WO1996010215A1

WO1996010215A1 - Negative working surprint colour proofing system

Info

Publication number: WO1996010215A1
Application number: PCT/EP1995/003423
Authority: WO
Inventors: Rudi De Busser; Eddie Daems; Luc Leenders
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1994-09-27
Filing date: 1995-08-30
Publication date: 1996-04-04
Anticipated expiration: 1997-03-27

Abstract

A negative working method is disclosed for the formation of an overlay colour proof in pre-press graphic arts. A pigmented photosensitive element provided with an adhesive layer is subjected image-wise to exposure by actinic radiation thereby undergoing photopolymerisation in the exposed areas. The exposed areas are transferred to a receiver base and form there a negative image. By repeating this procedure with different colorants, a full colour proof can be obtained.

Description

NEGATIVE WORKING SURPRINT COLOUR PROOFING SYSTEM

1. Field of the invention.

The present invention relates to a dry processable negative working surprint colour proofing system based on

photopolymerisation.

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 ctimbersome 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. However, this type of colour proofing material has the disadvantage that the superimposed polymeric supports tend to darken the background of the final full proof. As a result, the impression of the colour proofing composite thus prepared is vastly different from that of the prints actually obtained with conventional printing presses and with proof presses. 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 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 βilver halide containing hydrophilic layers onto a permanent support. After the transfer and the image-wise exposure a wet processing cyclus is performed consecutively 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 image-wise exposing 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 surprint (or single sheet) colour proofing systems based on photopolymerisation.

It is an object of the present invention to provide a surprint 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 amount monomer / amount binder ratio being at least 4:1 by weight,

(4) an adhesive layer comprising a thermoplastic polymer having a T_g between 20 °C and 110 °C,

(B) either,

(i) providing a receiver base and laminating said receiver base at elevated temperature to said adhesive layer (4) of said photosensitive element, and then image-wise exposing this composition to actinic radiation, or,

(ii) image-wise exposing said photosensitive element to actinic radiation, and then providing a receiver base and laminating said receiver base at elevated temperature to said adhesive layer (4) of said photosensitive element,

(C) peeling apart said transparent support and said receiver base whereby the adhesive layer and the image-wise exposed areas of said photosensitive layer are transferred to the receiver base while the image-wise non-exposed areas remain adhered to said subbing layer (b) on top of said transparent support,

(D) optionally repeating steps (A) through (C) at least once with another photosensitive element containing a different colorant thus forming a negative multicolour image on the receiver base.

Essential for obtaing the negative image polarity as it is the case in this invention is the high monomer / binder ratio being at least 4 : 1 by weight.

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 0.025 and 0.20 mm.

In a most preferred embodiment the support is a polyethylene terephthalate 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(styrene- 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 suitable binders for the photosensitive layer are styrene/maleic anhydride copolymers and their half esters, acrylic polymers and copolymers, polyarnides, polyvinyl pyrrolidones, cellulose and its derivatives and phenolic resins. 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. 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, stearylaσrylate, 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) :

AC-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 O-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 R 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-)_q- ;

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 QJ 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 NR ,

L* represents an aliphatic hydrocarbon residue that is at least divalent and that may comprise 1 to 3 0-atoms,

1> 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 O or NR¹⁰ with R¹⁰ representing alkyl containing 1 to

12 C-atoms ;

R 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 O 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 hexaarylbisimidazoles (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-mercaptobenzooazole 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, acyloin ethers, e.g. benzoin methyl and ethyl ethers, and CC-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 0.25 g/m² to 10 g/m².

In principle the thermoadhesive layer can be coated directly on top of the photosensitive layer, but the preferred method for applying the adhesive layer to the photosensitive layer is by laminating it to this photosensitive layer under elevated pressure and/or temperature. In the latter case the adhesive layer is

initially coated onto a temporary support. Then the dried adhesive layer can be brought in close contact with the photosensitve layer. The temporary support is removed, and then the adhesive layer with the photosensitive layer and the original temporary support is laminated to a receiver base. Alternatively, the dried adhesive layer may be laminated to a receiver base. The temporary support is removed, and the photosensitive layer with support is laminated to the adhesive layer on the receiver base. For the temporary support the same polymeric resins can be chosen as for the transparent support of the photosensitive element, e.g. polyethylene

terephthalate preferably unsubbed in this case.

The coating of the adhesive layer may be accomplished in several different ways. In the case of direct coating on top of the

photosensitive layer some adhesives may be coated out of organic solvents which do not have any solubilizing or deleterious effect on the photosensitive layer. Such solvents can include cyclohexane, n- heptane, and n-hexane. However, thermoadhesive polymers are preferably coated in this case as aqueous solutions for ecological reasons. They must show a T_g between 20 and 110 °C. Useful latices include 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). Other 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. still 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.

Acrylic polymers and copolymers are preferred for the lamination method of applying the thermoadhesive layer to the photosensitive layer. Vinyl acetate polymers and copolymers are most preferred.

