Glass panel resistant to coloring when irradiated with electron rays

Abstract: A glass panel for a face plate of a cathode ray tube which is characterized in that said glass panel has the following glass composition: SiO 2 64 -75% by weight Al 2 O 3 1.5-2.0% by weight CaO 6.5-9.0% by weight MgO 0 -5.0% by weight Li 2 O 0.5-2.5% by weight Na 2 O 7.0-12% by weight K 2 O 2.1-5.0% by weight SrO + BaO + ZrO 2 0 -10% by weight CeO 2 0 -0.5% by weight and wherein part of the Na ion in at least the surface layer of the panel region to be irradiated with electron rays, is substituted with K ion by an ion exchange treatment in a K-containing molten salt and resultantly the molar ratio of Na ion and K ion expressed by Na 2 O/(Na 2 O + K 2 O) in at least the glass layer ranging from 0.5 µm to 6.5 µm in depth from the panel surface, of said panel region to be irradiated with electron rays is 0.3-0.75; and the glass panel has a shape of a tray comprising a rectangular top portion and side portions, the shorter side of said rectangular top having a length (L 1 ) of 100 to 530 mm, the longer side having a length (L 2 ) of 1.3 to 3 times L 1 mm, the depth (H) of the tray being 20 to 40 mm, and the thickness (t) of the rectangular top expressed in mm and the length (L 1 ) of said shorter side expressed in mm satisfying the relationship as defined below: 25.45 t -1.8 > L 1 > 25.45 t - 52.7

Description: Field of the Invention The present invention relates to a glass panel used in image display devices (e.g. cathode ray tube, plasma display panel) or fluorescent indicator tubes which are irradiated with electron rays, particularly to a glass panel suitably used in cathode ray tubes which are irradiated with electron rays of high energy density. Prior Art Currently, display devices using a cathode ray tube or a plasma display are required to use electron rays of high acceleration voltage or high energy density in order to increase the luminance, or to make smaller the thickness of the glass panel in order to reduce the total weight. Hence, the glass panels used in said display devices are required to (a) undergo low coloring when irradiated with electron rays, (b) have a high strength even when the panels are thin and (c) be resistant to dielectric breakdown even when a high voltage is applied thereto. Further, it is necessary that material glass plates be available sufficiently in order to enable production therefrom, of glass panels meeting the above requirements, in a large amount. For these reasons, studies are under way on various glass compositions containing no PbO (PbO increases the weight of resulting glass). That is, it is under study to heat-form a material glass plate containing no PbO, then substitute the Na ion present at the plate surface with K ion to generate a compression layer at the plate surface and thereby produce a glass panel having a higher mechanical strength and incurring low coloring when irradiated with electron rays. In such a glass panel, the glass composition, particularly the Na 2 O, K 2 O and Li 2 O contents and the viscosity characteristic of material glass plate are important. Japanese Patent Application Kokai (Laid-open) No. 288134/1987 discloses a glass panel containing SrO and BaO and having a controlled ratio of Na 2 O and K 2 O contents. This glass panel has a high K 2 O content and therefore has a good electrical insulating property; however, even after being subjected to an ion exchange treatment (for substitution of Na ion in panel with K ion) in a molten salt containing K ion, the panel is unable to have a higher mechanical strength and shows no sufficient suppression for coloring caused by irradiation with electron rays. Further, the material glass plate used for the production of the above glass panel cannot be produced by shaping a molten material glass directly into a flat plate according to the float process, and must be produced by subjecting a glass gob to press molding, followed by surface grinding. Meanwhile, Japanese Patent Application Kokai (Laid-open) No. 153143/1987 discloses an material glass plate which can be produced as a flat plate directly by the float process. The glass panel produced therefrom has a high electrical insulating property but, even after being subjected to an ion exchange treatment, is unable to show improvements in resistance to coloring caused by irradiation with electron rays as well as in mechanical strength. Further, Japanese Patent Application Kokai (Laid-open) No. 195146/1988 discloses a glass panel which is produced by subjecting a material glass plate of soda-limesilica composition for commercially available window glass, obtained