

The polymerization inhibitor having no hydroquinone structure may be removed from the 2-cyanoacrylate compound used during the depolymerization and distillation, and may be included in the photocurable adhesive composition of the present invention without being completely removed from the 2-cyanoacrylate compound.

>Group VIII transition metal metallocene compound> The photocurable adhesive composition of the present invention contains: Group VIII transition metal metallocene compound. We speculate that the aforementioned Group VIII transition metal metallocene compound will function as a photopolymerization initiator. In addition, since the light absorption wavelength of the aforementioned Group VIII transition metal metallocene compound is also on the long-wavelength side of a wavelength of more than 500 nm, the photocurable adhesive composition of the present invention can still be photocured even with light in a wider wavelength range, i.e., the ultraviolet region and the visible light region.

As the aforementioned Group 8 transition metal metallocene compound, for example, there can be cited Group 8 transition metal elements having the periodic table, such as the following metallocene compounds: ferrocene compounds with iron as the transition metal, benzene-based compounds with benzene as the transition metal, ruthenium-based compounds with ruthenium as the transition metal, cobalt-based compounds with cobalt as the transition metal, and nickel-based compounds with nickel as the transition metal. Among these, at least one compound selected from the group consisting of ferrocene and ruthenium is preferred from the viewpoint of light curing property and storage stability.

Specific examples of the Group 8 transition metal metallocene compound include ferrocene, ethylferrocene, n-butylferrocene, benzoylferrocene, acetylferrocene, tert-pentylferrocene, 1,1'-dimethylferrocene, 1,1'-di(tert-butyl)ferrocene, 1 ,1'-bis(benzoyl)ferrocene, 1,1'-bis(acetylcyclopentadienyl)iron, bis(pentamethylcyclopentadienyl)iron, bis(cyclopentadienyl)barium, bis(pentamethylcyclopentadienyl)barium, ruthenocene (bis(cyclopentadienyl)ruthenium), bis(pentamethylcyclopentadienyl)ruthenium. In addition, as the aforementioned Group 8 transition metal metallocene compound, the Group 8 transition metal metallocene compound having an aromatic electron system ligand described in Japanese Patent Publication No. 2003-277422 can be preferably used. Among these, from the viewpoints of photocurability, adhesion speed, cost-effectiveness and storage stability, the Group 8 transition metal metallocene compound is preferably at least one compound selected from the group consisting of ferrocene, ethylferrocene, n-butylferrocene, benzoylferrocene and ruthenocene, and particularly preferably at least one compound selected from the group consisting of ferrocene, benzoylferrocene and ruthenocene.

The photocurable adhesive composition of the present invention may contain one or more Group 8 transition metal metallocene compounds. From the viewpoint of photocurability and storage stability, the content of the Group 8 transition metal metallocene compound in the photocurable adhesive composition of the present invention is preferably 1 ppm to 50,000 ppm, more preferably 10 ppm to 10,000 ppm, and particularly preferably 20 ppm to 2,000 ppm.

>2-cyanoacrylate compound> The photocurable adhesive composition of the present invention contains a 2-cyanoacrylate compound. The 2-cyanoacrylate compound used in the present invention is not particularly limited, and the compound represented by the following formula (C) is preferred.

In formula (C), R represents a saturated or unsaturated linear hydrocarbon group having 1 to 20 carbon atoms and which may have a halogen atom, a branched chain hydrocarbon group, or a cyclic hydrocarbon group, or an aromatic hydrocarbon group having 1 to 20 carbon atoms and which may have a halogen atom. Wherein, when R contains an ether bond, one or both of the hydrocarbon residues bonded to the ether bond are saturated or unsaturated linear hydrocarbon groups having 5 to 20 carbon atoms and which may have a halogen atom, a branched chain hydrocarbon group, or a cyclic hydrocarbon group, or an aromatic hydrocarbon group having 5 to 20 carbon atoms and which may have a halogen atom.

