Ternary photoinitiator system for addition polymerization

Abstract: Photocurable addition-polymerizable compositions containing a free-radically-polymerizable monomer and a photoinitiator system containing i) an arylidonium salt, ii) a sensitizing compound, and iii) an electron donor having an oxidation potential that is greater than zero and less than or equal to that of p-dimethyoxybenzene (1.32 volts vs. S.C.E.). The compositions cure rapidly and deeply under ultraviolet or visible light.

Description: TECHNICAL FIELD This invention relates to photoinitiator systems for use in addition (free radically-initiated) polymerization. BACKGROUND ART Aryliodonium salts have been previously described for use as photoinitiators in addition-polymerizable compositions. References relating to such compositions include U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,228,232, 4,250,053 and 4,428,807; H.J. Timpe and H. Baumann, Wiss. Z. Tech. Hochsch. Leuna-Merseburg , 26 , 439 (1984); H. Baumann, B. Strehmel, H. J. Timpe and U. Lammel, J. Prakt. Chem , 326 (3), 415 (1984); and H. Baumann, U. Oertel and H. J. Timpe, Euro. Polym. J. , 22 (4), 313 (April 3, 1986). US-A-4,228,232 (Rousseau) mentioned above discloses photopolymerizable compositions which comprise an ethylenically unsaturated carboxylic acid containing monomer, various other monomers and a free radical photoinitiator system. In one example Rousseau discloses that a solution was prepared from 0.3 grams 0.4 grams pentaerythritol tetraacrylate, 0.22 grams of a carboxy substituted urethane oligomer (P-II), 0.04 grams diphenyliodonium hexafluorophosphate 0.02 grams of (CEBH free radical initiator), 1.28 grams methylethyl ketone, 0.47 grams water, 7.36 grams isopropanol and 6 drops of a 20% by weight solution of triethylamine in n-propanol. The solution was coated with wire wound rods onto grained and anodized aluminum at coating weights of 0.95, 1.36 and 2.04 grams/m 2 . Some of the coatings were overcoated with a 10% by weight solution of poly(vinyl alcohol) using a #8 wire wound rod. The coatings were exposed using an argon laser at 488nm and developed by wiping with a pad and an aqueous solution of 0.35% sodium metasilicate and 0.25% sodium (lower alkyl)naphthalene sulfonate. The laser beam was passed through a 21 step sensitivity guide. However the small amount of triethylamine is just sufficient to neutralize the acid in oligomer P-II, and thus the triethylamine does not serve as a donor. Where bases are used in Rousseau the amount is always just sufficient to neutralize the acid and there is no donor action. Mono- and di-ketones have also previously been described for use as photoinitiators in addition-polymerizable compositions. References relating to such compositions include U.S. Pat. Nos. 3,427,161, 3,756,827, 3,759,807 and 4,071,424; U.K. Pat. Specification No. 1,304,112; European Published Pat. Appl. No. 150,952 and Chem. Abs. 95:225704U. SUMMARY OF THE INVENTION The subject invention provides an addition-polymerizable composition comprising: a) free-radically-polymerizable monomer ("monomer"), and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts of i) aryliodonium salt ("iodonium salt"), ii) sensitizing compound ("sensitizer") capable of absorbing light somewhere within the range of wavelengths between 300 and 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound ("donor") having oxidation potential E ox wherein said donor is different from said sensitizing compound and zero < E ox (donor) ≤ E ox (p-dimethoxybenzene), wherein said electron donor compound acts as an electron donor to at least one of said aryliodonium salt (i) and said sensitizing compound (ii). The compositions of the invention provide a very useful combination of cure speed, cure depth and shelf life. They cure well even when loaded with large amounts of fillers, and can be used in a variety of applications including graphic arts imaging (e.g., for color proofing systems, curable inks, or silverless imaging), printing plates (e.g., projection plates or laser plates), photoresists, solder masks, coated abrasives, magnetic media, photocurable adhesives (e.g., for orthodontics) and photocurable composites (e.g., for autobody repair or dentistry). The invention also provides a process for preparing photopolymerized compositions, and polymerized articles made therefrom. DETAILED DESCRIPTION OF THE INVENTION A wide variety of monomers can be photopolymerized using the photoinitiator system of the invention. Suitable monomers contain at least one ethylenically-unsaturated double bond, can be oligomers, and are capable of undergoing addition polymerization. Such monomers include mono-, di- or poly- acrylates and methacrylates such as methyl acrylate, methyl methacrylate, ethyl acrylate, isopropyl methacrylate, n-hexyl acrylate, stearyl acrylate, allyl acrylate, glycerol diacrylate, glycerol