RADIATION CURED RADIOGRAPHIC INTENSIFYING SCREEN

Abstract: The present invention describes a method for the manufacture of a phosphor intensifying screen which eliminates the use of solvents and high processing temperatures. The method comprises blending a phosphor in a hardenable system (i.e., that is, as defined herein, a polymerizable or curable system) comprising phosphor in a radiation curable binder composition, coating the phosphor in a radiation curable binder composition onto a substrate, and polymerizing (i.e., hardening) the system by exposure to actinic radiation. The radiation curable binder composition comprises an organopolysiloxane or a mixture of organopolysiloxanes.

Description: RADIATION CURED RADIOGRAPHIC INTENSIFYING SCREEN BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to processes for ihe manufacture of intensifying screens, particularly intensifying phosphor screens for use in radiographic imaging 2. Background of the Art There are at least two critical objectives in the production of radiographic images, particularly in medical radiographic images One desired aspect of radiographic imaging is the faithfulness of the generated image to the object through which radiation was passed during imaging Another important aspect, particularly during medical radiographic imaging, is the reduction of the level of exposure of the object (patient) to radiation during the imaging process One significant means of reducing the level of X-ray exposure during imaging has been the use of "intensifying screens" during the imaging process These screens usually comprise phosphors in a binder on a carrier layer. The phosphors absorb X-ray radiation at a higher efficiency than does silver halide which is normally used in the hard-copy output of radiographic images The phosphors not only absorb X-rays at an efficient rate, but can also phosphoresce (or fluoresce), emitting radiation at a wavelength other than the wavelength of X- rays which the phosphor absorbed. Depending upon the chemical nature and properties of the phosphor, the emitted radiation may be al essentially any wavelength between and including the infrared and ultraviolet wavelengths of the electromagnetic spectrum Silver halide naturally absorbs radiation in the ultraviolet and near blue wavelengths, and can be spectrally sensitized to efficiently absorb radiation in other portions of the visible and the infrared spectrum. By using an X-ray absorbing phosphor intensifying screen and a silver halide emulsion spectrally sensitized to the wavelength of emission of the phosphor screen, the efficiency of an X-ray imaging system can be greatly enhanced This allows for the use of lower doses of X-rays during exposure of the object The use of such phosphors is well known in the art as exemplified by U S Pat. Nos 3,883,747 and 4,204, 125, where there is direct emission of phosphorescent radiation upon X-rav stimulation Other examples of such phosphors may be found in U S Pat Nos 3,859,527 and 5, 164,224, where there is exposure to X-rays, storage of the absorbed energy by the phosphor, and subsequent stimulation by stimulating radiation to cause the phosphor to emit the stored energy as UV to infrared radiation These phosphor systems are commercially successful and provide a significant benefit to the radiographic art In these types of system-, hυwe . ei thei e is a ti de-υff between speed and sharpness The screen can be made thicker in order to absorb more X-rays and emit more light Unfortunaielv light generated w ithin the screen is scattered b\ the phosphor grains to a greater extent in thicker screens, thereby reducing the resulting image sharpness recorded on the film Conversely, to improve sharpness, a thinner screen is desirable, but a thinner screen requires a higher dosage to the patient or object being X-rayed due to reduced X-ray absorbing capability Many methods of impiO . ing the image qualιt\ . particularly the sharpness of images generated from phosphor screens without adversely affecting the sensitivity or speed of the system, have been proposed Reflective particulates, dyes, pigments and other light affecting materials have been proposed as additives to phosphor layers to improve sharpness as shown in EPO 102 790 (powdered glass), Japanese Application 146,447/ 1980 (white pigments), Japanese Patent Application 163.500/1980 (colorants), and EPO 175 578 (sputtering or vacuum evaporation of phosphoi s) I he objective of these methods is primarily to provide a high concentration of phosphor in the active layer of the screen and provide a screen of uniform properties U S Pat No 5,306,367 produces a storage phosphor screen by dispersing phosphor particles in a thermoplastic binder diluted with a solvent, then coats the mixture, dries to remove the solvent, and compresses the coating at a temperature above the melting point of the binder -j - U S Pat No 5,296, 1 17 deposits phosphor particles in a binder by electrophoretic deposition of a dispersion of the phosphor particles in a solution of polymeric binder The solution is coated onto a substrate, dried and the phosphor screen thus produced Each of these types of systems has shown some benefits, but there is still significant room for improvement in the sharpness of radiographic phosphor screens In particular, elimination of complicated or costly deposition processes, elimination of the use of environmentally harmful solvents, and reduction or elimination of high processing temperatures would be desirable Many radiation curable polymer systems are known However, many of these systems do not piovide the piopeπies required of a binder for phosphor particles in intensifying screens U S Patent 5,091 ,483 discloses radiation curable organopolysiloxanes which may cure to an elastomer or be used in a