by the float process, to an ion exchange treatment in a K-containing molten salt to form a K-ion-rich layer at the panel surface and which resultantly has sufficient resistance to coloring when irradiated with electron rays and sufficient mechanical strength. The glass panel, however, has a low electrical insulating property and, when receiving a high voltage locally, gives rise to dielectric breakdown. JP-A-3290335 discloses a glass panel having an unspecified size in which part of the Na + ions in a surface layer of unspecified depth are exchanged for K + ions by ion exchange treatment using a K-containing salt. GB-A-2262653 which was published between the priority and filing dates of the present application, relates to a glass pressure vessel for a cathode ray tube. The glass pressure vessel comprises a substantially rectangular flat portion, a side wall portion connected to the flat portion, and an annular flange portion whose outer periphery is substantially rectangular, and the flange portion of the concave glass is bound, with glass frit and in a predetermined bond width, to a rectangular glass back-plate, whose periphery is in accordance with the outer periphery of the flange portion. The following inequalities are satisfied, where t 1 (mm) is the thickness of the concave glass, L (mm) is the length of the short side of the concave glass and the glass back-plate, t 2 (mm) is the thickness of the glass back-plate and W (mm) is the bond width 1,000 W/(Lt 2 ) ≥ 2.8 t 1 ≥ 0.8 t 2 . The present invention has been made in order to solve the above-mentioned problems of the prior art. The object of the present invention is to provide a glass panel characterized by (a) enabling suppression of the colouring caused by electron ray irradiation, (b) having a high mechanical strength owing to the presence of a compression stress layer formed at the panel surface by an ion exchange treatment, and (c) having a high electrical insulating property and therefore being resistant to dielectric breakdown even when a high voltage is applied to the panel, and further characterized in that the material glass plate used for production of said glass panel can be produced directly from a molten material glass by the float process. The present application is a divisional application from EP-A-0604094. The parent application is concerned with a glass panel for a face plate of a cathode ray tube having the same glass composition as specified herein and having a surface layer in which the molar ratio Na 2 /(Na 2 + K 2 O) is from 0.3 to 0.75 at a depth of glass layer of from 0.5 µm to 1.5 µm in depth from the panel surface when the panel thickness is from 4 to 7 mm, at a depth of the glass layer of from 1.5 µm to 3.5 µm from the panel surface when the panel thickness is from 14 to 18 mm and a depth from the surface of from 1.5 µm to 6.5 µm when the panel thickness is from 19 to 23 mm. Summary of the Invention: The present invention provides a glass panel, resistant to colouring when irradiated with electron rays, which has the following glass composition: SiO 2 64-75% by weight Al 2 O 3 1.5-2.0% by weight CaO 6.5-9.0% by weight MgO 0-5.0% by weight Li 2 O 0.5-2.5% by weight Na 2 O 7.0-12% by weight K 2 O 2.1-5.0% by weight SrO + BaO + ZrO 2 0-10% by weight CeO 2 0-0.5% by weight and wherein part of the Na ion in at least the surface layer of the panel region to be irradiated with electron rays, is substituted with K ion by an ion exchange treatment in a K-containing molten salt and resultantly the molar ratio of Na ion and K ion expressed by Na 2 O/(Na 2 O + K 2 O) in at least the glass layer ranging from 0.5 µm to 6.5 µm in depth from the panel surface, of said panel region to be irradiated with electron rays is 0.3-0.75; and the glass panel has a shape of a tray comprising a rectangular top portion and side portions, the shorter side of said rectangular top having a length (L 1 ) of 100 to 530 mm, the longer side having a length (L 2 ) of 1.3 to 3 times L 1 mm, the depth (H) of the tray being 20 to 40 mm, and the thickness (t) of the rectangular top expressed in mm and the length (L 1 ) of said shorter side expressed in mm satisfying the relationship as defined below: 25.45 t -1.8 > L 1 > 25.45 t - 52.7 the glass panel having a thickness of from 4 to 23 mm; and in which the temperature and time of the ion exchange treatment fall within a polygonal area within the lines in a temperature-time diagram obtained by connecting the following points: (500 °C, 90 min.), (500 °C, 120 min.), (460 °C, 180 min.), (460 °C, 150 min.) and (480 °C, 90 min.), provided that the point (460 °C, 180 min.) is excluded. Brief Description of the