WO1996010214A1

WO1996010214A1 - Negative working overlay colour proofing system

Info

Publication number: WO1996010214A1
Application number: PCT/EP1995/003422
Authority: WO
Inventors: Luc Leenders; Eddie Daems; Rudi De Busser
Original assignee: Agfa-Gevaert Naamloze Vennootschap
Priority date: 1994-09-27
Filing date: 1995-08-30
Publication date: 1996-04-04

Abstract

A negative working method is disclosed for the formation of an overlay colour proof in pre-press graphic arts. A photosensitive element is subjected image-wise to exposure by actinic radiation thereby undergoing photopolymerisation in the exposed areas. The non-exposed non-polymerised areas are removed either by a rub off treatment, or by gumming, or by means of a cleaning foil.

Description

NEGATIVE WORKING OVERLAY COLOUR PROOFING SYSTEM

DESCRIPTION

1. Field qf the invention.

The present invention relates to a dry processable negative working overlay colour proofing system based on photopolymerization.

2. Background of the invention.

In the field of pre-press graphic arts there has been a long felt need for a simple and fast technique offering "colour proofs" of high quality and reproducibility.

Photographically produced colour proofs are a simulation 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 cumbersome and expensive procedure and therefore photoimaging processes and materials 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.

For colour proofing there are two general types of photoimaging methods, namely the overlay type and the single sheet or surprint type.

In the overlay type of colour proofing method, an independent transparent plastic support is used as base for the production of a proof image of each color separation film by applying a

photosensitive solution containing the corresponding colorant and by performing a process for image formation. A plurality of such supports carrying images of the corresponding colours are then superimposed upon each other over a white sheet to produce a colour proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two or three colours in register. Examples of such overlay approaches are contained in U.S. Patents 3,136,637,

3,211,553, and 3,326,682. A commercial overlay colour proofing system is CROMACHECK, marketed by Du Pont Co. In the single sheet or surprint type of colour proofing method, a colour proof is prepared by successively producing images of different colours from different colour separation films on a single receiver sheet. This can be accomplished by sequentially applying colorants or coloured, photosensitive layers to a single opaque support. This method more closely resembles the actual printing process and eliminates the colour distortion inherent in the overlay system. Examples of such single sheet approaches are contained in U.S. Patents 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642.

Colour proofing systems can also be subdivided in dry processes and in methods involving one or more wet processing steps.

As an example for the latter, in US 4,933,258 and US 4,766,053 a method is disclosed for forming a colour proof based on the

successive transfer to a temporary support of several colour images obtained by exposing and developing several photosensitive elements comprising photosensitive resin layers. Finally the complete image is transferred to a permanent support being print paper stock.

Products related to these method are marketed as the FUJI COLOR ART system.

Another wet process for producing a multicolour pattern using silver halide emulsion materials is described in European Patent Specification EP 0 185 410. This process involves the successive transfer of differently coloured hardening developable silver halide containing hydrophilic layers onto a permanent support. After the transfer and the image-wise exposure a wet processing cyclus is performed cosecutively for each colour, each involving a hardening development step, a bleach-fix step, and a wash-off step of the unhardened parts. Further variations and improvements on this process, which is commercialized by Agfa-Gevaert N.V. under the trade name AGFAPROOF, are disclosed in EP 0 305 599 and European patent application Appl. No. 92203339.

US 4,596,757 provides a method for transferring images or solid colours which are subsequently imaged. The photosensitive material comprises a carrier support having sequentially disposed thereon a release layer, a coloured photopolymerizable layer and an adhesive layer. The material can undergo exposure, lamination to a temporary support, wet development, and then lamination to a receptor sheet. Alternatively, the photosensitive material can be laminated to a receptor sheet, undergo exposure and then wet processing. Both processes require development in an aqueous medium. In EP 0 339 860 a proofing system is described involving a development step by alkali.

However in recent years, for reason of convenience,

easy-handling, and for ecological reasons, dry photoimaging

processes are preferred.

In U.S. Patent 4,489,154 a process is claimed which produces a single layer colour proof without wet development. The

photosensitive material comprises a stripable cover sheet, a

coloured photoadherent layer, a non-photosensitive organic

contiguous layer and a sheet support. The material is exposed and peel developed. The positive or negative image is transferred to a receiver base. A fresh layer of adhesive must be applied to the receptor for each subsequent transfer.

