Light-screening film paint for lamps, and light-screening film for lamps and producing method thereof

Abstract: In the case of a highly luminous discharge lamp for automobiles, since a temperature rise to about 700°C in the glass surface of a lamp during the lighting will lead to an exposure of the light-screening film 4 formed thereon also to a temperature of 700°C, the temperature of the glass surface of a lamp rises and accordingly that of the light-screening film 4 formed thereon also rises. Especially with a highly luminous discharge lamp for automobiles, the temperature of the glass surface of the lamp rises to about 700°C during the lighting and necessarily the light-screening film on the glass surface is also exposed to a temperature of 700°C, a light-screening film material free of occurrence of discoloration or peeling even after exposure to a temperature of 700°C has been desired. It is the object of the present invention to prepare a light-screening film paint for lamps comprising: at least one compound of either manganese oxide or iron oxide containing 5 mol% to 30 mol% of manganese; and powder glass containing at least one of compounds respectively of silica, zinc oxide, boron oxide and aluminum oxide, and moreover to produce a light-screening film by applying the above paint to the surface of a lamp, followed by the firing at a temperature of not higher than 1,200°C.

Description: Field of the Invention The present invention relates to a light-screening film paint for lamps, and to a method for producing a light-screening film having the features of the preambles of claims 1 and 13, respectively. Background of the Invention A film paint and a method having the features of the preambles of claims 1 and 13, respectively, are known from both prior art documents DD-A-102 010 and of EP-A-0 739 534, which is relevant pursuant to Articles 54(3) and (4) EPC. FIG. 1 shows a structural drawing of a highly luminous D2R-type discharge lamp for automobile front lamp. The discharge lamp for automobiles is so arranged as that a metal iodide sealed in a quartz emissive section 5 emits a light under application of a high tension between the metal tungsten electrodes 1a and 1b. The emissive section 5 is covered with an external quartz tube 2. As compared with a halogen lamp chiefly employed at present as front lamp for automobiles, this halogen lamp is advantageous in providing a threefold luminosity at a 70% consumed power. Beside, because of using no filament unlike halogen lights, the service life is very long and not shorter than 1,500 hours. In this discharge lamp, a light-screening film 4 is formed on the surface of a outer quartz tube 2 to control the project region of light. This light-screening film is about 20 µm thick, the shape and size of which are specified in according with the International Standard. The present invention refers also to a producing method for this light-screening film. Using ferric oxide or cupric oxide as pigment and sodium silicate or aluminum phosphate as binder, a conventional light-screening film for lamps has been formed by mixing a pigment and a binder to form a paint, applying this paint to the glass surface of a lamp and firing the coat at a temperature of 100°C to 250°C On lighting a lamp, the temperature of the glass surface of a lamp rises and accordingly that of the light-screening film formed thereon also rises. Especially with a highly luminous discharge lamp for automobiles, the temperature of the glass surface of the lamp rises to about 700°C during the lighting and necessarily the light-screening film on the glass surface is also exposed to a temperature of 700°C. As mentioned above, a light-screening film according to the prior art uses ferric oxide or the like as pigment. Though being black at room temperature, cupric oxide is known to turn into red powder with the progress of oxidation at about 350°C. Thus, if a light-screening film was formed on a highly luminous discharge lamp for automobiles according to the prior art, there has happened a phenomenon that the color of the light-screening film changes from black to red or white due to a rise in temperature during the lighting. When the color of the light-screening film turns from black to red or white, the absorbance of light changes, thereby leading to a decline in light-screening performance, which has given rise to troubles. Discoloration of a light-screening film leads not only to a decline in light-screening performance but a bad appearance of the lamp impressed on a user and accordingly has been a serious problem. Thus, a light-screening film material has been desired which undergoes no discoloration for a 1,500-hours period of lighting. Besides, if a light-screening film was formed on a highly luminous discharge lamp for automobiles according to the prior art, there has been another problem that a heat cycle comprising ups and downs of temperature due to the repetition of lighting and extinction of a lamp causes a cracking or peeling of the light-screening film, thereby resulting in loss of light-screening performance. Also regarding this problem, a light-screening film material free of occurrence of cracking or peeling due to lighting and extinction over 1,500 hours has been desired. Disclosure of the Invention In order to solve these problems, the present invention employs a light-screening film paint according to claim 1 and a method according to claim 13. Preferred embodiments are disclosed in the dependent claims. After a tentative drying process subsequent to the application of a paint, a paint film is formed on the glass surface by binder action of methyl cellulose, vinyl