Photoimaging and color proofing

Abstract

This invention concerns a procedure for photoimaging photosensitive coatings, development by selectively removing part of the coating, and transfer of the image to another support. The procedure is useful for production of multicolor images by multiple transfers of images from color separations.

Description

The present invention is concerned with photoimaging and photoprinting, particularly in aspects thereof useful in color proofing.

Various methods of image reproduction are known, including various ways of reproducing a picture by photoprinting. Some methods involve transfer of an image from one support to another, and effective ways of achieving such transfer have recognized value.

An object of the invention is to provide new and improved image transfer processes. A further object is to provide photoimaging procedures and compositions which are especially useful in such transfer processes.

SUMMARY OF THE INVENTION

The present invention involves image transfer processes in which images are formed and solvent-developed on a support and then transferred to another support. The invention also involves the use in such processes of photoimaged materials which are wellsuited to such transfer processes. A system particularly wellsuited is one involving a polymeric substrate in which an image is formed by peroxy groups bonded to the polymer. Photo-crosslinking or polymerizing systems can also be employed, such as systems involving iodoform and a diene polymer, or an acrylonitrile/diene polymer and a polynuclear aromatic-carbonyl compound sensitizer. The transfer processes are particularly useful for reproducing multicolor images, utilizing subtractive color components. Pigments are useful in providing the desired colors. Preparation of such multicolor images is particularly useful in color proofing, which involves preparing a photoproof from color separation transparencies, as a check on the accuracy of the transparencies before the transparencies are utilized in making a printing plate. Such procedures generally involve preparation of line or half-tone images, but present procedures can also be used for continuous tone images.

In another aspect the present invention is directed to a procedure for making multi-color images, in which images are separately formed on separate supports by a photoimaging procedure, solvent-developed, and then the developed images transferred in register to a single support. Alternatively, one of the images can be formed and developed on the final support, and the other images then transferred in register to it.

In the present invention a photooxidation imaging technique is particularly useful, the technique being one which involves formation of images in a polymer by oxidation of the polymer to form hydroperoxy groups in the imaged areas. The aforesaid techique is described in a copending application of Robert A. Heimsch and Eric T. Reaville, Ser. No. 644,121 filed June 7, 1967, and a continuation-in-part thereof Ser. No. 15,727, filed Feb. 16, 1971 and issued Feb. 5, 1974 as U.S. Pat. No. 3,790,389. The aforesaid photoimaging procedure produces latent images in terms of hydroperoxidized areas of the polymers. The hydroperoxidized areas differ in various properties from non-hydroperoxidized areas, and this makes possible various means of developing visible images, such as those depending on dye receptivity, solubility, conductivity, permeability, etc. In the present transfer processes, a photosensitive coating is provided on a temporary support. The coating is adherent to such support, but by a bond which is readily broken. Release paper can suitably be used for such support. In some aspects, the present invention involves solvent development techniques. The solvent is applied to the photosensitive coating after photoimaging, and either imaged or non-imaged areas of the coating are selectively removed, depending upon the type of solvent. The procedure does not require particularly good solvents, as it is only necessary to have solvents sufficient to swell or penetrate through the coating and fragment the coating or loosen its bond to the support, thereby permitting portions of the coating to be washed away. After portions of the coating are selectively washed away, leaving in effect, a developed relief image, the image is then transferred to another support. The transfer is by contact, ordinarily with heat and pressure. In one aspect the present invention involves a procedure in which a coating containing a latent image comprised of polymer containing hydroperoxy groups, and polymer not containing such groups, is developed with a solvent to remove selectively one of such areas, and the remaining coating in the form of an image is then transferred to another support by contact therewith, ordinarily with the application of heat and pressure. In the transfer process heat is applied to make the coating surface tacky. The heated coating then becomes adhesively bonded to the surface to which it is to be transferred. The resulting sandwich is then separated, ordinarily after cooling, and the coating adheres to the surface to which it is to be transferred, rather than to the original surface.

In carrying out the present invention a photosensitive coating is present on a surface to which it has low to moderate adhesion, and the coating is photoimaged while on such surface. The coating should have some adherence to the surface to permit some handling without removing the coating, and to give some stability and support to the image during development. If a coating has insufficient adherence to a particular support, the mere mechanical force of a solvent spray can dislodge it without regard to any solvent effect of the solvent employed. However, the adherence should be low enough to permit transfer therefrom. Thus the adherence to the photoimaging support should be less than the adherence to the receptive surface to which the image is to be transferred. Thus low energy surfaces should be used as the support for photoimaging, such as those characterized as release surfaces, or having release coatings or parting agents. Various materials are commonly used as release coatings, e.g., fluorocarbons, silicones, paraffins, soaps, mold release agents, etc.

In preferred aspects, the present invention involves use of pigmented photosensitized coatings. The pigments are primarily employed as colorants. However the pigments also have an effect on the polymeric film properties, contributing greatly to the action of various solvents in effecting removal of areas of the polymer from its support in developing images.

The present process has a number of advantages. A number of the advantages are mainly concerned with the transfer step. The present transfer process is a relatively simple, dry contact transfer. Prior art procedures have often involved the use of solvents, including water, in the transfer step. It can be seen that a dry contact procedure is more convenient than one involving use of a solvent to loosen the image or even float it away from its support prior to transfer. Moreover when it is necessary to use aqueous systems, there is a resulting problem of the dimensional stability of some support materials, particularly paper materials, including paperboard, as well as of the image materials themselves. Similarly, the development procedure in the present invention can utilize non-aqueous solvents, thereby avoiding problems of dimensional instability or disintegration of paper support materials as a result of spraying or other treatment with water.

The present invention includes both positive and negative working procedures. It can be seen that there is an advantage in being able to use the same photosensitive coatings in either a positive or negative system, depending upon convenience in a particular shop or with respect to a particular job. Also the present procedures have the proper combination of sensitivity and controllability. Thus, reasonably short exposures can be used in the photoimaging to obtain the desired degree of resolution and fidelity, but the systems are not so sensitive as to require extremely careful control of exposure times, development times, etc. or other variables, which would be a problem if carried out by various different people in various shops. The present invention is suitable for use in proofing the fine work done in off-set printing, which has around 150 lines to the inch, and there appears to be no reason for not employing it for up to 300 lines per inch or more. It can, of course, be used in less demanding applications, but most uses will probably involve imaging from half-tone transparencies having at least 60 lines per inch, and ordinarily 150 lines per inch. It will be noted that to hold high light dots at 150 lines per inch would involve producing dots of around 30 to 40 microns or less, assuming about 10 percent coverage in such areas.

In the preparation of multi-color prints, the present procedure has some time saving advantages. For example, some prior procedures involve a transfer to the final support, followed by the exposure and development step, for the image from each color separation. Thus for a 4-color proof, the procedure must be carried out four times, in series. In the present invention four exposures can be carried out simultaneously on separate supports, followed by four development procedures, and it is only the transfer which must be done in series.

In general the procedures for obtaining color separations from originals, making color separation half-tones therefrom, and the general use of such half-tones in producing colored images are well established. The present invention is mainly concerned with aspects of the procedure involving the use of color separations, particularly use of color separation halftones, but there is an interrelationship between such aspects of the procedure and the rest of the reproduction procedure. Color separations can be made from the original, employing red, green and blue filters. The color separation negative from the red flter, i.e. red light record negative, has the highest optical density in areas of the original transmitting red and the lowest optical density in areas absorbing red. This color separation negative can be converted to a positive in which the densities are reversed from the foregoing, and the positive can then be employed with a screen to produce a half-tone negative. In the half-tone negative the image is formed of dots which do not vary greatly in their optical density, the dots being of high density, substantially opaque, while non-dot areas are of low density, substantially transparent. The tone density depends upon the proportion of an area covered by dots. The half-tone negative resulting from the red filter will have the highest density, i.e., proportion of dots, in the area corresponding to the red transmitting area of the original, and the lowest optical density in areas of the original absorbing red. The red light record half-tone negative is then used to produce an image by the exposure, development and transfer steps taught herein. The resulting image has its lowest optical density in areas corresponding to those of the original which transmit red, and its highest optical density in the areas of the original absorbing red. The foregoing is accomplished by employing the photooxidation imaging as a negative working system, i.e., by removing parts of the photosensitive coating which are not photooxidized. Since the image has its highest density in red absorbing areas, a color subtracting red is used in such areas, namely cyan. The cyan pigmented photosensitive coating remains in the photooxidized areas following the solvent development. The image as developed then consists of cyan colored dots comprised of polymeric coating material and cyan colorant. The dots are then transferred to a second support. In use of half-tones, the dot coverage can run from less than 5 percent of an area to more than 95 percent of an area, and it should be recognized that the dot terminology is applicable even though the dot coverage in high density areas may be substantially contiguous. However, there will generally be some separate, discrete dots in an image, and the present procedures for half-tone imaging generally involve transfer of separate, discrete dots of coating material from one surface to another. In this connection it should be noted that the dot of coating material itself is transferred; that is, it is not a case of using the coating material as a printing surface from which to transfer ink or other coloring material to another surface.

