BACKGROUND OF THE INVENTION Epoxy compounds have previously been cured by various cationic initiator systems. U. S. Pat. No. 4,256,828 describes photoco- polymerizable compositions containing epoxides, organic mate- rial with hydroxyl functionality, and a photosensitive aromatic sulfonium or iodonium salt of a halogen-containing complex ion.

This patent also describes coated substrates. U. S. Pat. No.

4,250,053 describes sensitized aromatic iodonium or aromatic sulfonium salt photoinitiator systems for cationic reactions.

U. S. Pat. No. 4,026,705 describes epoxy compositions that can be cured with visible radiant energy based on the use of cer- tain organic dyes in combination with diarylhalonium salts.

Cationic polymerization of various olefinic and cyclic organic compounds and organosilicon cyclics is also described. WO 95/30402 describes systems of iodonium salts and ferrocenium salt for visible light cure of epoxides. U. S. Pat. No.

5,856,373 describes systems of iodonium salts and visible light sensitizers that yield in sufficient cure in the presence of hydroxyl-containing materials.

SUMMARY OF THE INVENTION The present invention provides a composition comprising a) an aryliodonium salt and b) a titanocene compound. This composi- tion can additionally comprise c) a cationically polymerizable epoxy compound (like a monomer) or resin.

DETAILED DESCRIPTION It has been surprisingly found that the photoinitiator composi- tions of the present invention can be used as photoinitiators for systems which comprise cationically polymerizable epoxy resins: Such systems can be cured using visible light cure techniques. Previous initiators provided rapid cure only for thin epoxy-based resin samples, such as utilized in coating and film technologies. Additionally, powerful and potentially harm- ful sources of UV-radiation were previously required to achieve polymerization to a significant depth. Using visible light sources the present invention achieves enhanced depth of cure in a surprisingly short time, so that safer and less expensive equipment may now be utilized to cure epoxy resins to thick- nesses not previously attainable. Furthermore, the new initia- tor combination of aryliodonium salts and titanocene derivates is new, very efficient and not depending on using compounds containing hydroxyl groups described in U. S. Pat. No. 5,856,373 which yields in non desirable high amounts of water sorption when used under humid conditions.

The photoinitiator systems and the photopolymerizable composi- tions of the invention are sensitive to visible light, and pho- tocure rapidly, without the use of heat, to polymers having de- sirable properties. For purposes of the present invention, visible light is defined as light having a wavelength of be- tween about 400 and 700 nanometers. The photopolymerization of the compositions of the invention occurs on exposure of the compositions to any source of radiation emitting actinic radia- tion at a wavelength within the visible spectral region. Expo- sures may be from less than about 1 second to 10 minutes or more, depending upon the amounts and particular components of the compositions utilized and depending upon the radiation source and distance from the source and the thickness of the composition to be cured. The compositions of the invention are one-part, stable compositions having very good shelf life and good thermal stability.

The cationically polymerizable epoxy resins useful in the com- positions of the invention comprise organic compounds having an oxirane ring, i. e., polymerizable by ring opening. Such materials, broadly called epoxides, include monomeric epoxy compounds and epoxides of the polymeric type and can be aliphatic, cycloaliphatic, aromatic or heterocyclic. These materials generally have, on the aver- age, at least 1 polymerizable epoxy group per molecule, and preferably at least about 1.5 polymerizable epoxy groups per molecule. The polymeric epoxides include linear polymers having terminal epoxy groups (e. g., a diglycidyl ether of a polyoxyal- kylene glycol), polymers having skeletal oxirane units (e. g., polybutadiene polyepoxide), and polymers having pendent epoxy groups (e. g., a glycidyl methacrylate polymer or copolymer).

The epoxides may be pure compounds or may be mixtures contain- ing one, two, or more epoxy groups per molecule. The"average" number of epoxy groups per molecule is determined by dividing the total number of epoxy groups in epoxy-containing material by the total number of epoxy molecules present.

These epoxy-containing materials may vary from low molecular weight monomeric materials to high molecular weight polymers and may vary greatly in the nature of their backbone and sub- stituent groups. For example, the backbone may be of any type and substituent groups thereon can be any group that does not substantially interfere with cationic cure at room temperature.

Illustrative of permissible substituent groups include halo- gens, ester groups, ethers, sulfonate groups, siloxane groups, nitro groups, phosphate groups, and the like. The molecular weight of the epoxy-containing materials may vary from about 58 to about 100,000 or more.

Useful epoxy-containing materials include those which contain cyclohexene oxide groups such as the epoxycyclohexanecarboxy- lates, typified by 3,4-epoxycyclohexylmethyl-3,4-epoxycyclo- hexanecarboxylate, 3,4-epoxy-2-methylcyclohexylmethyl-3,4- epoxy-2-methylcyclohexane carboxylate, and bis (3,4-epoxy-6- methylcyclohexylmethyl) adipate. For a more detailed list of useful epoxides of this nature, reference is made to the U. S.

Pat. No. 3,117,099, incorporated herein by reference.