Specific examples of the 2-cyanoacrylate compound include ester compounds such as the following esters of 2-cyanoacrylate: methyl ester, ethyl ester, chloroethyl ester, n-propyl ester, isopropyl ester, allyl ester, propargyl ester, n-butyl ester, isobutyl ester, n-pentyl ester, n-hexyl ester, pentyl ester, 2-methyl-3-butenyl ester, 3-methyl-3-butenyl ester, 2-pentenyl ester, 6-chlorohexyl ester, cyclohexyl ester, phenyl ester, tetrahydrofurfuryl ester, 2-hexenyl ester, 4-methylpentenyl ester, 3-methyl-2-cyclohexenyl ester, norbornyl ester, heptyl ester, cyclohexane methyl ester, cycloheptyl ester, 1-methylcyclohexyl ester, 2-methylcyclohexyl ester, 3-methylcyclohexyl ester, 2-ethylcyclohexyl ester, n-octyl ester, 2-octyl ester, cyclooctyl ester, Cyclopentane methyl ester, 2,3-dimethylcyclohexyl ester, n-nonyl ester, isononyl ester, ketononyl ester, n-decyl ester, isodecyl ester, n-dodecyl ester, 2-methoxyethyl ester, 2-ethoxyethyl ester, 2-ethoxy-2-ethoxyethyl ester, butoxyethoxyethyl ester, 1-(2-methoxy-1-methylethoxy)propyl ester, 2,2,2-trifluoroethyl ester, hexafluoroisopropyl ester, lauryl ester, isotridecyl ester, cinnamyl ester, cetyl ester, stearyl ester, oleyl ester, behenyl ester, hexadecyl ester, octyldodecyl ester, benzyl ester, chlorophenyl ester, 2-pentyloxyethyl ester, 2-hexyloxyethyl ester, 2-cyclohexyloxyethyl ester, 2-(2-ethylhexyloxy)ethyl ester, and 2-phenoxyethyl ester.

Among these, the 2-cyanoacrylate compound preferably includes, for example, the following esters of 2-cyanoacrylic acid: methyl ester, ethyl ester, n-propyl ester, isopropyl ester, n-butyl ester, isobutyl ester, cyclohexyl ester, phenyl ester, tetrahydrofurfuryl ester, 2-ethylhexyl ester, n-octyl ester, 2-octyl ester, 2-methoxyethyl ester, 2-ethoxyethyl ester, or 1-(2-methoxy-1-methylethoxy)propyl ester.

The photocurable adhesive composition of the present invention may contain one or more 2-cyanoacrylate compounds. From the viewpoint of curability, adhesion speed, and adhesion strength, the content of the 2-cyanoacrylate compound in the photocurable adhesive composition of the present invention is preferably 40% by mass or more, and more preferably 60% by mass or more.

>Photoradical generator> From the viewpoint of photocurability, the photocurable adhesive composition of the present invention preferably further contains a photoradical generator. As the aforementioned photoradical generator, a known photoradical generator used when photopolymerizing a radical polymerizable compound can be used. As the photoradical generator, for example: acylgeranyl compounds; acylphosphine oxide compounds; acetophenone compounds without hydroxyl groups, nitrogen atoms and thioether bonds; benzoin compounds without hydroxyl groups, nitrogen atoms and thioether bonds, etc. Among them, from the viewpoint of photocurability, adhesion speed, and storage stability, the photoradical generator is preferably an acylgeranyl compound.

As the acylgeranium compound, preferably, for example, a monoacylgeranium compound or a diacylgeranium compound is mentioned, and more preferably, for example, a diacylgeranium compound is mentioned. As the acylgeranium compound, preferably, for example, Ivocerin (manufactured by Ivoclar Vivadent) is mentioned.

As the acylphosphine oxide compound, preferably, for example, a monoacylphosphine oxide compound, a bisacylphosphine oxide compound, and more preferably, a bisacylphosphine oxide compound. As the monoacylphosphine oxide compound and the bisacylphosphine oxide compound, preferably, for example, a compound represented by the following formula (A-1) can be cited.

In formula (A-1), ^RA1 and ^RA2 each independently represent an alkyl group having 1 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, a phenyl group, or a phenyl group substituted with 1 to 3 alkyl groups having 1 to 8 carbon atoms or alkoxy groups having 1 to 8 carbon atoms, and ^RA3 represents an unsubstituted or acetyloxy-substituted linear or branched alkyl group having 1 to 18 carbon atoms or a cycloalkyl group having 3 to 12 carbon atoms; an alkyl group having 1 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, an unsubstituted or halogen-substituted aryl group; or a group represented by the following formula (A-2).

In formula (A-2), ^RA4 and ^RA5 each independently represent an alkyl group having 1 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms, a phenyl group, or a phenyl group substituted with 1 to 3 alkyl groups having 1 to 8 carbon atoms or alkoxy groups having 1 to 8 carbon atoms, and ^XA1 represents a para-phenyl group.