In EP 0 437 343 a dry proofing method is disclosed comprising, in order, (A) providing a photosensitive element comprising (i) a support, (ii) a coloured photopolymerizable layer, (iii) an adhesive layer, (B) in either order, providing a receiver base to which said adhesive layer is laminated, and exposing image-wise this

photosensitive composition to actinic radiation, (C) peeling apart said support and said receiver base, thereby transferring the adhesive layer and the non-exposed parts to the receiver base, while the exposed parts remain on the support, and (D) optionally

repeating steps (A) to (C) with different colorants. This method is only positive working.

The present invention extends the teachings on dry colour proofing systems based on photopolymerisation.

It is an object of the present invention to provide an overlay colour proofing system which involves only dry treatment steps.

It is a further object of the present invention to provide a method for colour proofing which is negative working.

3. Summary of the invention.

The objects of the present invention are realized by providing a method for the formation of a negative coloured image comprising, in order, the following steps :

(A) providing a photosensitive element comprising, in order :

(1) a transparent support,

(2) a subbing layer,

(3) a photosensitive layer comprising a polymeric binder, a colorant, a photoinitiating system, and a photopolymerisable ethylenically unsaturated monomer, the ratio monomer / binder being at least 1 : 1 by weight,

(B) image-wise exposing to actinic radiation said photosensitive element,

(C) removing the image-wise unexposed areas by, either,

(a) a rub off treatment, or

(b) lamination and delamination by means of a cleaning foil optionally provided with a permanent adhesive or thermoadhesive layer.

Since the method is negative working, a positive overlay colour proof is obtained when using colour separation negatives as

original. Colour proofs exhibiting a cyan, a magenta, a yellow and a black image respectively can be overlaid to form a full colour proof.

4. Detailed description of the invention.

The particular features and ingredients of the photosensitive element for use in the method of the present invention will now be explained in detail hereinafter.

Suitable transparent supports include e.g. cellulose nitrate film, cellulose acetate film, poly (vinyl acetal) film, polystyrene film, poly (ethylene terephthalate) film, polycarbonate film, polyvinylchloride film or poly-α-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm.

In a most preferred embodiment the support is a polyethylene terephthalate support.

As is well known in the art a subbing layer promotes the ahhesion of the next coated hydrophylic layer to the hydrophobic support. An example of a suitable subbing layer is a layer

containing a polymer containing covalently bound chlorine. Suitable chlorine containing polymers are e.g. polyvinyl chloride,

polyvinylidene chloride, a copolymer of vinylidene chloride, an acrylic ester and itaconic acid, a copolymer of vinyl chloride and vinylidene chloride, a copolymer of vinyl chloride and vinyl acetate, a copolymer of butylacrylate, vinyl acetate and vinyl chloride or vinylidene chloride, a copolymer of vinyl chloride, vinylidene chloride and itaconic acid, a copolymer of vinyl

chloride, vinyl acetate and vinyl alcohol, chlorinated polyethylene, polychloroprene and copolymers therof, chlorosulfonated

polyethylene, polychlorotrifluoroethylene, polymethyl-alpha- chloroacrylate etc. A preferred chlorine containing polymer is co(vinylidenechloride-methylacrylate-itaconic acid ; 88 % / 10 % : 2 %).

Suitable polymers not containing chlorine include co(styrerie- butadiene-carbonic acid), polyvinyl acetate, and

co (methylmethacrylate-butadiene-itaconic acid). In the latter case the amount of the itaconic acid part is preferably comprised between 2 and 15 %. The T_g of the polymer can be adjusted by varying the relative amounts of the methylmethacrylate and the butadiene parts while keeping the itaconic acid part constant at about 5 %. In a most preferred embodiment the copolymer is composed of 47.5 % of methylmethacrylate, 47.5 % of butadiene and 5 % of itaconic acid.

The essential ingredients of the photosensitive layer are a binder, a monomer, a colorant and a photoinitiator.

Examples of useful binders include organic solvent-soluble polymers, e.g. polymers derived from α, β-ethylenically unsaturated compounds such as e.g. polyvinyl acetate, a vinyl acetate-vinyl chloride copolymer, a styrene-butadiene copolymer, polyethylene, and polypropylene. Other preferred binders are σopolyesters with a T_g higher than -25 °C and lower than 50 °C, most preferably between -25 °C and 20 °C, e.g. DYNAPOLL S1420, marketed by Huls AG, which has a T_g of -10 °C. Still other preferred binders are polyethylacrylater copoly (styrene-butylacrylate) and poly (butylmethacrylate).