Drawings Fig. 1 is a graph showing the depth of each coloured layer formed at the surface of a glass plate having the glass composition of the present invention, when the glass plate is irradiated with electron rays accelerated by various voltages. Transmittance was measured after the glass was etched from the coloured side step by step. Fig. 2 is a graph showing the ranges of the temperature and time employed for ion exchange treatment of glass panel. Fig. 3 is a perspective view showing an example of the cathode ray tube used in flat surface televisions, and Fig. 4 is a sectional view taken at the A-A' line of Fig. 3. Fig. 5 is a sectional view showing an example of the shape of the glass panel of the present invention used as the flat plate of cathode ray tube of flat surface television. Fig. 6 is a graph showing the preferred ranges of the thickness (t) of top plane and the short side (L 1 ) of top plane, of the glass panel of the present invention used as the flat plate of cathode ray tube of flat surface television. Detailed Description of the Invention The present invention is hereinafter described in detail. In the following description, % refers to % by weight unless otherwise specified. In the present invention, the glass composition is as follows: SiO 2 64-75% by weight Al 2 O 3 1.5-2.0% by weight CaO 6.5-9.0% by weight MgO 0-5.0% by weight Li 2 O 0.5-2.5% by weight Na 2 O 7.0-12% by weight K 2 O 2.1-5.0% by weight SrO + BaO + ZrO 2 0 -10% by weight CeO 2 0 -0.5% by weight The glass composition is preferably as follows: SiO 2 64 -72% by weight Al 2 O 3 1.5-2.0% by weight CaO 6.5-9.0% by weight MgO 3.0-4.0% by weight Li 2 O 0.5-1.5% by weight Na 2 O 8.5-10.5% by weight K 2 O 2.1-3.0% by weight SrO + BaO + ZrO 2 4.5-10.0% by weight The above glass composition has been selected so as to satisfy the following requirements: (1) The content of alkali metal oxides, i.e. Na 2 O, K 2 O and Li 2 O is selected so that (a) the ion-exchange layer formed by ion exchange between the Na ion in the glass and the K-ion-containing molten salt can effectively prevent coloring when the glass panel of the present invention is irradiated with electron rays, (b) the ion-exchanged layer gives a high compression stress (40 kg/mm 2 or more), and (c) said ion-exchange layer is formed in a depth range from 0.5 µm to 6.5 µm measured from the panel surface (that is, the layer has a thickness of 6 µm). (2) The glass overall composition is selected so that it has a viscosity-temperature characteristic similar to that of the soda-lime-silica glass for window use, in order to enable shaping into a plate by the float process. That is, when the viscosity η (unit: poise) of glass is expressed as logη, logη is determined to be 9.5-11 at 600°C, 5.0-6.5 at 800°C, 3.5-4.5 at 1,000°C, 2.5-3.0 at 1,200°C and 1.5-2.5 at 1,400°C. (3) The electrical resistivity of glass must be at least 1x10 10 Ωcm in order for the glass to give rise to no dielectric breakdown even when electron rays and a high voltage are applied to the glass. When the content of Na 2 O is lower than 7%, the layer generated in the vicinity of the glass surface by a Na ion - K ion exchange treatment has a low compression stress and the resulting glass panel does not have high strength; moreover, the material glass has too high a viscosity and, in producing a glass plate therefrom, the melting and shaping are difficult. When the content of Na 2 O is higher than 12%, the resulting glass panel undergoes remarkable coloring when irradiated with electron rays. A preferable content of Na 2 O is in the range of 8.5-10.5%. When the content of K 2 O is lower than 2.1%, the resulting glass panel undergoes remarkable coloring when irradiated with electron rays, has a low electrical resistance, and tends to give dielectric breakdown when a high voltage is applied thereto locally. When the content of K 2 O is higher than 5%, the layer generated at the surface of the untreated glass panel by an ion exchange treatment has a low compression stress and the resulting glass panel does not have high strength; moreover, the glass material has too high a viscosity and, in producing a glass plate therefrom, melting and shaping are difficult. A preferable content of K 2 O is in the range of 2.1-3.0% and a more preferable content is in the range of 2.5-3.0%. Addition of Li 2 O to the Na 2 O and K 2 O contained in the above ranges can further suppress the coloring of the resulting glass panel when the panel is irradiated with electron rays, and can allow the glass panel to have an electrical resistivity of 1x10 10 Ωcm or more. When the content of Li 2 O is lower than 0.5%, the resulting glass