Patent Number: 060752505
Section: description

DETAILED DESCRIPTION OF THE INVENTION Typical examples of the radiation image storage panels of the invention are explained below by referring to the attached drawings. FIG. 2 shows an example of the radiation image storage panel of the invention. The panel comprises a transparent support 21 and a phosphor layer 22 provided thereon containing stimulable phosphor particles, and protective films 23, 24 having a high scratch resistance and a large contact angle are provided on both of the top surface of the phosphor layer 22 and the back (bottom) surface of the support 21. The scratch resistance and the contact angle of the protective films are higher and larger than those on the surface of the support. FIG. 3 shows another example of the radiation image storage panel of the invention. The panel comprises a transparent support 31 and a phosphor layer 32 provided thereon containing stimulable phosphor particles, and a protective film 33 having a high scratch resistance and a large contact angle (which are superior to those of the surface of the support 31) is formed on the back (bottom) surface of the support 31. On the other hand, on the phosphor layer 32 there are provided a transparent resin film 34a and a protective film 34b having a scratch resistance and a contact angle which are higher and larger than those of the surface of the support 31. The radiation image storage panel of the invention has a basic structure which comprises a transparent support and a phosphor layer provided thereon, and on both surfaces (i.e., the top surface and the bottom surface) of which protective film having the specific properties is provided. The radiation image storage panel of the invention can be prepared in the following manner. As the transparent support of the panel, a transparent plastic film (or sheet) is usually used. The transparent support film can be optionally selected from the known materials employed for the conventional radiation image storage panel. Examples of the known materials include films of plastic materials such as polyethylene terephthalate, polyethylene naphthalate, polyamide and polyimidoamide. Other materials also can be employed, provided that the materials have enough strength and high transparency. The thickness of the transparent support film generally in the range of 10 to 1,000 .mu.m. The stimulable phosphor gives a stimulated emission (i.e., light emission) when it is irradiated with stimulating rays after it is exposed to radiation. In the preferred radiation image storage panel, a stimulable phosphor gives a stimulated emission of a wavelength in the range of 300 to 500 nm when it is irradiated with stimulating rays of a wavelength in the range of 400 to 900 nm. Examples of the preferred stimulable phosphors include divalent europium activated alkaline earth metal halide phosphors and a cerium activated alkaline earth metal halide phosphors. Both stimulable phosphors favorably give the stimulated emission of high luminance. However, the stimulable phosphors employable in the radiation image storage panel of the invention are not limited to the above-mentioned preferred stimulable phosphors. A usual stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed therein, and the binder can be optionally selected from the known materials employed for the conventional radiation image storage panel. A stimulable phosphor layer can be formed on the support in the known manner as follows. The stimulable phosphor and a binder are added to an appropriate solvent, and they are well mixed to prepare a coating dispersion for the formation of a phosphor layer in which the stimulable phosphor particles are homogeneously dispersed in a binder solution. A ratio between the binder and the phosphor in the coating dispersion may be determined according to the characteristics of the desired radiation image storage panel and the nature of the employed phosphor. Generally, the ratio is within the range of from 1:1 to 1:100 (binder:phosphor, by weight), preferably from 1:8 to 1:40. The dispersion thus prepared is coated evenly over the surface of a support to form a coated layer of the dispersion. The coating procedure can be carried out by a conventional method such as a method of using a doctor blade, a roll coater, or a knife coater. Then the coated layer of the dispersion is dried to form a stimulable phosphor layer on the support. The thickness of the phosphor