CA2019366A1

CA2019366A1 - Photopolymerizable mixture and recording material produced therefrom

Info

Publication number: CA2019366A1
Application number: CA002019366A
Authority: CA
Inventors: Waltraud Mueller-Hess; Dieter Mohr; Karl-Josef Rauterkus; Matthias Kroggel
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1989-06-21
Filing date: 1990-06-20
Publication date: 1990-12-21
Also published as: EP0406599A1; EP0406599B1; BR9002931A; AU5772590A; JPH03135566A; DE59005884D1; KR910001467A; DE3920229A1

Abstract

Abstract Photopolymerizable mixture and recording material produced therefrom The invention discloses a photopolymerizable mixture which contains, as essential constituents a) a graft copolymer having a polyurethane as the graft backbone, onto chains containing vinyl alcohol units and vinyl acetal units are grafted, b) a free-radically polymerizable compound possessing at least one terminal ethylenically unsaturated group and having a boiling point of more than 100 °C at normal pressure, and c) a compound or a combination of compounds which under the action of actinic light is capable of initiating the polymerization of compound b).

The mixture is suitable for use in the production of printing plates and photoresists. It is distinguished by high photospeed and can be developed with aqueous solutions. It yields printing plates producing large print runs.

Description

2~ ~3~

89/X 032 Tran~lation Photopolymerizable mixture and recording material produced therefrom The present invention relates to a photopolymerizable mixture which can be developed with aqueous solutions, wich contains a polymeric binder, a polymerizable compound having at least one ethylenically unsaturated double bond in the molecule, and a photoinitiator or photoinitiator system, and which is suitable for the production of photosensitive recording materials, in particular of planographic printing plates and photoresists.

Photopolymerizable mixtures containing water-soluble binders are known from DE-B 15 22 362 and DE-A 19 17 917. As a rule, these mixtures can be developed readily and without scumming, but they suffer from the disadvantage that in planographic printing the hardened image areas, which contain the water-soluble binder, are wetted more easily by the dampening solutions and are attacked by the latter during the production of high print runs.
Mixtures of the above generic type are also described in DE-A 20 53 363. The binders contained therein comprise reaction products obtained from a polymer containing hydroxyl groups or amino groups and at least one saturated alkyl sulfonyl isocyanate, alkoxy sulfonyl isocyanate, aryl sulfonyl isocyanate or aryloxy sulfonyl isocyanate. In combination with diazonium salt condensation products or photopolymerizable mixtures, the binder is processed to give photosensitive layers. However, the resulting mixtures can only be developed with aqueous-alkaline solutions if the 2 ~

binders employed have high acid numbers, which adversely affect the abrasion resistance and printing properties of the hardened layers.

EP-A 167 963 describes a negative working photosensitive mix-ture which is suitable for the production of planographic printing plates and comprises a diazonium salt polyconden-sation product, an ethylenically unsaturated compound which can be polymerized by a free-radical process, a photo-initiator and a water-insoluble ~olymeric binder. High print runs can be achieved when polyvinyl acetals are used as bin-ders. In this case development can, however, be only per-formed with solutions comprising a predominant amount of organic solvents.
In DE-A 37 32 089 graft polymers are described, which com-prise a polyurethane as the graft backbone and grafted-on vinyl ester units, which are at least partially saponified to give vinyl alcohol units. The polymers are suitable as binders for pigments; for preparing printing inks, thermoplastic adhesives and solvent-containing adhesives;
as constituents of varnishes or coatings for fibers, films and metals, and for thermoplastic shaped articles.

JP-A 246,047/87 describes photopolymerizable mixtures wherein graft polymers of polyurethanes and polyvinyl alcohol with mercapto groups are contained as binders.

Graft polymers obtained from the graft polymers described in DE-A 37 32 089 above by acetalizing with aldehydes, are described in the prior German Patent Application P 38 35 840.9.

The prior German Patent Application P 38 24 146.3 discloses photocurable elastomeric mixtures which contain a compound being polymerizable by a free-radical process, a pho~o-2 ~

initiator and, as the binder, a graft polymer according to DE-A 37 32 089, which is soluble or dispersible in an aqueous solution.

It is an object of the present invention to provide a photo-polymerizable mixture which is suitable for use in the production of printing plates, particularly lithographic printing plates, or of photoresists; which has all the advan-tages of the known photopolymerizable compositions; which can be developed by means of virtually solvent-free, aqueous solutions, without resulting in disturbing deposits when developer solutions containing tap water are employed; and which, at the same time, yields printing plates of high photospeed and image resolution, which give high print runs and have a good ink acceptance and long shelf life, proper-ties which could up to now only be achieved with printing plates requiring the addition of relatively large amounts of organic solvents in the developing process.

In accordance with the instant invention, a photopoly-merizable mixture is provided which contains, as essential constituents, a) a polymeric binder possessing repeating vinyl acetal units, b) a free-radically polymerizable compound possessing at least one terminal ethylenically unsaturated group and having a boiling point of more than 100 C at normal pressure, and c) a compound or a combination of compounds which under the action of actinic light is capable of initiating the polymerization of compound b).

2~ 3~6 The mixture of this invention is characterized in that the binder is a graft polymer where the graft backbone is a polyurethane onto which chains containing vinyl alcohol units and vinyl acetal units are grafted.

In accordance with the instant invention, there is also pro-posed a photopolymerizable recording material comprising a layer support and a photopolymerizable layer wherein said photosensitive layer is comprised of the mixture defined above.

The graft polymers contained in the mixture of the instant invention are described in the prior German Patent Applica-tion P 38 35 840.9. For preparing them, a carboxylic acid vinyl ester and optionally another ethylenically unsaturated compound copolymerizable therewith is grafted onto a polyurethane graft backbone and saponified completely or partially. The polymers having vinyl alcohol units, which are obtained in this way, are further reacted with aldehydes to give polyvinyl acetals.

