DE69116191T2 - Process for improving the adhesion to radiation-hardened layers - Google Patents

Description

The present invention relates to radiation-cured layers, in particular those based on multifunctional acrylates and epoxies, and in particular to radiation-cured layers in X-ray screens, more particularly to a method for improving the adhesion to radiation-cured layers.

Radiation-cured coatings are made from polymerizable compositions, usually mixtures of unsaturated prepolymers diluted with monomers or liquid oligomers, which are cured (polymerized) with electromagnetic radiation such as ultraviolet radiation (UV) (wavelength 150 to 450 nm, 4-6 eV) or accelerated electron beams (EB) (150 to 350 keV). These two types of high-energy radiation trigger the formation of free radicals during their interaction with organic material, starting the chain reaction polymerization. The mechanisms of propagation and termination are the same whether UV light or an EB is used, only the initiation mechanism is different in the two processes. UV-curable compositions contain a photoinitiator that produces initiating radicals during UV exposure. EB curing is initiated by the indiscriminate interaction of accelerated electrons with organic material, causing the crime of chemical bonding.

The different types of radiation-curable systems commonly used are unsaturated polyester resins, thiolpolyene systems, polyfunctional acrylates and epoxies. The latter system involves cationic polymerization, the other systems involve free radical polymerization.

Common commercial applications of radiation-curable systems include protective and decorative coatings on wood and cardboard, metal (e.g. beverage cans), plastic (e.g. vinyl covering), paper and cardboard (e.g. packaging, stapled books), paints, photopolymers (e.g. photoresist samples, printing plates), adhesives.

An overview of radiation-curable compositions and their applications can be found in "Fundamentals of Radiation Curing" by Degussa AG (May 1980), in Farbe + Lack, 9/1985, pages 812-815 by W. Brushwell, in Radiation Curing, February 1986, pages 4-10 by G.W. Gruber, in Polymers Paint Colour, Oct. 5. 1988, pages 81-89 by C. Decker, in Farbe + Lack, 5/1989, pages 324-326 by H. Sellmer and in the Lectures Notes of the "Symposium on Radiation Curing" organized by The Center for Professional Advancement in Amsterdam, 3-5 October 1988.

Radiation-cured layers are also used as protective layers for X-ray screens. The term X-ray screen includes conventional fluorescent screens as well as stimulable fluorescent screens.

In conventional X-ray photography, especially medical X-ray photography, an X-ray image is obtained by irradiating an object with X-rays that pass through a fluorescent screen that contains a layer of fluorescent phosphor on a support that, when excited by X-rays, absorbs the X-rays and converts them into visible light and/or ultraviolet radiation, by irradiating a film coated with a silver halide emulsion layer with visible light and subsequently developing the film. The X-ray films used generally contain a transparent film support coated on both sides with a silver halide emulsion layer. Normally two fluorescent screens are used, each attached to one side of the double-sided coated film. The film and the two screens are housed in a suitable cassette. Combinations of single-sided emulsion-coated films with a screen are often used in high-resolution applications (extremities, mammography).

In recent years, methods have been devised whereby images are produced directly from the phosphor layer without using a film coated with a silver salt.

A description of such methods can be found in US 3859527, US 4239968, US 4258264, US 4654533 and US 4710626. In such a method, a stimulable phosphor layer of a stimulable fluorescent screen is irradiated with X-rays transmitted through an object to accumulate radiant energy at certain areas of the object in accordance with the degree of radiation transparency and to form a latent image, after which the stimulable phosphor layer is scanned with a stimulating excitation light, the radiant energy accumulated at the respective areas is released as light emission, and thereafter an image is obtained according to the optical signals depending on the intensity of the emitted light. The image can be processed and the final image obtained can be reproduced as a hard copy or on CRT. The stimulable phosphor sheet does not serve to ultimately record image information, but only temporarily stores the information to produce the image or a similar element on an independently prepared final recording medium. Consequently, the stimulable phosphor sheet, which is erased after stimulation, can be used repeatedly.

Traditional (stimulable) fluorescent screens contain, in order, a support, an active layer containing a (stimulable) fluorescent phosphor dispersed in a suitable binder, and a protective topcoat or protective layer applied over the active layer to protect the active layer during use.

Because every X-ray screen is used frequently, it is important to provide it with an adequate protective cover that effectively protects the active layer from physical and chemical damage. This is especially important in stimulable X-ray screens where the screen is not encased in a cassette but is handled and used unprotected. The use of a radiation-cured layer as a protective covering layer in a (stimulable) X-ray screen, as described e.g. in EP 209358 and JP 86/176900, produces satisfactory results.

A noted disadvantage of radiation-cured layers, especially those made from multifunctional acrylates and epoxies, is the weak adhesion to these layers. This is important when, for example, in the packaging industry, packaging information is produced by a printing process with UV-curable inks and an overprint layer is used to protect the ink from abrasion and absorption, where the adhesion of the overprint layer to the UV-curable ink is weak. In X-ray screens, this weak adhesion also presents a problem if one wants to print certain information, e.g. regarding the type and speed of the screen, the manufacturing number or a bar code label, on the radiation-cured layer. The ink used to print this information and the outer layer used to protect the ink do not adhere strongly enough to the radiation-cured layer.

This weak adhesion applies to all types of materials that are to be adhered to the radiation-cured layers. The most significant extreme case is weak auto-adhesion, i.e. the adhesion of a radiation-curable layer to a radiation-cured layer.

In order to achieve adequate adhesion to a polymeric substrate, surface pretreatment is often required. A description of various surface pretreatments for plastics is given in "Surface Analysis and Pretreatment of Plastics and Metals", edited by D.M. Brewis, Applied Science Publishers Ltd., London, 1982. Pretreatment methods applied to radiation-cured films to improve the adhesion properties of polyolefin surfaces, such as treatment with oxidizing solutions (etching liquids) and electrical ('corona') discharge treatment, do not improve adhesion thereto.

The object of the present invention is to improve the adhesion to radiation-cured layers, in particular the layers made from multifunctional acrylates and epoxies.

A further object of the present invention is to improve the printability of radiation-cured cover layers of X-ray screens.

Other items will be apparent from the description below.

The present invention provides a method for improving the adhesion of a radiation-cured protective layer on a X-ray screen, wherein the layer is processed with a composition comprising an inorganic base, an organic base or a compound capable of generating a base in situ and having a pH of at least 7, so that the surface energy of the radiation-cured layer increases to at least 37 mN/m.

