Halftone image production

Abstract: A process for the production of a halftone image on a paper base support coated with a polyolefin layer, wherein for dot gain control the polyolefin layer has a coverage in the range of 5 to 40 g/m2 and is coated with a binder layer from an aqueous liquid containing a hydrophilic colloid binding agent and white titanium dioxide pigment particles having an average grain size in the range of 200 nm to 450 nm being present in a weight percent of at most 300 % with respect to the total binder content, the titanium dioxide being applied in a coverage of at least 0.5 g per m2.

Description: The present invention relates to a process for the production of halftone patterns, particularly multicolour colloid patterns on a support providing optical dot gain control. The production of linework or halftone multicolour colloid patterns is of interest in the field of design, e.g. in the production of colour decorative patterns, colour wiring and circuit diagrams, cartography, colour proofing and the preparation of transparencies for transmission projection or opaque prints for overhead projection. Especially in the colour field of the graphic art there is a great need for a simple and fast technique offering "colour proofs" of high quality and reproducibility. Photographically produced colour proofs are a substitute for multicolour halftone reproductions as will be produced by successive printing in register with the separate standard inks : magenta, yellow, cyan and black on a conventional printing press. Press proofing for the production of colour proofs by preparing a printing plate and running the plate on the press to produce only a few copies as proof of the quality of the halftone separation transparencies used in the plate production is a very expensive procedure and therefore photographic processes have been developed to obtain a similar result by means of which the appearance of a print starting from particular colour separation negatives or positives can be judged by the printer and client. According to a process known from US-P 3,642,474 the production of such a proof in the form of superposed coloured colloid patterns, proceeds with coloured hydrophilic colloid layers, which are insolubilized in the irradiated portions by means of an active species formed during or after the informationwise exposure to active electromagnetic radiation of a photosensitive substance, and comprises the following steps : (1) transferring a coloured hydrophilic colloid layer containing a said photosensitive substance and a hydrophilic colloid, which undergoes a reduction in solubility in water by said active species, from a temporary support, which is more hydrophobic than a permanent support to which said layer has to be transferred, to said permanent support by pressing the latter in the presence of an aqueous liquid against said colloid layer, and removing the temporary support, thus leaving said layer on the permanent support; (2) exposing the transferred colloid layer in substantially dry state to actinic electromagnetic radiation, which is modulated according to the information to be recorded, (3) developing the exposed layer by means of an aqueous liquid followed by a wash-off processing resulting in a coloured relief pattern, and repeating the steps (1), (2) and (3) with said hydrophilic colloid layers having a colour as desired to produce superposed coloured colloid patterns on the permanent support. In said process for producing a multicolour pattern the exposures proceed in register on the same permanent support which received the unexposed coloured hardenable colloid layers by transfer from a temporary support, each transfer and exposure being followed by hardening development, wash-off processing and a drying step before a next transfer and exposure leading to a further selection colour pattern can be carried out. In most of the examples of said US-P 3,642,474 an ultraviolet radiation sensitive iron (III) complex is used which yields iron (II) upon UV-irradiation forming by reaction with hydrogen peroxide hydroxyl radicals that imagewise harden the gelatin binder of each coloured layer. Said iron (III) complex, e.g. ammonium iron (III) oxalate, has a poor photosensitivity in comparison with silver halide which is preferred for use in combination with low intensity exposure sources as are used e.g. in colour scanners operating with an imagewise modulated laser beam. A process for producing a multicolour pattern using silver halide emulsion materials is described in published European Patent Application (EP-A) 0 185 410. As can be learned from said published EP-A a hardening developable coloured silver halide emulsion layer on a temporary support is imagewise exposed whereupon the layer containing a latent image of exposed silver halide is transferred onto a permanent support and then hardening development and wash-off processing takes place. For obtaining a multicolour print red, green and blue filter exposures representing halftone selection negatives or positives are made on separate silver halide emulsion layers that are complementary in colour (cyan, magenta and yellow). The succesive transfer, hardening development and wash-off processing on a same permanent support of said layers yields a subtractive multicolour relief image of at least partly superposed cyan, magenta and yellow relief portions. In the above processes the wash-off processable coloured hydrophilic colloid layers are transferred from a temporary support onto a permanent support which may or may not carry relief portion(s) of precedingly formed colour image(s). GB-A-2 138 964 relates to a photographic paper material comprising : (1) a support comprising a paper substrate, with a synthetic resin film provided on both sides thereof; (2) a hydrophilic colloid layer containing a white