The procedure for the other color separations corresponds to that for the red record, with the changes being in the filter used and in the colorant employed. Thus a blue filter is used to make the blue record negative, and the color subtracting blue which is employed in the photosensitive coating is yellow. A green filter is used to make the green record negative, and the color subtracting green which is employed in the photosensitive coating is magenta. Similarly, when four color pictures are made employing black in addition to the other colors, the appropriate color separation negative can be made in recognized manner, as by employing white light, or successive exposures with red, blue and green filters, and the color subtracting white, i.e. black, is then employed as colorant in the photosensitive coating.

In the procedural steps exemplified above, negative working was employed in that half-tone negatives were utilized to obtain the yellow, magenta and cyan images. If desired, halftone positives can be used with positive working to obtain the positive yellow, magenta and cyan image when photooxidation photoimaging is employed. This provides an advantage in the adaptability and flexibility of the procedures in that one proofing system can be used as either positive or negative working, as happens to be convenient to fit in with procedures of any particular printing or other copying system being employed. To have positive working, the exposed, i.e., photooxidized areas are removed. This can be accomplished by use of a polar solvent. Since the positive working is used with a half-tone positive separation, the photooxidized area which is removed in positive working is the same area of the coatig which is removed as nonoxidized area when negative working is employed with half-tone negatives. Thus the color relationship is same for the positive working system as for the negative, i.e., cyan is used with the red light record, magenta with the green light record, and yellow with the blue light record. It will be recognized, of course, that the transfer involves a lateral reversal of the image. This can be readily controlled by reversing or non-reversing the transparency in the exposure step in known manner, depending upon the direction from which the photosensitive coating is to be imaged, and the direction from which the transferred image is to be viewed, i.e., directly or through a transparent support. For example, when both the imaging and the viewing are to be done on opaque supports, the transparency is reversed for the imaging step to allow for the lateral reversal upon transfer. Also, if for some reason one of the images is produced directly upon the final support, it will have to be the reverse image of the other images on temporary supports which are then transferred in register to such image.

In the above description it will be recognized that there can be variations in the order of step from the original to color separation negatives, positives, half-tones, etc., and that the half-tone can be produced simultaneously with production of negatives or positives. Moreover there can be additional conversions from negative to positive to negative etc., for converting images, masking purposes, or other reasons. It will also be recognized that the present process is adaptable to various color correction techniques, such as those involving additional masking exposures known to the art. Moreover additional dyes or coloring materials can be used to give background colors, amplify images, etc., or for other reasons. The colors in the present procedure are ordinarily determined primarily by the colors of coating material, from dyes or pigments, which are not removed in a washing development step. However, additional colorants can be added, as for example dyes in solvent solution, and this addition can be selective as in the case of photooxidation photoimaging the photooxidized areas tend to accept polar materials and the nonphotooxidized areas to accept non-polar materials. While such additional colorants can be added at any time, ordinarily they will be added prior to the solvent-development step. Then the colorant will selectively absorb, e.g., on photooxided areas, and any excess can be washed off with the development solvent. The colorants can be added after development, but then selectivity depends on the imaged areas compared to the bare support in developed areas. Colors are more difficult to wash out of the voids in a developed image, and if left, they may transfer to some extent in the transfer step.

The present invention involves use of a photosensitive layer containing carbon-to-carbon double bond unsaturation. Such layers are capable of undergoing crosslinking, polymerization, or oxidation reactions under the influence of light to effect a change in properties which can produce a latent image which is subject to development and transfer as taught herein. The photosensitive layer should be relatively stable in form to prevent undue migration of compounds containing the unsaturated group, if such compound should be of relatively low molecular weight or liquid. Ordinarily any film-forming materials will provide a stable matrix to prevent migration of the oxygen containing substituents with polymeric materials generally being used. Of course, some of the layer materials disclosed herein have both the unsaturated groups and the properties for forming stable films or coatings, e.g., high molecular weight polymers containing residual unsaturation, such as styrene/butadiene copolymers. Thus in general there will be no need for using separate binder materials, but the use of such are fully consonant with the present invention. Even though a particular polymer in itselt has all the properties necessary to serve as a photosensitive substrate, it may nevertheless also be used in conjunction with other polymeric binder materials which do not have unsaturated groups, or in conjunction with other materials which do have such groups.

The photosensitive layers include photopolymerizable layers containing carbon-to-carbon double bond unsaturation, i.e., ethylenically unsaturated compounds, and suitable binders therefor, such as the photopolymerizable materials disclosed in Planbeck U.S. Pat. No. 2,760,863, and Celeste U.S. Pat. No. 3,469,982. Such photopolymerizable materials can be activated as taught in the aforesaid patents, for example by actinic light and various free radical catalysts, for example polynuclear quinones which are compounds having two intracyclic carbonyl groups attached to intracyclic carbon atoms in a conjugated carbocyclic ring system. Suitable such initiators include 9,10-anthraquinone, 1-chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone, 2-methyl-1,4-naphthoquinone, 2,3-dichloronaphthoquinone, 1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone, 2-phenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt of anthraquinone alphasulfonic acid, 3-chloro-2-methylanthraquinone, retenequinone, 7,8,9,10-tetrahydronaphthacenequinone, and 1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione. Other photoinitiators which are also useful, even though some may be thermally active at temperatures as low as 85.degree.C., are described in Plambeck U.S. Pat. No. 2,760,863 and include vicinal ketaldonyl compounds, such as diacetyl, benzil, etc.; .alpha.-ketaldonyl alcohols, such as benzoin, pivaloin, etc., acyloin ethers, e.g., benzoin methyl and ethyl ethers, etc., .alpha.-hydrocarbon substituted aromatic acyloins, including .alpha.-methylbenzoin, .alpha.-allylbenzoin and .alpha.-phenylbenzoin. The photosensitive layers also include layers containing ethylenic unsaturation which are photocrosslinkable, such as materials disclosed in Celeste U.S. Pat. No. 3,526,504, including polyvinyl cinnamate and activators therefor. It is not seen to be necessary to determine whether particular materials would react or be classed as photopolymerizable or photocrosslinkable, or both, as the consequent change in properties permits development of images, aside from the particular mechanism. the photosensitive layers include the photosensitive compositions disclosed in the copending application Ser. No. 15,856 of Richard M. Anderson and Robert A. Heimsch filed Mar. 2, 1970, involving ethylenically unsaturated polymer and a halogen compound sensitizing agent, and can also include the metal organic compounds there disclosed; suitable sensitizing agents are, for example, iodoform or carbon tetrabromide, and metal organic compounds include various chelates, enolates, and fatty acid salts, e.g., copper octoate. The photosensitive layers also include the acrylonitrile/diene interpolymers with activated aromatic ketone sensitizers, as disclosed in application Ser. No. 40,417 of Robert A. Heimsch, John S. Bartosczewicz, and Robert J. Slocombe filed May 25, 1970. The acrylonitrile/isoprene, acrylonitrile/butadiene and other copolymers disclosed therein, as well as the polynuclear quinones and other sensitizers there disclosed can suitably be employed. It will be noted that need for or suitability of various sensitizers will vary with the particular unsaturated material, but suitable selections can be made in view of the foregoing teachings.

It is particulary preferred to use photooxidation photoimaging in the present invention because of versatility, convenient workability, and various other features of such procedures as pointed out in the present application. In such procedures the various layer materials containing unsaturation referred to herein can be employed. Any of the materials disclosed in the aforesaid applications Ser. No. 644,121 and Ser. No. 15,727 (C-11-21-0189) can be employed. The photooxidizable layer may be any natural or synthetic material containing suitable carbon-to-carbon unsaturation, which material is spreadable on a suitable base support such as a glass or metal plate, a plastic solid or sheet, or a paper sheet or board surface, etc., and is sufficiently non-volatile at the temperature used. For use of this invention at ordinary room temperature the substrate material should have a molecular weight above about 140 so that it will not be removed from the surface or from the reaction site by migration in the oxidized form or by evaporation from the treated surface. The photooxidizable substrate may contain the suitable carbon-to-carbon unsaturation as part of its structure or molecules containing suitable carbon-to-carbon unsaturation may be added thereto. Higher molecular weight polymers or other materials may be used as a binder for low molecular weight materials containing the unsaturation to provide a suitably stable layer or film for imaging.

Natural materials which may be used in photooxidation include rosin and the double bond containing components thereof, terpenes such as abietic-acid, neoabietic acid, maleopimaric acid, levopimaric acid, .alpha.-pinene, camphene, 3-carene, citronellol, aldehyde modified rosin materials such as formaldehyde modified rosins, and fortified rosin materials such as those obtained by reacting the rosin with alpha, beta-olefinically unsaturated polycarboxylic acids and anhydrides thereof, and partial and complete esters of such acids as maleic acid, fumaric acid, itaconic acid, aconitic acid, citraconic acid, etc., both saponified or unsaponified with an alkaline material. Other examples include the use of unsaturated fatty oils either in the glyceride ester form or in the free acid form. A few examples of such oils include olive, peanut, almond, neat's foot, pecan nut, lard, tung, safflower, cottonseed and soybean oils. Nonglyceride source unsaturated oils such as tall oil may also be used.

Unsaturated hydrocarbons of natural and synthetic origin may also be used in photooxidation. Examples of such materials include the aliphatic olefinically unsaturated hydrocarbons having an average of at least about 10 carbon atoms, e.g., 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-docosene, docosene, 1-pentacosene and the internally unsaturated olefins such as 7-heptadecene, 7,10-heptadecadiene, etc., the aromatic olefinically unsaturated hydrocarbons in the unpolymerized form such as isopropenyl toluene, phenylisobutene, phenylhexadiene and isopropenyl naphthalene.

