Improved radiographic screen construction

Description

FIELD OF THE INVENTION

[0001] This invention relates to X-ray intensifying screens. More specifically, the invention relates to X-ray intensifying screens having a cured protective topcoat and methods of manufacturing same.

BACKGROUND OF THE ART

[0002] In the field of X-ray photography, particularly medical radiography, it is normal practice to employ one or more phosphor screens, also known as fluorescent screens, intensifying screens, or (in special circumstances) storage screens, comprising a substrate and a layer of phosphor particles dispersed in a binder. Alternatively, the layer of phosphor particles dispersed in a binder may be self-supporting. The phosphor has the property of absorbing X-rays and, in response, emitting visible or UV light. In the case of storage screens, the light is not emitted immediately, but instead the energy is stored by the phosphor until appropriate stimulation is applied (such as heat or laser radiation), whereupon light is emitted, and is typically collected by electronic means, such as a CCD. More commonly, the phosphor emits light directly and the light exposes a silver halide photographic film placed in contact with the screen. Most commonly, the photographic film is double sided (i.e. has a coating of emulsion on both sides of the film base) and separate phosphor screens are contacted with the separate emulsion layers. The entire assembly is enclosed in a light-proof cassette, and sheets of film are typically loaded and unloaded in an entirely automated process using a film handling system such as that supplied by IMATION CORP under the tradename Trimatic™.

[0003] To facilitate the automated film handling, it is essential that the surface of the screen which contacts the film has the appropriate frictional properties. The frictional properties should remain constant over an extended period of time (e.g. many thousands of film changes) under varying conditions of temperature and humidity. In addition, the screens should resist the build-up of static charge, which (by discharging) can lead to fogging of the film. In pursuit of these properties, the screens are normally provided with a protective topcoat, and many different methods and materials have been employed for this purpose, but there are three main classes: (a) laminated films; (b) coated films; and (c) coated and cured films.

[0004] In the case of laminated films, a preformed film of a transparent plastic material such as cellulose acetate, polyethylene, polyesters, polyamides, etc. is laminated on top of the phosphor layer, optionally by means of a separate pressure-sensitive or hot-melt adhesive layer, as described in, for example, EP 0721192; EP 0102085; EP 0392474; US 5,153,078; US 4,983,848; US 4,979,200; US 4,952,813; US 4,665,003; US 4,839,243; and US 4,816,369. While this method has the benefit of simplicity and may avoid the use of coating solvents, it is also inflexible. For example, if it is desired to incorporate various additives such as dyes and antistatic agents in the topcoat (as is frequently the case), this cannot readily be achieved by the simple film lamination technique. Furthermore, since the resolution of the screen is inversely proportional to the thickness of the topcoat, relatively thin films (e.g. less than 10 microns) are desirable, and handling such thin films under factory conditions may require special techniques and equipment.

[0005] Additives may readily be incorporated if the topcoat is coated directly on to the phosphor layer as a solution of a resin in a solvent and allowed to dry. Topcoat materials applied in this manner include cellulose esters, poly(methyl methacrylate), poly(vinyl butyral), poly(vinyl formal), polycarbonate, poly(vinyl acetate) and copolymers of vinyl acetate with vinyl chloride ( see, for example, EP 0102085 and US 5,268,125), styrene-acrylonitrile copolymers (e.g. US 4,711,827 and US 5,017,440) and fluorinated polymers (e.g. US 4,666,774). The use of organic coating solvents is a disadvantage, particularly when the phosphor layers are porous, owing to penetration of the coating solution into the voids. Furthermore, the resulting topcoat may lack sufficient durability towards scratching etc., and may even wear away gradually over the lifetime of the screen. This is a particular problem if the topcoat contains a dye, such as those taught in US 4,012,637; US 4,696,868; US 4,362,944 and DE 3,031,267, as the optical density of the topcoat, and hence the sensitometric properties of the screen, will change with time.

[0006] A more durable topcoat may be obtained though the use of curable (i.e. crosslinkable) resins. US 4,205,116 and US 5,475,229 disclose solvent-coated curable layers. However, an alternative approach avoids the use of solvents by applying the topcoat as a solvent-free radiation curable formulation, as disclosed in EP0510753, EP0510754; US 4,292,107 and US 5,340,661. Highly durable topcoats may be formed by this route, but care must he taken to ensure that the coating is fully-cured at its outermost surface, since leaching or migration of residual monomers into the adjacent photographic film would have serious consequences. Due to inhibition by atmospheric oxygen at the surface, multiple passes through the curing station and/or blanketing with inert gas may be required to achieve a fully cured coating, which adds to the cost of the process.

[0007] US 5,411,806; US 5,607,774; and US 5,520,965 disclose methods of manufacturing radiographic screens in which a dispersion of phosphor particles in a curable binder system, essentially free from solvents and low molecular weight monomers, is coated on a substrate, and a coversheet applied to the coating. After curing of the coating (e.g. by UV irradiation), the coversheet is removed. If the coversheet is suitably thin and transparent, it may be left in place to act as a protective topcoat, but this is said to be less preferred than other (unspecified) methods of providing such a layer.

[0008] US 5,569,485 describes the application of an antistatic topcoat to radiographic screens comprising a UV-cured phosphor layer.

[0009] Photocurable layers which are releasably attached to a transparent carrier have previously found use in various imaging systems, wherein the photocurable layer is irradiated in an imagewise fashion so that only certain areas become crosslinked. This results in the exposed and unexposed areas of the coating having different physicochemical properties (such as solubility in a developer, or affinity for toner powders) which may be exploited in the production of a visible image. Examples of this use include the color proofing systems described in patents such as US 3,649,268 and US 4,806,451 and exemplified by the Chromalin™ and Eurosprint™ proofing systems.

[0010] There is a continuing need for alternative methods of applying a protective topcoat, especially a cured topcoat, to a radiographic screen.

SUMMARY OF THE INVENTION

[0011] The present invention provides a method for manufacturing a radiographic screen comprising the steps of (a) providing a first element comprising a carrier having deposited thereon a releasable curable resin layer; (b) contacting the curable resin layer of the first element with a phosphor-containing layer, wherein the phosphor-containing layer is either self-sustaining or deposited upon a support and emits light in response to irradiation with X-rays; (c) curing the curable resin layer and optionally the phosphor-containing layer with radiation energy or heat; and (d) removing the carrier. Alternatively, step (d) may be carried out before step (c). In a preferred embodiment of the invention, the phosphor-containing layer comprises a photocurable resin as a binder and the radiation energy is UV radiation.

[0012] In another embodiment of the present invention, there is provided a radiographic screen comprising (in the following order): (a) a substrate; (b) a phosphor-containing layer, wherein the phosphor emits light in response to irradiation with X-rays; and (c) a cured topcoat; wherein the cured topcoat comprises at least 25% by weight of one or more thermoplastic film-forming polymers and up to 75% by weight of a crosslinked acrylic resin. Preferably, the cured topcoat is chemically bonded to the phosphor-containing layer, and the phosphor-containing layer comprises a phosphor dispersed in a curable binder.

DETAILED DESCRIPTION OF THE INVENTION

[0013] In the practice of the invention, a radiographic screen is prepared by a method comprising the steps of (a) providing an element having a releasable curable (preferably photocurable) resin layer deposited on a carrier; (b) contacting the resin layer with a phosphor layer (which is normally carried on a substrate); (c) curing the resin layer with radiation energy (e.g. UV radiation) or heat; and (d) removing the carrier. Optionally, step (d) may be carried out prior to step (c). If step (c) is carried out before step (d), and radiation is employed to cure the resin layer, then the carrier must be translucent (and preferably transparent) to that radiation.