Examples of the acylphosphine oxide compounds include methyl isobutylyl methyl phosphinate, methyl isobutylyl phenyl phosphinate, methyl trimethyl acetyl phosphinate, methyl-2-ethylhexyl phosphinate, isopropyl-2-ethylhexyl phenyl phosphinate, methyl p-tolyl phenyl phosphinate, methyl o-tolyl phenyl phosphinate, methyl-2,4-dimethylphenyl phosphinate, Formylphenyl phosphinate, methylacylphenyl phosphonate, isobutyldiphenylphosphine oxide, 2-methylhexyldiphenylphosphine oxide, o-toluoyldiphenylphosphine oxide, p-(tertiary butylbenzyl)diphenylphosphine oxide, 3-pyridylcarbonyldiphenylphosphine oxide, acryldiphenylphosphine oxide, benzoyldiphenylphosphine oxide, adipylbis(diphenylphosphine oxide), and the like.

Preferred examples of the bisacylphosphine oxide compound include compounds represented by the following formula (A-3).

In formula (A-3), R ^p is unsubstituted, or an alkyl group having 1 to 12 carbon atoms, an alkylthio group having 1 to 8 carbon atoms, or a halogen atom, and each R ^p may be the same or different.

In formula (A-3), the aforementioned alkyl group with 1 to 12 carbon atoms in ^Rp may be a straight chain or a branched chain, for example: methyl, ethyl, propyl, isopropyl, butyl, isobutyl, dibutyl, tertiary butyl, pentyl, isopentyl, hexyl, heptyl, octyl, nonyl, decyl, or dodecyl. Preferably, it is an alkyl group with 1 to 6 carbon atoms, and more preferably, it is an alkyl group with 1 to 4 carbon atoms. In formula (A-3), the aforementioned alkylthio group with 1 to 8 carbon atoms in ^Rp may be a straight chain or a branched chain, for example: methylthio, ethylthio, propylthio, isopropylthio, butylthio, tertiary butylthio, hexylthio, or octylthio. Among them, methylthio is preferred. Examples of halogen atoms include: chlorine atoms, bromine atoms, and iodine atoms. Among them, chlorine atoms are preferred.

Examples of the bisacylphosphine oxide-based compound include bis(2,4,6-trimethylbenzyl)phenylphosphine oxide and bis(2,6-dimethoxybenzyl)-2,4,4-trimethylpentylphosphine oxide.

Examples of acetophenone compounds include 4-phenoxydichloroacetophenone, 4-tert-butyldichloroacetophenone, 4-tert-butyltrichloroacetophenone, and diethoxyacetophenone. Examples of benzoin compounds include benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and benzil dimethyl ketal.

From the viewpoint of photocurability, adhesion speed, and storage stability, the photoradical generator is preferably selected from diphenylformyldiethylgermanium, bis(4-methoxybenzyl)dimethylgermanium, bis(4-methoxybenzyl)diethylgermanium, ferrocenylacylgermanium, benzoyldimethylgermanium, bis(4-methylbenzyl)diethylgermanium, bis(4-methylbenzyl)dimethylgermanium, and Preferably, the compound is at least one compound selected from the group consisting of diphenylformyldibutylgermanium, more preferably at least one compound selected from the group consisting of diphenylformyldiethylgermanium, bis(4-methoxybenzyl)dimethylgermanium, bis(4-methoxybenzyl)diethylgermanium, bis(4-methylbenzyl)diethylgermanium, bis(4-methylbenzyl)dimethylgermanium and diphenylformyldibutylgermanium.

The photocurable adhesive composition of the present invention may contain one or more photoradical generators. From the viewpoint of photocurability, adhesion speed, and storage stability, the content of the photoradical generator in the photocurable adhesive composition of the present invention is preferably 0.01 mass% to 5 mass%, more preferably 0.05 mass% to 2 mass%, and particularly preferably 0.05 mass% to 1 mass%, relative to the total mass of the photocurable adhesive composition.

>Polymerization inhibitor> From the perspective of storage stability, the photocurable adhesive composition of the present invention preferably further contains a polymerization inhibitor. As the aforementioned polymerization inhibitor, a known polymerization inhibitor can be used.

Preferred examples of the polymerization inhibitor include anionic polymerization inhibitors having no hydroquinone structure, such as phosphorus pentoxide, SO ₂ , p-toluenesulfonic acid, methanesulfonic acid, propane sultone, and BF ₃ complex.

Furthermore, from the viewpoint of storage stability, it is preferred to use a phenolic radical polymerization inhibitor as the polymerization inhibitor.

