GB2068006A - Photopolymerizable Composition Containing an O- Nitroaromatic Compound as Photoinhibitor - Google Patents

Abstract

A photopolymerizable coating composition comprises (1) a nongaseous, ethylenically unsaturated, polymerizable compound, (2) a specified nitrophenyl compound in which the nitro group is ortho to a hydrogen bearing alpha carbon substituent, (3) an organic free-radical photoinitiator and is useful for making a positive or negative polymeric image on a substrate.

Description

SPECIFICATION Photopolymerizable Composition Containing an O-nitroaromatic Compound as Photoinhibitor Background of the Invention (1) Field of the Invention This invention relates to photopolymerizable compositions, and to methods of producing positive and negative polymeric images from these compositions.

(2) Description of the Prior Art Conventional polymeric imaging systems are of the negative polymeric image type; that is, the photopolymerizable coating is exposed to radiation through a process transparency and a negative polymeric image is produced. By "negative polymeric image" is meant a polymeric image which corresponds to the transparent portions of the process transparency.

Recently systems which provide a positive polymeric image have been developed. One such system is described by Nebe in Belgian Patent 818,371. In this system: (a) a substrate is coated with a photopolymerizable composition containing: (1 ) a nongaseous, ethylenically unsaturated, polymerizable compound, (2) 0.001 to 1.0 part by weight per part of polymerizable compound of an organic, radiation sensitive, free-radical generating system, and (3) 0.1 to 10% by weight, based on the photopolymerizable composition, of a nitroso dimer, (b) the photopolymerizable coating is exposed through a process transparency to radiation at least some of which has a wavelength less than 3400 A whereby the nitroso dimer is dissociated to a polymerization-inhibiting nitroso monomer in the radiation-struck areas, and (c) a greater portion of the photopolymerizable coating is exposed to radiation substantially limited to wavelengths greater than 3400 A whereby a positive polymeric image is formed in the areas struck by the second radiation but not struck by the first imagewise radiation.

Although this system can produce excellent positive polymeric images, a limited time factor is involved. The second exposure must be carried out soon after the first exposure since the nitroso monomer formed during the first exposure can recombine to form inactive nitroso dinner. It would also be desirable to have a system which is not dependent on exposure to such short wavelength radiation.

Another system for producing positive polymeric images is described by Margerum in U.S.

3,556,794. In this system, a photopolymenzable material comprising: (1) an ethylenically unsaturated monomer, (2) a photopolymerization initiator system comprising a photo-oxidant dye and a reducing agent for the dye, and (3) â an ionizable desensitizing agent having a nitrobenzyl moiety in combination with a carboxylste group, is imagewise exposed to radiation in the wavelength range of 2000-4000 A, thereby inhibiting polymerization by the desensitization of the initiator system, and exposing the photopolymerizable material to radiation in the wavelength range of 3800 to 7200 A, thereby producing a polymeric image in the #area not exposed to the imagewise radiation.This system suffers from the drawback that the desensitizing effect of the desensitizing agent is only temporary. Margerum states that the desensitization lasts "for up to about fifteen or more minutes".

British Patent Specification No. 1,547,548 discloses a photopolymerizable coating composition comprising: (1) a nongaseous, ethylenically unsaturated, polymerisable compound, (2) a specified nitroaromatic compound, and (3) an organic, radiation-sensitive, free radical generator.

It has now been found that certain other nitroaromatic compounds may be used in compositions of the kind disclosed in Specification No. 1,547,548.

According to the present invention, therefore, there is provided a photopolymerizable coating composition which comprises: (a) nongaseous, ethylenically unsaturated compound capable of addition polymerization by freeradical initiated chain propagation, (b) 0.004 to 0.7 part by weight, per part of component (a), of nitroaromatic compound of the formula

wherein:: R', R2, R3 and R4, alike or different are H, OH, halogen, NO2, CN, alkyl of 1 to 18 carbons, alkoxy in which the alkyl is of 1 to 18 carbons, acyloxy of 2-7 carbons, aryl of 6 to 18 carbons, benzyl, halogensubstituted phenyl, polyether of 2 to 18 carbons and 2 to 10 oxygens, dialkylamino in which each alkyl is of 1 to 18 carbons, thioalkyl in which the alkyl is of 1 to 1 8 carbons, or thioaryl in which the aryl is of 6 to 18 carbons, R2 and R3, taken together, are -OCH2O- or O#(#CH2CH2O#)q# in which q is an integer from 1 to 5, or any two vicinal R', R2, R3 and R4 groups, taken together, are the residue of a second benzene ring fused onto the benzene nucleus, with the proviso that not more than one of R1, R2, R3 and R4 is OH or NO2, R5 is H, alkyl of 1 to 18 carbons, halogen, phenyl, or alkoxy in which the alky is of 1 to 18 carbons, R6 is aryloxy of 6 to 18 carbons unsubstituted or substituted with halogen, with alkyl of 1 to 6 carbons or with alkoxy of 1 to 6 carbons, or R5 and R8 taken together are =N(alkyl) in which the alkyl is of 1 to 6 carbons or R', R2, R3, R4 are alike or different and are as defined above, with the proviso that at least one of R1 to R4 is acyloxy of 2 to 7 carbons, polyether of up to 18 carbons and 7 to 10 oxygens or R2 and R3 taken together are -O-CH2-O- or -O-(-CH2CH2O-)-q, in which q is an integer from 1 to 5, R5 is H, alkyl of 1 to 18 carbons, halogen, phenyl, or alkoxy in which the alkyl is of 1 to 18 carbons, R6 is H, OH, alkyl of 1 to 18 carbons, phenyl, alkoxy in which the alkyl is of 1 to 18 carbons, or aryloxy of 6 to 18 carbons unsubstitued or substituted with halogen, alkyl of 1 to 6 carbons, or alkoxy of 1 to 6 carbons, with the proviso that only one of R5 and Rff is H, or R5 and R6 taken together are =0, =CH2, -O-CH2-, =NCeH5, =NC6H5N(alkyl)2 in which each alkyl is of 1 to 18 carbons, -0-C2H4-0-, =N (alkyl in which the alkyl is of 1 to 6 carbons, =N(hydrocarbylene)

