DE69713565T2 - Thermally imageable material - Google Patents

Description

The invention relates to a thermally imageable material and a method for producing a printing form for offset printing using the material according to the invention.

The traditional printing plate production for offset printing is carried out by placing a film containing text and image information on a light-sensitive coated carrier material and then exposing it to radiation that contains components in the near UV and visible range. In positive-working materials, o-quinone diazides are mainly used as light-sensitive coatings, while in negative-working materials, polycondensates of diazonium salts or radically cross-linking photopolymer systems are common. These plates are normally processed under yellow light.

Newer developments make it possible to image pre-coated offset printing plates directly from digital data sets. In these so-called "computer-to-plate" processes, highly light-sensitive materials are irradiated image-wise with lasers that emit radiation in the visible range (488 nm, 532 nm). When processing such photopolymer-based printing plates, as described in BP 0 537 805 and EP 0 704 764, care must be taken to ensure that daylight or yellow light is strictly excluded. Other silver-based materials also require the printer to change their usual working methods, as these plates exhibit unconventional behavior in the printing machine (ink acceptance, print run).

Recently, the possibility of using longer-wave emitting lasers for digital printing plate imaging has opened up, with laser diodes in the emission range of 780 to 830 nm and solid-state lasers in the emission range of 1060 or 1064 nm being the focus of particular interest, as they are available with high performance at relatively low cost. The thermally imageable offset printing plates required for this type of exposure can in principle be processed under daylight if they are appropriately sensitized. The aim is to achieve a conventional printing technology behavior of printing forms produced in this way in the printing machine, so that the printer can retain tried and tested working methods.

Thermally imaged materials for the production of offset printing plates are known.

The printing plate material described in EP 0 580 393 is a two-layer material for waterless offset printing that does not require a development step, in which a silicone top layer is removed (ablated) from the hydrophilic base layer by infrared irradiation.

EP 0 625 728 and EP 0 672 954 describe a printing plate material consisting of one layer that can be imaged in both the infrared and UV ranges and can be used for reversal processing. The acid released from a photoacid donor sensitized to the IR range upon exposure initiates the cross-linking of a mixture of resole resin and novolak and is completed by a post-heating step. For longer runs, it is necessary to bake this material, as is known for diazo layers from British patent specification No. 1 154 749.

EP 0 720 057 claims a two-layer material which consists of a support, a photosensitive layer and a cover layer. The support preferably consists of surface-roughened and anodically oxidized aluminum. The photosensitive layer can be a positive or negative working layer. The negative working layer generally comprises a diazonium salt, an addition-polymerizable, unsaturated compound and a photopolymerization initiator. On this photosensitive layer there is a cover layer which has as essential components a binder which is soluble in water or in aqueous alkali and an IR-absorbing Dye and another dye that absorbs in the wavelength range in which the light-sensitive layer is sensitive. The cover layer is removed (ablated) by the action of IR radiation. The non-removed areas of the cover layer act as an exposure mask during the subsequent flood exposure. In the non-masked areas of the light-sensitive layer, a photochemical reaction is triggered that changes the solubility of the corresponding layer areas in a developer liquid. In the masked areas, a photochemical reaction of the light-sensitive layer is prevented. During subsequent development, the mask and the soluble parts of the light-sensitive layer are removed.

The object of the invention is to provide an image-forming element for producing printing plates for waterless offset printing in a new and simple manner.

The present invention describes a thermally imageable material consisting of a carrier material and two layers (a) and (b) built up thereon in the order given:

a) a photopolymer layer (a) which is sensitive to radiation in the near UV or visible range, but is less sensitive to radiation in the infrared range, and

b) a layer (b) that is permeable to radiation in the near UV and visible range, but only slightly permeable to oxygen, which becomes more permeable to oxygen after infrared irradiation.

The following can be used as carrier materials for offset printing plates: mechanically or electrochemically roughened and, if necessary, anodized aluminum, which can also be chemically pretreated, e.g. with polyvinyl phonic acid, silicates or phosphates, as well as multi-metal plates with Cu/Cr or brass/Cr as the top layer. Films made of other materials that have been hydrophilized by appropriate pretreatment, such as polyester films, are also suitable.

Radically crosslinkable photopolymer layers that absorb in the near UV or visible spectral range for use in offset printing plate layers have been known for a long time and have been widely used (see, for example, J. P. Fouassier, J. F. Rabek, Radiation Curing Science and Technology, Plenum, New York, 1993). Compared to other technologies used, photopolymer systems are characterized by short exposure times and high achievable print runs.