triacrylate, ethyleneglycol diacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, 1,3-propanediol diacrylate, 1,3-propanediol dimethacrylate, trimethylolpropane triacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, sorbitol hexacrylate, bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane, bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane, trishydroxyethyl-isocyanurate trimethacrylate; the bis-acrylates and bis-methacrylates of polyethylene glycols of molecular weight 200-500, copolymerizable mixtures of acrylated monomers such as those of U.S. Pat. No. 4,652,274, and acrylated oligomers such as those of U.S. Pat. No. 4,642,126; unsaturated amides such as methylene bis-acrylamide, methylene bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine tris-acrylamide and beta-methacrylaminoethyl methacrylate; and vinyl compounds such as styrene, diallyl phthalate, divinyl succinate, divinyl adipate and divinylphthalate. Mixtures of two or more monomers can be used if desired. The monomer is combined with a three component or ternary photoinitiator system. The first component in the photoinitiator system is the iodonium salt, i.e., a diaryliodonium salt. The iodonium salt should be soluble in the monomer and preferably is shelf-stable (i.e., does not spontaneously promote polymerization) when dissolved therein in the presence of the sensitizer and donor. Accordingly, selection of a particular iodonium salt may depend to some extent upon the particular monomer, sensitizer and donor chosen. Suitable iodonium salts are described in U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,250,053 and 4,394,403. The iodonium salt can be a simple salt (e.g., containing an anion such as Cl - , Br - , I - or C 6 H 5 SO 3 - ) or a metal complex salt (e.g., containing an anion such as BF 4 - , PF 6 - , SbF 6 - , SbF 5 OH - or AsF 6 - ). Mixtures of iodonium salts can be used if desired. Preferred iodonium salts include diphenyliodonium salts such as diphenyliodonium chloride, diphenyliodonium hexafluorophosphate and diphenyliodonium tetrafluoroborate. The second component in the photoinitiator system is the sensitizer. The sensitizer should be soluble in the monomer, and is capable of light absorption somewhere within the range of wavelengths between about 300 and about 1000 nanometers, more preferably about 400 and about 700 nanometers and most preferably about 400 to about 600 nanometers. The sensitizer is also capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, using the test procedure described in U.S. Pat. No. 3,729,313. Using currently available materials, that test is carried out as follows. A standard test solution is prepared having the following composition: 5.0 parts of a 5% (weight by volume) solution in methanol of 45,000-55,000 molecular weight, 9.0-13.0% hydroxyl content polyvinyl butyral ("Butvar B76", Monsanto) 0.3 parts trimethylolpropane trimethacrylate 0.03 parts 2-methyl-4,6-bis(trichloromethyl)-s-triazine (see Bull. Chem. Soc. Japan , 42 , 2924-2930, 1969). To this solution is added 0.01 parts of the compound to be tested as a sensitizer. The solution is knife-coated onto a 0.05mm clear polyester film using a knife orifice of 0.05mm, and the coating is air dried for about 30 minutes. A 0.05mm clear polyester cover film is carefully placed over the dried but soft and tacky coating with minimum entrapment of air. The resulting sandwich construction is then exposed for three minutes to 161,000 Lux of incident light from a tungsten light source providing light in both the visible and ultraviolet range ("FCH" 650 watt quartz-iodine lamp, General Electric). Exposure is made through a stencil so as to provide exposed and unexposed areas in the construction. After exposure the cover film is removed and the coating is treated with a finely divided colored powder, such as a color toner powder of the type conventionally used in xerography. If the tested compound is a sensitizer, the trimethylolpropane trimethacrylate monomer will be polymerized in the light-exposed areas by the light-generated free radicals from the 2-methyl-4,6-bis(trichloromethyl)-s-triazine. Since the polymerized areas will be essentially tack-free, the colored powder will selectively adhere only to the tacky, unexposed areas of the coating, providing a visual image corresponding to that in the stencil. Preferably, in addition to passing the above test, a sensitizer is also selected based in part upon shelf stability considerations. Accordingly, selection of a particular sensitizer may depend to some extent upon the particular monomer, iodonium salt and donor chosen. Suitable sensitizers are believed to include compounds in the following categories: ketones, coumarin dyes (e.g., ketocoumarins), xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes and pyridinium dyes. Ketones (e.g., monoketones or alpha-diketones), ketocoumarins, aminoarylketones