Patent Number: 062460633
Section: description

[EXAMPLE 1] (I) Production of Radiation Image Storage Panel 1) 200 g of stimulable phosphor (BaFBr.sub.0.85 I.sub.0.15 :0.001 Eu.sup.2+), 8.0 g of polyurethane resin (Pandex T5265M, trade name, available from Dainippon Ink & Chemicals, Inc.), and 2.0 g of epoxy resin (anti-yellowing agent, Epikote 1001, trade name, available from Yuka Shell Epoxy Kabushiki Kaisha) were added to methyl ethyl ketone, and mixed by means of a propeller mixer to prepare a coating liquid having a viscosity of 30 PS (at 25.degree. C.). The prepared coating liquid was applied onto a temporary support (polyethylene terephthalate sheet having a surface beforehand coated with silicone-releasing agent) of 150 .mu.m thickness, and dried to form a phosphor layer. The phosphor layer thus formed was then peeled off from the temporary support to give a stimulable phosphor sheet (thickness: 430 .mu.m). 2) Independently, 90 g (in terms of solid content of soft acrylic resin and 50 g of nitrocellulose were added to methyl ethyl ketone, and mixed to prepare a coating dispersion for subbing layer [viscosity at 25.degree. C.: 3-6 PS]. On a glass plate, a film of polyethylene terephthalate (support) was placed. The dispersion was then coated on the polyethylene terephthalate film by means of a doctor blade to form a layer of 15 .mu.m thick, and gradually heated from 25.degree. C. to 100.degree. C. Thus, the coated layer was dried to form a subbing layer on the support. 3) The above-prepared phosphor sheet was placed on the subbing layer, and then continuously pressed with heating by means of a calender roll under a pressure of 500 kgw/cm.sup.2 (temperature of the upper and lower rolls: 45.degree. C., moving speed: 0.3 m/minute). By this pressing procedure, the phosphor sheet was fixed onto the transparent support via the subbing layer. Thus, a stimulable phosphor layer (thickness: 230 .mu.m) is formed. 4) 70 g of fluorocarbon resin (copolymer of fluoro-olefin and monovinyl ether, Lumiflon LF504X, trade name, available from Asahi Glass Co., Ltd.), 5.2 g of isocyanate (crosslinking agent, Sumidule N3500, trade name, available from Sumitomo Bayer Urethane Co., Ltd.), 6.7 g of silicon resin (lubricating agent, X-22-2809, trade name, available from The Shin-Etsu Chemical Co., Ltd.), 0.3 g of dibutyltin laurate (catalyst, KS-1269, trade name, available from Kyodo Chemical Co., Ltd.), 2.8 g of light-scattering particles (anatase type titanium dioxide, A220, trade name, available from Ishihara Industries Co., Ltd., mean particle size: 0.15 .mu.m, refractive index: about 2.6) and 0.12 g of titanate type coupling agent (Plane-act AL-M, trade name, available from Ajinomoto Co., Inc.) were added to methyl ethyl ketone, and mixed to prepare a coating liquid (solid content: 12%). The coating liquid was then applied onto the phosphor layer by means of a doctor blade, and dried to form a surface protective film of approximately 7 .mu.m thick. The content of titanium dioxide in the protective film was found to be 3 wt. %. (II) Calculation of Scattering Length and Absorption Length of Surface Protective Film The coating liquid of the 4) above was applied onto a transparent support (thickness: 180 .mu.m) so that the formed layer might have a thickness of 5 to 50 .mu.m. The diffuse transmittance (%) of the formed layer was measured at wavelength of 400 nm (which corresponds to the main peak of the stimulated emission which was emitted from the aforementioned BaFBR.sub.0.85 I.sub.0.15 :0.001Eu.sup.2+ phosphor), by means of an automatic recording spectrophotometer (U-3210, manufactured by HITACHI, Ltd.) equipped with integrating sphere of 150 .phi. (150-0910). The results are set forth in Table 1. TABLE 1 thickness (.mu.m) 7 11 24 40 diffuse 70.3 62.6 48.4 40.2 transmittance (%) In accordance with the aforementioned formulas, the values of K and S were calculated from the data shown in Table 1. From the calculated values of K and S, the scattering length and the absorption length were calculated to be 23 .mu.m (scattering length=1/S) and 10,000 .mu.m (absorption length=1/K), respectively. [EXAMPLE 2] The procedures of Example 1 were repeated except that 0.9 g of titanium dioxide was added to the coating liquid for the preparation of a protective film, to prepare a radiation image storage panel of the invention. [EXAMPLE 3] (I) Production of Radiation Image Storage Panel A radiation image storage panel was prepared in the manner as described in Example 1, except that the protective film was replaced with the following two-layered protective films. 1) Lower protective film (Protective film I) 70 g of fluorocarbon resin (copolymer of fluoro-olefin and monovinyl ether, Lumiflon LF504X, trade name, available from Asahi Class Co., Ltd.), 5.2 g of isocyanate (crosslinking agent, Sumidule N3500, trade name, available from Sumitomo Bayer Urethane Co., Ltd.), 0.3 g of dibutyltin laurate (catalyst, KS-1269, trade name, available from Kyodo Chemical Co., Ltd.), 2.8 g of light-scattering particles (anatase type titanium dioxide, A220, trade name, available from Ishihara Industries Co., Ltd., mean particle: 0.15 .mu.m, refractive index: about 2.6) and 0.12 g of titanate type coupling agent (Plane-act AL-M, trade name, available from Ajinomoto Co., Inc.) were added to methyl ethyl ketone, and mixed to prepare a coating liquid (solid content: 12%). The coating liquid was then applied onto the phosphor layer by means of a doctor blade, and dried to form a protective film I of approximately 2 .mu.m thick. 