in which the hydrncarbylene group is of 1 to 18 carbons, or

in which R8 and R9, alike or different, are H or alkyl of 1 to 4 carbons, and R7 and R10, alike or different, are -CN,-C0R11 in which R11 is alkyl of 1 to 5 carbons, or-COOR12 in which R12 is alkyl of 1 to 6 carbons which may be interrupted by an oxygen atom, alkenyl of 2 to 5 carbons, or alkynyl of 2 to 5 carbons, or R7 and R8 together, or R9 and R' together, complete a 6-membered carbocyclic ring containg a keto group, and (c) 0.001 to 10 parts by weight, per part of component (a), of an organic, radiation-sensitive, freeradical generator, activatable by actinic radiation that does not significantly convert the nitroaromatic compound to an inhibitor of free-radical polymerization.

Preferably, components (a), (b) and (c), in combination, comprise 15~100% by weight of the photopolymerizable composition.

Positive polymeric images are produced on a substrate by the process which comprises: (1) coating the substrate with the above photopolymerizable composition, (2) imagewise exposing the photopolymerizable coating through an image-bearing transparency to radiation at least about 20% of which has a wavelength of about 200 to about 380 nm, thereby converting at least some of the nitroaromatic compound to polymerization-inhibiting nitrosoaromatic compound, and (3) subjecting the coating in a second exposure to actinic radiation substantially limited to wavelengths greater than about 380 nm, whereby a positive polymeric image is formed in the areas exposed during the second exposure, but not exposed during the imagewise exposure.

The image formed in step (3) is then developed. This can be effected by removing the nonpolymerized portion of the photopolymerizable coating in the areas exposed to the imagewise exposure radiation, or by differential adhesion of a pigment toner to the unpolymerized portion of the photopolymerizable coating in the areas exposed to the imagewise exposure radiation.

Negative polymeric images are produced on a substrate by the process which comprises: (1) coating the substrate with the above photopolymerizable composition, (2) imagewise exposing a portion of the photopolymerizable coating through an image-bearing transparency to actinic radiation substantially limited to wavelengths greater than about 380 nm, whereby a negative polymeric image is formed in the areas exposed to the radiation, and (3) developing the image formed in Step (2).

Description of the Preferred Embodiments In a preferred embodiment of the photopolymerizable coating composition, component (a) is an unsaturated ester of a polyol and an a-methylenecarboxylic acid selected from the group consisting of acrylic and methacrylic acids, any alkyl groups in component (b) are of 1 to 6 carbons, and component (c) has at least one component having an active radiation absorption band with a molar extinction coefficient of at least about 50 within the range of greater than 380 to 800 nm, and most preferably within the range of 400 to 600 nm, and is selected from the group consisting of phenanthrenequinones and 2,4,5-triarylimidazole dimers.Preferably component (b) is present in the amount of 0.04 to 0.15 part by weight of component (a), and component (c) is present in the amount of 0.01 to 2 parts by weight per part of component (a).

In a preferred embodiment of the method of making positive polymeric images at least about 30% of the radiation in the imagewise exposure has a wavelength of 200 to 380 nm, and the radiation in the second exposure has wavelengths substantially limited to greater than 380 to 800 nm, and most preferably substantially limited to 400 to 600 nm.

This invention is based on the discovery that certain nitroaromatic compounds in which the nitro group is ortho to a hydrogen-bearing alpha-carbon substituent do not significantly retard or inhibit freeradical polymerization in certain photopolymerizable systems, but are photochemically rearranged to nitrosoaromatic inhibitors of free-radical polymerization by exposure to radiation having a wavelength of 200 to 380 nm. These nitroaromatic compounds are relatively unaffected by radiation of longer wavelength. On the other hand, certain radiation sensitive, free-radical initiators absorb radiation of longer wavelength, especially in the presence of added sensitizers, to provide sufficient radicals for polymerization of a polymerizable monomer in the absence of an appreciable concentration oF inhibiting nitrosoaromatic species.

The nitroso compounds formed by irradiation of the nitroaromatic compounds described herein with short wavelength radiation interfere with the normal free-radical induced polymerization process.

Thus, when using the shorter wavelength region of The spectrum in the presence of a nitrosoaromatic compound, an insufficient number of initiating and propagating free radicals is available, and polymerization does not occur. When a composition of this invention is exposed to radiation of wavelength greater than about 380 nm, the nitroaromatic compound is relatively unaffected, and the photoinitiator system operates to produce initiating radicals. These radicals are able to effect chain propagation in the usual way and polymerization occurs.

Suitable polymerizable compounds for use as component (a) of the photopolymerizable coating compositions of this invention are the nongaseous, ethylenically unsaturated compounds described by Burg et al. in U.S. Patent 3,060,023; Martin et al., in U.S. Patent 2,927,022; and Hertler in Belgian Patent 769,694. By "nongaseous" is meant compounds which are not gases under atmospheric conditions. They are preferably monomeric, have a boiling point above 900C at normal atmosphere pressure, and contain at least one terminal ethylenic group, but may contain 2-5 terminal ethylenic groups. Monomers which contain two or more terminal ethylenic groups are particularly preferred.

