Photopolymerizable Epoxy-Containing Compositions

1. Field of the Invention

This invention relates to photopolymerizable compositions. More particularly, this invention relates to compositions which comprise an epoxy-containing material and an aromatic iodonium complex salt as photoinitiator and which can be cured by exposure to actinic radiation.

2. Description of the Prior Art

Although photopolymerizable epoxy compositions comprising epoxy resin materials and photosensitizers have been desired for some time, those which have previously been proposed suffer from one or more drawbacks. Thus, in U.S. Pat. No. 3,074,869 there are disclosed photosensitive epoxy compositions containing a nitrosoamine as photosensitizer. Compositions of this type require relatively long exposure to a high intensity light source to produce complete polymerization. In U.S. Pat. Nos. 3,205,157 and 3,708,296, there are disclosed photosensitive epoxy compositions containing respectively aryldiazonium salts and aryldiazonium salts of halogen-containing complex anions. Such compositions have limited usefulness because they have poor thermal stability, because their spectral response is limited to the ultraviolet region of the spectrum, and because nitrogen is evolved during photopolymerization causing pinholes and bubbles in heavy coatings of the composition. In U.S. Pat. No. 3,450,613 there is described another photopolymerizable epoxy composition comprising the reaction product of an epoxy resin prepolymer and an ethylenically unsaturated organic acid, a photosensitizer, and optionally polyfunctional acids or bases. This composition on exposure to ultraviolet light gels by reason of the photoinduced polymerization of the ethylenically unsaturated portion of the reaction product. Completion of the cure of the composition is effected by heating to bring about reaction of the epoxy resin portion of the composition. Such compositions as this have limited usefulness because of the requirement of both light and heat to effect complete polymerization of the composition. The composition furthermore is oxygen sensitive and has poor thermal stability.

The present invention provides novel photopolymerizable compositions, comprising epoxy-containing materials and a photoinitiator, which are sensitive throughout the ultraviolet and visible spectral regions, i.e., wavelengths from about 300 to 700 millimicrons and which may be photocured by exposure for relatively short periods of time to radiation within this range of wavelengths.

In accordance with the present invention there are provided photopolymerizable epoxy compositions which are readily photocured by exposure to actinic radiation comprising an organic compound having an epoxy functionality of at least 1 and certain photosensitive aromatic iodonium complex salts as photoinitiator and, optionally, a sensitizer for said photoinitiator. The photopolymerizable epoxy compositions of the invention are sensitive throughout the ultraviolet and visible spectral regions and photocure rapidly, without use of heat, to epoxy polymers which possess inherent superior toughness; abrasion resistance; adhesion to metal, glass, plastic, wood and other surfaces; and resistance to chemical attack. The aromatic iodonium complex salt photoinitiator suitable for use in the compositions of the invention can be defined by the formula ##STR1## wherein Ar.sup.1 and Ar.sup.2 are the same or different aromatic groups known in the art and Z is an oxygen atom, a carbon-to-carbon bond joining Ar.sup.1 and Ar.sup.2, or ##STR2## where R.sub.1 and R.sub.2 are selected from the group consisting of hydrogen, an alkyl radical having 1 to 4 carbon atoms, and an alkenyl radical having 2 to 4 carbon atoms, and n is zero or 1; X.sup.- is a halogen-containing complex anion selected from the group consisting of tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate, hexachlorostannate, hexafluorostannate, hexachloroantimonate, and hexafluoroantimonate.

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 XXIII, J. Chem. Soc. 1964, 442-51.

Representative Ar.sup.1 and Ar.sup.2 groups are aromatic groups having 4 to 20 carbon atoms selected from phenyl, thienyl, and furanyl groups. These aromatic groups may optionally have one or more fused benzo rings (e.g., naphthyl and the like; benzothienyl, dibenzothienyl; benzofuranyl, dibenzofuranyl; etc.). Such aromatic groups may also be substituted, if desired, by one or more of the following groups: halogen, nitro, hydroxyl, carboxyl, anilino or N-alkylanilino groups, ester groups (e.g., alkoxycarboxyl such as methoxycarbonyl and ethoxycarbonyl, phenoxycarbonyl), sulfo ester groups (e.g., alkoxysulfonyl such as methoxysulfonyl and butoxysulfonyl, phenoxysulfonyl, and the like), amido groups (e.g., acetamido, butyramido, ethylsulfonamido, 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), alkyl 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., phenylsulfonyl groups), perfluoroalkyl groups (e.g., trifluoromethyl, perfluoroethyl, and the like), and perfluoroalkylsulfonyl groups (e.g., trifluoromethylsulfonyl, perfluorobutylsulfonyl, and the like).