Especially preferred binders are polyvinyl acetals, such as

polyvinyl butyral and polyvinyl propional. Still other preferred binders are polyvinyl formals which are commercially available from Monsanto as FORMVAR. The formal content of the polyvinyl formals is approximately 65 % to 86 % expressed as percent polyvinyl formal. The acetate content is approximately 9 % to 30 % expressed as percent polyvinyl acetate. The hydroxyl content is approximately 5 % to 7 % as expressed as percent polyvinyl alcohol. The average molecular weight is between 10,000 and 40,000.

A wide variety of photopolymerisable and photocrosslinkable compounds can be used in the present invention. Preferred monomers are those with a melting viscosity higher than 100 mPa.s, most preferred those with a melting viscosity above 1000 mPa.s at 25 °C. Suitable monomers include e.g. the monomers disclosed in DE-OS Nos. 4005231, 3516256, 3516257, 3632657 and US 4,629,676, unsaturated esters of polyols, particularly such esters of the alpha-methylene carboxylic acids, e.g. ethylene diacrylate, glycerol

tri(meth)acrylate, ethylene dimethacrylate, 1,3-propanediol

di(meth)acrylate 1,2,4-butanetriol tri(meth)acrylate,

1,4-cyclohexanediol di(meth)acrylate, 1,4-benzenediol

di (meth) acrylate, pentaerythritol tetra(meth)acrylate,

pentaerythritol triacrylate, dipentaerythritol pentacrylate, trimethylolpropane triacrylate, 1,5-pentanediol di(meth)acrylate, the bis acrylates and methacrylates of polyethylene glycols of molecular weight 200-500, and the like : unsaturated amides, particularly those of the alphamethylene carboxylic acids, and especially those of Alpha-Omega-diamines and oxygen-interrupted omega-diamines, such as methylene bis-acrylamide, methylene

bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine tris-methacrylamide,

bis(gamma-methacrylamidopropoxy)ethane, beta-methacrylamidoethyl methacrylate, N-(beta-hydroxyethyl)-beta-(methacrylamido)ethyl acrylate and N,N-bis(beta-methacrylolyoxyethyl)acrylamide, vinyl esters e.g. divinyl succinate, divinyl adipate, divinyl phthalate, divinyl butane-1,4-disulphonate; and unsaturated aldehydes, e.g. sorbaldehyde (hexadienal). The photopolymerizable composition may also comprise polymers and/or oligomers comprising 2 or more polymerizable functions e.g. acrylated epoxies, polyester acrylates, urethane acrylates etc.. It will be clear that these monomers and/or polymers and/or oligomers can be used in admixture.

It is also possible to use monofunctional (meth) acrylic acid esters as monomer provided they are not to volatile and do not spread an unwanted odour. Suitable compounds include n- octylacrylate, n-octylmethacrylate, decylacrylate,

decylmethacrylate, stearylacrylate, stearylmethacrylate,

cyclohexylacrylate, cyclohexylmethacrylate, phenylethylacrylate, phenylethylmethacrylate.

The most preferred polymerizable compounds comprise one or more (meth) acrylate functional groups.

Particular preferred classes of photopolymerizable compounds containing (a) (meth) acrylate group (s) are reactive multifunctional monomers disclosed in EP 0 502 562 and represented by general formula (I) or (II) :

A [-NHCO-X-L¹(-(L²)_u-OCO-CR¹=CH₂)_n]_m (I) wherein n represents an integer from 1 to 3, m equals an integer of 3 to 6 when n equals 1, and 2 to 6 when n equals 2 or 3, and u equals 0 or 1;

A represents an organic group of the following nature being 3 to 6 valent when n equals 1 and being 2 to 6 valent when n equals 2 or 3 a) a hydrocarbon residue containing 5 to 25 carbon atoms which may be interrupted by one or more ether, ester or amide functions; b)

with A¹ representing a linear or branched aliphatic residue that may contain 0 to 3 0-atoms and 2 to 20 C-atoms, an aromatic residue containing 6 to 24 carbon atoms, an aromatic aliphatic residue containing 7 to 28 C-atoms or an cycloaliphatic residue containing 6 to 26 C-atoms, R³ and R⁴ each independently representing a hydrogen or a methyl group, A representing a hydrocarbon residue containing 5 to 25 carbon atoms, o

represents an integer of 0 to 5 and p represents an integer of 2 to 6 when n equals 2 or 3 and represents an integer of 3 to 6 when n equals 1 ;