and p-substituted aminostyryl ketone compounds are preferred sensitizers. For applications requiring high sensitivity (e.g., graphic arts), it is preferred to employ a sensitizer containing a julolidinyl moiety. For applications requiring deep cure (e.g., cure of highly-filled composites), it is preferred to employ sensitizers having an extinction coefficient below about 1000, more preferably below about 100, at the desired wavelength of irradiation for photopolymerization. By way of example, a preferred class of ketone sensitizers has the formula: ACO(X) b B where X is CO or CR 1 R 2 , where R 1 and R 2 can be the same or different, and can be hydrogen, alkyl, alkaryl or aralkyl, b is zero or 1, and A and B can be the same or different and can be substituted (having one or more non-interfering substituents) or unsubstituted aryl, alkyl, alkaryl, or aralkyl groups, or together A and B can form a cyclic structure which can be a substituted or unsubstituted cycloaliphatic, aromatic, heteroaromatic or fused aromatic ring. Suitable ketones of the above formula include monoketones (b=0) such as 2,2-, 4,4- or 2,4-dihydroxybenzophenone, di-2-pyridyl ketone, di-2-furanyl ketone, di-2-thiophenyl ketone, benzoin, fluorenone, chalcone, Michler's ketone, 2-fluoro-9-fluorenone, 2-chlorothioxanthone, acetophenone, benzophenone, 1- or 2-acetonaphthone, 9-acetylanthracene, 2-, 3- or 9-acetylphenanthrene, 4-acetylbiphenyl, propiophenone, n-butyrophenone, valerophenone, 2-, 3- or 4-acetylpyridine, 3-acetylcoumarin and the like. Suitable diketones include aralkyldiketones such as anthraquinone, phenanthrenequinone, o-, m- and p-diacetylbenzene, 1,3-, 1,4-, 1,5-, 1,6-, 1,7- and 1,8-diacetylnaphthalene, 1,5-, 1,8- and 9,10-diacetylanthracene, and the like. Suitable α-diketones (b=1 and x=CO) include 2,3-butanedione, 2,3-pentanedione, 2,3-hexanedione, 3,4-hexanedione, 2,3-heptanedione, 3,4-heptanedione, 2,3-octanedione, 4,5-octanedione, benzil, 2,2'-, 3,3'- and 4,4'-dihydroxylbenzil, furil, di-3,3'-indolylethanedione, 2,3-bornanedione (camphorquinone), biacetyl, 1,2-cyclohexanedione, 1,2-naphthaquinone, acenaphthaquinone, and the like. Preferred ketocoumarins and p-substituted aminostyryl ketone compounds are listed in TABLE II, below. The third component in the photoinitiator system is the electron donor. A wide variety of donors can be employed. The donor is soluble in the monomer, and should meet the oxidation potential (E ox ) limitation discussed in more detail below. Preferably, the donor also is selected based in part upon shelf stability considerations. Accordingly, selection of a particular donor may depend in part on the monomer, iodonium salt and sensitizer chosen. Suitable donors are capable of increasing the speed of cure or depth of cure of a composition of the invention upon exposure to light of the desired wavelength. Also, the donor has an E ox greater than zero and less than or equal to E ox (p-dimethoxybenzene). Preferably E ox (donor) is between about 0.5 and 1 volts vs. a saturated calomel electrode ("S.C.E."). E ox (donor) values can be measured experimentally, or obtained from references such as N. L. Weinburg, Ed., Technique of Electroorganic Synthesis Part II Techniques of Chemistry , Vol. V (1975), and C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems (1970). Preferred donors include amines (including aminoaldehydes and aminosilanes), amides (including phosphoramides), ethers (including thioethers), ureas (including thioureas), ferrocene, sulfinic acids and their salts, salts of ferrocyanide, ascorbic acid and its salts, dithiocarbamic acid and its salts, salts of xanthates, salts of ethylene diamine tetraacetic acid and salts of tetraphenylboronic acid. The donor can be unsubstituted or substituted with one or more non-interfering substituents. Particularly preferred donors contain an electron donor atom such as a nitrogen, oxygen, phosphorus, or sulfur atom, and an abstractable hydrogen atom bonded to a carbon or silicon atom alpha to the electron donor atom. Preferred amine donor compounds include alkyl-, aryl-, alkaryl- and aralkyl-amines such as methylamine, ethylamine, propylamine, butylamine, triethanolamine, amylamine, hexylamine, 2,4-dimethylaniline, 2,3-dimethylaniline, o-, m- and p-toluidine, benzylamine, aminopyridine, N,N'-dimethylethylenediamine, N,N'-diethylethylenediamine, N,N'-dibenzylethylenediamine, N,N'-diethyl-1,3-propanediamine, N,N'-diethyl-2-butene-1,4-diamine, N,N'-dimethyl-1,6-hexanediamine, piperazine, 4,4'-trimethylenedipiperidine, 4,4'-ethylenedipiperidine, p-N,N-dimethylaminophenethanol and p-N,N-dimethylaminobenzonitrile; aminoaldehydes such as p-N,N-dimethylaminobenzaldehyde, p-N,N-diethylaminobenzaldehyde, 9-julolidine carboxaldehyde and 4-morpholinobenzaldehyde; and aminosilanes such as