panel has a small effect on the suppression of coloring caused by electron rays. When the content is higher than 2.5%, the material glass has a low viscosity at a low temperature range (500-800°C), which is not preferable for production of a glass plate. A preferable content of Li 2 O is in the range of 0.5-1.5%. SiO 2 is a major component for glass formation. When the content of SiO 2 is lower than 64%, the resulting glass has too low a viscosity. When the content is higher than 75%, the glass has too high a viscosity and melting and shaping for production of a glass plate therefrom is difficult. A preferable content of SiO 2 is in the range of 64-72%. When the content of Al 2 O 3 is lower than 1.5%, the resulting glass has poor chemical durability. When the content is higher than 2%, the glass has a high viscosity and is difficult to melt for production of a glass plate. When the content of CaO is lower than 6.5%, the resulting glass has a low electrical resistance and, when a high voltage is applied thereto, tends to give rise to dielectric breakdown; further, the glass has a high viscosity at high temperatures and is difficult to melt. When the content of CaO is higher than 9%, the glass has a high viscosity at a lower temperature range (500-800°C) and too low a viscosity at a high temperature range (800-1,500°C); this causes, when a material glass plate is produced from the glass on a molten tin bath by the float process, a variation in panel thickness owing to the temperature distribution of said bath. MgO is used together with CaO. When the content of MgO is higher than 5%, the resulting glass has a high viscosity at a high temperature range (800-1,500°C) and is difficult to melt for production of glass plate therefrom. A preferable range of MgO is in the range of 3.0-4.0%. When the glass panel of the present invention is irradiated with electron rays at a high acceleration voltage, the present glass panel may contain SrO 2 , BaO, ZrO 2 , etc. in a total amount not exceeding 10% in order for the generated X-rays not to leak to outside; and the glass panel may further contain CeO 2 and TiO 2 each in amount not exceeding 0.5% in order to suppress the coloring of panel by said X-rays. Besides, Fe 2 O 3 , etc. may be contained in the glass panel, as impurities of material glass. It is possible to add coloring components such as NiO, CoO, Cr 2 O 3 , Nd 2 O 3 , MnO 2 and the like, for tinting of the glass panel. A preferred content of (SrO + BaO + ZrO 2 ) is in the range of 4.5-10.0%. When SrO, Bao and ZrO 2 are contained in a material glass in a total amount of 4.5% or more, the material glass can have an increased electrical resistivity of 1x10 10.7 cm or more at 150°C, whereby the glass panel for cathode ray tube produced therefrom has a very low risk of giving dielectric breakdown even when a high voltage is applied thereto. With respect to the use of SrO, Bao and ZrO 2 , single use of BaO is most preferable. In the glass panel of the present invention, part of the Na ion in at least the surface layer of the panel region to be irradiated with electron rays, is substituted with K ion by an ion exchange treatment in a K-containing molten salt and, resultantly, the molar ratio of Na ion and K ion expressed by Na 2 O/(Na 2 O + K 2 O) in at least the glass layer ranging from 0.5 µm to 6.5 µm in depth from the panel surface of said panel region to be irradiated with electron rays is 0.3-0.75. In the present invention, by maintaining the molar ratio of Na ion and K ion in said glass surface layer, in the above range, the glass panel has a high strength and further is effectively protected from coloring caused by irradiation with electron rays. Incidentally, the molar ratio of Na ion and K ion at a desired depth range measured from the panel surface can be measured by an X-ray analyzer. Fig. 1 shows the depths of colored layers formed in a material glass plate having the glass composition of the present invention but subjected to no ion exchange treatment, when the surface of said glass plate was irradiated with electron rays accelerated by various voltages. In Fig.1, the ordinate indicates the light transmittance measured after the material glass plate irradiated with electron rays was etched from the irradiated surface little by little. As appreciated from Fig. 1, when the material glass panel was irradiated with electron rays accelerated by a voltage of 10 KV, there was formed a coloured layer having a depth range of 0.5-1.5 µm (the depth is a distance from the panel surface); that is, the electrons penetrated as far as the above depth range. Similarly, when the material glass plate was irradiated