Suitable examples of the aromatic iodonium complex salt photoinitiators include:

diphenyliodonium tetrafluoroborate

di(4-methylphenyl)iodonium tetrafluoroborate

phenyl-4-methylphenyliodonium tetrafluoroborate

di(4-heptylphenyl)iodonium tetrafluoroborate

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 tetrafluoroborate

diphenyliodonium hexachlorostannate

phenyl-2-thienyliodonium hexafluorophosphate

diphenyliodonium hexafluorostannate

diphenyliodonium hexachloroantimonate

diphenyliodonium hexafluoroantimonate

2,2'-diphenyliodonium tetrafluoroborate

di(2,4-dichlorophenyl)iodonium hexafluorophosphate

di(4-bromophenyl)iodonium hexafluorophosphate

di(4-methoxyphenyl)iodonium hexafluorophosphate

di(3-carboxyphenyl)iodonium hexafluorophosphate

di(3-methoxycarbonylphenyl)iodonium hexafluorophosphate

di(3-methoxysulfonylphenyl)iodonium hexafluorophosphate

di(4-acetamidophenyl)iodonium hexafluorophosphate

di(2-benzothienyl)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 such as diphenyliodonium hexafluorophosphate. These salts are preferred because, in general, they 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 metathesis 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). Thus, for example, the complex salt diphenyliodonium tetrafluoroborate is prepared by the addition at 60.degree. C. of an aqueous solution containing 29.2 g. (150 millimoles) silver fluoroborate, 2 g. fluoroboric acid, and 0.5 g. phosphorous acid in about 30 ml. of water to a solution of 44 g. (139 millimoles) of diphenyliodonium chloride. The silver halide that precipitates is filtered off and the filtrate concentrated to yield diphenyliodonium fluoroborate which may be purified by recrystallization.

The aromatic iodonium simple salts may be prepared in accordance with Beringer et al., above, by various methods including (1) coupling of two aromatic compounds with iodyl sulfate in sulfuric acid, (2) coupling of two aromatic compounds with an iodate in acetic acid-acetic anhydride-sulfuric acid, (3) coupling of two aromatic compounds with an iodine acylate in the presence of an acid, and (4) condensation of an iodoso compound, an iodoso diacetate, or an iodoxy compound with another aromatic compound in the presence of an acid. Diphenyliodonium bisulfate is prepared by method (3), for example, by the addition over a period of eight hours at below 5.degree. C. of a mixture of 35 ml. of conc. sulfuric acid and 50 ml. of acetic anhydride to a well-stirred mixture of 55.5 ml. of benzene, 50 ml. of acetic anhydride, and 53.5 g. of potassium iodate. The mixture is stirred for an additional four hours at 0.degree.-5.degree. C. and at room temperature for 48 hours and treated with 300 ml. of diethyl ether. On concentrating, crude diphenyliodonium bisulfate precipitates. If desired, it may be purified by recrystallization.

Epoxy-containing material useful in the compositions of the invention are any organic compounds having an oxirane ring 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 at least one polymerizable epoxy group per molecule (preferably two or more epoxy groups per molecule) and, in the polymeric type there are many pendent epoxy groups (e.g., a glycidyl methacrylate polymer could have several thousand pendent epoxy groups per average molecular weight).

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 substituent groups. For example, the backbone may be of any type and substituent groups thereon can be any group not having an active hydrogen atom which is reactive with an oxirane ring. Illustrative of permissible substituent groups include halogens, ester groups, ethers, sulfonate groups, siloxane groups, nitro groups, amide groups, nitrile groups, phosphate groups, etc. The molecular weight of the epoxy-containing materials may vary from 58 to about 100,000 or more. Mixtures of various epoxy-containing materials can also be used in the compositions of this invention.

Such epoxy-containing materials are well known and include such epoxides as epichlorohydrins, e.g., epichlorohydrin; alkylene oxides, e.g., propylene oxide, styrene oxide; alkenyl oxides, e.g., butadiene oxide; glycidyl esters, e.g., ethyl glycidate, glycidyl-type epoxy resins, e.g., the diglycidyl ethers of Bisphenol A and of novolak resins, such as described in "Handbook of Epoxy Resins" by Lee and Neville, McGraw-Hill Book Co., New York (1967).

Other useful epoxy-containing materials which can be used in this invention are those which contain one or more cyclohexene oxide groups such as the epoxycyclohexanecarboxylates, typified by 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, 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 useful in the practice of this invention include glycidyl ether monomers of the formula ##STR3## where R is alkyl or aryl and n is an integer of 1 to 6. Examples are the 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-epoxy propoxyphenol)-propane). Further examples of epoxides of this type which can be used in the practice of this invention are described in U.S. Pat. No. 3,018,262, incorporated herein by reference.