The proportion of the grafted-on components is in general 10 to 95, preferably 30 to 90 and in particular 40 to 80 %
by weight, based on the total graft polymer.
The graft backbones consist of polyurethanes having at least two urethane groups in the molecule, the number of urethane groups per molecule being subject to no particular upper limit and in general having values higher than 2.
The polyurethanes employed as graft backbone can be produced from diols and diisocyanates by conventional processes of polyurethane synthesis. In principle, all of the diols custo-marily used in polyurethane synthesis can be employedO
Cycloaliphatic diols, such as cyclohexanediols, and in particular aliphatic diols having 2 to 12 carbon atoms are preferred. Polyetherdiols, for example polypropylene oxides, polybutylene oxides and copolymers of ethylene oxide, propy lene oxide and butylene oxide, preferably the block copoly-mers thereof, are also preferred; polyethylene oxides with molecular weights of between 200 and 10,000, and more pre-ferably of between 400 and 1,500 are particularly preferred.
The polyetherdiols are advantageously employed in combination with low-molecular aliphatic diols, for example 1,4-butane-diol, 1,3-propanediol, ethylene glycol, diethylene glycol, 1,2-hexanediol, 1,2-propanediol, pentanediol or cyclohexane-diol. The molar ratio of polyetherdiol to low-molecular aliphatic diol is preferably from 1 : 0.1 to 1 : 0.7.

Diisocyanate components which can be employed are aromatic diisocyanates. Aliphatic and/or cycloaliphatic diisocyanates are preferred. Preferred aliphatic diisocyanates are those having 2 to 12 carbon atoms in the aliphatic radical, for example ethylene diisocyante, propylene diisocyanate, tetra-methylene diisocyante and 2,2,4-trimethylhexamethylene diisocyante. Preferred cycloaliphatic diisocyantes are, for example, 1,4-diisocyanato-cyclohexane, dicyclohexylmethane-4,4'-diisocyante and isophorone diisocyanate. Hexamethylene diisocyanate and isophorone diisocyanate are particularly preferred.
The molar ratio of diol component to diisocyanate component is preferably between 1 : 0.99 and 1 : 0.5, in particular between 1 : 0.98 and 1 : 0.7. The average molecular weights of the polyurethanes are preferably between 200 and 100,000, in particular between 1,000 and 50,000 and particularly preferentially between 3,000 and 25,000.

Carboxylic acid vinyl esters having 3 to 20 and preferably 4 to 14 carbon atoms are employed for grafting onto the polyurethane. Vinyl acetate and/or vinyl propionate, in particular vinyl acetate, are particularly preferred.

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Mixtures of vinyl acetate and/or vinyl propionate and vinyl versatate are also preferred. In particular in the case of partial or complete saponification of the products following the graft polymerization, the co-use of vinyl propionate in addition to vinyl acetate during grafting is advantageous.
Moreover, copolymerizable mixtures of carboxyli~ acid vinyl esters can be grafted, preferably mixtures of vinyl acetate and minor amounts of vinyl versatate.

Grafting with different carboxylic acid vinyl esters in the form of block copolymers, optionally in combination with further ethylenically unsaturated and copolymerizable mono-mers, can also be advantageous. Furthermore, the carboxylic acid vinyl esters can also be grafted together with other ethylenically unsaturated and copolymerizable monomers, in particular acids, such as maleic acid, itaconic acid, mesaconic aicd, crotonic acid, acrylic acid or the esters thereof.

The graft polymers obtained can be converted by hydrolysis, alcoholysis or transesterification into partially or comple-tely saponified products, the degree of hydrolysis being at least 30 mol%, preferably 45 to 99 mol~, based on the mole number of saponifiable monomer units in the graft polymer.
The production of graft polymers with a polyurethane graft backbone is described in DE-A 37 32 089.

The saponified graft polymers can be acetalized in an acidic medium by means of known methods.
Aliphatic aldehydes having 1 to 20 carbon atoms, which may be substituted, and aromatic aldehydes, which may also be sub-stituted, are employed for acetalizing. Preference is given to aliphatic aldehydes having 1 to 5 carbon atoms, such as n-butyraldehyde, isobutyraldehyde, propionaldehyde or formaldehyde. Substituted or unsubstituted benzaldehydes, 2 ~

such as benzaldehyde, p-chlorobenzaldehyde or p-methoxybenzaldehyde are also suited. It is also possible to employ combinations of several of these aldehydes.

The degree of acetalization of the graft polyvinyl acetals used as binders in accordance with this invention is pre-ferably selected such that the content of non-acetalized polyvinyl alcohol units in the graft polyvinyl acetals is 18 to 60 mole %, and particularly preferably 20 to 45 mole ~, each time relative to the molar amount of vinyl alcohol units contained in the saponified graft polymers used, it being possible for up to 55 mole %, relative to the original amount, to be present as vinyl ester units. The hydroxyl number of the ready-for-use binder should be in the range of 100 to 600, preferably of 200 to 500.

The acetalization can be performed employing two different methods:

In accordance with the first process variant, the graft poly-mer is dissolved or dispersed in an alcohol or in a water/alcohol mixture, mixed with a catalytic amount of an organic or inorganic acid and an aldehyde or aldehyde mix-ture, and heated. The resulting polymer solution, which where appropriate also contains an anti-oxidant, can either be used directly for preparing the mixtures according to this inven-tion, or alternatively, the polymer can be precipitated and purified by dropwise adding the solution to a non-solvent.

In accordance with the second process variant, the graft polymer is dissolved in water and mixed with an aldehyde or aldehyde mixture. Subsequently, an aqueous solution of an inorganic or strong organic acid - if appropriate with the addition of a surfactant and of an antioxidant - is added dropwise at a low temperature. Thereby, the acetalized graft polymer is often precipitated. The reaction is completed at an increased temperature of about 20 to 60 C. The isolated polymer is purified by washing with water or re-precipita-tion.