As the aforementioned phenolic radical polymerization inhibitor, preferably, for example, the aforementioned radical polymerization inhibitor not having a hydroquinone structure, such as the compound represented by formula (1) or formula (2), etc. From the viewpoint of storage stability, as the free radical polymerization inhibitor having no hydroquinone structure, at least one selected from the group consisting of p-methoxyphenol, butylhydroxyanisole, dibutylhydroxytoluene, di(tert-butyl)hydroxytoluene, 6-tert-butyl-4-xylenol, 2,6-di(tert-butyl)phenol, 2,2'-methylenebis(6-tert-butyl-p-cresol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 2,2'-methylenebis(4-methyl-6-tert-butylphenol) monoacrylate, 2,2'-ethylidenebis(4,6-di(tert-pentyl)phenol) monoacrylate and 2,2'-methylenebis(6-(1-methylcyclohexyl)-p-cresol) is preferred.

In addition, from the perspective of storage stability, the aforementioned phenolic radical polymerization inhibitor preferably includes a radical polymerization inhibitor having a hydroquinone structure, and more preferably includes a radical polymerization inhibitor having a 1,4-hydroquinone structure. From the viewpoint of storage stability, as the radical polymerization inhibitor having a hydroquinone structure, preferably, at least one compound is selected from the group consisting of 1,4-hydroquinone, 1,2-hydroquinone, methyl hydroquinone, 2,6-dimethyl hydroquinone, 2,6-di(tertiary butyl) hydroquinone, 1,4-dihydroxynaphthalene, 1,2-dihydroxynaphthalene and 9,10-dihydroxyanthracene; more preferably, at least one compound is selected from the group consisting of 1,4-hydroquinone, 1,2-hydroquinone, methyl hydroquinone, methoxy hydroquinone, 2,6-dimethyl hydroquinone and 2,6-di(tertiary butyl) hydroquinone; and particularly preferably, at least one compound is selected from the group consisting of hydroquinone, methyl hydroquinone and methoxy hydroquinone.

Furthermore, the addition of the radical polymerization inhibitor having a hydroquinone structure is preferably performed during the preparation of the photocurable adhesive composition, or after distillation purification during the production of the raw material, that is, the 2-cyanoacrylate compound, more preferably during the preparation of the photocurable adhesive composition.

The photocurable adhesive composition of the present invention may contain one or more polymerization inhibitors. Among them, from the perspective of storage stability, it is preferred to contain a cationic polymerization inhibitor without a hydroquinone structure, more preferably a free radical polymerization inhibitor without a hydroquinone structure and a cationic polymerization inhibitor without a hydroquinone structure, and particularly preferably a free radical polymerization inhibitor with a hydroquinone structure, a free radical polymerization inhibitor without a hydroquinone structure, and a cationic polymerization inhibitor without a hydroquinone structure. Relative to the total mass of the photocurable adhesive composition, the above-mentioned polymerization inhibitor is preferably 50 ppm to 1 mass%, and preferably 20 ppm to 5,000 ppm. Furthermore, the content of the radical polymerization inhibitor having a hydroquinone structure is preferably 10 ppm or more and 1000 ppm or less, and more preferably 20 ppm or more and 500 ppm or less.

>Other additives> The photocurable adhesive composition of the present invention may contain other additives other than the aforementioned components. There are no particular restrictions on other additives, and known additives can be used. As other additives, for example, the following can be formulated in appropriate amounts according to the purpose, within the range that does not impair the curability and adhesive strength of the photocurable adhesive composition: anionic polymerization accelerator, plasticizer, thickener, fuming silica, particles, filler, colorant, fragrance, solvent, strength enhancer, etc. There are no particular restrictions on the content of other additives, and it is preferably 20% by mass or less, and more preferably 10% by mass or less, relative to the total mass of the photocurable adhesive composition.