Polyvinyl acetates are available from Hoechst AG as Mowilith. These resins have a average molecular weight between 35,000 and 2,000,000. In the preferred embodiment, the polyvinyl acetate is present in the adhesive layer in an amount of greater than 60 percent by weight. In this case organic solvents can also be used for coating the adhesive layer to the temporary support. This temporary support can be chosen from the same list of polymeric resins suitable as support for the photosensitive element.

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. Further this layer may contain components as UV absorbers, antistatic compositions, optical brighteners and plasticizers.

Suitable plasticizers include phthalates, non-exclusively including dibutyl phthalate, butyl benzyl phthalate, and dimethyl phthalate. Polymeric plasticizers, such as RESOFLEX R-296 available from

Cambridge Industries, may also be used. The plasticizer may be present in the adhesive layer in an amount of up to approximately 25 percent by weight.

In the preferred embodiment, the dry adhesive layer has a coating weight range between approximately 3 and 30 g/m². The more preferred coating weight is between approximately 5 and 15 g/m². The thickness of the adhesive may be adjusted to regulate the apparent dot size of the final proof.

Receiver bases may comprise virtually any material which can withstand the laminating and dry development processes. Plastic sheets such as polyethylene terephthalate are useful for this purpose. Preferably the plastic sheet serving as receiver base is thicker than the transparent support of the original photosensitive element ; its thickness is preferably comprised between 50 and 400 μm. In a preferred embodiment this polymeric receiver base is be pigmented e.g. by means of titanium dioxide or they can carry a precoat containing titanium dioxide. Plastic coated paper sheets, such as polyethylene coated paper, may also be used. Optionally, they can also be provided with a precoat containing titanium dioxide. Still other bases may include wood, glass, metal, cardboard and the like. Preferably, the receiver sheet in ordinary printing paper.

Apart from the receiver base also the thermoadhesive layer, belonging to first colour transferred, can contain titanium dioxide. An appropriate amount of titanium dioxide can be chosen in order to regulate the apparent dot size (so-called "dot gain") in the final proof, as will be illustrated by example 2 furtheron. Preferably this amount is limited to 50 % vis-a-vis the amount of

thermoadhesive polymer.

Lamination may be conducted by putting the two materials in contact and then introducing the materials into the nip of a pair of heated laminating rollers under suitable pressure. Suitable

laminating temperatures usually range from approximately 50°C to 130ºC, preferably from 75ºC to 110ºC.

The photosensitive layer is exposed by means well known in the art. Exposure can be performed after the adhesive is applied to the photosensitive layer, but preferably it is done after the adhesive and photosensitive layer have been laminated to the receiver base . Such exposure may be performed by actinic radiation from a light source through a conventional halftone negative or positive colour separation under vacuum frame conditions. Mecury vapor discharge lamps are preferred over metal halide lamps. Other radiation sources, such as carbon arc and pulsed xenon may also be used. Light absorbing filters may be used to reduce light scattering in the materials. When the photosensitive element has been sensitized to the visible spectral region as explained above electronically stored information can be recorded directly on the photosensitive element by a laser source, e.g. an Ar ion laser (so-called Direct Digital Colour proofing, DDCP).

Finally the photosensitive composition is dry developed by peeling apart the support from the receiver base at room temperature with a steady, continuous motion. No devices are necessary to hold down the receiver base during peeling because only moderate manual peeling forces are needed to separate the materials. The preferred peel angle relative to the peel direction is greater than 90°.

Contrary to the practice of EP 0 437 343 cited above the

delamination leaves the non-exposed areas on the original support and the exposed areas on the adhesive layer which remains with the receiver base. This is due to the high monomer / binder ratio in the photosensitive layer. Thus a negative coloured image remains on the adhesive layer on the receiver base. So when the original was a negative colour separation a positive proof image is obtained.

Another photosensitive layer can be laminated via another adhesive layer (this time containing no titanium dioxide) to the first image on the receiver base. This second photosensitive layer has usually a different colour than the first and is exposed through the appropriate color separation. After lamination to the receiver and exposure, the support of the second photosensitive layer is removed as was done with the first support. The second negative image with its adhesive remains with the first image. A third and a fourth image may be added in a manner similar to that used to produce the second image. In the usual case, four coloured layers are used to produce a full colour proof. These four colours are cyan, magenta, yellow, and black.

A matte finish of the final image may be obtained by embossing the shiny, top surface of the image with a matte material, such as Melinex 377 from ICI. This is done by laminating together the final image and matte material. The matte material is then generally removed after lamination. The advantage of this method is that the finish of the final proof can be determined by careful selection of the matting material.

A protective layer may be laminated on top of the last dry developed layer.

The invention will now be illustrated by the following examples without however being limited 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 photopolymerisable photosensitive layer a coating composition was prepared containing following ingredients :

{*) ketoxime ester is represented by following formula

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 on a warm coating plate (45 °C) giving rise to a wet coating thickness of 50 μm. After drying on air the layer was further dried for 30 minutes.