trimethylsilylmorpholine, trimethylsilylpiperidine, bis(dimethylamino)diphenylsilane, tris(dimethylamino)methylsilane, N,N-diethylaminotrimethylsilane, tris(dimethylamino)phenylsilane, tris(methylsilyl)amine, tris(dimethylsilyl)amine, bis(dimethylsilyl)amine, N,N-bis(dimethylsilyl)aniline, N-phenyl-N-dimethylsilylaniline and N,N-dimethyl-N-dimethylsilylamine. Tertiary aromatic alkylamines, particularly those having at least one electron-withdrawing group on the aromatic ring, have been found to provide especially good shelf stability. Good shelf stability has also been obtained using amines that are solids at room temperature. Good photographic speed has been obtained using amines that contain one or more julolidinyl moieties. Preferred amide donor compounds include N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-N-phenylacetamide, hexamethylphosphoramide, hexaethylphosphoramide, hexapropylphosphoramide, trimorpholinophosphine oxide and tripiperidinophosphine oxide. Suitable ether donor compounds include 4,4'-dimethoxybiphenyl, 1,2,4-trimethoxybenzene and 1,2,4,5-tetramethoxybenzene. Suitable urea donor compounds include N,N'-dimethylurea, N,N-dimethylurea, N,N'-diphenylurea, tetramethylthiourea, tetraethylthiourea, tetra-n-butylthiourea, N,N-di-n-butylthiourea, N,N'-di-n-butylthiourea, N,N-diphenylthiourea and N,N'-diphenyl-N,N'-diethylthiourea. The three components of the photoinitiator system are present in "photochemically effective amounts", that is, amounts of each component sufficient to enable the monomer to undergo photochemical gelation or hardening upon exposure to light of the desired wavelength. Preferably, for every 100 parts of monomer, the photocurable composition contains 0.005 to 10 parts (more preferably 0.1 to 4 parts) each of iodonium salt, sensitizer and donor. The amounts of each component are independently variable and thus need not be equal, with larger amounts generally providing faster cure, but shorter shelf life. Sensitizers with high extinction coefficients (e.g., above about 10,000) at the desired wavelength of irradiation for photopolymerization generally are used in reduced amounts. The photocurable composition can contain a wide variety of adjuvants depending upon the desired end use. Suitable adjuvants include solvents, diluents, resins, binders, plasticizers, pigments, dyes, inorganic or organic reinforcing or extending fillers (at preferred amounts of about 10% to about 90% by weight, based on the total weight of the composition), thixotropic agents, indicators, inhibitors, stabilizers, UV absorbers, medicaments (e.g., leachable fluorides) and the like. The amounts and types of such adjuvants, and their manner of addition to the composition will be familiar to those skilled in the art. The photocurable composition can be cured using a variety of methods. It is convenient to employ light sources that emit ultraviolet or visible light such as quartz halogen lamps, tungsten-halogen lamps, mercury arcs, carbon arcs, low-, medium-, and high-pressure mercury lamps, plasma arcs, light emitting diodes and lasers. Electron beam ("E-beam") irradiation and other curing devices that do not depend on light emission can also be employed. In general, heat or an inert atmosphere will accelerate cure. The following examples are offered to aid in understanding the invention and are not to be construed as limiting the scope thereof. Unless otherwise indicated, all parts and percentages are by weight. When used herein C.EX. means Comparative Example. EXAMPLE 1 Three stock solutions were prepared from 0.25 parts camphorquinone (CPQ), 50 parts triethyleneglycol dimethacrylate (TEGDMA) and 50 parts bisphenol A diglycidyl ether dimethacrylate (BisGMA). 0.50 Part diphenyliodonium hexafluorophosphate ( 2 I + PF 6 - ) was added to the first solution. 0.25 Part sodium p-toluenesulfinate (STS) was added to the second solution. 0.50 Part  2 I + PF 6 - and 0.25 part STS were added to the third solution. Each solution was poured into a 6mm diameter "Teflon" mold to a depth of 2.5 mm, covered with polyester film and irradiated for 10 seconds using a handheld visible light curing lamp ("Visilux", 3M) whose lightguide output end was placed directly on the polyester film. The solutions containing only CPQ and  2 I + PF 6 - or CPQ and STS formed a soft gel. The solution containing CPS,  2 I + PF 6 - and STS hardened to a solid having a Barcol hardness of 40 (ASTM D-2583) on both its top and bottom surfaces. In a further experiment, three stock solutions were prepared from 11.85 parts each of the above monomers, 76 parts filler, and 0.25 part CPQ. 0.25 Part  2 I + PF 6 - was added to the first solution. 0.25 Part N,N-dimethylaminophenethyl alcohol ("D-1") was added to the second solution. 