To prepare the graft polyvinyl acetals in an aqueous medium, known methods are employed to produce 1 to 50 % strength, preferably 5 to 20 % strength, aqueous solutions of the graft polyvinyl alcohols, preferably at elevated temperatures, the acid catalyst is added, the solutions are then cooled to tem-peratures of less than 25 C, and the acetalization reactionis finally performed by metering in the aldehyde with agitating, preferably within a time of 3 to 300 minutes.
As is known, the aldehyde conversion is incomplete in most cases, and therefore an excess of aldehyde, preferably of 10 to 20 mole percent, is usually added.

In a preferred process variant, the aqueous solution is allowed to stand for at least about 30 minutes at tempera-tures of 0 to 5 ~C prior to the start of the reaction, whereupon the graft polyvinyl acetal formed is generally separated off after a short time, as a pulverulent substance.
To complete the reaction, the reaction mixture is slowly heated to room temperature, and where appropriate it is post-reacted at elevated temperatures, for example at 25 to 70 C, during about one to three hours. The added amount of acid catalyst depends, inter alia, on the degree of acetalization to be achieved and may preferably be up to 1.1 mole, relative to the molar content of vinyl alcohol units.

The resulting graft polyvinyl acetal is isolated by suction, washed with weakly alkaline water (pH 9 to 12) and dried.
Acetalization products which do not precipitate from the aqueous reaction solution can be isolated by the addition of precipitating agents, purified and dried.

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The acetalization can also be performed in organic solvents.
Suitable solvents include water-miscible solvents, in parti-cular water-soluble alcohols, such as ethanol and/or metha-nol, to which water may be added.

Preferred acid catalysts are organic sulfonic acids, for example, toluene sulfonic acids, and also mineral acids, for example sulfuric acid, phosphoric acid, hydrochloric acid or nitric acid. Among these, perference is given to phosphoric acid and hydrochloric acid.

For performing the preparation in organic solvents, the acid catalyst, the aldehyde and the graft polyvinyl alcohol are dispersed or dissolved in the solvent, and the mixture is refluxed. It may also be expedient to add the aldehyde in the course of the reaction. The graft polyvinyl alcohols which are insoluble in the organic solvents gradually dis-solve as a consequence of the acetalization in progress.

When the acetalization reaction is completed the reaction product is precipitated by adding non-polar solvents, for example, aliphatic hydrocarbons, or by pouring the reaction solution into ice-cold water or an ice-cold water/alcohol mixture, isolated by suction, washed with weakly alkaline water (pH 9 to 12), and dried.

The preparation of the graft polyvinyl acetals is also described in the former German Patent Application P 38 35 ~40.9.
In combination with polymerizable compounds and photoinitia-tors, the polymers obtained in this way result in layers which can be developed easily and without scumming. Depending on their individual composition, the layers can be developed with aqueous solutions of inorganic salts and/or surfactants or with aqueous-alkaline solutions. The layers are distin-lo ~ 6~

guished by a high abrasion resistance, good ink acceptanceand adequate shelf lives and can therefore be used for nume-rous applications, in particular for producing planographic printing plates, screen printing stencils and photoresists.

In general, the photopolymerizable mixtures contain 20 to 90, preferably 30 to 75, % by weight of polymeric binder, rela-tive to the weight of all non-volatile constituents of the mixture. Up to 50 % by weight, preferably up to 20 % by weight, of the total amount of binder can be replaced by any of a great number of customary binders. Examples of suitable compounds are: polyamides, polyvinyl esters, polyvinyl acetals, polyvinyl ethers, epoxide resins, polyacrylic acid esters, polymethacrylic acid esters, polyesters, alkyd resins, polyacryl amides, polyvinyl alcohols, polyethylene oxides, polydimethylacrylamide, polyvinylpyrrolidone, poly-vinylmethylformamide, polyvinylmethylacetamide, and copoly-mers of the monomers forming the homopolymers enumerated.

Advantageously, these binders, too, are water-insoluble but soluble or at least swellable in aqueous-alkaline solutions.
Examples of polymers of this generic type include: maleate resins, polymers of ~-(methacryloyloxy)-ethyl N-)p-tolyl-sulfonyl)-carbamate and copolymers of these and similar mono-mers with other monomers, as well as vinyl acetate/crotonicacid copolymers, styrene/maleic anhydride copolymers, alkyl methacrylate/methacrylic acid copolymers and copolymers of methacrylic acid, higher alkyl methacrylates and methyl methacrylate and/or styrene, acrylonitrile and others.
The mixtures and materials according to the invention contain free-radically polymerizable compounds with at least one terminai ethylenic double bond. Preferably, esters of acrylic or methacrylic acid with mono- or polyhydric, preferably primary, alcohols are used as the polymerizable compounds.
Preferably, the polymerizable compounds should possess more 3 ~ ~

than one, in particular 2 to 4, polymerizable groups.
Examples of suitable polyhydric alcohols are ethylene glycol, propylene glycol, butane-1,4-diol, butane-1,3-diol, diethy-lene glycol, triethylene glycol or polyethylene glycols or polypropylene glycols with molecular weights from about 200 to 1,000, neopentyl glycol, glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, bisphenol-A derivatives and reaction products of these compounds with ethylene oxide and/or propylene oxide. Bis-acrylates and bis-methacrylates which contain urethane groups and which are obtained by the reaction of 1 mole of a diisocyanate with 2 moles of a hydroxyalkyl acrylate or hydroxyalkyl methacrylate are parti-cularly suited. The diisocyanate may also be an oligomeric product obtained by the reaction of a diol with a molar excess of a monomeric diisocyanate. These and similar monG-mers containing urethane groups are described in DE-A
20 64 079, DE-A-28 22 190, DE-A-30 48 502 and DE-A-35 40 480.