alcohol and acrylic resin. Besides, methyl cellulose functions to inhibit the sedimentation, coagulation or separation of pigment powder in a paint, whereby the paint can be stabilized by adding 0.2 to 2 parts thereof to 100 parts of powder quantity. By setting the solid fraction in the paint to not less than 40 % by weight, dry contraction or liquid dripping after the application of a paint can be suppressed, thereby enabling a high precision light-screening film pattern to be formed. And preferably, by adding 0.2 to 2 parts of either silica having a primary grain size of not greater than 100 nm or clay mineral containing silica to 100 parts of powder quantity, thixotropy can be afforded to a paint, so that a stable application of the paint by using a coater becomes possible. During the firing at a temperature not higher than 150°C, methyl cellulose, vinyl alcohol and acrylic resin are thermally decomposed and consequently their residue in the paint film vanishes. During the firing at a temperature not higher than 1500°C, powder glass contained in the paint melts and functions as a binder. More preferably, by using glass mainly comprising zinc oxide and silica as powder glass, a light-screening film is obtained which has a high adhesion strength to the surface of quartz glass. And more preferably, by using powder glass having a melting point of not higher than 1000°C and setting the firing temperature at not higher than 1000°C, the deterioration of lamp performance can be prevented. More preferably, by using powder glass having a thermal expansion coefficient not higher than 10 -6 , the difference in thermal expansion coefficient from a quartz glass tube decreases and the peeling of a light-screening film due to local ups and downs of temperature caused by the lighting and extinction of a lamp hardly occurs. Still more preferably, by using powder glass that will crystallize under a temperature not lower than 600°C, the deterioration of light-screening film strength due to a rise in temperature caused by the lighting of a lamp after the firing can be prevented. By allowing the firing to proceed under the nitrogen atmosphere, or under the inert atmosphere, or under the vacuum atmosphere of not more than 10 -2 Torr, the metal electrode material for a main lamp body is kept from being oxidized, so that a light-screening film can be formed without deterioration of the lamp performance. Brief Description of the Drawings FIG. 1 is an outside view of a discharge lamp; FIG. 2 is an illustration of the measured results of discoloration degree for a light-screening film according to the present invention and a conventional light-screening film; FIG. 3 is a table showing the mixing ratios in the mixture of manganese oxide powder and manganese-contained iron oxide powder and the estimation results of obtained light-screening films; FIG. 4 is a table showing the manganese contents of manganese-contained iron oxide powder and the estimation results of obtained light-screening films; FIG. 5 is a table showing the softening points of powder glass and the estimation results of obtained light-screening films; FIG. 6 is a table showing the added amounts of clay mineral containing silica and the estimation results of obtained paints; and FIG. 7 is a talbe showing the added amounts of methyl cellulose and the estimation results of obtained light-screening paints. Detailed Description of the Preferred Embodiments Next, one embodiment of the present invention will be described. 100g of manganese oxide having a grain size distribution of 1 µ m to 20µm, 100g of iron oxide powder containing about 20 % by weight of manganese added for obtaining iron oxide from iron hydroxide and having a grain size distribution of 0.3 µ m to 3 µ m, and 100g of powder glass containing about 70 % by weight of zinc oxide, aluminum oxide and silica were dry-mixed in an agate mortar for one hour to make a mixture. The powder glass employed is one which melts at about 700°C and crystallizes as the temperature is raised up to 750°C. Then, 300g of water containing 3% by weight of methyl cellulose and the above mixture were mixed in a disper mill, and the powder obtained was dispersed into water by a high-speed disperser. The mixing was performed at a surface speed of the high-speed disperser being not lower than 5m/s. The solid ratio of a paint was set to 50% by weight. And by adding one part of silica powder having a grain size of not larger than 100nm to 100 parts of powder component and dispersing it, a good thixotropy can be afforded to the coating. The light-screening film paint prepared in this way was applied to the quartz glass surface of a lamp by means of a coater and fired at 800°C for an hour after transient drying at 100°C. Firing was conducted under the vacuum atmosphere of 1 x 10 -4 Torr. The light-screening film formed in this way on the glass surface of a highly luminous discharge lamp has a high adhesion strength to the lamp and the color of the whole film was black. The light-screening film obtained was estimated by discoloration measurement after the lighting, film strength measurement, transmissibility measurement, and surface observation after the lighting. And, a change in the color of a light-screening film was measured with a chroma meter and estimated in accordance with the Lab method. From changes in the respective values of L, a and b, ΔEab was calculated. The passable levels for estimation were set at a ΔEab value determined to be not greater than 1, calculated from measurements of the color of a light-screening film before lighting and after an 1,500-hours duration of lighting. The film strength, determined by