AU633291B2 - Photocurable elastomeric mixture and recording material obtained therefrom for producing relief printing plates - Google Patents

Description

Our Ref: 303884 1 -3

AUSTRALIA

Patents Act COMPLETE SPECIFICATION FORM

(ORIGINAL)

Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: Related Art: Applicant(s): Address for Service: Hoechst Aktiengesellschaft Rheingaustrasse 190 6200 WIESBADEN FEDERAL REPUBLIC OF GERMANY ARTHUR S. CAVE CO.

Patent Trade Mark Attornerys Level 10, 10 Barrack Street SYDNEY NSW 2000 Complete specification for the invention entitled "Photocurable elastomeric mixture and recording material obtained therefrom for producing relief printing plates".

The following statement is a best method of performing it full description of this invention, including the known to me:- 1 5020 PHOTOCURABLE ELASTOMERIC MIXTURE AND RECORDING MATERIAL OBTAINED THEREFROM FOR PRODUCING RELIEF PRINTING PLATES Background of the Invention The invention relates to a photocurable negative-working mixture and a recording material obtained therefrom which is suitable for producing elastic relief printing plates.

Mixtures of the said class are known and normally contain an elastomeric binder, a radicalpolymerizable compound and a polymerization initiator activatable by actinic radiation. Mixtures of this type, which are described, for example, in DE-B-2,215,090 US-A-4,423,135), can be developed after exposure only with organic solvents to form the relief image.

Although photosensitive copying materials which can be aqueously developed and are based on partially hydrolyzed polyvinyl acetate are described for producing relief printing plates in US-Ala 4,042,386 and DE-B-1,902,639/1,917,917 US-A- 3,630,746) and 2,402,808 US-A 3,936,254), an addition of water, which makes a subsequent drying process necessary, is required for the purpose of homogenization and layer formation. Furthermore, these relief printing forms do not have the elasticity or low Shore A hardness required for flexographic printing.

DE-A-3,541,162 AU 86/65,535) and EP-B- 0,080,664 US-A-4,493,807) describe internally plasticized partially hydrolyzed polyvinyl acetates as binders for photosensitive copying materials.

Although these graft polymers, which are obtained by polymerization of vinyl esters in the presence of polyalkylene oxides and subsequent partial hydrolysis, are more elastic than pure partially hydrolyzed polyvinyl acetates, water addition is likewise necessary in processing the corresponding photopolymer mixtures to form laminar copying materials.

German Patent Application P 38 24 146.3, which is not prepublished, describes a photocurable elastomeric mixture for producing relief printing plates which contains, as elastomeric binder, a graft polymer which has, as grafting base, a polymer of diol components and diisocyanate components containing at least two urethane groups in the molecule onto which are grafted polymer chains composed of units of carboxylic acid vinyl esters containing 3 to 20 carbon atoms and/or their hydrolysis products and/or of further ethylenically 1 0504e:mmb unsaturated monomers and/or their hydrolysis products.

The photocurable elastomeric mixtures which contain said binders can be developed after imagewise exposure with water or aqueous solutions. The stability of the flexographic printing plate in the printing operation appears to be worth improving.

It is assumed that (since water is employed in printing) the cured regions of the flexographic printing plate could also be attacked by water.

Summary of the Invention The present invention provides in one aspect, a photocurable elastomeric mixture which comprises: about 5 to 70% by weight of at least one radical-polymerizable compound compatible with the binder mentioned below and containing at least one terminal ethylenically unsaturated group and having a boiling point at normal pressure above 100°C; about 0.01 to 10% by weight of a compound or combination of compounds capable of initiating the polymerization of the polymerizable compound on exposure to actinic light; and about 20 to 95% by weight of elastomeric binder, comprising a graft polymer which comprises the 3 esterification product of a hydroxyl-containing graft polymer with an acylating reagent, esterified in the melt in homogeneous phase, wherein said hydroxyl-containing graft polymer has a graft base which comprises a polymer comprising diol components and diisocyanate components, said polymer having at least two urethane groups in the molecule, onto which graft base have been grafted a plurality of polymer chains comprising units of vinyl esters of carboxylic acids, said vinyl esters having 3 to 20 carbon atoms, and/or hydrolysis products thereof.

