METHOD OF FORMING A GLASS SHEET WITH AN OIL-BASE CERAMIC PAINT THEREON

Abstract: This specification is directed to a method of forming a glass sheet with an oil-base ceramic paint thereon, the method having the following steps. An oil-base ceramic paint is applied to the glass sheet. The oil-base ceramic paint includes normal ceramic paint components along with, as a new component thereof, finely divided zinc metal powder. The glass sheet is heated to a temperature which softens the glass sheet sufficiently so that the glass sheet can be formed. The glass sheet and the ceramic paint thereon are engaged with a fiberglass covered forming die to form the heated glass sheet to a desired shape. The forming die is removed from engagement with the glass sheet. The finely divided zinc metal powder addition to the ceramic paint is credited with being effective to prevent sticking of any of the ceramic paint to the forming die. The glass sheet is cooled to obtain a formed glass sheet with a ceramic paint thereon.

Description: This invention relates to a method forming a glass sheet with an oil-base ceramic paint thereon. The oil-base ceramic paint composition is one which does not stick to fiberglass materials at elevated temperatures when the fiberglass is brought into contact therewith. The method of forming the glass sheet is one which allows an oil-base ceramic painted glass sheet to be contacted by a fiberglass covered forming die which forms the glass in a heated condition into a desired shape. The forming operation may be carried out without any sticking of the oil-base ceramic paint to the fiberglass forming die. Ceramic paint compositions are generally well known to the skilled artisan. These ceramic paint compositions can be used, for example, to form borders around the edges of glass sheets which are used as windshields, side lights and back lights in motor vehicles. Such ceramic paints usually include a mixture of metal oxides which together act as a coloring agent for the ceramic paint. The metal oxides are nonreactive with one another and nonreactive with any elements or compounds they come in contact with while being heated to temperatures up to 704°C (1300°F). The mixture of metal oxides can be controlled so as to get a selected color from the ceramic paint composition. Normally, in automotive applications, the selected color is black, although shades of grey are now becoming popular. Such ceramic paint compositions also include a low melting point glass frit which generally melts at a temperature below 704°C (1300°F). The low melting point glass frit is the material which bonds the mixture of metal oxides to the glass sheet and ensures that it remains after the glass sheet has been cooled back to room temperature. A vehicle is normally mixed with the metal oxides and low melting point glass frit to allow the ceramic paint composition to be applied in a paint application process. For example, if the paint application process is a silk screen printing operation, the vehicle is an oil-base organic medium for carrying the other materials during the silk screen printing operation. Many different types of ceramic paint compositions of the above general type are well known to skilled artisans. The exact metal oxides, low melting point glass frit, and oil-base vehicle which are selected for such a paint, are well within the skill of artisans in this area. The manner in which the different materials may be varied in order to achieve the results desired in a particular application is also well within the skill of an artisan. As was discussed in US-A-4,596,590, in the situation where a sheet having a ceramic paint thereon was contacted by a fiberglass forming die, certain paints of the above-described general composition had a great tendency to stick on the fiberglass covered die. The sheet of glass, of course, was being heated to an elevated temperature, generally less than 704°C (1300°F) so that it may be engaged by a forming die and formed into a configuration other than flat. As further discussed in US-A-4,596,590, the difficulty arose that the ceramic paint compositions generally known to skilled artisans would not operate in a glass forming process in which a heated glass sheet containing the ceramic paint composition was engaged by a fiberglass covered forming die. As US-A-4,596,590 patent, no other materials are known for covering the forming die because the fiberglass is the only material which will withstand the heat and rigorous requirements of the environment to which it is subjected. Other materials that have been used to cover forming dies quickly break down and wear out when repeatedly brought into engagement with the surface of a heated glass sheet. Thus, fiberglass is the material which is used to cover forming dies and this material, unfortunately, sticks to ceramic paint compositions then known to skilled artisans when those compositions are heated to an elevated temperature which is necessary to form glass by a forming die. US-A-4,596,590 discloses a method of forming glass sheets with a ceramic paint thereon which could withstand the high temperatures and repeated engagements of painted glass sheets by the same forming die without paint sticking to the forming die. The problem was solved by adding to the ceramic paint, as a new component thereof, a metal oxide having a low valence oxidation state and a high valence oxidation state. The metal oxide is in its low valence state when the ceramic paint is applied. US-A-4,596,590 teaches that, when this metal oxide powder is subjected to a heating operation, that portion of the powder on the upper surface of the ceramic paint composition readily oxidizes to form an excellent, fully oxidized metal oxide barrier across the top surface of the paint which insulates the low melting point glass frit and isolates the same from the fiberglass cloth. In particular, it teaches that stannous oxide is an excellent metallic oxide powder to use. In carrying on additional work in this area, particularly in working with oil-base ceramic paints, it is found that the utilization of stannous oxide did have some drawbacks. Stannous oxide has a tendency to oxidize rapidly when subjected to a heating operation. This rapid oxidation causes a fine, white dust to be generated which loosens from the surface of the paint and is moved about the glass sheet during the bending operation. The loose dust at the elevated temperature can cause marking and/or indentation of the glass sheet, which results in a scrappage thereof. This dusting problem was particularly severe when stannous oxide was used in conjunction with an oil-base ceramic paint. According to the invention there is provided a method of forming a glass sheet with a ceramic paint thereon, the method comprising applying an oil-base ceramic paint to said glass sheet, said oil-base ceramic paint including finely divided zinc metal powder as a constitutent thereof, said oil-base ceramic paint containing