the cross cut test according to the JIS (Japanese Industrial Standards) Z 1522, was examined respectively before lighting and after an 1,500-hours duration of lighting. A light-screening film was cut into specimens with a diamond cutter and tapes were pasted to the respective specimens and peeled. The peeling degree of the light-screening film observed at that time was examined. Only those in which no peeling whatever was observed were determined as the passable level. The passable level for transmittance measurements was taken at a leaking light ratio of not greater than 0.5%. FIG. 2 shows the result of discoloration measurements of the light-screening film obtained in the above embodiment. A light-screening film according to the present invention has a much smaller degree of discoloration than that of a conventional one using cupric oxide as pigment and manifested a ΔEab value of not greater than 1 after an 1,500-hours duration of lighting. In the above embodiment, 67 parts by weight of powder glass mainly containing zinc oxide was mixed to 100 parts in total of the mixture of manganese oxide powder and iron oxide powder containing about 20% by weight of manganese to prepare a paint, but light-screening films equivalent to the one obtained in the above embodiment were obtained also at other ratios. FIG. 3 shows mixing ratios of the mixture of manganese oxide powder and iron oxide powder containing about 20% by weight of manganese to powder glass mainly containing zinc oxide, and the estimation results of the obtained light-screening films. This result revealed that a good light-screening film can be obtained by mixing 30 to 100 parts of powder glass mainly containing zinc oxide with 100 parts of the mixture of manganese oxide powder and manganese-contained iron oxide powder. In the above embodiment, the manganese content in manganese-contained iron oxide was 20%, but results similar to the one obtained in the above embodiment were obtained also at other ratios. FIG. 4 shows these results. From FIG. 4 the most appropriate content of manganese was found to range from 5 mol% to 30 mol%. In the above embodiment, powder glass mainly containing zinc oxide was employed, but powder glass mainly comprising any of zinc oxide or boron oxide or aluminum oxide or silica provided a light-screening film equivalent to the one obtained in the above embodiment. On using powder glass containing a great amount of alkaline metal or alkaline earth metal, no good result was obtained because it reacted to cause an external quartz glass tube to be devitrifed. In the above embodiment, powder glass that begins to melt at 700°C and is crystallized at 750°C was employed, but a light-screening films equivalent to the one obtained in the above embodiment were obtained for powder glass having other softening points. FIG. 5 shows these results. To allow glass to melt sufficiently, the firing temperature was set to the softening temperature plus 100°C. From FIG. 5 it proved appropriate that the melting point of the powder glass employed ranges from 600°C to 1,100°C, both inclusive, and the firing temperature is not higher than 1,200°C. And with respect to adhesion strength of a film, using glass to be crystallized with elevating temperature provides a rather preferred result. In the above embodiment, thixocity was afforded to a paint by adding one part of silica powder having a grain size of not greater than 100nm to 100 parts of powder quantity, but equivalent results were obtained also on using swelling clay minerals such as smectite containing silica in place of silica powder. The powder quantity referred to here is the total quantity of a mixture comprising manganese oxide and manganese contained iron oxide and powder glass. Silica powder having a larger grain size that 100nm provides no desired effect. A most appropriate added amount of silica powder depends on a coater or a desired thickness, but the results examined in the present invention are shown in FIG. 6. FIG. 6 revealed that 0.2 part to 2 parts of added amount to 100 parts of powder quantity is suitable for the attainment of a stable coating by the prevention of liquid dripping or the like. FIG. 6 shows the results examined on swelling clay minerals containing silica, but equivalent results were obtained also for silica powder having a grain size of not greater than 100nm. In the above embodiment, methyl cellulose was employed, but equivalent paints were obtained also on using vinyl alcohol or acrylic resin in place of this. FIG. 7 shows the results of correspondence between the added amount of methyl cellulose. The results of FIG. 7 revealed that the added amount of methyl cellulose is preferably 0.2 to 2 parts to 100 parts of powder quantity. FIG. 7 shows the results in the case of methyl cellulose, but equivalent results were obtained also on using vinyl alcohol or acrylic resin in place of this. In the above embodiment, firing was conducted in the atmosphere of 1 x 10 -4 Torr. As a result of examinations on the degree of vacuum, firing in the vacuum atmosphere of not lower than 1 x 10 -2 Torr provided no favorable result because the tungsten electrode section of the lamp of FIG. 1 was oxidized. Thus, the degree of vacuum was found to be preferably not higher than 1 x 10 -2 Torr. According to the present invention, as described above, a favorable effect in a light-screening film formed on the glass surface of a highly luminous discharge lamp was obtained that the adhesion strength to a lamp is strong, the color of the whole film is black and neither peeling nor discoloration of the light-screening film occurs even after a 200-hours duration of lighting.