layer varies depending upon the characteristics of the desired radiation image storage panel, the nature of the phosphor, the ratio between the binder and the phosphor, etc. Generally, the thickness of the phosphor layer is in the range of 20 .mu.m to 1 mm, preferably in the range of 50 to 500 .mu.m. The phosphor layer can be provided on the support a method other than that given in the above. For example, the phosphor layer is initially prepared on a sheet (i.e., temporary support) such as a glass plate, metal plate or plastic sheet using the aforementioned coating dispersion and thus prepared phosphor layer is then placed on the genuine support by pressing or using an adhesive agent. For the stimulable phosphor layer of the radiation image storage panel of the invention, not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of only an agglomerate of a stimulable phosphor containing no binder can be also employable. Also employable is a phosphor layer in which voids of stimulable phosphor agglomerate are impregnated with a polymer material. In the above-described manner, a composite body comprising the phosphor layer and the transparent support can be prepared. The radiation image storage panel of the invention is characterized by providing the specific protective film on both surfaces (i.e., the top surface and the bottom surface) of the composite body, and the protective film has a higher scratch resistance and a larger contact angle, as compared with those of the surface of the support. Since the protective film has a high scratch resistance and a high anti-staining property, the storage panel of the invention is hardly stained and the stains on the panel are easily removed out with an organic solvent. The protective film of the invention is preferably made of a fluororesin alone or a resin composition mainly containing a fluororesin. The protective film may comprise a transparent film and a protective layer provided thereon which is made of a fluororesin alone or a resin composition mainly containing fluororesin. A preferred protective film of the invention can be produced in the following manner: a coating liquid (in the form of dispersion or solution) for the formation of the protective film is prepared by adding a film-forming resin composition (such as a fluororesin alone, a fluororesin and other film-forming resins, or a fluororesin and oligomer having a polysiloxane structure and/or having a perfluoroalkyl group) to an appropriate solvent, and the liquid mixture thus prepared is coated evenly on the back surface of a support and the surface of the phosphor layer by means of coating means such as a doctor blade, and then the coated layer of the liquid is dried to form the protective film. The protective film may be made in advance of forming the composite body comprising the support and the phosphor layer. For instance, the radiation image storage panel of the invention can be also produced in the following manner: a protective film is beforehand formed by coating the above-described coating liquid on one surface of the support, and independently another protective film is beforehand formed in the same manner on the surface of the phosphor sheet, and then the support and the phosphor sheet are combined to give the radiation image storage panel. The film-forming resins employable for forming the protective film in conjunction with the fluororesin are selected from known materials such as polyurethane resin, polyacryl resin, cellulose derivatives, polymethyl methacrylate, polyester resin and epoxy resin. The fluororesin can be a homopolymer of a fluorine atom-containing olefin or a copolymer of fluorine atomcontaining olefin and one or more other monomers. Examples of the fluororesins include polytetrafluoroethylene, polychlorotrifluoroethylene, polyfluorinated vinyl, polyfluorinated vinylidene, tetrafluoroethylene-hexafluoropropylene copolymer, and fluoroolefin-vinyl ether copolymer. Most of the fluororesins are insoluble in organic solvents. However, copolymers of the fluoroolefin and other polymerizable monomer can be made soluble in a certain organic solvent if an appropriate monomer is combined. Therefore, such soluble fluororesins can be dissolved in an appropriate organic solvent to prepare a coating solution, and thus prepared coating solution can be applied on the support or on the phosphor layer and dried to form a protective film containing the fluororesin. Examples of such soluble fluororesin