0.25 Part  2 I + PF 6 - and 0.25 part D-1 were added to the third solution. Each solution was cured in a mold as described above, but using a 6mm deep mold and a 20 second cure time. The solution containing only  2 I + PF 6 - did not cure. The solution containing only D-1 had top and bottom Barcol hardness values of 56 and 2, respectively. The solution containing both  2 I + PF 6 - and D-1 had top and bottom Barcol hardness values of 60 and 30, respectively. The above data illustrates that an increased degree of polymerization and depth of cure can be obtained using a composition cured according to the invention. EXAMPLE 2 Equimolar amounts of a variety of donors were added to monomer stock solutions containing 50 parts trimethylolpropane trimethacrylate, 50 parts 1,4-butanediol dimethacrylate, 0.25 part CPQ and optionally 0.5 part of the iodonium salt  2 I + PF 6 - . The resulting solutions were irradiated with visible light at an intensity of 60 mW/cm 2 (as measured by a United Detector Technology Model 351 portable photometer/radiometer) at 400-500 nm. The solutions were stirred using a glass rod and the time required to reach the gelation point was recorded. Set out below in TABLE I are the run number, donor compound, E ox (donor), weight percent donor, and gel times for solutions prepared with and without the iodonium salt. The donors are listed in TABLE I in order of generally decreasing oxidation potential. Gel time, seconds Run No. Donor E ox (donor) % donor Sensitizer/ donor Sensitizer/ donor/ iodonium salt 1 control (C.Ex) 0 >200 190 2 acetonitrile (C.Ex) 2.60 0.124 >200 >190 3 nitrobenzene (C.Ex) 0.373 >200 >190 4 methylethylketone (C.Ex) 0.218 >200 >190 5 2,5-dimethyl-2,4-hexadiene (C.Ex) 2.10 0.334 >200 >190 6 ethylmethylthioacetate (C.Ex) 1.70 0.407 >200 >190 7 p-bromothioanisole (C.Ex) 1.60 0.615 >200 >190 8 3,3'-dimethoxybiphenyl (C.Ex) 1.60 0.649 >200 >190 9 tetrahydrofuran (C.Ex) 1.60 0.220 >200 >190 10 hexaethylbenzene (C.Ex) 1.49 0.492 >200 >190 11 methoxyphenylphenylether (C.Ex) 0.604 >200 188 12 p-dimethoxybenzene 1.34 0.418 205 160 13 N,N-dimethylacetamide 1.32 0.264 204 150 14 phenylacetate 1.30 0.413 >200 >190 15 n-propylamine 1.30 0.200 90 24 16 aniline 1.28 0.282 >200 >190 17 1,3-dibutylthiourea 0.570 >200 137 18 tetramethylurea 0.352 94 101 19 tetrabutylthiourea 0.909 38 29 20 dipentylamine 1.22 0.477 159 19 21 1,2,4-trimethoxybenzene 1.12 0.509 >225 55 22 hexamethylphosphoramide 1.00 0.543 80 50 23 tripiperdinophosphine oxide 1.00 0.907 52 40 24 trimethylsilylmorpholine 0.483 112 21 25 N,N-dimethylbenzylamine 1.00 0.410 18 8 26 tris-dimethylsilylamine 0.580 108 32 27 triethanolamine 0.96 0.452 17 6 28 tris(dimethylamino)- 0.719 15 9 phenylsilane 29 triphenylamine 0.86 0.737 >200 >190 30 triphenylphosphine 0.794 >200 172 31 p-dimethylaminobenzaldehyde 0.70 0.452 13 11 32 N,N-dimethyl-p-toluidine 0.65 0.410 14 7 33 p-dimethylaminophenyl-alcohol 0.65 0.500 13 8 The above data illustrates that an increased cure rate is obtained using a composition of the invention, and demonstrates the advantage of using donors whose E ox value is less than or equal to that of p-dimethoxybenzene and that have an abstractable hydrogen atom on a carbon or silicon atom alpha to the donor atom. EXAMPLE 3 Solutions similar to those of U.S. Pat. No. 4,228,232 (Rousseau) were prepared from 4.3 parts pentaerythritol tetraacrylate, 5.6 parts "Oligomer P-II" (Rousseau, Col. 11, 61% by weight in MEK), 0.30 parts triethylamine, 14.9 parts of a dispersion containing 4.1% Pigment Red 48 (CI 15865) and 8.2% polyvinyl formal resin ("Formvar 12/85", Monsanto) in n-propanol-water azeotrope, and 74.2 parts n-propanol-water azeotrope. As was the case in Rousseau, the amount of triethylamine was just sufficient to neutralize the acid in oligomer P-II, and thus the triethylamine did not serve as a donor. To 5 parts of the above solutions were added 0.02 part  2 I + PF 6 - , varying amounts of various sensitizers, and optionally 0.015 part of the donor compound 9-julolidine carboxyaldehyde. The solutions were then coated with wire-wound rods onto grained and anodized aluminum at coating weights of 1-2 g/m 2 and dried at 66°C for 2 minutes. The resulting photosensitive printing plates were topcoated with 5% aqueous polyvinyl alcohol containing 0.26 parts surfactant ("Triton X-100", Rohm and Haas) as a coating aid at coating weights of 1-2 g/m 2 . The dried plates were exposed through a √ density increment, 21 step sensitivity guide (Stouffer Graphic Arts) for 2 sec. using a 172,000 Lux tungsten light source at a distance of 25.4 cm ("Model 70" Transparency Maker, 3M). The exposed plates were developed with an aqueous solution of 4% n-propanol, 2% sodium metasilicate and 0.06% surfactant ("Dowfax 2AL", Dow Chemical Company). Set out below in TABLE II are the run number, the type and amount of sensitizer and its λ max value, a statement ("yes" or "no") indicating whether or not the optional donor was added, and the number of solid steps retained after exposure and development. Higher step values indicate greater sensitivity. Exposure values (in (J/cm 2 ) (ergs/cm 2 ) required