pressure sensitive adhesiv e SUMM ARY OF 1 HE INN EM ION The present invention describes a method for the manufacture of a phosphor intensifying screen w hich eliminates the need foi soh ents and high processing temperatures The invention also encompasses the phosphor screen itself The method comprises blending a phosphor in a hardenable system (i e , that is, as defined hei eni, a polymei izable υi cuiable s . stem) comprising phosphor in a radiation curable binder composition, coating said phosphor in a radiation curable binder composition onto a substrate, and polymerizing or curing (i e , hardening) said system by exposure to actinic radiation The radiation curable silicon composition comprises an organopolysiloxane polymer or a mixture of organopolysiloxane polymers at least one of which has the following general formula wherein X is an organic group having ethylenic unsaturation, R and Y are independently divalent linking groups, m is an integer of 0 to 1 , D is selected from hydrogen, an alkyl group of I to preferably no more than 10 carbon atoms, and an aι \ l gi oup of up to 20 carbon atoms. R 1 are monovalent substituents which can be the same or different and are selected from an alkyl group of up to 20 carbon atoms and an aryl group of up to 20 carbon atoms, R 2 are monovalent substituents which can be the same or different and are selected from an alkyl group of up to 20 carbon atoms and an arvl group of up to 20 carbon atoms. R ' is a monovalent substiuient which can be the same υr different and is selected from an alkyl group of up to 20 carbon atoms and an aryl group of up to 20 carbon atoms, R 4 is a monovalent substituent which can be the same or different and is selected from an alkyl group of up to 20 carbon atoms and an aryl group of up to 20 carbon atoms, and n is an lntegei of lϊ m about ι to about I 0UU As is well undei tood in this aiea. substitution is not onl . toleiaied. but is often advisable and substitution is anticipated on the compounds used in the present invention As a means of simplifying the discussion and recitation of certain terminology used thioughout this application, the terms "group" and "moiety" are used to differentiate between chemical species that allow for substitution or which may be substituted and those which do not so allow or may not be so substituted " I hu . hen the tei ni "gi oup" is used to describe a chemical compound or substituent, the described chemical material includes the basic group and that group with conventional substitution Where the term "moiety" is used to describe a chemical compound oι substituent, only an unsubstituted chemical material is intended to be included For example the phrase l gi oup" is -v intended to include not onl\ pin e open-chain and cyclic saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, and the like, but also alkyl substituents bearing further substituents known in the art, such as hydroxyl, alkoxy, vinyl, phenyl, halogen atoms (F, Cl, Br, and 1), cyano, nitro, amino, carboxyl, etc On the other hand, the phrase "alkyl moiety" is limited to the inclusion of only pure open-chain and cyclic saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, t-butyl, cyclohexyl, adamantyl, octadecyl, and the like DETAILED DESCRIPTION OF THE INVENTION Any stimulable or fluorescing phosphor which absorbs X-rays and emits radiation between 200nm and I l OOnm can be used in the practice of the present invention Normally those phosphors are to be provided as particulates, particularly with average particle sizes between 0 3 and 50 microns, preferably between 0 5 and 40 microns, more preferably between 0 7 and 35 microns and most preferably between I and 30 microns Amongst the many phosphors known in the art which may be considered in the practice of the present invention are alkali halides, doped alkali ha des. rare earth and others such as are described in U S Pat No 5,302,423 which is included herein by reference for its disclosure of phosphors Other literature disclosing phosphors which are contemplated within the scope of the present invention include U S Pat Nos 4,258,264, 4,261 ,854, 5, 124. 64. 4.225,653, 4.3S7, 14 1 , 3 795.8 14. 3,974.389. The radiation curable binder compositions useful in this invention cure rapidly, completely, and reliably to form a translucent υi tianspaient binder upon exposure to UV radiation The binders may have to be particularly selected for use with individual phosphors as some polymei izable materials may react with active components in the phosphoi , reducing or destroying the efficiency of the phosphor in absorbing and emitting radiation The binder systems comprise at least one organopolysiloxane according to Formula I and preferably comprise a combination of a low moleculai weight organopolysiloxane. preferably having a molecular weight less than about 20 000 more preferably less than 15,000, and a high molecular weight organopolysiloxane, preferably having a molecular weight of more than about 20,000. more preferably more than 25.000 The precise molecular weight divisions vary amongst the oiganopolysiloxanes within the scope of Formula I The blending of high and low molecular weight difunctional organopolysiloxane may allow controlled variation of properties Desired properties can be obtained via variation in the nature, molecular weight, and amount of the material added In fact, the blending of the high molecular weight and low molecular weight difunctional organopolysiloxanes provides beneficial synergistic effects on mechanical properties of the cured material When blending the high and low molecular weight or ganopolysiloxanes. a compatibilizer may be used if necessary Preferably, the weight l atio of low molecular w eight difunctional organopolysiloxane to high moleculai w eight difunctional