In accordance with another aspect of the present invention, there is provided a photocurable recordina material which comprises a coating base and a photocurable coating comprising the foregoing mixture.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.

-4- Detailed Description of the Preferred Embodiments Graft polymers containing hydroxyl groups are understood to be those compounds which result from grafting onto a grafting base carboxylic acid vinyl esters and/or their hydrolysis products and optionally further monomer units from ethylenically unsaturated polymerizable compounds and their hydrolysis products, wherein the graft polymer is esterified with an acylating agent in a homogeneous phase in the melt. Partially esterified graft polymers are particularly preferred. The percentage of grafted-on components is about 10 to 99% by weight, preferably 30 to 90% by weight, in particular 40 to 80% by weight, based on the total graft polymer.

As grafting bases, mention is made of those which are composed of polyurethanes containing at least 2 urethane groups in the molecule, the number of urethane groups per grafting base molecule not being subject to any particular limitation in the upward direction and having in general values higher than 2.

In principle, any diol which can be used in polyurethane synthesis can be employed. Preferred are cycloaliphatic diols such as, for example, cyclohexanediols and also aliphatic diols containing preferably 2 to 12 carbon atoms. Polyether diols, for example polypropylene oxides, polybutylene oxides, copolymers of ethylene oxides, propylene oxide, butylene oxide, preferably their block c copolymers are furthermore preferred, polyethylene oxides being particularly preferred.

Preferably, polyether diols, in particular polyethylene glycols, having molecular weights between about 200 and 10,000 are used, polyethylene glycols having molecular weights between 400 and 1,500 being particularly preferred. The polyether diols are optionally employed in combination with low-molecular aliphatic diols such as, for example, 1,4-butanediol, 1,3-propanediol, ethylene glycol or diethylene glycol. Preferably the molar ratio of polyether diol to low-molecular aliphatic diol is from about 1:0.1 to 1:0.7.

Aromatic diisocyanates, for example m- and p-xylylene diisocyanate, tolylene 2,4-diisocyanate, tolylene 2,6-diisocyanate or mixtures of the last two isomers, naphthylene diphenylmethane-4,4'-diisocyanate and phenylbenzyl- 4,4'-diisocyanate are used as diisocyanate components.

Preferably aliphatic and/or cycloaliphatic diisocyanates are employed. Preferred aliphatic diisocyanates are, for example, those containing 2 to 12 carbon atoms in the aliphatic radical, for example ethylene diisocyanate, propylene diisocyanate, tetramethylene diisocyanate and 2,2,4trimethylhexamethylene diisocyanate. Preferred cycloaliphatic diisocyanates are, for example, 1,4diisocyanatocyclohexane, dicyclohexylmethanediisocyanate and isophorone diisocyanate.

-6- Particularly preferred is the use of hexamethylene diisocyanate and isophorone diisocyanate.

The molar ratio of diol component to diisocyanate component is preferably between about 1:0.99 and 1:0.5, in particular between 1:0.98 and 1:0.7. The mean molecular weights of the polyurethanes are preferably between about 200 and 100,000, in particular between 1,300 and 50,000, particularly preferably between 3,000 and 25,000.

To regulate the molecular weight in the preparation of the polyurethanes, use may be made of monohydroxyl compounds such as, for example, methanol, ethanol or other aliphatic alcohols, in particular those of the general formula CH 2 ,n+O where n 1 to 4, but also semi-esterified and semietherified diols or diol components. In addition, monoisocyanates are used to control the molecular weight.

It should be pointed out that, for example, with a specified molecular weight for the grafting base, the number of its urethane groups is directly dependent on the molecular weights of the diol components or diisocyanate components used.