with the electron rays accelerated by a voltage of 20 KV or 30 KV, there was formed a coloured layer having a depth range of 1-3.5 µm or 2-6.5 µm. In the glass panel of the present invention, the colouring caused by irradiation with electron rays is effectively prevented by maintaining the molar ratio of Na 2 O and K 2 O present in the surface layer corresponding to the above coloured layer, in a desired range as mentioned above. Further, by substituting the Na ion present in said surface layer, with K ion of larger ionic radius, a compression layer is formed at the glass surface and, as a result, the resulting glass panel has an increased mechanical strength. When the glass panel is subjected to an ion exchange treatment in a K-containing molten salt, whole areas of both sides (front and back sides) of the panel are subjected to the ion exchange treatment. As necessary, however, only one side of the glass panel or only the desired region of one side may be subjected to an ion exchange treatment. Such partial ion exchange can be conducted by, for example, covering the region requiring no ion exchange treatment, with a shielding layer with prevents ion exchange. According to our study, when the value of Na 2 O/Na 2 O + K 2 O) in the desired depth range of the glass panel measured from the panel surface is smaller than 0.3 or larger than 0.75, irradiation of the glass panel with electron rays tends to cause movement of alkali components such as Na 2 O and K 2 O in the glass, making it difficult to suppress the colouring brought about by irradiation with electron rays; further, the glass panel has a low light transmittance, making it difficult to efficiently take a transmitted light out of the panel. In the glass panel of the present invention, the ion exchange treatment is conducted by processing a material glass plate to a desired shape and then dipping the resulting glass panel in a K-ion-containing molten salt under conditions such that the temperature and time of the ion exchange treatment fall within polygonal area within the lines in a temperature - time diagram obtained by connecting the following points: (500°C, 90 min), (500°C, 120 min), (460°C, 180 min), (460°C, 150 min) and (480°C, 90 min) provided that the point (460°C, 180 min) is excluded. When there is used a molten salt whose cation consists of K ion alone, it is possible to form a K ion layer at the surface of the glass panel; then, the K ion and Na ion in the panel are subjected to mutual diffusion by, for example, heating in the air, whereby the above surface layer can have a molar ratio expressed by Na 2 O/Na 2 O + K 2 O), of 0.3 to 0.75. The temperature and time of the ion exchange treatment fall within the hatched region show in Fig. 2, except that the point (460°C, 180 min) is excluded, in order to effectively suppress the colouring of the resulting glass panel caused by irradiation with electron rays and maximize the mechanical strength of the panel. Within the hatched region of Fig. 2, the ion exchange treatment is conducted in the shortest time in order to achieve the prevention of colouring by electron rays and the improvement in mechanical strength. The glass panel of the present invention is used in production of image display devices or fluorescent indicator tubes irradiated with electron rays, such as cathode ray tubes and plasma display panels. Figs. 3 and 4 show an example 10 of the cathode ray tube used in flat surface televisions. In Figs. 3 and 4, 1 is a glass-made face plate of brimmed thin tray shape. The glass panel of the present invention is used as such a face plate. The face plate 1 is vacuum-tightly adhered to a glass back plate 2 with a low-melting frit 9. On the inner side of the face plate 1 is formed a fluorescent film 5, and a metallic thin film 6 is formed thereon. 3 is a stud pin; 4 is an anode; 7 is an electron gun unit; and 8 is signal-introducing line. As shown in Fig. 5, the face plate 1 consists of a top portion la of rectangular shape, a side 1b and a brim 1c. The thickness t (mm) of the top portion, the length L 1 (mm) of the short side of the top portion, the length L 2 (mm) of the long side of the top portion and the depth H (mm) are selected so as to satisfy the following relationships. L 1 = 100-530 L 2 = (1.3-3)L 1 H = 20-40 25.45t - 1.8 > L 1 > 25.45t - 52.7 the relationship between the thickness t and the length L 1 of the short side is in the range surrounded by the parallelogram shown in Fig. 6. As illustrated in Figs. 3 and 4, the glass panel of the present invention forms a cathode ray tube (CRT), as sealed at its flange portions with a flat back plate to which an electron gun is