organopolvsiloxane in the hardenable coating composition is in the range of fiom 10 90 to 90 10. more preferably from 30 70 to 70 30 High concentrations of low molecular weight difunctional organopolysiloxane yield materials with a higher crosslink density (low molecular weight between crosslinks) and may give less desirable mechanical properties Too much low molecular weight organopolysiloxane may also create difficulties in coating the binder/phosphor blend, e g a tendenc> to e voids or pinholes As an alternative to the low molecular weight difunctional organopolysiloxane, a monofunctional siloxane macromolecular monomer may be used See U S Patent 5,091 ,483, incorporated herein by reference The oiganopolysiloxanes have been chemically tailored to contain terminal functionality which provides l apid and complete cure even at high molecular weight, thereby overcoming the moleculai weight limitations of prior art radiation- cured systems Thus, an important ieatui e oi ' the invention is the use of organopolysiloxane. radiation curable binder systems hav ing terminal groups which not only contain ethylenic unsaturation (so as to be free-radically polymerizable) but which, in addition, possess both hydrogen bond donor and acceptor capabilities The use of such groups enables rapid and complete cure, such that prior art problems with stability of properties are also overcome, and renders achievable the uniform cure of thick films Such groups additionally enable careful regulation of crosslink density Other advantages of the silicone compositions of this invention include ease of preparation, ease of processing (which reduces or even eliminates the need for solvent), and as radiation-curable systems, the ability to cure without damage to heat-sensitive substrates The silicone composition of the inv ention is represented b> Formula I An example of a preferred organolysiloxane comprises the organopolysiloxane of Formula I wherein X comprises CH - CH ,=C Y comprises O H II I -COCH 2 CH 2 N — m=l , D=H, R comprises ACH .CHiCH -A, and R 1 . R'. R' and R 4 each comprise - CH 3 Acrylamidoamidosiloxane (herein also referred to as ACMAS) is another preferred embodiment According to this embodiment ACMAS as defined by Formula 1 has X comprising CH_-=CHA. Y comprising CH 3 m=l, D = H, R comprising ACH 2 CH 2 CH : A, and R 1 . R 1 R' and R 4 each comprising ACH . Another preferred organopolysiloxane comprises the organopolysiloxane of Formula 1 wherein X comprises CH 2 ICHA, m=0, DIH, R comprises λCH 2 CH 2 CH 2 λ, and R 1 , R 2 . R ' and R 4 each comprise ACH, The silicone binder compositions of this invention comprise terminally difunctional, i e , telechelic, silicones represented by Formula I above, which can be prepared by reaction of an oiganopolv siloxane diamine. represented by the general formula where n, R, R 1 , R 2 , R\ R "1 , and D are as defined above, with an electrophile having ethylenic unsaturation, X, and such other functιonalιt\ that, upon reaction with the organopolysiloxane diamine. not only a terminal X group but also an amide, urea, or urethane moiety is provided Examples of the types of functionality required in such electrophilic compounds include acid halide. acid anhydride, and cyclic anhydride (such as the azlactone ring), each of which provides an amide moiety upon reaction with the diamine, and isoc>anate, which provides a urea moiety Preferably. X comprises wherein R " is selected from the group consisting o\ ~ hydrogen and ACOOH and R 6 is selected from the group consisting of hydrogen, methyl, and ACH COOH Most preferably, R 3 comprises hydrogen and R" is selected from the group consisting of hydrogen and methyl The reaction of the organopolysiloxane diamine and the electi ophilc X can be cai πed out at tempei atures of about - I 0°C to about 50°C and undei atmosphenc pressure by combining the diamine and the electrophile while providing appropriate mixing A non-reactive organic solvent can be used as a diluent but is not necessary, and the two reactants can be charged to the reaction vessel in any order Alternatively an organopolysiloxane diamine according to Formula II above can be reacted first with a compound containing the two electrophilic groups, e g , a diisocyanate, (oi with a compound such as phosgene) and the lesultant pi oduct l eacted in a second step with a nucleophile, e g , an amine or an alcohol, to provide terminally difunctional silicone according to Formula I When an alcohol such as hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypiopyl methaciylate is utilized, the product organopolysiloxane contains urethane moieties Organopolysiloxane diamines useful in the preparation of the teleche c si cones of this invention can be piepaied in vaπous wav See U S Patent 5,0 1 ,483, lncorpoiated hei ein b\ l elei ence One pi elened method iυi preparation of organopolysiloxane involv es running the reaction in two stages utilizing a minimum amount of an essentially anhydrous ammo alkyl functional silanolate catalyst represented by the general formula D R 1 III I I H — N — R— Si — O ' where D, R, R 1 , and R 2 ai e as defined above and M is a cation selected from the group consisting of K . Na and tetiaorganoammonium ion with N(CH being preferred In the first stage of the reaction a low molecular weight organopolysiloxane diamine repiesented by the general formula where D, R, R 1 , R 2 , R\ and R are as defined above and x is an integer of about 4 to about 40, is prepared by reacting an amine functional disiloxane endblocker represented by the general formula where D, R, R 1 , and R 2 are as defined above with a cyclic organosiloxane represented by the general formula where R ' and R J are as defined abov e and k is a positive integer of 3 to 8 in the presence of a catalytic amount of essentially