GB2304719A - Lithographic plates; photopolymerisable compositions containing N-substituted N-phenyl glycines - Google Patents

Abstract

A lithographic plate comprises a base coated with a photopolymerisable composition which comprises a polymeric binder, at least one free radically polymerisable ethylenically unsaturated compound and as the photoinitiator the combination of an N-phenyl-glycine compound of the general formula:- where R 1 is a hydroxyalkoxyaryl group, an hydroxy alkoxyalkyl group having 2 to 12 carbon atoms, a hydroxyalkyl ester, a hydroxyalkylaryl ester, an alkyl carboxylic acid or salts thereof, R 4 , R 5 , R 6 , R 7 and R 8 are each a hydrogen atom, an alkyl group having 1 to 12 carbon atoms or a halogen atom, or at least one R 4 , R 5 , R 6 , R 7 and R 8 are a nitro or alkoxy group the remainder being hydrogen, R 2 and R 3 are each a hydrogen, a halogen atom, or an alkyl group having 1 to 8 carbon atoms and X is hydrogen or an organic or inorganic cation together with a light absorbing compound.

Description

This invention relates to novel photopolymerisable compositions.

A large variety of photopolymerisable compositions useful for preparing lithographic plates have been described in the patent literature. A number of these comprise a polymerisable compound and a binary photosensitive combination which when photo-exposed causes the polymerisable compound to harden in the exposed areas. A number of these prior art patent applications comprise a polymerisable compound and a binary photosensitive composition which comprises N-phenyl glycine or a derivative thereof together with a compound which absorbs light. Exemplary of these prior art patent specifications are G.B. 2189496, E.P. 497552 and U.S. 5378579.In published P.C.T. patent application W.O. 95/09383 there is described a lithographic plate which comprises a base coated with a photopolymerisable composition which comprises a polymeric binder, at least one free-radically polymerisable ethylenically unsaturated compound and as the photoinitiator combination a metallocene compound, N-phenyl glycine or a substituted N-phenyl glycine or an N-phenyl glycine derivative together with a third component which is a substance which helps in the reaction initiation but also increases the sensitivity of the photopolymerisable composition to a desired region of the spectrum.

The N-phenyl glycine derivatives described in E.P. 497552 and G.B. 2189496 are chemically unstable. Thus they break down on storage and when used produce a very offensive smell. Therefore these N-phenyl glycine derivatives are of limited practical use. On the other hand the N-phenyl glycine derivatives described in U.S. 5378579 have been found to be very difficult to manufacture. Thus lithographic plates which used the N-phenyl-glycine derivatives described in U.S. 5378579 would be too expensive to manufacture. Also these compounds as well have been found to be unstable in the photosensitive composition.

We have found a class of N-phenyl-glycine compounds which are comprehended in formula 1 of WO 95/09383 are stable on storage and do not break down to yield an evil smelling residue, are easy to make and surprisingly in view of the disclosure in WO 95/09383 can form the basis of a two part photoinitiator in a photopolymerisable composition.

Therefore according to the present invention there is provided a lithographic plate which comprises a base coated with a photopolymerisable composition which comprises a polymeric binder, at least one free radically polymerisable ethylenically unsaturated compound and as the photoinitiator the combination of an N-phenyl-glycine compound of the general formula I::

where R, is a hydroxyalkoxyaryl group, an hydroxy alkoxyalkyl group having 2 to 12 carbon atoms, a hydroxyalkyl ester, a hydroxyalkylaryl ester, an alkyl carboxylic acid or salts thereof, R4, R5, R6, R7 and R8 are each a hydrogen atom, an alkyl group having 1 to 12 carbon atoms or a halogen atom, or at least one R4, R5, R6, R, and R8 are a nitro or alkoxy group the remainder being hydrogen, R2 and R3 are each a hydrogen, a halogen atom, or an alkyl group having 1 to 8 carbon atoms and X is hydrogen or an organic or inorganic cation together with a light absorbing compound.

It is an important feature of the present invention that neither a vinyl moiety nor an acrylate moiety nor a methacrylate moiety should be present in the substituent group Rl.

Preferably each of Re - Re is a hydrogen atom or Re is a halogen atom and the Re, R5, R, and Re are hydrogen atoms.

Preferably R2, and Re are both hydrogen atoms.