Natural and synthetic polymeric materials containing unreacted carbon-to-carbon double bonds therein may also be used as the photooxidizable substrate material in practicing this invention. The carbon-to-carbon unsaturation may be intralinear, e.g., --CH.sub.2 --CH=CH--CH.sub.2 --, a vinylene linkage, terminal, e.g., --CH.sub.2 --CH=CH.sub.2 vinyl, ##SPC1##

vinylidene and the like. Attached groups to the aforedescribed entities may be linear or branched. In general, the polymeric backbone will be hydrocarbon in structure with any halide, ester, ether, hydroxyl, nitrile, phenyl or other group present in the polymer molecule appended to the polymeric backbone.

It will be understood that the vinyl compounds are a species of vinylidene compounds since they contain the characteristic CH.sub.2 CH-- group, the indicated free carbon valence being satisfied by another atom in the polymer molecule. The term "vinylidene" is used herein to include both vinylidene and vinyl unsaturation.

Illustrative examples of these olefinically unsaturated polymers useful in photooxidation include natural rubbers; homopolymers, copolymers, and polymers from three or more monomers, prepared from diolefins such as butadiene, isoprene, 2,3-dimethyl- 1,3 -butadiene, piperylene, chloroprene, bromoprene, 2-acetoxy-butadiene - 1,3,2-methyl-pentadiene, 2-ethylhexadiene; and polymers prepared from diolefins such as those aforementioned and compounds containing a vinyl or a vinylidene group such as

a. Vinyl ethers, e.g. vinyl alkyl ethers such as vinyl ethyl ether, vinyl butyl ether, vinyl octyl ether, vinyl dodecyl ether, vinyl tetradecyl ether, vinyl hexadecyl ether, viny octadecyl ether and vinyl alkenyl ethers, e.g., vinyl ether, vinyl octenyl ether, vinyl tetradecenyl ether, vinyl octadecenyl ether;

b. Vinyl esters, e.g. vinyl acetate, vinyl butyrate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate;

c. Vinyl halides, e.g. vinyl chloride, vinyl bromide;

d. Vinyl ketones, e.g. vinyl methyl ketone;

e. Vinyl sulfides, sulfoxides and sulfones, e.g. vinyl ethyl sulfide; vinyl propyl sulfoxide, vinyl tert-butyl sulfone;

f. Vinylidene compounds, e.g. vinylidene chloride;

g. Acrylic, methacrylic acids or crotonic acids and their derivatives, e.g. acrylic acid, acrylonitrile, methacrylamide, crotonamide;

h. Acrylic, methacrylic esters or crotonic esters, e.g. methyl methacrylate, ethyl acrylate, propyl acrylate, amyl acrylate, heptyl acrylate, octyl methacrylate, nonyl acrylate, undecyl acrylate,tetradecyl acrylate, hexadecyl acrylate, octadecyl acrylate, ethenyl acrylate, hexenyl methacrylate, dodecenyl acrylate, octadecenyl acrylate, ehtyl crotonate;

i. Allyl esters, e.g. allyl acetate, allyl butyrate, allyl caprylate, allyl caprate, allyl laurate, allyl myristate, allyl palmitate, allyl stearate;

j. Allyl alkyl ethers, e.g. allyl ethyl ether, allyl octyl ether, allyl dodecyl ether, allyl tetradecyl ether, allyl hexadecyl ether, allyl octadecyl ether, and vinyl alkenyl ethers, e.g. allyl ethenyl ether, allyl octenyl ether, allyl tetradecenyl ether, allyl octadecenyl ethers;

k. Cycloaliphatic vinyl compounds, e.g. vinyl cyclohexane;

l. Aryl vinyl compounds, e.g. styrene, vinyltoluene, vinylbiphenyl, vinyl naphthalene and the ar-chloro substituted styrenes;

m. Heterocyclic vinyl compounds, e.g. vinyl pyridine and vinyl dihyropyrane;

n. Alpha-olefins, e.g. ethylene, propylene, butene-1, octene-1, dodecene-1, tetradecene-1, hexadecene-1 and heptadecene-1, dichloroethylenes, tetrafluoroethylene; and

o. Branched olefins, e.g. isobutylene, isoamylene, 2,3,3 -trimethyl-1-butene.

It is to be understood that the unsaturated polymers which are used in the practice of this invention can also be prepared by copolymerization of two or more different diolefins, e.g. from a mixture of butadiene and piperylene, either in the presence or absence of one or more non-dienic copolymerizable monomers.

The physical characteristics of the olefinically unsaturated polymers which can be photooxidized in accordance with the present invention may vary from low molecular weight polymer oils containing relatively few olefinic bonds to high molecular weight rubbers and resins such as those resulting from the polymerization or copolymerization of diolefins in the presence or absence of one or more non-dienic copolymerizable monomers.

The photoimaging by photooxidation utilized in the present invention depends upon the change in properties caused by addition of oxygen to molecular structures of the photosensitive substrate, and appears to a considerable extent to be due to the polarity of hydroperoxy groups in such molecules. The photooxidation makes possible differentially profound point-by-point property changes in a film due to differential distribution of non-migrating hydroperoxy groups in such film. The differential distribution of hydroperoxide accurately reproduces the intensity of light to which each differential area was exposed and the difference in chemical and physical properties of the hydroperoxidized regions compared to the non-exposed regions results in a latent image which can be rendered visible by difference in dye sorption, solubility, surface tack or other properties. For the differences caused by the photooxidation to be most evident, it is desirable that the original photosensitive composition not have groups with properties similar to hydroperoxides, although some such groups can be tolerated, even though contributing to an objectionable background in some photoimaging procedures. Such polar groups as carboxy, hydroxyl, nitrile, etc. may mask the photoimaging effect of hydroperoxy groups to some extent. For this reason some of the preferred polymers for use herein are hydrocarbon polymers with residual unsaturation, e.g., styrene/ butadiene copolymers, etc. However, for the effect upon the original solvent solubility, or upon mechanical film properties it may at times be desirable to use polymers having various substituents, such as halogen, nitrile, carboxyalkyl, etc, and polymers containing such substituents can be used if some loss in photoimaging sensitivity can be tolerated in the particular application in view. In general it will be preferred that polar monomers not constitute more than three-fourths of the monomer content of a copolymer used as the photooxidizable substrate in the present invention.

Aside from characteristics of the photosensitive compositions for forming latent photoimages in accordance herewith, various other mechanical or other properties of the polymers will have significance with respect to ease of coating application, durability and handling properties of the film, but in general those skilled in the art will be able to select appropriate materials for particular applications, particularly in view of the present disclosure.

In general solid polymers having carbon-to-carbon double bonds with allylic hydrogen adjacent thereto are suitable for photooxidation usse herein. Such polymers will preferably be substantially linear and have a weight average molecular weight of at least 50,000, and at least one such carbon-to-carbon double bond for about every 50 carbon atoms; a more limited class of such polymers are hydrocarbon in structure to avoid possible adverse effects of polar or other substituents.

Photosensitizers used herein for catalyzing the preferred photooxidation type of reaction are referred to generally as being of the porphyrin type. The porphyrin type of photosensitizer may be described as any compound having the porphin structure, i.e., four pyrrole rings connected by single carbon or nitrogen atoms, which includes related compounds such as the porphyrazines, phthalocyanines or chlorophylls.

Particular photosensitizers useful in practicing photooxidation in the present invention are the aromatic group mesosubstituted porphin compounds. Among such aromatic substituted porphins are the ms-tetraarylporphins (ms-meso). These compounds are those porphins in which aryl groups having from 6 to 24 carbon atoms are substituted on the bridging carbon atoms of the porphin ring structure which contains four pyrrole nuclei linked together in a circular pattern by four bridging carbon atoms to form a great ring. Examples of aryl groups which may be substituted in the meso-position of these compounds are phenyl, chlorophenyl, dichlorophenyl, methylphenyl, N,N-dimethylaminophenyl, hydroxyphenyl, naphthyl, biphenyl, anthracyl, phenanthryl, etc. In addition to the substituents in the aryl group substituents noted above, the aryl groups can also have any or a combination of such substituents, e.g., as alkyloxy (1 to 20 carbon atoms) substituents such as methoxy, ethoxy, isopropoxy, butoxy, hexyloxy, etc., as well as other substituents, particularly those which do not change the fundamental aromatic character of the groups. These porphin sensitizers including the above exemplified arylporphin, can have various other substituents, particularly at the beta and beta' positions of the pyrrole rings, e.g., such substituents as lower alkyl (1-20 carbon atoms) such as vinyl or allyl or alkanoic acid groups such as methylcarboxy or ethylcarboxy.