In principle, any photopolymer system that absorbs in the near UV or visible spectral range is suitable as a radically crosslinkable photopolymer layer (a), provided that its crosslinking ability in the presence of atmospheric oxygen is lower than in its absence. Suitable systems include

- a polymeric binder (A),

- a polymerizable compound (B) containing at least two ethylenically unsaturated units and

- a photoinitiator system (C) that generates radicals under the influence of radiation in the near UV or visible range as essential components.

Examples of usable polymeric binders (A) are poly(meth)acrylic acid alkyl esters in which the alkyl group is, for example, methyl, ethyl, n-butyl, isobutyl, n-hexyl or 2-ethylhexyl; copolymers of the above-mentioned (meth)acrylic acid alkyl esters with at least one monomer such as acrylonitrile, vinyl chloride, vinylidene chloride, styrene or butadiene; polyvinyl acetate, vinyl acetate copolymers, polyurethanes, methylcellulose, ethylcellulose, polyvinyl formal and polyvinyl butyral.

Particularly suitable are binders which are insoluble in water, soluble in organic solvents and soluble or at least swellable in aqueous alkaline solutions. Special mention should be made of binders which contain acidic groups, e.g. carboxyl groups or phenolic OH groups, such as copolymers of (meth)acrylic acid and/or its unsaturated homologues such as Crotonic acid, copolymers of maleic anhydride or its half esters, reaction products of hydroxyl-containing polymers with di- or tricarboxylic anhydrides and their mixtures, homo- or copolymers of hydroxystyrene.

The polymers described above are particularly suitable if they have a molecular weight between 500 and 200,000 or more, preferably 1,000 to 100,000 and either acid numbers between 10 and 250, preferably 20 to 200, or hydroxyl numbers between 50 and 750, preferably 100 to 500.

Preferred polymeric binders (A) are

- Copolymers of (meth)acrylic acid with alkyl (meth)acrylates, alkenyl (meth)acrylates, (meth)acrylonitrile or the like;

- vinyl acetate copolymers;

- Copolymers of crotonic acid with vinyl acetate, alkyl (meth)acrylates, alkenyl (meth)acrylates, (meth)acrylonitrile or the like;

- Copolymers of vinylacetic acid with alkyl (meth)acrylates and/or alkenyl (meth)acrylates;

- Copolymers of maleic anhydride with optionally substituted styrenes, unsaturated hydrocarbons, unsaturated esters or ethers;

- Esterification products of copolymers of maleic anhydride;

- Esterification products of polymers containing hydroxyl groups with anhydrides of di- or polycarboxylic acids, e.g. of copolymers of hydroxyalkyl (meth)acrylates with alkyl (meth)acrylates, (meth)acrylonitrile or the like, of polyurethanes with a sufficient number of OH groups, of epoxy resins, polyesters, partially saponified vinyl acetate copolymers, polyvinyl acetals with free OH groups, polyhydroxystyrene, copolymers of hydroxystyrenes with alkyl (meth)acrylates or the like;

- Phenol-formaldehyde resins, e.g. novolaks;

- as well as homo- or copolymers of hydroxystyrene.

The amount of the polymeric binder (A) in the radically crosslinkable photopolymer layer (a) is generally 5 to 80, preferably 25 to 60 wt.%.

Suitable polymerizable compounds having at least two ethylenically unsaturated units (B) are known and are described, for example, in US 2,760,863 and US 3,060,023.

Preferred examples are acrylic and methacrylic acid esters of di- or polyhydric alcohols such as ethylene glycol diacrylate, acrylates or methacrylates of trimethylolethane, trimethylolpropane, pentaerythritol and dipentaerythritol and of polyhydric alicyclic alcohols or N-substituted acrylic and methacrylic acid amides. Reaction products of mono- or diisocyanates with partial esters of polyhydric alcohols are also advantageously used. Such monomers are described in DE 2064079, DE 2361041 and DE 2822190.

Compounds which contain at least one photooxidizable and optionally at least one urethane group in the molecule can also be used. Examples of such polymerizable compounds are described in EP 287 818, EP 355 387 and EP 364 735.

The proportion of the polymerizable compounds (B) in the photopolymer layer (a) is generally about 20 to 90, preferably 35 to 70 wt.%.