mounted. In that occasion, the inside of the CRT is under reduced pressure. Consequently, the central part of the glass panel tends to cave in under the atmospheric pressure. Fig. 6 indicates such panel dimensions as would make the deformation negligible for the functions of CRT. The dimensions are found to solve the problems inherent in preparing a material glass plate of a fixed thickness through bending processing and chemical reinforcing treatment. The glass panel of the present invention to form CRT basically has the same, fixed thickness similarly to its material glass plate, at the central and peripheral portions of the display device and also at the side walls. In contrast thereto, conventional CRT's are formed by gob method (a method comprising pouring molten glass into a mold), in which the thickness of the glass continuously increases from the central portion to the peripheral portion of the display part, in order to secure satisfactory panel strength. A CRT formed from a glass panel of the present invention and a flat back plate has the following characteristic features. 1) The CRT depth becomes thin. The distance between the inner plane of the back plate and the inside wall of the front panel can be made as short as 20-40 mm, in contrast to several tens centimeters' depth of conventional CRT. 2) Because the electron beam irradiation unit is mounted on the flat back plate, the distance between the electron gun to the fluorescent painted plane can be accurately set. This feature has particularly high practical value for multi-electron gun-type CRT. In shaping a material glass plate into a glass panel of the present invention, there can be used a known method, for example, a method disclosed in Japanese Patent Application Kokai (Laid-open) No. 122932/1989 which comprises mounting a material glass plate on a male die and heating the plate so that the peripheral portion be heated to a higher temperature, to obtain a glass panel of desired shape. In the glass panel of the present invention, the contents of Na 2 O, K 2 O and Li 2 O are controlled so that the panel has a high electrical insulating property, whereby the glass panel is resistant to dielectric breakdown even when the panel is irradiated with electron rays and receives a high voltage. Further in the present glass panel, there is formed, at least at the glass surface region corresponding to the panel region to be irradiated with electron rays, a surface layer which has a compression stress and wherein the contents of Na 2 O and K 2 O are controlled, whereby the glass panel is resistant to coloring even when irradiated with electron rays and has a high mechanical strength. Thus, in the glass panel of the present invention, the reduction (ΔT) in transmittance of the panel after irradiation with electron rays as compared with before irradiation is smaller than 40%, preferably smaller than 20% when the panel is irradiated with electron rays of 10 KV and 40 µA/cm 2 for 100 hours in vacuum and the transmittance after and before irradiation are measured using a light of 400 nm wavelength. Examples Preparation Example 1 A glass having a composition shown in the preparation sample 1 column of Table 1 was melted in a platinum crucible. The molten material glass was cast onto a metal plate and cooled. The cooled glass was ground to obtain a glass plate of 5 mm in thickness. The glass plate was mounted on a male die and placed in an electric furnace, wherein the plate was heated so that the peripheral portion became hotter, to prepare a glass panel as shown in Fig. 5, whose image display area had a rectangular shape of 15 cm in diagonal. The glass panel was subjected to an ion exchange treatment at 460°C for 180 minutes in molten potassium nitrate and then subjected to a heat treatment at 450°C for 3 hours. The thus treated glass panel was measured for logarithm of volume resistivity (log ρ), reduction (ΔT) in transmittance after irradiation with electron rays (ΔT = transmittance before irradiation minus transmittance after irradiation), and compression stress of surface layer. The results are shown in Table 3. The volume resistivity was measured at 150°C. The reduction (ΔT) in transmittance after irradiation with electron rays is a transmittance of glass panel before irradiation with electron rays minus a transmittance of the glass panel after irradiation with electron rays of 10 KV and 40 µA/cm 2 for 100 hours in vacuum (the transmittance was measured using a light of wavelength of 400 nm). The surface compression stress was measured using a polarizing microscope. Preparation samples 2-13 were prepared in the same manner as for the preparation