pigment such as titanium dioxide on one side of the support; and (3) a light-sensitive silverhalide emulsion layer on the hydrophilic colloid layer (2). The synthetic resin film can be made of polyolefin and the white pigment content of the hydrophilic colloid layer (2) is at least about 40% by volume. Titanium dioxide may be present at coverages of e.g. 3.1 and 5.2 g/m² according to Table 2 of said GB-A. The presence of said white pigment has been intended for improving the image sharpness as is shown by better modulation transfer (MT) values. In the photomechanical production of halftone prints there exists a typical phenomenon known as "dot gain" which has to be taken into account in the production of colour proofs for ressembling as close as possible the prints made on the press. As is known a halftone image consists of numerous dots of different areas that are created when a continuous-tone image is photographed through a halftone screen or obtained by halftone modulated scanning exposure. Due to a number of factors - some of which are controllable, others of which are not - the dot increases in size from the time it is first created on the photographic material serving as intermediate original in the formation of the printing form until it is finally reproduced as an ink dot in the printing step. If the dot grows (gains in size) more than it should, the colour in the print will be more saturated than it should and true colour would not be obtained for it is dot size dependent. Dot gain is made up of two components, physical and optical dot gain. Physical or mechanical dot gain is the actual physically measurable enlargement of the dot size which occurs during the printing process wherein individual ink dots are spread on the printing paper. Optical dot gain results when light penetrates the printing paper in the clear areas, becomes internally scattered therein and partially absorbed in the ink dots making the dots visually appear larger. Optical dot gain can be influenced by changing the paper stock (see the article "Gaining on Dot Gain" by Johan Strashun in Graphic Arts Montly, January 1985, p. 69). In offset printing the greatest dot gain occurs in the mid tones, i.e. around the 50 percent dot value. Good colour balance is obtained when all colours have equal dot gain. In simulating the printing result with a colour proof it is important to obtain a total dot gain in the colour proof as close as possible to the total dot gain obtained in the actual print, whatever the printing stock or printing method may be. It is an object of the present invention to provide a process for the production of halftone patterns, i.e. images built up by screen dots, on a support controlling or influencing optical dot gain. It is more particularly an object of the present invention to provide a process for the production of halftone multicolour colloid patterns serving as colour proof on a support with optical dot gain control. It is a further object of the present invention to provide a sheet material or a material in the form of a web suited for producing thereon a halftone image, more particularly a material having a multilayer structure adapted to yield in colour proofing of multicolour halftone images an optical dot gain as required for correct simulation of printed multicolour halftone image. Other objects and advantages of the present invention will become clear from the further description. The present invention is directed to the use of a paper base support in a process for the production of a multicolour halftone colloid pattern with controlled dot gain, wherein said process comprises the following steps : (1) transferring a substantially non-hardened gelatin silver halide emulsion layer containing coloured pigment particles dispersed therein from a temporary support onto a permanent support being said paper base support which support may carry already a halftone relief image containing coloured hardened gelatin, (2) imagewise exposing the non-hardened gelatin silver halide emulsion layer prior to or after carrying out said step (1), (3) forming by hardening development a halftone image in said transferred silver halide emulsion layer, and (4) wash-off processing to remove selectively unhardened portions of the transferred layer, and (5) repeating steps (1), (2) and (3) to form at least partly superposed portions of differently coloured gelatin containing layers, and wherein the transferred silver halide emulsion layer before or after its transfer is exposed through a halftone selection image of a multicolour original, characterized in that as permanent support a paper base support is used which is coated with a polyolefin layer having a coverage in the range of 5 to 40 g/m² which layer is itself coated with a binder layer containing a hydrophilic colloid binding agent and white titantium dioxide pigment particles having an average grain size in the range of 200 nm to 450 nm, being present in a weight percent of at most 300 % with respect to the total binder content, and being applied at a coverage of at least 0.5 g per m² to provide a dot gain up to 22 % with respect to the original dots obtained at 50 % dot value. A practically useful range of titanium dioxide coverage is in the range of 0.5 to 4.5 g per m². According to an embodiment of the process according the present invention a halftone multicolour colloid pattern, which may serve as colour proof, is produced on a paper base support, the process comprising the steps of : (1) transferring a substantially non-hardened gelatin silver halide emulsion layer containing coloured pigment particles dispersed therein from a temporary support onto a permanent support being said paper base support which support may carry already a halftone relief image containing coloured hardened gelatin, (2) imagewise exposing the non-hardened gelatin