The coating for the thermoadhesive layer (TAX) was composed of : n.-butylacetate 82 g

MOWILITH 30 (vinyl acetate compolymer, Hoechst AG) 18 g

This solution was coated onto the untreated side of polyethylene terephthalate sheet having a thickness of 63 μm and serving as temporary support. The layer was dried in the same way as the photosensitive layer. The dry tickness of the layer was 12 μm. The thermoadhesive layer was applied to the photosensitive layer at 80 °C. Then the temporary support was removed by delamination.

Thereafter the photosensitive element was laminated to a receiver base serving as permanent support in a CODOR LAMIPACKER LPP 650 (Domed B.V., Amsterdam, NL). The receiver base consisted of a 180 μm thick polyethylene terephthalate sheet, pigmented with titanium dioxide and covered with a copolyester (VITEL PE200, Goodyear Co.) subbing layer. This composite material was then image-wise exposed through its original support by means of a UV source in a

contactexposure apparatus. Peeling apart was performed at an angle of 180° at a constant speed of 200 cm/min. As a result the image- wise unexposed parts were removed with the original support while the image-wise exposed areas remained adhered to the thermoadhesive layer on the receiver. In this way a negative coloured image was formed on the receiver base.

Example 2

A coating solution was prepared according to the following composition :

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

This coating solution was applied to the same subbed support as in example 1 by means of a BRAIVE coating knife giving rise to a wet coating thickness of 50 μm. After drying on air the layer was further dried in an oven at 60 °C.

Four different coating solutions for the thermoadhesive layer were prepared containing 18 g of MOWILITH 30, titanium dioxide in amounts varying between 0 and 7.5 g and n.-butyl acetate as to give a total weight of 30 g. The layers were coated to a dry coverage of 12 g MOWILITH per m².

Each solution was coated onto the untreated side of polyethylene terephthalate sheet having a thickness of 63 μm and serving as temporary support. The layer was dried in the same way as the photosensitive layer. Each thermoadhesive layer was applied to the photosensitive layer at 80 °C. Then the temporary support was removed by delamination. Thereafter the photosensitive element was laminated to a receiver base serving as permanent support in a CODOR LAMIPACKER LPP 650 (Domed B.V., Amsterdam, NL). This receiver base consisted of polyethylene coated paper. This composite material was then image-wise exposed at the original support side through a 50 % dot screen original by means of a UV source in a contactexposure apparatus. Peeling apart was performed at an angle of 180° at a constant speed of 200 cm/min. As a result the image-wise unexposed parts were removed with the original support while the image-wise exposed areas remained adhered to the thermoadhesive layer on the receiver base. In this way a negative coloured image was formed on the receiver element.

In table 1 the dot gain results as a function of the TiO₂ concentration in the TAL are summarized.

It is clear from table 1 the dot gain diminishes with increasing amount of titanium dioxide. Within certain limits a continuous adjustment of the dot gain is possible.

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,

(4) an adhesive layer comprising a thermoplastic polymer having a T between 20 °C and 110 °C,

(B) either,

2. Method according to claim 1 wherein said photoinitiating system comprises 2,2'-bis-(o-chloro-phenyl)-4,4,5,5',tetraphenyl-1,1'- bisimidazole as photoinitiator.

3. Method according to claim 1 or 2 wherein said photosensitive layer is coated at a dry coverage between 0.25 g/m and 10 g/m .

4. Method according to any of claims 1 to 3 wherein the

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

5. Method according to any of claims 1 to 4 wherein said colorant is present in said photosensitive layer in an amount ranging from 10 % to 50 % by weight.

6. Method according to any of claims 1 to 5 wherein said adhesive layer (4) is applied to said photosensitive layer (3) by (a) coating said adhesive layer on a temporary support, (b)

laminating the adhesive layer to the photosensitive layer, and (c) removing the temporary support.

7. Method according to any of claims 1 to 6 wherein said adhesive layer (4) contains a poly (vinyl acetate) or a copolymer of vinyl acetate.

8. Method according to any of claims 1 to 7 wherein the dry coating weight of said adhesive layer (4) ranges from 3 to 30 g/m².

9. Method according to any of claims 1 to 8 wherein the adhesive layer (4) of the photosensitive element contains titanium dioxide, with the proviso that this is only the case for the layer (4) of that photosensitive element subjected as the first to steps (B) and (C).

10. Method according to claim 9 wherein said titanium dioxide is present in said adhesive layer (4) in an amount ranging between 0 % and 50 % vis-a-vis the amount of thermoadhesive polymer.

11. Method according to any of claims 1 to 10 wherein said receiver base is a polyethylene terephthalate sheet.

12. Method according to claim 11 wherein said polyethylene

terephthalate sheet is pigmented with titanium dioxide.

13. Method according to any of claims 1 to 10 wherein said receiver base is a resin coated paper base.

14. Method according to any of claims 1 to 10 wherein said receiver base is printing paper.