to form one solid step; last column in TABLE II) for many of the plates were also measured by exposing the plates to coherent light using an argon-ion laser (488 nm), or by exposing the plates to incoherent light at the absorption maximum of the sensitizer using a one Kw high pressure mercury-xenon light source directed through a variable frequency monochromator having a 20 nm bandpass. The monochromator output was measured using a radiometer. Numerically lower exposure values indicate increased sensitivity. From the data in TABLE II it can be seen that plates containing iodonium salt, sensitizer and donor are more sensitive than plates containing only iodonium salt and sensitizer. An exposure value of 450 x 10 -7 J/cm 2 (450 ergs/cm 2 ) at 488 nm (argon-ion laser) was obtained using a combination of diphenyliodonium salt, ketocoumarin sensitizer, and 9-julolidine carboxaldehyde (Run 8). For comparison, the relatively sensitive laser-imageable compositions of Rousseau Exs. 18 and 19 (cols. 18-19) have an exposure value at 488 nm of only 3,200 x 10 -7 J/cm 2 (3,200 ergs/cm 2 ). EXAMPLE 4 A coating formulation was prepared from the following ingredients: % Solids acrylamide 43.3 N,N'-methylenebisacrylamide 4.3 polyvinyl alcohol (m.w. 2000, 75% hydrolyzed) 51.9 surfactant ("Triton X-100") 0.5 made up to 11.5% solids in a 1/1 v/v acetonitrile/water mixture. Using a red safelight, coating samples were prepared by combining 25ml portions of the above stock solution with 0.01g of the sensitizer and optionally adding 0.1g  2 I + PF 6 - and/or 0.1g STS. The samples were coated onto gelatin-subbed polyester film using a #18 wire wound rod, dried with a heat gun, then oven-dried for 2 minutes at 60°C. The coated films were exposed under vacuum through a 21 step sensitivity guide, using a tungsten ("Model 70" Transparency Maker, 3M) or ultraviolet (2Kw Berkey Ascor, Berkey Technical Company) light source. The exposed samples were developed using a 3/20, v/v water/methanol solvent mixture. Relative speed was determined by the number of steps (average of 3 samples) remaining after development. Set out below in TABLE III are the results for the samples exposed to visible light, and set out below in TABLE IV are the results for the samples exposed to ultraviolet light. Each exposure was 30 sec. in the visible region or 60 sec. in the ultraviolet region except as noted. The above data illustrates that combination of an iodonium salt, sensitizer and donor can increase imaging speed by one to two orders of magnitude compared to compositions containing only sensitizer and donor or only sensitizer and iodonium salt. EXAMPLE 5 To illustrate the effect of E ox (donor) upon cure speed, a series of compositions was evaluated as follows. A monomer stock solution was prepared from 10% pentaerythritol tetraacrylate in 4/1, w/w, acetonitrile/water. To 3 ml portions of this solution in 13 × 100mm "Pyrex" test tubes were added about 0.02g of  2 I + PF 6 - and/or a donor compound and enough sensitizer to give an optical density of between 1 and 2, as evaluated visually. The solutions were purged with N 2 for 2 minutes before and continuously during light irradiation. The light source was a Kodak "Carousel" Projector lamp equipped with a 440nm filter. Relative speed was determined by measuring gelation time. Set out below in TABLE V are the run number, the sensitizers and their λ max values, the donor compounds and their E ox values, and the gelation times for solutions containing iodonium salt plus sensitizer, sensitizer plus donor, or iodonium salt plus sensitizer plus donor. The above data illustrates that when E ox (donor) is decreased, cure speed generally increases (see also TABLE I). EXAMPLE 6 A polyester cloth backing (woven, spun polyester cloth, 0.03 g/cm 2 , Milliken) was impregnated with resin by saturating the backing with an ultraviolet-light-curable resin mixture made by combining 75 parts epoxy-acrylate resin ("Novacure 3500", Interez), 15 parts pentaerythritol triacrylate, 9 parts n-vinyl pyrrolidone and 1 part α,α-dimethoxy-α-phenyl acetophenone, and curing the resin under ultraviolet light in air using an energy density of 0.3 J/cm 2 . A coated abrasive binder resin was prepared from a 50:50 mixture of the triacrylate of tris-hydroxyethylisocyanurate and the triacrylate of trimethylolpropane, filled to 50% with calcium carbonate. 