sample 1 except that there were used glasses having compositions shown in the columns of preparation samples 2-13 of Table 1 and that there were used the conditions for ion exchange treatment shown in Table 3. The preparation samples 2-13 were measured for the same items as above. The results are shown in Table 3. Comparative Examples Glass panels having the glass compositions shown in Table 2, disclosed in the prior art were produced and subjected to ion exchange under the ion exchange conditions shown in Table 4. The resulting comparative samples were measured for logarithm of volume resistivity (logρ), reduction (ΔT) in transmittance after irradiation with electron rays (ΔT = transmittance before irradiation minus transmittance after irradiation), and compression stress of the surface layer obtained by the ion exchange treatment. The results are shown in Table 4. Example 2 A glass plate of 5 mm in thickness was produced in the same manner as for the preparation sample 1, using a glass having a composition shown in the sample 6 column of Table 1. The glass plate was treated under the conditions for ion exchange treatment shown in Table 5 and then measured for resistance to coloring by electron rays (ΔT) and mechanical strengths [maximum stress (kg/mm 2 ) and depth of stress (µm)]. The results are shown in Table 5. The maximum stress and depth of stress were measured according to the following methods. The maximum stress was determined by grinding a glass plate to a thickness of 2-3 mm and measuring the optical pass difference of the plate by the retardation method. The depth of stress was determined by measuring the positions from glass surface, of the interference fringes appearing in the same method. Ion exchange treatment Resistance to coloring by electron rays ΔT (%) Mechanical strength Temp. (°C) Time (min) Maximum stress (Kg/mm 2 ) Depth of stress (µm) 460 150 10 45 6.0 460 360 15 62 8.6 490 90 13 64 7.7 490 120 14 56 8.0 490 300 14 47 8.5 500 90 14 53 8.6 500 120 15 48 9.0 Example 3 Two glass plates of 10 mm and 15 mm in thick-ness, each having a composition shown in the preparation sample 6 column of Table 1 were produced by the float process of shaping a material glass into a glass plate on a molten tin bath. Using the two glass plates, front glass panels having dimensions shown in the sample A to D columns of Table 6 were produced by a known heat-bending shaping method using a male die and a female die. Each of these glass panels was subjected to an ion exchange treatment (ion exchange between K ion in molten potassium nitrate and Na ion in glass) in molten potassium nitrate under the temperature and time shown in Table 6, to form a layer for prevention of coloring by electron rays and a compression stress in the vicinity of the glass surface. The thus obtained comparative sample A and samples B to D were irradiated, at the image display regions, with electron rays of 40 µA/cm 2 accelerated by 10 KV, for 100 hours, and then measured for the resulting reduction in transmittance of 400 nm light. The results are shown in Table 6. Comparative Example 4 Two glass plates each of 5 mm in thickness were produced from the glass having a composition shown in the preparation sample 6 column of Table 1, in the same manner as for the preparation sample 1. One of the glass plates was subjected to an ion exchange treatment at 460°C for 180 minutes to obtain a sample X. The other glass plate was subjected to an ion exchange treatment at 460°C for 360 minutes to obtain a sample Y. The samples X and Y were measured for mole % of Na 2 0/(Na 2 0 + K 2 0) at various depths (0 to 7 µm) from the plate surface, using an X-ray microanalyzer. The results are shown in Table 7. The samples X and Y were irradiated with electron rays of 40 µA/cm 2 accelerated by 10 KV, in vacuum for 100 hours, and then measured for reduction in transmittance of 400 nm light, i.e. ΔT. The ΔT was 23% in the sample X and 30% in the sample Y. The glass before ion exchange treatment gave a ΔT of 60%. Mole % of Na 2 0/(Na 2 0 + K 2 0) at various depths Mole % Depth (µm) 460°C x 180 minutes 460°C x 360 minutes 0 0.17 0.19 0.1 0.21 0.19 0.3 0.23 0.2 0.5 0.30 0.25 1 0.34 0.31 2 0.44 0.4 3 0.56 0.51 4 0.68 0.62 5 0.78 0.66 6 0.83 0.68 7 0.85 0.76 Example 5 From the glasses having the compositions shown in the preparation samples 4, 6 and 12 columns of Table 1, seven kinds of glass panels having the thickness and dimensions as indicated in Table 8 and the shape as shown in Fig. 5 were prepared by the method as employed in Preparation Example 1, which were labelled Comparative Samples 14-16, 18 and 20 and Samples 17 and 19. Those glass panels