Examples of porphin compounds which are useful as photochemical sensitizers in practicing this invention are the arylporphins such as the tetraphenyltetrazoporphins and the complexes thereof, such as diamagnetic complexes, e.g., magnesium tetraphenyltetrazoporphin, tetraphenyl tetrazoporphin acetate, tetraphenyltetrazoporphin sufate, zinc tetraphenyltetrazoporphin; the meso-aryl porphins including alpha, beta, gamma, deltanaphthylporphin and the diamagnetic metal chelates thereof, e.g.,

tetraphenylporphin

tetrakis( 2,4 -dichlorophenyl)porphin

tetrakis( 2-furyl)porphin

tetrakis( 4-methoxyphenyl)porphin

tetrakis( 4-methylphenyl)porphin

tetrakis( 2-thienyl)porphin

tetraphenylporphin zinc complex

tetrakis( 4-nitrophenyl)porphin

tetrakis( 4-dimethylaminophenyl)porphin

zinc complex;

the tetrabenzomonoazo- and tetrabenzodiazo porphins, the 1,2,3,4,5,6,7,8 -octaphenylporphins and azoporphins such as octaphenylporphyrazine, the tetrabenzoporphins, e.g., tetrabenzoporphin and the zinc complex of tetrabenzoporphin.

Other useful porphin types of photosensitizing materials which may be used include chlorophyll, such as chlorophyll a and chlorophyll b, hemin, the tetrazoporphins, chlorophyllin salt derivatives such as the reaction product of an alkaline metal chlorophyllin salt and sodium bisulfite, hematoporphin, mercury proto- and hemato-porphins, vitamin B.sub.12 and its derivative tetrakis (1-naphthyl) porphin, porphin, ms-tetraethylporphin, ms-tetramethylporphin, and other ms-alkyl and alkoxy prophins.

Related porphin type materials which may be used include the phthalocyanines including the metal-free phthalocyanine and metal complexes of phthalocyanine such as the zinc and magnesium, complexes of phthalocyanine, as well as phthalocyanine derivatives such as the barium or calcium salts of the phthalocyanine sulfonic acid, acetylated phthalocyanine, alkoxy- and aryloxy-benzosubstituted phthalocyanines, 5,5' , 5", 5'"tetraamino-metal phthalocyanine- 4, 4', 4', 4'"-tetrasulfonic acid; magnesium tetra(4) methylthiophthalocyanine, arylthioethers of phthalocyanines, vinyl group containing tetraazoporphins and polymers thereof, mercaptoamino phthalocyanine derivatives and phthalocyanine.

Other useful photosensitizers which can be used include fluorescein type dyes and light absorber materials based on a triarylmethane nucleus. Such compounds are well known and include Crystal Violet, Malachite green, Eosin, Rose Bengal and the like.

Another group of photosensitizers particularly useful in the ultraviolet region include the aromatic compounds such as acetophenone, benzophenone, benzoin, benzil and triphenylene.

Light necessary in the practice of this invention can vary considerably in wave lengths, depending on the sensitizer in this system. The light can be monochromatic or polychromatic. Light of wave lengths in the range of 3600 to 8000 Angstroms has been found very suitable.

Light sources suitable for use in the practice of this invention include carbon arcs, tungsten and mercury-vapor arcs, fluorescent lamps, argon glow lamps, electronic flash units, photographic flood lamps and sunlight.

The present invention is particularly concerned with imaging procedures utilizing photosensitized oxygen transfer reactions in which light in the presence of a sensitizer causes the oxygen to oxidize the carbon-to-carbon unsaturated substrate by being added to one carbon atom of a double bond with shift of the double bond to the allyl position and movement of the allylic hydrogen atom to the oxygen atom of the oxygen molecule which is not attached to the carbon atom. The reaction can be postulated: ##SPC2##

The aforesaid oxygen transfer reaction does not include autooxidations, proceeding by a free radical mechanism in which irradiation with light serves to initiate free radicals and cause the formation of free radical sites in the substrate by hydrogen abstraction. The true photosensitized oxidation or oxygen transfer reactions used herein are characterized by the fact that they can proceed using wavelengths of light which may be ineffective for autooxidation and by the fact that in general ordinary oxidation inhibitors do not retard the reaction.

The photosensitized oxygen utilized herein involves what is referred to be Gollnick and Schenck (K. Gollnick and G. O. Schenck, Pure and Applied Chemistry, Vol. 9, 507 [1964]) as a Type 2 reaction, or "photosensitized oxygen transfer." The reaction involves some excited oxygen species, whether pictured as an oxygen molecule itself in an excited singlet state, or an excited sensitizer-oxygen adduct. Irradiation with light appears to transform the sensitizer to an excited state, such as a triplet state: ##SPC3##

The sensitizer then transfers its energy to oxygen. This can be postulated as ##SPC4##

and the excited singlet delta oxygen can then add to the double bond as pictured above. It should be understood that if the triplet energy of the sensitizer is above 37 kcal singlet sigma oxygen may also be produced. It may react directly or decay to the lower energy delta species. The reaction involves light energy to excite the sensitizer and produce an excited state oxygen which reacts with the substrate to cause addition of oxygen to one of the doubly-bonded carbons thereof. The reaction ordinarily does not include any chain propagation, but only one oxygen addition per photon absorbed at quantum yield of unity. The photosensitized reaction can proceed using wave lengths of light other than ultraviolet, and ordinary oxidation inhibitors do not inhibit the reaction. A sensitizer-oxygen adduct is presumably formed but is apparently short-lived; however it should be understood that the oxidation is effective regardless of what the mechanism and exact contribution of the adduct to the oxidation of the olefin may be.

In another aspect, the present invention can utilize an oxidation reaction in which a hydrogen atom is abstracted to give a radical and oxygen is then added. For example the Type 1 reaction in the foregoing Gollnick and Schenck article involves a reaction in which the allylic hydrogen atom at C-3 is abstracted to give a mesomeric mono-radical ##SPC5##

and oxygen is then attached at either of radical sites C.sub.1 or C.sub.3, and the peroxyradicals thus formed extract hydrogen from the C-3 position of the olefin to give hydroperoxides and a new mesomeric radical, thus permitting chain propagation. Sensitizers which are voracious hydrogen abstractors when excited by light energy are suitable for use in such reaction. One group in such a class is the carbonyl compounds, as for example, benzophenone, acetophenone, etc. Aside from olefins containing allyl hydrogen, other types of materials containing labile hydrogen can undergo reactions similar to that of Type 1 to produce materials containing hydroperoxy groups, and such materials can be used for photoimaging in accordance with this aspect of the invention, for example, such materials as ditetrahydrofurfuryl phthalate, di-2-ethylhexyl phthalate, compounds containing tertiary hydrogen, etc. If such materials are liquid they will generally be used in conjunction with a polymer or some other high molecular weight binder to provide a suitable matrix. In oxidations involving abstraction of labile hydrogen to generate free radicals, other reactions such as crosslinking can also occur and may interfere with the desired photoimaging reaction, and such oxidations, while part of the present invention and useful to some extent, are not in accord with the preferred aspects of the present invention.

Sensitizers which can function in both Type 1 and Type 2 reactions include benzophenone, acetophenone, benzil, benzoin, etc. These carbonyl type sensitizers require ultraviolet light of suitable wave length for excitation. Sensitizers which also function in both Type 1 and Type 2 reactions, but are active in the visible region as well as the ultraviolet region of the spectrum include eosin, fluorescein, rose bengal, etc. Sensitizers which are not hydrogen abstracters and which function largely or wholly by energy interchange as in Type 2 reactions, include the various porphyrin type photosensitizers. In general photosensitizers capable of Type 2 sensitization are much preferred, and ordinarily the better ones of this Type are not very effective as Type 1 photosensitizers. Moreover, the absence of Type 1 activity may make it easier to minimize side reactions. There will be variations in the effectiveness of sensitizers with the physical type of phase involved and it will be understood that the sensitizers utilized herein will be those effective under the conditions of use. The sensitizers described above are known to function in liquid phase. In the practice of this invention some systems have involved the post-application of a sensitizer solvent system to an already pre-formed film. the resulting system can be called a quasi-solid phase as it consists of a solid polymer lightly swollen with a swelling solvent system. In such a quasi-solid phase, the sensitizers listed above and similar photosensitizers are found to effect photochemical addition of oxygen, although with great variation in effectiveness. In the practice of the invention it is also feasible to use solid phase systems in which the sensitizer has been incorporated into a polymeric film composition which has not been swollen by solvent. In the solid phase such sensitizers as rose bengal, fluorescein, eosin, methylene blue, etc. did not exhibit significant photoimaging capability in reasonable exposure times. Benzophenone, acetophenone, benzil, etc., were found to require high level of application, circa 10 percent, and prolonged exposure, circa 30 minutes at ultraviolet flux of 7 .times. 10.sup.5 ergs/cm.sup.2 /sec. in order to show substantial photoimaging capability. In contrast to this, porphyrins, such as tetraphenyl porphin are very active at levels as low as 0.1 percent at a flux of 2.5 .times. 10.sup.5 ergs/cm.sup.2 /sec. for 1 to 2 minutes. Triphenylene in the solid phase seems to function largely by crosslinking, making it of little value for continuous tone image production.

The photosensitizers can be placed in solid phase by incorporation into a coating solution, emulsion, melt, or suspension etc. and application to support. After evaporation, drying, or other means of removing volatile solvents or other liquid medium, the photosensitizer remains in the residual coating composition dispersed in solid form in the solid composition. Even if the photosensitizer is incorporated by post-application with a solvent, it can be converted to the solid state by permitting the solvent to evaporate. For ease of handling and reproducibility, it is preferred that the photosensitive compositions be dry at the time of photoimaging. It will be recognized that in referring to the photosensitizer as being in a solid medium, it is not meant to exclude such resilience, elasticity or other properties as may be desirable in photosensitive films for various purposes, and that plasticizers or low molecular weight materials may be present for such purposes. Thus the solid state contrasts with the quasi solid state in which liquid is present to swell the polymer structure as in a plastisol or organosol and to provide a liquid medium in which the photosensitizer can be present, aside from how much of the photosensitizer is actually present in such medium.