The method for determining the surface energy of the layer is to measure the contact angle that a liquid forms with the layer, i.e. the angle between the liquid/vapor interface and the liquid/solid interface from the point of three-phase contact of a drop of liquid on the layer surface in equilibrium.

A review of the contact angle measurement procedures and an analysis of the relevant thermodynamic equations is given by D.G. Rance in "Surface Analysis and Pretreatment of Plastics and Metals", edited by D.M. Brewis, Applied Science Publishers, London, 1982, pages 121-152 and by R.J. Good and A.W. Neumann in "Surface and Colloid Science", Vol 11, edited by R.J.Good and R.R. Stromberg, Plenum Press, New York, 1979, pages 1-91.

The contact angle can be obtained by drawing a tangent to the profile at the point of free-fiber contact, with the drop profile magnified either by image projection or photography. The contact angle is measured directly with a telescope fitted with a goniometer eyepiece or indirectly by measuring the angle, at which light emitted by a point source of radiation is reflected from the surface of a liquid drop at its point of contact with the solid. Another useful method for indirect measurement is the measurement of the drop dimensions, ie the height and base diameter of the drop. A suitable method is to measure the contact angle as the average value of the advancing and receding contact angles. This method is described by H. Bracke, F. De Bisschop and P. JOOS in Progr. Colloid Polym. Sc., no. 76 (1988), pages 251-259.

Below is the relevant equation used to calculate the surface energy of the layer (s), which is represented by the sum of the surface energy excited by dispersion interactions (ds) and the surface energy excited by polar interactions (ps) when measuring the contact angle

s(1 + cos) = 2(ds d1)0.5 + 2(ps p1)0.5

in which d1 and p1 are the disperse and polar components, respectively, of the surface energy of the liquid.

The measurement of equilibrium contact angles of two liquids of which d1 and p1 are known provides all information needed to solve the two equations formed by the above equation and to obtain the values ds and ps.

The Ps and ds values for the layer can also be determined by a graphical method based on contact angles of a number of clearly identified liquids.

Preferably, a processing composition with a pH greater than 10 is used. If a composition with a pH less than 10 is used, a longer processing time is required to obtain the desired surface energy.

If a composition with a weak physical self-spreading ability is used, processing also takes longer before a sufficient proportion of the surface of the radiation-cured layer comes into molecular contact with the processing liquid. For this purpose, a spreading agent such as a surfactant or alcohols can be added to the processing composition to reduce the surface tension of the composition.

The processing composition can be carried out by rubbing the surface of the layer with the composition or by immersing the layer in the composition. Depending on the contact time required to allow the surface energy of the layer to rise to the desired value, the layer is rubbed several times or immersed in the composition for a longer period of time. Not only the contact time in the wet state, but also the contact time in the dry state is important.

If the radiation-cured layer is a protective cover layer of an X-ray screen which is, for example, printed with some manufacturing information, the printing side of the ink-bearing ribbon can be treated with the composition of the invention prior to printing in an on-line or off-line configuration so that the processing and printing of the radiation-cured cover layer are carried out at the same time, with the ribbon and the cover layer being in contact with each other during the printing process.

Examples of alkaline compounds for use in the present invention include inorganic bases such as LiOH, KOH, NaOH, Ca(OH)2, Mg(OH)2, Ba(OH)2, Sr(OH)2, Pb(OH)2, Zn(OH)2, AgOH, NH4OH, NH2NH2 and organic bases such as primary, secondary and tertiary amines, quaternary ammonium hydroxide, urea, alkanediamines, hydroxyalkanamines, piperidine, morpholine, p-aminopyridine. Compounds which generate a base in situ can also be used, e.g. sodium, potassium and barium alkylate, Na2CO3, Na2SO3.

Preferred alkaline compounds are alkali and Alkaline earth metal hydroxides, preferably Ba(OH)₂ and quaternary ammonium hydroxides, preferably (CH₃)₄NOH.

Preferably, a surfactant such as C₇F₁₅COONH₄ or alcohols such as methanol, ethanol and isopropyl alcohol are added to the aqueous alkaline composition to improve the spreading of the processing liquid on the surface of the radiation-cured layer. The composition is applied to the radiation-cured layer by rubbing the layer one or more times or by immersing it in the composition for a while. The composition is then allowed to interact with the surface in a "wet" state for a while and with the surface in a "dry" state for a while. The application intensity and interaction time required to obtain the desired adhesion improvement depend on the pH of the composition, the spreadability of the composition, the type of radiation-cured layer and the type of substrate to be adhered.

The bond strength between an adherent substrate and a processed radiation-cured layer increases as the processing composition has a higher pH value and greater spreading ability and the processing time and processing temperature increase.

A typical radiation-curable composition contains a prepolymer, a reactive diluent monomer or oligomer, and in the case of a UV-curable composition, a photoinitiator. Typical amounts of these primary components are 30-100 wt% for the prepolymer, 0-70 wt% for the reactive diluent, and 0-10 wt% for the photoinitiator.

In addition to these primary components, additives can be added in an amount between 0 and 2% by weight. Surfactants, waxes, defoamers, plasticizers and stabilizers can be used as additives. Pigments can be added in an amount between 0 and 60% by weight.

Examples of prepolymers or oligomers with two or more unsaturated double bonds are: unsaturated polyesters, modified unsaturated polyesters such as urethane-modified unsaturated polyester, acrylurethane-modified unsaturated polyester and a liquid unsaturated polyester having an acrylic group as an end group, an acrylic polymer such as polyester acrylate, epoxy acrylate, silicone acrylate and urethane acrylate, a butadiene series polymer, an epoxy series polymer such as polyglycidyl ether of aliphatic polyol, bisphenol A (or F, S), diglycidyl ether, dicarboxylic acid epoxy cyclohexylalkyl and an epoxy having two or more cyclopentene oxide groups, polythiolpolyene resin.

The above-mentioned radiation-curable type resin can be used alone or as a mixture of two or more resins.

Diluent monomers are used in radiation-curable compositions to reduce viscosity and improve flow properties. These monomers play a role in the free radical curing process. Examples of the above-mentioned reactive diluent used for reducing the viscosity of the composition and improving the rate of radiation curing are monofunctional monomers such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, glycidyl methacrylate, n-hexyl acrylate, lauryl acrylate, etc., difunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, silicone diacrylate, neopentyl glycol, 1,4-butanediol diacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, pentaerythritol diacrylate, divinylbenzene, etc., tri- or polyfunctional monomers such as trimethylolpropane triacrylate, Trimethylolpropane trimethacrylate, pentaerythritol triacrylate, dipentaerythritol hexaacrylate, acrylates of ethylenediamine, aliphatic and aromatic urethane acrylates, etc.