[0014] Preferably, the curable resin layer comprises a blend of one or more thermoplastic film-forming polymers with one or more polyfunctional free-radical polymerizable monomers, (preferably one or more polyfunctional acrylic monomers). Consequently, the finished screen has a topcoat comprising a blend of at least one thermoplastic film-forming polymer and a crosslinked resin formed by the polymerization of the one or more polyfunctional free-radical polymerizable monomers. Preferably, the crosslinked resin is a photocured acrylic resin.

[0015] Preferably the thermoplastic film-forming polymer(s) constitute at least 25% by weight of the curable resin layer and finished screen topcoat. "Thermoplastic film-forming polymers" refers to polymers which form films (e.g. by solvent casting or extrusion) which are essentially tack-free at 25°C. Preferably, they do not contain groups which participate in the curing reaction. The presence of the thermoplastic polymer(s) in the cured topcoat of the finished screen improves the flexibility of the topcoat. Furthermore, it is believed that the addition of a substantial amount of thermoplastic film-forming polymer to a photocurable monomer mixture provides a composition which cures more efficiently, in that a lower proportion of unreacted monomer remains for a given input of energy. However, the main advantage of the compositions used for the topcoat in the present invention is the fact that they can exist (in their uncured state) as solid, tack-free coatings at ambient temperature. Whereas the photocurable compositions disclosed in the prior art typically are liquids prior to curing, the incorporation of substantial amounts of thermoplastic film-forming polymer(s) enables the curable resin layer to be deposited as a tack-free coating on the carrier. In such as a state, it can be handled and stored (e.g. on a roll) without damage. Thus, bulk quantities of the uncured topcoat on its carrier may be prepared in advance, stored, and used as required over a period of time, which is highly convenient for the manufacturer.

[0016] The radiographic screens prepared by the method of the invention possess an extremely durable topcoat and display excellent surface properties when used in automated film handling equipment. Because the carrier provides an effective barrier against atmospheric oxygen, uniform curing of the curable resin layer is readily achieved when step (c) is carried out before step (d). Furthermore, the manufacturing process enables additives such as dyes and antistats to be readily incorporated in the topcoat. Compared to conventional manufacturing methods in which the topcoat is solvent-coated directly on to the phosphor layer, the method of the invention can provide less opportunity for such additives to diffuse out of the topcoat and into the phosphor layer. Also, because the topcoat is coated initially on a separate substrate (the carrier), quality control may be performed, and defective parts discarded, without wasting expensive phosphor. The processes embodied in steps (b) - (d) are readily adaptable to automation, and do not involve the use of solvents.

[0017] The curable resin layer may readily be formulated so as to show adhesive properties towards a wide variety of phosphor layers, and in the case of simultaneous curing of the topcoat and phosphor layer (described in detail later herein), chemical bonding of the topcoat to the phosphor layer is possible.

[0018] Preferred embodiments of the present invention utilize a photocurable layer releasably attached to a transparent carrier, where the entire area of the photocurable layer is irradiated. However, one could conceptually provide identification marks or other visible information on the screen by selectively masking off areas during the photocuring process. The masked areas would remain uncured and could then be selectively toned in a subsequent process to form an image. In normal circumstances, such areas would be confined to the edges or corners of the screen so as not to impair its normal functioning.

[0019] The carrier may comprise any suitably flexible and dimensionally-stable sheet form material. An opaque material may be used if curing via heat or electron beams is intended; however, when UV curing is employed through the carrier, the carrier must be transparent to sufficient UV radiation to accomplish curing of the photocurable layer. Suitable materials include cellulose acetate, polyamide, polyester, polycarbonate, and polyethylene. A preferred material is poly(ethylene terephthalate) (PET) film with a thickness in the range 20 - 200 µm, preferably about 100 µm. The surface of the film may be subjected to physical or chemical treatments (such as corona discharge, or the application of subbing or release layers) to modify the wetting or adhesion behavior of coatings subsequently applied thereto, in accordance with known techniques. In particular, a release layer, such as a silicone- or fluorochemical- or polyvinylalcohol-containing layer, may provide the surface properties consistent with releasable attachment of the curable resin layer. However, in the preferred embodiments described herein, a release layer is not required.

[0020] Releasably attached to a surface of the carrier is a curable resin layer. The curable resin layer, in its uncured state, is thermoplastic and has the ability to bond adhesively under conditions of heat and/or pressure to a variety of substrates, and in particular to a phosphor layer. Subsequent to such adhesive bonding, peeling of the carrier (either before or after curing) leaves the curable resin layer attached to the new substrate.

[0021] For ease of handling, the curable resin layer is preferably non-tacky at ambient temperatures, but softenable at moderately elevated temperatures, e.g. of about 50°C or greater. In its cured state, however, the resin layer remains hard and non-tacky at such temperatures.

[0022] Such properties are most readily achieved by formulating the curable resin layer as a blend of one or more thermoplastic film-forming polymers with one or more polymerizable monomers. The thermoplastic film-forming polymer dissolves or disperses the other ingredients and provides the uncured composition with solidity and structural integrity, while the monomers provide a plasticizing action prior to curing. Thermoplastic film-forming polymers which are selected for their ability to confer durability and abrasion-resistance on the final product frequently have properties (such as high Tg and high melting point) which make them difficult to laminate by a transfer process. However, the plasticizing action of the monomers counteracts this effect, and improves the lamination properties. The monomers polymerize to a crosslinked three-dimensional network as a result of the curing process, and hence cease to exert a plasticizing action, which would be undesirable in the final product. By varying the identity of the thermoplastic film-forming polymer(s) and the monomer(s), and their relative proportions, it is possible to tailor the physical and chemical properties of the curable resin layer as desired.

[0023] Thus, the thermoplastic film-forming polymer may be selected to provide properties such as toughness, durability and abrasion-resistance, but should preferably be soluble in common solvents. The thermoplastic film-forming polymer is generally selected from thermoplastic polymers such as polyesters, polycarbonates, thermoplastic polyurethanes, poly(meth)acrylate resin, cellulose esters and ethers, phenolic resins (including modified versions thereof), poly(vinylalcohol), poly(vinylbutyral), and polymers and copolymers of vinyl monomers such as vinyl chloride, vinyl esters, vinyl ethers, styrene etc. Fluorinated polymers may be employed as the thermoplastic film-forming polymer or as a component thereof in order to confer non-stick and/or antistatic properties on the coating. Preferred thermoplastic film-forming polymer materials include one or more of Elvacite™ 2008 and 2014 (polyacrilate resins supplied by ICI), cellulose acetate butyrate, and Fluorel™ 2330 (a fluoropolymer supplied by 3M). The thermoplastic film-forming polymer typically may constitute at least 25 wt% of the curable resin layer, preferably from about 25 to about 75 wt%, and most preferably from about 45 to about 65 wt% of the curable resin layer.

[0024] Suitable monomers include those capable of undergoing polymerization by free-radical processes, such as vinyl monomers exemplified by vinyl ethers, vinyl esters, styrenes etc., as well as those capable of undergoing polymerization by cationic processes, such as epoxides, but the preferred monomers are acrylate or methacrylate esters or amides. Polyfunctional derivatives, possessing two or more polymerizable groups per molecule, are preferred over the monofunctional counterparts, although mixtures of mono- and polyfunctional monomers (or of different polyfunctional monomers) may be used. Highly preferred monomers include ethylene glycol dimethacrylate, hydantoin hexa-acrylate, trimethylolpropane triacrylate, pentaerythritol tetra-acrylate, and dipentaerythritol tetra-acrylate. One or more fluorinated monomers may be included to provide non-stick and/or antistatic properties, e.g. dihydroperfluorooctyl acrylate, perfluorocyclohexyl acrylate or N-ethylperfluorooctylsulphonamidoethyl acrylate. The monomer or mixture of monomers generally constitutes from about 10 to about 75 wt% of the curable resin layer, preferably from about 15 to about 60 wt%, and most preferably from about 20 to about 40 wt% of the curable resin layer.