Carboxylic acid vinyl esters containing 3 to 20 carbon atoms, preferably containing 4 to 14 carbon atoms are employed for grafting onto the polyurethane. Particularly preferred are vinyl acetate and/or vinyl propionate, in particular vinyl acetate. Mixtures of vinyl acetate and/or vinyl propionate and vinyl versatate are furthermore preferred. In particular, with partial or complete -7hydrolysis of the products following the graft polymerization, the concomitant use of vinyl propionate in addition to vinyl acetate in grafting is advantageous. In addition, copolymerizable mixtures of carboxylic acid vinyl esters, preferably mixtures of vinyl acetate and vinyl versatate, can be grafted, the proportion of vinyl versatate being about 0.2 to 10% by weight, preferably 0.5 to 5% by weight, based on the proportion of vinyl acetate.

Grafting with various carboxylic acid vinyl esters in the form of block copolymers, optionally in combination with further ethylenically unsaturated and copolymerizable monomers, may also be advantageous.

Furthermore, the carboxylic acid vinyl esters can also be grafted together with other ethylenically unsaturated and copolymerizable monomers such as, for example, maleic acid, itaconic acid, mesaconic acid, crotonic acid, acrylic acid or their esters.

Grafting is carried out using grafting catalysts which initiate radical chains. Preferably, any radical former which is soluble in the monomers, the monomer mixture or the monomer solution is suitable. In particular mention may be made of organic percompounds such as peroxides and percarbonates and organic azo compounds.

Azobisisobutyronitrile and particularly dibenzoyl peroxide are preferred. The grafting reaction is carried out in the presence of preferably about 0.013 to 1.3 mol in particular 0.026 to 0.27 mol 1 of a radical-forming catalyst, based on the quantity of monomer.

The graft polymers containing hydroxyl groups obtained can be converted into partially or fully hydrolyzed products by hydrolysis, alcoholysis or transesterification, the degree of hydrolysis being at least 1 mol preferably about 70 to 99 mol based on the number of moles of hydrolyzable monomer units in the graft polymer.

The process for preparing the graft polymers containing hydroxyl groups is described in German Patent Application P 37 32 089.

The process in which the graft polymers containing hydroxyl groups mentioned in P 37 32 089 are reacted with an acylating reagent and also the products resulting therefrom are described in the simultaneously filed German Patent Application P 38 43 505.5,jQo. 4P4-Rdng to U.S. Application NQ As acylating reagents for preparing said graft polymers containing hydroxyl groups, mention is made there, in particular, of carboxylic acid anhydrides. Preferably, alkanecarboxylic, alkenecarboxylic and/or acrylic acid anhydrides containing 2 to 12 carbon atoms are used. Mention may be made, for example, of succinic anhydride, maleic anhydride, citraconic anhydride, glutaric anhydride, phthalic anhydride, cis-4-cyclohexene-l,2dicarboxylic anhydride, acrylic anhydride, methacrylic anhydride, crotonic anhydride and itaconic anhydride. Mixtures of carboxylic acid anhydrides may also be used.

The degree of reaction of free hydroxyl groups of the graft polymer with the said acylating reagents for preparing the esterified product depends on its desired properties, in particular the solution properties of the product. Either hydrolyzed or partially hydrolyzed graft polymers may be employed as starting material for the esterification reaction. Partially hydrolyzed graft polymers are, in general, understood to be products which are hydrolyzed to an extent of about 80 to mol (degree of hydrolysis) Less usual are partially hydrolyzed products which have a degree of hydrolysis of about 50 to In general, the degree of reaction of the graft polymer containing hydroxyl groups with the acylating reagent may be so described that the weight ratio of graft polymer to acylating reagent, particularly if a carboxylic acid anhydride is used, is about 1:0.01 to 1:5, preferably 1:0.05 to 1:2.

The basis of this specification is formed by the partially hydrolyzed graft polymers which are hydrolyzed to an extent of about 80 to Finally, the reaction must be adjusted in such a way that the resulting product can be used as binder in a photosensitive mixture. This implies that it is possible to develop the coating in aqueous alkali (in particular, in 1% strength aqueous soda solution), but equally that. the developed relief form is not destroyed by water.

r The mean molecular weight of the at least partially esterified graft polymers containing hydr-.xyl groups is preferably between about 200 and 100,000, in particular between 1,300 and 50,000, particularly preferably between 3,000 and 25,000.