silver halide emulsion layer prior to or after carrying out said step (1), (3) forming by hardening development a halftone image in said transferred silver halide emulsion layer, (4) wash-off processing to remove selectively unhardened portions of the transferred layer, and (5) repeating steps (1), (2) and (3) to form at least partly superposed portions of differently coloured gelatin containing layers, characterized in that the polyolefin layer of said permanent support has a coverage in the range of 5 to 40 g/m2 and is coated for carrying the halftone multicolour colloid pattern with a binder layer containing a hydrophilic colloid binding agent and white titanium dioxide pigment particles having an average grain size in the range of 200 nm to 450 nm and being present in a weight percent of at most 300 % with respect to the total binder content, the titanium dioxide being applied at a coverage of at least 0.5 g/m². Polyolefin resins suited for use in the polyolefin layer of the support material used according to the present invention include ethylene homopolymers such as high density polyethylene and low density polyethylene, propylene homopolymers and copolymers of ethylene with one or more copolymerizable monomers. Copolymers preferably do not contain more than 10 % by weight of a polymerized monomer other than polymerized ethylene. Examples of copolymerizable monomers include alfa-olefins such as styrene, vinyl stearate, vinyl acetate, acrylic acid, ethyl acrylate, methyl methacrylate, methacrylic acid and diene compounds such as butadiene and isoprene. The titanium dioxide may be either of the rutile type or anatase type and may be treated with an inorganic material and/or a polyhydric alcohol examples as described e.g. in US-P 4,443,535. In accordance with the present invention a sheet material or material in the form of a web is provided which material consists of a paper base support coated with a polyolefin layer having a coverage in the range of 5 to 40 g/m² and being coated with a binder layer containing gelatin and white titanium dioxide pigment particles having an average grain size in the range of 200 nm to 450 nm and being present in a weight percent of at most 300 % with respect to the total binder content, the titanium dioxide being applied at a coverage in the range of 0.5 to 6.4 g/m², and characterized in that said gelatin is present in combination with a hydrophobic polymer in latex form. The hydrophobic polymer provides good adhesion of the titanium dioxide containing layer to the hydrophobic polyolefin layer upon its stratum during drying. A particularly useful latex formation of polymer is copoly(methylmethacrylate-1,3 butadiene (50/50 by weight) having a glass transition temperature (Tg) of 10 °C. The gelatin and latex polymer solids are preferably used in a weight ratio range of 1 : 5 to 5 : 1. The gelatin of the pigment-binder layer may be uniformly pre-hardened to some extent with a classical hardening agent therefor, e.g. formaldehyde. The white titanium dioxide pigment particles may be used in combination with minor amounts of matting particles, e.g. a matting agent on the basis of hard polymethylacrylate beads having an average particle size of 2 to 5 micrometer and/or Alpha-quartz particles having an average particle size of 0.5 to 3 micrometer. In the accompanying drawing a graph illustrates the percent dot gain (% DG) as a function of the content of white titanium dioxide (g/m2) in a hydrophilic colloid binder layer applied on a polyethylene coated paper support according to the present invention. In said graph dot gain for a halftone coloured colloid pattern obtained as described in Example 1 is plotted as percental dot size increase at 50 percent dot value at 150 lines per 2.54 cm (inch) in the ordinate versus the coverage of titanium dioxide in g/m2 in the abscissa. Details about dot gain determination are given at the end of said Example 1. In a particular embodiment of the process according to the present invention in the preparation of superposed multicolour patterns photographic materials are used which comprise on a temporary hydrophobic resin support a silver halide emulsion layer incorporating a substantially unhardened gelatin as binder, a silver halide hardening (tanning) developing agent, photosensitive silver halide grains and pigments providing the desired colour. The unhardened gelatin is e.g. of the type described in US-P 3,364,024. The pigments or dyes are preferably used in diffusion-resistant form and can have all kinds of colour, e.g. are cyan, light-cyan, magenta, warm magenta, black, yellow, green, brown, orange, red, white or blue. Considered are likewise metallic colours such as pale gold, rich gold, copper, and silver. In other words the term "colour" in the present invention encompasses the use of all pure and mixed colours as well as black and white. In the production of superposed multicolour colloid patterns used in colour proofing the pigments have to match with the absorption spectrum of the standard process inks as close as possible. Information about standard colour inks can be found in H.M.Cartwright - Ilford Graphic Arts Manual (1962) VoL. I - pages 502 to 504. In the production of colour proofs the dyes used in the coloured layers are selected to correspond in spectral properties as close as possible to the colour of the printing inks, which for letterpress printing have colour tones as defined in DIN 16538 and for offset printing the colour tones defined in DIN 16539. Further information about colour tones can be found for the USA in the GATF-Colour Charts. It has been found experimentally that pigments, which are insoluble or very poorly soluble in water and in organic liquids of the alcohol or polyhydric alcohol type, e.g. glycerol, fulfil the requirements of resistance to diffusion. For colour proofing