Further epoxy-containing materials which are particularly use- ful in the practice of this invention include glycidyl ether monomers of the formula where R'is alkyl, especially C1-C6 alkyl, or aryl and n is an integer of 1 to 6. Examples are glycidyl ethers of polyhydric phenols obtained by reacting a polyhydric phenol with an excess of chlorohydrin such as epichlorohydrin (e. g., the diglycidyl ether of 2,2-bis- (2,3-epoxypropoxyphenol)-propane). Further ex- amples of epoxides of this type which can be used in the prac- tice of this invention are described in U. S. Pat. No.

3,018,262, incorporated herein by reference, and in"Handbook of Epoxy Resins"by Lee and Neville, McGraw-Hill Book Co., New York (1967).

There are a host of commercially available epoxy resins which can be used in this invention. In particular, epoxides which are readily available include octadecylene oxide, epichlorohy- drin, styrene oxide, vinyl cyclohexene oxide, glycidol, gly- cidylmethacrylate, diglycidyl ether of Bisphenol A (e. g., those available under the trade designations"Epon 828","Epon 825", "Epon 1004"and"Epon 1010"from Shell Chemical Cc.,"DER-331", "DER-332", and"DER-334", from Dow Chemical Co.), vinylcyclo- hexene dioxide (e. g.,"ERL-4206"from Union Carbide Corp.), 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexene carboxylate (e. g.,"ERL-4221"or"UVR 6110"or"UVR 6105"from Union Car- bide Corp.), 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6- methyl-cyclohexene carboxylate (e. g.,"ERL-4201"from Union Carbide Corp.), bis (3,4-epoxy-6-methylcyclohexylmethyl) adipate (e. g.,"ERL-4289"from Union Carbide Corp.), bis (2,3- epoxycyclopentyl) ether (e. g.,"ERL-0400"from Union Carbide Corp.), aliphatic epoxy modified with polypropylene glycol (e. g.,"ERL-4050"and"ERL-4052"from Union Carbide Corp.), di- pentene dioxide (e. g.,"ERL-4269"from Union Carbide Corp.), epoxidized polybutadiene (e. g.,"Oxiron 2001"from FMC Corp.), silicone resin containing epoxy functionality, flame retardant epoxy resins (e. g.,"DER-580", a brominated bisphenol type ep- oxy resin available from Dow Chemical Co.), 1,4-butanediol di- glycidyl ether of phenolformaldehyde novolak (e. g.,"DEN-431" and"DEN-438"from Dow Chemical Co.), and resorcinol diglycidyl ether (e. g.,"Kopoxite"from Koppers Company, Inc.). bis (3, 4- epoxycyclohexyl) adipate (e. g.,"ERL-4299"or"UVR-o128", from Union Carbide Corp.), 2- (3, 4-epoxycyclohexyl-5, 5-spiro-3,4- epoxy) cyclohexane-meta-dioxane (e. g.,"ERL-4234"from Union Carbide Corp.), vinylcyclohexene monoxide (from Union Carbide Corp.), 1,2-epoxyhexadecane (e. g.,"UVR-6216"from Union Car- bide Corp.), alkyl glycidyl ethers such as alkyl C. sub. 8- C. sub. 10 glycidyl ether (e. g.,"HELOXY Modifier 7"from Shell Chemical Co.), alkyl C. sub. 12-C. sub.. sub. 14 glycidyl ether (e. g.,"HELOXY Modifier 8"from Shell Chemical Co.), butyl gly- cidyl ether (e. g.,"HELOXY Modifier 61"from Shell Chemical Co.), cresyl glycidyl ether (e. g.,"HELOXY Modifier 62"from Shell Chemical Co.), p-tert butylphenyl glycidyl ether (e. g., "HELOXY Modifier 65"from Shell Chemical Co.), polyfunctional glycidyl ethers such as diglycidyl ether of 1,4-butanediol (e. g.,"HELOXY Modifier 67"from Shell Chemical Co.), digly- cidyl ether of neopentyl glycol (e. g.,"HELOXY Modifier 68" from Shell Chemical Co.), diglycidyl ether of cyclohexanedi- methanol (e. g.,"HELOXY Modifier 107"from Shell Chemical Co.), trimethylol ethane triglycidyl ether (e. g.,"HELOXY Modifier 44"from Shell Chemical Co.), trimethylol propane triglycidyl ether (e. g.,"HELOXY Modifier 48"from Shell Chemical Co.), polyglycidyl ether of an aliphatic polyol (e. g.,"HELOXY Modi- fier 84"from Shell Chemical Co.), polyglycol diepoxide (e. g., "HELOXY Modifier 32"from Shell Chemical Co.), bisphenol F ep- oxides (e. g.,"EPN-1138"or"GY-281"from Ciba-Geigy Corp.), 9,9-bis>4- (2,3-epoxypropoxy)-phenyl fluorenone (e. g.,"Epon 1079"from Shell Chemical Co.).