The photosensitizer as employed will generally be of a type suitable for producing a desired amount of oxygen addition in the particular photosensitive composition upon degrees of light exposure within ranges of practicality for some photoimaging applications. For most applications it will be desirable that a required energy absorption be no greater than 10.sup.7 ergs/cm.sup.2, or possibly 10.sup.8 ergs/cm.sup.2, in some applications, and preferred that such absorption be sufficient with a concentration of photosensitizer no greater than 1% by weight of the photosensitive composition, for example with a flux sufficient to obtain such absorption over a 2-minute exposure period. The energy required for imaging will generally lie in the range of 1 to 10 .times. 10.sup.6 ergs/cm.sup.2. Absorptions with usual imaging procedures are frequently of the order of 2.5 .times. 10.sup.4 ergs/cm.sup.2 /sec. The foregoing energy ranges are those generally employed with the concentrations of photosensitizer generally used, but increasing the concentration makes it possible to use lower energy input for the photoimaging as the imaging requirements are approximately in inverse proportion to the photosensitizer concentration. The use of various amplifying means also makes it possible to lower the energy requirements. In order to have practical value in a photoimaging system, a photosensitizer as used should produce an oxygen uptake of at least about 10.sup.-.sup.9 moles/cm.sup.2 with an absorbed energy of 10.sup.7 ergs. A styrene-butadiene polymer (40 wt. parts styrene/60 wt. parts butadiene) film can suitably be used for determining oxygen uptake at particular energy absorptions.

Only a portion of the applied light flux will be absorbed by the photoimaging substrate, with that portion varying with the absorption characteristics of such substrate, as well as with the wave length of the light. Absorptions may, for example, be less than 10 percent, or of the order of 7 percent in some cases. Visible light can effectively be used in the present invention, and this is a definite advantage as it avoids the cost and loss in efficiency which results from having to produce light in particular ranges, such as the ultra violet range. In the present invention it is not necessary to utilize the ultra violet range, and in fact, the range of 4000 to 4500 angstroms appears most efficient. If desired, light in ranges above 4000 angstroms can be used to avoid possible ultra violet catalysis of competing reactions, although this is not ordinarily necessary. While visible light is effective, the present materials can generally be handled in ordinary daylight, such as by removing the photosensitive material from the intense light used for exposure, and carrying out development or other steps without special precaution to avoid further exposure to ordinary ambient light.

The preferred sensitizers for use herein belong to the class of porphyrins which are compounds with pyrrole rings linked together by carbon atoms to form a conjugated double bond structure, and in which one or more of the carbon atoms, i.e. methine groups, can be replaced by a nitrogen atom, and which class also includes the phthalocyanines and benzoporphyrins, as well as the meso-arylporphyrins, or other compounds having various substituents on the basic porphyrin structure. The tetraaryl porphyrins, particularly tetraphenyl porphyrin, are characterized by the capability to utilize energy from three of the main areas of the visible light spectrum, i.e., the violet, yellow and red areas and therefore to make efficient use of light energy. Such sensitizers can be termed panchromatic photooxidation sensitizers and are particularly valuable. Some other sensitizers are active only in limited areas of the spectrum, for example chlorophyll absorbs mainly in the red region. Aside from efficiency, the panchromatic sensitizers are advantageous for multi-color work in that it makes it possible to use a single sensitizer for exposures to reproduce different colors from an original, rather than having to change sensitizers for each of the main segments of the color spectrum. Tetraphenyl porphyrin absorbs most strongly in the violet, and less in the yellow and least in the red, which is advantageous in that visible light is the reverse, being strong in the red, etc., and therefore the sensitizer compensates for the variation in light-intensity and tends to equalize the effect of different intensities. Utilizing the panchromatic sensitizers, the present invention produces images from multi-color transparencies. If polychromatic light is used with a single exposure, the resultant image will not ordinarily exhibit appreciable differentiation between the colors, but the image produced will have parts corresponding to the various colors. The image can be developed by applying a single dye to give a monochromatic image e.g., red and white, or by using a solvent to remove areas either corresponding to the colored regions or the non-colored regions.

In preparing 4-color proofs in accord with the present invention, color separation exposures are formed on photosensitive polymeric coatings on supports. The coated supports are prepared by coating supports with a material comprising polymer, photosensitizer and pigment. Various procedures can be employed for coating the support, but the following exemplifies a suitable procedure. Mastergrinds of pigments are prepared containing in parts by weight, 30 parts pigment solids, 170 parts toluene, 0.2 part of a surfactant and 25 parts of a 20 percent solution of styrene/butadiene copolymer in benzene. The pigments are ground for a number of hours with ceramic balls. The polymer to be employed is placed in solution as a 20 percent solution in benzene by rolling pieces of the polymer in benzene on a ball mill. The polymer solution is sensitized by adding sensitizer thereto, e.g. tetraphenylporphin for photooxidation photoimaging. The desired amount of sensitizer can be added dissolved in 60 parts by weight of chloroform which is added to 50 parts of the polymer solution, along with 50 additional parts benzene. Varying amounts of sensitizer can be used, for many applications a suitable amount is 0.25 percent by weight of tetraphenylporphin on the total polymer for the yellow, magenta and cyan coatings, and 0.5 percent for the black coating. Coating slurries are made from the pigment grinds and sensitizer, polymer solution, coveniently by adding the polymer solution slowly with stirring to the pigment grind in a brown bottle, and adding benzene to obtain the desired solids content. The slurries can conveniently be used as 6 percent solids concentrations (polymer and pigment). Suitable pigment loadings, are, for example, 25 parts per hundred yellow and cyan, and 20 parts per hundred of magenta and black, the parts being by weight on the polymer. It may be desirable to filter the slurry to insure absence of large particles, e.g. through a seven micron opening nylon screen. The coating slurries are then used to coat a support appropriate for transfer, e.g., a release paper. The coating can be accomplished by usual procedures for applying liquid coatings, for example by using wire wound rods, such as a No. 10 for yellow and cyan, No. 14 for magenta, and a No. 12 for the black coating. If the coated papers are to be stored before use, they can be faced with another sheet of release paper to prevent damage to the coating.

The color separation exposures are formed on the coated release papers using the appropriate color separation transparency. A 4000 watt pulsed Xenon light can conveniently be used. During the exposure when a photooxidation is involved, the photosensitive coating must have access to oxygen. This can conveniently be provided for by having an air gap between the transparency and the photosensitive surface. For example, the coated support can be held flat on a vacuum plate. The transparency is on top of the coated support and held in register by register pins. The transparency is topped by a glass plate which is supported by spacers one-half mil thicker than the transparency. An air stream is then forced between the transparency and the coated support, forcing the transparency against the glass plate, thereby providing uniform spacing from the photosensitive coating, and providing oxygen for the photoxidation reaction. With the above light coil held 32 inches from the coated film, the flux as measured by a radiometer was 5.2 .times. 10.sup.5 ergs/cm.sup.2 /sec. Typical exposure times for photooxidation imaging with this flux are 2.5 minutes for yellow and magenta coatings, 2.75 minutes for cyan coatings, and 7 minutes for black coatings.

For development of the images, areas of the image are selectively removed by treatment with fluids. For negative working, the non-exposed areas are removed. This is generally accomplished by applying a liquid having some affinity for the polymeric coating but not very great solvent power for it, such as an aliphatic hydrocarbon when the polymer is a styrene/butadiene copolymer. For example, a varnish makers and painters naphtha with a KB value of 36 (Skellysolve V) can be used with a styrene/butadiene copolymer. Any type of washing procedure with such fluids can be employed, provided that care is exercised to avoid excessive mechanical stress on the coating. Spraying with the liquid has been found suitable, whether by hand with a nozzle, or mechanically with a gang of nozzles. Spray nozzles can, for example, be about 5 inches from the coating and operated from a tank under 25 psi. Passing back and forth before the nozzles for about 20 seconds is usually sufficient. In general the fluids employed in the development step will be liquids. However, materials having the proper affinity for the coating can also be employed in gaseous or vapor form, employing mild heating if desired. The spray nozzles can also employ air or other gases under pressure as an aid in spraying, or various other expedients employed in spraying paints or cleaning solutions. It is also feasible to employ other washing methods, such as immersing or dipping the coatings in the solutions, with mild agitation. This can at times be combined with blotting or wiping dry, although care must be exercised to avoid damaging the image and it is generally advisable to avoid rubbing the coating. In the dipping procedure the wash solvent becomes discolored with the pigment, and presents some redeposition problem, but this can ordinarily be overcome by changing the wash solution in the later stages.