[0025] The curable resin layer further comprises an initiator capable of initiating polymerization within the layer in response to energy, especially thermal energy or UV radiation. Depending on the choice of monomer(s), suitable initiators include cationic initiators, which decompose to provide a strong Bronsted acid or Lewis acid, and radical initiators, which decompose to provide free radicals. Examples of initiators which release free radicals on thermal decomposition include peroxides (such as benzoyl peroxide, dicumyl peroxide etc.) and azo compounds (such as azobisisobutyronitrile). However, photoinitiators which decompose in response to photoirradiation (especially UV irradiation) are preferred. Depending on the type of monomer present in the curable resin layer, any of the known cationic or free-radical photoinitiators may be used, as described, for example, in "UV Curing: Science and Technology", ed. S.P. Pappas (Technology Marketing Corporation), Vol. I chapters 1 and 2, and Vol. II chapter 1. Suitable cationic photoinitiators include aryldiazonium salts, diaryliodonium salts and triarylsulphonium salts. Suitable free-radical photoinitiators include benzophenones, benzoins, benzoin alkyl ethers, benzyl dialkylketals, acylphosphine oxides, trichloromethyltriazines, etc. The photoinitiator may be intrinsically UV-sensitive, or may be used in combination with a separate photosensitizer, as described in the above-referenced textbook. A preferred class of photoinitiator is that of the acylphosphine oxides (described, for example, in US4265723, US5210110 and WO96/07662), exemplified by commercially available materials such as Lucirin™ TPO and Darocur™ 4265 (both supplied by Ciba Geigy). Acylphosphine oxide initiators have a relatively weak absorption band in the near UV, but it is sufficient to enable direct photolysis without the need for a separate sensitizer. However, if a greater photosensitivity is desired, a sensitizer of the type disclosed in WO97/35232 may be added. Such sensitizers are optical brighteners, such as coumarins, pyrazolines and 2,2'-bisbenzoxazolyl derivatives, preferred examples including Uvitex™ OB, supplied by Ciba Geigy, and Blankophor MAN-01 ™, supplied by Bayer.

[0026] When an acylphosphine oxide is employed as photoinitiator in the curable resin layer (which becomes the protective topcoat for the screen), the decision whether or not to add a separate photosensitizer may be governed by the performance required from the screen. For example, if the phosphor in the screen is of the UV-emitting type, it may be undesirable to add a photosensitizer to the topcoat, as this may absorb too much of the radiation emitted by the phosphor during normal use. On the other hand, if the phosphor is of the green-emitting type, the presence of a UV-absorbing sensitizer in the topcoat need not have a deleterious effect.

[0027] The quantity of initiator incorporated in the curable resin layer is typically in the range 0.5 - 15.0 wt%, and from about 1.0% to about 12.0% in the preferred embodiments.

[0028] Other ingredients which may optionally be included in the curable resin layer (and hence in the topcoat of the screen) include antistats to control the build up of static charge during use. Any of the known antistatic compounds may be used, such as the perfluoroalkylsulfonyl methide salts and the perfluoroalkylsulfonyl imide salts disclosed in EP 0692796, and commercially-available materials such as Catanac™ 609 and Cyastat™ SN, both supplied by American Cyanamid.

[0029] Minor amounts of other ingredients may be incorporated in the curable layer, e.g. powdered silica or other particulates to enhance vacuum drawdown and reduce blocking, plasticizers which improve the flexibility of the cured coating, and surfactants or other coating aids which may improve the quality of coating on the transparent carrier and/or improve the release properties, in accordance with known techniques. Suitable plasticizers include Santiciser™ 278 (benzyl phthalate, supplied by Monsanto). Suitable coating aids include BYK™ 307 and Disperbyk™ 161 (both supplied by Byk-Chemie).

[0030] Such optional ingredients may together constitute up to about 20 wt% of the total solids of the curable resin layer, typical ranges being 0.5 - 10.0 % for the antistat, 0.1 - 2.0% for the silica, 1.0 - 10.0% for the plasticizer, and 0.1 - 2.0% for the coating aid (all percentages by weight).

[0031] Further optional ingredients include dyes or pigments to alter the sensitometric properties of the screen. For example, one or more dyes or pigments may be added which absorb at the wavelength of peak emission of the phosphor, as described in US 4,012,637; US 4,696,868; US 4,362,944 and DE 3,031,267. This reduces the speed of the screen, but also improves the resolution and image quality. For some diagnoses, this is a useful trade-off. Alternatively one or more dyes or pigments may be added which do not absorb at the primary output wavelength of the phosphor, but absorb any secondary emissions. For example, a green phosphor will typically emit mainly in the green region of the spectrum, but may have significant output also in the blue and UV regions. By including in the topcoat an absorber for these secondary emissions, a more monochromatic output is achieved, leading to improved image quality.

[0032] For some applications, it is useful to prepare a "gradual" screen where the intensity of the output varies gradually across the screen surface (see, for example, US 4,816,350 and US 4,772,803). In the prior art, such screens are prepared by imbibing an attenuating dye into the topcoat to varying extents, or by printing an absorbing halftone dot pattern on to the screen. The present invention may readily be adapted to provide a gradual screen by incorporating a suitable dye or pigment in the curable resin layer, and coating the layer at a non-uniform thickness, so that the optical density of the resulting topcoat varies across the screen.

[0033] The curable resin layer is typically coated on the carrier as a solution in an organic solvent, e.g. at about 20 wt% solids in methyl ethyl ketone (MEK), in which case any of the standard coating methods may be used, such as roller coating, bar coating, die coating etc. The coating is typically dried at moderately elevated temperatures, e.g. of about 80°C. Alternatively, the layer may be hot-melt extrusion coated or coated as an aqueous dispersion. The thickness of the resulting curable resin layer ranges from 1 to 15 µm, preferably from 2 to 10 µm, more preferably from 3 to 7 µm.

[0034] The second step in the fabrication of the screens is the assembly of the curable resin layer in adhesive contact with a phosphor-containing layer. The phosphor-containing layer emits light in response to irradiation with X-rays. The phosphor may be of the "direct" variety, where emission of light occurs spontaneously within a fraction of a second of X-rays being absorbed by the phosphor, or of the "storage" variety, where emission of light subsequent to absorption of X-rays is delayed until further stimulation in the form of heat or laser radiation is applied.

[0035] The phosphor-containing layer is usually formed on a support, which may be any suitable flexible or rigid dimensionally-stable sheet-form material, but is preferably a plastic film such as PET, e.g. 250 µm in thickness. The surface of the support which bears the phosphor layer may be primed or otherwise treated so as to improve the adhesion of the phosphor layer thereto. In addition, the surface of the support which bears the phosphor layer may be provided with a reflective layer to ensure that all light emitted by the phosphor emerges in the forward direction (i.e. away from the support). Such reflective layers are well known in the field of radiographic screens, and typically comprise a white pigment such as titanium dioxide, barium sulphate, zinc oxide, alumina etc, dispersed in a binder, or may comprise a vapor deposited metallic layer (e.g. of aluminum). Alternatively, the support may be intrinsically reflective by virtue of a filler such as titanium dioxide, barium sulphate, zinc oxide, or alumina dispersed within the support itself.

[0036] The phosphor screen may be formed using a two-step process. In the two-step process, the phosphor-containing layer is formed on a support and then subjected to drying, curing, calendering, etc as necessary in a separate operation prior to its assembly in adhesive contact with the curable resin layer. Preferably, the screen is formed in a one-step process whereby the phosphor-containing layer is formed on a support and placed in adhesive contact with the curable resin layer in a single operation.