The acid number of the product should be in the range from about 80 to 250 mg KOH/g of polymer, in particular from 100 to 200 mg KOH/g of polymer.

According to the method mentioned in the simultaneously filed German Patent Application P 38 43 505.5, the graft polymer containing hydroxyl groups is fused either without acylating agent or simultaneously with the acylating agent, preferably without adding a solvent. Carboxylic acid anhydrides are preferably used as acylating reagents. The melting point is, in general, about to 200° C, in particular 100 to 180° C, and also particularly preferably 120 to 160°C.

The reaction is carried out without adding a catalyst which may result in coloration of the product, in particular of amines. The reaction time may be made extremely short and is in the region of a few minutes. Preferably, the reaction is already complete after a reaction time of less than 10 min.

Even if the acylating reagent is not added simultaneously with the fusion, the esterification can be carried out in a conventional reaction vessel or processing apparatus for carrying out thermoplastic reactions. Such apparatuses are, for example, two roll mills, kneading machines and extrusion appliances. Surprisingly, it was found -11that, for example, for a planned maximum of reaction of the secondary hydroxyl groups contained in the graft polymer (by using a corresponding molar ratio of the starting materials), the reaction has already proceeded to completion after a few minutes even without using the catalyst. The product is obtained in a homogeneous phase. It does not have any locally varying degrees of reaction in the graft polymer which otherwise have to be tolerated; the product has consequently reacted uniformly and can therefore be prepared reproducibly.

The photosensitive mixtures according to the invention contain, in general, about 20 to preferably 30 to 70, by weight of binder. They furthermore contain at least one radicalpolymerizable olefinically unsaturated compound and also at least one photoinitiator.

Suitable monomers having one or more polymerizable olefinic double bonds are, in particular, esters and amides of acrylic and methacrylic acid. Examples are the compatible monoand diacrylates and mono- and dimethacrylates of monohydric or polyhydric alcohols such as ethylene glycol, di-, tri-, tetra- or polyethylene glycols, the latter preferably containing 10 to 15 ethylene glycol units, 1,3-propanediol, glycerol, 1,1,1trimethylolpropane, 1,2,4-butanetriol or pentaerythritol, for example ethylene glycol monomethacrylate, 1,3-propanediol monomethacrylate, glycerol mono- and diacrylate, 1,2,4-butanetriol monomethacrylate, pentaerythritol triacrylate, -12polyethylene glycol methyl ether acrylate, tetradecaethylene glycol dimethacrylate or the triether formed from glycerol and 3 mols of Nmethylolacrylamide or -methacrylamide. The quantity of monomersis, in general, about 5 to 70, preferably about 10 to 50, by weight of the nonvolatile constituents of the mixture.

Suitable photoinitiators for the mixtures according to the invention are the known compounds which have an adequate thermal stability during the processing of the copying materials and also an adequate radical formation on exposure to initiate the polymerization of the monomers. They should absorb light in the wavelength range from approximately 250 to approximately 500 nm to form radicals. Examples of suitable photoinitiators are acyloins and their derivatives such as benzoin, benzoin alkyl ethers, for example benzoin isopropyl ether, vicinal diketones and their derivatives, for example benzil, benzil acetals such as benzildimethyl ketal, fluorenones, thioxanthones, polynuclear quinones, acridines and quinoxalines; also trichloromethyl-s-triazines, 2-halomethyl-4styryl-1,3,4-oxadiazole derivatives, halooxazoles substituted with trichloromethyl groups, carbonylmethyleneheterocyclics according to DE-A 33 33 450 containing trihalomethyl groups, acylphosphine oxide compounds such as are described, for example, in DE- A-31 33 419, and other phosphorus-containing photoinitiators, for example 6-acyl-(6H)-dibenz[c,e] [1,2]oxaphosphorine-6-oxides, in particular the 6- -13- .r (2,4,6-trimethylbenzoyl)-(6H)-dibenz{c,e] oxaphosphorine- 6-oxide described in the older German Patent Application P 38 27 735.2. The photoinitiators may also be used in combination with one another or with coinitiators or activators, for example with Michler's ketone and its derivatives or 2-alkylanthraquinones. The quantizy of photoinitiator is, in general, about 0.01 to preferably about 0.5 to 5, by weight of tha recording material.