copolymer include fluoroolefin-vinyl ether copolymer. Besides fluoroolefin-vinyl ether copolymer, tetrafluoroethylene and its modified polymer are also employable because they are soluble in fluorine atom-containing organic solvents such as a perfluoro solvent. The film-forming composition may contain a cross-linking agent, a hardening agent and an anti-yellowing agent. If a fluororesin is employed for forming the protective film, the fluororesin is preferably crosslinked to increase strength and durability of the film. Examples of the preferred crosslinking agents include a compound having a plural number of isocyanate groups (e.g., polyisocyanate) and melamine derivatives. The oligomer having polysiloxane structure employable for forming the protective film with the fluororesin is, for example, an oligomer having dimethylpolysiloxane structure. The oligomer preferably has at least one functional group (e.g., hydroxyl group), and the molecular weight preferably is in the range of 500 to 100,000 (weight average), more preferably 1,000 to 100,000 and particularly preferably 3,000 to 100,000. The oligomer having a perfluoroalkyl group (e.g., tetrafluoroethylene group) preferably has at least one functional group (such as hydroxyl group), and the molecular weight is preferably in a range of 500 to 100,000 (weight average), more preferably 1,000 to 100,000 and particularly preferably 3,000 to 100,000. The oligomer having functional group is preferably employed for forming the protective film. Such oligomer is incorporated into the molecular structure of the film-forming resin comprising a fluororesin during the crosslinking reaction between the oligomer and the resin for the formation of the protective film, and therefore the oligomer is hardly removed by cleaning the film surface with an organic solvent or by repeated use of the radiation image storage panel for a long period. Accordingly, the effect of the incorporation of the oligomer continues for long time. The film-forming composition comprising a fluororesin contains the above oligomer preferably in an amount of 0.01-10 wt. %, more preferably 0.1-2 wt. %. The film-forming composition may contain a perfluoroolefin resin powder or a silicone resin powder. The mean particle size of the powder is preferably in a range of 0.1 to 10 .mu.m, more preferably 0.3 to 5 .mu.m. The amount of the powder is preferably in a range of 0.5 to 30 wt. %, more preferably 2 to 20 wt. %, and further preferably 5 to 15 wt. %. The protective film, particularly the protective film on the transparent support side, preferably comprise a fluororesin and light-scattering particles. The protective film preferably further contains a dispersing agent such as a coupling agent of a titanate type or an aluminate type. The light-scattering particles preferably contains in an amount of 1 to 30 weight % (more preferably 5 to 20 weight %, most preferably 10 to 20 weight %) in the fluororesin-containing protective layer. The light-scattering particles preferably have a mean particle size smaller than the thickness of the protective layer into which the particles are incorporated. For instance, the light-scattering particles preferably have a mean particle size of 0.05 to 5 .mu.m, particularly 0.1 to 1.0 .mu.m. The light-scattering particles preferably a refractive index higher than that of the fluororesin or a mixture of a fluroresin and other polymers in the protective layer, so that effective light-scattering proprerty is introduced into the protective layer. In view of the advantageous high light-scattering property provided by the incorporated light-scattering property and disadvantageous features such as decrease of strength of the coated film and decrease of uniformity of the protective film, the light-scattering particles are preferably incorporated into the fluororesin-containing protective layer in an amount of 1 to 30 weight %, more particularly 5 to 20 weight %, most preferably 10 to 20 weight %. There are no specific limitations with respect to the light-scattering particles incorporated into the protective layer, so long as the particles have a refractive index higher than that of the fluororesin or a mixture of the fluororesin and other polymers employed for the preparation of the protective layer. Organic fine particles and inorganic fine particles are both employable. Examples of the preferred light-scattering particles include benzoquanamine resin particles having a mean