anhydrous amino alkyl functional silanolate represented by Formula 111 in an inert atmosphere such as nitrogen or argon The preferred catalyst f i use in this reaction is 3-amιnopropyl dimethyl tetramethylammonium silanolate. which can be obtained as a crystalline solid from the reaction of one molar equivalent of l ,3-bis(3-amιnopropyl) tetramethyldisiloxane with two molar equivalents of tetramethylammonium hydroxide pentahydrate in tetrahydrofuran under reflux, followed by drying under vacuum for five hours (0 1 mm) at 60°C The amount of catalyst employed should be less than about 0 05%. preferably about 0 005 to about 0 03%, by weight of the resultant organopolysiloxane diamine The reaction can be carried out in bulk - I 1 - at a temperature of 80°-90°C, and under these conditions is usually complete in about 0 5-2 hours, as judged by substantially complete disappearance of the endblocker of the reaction mixture as determined by vapor phase chromatography The second stage of the reaction involves the slow addition of the remainder of the cyclic organosiloxane required to achieve the desired molecular weight This addition is preferably carried out dropwise at such a rate that the cyclic organosiloxane is incorporated into the polymer about as fast as it is added, usually in about five to seven hours at the reaction temperature of 80°-90°C By utilizing this two-stage method with a minimum amount of essentially anhydrous catalyst, organopolysiloxane diamines represented by Formula II above can be consistently prepared having excellent difunctiona tv with little contamination from monofunctional and non-functional polysiloxane impurities Preferred organopolysiloxane diamines for use in preparing the teleche c silicones of this invention are those foi which n is an integer of about 35 to about 1000, R is selected from the group consisting of alkylene of one to about twelve carbon atoms, alkylarylene of up to 20 carbons, and arylene of up to 20 carbon atoms, R 1 and R 2 are independently selected from the group consisting of alkyl of one to about twelve carbon atoms, substituted alkyl of one to about twelve carbon atoms, aryl of up to 20 carbon atoms, and substituted aryl of up to 20 carbon atoms R ' and R 4 are at least 50% methyl with the remainder selected from the group consisting of alkyl of two to about twelve carbon atoms, substituted alkyl of two to about twelve carbon atoms, vinyl, aryl of up to 20 carbon atoms, and substituted aryl of up to 20 carbon atoms, and D is hydi ogen Most preferably, R is alkylene of one to about twel e carbon atoms and R' R " . R . and R "1 are methyl as polydimethylsiloxanes ai e the most l eadily av ailable and the most men Examples of electrophiles suitable for reaction with organopolysiloxane diamines to produce the telechehc silicones of the invention include but are not limited to isocyanatoethyl methacrylate, alkenyl azlactones such as vinyl dimethyl azlactone and lsopropenyl dimethyl azlactone, m-isopropenyl-a, a-dimethyl benzyl isocyanate, and acryloyl ethyl carbonic anhydride Some electrophiles, e g , isocyanatoethyl methaciylate aie commercially available and others can be prepared via literature methods Alkenyl azlactones and their preparation are described in U S Pat No 4,777.276 (Rasmussen et al ). the disclosure of which is incorporated herein by reference Acryloyl ethvl carbonic anhydride can be prepared from ethyl chloroformate and acrylic acid by the method of R Hatada and H Kondo given in Bull Chem Soc Japan 41 ( 10). 252 l ( 1968), the disclosure of which is also incorporated herein by reference Preferred electrophiles include those selected from the group consisting of isocyanatoethyl methacrylate, vinyl dimethyl azlactone, and acryloyl ethyl carbonic anhydride The silicone compositions of the invention can, depending upon their viscosity, be coated, extruded, or poured, and rapidly completelv. and reliably radiation cured (even at high molecular weight) by exposure to electron beam, visible, or ultraviolet l adiation Curing preferablv should be carried out in as oxygen-free an environment as possible, e g , in an inert atmosphere such as nitrogen gas or by utilizing a barrier of radiation-transparent material having low oxygen permeability Curing can also be carried out under an inertmg fluid such as water When visible oi ultra iolet radiation is used foi curing the silicone compositions also contain photυinitiatoi Suitable photoinitiators include benzoin ethers, benzophenone and deriv ativ es thereof, acetophenone derivatives, camphorquinone, and the like Phoioiniuaioi is geneiallv used at a concentration of from about 0 1 % to about 5% by weight of the total polymeπzable composition, and, if curing is carried out under an inertmg fluid, the fluid is preferably saturated with the photoinitiator or photoinitiators being utilized in order to avoid the leaching of initiator from the silicone composition The rapid cure observed for these materials allo s for the use of ery low levels of photoinitiator relative to the pnoi art, hence unifoi m cui e of thick sections can be achieved due to deeper penetration of radiation If desired, the silicone compositions of the invention can also be cured thermally, requiring the use of thermal initiator such as peroxides, azo compounds, or persulfates generally at a concentration of from about 1 % to about 5% by weight of the total polymeπzable composition It is preferable that any initiator (thermal or photo- ) utilized be soluble in the silicone compositions themselves, requiring no use of solvent Liquid initiators are especially preferred The radiation curable material, when blended with the phosphors in forming the polymerizable compositions used in the practice of the present invention, preferably contains less than 5% each by