It is often advantageous to add still further auxiliaries and additives to the recording material, for example thermal polymerization inhibitors such as hydroquinone and its derivatives, 2,6-di-tert.butyl-p-cresol, nitrophenols, nitrosamines such as N-nitrosodiphenylamine or salts of N-nitrosocyclohexylhydroxylamine, for example its alkali metal or aluminum salts. Further common additives are dyestuffs, pigments, processing auxiliaries and plasticizers.

The mixtures according to the invention can be used for producing relief printing plates and flexographic printing plates by casting from solution or extrusion and calendering to form coatings having a thickness of about 0.02 to 6 mm, preferably from 0.2 to 2 mm. The coating may be laminated onto the surface of a suitable base or a solution of the mixtures according to the invention can be applied to a coating base.

The mixtures according to the invention can be used not only for producing relief printing -14- L plates, but also, for example, for producing lithographic printing plates, gravure cylinders, screen printing stencils and photoresists.

Suitable substrates are, depending on the intended use, for example, polyester films, steel or aluminum sheets, copper cylinders, bases for screen printing stencils, foam layers, rubber-elastic bases or p.c. boards. It may also be advantageous to apply a topcoating or protective coating, for example a thin coating of polyvinyl alcohol or a peelable top film, for example of polyethylene glycol terephthalate, to the photosensitive recording coating. Furthermore, a preliminary coating of the base may be advantageous. The additional coating between the base and the photosensitive coating may be effective, for example, as an antihalation coating or as an adhesive coating.

The recording materials according to the invention can be subjected to imagewise exposure with actinic light from light sources such as mercury vapor lamps or fluorescent tubes, the emitted wavelength-preferably.being.between 300 and 420 nm. The unexposed and uncrosslinked coating areas can be removed by spraying, washing or brushing,.. with ,aqueous alkaline ,solutions, for example of wetting or emulsifying agents. The crosslinked coating areas are resistant to water. Small quantities of foam inhibitors or organic solvents miscible with water, for example low aliphatic alcohols, may also be added to the solution. The quantitative proportion of organic c solvents is, in general, less than 10%, preferably less than Advantageously, the developed relief forms are dried at temperatures up to 120° C and optionally post-exposed at the same time or later to actinic light.

The recording materials according to the invention are suitable in particular for producing printing forms, especially letterpress or relief printing forms which are particularly suitable for flexographic printing.

The invention is explained by the examples below. The limiting viscosity numbers J. [ml/g] were determined at 25 C in an Ostwald viscometer. The degrees of hydrolysis (in mol relate to the hydrolyzable monomer units in the unhydrolyzed graft polymer.

Preparation Examples: Example 1 Preparation of the graft polymer containing hydroxyl groups: a) Synthesis of the polyurethane grafting base 3,000 g of polyethylene glycol 600 and 193.1 g of 1,4-butanediol are heated together with ml of 1,4-dimethylpiperazine to 75 0 C and 1,428.2 g of isophorone diisocyanate are added in the course of 6 hours. The reaction -16is then allowed to proceed for two hours at

C.

The limiting viscosity number J, in methanol of the polyurethane prepared in this manner is 10.6 ml/g.

b) Synthesis of the graft polymer 1,500 g of the polyurethane grafting base are heated to 80*C and a mixture of 3,480 g of vinyl acetate and 26.1 g of dibenzoyl peroxide (75% strength) is added in the course of 5 hours while stirring. After further reaction for 45 minutes at 80°C, the unreacted vinyl acetate is removed by azeotropic distillation with methanol. The limiting viscosity number Jo in THF of the graft polymer prepared in this manner is 17.5 ml/g.

c) Synthesis of the graft polymer containing hydroxyl groups 172.1 g of a 10% strength methanolic NaOH and 172.1 g of water are added to 9,178 g of a strength methanolic solution of the graft polymer at room temperature. The mixture gels after some time and is then granulated. The granulated material obtained is suspended in methanol and a quantity of acetic acid -17equivalent to the NaOH is added. The product is filtered off, washed several times with methanol and dried. The limiting viscosity number J. in water is 10.1 ml/g, the degree of hydrolysis is 85.8% and the proportion of vinyl alcohol groups is 42.2%.