C)

wherein A¹, A², R³, R⁴, o and p have the same meaning as defined above

d)

wherein A¹, A², R³, R⁴, o and p have the same meaning as defined above;

G represents -O-CO-NH-Y(-COO-) - ;

wherein Y represents a divalent (cyclo) aliphatic residue containing 2 to 15 C-atoms and that may contain an ester, ether or urethane function, and q represents 0 or 1

Q represents a linear or branched aliphatic hydrocarbon residue containing 3 to 15 carbon atoms and which may comprise 1 to 3 oxygen bridges and r equals 0 or 1,

X represents O or NR²,

L¹ represents an aliphatic hydrocarbon residue that is at least divalent and that may comprise 1 to 3 O-atoms,

L² represents a lower alkylene of 1 to 6 C-atoms which may be branched or linear,

R¹ represents hydrogen or a methyl group,

R² represents hydrogen or a lower alkyl group of 1 to 6 C-atoms;

wherein

Ur represents a divalent or trivalent condensed urea residue ;

Z represents 0 or NR with R representing alkyl containing 1 to

12 C-atoms ;

R⁷ represents a divalent hydrocarbon residue containing 2 to 25 C- atoms ;

R⁸ represents a hydrocarbon residue with a valence between 2 and 6, and containing 2 to 18 C-atoms, which can be linear or branched and which can be interrupted by upto 3 0 atoms ;

R⁹ represents hydrogen or methyl ;

α represents an integer from 1 to 5, and

β equals 2 or 3.

Preferably used monomers comprise one of the following residues as hydrocarbon residue A and/or A² of general formula (I) :

wherein R⁵ and R⁶ each independently represent hydrogen or a lower alkyl of 1 to 6 C-atoms, s and t independently represent an integer from 1 to 6 and wherein the aliphatic hydrocarbon residues la, Ic and Id comprise 2 to 6 free valences.

Examples of monomers according to formula (I) suitable for use in accordance with the present invention are shown in table 1.

i and j are respectively 3.5 and 0.5 indicating that compound 6 is a mixture of compounds obtained by reacting i equivalents of glycerine- dimethacrylate and j equivalents of hydroxyethyl methacrylate as disclosed in DE 3,703,130.

i and j are respectively 2.5 and 1.5 indicating that compound 7 is a mixture of compounds obtained by reacting i equivalents of glycerine- dimethacrylate and j equivalents of hydroxyethyl methacrylate as disclosed in DE 3,703,130.

The fractal indexes in the formulas 1, 2 and 10 indicate that these formulas represent a mixture of compounds having a different length of the ethylene-oxide piece in said formulas the indexes thus representing an avarage of said ethylene-oxide piece. The formulas 14 to 23 represent a mixture of structural isomers and can be used in accordance with the present invention without separation of the isomers. The monomers corresponding to general formula (I) are known and can be prepared according to the German patent application numbers 3,522,005, 3,703,080, 3,643,216, 3,703,130, 3,917,320 and 3,743,728.

In general monomer formula (II) preferred condensed urea residues represented by Ur are following structural units :

Examples of preferred useful monomers according to general formula (II) are listed below in table 2 :

The photoinitiator system, present in the photosensitive layer, comprises one or more compounds which directly furnish free-radicals when activated by actinic radiation. It can also comprise a

plurality of other compounds, e.g. spectral sensitizers, hydrogen donors.

Numerous conventional photoinitiators systems may be used provided they are compatible with the other ingredients of the element. Useful photoinitiators are ketoxime-esters. Preferred photoinitiator systems are 2,4,5-triphenylimidazolyl dimers in combination with chain transfer agents, or hydrogen donors, such as are disclosed in US 3,479,185, US 3,784,557, US 4,311,783 and

US 4,622,286. Preferred hexaarvlbisimidazoles (HABI) are 2-o-chloro- substituted hexaphenylbisimidazoles in which the other positions on the phenyl radicals are unsubstituted or substituted with chloro, methyl or methoxy. The most preferred initiator is o.-Cl-HABI, i.e., 2,2'-bis-(o-chloro-phenyl)-4,4,5,5',tetraphenyl-1,1'-bisimidazole (or simply "bisimidazole") corresponding to following chemical formula