Photoinitiator systems (C) which generate radicals under the influence of radiation in the near UV or visible range are known and are described, for example, in Chemistry and Technology of UV and EB formulations for coatings, inks and paints, Vol. 3, Sita Technology Ltd., London, 1991. The following can be used in the mixture according to the invention: α-carbonyl compounds (e.g. described in US 2 367 660); hexaarylbisimidazoles, also in a mixture with other photoinitiators (DE 40 09 700); triazines (US 4 189 323, EP 0 137 452) and acridine and phenazines (US 3 751 259) also in combination with each other or with other photoinitiators.

Particularly preferred for use in the photopolymerizable layer (a) are the representatives of the class of trihalomethyl-striazines, especially bistrichloromethyl-striazines, and 9-phenylacridines, also in combination with one another. If particularly high sensitivities in the visible spectral range are desired, the addition of a metallocene initiator system is recommended, especially a combination of metallocene and a trihalomethyl compound that can be split by radiation, as described in EP 364 735.

The proportion of the photoinitiator system (C) in the photopolymer layer (a) is generally about 2 to 15, preferably 5 to 10 wt.%.

The photopolymerizable layer (a) can contain small amounts of various substances as additives for special requirements such as flexibility, adhesion, gloss, etc. Examples are: inhibitors to prevent thermal polymerization of the monomers, hydrogen donors, dyes, colored and uncolored pigments, color formers, indicators, plasticizers, chain transfer agents and wetting agents.

Finally, soluble or finely dispersible dyes and, depending on the application, UV absorbers can be added to the light-sensitive mixtures. Triphenylmethane dyes have proven particularly useful as dyes. The most suitable proportions of the components can easily be determined in individual cases by preliminary tests.

Suitable solvents for applying the photopolymerizable layer (a) are ketones such as methyl ethyl ketone, chlorinated hydrocarbons such as trichloroethylene and 1,1,1-trichloroethane, alcohols such as n-propanol, ethers such as tetrahydrofuran, alcohol ethers such as ethylene glycol monoethyl ether and esters such as Butyl acetate. Mixtures can also be used which may also contain solvents such as acetonitrile, dioxane or dimethylformamide for special purposes. In principle, all solvents can be used which do not react irreversibly with the layer components.

However, the choice of solvent must be tailored to the intended coating process, the layer thickness and the drying device. Thin layers of up to approx. 5 µm are preferably applied for test quantities using a centrifuge by pouring. With solutions with a solids content of up to approx. 40%, layer thicknesses of more than 60 µm can be achieved with a single application or with a doctor blade. Dip coating is preferred for double-sided coating, whereby the use of low-boiling solvents allows for rapid drying. Coil coating is carried out by applying using rollers, slot nozzles or spraying; individual plates such as zinc and multi-metal plates can be coated using a curtain coater.

For drying after coating the radically crosslinkable photopolymer layer (a), the usual equipment and conditions can be used; temperatures around 100ºC and briefly up to 120ºC are tolerated without loss of radiation sensitivity.

The layer (b) which is permeable to radiation in the near UV and visible range, but only slightly permeable to oxygen, and which becomes more permeable to oxygen under the influence of radiation in the infrared range than before irradiation, preferably consists of a material which dissolves in the developer liquid or can be removed at least from the parts affected by infrared radiation during development. Suitable materials for this are, for example, polyvinyl alcohol, polyvinylpyrrolidone, polyphosphates, sugar, etc.

Suitable sensitizers are required to make the cover layer (b) sensitive to infrared radiation. One possible option is to use dyes that absorb in the infrared spectral range, such as those described in M. Matsuoka, Infrared Absorbing Dyes, Plenum Press, 1990, or in J. Fabian, Chem. Rev. 1992, 92, 1197 - 1226. Carbon black is preferred because this allows sensitization to be achieved with the same recipe for both the diode laser range (780-830 nm) and the solid-state laser range (1060 or 1064 nm).

The cover layer (b) is produced by dissolving or dispersing a preferably water-soluble resin and the infrared radiation-absorbing component in a suitable solvent and coating the resulting solution onto the photopolymer layer (a). Suitable solvents are those which do not react with the photopolymer layer (a) or do not dissolve it. A particularly preferred solvent is water, to which additives such as adhesion promoters or dispersing agents can be added in small amounts.