Claims: A light-screening film paint for lamps comprising: at least one compound of either manganese oxide or iron oxide; and powder glass containing at least one of compounds respectively of silica, zinc oxide, boron oxide and aluminum oxide, characterized in that said compound contains 5 mol% to 30 mol% of manganese. A light-screening film paint for lamps as set forth in claim 1, characterized by a compound of manganese oxide, iron oxide and zinc oxide; and glass containing at least one compound respectively of silica, zinc oxide, boron oxide, and aluminum oxide. A light-screening film paint for lamps as set forth in claim 1, wherein water is employed as solvent. A light-screening film paint for lamps as set forth in claim 1, further comprising at least one of compounds respectively of methyl cellulose, vinyl alcohol, and acrylic resin. A light-screening film paint for lamps as set forth in claim 1, wherein silica having a primary grain size of not greater than 100 nm is added. A light-screening film paint for lamps as set forth in claim 1, wherein clay mineral containing silica having a primary grain size of not greater than 100 nm is added. A light-screening film paint for lamps as set forth in claim 1, wherein the solid fraction is not smaller than 40 % by weight. A light-screening film paint for lamps as set forth in claim 1, wherein the melting point of powder glass is not higher than 1,200°C. A light-screening film paint for lamps as set forth in claim 4, wherein the amount of the compound of at least one of methyl cellulose, vinyl alcohol and acrylic resin ranges from 0.2 part to 2 parts relative to 100 parts of powder quantity. A light-screening film paint for lamps as set forth in claim 1, wherein the thermal expansion coefficient of the powder glass is not higher than 10 -6 . A light-screening film paint for lamps as set forth in claim 1, wherein the powdered glass to be crystallized by heating is employed as powder glass for preparing said light-screening film paint. A light-screening film paint for lamps as set forth in claim 1, wherein relative to 100 parts of at least one compound of either manganese oxide or iron oxide containing 5 mol% to 30 mol% of manganese, the mixing ratio of said powder glass containing at lest one of the compounds respectively of silica, zinc oxide, boron oxide, and aluminum oxide ranges from 30 parts by weight to 100 parts by weight. A method for producing a light-screening film (4) for lamps, comprising the steps of: preparing a light-screening paint for lamps; said light-screening paint comprises at least one compound of either manganese oxide or iron oxide and moreover powder glass containing at least one compound, respectively, of silica, zinc oxide, boron oxide and aluminum oxide. applying the above paint to the surface of a lamp, forming a light-screening film (4) by firing at not higher temperatures than 1200°C, characterized in that said compound contains 5 mol% to 30 mol% of manganese. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein water is employed as solvent. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein added to said light-screening film paint is a compound of at least one of methyl cellulose, vinyl alcohol and acrylic resin. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein silica having a primary grain size of not greater than 100 nm is added. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein clay mineral containing silica having a primary grain size of not greater than 100 nm is added. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein the solid fraction is not smaller than 40 % by weight. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein the melting point of powder glass is not higher than 1,200°C. A producing method of a light-screening film 4 for lamps as set forth in claim 15, wherein the amount of the compound of at least one of methyl cellulose, vinyl alcohol and acrylic resin ranges from 0.2 part to 2 parts relative to 100 parts of powder quantity. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein the firing of said paint is allowed to proceed in the vacuum atmosphere of not more than 10 -2 Torr. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein the thermal expansion coefficient of the powder glass is not higher than 10 -6 . A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein the powder glass to be crystallized by heating is employed as powder glass for preparing said light-screening film paint. A producing method of a light-screening film 4 for lamps as set forth in claim 13, wherein relative to 100 parts of at least one compound of either manganese oxide or iron oxide containing 5 mol% to 30 mol% of manganese, the mixing ration of said powder glass containing at least one of the compounds respectively of silica, zinc oxide, boron oxide and aluminum oxide ranges from 30 parts by weight to 100 parts by weight.

Inventors: MIFUNE TATSUO, OHATA TSUMORU, SAKAMOTO KAZUNORI, WATANABE MASARU, MIFUNE, TATSUO, OHATA, TSUMORU, SAKAMOTO, KAZUNORI, WATANABE, MASARU, Mifune, Tatsuo, c/o Matsushita El.Ind.Co.Ltd., Ohata, Tsumoru, c/o Matsushita El.Ind.Co.Ltd., Sakamoto, Kazunori, c/o Matsushita El.Ind.Co.Ltd., Watanabe, Masaru, c/o Matsushita El.Ind.Co.Ltd.
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H01J61
IPC codes: H01J61, H01J9