There are a host of commercially available epoxy-containing materials which can be used in this invention. In particular, epoxides which are readily available include propylene oxide, epichlorohydrin, styrene oxide, vinyl cyclohexene oxide, glycidol, glycidylmethacrylate, diglycidyl ether of Bisphenol A (e.g., those available under the trade designations "Epon 828" from Shell Chemical Co., "DER 331", "DER 332", and DER 334", from Dow Chemical Co.), vinylcyclohexene dioxide (e.g., "ERL-4206" from Union Carbide Corp.), 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexene carboxylate (e.g., "ERL 4221" from Union Carbide Corp.), 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexene 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-epoxy-cyclopentyl)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.), dipentene 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 epoxy resin available from Dow Chemical Co.), 1,4-butanediol diglycidyl ether (e.g., "Araldite RD-2" from Ciba-Geigy), polyglycidyl 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.).

Still other epoxy-containing materials are copolymers of acrylic acid esters of glycidol such as glycidylacrylate and glycidylmethacrylate with one or more copolymerizable vinyl compounds. Examples of such copolymers are 1:1 styrene-glycidylmethacrylate, 1:1 methylmethacrylate-glycidylacrylate and a 62.5:24:13.5 methylmethacrylate-ethyl acrylate-glycidylmethacrylate.

Still other epoxy-containing materials are the polyurethane polyepoxides which are obtained by reacting an organic polyisocyanate with a triol or a mixture of a triol and diol to form an isocyanate terminated polyurethane prepolymer and reacting the prepolymer with a hydroxy aliphatic epoxide compound. Further examples of epoxy-containing material of this type which can be used in the practice of this invention are described in U.S. Pat. No. 3,445,436, incorporated herein by reference.

The photopolymerizable compositions of the invention comprising epoxy-containing materials and aromatic iodonium complex salt photoinitiators can be used as adhesives, caulking and sealing compounds, casting and molding compounds, potting and encapsulating compounds, impregnating and coating compounds, etc., depending on the particular epoxy materials and aromatic iodonium complex salt used. The photopolymerizable composition can be used as a one-component cured-in-place composition. The photopolymerizable compositions are particularly suitable in a variety of applications in the field of graphic arts due to their superior abrasion-resistance and adhesion to rigid, resilient and flexible substrates such as metal, plastic, rubber, glass, paper, wood, and ceramics; their excellent resistance to most solvents and chemicals; and their capability of forming high resolution images. Among such uses are in making acid- and alkali-resist images for chemical milling, gravure images, offset plates, flexographic printing, screenless lithography, printing plates, stencil making, microimages for printed circuitry, microimages for information storage, decorations of paper, glass, and metal surfaces, and light-cured coatings.

The photopolymerization of the compositions of the invention occurs on exposure of the compositions to any source of radiation emitting actinic radiation at a wavelength within the ultraviolet and visible spectral regions. Suitable sources of radiation include mercury, xenon, carbon arc and tungsten filament lamps, sunlight, etc. Exposures may be from less than about 5 seconds to 10 minutes or more depending upon the amounts and particular epoxy materials and aromatic iodonium complex salts being utilized and depending upon the radiation source and distance from the source.

While not desiring to be bound by theory, the photopolymerization apparently takes place by reason of the photoinduced degradation of the aromatic iodonium complex salt with the production of a Lewis acid which catalyzes the polymerization of the epoxy material. The curing is a triggered reaction, i.e., once the degradation of the aromatic iodonium complex salt has been initiated by exposure to a radiation source, the curing reaction proceeds and will continue even after the radiation source is removed.

The aromatic iodonium complex salts useful in the photopolymerizable epoxy compositions of the invention are of themselves photosensitive only in the ultraviolet. They, however, are sensitized to the near ultraviolet and the visible range of the spectrum by sensitizers for known photolyzable organic halogen compounds in accordance with the teachings of U.S. Pat. No. 3,729,313 which is incorporated herein by reference. Illustrative sensitizers are found in the following categories: aromatic amines, aminoketones, and colored aromatic polycyclic hydrocarbons. The use of basic amino compounds is avoided since such compounds tend to inhibit the polymerization of the epoxy material.

The amount of aromatic iodonium complex salt that may be employed in the compositions of the invention is from about 0.5 to 30 parts per 100 parts of epoxy-containing material and preferably from about 2 to 7 parts per 100 parts of epoxy-containing material. For those compositions wherein a sensitizer is used to make the composition sensitive to radiation in the visible range, about 0.01 to 1.0 parts and preferably about 0.1 to 1.0 parts by weight of sensitizer per part of aromatic iodonium complex salt may be employed.