Two compounds of particular use in the composition used in the present invention are

This compound is hereinafter referred to as compound A. The presence of the -OCH3 terminal group is of particular benefit in this group to increase the sensitivity of the composition in which this compound is present.

This compound is hereinafter referred to as compound B. Preparation of N-Phenylglyane Derivatives of formula I Dissolve N-Phenylglycine (0.05 mol) in rapidly stirred methanol (65 ml).

Dissolve NaOH (2.0g, 0.05 mol) in water (75 ml).

Slowly add the NaOH solution to the NPG solution.

Allow to stir for thirty minutes.

Over a period of 1 hour, slowly add the appropriate glycidyl reactant (0.05 mol) to the rapidly stirred NPG salt solution.

Heat the mixture to 500C and maintain at that temperature for 3 hours.

After this time, remove the MeOH under vacuum.

Allow the solution to cool and pour into water (250ml).

Stir rapidly and slowly acidify with concentrated HC1 to pH2.

Collect the solid vacuum filtration and wash with water.

Dry at room temperature under vacuum.

This is a much simpler preparation than that described in USP 5378579 for the N-phenylglycine derivatives described therein.

A range of materials are suitable as the second component to tailor the spectral sensitivity of the photopolymerisable composition to the desired light source, be this near, visible or near-IR, or as additional sensitisers.

If the composition is to be exposed to the near ultra-violet region of the spectrum then the second component of the composition could be a compound which comprises an acridine, phenazine or quinoxaline moiety. Examples are acridine substituted in the 9 position, such as 9-phenyl-, 9-p-methoxyphenylor 9-acetylaminoacridine, or acridine derivatives having fused-on aromatic rings, for example benz[a]acridine. A suitable phenazine derivative is, for example, 9,10-dimethylbenzEaaphenazine. Suitable quinoxaline derivatives are in particular 2,3-diphenyl derivatives which are preferably further substituted in the two phenyl radicals by methoxy groups. In general, the acridine derivatives are preferred.The amount of this component in the mixture is in the range from about 0 to about 10% by weight, preferably between about 0.05 and 5% by weight.

For sensitisation to the visible region compounds of the dibenzalacetone or coumarin type are particularly useful. Suitable representatives of these compounds are 4,4'-disubstituted dibenzalacetones, for example diethylamino-4'-methoxydibenzalacetone, or coumarin derivatives, such as 3 -acetyl-7-diethylamino-, 3 -benzimidazolyl-7-diethylamino- or carbonyl-bis(7-diethylaminocoumarin). The amount of these compounds in the mixture are in the range from 0 to about 10, preferably about 0.05 to 4% by weight.

Alternatively, many dye types are particularly useful for spectral sensitisation.

Suitable dyes are in particular xanthene, benzoxanthene, benzothioxanthene, thiazine, pyronine, porphyrin or acridine dyes.

Suitable xanthene and thiazine dyes are described, for example, in EP-A 287,817. Suitable benzoxanthene and benzothioxanthene dyes are described in DE-A 2,025,291, and in EP-A 321,828.

An example of a suitable porphyrin dye is haematoporphyrin and an example of a suitable acridine dye is acriflavinium chloride hydrochloride.

Examples of xanthene dyes are Eosin B (C.I. No. 45400), Eosin J (C.I. No. 45380), Eosin alcohol-soluble (C.I. 45386), Cyanosine (C.I. No. 45410). Rose Bengal, Erythrosine (C.I. No. 45430), 2,3,7-trihydoxy-9-phenylxanthen-6-one and Rhodamin 6 G (C.I. No. 45160).

Examples of thiazine dyes are thionine (C.I. No. 52000), Azure A (C.I. No. 52005) and Azure C (C.I. No. 52002).

Examples of pyronine dyes are Pyronin B (C.I. No. 45010) and Pyronin GY (t'.1. No. 45005). The amount of these dyes is in general between about 0.01 and 10, preferably between about 0.05 and 4, % by weight, relative to the non-volatile portions of the mixture.

Alternatively instead of using a dye as the second component a metallocene compound may be used as an additional sensitiser.

The preferred metallocenes are formed from elements of Group IVB of the periodic table for example titanium as described above and zirconium.

Titanocene compounds and their use as photoinitiators for the photopolymerisation of polymerisable ethylenically unsaturated compounds are described in a number of Ciba-Geigy A.G. patents including USP 4590287, USP 4548891 and EP 318894. Ciba-Geigy are the main suppliers of titanocenes for use as photoinitiators in photopolymerisable compositions.