The support utilized herein from which the image is to be transferred should have a surface to which the photosensitive coating has low to moderate adherence. Release papers used in casting of plastics in general have suitable surfaces if they can be coated with the photosensitive material. Such papers can have matt, dull, gloss or patent finishes, so long as the proper release properties are present. In paper or other materials, the release properties are generally provided by release or parting agents, such as waxes, silicones, fluorocarbons, metallic stearates, or other soaps, paraffins or other hydrocarbons, e.g., polyethylenes, or inorganic materials such as silica or silicates, e.g., calcium silicate. Such materials can be applied as a solution or suspension in a volatile solvent which evaporates, leaving a film of the release material on the surface. Such materials need not necessarily be a completely contiguous film as such materials can frequently be effective if incorporated into the body of the support material, as practiced with anti-blocking materials in plastics manufacture, or of various agents in paper manufacture. There are a number of proprietary silicones used as release agents, some of which are dimethyl siloxane resins. In general such resins, or lower molecular weight materials, having alternating silicon and oxygen atoms, with hydrocarbon substituents on the silicon, have release properties. The series of silicones manufactured by Dow Corning as release agents give release surfaces, including those of the 200 series, e.g., Dow Corning 203 fluid, or Dow Corning 230 Fluid, which is an alkylaryl polysiloxane fluid, or emulsions such as Dow Corning HV-490 emulsion which is a silicone emulsion prepared from 100,000-centistoke dimethyl polysiloxane fluid. Possibly the foregoing will be modified by additives, e.g., as Dow-Corning No. 23 silicone release coating has its release properties degraded by Dow-Corning C-4-2109 release additive. The desirable release coatings will vary somewhat with the properties of the photosensitive coatings employed, the surface properties of the support to which the image is to be transferred, and the development liquids and washing procedure employed. However in general the release surface will have a relatively low surface energy so that the developed image can be transferred to the higher surface energy receptive support. At the same time the photosensitive coating has to be sufficiently adherent to the release surface that it is not completely removed therefrom in the washing development. Any surface can be used for the receptive, i.e., transfer support, provided only that the photosensitive coating can be bonded to it. Paper, including paperboard, can be conveniently used for the transfer surface, either as uncoated paper stock, or coated paper. For example, high quality papers, bond paper, etc. are suitable, and in general most papers having a glossy surface. Cast coat, a high quality paper provided by the Albermarle Paper Company is satisfactory. Papers having the usual titanium dioxide, clay and synthetic binder, e.g. styrene-butadiene resin, coatings are suitable. Various other surfaces can be used for transfer surfaces, e.g., plastic film, metals in general, etc. For reasons of economy and convenience, paper surfaces will generally be preferred.

After development, the images are dried and transferred. When a composite proof is made, a number of images are separately transferred in register from their supports to a single transfer support, and the images superimposed thereon constitute the composite proof. The transfer is accomplished by contacting the second support with the image, and stripping the original support from the image. Simple lamination procedures can be employed for the contacting, such as pressing the materials together, and, if desired, applying heat. A platen press can conveniently be employed with pressures of the order of 100 to 500 lbs/in.sup.2. Heat can conveniently be used by heating the platen upon which the receptive transfer support rests, while the upper platen is cooled by tap water. Temperatures employed will be well below the melting point of the coating materials, but can make the surface more tacky or soften it to promote bonding to the receptive surface. A rubber blanket or other resilient material may be used between the upper platen and the release support to aid in equal distribution of the pressure. For styrene/butadiene copolymer materials, a bottom platen temperature, for example, around 80.degree. C. is satisfactory. A short sojourn under pressure is sufficient, say five seconds or so. The sheets are then cooled and the original support is separated, leaving the image on the receptive transfer support. Ordinarily a clean transfer is obtained and no residue on the release support is observed. At times there may be some residual color left, or even some background color in areas where the coating has been removed by development, but this is ordinarily not harmful as it does not usually transfer. In the transfer the heating is ordinarily very brief and limited. Heating can affect the photoreaction products, e.g., hydroperoxy groups, and promote further reactions thereof, or affect the colorants, and this should be taken into consideration when employing heat in the development or transfer steps. In some cases such further efforts may be desirable, but if not, only mild heating conditions should be used. Platen temperatures are usually in the range of 50.degree. to 150.degree.C.

When half-tone images are transferred, the coating is being transferred in the form of dot areas. There may be some large areas in which dots are contiguous, but in general there will be discontinuities and individual discrete dots being transferred. As the dots in an image of one color will not necessarily be in the same areas as those of another image, some parts of one of the images may rest on a previously transferred lower image, while other parts rest directly upon the receptive support. Thus the composite proofs do not necessarily have several discrete superimposed layer images, but some portions of the upper layer may penetrate into or through lower layers. Also, when continuous tone images are involved, the selective removal in developing such images includes removal of portions to varying depth of the coating, so the transferred layers may vary in thickness. Other devices can be used to effect transfer, such as a heated platen and a rubber roll, or the nip between two opposing rolls, such as a heated metal roll and a metal roll or rubber roll, or various other laminating devices. While the heated platen will generally be next to the support to which the image is being transferred, transfers can also be effected when the platen is adjacent to the release support. Also, if desired, the original support can be removed without permitting the laminate to cool first.

EXAMPLE 1

Photosensitized coated papers were prepared in accordance with the procedure described above utilizing a heterogeneous styrene/butadiene copolymer of 48 parts styrene to 52 parts butadiene (Solprene 303 from Phillips Petroleum Company) with 25 parts per hundred loadings of the yellow and cyan pigments and 20 parts per hundred for magenta and black pigments, based on the polymer. The sensitizer concentration was 0.25% for the colored coatings and 0.5 percent for the black coatings. The coatings were formed on release paper, Transkote ER (S. D. Warren Co.) release paper for yellow, magenta and black, and Transkote FER (S. D. Warren Co.) release paper for cyan. The coated release papers were separately exposed utilizing a 4000 watt pulsed Xenon light 32 inches from the coatings, utilizing color separation negatives with an air gap between the negative and the coating surface. The yellow pigmented coating was exposed through a color separation half-tone negative obtained by use of a blue filter, the magenta through such a negative from a green filter and the cyan through a negative from the red filter, with the black coating being exposed through a negative appropriate for printing black areas. The exposure times were 2.5 minutes for yellow and magenta, 2.75 minutes for cyan and 7 minutes for black. The images were developed by passing in front of a gang of three spray nozzles spraying an aliphatic naphtha (Skellysolve V, KB value 36), with 9 passes for a total of about 20 seconds. The images were individually transferred by pressing in a platen press onto a sheet of paperboard (Castcoat), employing a press pressure of 200 psi. and a bottom platen temperature of 80.degree.C. The release supports were stripped from the resulting sandwiches after cooling. The four color images were superimposed in register to give a good four color print. The images were transferred in the order, yellow, magenta, cyan, and black but this order can be changed as the images of one color do not prevent viewing those of the other colors to obtain the overall effect. A picture was obtained with good reproduction of blue, red, and yellow colors and combinations thereof, as well as blacks, with good highlight and shadow areas. Either of the above release papers can be used with any of the colored coatings.

In the foregoing Example 1 the pigments employed were as follows. The yellow was Diarylide Yellow OT YT-564-D, brand of duPont. The magenta was 13-3150 Hostaperm Carmine FBB Powder brand of Farbwerke Hoescht Ag. The cyan was 9 to 1 mixture of Monastral Blue B BT-284-D/Monostral Green Y GT-805-D brands of duPont. The black was Black Shield No. 8537 carbon black marketed as a 20 percent solids dispersion in toluene by Carbon Dispersions, Incorporated.

EXAMPLE 2

Sensitized pigment polymer mixtures were coated on a release paper (Transkote ER). The coating slurries were made by mixing sensitized polymer solutions with 15 percent pigment slurries in toluene. For the magenta, cyan and black coatings, the polymer solution was 100 grams of 10 percent styrene/butadiene copolymer in benzene, 25 mg. of tetraphenyl porphin in 25 ml. chloroform, and an additional 22.5 grams chloroform. For 25 grams of the solution, 1.4 grams of the magenta pigment slurry was used for the magenta coating while 0.8 gram of the cyan slurry was used, and 1.25 gram of the black. For the yellow coating, the polymer solution varied by using only 10 mg. of tetraphenylporphin in 10 ml. chloroform, and having an additional 45 grams chloroform, and 1.4 grams of the yellow pigmented slurry was used with 25 grams of the solution. The coatings were imaged with appropriate color separation negatives with a flux of 2.5 ergs/cm.sup.2 /sec. The exposure times were 1.75 minutes for yellow, 1.25 minutes for magenta, 0.75 minutes for cyan, and 3 minutes for black. The resulting latent images were developed by spraying with a V, M and P naphtha (Skellysolve C) to wash away non-exposed area and transferred individually in register to a receptive paperboard to give a good, colored image.

EXAMPLE 3

An isoprene/acrylonitrile copolymer was dissolved to form a 10 percent solution of benzene. A 20 percent amount of iodoform, based on polymer, was added, and 10 percent of a pigment, based on the polymer. The pigments were added as 10-15 percent slurries in hydrocarbon solvent. A polyethyl terephthalate film (Mylar) was coated with a fluorocarbon resin, and separate pieces were coated, the coatings employing yellow, magenta, cyan and black pigments. The coated films were separately exposed through appropriate color separation negatives employing a flux of about 2.5 ergs/cm.sup.2 /sec. for about one minute. The exposed films were developed by washing with toluene to remove non-exposed areas of the coatings. The resulting images were transferred in register to a coated Bristol board to provide successfully a four-color proof.