particle size of 0.1 to 0.5 .mu.m, melamine-formaldehyde condensation resin particles having a mean particle size of 0.1 to 0.5 .mu.m, and titanium dioxide particles having a mean particle size of 0.1 to 0.5 .mu.m. It is preferred that the light-scattering particles are uniformly dispersed in the fluororesin-containing protective layer. Therefore, the light-scattering particles are preferably treated on their surfaces so as to improved dispersibility. Otherwise, a dispersing agent is incorporated into a coating solution containing the fluororesin or a mixture of the fluororesin and other polymers and the light-scattering particles. Examples of the employable dispersing agents include cationic dispersants, anionic dispersants, nonionic dispersants, and amphoteric dispersants (e.g., betaine-type surfactants). Also employable are coupling agent-type dispersants such as silane-coupling agents, titanate-coupling agents, and aluminum-coupling agents. The titanate-coupling agents and the aluminum-coupling agents are most preferred. The coupling agent is preferably incorporated into the protective layer in an amount of 0.5 to 5.0 weight %, based on the amount of the light-scattering particles. As is described above, the protective film may comprise a transparent film and a protective layer provided thereon. The transparent film can be optionally selected from those known as a protective film of the conventional radiation image storage panel, for instance, films of polyethylene terephthalate, polyethylene naphthalate, polyamide, polycarbonate, polyvinylidene chloride, polyimide and aramide. Other plastic materials also can be employed, provided that the plastic materials have enough strength and high transparency. The thickness of the transparent film of plastic material generally ranges from 1 to 10 .mu.m. EXAMPLE 1 The radiation image storage panel of the invention was prepared in the following manner. (1) 200 g of a stirnulable phosphor (BaFBr.sub.0.9 I.sub.0.1 :0.001Eu.sup.2+), 8 g of a polyurethane elastomer (Pandex T-5265H, product of Dai-Nippon Ink Chemical Industries Co., Ltd.) and 2 g of an epoxy resin (Epikote 1001, product of Yuka Shell Epoxy Co., Ltd.) were placed in methyl ethyl ketone and dispersed by means of a propeller mixer to give a coating dispersion of a viscosity of 25-30 PS (at 25.degree. C.). The coating dispersion was coated on a polyethylene terephthalate temporary support having silicone release coating. After the coated layer was dried at 100.degree. C. for 15 minutes, the dried layer was peeled from the temporary support to prepare a stimulable phosphor sheet having a thickness of 300 .mu.m. (2) 70 g of a fluororesin (fluoroolefin-monovinyl ether copolymer: Lumiflon LF504X, product of Asahi Glass Co., Ltd.), 12 g of an isocyanate crosslinking agent (Olestar NP38-70S, product of Mitsui Toatsu Chemicals, Inc.), 0.55 g of a lubricant (silicone resin: X-22-2809, product of The Shin-Etsu Chemical Co., Ltd.) and 0.0004 g of a catalyst (dibutyltin laurate, KS-1260, product of Kyodo Yakuhin Co., Ltd.) were dissolved in a mixture of methyl ethyl ketone and cyclohexane to prepare a coating solution containing the resin in an amount of 14 wt. %, and then the viscosity of coating solution was adjusted to 2-3 PS. The prepared solution was then coated and dried on a polyethylene terephthalate film (thickness: 300 .mu.m) to give a dry coated layer of 7 .mu.m thick. Thus, a transparent support having a protective film on one side was produced. The scratch resistance and the contact angle of the surface of the protective film provided on the support (polyethylene terephthalate film) and those of the unprotected surface of the same support were measured in the following manner; contact angle: methylene iodide was dropped onto the sample surface, and after 60 seconds the value of the contact angle was measured; PA1 scratch resistance: the sample surface was scratched with a pencil and the scratch value was determined in accordance with JIS (Japanese Industrial Standard). PA1 surface of the protective film: PA1 surface of the protective film: PA1 surface of the protective film: PA1 surface of the protective film: The results are as follows: contact angle: 75.degree., scratch resistance: 3B unprotected surface of the same support: PA2 contact angle: 78.degree., scratch resistance: 3B unprotected surface of the same support: PA2 contact angle: 32.degree., scratch resistance: 4B. PA2 contact angle: 67.degree., scratch