The photopolymerizable compositions of the invention are prepared by simply admixing, under "safe light" conditions, the aromatic iodonium complex salt and the sensitizer, when used, with the epoxy-containing material. Suitable inert solvents may be employed if desired when effecting this mixture. Examples of suitable solvents are acetone, acetonitrile, methanol and includes any solvent which does not react appreciably with the epoxy-containing material, the aromatic iodonium complex salt or the sensitizer. A liquid epoxy material may be used as a solvent for another liquid or solid epoxy material. A solvent, however, is generally used only to aid in providing a suitable viscosity to the composition for purposes of coating. Solventless compositions can be prepared by simply dissolving the aromatic iodonium complex salt and sensitizer in the epoxy-containing material with or without the use of mild heating.

In the following examples which will serve to illustrate the present invention, all parts are parts by weight and all percentages are given as percentages by weight, unless otherwise indicated.

In separate examples the various amounts of diphenyliodonium hexafluorophosphate (designated .phi..sub.2 IPF.sub.6) shown in Table I were added to 5 parts of "DER-331" (a diglycidyl ether of Bisphenol A available from Dow Chemical Co. having an epoxy equivalency of about 190) and well mixed at about 50.degree. C. Each portion was then wire rod coated at 2.3 mils (0.006 cm.) thick onto a 2 mil (0.005 cm.) polyester film. Samples of the coating were then exposed to a 275 watt General Electric RS sunlamp at a distance of 5 inches (12.7 cm.). The time required for the surface of the coating to become tack-free is listed in Table I.

It may be observed by inspection of Table I that the time required for the epoxy composition to become tack-free decreases from 120 seconds at 1% concentration of diaryliodonium complex salt to 25 second at 5% of the salt. Increase of the salt concentration to 6% does not further decrease the tack time because at 6%, the solubility of the salt in "DER-331" has been exceeded.

In separate examples, coatings were prepared as in Examples 1-6 from photopolymerizable compositions containing the various epoxy materials and the various amounts of the various aromatic iodonium complex salts shown in Table II, the coatings then being exposed to a General Electric sunlamp at 5 inches (12.7 cm.) or to a General Electric "H3T7" mercury lamp at 7 inches (17.8 cm.). The time required for the surface of the coating to become tack-free is also reported in Table II.

Photopolymerizable compositions were prepared and coated as described in Examples 1-6 using the epoxy-containing materials, aromatic iodonium complex salt, and sensitizing dye (wt. % based on wt. of complex salt) shown in Table III. The time required for the surface of the coating of each composition to become tack-free is presented in Table III.

Solution of 20 grams "DER-331", 1.0 gram diphenyliodonium hexafluorophosphate, and 0.1 gram 2-ethyl-9,10-dimethoxyanthracene was coated at 30.0 mils (0.076 cm.) thick on 2 mil (0.005 cm.) thick polyester film. After exposure of the coated film for 5 minutes at 5 inches (12.7 cm.) from a General Electric sunlamp, a hard cured sample was obtained, the coating being firmly bonded to the film. The cured sample was completely transparent and contained no bubbles. Unexposed portions of the coating can be washed away with acetone leaving a sharp, clearly defined image area.

A solution was prepared containing 5 grams of a 5% solution of a 1:1 styrene-glycidylmethacrylate copolymer in acetone, 0.01 gram of diphenyliodonium hexafluorophosphate, and 0.005 grams 2-ethyl-9,10-dimethoxyanthracene. It was coated on 2 mil (0.005 cm.) thick polyester film at a wet thickness of 2.3 mils (0.006 cm.) and air dried. A sample was exposed to a General Electric H3T7 lamp at 7 inches (17.8 cm.) distance through a .sqroot.2 photographic step tablet for 3 minutes and dipped in acetone to remove the unexposed areas. Seven steps of polymerized material remained.

As another example of the utility of the compositions described herein a photopolymerizable varnish is prepared which is useful for coating fishing rod windings. The composition is prepared with the following ingredients:

The "DER-334" and phenyl glycidyl ether are mixed with gentle heating while the diphenyl iodonium complex salt and sensitizer are added thereto with stirring. A clear photopolymerizable solution is obtained.

The photopolymerizable solution is coated onto the nylon thread windings of a fishing rod guide. The thread had been previously heated to about 50.degree. C. so that the solution would saturate rapidly into the windings when applied thereto with a brush. The impregnated windings are then exposed to a 275 watt sunlamp at a distance of about 7.5 centimeters for about two minutes, the fishing rod being rotated slowly during exposure. After exposure in the manner described a second application of photopolymerizable solution to the windings is made, followed by exposure to a sunlamp to cure the coating. A smooth, varnished winding with high gloss and durability is obtained.

This technique for coating windings on fishing rods is much faster and simpler than with conventional techniques wherein solvent-based lacquers and varnishes are used.