In addition to the esters mentioned above, amides of acrylic or methacrylic acid can be employed. Examples are methylene-bis-~meth)acrylamide and m-xylylene-bis-(meth)acrylamide.

The amount of monomers contained in the mixture generally is about 10 to 80 % by weight, preferably 25 to 70 % by weight, of the non-volatile constituents.

A large number of substances can be used as photoinitiators.
Examples include benzoins; benzoin ethers; polynuclear quino-nes, such as 2-ethyl-anthraquinone; acridine derivatives, such as 9-phenylacridine or benzacridine; phenazine derivati-ves, such as 9,10-dimethylbenz(a)phenazine; quinoxaline deri-vatives or quinoline derivatives, such as 2,3-bis-(4-methoxy-phenyl)quinoxaline or 2-styrylquinoline; quinazoline com-pounds; or acyl-phosphineoxide compounds. Photoinitiators of this type are described in DE-C 20 27 467, DE-C 20 39 861, DE-A 37 28 168, EP-B 0,011,786 and EP-A 0,220,589. Hydrazo-12 ~ 3~

nes, mercapto compounds, pyrylium salts or thiopyrylium salts, xanthones, thioxanthones, benzoquinones, acetopheno-nes, benzophenones, synergistic mixtures with ketones or hydroxy ketones and dyestuff redoxy systems may also be used.
Particular preference is given to photoinitiators possessing photo-cleavable trihalomethyl groups, especially to corresponding compounds of the triazine or thiazoline series.
Compounds of this type are described in DE-A 27 18 259, DE-A
33 33 450 and DE-A 33 37 024. 2-(4-Methoxystyryl)-4,6-bis-trichloromethyl-s-triazine is a preferred example. It is advantageous to combine these compounds with photooxidizable dyes, photoreducible compounds and optionally further coinitiators, as is described, for example, in EP-A 0,284,939 and EP-A 0,287,817.
The photoinitiators are generally employed in a quantity from 0.1 to 15, and preferably from 0.5 to 10, % by weight, relative to the non-volatile constituents of the mixture.

Depending on their intended use and on their desired proper-ties, the photopolymerizable mixtures may contain a number of various additives, such as, for example, inhibitors to pre-vent thermal polymerization of the monomers, hydrogen donors, dyes, colored and uncolored pigments, color formers, indica-tors, plasticizers and chain transfer agents.

The types and quantities of such additions depend upon thefield of application for which the photosensitive mixture is intended. In principle, care must be taken that the added substances do not absorb an excessive portion of the actinic light which is required for cross-linking, because this would result in a reduction of the practical sensitivity to light.

The photopolymerizable mixtures can also be combined with other, negative working photosensitive compounds, in particu-lar with diazonium salt polycondensation products. Suitable 3i~

diazonium salt polycondensation products are condensation products of condensible aromatic diazonium salts, for example, of diphenylamine-4-diazonium salts, with aldehydes, preferably with formaldehyde. It is particularly advantageous to use cocondensation products containing, in addition to the diazonium salt units A-N2X, other, non-photosensitive units B
which are derived from condensible compounds, particularly from aromatic amines, phenols, phenol ethers, aromatic thioethers, aromatic hydrocarbons, aromatic heterocyclic com-pounds and organic acid amines. These cond~nsation productsare described in DE-A 20 24 244. Generally, all diazonium salt polycondensation products described in DE-A 27 39 774 are suitable.

The diazonium salt units A-N2X are preferably derived from compounds corresponding to the formula (R1-R2-)pR3-N2X, in which X is the anion of the diazonium compound, p is an integer from 1 to 3, Rl is an aromatic radical which is capable, in at least one position, of condensation with an active carbonyl compound, R3 is an arylene group, preferably a phenylene group which may be substituted, R2 is a single bond or one of the groups:

~(CH2)q~NR4~l -O- ( CH2 ) r-NR4 -, -S-(CH2)r-NR4-, -S-CH2CO-NR -, -o-R5_0_ --O--, -S-, or -Co-NR4-where 2~ ~ 93~

q is a number from O to 5, r is a number from 2 to 5, R4 ~s a hydrogen atom, an alkyl group having from 1 to 5 carbon atoms, an aralkyl group having from 7 to 12 carbon atoms or an aryl group havir.g from 6 to 12 carbon atoms, and R5 is an arylene group having from 6 to 12 carbon atoms.

10 Further advantageous polycondensation products are obtained by condensing an optionally substituted diphenylamine diazo-nium salt first with an aromatic compound R'-O-CH2-B and then with an aromatic compound R'-O-CH2-B-CH2-O-R', R' denoting a hydrogen atom, an alkyl radical or an aliphatic acyl radical, and B denoting the radical of any one of the condensible com-pounds listed above. These condensation products are described in detail in EP-A 126,875.

The diazonium salt polycondensation products are contained in the mixture in an amount of 0 to 40 ~ by weight, preferably of O to 30 % by weight, relative to the non-volatile consti-tuents of the mixture.

In addition, the photopolymerizable mixtures can contain dyes and/or pigments which may serve to enhance the contrast upon exposure and also to harden the layer. Suitable dyes are, for example, specified in US-A 3,218,167 and US-A 3,884,693. Par-ticularly suitable are, for example, Victoria Pure Blue FGA, Victoria Pure Blue BO (C.I. 42,595), Malachite Green, Victoria Blue B (C.I. 44,045), Renol Blue B2G-H (C.I.
74,160), Crystal Violet, Fatty Red 5B (C.I. 26,125), Neo7apon Blue FLE (C.I. Solvent Blue 70), Brilliant Blue Salt Acetate, Samarone Navy-Blue, Orasol Blue GN, Zapon Fast Fire-Red B
(C.I. 13,900:1) or Rhodamine 6 GDN (C.I. 45,160). To enhance 3 ~ ~

the image contrast after exposure, Metanil Yellow (C.I.
13,065), Methyl Orange (C.I. 13,025) or phenylazo-diphenylamine can be used.