[0037] In a two-step process, the phosphor-containing layer may be formed by any method known in the art. For example, it may be formed as a binder-free layer by a reactive spray pyrolysis technique (as described in US 5,540,947) or by a spray-sintering technique (as described in EP Appl. 97-111240.4), but more commonly it takes the form of a dispersion of inorganic phosphor particles in an organic binder. Typically, the binder is a thermoplastic polymer, such as polyvinylbutyral, poly(vinyl acetate), nitrocellulose, ethyl cellulose, vinylidene chloride/vinyl chloride copolymer, poly(methyl methacrylate), vinyl chloride/vinyl acetate copolymer, polyurethanes, and cellulose acetate butyrate. The phosphor particles are dispersed in a solution of the binder in an organic solvent, then coated on the support and dried in accordance with standard manufacturing processes. Optionally, the dried coating may be subjected to a calendering process as taught in US 4,952,813.

[0038] Alternatively, the phosphor particles may be dispersed in a curable binder composition which is essentially free from solvent or other volatiles, and the dispersion coated on the support, then cured (e.g. by UV radiation or E-beam). Optionally, a cover sheet may be applied to the coating prior to curing and removed afterwards as described in US 5,569,485; US 5,507,774; US 5,411,806; US 5,520,965; and European Patent Application No. 97-......entitled "Improved X-Ray Intensifying Screen" filed concurrently herewith.

[0039] In the second step of the two-step method of screen fabrication, the curable resin layer of the first element and the phosphor-containing layer are brought into adhesive contact, so that the curable resin layer adheres more strongly to the phosphor layer than to the carrier. This may be achieved by any of the well-known techniques of web lamination, e.g. using a hot press, calender rolls or a heated roller laminating device such as the Matchprint™ 447 laminator available from IMATION CORP. If the curable resin layer has pressure-sensitive adhesive properties (i.e. is tacky at ambient temperature), then pressure lamination may be sufficient. Preferably, the curable resin layer is non-tacky at ambient temperatures and softenable at moderately elevated temperatures (i.e. acts as a hot melt adhesive), in which case both heat and pressure are applied during lamination. For example, the first element (comprising a curable resin layer on a transparent carrier) may be assembled in face-to-face contact with the support bearing the phosphor-containing layer, and the assembly fed into a Matchprint™ 447 laminator at a transport speed of about 45 cm/min, with the upper roller (which contacts the transparent carrier) set at about 130°C and the lower roller set at about 60°C. The lamination conditions can, of course, be varied to suit the requirements of different formulations of the curable resin layer.

[0040] Curing of the topcoat is effected by the appropriate means, i.e. heat treatment in the case of thermally-curable layers, or irradiation (e-beam or UV) in the case of radiation-curable layers. Preferably, the topcoat is UV-curable, and curing is carried out by using the equipment and techniques well known in the coatings industry. Curing may be performed before or after removal of the carrier, but UV curing is more rapid and demands less energy if the carrier is removed after curing. The carrier acts as a barrier to atmospheric oxygen and hence promotes more efficient curing. However, some curable resins are not inhibited by oxygen in which case the carrier may be removed prior to curing. The thickness of the topcoat after curing is in the range of from 1 to 15 µm, preferably from 2 to 10 µm, most preferably from 3 to 7 µm.

[0041] If the carrier is removed prior to curing, a surface texture may be applied to the topcoat, e.g. using an embossing roller, while the curable resin layer is still in a thermoplastic, easily-deformable state. Such a texture may provide improved handling properties in the final product, and so this technique may be preferred in certain circumstances, even though the UV curing operation may require blanketing the surface with an inert atmosphere, longer curing times or higher intensity output radiation. A preferred method of creating a surface texture in the topcoat is by using a textured transparent carrier which is replicated in the topcoat regardless of whether the carrier is removed before or after curing.

[0042] In a preferred embodiment of the invention, the phosphor-containing layer is formed on the support and placed in adhesive contact with the curable resin layer in a single operation. This can be achieved by modifying any of the processes taught in US 5,607,774; US 5,411,806; US 5,520,965; and European Patent Application No. 97-...... entitled "Improved X-Ray Intensifying Screen" filed concurrently herewith. The phosphor particles are dispersed in a curable binder mixture which is preferably free of solvents. The dispersion having a paste-like consistency is applied to the support followed by contacting the phosphor dispersion layer with the first element (comprising a curable resin layer on a carrier) such that the curable resin layer is in contact with the phosphor dispersion layer. The assembly is fed through one or more pairs of nip rollers until the desired layer thickness is achieved, then energy (e.g. heat, e-beam or UV radiation) is applied to cure both the phosphor-containing layer and the curable resin layer simultaneously. Preferably, UV radiation is used as the energy source. Finally, the carrier is peeled off leaving a cured topcoat bonded to the cured phosphor layer. Prior to curing, the curable components of the two layers at the interface between the layers have an opportunity to inter-mix, thus enabling the monomer(s) of the topcoat to copolymerize with the curable binder of the phosphor layer. As a result, the two layers become chemically bonded to each other which enhances the durability of the radiographic screen and improves its tolerance to flexing.

[0043] Preferably, the curable binder medium for the phosphor layer comprises a mixture of at least one urethane acrylate prepolymer, at least one photopolymerizable monomer and/or oligomer, and a photoinitiator. Such a photocurable resinous binder basically comprises a photoinitiator solubilized into a mixture of at least one photopolymerizable monomer and/or oligomer and at least one urethane acrylate prepolymer.

[0044] The urethane acrylate prepolymer imparts most of the basic properties to the coating dispersion (such as viscosity, stability, and wetting power) and to the final cured phosphor layer (such as hardness, adhesion, and flexibility). The urethane acrylate prepolymer is preferably chosen among polyether urethane acrylates, polyester urethane acrylates, and polyol urethane acrylates. More preferably, aliphatic polyether or polyester urethane acrylates are used which crosslink into a tough and flexible material and do not have the yellowing characteristics of the aromatic derivatives. The urethane acrylates are preferably added in an amount ranging from 30% to 40% by weight of the final coating dispersion.

[0045] The urethane acrylates can be formulated alone or in combination with a low percentage of at least one additional prepolymer to improve the above mentioned basic properties of the coating dispersion and the final cured phosphor layer. Useful additional prepolymers are epoxy acrylates and polyester acrylates which can reduce the viscosity of the coating dispersion, improve the wetting power of the binder versus the phosphor particles (giving a more stable dispersion) and produce a harder material. The additional prepolymers are preferably added in an amount ranging from 1 to 30 %, more preferably from 5 to 20% by weight. The additional prepolymers are added to the formulation such that flexibility of the final cured phosphor layer is maintained.

[0046] Both urethane acrylate prepolymers and other additional prepolymers are characterized by a molecular weight higher than 500, preferably higher than 1000.

[0047] The photopolymerizable monomer or oligomer performs two primary functions. First, the monomers or oligomers form links between the prepolymer molecules and other monomer units. Second, the monomers or oligomers act as a diluent for the prepolymer and reduces the viscosity of the photocurable composition. The monomer can be di-, tri- or tetra-functional. Suitable monomers include acrylic or methacrylic esters of polyhydroxy alcohol derivatives, such as diethyleneglycol diacrylate, tripropyleneglycol diacrylate, polyethyleneglycol diacrylate or dimethacrylate, neopentylglycoldiacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, trimethylolpropane ethoxy triacrylate, and di-trimethylolpropane tetracrylate. Ethoxylated and propoxylated polyol monomers are preferred which have low toxicity, exhibit low volatility, low viscosity, high reactivity and good mechanical properties. The photopolymerizable monomer or oligomer is characterized by a molecular weight lower than 500, preferably lower than 300.