Still other epoxy resins contain copolymers of acrylic acid es- ters or glycidol such as glycidylacrylate and glycidylmetha- crylate with one or more copolymerizable vinyl compounds. Exam- ples of such copolymers are 1 : 1 styrene-glycidylmethacrylate, 1: 1 methylmethacrylate-glycidylacrylate and a 62.5: 24: 13.5 methylmethacrylate-ethyl acrylate-glycidylmethacrylate.

Other useful epoxy resins are well known and contain such epox- ides as epichlorohydrins, e. g., epichlorohydrin; alkylene ox- ides, e. g., propylene oxide, styrene oxide; alkenyl oxides, e. g., butadiene oxide; glycidyl esters, e. g., ethyl glycidate.

Other useful epoxy resins are silicones with epoxyfunctional- ity, especially cyclohexylepoxy groups, especially those with a silicone backbone. Examples are UV 9300, UV 9315, UV 9400, UV 9425, all delivered by GE Bayer Silicones.

The amounts of the cationically polymerizable epoxy compounds or resin useful in the photopolymerizable compositions of the invention may vary over broad ranges. Preferred amounts are 40 to 99,98% by weight, more preferred amounts are 80 to 99,9% by weight and especially preferred amounts are 90 to 99% by weight of the photopolymerizable composition.

The polymers of the epoxy resin may optionally contain other functionalities that do not substantially interfere with cati- onic cure at room temperature.

Blends of various epoxy-containing materials are particularly contemplated in this invention. Examples of such blends include two or more molecular weight distributions of epoxy-containing compounds, such as low molecular weight (below 200), intermedi- ate molecular weight (about 200 to 10,000) and higher molecular weight (above about 10,000). Alternatively or additionally, the epoxy resin may contain a blend of epoxy-containing materials having different chemical nature, such as aliphatic and aro- matic, or functionality, such as polar and non-polar. Other ca- tionically polymerizable monomers or polymers may additionally be incorporated.

Hydroxyl-containing materials can be added but are not a neces- sary embodiment of the invention. If they are used they may be any liquid or solid organic material having hydroxyl function- ality.

Preferably the hydroxyl-containing material contains two or more primary or secondary aliphatic hydroxyl groups (i. e., the hydroxyl group is bonded directly to a non-aromatic carbon atom). The hydroxyl group may be terminally situated, or they may be pendent from a polymer or copolymer. The molecular weight of the hydroxyl-containing organic material may vary from very low (e. g., 32) to very high (e. g., one million or more). Suitable hydroxyl-containing materials may have low mo- lecular weights, i. e. from about 32 to 200, intermediate mo- lecular weight, i. e. from about 200 to 10,000, or high molecu- lar weight, i. e. above about 10,000. As used herein, all mo- lecular weights are weight average molecular weights.

The hydroxyl-containing material may optionally contain other functionalities that do not substantially interfere with cati- onic cure at room temperature. Thus, the hydroxyl-containing materials may optionally be nonaromatic in nature or may com- prise aromatic functionality. The hydroxyl-containing material may optionally contain heteroatoms in the backbone of the mole- cule, such as nitrogen, oxygen, sulfur, and the like, provided that the ultimate hydroxyl-containing material does not sub- stantially interfere with cationic cure at room temperature.

The hydroxyl-containing material may, for example, be selected from naturally occurring or synthetically prepared cellulosic materials. Of course, the hydroxyl-containing material is also substantially free of groups which may be thermally or photo- lytically unstable ; that is, the material will not decompose or liberate volatile components at temperatures below about 1000'- or in the presence of actinic light which may be encountered during the desired curing conditions for the photocopolymeri- zable composition.

Representative examples of suitable hydroxyl-containing materi- als having a hydroxyl functionality of 1 include alkanols, monoalkyl ethers of polyoxyalkyleneglycols, monoalkyl ethers of alkylene-glycols, and others known to the art.

Representative examples of useful monomeric polyhydroxy organic materials include alkylene glycols (e. g., 1, 2-ethanediol; 1,3- propanediol ; 1,4-butanediol ; 1,6-hexanediol; 1,8-octanediol ; 2- ethyl-1, 6-hexanediol ; bis (hydroxymethyl) cyclohexane; 1,18- dihydroxyoctadecane ; 3-chloro-1, 2-propanediol); polyhydroxyal- kanes (e. g., glycerine, tri-methylolethane, pentaerythritol, sorbitol) and other polyhydroxy compounds such as N, N- bis (hydroxyethyl) benzamid ; 2-butyne-1, 4-diol ; 4,4'- bis (hydroxymethyl) diphenylsulfone ; castor oil; and the like.