The support material is coated from appropriate organic solvents or solvent mixtures, generally by flow-coating, spraying or dipping. The coating method depends on the desired layer thickness, the dried layers usually having thicknesses between 0.5 and 200 ~m.
Suitable supports are, for example, magnesium, zinc, copper, mechanically, chemically and electrochemically grained aluminum, anodically oxidized aluminum, steel, and also poly-ester film or cellulose acetate film, Perlon gauze etc., the surface of which may have been subjected to a pretreatment.
The support material may function as the final support or as a temporary support material from which the photosensitive layer is transferred by lamination to the workpiece to be processed.
Employing the mixture according to the invention, it is not absolutely necessary to exclude the access of atmospheric oxygen during exposure. Nevertheless, it is generally favorable to keep the mixture away from the influence of atmospheric oxygen during the photopolymerization. When the mixture is used in the form of thin copying layers, it is advisable to apply a suitable protective film which is substantially impermeable to oxygen. This film can be self-supporting and can then be peeled off before development of the copying layer. For this purpose, polyester films, for example, are suitable. The protective film can also comprise a material which dissolves in the developer liquid or can be removed during development, at least from the non-hardened areas. Materials which are suitable for this purpose are, for example, polyvinyl alcohol, vinyl alcohol/vinyl acetate copolymers, polyvinylpyrrolidone, vinylpyrrolidone/vinyl acetate copolymers, polyacrylic acid, butadiene/maleic acid copolymers, polyvinylmethyl ethers, polyphosphates, sugars etc.. Such protective layers generally have a thickness of 0.1 to 10 ~m, preferably of 0.5 to 5 ~m.

The recording material which is prepared using the photopoly-merizable mixtures of the invention serves, on the one hand, to produce images on suitable supports or receptor sheets and, on the other hand, to produce reliefs which are used as printing plates, screens, resists, and the like.

In addition, it is also possible to use the photosensitive mixtures for the formulation of UV-hardenable printing inks or for the preparation of lacquers which are hardenable by ultraviolet radiation and may be used for the protection of surfaces.

Preferably, the mixtures are used for the production of lithographic printing plates, in which aluminum is the pre-ferred support material. It is particularly preferred topretreat the aluminum used for this purpose in the usual manner, for example, by a mechanical, chemical or electro-chemical graining process which is, optionally, followed by an anodic oxidation. A further treatment of this support material, for example, with polyvinyl phosphonic acid, alkali metal silicate, phosphate, hexafluorozirconate, chromate, borate, polyacrylamide and ceilulose derivatives is advanta-geous.

The recording materials obtained from the mixtures are processed in the conventional manner, by exposing it imagewise and washing-out the unexposed areas of the layer with a suitable developer.

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The recording material is exposed under an original, as is known in the art, using light sources which emit light with the highest possible spectral fraction in the near ultra-violet region. The material can also be exposed by laserirradiation. Suitable lasers for irradiation are shorter-wave lasers of adequate performance, for example, Ar lasers, krypton ion lasers, helium/cadmium lasers, emitting in the region between about 300 and 600 nm and, for some layers, even CO2 lasers, which emit at about 10.6 ~m, or YAG lasers emitting at about 1.06 ~m.

As the developer solutions, neutral or preferably alkaline aqueous solutions are used, which have a pH value in the range from 8 to 14, preferably from 8.5 to 13, and which con-tain buffer salts, for example, water-soluble alkali metal phosphates, alkali metal silicates, alkali metal borates, alkali metal carbonates, alkali metal acetates or alkali metal benzoates. Additional constituents used are wetting agents, preferably anionic wetting agents and, if appro-priate, water-soluble polymers. The solution can also contain minor amounts, for example, up to 5 percent by weight, pre-ferably not more than 2 percent by weight, of water-miscible organic solvents. It is preferred to use difficultly volatile solvents, for example, araliphatic alcohols, the vapor pressure of which is of no consequence in the handling of the developer. Development can be performed in the conventional manner by dipping, spraying, brushing or wiping--over with a pad. If desired, the developed material can be treated with a gumming solution.

The photosensitive recording materials of this invention are distinguished by good reproduction properties and an adequate shelf life. They can be easily developed without scum, using developer solutions which from an ecological point of view are virtually neutral.

2~

The large print runs and the good ink acceptance obtained with printing forms prepared using the material of the present invention are particularly advantageous. The printing stencils are also highly resistant to alkaline developers and other processing solutions. To achieve even higher print runs, the plates can be post-heated after exposure and/or the developed plates can be post-exposed.

The preparation of the polyvinyl acetals used in the Examples is described below.

a) PreParation of Polvurethane qraft backbones In each case, the diol component and the catalyst were first introduced into a reaction vessel equipped with a stirrer and supplied with a nitrogen atmosphere, and the mixture was heated to a reaction temperature of between ~5 C and 100 C. Then the diisocyanate component was metered in whereby care was taken that the temperature of the reaction mixture did not exceed 120 C, and preferably did not exceed 100 C. After the complete addition of the diisocyanate component, the mixture was post-heated to a temperature between 80 C and 100 C
for a period of up to two hours in order to complete the reaction. The completion of the conversion and thus the end of the reaction were determined by an analysis of the diisocyanate consumption with the aid of known methods (p.e., IR-spectroscopy, titration).

The detailed composition of the reaction mixtures used in the Examples and the calculated mean molecular weights (MCalc) of the resulting polyurethanes (PU) are compiled in Table 1 below. The molecular weights result from the molar ratio diol component/diisocyanate, assuming a complete conversion of the NCO groups.