weight of low molecular weight non-polymerizable organic materials other than phosphors and low molecular weight polymerizable ingredients These low molecular weight, non- polymerizable materials may be defined as those having a molecular weight of less than 300, and more preferably less than 500, still more preferably less than 2,000, and most preferably having a molecular weight less than 5,000 The low molecular weight polymerizable materials may be defined as having a molecular weight less than 300, preferably less than 500. more preferably less than 1 ,000. and most preferably less than 2000. Note that photoinitiators present in the composition are not included in determining the total weight for assessing concentrations of lower molecular weight materials The intensifying screens of this invention provide high resolution images without a loss of screen speed How the inventive screen provides these improvements is not fully understood, but it may be a combination of such factors as greater uniformity in the distribution of the binder aft r polymerization, a higher packing density of the phosphor particles, less redistribution of ingredients within the phosphor layer due to substantial elimination of solvent migration out of the system, and reduced stress on the system by reduction of dimension changes during solidification of the phosphor layer Conventional additives to the phosphor layer may be present in the practice of the present invention so long as the additives do not interfere with cure or other desired properties For example, brighteners; white pigments; reflective particulates; colorants; coating aids; surfactants, especially polymerizable surfactants; antistats; lubricants, and the like may be present within the coating composition and the final phosphor layer. Particularly useful additions to the coating compositions are antistatic agents such as functionalized derivatives of Jeffamine I antistats as described in U S Pat. No 5,217.767 and property modifying agents such as reactive silicones used as hardness modifiers (available from Th Goldschmidt AG) A preferred method for manufacturing the phosphor screens according to the present invention comprises the steps of blending the phosphor and binder (and optional ingredients) to form a coating mixture comprising less than 5% each by total weight of non-polymerizable organic components and polymerizable components having a molecular weight of less than >ϋυ υi 500. coating the mixture onto a substrate, optionally covering the substrate with a smooth layer or a microtextured layer thereby forming a laminate or a surface with controlled roughness, polymerizing said composition by exposure to actinic radiation, and stripping any optional cov r layer The present invention is particularly effective while using microtextured cover sheets, which impart texture to the screen surface when the sheet is removed from the polymerized composition This microtextuπng can serve to prevent "blocking" (l e , non-uniform sticking) of the screen and X-ray film, by providing a smaller contact surface along with sufficient channels for air bleed during lamination of the screen to the film Typically, the surface features imparted by texturing range up to 25 microns in height, created by using a microtextured cover sheet with featuies up to 2"> micions in depth It is also practical in the pi sent inv ention to pi υduced prestructured' phosphor screens, that is, screens with a built-in raster orientation of the phosphor so that stimulation of the screen, when used in a stoi age phosphor mode, can be effected by an entire surface irradiation rather than by only a point-by-point irradiation This can be accomplished b\ etching the desired pattern of phosphor distribution onto the surface of a carriei element, the usual pattern being columns and rows of closely spaced dots, filling the pattern with the phosphor containing radiation curable compositions of the invention and radiation curing the composition of the invention within the pattern The composition may be applied to the patterned surface by conventional coating processes (e g , curtain coating, roller bead coating, knife edge coating, spin coating, extrusion coating, sheet - 1 S- coating, etc ) and the excess w iped oil so thai essentially only the pattern and not the flat surface is coated with the composition The phosphor screens produced according to the present invention are characterized by a high phosphor gram loading (phosphor to binder ratios in excess of 6 I , preferably greatei than 8 1, more preferably greater than 10 1 , most preferably in the range ftom 10 I to 18 1 ) and high phosphor packing density in the cured screen The procedure for producing the phosphor screens of the present invention can be summarized as being a series of four distinct steps The components of the photopolymer mixtuie and the phosphor particles are weighed out and blended together, for example, by successive passes through a commercially available 3- roll mill, such as a paint mill Typically, several passes of the mixture through the mill are required to homogeneously blend the material The blended mixture is then dispensed onto a suitable substrate and a cov er sheet is preferably applied over the mixture, producing a laminated or covered structure to protect the material during subsequent processing steps The cover sheet may be any material which does not bond to the phosphor layer during hardening Sheets with release coatings thereon (e g , paper or film with low-adhesion coatings of fluorocarbons) are preferred It is possible to use a very thin cover sheet which will bond to the phosphor layer and use that as a piotective cov er lavei and/oi release surface on the phosphor, but other means of applying such layeis are preferred The laminate