Representative examples of useful polymeric hydroxyl-containing materials include polyoxyethylene and polyoxypropylene glycols, and particularly the polyoxyethylene and polyoxypropylene gly- col diols and triols of molecular weights from about 200 to about 10,000 corresponding to equivalent weight of 100 to 5000 for the diols or 70 to 3300 for triols; polytetramethylene ether glycols (polytetrahydrofuran"poly THF") of varying mo- lecular weight; copolymers of hydroxypropyl and hydroxyethyl acrylates and methacrylates with other free radical- polymerizable monomers such as acrylate esters, vinyl halides, or styrene; copolymers containing pendent hydroxy groups formed by hydrolysis or partial hydrolysis of vinyl acetate copoly- mers, polyvinylacetal resins containing pendent hydroxyl groups; modified cellulose polymers such hydroxyethylated and hydroxypropylated cellulose; hydroxy-terminated polyesters; hy- droxy-terminated polylactones, and particularly the polycapro- lactones; fluorinated polyoxyethylene or polyoxypropylene gly- cols; and hydroxy-terminated polyalkadienes.

Useful commercially available hydroxyl-containing materials in- clude the"Terathane"series (available from du Pont de Ne- mours) of polytetramethylene ether glycols such as"Terathane" 650, 1000,2000 and 2900;"PeP" (available from wyandotte Chemicals Corporation) of polyoxyalkylene tetrols having secon- dary hydroxyl groups such as"PeP"450, 550 and 650 ;"Butvar" series (available from Monsanto Chemical Company) of polyvi- nylacetal resins such as"Butvar"B-72A, B-73, B-76, B-90 and B-98 ; and"Formvar"7/70,12/85,7/95S, 7/95E, 15/95S and 15/95E ;"Tone"series (available from Union Carbide) of poly- caprolactone polyols such as"Tone"0200,0210,0230,0240, 0300 ;"Paraplex U-148" (available from Rohm and Haas), and ali- phatic polyester diol;"Multron"R series (available from Mobay Chemical Co.) of saturated polyester polyols such as"Multron" R-2, R-12A, R-16, R-18, R-38, R-68 and R-74 ;"Klucel E" (avail- able from Hercules Inc.) a hydroxypropylated cellulose having an equivalent weight of approximately 100; and"Alcohol Soluble Butyrate" (available from Eastman Kodak) a cellulose acetate butyrate ester having a hydroxyl equivalent weight of approxi- mately 400; polyether polyols such as polypropylene glycol diol (e. g.,"Arcol PPG-425","Arcol PPG-725","Arcol PPG-1025","Ar- col PPG-2025", Arcol PPG-3025","Arcol PPG-4025"from ARCO Chemical Co.); polypropylene glycol triol (e. g.,"Arcol LT-28", "Arcol LHT-42","Arcol LHT 112","Arcol LHT 240","Arcol LG- 56","Arcol LG-168","Arcol LG-650"from ARCO Chemical Co.) ; ethylene oxide capped polyoxypropylene triol or diol (e. g., "Arcol 11-27","Arcol 11-34","Arcol E-351","Arcol E-452", "Arcol E-785","Arcol E-786"from ARCO Chemical Co.); ethoxy- lated bis-phenol A; propylene oxide or ethylene oxide-based polyols (e. g.,"Voranol"polyether polyols such as"Voranol 230-056","Voranol 220 series","Voranol 230 series","Voranol 240 series"from the Dow Chemicals Co.).

The amount of hydroxyl-containing organic material which may be used in the compositions of the invention may vary over broad ranges, depending upon factors such as the compatibility of the hydroxyl-containing material with the epoxide, the equivalent weight and functionality of the hydroxyl-containing material, the physical properties desired in the final cured composition, the desired speed of photocure, and the like. Preferred amounts are up to 20% by weight, especially preferred amounts are 0.5 to 5% by weight of the photopolymerizable compositions.

Blends of various hydroxyl-containing materials are also con- templated in this invention. Examples of such blends include two or more molecular weight distributions of hydroxyl- containing compounds, such as low molecular weight (below 200), intermediate molecular weight (about 200 to 10,000) and higher molecular weight (above about 10,000). Alternatively or addi- tionally, the hydroxyl-containing material may contain a blend of hydroxyl-containing materials having different chemical na- ture, such as aliphatic and aromatic, or functionality, such as polar and non-polar. As an additionally example, one may use mixtures of two or more poly-functional hydroxy materials or one or more mono-functional hydroxy materials with poly- functional hydroxy materials.

The photopolymerizable (photocurable) composition of the pres- ent invention may additionally comprise a radically curable monomer or resin.

Such radically curable monomers or resins can, for example, be selected from acrylates, vinyl benzene derivatives, allyl de- rivatives, vinyl ketones.

Addition of vinylether compounds to the epoxy resin may be use- ful to speed up the photocure or trigger the desired physical properties of the final cured composition. Examples of vinyle- ther compounds are Rapicure DVE-3 (triethylenglykoldivinyle- ther), Rapicure CHVE (1,4-cyclohexan dimethanoldivinylether) and Rapicure HBVE (butandiolmonovinylether), all available from ISP Global Technologies Deutschland GmbH, Frechen, Germany. The ratio of epoxy compounds or resins to radically curable mono- mers or resins can be 10: 90 to 90: 10 based on the weight of the mixture of these components.