Suitable polymerizable compounds include unsaturated esters of polyols, particularly such esters of a-methylenecarboxylic acids, for example, ethylene glycol diacrylate, diethylene glycol diacrylate, glycerol diacrylate, glyceryl triacrylate, mannitol polyacrylate, sorbitol polyacrylates, ethylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1 ,2,4-butanetriol tnmethacrylate, 1 1,1 - trimethylolpropane triacrylate, triethylene glycol diacrylate, 1,4-cyclohexanediol diacrylate, 1,4benzenediol dimethacrylate, pentaerythritol di-, tri-, and tetramethacrylate, dipentaerythritol polyacrylate, pentaerythritol di-, tri-, and tetraacrylates, 1 3-propanediol diacrylate, 1 ,5-pentanediol dimethacrylate, the bis-acrylates and methacrylates of polyethylene glycols of molecular weight 200- 4000, and the like; unsaturated amides, particularly those of -methylenecarboxylic acids, and especially those of a,w-diamines and oxygen-interrupted ss-diamines, such as methylene bisacrylamide, methylene bis-methacrylamide, ethylene bis-methacrylamide, 1 6-hexamethylene bisacrylamide, bis(y-methacrylamidopropoxy)-ethane and p-methacrylamidoethyl methacrylate; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate, divinyl benzene-1 ,3-disulfonate and divinyl butane-1 ,4-disulfonate; styrene and derivatives thereof; unsaturated aldehydes, such as hexadienal; and the like; and mixtures thereof.

A preferred group of polymerizable compounds, because of the good physical properties of compositions containing them, includes N-phenyl-N-methylacrylamide, N-vinylphthalimide, diacetone acrylamide, N-vinylsuccinimide, p-xylylene diacrylate, 1 ,4-bis(2-acryloxyethyl)benzene, pentaerythritol triacrylate, 4-acryloxybenzophenone, 4-methacryloxybenzophenone, N-(2-acryloxyethyl)succinimide, 1,1,1 -trimethylolpropane triacrylate, pentaerythritol tetraacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, 1,1 ,1 -trimethylolpropane trimethacrylate, 4-acryloxydiphenylmethane, N-(2-acryloxypropyl)succinimide, 2,4-diacryloxybenzophenone, 4-(,-dimethylbenzyl)phenyl acrylate, 3-acryloxybenzophenone, 2,acryloxybenzophenone, 2-acryloxy-4-octyloxybenzophenone, and mixtures thereof. The most preferred polymerizable compounds are esters of a-methylenecarboxylic acids selected from the group consisting of acrylic and methacrylic acids.

Many of the polymerizable compounds listed above are subject to thermal polymerization, especially when stored for long periods or at elevated temperatures. When such compounds are supplied commercially, it is customary for them to contain a small, but effective, amount of a thermal polymerization inhibitor. These inhibitors may be left in the monomers when the photopolymerizable coating compositions of this invention are prepared, as was done in the examples which follow. The resulting compositions usually have satisfactory thermal stability.If unusual thermal exposure is anticipsted, or if monomers containing little or no thermal polymerization inhibitor are employed, compositions with adequate shelf life can be obtained by incorporating up to 5 percent, by weight of monomer, of a thermal polymerization inhibitor such as hydroquinone, methylhydroquinone, p methoxyphenol, and the like.

The photopolymerizable coating compositions also contain a nitroaromatic compound of the formula described above. In this formula, the preferred alkyl groups are lower alkyl groups containing 1 to 6 carbon atoms. In the bis compounds, the term "hydrocarbylene" represents any diva lent radical composed solely of carbon and hydrogen containing 1 to 18 carbon atoms. Typical radicals include o-, m-, and p-phenylene, vinylene, 2-butynylene, 1 3-butadienylene, hexamethylene, octamethylene, octadecamethylene, naphthylene (1,2; 2,3; 1,4; and 1,5),

and the like.

It has been found that the nature of the R5 and R6 substituents in the nitroaromatic compounds is very important. The unsubstituted compounds in which R5 and Rss are H do not rearrange to phstoinhibitor and thus do not work. Furthermore, some R5 and Re substituents deactivate the CH moiety toward rearrangement, for example, subst uents which are normally considered to destabilize positive charges, such as nitro, cyano, carboxy and 2-pyridyl. It has been found, for instance, that

do not work in accordance with this invention.

Suitable nitroaromatic compounds include: 1 -(2'-nitro-4',5'-dimethoxy)phenyl 1-1 -(4#rrc'thoxyphenoxy)ethane, 1 -(2'-nitro-4',5 '-dimethoxy)phenyl- 1 -phenoxyethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl 1 -(2,4-di'nethylphenoxy-ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl- 1 -(4-chlorophenoxy)ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl-1 -(4-bromophenoxy)ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl- 1 -(2-naphthyloxy)ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl 1 - (2,4-dimethoxy)phenoxy-ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl-1 -(4-t-butylphenoxy)ethane, 1 -(2'-nitro-4',5'-dimethoxy)phenyl-1 -(2-t-butylphenoxy)ethane, 2-nitrn-4,5-dimethoxy-a-methyliminotoluene, 2-nitro-4,5-dimethoxy-or-t-butyliminotoluene, 2-nitro-4,5-dimethoxy-a-n-butyliminotoluene, 2-nitro-4,5-dimethoxy-~-n-hexyliminotoluene, and 1 -(2'-nitro-4,5'-dimethoxy)phenyl-1 -(2,4,5-trimethylphenoxy)ethane.