If ultraviolet radiation is used as a radiation agent, the following elements are added: a Photoinitiator, which acts as a catalyst to start the polymerization by absorbing ultraviolet radiation energy, and a photosensitizer to accelerate the action of the photoinitiator by transferring energy and forming free radicals by interaction.

As the above-mentioned photoinitiator, carbonyl compounds are often used, for example, benzom ether series compounds such as benzomisopropyl, isobutyl ether, benzil ketal series compounds, ketoxime esters, benzophenone series compounds such as benzophenone, o-benzoylmethylbenzoate, acetophenone series compounds such as acetophenin, trichloroacetophenone, 1,1-dichloroacetophenone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2phenylacetophenone, thioxanthone series compounds such as 2-chlorothioxanthone, 2-ethylthioxanthone, and compounds such as 2-hydroxy-2-methylpropiophenone, 2-hydroxy-4-isopropyl-2-methylpropiophenone, 1-hydroxycyclohexylphenyl ketone, etc.

The following elements can be used as photoinitiators, particularly for polymers of the epoxy series: an aromatic onium salt, namely a diazonium salt such as a diazonium salt of a Lewis acid, a phosphonium salt such as a hexafluorophosphoric triphenylphenacylphosphonium salt, a sulfonium salt such as a tetrafluoroboric triphenylsulfonium, a hexafluoroboric triphenylsulfonium, and an iodonium salt such as a chlorodiphenyliodonium, etc. Sulfur compounds, azo compounds, halogen compounds, organic peroxides, etc. can also be used as photoinitiators.

The above photopolymerization initiator may be used alone or as a mixture of two or more.

Examples of photosensitizer are amine, nitrile and compounds of sulfur, phosphorus, nitrogen, chlorine, etc.

Equipment and methods for curing the curable coating compositions described herein using suitable radiation forms are well known, and any suitable radiation curing process may be used. For example, the layer may be cured using medium pressure Mercury vapor lamps or other sources of ultraviolet radiation of sufficient intensity. High energy ionizing radiation such as X-rays, π-radiation, β-radiation and accelerated electron beams may also be used to cure the coating. The radiation applied should typically be of sufficient intensity to penetrate substantially throughout the entire coating being cast. The total dose used should be sufficiently high to enable the radiation-curable coating composition to cure and thus form a solid coating.

UV radiation is more suitable for non-pigmented or lightly pigmented systems whose fairly thin films allow complete penetration of the radiation. For heavily pigmented layers, polymerization is best achieved by electron beam curing because EB curing can penetrate through thicker and light-tight films.

UV irradiation is normally carried out using medium pressure mercury vapor lamps or pulsed xenon lamps. These ultraviolet sources normally have a cooling system, a system for removing the ozone produced and a nitrogen inlet which ventilates the surface of the product to be cured during the irradiation process. Normally an intensity of 40 to 120 W/cm in the range of 200-400 nm is applied. An example of a commercially available ultraviolet source is the IST sold by Strahlentechnik, Oberboihingen, Germany.

Two types of electron beam accelerators are: high energy scanning types and low energy linear cathode types, also called electric curtain type accelerators. These accelerators are usually equipped with a nitrogen inlet. A dose in the range of 0.01 to 10 megarads is used. Examples of commercially available EB accelerators are the PILOT 200 and the CB175/60/380, both sold by Energy Sciences Inc., Geneva, Switzerland.

Curing times can be reduced to a minimum if the radiation source, the photoinitiator, the photoinitiator concentration, the prepolymer, the reactive diluent and the distance between the radiation source and the product to be cured are correctly selected. Curing times of about 1 second are possible and are particularly required in thin film layers. For thicker cured products, curing times of up to 5 minutes, preferably 1-2 minutes, are achievable.

An important application of radiation-cured layers is the formation of a protective coating on the phosphor layer of an X-ray screen.

An X-ray screen essentially contains a carrier and a phosphor layer cast thereon, whereby the phosphor layer contains a binder and a (stimulable) phosphor dispersed therein.

In a conventional fluorescent screen, the phosphor used is a fluorescent substance that emits ultraviolet radiation and/or visible light when excited by penetrating X-rays.

The phosphor may be selected from a variety of well-known X-ray fluorescent phosphors or phosphor particles which are state of the art. The phosphors include, for example, calcium tungstate, zinc sulfide, zinc cadmium sulfide, zinc oxide and calcium silicate, zinc phosphate, alkali halides, cadmium sulfide, cadmium selenide, cadmium tungstate, magnesium fluoride, zinc fluoride, strontium sulfide, zinc sulfate, barium lead sulfate, barium fluorohalides and mixtures of two or more of the above elements. The above phosphors may be activated with, for example, europium, silver, copper and nickel. Phosphors which are particularly suitable for use in high sensitivity fluorescent screens are fluorescent substances which contain elements with an atomic number of 39 or 57 to 71, which include rare earth elements such as yttrium, gadolinium, lanthanum and cerium. Particularly suitable are the fluorescent rare earth oxysulphide and oxyhalide materials activated with other certain rare earth elements, e.g. lanthanum and gadolinium oxybromide and oxychloride activated with terbium, ytterbium or dysprosium, lanthanum and gadolinium oxysulfides activated with terbium, europium, or a mixture of europium and sulfur, yttrium oxide activated with gadolinium, europium, terbium or thulium, yttrium oxysulfide activated with terbium or a mixture of terbium and dysprosium, yttrium tantalate doped with small amounts of terbium or strontium or lithium or a mixture of these elements, and activated with thulium, niobium, europium, gadolinium, or neodymium. These and other fluorescent rare earth materials are described in detail in the literature, for which we refer, for example, to the following publications EP 11909, EP 202875, EP 257138, DE 1282819, DE 1952812, DE 2161958, DE 2329396, DE 2404422, FR 1580544, FR 2021397, FR 2021398, FR 2021399, UK 1206198, UK 1247602, UK 1248968, US 3546128, US 3725704, US 4220551, US 4225653, to KA Wickersheim et al. "Rare Earth Oxysulfide X-ray Phosphors", in the Proceedings of the IEEF Nuclear Science Symposium, San Francisco, October 29-31, 1969, on SP Wang et al., IEEE Transactions on Nuclear Science, February 1970, pp. 49-56, and RA Buchanan, IEEE Transactions on Nuclear Science, February 1972, pp. 81-83. A review of blue- and green-emitting phosphors is given in EP 88820.