is then passed through a series of rollers at ever decreasing gap space, so that the final desired thickness of the phosphor is obtained The laminate is then cured using radiation and the cover sheet is removed to expose the final phosphor screen The cover sheet may remain on the suiface during exposure if it is transparent, does not bond to the phosphυi layei surface, or is intended to bond to the phosphoi lav ei sui lace Tπmax 1 (3M Company) radiographic screens are designated by grades T2, T6, T I 6, etc The lower the " ' I " number, the higher the resolution, the slower the speed, and the finer the paniculate size of the phosphor which makes up the screen The object in is to minimize the exposure to X-ravs (faster speed), while obtaining the highest resolution possible The comparative examples which will follow compare standard commercial screen performance to the performance of the screens of this invention There are several measurements which are made on the X-ray film image during the comparison of the performance of the phosphor screens The optical density is measured using a commercially available optical densitometer A silver halide emulsion will develop to some extent without exposure to X-rays, without exposure to any radiation (because of fog centers in the silver halide) or with exposure to X-rays without an associated phosphoi layer due to absorption of X- rays by the silver halide grains (fog) The X-ray dosage for comparison of phosphor screens is set to a value to achieve an optical density of " I over fog" (e g , if the optical density of a fogged film is 0 24, the dosage will be set to achieve an optical density when using a screen, of 1 24) The relative speed of the phosphor screen and film combination is a measure of how efficiently the film is exposed to achiev e the required optical density, i e , how much dosage is required In the examples, this relative speed is the dosage required by a standard screen divided by the dosage required by an inventive screen, to obtain the optical density of " I over fog " The CTF (Contrast Transfer Function) is a measurement used in the industry to quantify the resolving power exhibited bv the X-rav image As features to be imaged decrease in size the scattering of the radiation convened bv the phosphor screen becomes more significant Foi example, two small features in close proximity will often appear as a larger indistinct feature since the scattering from the phosphor layer merges information from each of the-smaller features The CTF can be used as a way to quantify the qualitative clarity of an X-ray image as practiced by the radiologist The CTF is a function of line pairs resolved per millimeter, and as used in this discussion, it is defined bv (the difference in the optical density of the dark and light ai eas of the measured line pair) , (the difference in the optical density of the dark and light regions of the largest line pair) Optical density measurements used in the determination of the CTF of a film/screen combination are obtained by using a microdensitometer The maximum CTF is equal to I 0. and screens with a better resolving power will have a higher CTF EXAMPLE PREPARATION OF DIFUNCTIONAL ORGANOPOLYSILOXANES Difunctional polysiloxanes terminated on both ends with ethylenic unsaturated groups were prepared, having a molecular weights of 35,000 g/mol and 10,000 g/mol A reactor was purged with nitrogen and was charged with 170 grams of bis-(3-aminopropyl)tetramethyldιsιloxane (end-blocker) and 858 grams of octamethylcyclotetrasiloxane (D ) The reactor was heated to 80"C and 5 grams of anhydrous 3-amιnopropyl dimethyl tetramethylammonium silanolate (catalyst) was added The reaction mixture was stirred at 80°C until the end-blocker disappeared (about 1 hour - checked by GC) A further 26 4 kg of nitrogen-purged D was added to the resultant reaction mixture (which consisted of a 1 ,500 molecular weight polysiloxane with ammopropvl endgroups. cyclic siloxanes and active catalyst) over a period of 5 hours The reaction mixture was maintained at 80"C while stirring overnight Then, the catalyst was decomposed by heating at 150"C for 1/2 houi, and the product was stripped at I40 "C and 0 I mm pressure until no more volatiles distilled (about 5 hours) After cooling the reaction mixture to room temperature, 191 grams of vinyl dimethylazlactone (VDM) was added slowly while stirring, to result in 25 kg (95% solids) acrylamidoamidosiloxa c ( ΛCΛ1 ΛS) hav ing a numbei av eiage molecular weight of 35,000 g/mol (determined by acid tπration of silicone diamine before functionahzation with VDM) ACMAS having a 10,000 g/mol molecular weight was made by an analogous procedure, using the following amounts of materials (the only difference being that a different relative amount of D4 was added in the second step of polymerization) 6S I grams of end-blocker, 3405 grams of D4, 5 grams of catalyst, 28 4 kg of D 4 , and 763 grams of VDM COMPARΛTIVE EXAMPLE 1 Trimax ι vι T2 and Trimax I M T6 phosphor screens (3M Company, St Paul, MN) were exposed conventionally using XD/a+ radiographic film (3M Company, St Paul, MN) and a standard target The conditions of exposure and resulting measured CTF are summarized below The exposure of a film without having an associated phosphor screen yielded an optical density of 0 29 The applied dose was adjusted to yield an optical density for all exposures of 1.29 (a " I over fog" condition). Trimax™ Phosphor Dose mR CTF @2 CTF @4 CTF @6 Screen kVp I p/mm 1 p/mm I p/mm T2 40 3 45 0 70 0 39 0 18 T2 60 2 137 0 6 0 33 0 1 5 T2 80 1 2 1 5 0 43 0 14 — T6 40 1 82 0 67 0 28 — T6 60 0 879 0 49 0 19 0 04 T6 80 0 501 0 32 0 09 — EXAMPLE I A phosphor screen comprising T6 Trimax 1 M phosphor panicles (3M Company, St Paul, MN) and a radiation curable binder, was