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Table 1 Poly- Diol Reaction Molar ratioMcalc urethane component temperature diol com-(PU) PEG/PU (o C) ponent/
(molar ratio) diisocyanate A 7 : 3 80 1 : 0.96,470 B 7 : 3 72 1 : 0.96,470 C 3 : 2 78 1 : 0.97 18,300 D 7 : 3 75 1 : 0.97 19,900 E 7 : 3 78 l : 0.95 13,200 PEG = Polyethylene glycol, molecular weight 600 Bu = 1,4-butanediol All products were prepared using isophorone diisocyanate as the diisocyanate component. In each case, 1,4-dimethyl-piperazine was used as the catalyst in an amount of 0.48 mole %, relative to the diisocyanate.

b) Preparatlon of the qraft Polvmers In each case, the polyurethane graft backbone was melted in a reaction vessel in a nitrogen atmosphere or was dissolved, respectively, by adding small amounts of methanol, and heated to a temperature between 60 ~C and 100 C. The monomers to be grafted on, which had optionally been dissolved in a solvent (e.g. methanol), including the free-radical initiator dissolved in the monomer, were then slowly metered to the polyurethane graft backbone, in a way such that the homopolymer formation was largely suppressed.

2 ~

The maximum temperature of the reaction mixture should be 120 C, more preferably 100 C.

When the post-reaction was completed, excess monomer remainders were removed by azeotropic distillation with methanol. The compositions of the individual reaction mixtures and the reaction parameters are compiled in Table 2 below.
Table 2 Graft PU VAc Cr Start Meter- Post- Graft- JO calc.
poly- g/g g/g temp. ing react. ed-on mole-mer PU PU C time min monom. ml/ cular min wt %g weight _ AV A 4 0 75 420 45 79.821.9 31,300 BV B 2.32 0 73 420 45 69.318.3 21,000 CV C 3.16 0 75 300 45 74.726.3 72,500 DV D 3.16 0 75 420 45 74.528.7 78,000 EVCr E 3.03 0.126 75 300 45 75.3 31.5 53,000 VAc = Vinyl acetate JO = intrinsic viscosity Cr = Crotonic acid All products were prepared employing 0.2 mole % of dibenzoyl peroxide (relative to the monomer used in each case). The intrinsic viscosities were determined at 25 C in tetrahydrofuran, using an Ostwald Viscosimeter, with the measured concentrations being selected such that a Hagenbach correction was not necessary. The grafted-on amount of monomer, in % by weight, is related to the weight of the total polymer.

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c) Saponification of the qraft ~olvmers The graft polymers of Table 2 were transesterified or saponified within two hours at room temperature. For this purpose, the products were dissolved in methanol to give 50 % strenghth solutions and mixed with methanolic soda lye (10 % strength). Depending on the added amount of alkali and the degree of grafting of the graft polymer, polymeric hydrolysis products having different degrees of hydrolysis were obtained. The resulting gels were granulated using conventional mills, the granules were washed with methanol (where appropriate with an addition of acetic acid to neutralize the soda lye) and dried. The process parameters and results are compiled in Table 3 below.
Table 3 .

Graft Graft mole % of Degree of Mcalc polyvinyl polyvinyl NaOH per hydrolysis alcohol acetate ester unit in %
F AV 1.92 98.2 22,000 G BV 5.00 98.9 14,000 H CV 1.92 98.0 47,000 I DV 1.97 98.4 44,000 _ EVCr 2.00 94.5 * 36,000 0 * found by determination of the acid present following saponification; the crotonic acid present in the polymer is also measured.

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~2~ ~3~

d) Pre~aration of the araft polw inyl acetals The graft polymers listed in Table 3 were dissolved in about eight times their amount of distilled water. The corresponding amount of aldehyde and a small amount of 2,6-di-tert.-butyl-4-methylphenol were added at room temperature. A solution comprising a small amount of sodium octyl sulfate, 1/3 of the weight amount of the aldehyde of concentrated hydrochloric acid and water was dropwise added to this solution, with agitating.
The mixture was stirred for one hour at room temperature, then heated to 40 ~C, and stirring was continued for another two hours. Thereafter concentrated hydrochloric acid (the same weight amount as the aldehyde) was added and stirring was continued at 40 C
for a further two hours. When the mixture had cooled down to room temperature the aqueous phase was decanted from the precipitated`polymer, the polymer was dissolved in ethanol and precipitated by pouring it into an excess amount of water. The polymer was dried in a vacuum drier at 40 C until its weight remained constant.

Table 4 Graft Graft vinyl poly Aldehyde OH-number acetal alcohol L F Pr+4-ClBz 3) 302 M G Bu 290 N H Pr 336 O I i-Bu 268 P K Bu 401 Q I 4-CH3OBz+Bu 1) 214 R Pr+Bu 2) 312 Bu = n-butyraldehyde i-Bu = isobutyraldehyde 4~CHqOBz = 4-methoxy-benzaldehyde 4-ClBz = 4-Chloro-benzaldehyde Pr = propionaldehyde ~ 23 1) Molar ratio 1 : 3 2) Molar ratio 1 : 2 3) Molar ratio 3 : 4 Preferred embodiments of the invention are described in the Examples which follow. In most cases, amounts are indicated in parts by weight ~pbw). Unless otherwise specified, percentages and proportions are given in weight units.
Example 1 A coating solution comprised of 1.20 pbw of polymer N, 1.00 pbw of a technical-grade mixture of pentaerythritol triacrylate and pentaerythritol tetraacrylate, 0.15 pbw of 2-(p-trichloromethyl-benzoylmethylene)-3-ethyl-benzothiazoline and 0.03 pbw of a blue azo dye obtained by coupling 2,4-dinitro-6-chlorobenzenediazonium salt with 2-methoxy-5-acetylamino-N-cyanoethyl-N-hydroxyethyl-aniline, in 50.00 pbw of 2-methoxyethanol was applied to a 0.3 mm thick aluminum foil which had been electrochemically grained in nitric acid, anodically oxidized in sulfuric acid and post-treated with a 0.1 % strength aqueous solution of polyvinyl phosphonic acid. Application was performed such that a dry layer weight of 1.1 g/m2 resulted.