Thus, particularly useful titanocene compounds include titanocenes having two optionally substituted cyclopentadienyl ligands and one or two unsubstituted or substituted 6-membered carbocyclic or 5- or 6-membered heterocyclic aromatic radicals attached to the titanium atom. In preferred compounds, the aromatic radicals have a fluorine substituent in at least one of the two ortho positions relative to the titanium-carbon bond and optionally have further substituents.

Substituents which may optionally be present on the cyclopentadiene rings include C,-Cl,-alkyl and alkoxy, especially C1-C8-cycloalkyl and cycloalkenyl, C6 C,raryl, CrC,6"aralkyl and halogen, for example chlorine.

The same substituents may optionally be present on the carbocyclic or heterocyclic aromatic radicals as may also other substituents such as amino groups, especially tertiary amino groups which may optionally be quaternised and may be derived from heterocyclic amines such as pyrrolidine, piperidine, piperazine, N-methylpiperazine, morpholine, pyrrole and pyridine.

Specific examples of useful titanocene compounds include bis(cyclopentadienyl) -bis[2, 6-difluoro-3 -(1 -pyrrolyl) phenyl]-titahium available from Ciba-Geigy as CGI-784,bis (cyclopentadienyl) -bis(pentafluorophenyl)-titaaium and bis (cyclopentadienyl) -bis(2,3,5,6-tetrafluoro4-decyloxyphenyl) Another class of compound which can be used as the second component is a compound comprising trihalomethyl groups which compound can be cleaved photolytically to release free radical groups.

With regard to the trihalo compounds, compounds containing chlorine and bromine, in particular chlorine, as halogens are particularly useful. The trihalomethyl groups can be bound directly to an aromatic carbocyclic or heterocyclic ring or via a conjugated chain. Preferred compounds are those containing a triazine ring in the basic structure, which preferably carries 2 trihalomethyl groups, in particular chlorine, as halogens are particularly successful. The trihalomethyl groups can be bound directly to an aromatic carboxyclic or heterocyclic ring or via a conjugated chain. Preferred compounds are those containing a triazine ring in the basic structure, which preferably carries 2 trihalomethyl groups, in particular those which are described in EP-A 137 452, DE-A 27 18 259 and DB-A 2243 621.These compounds exhibit strong light absorption in the near Lw region, at 350400no.

Preferred azines have the general formula:

where X is Cl or Br, R3, R4 are independently CX3, R5, R5, RSNH-, or R5CONH-, wherein R5 is alkyl of 1 to 8 carbons, substituted alkyl of not more than 12 carbons, phenyl, substituted phenyl of not more than 12 carbon atoms, naphthyl, or substituted naphthyl of not more than 14 carbons.

Especially preferred azines have the general formula

wherein n, m, p and q are each independently 0, 1, 2 or 3, the sum of n+m is not greater than 3, the sum of p+q is not greater than 3 and the sum of n+m+p+q is at least one, X is a sulphur or selenium and R is an alkyl group or an aryl group.

Iodonium compounds can also be used for the second component as a sensitiser. For example a salt of diphenyl iodonium can be used.

The free-radically polymerisable ethylenically unsaturated compound or compounds contain at least one ethylenically unsaturated double bond per molecule. When free-radicals are generated by the photo-initiator composition these compounds polymerise to give higher molecular weight compounds which are insoluble in selected solvents.

Preferably two free-radically polymerisable unsaturated compounds are present in the photopolymerisable composition, one of which is an oligomer with acrylate functionality and the other of which is a monomer with acrylate functionality.

An example of a class of oligomers with acrylate functionality is a polyester with four acrylate groups.

Commercially available compound of this class are Ebecryls which are marketed by UCB, S.A. SPECIALITIES DIVISION, DROGENBOS, BELGIUM An example of a class of monomers with acrylate functionality is dipentaerythritol penta-acrylate. Certain Ebecryl compounds also fall within this class.

Polymeric binders for inclusion in the composition may have useful effects on solubility, film formation, photoreactivity and adhesive strength, and may be chosen according to the particular application. For example, a copolymer of acrylic acid (or methacrylic acid) and an alkyl acrylate (or methacrylate) such as the methyl, ethyl or butyl esters, itaconic acid copolymers, partially esterfied maleic acid copolymers, cellulose acetate derivatives having carboxyl substituents, poly(ethylene oxide) and poly(vinylpyrrolidone) are useful in aqueous processing systems.