Hydrogen donor compounds useful as chain tranfer agents in the photopolymer layer include : 2-mercaptobenzoxazole, 2- mercaptobenzothiazole, 4-methyl-4H-1,2,4,-triazole-3-thiol, and the like. A preferred hydrogen donor is 2-mercaptobenzoxazole with following formula :

Although the HABI initiating systems described above are preferred, other initiating systems may be used in practicing this invention. Useful photoinitiators described in US 2,760,863 include vicinal ketaldonyl alcohols, such as benzoin, pivaloin, aσyloin ethers, e.g. benzoin methyl and ethyl ethers, and α-hydrocarbon- substituted aromatic acyloins, such as α-methylbenzoin. Further useful photoinitiators include quinoxaline compounds as described in U.S. Patent 3,765,898, the vicinal polyketaldonyl compounds in U.S. Patent 2,367,660, the α-carbonyls in U.S. Patent 2,367,661 and 2,367,670, the acyloin ethers in U.S. Patent 2,448,828, the

triarylimidazolyl dimers in U.S. Patent 3,479,185, the α-hydrocarbon substituted aromatic acyloins in U.S. Patent 2,722,512, polynuclear quinones in U.S. Patents 2,951,758 and 3,046,127, and s-triazines in U.S. Patent 4,656,272.

The photoinitiator is preferably present in the photosensitive layer in an amount ranging from 2 to 30 % by weight.

Many of the well known photoinitiator systems have limited applicability because they are activated only by UV radiation. For exposure in the visible region, e.g. by lasers, the use of so-called sensitizers is indispensable. A large number of free-radical generating systems have been used as sensitizers for the visible region for photopolymerizable compositions. US-P 3,652,275 discloses selected bis (p-dialkylaminobenzylidene) ketones as sensitizers for HABI initiator systems. US-P 4,162,162 discloses selected

sensitizers derived from aryl ketones and p-dialkylaminoaldehydes. US-P 4,987,230 and US-P 4,987,230 also disclose sensitizers for HABI systems.

A preferred sensitizer is 7-diethylamino-4-methylcoumarin.

Dyes and/or pigments are included in the photosensitive layer. Preferred colorants for this invention are pigments rather than dyes. Light fast colorants are preferred. The pigments are typically dispersed with an organic binder in an organic solvent or mixture of organic solvents. The pigments may be organic or inorganic. They are ground to a small enough particle size to duplicate the particle size and color of equivalent inks. The median diameter is generally less than 1 μm.

Non-exclusive examples of colorants usable in the present invention are as follows : Permanent Yellow G (C.I. 21095);

Permanent Yellow GR (C.I. 21100), Permanent Yellow DHG (C.I. 21090), Permanent Rubine L6B (C.I. 15850:1), Permanent Pink F3B (C.I.

12433), Hostaperm Pink E (73915), Hostaperm Red Violet ER (C.I.

46500), Permanent Carmine FBB (12485), Hostaperm Blue B2G (C.I.

74160), Hostaperm Blue A2R (C.I. 74160), and Carbon Printex 25. Most of these pigments are products of Hoechst AG. They can be used separately or blended for a desired colour.

The colorant is present in the photosensitive layer in an amount preferably ranging from 10 % to 50 % by weight.

Other ingredients which may be present in the photosensitive layer include thermal polymerization inhibitors, plasticizers, residual solvents, surfactants, inert fillers, antihalation compounds and optical brightening agents.

The photosensitive layer is preferably coated at a dry coverage ranging from 0.1 g/m² to 5 g/m².

The exposure to actinic radiation of the original, e.g. a colour separation negative, is performed preferably in a contact apparatus under vacuum frame conditions using a UV-rich light source, such as a mercury vapour discharge lamp, a metal-halide source or a quartz- halide source. Alternatively, a carbon arc, a pulsed xenon source, a cathode ray tube or a laser can be used.

Due to this actinic radiation photopolymerisation accompanied by hardening takes place in the image-wise exposed areas. As a

consequence these areas adhere strongly to the subbing layer and will not be removed in the following removal step. At the same time the cohesive strenght of the photosensitive layer is enhanced. This removal of the unhardened unexposed parts can be performed in two different ways, i.e. (a) a rub off treatment, or (b) lamination and delamination by means of a cleaning foil optionally provided with a permanent adhesive or thermoadhesive layer.