formulated having a phosphor to binder ratio of approximately 12 1 A mixture of 3 1 35 grams of T6 Trimax I phosphor particles, 1 6 grams of acrylamidoamidosiloxane (ACMAS) blend comprising 50 parts by weight of 35,000 molecular weight acrylamidoamidosiloxane. ) parts bv weight of 1 ,000 molecular weight acrylamidoamidosiloxane with 0 5 parts Darocure 1 M 1 173 (free radical initiator from EM Industries), 0 9 grams of a hardness modifier comprising 25 parts by weight TEGO l l RC726, 25 pans by weight TEGO' M RC7 I I (both from Th Goldschmidt AG), and 1 part by weight Darocure 1 173 (mixture designated by 71 1/726/1 173); and 0 I I grams of functionalized Jeftamine I I antistat (FX-8 derivative of Jeffamine ED-900, prepared according to the method described in U.S. Pat. No 5,217,767, perfiuorooctanesulfonyl fluoride, having a molecular weight of -502), was placed in a 3-roll mill The gap between the first two rolls was adjusted to 0.005 inches (0 127 mm), and the gap between the second and third roll was set to 0.002 (0 05 I mm) inches The rotational speed of the first roll was 3 rpm, the second roll was rotated at 9 rpm. and the third roll at 28 25 rpm The mixture was passed through this mill 10 time--, befoi e remov ing from the mill and spreading onto a 0 007 (0 18 mm) inch thick polyester substrate A 0.0023 (0.058 mm) inch thick polyester cover sheet was placed over the mixture to form a laminate, which was then passed through a pair of rollers initially gapped to 0.0243 inches (0.06 mm), resulting in a coating thickness within the laminate of 0.015 inches (0.38 mm) The gap between the rollers was then decreased by approximately 0 003 inches (0 076 mm) and the laminate again passed through the rollers to further compress the mixture This step was repeated until the resulting coating thickness was 0 005 inches (0 127 mm) or 0 004 inches (0 100 mm) The laminate was then cured using ultraviolet light, and the cover sheet removed A second screen of each thickness was made using the same procedure as above, like screens were placed on opposite sides of a commercial X-ray film (XD/A+ film, 3M company) with the phosphor layer in contact with the film surface, forming a screen film/screen laminate, and an exposure mask was placed over the top phosphor screen Exposure Data kVp Dose Mils Thick CTF @2 CTF 4 CTF @6 Speed std Mr Top/ Bot 1 p/mm 1 p/mm 1 p/mm above T2 40 1 .126 4/4 0 64 0.32 — 3 06 40 1 .346 5/5 0 69 0 25 0 12 2 56 60 0 680 4/4 0 56 0 27 — 3 14 60 0.688 5/5 0 46 0 1 0 01 3 I I EXAMPLE 2 A phosphor screen comprising T6 Trimax phosphor particles (3M Company, St Paul, Mn) and a radiation curable binder, was formulated having a phosphor to binder ratio of approximately 9 1 The same method as described in Example 1 was used, however, the formulation of the mixture was as follows 23 5 grams Trimax T6 Phosphor 1 6 grams ACMAS blend as in Example 1 0 9 grams 71 1/726/ 1 173 0 1 1 iirams functionalized Jeffammc I antistat Two different thicknesses of screens were made with this formulation using the procedure outlined in Example 1 one set of screens having a thickness of 0 003 inches (0.076 mm), and another set of screens having a thickness of 0 005 inches (0.127 mm) A screen/film screen laminate was formed as in Example 1 , and exposed to X-rays Expos ure Data kVp Dose Mils Thick CTF @2 CTF @4 CTF @6 Speed std Mr Top/ Bot 1 p/mm l p/mm lp/mm above T2 40 1 .558 3/3 0 73 0 42 0.25 2 21 40 1.462 5/5 0 70 0.31 0 16 2.36 60 0 793 3/3 0 58 0 3 1 0 15 2.69 60 0 739 5/5 0 5 1 0 2 1 0 12 2 89 Exposure Data kVp Dose Mils Thick CTF @2 CTF @4 CTF @6 Speed std Mr Top/ Bot 1 p/mm 1 p/mm 1 p/mm above T6 80 0 442 3/3 0 50 0 21 0 10 1 13 80 0.365 5/5 0 38 0 12 0 05 1 37 EXAMPLE 3 A phosphor screen comprising T6 Trimax phosphor particles (3M Company, St Paul, MN) and a radiation curable binder, was formulated having a phosphor to binder ratio of approximately 12 I The same method as described in Example I was used, however, TEGO RC7 I 5 was substituted for TEGO RC71 1 , and the rest of the formulation was as follows 3 I 2 grams Trimax T6 Phosphor 1 60 grams ACMAS blend as in Example I 0 9 grams 715/726/1 173 0 I mams functionalized Jeffaιnine I antistat . • - '.. One set of screens with this formulation was made using the procedure outlined in Example 1 , each screen having a thickness of 0 004 inches (0 1 I mm) A screen/film/screen laminate was formed as m Example 1 , and exposed to X-rays Exposure Data kVp Dose Mils Thick CTF @2 CTF @4 CTF @6 Speed std Mr Top/ Bot 1 p/mm 1 p/mm l p/mm above T6 80 0 367 4/4 0 41 0 14 — 1 37 Comparison of the data of Examples 1 -3 with the standard screen data presented in Comparative Example I clearly shows that with proper choice of the thickness of the screen and phosphor to binder ratio at 40 kVp the CTF of the inventive screen is comparable or higher at every resolution (lp/mm) than a T2 screen at ovei double the speed, and that at 60 kVp the screen has comparable resolution to a T2 screen again at over double the speed Similar comparisons are made on penetrations of 60 and 80 k\ p w ith a Tϋ sci een wheie comparable or higher CTF values ai e associated w ith the fastei inv entiv e screen There is a wide range of vanables that can be considered in comparing the performance of the inventive screens herein described, in particular there is a tradeoff .between the speed and the resolving power of the screen, and each is dependant on the type and granular size of the phosphor, the phosphor to binder ratio, and the thickness of the screen It has been shown that the inventive screen described herein exhibits the resolving power of a standard screen while at a much improved speed, oi a highei i evolving powei at the ->ame speed, which in turn leads to a lower dose of X-iays to which a patient is exposed in order to obtain the necessary information requn ed by the physician