Useful binders for the photopolymerisable composition are partially hydrolysed cellulose acetates or cellulose acetate-butyrate polymers. The partial hydrolysis is required to increase their water developability. Most preferably cellulose acetate-butyrates which have been reacted with an acrylamide to provide polymers with acrylate functionality are used.

However, the preferred binder combination for the photopolymerisable composition consists of a cellulose acetate butyrate, a cellulose acetate hydrogen phthalate and a polymer consisting of an acrylic backbone functionalised with pendent dimethylmaleiimide photoreactive groups.

Most preferably cellulose acetate butyrates which have been reacted with an acrylamide to provide polymer with acrylate functionality are used.

USP 4490516 and USP 4557951 to the A.E. Staley Manufacturing Company describe such CAB polymers.

Most preferably the dimethylmaleiimide containing polymer is R0300 manufactured by Rohner and known as Rohnercoat.

A desirable feature of the lithographic plate of the present invention is that an oxygen inhibition means or an additive inhibitor be present in or on the lithographic plate when the photopolymerisable composition is imagewise exposed. In some circumstances if no oxygen inhibition means or additive is present, little photohardening of the photopolymerisable composition occurs.

Preferably the oxygen inhibition means is a coating of a water-soluble polymer such as polyvinyl alcohol over the photopolymerisable composition.

This coating is removed during the development process. Alternatively but less usefully oxygen inhibition additives such as behenic acid are added to the photopolymerisable composition. When the composition is coated the behenic acid floats to the surface of the composition.

Another suitable oxygen inhibition means is a layer which is self supporting and which can be peeled off the plate after exposure and before development.

Suitable thickness of oxygen inhibiting layers are from 0.1 to lOum and preferably 1 to Sum.

However exposure to the plate in a vacuum frame obviates the need to provide oxygen inhibition.

A suitable composition of the dried layer of the photopolymerisable composition is per 100g of composition:binder 70g to 20g unsaturated compound or compounds 80g to 20g second component 10g to Ig N-phenyl glycine derivative 10g to ig Optionally a suitable pigmented resin or dye may also be present in the composition and this may be present to the extent of from 20g to 2g per 100g of the composition.

A useful solvent from which to coat the composition is for example a mixture of glycol ethers and dimethyl formamide. Another useful solvent is MEK.

There may be present in the composition, in addition to the already listed ingredients inhibitors to prevent thermal polymerisation of the composition, indicators and plasticisers. Only a small amount of such additives are present if any so as to absorb as little as possible of the exposing light.

The photosensitive materials are fabricated in a conventional manner. For instance, it is possible to take up the layer constituents in a solvent, to apply the solution or dispersion to the intended support material by casting, spraying, dipping, roller coating and the like, and then to dry.

Owing to the broad spectral sensitivity of the polymerisable composition according to the invention which can be achieved, it is possible to use any light source familiar to those skilled in the art, for example fluorescent tubes, pulsed xenon lamps, metal-halide-doped high-pressure mercury vapor lamps and carbon arc lamps, metal-halide-doped high-pressure mercury vapor lamps and carbon arc lamps. In addition, it is possible to expose the lithographic plates of the invention in conventional projection and magnification equipment with the light from metal filament lamps and by contact exposure with customary incandescent lamps. Exposure can also be effected with the light of a laser.

Suitable for the purposes of the present invention are lasers of appropriate power, for example the argon ion laser, which emits at 488nm. The laser beam can be controlled by means of a pre-programmed linewise scanning movement.

To develop the photopolymerisable compositions they are treated with a suitable developer solution, for example with organic solvents, but preferably with a neutral or weakly alkaline aqueous solution, which serves to remove the unexposed areas of the layer, leaving behind the exposed areas of the copying layer on the support material. The developer solutions may contain a small amount, preferably less than 5% by weight, of water-miscible organic solvents. They may further contain wetting agents, dyes, salts and other additives. In the course of development, the entire cover layer if present is removed together with the unexposed areas of the photopolymerisable layer.

The accompanying Example will serve to illustrate the invention.