The aromatic iodonium complex salts or aryliodonium salts are of the formula: where Arl and Ar2 are aromatic groups having 4 to 20 carbon at- oms and can, for example, be selected from the group consisting of phenyl, thienyl, furanyl and pyrazolyl groups; Z can, for example, be selected from the group consisting of oxygen; sul- fur; where R is aryl (of 6 to 20 carbons, such as phenyl) or acyl (of 2 to 20 carbons, such as acetyl, benzoyl, and the like.) ; a carbon-to-carbon bond; or where R1 and R2 are selected from hydrogen, alkyl radicals of 1 to 4 carbons, and alkenyl radicals of 2 to 4 carbons; and n is zero or 1; and wherein X-is a halogen-containing complex anion which can, for example, be selected from tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate, and hexafluoroanti- monate.

The aromatic iodonium cations are stable and are well known and recognized in the art. See for example, U. S. Pat. Nos.

3,565,906; 3,712,920; 3,759,989; and 3,763,187; F. Beringer, et al., Diaryliodonium Salts IX, J. Am. Chem. Soc. 81,342-51 (1959) and F. Beringer, et al., Diaryliodonium Salts XXII, J.

Chem. Soc. 1964,442-51; F. Beringer, et al., Iodonium Salts Containing Heterocyclic Iodine, J. Org. Chem. 30,1141-8 (1965).

Representative Arl and Ar2 groups are aromatic groups having 4 to 20 carbon atoms selected from phenyl, thienyl, furanyl, and pyrazolyl groups. These aromatic groups may optionally have one or more fused benzo rings (e. g., naphthyl and the like; benzo- thienyl, dibenzothienyl ; benzofuranyl, dibenzofuranyl; and the like.). Such aromatic groups may also be substituted, if de- sired, by one or more of the following non-basic groups which are essentially non-reactive with epoxide vinylether or hy- droxy: halogen, nitro, N-arylamino groups, ester groups (e. g., alkoxycarbonyl such as methoxycarbonyl and ethoxycarbonyl, phe- noxycarbonyl), sulfo ester groups (e. g., alkoxysulfonyl such as methoxysulfonyl and butoxysulfonyl, phenoxysulfonyl, and the like), amido groups (e. g., acetamido, butyramido, ethylsulfona- mido, and the like), carbamyl groups (e. g., carbamyl, N- alkylcarbamyl, N-phenylcarbamyl, and the like), sulfamyl groups (e. g., sulfamyl, N-alkylsulfamyl, N, N-dialkylsulfamyl, N- phenylsulfamyl, and the like), alkoxy groups (e. g., methoxy, ethoxy, butoxy, and the like), aryl groups (e. g., phenyl), al- kyl groups (e. g., methyl, ethyl, butyl, and the like), aryloxy groups (e. g., phenoxy) alkylsulfonyl (e. g., methylsulfonyl, ethylsulfonyl, and the like), arylsulfonyl groups (e. g., phenylsulfuonyl groups), perfluoroalkyl groups (e. g., trifluo- romethyl, perfluoroethyl, and the like), and perfluoroalkysul- fonyl groups (e. g., trifluoromethylsulfonyl, perfluorobutylsul- fonyl, and the like).

Suitable examples of the aromatic iodonium complex salt photo- initiators include: diphenyliodonium tetrafluoroborate; di (4- methylphenyl) iodonium tetrafluoroborate; phenyl-4-methylphenyl- iodonium tetrafluoroborate; di (4-heptylphenyl) iodonium tetra- fluoroborate; di (3-nitrophenyl) iodonium hexafluorophosphate ; di (4-chlorophenyl) iodonium hexafluorophosphate; di (naphthyl)- iodonium tetrafluoroborate; di (4-trifluoromethylphenyl) iodonium tetrafluoroborate; diphenyliodonium hexafluorophosphate; di (4- methylphenyl) iodonium hexafluorophosphate; diphenyliodonium hexafluoroarsenate; di (4-phenoxyphenyl) iodonium tetrafluorobo- rate; phenyl-2-thienyliodonium hexafluorophosphate; 3,5-di- methylpyrazolyl-4-phenyliodonium hexafluorophosphate; di- phenyliodonium hexafluoroantimonate; di (2,4-dichlorophenyl)- iodonium hexafluorophosphate; di (4-bromophenyl) iodonium hexafluorophosphate; di (4-methoxyphenyl) iodonium hexafluoro- phosphate; di (3-carboxyphenyl) iodonium hexafluorophosphate; di (3-methoxycarbonylphenyl) iodonium hexafluorophosphate ; di (3- methoxysulfonylphenyl) iodonium hexafluorophosphate; di (4- acetamidophenyl) iodonium hexafluorophosphate and di (2-benzo- thienyl) iodonium hexafluorophosphate.

Of the aromatic iodonium complex salts which are suitable for use in the compositions of the invention the preferred salts are the diaryliodonium hexafluorophosphate and the diaryliodo- nium hexafluoroantimonate. These salts are preferred because, in general, they are more thermally stable, promote faster re- action, and are more soluble in inert organic solvents than are other aromatic iodonium salts of complex ions.

