Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
  • G21K4/00—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
  • G21K4/00—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens
  • G21K2004/08—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens with a binder in the phosphor layer
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
  • G21K4/00—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens
  • G21K2004/10—Conversion screens for the conversion of the spatial distribution of X-rays or particle radiation into visible images, e.g. fluoroscopic screens with a protective film

Definitions

• 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.
• 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.
• 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.
• 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.
• the phosphor emits light directly and the light exposes a silver halide photographic film placed in contact with the screen.
• 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 TrimaticTM.
• 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.
• the screens should resist the build-up of static charge, which (by discharging) can lead to fogging of the film.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• US 5,569,485 describes the application of an antistatic topcoat to radiographic screens comprising a UV-cured phosphor layer.
• 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 ChromalinTM and EurosprintTM proofing systems.
• 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).
• the phosphor-containing layer comprises a photocurable resin as a binder and the radiation energy is UV radiation.
• 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.
• the cured topcoat is chemically bonded to the phosphor-containing layer, and the phosphor-containing layer comprises a phosphor dispersed in a curable binder.
• 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.
• 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.
• 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.
• the crosslinked resin is a photocured acrylic resin.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a photocurable layer releasably attached to a transparent carrier, where the entire area of the photocurable layer is irradiated.
• 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 ⁇ m, preferably about 100 ⁇ m.
• 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.
• 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.
• a release layer is not required.
• 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.
• 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.
• thermoplastic film-forming polymers 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.
• 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.
• thermoplastic film-forming polymer 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 ElvaciteTM 2008 and 2014 (polyacrilate resins supplied by ICI), cellulose acetate butyrate, and FluorelTM 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.
• 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.
• 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.
• 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.
• initiators which release free radicals on thermal decomposition include peroxides (such as benzoyl peroxide, dicumyl peroxide etc.) and azo compounds (such as azobisisobutyronitrile).
• photoinitiators which decompose in response to photoirradiation especially UV irradiation
• photoinitiators which decompose in response to photoirradiation (especially UV irradiation) are preferred.
• 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 LucirinTM TPO and DarocurTM 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.
• sensitizers are optical brighteners, such as coumarins, pyrazolines and 2,2'-bisbenzoxazolyl derivatives, preferred examples including UvitexTM OB, supplied by Ciba Geigy, and Blankophor MAN-01 TM, supplied by Bayer.
• 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.
• 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.
• 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.
• 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 CatanacTM 609 and CyastatTM SN, both supplied by American Cyanamid.
• plasticizers which improve the flexibility of the cured coating
• 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 SanticiserTM 278 (benzyl phthalate, supplied by Monsanto).
• Suitable coating aids include BYKTM 307 and DisperbykTM 161 (both supplied by Byk-Chemie).
• 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).
• Further optional ingredients include dyes or pigments to alter the sensitometric properties of the screen.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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.
• MEK methyl ethyl ketone
• the coating is typically dried at moderately elevated temperatures, e.g. of about 80°C.
• 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 ⁇ m, preferably from 2 to 10 ⁇ m, more preferably from 3 to 7 ⁇ m.
• 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.
• 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 ⁇ m in thickness.
• 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 ⁇ m 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.
• 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).
• 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.
• the phosphor screen may be formed using a 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.
• 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.
• the phosphor-containing layer may be formed by any method known in the art.
• 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.
• 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.
• the dried coating may be subjected to a calendering process as taught in US 4,952,813.
• 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).
• 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.
• 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 MatchprintTM 447 laminator available from IMATION CORP.
• the curable resin layer has pressure-sensitive adhesive properties (i.e. is tacky at ambient temperature), then pressure lamination may be sufficient.
• the curable resin layer is non-tacky at ambient temperatures and softenable at moderately elevated temperatures (i.e.
• 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 MatchprintTM 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.
• 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.
• 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 ⁇ m, preferably from 2 to 10 ⁇ m, most preferably from 3 to 7 ⁇ m.