0.25 Part each of  2 I + PF 6 - , CPQ and D-1 were mixed into the binder resin. The resin-impregnated backing was knife-coated with the binder resin at a coating thickness of 0.1mm, then drop-coated with grade 50 Al 2 O 3 mineral. The binder was cured under nitrogen in 5 to 10 seconds using a high intensity visible light source (Model F440 with 4V678 lamp, Fusion Systems) operated at a distance of about 15 cm. Microscopic examination of the cured abrasive showed that cure took place throughout the binder, even underneath individual mineral granules. By comparison, if the photoinitiator system was excluded from the resin and a 5Mrad dose of E-beam irradiation (250 Kv acceleration potential) was employed to effect cure, pools of wet uncured resin remained under individual mineral granules and the granules were poorly adhered on the backing. A sample of the coated abrasive was size coated with the same resin system, using a coating weight just sufficient to coat the mineral granules. The size coat was cured under the visible light source used to cure the make coat. The resulting coated abrasive was evaluated using a reciprocating grinding apparatus ("rocker drum") on a 6mm thick 1018 carbon steel workpiece. After 500 cycles, no shelling was observed and an average of 0.77g of steel was removed. A comparison abrasive sample was prepared using a make coat that contained the same photoinitiator system (cured using visible light), and a size coat that did not contain the photoinitiator system (cured using E-beam). The comparison abrasive shelled after only 20 cycles and removed only 0.02g of steel. Comparable results were obtained when sensitizers such as benzil, 2-chlorothioxanthone and fluorenone were substituted for camphorquinone. Improved uncured resin shelf life and ambient light stability were obtained when donor compounds such as ethyl p-dimethylaminobenzoate, p-dimethylaminobenzaldehyde and p-dimethylaminobenzonitrile were substituted for p-dimethylaminophenethyl alcohol. The above-described abrasive binder system should be a useful substitute for standard phenolic-based binders, and could offer reduced energy consumption and higher throughput during manufacture. EXAMPLE 7 A visible-light-curable orthodontic bracket adhesive was prepared by mixing together 197 parts BisGMA, 197 parts TEGDMA, 1,618 parts silane-treated microparticles (Example 1, U.S. Pat. No. 4,503,169), 33 parts pyrogenic silica ("R-972", DeGussa), 2 parts  2 I + PF 6 - , 1 part CPQ and 3 parts triethanolamine to yield a paste-like composition. A bovine incisal tooth was potted in a circular acrylic cylinder, and the labial surface of the tooth cleaned with a pumice slurry, rinsed, dried and etched with gelled 37% phosphoric acid for 1 minute. The tooth was rinsed with distilled water for 45 seconds, air dried, coated on its etched surface with a methacrylate-based primer ("Light-Curable Enamel Bond", 3M), blown with air to level the primer, and irradiated for 20 seconds with a handheld curing light ("Visilux", 3M). A thin film of the above-described orthodontic bracket adhesive was applied to the entire back surface of a standard stainless steel orthodontic bracket (4.0 × 4.0 mm, 100 mesh backing). The bracket was then pressed firmly against the etched, primed bovine tooth. The lightguide output end was oriented orthogonally with respect to the face of the bracket, at a distance of about 3mm, and the light energized for 20 seconds. Fifty brackets were bonded to teeth using the above procedure. The resulting cured bracket-tooth assemblies were stored for various periods of time, then clamped in the fixed jaw of an "Instron" tensile tester (Instron Engineering Corporation). The movable jaw of the tester was fastened to the bracket using a length of orthodontic wire. The shear force required to cause bond failure was measured at a jaw separation rate of 2mm/minute, and an average value determined for five samples at each storage time. Set out below in TABLE VI are the storage times and measured average shear bond strengths for the above-described adhesive, along with comparison values for a conventional two-part chemical cure orthodontic adhesive ("Concise" No. 1960, 3M) and a commercially available one-part light cure orthodontic adhesive ("Heliosit", Vivadent, cured according to the manufacturer's instructions). Average shear bond strength, kg/cm 2 Storage time Adhesive of EXAMPLE 7 Concise Heliosit 30 seconds 103.3 (c) 53.6 2 minutes 112.3 (c) 68.5 5 minutes 102.9 82.0 77.2 10 minutes 99.1 105.6 80.0 1 hour 142.9 150.0 86.1 24 hours 134.9 115.6 93.4 3 months 160.1 6 months 186.5 12 months 172.2 18 months 161.6 Notes to TABLE VI: The commercial chemical cure adhesive requires at least 5 minutes waiting time before archwire application. The commercial light cure adhesive also requires some waiting time before archwire application, and its ultimate strength is believed to be insufficiently high to prevent occasional bracket adhesion failure. In contrast, the adhesive made from the photoinitiator system of this invention exhibits rapid strength buildup and high ultimate bond strength. It should enable immediate archwire application, and reduce the incidence of early bracket adhesion failure. Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope of this invention. It should be understood that this invention is not limited to the illustrative embodiments set forth herein.