Example 2 Esterification of the graft polymer containing hydroxyl groups: After 100 g of the partially hydrolyzed graft polymer described in Example Ic) have been fused at 140° C in a two-roll mill 47.8 g of succinic anhydride are added at this temperature. After reacting for 6 min. at 140° C, the reaction is complete; a colorless homogeneous product is obtained which has an acid number of 181 mg KOH/g of polymer. The product is not soluble in water, but it dissolves to form a clear solution both in a 1% strength aqueous soda solution and also in ethanol.

Example 3 Esterification of the graft polymer containing hydroxyl groups: g of the partially hydrolyzed graft polymer described in Example Ic) are fused in a two-roll mill at 140°C. Then 23.5 g of maleic anhydride and -18- 0.8 g of 2,6-di-tert.-butyl-4-methylphenol are added at 140° C. After 4 min. the reaction is complete.

The product obtained is colorless and has an acid number of 133 mg KOH/g of polymer. It is only sparingly soluble in water, producing turbidity, but a clear solution is obtained in 1% strength aqueous soda solution or in ethanol.

Example 4 Esterification of the graft polymer containing hydroxyl groups: 200 g of the partially hydrolyzed graft polymer described in Example Ic), 94 g of maleic anhydride and 3.2 g of 2,6-di-tert.-butyl-4-methylphenol are mixed and then jointly extruded at 140"C and with a retention time of 3 min. in a counter-rotating twinscrew extruder having a screw length to screw diameter ratio of 15. The colorless homogeneous extrudate has an acid number of 142 mg KOH/g of polymer and, in addition, exhibits the same properties as the polymer from Example 3.

Working examples: Example 1 73.15 pbw (parts by weight) of the graft polymer containing hydroxyl groups reacted with succinic anhydride (Example 2) are premixed with 20.0 pbw of -19triethylene glycol dimethacrylate, 4.0 pbw of trimethylolpropane triacrylate, 2.0 pbw of benzil dimethyl ketal, 0.8 pbw of 2,6-di-tert.-butyl-4methylphenol and 0.05 pbw of 2,4dihydroxybenzophenone, and extruded in a single screw extruder (Brabender Plasticorder) at 140°C and 150 rev/min via a flat film extrusion die to form a transparent melt. The extrudate is hot-pressed in a plate press for one minute at 100 C and 15 bar to form a 0.6 mm thick photopolymer coating between a 0.125 mm thick polyester film and a 0.3 mm thick anodically oxidized aluminum sheet which is provided with a polyurethane adhesive coating. After peeling off the polyester film, imagewise exposure is carried out for five minutes in a commercial UVA flat-bed exposure apparatus and development is carried out for two minutes with 1% strength aqueous soda solution to produce a relief printing plate having a Shore A hardness of 90 which is resistant to water (storage for 24 hours at If development is carried out with pure water, it is only possible to obtain incompletely developed relief coatings.

Example 2 (Comparison example) 73.15 pbw of a graft polymer containing hydroxyl groups (according to Example lc) which has not been reacted with a carboxylic acid anhydride are processed similarly to Example 1 to produce a photopolymer coating. After exposure, development is carried out with water. The relief printing plate obtained is destroyed during several hours of storage in water thermostated to Example 3 57.8 pbw of the binder described in Example 3 are homogenized for 2 minutes in a two-roll mill at 130° C with 35.8 pbw of 2-hydroxypropyl methacrylate, pbw of trimethylolpropane triacrylate, 2.0 pbw of benzildimethyl ketal and 0.8 pbw of 2,6-ditert.-butyl-4-methylphenol, and the mixture is i processed similarly to Example 1 to produce a relief printing plate having a Shore A hardness of 92. It was possible to develop the plate well with 1% strength aqueous soda solution. In addition, the relief coating is resistant to pure water (storage for 24 hours at Example 4 (Comparison example) 57.8 pbw of a graft polymer containing hydroxyl groups (according to Example lc) which has not been reacted -with a carboxylic acid anhydride are processed similarly to Example 3 to produce a photopolymer coating. After exposure, development is carried out with water. The relief printing plate obtained is destroyed on storing for several hours in water thermostated to -21-