resistance: 3B unprotected surface of the same support: PA2 contact angle: 32.degree., scratch resistance: 4B. PA2 contact angle: 70.degree., scratch resistance: 3B unprotected surface of the same support: contact angle: 32.degree., scratch resistance: 4B. PA3 contact angle: 32.degree., scratch resistance: 4B. (3) The phosphor sheet prepared in the above procedure (1) was overlaid on the other surface (on which the protective film was not provided) of the support of (2) via an adhesive agent, and then pressed and heated at 60-70.degree. C. by means of a heating roll. Thus, a phosphor layer (thickness: 200 .mu.m) was provided, via a subbing layer, on the support having protective film was obtained. (4) A transparent polyethylene terephthalate film (thickness: 6 .mu.m, having an adhesive layer of a polyester adhesive agent on one surface) was overlaid on the above phosphor layer under the condition that the adhesive layer was brought into contact with the phosphor sheet, and then pressed and heated at 90-100.degree. C. by means of a heating roll. Independently, 50 g of 50 wt. % xylene solution of a fluororesin (fluoroolefin-vinyl ether copolymer: Lumiflon LF100, product of Asahi Glass Co., Ltd.), 5 g of an isocyanate crosslinking agent (Colonate HX, solid content 100 wt. %, product of Nippon Polyurethane Co., Ltd.) and 0.5 g of an alcohol-modified silicone oligomer (X-22-2809, solid content 66 wt. %, product of Shin-Etsu Chemical Co., Ltd., which had dimethylpolysiloxane structure and had hydroxyl groups (carbinol groups) in both terminals), were dissolved in methyl ethyl ketone to prepare a coating solution (viscosity: 0.1-0.3 PS). The prepared coating solution was coated on the above polyethylene terephthalate film provided on the phosphor layer by means of a doctor blade, and then heated and dried to cure at 120.degree. C. for 20 minutes to prepare a protective layer (thickness: about 2 .mu.m) on the transparent film. Thus, a radiation image storage panel of the invention was prepared. EXAMPLE 2 The procedure of Example 1 was repeated, except for preparing the protective film on the support in the following manner, to produce a radiation image storage panel of the invention. 70 g of a fluororesin (fluoroolefin-monovinyl ether copolymer: Lumiflon LF504X, product of Asahi Glass Co., Ltd.), 7 g of a melamine crosslinking agent (Cymel 303, product of Mitsui Cytec Co., Ltd.), 0.55 g of a lubricant (silicone resin: X-22-2809, product of The Shin-Etsu Chemical Co., Ltd.) and 0.5 g of a catalyst (Catalyst 4040, product of Mitsui Cytec Co., Ltd.) were dissolved in a mixture of methyl ethyl ketone and cyclohexane to prepare a coating solution containing the resin in an amount of 18 wt. %, and then a viscosity of the coating solution was adjusted to 2-3 PS. The prepared solution was then coated and dried on a polyethylene terephthalate film (thickness: 300 .mu.m) to give a dried layer of 10 .mu.m thick. Thus, a transparent support having a protective film on one surface was provided. The scratch resistance and the contact angle of the surface of the protective film provided on the support (polyethylene terephthalate film) and those of the unprotected surface of the same support were measured in the manner as in Example 1. The results are as follows: EXAMPLE 3 The procedure of Example 1 was repeated, except for preparing the protective film on the support in the following manner, to produce a radiation image storage panel of the invention. 70 g of a fluororesin (fluoroolefin-monovinyl ether copolymer: Lumiflon LF504X, product of Asahi Glass Co., Ltd.), 5.2 g of an isocyanate cross-linking agent (Sumijule N3500, product of Sumitomo Bayer Urethane Co., Ltd.), 6.7 g of a lubricant (silicone resin: X-22-2809, product of The Shin-Etsu Chemical Co., Ltd.), 0.3 g of a catalyst (dibutyltin laurate, KS-1260, product of Kyodo Yakuhin Co., Ltd.), melamine-formaldehyde condensation resin particles (mean diameter: 0.6 .mu.m, refractive index: 1.57, Eposter S-6, product of Nihon Catalyst Co., Ltd.), and 0.12 g of a titanate-coupling agent (Prenact AL-M, product of Azinomoto Co., Ltd.) were dissolved or dispersed in a mixture of methyl ethyl ketone and cyclohexane to prepare a coating solution containing the resin in an amount of 12 wt. %. The prepared solution was then coated and dried on a polyethylene terephthalate film (thickness: 300 .mu.m) to give a dried layer of 7 .mu.m thick. Thus, a transparent support having a protective film on one surface was provided. The resin composition comprising the fluororesin and other binder compositions had a refractive index of 1.45. The scratch resistance and the contact angle of the surface of the protective film provided on the support (polyethylene terephthalate film) and those of the unprotected surface of the same support were measured in the manner as in Example 1. The results are as follows: EXAMPLE 4 The procedure of Example 1 was repeated, except for preparing the protective film on the support in the following manner, to produce a radiation image storage panel of the invention. 