purposes a hardenable colloid layer in the present photographic material should preferably contain said pigments in a concentration sufficiently high for obtaining an optical density of at least 0.35 in the wavelength range of maximum absorption. Pigments particularly suitable for use in the present invention are known organic non-migratory pigment type dyes, e.g. obtainable under the Trade Marks "HELIO-echt", "PIGMOSOL" and "COLANYL" dyes. "HELIO-echt", "PIGMOSOL" and "COLANYL" are diffusion resistant organic pigments that can be dispersed in aqueous medium with the aid of a dispersing agent. These pigments excel in resistance to light, heat, acids, bases, oxidizing agents, and solvents. They are insoluble in hydrophilic colloids such as gelatin. When in addition to cyan, magenta and yellow relief patterns a black relief pattern is formed in colour proofing, preferably carbon black is used. Apart from carbon black mixtures of coloured pigments may be applied as described e.g. in US-P 4,427,757. In order to obtain images with a good resolution, relatively thin hardenable coloured gelatin containing coatings are used. Preferably such coatings having a thickness in the range of 1 µm and 15 µm are used and good results are obtained with pigment coloured layers containing 1 to 10 g of gelatin per sq.m. Very good results from the viewpoint of image sharpness and mutual adherence of the differently coloured relief portions are obtained with a composite layer structure of a pigmented gelatin layer containing silver halide coated in combination with a non-pigmented gelatin top layer having a combined thickness preferably in the range of 1 to 3 micrometer. The thickness of the pigmented gelatin silver halide emulsion layer is preferably twice that of the non-pigmented gelatin silver halide emulsion top layer. The total gelatin coverage of said composite layer structure is preferably in the range of 1.0 to 3 g of gelatin per sq.m. The pigmented gelatin containing silver halide emulsion layer contains preferably at least 50% by weight of gelatin. The adhering power of said composite layer structure to its temporary support, preferably a flexible one, should be such that an easy stripping off from the temporary support is possible after pressing said composite layer structure into contact with a wetted permanent support used according to the present invention. The temporary support is e.g. an unsubbed cellulose triacetate sheet, a polystyrene sheet or a sheet of copoly(vinyl acetate/vinylchloride). Other temporary supports having a repelling power for wet gelatin coatings are, e.g. a paper base coated with a polyethylene layer, a paper base impregnated with wax, a paper base coated with a layer of cellulose nitrate or a paper base coated with a layer of insolubilized polyvinyl alcohol or a layer of alginic acid insolubilized with an alkaline earth metal salt. The photosensitive silver halide used in the silver halide emulsion layers of the photographic material used according to the present invention is e.g. silver chloride, silver bromide, silver bromoiodide, silver chlorobromoiodide, or mixtures thereof. The silver halide emulsions may be coarse or fine grain and can be prepared by any of the well known procedures, e.g. as single jet emulsions or double jet emulsions. The silver halide emulsions may be Lippmann emulsions, ammoniacal emulsions, thiocyanate-or thioether-ripened emulsions such as those described in U.S. Patent Specifications 2,222,264 of Adolph H. Nietz and Frederick J. Russell, issued November 19, 1940, 3,320,069 of Bernard D. Illingsworth, issued May 16, 1967, and 3,271,157 of Clarence E. McBride, issued September 6, 1966. Surface image emulsions may be used or internal image emulsions may be used such as those described in U.S. Patent Specifications 2,592,250 of Edward Philip Davey and Edward Bowes Knott, issued April 8, 1952, 3,206,313 of Henry D. Porter, Thomas H. James and Wesley G. Lowe, issued September 14, 1965, and 3,447,927 of Robert E. Bacon and Jean F.Barbier, issued June 3, 1969. The emulsions may be tabular grain or regular-grain emulsions such as those of the type described by Klein and Moisar in J. Photogr. Sci., Vol. 12, No. 5, Sept./Oct. 1964, pp. 242-251. If desired, mixtures of surface and internal image emulsions may be used as described in U.S. Patent Specification 2,996,382 of George W. Luckey and John C. Hoppe, issued August 15, 1961. Apart from negative working silver halide emulsions that are preferred for their high light sensitivity, direct positive silver halide emulsions may be used that produce a positive silver image and a corresponding imagewise distribution of developing agent oxidation products during their development. For example those direct positive silver halide emulsions are considered wherein by exposure or by a chemical treatment a developable fog has been produced, which is destroyed imagewise during the imagewise exposure when certain conditions are fulfilled. In the unexposed areas the fog is maintained so that during the subsequent development a direct positive silver image is obtained and in correspondence therewith an imagewise distribution of oxidized hardening developing agent. For example, direct positive emulsions of the type described in P.J.Hillson, U.S. Patent 3,062,651, may be utilized to obtain direct positive images. In emulsions of this type, a nonhardening fogging agent such as stannous chloride, formamidine sulfinic acid, or the like is used. More details about composition, preparation and coating of silver halide emulsions are described, e.g., in Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109 and Research Disclosure, of December 1978, publication 17643. The silver halide emulsions can be chemically sensitized, e.g. by adding sulphur-containing compounds, e.g. allyl isothiocyanate, allyl thiourea, sodium