[0048] When UV radiation is used as the energy source, the photoinitiator preferably absorbs between 200 nm and 500 nm. Upon absorption of the UV radiation, the photoinitiator generates free radicals which promote the crosslinking reaction between the prepolymer and the monomer. Suitable photoinitiators include benzoin and acetophenone derivatives, benzylketals, α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides, acylphosphonates, benzophenones, xanthones, thioxanthones, aromatic 1,2-diketones, triazines and combination thereof. The amount of photoinitiator added to composition is preferably from 0.01 to 10% weight of the resin, more preferably from 0.1 to 4%. Acyl phosphine oxides and their derivatives having an absorption peak shifted towards the visible region are preferred because they give a more efficient curing of the highly pigmented layer at the lowest concentrations (0.5-1%).

[0049] An additional reacting monomer and other additives can be optionally added to match the coating dispersion physical and rheological characteristics required by the manufacturing method.

[0050] A monofunctional (meth)acrylate or a non-acrylate monomer can optionally be added as additional reacting monomer. Monofunctional monomers are used primarily to adjust the viscosity of the final dispersion. They also affect the properties of the cured coating by enhancing the adhesion to the substrate and by flexibilizing the cured material. Suitable non-acrylated monomers include styrene, vinyl acetate, and N-vinyl pyrrolidone. Suitable monofunctional (meth)acrylates include alkyl (meth)acrylates, hydroxyalkyl (meth)acrylates, β-carboxyethyl (meth)acrylate, isobornyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-(2-ethoxyethoxy)ethyl (meth)acrylate, and aliphatic urethane acrylates. Alkyl acrylates and monomers having a low vapor pressure are preferred.

[0051] Other useful additives include wetting agents, dispersing agents, slipping agents, plasticizers, and stabilizers. Wetting and dispersing agents can be particularly useful in promoting the dispersion of phosphor particles into the photopolymerizable resinous binder. High and low molecular weight, anionic, cationic or neutral polymers comprising groups having pigment affinity (like polyacrylate or polyether polymers with phosphates or carboxylic acid groups in the side chains) can be adsorbed onto the phosphor surface giving the dispersion reduced viscosity and good flow by electrostatic repulsion or steric hindrance. The reduced viscosity and good flow properties provide easier handling of the dispersion and improved processability. Acrylate modified silicone and siloxane polymers and copolymers may serve as slipping agents and plasticizers as well as coating aids. Stabilizers are added to prevent unwanted bulk polymerization during the process. Preservatives are added to retard degradation caused by environmental attack. Antistatic agents are added to dissipate unwanted electronic charges during handling of the cured film. The combined additives are added to the formulation in an amount from 0.01 to 10%, more preferably 0.1 to 5%, and most preferably from 0.5 to 1% by weight of the total binder. Acrylated silicones can improve the flexibility of the final cured phosphor layer but give poor adhesion to the base and insufficient hardness and scratch resistance. For this reason, they are used at very low concentration preferably from 0.1 to 1% by weight.

[0052] The preferred procedure for manufacturing radiographic screens by the one step process includes the following steps.

[0053] First, the prepolymer is dispersed in a monomer solution containing all additional components of the photopolymerizable mixture (additional prepolymer, additional monomer, additives, and the like). The dispersion is prepared by adding the phosphor particles to the photopolymerizable mixture in single or multiple steps and mixing it homogeneously. Alternatively, a phosphor predispersion in a solution of a wetting polymer in the monomers is prepared first, and the remaining components are added later. The resinous binder and the phosphor particles are then blended intimately by means of a three-roll mill, a double planetary, or other mixing system.

[0054] The resulting dispersion is characterized by a high phosphor to unsaturated resin weight ratio, typically in the range of from 1:1 to 20:1, preferably from 10:1 to 15:1, and by a viscosity value typically in the range from 10,000 to 1 million cps, preferably of 20,000 to 400,000 cps at 35°C. Phosphor to binder ratio and final viscosity of the dispersion depend on the binder composition, the phosphor grain size employed and the grain surface. A three-roll mill is preferred when the viscosity of the dispersion is high and several passes are required to homogeneously blend the material. A double planetary mixer is used when the viscosity is low and a paste having good flow characteristics particularly suitable to the process is obtained. The mixing is made at controlled temperature by keeping constant the temperature in the range of from 10 to 50°C, preferably from 10 to 30°C. The temperature control can be helpful to make the workflow easier and to avoid unwanted polymerization in the bulk when long time mixing is required. The dispersion is then kept under vacuum for a period depending on the amount of dispersion (for example, about 30 minutes for an amount of five kilograms) to get rid of the air generated by the mixing step and dispersed into the bulk. The reduced viscosity plays an important role in facilitating the removal of the bubbles which is fundamental for the following step.

[0055] The obtained dispersion is then dispensed by means of a syringe, by extrusion or by other feeding method, between a substrate and a photosensitive element (which comprises a transparent carrier sheet releasably attached to a photocurable resin layer) which are fed through the gap of a calender and laminated to the desired final thickness. The lamination is preferably conducted vertically in a single step. If the viscosity of the dispersion is too high and the air cannot be removed completely or if the degassing step is not carried out, it is not possible to laminate the dispersion in a single step without worsening the coating quality of the resulting phosphor layer. Multiple passes may be required to remove the bubbles marks to obtain a good coating quality, thus making the process less suitable for a continuous industrial application.

[0056] The laminate is then exposed to radiation and cured. The exposure is performed through the transparent carrier sheet. After curing, the carrier sheet is peeled off while the phosphor layer firmly sticks to the support. The curing time is typically very short (in the order of a few seconds), depending on the coating thickness and composition, and on the characteristics of the equipment used. A medium pressure mercury lamp or an electrodeless lamp can be used as a source of UV-visible radiation. An electrodeless doped lamp type D or V which is characterized by a main output in the region between 360 nm and 420 nm and reduced IR emission is preferred, since the emission around 400 nm will not be absorbed by the pigments but will be absorbed mainly by the photoinitiator, and hence efficient photolysis will occur and a deeper and faster curing will result. A thermal post-cure, by means of a short heating step subsequent to UV exposure, may be employed to further promote the crosslinking reaction.

[0057] Regardless of whether the one-step or two-step process is followed, there are no particular restrictions on the identity of the phosphor, its particle size, or on the thickness of the resulting phosphor-containing layer. For example, phosphors having emission maxima in one or more of the UV, blue, green, red or infrared regions of the spectrum may be used. Storage phosphors and direct-emitting phosphors are both within the scope of the invention. Among the many phosphors known in the art which may be considered in the practice of the present invention are alkali halides, doped alkali halides, rare earth oxy-halides, and others such as are described in US 5,302,423 and in Research Disclosure, Vol.184, Aug.1979, Item 18431, Section IX. Preferably, the phosphor emits predominantly in the green or UV-blue regions. Green-emitting phosphors which may be used include rare earth activated rare earth oxysulfide phosphors comprising at least one rare earth element selected from yttrium, lanthanum, gadolinium and lutetium; rare earth activated rare earth oxyhalide phosphors comprising at least one rare earth element selected from the same list; and phosphors comprising a borate, phosphate or tantalate of one or more of the above-listed rare earth elements. Rare earth green-emitting phosphors are extensively described in the patent literature, e.g. in US Pat. Nos. 4,225,653; 3,418,246; 3,418,247; 3,725,704; 3,617,743; 3,974,389; 3,591,516; 3,607,770; 3,666,676; 3,795,814; 4,405,691; 4,311,487 and 4,387,141. Any of the known UV-blue emitting phosphors may be used, such as lead or lanthanum activated barium sulfate phosphors, barium fluorohalide phosphors, lead activated barium silicate phosphors, gadolinium activated yttrium oxide phosphors, barium fluoride phosphors, alkali metal activated rare earth niobate or tantalate phosphors, etc. Such phosphors are disclosed, for example, in Belgian Pat. Nos. 703,998 and 757,815, and in EP 202,875.