EXAMPLE 4

A polyethyl terephthalate film (Mylar) was coated with a trans polyisoprene polymer containing 0.26 percent tetraphenyl porphyrin and 10 percent pigment, based on the polymer. Both yellow and cyan pigments were used. Exposures were made through negatives for 6 minutes, and development was by immersion in a tray of cyclohexane-toluene. An image was obtained. The procedure was repeated employing a medium viscosity polybutadiene as polymer. Cyclohexane development produced a fair image by removing unexposed parts, while toluene tended to remove exposed areas also. With a 40/60 styrene/butadiene black copolymer, the procedure was repeated as above with four pigments, and transfers were made in register to give a full color print. An aliphatic VM and P naphtha was used for development (Skellysolve L).

In the present invention it is feasible to use relatively thin coatings. This has some slight advantage in economy of materials. Also, since in half-tone procedures it is necessary to effect complete selective removal of the coatings during development to achieve the dot structure, it is desirable to have the coating relatively thin to contribute to ease of removal and to minimize exposure times, etc., to achieve reaction through the depth of the coating, although surface reaction may be sufficient to give the desired protection against the development fluids. The coatings are usually of the order of 1 to 2 microns, say 0.5 to 3 microns, and seldom exceed 5 microns. However, thicker coatings, such as up to 10 or 20 microns or more can be used. Relatively thick coatings may be easier to transfer and handle. For a balance, one can select the thicker coating having suitable developability, and employ lower pigment loadings to compensate for the thicker image.

Utilizing a slurry as above, with a styrene/butadiene random copolymer (48 styrene to 52 butadiene) and 6 percent solids content, in which the pigment was 15 parts of yellow pigment for 100 parts of polymer, coatings were drawn with different rods and thickness calculated from weight and density values. The thicknesses were in the range of about 1 micron for a No. 8 rod, to about 2 microns for a No. 16 rod, and about 4.5 microns for a No. 40 rod. In general, exposure and development did not change the film thickness appreciably.

The pigments employed herein can in general be any pigments which have the desired colors. For the most part the pigments will be organic compounds and these are most suitable although inorganic colorants, for example chrome yellows, can also be used. As used herein the term pigment refers to solid, colorant materials which are insoluble, or at least of very limited solubility, in the polymer solvent systems employed in coating and development. The term dye is used to designate colorant materials employed in solution. The pigments as used are generally in very finely divided, non-agglomerated form, such as in ranges up to 0.5 micron, say 0.1 to 0.5 micron, and seldom exceed 1 micron in diameter. The advantage of the small particle size can be seen with reference to obtaining fine resolution, and also for production of thin, smooth coatings. If not already in fine particle size, the pigments should be reduced to such size, and it may be necessary to break up any agglomerates in preparing the coating formulations. The pigment loading will be regulated to give the desired color depth with the particular coating thickness and other factors. However, the pigment also has an effect upon the developability of the coatings as it tends to make the coatings fracture upon washing with appropriate liquids. A 5 to 10 percent by weight amount of pigment, based on polymer, is usually sufficient for this purpose, and loadings up to the amount the polymer can bind can be employed, but it will generally be convenient to operate in the range of 10 to 50 percent by weight, based on polymer. If for some reason a particular coating is to have little or no color, it may be advisable to add some insoluble solids to the coatings for the effect upon developability, the amounts possibly being sufficient to provide 5 to 10 percent or so of solids, taking any pigments into account as well as any white, colorless, or low-color solids. For example, various amounts of silica or other fillers can be added along with pigments.

EXAMPLE 5

A magenta pigmented styrene-butadiene copolymer slurry in benzene, containing 0.25% tetraphenylporphin on the polymer was coated on a release paper. The coated sheet was imaged 2.5 minutes through a magenta half-tone color separation with a 4000 watt pulsed Xenon light 32 inches from the surface. The exposed coating was then immersed in a yellow dye solution for 2 minutes, and the dye was wiped off, and the image developed by spraying with a V, M and P Naphtha (Skellysolve V), washing away the non-exposed area. The usual magenta image had been shifted to red to orange by the yellow dye. The image was transferred to a receptive paper support. The use of a dye makes possible the shifting of hues in the images. The magenta pigment used above was at a loading of 20 parts per hundred, based on polymer, and was a commercially available magenta (13-3150 Perm Carmine FBB (Hoechst)). The yellow dye was a 2 percent (weight/volume) solution of a yellow dye, Alcian Yellow GX (Imperial Chem. Ind.) in a 3:7 solution of ethanol and dodecyl alcohol, with 5 percent by weight of added butanediol. Dyes can be used in this manner to modify the colors of any of the color separations images, or even of a composite formed therefrom. After modification of the magenta image as shown above, the images from the other color separations can be transferred in register to the same support as the magenta image to form the composite colored picture. This procedure may be particularly convenient for modifying the magenta image, since some printers inks are more red than magenta, and this procedure makes it possible to use pure magenta and to obtain the desired match by dye modification. Modification of a cyan image with a yellow dye may also have frequent use.

EXAMPLE 6

A magenta-pigmented coating was used for imaging. The coating contained 12.5 parts per hundred loading of the magenta pigment, Monastral Red B, and had been drawn on a release paper with a No. 8 rod. The coating was exposed through a green record (magenta) half-tone color separation negative and developed with an aliphatic naphtha (Skellysolve C) to wash away non-exposed areas. The image was transferred to a coated paper. An image so obtained had measured optical densities of 0.08 through a red filter, 0.50 through a green filter, and 0.34 through a blue filter. Another image, after development, was soaked in a blue dye solution for 30 seconds, followed by a wash with the same naphtha used for development. The image was transferred and had optical densities of 0.31 through a red filter, 0.61 through a green filter and 0.36 through a blue filter. Another image was similarly treated with the blue dye, but ethanol was used for the wash, with results of optical density 0.1 through the red, 0.48 through the green, and 0.31 through the blue. It is apparent that the blue dye has shifted the hue, especially when the naphtha wash was used, causing greater absorption of the light which had passed the red and green filters, i.e., higher density, because of the shift of the magenta toward the blue. The dye solution used in the dye treatments was a 1% (weight/volume) solution of blue dye (Victoria Blue R) in ethanol. Images were similarly treated with 1 percent red dye solution (Neozapon RED GE 216) in ethanol. A 30 second soak followed by washing with aliphatic naphtha gave optical densities of the transferred image of, 0.11 through the red, 0.61 through the green, and 0.43 through the blue. With a 1 minute soak the values were 0.11 through the red, 0.64 through the green, and 0.45 through the blue. The higher density in the green and blue shows greater red absorption and that the dye has made the hue more red.

EXAMPLE 7

A polymer solution was prepared from 100 grams of chloroform, 100 grams of a 20% solution of a butadiene/styrene random copolymer (Solprene 303) in benzene and 100 mg. tetraphenyl porphyrin. A finely divided silica (Santocel Z brand of Monsanto Company) was dispersed in benzene at a rate of 0.75 grams to 45 grams. Approximately equal parts of the polymer solution and silica dispersion were mixed, and a release paper was coated with the slurry. Coatings were exposed through color separation negatives, dyed, and then developed by washing with VM and P naphtha (Skellysolve V) to give images. The exposure times with a 4000 watt pulsed Xenon light 32 inches away were 2.5 minutes for yellow and magenta, and 1.5 minutes for cyan. The images were transferred in register to make a good composite picture, but with too much yellow tone. A repeat of the procedure, but with a 1.5 minute exposure for all color separations produced a picture with much truer color. Utilizing the latter procedure above, but with omission of the silica in the coating solution, produced a good composite picture, but the transfers were more difficult to accomplish. In the above procedures, the yellow dye was a 2 percent (weight-volume) solution of a yellow dye, Alcian Yellow GX, in a 3:7 solution of ethanol and dodecyl alcohol, with 5 percent by weight of added butanediol. The magenta was 85 grams of a 2 percent solution of magenta dye (Sevron Brilliant Red D (duPont)) in 7:3 hexadecyl alcohol and ethanol, with 15 grams of a 2 percent solution of yellow dye (Maxilon Brilliant Yellow 7 GLA (Geigy Chemical)) in the same solvents, and 10 grams of butanediol. The cyan dye was 44 grams of a 2 percent solution of a cyan dye (Genacryl Blue 5B (General Aniline and Film)) in 7:3 hexadecyl alcohol and ethanol, with 40 additional grams of the solvent mixture and 4 grams butanediol. The black was 120 grams of a 2% solution of black (Nigrosin SS J (duPont)) in 100 ml 7:3 hexadecyl alcohol and ethanol with 50 grams of the dye solution used for yellow, except the butanediol was omitted. The soak for the dye application was 2 minutes. Colorless "pigments" such as silica can be used in this procedure for their effect upon the physical properties of the coatings, particularly with respect to developability and transferability.

EXAMPLE 8

A release paper (Transkote FER) was coated with a styrene/butadiene copolymer containing a 25 parts per hundred loading of blue green pigment comprised of 9 parts blue (Monastral Blue B BT 284D) to one part green (Monastral Green Y GT 805D). Coated samples were imaged through a chrome tint and treated by aerosol sprays of liquids with different KB values. The coating was resistant to n-hexane having a KB value of 30.5. A VM and P naphtha (Skellysolve V) of KB value 36, and a similar aliphatic (Quick Solution H) of KB value 33.8, both developed good dot structures, although the latter gave somewhat higher print density. Another VM and P naphtha (Shell V), KB value 39, was fairly strong for the conditions of use, tending to cause loss of some of the dot structure.