The nitroaromatic compounds are ordinarily employed in concentrations of about 0.004 to about 0.7 part by weight per part of polymerizable compound. The preferred amount in any specific case will depend upon the particular monomer/free-radical generating system employed. In general, the preferred amount of nitroaromatic compound is 0.04 to 0.1 5 part by weight per part of polymerizable compound.

The third component which the photopolymerizable coating composition must contain is an organic, radiation-sensitive, free-radical generator which initiates polymerization of the monomer and does not subsequently terminate the polymerization. The word "organic" is used here and in the claims to designate compounds which contain carbon, and one or more of oxygen, hydrogen, nitrogen, sulfur and halogen, but are free of metal.

The free-radical generator absorbs actinic radiation with wavelengths within the range of 200 to 800 nm that does not significantly rearrange the nitroaromatic compound to an inhibitor of free-radical polymerization. By "actinic radiation" is meant radiation which is active to produce the free radicals necessary to initiate polymerization of the monomeric material. The free-radical generator can comprise one or more compounds which directly furnish free radicals when activated by radiation. It can also comprise a plurality of compounds, one of which yields the free radicals after having been caused to do so by a sensitizer which is activated by the radiation.Preferably the free-radical generator has at least one component having a radiation absorption band with a molar extinction coefficient of at least about 50 within the range of greater than 380 to 800 nm, and more preferably 400 to 600 nm.

A large number of free-radical generating compounds can be utilized in the practice of this invention including aromatic ketones such as benzophenone, hAichler's ketone (4,4' bis(dimethylamino)benzophenone), 4,4'-bis(diethylamino)benzophenone, 4-acryloxy-4'- dimethylaminobenzophenone, 4-acryioxy-4'-diethylaminobenzophenone, 4-methoxy-4'- dimethylaminobenzophenone, phenanthrenequinone, 2,7-di-t-butylphenanthrenequinone, and other aromatic ketones; benzoin ethers such as benzoin methyl ether, benzoin ethyl ether and benzoin phenyl ether, methylbenzoin, ethylbenzoin and other benzoins; 2,4,5-triaiylimidazole dimers such as chlornphenyl)-4,5-diphenylimidazole dimer, 2-(e-chlorophenyl)-4,5-di(m-methoxyphenyl)imidazole dimer, 2,2'-bis(2-chlornphenyl)-4,4',5,5'-tetrakis(3-msthoxyphenyl)biimidazole, 2-(o-fluorophenyl) 4,5-di-phenylimidazole dimer, 2-(o-metlioxyphenyl)-4,5-diphenylimidazole dimer, 2-(p methoxyphenyl)-4,5-diphenylin'idazole dimer, 2,4-di(p-methoxyphenyl)-5-phenylimidazole dimer, 2 (2,4-dimethoxyphenyl )-4,5-diphenylimidazole dimer, 2-(p-methyl-mercaptophenyl)-4,5- diphenylimidazole dimer, and the like disclosed in U.S. Patent 3,479,1 85 and in British Patents 997,396, published July 7, 1965, and 1,047,569, published November 9, 1966.

The imidazole dimers may be used with hydrogen donors such as 2-mercaptobenzoxazole or 2mercaptobenzothiazole, with or without sensitizers such as Michler's ketone, 2,5-bis(4'diethylamino-2'-methylbenzylidene)cyclopentanone, and various dyes. Additional examples of suitable initiators are disclosed by Plambeck in U.S. Patent 2,760,863. Redox systems, especially those involving dyes, may also be used. These include combinations such as Rose Benyal/2dibutylaminoethanol; 2-o-chlorophenyl-4,5-di(m-methoxyphenyl)imidazole dimer/2mercaptobenzoxazole; 2-o-chlorophenyl-4,5-di(rn-methoxyphenyl)imidazole dlmer/2- mercaptobenzothiazole; and the like.

A preferred group of free-radical generators characterized by good efficiency includes the phenanthrenequinones and 2A,5-triarylimidazole dimers, with or without hydrogen donors such as 2mercaptobenzoxazole, and .2-mercaptoben#hiazole, especially in the presence of sensitizers. The concentration of the free-radical generator is 0.001 to 10.0 parts by weight per part of polymerizable compound, and preferably 0.01 to 2.0 parts by weight The coating compositions used herein can also contain other components, if desired. For example, the coating can be of the monomer/binder type containing addittionally a thermoplastic macromolecular organic polymer binder.The coating can also be of the substantially dry, predominately crystalline type, described by Hertler in Belgian Patent 769,694, wherein the coating contains a solid ethylenically unsaturated polymerizable compound, an organic radiation-sensitive, free-radical generator and a nonpolymeric normally liquid or solid organic substance which does not inhibit polymerization, in addition to a suitable nitroaromatic compound.

Suitable thermoplastic macromolecular organic polymer binders for use in a monomer/binder system are described by Chang in U.S. 3,661,588, and include such polymeric types as (a) copolyesters based on terephthalic, isophthalic, sebacic, adipic and hexahydroterephthalic acids; (b) nylons or polyamides; (c) vinylidene chloride copolymers; (d) ethylene/'inyl acetate copolymers; (e) cellulosic ethers; (f) polyethylene ; (g) synthetic rubbers; (h) cellulose esters; (i) polyvinyl esters including polyvinyl acetate//acrylate and polyvinyl acetate/methacrylate copolymers; (j) polyacrylate and poly-#-alkylacrylate esters, e.g., polymethyl methacrylate and polyethyl methacrylate; (k) high molecular weight ethylene oxide polymers (polyethylene glycols) having average molecular weights from 4,000-4,000,000; (1) polyvinyl chloride and copolymers; (m) polyvinyl acetal; (n) polyformaldehydes; (o) polyurethanes; (p) polycarbonates; and (q) polystyrenes.