Examples of anionic polymerization accelerators include polyoxadienes, crown ethers, silyl crown ethers, calixarenes, cyclodextrins, and gallic phenol-based cyclic compounds. The polyoxadienes refer to polyoxadienes and their derivatives, and examples thereof include those disclosed in Japanese Patent Publication No. 60-37836, Japanese Patent Publication No. 1-43790, Japanese Patent Publication No. 63-128088, Japanese Patent Publication No. 3-167279, U.S. Patent No. 4386193, and U.S. Patent No. 4424327. Specifically, examples include: (1) polyalkylene oxides such as diethylene glycol, triethylene glycol, polyethylene glycol, and polypropylene glycol; (2) derivatives of polyalkylene oxides such as polyethylene glycol monoalkyl esters, polyethylene glycol dialkyl esters, polypropylene glycol dialkyl esters, diethylene glycol monoalkyl ethers, diethylene glycol dialkyl ethers, dipropylene glycol monoalkyl ethers, and dipropylene glycol dialkyl ethers. Examples of crown ethers include those disclosed in Japanese Patent Publication No. 55-2236 and Japanese Patent Application Laid-Open No. 3-167279. Specifically, there are 12-crown-4, 15-crown-5, 18-crown-6, benzo-12-crown-4, benzo-15-crown-5, benzo-18-crown-6, dibenzo-18-crown-6, dibenzo-24-crown-8, dibenzo-30-crown-10, tribenzo-18-crown-6, asymmetric dibenzo-22-crown-6, dibenzo-14 -crown-4, dicyclohexyl-24-crown-8, cyclohexyl-12-crown-4, 1,2-decalyl-15-crown-5, 1,2-naphtho-15-crown-5, 3,4,5-naphthyl-16-crown-5, 1,2-methylbenzo-18-crown-6, 1,2-tert-butyl-18-crown-6, 1,2-vinylbenzo-15-crown-5, etc. Silica-doped crown ethers include, for example, those disclosed in Japanese Unexamined Patent Publication No. 60-168775, etc. Specifically, dimethylsilica-doped-11-crown-4, dimethylsilica-doped-14-crown-5, dimethylsilica-doped-17-crown-6, etc. Examples of the cycloaromatic hydrocarbons include those disclosed in Japanese Patent Application Laid-Open No. 6-179482, Japanese Patent Application Laid-Open No. 62-235379, Japanese Patent Application Laid-Open No. 63-88152, and the like. Specific examples include 5,11,17,23,29,35-hexa(tert-butyl)-37,38,39,40,41,42-hexahydroxycyclo[6]arene, 37,38,39,40,41,42-hexahydroxycyclo[6]arene, 37,38,39,40,41,42-hexa(2-keto-2-ethoxy)ethoxycyclo[6]arene, 25,26,27,28-tetra(2-keto-2-ethoxy)ethoxycyclo[4]arene, tetrakis(4-tert-butyl-2-methylenephenoxy)ethyl acetate, etc. Examples of cyclodextrins include those disclosed in Japanese Patent Application No. 5-505835, etc. Specifically, it includes α-, β- or γ-cyclodextrin. Examples of gallic phenol-based cyclic compounds include those disclosed in Japanese Patent Application Laid-Open No. 2000-191600. Specifically, it includes 3,4,5,10,11,12,17,18,19,24,25,26-dodecaethoxycarbomethoxy-C-1,C-8,C-15,C-22-tetramethyl[14]-metacyclophane. Only one of these cationic polymerization accelerators may be used, or two or more of them may be used in combination.

The plasticizer may be contained as long as it does not impair the effect of the present invention. Examples of such plasticizers include triethyl acetyl citrate, tributyl acetyl citrate, dimethyl adipate, diethyl adipate, dimethyl sebacate, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diisodecyl phthalate, dihexyl phthalate, diheptyl phthalate, dioctyl phthalate, bis(2-ethylhexyl) phthalate , diisobutyl phthalate, diisododecyl phthalate, dipentadecyl phthalate, dioctyl terephthalate, diisobutyl isophthalate, decyl toluate, di(2-ethylhexyl) camphorate, 2-ethylhexyl cyclohexanecarboxylate, diisobutyl fumarate, diisobutyl maleate, tricaproyl glycerol, 2-ethylhexyl benzoate, dipropylene glycol dibenzoate, etc. Among them, tributyl acetyl citrate, dimethyl adipate, dimethyl phthalate, 2-ethylhexyl benzoate, and dipropylene glycol dibenzoate are preferred from the viewpoint of good compatibility with 2-cyanoacrylate and high plasticization efficiency. Only one of these plasticizers may be used, or two or more of them may be used in combination. In addition, the content of the plasticizer is not particularly limited, but when the content of the 2-cyanoacrylate compound is set to 100 parts by mass, it is preferably 3 parts by mass to 50 parts by mass, more preferably 10 parts by mass to 45 parts by mass, and even more preferably 20 parts by mass to 40 parts by mass. If the content of the plasticizer is 3 parts by mass to 50 parts by mass, the retention rate of the adhesive strength after the cold and hot cycle test can be improved.

In addition, examples of the thickener include polymethyl methacrylate, copolymers of methyl methacrylate and acrylate, copolymers of methyl methacrylate and other methacrylates, acrylic rubber, polyvinyl chloride, polystyrene, cellulose ester, poly-2-cyanoacrylate alkyl ester, and ethylene-vinyl acetate copolymer, etc. These thickeners may be used alone or in combination of two or more.