[0058] The average particle size of the phosphor is preferably within the range 0.3 - 50 µm, more preferably within the range 1 - 30 µm, and the thickness of the phosphor-containing layer is preferably within the range 10µm - 1mm, but both of these parameters may be varied so as to provide screens with different imaging characteristics. For example, increasing phosphor particle size and/or layer thickness leads to improved sensitivity, so that lower doses of X-rays may be used, but this is at the expense of poorer resolution. Conversely, thinner layers and/or smaller phosphor particles give improved resolution, but require higher X-ray doses. It is customary to manufacture a range of screens using the same phosphor but at varying loadings and/or particle sizes, to enable a radiologist to select the optimum balance between sensitivity and resolution for a given diagnostic problem.

[0059] The invention is illustrated by the following non-limiting examples, in which the following tradenames, abbreviations etc. are used:

Elvacite™ 2008

acrylate polymer (ICI)

Elvacite™ 2014

acrylate polymer (ICI)

Fluorel™ 2330

fluoroelastomer (3M)

CAB 381-20

cellulose acetate butyrate (Eastman Chemical)

Sartomer™ 399

polyfunctional acrylate monomer (Cray Valley)

Sartomer™ 344

polyethyleneglycol 400 diacrylate (Cray Valley)

Ebecryl™ 1800

polyester acrylate oligomer (UCB Chemicals)

Ebecryl™ 270

urethane diacrylate (UCB Chemicals)

Darocur™ 4265

acylphosphine oxide photoinitiator (Ciba Geigy)

Darocur™ 1173

photoinitiator (Ciba Geigy)

Irgacure™ 1800

acylphosphine oxide/alkylphenone photoinitiator (Ciba Geigy)

Uvitex™ OB

optical brightener (sensitizer) (Ciba Geigy)

Blankophor™ MAN 01

optical brightener (sensitizer) (Bayer)

Syloid™ ED5O

powdered silica (antiblocking agent) (W.R. Grace)

Syloid™ 74

powdered silica (antiblocking agent) (W.R. Grace)

Catanac™ 609

antistat (American Cyanamid)

Cyastat™ SN

antistat (American Cyanamid)

Jeffamine™ ED-900

polyoxyalkylene diamine (Texaco)

Disperyk™ 110

dispersant (Byk-Chemie)

Byk™ 307

coating aid/release agent (Byk-Chemie)

Disperbyk™ 161

dispersant/release agent (Byk-Chemie)

Santiciser™ 278

benzyl phthalate (plasticizer) (Monsanto)

MEK

methyl ethyl ketone (butan-2-one)

PET

poly(ethylene terephthalate) film base

Trimatic™ film loader

automated X-ray film loader (Imation)

Example 1

[0060] The following composition (as a 20 %w/w solution in MEK) was coated on 50 µm PET at a web speed of approximately 40 m/min using a wrap cast roll, and passed through a forced air drying oven at 85°C (approx. 1 min dwell time), to provide a photocurable resin layer releasably attached to a transparent carrier (all parts by weight):

Elvacite™ 2008

- 55.0 parts

Sartomer™ 399

- 32.3 parts

Darocur™ 4265

- 10.0 parts

Uvitex™ OB

- 0.5 parts

Syloid™ ED50

- 0.5 parts

Catanac™ 609

- 2.0 parts

Cyastat™ SN

- 2.0 parts

Disperbyk 161

- 2.0 parts

[0061] The dried coating weight was approx. 5.2g/m², and this material was used in all subsequent experiments, unless otherwise indicated.

Example 2

[0062] This example illustrates the manufacture of screens in accordance with the invention by the two-step process.

[0063] A reflective support comprising a TiO₂-filled polyurethane coating (approx. 25 µm thick) on 250 µm PET was further coated with a dispersion of a terbium-doped gadolinium oxysulfide phosphor (average particle size approx. 8 µm) and hydrophobic polymer binder in a mixed solvent to give a phosphor coating weight of approx. 700 g/m², a dry coating thickness of approx. 150 µm, and a phosphor/binder weight ratio of 15:1. The hydrophobic polymer binder was a 1:1 w/w blend of methyl acrylate - ethyl acrylate copolymer and vinyl chloride - vinyl propionate copolymer. The solvent was an approx. 3:6:1 (by weight) mixture of acetone, ethyl acetate and methyl isobutyl ketone. After drying, samples of this material were assembled in face-to-face contact with samples of the photocurable coating of Example 1, and fed through a Matchprint™ 447 laminator at 45 cm/min with the rollers set at 130°C (upper) and 60°C (lower). Samples thus prepared were subjected to further treatment as follows:

Sample 2(a): (i) 50 units UV exposure on a Sack Model OR-30 printing frame, fitted with a 3kW metal halide lamp, (ii) a second pass through the laminator (giving a short thermal post-cure), (iii) removal of carrier sheet.

Sample 2(b): (i) 50 units UV exposure as above, (ii) removal of carrier sheet.

Sample 2(c): (i) removal of carrier sheet, (ii) second pass through laminator (at 70 cm/min) for additional matting of surface, (iii) 100 units UV exposure as above.

[0064] The resulting screens were tested as described below, along with the following comparison or control screens:

C1 - a screen prepared as above, but lacking any topcoat.

C2 - a screen similar to the above, but with a phosphor/binder weight ratio of 8:1, and with a conventional uncured solvent-coated topcoat of cellulose acetate and polyvinylacetate approx. 5 µm thick.

C3 - a screen prepared as above, but with a wet-coated, cured topcoat. (The wet coating consisted of 90 parts Sartomer™ 399, 10 parts Ebecryl™ 1800, 2 parts Darocur™ 1173 and 4 parts antistat (perfluorooctylsulphonamide derivative of Jeffamine™ ED-900, prepared by the method described in US5,217,767), and was applied as a bead across the phosphor layer by pipette. A PET coversheet was placed on top, and the assembly fed through a laminator (with rollers set to provide a 3 - 5 µm topcoat), then exposed to UV radiation sufficient to cure the topcoat (approx. 1 minute exposure to 400 watt mercury lamp.) Finally, the coversheet was removed.

Photographic Speed and Image Quality

[0065] Samples of a conventional green-sensitive double-sided X-ray film were exposed to X-rays (80 kVp, 0.1s) using pairs of the screens 2(b), C1 and C2, developed, and subjected to conventional sensitometric analysis. All three types of screen showed the same photographic speed, within the experimental error of ± 0.02 logE.

[0066] The modulation transfer function (MTF) at 2 Ip/mm (a measure of image resolution) was calculated according to the edge method described in "Image Science", J. C Dainty & R. Shaw, Academic Press, 1974, pages 244-246 'Spread Function Methods'. The resulting MTF values were 0.22 for screens 2(b) and C2, and 0.24 for C1, which lacked a topcoat. Thus, the provision of a topcoat caused a very slight loss of resolution, but the performance of the topcoat of the invention was equivalent to that of a conventional solvent coated topcoat in this test.

Film/Screen Transport Tests

[0067] One pair each of screens 2(a), 2(b), 2(c), C2 and C3 were tested in an Imation Trimatic™ Automatic Daylight Film Loader. Cassettes containing the screens were subjected to 100 load/unload cycles using standard double-sided X-ray film, at both high (75%) and low (15%) relative humidity, and the number of errors noted. (High RH tests were run at 31°C, low RH tests at 23°C).

[0068] All three screen pairs of the invention gave excellent results, with no errors recorded at high RH and 1% or fewer at low RH, comparing favorably with the reference screens C2 which gave 4% errors at high RH and 2% at low RH. Screens C3 performed well at low RH, with no errors recorded, but failed at high RH, because the screens adhered to one another and the cassette could not be opened to load the first film.

Hardness Tests

[0069] The topcoats of screens 2(a), 2(b), 2(c) and C3 were tested for relative hardness using a weighted stylus device. The point of the stylus was placed on the surface of the test sample and weights applied until a visible indentation was made on the surface. The results record the maximum weight which could be applied without causing an indentation:

2(a)

- 160g

2(b)

- 140g

2(c)

- 160g

C3

- 120g

[0070] Thus, the screens in accordance with the invention show greater resistance to surface damage.