Claims: An addition-polymerizable composition comprising: a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts of i) aryliodonium salt, ii) sensitizing compound capable of absorbing light somewhere within the range of wavelengths between 300 and 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound having oxidation potential E ox characterized in that said donor is different from said sensitizing compound and zero < E ox (donor) ≤ E ox (p-dimethoxybenzene), wherein said electron donor compound acts as an electron donor to at least one of said aryliodonium salt (i) and said sensitizing compound (ii). A composition according to claim 1, characterized in that said E ox (donor) is between 0.5 and 1 volts vs. a saturated calomel electrode. A composition according to any preceding claim, characterized in that said sensitizing compound has the formula: ACO(X) b B where X is CO or CR 1 R 2 , where R 1 and R 2 can be the same or different, and can be hydrogen, alkyl, alkaryl or aralkyl, b is zero or 1, and A and B can be the same or different and can be substituted or unsubstituted aryl, alkyl, alkaryl or aralkyl groups, or together A and B can form a cyclic structure which can be a substituted or unsubstituted cycloaliphatic, aromatic, heteroaromatic or fused aromatic ring. A composition according to any preceding claim, characterized in that said sensitizing compound is selected from the group consisting of ketones, coumarin dyes, xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes and pyridinium dyes and said donor is selected from the group consisting of amines, amides, ethers, ureas, ferrocene, sulfinic acids and their salts, salts of ferrocyanide, ascorbic acid and its salts, dithiocarbamic acid and its salts, salts of xanthates, salts of ethylene diamine tetraacetic acid and salts of tetraphenylboronic acid. A composition according to any preceding claim, characterized in that said donor contains a nitrogen, oxygen, phosphorus or sulfur donor atom and an abstractable hydrogen atom bonded to a carbon or silicon atom alpha to said donor atom. A composition according to any preceding claim, characterized in that said donor comprises a tertiary amine containing an aromatic ring having at least one electron-withdrawing group on said ring. A composition according to any preceding claim, characterized in that said composition contains, for every 100 parts by weight of said monomer, 0.005 to 10 parts by weight each of said aryliodonium salt, said sensitizing compound and said donor. A composition according to any preceding claim, characterized in that said sensitizing compound comprises camphorquinone, said composition contains 10% to 90% reinforcing and/or extending fillers, based on the total weight of said composition, and said composition is in the form of a photocurable adhesive applied to an orthodontic bracket or in the form of a photocurable composite suitable for use as a dental restorative. A composition according to any of claims 1 to 7, characterized in that said composition contains at least one julolidinyl compound and is in the form of a photocurable or photocured image-bearing coating atop a support. A process for addition photopolymerization of a composition according to claim 1, characterized in that said composition is irradiated with light having a wavelength between 300 and 1000 nanometers until said composition gels or hardens, and further characterized in that said composition comprises: a) said free-radically-polymerizable monomer, and b) said photoinitiator system, comprising photochemically effective amounts of i) diphenyliodonium salt, ii) said sensitizing compound, and iii) said electron donor compound, said donor being different from said sensitizing compound and 0.5 < E ox (donor) ≤ 1 volt.

Inventors: ALI ZAKI M, OXMAN JOEL D, PALAZZOTTO MICHAEL C, UBEL III ANDREW F, ALI, ZAKI M., OXMAN, JOEL D., PALAZZOTTO, MICHAEL C., UBEL III, ANDREW F., ALI, ZAKI M. C/O MINNESOTA MINING AND, OXMAN, JOEL D. C/O MINNESOTA MINING AND, PALAZZOTTO, MICHAEL C. C/O MINNESOTA MINING AND, UBEL III, ANDREW F. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1988
Main classification: C08F2
IPC codes: A61K6, C08F2, G03F7, C09D11