70 g of a fluororesin (fluoroolefin-monovinyl ether copolymer: Lumiflon LF504X, product of Asahi Glass Co., Ltd.), 7 g of a melamine crosslinking agent (Cymel 303, product of Mitsui Cytec Co., Ltd.), 6.7 g of a lubricant (silicone resin: X-22-2809, product of The Shin-Etsu Chemical Co., Ltd.), 6.62 g of melamine-formaldehyde condensation resin particles (mean diameter: 0.6 .mu.m, refractive index: 1.57, Eposter S-6, product of Nihon Catalyst Co., Ltd.), 0.12 g of a titanate-coupling agent (Prenact AL-M, pro-duct of Azinomoto Co., Ltd.), and 0.5 g of a catalyst (Catalyst 4040, product of Mitsui Cytec Co., Ltd.) were dissolved or dispersed in a mixture of methyl ethyl ketone and cyclohexane to prepare a coating solution containing the resin in an amount of 12 wt. %. The prepared solution was then coated and dried on a polyethylene terephthalate film (thickness: 300 .mu.m) to give a dried layer of 7 .mu.m thick. Thus, a transparent support having a protective film on one surface was provided. The resin composition comprising the fluororesin and other binder compositions had a refractive index of 1.45. The scratch resistance and the contact angle of the surface of the protective film provided on the support (polyethylene terephthalate film) and those of the unprotected surface of the same support were measured in the manner as in Example 1. The results are as follows: COMPARISION EXAMPLE 1 The procedure of Example 1 was repeated except for not providing the protective film on the support to produce a radiation image storage panel for comparison. Evaluation of Radiation Image Storage Panel The scratch resistance and the anti-staining property of the panels of Examples 1 and 2 and Comparison Example 1 were evaluated by the following tests. (1) Durability test for conveying The radiation image storage panel was cut to prepare a rectangular sample piece (100 mm.times.250 mm), and then the sample piece was subjected to a durability test for conveying in a model conveying system (a miniature of conveying system of a commercially available radiation image recording and reproducing apparatus). The durability test was carried out by repeating a series of procedures consisting of: conveying the sample piece between a guide plate and nip rolls, bending by force once upward and then downward around a rubber roll (diameter: 40 mm) by a conveying belt, and then conveying back between the guide plate and nip rolls to the initial position. After those procedures were repeated 3,000 times, cracks occurring on the protective film of the sample were observed. If there was no crack on the film, the same procedures were further repeated another 7,000 times and then cracks were observed. The results are shown in Table 1. (2) Deterioration Test of Sensitivity X-rays were imagewise applied on the radiation image storage panel having been subjected to the above durability test (the procedures had been repeated 3,000 times), and the storage panel was scanned with He--Ne laser beam to stimulate the phosphor. Light emitted from the phosphor was detected from both the upper and the lower surface sides to obtain image data. According to the obtained data, sensitivity (an amount of stimulated emission) of the area having been in contact with parts of the conveying system (belt, rolls, etc.,) was calculated to evaluate deterioration by conveying procedures. The results are shown in Table 1. TABLE 1 ______________________________________ conveying durability (cracks on the film) sensitivity decrease ______________________________________ Ex. 1 not observed 0% (after 4,000 times procedures) Ex. 2 not observed 0% (after 4,000 times procedures) Ex. 3 not observed 0% (after 4,000 times procedures) Ex. 4 not observed 0% (after 4,000 times procedures) Comp. Ex. 1 observed 3% (after 2,000 times procedures) ______________________________________