However, the choice of solvent must be tailored to the intended coating process, the layer thickness and the drying device. Thin layers of up to approx. 5 μm are preferably applied for test quantities using a centrifuge by pouring. With solutions with a solids content of up to 40%, layer thicknesses of more than 60 μm can be achieved with a single application or with a doctor blade. Dip coating is preferred for double-sided coating, whereby the use of low-boiling solvents allows for rapid drying. Coil coating is carried out by applying it using rollers, slot nozzles or spraying; individual plates such as zinc and multi-metal plates can be coated using a curtain coater.

For the top layer (b), basically any material that is permeable to radiation in the near UV and visible range and that changes its properties under the influence of infrared radiation appears to be suitable. Oxygen permeability is modified. Self-supporting cover layers made of suitably pretreated polyester film, which are removed before the radically crosslinkable photopolymer layer is developed, are also conceivable.

The material according to the invention is, in contrast to the described prior art, the concept of a chemical mask. In contrast to optical mask systems, as described in EP 0 720 057, for example, the cover layer (b) has a permeability to radiation in the wavelength range to which the photopolymer layer (a) is sensitive. The required differentiation for the flood exposure following the IR exposure is achieved by a chemical-physical change in the mask - the permeability to oxygen. The change in the solubility of the photopolymer layer (a) takes place in the areas protected by the cover layer (b), which has remained unchanged.

The processing of the material according to the invention takes place according to the following process, which is also part of the invention:

- The material according to the invention is irradiated imagewise with infrared radiation, e.g. a Nd:YAG laser (emission at 1064 nm), which is controlled by programmed line or raster movement, in order to increase the oxygen permeability of the cover layer (b).

- Flood exposure is then carried out using radiation in the near UV and/or visible spectral range in order to crosslink the radically crosslinkable photopolymer layer (a) in the areas that were not hit by the infrared radiation. In the areas where the cover layer (b) was hit by infrared radiation, crosslinking of the photopolymer layer (a) is not possible due to the increased access of oxygen to the same.

- If necessary, a post-heating step follows in order to complete the cross-linking of the photopolymer layer (a).

- In the development step, the cover layer (b) and the non-crosslinked parts of the photopolymer layer (a) are removed.

This can be done together in one developer fluid or in two separate steps, possibly with two different developer fluids.

For imaging the material according to the invention, an apparatus such as that described in EP 0 580 394 can be used. Commercially available plate or film exposure units are also suitable, provided they are equipped with a laser or another suitable light source that emits in the near IR range (780-1100 nm).

For the flood exposure with radiation in the near UV and/or visible spectral range required after image exposure with infrared radiation, the usual copying devices such as tube lamps, xenon pulse lamps, metal halide-doped mercury vapor high-pressure lamps and carbon arc lamps can be used. Contact exposure with ordinary light bulbs is also possible. The exposure can also take place in a continuous process using a flood exposure unit installed upstream of the actual developing machine.

The flood-exposed layer (a) can - if necessary after a thermal post-treatment step to complete the cross-linking reaction - be removed together with the top layer (b) with practically the same developers as for commercially available photopolymer systems, or the layers according to the invention can be advantageously adapted in their exposure conditions to the known aids such as developers and programmed spray or immersion bath development devices. The aqueous developer solutions can contain, for example, alkali phosphates, silicates or hydroxides and also wetting agents and small amounts of organic solvents. In certain cases, solvent/water mixtures can also be used as developers. The most favorable developer can be determined by experiments with the layer used in each case. If necessary, development can be mechanically assisted.

It can be advantageous to remove the top layer in a step prior to the actual development. In the simplest case, this can be done by rinsing with water, to which additives such as surfactants can be added in small quantities if necessary. If necessary, the rinsing can be assisted mechanically.

The following examples show the preferred embodiments of the thermally imageable mixture according to the invention, without restricting the invention to these.

example 1

A solution from

5.5 pbw of a copolymer of methyl methacrylate and methacrylic acid (ratio 64:36), which was reacted with glycidyl methacrylate in a molar ratio of 3:1 (acid number 85),

6.5 pbw of an aliphatic urethane acrylate (Ebecryl 1290 from UCB Chemicals),

0.2 Gt 2,4-bistrichloromethyl-6(4-styrylphenyl)-s-triazine

0.3 Gt 9-phenylacridine,

0.2 pbw of bis(cyclopentadienyl)-bis(2,6-difluoro-3-[pyrr-1-yl]-phenyl)titanium,

0.1 Gt Leucocrystal violet

60 Gt propylene glycol monomethyl ether and

27 Gt butanone

is applied to electrolytically roughened and anodized aluminum foil. The layer is dried for 2 minutes in a circulating air drying cabinet at 100ºC, resulting in a layer weight of 2 g/m². A 7% aqueous solution of polyvinyl alcohol, to which 0.1% carbon black in water-dispersible form (e.g. carbon black paste Derussol P130 from Degussa) has been added, is then applied in such a thickness that after drying (2 minutes in a circulating air drying cabinet at 100ºC) a layer weight of 1.5 g/m² is obtained.