In a particularly preferred embodiment of the invention, the polymeric binder is selected so that the unexposed photopolymerizable coating is soluble in predominantly aqueous solutions, for example dilute aqueous alkaline solutions, but upon exposure to actinic radiation becomes relatively insoluble therein. Typically, polymers which satisfy these requirements are carboxylated polymers, for example vinyl addition polymers containing free carboxylic acid groups. Another preferred group of binders, includes polyacrylate esters and poly-a-alkylacrylate esters, particularly polymethyl methacrylate.

When a monomer/binder system is employed, the amount of polymeric binder present is about 10 to about 80% by weight based on the total solids content, and preferably about 25% to about 75%.

Polymerizable compounds which contain only one site of ethylenic unsaturation are generally not satisfactory for use in a monomer/binder system.

When the substantially dry, predominantly crystalline system, described in the Hertler Belgian Patent, is employed, in one aspect of the invention, the system may contain in addition to the polymerizable compound, about 0.01 to about 0.25 part by weight, per part of polymerizable compound of a nonpolymeric, normally liquid organic compound which does not inhibit the polymerization of the polymerizable material and does not absorb so much of the incident radiation as to prevent the initiation of the polymerization by the free-radical generator.In another aspect of the invention, the system may contain about 0.01 to about 250 parts by weight, per part of polymerizable compound, of a nonpolymerizable, crystalline organic solid which does not inhibit polymerization of the polymerizable compound and also does not absorb the incident radiation to such an extent as to prevent initiation of polymerization by the free radical generator.

Illustrative examples of suitable organic compounds which may be added include octadecanol, triethanolamine, stearic acid, cyclododecane, 1,1 0-decanediol, dimethylaminobenzonitrile, acetone oxime, desoxybenzoin, naphthalene, N,N'-dimethylhexamethylenediamine, p-diethoxybenzene, 1,2diphenylethane, biphenyl, dotriacontane, tetramethylurea, tributylamine, 2-dimethylaminoethanol, pentamethylbenzene, 1,1 2-dodecanediol, 1 ,2-diphenoxyethane, octacosane, trichloroxylene, cyclododecanol, and the like. A preferred group of solid compounds includes bibenzyl, biphenyi, 1,2diphenoxyethane, p-diethoxybenzene, octacosane, 1 -octadecanol and cyclododecanol.

The photopolymerizable compositions described herein may be coated on a wide variety of substrates. By "substrate" is meant any natural or synthetic support, preferably one which is capable of existing in a flexible or rigid film or sheet form. For example, the substrate could be a metal sheet or foil, a sheet or film of synthetic organic resin, cellulose paper, fiberboard, and the like, or a composite of two or more of these materials. Specific substrates include alumina-blasted aluminum, alumina-blasted polyethylene terephthalate film, polyethylene terephthalate film, polyvinyl alcohol-coated paper, crosslinked polyester-coated paper, nylon, glass, cellulose acetate film, heavy paper such as lithographic paper, and the like.

The particular substrate will generally be determined by the use application involved. For example, the compositions and method of this invention are particularly useful for producing printed circuits using as the substrate a plate which is a copper coating on fiberboard. When the photopolymerizable compositions are coated on metal surfaces, they are useful for making presensitized lithographic printing plates. For example, such a plate can be prepared from grained aluminum base in combination with a photopolymerizable coating. After the image has been developed, the plate is first coated with water and then contacted with a roller which wets only the photopolymer image with ink. The inked plate can then be used in lithographic printing in the usual way. Preferably the substrate is impermeable to oxygen.

The coated compositions can also serve as photo-resists in making etched or plated circuits or in chemical milling applications. They are also useful for preparing colored images from color separation negatives suitable for color-proofing. The images formed with these elements may also be used for making copies by thermal transfer to a substrate. Specific uses will be evident to those skilled in the art; many uses for positive images on substrates are disclosed in U.S. Patents 2,760,863; 3,060,023; and 3,060,026.

Processes for coating the substrate are described in the patents listed in the preceding paragraph.

Processes using coating compositions of the substantially dry, predominantly crystalline type are of five general types: those in which (1) the components of the coating composition are melted together generally to form a homogeneous melt which is coated onto the substrate; (2) the components of the coating composition are dissolved together in a solvent in which the components are preferably completely soluble and the resulting solution is poured or painted onto the substrate; (3) the components of the coating composition are dissolved in a volatile solvent and the resulting solution is sprayed as a fine mist against the substrate; (4) the components of the coating composition are melted together and the melt is sprayed as a fine mist onto the substrate; (5) the components of the coating composition are mixed together in a heated vessel which contains an inner surface that is cooled in which the distance from the mixture to the cooled, surface can be varied, whereby the components are sublimed onto the cooled surface further details of the these processes can be found in the Belgain patent of Hertler, cited above. A preferred coating method involves solution coating as in (2) above.

One of the special features of the photopolymerizable coating compositions of this invention is that they can be used to prepare either positive or negative polymeric images. Positive polymeric images are prepared by a two exposure procedure while negative polymeric images are prepared by the one exposure technique.