were subjected to the ion exchange treatment under the conditions indicated in Table 8, followed by three hours' heat treatment at 450°C. Thereafter the glass panels were measured of their volume resistivity (log ρ), reduction (ΔT) in transmittance after irradiation with electron rays, and compression stress of surface layer, by the methods as described in Preparation Example 1. The results are shown in Table 8. Each of Comparative Samples 14-16, 18 and 20 and Samples 17 and 19 glass panels were used as a front panel which was sealed at its flange portions with a glass back plate of identical thickness with the front panel, using a low-temperature melting glass. The inside of the system was made the vacuum of not higher than 10 -7 torr, to form a vacuum container as illustrated in Fig. 4. The central portions of the image display devices caved in under the atmospheric pressure, but the volume of depression (volume of displacement) in all of Comparative Samples 14-16, 18 and 20 and Samples 17 and 19 was invariably within 150 µm, not interfering with the practical use of the CRT's. As is appreciated from the above Examples, the glass panel of the present invention has, as an electrical insulating property, a volume resistivity larger than 1x10 10 Ωcm which is a yardstick inviting no dielectric breakdown; has a high mechanical strength of at least 40 kg/mm 2 ; and gives a low reduction in transmittance when irradiated with electron rays. As compared with the glass panels of the prior art, the present glass panel is improved in the compression stress of the surface layer (the compression stress can be used as a yardstick for mechanical strength) and resistance to coloring by electron rays. The glass panel of the present invention has, at the surface, a compression stress layer capable of suppressing the appearance of coloring by irradiation with electron rays; therefore, when used in image display devices wherein irradiation with electron rays takes place, the present glass panel can emit a transmitted light efficiently. Further, being strong to the mechanical impact applied from outside, the glass panel can be made in a small glass thickness, making it possible to provide a lightweight image display device. Furthermore, the glass panel of the present invention has a large volume resistivity and therefore is resistant to dielectric breakdown even when a high voltage is applied thereto. Moreover, the material glass plate from which the glass panel of the present invention is to be shaped, can be produced directly from a molten material glass by the float process and therefore can be obtained inexpensively in a large amount.

Claims: A glass panel for a face plate of a cathode ray tube which is characterized in that said glass panel has the following glass composition: SiO 2 64-75% by weight Al 2 O 3 1.5-2.0% by weight CaO 6.5-9.0% by weight MgO 0-5.0% by weight Li 2 O 0.5-2.5% by weight Na 2 O 7.0-12% by weight K 2 O 2.1-5.0% by weight SrO + BaO + ZrO 2 0-10% by weight CeO 2 0-0.5% by weight and wherein part of the Na ion in at least the surface layer of the panel region to be irradiated with electron rays, is substituted with K ion by an ion exchange treatment in a K-containing molten salt and resultantly the molar ratio of Na ion and K ion expressed by Na 2 O/(Na 2 O + K 2 O) in at least the glass layer ranging from 0.5 µm to 6.5 µm in depth from the panel surface, of said panel region to be irradiated with electron rays is 0.3-0.75; the glass panel has a shape of a tray comprising a rectangular top portion and side portions, the shorter side of said rectangular top having a length (L 1 ) of 100 to 530 mm, the longer side having a length (L 2 ) of 1.3 to 3 times L 1 mm, the depth (H) of the tray being 20 to 40 mm, and the thickness (t) of the rectangular top expressed in mm and the length (L 1 ) of said shorter side expressed in mm satisfying the relationship as defined below: 25.45 t -1.8 > L 1 > 25.45 t - 52.7 the glass panel having a thickness of from 4 to 23 mm; and in which the temperature and time of the ion exchange treatment fall within a polygonal area within the lines in a temperature-time diagram obtained by connecting the following points: (500°C, 90 min.), (500°C, 120 min.), (460°C, 180 min.), (460°C, 150 min.) and (480°C, 90 min.), provided that the point (460°C, 180 min.) is excluded.

Inventors: SHIBAOKA KAZUO, SUZUKI KOUICHI, SHIBAOKA, KAZUO, SUZUKI, KOUICHI, Shibaoka, Kazuo, c/o Nippon Itagarasu K.K., Suzuki, Kouichi, c/o Nippon Itagarasu K.K.
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO. LTD.
Application year: 1993
Main classification: C03C21
IPC codes: H01J29, C03C21, C03C3