The pre-coated printing plate produced in this way is imaged with an Nd:YAG laser with infrared radiation of wavelength 1064 nm in an external drum exposure device with an energy of 1500 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and in the visible range with an energy of 25 mJ/cm² and in a developer of the composition

1 part by weight of trisodium citrate · 2 H2 O,

2 parts by weight 1-aminopropan-2-ol,

1.4 Gt benzyl alcohol,

0.6 Gt sodium cumene sulfonate,

0.05 pbw of sodium metasilicate·5H2O,

0.02 Gt fatty alcohol polyglycol ether and

94 Gt water

developed. You get a perfect reproduction of the image parts, while the non-image areas are developed away without leaving any residue.

Example 2

A solution from

4.5 pbw of a copolymer of methyl methacrylate and methacrylic acid (weight ratio 85:15, acid number 115),

3.2 pbw of the reaction product of 1 mole of hydroxyethylpiperidine, 2 moles of hexamethylene diisocyanate and 2 moles of hydroxyethyl methacrylate,

2.4 pbw of the reaction product of 1 mole of 2,2,4- trimethylhexane-1,6-diisocyanate and 2 moles of hydroxyethyl methacrylate,

0.2 Gt 2,4-bistrichloromethyl-6-(4-styrylphenyl)-s-triazine,

0.2 Gt 9-phenylacridine,

0.1 Gt leuco crystal violet,

60 Gt propylene glycol monomethyl ether and

27 Gt butanone

is applied to electrolytically roughened and anodized aluminum foil. The layer is dried for 2 minutes in a circulating air drying cabinet at 100ºC, resulting in a layer weight of 2.3 g/m². A 7% aqueous solution of polyvinyl alcohol, to which 0.5% carbon black in water-dispersible form (e.g. carbon black paste Derussol P130 from Degussa) has been added, is then applied in such a thickness that after drying (2 minutes in a circulating air drying cabinet at 100ºC) a layer weight of 1.5 g/m² is obtained.

The resulting pre-coated printing plate is imaged with an Nd:YAG laser using infrared radiation with a wavelength of 1064 nm in an external drum exposure device with an energy of 300 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and visible range with an energy of 50 mJ/cm², reheated in a circulating air drying cabinet for 1 minute at 100ºC and developed in a developer with the composition of Example 1. When clamped into a printing machine, the printing plate produced in this way produces more than 50,000 perfect prints.

Example 3

A 7% aqueous solution of polyvinyl alcohol, to which 0.5% by weight of carbon black in water-dispersible form (e.g. carbon black paste Derussol P130 from Degussa) and 0.2% by weight of nitrocellulose dissolved in propylene glycol monomethyl ether have been added, is applied to a photopolymer layer produced and applied analogously to the first coating step in Example 2, in such a thickness that after drying (2 minutes in a circulating air drying cabinet at 100°C) a layer weight of 1.5 g/m² is obtained.

The resulting photosensitive coating is imaged with an Nd:YAG laser using infrared radiation of wavelength 1064 nm in an external drum exposure device with an energy of 300 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and visible range with an energy of 25 mJ/cm², in the The paper is then reheated in a circulating air drying cabinet for 1 minute at 100ºC and developed in a developer having the composition of Example 1, whereby a perfect reproduction of the digital image is achieved.

Example 4

The pre-coated printing plate produced analogously to Example 2 is imaged with a laser diode with infrared radiation of wavelength 830 nm in an external drum exposure device with an energy of 250 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and visible range with an energy of 50 mJ/cm², reheated in a circulating air drying cabinet for 1 minute at 100ºC and developed in a developer with the composition of Example 1. When clamped into a printing machine, the printing form produced in this way produces more than 50,000 perfect prints.

Example 5

The pre-coated printing plate produced analogously to Example 2 is imaged with an Nd:YAG laser with infrared radiation of wavelength 1064 nm in an internal drum exposure device with an energy of 360 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and visible range with an energy of 50 mJ/cm², reheated in a circulating air drying cabinet for 1 minute at 100ºC and developed in a developer of the composition of Example 1, whereby a perfect printing form is obtained.