The use of a plurality of phosphor layers with a different composition or of an X-ray photographic screen containing a mixture of different phosphors makes it possible to obtain fluorescence over the entire visible spectrum. Consequently, such a combination is particularly suitable for a recording process with silver halide recording elements which have been made sensitive to light over the entire visible spectrum.

The one used in a stimulable fluorescent screen, A stimulable phosphor is a phosphor that can emit stimulated fluorescent light when it is irradiated with a stimulating excitation light after X-ray irradiation. In terms of practical use, a stimulable phosphor is preferred that should produce a stimulated emission in the wavelength range of 300 to 500 nm when it is excited by stimulating radiation in the wavelength range of 400 to 900 nm. Stimulable phosphors described in US 4,825,085, which emit in the wavelength range of around 600, can also be used. Suitable stimulable phosphors are described, for example, in the following patent specifications: EP 304121, EP 345903, EP 353805, EP 382295, US 3859527, US 4236078, US 4239968, JP 73/80487, JP 73/80488, JP 73/80489, JP 76/29889, JP 77/30487, JP 78/39277, JP 79/47883, JP 80/12142, JP 80/12143, JP 80/12144 = US 4236078, JP 80/12145, JP 80/84389, JP 80/160078, JP 81/116777, JP 82/23673, JP 82/23675, JP 82/148285, JP 83/69281 and JP 84/56479. The divalent europium activated alkaline earth metal halide phosphors and rare earth element activated rare earth oxyhalide phosphors are particularly preferred because of their stimulated emission of high luminance.

The stimulable fluorescent screen can contain a group of stimulable phosphor layers in which one or more stimulable phosphor layers containing at least one of the above-mentioned stimulable phosphors are incorporated. The stimulable phosphors contained in different stimulable phosphor layers can be either identical or different.

The (stimulable) phosphors can be used in any conventional particle size range and distribution. It is well known that a smaller average particle size results in sharper images and reduced noise, but also results in weaker light emission. The optimal average particle size for a given Application is therefore a compromise between the desired image sharpness and image generation sensitivity.

The (stimulable) phosphors are present in the form of a layer cast on a carrier or are applied as a self-supporting layer or a self-supporting sheet.

Although it is recognized that the (stimulable) phosphor layer should not contain a separate binder, in most applications the phosphor layers contain sufficient binder to impart structural coherence to the layer. The binders that can be used in practice are generally those traditionally used in the art and include proteins such as gelatin, polysaccharides such as dextran, gum arabic, and synthetic polymers such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethyl cellulose, a vinylidene chloride-vinyl chloride copolymer, polyalkyl (meth)acrylate, a vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate, cellulose acetate butyrate, polyvinyl alcohol, polystyrene, polyester, etc. These and other useful binders are described in US 2502529, US 2887379, US 3617285, US 3300310, US 3300311, US 3743833 and RD 15444. A mixture of two or more of these binders, e.g. a mixture of polyethyl acrylate and cellulose acetobutyrate, can be used.

Typically, the active layer contains (stimulable) phosphor particles in a significantly larger amount than a binder and is considerably thick in terms of sensitivity.

The ratio between the binder and the (stimulable) phosphor can be determined according to the properties of the X-ray screen to be obtained and the type of phosphor used. In general, the weight ratio is between 1:1 and 1:100 (binder:phosphor), preferably between 1:10 and 1:25.

The thickness of the (stimulable) phosphor layer, which can vary depending on the radiation sensitivity of the X-ray screen, the type of (stimulable) phosphor, etc., can be between 10 and 1,000 µm, preferably between 50 and 500 µm and even better between 150 and 250 µm.

Two or more phosphor layers with a different thickness and/or a different binder-phosphor ratio and/or a different phosphor particle size can be used.

Examples of the support material include plastic films such as films of cellulose acetate, polyvinyl chloride, polyvinyl acetate, polyacrylonitrile, polystyrene, polyester, polyethylene terephthalate, polyamide, polyimide, cellulose triacetate and polycarbonate, metal foils such as aluminum foil and aluminum alloy foil, common papers, baryta paper, resin-coated papers, pigment papers containing titanium dioxide or the like, and paper sized with polyvinyl alcohol or the like. Preferably, a plastic film is used as the support material.

The plastic film may contain a light-absorbing material such as carbon black or a light-reflecting material such as titanium dioxide or barium sulfate. The light-absorbing material is suitable for making an X-ray photography screen with a high resolving power, while the light-reflecting material is suitable for making a high-sensitivity X-ray photography screen.

Examples of preferred supports include clear or blue-colored or black-colored polyethylene terephthalate (e.g., LUMIRROR C, Type X30 sold by Toray Industries, Tokyo, Japan) and TiO2 or BASO4-filled polyethylene terephthalate.

The thickness of these supports can vary depending on the carrier material and can generally be between 60 and 1,000 µm, preferably between 80 and 500 µm to ensure easy handling.

The (stimulable) phosphor layer can, for example, formed on the carrier by the following process.

(Stimulable) phosphor particles and a binder are added to a suitable solvent and then mixed to form a casting dispersion containing the phosphor particles homogeneously dispersed in the binder solution.

Examples of the solvent usable in preparing the casting dispersion include lower alcohols such as methanol, ethanol, n-propanol and n-butanol, chlorinated hydrocarbons such as methylene chloride and ethylene chloride, ketones such as acetone, butanone, methyl ethyl ketone and methyl isobutyl ketone, esters of lower alcohols with lower aliphatic acids such as methyl acetate, ethyl acetate and butyl acetate, ethers such as dioxane, ethylene glycol monoethyl ether, methyl glycol, and mixtures of the above compounds.

The casting dispersion may contain a dispersant to improve the dispersibility of the phosphor particles in the dispersion, and may contain various additives such as a plasticizer to enhance the adhesion between the binder and the phosphor particles in the phosphor layer.