The plate was then coated with a 3.5 ~ strength aqueous solution of polyvinyl alcohol (12 % of residual acetyl groups, K value 4). After drying, a protective layer having a weight of 0.8 g/m2 was obtained. The printing plate obtained was exposed for 40 seconds using a 5 kW metal halide lamp at a distance of 110 cm under a 13-step exposure wedge having density increments of 0.15 and on which a silver film of uniform optical density (density 1.57) and uniform absorption over the effective spectral range had additionally been mounted as neutral density filter. After exposure, the plate was developed in a developer of the following composition with the aid of a plush pad:

5.0 pbw of sodium octyl sulfate, l.0 pbw of sodium metasilicate x 5 H2O, and 94.0 pbw of distilled water.

The non-exposed layer areas were completely removed within a few seconds. Step 4 of the wedge step was rendered solid on the copy. Even the finest elements of the original were reproduced.

Example 2 A coating solution was prepared from 1.000 pbw of polymer M, 1.500 pbw of trimethylolethane triacrylate, 0.200 pbw of 2,4-bis-trichloromethyl-6-(4-styryl phenyl)-s-triazine and 0.015 pbw of Crystal Violet (C.I. 42,555), in 50.000 pbw of 2-methoxyethanol and applied to the layer support of Example 1, in a way such that a dry layer weight of 1.5 g/m2 was obtained.
A protective cover layer as described in Example 1 and having a weight of 0.6 g/m2 was applied. Exposure and development were performed as in Example 1, with the exception that the exposure time was 18 seconds only.

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Example 3 A coating solution was prepared as in Example 1, except that polymer N was replaced by the same amount of polymer P. The solution was applied to a support material as described in Example 1, such that a dry layer weight of 1.3 g/m2 was obtained. The cover layer had a weight of 0.8 g/m2. Exposure and development were performed as in Example 1. A copy exhibiting excellent resolution and good ink acceptance was obtained~
Example 4 A coating solution was prepared from 1.560 pbw of polymer L, 0.910 pbw of a diazonium salt polycondensation product prepared from 1 mole of 3-methoxy-diphenylamine-4-diazonium sulfate and l mole of 4,4'-bis-methoxy-methyl-diphenylether, isolated as mesitylene sulfonate, 0.049 pbw of phosphoric acid (85 % strength), 0.162 pbw of 2-(4-methoxystyryl)-4,6-bis-trichloro-methyl-s-triazine, 1.750 pbw of a technical-grade mixture of pentaerythritol tri- and tetraacrylate and 0.550 pbw of the azo dye described in Example 1, in 70.000 pbw of butanone, 4.000 pbw of ethanol and 2.000 pbw of butyl acetate and applied to an aluminum foil which had been electrochemically grained, anodically oxidized and post-treated with polyvinylphosphonic acid. Application took place such that a dry layer weight of 2.1 g/m2 resulted.

Q~ ~' The layer was exposed for 25 seconds through a negative original, with a solid step 4 being obtained on the test wedge. Development was performed with a developer having the following composition~

5.0 pbw of sodium octyl sulfate, 1.5 pbw of sodium metasilicate x 5 H2O, 1.5 pbw of trisodium phosphate x 12 H2O, and 92.5 pbw of distilled water.
The printing plate was clamped into a sheet-fed offset press where it readily accepted printing ink. 280,000 good prints could be run.

Example 5 A coating solution was prepared from 2.50 pbw of polymer O, 5.60 pbw of a technical-grade mixture of pentaerythritol tri- and tetraacrylate, 0.20 pbw of the triazine specified in Example 4 and 0.03 pbw of the azo dye specified in Example 1, in 25.00 pbw of butanone, 2.00 pbw of ethanol and 1.00 pbw of butyl acetate and spin-coated onto a 25 ~m thick biaxially stretch-oriented and heat-set polyethylene terephthalate film, such that a layer weight of 35 g/m2 resulted after drying at 100 C.
The dry resist film prepared in this way was laminated at 120 ~C onto a phenoplast laminate board clad with a 35 ~m thick copper foil, employing a customary laminating device.
Exposure was performed for 25 seconds using a customary exposure device. The original used was a line original having line widths and spacings down to 80 ~m. Following exposure the polyester film was carefully peeled off, and the layer 2 ~ 6 ~

was developed in a spray developing station with the following developer, during 90 seconds:

5.0 pbw of sodium octyl sulfate, 1.5 pbw of sodium metasilicate x 5 H2O, 1.5 pbw of disodium hydrogen phosphate x 12 H2O, in 92.0 pbw of distilled water.

After development, the plate was rinsed with tap water for 30 seconds, etched for 30 seconds in a 15 % strength ammonium peroxydisulfate solution and then electroplated in the following electroplating baths:

1. for 30 minutes in a copper bath from Messrs. Schloetter, Geislingen/Steige, West ~ermany, type "Glanzkupfer-Bad"
(Brilliant Copper Bath).
current density: 2.5 A/cm2 metal build-up: about 12.5 ~m 2. for 30 minutes in a nickel bath from the same manufacturers, type "Norma".
current density: 4.0 A/cm2 metal build-up: 9.0 ~m The plate did not exhibit any damage or undercutting.
Decoating was performed at a temperature of 50 C, using 5 % strength KOH solution. The bared copper was etched away with customary etchants.

Example 6 A coating solution comprised of 1.00 pbw of polymer Q, 1.00 pbw of technical-grade mixture of pentaerythritol tri- and tetraacrylate, 0.10 pbw of 2,4-bis-trichloromethyl-6-(4-ethoxy-naphth-l-yl)-s-triazine and 0.02 pbw of Victoria Pure Elue FGA
(C.I. Basic Blue 81), in 50.00 pbw of propylene glycol monomethyl ether was applied onto the aluminum support material described in Example 4, such that a dry layer weight of 1.3 g/m2 was obtained. The resulting printing plate was exposed through a test original and developed with the developer specified in Example 5.