The rub off treatment can be performed, for instance, by rubbing with a cotton pad, by rubbing manually with a brush or by bringing the exposed element in contact with an apparatus equipped with a rotating brush. Gumming is e.g. performed by a STAEDLER gum MARS PLASTIC ref. 52650.

The cleaning foil for the lamination/delamination procedure can be a paper sheet, optionally provided with a polyolefine coating, but, more preferably, it is a thin transparent polymeric resin foil, such as a polyethylene terephthalate foil having a thickness preferably comprised between 23 and 175 μm. The cleaing foil can be provided with an adhesive layer containing a permanent adhesive, also called pressure-sensitive adhesive polymer, or with a

thermoadhesive, also called heat-sensitive polymer. A survey of pressure and/or thermal adhesives is given by J. Shields in

"Adhesives Handbook", 3rd. ed. (1984), Butterworths - London,

Boston, and by Ernest W. Flick in "Handbook of Adhesive Raw

Materials" (1982), Noyens Publications, Park Ridge, New Jersey - USA.

Examples of pressure-sensitive adhesive resins are described in US-P 4,033,770 for use in the production of adhesive transfers

(decalcomanias) by the silver complex diffusion transfer process, in the Canadian Patent 728,607 and in the United States Patent

3,131,106. Pressure-sensitive adhesives are usually composed of (a) thermoplastic polymer (s) having some elasticity and tackiness at room temperature (about 20°C), which is controlled optionally with a plasticizer and/or tackifying resin. A thermoplastic polymer is completely plastic if there is no recovery on removal of stress and completely elastic if recovery is instantaneous and complete.

Particularly suitable pressure-sensitive adhesives are selected from the group of polyterpene resins, low density polyethylene, a copoly (ethylene/vinyl acetate), a poly (C₁-C₁₆) alkyl acrylate, a mixture of poly (C₁-C-₁₆) alkyl acrylate with polyvinyl acetate, and copoly (vinylacetate-acrylate) being tacky at 20°C.

In the production of a pressure-adhesive layer an intrinsically non-tacky polymer may be tackified by the adding of a tackifying substance, e.g. plasticizer or other tackifying resin.

Examples of suitable tackifying resins are the terpene

tackifying resins described in the periodical "Adhesives Age", Vol. 31, No. 12, November 1988, p. 28-29.

Thermoadhesive polymers are preferably coated from aqeous solutions for ecological reasons. Therefore the polymers are preferably incorporated as latices. Preferably they show a T between 10 and 100 °C. Preferred latices are styrene-butadiene latices. These latices can contain other comonomers which improve the stablitity of the latex, such as acrylic acid, methacrylic acid and acrylamide. Other possible polymer latices include

polyvinylacetate, copoly(ethylene-vinylacetate),

copoly(acrylonitrile-butadiene-acrylic acid), copoly(styrene- butylacrylate), copoly(methylmethacrylate-butadiene),

copoly(methylmethacrylate-butylmethacrylate),

copoly(methylmethacrylate-ethylacrylate), copolyester(terephtalic acid-sulphoisophtalic acid-ethyleneglycol), copolyester(terephtalic acid-sulphoisophtalic acid-hexanediol-ethyleneglycol).

Particularly suitable polymers for use in the thermoadhesive layer are the BAYSTAL polymer types, marketed by Bayer AG, which are on the basis of styrene-butadiene copolymers. Other useful polymers are the EUDERM polymers, also from Bayer AG, which are copolymers comprising n.-butylacrylate, methylmethacrylate, acrylonitrile and small amounts of methacrylic acid.

Other additives can be present into the thermoadhesive layer to improve the layer formation or the layer properties, e.g. thickening agents, surfactants, levelling agents, thermal solvents and

pigments.

Depending on the permanent adhesive or thermoadhesive character of its adhesive layer the cleaning foil is laminated to the exposed photosensitive element at room temperature or at elevated

temperature. Preferably the lamination is performed by means of a pair of rollers such as in the LAMIPACKER device. The delamination can be performed manually or by means of an apparatus.

When using a colour separation negative as original a positive overlay colour proof is otained. Colour proofs exhibiting a cyan, a magenta, a yellow and a black image respectively can be overlaid to form a full colour proof. Alternatively, the proofing procedure can be be limited to the production of just one single "false" colour proof serving as a so-called "position proof", for instance, in order to check a page lay-out.