The compositions of the invention are exposed to radiation of wavelength in the 200~800 nm range. Suitable sources of such radiation, in addition to sunlight, include carbon arcs, mercury-vapor arcs, fluorescent lamps with ultraviolet radiation-emitting phosphors, argon and xenon glow lamps, electronic flash units, and photographic-flood lamps. Other fluorescent radiation sources such as the tracings on the face of a cathode ray tube may be used. Electron accelerators and electron beam sources through an appropriate mask may also be used.

Where artificial radiation sources are used, the distance between the photosensitive layer and the radiation source may be varied according to the radiation sensitivity of the composition and the nature of the photopolymerizable polymer. Customarily, mercury-vapor arcs are used at a distance of 1.5 to 20 inches from the photopolymerizable layer. Radiation fluxes of 1010,000 yw/cm2 are generally suitable for use.

During the first exposure in preparing a positive polymer image, radiation having a wavelength of 200 to 380 nm is used, but it is not necessary that the wavelength be limited to this range. The radiation may have wavelengths over the entire range of 200 to 800 nm. In order to form an effective amount of inhibitor in the first exposure, at least about 20% of the radiation should be between 200 and 380 nm; and preferably at least about 30% of the radiation is within this range.

The radiation used during the second exposure should be substantially limited to wavelengths greater than about 380 nm. By "substantially limited" it is meant that the radiation is filtered to exclude greater than about 95% of the radiation at about 380 nm and below, or is otherwise limited to radiation greater than about 380 nm. Preferably, the radiation in the second exposure has wavelengths substantially limited to greater than 380 to 800 nm, and most preferably 400 to 600 nm.

Although all of the photopolymerizable compositions of this invention can be imaged with the second exposure radiation having wavelengths down to about 380 nm, shorter exposure times are generally experienced when wavelengths up to about 420 nm are filtered out since many of the nitroaromatic compounds absorb radiation that far out into the visible range. During the second exposure, a greater portion of the coating, typically the entire coated area, is struck by radiation with the result that free radicals are generated and polymerization takes place in the areas struck by radiation during the second exposure but not during the first exposure.

The length of time for which the compositions are exposed to radiation may vary upward from fractions of a second. The exposure times will vary, in part, according to the nature and concentration of the polymerizable compound and initiator, and the type of radiation. Exposure can occur over a wide range of temperatures, as for example, from -800C up to + 1 500C with selected compositions.

Preferred exposure temperatures range from 30 to +350C. There is an obvious economic advantage to operating the process at room temperature.

Imayewise exposure, for example in preparing printing plates, is conveniently carried out by exposing a layer of the photoactive composition to radiation through a process transparency, that is, an image-bearing transparency consisting solely of areas substantially opaque and substantially transparent to the radiation being used where the opaque areas are substantially of the same optical density; for example, a so-called line or halftone negative or positive. Process transparencies may be constructed of any suitable materials including cellulose acetate film and polyethylene terephthalate film. An example is the preparation of a positive working lithographic plate using the novel system of this invention.In a positive imaging system, polymer is ultimately formed under the opaque areas of the process transparency; that is, the areas not struck by radiation passing through the transparency.

Exposure of a plate coated with the nitroaromatic compound-containing photoactive composition to the full spectrum of a mercury-vapor lamp through a cellulose acetate or polyethylene terephthalate film negative causes rearrangement of the nitroaromatic compound to a nitrosoaromatic compound in the radiation-struck areas. The areas struck by radiation during the first exposure will become nonimage areas since no polymerization will be initiated in these areas.

Removal of the process transparency followed by a second exposure of the plate to radiation substantially limited to wavelengths greater than about 380 nm causes polymerization to occur in the areas which were not struck by radiation during the first exposure. Radiation of this wavelength is insufficiently absorbed by the nitroaromatic compound to rearrange it to a nitroso compound. The portions of the coating exposed in this manner become the polymeric image areas. Development of the doubly exposed plate provides a positive working plate suitable for use in lithography.

The exposed photosensitive layer may be developed by removing the unpolymerized, ethylenically unsaturated compound from the coating, leaving behind a polymeric replica of the original. This may be accomplished by heating the coating under conditions which result in some or all of the volatile components being vaporized, whereby the photopolymer is left behind. The conditions of thermal development selected will depend upon the nature of the substrates, the volatility of the components to be removed, and the thermal stability of the components. Alternatively, development may be achieved by solvent washout, thermal transfer, pressure transfer, or differential adhesion of the exposed versus unexposed areas, and the like. Preferably, polymeric images are developed by solvent washout.Alternatively, they may be developed without washout by differential adhesion of a pigment toner to the tacky unpolymerized areas.

The photopolymerizable compositions of this invention have the advantage that the same compositions used for the two-exposure positive-working applications described above may also be used to form negative polymeric images by the single exposure procedure. For this application, a layer of any of the photopolymerizable nitroaromatic compound-containing compositions described above is exposed through an image-bearing transparency to radiation substantially limited to wavelengths above about 380 nm until polymer is formed in the exposed areas. Unpolymerized portions of the photopolymerizable layer are then removed by solvent washout, or any of the other removal methods described above, to leave a negative polymeric image of the pattern of the transparency employed.

The photopolymerizable compositions of this invention have the very special advantage that the imagewise exposure of the positive-working process can be carried out using polyethylene terephthalate process transparencies. This is possible because the nitroaromatic compounds are sensitive to radiation of wavelength about 366 nm which is readily passed by polyethylene terephthalate film. Most of the previously known sources of nitroso inhibitors in positive-working photopolymerizable compositions have required activation by radiation having wavelengths below about 330 nm which is screened out by polyethylene terephthalate film. Thus, the present invention is useful with the latest, and most preferred, image-bearing transparencies.