The aromatic iodonium complex salts may be prepared by metathe- sis of corresponding aromatic iodonium simple salts (such as, for example, the diphenyliodonium bisulfate) in accordance with the teachings of Beringer, et al., J. Am. Chem. Soc 81,342 (1959).

The aryliodonium complex salt is preferably present in the pho- toinitiator composition in amounts of about 10 to 90% by weight, preferably of about 30 to 70% by weight and most pref- erably of about 40 to 60% by weight. It is preferably present in the photomerizable composition in amounts of about 0.01-10% by weight, more preferably of about 0.02-5% by weight, and most preferably of about 0.05-4% by weight.

The (di) aryliodonium salts alone are not initiating the light cure of epoxy resins.

The sensitizer of the invention are titanocene compounds or de- rivatives. Titanocene senzitizers are known as light cure ini- tiators for acrylic resins. Nothing is described about their ability to act as a sensitizer for diaryliodonium compounds.

Preferably the titanocene compound is bis-cyclopentadienyl- bis (2,6-difluoro-3-pyrrolyl-phenyl) titanium. Preferably the ti- tanocene derivate should have an extinction coefficient of less than about 1000 1 mol-1 cm-1. The derivate can be prepared as described in Macromolecules 1993,26 (9), 2216-2221 and ibidem, 1994 27 (13), 3472-3477.

The sensitizer is selected based in part upon shelf stability considerations. Accordingly, selection of a particular sensi- tizer may depend to some extent upon the particular epoxy, vi- nyl ether, hydroxy-containing material and iodonium salt cho- sen.

The visible light sensitizer is preferably present in the pho- toinitiator composition in amounts of about 10 to 90% by weight, preferably of about 30 to 70% by weight and most pref- erably of about 40 to 60% by weight. It is preferably present in the photopolymerizable composition in amounts of about 0.01- 10% by weight, more preferably of about 0.02-5% by weight, and most preferably of about 0.05-4% by weight.

The photopolymerizable compositions of the invention are pre- pared by simply admixing, under"safe light"conditions, the components of the inventive compositions. Suitable inert sol- vents may be employed if desired when effecting this mixture.

Examples of suitable solvents are acetone and acetonitrile, and include any solvent which does not react appreciably with the components of the inventive compositions. A liquid material to be polymerized may be used as a solvent for another liquid or solid material to be polymerized. Solventless compositions can be prepared by simply dissolving the aromatic iodonium complex salt and sensitizer in the epoxy resin or vinyl ether with or without the use of mild heating.

Dental applications particularly benefit from the present in- vention. Until now, acrylate and methacrylate chemistry has been used extensively for adhesive and restorative dental com- positions. This chemistry advantageously can be cured with visible light using photoinitiator systems. However, this chem- istry has the disadvantage of a relatively high degree of shrinkage during the polymerization process. In contrast, dur- ing polymerization, the epoxy resins of the present invention shrink significantly less than the acrylate and methacrylate resins of the prior art. The present invention provides a sys- tem for curing epoxy resins in an acceptable time frame and to sufficient depth using visible light source equipment already available in the dental office.

The dental materials of the present invention may be filled or unfilled and include dental materials such as direct esthetic restorative materials (e. g., anterior and posterior restora- tives), prostheses, adhesives and primers for oral hard tis- sues, sealants, veneers, cavity liners, orthodontic bracket ad- hesives for use with any type of bracket (such as metal, plas- tic and ceramic), crown and bridge cements, artificial crowns, artificial teeth, dentures, and the like. These dental materi- als are used in the mouth and are disposed adjacent to natural teeth. The phrase"disposed adjacent to"as used herein will refer to the placing of a dental material in temporary or per- manent bonded (e. g., adhesive) or touching (e. g., occlusal or proximal) contact with a natural tooth. The term"composite"as used herein will refer to a filled dental material. The term "restorative"as used herein will refer to a composite which is polymerized after it is disposed adjacent to a tooth. The term "prosthesis"as used herein will refer to a composite which is shaped and polymerized for its final use (e. g., as crown, bridge, veneer, inlay, onlay or the like) before it is disposed adjacent to a tooth. The term"sealant"as used herein will re- fer to a lightly filled composite or to an unfilled dental ma- terial which is cured after it is disposed adjacent to a tooth.

"Polymerizable", as used herein, refers to curing or hardening the dental material, e. g., by free-radical, ionic or mixed re- action mechanisms.

In certain applications, the use of a filler may be appropri- ate. The choice of filler affects important properties of the composite such as its appearance, radiopacity and physical and mechanical properties. Appearance is affected in part by ad- justment of the amounts and relative refractive indices of the ingredients of the composite, thereby allowing alteration of the translucence, opacity or pearlescence of the composite. Ep- oxy resin compositions of the invention, either alone or in ad- mixture with diluent monomer, can be prepared with refractive indices which approach or approximate the refractive indices of fillers such as quartz (refractive index 1.55), submicron sil- ica (1.46), and 5.5: 1 mole ratio Si02 : ZrO2 non-vitreous micro- particles (1.54). In this way the appearance of the dental ma- terial can, if desired, be made to closely approximate the ap- pearance of natural dentition.