When being clamped into the sheet-fed offset press, the printing form immediately accepted greasy printing ink.
Fven after a prolonged press standstill the plate excellently accepted ink after printing 2 to 8 sheets.
95,00Q good prints could be run, and even the finest lines were reproduced.
Four of the plates prepared as described above were stored at 100 C in a drying oven for a period of 1 to 4 hours.
After this heat treatment the plates were imagewise exposed and developed with the developer solution specified above.
To render visible any layer residues (toning) remaining in the non-image areas, the dried plates were dyed with a protective ink.

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The plates stored in the oven for 1 and 2 hours could be developed satisfactorily. They do not exhibit any substantial reduction of the continuous tone step wedge. After a storage time of 3 hours, the continuous tone step wedge was reduced by one step. Development was insignificantly retarded.
Development of the plate stored for 4 hours was slightly retarded. These results show that the mixtures according to this invention have a relatively good resistance to storage in the heat.
Example 7 A coating solution comprised of 2.280 pbw of polymer R, 2.280 pbw of pentaerythritol triacrylate, 0.010 pbw of phenylazodiphenylamine 0.211 pbw of the triazine of Example 2, and 0.350 pbw of Renol Blue B2G-H (C.I. 74,160) in 70.000 pbw of propylene glycol monomethyl ether was applied to the support material described in Example 1, in a way such that a dry layer weight of 2.5 g/m2 was obtained. The photosensitive layer was exposed through a standard negative original for 12 seconds, using a 5 kW metal halide lamp. The photosensitive layer, which displayed good differention between exposed and non-exposed areas, was developed with the developer solution of Example 5 with the aid of a plush pad. The non-exposed layer areas were removed within 10 seconds after wetting with the developer solution.
The plate was then rinsed with water and dried.

Step 4 of a silver film continuous tone step wedge having an optical density range of 0.05 to 3.05 and density increments of 0.15 was reproduced solid on the copy. Even the finest screen dots and lines of the original were reproduced.

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A layer of polyvinyl alcohol (12 % residual acetyl groups, X-value 4) was applied to a printing plate prepared as described above, such that a dry layer weight of 0.8 g/m2 was obtained. The photospeed of the photocurable layer was increased by 30 % as a result of applying this oxygen barrier layer. When the two printing plates were clamped in a sheet-fed offset press, the plate provided with the protective cover layer produced much more prints than the plate without a cover layer, i.e. the print run was increased by 250 %.
Exam~le 8 Following exposure, plates produced as in Example 7 (with and without an oxygen barrier layer) were subjected to a heat treatment at 100 C for 1 minute and then developed. Two additional solid steps were obtained on the test wedge in each case, compared to the printing plates of Example 7, which had not been thermally post-treated.

Post-heating increased the print run as well. The increase was about 200 % for the plate without a cover layer and about 100 % for the plate with the cover layer. A post-exposure step represents another possibility of increasing the number of prints in a comparably way.

Claims

1. A photopolymerizable mixture which contains, as essential constituents, a) a polymeric binder possessing repeating vinyl acetal units, b) a free-radically polymerizable compound possessing at least one terminal ethylenically unsaturated group and having a boiling point of more than 100 °C at normal pressure, and c) a compound or a combination of compounds which under the action of actinic light is capable of initiating the polymerization of compound b), wherein said binder is a graft polymer obtained from a polyurethane graft backbone, onto which chains containing vinyl alcohol units and vinyl acetal units are grafted.

2. The photopolymerizable mixture as claimed in claim 1, wherein the grafted-on chains furthermore contain vinyl ester units.

3. The photopolymerizable mixture as claimed in claim 1, wherein the vinyl acetal units are derived from an aliphatic or cycloaliphatic aldehyde.

4. The photopolymerizable mixture as claimed in any of claims 1 to 3, wherein the grafted-on chains furthermore contain units of other ethylenically unsaturated monomers which are copolymerizable with vinyl esters.

5. The photopolymerizable mixture as claimed in claim 1, wherein the polyurethane is a polyaddition product obtained from diisocyanates and diols.

6. The photopolymerizable mixture as claimed in claim 5, wherein the diol is an aliphatic diol having 2 to 12 carbon atoms, a cycloaliphatic diol having 5 to 10 carbon atoms or an aliphatic polydiol having a molecular weight between 200 and 10,000.

7. The photopolymerizable mixture as claimed in claim 6, wherein the diol is a mixture comprising 1 mol of polydiol and 0.1 to 0.7 mol of low molecular weight aliphatic diol.

8. The photopolymerizable mixture as claimed in claim 5, wherein the diisocyanate is an aliphatic diisocyanate having 4 to 15 carbon atoms or a cycloaliphatic diisocyanate having 7 to 15 carbon atoms.

9. The photopolymerizable mixture as claimed in claim 1 or 5, wherein the polyurethane has a molecular weight between 200 and 100,000.

10. The photopolymerizable mixture as claimed in claim 1, wherein the graft polymer has a hydroxyl number in the range of 100 to 600.

11. The photopolymerizable mixture as claimed in claim 1, wherein the compound which can be polymerized by a free-radical process is an acrylate or methacrylate of a mono- or polyhydric alcohol or an acrylic or methacrylic acid amide.

12. The photopolymerizable mixture as claimed in claim 1, wherein 20 to 95 % by weight of graft polymer, 5 to 70 %
by weight of polymerizable compounds and 0.1 to 15 % by weight of photopolymerization initiator are contained.

13. The photopolymerizable mixture as claimed in claim 1, wherein a diazonium salt polycondensation product is additionally contained.

14. A photopolymerizable recording material comprising a layer support and a photopolymerizable layer, wherein the photopolymerizable layer comprises a mixture as claimed in any of claims 1 to 13.