The following example further illustrate the compositions and methods of this invention. All parts and percentages are by weight unless otherwise specified.

Example A photosensitive composition was prepared containing the following ingredients: Components Parts Methyl methacrylate/methacrylic acid copolymer (9:1) 60.86 Trimethylolpropane triacrylate (monomer) containing 235-265 ppm hydroquinone inhibitor 21.31 Triethylene glycol dicaproate and triethylene glycol dicaprylate 6.75 2-(o-chlornphenyl)-4,5-diphenylimidazolyl dimer 6.18 1 -(2'-nitro-4',5'-dimethoxy)phenyl-1 -(4-methoxyphenoxy)ethane 2.81 2,5-bis(4'-diethylamino-2'-methylbenzylidene)-cyclopentanone 0.90 Cl. Solvent Red 109 0.95 Leuco Crystal Violet 0.19 1 ,4,4-trimethyl-2,3-diazabicyclo(3.2.2)non-2-ene-N,N'-dioxide 0.05 A coating solution was prepared by dissolving the components in enough 2-ethoxyethanol to obtain a solution containing 19% solids. The solution was spin-coated onto an anodized aluminum plate; after drying at 380C, the dried photopolymerizable coating weight was 20 mg/dm2. After the plate was cooled to room ternperature, it was overcoated with an oxygen-barrier polymer using an aqueous solution (10% solids) of polyvinyl alcohol (medium visocity, 87 -89% hydrolyzed). Afer drying at 380C, the overcoat weight was 19 mg/dm2.

The negative- and positive-working modes for the plate were then evaluated by exposure, through a step wedge transparency in contact with the plate in a vacuum frame, for 20 units with a 2 Kw Hg photopolymer lamp in a Berkey Askor unit where the distance from plate to bulb is 96.5 cm. Part of the plate was blocked off with a piece of blank polyethylene film. The transparency was then removed and placed in the area where the black film had been, a UV filter that transmits light above 420 nm was placed over the plate, and an overall exposure to visible light with a 100 unit exposure with the same source was made. The plate was developed using a solution of sodium carbonate (1.8%) and diethylene glycol monobutyl ether (8%) in distilled water for 30 seconds at 220C, rinsed with water while lightly rubbing with a cotton pad, and blotted dry. The positive image consisted of two completely unpolymerized steps (steps 1 and 2). The negative image consisted of an image where the last partially polymerized step was step 12.

The polymerized areas on the developed plate accepted conventional lithographic greasy inks and the clear areas of the aluminum support were readily wetted with water to yield a high quality positive and negative working lithographic plate.

Claims (31)

Claims

1. A photopolymerizable coating composition which comprises: (a) nongaseous, ethylenically unsaturated compound capable of addition polymerization by freeradical initiated chain propagation, (b) 0.004 to 0.7 part by weight, per part of component (a), of nitroaromatic compound of the formula

wherein:: R1, R2, R3 and R4, alike or different, are H, OH, halogen, NO2, ON, alkyl of 1 to 18 carbons, alkoxy in which the alkyl is of 1 to 18 carbons, acyloxy of 2-7 carbons, aryl of 6 to 18 carbons, benzyl, halogensubstituted phenyl, polyether of 2 to 18 carbons and 2 to 10 oxygens dialkylamino in which each alkyl is of 1 to 18 carbons, thioalkyl in which the alkyl is of 1 to 18 carbon's, or thioaryl in which the aryl is of 6 to 18 carbons, R2 and R3, taken together, are -0CH20- or O#(#CH2CH2O#)q in which q is an integer from 1 to 5, or any two vicinal R1, R2, R3, and R4 groups, taken together, are the residue of a second benzene ring fused onto the benzene nucleus, with the proviso that not more than one of R" R2, R3 and R4 is OH or NOo, R5 is H, alkyl of 1 to 18 carbons, halogen, phenyl or alkoxy in which the alkyl is of 1 to 18 carbons, R6 is aryloxy of 6 to 18 carbons, unsubstituted or substituted with halogen, with alkyl of 1 to 6 carbons or with alkoxy of 1 to 6 carbons, or R5 and Re taken together are =N(alkyl) in which the alkyl is of 1 to 6 carbons or R1, R2, R3, R4 are alike or different and are as defined above, with the proviso that at least one of R1 to R4 is acyloxy of 2 to 7 carbons, polyether of up to 18 carbons and 7 to 10 oxygens or R2 and R3 taken together are -0-CH2- -0- or #O#(#CH2CH2O#)q# in which q is an integer from 1 to 5, R5 is H, alkyl of 1 to 18 carbons, halogen, phenyl, or alkoxy in which the alkyl is of 1 to 18 carbons.

R6 is H, OH, alkyl of 1 to 18 carbons, phenyl, alkoxy in which the alkyl is of 1 to 18 carbons, or aryloxy of 6 to 18 carbons unsubstituted or substituted with halogen, alkyl of 1 to 6 carbons, or alkoxy of 1 to 6 carbons, with the proviso that only one of R5 and R6 is H, or R5 and R6 taken together are =O,=CH2, --O--CH2--,=NC6H5' =NC@H5N(alkyl)2 in which each alkyl is of 1 to 18 carbons,--O--C2H4--O-, =N (alkyl) in which the alkyl is of 1 to 6 carbons, =N (hydrocor6yLens)

in which the hydrocarbylene group is of 1 to 18 carbons, or

in which R8 and R9, alike or different, are H or alkyl of 1 to 4 carbons, and R7 and Ft' , alike or different, are --CN,-- COR11 in which R11 is alkyl of 1 to 5 carbons, or --COOR12 in which R12 is alkyl of 1 to 6 carbons which may be interrupted by an oxygen atom, alkenyl of 2 to 5 carbons, or alkynyl of 2 to 5 carbons, or R7 and R8 together, or R9 and R10 together complete a 6-membered carbocyclic ring containing a keto group, and (c) 0.001 to 10 parts by weight, per part of component (a), of an organic radiation-sensitive, freeradical generator, activatable by actinic radiation that does not significantly convert the nitroaromatic compound to an inhibitor of free-radical polymerization.