Examples of the dispersant include known ionic and non-ionic dispersants or combinations thereof, e.g. GAFAC RM 610 sold by General Aniline and Film Company (GAF), New York, USA, a sorbitan monopalmitate and monolaurate of polyoxyethylene (20), a polymeric surfactant such as an acrylic graft copolymer, PHOSPHOLIPON 90 sold by Nattermann-Phospholipid GmbH, Cologne, Germany, silane-based dispersants e.g. SILANE Z6040 sold by Dow Corning Corporation, Midland, Michigan, USA, or glymo-3-glycidyloxypropylmethoxysilane or organosulfate polysilanes, unsaturated p-amine amide salts and high molecular weight acid esters such as ANTI TERRA U 80 sold by BYK-Chemie GmbH, Wesel, Germany). high molecular weight, unsaturated polyesters. Dispersants are added in an amount of 0.05 to 10% by weight in relation to the phosphor.

Examples of the plasticizer include phosphates such as triphenyl phosphate, tricresyl phosphate and diphenyl phosphate, phthalates such as diethyl phthalate and dimethoxyethyl phthalate, glycolates such as ethyl phthalylethyl glycolate and butyl phthalylbutyl glycolate, polymeric plasticizers, and polyesters of polyethylene glycols with aliphatic dicarboxylic acids such as polyester of triethylene glycol with adipic acid and polyester of diethylene glycol with succinic acid.

The casting dispersion may also contain a filler (reflective or absorbing) or may be colored with a dye capable of absorbing light within the spectrum emitted by the phosphor or, in the case of a stimulable fluorescent screen, of absorbing excitation light. Examples of dyes include Solvent Orange 71 (Diaresin Red 7), Solvent Violet 32 (Diaresin Violet A), Solvent Yellow 103 (Diaresin Yellow C) and Solvent Green 20 (all four sold by Mitsubishi Chemical Industries, Japan), Makrolex Red GS, Makrolex Red EG, Makrolex Red E2G, Helioechtgelb 4G and Helioechtgelb HRN (all five sold by Bayer AG, Leverkusen, Germany), Neozaponfeuerrot G and Zaponechtbraun BE (both sold by BASF, Ludwigshafen, Germany).

The coating dispersion containing the phosphor particles and the binder prepared as described above is evenly coated on the support surface to form a layer of the coating dispersion. The coating can be carried out by any conventional method such as the doctor blade coating method, the dipping method or the roller coating method.

After the casting dispersion has been cast onto the carrier, it is slowly heated to dryness in order to complete the formation of a phosphor layer.

Ultrasonic treatment can be performed to improve the volume factor and deaerate the phosphor-binder combination.

The phosphor layer may be coated on the support by methods other than those mentioned above. For example, the phosphor is initially prepared using the above-mentioned coating dispersion on a sheet (false support) such as a glass plate, metal plate or plastic sheet, after which the phosphor layer thus prepared is applied to the real support by pressing or with an adhesive. The phosphor layer may also be applied to the support by a method such as vapor deposition, spraying, sputtering, etc.

In the manufacture of an X-ray screen, one or more additional layers are optionally incorporated between the support and the phosphor layer to improve the adhesion between the support and the phosphor layer or to improve the screen sensitivity or the quality of an image thereby provided. For example, an adhesive layer can be incorporated by coating a polymeric material such as gelatin over the support surface on the phosphor layer side. A light-reflecting or a light-absorbing layer can also be incorporated by forming a polymeric material layer containing a light-reflecting material such as titanium dioxide or a light-absorbing material such as carbon black. Such a layer can be coated either as a backing layer on the support or as an intermediate layer between the support and the active layer. One or more of these additional layers can be coated on the support.

The radiation-cured protective cover layer is cast onto the surface of the (stimulable) phosphor layer not opposite the carrier.

This protective coating is applied by a process in which a casting liquid containing at least one resin of the radiation-curable type is applied to the front side of the (stimulable) phosphor layer is applied and irradiated with ultraviolet radiation or an electron beam to dry the casting liquid.

The monomers and prepolymers are selected so that they enter into a cross-linking reaction to form a solvent-insoluble layer with good abrasion resistance.

The radiation-curable type resin is preferably a urethane acrylate, even better would be an aliphatic urethane acrylate. The radiation-cured layer based on urethane acrylates is known for its abrasion resistance.

To the polymer, i.e., the above-mentioned radiation-curable type resin, a vinyl monomer as a reactive diluent, a non-reactive binder, a crosslinking agent, a photoinitiator, a photosensitizer, a storage stabilizer, a dye and other additives may be added if necessary and then dispersed therein to form the coating liquid for the protective layer. Examples of dyes that can be used in the protective layer include MAKROLEX ROT EG, MAKROLEX ROT GS and MAKROLEX ROT E2G, all three of which are sold by Bayer, Leverkusen, Germany.

As a reactive diluent, preferably a bifunctional acrylate, e.g. hexanediol diacrylate, is added in an amount between 0 and 50% by weight, preferably 10 and 30% by weight. Multifunctional acrylates would produce hard and brittle layers.

Curing by an electron beam is particularly preferred in X-ray screen applications. The photoinitiator to be added to the casting solution if ultraviolet radiation is used as a curing source will also absorb the light emitted by the phosphor to some extent and thus reduce the sensitivity of the X-ray screen, especially if a blue-emitting or ultraviolet-emitting phosphor is used. In this case, the electron beam is preferred. If a green-emitting phosphor is used, a photoinitiator should therefore be selected whose Absorption spectrum overlaps the emission range of the phosphor only to a small extent. A preferred photoinitiator is 2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR 1173 sold by E. Merck, Darmstadt, Germany).

The amount of photoinitiator used is preferably between 0.01 and 5 parts by weight relative to 100 parts by weight of prepolymer. The amount of photoinitiator used is particularly preferably between 0.5 and 3 parts by weight and in the range of 3 to 7 times the amount of radical generating compound used.

A variety of other additional materials may be incorporated into the surface of the X-ray screen, e.g. agents for reducing the accumulation of static electric charge, plasticizers, matting agents, lubricants, defoaming agents and the like.

Examples of lubricants to be added include silicones such as SURFACTANT 190, distributed by Dow Corning Gorporation, Midland, Michigan, USA, fluorine-containing compounds such as polytetrafluoroethylene and LANCO WAX, distributed by Georg M. Langer & Co., Bremen, Germany, waxes such as ACRAWAX, distributed by Glyco Products, New York, USA and LANCO GLIDD, distributed by Georg M. Langer & Co., Bremen, Germany. The lubricants are added in amounts between 0.01 and 0.5 wt.%.

