Plate glass for automobile windows, such as windshield glass, side window glass, rear window glass, and sunroof glass, is attached to car bodies using an organic adhesive. In order to prevent deterioration of the organic adhesive caused by sunlight, to make the excess portion of the adhesive inconspicuous, and to improve the appearance of the window, the portion (usually, a periphery) of each glass attached to a car body is colored in black or dark gray by screen printing (coating) a ceramic color paste composition and firing.
Usually, automobile window glass is manufactured by screen printing a paste form ceramic color composition in the manner as described above on the periphery of a flat glass (glass plate), which has been cut into a predetermined shape, and then subjecting it to a molding process or a tempering process by heating. Among the types of window glass, side window glass, rear window glass, and sunroof glass are usually bended by forcibly pressed in a press-mold.
Therefore, the ceramic color composition is primarily required to have an excellent mold release property during the formation process in view of the above feature of the manufacturing process. This is because the shape of the glass is defined by the mold having ceramic paper, glass fiber cloth, stainless steel cloth or the like on its surface, and the mold and the periphery of the glass plate to which the ceramic color composition is applied are kept in constant contact with each other. The use of a ceramic color composition having a poor mold release property adversely affects the dimensional stability, and, in an especially troublesome case, the glass plate cannot be removed from the mold. This inevitably interrupts the manufacturing operation and significantly lowers the productivity. This is a very serious problem.
One of the other properties required of the ceramic color composition is an acid resistance of the fired color. The ceramic color fired onto a glass plate for use in an automobile may be whitened by being exposed to acid rain or the like during long-term use, especially in areas on the glass plate where rain accumulates without readily flowing off. In order to prevent this phenomenon, the ceramic color must have an acid resistance higher than a predetermined level.
Furthermore, in almost all cases, silver paste is fired onto the rear window glass to serve as preventing fogging or an antenna, and the areas where the electrodes of the silver electric circuit are extended (bus bar portion) are printed in a wide area onto the ceramic color. However, in some cases, silver in the silver paste reaches the glass plate surface by migrating through the ceramic color layer. Such migration of silver causes irregular coloration of white, yellow, blue, green, etc., on the glass plate, adversely affecting its appearance. The irregular coloration is greatly affected by the ratio of the glass components of the ceramic color, and the ceramic color is preferably required such that it can develop a color even on a dark amber color.
The above-described silver circuit also functions as electric resistance when an electric current is sent thereto, and, by generating heat, exhibits an antifogging effect. However, its electric resistance may become too high depending on its design and cause overheating. To prevent overheating, electrolytic copper plating and electrolytic nickel plating are commonly applied to the silver circuit. The bath used for this electrolytic copper plating is strongly acidic. Therefore, the ceramic color must be able to withstand such a plating bath. In other words, the ceramic color is required to have a property that prevents its external appearance from being degraded when it is dipped into the bath and a current is applied during the electrolytic copper plating process. Furthermore, the ceramic color should have a property such that the adhesiveness of the plated film is not lowered when terminals are soldered thereon. The properties required of the ceramic color as described above also vary depending on the ratio of the glass components of the ceramic color.
Heretofore, to meet each requirement in the property of the ceramic color as described above, several recipes that are effective to some degree have been proposed. For example, the mold release property can be improved by using PbO—SiO2—B2O3 based glass, which has relatively strong acid resistance, and adding a large volume of inorganic pigments and inorganic fillers, which will not melt during the heating process, or by lowering the fluidity of the ceramic color during the molding process by changing the glass components so as to make them crystallizable during the heating process. With regard to acid resistance, coloration in the bus bar portion, and resistance to plating solutions, in most cases, it is possible to obtain the required properties by using the above-described PbO—SiO2—B2O3 based glass in a similar manner (for example, Japanese Unexamined Patent Publication No. 1991-285844 (Japanese Patent No. 2748647), Japanese Unexamined Patent Publication No. 1993-85770, and Japanese Unexamined Patent Publication No. 1994-239647).
However, in recent years, prompted by the trend to be environmentally friendly, severe legal controls have been imposed on the use of not only cadmium but also lead and lead compounds. Accordingly, there have been strong demands to stop using or reduce the amount of lead contained in the ceramic color. In order to meet these demands, several kinds of research and development have been conducted to obtain a lead-free glass composition to replace the known PbO—SiO2—B2O3 based glass.
Specific examples include Japanese Unexamined Patent Publication No. 1996-133784 and Japanese Unexamined Patent Publication No. 1999-157873, which disclose crystalline ZnO—SiO2—B2O3 based glass compositions. Such compositions generally meet the requirements in the mold release property, coloration in the bus bar portion, and acid resistance. However, they have a drawback in that the adhesive strength of the soldered terminals is significantly reduced and peeling-off occurs at the interface between the silver coating and the ceramic color during adhesive strength measurements.
Japanese Unexamined Patent Publication No. 1994-234547, Japanese Unexamined Patent Publication No. 1995-144933, Japanese Unexamined Patent Publication No. 1996-34640, Japanese Unexamined Patent Publication No. 1997-227214, etc., disclose Bi2O3—SiO2—B2O3 based glass. By selecting the composition for crystallization, such types of glass exhibit a satisfactory mold release property, and, to some extent, strong resistance to a plating solution, and acid resistance. However, such types of glass are disadvantageous in that they always have undesirable yellow or blue coloring on the bus bar portion. In addition, they have a drawback such that, when they are fired with a black pigment added, the shade of black seen through the glass is too weak and appears grayish, and this adversely affects its beauty of appearance and other design-related characteristics.
Japanese Unexamined Patent Publication No. 1998-87340 discloses a ZnO—Bi2O3—SiO2—B2O3 based glass composition. This type of glass has fine coloration in the bus bar portion, strong resistance to plating solutions, and sufficient acid resistance. However, it has an inferior mold release property because of the lowered crystallinity. The inventors conducted an additional examination and verified that the above-described group of glass (crystallizable) can deposit crystals based on either zinc or bismuth. However, it is difficult to deposit both crystals at the same time. Furthermore, even when crystals of either zinc or bismuth are deposit, the amount is insufficient and the desired mold release property cannot be obtained.
Table 1 shown below summarizes the-above explanation. Note that each glass listed in Table 1 is crystalline, and, in the table, “A” indicates that a satisfactory result was obtained regarding the properties required of the ceramic color for use in automobile window glass, especially, in the rear window glass, and “B” indicates that a satisfactory result was not obtained.
TABLE 1PropertiesResistanceColorationMoldto metalin theType ofreleaseAcidplatingbus barGlasspropertyresistancesolutionportionZnO—SiO2—B2O3AABABi2O3—SiO2—B2O3AAABZnO—Bi2O3—SiO2—B2O3BAAB
As shown in the table, several lead-free ceramic color compositions have been proposed. However, these ceramic color compositions are still deficient in one or more properties required to the ceramic color of this kind. Heretofore, there has been no ceramic color composition developed, that meets all of the required properties.