thiosulphate and the like, during the chemical ripening stage. Also reducing agents, e.g. the tin compounds described in the Belgian Patent Specifications 493,464 and 568,687, and polyamines such as diethylenetriamine or derivatives of aminomethane-sulphonic acid, e.g. according to the Belgian Patent Specification 547,323, can be used as chemical sensitizers. Other suitable chemical sensitizers are noble metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium. This method of chemical sensitization has been described in the article of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951). Further it is possible to sensitize the emulsions with polyalkylene oxide derivatives, e.g. with polyethylene oxide having a molecular weight between 1000 and 20,000, or with condensation products of alkylene oxides and aliphatic alcohols, glycols, cyclic dehydration products of hexitols, alkyl substituted phenols, aliphatic carboxylic acids, aliphatic amines, aliphatic diamines and amides. The condensation products have a molecular weight of at least 700, preferably of more than 1000. For obtaining special effects these sensitizers of course can be combined with each other as described in Belgian Patent Specification 537,278 and UK Patent Specification 727,982. For a proper spectral sensitization, e.g. with respect to laser beam light, the usual mono- or polymethine dyes such as acidic or basic cyanines, hemicyanines, oxonols, hemioxonols, styryl dyes or others, also tri- or polynuclear methine dyes, e.g. rhodacyanines or neocyanines may be used. Such spectral sensitizers are described, e.g., by F.M. HAMER in "The Cyanine Dyes and Related Compounds" (1964) Interscience Publishers, John Wiley & Sons, New York. The silver halide emulsions may contain the usual stabilizers, e.g. homopolar or salt-like compounds of mercury with aromatic or heterocyclic rings such as mercaptotriazoles, simple mercury salts, sulphonium mercury double salts and other mercury compounds. Other suitable stabilizers are azaindenes, preferably tetra- or penta-azaindenes, especially those substituted with hydroxyl or amino groups. Compounds of this kind are described by BIRR in Z. Wiss. Photogr. Photophys. Photochem. 47,, 2-27 (1952). Still other suitable stabilizers are amongst others heterocyclic mercapto compounds, e.g. phenylmercaptotetrazole, quaternary benzothiazole derivatives and benzotriazole. The hardening development of a latent silver image proceeds with commonly used hardening developing agents, also called tanning developing agents for effecting the development of the silver halide and producing oxidized developing agent acting as hardening agent for gelatin. Suitable hardening developing agents are : 1,4-dihydroxy benzene compounds such as hydroquinone, chlorohydroquinone, bromohydroquinone, toluhydroquinone, morpholinemethyl hydroquinone and sulfohydroquinone. The developing agent(s) may be incorporated in the photographic silver halide material and/or in a developing bath. When incorporated in the photographic material it or they may be present in the silver halide emulsion layer or in a waterpermeable non-silver halide containing layer subjacent thereto so that development can be attained by using an alkaline activator liquid. According to a special embodiment a hydrophilic gelatin layer containing photosensitive silver halide and coloured pigment particles is combined with an underlying additional layer containing an auxiliary developing agent forming a superadditive developing system with a hardening developing agent present in an overlying silver halide emulsion layer. In said embodiment the development is carried out with a so-called activator liquid being an aqueous alkaline solution free from developing agents. Typical activator liquids for a hardening developable photographic silver halide emulsion material comprise, for example, an aqueous solution of an alkaline material, such as sodium carbonate, sodium hydroxide, potassium carbonate, potassium hydroxide, mixtures of sodium hydroxide and sodium sulfite, organic alkaline substances, e.g. alkanolamines, etc. A suitable activator bath comprises e.g. about 2 percent by weight of sodium hydroxide and 0.5 percent by weight of sodium sulfite. Typical auxiliary developing agents include 3-pyrazolidinone developing agents, e.g. 1-phenyl-3-pyrazolidinone, 1-phenyl-4,4-dimethyl-3-pyrazolidinone, 1-phenyl-4-methyl-4′-hydroxymethyl-3-pyrazolidinone and N-methyl-p-aminophenol sulphate. Said auxiliary developing agents or a mixture thereof may be incorporated in the photographic material and/or in the developing liquid in which the hardening developing agent may be absent when using an alkaline activator bath. In order to obtain more bright and colour-pure images in the obtained relief images it is necessary to apply a bleachfix treatment to remove image silver after the development step. A detailed description is hereinafter set forth of a composition and structure of a coloured light-sensitive silver halide material and its use in the production of a multicolour image according to an embodiment of the present invention. A coating composition is prepared containing gelatin dissolved in water wherein (a) selected pigment(s) is (are) dispersed in a concentration to yield after coating and drying a layer having an optical density in the wavelength range of maximal absorption of at least 0.35. The coating composition contains preferably at least 50% by weight of gelatin with respect to the pigment particles, optionally a plasticizing agent and a water attracting compound, e.g. glycerol to give the coating a sufficient adherence to its temporary support and to enable an easy wet (aqueous) stripping off from the temporary support. The said coating contains preferably 1 to 10 g of gelatin per m2. On said silver halide emulsion layer a top layer free from