Claims: WHAT IS CLAIMED IS 1 A phosphor screen comprising paniculate phosphors in a binder on a substrate wherein said binder is a radiation cured product of a radiation curable binder composition comprising at least one organopolysiloxane polymer having the following general formula wherein X is an ethylenically unsaturated hydrocarbon group, Y and R independently are a divalent linking groups, m is an integer of 0 to I , D is selected from the group consisting ofhydrogen. an alkyl group of 1 to about 10 carbon atoms, and an aryl group of 1 to 20 carbon atoms, R 1 are monovalent substituents which can be the same or different and are selected from an alkyl group and an aryl group, R 2 are monovalent substituents which can be the same or different and are selected from an alkyl group and an aryl group, R' is a monovalent substituent which can be the same or different and is selected from an alkyl group and an aryl group, R 4 is a monovalent substituent which can be the same or different and is selected from an alkyl group and an arvl group, and n is an integer of from 35 to 1000 2 The phosphor screen ot claim 1 wherein the radiation curable binder composition comprises a blend ot a low molecular weight organopolysiloxane accoiding to Formula 1 and a high molecular weight organopolysiloxane according to Formula 1 3 The phosphoi screen of claim 2 wherein the low molecular weight organopolysiloxane has a molecular weight less than 20 000 and the high molecular weight organopolysiloxane has a moleculai weight more than 20 000 4 The phosphoi screen of claim 2 wherein the low molecular weight organopolysiloxane has a molecular weight less than 1 5,000 and the high molecular weight organopolysiloxane has a molecular weight more than 25,000 5 The phosphoi scieen of claim 2 wheiein the weight ratio of low molecular weight matenal to high molecular weight material is in the range of from 10 90 to 90 10 6 The phosphoi scieen of claim 2 wherein the weight ratio of low molecular weight material to high molecular weight material is in the range of from 30 70 to 70 30 7 The phosphoi sci een ol claim I wheiein said radiation curable binder composition compiises less than 5% by weight ot the total composition of organic materials having a moleculai weight less than ^00 8 The phosphor scieen of claim I wherein said radiation curable binder composition comprises less than * ^% by weight of the total composition of organic materials hav ing a moleculai weight less than 2000 9 The phosphoi scieen of claim 1 wherein the organopolysiloxane comprises an acrylamidoamidosiloxane 10 T he phosphor screen of claim I wherein said binder further comprises an antistat 1 1 A process for preparing the phosphor screen of any of claims 1 - 10 comprising the steps of a) mixing paniculate phosphors with a radiation curable binder composition to form a polymei izable mixture, b) coating said pol menzable mixture onto a substrate, and c) cui ing said pol \ menzable mixtui e bv exposure to actinic radiation, wherein the radiation curable binder composition comprises at least one organopolysiloxane polγmeι having the following genei al formula wherein X is an ethylenically unsaturated hydrocarbon group. Y and R independently are a divalent linking groups, m is an integei of 0 to 1 D is selected fiom the gioup consisting of hsdiυgen, an alkv group of I to about 10 caibon atoms and an aiyl group, R 1 are monovalent substituents which can be the same or different and are selected from an alkyl group and an aryl group, R 2 are monovalent substituents which can be the same or different and are selected from an alkyl gi oup and an ai l gi oup, R' is a monovalent substituent w hich can be the same or different and is selected from an alkyl gioup and an aryl group, R is a monovalent substituent w hich can be the same or different and is selected from an alkyl group and an aryl group and n is an integei of to 1000 12 The process of claim 1 1 wherein a cover sheet is placed over said polymenzable mixture after coating onto said substrate and before polymerization, and said polymerizable mixture is exposed to radiation through said cover sheet to effect polymerization 13 The piocess of claim 1 1 w herein a cov er sheet is placed over said polymenzable mixture attei coating onto said substi te and before polymerization of said composition, and the polymenzable mixture is irradiated to effect polymerization of said composition 14 The process of claim I I wheiein said substi ate comprises a layer having a surface with pieposilioned spaces in said surface said radiation curable bindei composition and phosphoi fill said spaces when coated on said substrate, and excess radiation curable binder composition is removed from said surface before said curing 15 The pi ocess of claim 1 I wherein aftei coating said substrate with said polymenzable mixtui e said substi te with coating composition thereon is moved between a piessui e applv ing means to i educe the thickness of said polymerizable mixture 16 The process of claim 1 5 wherein said pressure applying means comprise nip rollers 17 The process ot claim 16 wherein at least two different pairs of nip rollers compnsing a fu st and second pan ol nip i oilers ai e used in sequence, said second pair having a narrower gap between rollers than said first pair 18 The process of claim 12 wherein said cover sheet is a microtextured cover sheet 19 The process of claim 1 wherein said microtextured cover sheet has at least some surface features from I micron up to 25 microns in depth

Inventors: DALHQUIST JOHN C, MAZUREK MIECZYSLAW H, DALHQUIST, JOHN, C., MAZUREK, MIECZYSLAW, H.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: G21K4
IPC codes: C09D183, C09K11, G03F7, G21K4, H01J29