Examples of defoamers to be added include LANCO ANTIBUBBLE L and LANCO FOAMSTOP PL, both sold by Georg M. Langer & Co., Bremen, Germany. Although antistatic agents are normally incorporated into the X-ray element that comes into contact with the X-ray screen, small amounts of conventional antistatic agents can be added to the cover layer or active layer. For the conventional fluorescent screens in particular, static electricity normally builds up during insertion into and removal of the film from the area or cassette containing the X-ray screens. Exposure of the sensitive photographic film is known to cause static marks, which is unacceptable.

Particularly preferred antistatic agents for incorporation into the phosphor layer or into the radiation-cured protective top layer and also for incorporation into other types of protective layers made from an organic film-forming polymer such as cellulose nitrate, cellulose acetate, polymethyl methacrylate and similar products are polyethylene oxides, which preferably correspond to the formula RO-(CH₂CH₂O)n-H, where n = 2 and R = cetyl or stearyl or oleyl. These compounds are added in an amount of 0-10% by weight, preferably 2-4% by weight. The use of these compounds in combination with anionic or cationic antistatic agents (e.g. quaternary ammonium salt) results in a synergistic effect.

The thickness of the protective layer produced by the above-mentioned process can be between 1 and 100 µm, preferably between 2 and 20 µm.

To form the protective layer, the composition is applied to a suitable wet thickness (preferably between 60 and 100 µm) and dried by irradiation to convert it into a solid form. Any suitable method for applying a thin, uniform layer of the composition may be used. Examples of suitable coating methods include the dipping method, the air knife method, the roll coating method, the extrusion coating method, the wave coating method, the curtain coating method, and so on.

X-ray screens as described above can also be in the form of stepwise screens, i.e. screens that have a stepwise or imagewise varying intensity across their width and/or length. This can be achieved by gradually varying the thickness of the phosphor layer or the amount of dye that can absorb the light emitted by the phosphor into the protective layer.

The following examples illustrate the present invention without limiting it thereto.

EXAMPLE 1 : Determination of surface energies

An X-ray screen provided with a radiation-hardened layer (A) is manufactured as follows.

Green-emitting gadolinium oxysulfide phosphor (80 wt.%) is dispersed with a dispersant in a slightly viscous pre-solution (20 wt.%) with a low binder content while stirring for 1 minute at a speed of 1700 rpm. The pre-solution consists of 7 wt.% polyethyl acrylate binder (PLEXISOL B 372 sold by Röhm GmbH, Darmstadt, Germany), 18 wt.% ethyl acetate, 50 wt.% methyl ethyl ketone, 25 wt.% methyl glycol and 0.5 wt.% GAFAC RM 610 (sold by GAF, New York, USA); The remaining amount of polyethyl acrylate binder and ethyl acetate solvent are then added to the phosphor pre-dispersion to obtain a solution with a solids content of 70%, with 89% by weight of phosphor based on 11% by weight of binder.

The resulting phosphor dispersion is applied to a black, subbed polyethylene terephthalate carrier (wet layer thickness 900 µm) using the doctor blade method. After evaporation of the solvent, a 160 µm thick phosphor layer is obtained.

A radiation-curable layer comprising 80% by weight of a mixture of 80% of an aromatic urethane acrylate (EBC 220 sold by UCB, Drogenbos, Belgium) and 20% of an aliphatic urethane acrylate (EBC 264 sold by UCB, Drogenbos, Belgium), 15% by weight of hexanediol diacrylate, 4.5% by weight of photoinitiator (DAROCUR 1173 sold by E. Merck, Darmstadt, Germany) and 0.5% by weight of a silicone surface-active compound is prepared. To this composition is added ethanol as solvent in an amount such that a 25% by weight solution is obtained.

The composition is coated onto the phosphor layer by the immersion method (wet layer thickness of about 100 µm). After evaporation of the solvent, a A top layer with a thickness between 10 and 15 µm is obtained. The layer is cured by UV radiation using a Labcure unit (O₃ removal, air cooling, power output of 80 W/cm, speed 5 m/min, distance UV source-substrate 11 cm) distributed by Technigraf Gmbh, Grävenwiesbach, Germany.

The X-ray screen itself, i.e. the untreated X-ray screen, is referred to below as (I).

A X-ray screen prepared in the same way is prepared with a composition containing a saturated solution of barium hydroxide in water (50% by volume), isopropanol (20% by volume) and methanol (30% by volume) as follows. A cotton cloth is soaked in the processing liquid, after which the cloth is rubbed once over the radiation-cured layer. The interaction time is 5 minutes in the wet state and 12 hours in the dry state. The processed X-ray screen thus obtained is referred to below as (II).

A X-ray screen similar to (I) is processed with a composition containing 5% urea in water as follows. The screen is immersed in the processing liquid at 50ºC for 15 minutes and then rubbed to dryness. The interaction time is 12 hours in a dry state. The processed X-ray screen thus obtained is hereinafter referred to as (III).

A X-ray screen similar to (I) is processed with a composition containing 5% urea in water as follows. A cotton cloth soaked in the processing liquid is rubbed over the screen once, after which the screen is dried. The interaction time is 12 hours in a dry state. The processed X-ray screen thus obtained is referred to below as (IV).

A X-ray screen similar to (I) is processed with a composition containing 25 ml of tetramethylammonium hydroxide 25%, 25 ml of water and 50 ml of ethanol as follows. The screen is immersed in the processing liquid for 5 minutes immersed, then dried and rinsed with water. The interaction time is 12 hours in the dry state. The processed X-ray screen thus obtained is referred to below as (V).

A X-ray screen similar to (I) is processed with a composition containing a saturated solution of barium hydroxide in water (50% by volume), isopropanol (20% by volume) and methanol (30% by volume) as follows. A cotton cloth is soaked in the processing liquid, after which the cloth is rubbed once over the radiation-cured layer. The interaction time is 5 minutes in the wet state and 1 minute in the dry state. The processed X-ray screen thus obtained is referred to below as (VI).

The surface energy of each of the above radiation-cured screens (I) to (VI) is determined by measuring the contact angle between the surface of the radiation-cured layer and a series of well-defined liquids. The contact angle is obtained by drawing a tangent to the magnified profile of a drop of each liquid present on the surface.

The contact angles in o obtained for each processed layer and the reference liquids are listed in Table 1 below. Table 1 Screen Liquid Water Glycerol Formamide Methylene iodide Tricresyl phosphate Hexadecane

The obtained surface energy in mN/m of each radiation-cured layer (s) and its disperse (ds) and polar (ps) components as calculated based on the measured contact angles listed in Table 1 above and the values d1 and p1 of the reference liquids mentioned below in mN/m are listed in Table 2.