Radiopacity is a measurement of the ability of the composite to be detected by x-ray examination. Frequently a radiopaque com- posite will be desirable, for instance, to enable the dentist to determine whether or not a filling remains sound. Under other circumstances a non-radiopaque composite may be desir- able.

The amount of filler which is incorporated into the composite (referred to herein as the"loading level"and expressed as a weight percent based on the total weight of the dental mate- rial) will vary depending on the type of filler, the epoxy resin and other components of the composition, and the end use of the composite.

For some dental materials (e. g., sealants), the epoxy resin compositions of the invention can be lightly filled (e. g., hav- ing a loading level of less than about 40 weight percent) or unfilled. Preferably the viscosity of the dental material is sufficiently low to allow its penetration into pits and fis- sures of occlusal tooth surfaces as well as into etched areas of enamel, thereby aiding in the retention of the dental mate- rial. In applications where high strength or durability are de- sired (e. g., anterior or posterior restoratives, prostheses, crown and bridge cements, artificial crowns, artificial teeth and dentures) the loading level can be as high as about 95 weight percent. For most dental restorative and prosthetic ap- plications a loading level of between about 70 and 90 weight percent is generally preferred.

Fillers may be selected from one or more of any material suit- able for incorporation in compositions used for medical appli- cations, such as fillers currently used in dental restorative compositions and the like. The filler is finely divided and preferably has a maximum particle diameter less than about 50 micrometers and an average particle diameter less than about 10 micrometers. The filler can have a unimodal or polymodal (e. g., bimodal) particle size distribution. The filler can be an inor- ganic material. It can also be a crosslinked organic material that is insoluble in the polymerizable resin, and is optionally filled with inorganic filler. The filler should in any event be non-toxic and suitable for use in the mouth. The filler can be radiopaque, radiolucent or nonradiopaque.

Examples of suitable inorganic fillers are naturally-occurring or synthetic materials such as quartz, nitrides (e. g., silicon nitride), glasses derived from, for example Ce, Sb, Sn, Zr, Sr, Ba and Al, colloidal silica, feldspar, borosilicate glass, kao- lin, talc, titania, and zinc glass; low Mohs hardness fillers such as those described in U. S. Pat. No. 4,695,251; and submi- cron silica particles (e. g., pyrogenic silicas such as the "Aerosil"Series"OX 50","130","150"and"200"silicas sold by Degussa and"Cab-O-Sil M5"silica sold by Cabot Corp.). Ex- amples of suitable organic filler particles include filled or unfilled pulverized polycarbonates, polyepoxides, and the like.

Preferred filler particles are quartz, submicron silica, and non-vitreous microparticles of the type described in U. S. Pat.

No. 4,503,169. Metallic fillers may also be incorporated, such as particulate metal filler made from a pure metal such as those of Groups IVA, VA, VIA, VIIA, VIII, IB, or IB, aluminum, indium, and thallium of Group IIIB, and tin and lead of Group IVB, or alloys thereof. Conventional dental amalgam alloy pow- ders, typically mixtures of silver, tin, copper, and zinc, may also optionally be incorporated. The particulate metallic filler preferably has an average particle size of about 1 mi- cron to about 100 microns, more preferably 1 micron to about 50 microns. Mixtures of these fillers are also contemplated, as well as combination fillers made from organic and inorganic ma- terials. Fluoroaluminosilicate glass fillers, either untreated or silanol treated, are particularly preferred. These glasses have the added benefit of releasing fluoride at the site of dental work when placed in the oral environment.

Optionally, the surface of the filler particles may be treated with a surface treatment such as a coupling agent in order to enhance the bond between the filler and the polymerizable resin. The coupling agent may be functionalized with reactive curing groups, such as epoxies, vinylethers, acrylates, metha- crylates and the like. Examples of coupling agents include gly- cidyltrimethoxysilane, O (vinyloxyethyl)-N- (triethoxysilyl- propyl) urethane, gamma-methacryloxypropyltrimethoxysilane, gamma-mercaptopropyltriethoxysilane, gamma-aminopropyltri- methoxysilane, beta- (3, 4-epoxycyclohexyl) ethyltrimethoxysilane, and the like.

The dental materials of the present invention can also contain suitable adjuvants such as accelerators, inhibitors, absorbers, stabilizers, pigments, dyes, viscosity modifiers, surface ten- sion depressants and wetting aids, antioxidants, and other in- gredients well known to those skilled in the art.

The amounts and types of each ingredient in the dental material should be adjusted to provide the desired physical and handling properties before and after cure. For example, the cure rate, cure stability, fluidity, compressive strength, tensile strength and durability of the dental material typically are adjusted in part by altering the types and amounts of polymeri- zation initiator (s) and, if present, the loading and particle size distribution of filler (s). Such adjustments typically are carried out empirically based on experience with dental materi- als of the prior art.