The photocurable adhesive composition of the present invention can also contain fuming silicon oxide. This fuming silicon oxide is an ultrafine powder (preferably a primary particle size of 500 nm or less, particularly preferably a primary particle size of 1 nm to 200 nm) of anhydrous silicon oxide, and this anhydrous silicon oxide is, for example: an ultrafine powder (preferably a primary particle size of 500 nm or less, particularly preferably a primary particle size of 1 nm to 200 nm) of anhydrous silicon oxide produced by oxidation in a gas phase state using silicon tetrachloride as a raw material in a high-temperature flame, and there are hydrophilic silicon oxide with high hydrophilicity and hydrophobic silicon oxide with high hydrophobicity. As the fumed silica, any kind can be used, but hydrophobic silica is preferred from the viewpoint of good dispersibility in the 2-cyanoacrylate compound.

As the hydrophilic silicon oxide, various commercially available products can be used, for example, AEROSIL 50, 130, 200, 300 and 380 (the above are trade names, manufactured by Japan AEROSIL Co., Ltd.). The specific surface areas of these hydrophilic silicon oxides are 50±15 m ² /g, 130±25 m ² /g, 200±25 m ² /g, 300±30 m ² /g, and 380±30 m ² /g, respectively. In addition, commercially available hydrophilic silicon oxides that can be used include REOLOSIL QS-10, QS-20, QS-30 and QS-40 (the above are trade names, manufactured by TOKUYAMA Co., Ltd.). The specific surface areas of these hydrophilic silicon oxides are 140±20 m ² /g, 220±20 m ² /g, 300±30 m ² /g, and 380±30 m ² /g, respectively. In addition, commercially available hydrophilic silicon oxides such as those manufactured by CABOT can also be used.

Furthermore, as the hydrophobic silicon oxide, a product produced in the following manner can be used: a specific compound is brought into contact with a hydrophilic silicon oxide in the presence or absence of a solvent, and preferably heated to treat the surface of the hydrophilic silicon oxide, wherein the specific compound is a compound that can react with a hydroxyl group present on the surface of the hydrophilic silicon oxide to form a hydrophobic group; or a compound that can adsorb on the surface of the hydrophilic silicon oxide and form a hydrophobic layer on the surface.

Examples of compounds used for surface treatment of hydrophilic silica to make it hydrophobic include: various silane coupling agents of alkyl, aryl, and aralkyl groups having a hydrophobic group such as n-octyltrialkoxysilane; silane agents such as methyltrichlorosilane, dimethyldichlorosilane, and hexamethyldisilazane; silicone oils such as polydimethylsiloxane; higher alcohols such as stearyl alcohol; and higher fatty acids such as stearic acid. As hydrophobic silica, products made hydrophobic using any of these compounds can be used.

Examples of commercially available hydrophobic silica include AEROSIL RY200 and R202 which have been hydrophobized by surface treatment with silicone oil, AEROSIL R974, R972 and R976 which have been hydrophobized by surface treatment with a dimethyl silylating agent, AEROSIL R805 which has been hydrophobized by surface treatment with n-octyltrimethoxysilane, AEROSIL R811 and R812 which have been hydrophobized by surface treatment with a trimethyl silylating agent (these are trade names, manufactured by AEROSIL Co., Ltd. of Japan); and REOLOSIL MT-10 which has been hydrophobized by surface treatment with methyltrichlorosilane (trade name, manufactured by TOKUYAMA Co., Ltd.). The specific surface areas of the hydrophobic silicon oxides are 100±20 m ² /g, 100±20 m ² /g, 170±20 m ² /g, 110±20 m ² /g, 250±25 m ² /g, 150±20 m ² /g, 150±20 m ² /g, 260±20 m ² /g, and 120±10 m ² /g, respectively.

In the photocurable adhesive composition of the present invention, when the content of the 2-cyanoacrylate compound is set to 100 parts by mass, the preferred content of the fumed silica is 1 part by mass to 30 parts by mass. Although it varies depending on the type of the 2-cyanoacrylate compound and the type of the fumed silica, the preferred content of the fumed silica is 1 part by mass to 25 parts by mass, and the particularly preferred content is 2 parts by mass to 20 parts by mass. When the content of the fumed silica is 1 part by mass to 30 parts by mass, it is possible to produce an adhesive composition having good workability without impairing the curability and adhesive strength of the photocurable adhesive composition.

The curing method of the photocurable adhesive composition of the present invention is not particularly limited as long as it can be cured by polymerization by the 2-cyanoacrylate compound, and can be cured by light or by moisture such as humidity. When the photocurable adhesive composition of the present invention is cured by light, it can be cured by the following methods: irradiating ultraviolet light and visible light using a high-pressure mercury lamp, a halogen lamp, a xenon lamp, an LED (light-emitting diode) lamp, sunlight, etc.