pigments is coated preferably. Said top layer preferably contains 0.2 to 2.5 g of gelatin per m2. The top layer and underlying layer form firmly bound together a composite layer structure that can be transferred as a whole from the temporary support to the permanent support. A set of materials each containing such a composite layer structure of different colour is preferably used for preparing a multicolour image. For colour proofing a usual set contains yellow, magenta, cyan, and black pigment coatings on separate cellulose triacetate supports. According to one embodiment in the production of colour proofs the imagewise exposure of the pigmented silver halide emulsion layer proceeds in a vacuum frame with the particular halftone separation negative or positive of the multicolour pattern to be reproduced in contact with the coloured colloid layer on the permanent support. According to an other embodiment the image-wise exposure in the form of screen dots is made scanning-wise, e.g. in a commercial colour scanner on the permanent or temporary support. Colour scanners operate with a rotating scanner drum and mostly with a laser light source of a definite narrow spectral composition. Therefore, scanningwise exposure requires photographic materials with a specific spectral photosensitivity and in order to have a more economic application requires a sequence of exposure and processing steps such that the scanner can operate without interruptions caused by processing. In a preferred scanningwise exposure called "a single scan operation" the exposure of all the differently coloured hardening developable photographic materials comprising on a temporary support a silver halide emulsion layer proceeds on the same scanner drum by arranging the materials on the drum in adjacent order and effecting the scanning without interrupting the rotation of the drum. From the viewpoint of image-sharpness the best results are obtained by effecting the scanningwise exposure of the light-sensitive silver halide from the front side, so not through a transparent temporary support. Such requires, however, a reverse reading exposure to obtain right reading copies after transfer of the composite layer structure on the permanent support. In modern scanner exposure apparatus such does not form a problem since the computer software allows electronic image-reversing without special optics such as mirrors and the like. The transfer of the imagewise hardenable silver halide emulsion layer or composite layer including such emulsion layer from its temporary support to a permanent support can be carried out in an apparatus, in which the materials involved are pressed together between rollers in the presence of an aqueous liquid and peeling off the temporary support. The imagewise exposure and the hardening development step is followed by a bleach-fixing step in order to obtain pure colours , wherein silver metal is bleached and residual unexposed silver halide is removed. The nonhardened portions of the hardening developed layers on the permanent support are washed off without mechanical rubbing by means of running water at a temperature preferably between 35 and 50°C. Excess of liquid is preferably removed by squeezing the relief between two smooth soft rollers, e.g. rubber rollers. In a preferred procedure a multicolour image, e.g. multicolour printing proof, is made according to the principles of subtractive colour photography starting with the production of a first relief image being a cyan part image of the multicolour original. In successive order a magenta, yellow and black relief image is produced on the same support. However, the order wherein the colour relief images are made can be chosen arbitrarily. In a particular case of forming a multicolour image the cyan layer or other first monochrome layer is applied from a coating solution in unexposed state on the permanent support and dried thereon before its imagewise hardening to form a direct reading relief image that is covered with other hardening developable silver halide emulsion layers wherein in their turn the unhardened portions are washed off after their imagewise hardening development. The obtained multicolour relief can be protected by a transparent resin topcoat, which according to a preferred embodiment is applied by spraying a solution of film forming resin onto the relief and drying. A suitable "spray-cover" consists of polyisobutyl methacrylate. The production of multicolour images of different dot gain on different permanent supports used according to the present invention is illustrated in more detail in the following examples. The percentages and ratios are by weight if not otherwise indicated. EXAMPLE 1 - Preparation of photographic material with temporary support. On a temporary support of unsubbed cellulose triacetate provided at the rear side with a black anti-halation layer and having a thickness of 0.1 mm the following layers were applied in the order given : (i) a silver chlorobromide-iodide emulsion (molar ratio 83.6/16/0.4) containing diffusion-resistant cyan coloured pigment and having a silver halide coverage equivalent with 0.25 g of silver per sq.m. and gelatin coverage of 1.8 g per sq.m. The emulsion was spectrally sensitized for argon ion laser light. The cyan pigment was FLEXONYL BLUE Paste (C.I. Index Nr. 74,160) which was applied at 6.3 g per liter of coating composition corresponding with 0.126 g of pigment solid per m2. The pH of the coating composition was 5.0; and (ii) a non pigmented gelatin binder layer containing per sq.m 0.7 g of gelatin, 0.066 g of hydroquinone and 0.095 g of matting agent on the basis of silica particles coated with urea formaldehyde resin. Magenta, yellow and black pigment coatings were applied to unsubbed cellulose triacetate temporary supports as defined for the cyan pigmented coating above. The