Suitable support materials for the lithographic plates according to the invention are, for example, aluminium, steel, zinc and copper foils, plastic films, for example polyethylene terephthalate or cellulose acetate films, and support materials for screen printing, such as Perlon gauze. It is in many cases advantageous to subject the surface of the support material to a pretreatment (electrochemical thermal or mechanical) with the aim of correctly adjusting the adhesion of the layer, improving the lithographic properties of the surface of the support material, or reducing the reflectivity of the support material in the actinic region of the copying layer (anti-halation).

Example The following photopolymerisable composition was prepared in which the binder was 19.25 parts of the modified cellulose acetate-butyrate as prepared in Example 6 of USP 4557951 1.5 parts of cellulose acetate hydrogen phthalate and 19.25 parts of the polymer RO300 from Rohner.

parts by weight Binder 40 Ebecryl 140 30 Ebecryl 1290 30 Photoinitiators 8 Methyl ethyl ketone 22.10 Three sets of photoinitiators were used: Set 1 Compound A as hereinbefore set forth -' Titanocene compound (Ciba-Geigyls CGI-784) Set 2 Compound B as hereinbefore set forth Titanocene compound (as above) Set 3 Compound 1 from US 5378579 Titanocene compound as above All the compositions were easily soluble in the methyl ethyl ketone. They were coated by bar coating on 0.3mm grained and anodised aluminium plate at a dry coating weight of lg/m2.

After coating and drying they were supercoated with an aqueous solution of polyvinyl alcohol to provide a coating weight of at least 1g(m2 as an oxygen inhibition coating.

All the lithographic plates were imagewise exposed to a Lw source in a frame through a photographic negative. Each plate was exposed for sufficient time to fully polymerise the image area of the plate to achieve optimum image reproduction. These plates were then developed in a solution of sodium hexyl sulphate at 20" for one minute. The non-exposed areas of all the plates were removed by this process.

A clear hardened image which could be used for lithographic printing was obtained using all three sets.

In another test a number of plates coated with Sets 1, 2 and 3 were coated on the aluminium plate. They were subjected to an accelerated ageing test by being placed in an oven at 600C.

One each of Sets 1, 2 and 3 plates were exposed and developed after 1 week.

Sets 1 and 2 plates produced as good an image as the initial test which was carried out immediately after coating, but Set 3 did not produce any image.

Sets 1 and 2 plates were tested weekly and both sets continued to produce good images. After 1 month in the oven which can be considered to represent several years storage Sets 1 and 2 produced a good image.

Claims (9)

Claims

1. A lithographic plate which comprises a base coated with a photopolymerisable composition which comprises a polymeric binder, at least one free radically polymerisable ethylenically unsaturated compound and as the photoinitiator the combination of an N-phenyl-glycine compound of the general formula I::

where R, is a hydroxyalkoxyaryl group, an hydroxy alkoxyalkyl group having 2 to 12 carbon atoms, a hydroxyalkyl ester, a hydroxyalkylaryl ester, an alkyl carboxylic acid or salts thereof, R4, R5, R5, R, and Ra are each a hydrogen atom, an alkyl group having 1 to 12 carbon atoms or a halogen atom, or at least one Ra, R5, R5, R7 and Ra are a nitro or alkoxy group the remainder being hydrogen, R2 and R3 are each a hydrogen, a halogen atom, or an alkyl group having 1 to 8 carbon atoms and X is hydrogen or an organic or inorganic cation together with a light absorbing compound.

2. A lithographic plate according to claim 1 wherein the N-phenyl glycine compound of general formula I each of Ra, R5, R, and Ra are hydrogen atoms.

3. A lithographic plate according to claim 1 R2 and R3 are both hydrogen atoms.

4. A lithographic plate according to claim 1 wherein the N-phenyl glycine is of the formula

5. A lithographic plate according to claim 1 wherein the N-phenyl glycine is of the formula:

6. A lithographic plate according to claim 1 suitable for exposure to the near ultra-violet region of the spectrum which comprises as a light absorber an acridine, phenazine or quinoxaline based compound.

7. A lithographic plate according to claim 1 suitable for exposure to the visible region of the spectrum which comprises dibenzylactone or coumarin compound.

8. A lithographic plate according to claim 1 wherein the light-absorbing compound is a metallocene.

9. A lithographic plate according to claim 1 wherein the light-absorbing compound is a compound comprising a trihalomethyl group which can be clear to photolytically release free radical groups.