• 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.
• 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.
• energy e.g. heat, e-beam or UV radiation
• UV radiation is used as the energy source.
• the carrier is peeled off leaving a cured topcoat bonded to the cured phosphor layer.
• the curable components of the two layers at the interface between the layers 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.
• 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.
• 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.
• 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.
• 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.
• Both urethane acrylate prepolymers and other additional prepolymers are characterized by a molecular weight higher than 500, preferably higher than 1000.
• 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.
• the photoinitiator 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, ⁇ -hydroxyalkylphenones, ⁇ -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%).
• 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.
• 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, ⁇ -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.
• 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.
• the preferred procedure for manufacturing radiographic screens by the one step process includes the following steps.
• 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.
• 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.
• 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.
• 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.
• the laminate is then exposed to radiation and cured.
• the exposure is performed through the transparent carrier sheet.
• 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.
• 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.
• 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.
• 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.
• the average particle size of the phosphor is preferably within the range 0.3 - 50 ⁇ m, more preferably within the range 1 - 30 ⁇ m, and the thickness of the phosphor-containing layer is preferably within the range 10 ⁇ m - 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.
• composition (as a 20 %w/w solution in MEK) was coated on 50 ⁇ m 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):
• the dried coating weight was approx. 5.2g/m 2 , and this material was used in all subsequent experiments, unless otherwise indicated.
• This example illustrates the manufacture of screens in accordance with the invention by the two-step process.
• a reflective support comprising a TiO 2 -filled polyurethane coating (approx. 25 ⁇ m thick) on 250 ⁇ m PET was further coated with a dispersion of a terbium-doped gadolinium oxysulfide phosphor (average particle size approx. 8 ⁇ m) and hydrophobic polymer binder in a mixed solvent to give a phosphor coating weight of approx. 700 g/m 2 , a dry coating thickness of approx. 150 ⁇ m, 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 MatchprintTM 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:
• the modulation transfer function (MTF) at 2 Ip/mm 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.
• 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.
• 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:
• the screens in accordance with the invention show greater resistance to surface damage.
• This example illustrates the manufacture of screens by the one-step process in accordance with the invention.
• a roll of this material was fed between calender rolls (500 ⁇ m gap) together with the reflective support described in Example 2 so that the coated sides faced each other.
• 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.
• the 100 ⁇ m PET carrier sheet was peeled off, to give the radiographic screen 3(a) in accordance with the invention.
• the photocurable phosphor dispersion was prepared by dissolving photoinitiator IrgacureTM 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 SartomerTM 344 (20%), acrylate prepolymer Ebecryl 270 (37%) and additive DisperbykTM 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.
• Screen 3(b) of the invention was prepared similarly, except that DisperbykTM 110 was omitted from the phosphor layer formulation (being replaced by an extra 1% of EbecrylTM 270), the phosphor dispersion was mixed using a three roll mill, and the gap between the calender rolls was 450 ⁇ m.
• 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 ⁇ m, and UV curing was effected with a V bulb at a web speed of 5 m/min.
• 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).
• a topcoat i.e., plain PET was used as coversheet during the calendering and curing of the phosphor dispersion.
• 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.
• ST is the coefficient of static friction
• KI is the coefficient of kinetic friction
• delta is the variation in the amplitude of the force vs. time curve. Figures in brackets are for the controls lacking a topcoat.
• topcoat in accordance with the invention clearly improved both the frictional and antistatic properties.
• This example illustrates the incorporation of fluorinated resins and monomers in screen topcoats in accordance with the invention.
• a series of photocurable resin layers of differing composition were coated on to separate transparent carriers (50 ⁇ m 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 ⁇ m wet thickness using a wire-wound bar, and dried in an oven at 85°C for 5 minutes.
• the resin layer had essentially the same composition as in Example 1, with the exception that UvitexTM OB and SyloidTM ED5O were omitted.
• Screens 4(b) - 4(f) used the same formulation as 4(a) but with substitution of the ElvaciteTM 2008 by FluorelTM 2330 to the extent (respectively) of 5%, 15%, 25%, 50% and 75%.
• 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).
• This example illustrates the use of plasticizer in the curable resin layer, and different thermoplastic resins, to improve the flexibility of the cured layer.
• 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) ElvaciteTM 2014 (pbw) 60 - 40 CAB 381-20 (pbw) - 60 20 SanticiserTM 278 (pbw) 5 5 5 5 SartomerTM 399 (pbw) 25 25 25 25 DarocurTM 4265 (pbw) 6 6 6 6 BlankophorTM MAN 01 (pbw) 1 1 1 1 CatanacTM 609 (pbw) 1 1 1 1 1 CyastatTM SN (pbw) 1 1 1 1 BYKTM - 307 (pbw) 1 1 1 % solutes in coating solution 25 8 14.3 Dry coating weight (g/m 2

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• 1997
  • 1997-11-05 EP EP97119309A patent/EP0915483A1/de not_active Ceased

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