Example 3

[0071] This example illustrates the manufacture of screens by the one-step process in accordance with the invention.

[0072] The following formulation was coated as a 22% w/w solution in acetone on to 100 µm PET at 5.7 g/m² dry coating weight to provide a photocurable layer releasably attached to a transparent carrier (all parts by weight):

Elvacite™ 2008

- 53 parts

Sartomer™ 399

- 32 parts

Darocur™ 1173

- 5 parts

Cyastat™ SN

- 1 part

Catanac™ 609

- 1 part

Syloid™ 74

- 1 part

[0073] A roll of this material was fed between calender rolls (500 µm gap) together with the reflective support described in Example 2 so that the coated sides faced each other. As the webs entered the nip, a dispersion of phosphor particles in a photocurable binder (described below) was applied between them, and the resulting sandwich was passed directly at a web speed of 4 m/min under a UV lamp situated 10 cm above the laminate and equipped with an H bulb, thereby curing the phosphor layer and the topcoat simultaneously. Finally, the 100 µm PET carrier sheet was peeled off, to give the radiographic screen 3(a) in accordance with the invention.

[0074] The photocurable phosphor dispersion was prepared by dissolving photoinitiator Irgacure™ 1800 (2% by weight of the total solids) into photopolymerizable monomer tripropyleneglycol diacrylate (40% of total solids), mixing for 15 min then adding to the solution additional prepolymer Sartomer™ 344 (20%), acrylate prepolymer Ebecryl 270 (37%) and additive Disperbyk™ 110 (1%). Tb-doped gadolinium oxysulfide phosphor was added to the resin in a weight ratio of 11:1, and the blend mixed by hand.

[0075] Screen 3(b) of the invention was prepared similarly, except that Disperbyk™ 110 was omitted from the phosphor layer formulation (being replaced by an extra 1% of Ebecryl™ 270), the phosphor dispersion was mixed using a three roll mill, and the gap between the calender rolls was 450 µm.

[0076] Screen 3(c) of the invention was prepared as follows:

[0077] The following formulation was coated as a 25% solids solution in MEK on to 50 µm PET at 5.8 g/m² dry coating weight to provide a photocurable layer releasably attached to a transparent carrier (all parts by weight):

Elvacite™ 2014

- 60 parts

Santiciser™ 278

- 5 parts

Sartomer™ 399

- 25 parts

Darocur™ 4265

- 6 parts

Blankophor™ MAN-01

- 1 part

Catanac™ 609

- 2 parts

Cyastat™ SN

- 2 parts

[0078] This material was calendered and cured in a similar manner as for Screens 3(a) and 3(b), but with the following modifications. The phosphor dispersion had a phosphor/binder ratio of 11.3 and was mixed using a double planetary mixer, then held under vacuum for 30 minutes. The gap between the calender rolls was 425 µm, and UV curing was effected with a V bulb at a web speed of 5 m/min.

[0079] For each of screens 3(a)-3(c), a control screen was prepared which lacked a topcoat (i.e., plain PET was used as coversheet during the calendering and curing of the phosphor dispersion).

[0080] Film/screen friction was assessed for screens 3(a) and 3(b) using a Thwing-Albert friction tester (50% RH at 22°C, 200g sled at 13 cm/min) and standard X-ray film. Surface resistivity and static charge decay time were measured for screen 3(c) at 25% RH and 21°C. Surface resistivity measurements were obtained using a Surface Resistance Meter Model 482, supplied by Industrial Development Bangor Ltd. (Bangor, Wales, UK). Charge decay times were measured using a JCI 155 Charge Decay Test Unit supplied by John Chubb Instrumentation (Cheltenham, UK). This instrument applies a corona discharge to the specimen surface and then monitors the surface voltage as a function of time. The charge decay time is taken as the time (in seconds) for the surface voltage to fall to the fraction 1/e of the value at discharge. The results are presented below, where ST is the coefficient of static friction, KI is the coefficient of kinetic friction, and delta is the variation in the amplitude of the force vs. time curve. Figures in brackets are for the controls lacking a topcoat.

	Screen 3(a)	Screen 3(b)	Screen 3(c)
ST	0.45 (0.49)	0.33 (0.40)	nd
KI	0.34 (0.48)	0.32 (0.38)	nd
Delta (g)	2.0 (5.5)	3.0 (4.5)	nd
Resistivity (Ω/sq)	nd	nd	1.7x1010 (3.0x1013)
Decay Time (sec)	nd	nd	2.2 (*)

nd - not determined

* - not measurable for the control

[0081] The provision of the topcoat in accordance with the invention clearly improved both the frictional and antistatic properties.

Example 4

[0082] This example illustrates the incorporation of fluorinated resins and monomers in screen topcoats in accordance with the invention.

[0083] A series of photocurable resin layers of differing composition were coated on to separate transparent carriers (50 µm PET), then used to manufacture screens 4(a) - 4(i) by the method described in Example 2 for screens 2(b). The resin layers were handcoated from 25%w/w solutions in MEK at 24 µm wet thickness using a wire-wound bar, and dried in an oven at 85°C for 5 minutes.

[0084] In Screen 4(a), the resin layer had essentially the same composition as in Example 1, with the exception that Uvitex™ OB and Syloid™ ED5O were omitted.

[0085] Screens 4(b) - 4(f) used the same formulation as 4(a) but with substitution of the Elvacite™ 2008 by Fluorel™ 2330 to the extent (respectively) of 5%, 15%, 25%, 50% and 75%.

[0086] Screens 4(g) - 4(i) used the same formulation as 4(a), but with substitution of 25% of the Sartomer™ 399 by a fluorinated monomer, as follows:

4(g)

- perfluorocyclohexyl acrylate

4(h)

- 1,1-dihydroperfluorooctyl acrylate

4(i)

- N-ethylperfluorooctylsulphonamidoethyl acrylate

Photographic Speed

[0087] The screens 4(a) and 4(d) - 4(i) were tested for photographic speed as described in Example 2, and all were found to be at least as fast as the control C2 having a conventional solvent-coated topcoat. Those with a relatively high content of fluorinated resin or monomer in the topcoat [4(d) - 4(i)] consistently gave a slight speed increase (0.01 - 0.04 logE) relative to the fluorine-free Screen 4(a).

Electrostatic Properties

[0088] The surface resistivity and charge decay time were measured for Screens 4(a) - 4(d) and 4(g) - 4(i) at 21°C and 42% RH by the method described in Example 3.

[0089] The results obtained were as follows:

Screen	log SR	Decay Time (sec)
4(a)	10.90	0.64
4(b)	10.80	0.43
4(c)	10.80	0.36
4(d)	10.77	0.47
4(g)	10.77	0.42
4(h)	10.90	0.51
4(i)	10.70	0.42
N. B. log SR = log10(surface resistivity in ohms/square)

[0090] All the screens showed good electrostatic properties, but those with fluorochemical additives showed slightly lower surface resistivities and/or shorter charge decay times.

Example 5

[0091] This example illustrates the use of plasticizer in the curable resin layer, and different thermoplastic resins, to improve the flexibility of the cured layer.