When the dental material is applied to a tooth, the tooth can optionally be pre-treated with a primer such as dentin or enamel adhesive by methods known to those skilled in the art.

Particularly useful compositions of the present invention are visible-light photopolymerizable dental composition useful for polymerization in the oral environment. These compositions com- prise a) a cationically polymerizable epoxy resin, b) an aryli- odonium salt, c) a titanocene derivate and optionally d) a den- tal filler present in the composition. These components are present in amounts sufficient to provide cure of said photo- polymerizable resin by exposure to visible light to a cure depth of at least about 1 mm. The compositions may additionally comprise cationically curable resins selected from oxetanes, oxolanes, cyclic acetals, lactams, lactones and vinyl ethers.

The compositions may additionally comprise radically curable resins selected from acrylates, methacrylates, allyl compounds, vinyl benzene compounds or other unsaturated compound suitable for radical polymerisation.

The inventive compositions can be used for the restauration of teeth and for fixing brackets, crowns, prostheses or inlays in teeth. Unless otherwise indicated, all parts and percentages are by weight, and all molecular weights are weight average mo- lecular weight.

The compositions of the invention are as well very useful for industrial applications, e. g. for coatings on metal, glass, fabrics, paper, creating negative print reliefs for the print- ing industry, conductive plates, visible light cure glues or sealants, building up moulds by visible light. An important ad- vantage is the deeper depth of cure in comparison to UV light, especially in pigmented systems and the less harmfulness of visible light.

EXAMPLES EXAMPLE 1 A mixture of 17.8 grams epoxy resin ERL4221 fom Union Carbide, 3.35 grams vinylether DVE-3 from ISP TECHNOLOGIES, INC., 0.2200 grams bis-cyclopentadienyl-bis [2, 6-bifluoro-3-N-pyrrolylphenyl]- titanium and 0.2935 grams of (4-n-Dodecyloxyphenyl) phenyl- iodonium-hexafluoroantimonate was combined and stirred in the absence of light. 5.60 grams of the above obtained light cur- able epoxy system was filled with 12 grams of a silane-treated filler (Barium-Glas GM27884 SCHOTT Landshut). The composite is stable at room temperature in the absence of light.

EXAMPLE 2 A mixture of 17.8 grams epoxy resin ERL4221 fom Union Carbide, 3.35 grams vinylether DVE-3 from ISP TECHNOLOGIES, INC., 0.2200 grams bis-cyclopentadienyl-bis [2, 6-bifluoro-3-N-pyrrolylphenyl] titanium and 0.2935 grams of (4-n-Dodecyloxyphenyl) phenyl- iodonium-hexafluoroantimonate was combined and stirred in the absence of light. 5.60 grams of the above obtained light cur- able epoxy system was filled with 12 grams of a silane-treated filler (F-Quarz-066, Industrial Corporation). The composite is stable at room temperature in the absence of light.

EXAMPLE 3 A mixture of 17.8 grams epoxy resin ERL4221 fom Union Carbide, 3.35 grams vinylether DVE-3 from ISP TECHNOLOGIES, INC., 0.2200 grams bis-cyclopentadienyl-bis [2,6-bifluoro-3-N-pyrrolylphenyl] titanium and 0.2935 grams of (4-n-Dodecyloxyphenyl) phenyl- iodonium-hexafluoroantimonate was combined and stirred in the absence of light. 5. 60 grams of the above obtained light cur- able epoxy system was filled with 13 grams of a silane-treated filler (SILBOND FW12 EST, Quarzwerke). The composite is stable at room temperature in the absence of light.

Example for Titan Catalyst Mixture A catalyst mixture is prepared by dissolving 2,9 g (4-n- Dodecyloxyphenyl) phenyliodoniumhexafluoroantimonat and 2,2 g Bis-cyclopentadienyl-bis (2,6-bifluoro-3-N-pyrollylphenyl)- titanium in 10 g acetone (thoroughly stirring in the absence of light).

This catalyst mixture is able to harden the mentioned monomers.

For example, 1,51 g of this catalyst mixture is able to cure a monomer mixture of 8,9 g epoxy resin ERL4221 (Union Carbide) and 1,68 g vinylether DVE-3 (ISP Technologies) to a depth of cure of 4,6 mm (90s, lamp: Kulzer UniXS).

Table 1: Properties of Examples Example 1 Example 2 Example 3 Starter: Ti-Kat/Iodonium Ti-Kat/Iodonium Ti-Kat/Iodonium Monomer: ERL 4221/DVE-3 ERL 4221/DVE-3 ERL 4221/DVE-3 Filler : Ba-Glas (epoxi-F-Quarz (epoxi-Silbond FW12EST dized) dized) Depth of Cure 1 3. 0 4. 7 4. 1 min light cure [mm] Translucency very good low very low 2mm Transverse 56. 8 78. 5 86. 1 strength [MPa] Compressive 190 240 140 strength [N/mm2] Diamet.tensile 32 28 46 strength [N/mm2] Estetic toothlike opaque greyopaque The examples show similar mechanical strength like widely used light cure compositions based on methacrylate resins.