The photocurable adhesive composition of the present invention can be stored by a known storage method, for example, preferably in an environment with no or little (for example, less than 0.01 volume %) moisture and oxygen, and more preferably in an inert gas environment. As an inert gas, nitrogen, argon, etc. can be cited. In addition, the photocurable adhesive composition of the present invention is preferably stored under light shielding.

The photocurable adhesive composition of the present invention can be used for the known uses of 2-cyanoacrylate compositions and the uses of photocurable adhesive compositions. For example, it can be used as a so-called instant adhesive and can be used as a photocurable instant adhesive. The photocurable adhesive composition of the present invention can be used in a wide range of fields such as general use, industrial use, and medical use because it has light curing and moisture curing properties and excellent storage stability. Specifically, it can be suitably used for bonding and fixing or coating of the same or different kinds of objects, such as: sealing of electronic parts, installation of reels and wire loops in fishing rods, fixing of wires such as coils, fixing of magnetic heads on pedestals, filling agents for treating teeth, bonding and decoration of artificial nails, etc. [Example]

The present invention is specifically described below based on the embodiments. Furthermore, the present invention is not limited to these embodiments. In addition, in the following, "parts" and "%" mean "parts by mass" and "% by mass" respectively unless otherwise specified.

(Example 1) The photocurable adhesive composition of Example 1 was obtained by mixing with 2-octyl 2-cyanoacrylate (2-OctCA) having an acid content (acid content) of 0.20 μg equivalent in the following manner: BF ₃ -methanol complex (polymerization inhibitor) was added to the composition at 20 ppm; SUMILIZER MDP-S (polymerization inhibitor, the following compound) was added to the composition at 1,000 ppm; ferrocene (FeCp ₂ , a Group 8 transition metal metallocene compound, manufactured by Wako Pure Chemical Industries, Ltd.) was added to the composition at 600 ppm; and Ivocerin (a photoradical generator, the following compound, manufactured by Ivoclar Vivadent) was added to the composition at 3,000 ppm.

SUMILIZER MDP-S

Ivocerin

>Viscosity> Measured using an E-type viscometer at 25°C and at any rotation speed.

> Storage stability evaluation> 1.5 g of the obtained photocurable adhesive composition was added to a light-shielding sealed container and stored in a constant temperature room at 60°C. The viscosity was measured at the initial stage and after a period of time as shown in Table 1. The smaller the viscosity change, the better the storage stability. The evaluation results are shown in Table 1.

> Adhesion speed > After adding the obtained photocurable adhesive composition to a light-shielding sealed container and storing it in a constant temperature room at 60°C, the adhesion speed of the photocurable adhesive composition at the initial stage and after the time points recorded in Table 1 were measured in an environment of 23°C and 60% RH in accordance with JIS K 6861 "Test methods for α-cyanoacrylate adhesives". The test pieces used are as follows. Test piece: ABS resin manufactured by UMG ABS, trade name "GSE" The evaluation results are shown in Table 1.

> Photocuring evaluation> The obtained photocurable adhesive composition was placed in a light-shielding sealed container and stored in a constant temperature room at 60°C. The photocurable adhesive composition was taken at the initial stage and at the time points as shown in Table 1 to form a liquid film with a thickness of 1 mm in the polyethylene container. After irradiation with an intensity of 150 W/cm ² (wavelength 405 nm) using a spot UV irradiation device (Spot Cure UIS-25102 manufactured by USHIO Electric Co., Ltd.) equipped with a longpass filter (GG395 manufactured by Schott), the irradiation time until the sample was completely cured was measured. Furthermore, Example 7 and Comparative Example 5 were irradiated at an intensity of 150 W/cm ² (UV-A: value at wavelength 320 nm to 390 nm) in consideration of the absorption wavelength of the photo-radical generator without a long-wave filter. The smaller the value, the better the photocurability. The evaluation results are shown in Table 1.

(Examples 2 to 5, and Comparative Examples 1 and 2) Except that 1,4-hydroquinone (HQ), 1,4-benzoquinone (BQ), and 2-hydroxy-1,4-naphthoquinone were added to the composition in the amounts shown in Table 1, the rest was carried out in the same manner as in Example 1 to prepare photocurable adhesive compositions. The obtained photocurable adhesive compositions were used to perform various evaluations in the same manner as in Example 1. The evaluation results are summarized in Table 1.