Example 6

A solution from

6 pbw of a copolymer of methyl methacrylate and methacrylic acid (ratio 64:36), which was reacted with glycidyl methacrylate in a molar ratio of 3:1 (acid number 85),

3 pbw of the reaction product of 1 mole of hydroxyethylpiperidine, 2 moles of hexamethylene diisocyanate and 2 moles of hydroxyethyl methacrylate,

0.04 Gt 2,4-bistrichloromethyl-6-(4-styrylphenyl)-s-triazine,

0.2 pbw of bis(cyclopentadienyl)-bis(2,6-difluoro-3-[pyrr-1-yl]-phenyl)titanium,

0.3 Gt Flexo Blue,

20 pbw of diethylene glycol dimethyl ether,

57 Gt propylene glycol monomethyl ether and

12 Gt butanone

is applied to electrolytically roughened and anodized aluminum foil. The layer is dried for 2 minutes in a circulating air drying cabinet at 100ºC, resulting in a layer weight of 2.1 g/m². A 7% aqueous solution of polyvinyl alcohol, to which 0.1% carbon black in water-dispersible form (e.g. carbon black paste Derussol P130 from Degussa) has been added, is then applied in such a thickness that after drying (2 minutes in a circulating air drying cabinet at 100ºC) a layer weight of 1.7 g/m² is obtained.

The resulting light-sensitive coating is imaged with an Nd:YAG laser with infrared radiation of wavelength 1064 nm in an external drum exposure device with an energy of 300 mJ/cm², flood-exposed in a copying frame using radiation in the near UV and visible range with an energy of 50 mJ/cm² and developed in a developer of the composition of Example 1, whereby a perfect image reproduction is obtained.

Claims (8)

1. Thermally imageable material consisting of a support material and two layers (a) and (b) built up thereon in the order given: a) a photopolymer layer which is sensitive to radiation in the near UV or visible range, but less sensitive to radiation in the infrared range, and b) a layer that is permeable to radiation in the near UV and visible range, but only slightly permeable to oxygen, and that becomes more permeable to oxygen after infrared irradiation. 2. Material according to claim 1, characterized in that the photopolymer layer (a) has a limited crosslinking ability when irradiated with access to oxygen compared to irradiation without access to oxygen. 3. Material according to one or more of claims 1 and 2, characterized in that the photopolymer layer (a) contains as essential components A) a polymeric binder, B) a polymerizable compound containing at least two ethylenically unsaturated units and B) contains a photoinitiator system that generates radicals under the influence of radiation in the near UV or visible range. 4. Material according to one or more of claims 1 to 3, characterized in that the layer (b) which is only slightly permeable to oxygen contains as essential components I) a polymeric binder and II) contains a substance that absorbs infrared radiation. 5. Material according to claim 4, characterized in that the infrared radiation absorbing substance (II) contains soot particles or consists essentially of soot. 6. Method for producing a printing form for offset printing, characterized in that a material according to one or more of claims 1 to 5 - is irradiated image-wise with infrared radiation in order to increase the oxygen permeability of the upper layer (b) at the irradiated areas; - is then flood-exposed to radiation in the near UV and/or visible range in order to trigger cross-linking of the lower layer (a) at the points where the oxygen permeability of the upper layer (b) is unchanged and - is developed in an aqueous alkaline developer to completely remove the upper layer (b) as well as the non-crosslinked areas of the lower layer (a). 7. A method for producing a printing form for offset printing, characterized in that a material according to one or more of claims 1 to 5 - is irradiated image-wise with infrared radiation in order to increase the oxygen permeability of the upper layer (b) at the irradiated areas; - is subsequently flood-exposed to radiation in the near UV and/or visible range in order to trigger cross-linking of the lower layer (a) at the points where the oxygen permeability of the upper layer (b) is unchanged; - is developed in an aqueous developer liquid or pure water to completely remove the upper layer (b) and - is developed in an aqueous alkaline developer to remove the non-crosslinked areas of the lower layer (a). 8. A method for producing a printing form for offset printing, characterized in that in the method according to one or more of claims 6 and 7 after the flood exposure step a post-heating step is inserted in which the material reaches a temperature of 70ºC to 150ºC, preferably 80ºC to 130ºC, particularly preferably 90ºC to 120ºC.