magenta pigment was used as a paste sold under the name HELIOECHTCARMIN BB TEIG (Colour Index Nr. 12,485) and the yellow pigment was used as a paste sold under the name HELIOECHTGELB GRN 7476 Feinteig (Colour Index Nr. 21,100). Said pigment-pastes were used in an amount of 14.5 g and 17.6 g respectively per liter of coating composition. In the black pigment coating a carbon black paste was incorporated in admixture with the above mentioned FLEXONYL BLUE-paste in an amount of 15.75 g and 1.26 g respectively per liter of coating composition. - Preparation of the permanent supports with medium, high and low dot gain, which are called respectively SM, SH and SL. Support SM (for medium dot gain) A double-side polyethylene coated paper with paper weight of 180 g/m2 and a polyethylene layer weight of 30 g per m2 was air-knife coated at a wet coverage of 55 g per m2 with the following coating composition : Matting agent A is an aqueous dispersion containing 10.7 % of amorphous silica, 6.4 % of gelatin and 4.3 % of polyethylacrylate latex particles. Matting agent B is an aqueous dispersion containing 41.8 % of Alpha-quartz and 4.2 % of gelatin. The weight % of TiO₂ with respect to the total layer composition was 29.5 in the dried layer. The coverage of TiO₂ was 1.54 g/m2. Support SH (for high dot gain) A double-side polyethylene coated paper with a paper weight 180 g/m2 and a polyethylene layer weight of 30 g per m2 was air-knife coated at a wet coverage of 56 g per m2 with the following coating composition : The weight % of TiO₂ with respect to the total layer composition was 19.6 in the dried layer. The coverage of TiO₂ was 0.86 g/m2. Support SL (for low dot gain) A double-side polyethylene coated paper wherein the paper weights 180 g/m2 and each polyethylene layer weights 30 g per m2 was slide-hopper coated (for slide-hopper coating see e.g. DE-OS 2 224 872) at a wet coverage of 55 g per m2 with the following coating composition : The weight % of TiO₂ with respect to the total layer composition was 56.3 in the dried layer. The coverage of TiO₂ was 6.40 g/m2. -Exposure. The photographic layers on their temporary support were scanningwise halftone exposed on a commercial argon ion laser scanner operating with electronic dot generation. The four photographic materials containing the differently pigmented coatings, having DIN A4 format, were exposed successsively for obtaining the corresponding colour selections. - Transfer. In the exposed state first the cyan pigment containing multilayer structure was transferred respectively onto a permanent support SM, SH and SL as described hereinbefore by soaking the permanent support in water for 30 s and then pressing it into contact with layer (ii). By stripping apart the temporary support the composite layer structure of layers (i) and (ii) was left on the permanent support and the transferred composite layer was dried. - Hardening development. The transferred composite coating was hardening developed by dipping it for 30 s in an aqueous liquid, called activator bath, having following composition : - Bleach-fixing The developed material was led through a bleach-fix bath containing sodium thiosulphate and the mono sodium,iron(III) salt of ethylenediamine tetra-acetic acid. - Wash-off processing. To obtain a cyan coloured relief image the hardening developed and bleach-fix processed material was wash-off processed with a warm (40 °C) water spray. The relief image was dried. The percental dot gain with respect to the original dots obtained at 50 % dot value on permanent support materials SM, SH and SL was with respect to a non-pigment coated polyethylene paper support respectively 17, 21, and 12 %. The halftone dot area percentage was calculated with the Murray-Davies equation (ref. Murray A., "Monochrome Reproduction in Photoengraving", J. Franklin Institute, vol. 221, p. 721-744, reading as follows : a = 1 - 10 -D t 1 - 10 -D s x 100 wherein : D t represents the density of the halftone tint, D s represents the density of the solid, and a represents the halftone dot area percentage. The dot gain is obtained by subtracting the dot area percentage of the original halftone separation image from the above calculated one obtained on the titanium dioxide coated paper supports used according to the present invention. EXAMPLE 2 - Preparation of cyan coloured photographic silver halide emulsion layer on temporary support. On an unsubbed cellulosetriacetate film having a thickness of 130 um the following layers (i) and (ii) were applied in the order given. Layer (i) was coated from a coating composition at the indicated coverages : (1) a silver chlorobromide-iodide emulsion (molar ratio 97.65/2.0/0.35) equivalent with 0.3 g of silver nitrate per m2, (2) a gelatin layer at 1.4 g per m2, (3) diffusion-resistant cyan coloured pigment FLEXONYL BLUE paste (C.I. 74,160) at 0.267 g per m2; and Layer (ii) was coated as a non-pigmented gelatin binder layer containing per m2 0.6 g of gelatin, 0.08 g of poly(methyl methacrylate) as a matting agent and a poly(ethyl acrylate) latex at 0.02 g of solids per m2. - Preparation of magenta coloured photographic silver halide emulsion layer on temporary support. The preparation was identical to that of the cyan layer described above with the difference that the cyan pigment was replaced by HELIOECHTCARMIN BBN TEIG paste (Colour Index Nr. 12,485) at a coverage of 0.6 g per m2. - Preparation of yellow coloured photographic silver halide emulsion layer on temporary support. The preparation was identical to that of the cyan layer described above with the difference that the cyan pigment was replaced by HELIOECHTGELB GRN 7476 Feinteig paste (Colour Index Nr. 21,100) at a coverage of 0.467 g per m2. - Preparation of black photographic silver halide emulsion layer on temporary support. The preparation was identical to that of the