The contact angles obtained for water and formamide are not used to calculate the surface energy of the layer. Reference liquid Water Glycerol Formamide Methylene iodide Tricresyl phosphate Hexadecane Table 2

For comparison: s of a non-radiation-cured protective layer of an X-ray screen (i.e. a cellulose acetate butyrate layer) is 42 mN/mj, ds = 29 mN/m and ps = 12 mN/m. There are no problems with the adhesion to the layers.

By immersing the unprocessed layer (I) in water, a break-off into separate water drops is obtained. Immersing the above-mentioned processed layers (II) to (VI) in water, on the other hand, leads to spreading and the formation of a homogeneous water film on the surface, which indicates that the surface energy of the layer increases compared to (I).

EXAMPLE 2 : Correlation with the peel test

On each of the above layers (I) to (VI) is sprayed a thin layer (about 10 µm) consisting of a polymethyl methacrylate varnish (containing 10 g of DEGALAN 62/03 distributed by Degussa AG, Frankfurt, Germany, 40 g of methyl ethyl ketone, 40 g of toluene, 20 g of butyl acetate and 0.15 g of surfactant) or a nitrocellulose varnish (containing 10 g of WALSRODER NITROCELLULOSE E510 distributed by Wolff Walsrode AG, Walsrode, Germany, 20 g of ethyl acetate, 40 g of toluene, 40 g of butyl acetate and 0.15 g of surfactant). After drying, a piece of tape (Tesatape Type 4101, distributed by Beiersdorf AG, Hamburg, Germany) is firmly applied to this layer and then abruptly peeled off.

The adhesion of the paint to the radiation-cured layer is checked visually by comparing the processed and non-processed layers by peeling them off the paint layer together with the tape.

No improvement in adhesion compared to the untreated layer is indicated with - (the entire paint layer under the tape is removed directly and easily). A strong improvement is indicated with +++ (no removal of the paint layer under the tape). Table 3 Peel test

EXAMPLE 3 : Correlation with printability

The manufacturing number is printed on each of the above layers (II) and (V) in a thermal printing press of the GEBA 1P type distributed by Gebr. Baier KG, Schlechtbach, Germany, or of the Pressar P16 type distributed by MADAG Maschinen und Apparatebau Dietikon AG, Dietikon, Switzerland, using a black ink of the type Colarit VL912 94er and a coding foil CO512 (both distributed by Leonard Kurz GmbH & Co., Fürth, Germany). The following parameters are set: temperature 150ºC, pressure 300 kg, time 0.35 sec.

The paint adheres sufficiently strongly to the layer. In addition, a layer of polymethyl methacrylate paint (DEGALAN as described in example 2) can be applied to protect the paint from the optional screen cleaning agent used afterwards.

EXAMPLE 4 : Influence of pH value

An X-ray screen provided with a radiation-cured layer (B) is prepared as in Example 1 with the difference that no surface-active silicone agent is added to the radiation-cured layer.

Several identical screens are processed with different compositions, the surface being rubbed with the composition a number of times (n). The composition is allowed to interact for some time with the film surface in the wet state (wet) and in the dry state (dry). The improvement in adhesion is checked by the peel test as described above in Example 2. Table 4 Composition wet dry Adhesion Ethanol Isopropanol Urea

The above results show that a greater improvement in adhesion is obtained when processing is carried out under identical conditions but with compositions with a higher pH value.

EXAMPLE 5 : Influence of the spreading ability of the processing composition

Various screens similar to the screen described in Example 4 are treated with different compositions, the surface being rubbed several times with the composition or the layer being immersed in the composition for some time at a certain temperature (*). The composition is allowed to interact for a while with the coating surface in wet (wet) and dry (dry) states. The improvement in adhesion is checked by the peel test as described above in Example 2. Table 5 Composition wet dry Adhesion Methanol/Isopropanol Urea Isopropanol

The The above results show that the improvement in adhesion depends on the spreading ability of the processing composition on the layer surface, ie on the surface energy of the composition itself. The spreading can be influenced by the addition of surface-active agents such as C₇F₁₅COONH₄ or alcohols such as isopropanol and methanol (ie the surface energy of the composition is reduced). The spreading can also be influenced artificially by immersing the layer in the composition or by wetting it several times.

EXAMPLE 6 : Influence of interaction time

Various screens, similar to either the screen provided with the radiation-cured layer (A) described in Example 1 or the screen provided with the radiation-cured layer (B) described in Example 4, are processed with various compositions, the surface being rubbed with the composition a number of times (n). The composition is allowed to interact with the layer surface for some time in the wet state (wet) and in the dry state (dry). The improvement in adhesion is checked by the peel test as described above in Example 2. Table 6 Composition Layer wet dry Adhesion Methanol/Isopropanol Methanol/Isopropanol

This Results show that the interaction time in wet condition is an important factor. While after a While a mediocre improvement in adhesion is obtained after a short interaction time, the improvement in adhesion after a longer interaction time is very good.

The results obtained by immersing the layer in the composition defined in Table 5 also demonstrate the effect of the interaction time.

EXAMPLE 7 : Influence of temperature

Several screens similar to the one described in Example 4 are treated with different compositions by rubbing the composition over the surface a number of times (n) or by immersing the layer in the composition for a certain time at a certain temperature (*). The composition is allowed to interact with the surface of the layer for a certain time in the wet state (wet) and in the dry state (dry). The improvement in adhesion is checked by the peel test as described above in Example 2. Table 7 Composition wet dry Adhesion Urea

The above results show that the improvement in adhesion increases as the temperature also increases.

EXAMPLE 8 : Influence of the radiation-cured layer

Various radiation-cured layers defined below are processed with different compositions by rubbing the composition over the surface a number of times (n) or by immersing the layer in the composition for some time at a certain temperature (*). The composition is allowed to interact with the layer surface for some time in the wet state (wet) and in the dry state (dry). In cases (**) the surface is then rinsed with water. The improvement in adhesion (Atr) is checked by the peel test as described above in Example 2. For comparison, the adhesion of the unprocessed layer (Antr) is also shown.

Layer A represents the layer described in Example 1.

Layer B represents the layer described in Example 4.

Layer C represents the layer used on the COPYPROOF packaging of Agfa-Gevaert N.V., Mortsel, Belgium.

Layer D represents a paint type UVN/70 from Entreprises Kurfürst, Brussels, Belgium.

Layer E is a urethane acrylate varnish of type B4655 distributed by Reliance Universal N.V., Mechelen, Belgium.