The following examples will illustrate the present invention without however limiting it thereto.

EXAMPLES

EXAMPLE 1

To a polyethylene terephthalate support of 63 μm thickness a subbing layer was applied containing copoly (methylmethacrylate- butadiene-itaconic acid ; 47.5 % / 47.5 % / 5 %) at a coverage of 50 mg/m . For the photosensitive layer a coating composition was prepared containing following ingredients :

Carbon printex 25 was incorporated as a 10 % dispersion containing also 5 % of FORMVAR from a 1:1 solvent mixture of 1-methoxy-2-propanol and γ-butyrolactone.

This coating solution was applied to the subbed support by means of a BRAIVE coating knife giving rise to a wet coating thickness of 50 μm. The photosensitive element was then image-wise exposed by UV radiation in a contact apparatus. After exposure the non-exposed parts were gummed off by means of STAEDLER gum, marketed by Mars- Plastic Co., ref. 52650. A black proof image of excellent quality was obtained.

EXAMPLE 2

A photosensitive element was prepared as follows. The same support as in example 1 was coated by means of a knife coater with a photosensitive composition consisting of a methylethylketone

solution of 1.5 % by weight of dipentaerythritol pentacrylate, 6.85 % by weight of copoly (styrene-butylmethacrylate), 0.45 % by weight of a blue dye (C.I. 61551), 1.33 by weight of bis-imidazole, 0.1 % by weight of Michler's ketone, and 0.07 % by weight of

mercaptobenzoxazole. The mixture was coated at a wet thickness of 25 μm.

A polyethylene terephthalate foil was overcoated by means of a knife coater with a thermoadhesive composition consisting of a 40 % by weight aqueous dispersion of BAYSTAL KA8522,, marketed by Bayer AG, being a copolymer of styrene-butadiene-acrylic acid with a glass transition temperature T of 34 °C (measured with the 1090

THERMOANALYZER of Du Pont Co.), a melt viscosity of more than 13420 mPa.s and an elasticity corresponding to a tg δ^-1 value of 3.54, both properties measured with the VISCOELASTIC MELT TESTER of

Rheometrics Co., Surrex, UK., in an amount of 10 g/m².

The photosensitive element was exposed by means of UV through a test pattern containing a 60 lines per cm screen as well as fine positive and negative lines. The exposed element was placed in face- to-face contact with the PET foil provided with the thermoadhesive layer. The contacting elements were conveyed through a roller laminator at 107 °C at a speed of 0.3 m/min. Immediately thereafter, the elements were peeled apart and the non-exposed parts of the photosensitive layer were removed and the exposed areas remained on the support thus giving rise to a negative image. A 20 μm line was faithfully reproduced.

Claims

1. Method for the formation of a negative coloured image

comprising, in order, the following steps :

(A) providing a photosensitive element comprising, in order :

(1) a transparent support,

(2) a subbing layer,

(3) a photosensitive layer comprising a polymeric binder, a colorant, a photoinitiating system, and a photopolymerisable ethylenically unsaturated monomer, the ratio monomer / binder being at least 1:1 by weight,

(B) image-wise exposing to actinic radiation said photosensitive element,

(C) removing the image-wise unexposed areas by, either,

(a) a rub off treatment, or

2. Method according to claim 1 wherein said polymeric binder has a glass transition temperature T_g between -25 °C and 50 °C.

3. Method according to claim 1 or 2 wherein said photopolymerisable ethylenically unsaturated monomer has a melt viscosity higher than 200 mPa.s at 25 °C.

4. Method according to claim 3 wherein said photopolymerisable

ethylenically unsaturated monomer has a melt viscosity higher than 1000 mPa.s at 25 °C.

5. Method according to any of claims 1 to 4 wherein said

photoinitiator is 2,2'-bis-(o-chloro-phenyl)- 4,4,5,5',tetraphenyl-1,1'-bisimidazole.

6. Method according to any of claims 1 to 5 wherein said

photosensitive layer is coated at a dry coverage between 0.1 g/m and 5 g/m .

7. Method according to any of claims 1 to 6 wherein the

photoinitiator is present in said photosensitive layer in an amount ranging from 2 % to 30 % by weight.

8. Method according to any of claims 1 to 7 wherin said colorant is present in said photosensitive layer in an amount ranging from 10 % to 50 % by weight.