EXAMPLE 9

A variation of the usual procedure employs a transferable polymer film having a high energy surface. The pigmented, sensitized coating is exposed and developed on the film, and the film is then transferred along with the developed image. For example a release paper is coated with a polar polymer solution to give a thin film, and this is then overcoated with a coating solution containing as polymer 95 parts styrene/butadiene (Solprene 303) to 5 parts cis-polybutadiene. The coating had a 20 parts per hundred loading of magenta pigment and 0.5 percent tetraphenylporphin. The coating was exposed through a screen tint transparency, and developed with an aliphatic naphtha, KB value 39. The image was then laminated to an acceptor paper at 10,000 psi and 120.degree. for 15 seconds in a 6 inch by 6 inch press. The release paper was removed, leaving the image covered by the transparent covered film. Fair quality images can be obtained and transferred in this manner, although there is some problem of background from the film. a carboxylated polymethyl methacrylate (Lucite 6012) gave lower background than either a low molecular weight polymethyl methacrylate (Elvacite 2008) or high molecular weight polymethyl metharcylate (Elvacite 2041). The development of cyan, yellow and black images in similar manner can be followed by their transfer in register to the same support to obtain a four color composite. The background presents some difficulty, but film backings can be selected and other adjustments made to overcome or minimize this problem.

EXAMPLE 10

A magenta-pigmented styrene/butadiene copolymer containing 0.25 percent tetraphenylporphin was coated on release paper (Transkote ER) in the usual manner. It was imaged with a positive half-tone, transparency, and developed with a solvent mixture having effective polarity, a 1:1 mixture of methanol and a VM & P naphtha (Skellysolve V). The exposed areas were washed away, and the image was transferred with heat and pressure to a paperboard. Similar results are obtained with butyl cellosolve as the developing liquid. Images can be prepared in similar manner from the other color separation half-tones, and transferred in register to form a four color positive replica. In general the solvents used herein for development by coating removal will be relatively poor solvents for the polymer substrate involved, as a selective removal is desired. For selective removal of non-exposed areas, solvents will be selected which are effective to soften or dissolve the non-exposed areas, but which are not sufficiently effective to remove exposed areas in which hydroperoxy groups or other groups affecting solubility are present. Solubility parameters present a suitable guide for choosing appropriate solvents, although there will be some variance with individual members of such classes. Such parameters .delta. based on cohesive energy density, are described in Solubility Parameters by Harry Burrell, Parts I and II, Interchemical Review, Vol. 14, No. 1, pages 3-16, and Vol. 14, No. 2, pages 31- 46 (1955). As the hydroperoxy groups increase the solubility parameter, the solvent will generally be selected to have a parameter slightly less than that of the polymer employed. Thus with a diene rubber having a solubility parameter around 8.5, a suitable solvent would probably have a solubility parameter less than 8. Many of the hydrocarbon diene polymers suitable for use herein have solubility parameters in the range of 8 to 9.5, and accordingly the developing solvents usually will have solubility parameters below 8, although higher parameters may at times be suitable. Most of the solvents suitable for use herein for removal of non-exposed areas will have solubility parameters in the range of 7 to 8 or 8.2. Poorer solvents when used sometimes require longer development times, but this may not be objectionable in some applications. Aliphatic hydrocarbons, including cycloaliphatics, provide a number of suitable solvents for use with particular polymer systems, e.g., kerosene, VM and P naphthas, methylcyclohexane, octane etc. Some aliphatic, i.e., non-benzenoid, unsaturation may be present in such solvents, but aromatic solvents generally have too high a solubility parameter for use with the preferred polymer systems herein. Solvents such as benzene, toluene, etc. will ordinarily dissolve both the exposed and non-exposed areas in a photooxidation imaging system. Thus it is not desirable to use really good solvents for the polymer for the development purposes, but to select solvents of lowe solubility parameter than the polymer, e.g. 0.2 to 0.5 or more units lower. If the starting polymer is readily soluble in toluene or benzene, as is usually the case herein, the exposed areas after photooxidation imaging are still readily soluble as the imaging does not crosslink or otherwise change such polymer so as to render it insoluble in such solvents. It will be recognized that the suitability of particular development solvents will vary with such factors as the molecular weight of polymers in the film substrate, characteristics of other additives of components in the film, desired speed of development, film thickness and strength, etc. The hydroperoxy groups are strongly polar, and therefore to avoid dissolution of the exposed areas, solvents lacking strong polar groups are ordinarily employed. Hydroxy, keto, etc. groups tend to increase the solubility parameter, and are therefore not generally used unless with a high solubility parameter polymer system, or if such groups are present in such proportions in the solvent as not to unduly affect the parameter. Similarly strongly halogenated solvents tend to have higher solubility parameters, and are not ordinarily employed as development solvents herein.

For the development of images by dissolution of the exposed photooxidized areas, solvents of higher solubility parameter, such as polar solvents, are employed. In broad terms, solvents will dissolve polymers if the solubility parameters match. Thus if a high solubility parameter solvent is used, it can dissolve the hydroperoxidized region having a similar solubility parameter, but has too high a parameter to dissolve the non-exposed regions of the polymer. The precise relationship between the number of hydroperoxy groups and the solubility parameter is not known, but the solvents chosen should have a solubility parameter above that of the original polymer so as not to dissolve non-exposed areas. Polar solvents, such as hydroxyl containing compounds, e.g., various alcohols, glycols, cellosolves, etc. can be used. Solvents will be selected so as to give the desired degree of solubility without unduly affecting the non-exposed area, and may at times have solubility parameters considerably above that of the original polymer substrate. It is to be recognized that the solvents herein can include mixtures of various solvents, and that descriptions herein are mainly concerned with the overall characteristics of solvents, rather than with characteristics of individual components thereof. For example, a particular naphtha may be considered as a low solubility parameter aliphatic hydrocarbon solvent even though it has some aromatics content. With some of the solvent for the hydroperoxidized areas it may be desirable to use small amounts of bases, for example, alkalies such as sodium hydroxide, or chemical reagents to aid in the removal.

Another useful guide in choice of development fluids is the KB value. The KB value, i.e., Kauri-butanol value, is a measure of the solvent power of a solvent as determined by dissolving Kauri resin in butanol, and titrating the amount of a given solvent necessary to titrate to incipient insolubility. In general, with hydrocarbons, the stronger the solvent, the higher the KB value. The KB value has a fairly good correlation with solubility parameter for aliphatic solvents. Reported KB values are available for many solvents. When a solvent of particular KB value has been found suitable for developing a particular photosensitive coating, with a particular application method, other solvents of similar KB value will also be suitable. The initial selection can be made by trying solvents on a scale of gradually changing KB value, generally in half-tone work using one which is just sufficiently effective to remove non-exposed areas well, but without disturbing the exposed areas. In the case of mixed solvents of vastly different character, e.g., hydrocarbons and polar solvents, effective KB value must be used, as dependent upon the KB values and relative amounts, as the ordinary method of determining KB values would not give the proper value in such case. With the unsaturated hydrocarbon resins preferred herein, particularly styrene/butadiene polymers, useful KB values are usually in the range of about 32 to 38, or up to 39 to 40 with some application methods.

Claims

What is claimed is:

1. The process of making colored prints which comprises forming a plurality of color separation exposures on photosensitive polymeric coatings on supports, said coatings comprising a solid polymer having carbon-to-carbon double bond unsaturation and a photooxidation photosensitizer capable of absorbing light energy and utilizing same in the presence of oxygen to cause formation of a bond between oxygen and the polymer, applying a liquid to develop images in said coatings by selectively removing either exposed or non-exposed areas from said support, placing one of said images in contact with a backing and removing said support, thereby transferring the part of the coating forming an image from said support to said backing, and thereafter transferring in similar manner another of said images to the first in registered relationship therewith.

2. The process of claim 1 in which the images are half-tone images.

3. The process of claim 1 in which the images are half-tone images of at least about 150 lines per inch.

4. The process of claim 1 in which yellow, cyan, magenta and black images are transferred in register to form a four color composite picture.

5. The process of claim 1 in which the supports have a release coating surface so that the polymeric coating has low to moderate adherence thereto.

6. The process of claim 5 in which the polymeric coatings contain pigments which make them more readily removable from the support by action of solvents.

7. The process of claim 6 in which the liquid used for development is a poor solvent for the polymer in the coating and removes the non-exposed areas of the coating to form an image.

8. The process of claim 6 in which the liquid is a non-aromatic hydrocarbon.

9. The process of claim 1 in which excited state oxygen reacts directly with the polymer and the photosensitive coatings contain pigments in 10 to 50 parts by weight based on polymer and in respective colors yellow, cyan, magenta and black.

10. The process of claim 1 in which the polymer is a butadiene-styrene interpolymer and a porphyrim photosensitizer is used.

11. The process of claim 1 in which a dye is added to at least one of the images after exposure.

12. The process of claim 1 in which portions of the coating in non-exposed areas are removed to form an image comprised of a discontinuous coating.

13. The process of claim 1 in which portions of the coating in exposed areas are removed by a polar solvent.

14. The process of claim 1 in which portions of the coating in non-exposed areas are removed by a non-aqueous solvent having a solubility parameter below 8.

15. The process of claim 1 in which the tranfer is accomplished by pressure contact without application of solvent.

16. The process of claim 15 in which heat is employed in the transfer.

17. The process of claim 1 in which the coating has a thickness no greater than 5 microns.