[Table 1]

(Examples 6 to 8, and Comparative Examples 3 to 5) Except that 2-octyl ester of 2-cyanoacrylic acid (2-OctCA) having an acid content (acid content) of 0.62 μg was used, and bis(2,4,6-trimethylbenzyl)phenylphosphine oxide (manufactured by iGM, trade name "Omnirad819") or 2,2-dimethoxy-2-phenylacetone (manufactured by iGM, trade name "Omnirad651") was added to the composition in the amount shown in Table 2 instead of Ivocerin, the rest was carried out in the same manner as in Example 1 to prepare photocurable adhesive compositions. Using the obtained photocurable adhesive compositions, various evaluations were carried out in the same manner as in Example 1. The evaluation results are summarized in Table 2.

[Table 2]

(Examples 9 to 11, and Comparative Examples 6 and 7) Photocurable adhesive compositions were prepared in the same manner as in Example 1, except that 2-OctCA was replaced with 2-ethoxyethyl 2-cyanoacrylate (EtOEtCA), the contents of ferrocene (FeCp ₂ , a Group 8 transition metal metallocene compound) and Ivocerin (photoradical generator) were changed to the amounts shown in Table 3, and 1,4-hydroquinone (HQ) or 1,4-benzoquinone (BQ) was added to the composition in the amounts shown in Table 3. Using the obtained photocurable adhesive compositions, various evaluations were performed in the same manner as in Example 1. The evaluation results are summarized in Table 3.

[Table 3]

(Examples 12 to 14, and Comparative Examples 8 and 9) Photocurable adhesive compositions were prepared in the same manner as in Example 1, except that 2-OctCA was replaced with isobutyl 2-cyanoacrylate (iBuCA), the contents of ferrocene (FeCp ₂ , a Group 8 transition metal metallocene compound) and Ivocerin (photoradical generator) were changed to the amounts shown in Table 4, and 1,4-hydroquinone (HQ) or 1,4-benzoquinone (BQ) was added to the composition in the amounts shown in Table 4. Using the obtained photocurable adhesive compositions, various evaluations were performed in the same manner as in Example 1. The evaluation results are summarized in Table 4.

[Table 4]

As shown in Tables 1 to 4, the photocurable adhesive composition of the present invention, that is, the photocurable adhesive composition of the embodiment, has better storage stability than the photocurable adhesive composition of the comparative example.

Domestic storage information (please note the storage institution, date, and number in order) None

Overseas storage information (please note the storage country, institution, date, and number in order) None

Claims (5)

A photocurable adhesive composition comprises: a Group 8 transition metal metallocene compound, a 2-cyanoacrylate compound, a radical polymerization inhibitor having no hydroquinone structure, a cationic polymerization inhibitor having no hydroquinone structure, and a radical polymerization inhibitor having a hydroquinone structure; wherein the composition does not contain a compound having a benzoquinone structure, or the content of the compound having a benzoquinone structure is more than 0 ppm and less than 4 ppm, the content of the radical polymerization inhibitor having a hydroquinone structure is more than 10 ppm and less than 1,000 ppm, the radical polymerization inhibitor having no hydroquinone structure is one or more radical polymerization inhibitors selected from the group consisting of a compound represented by the following formula (1) and a compound represented by the following formula (2), and the cationic polymerization inhibitor having no hydroquinone structure is selected from the group consisting of phosphorus pentoxide, SO ₂ , p-toluenesulfonic acid, methanesulfonic acid, propane sultone, and _BF3 complex,

In formula (1), _R1 and _R5 each independently represent a hydrogen atom or a substituent other than a hydroxyl group that can bond to each other to form a ring, but the hydroxyl group excludes a phenolic hydroxyl group, _R3 represents a hydrogen atom, an alkyl group or an alkoxy group, _R2 and _R4 each independently represent a hydrogen atom or an alkyl group, and in formula (2), _R6 represents a hydrogen atom or an alkyl group, _R7 to _R10 each independently represent an alkyl group, a cycloalkyl group, or an alkenyl group, and _R11 represents a hydrogen atom or a (meth)acryl group. The photocurable adhesive composition as described in claim 1, wherein the aforementioned Group 8 transition metal metallocene compound is at least one compound selected from the group consisting of ferrocene and ruthenocene. The photocurable adhesive composition as described in claim 1, wherein the aforementioned Group 8 transition metal metallocene compound is ferrocene. The photocurable adhesive composition as described in claim 1 further contains a photoradical generator. The photocurable adhesive composition as described in claim 4, wherein the photoradical generator is at least one compound selected from the group consisting of acylgermanium compounds, acylphosphine oxide compounds and alkylphenone compounds.