2) Upper protective film (Protective film II) 50 g of fluorocarbon resin (copolymer of fluoro-olefin and vinyl ether, Lumiflon LF100, trade name, available from Asahi Glass Co., Ltd., 50 wt. % xylene solution), 5 g of isocyanate (crosslinking agent, Colonate HX, trade name, available from Nippon Polyurethane Co., Ltd.), 0.5 g of silicon resin (lubricating agent, X-22-2809, trade name, available from The Shin-Etsu Chemical Co., Ltd., solid content: 66 wt. %), 0.0004 g of dibutyltin laurate (catalyst, KS-1260, trade name, available from Kyodo Chemical Co., Ltd.), 6 g of light-scattering particles (benzoguanamine resin particles, Epostar s6, trade name, available from Japan Catalyst Co., Ltd.; mean particle size: 6, refractive index: about 1.6), and 0.1 g of titanate type coupling magnet (Plane-act AL-M, trade name, available from Ajinomoto Co., Inc.) were added to methyl ethyl ketone, and mixed to prepare a coating liquid. The coating liquid was then applied onto the protective film I by means of a doctor blade, and heated to 120.degree. C. for 20 minutes to dry and thermally treat the coated liquid to form a protective film II of approximately 7 .mu.m thick. Accordingly, the combination of the protective film I and the protective film II (total thickness: 9 .mu.m was provided on the phosphor layer. (II) Calculation of Scattering Length and Absorption Length of Combined Protective Films The coating liquid for the protective film II was applied onto each of the transparent supports having protective films of different thickness (which were prepared in Example 1-(II), to have 2 .mu.m thick. The diffuse transmittance (%) of the formed layer was measured in the same manner as described in Example 1-(II). [COMPARISON EXAMPLE 1] The procedures of Example 1 were repeated except that titanium dioxide and the coupling agent were not added to the coating liquid for protective film, to prepare a radiation image storage panel for comparison having a protective film of approximately 7 .mu.m thick. [COMPARISON EXAMPLE 2] The procedures of Example 1 were repeated except that titanium dioxide and the coupling agent were not added to the coating liquid for protective film, to prepare a radiation image storage panel for comparison having a protective film of approximately 3.5 .mu.m thick. [EVALUATION OF RADIATION IMAGE STORAGE PANEL] With respect to sensitivity, sharpness and durability, each radiation image storage panel prepared above was evaluated in the following manners. (1) Measurement of sensitivity After the sample storage was exposed to X-rays (generated under 80 kVp), the stimulable phosphor was excited with He-Ne laser (wavelength: 632.8 nm). The stimulated emission produced from the panel was detected, and the sensitivity was evaluated from the relative intensity of the emission. (2) Measurement of sharpness After the sample storage panel was exposed to X-rays (generated under 80 kVp) through an MTF chart, the stimulable phosphor was excited with He-Ne laser (Wavelength: 632.8 nm). The stimulated emission produced from the storage panel was detected by a photomultiplier tube (S-5) to convert into electric signals. On the basis of the obtained signals, the radiographic image was reproduced on a display of an image-reproducing apparatus. The MTF (modulation transfer function) of the reproduced radiographic image was determined (spatial frequency: 2 cycle/mm). (3) Measurement of durability The storage panel was cut into pieces (size: 100 mm .times.250 mm), and one of them was repeatedly subjected to the durability test [described in Japanese Patent Provisional Publication No. 8(1996)-36099] until cracks appeared on the surface of the protective film. From the repeated number of the test, the durability of the panel was evaluated. The results are set forth in Table 2. TABLE 2 panel Ex. 1 Ex. 2 Ex. 3 C. Ex. 1 C. Ex. 2 particles 3 1 3 (20) 0 0 (wt. %) film 7 7 2 (7) 7 3.5 thickness (.mu.m) scattering 23 50 49 &gt;200 &gt;200 length (.mu.m) relative 99 100 98 100 100 sensitivity sharpness 38.0 36.9 37.1 36.5 38.2 (%) durability &gt;4,000 &gt;4,000 &gt;4,000 &gt;4,000 2,000 (times) Remarks: The value in the parenthesis of Ex. 3 is the value of the protective film II. The results set forth in Table 2 indicate the following facts. The radiation image storage panels of the invention (Examples 1, 2 and 3) give images of high sharpness while they have almost the same sensitivity and durability as the conventional radiation image storage panel (Comparison Example 1). Further, the conventional storage panel having a thin protective film (Comparison Example 2) gives high sharpness but shows apparently poor durability.