2. The composition as claimed in Claim 1 wherein the free-radical generator has at least one component having an active radiation absorption band with a molar extinction coefficient of at least 50 within the range of greater than 380 to 800 nm.

3. The composition as claimed in Claim 1 or 2 wherein the composition also contains 10 to 80% by weight, based on the total solids content, of a polymeric binder.

4. The composition as claimed in Claim 3 wherein the polymeric binder is selected from vinyl addition polymers containing free carboxylic acid groups, polyacrylate esters and poly-a-alkylacrylate esters.

5. The composition as claimed in any of the preceding claims wherein component (a) is an unsaturated ester of a polyol and an a-methylenecarboxylic acid.

6. The composition as claimed in Claim 5 wherein the ar-methylenecarboxylic acid is selected from acrylic and methacrylic acids.

7. The composition as claimed in any of the preceding claims wherein component (c) is present in the amount of 0.01 to 2 parts by weight per part of component (a).

8. The composition as claimed in any of the preceding claims wherein component (b) is present in the amount of 0.04 to 0.1 5 part by weight per part of component (a).

9. The composition as claimed in any of the preceding claims wherein any alkyl groups in component (b) are of 1 to 6 carbons.

10. The composition as claimed in any of the preceding claims wherein component (c) has at least one component that has an active radiation absorption band with a molar extinction coefficient of at least 50 within the range of 400 to 600 nm.

11. The composition as claimed in any of the preceding claims wherein component (c) includes a compound selected from phenanthrenequinones and 2,4,5-triarylimidazole dimers.

12. The composition as claimed in any of the preceding claims wherein component (a) is trimethylol propane triacrylate.

13. The composition as claimed in any of the preceding claims wherein component (b) is 1-(2' nitro-4',5'-dimethoxy)phenyl- 1 -(4-methoxyphenoxy) ethane.

14. The composition as claimed in any of the preceding claims wherein component (c) is 2,2 bis(2-chlorophenyl)-4,4',5,5'-tetraphenylbiimidazole.

15. Method of making a positive polymeric image on a substrate which comprises: (1) ) coating the substrate with the photopolymerizable composition claimed in any of the preceding claims, (2) imagewise exposing the photocopolymerizable coating through an image-bearing transparency to radiation at least 20% of which has a wavelength of 200 to 380 nm, thereby converting at least some of the nitroaromatic compound to polymerization inhibiting nitrosoaromatic compound, and (3) subjecting the coating in a second exposure to actinic radiation substantially limited to wavelengths greater than 380 nm, whereby a positive polymeric image is formed in the areas exposed during the second exposure but not exposed during the imagewise exposure, and (4) developing the image formed in step (3).

16. The method as claimed in Claim 1 5 wherein at least 30% of the radiation in the imagewise exposure has a wavelength of 200 to 380 nm.

17. The method as claimed in Claim 15 or 16 wherein the radiation in the second exposure has wavelengths substantially limited to greater than 380 to 800 nm.

18. The method as claimed in any of Claims 15 to 17 wherein the polymeric image formed in step (3) is developed by removing the nonpolymerized portion of the photopolymerizable coating in the areas exposed to the imagewise exposure radiation.

19. The method as claimed in any of Claims 15 to 1 7 wherein the image formed in step (3) is developed by differential adhesion of a pigment toner to the unpolymerized portion of the photopolymerizable coating in the areas exposed to the imagewise exposure radiation.

20. The method as claimed in any of Claims 15 to 19 wherein the radiation in the second exposure has wavelengths substantially limited to 400 to 600 nm.

21. The method of making a negative polymeric image on a substrate which comprises: (a) coating the substrate with the photopolymerizable composition claimed in any of Claims 1 to 14, (b) imagewise exposing a portion of the photopolymerizable coating through an image-bearing transparency to actinic radiation substantially limited to wavelengths greater than 380 nm, whereby a negative polymeric image is formed in the areas exposed to the radiation, and (c) developing the image formed in step (b).

22. The method as claimed in Claim, 21 wherein the radiation has wavelengths substantially limited to greater than 380 to 800 nm.

23. The method as claimed in Claim 21 wherein the radiation has wavelengths substantially limited to 400 to 600 nm.

24. A method as claimed in Claim 15 substantially as herein described.

25. A method as claimed in Claim 21 substantially as herein described.

26. A photopolymerizable coating composition according to Claim 1 substantially as described herein.

27. A photopolymerizable coating cornposition substantially as described herein with reference to the Example.

28. A method of making a positive polymerie image on a substrate according to Claim 15 substantially as described herein and exemplified.

29. A method of making a negative polymeric image on a substrate according to Claim 21 substantially as described herein and exemplified.

30. A substrate bearing a polymeric image prepared by a process as claimed in any of Claims 15 to 25. 28 or 29.

31. A lithographic film or plate, photoresist, relief printing plate or thin-film resist comprising a substrate according to Claim 30.