Layer F is an epoxy acrylate varnish of type V27885-1 (overprint varnish) from Jänecke & Schneemann, Drukinkten Arets, Wilrijk, Belgium.

Layer G represents an epoxy acrylate in tripropylene glycol diacrylate (EBC 605 distributed by UCB, Drogenbos, Belgium).

Layer H is an epoxy acrylate in hexanediol diacrylate (EBC 604 sold by UCB, Drogenbos, Belgium).

Layer I represents a type of varnish formulated by UCB, Drogenbos, Belgium for Velpa, Anderlecht, Belgium for use as a paper overprint varnish having the following composition: 19 parts epoxy acrylate EBC 605, 19 parts urethane acrylate EBC 210, 35 parts tripropylene glycol diacrylate, 14 parts hexanediol diacrylate, 5 parts benzophenone, 8 Parts of an unsaturated tertiary amine and a silicone lubricant.

Layer J represents an EB-cured protective coating for an X-ray screen as described in Example 1, except that instead of the gadolinium oxysulfide phosphor used in Example 1, a blue-emitting yttrium tantalate phosphor is used which contains 80% of a mixture of 20% of an aromatic urethane acrylate (EBC 220 sold by UCB, Drogenbos, Belgium) and 80% of an aliphatic urethane acrylate (EBC 264 sold by UGB, Drogenbos, Belgium), and 20% hexanediol diacrylate. The layer is cured by means of an electron beam in an ESI Labcure unit at 4 megarads, 150 kV and inerting with 100 ppm oxygen.

Layer K is an epoxy layer containing 53.3% CYRACURE UVR-6100 resin, 14.3% CYRACURE UVR-6200 diluent, 27.9% CYRACURE UVR-6379 flexibilizer, 4% CYRACURE UVI-6990 photoinitiator (all sold by Union Carbide Corporation, Danbury, USA) and 0.5% of a UV-cured flow control agent/surfactant (3 x 20 mlmin, 80 W/cm)

Layer L is a layer similar to layer K, with the difference that layer L is first UV-cured and then (to complete the curing) thermally cured at 130ºC for 7 minutes. Table 8 Composition layer wet dry Ethanol Isopropanol Urea urea Methanol/Isopropanol Ethanol

These results show that the process according to the invention can be used for a wide variety of radiation-cured layers. Radiation-cured layers based on epoxy (layers K and L) give rise to fewer problems with regard to adhesion than radiation-cured layers based on acrylate.

Layers containing silane compounds (layer A) have poorer adhesion properties compared to layers without this compound (layer B).

EXAMPLE 9 : Formation of the alkali compound in situ

Various screens similar to the screen provided with the radiation-cured layer (B) described in Example 4 are treated with various compositions by rubbing the surface with the composition several times (n). the composition is allowed to interact for some time with the coating surface in the wet state (wet) and in the dry state (dry). In certain cases, the surface is then wiped with water (**). The improvement in adhesion is checked by the peel test as described above in Example 2. Table 9 Composition wet dry Adhesion Methanol Isopropanol

In cases (**) or if H₂O is added to the processing composition, the alkali compound forms in situ.

EXAMPLE 10 : Adhesion to another radiation-cured layer

A radiation-curable composition containing 5 g of EBC 264 (sold by UCB, Drogenbos, Belgium), 95 g of methyl ethyl ketone and 0.5 g of DAROCUR 1173 (sold by E. Merck, Darmstadt, Germany) is sprayed onto the radiation-cured layer A or B which has been processed with a saturated aqueous Ba(OH)₂ solution containing isopropanol and methanol and then cured by UV radiation.

The adhesion of the radiation-cured top coat to the radiation-cured layer is tested using the peel test as described above in Example 2 and gave +++ as a result.

EXAMPLE 11 : common preprocessing procedures

Certain common pre-processing procedures to improve adhesion to plastic materials are tested on a UV-cured urethane acrylate layer.

The corona discharge treatment does not result in any improvement in adhesion as shown by the peel test and wetting with water.

Etching with diluted acids does not improve adhesion. Treatment for 5 seconds with a 10% or 30% aqueous H₂SO₄ solution or a 10% aqueous HNO₃ solution corrodes the layer, causing it to discolor. Treatment for 5 seconds with a 10% aqueous HCl or HF solution has no effect on the layer.

Claims (15)

1. A method for improving the adhesion of a radiation-cured protective layer on an X-ray screen, wherein the layer is processed with a composition which contains an inorganic base, an organic base or a compound which can generate a base in situ and has a pH of at least 7, so that the surface energy of the radiation-cured layer increases to at least 37 mN/m. 2. Process according to claim 1, characterized in that the processing is such that the surface energy increases to at least 45 mN/m. 3. Process according to claim 1 or 2, characterized in that the radiation-cured layer is made of multifunctional acrylates or epoxies. 4. Process according to any one of the preceding claims, characterized in that the composition has a pH greater than 10. 5. Process according to any one of the preceding claims, characterized in that the composition contains an alkali or alkaline earth metal hydroxide or a quaternary ammonium hydroxide. 6. Process according to claim 5, characterized in that the composition contains barium hydroxide or tetramethylammonium hydroxide. 7. A process according to any one of the preceding claims, characterized in that the composition contains a spreading agent. 8. Process according to claim 7, characterized in that the spreading agent is a surface-active compound or an alcohol. 9. Method according to any one of the preceding claims, characterized in that the composition is applied by repeatedly rubbing the surface of the radiation-cured layer with the composition or by immersing the radiation-cured layer in the composition for a while. 10. Method according to claim 1, characterized in that the X-ray screen is a conventional fluorescent screen or a stimulable fluorescent screen. 11. Process according to claim 1 or 10, characterized in that the radiation-cured layer is made of urethane acrylate. 12. A method according to any one of claims 1, 10 or 11, characterized in that the radiation-cured layer contains a silicone surfactant in an amount of between 0.01 and 0.5% by weight. 13. A method according to any one of claims 1, 10, 11 or 12, characterized in that the protective layer has been cured by means of electron beam radiation. 14. A method according to any one of claims 1, 10, 11 or 12, characterized in that the protective layer has been cured by means of ultraviolet radiation using 2-hydroxy-2-methyl-1-phenyl-propan-1-one as photoinitiator. 15. X-ray screen containing a radiation-hardened protective layer with printed information, characterized in that the protective layer is removed before or during the printing process according to the method of any one of the preceding claims to improve the adhesion of the printed information.