Claims (16)

1. A photocurable elastomeric mixture which comprises: from 5 to 70% by weight of at least one radical-polymerizable compound compatible with the binder and containing at least one terminal ethylenically unsaturated group and having a boiling point at normal pressure above 100 0 C; from 0.01 to 10% by weight of a compound or combination of compounds capable of initiating the polymerization of compound on exposure to actinic light; and from 20 to 95% by weight of elastomeric binder comprising a graft polymer which comprises the esterification product of a hydroxyl-containing graft polymer with an acylating reagent, esterified in the melt in homogeneous phase, wherein said hydroxyl-containing graft polymer has a graft base which comprises a polymer comprising diol components and diisocyanate components, said polymer having at least two urethane groups in the molecule, onto which graft base have been grafted a plurality of polymer chains comprising units of vinyl esters of carboxylic acids, said vinyl esters having 3 to 20 carbon atoms, and/or hydrolysis products thereof.

2. A photocurable mixture as claimed in claim 1, wherein said graft polymer comprises a graft base onto which a plurality of additional ethylenically unsaturated monomers or hydrolysis products thereof have been grafted. 22

3. A photocurable mixture as claimed in claim 1, wherein the graft polymer is a partially esterified product.

4. A photocurable mixture as claimed in claim 3, wherein said partially esterified hydroxyl- containing graft polymer is about 80 to 90 mol% hydrolyzed. A photocurable mixture as claimed in claim 4, wherein the weight ratio of the hydroxyl- containing graft polymer to the acylating reagent ranges from about 1:0.01 to

6. A photocurable mixture as claimed in claim 1, wherein a carboxylic acid anhydride is used as acylating reagent.

7. A photocurable mixture as claimed in claim 1, wherein said polymer has a mean molecular weight of about 200 to 100,000.

8. A photocurable mixture as claimed in claim 7, wherein said polymer has an average molecular weight of 1300 to 50,000.

9. A photocurable mixture as claimed in claim 1, wherein the polymer has an acid number of about 80 to 250 mg KOH/g of polymer. -23- A photocurable mixture as claimed in claim 1, wherein said graft polymer comprises about to 99% by weight of grafted-on constituents.

11. A photocurable mixture as claimed in claim 10, wherein said graft polymer comprises 30 to by weight of grafted-on constituents.

12. A photocurable mixture as claimed in claim 1, wherein the diisocyanate is an aliphatic or cycloaliphatic diisocyanate.

13. A photocurable mixture as claimed in claim 1, wherein the diol is a mixture of a polyetherdiol and a monomeric diol.

14. A photocurable mixture as claimed in claim 13, wherein about 0.1 to 0.7 mol of monomeric diol is employed per mol of polyether diol. A photocurable mixture as claimed in claim 1, wherein the molar ratio of diol component to diisocyanate component is about 1:0.99 to 1:0.5.

16. A photocurable mixture as claimed in claim 1, wherein said vinyl esters have 4 to 12 carbon atoms.

17. A photocurable mixture as claimed in claim 1, wherein the graft polymer is hydrolyzed to -24- c i 0504e:mmb an extent of about 70 to 99 mol based on the number of hydrolyzable monomer units.

18. A photocurable mixture as claimed in claim 1, wherein the radical-polymerizable compound is an acrylic or methacrylic acid ester of a mono or polyhydric alcohol or an acrylamide or methacrylamide.

19. A photocirable recording material comprising a coating base and a photocurable coating, wherein the photocurable coating comprises a mixture as claimed in any one of claims 1 to 18. A photocurable elastomeric mixture substantially as herein described with reference to any one or more of the foregoing Examples, (excluding the comparison examples). DATED this 23rd day of November, 1992. HOECHST AKTIENGESELLSCHAFT By Its Patent Attorneys DAVIES COLLISON CAVE 25