cyan layer described above with the difference that the cyan pigment was used at a coverage of of 0.058 g per m2 in combination with HELIOECHTPAPIERSCHWARZ NL-paste at a coverage of 0.486 g per m2. - Preparation of the permanent supports SM, SH and SL. These supports were prepared as described in Example 1. - Transfer The light-sensitive cyan-coloured composite layer structure (i) and (ii) being in unexposed state was transferred using a pair of pressure rollers onto a permanent support SM, SH and SL as described hereinbefore by soaking the permanent support prior to transfer in water. By stripping apart the temporary support said composite layer structure was left on the permanent support and the transferred composite layer was dried in a hot air stream. - Exposure In a contact-exposure apparatus provided with ultraviolet radiation source the transferred layer structure was exposed through the proper halftone selection negative. - Hardening development. The transferred composite coating was hardening developed by dipping it for 30 s in an aqueous liquid containing per liter : - Bleach-fixing The developed material was led through a bleach-fix bath containing sodium thiosulphate and the mono sodium,iron(III) salt of ethylenediamine tetra-acetic acid. - Wash-off processing To obtain a cyan coloured relief image the hardening developed and bleach-fix processed material was wash-off processed with warm water (40°C) water spray. The relief image was dried. The percental dot gain obtained at 50 % dot value on the permanent support materials SM, SH and SL with respect to the original dots was respectively 17, 22 and 11 %. Onto the cyan coloured relief pattern the composite magenta coloured layer structure was transferred, dried, image-wise exposed and processed so as to obtain a magenta coloured relief pattern, whereupon that procedure was repeated for the yellow and black coloured relief patterns respectively to form a complete dot size corrected colour proof on the same permanent support.

Claims: The use of a paper base support in a process for the production of a multicolour halftone colloid pattern, wherein said process comprises the following steps : (1) transferring a substantially non-hardened gelatin silver halide emulsion layer containing coloured pigment particles dispersed therein from a temporary support onto a permanent support being said paper base support which support may carry already a halftone relief image containing coloured hardened gelatin, (2) imagewise exposing the non-hardened gelatin silver halide emulsion layer prior to or after carrying out said step (1), (3) forming by hardening development a halftone image in said transferred silver halide emulsion layer, and (4) wash-off processing to remove selectively unhardened portions of the transferred layer, and (5) repeating steps (1), (2) and (3) to form at least partly superposed portions of differently coloured gelatin containing layers, and wherein the transferred silver halide emulsion layer before or after its transfer is exposed through a halftone selection image of a multicolour original, characterized in that a paper base support is used which is coated with a polyolefin layer having a coverage in the range of 5 to 40 g/m2 which layer is itself coated with a binder layer containing a hydrophilic colloid binding agent and white titanium dioxide pigment particles that have an average grain size in the range of 200 nm to 450 nm, are present in a weight percent of at most 300 % with respect to the total binder content, and are applied at a coverage of at least 0.5 g per m2. Use of a paper base support according to claim 1, wherein said polyolefin layer includes a polymer selected from the group consisting of an ethylene homopolymer being a high density polyethylene or a low density polyethylene, a propylene homopolymer or a copolymer of ethylene containing not more than 10 % by weight of styrene, vinyl stearate, vinyl acetate, acrylic acid, ethyl acrylate, methyl methacrylate, methacrylic acid, butadiene or isoprene. Use of a paper base support according to claim 1 or 2, wherein said hydrophilic colloid binding agent is gelatin. Use of a paper base support according to claim 3, wherein said gelatin is used in combination with a hydrophobic polymer in latex form. Use of a paper base support according to claim 4, wherein the gelatin and latex polymer as solids are used in a weight ratio range of 1 : 5 to 5 :1. A sheet material or material in the form of a web which material consists of a paper base support coated with a polyolefin layer having a coverage in the range of 5 to 40 g/m² and being coated with a binder layer containing gelatin and white titanium dioxide pigment particles having an average grain size in the range of 200 nm to 450 nm and being present in a weight percent of at most 300 % with respect to the total binder content, the titanium dioxide being applied at a coverage in the range of 0.5 to 6.4 g/m², characterized in that said gelatin is present in combination with a hydrophobic polymer in latex form. A material according to claim 6, wherein the polyolefin layer includes a polymer selected from the group consisting of an ethylene homopolymer being a high density polyethylene or a low density polyethylene, a propylene homopolymer or a copolymer of ethylene containing not more than 10 % by weight of styrene, vinyl stearate, vinyl acetate, acrylic acid, ethyl acrylate, methyl methacrylate, methacrylic acid, butadiene or isoprene. A material according to claim 6, wherein the gelatin and latex polymer as solids are present in a weight ratio range of 1 : 5 to 5 : 1.

Inventors: DAEMS EDDIE ROZA, LEENDERS LUC HERWIG, DAEMS, EDDIE ROZA, LEENDERS, LUC HERWIG
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP
Application year: 1987
Main classification: G03F3
IPC codes: G03F3, D21H19, G03C1, G03C5