[0092] Screens 5(a) - 5(c) of the invention were prepared similarly to screen 2(b), except that lamination of the photocurable layer was conducted at 50 cm/min with the upper roller at 132°C and the lower roller at 90°C, the laminate was exposed to 100 units UV radiation, and the following formulations were used for the curable resin layer:

	Screen 5(a)	Screen 5(b)	Screen 5(c)
Elvacite™ 2014 (pbw)	60	-	40
CAB 381-20 (pbw)	-	60	20
Santiciser™ 278 (pbw)	5	5	5
Sartomer™ 399 (pbw)	25	25	25
Darocur™ 4265 (pbw)	6	6	6
Blankophor™ MAN 01 (pbw)	1	1	1
Catanac™ 609 (pbw)	1	1	1
Cyastat™ SN (pbw)	1	1	1
BYK™ - 307 (pbw)	1	1	1
% solutes in coating solution	25	8	14.3
Dry coating weight (g/m2)	5.8	5.8	5.8
(pbw = parts by weight)

[0093] The flexibility of the resulting screens (and of screen 2(b)) was assessed using an apparatus which imposed a bend on a strip cut from the screen, the radius of the bend decreasing as the strip was pulled through the apparatus, and measuring the length of the strip which suffered from cracking as a result, after first swabbing the screen surface with an aqueous dispersion of carbon black so as to render the cracks more visible. In this test, screens 5(a) - 5(c) all showed a similar low susceptibility to cracking, whereas screen 2(b) showed relatively higher level of cracking.

[0094] The ease with which the carbon black could be removed from the screen surface provided an indication of how easily each screen might be kept clean during extended service. Those comprising cellulose acetate butyrate as part or all of the binder were markedly easier to clean than those containing Elvacite™ resins as the sole binder component.

Claims

1. A method for manufacturing a radiographic screen comprising the steps of:

(a) providing a first element comprising a carrier having deposited thereon a releasable curable resin layer;

(b) contacting said curable resin layer of said first element with a phosphor-containing layer, wherein said phosphor-containing layer is either self-sustaining or deposited upon a support and emits light in response to irradiation with X-rays;

(c) curing the curable resin layer and optionally the phosphor containing layer with radiation energy or heat; and

(d) removing said carrier.

2. The method of claim 1 wherein step (d) is carried out before step (c).

3. The method according to claim 1 wherein said radiation energy is UV radiation.

4. The method according to claim 1 wherein said radiation energy is electron beam radiation.

5. The method according to claim 3 wherein said carrier is transparent to UV radiation.

6. The method according to claim 1 wherein said curable resin layer comprises a blend of one or more thermoplastic film-forming polymers with one or more polymerizable monomers.

7. The method according to claim 6 wherein said one or more thermoplastic film-forming polymers constitute at least 25% by weight of said curable resin layer.

8. The method according to claim 6 wherein said one or more thermoplastic film-forming polymers constitute from 25 to 75% by weight of said curable resin layer.

9. The method according to claim 6 wherein said one or more thermoplastic film-forming polymers are selected from the group consisting of polyesters, polycarbonates, thermoplastic polyurethanes, poly(meth)acrylate resins, cellulose esters, cellulose ethers, phenolic resins, poly(vinylalcohol), poly(vinylbutyral), vinyl chloride (co)polymers, vinyl ester (co)polymers, vinyl ether (co)polymers, styrene (co)polymers, fluorinated polymers and combinations thereof.

10. The method according to claim 6 wherein said one or more thermoplastic film-forming polymers are selected from the group consisting of polyacrylate resins and cellulose acetate butyrate, and are present in an amount ranging from 45 to 65% by weight of said curable resin layer.

11. The method according to claim 6 wherein said one or more polymerizable monomers are present in an amount ranging from 10 to 75 % by weight of said curable resin layer.

12. The method according to claim 6 wherein said one or more polymerizable monomers are selected from the group consisting of epoxides, (meth)acrylates, (meth)acrylamides, vinyl ethers, vinyl esters, styrenes, and fluorinated monomers.

13. The method according to claim 6 wherein said one or more polymerizable monomers are polyfunctional (meth)acrylates or (meth)acrylamides, and are present in an amount ranging from 15 to 60 % by weight of said curable resin layer.

14. The method according to claim 6 wherein said curable resin layer additionally comprises an initiator.

15. The method according to claim 14 wherein said initiator is a photoinitiator selected from the group consisting of aryldiazonium salts, diaryliodonium salts, triarylsulphonium salts, benzophenones, benzoins, benzoin alkyl ethers, benzil dialkylketals, acylphosphine oxides, and trichloromethyltriazines, and is present in amount from 0.5 - 15.0 % by weight of said curable resin layer.

16. The method according to claim 15 wherein said curable resin layer additionally comprises a photosensitizer.

17. The method according to claim 6 wherein said curable resin layer additionally comprises a plasticizer.

18. The method according to claim 1 wherein said curable resin layer contains an antistat.

19. The method according to claim 1 wherein said curable resin layer comprises at least one dye or pigment.

20. The method according to claim 19 wherein said at least one dye or pigment absorbs at a wavelength corresponding to the emission peak of the phosphor.

21. The method according to claim 1 wherein said phosphor-containing layer comprises a phosphor that emits green, blue or UV light in direct response to irradiation with X-rays.

22. The method according to claim 1 wherein said phosphor-containing layer comprises a storage phosphor.

23. The method according to claim 1 wherein said phosphor-containing layer comprises a dispersion of phosphor particles in an organic binder and is deposited onto said support in a separate operation.

24. The method according to claim 23 wherein said curable resin layer and said phosphor-containing layer are laminated together using a heated roller laminating device.

25. The method according to claim 1 wherein said phosphor-containing layer is formed on a support and assembled in contact with said curable resin layer in a single operation.

26. The method according to claim 25 wherein said phosphor-containing layer comprises a dispersion of phosphor particles in a curable binder, and is cured simultaneously with said curable resin.

27. The method according to claim 26 in which curing is effected by UV radiation.

28. The method according to claim 26 wherein said phosphor-containing layer and said curable resin layer become chemically bonded to each other as a result of said simultaneous curing.

29. A radiographic screen comprising in the following order:

(a) a substrate;

(b) a phosphor-containing layer, wherein said phosphor emits light in response to irradiation with X-rays; and

(c) a cured topcoat;
characterized in that said cured topcoat comprises at least 25% by weight of one or more thermoplastic film-forming polymers and up to 75% by weight of a crosslinked acrylic resin.

30. The radiographic screen according to claim 29 wherein said cured topcoat is chemically bonded to said phosphor-containing layer.

31. The radiographic screen according to claim 29 wherein said phosphor emits green, blue or UV light in direct response to irradiation with X-rays.

32. The radiographic screen according to claim 29 wherein said phosphor is a storage phosphor.

33. The radiographic screen according to claim 29 wherein said cured topcoat comprises an antistat.

34. The radiographic screen according to claim 29 wherein said cured topcoat comprises at least one dye or pigment.

35. The radiographic screen according to claim 34 wherein said at least one dye or pigment absorbs at a wavelength corresponding to the emission peak of said phosphor.

36. The radiographic screen according to claim 29 wherein said cured topcoat comprises from 45 to 65 % by weight of one or more thermoplastic film-forming polymers.

37. The radiographic screen according to claim 29 wherein said one or more thermoplastic film-forming polymers are selected from the group consisting of polyesters, polycarbonates, thermoplastic polyurethanes, poly(meth)acrylate resins, cellulose esters, cellulose ethers, phenolic resins, poly(vinylalcohol), poly(vinylbutyral), vinyl chloride (co)polymers, vinyl ester (co)polymers, vinyl ether (co)polymers, styrene (co)polymers, fluorinated polymers and combinations thereof.

38. The radiographic screen according to claim 37 wherein said one or more thermoplastic film-forming polymers are selected from the group consisting of polyacrylate resins and cellulose acetate butyrate.

39. The radiographic screen according to claim 29 wherein said phosphor-containing layer comprises a dispersion of phosphor particles in an organic binder.

40. The radiographic screen according to claim 29 wherein said phosphor-containing layer comprises a dispersion of phosphor particles in a cured binder.

41. The radiographic screen according to claim 29 wherein said phosphor-containing layer is obtained by curing a dispersion of phosphor particles in a binder comprising a photopolymerizable mixture of at least one urethane (meth)acrylate prepolymer, at least one photopolymerizable monomer and/or oligomer, and a photoinitiator.