no stannous oxide, heating said glass sheet to drive off the volatiles contained in said oil-base ceramic paint and to bond said remaining portion of said oil-base ceramic paint to said glass sheet, engaging said glass sheet and said oil-base ceramic paint thereon with a fiberglass covered forming die to form said heated glass sheet to a desired shape, removing said forming die from engagement with said glass sheet and, cooling said glass sheet to obtain a formed glass sheet with a ceramic paint thereon. The remainder of ceramic paint may be made up from metal oxides, which together act as a coloring agent for the ceramic paint. These metal oxides are nonreactive with one another and nonreactive with any elements or compounds they come in contact with while being heated to temperatures up to 704°C (1300°F). The ceramic paint, in addition, includes a low melting point glass frit and the oil-base vehicle needed to transport the remaining materials so that they may be applied to the glass sheet. The low melting point glass frit is a glass material which melts below 704°C (1300°F) to act as a binding material to bind the metal oxides to a glass surface. The ceramic paint composition useful with the method of the invention comprises the following materials. The first material is a mixture of metal oxides which together act as a coloring agent for the oil-base ceramic paint. These metal oxides, because of their manufacturing process, are normally all in their highest oxidation state. A small proportion of the metal oxides may be in a lower valence state simply because of manufacturing variability. However, normally the mixture of metal oxides used as a coloring agent is in its highest oxidation state because of the manufacturing process, and thus the mixture of metal oxides is nonreactive with one another and nonreactive with any elements or compounds they come in contact with while being heated to a temperature in a range up to 704°C(1300°F) an oxygen containing atmosphere. The nonreactive oxides generally include oxides of material such as chrome, cobalt, nickel, manganese, iron and copper. Such a mixture is commonly known as black oxide powder and is well known in the industry. Any oxide included in this material is normally in its highest valence oxidation state because of the process of manufacture and because those who are purchasing the black oxide powder mixture desire a material which does not react when heated to moderate temperatures. Other mixtures of oxides may be put together in order to get different colors, as is well known in the industry. In normal practice, the mixture of nonreactive metal oxides makes up about 8-25% by weight of the ceramic paint composition, preferably 10-20% by weight, and most preferably 16% by weight. A second material forming the ceramic paint composition is a low melting point glass frit. Normally, the low melting point glass frit is a lead borosilicate glass frit high in lead content. Any other low melting point glass frit is also suitable, but the lead borosilicate glass is preferred because it is readily available and relatively inexpensive. This material is included in a range of 30-50% by weight. In accordance with the teachings of the preferred embodiment, the zinc metal powder is used in a 5-60% by weight amount in the composition, preferably 20-40%, and most preferably 25-35% by weight. As more and more zinc powder is used, the oil-base ceramic paint becomes more grey in color from the black color. If a black color is desired, zinc powder should not exceed 40%. If a grey color is desired, the higher amounts of zinc powder, up to 60% may be used. An oil-base vehicle is mixed with the metal oxides, the low melting point glass frit, and the finely divided zinc metal powder to form all of the mentioned materials into an oil-base ceramic paint which can be applied by a silk screen printing operation to a glass surface. Oils for making an oil-base paint are well known to the skilled artisan. A particularly good material is sold by Drakenfeld Incorporated as 900 Pine Oil Base. This organic medium would make up 20-35% by weight of a paint composition. The zinc metal powder is the thing that makes the ceramic paint composition nonstickable. It is believed that when the zinc metal powder is subjected to a heating operation, that portion of the zinc powder on the upper surface of the ceramic paint composition readily oxidizes to form an excellent, fully oxidized zinc oxide barrier across the top surface of the paint, which insulates the low melting point glass frit and isolates the same from the fiberglass cloth. Thus, the fiberglass cloth, while contacting the surface, does not adhere to the ceramic paint and the paint does not stick to the cloth. Also there is no dusting of the paint over the surface of the glass sheet. The ceramic paint includes a mixture of metal oxides which together act as a coloring agent for the ceramic paint. The metal oxides are oxides which are nonreactive with one another and nonreactive with any elements or compounds they come into contact with while being heated to temperatures up to 704°C (1300°F). A low melting point glass frit is also included in the ceramic paint composition. A principal ingredient from a functional standpoint in achieving nonstick capabilities for the oil-base ceramic paint is finely divided zinc metal powder. An oil-base vehicle is also mixed with the metal oxide, the low melting point glass frit, and the finely divided zinc metal powder to form all of the mentionned materials into an oil-base ceramic paint which can be applied to a glass surface. After the ceramic paint has been aplied to the glass sheet, the glass sheet is heated to a temperature which softens the glass sufficiently so that the glass sheet can be formed. Generally, this temperature is in a range of 635-677°C (1175-1250°F), which is below 704°C (1300°F.) After heating, the glass sheet and the ceramic paint thereon are engaged with a fiberglas covered forming die to form the heated glass sheet into a desired shape. The heating operation, of course, burns off the organic materials contained in the oil-base paint and bonds the remaining portion of the oil-base ceramic paint to the glass sheet. The zinc metal powder addition to the ceramic paint is effective to prevent sticking of any of the ceramic paint to the forming die and also eliminates the generation of finely divided dust during the oxidation of the zinc as the heating step is carried out. After the forming die has been removed from engagement with the glass sheet and the ceramic paint, without any sticking of the ceramic paint thereto, the glass sheet may be cooled to obtain a formed glass sheet with ceramic paint thereon. Normally, the glass sheet is rapidly cooled in a glass tempering operation to achieve a tempered glass product having the ceramic paint thereon. While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made to my invention without departing from my invention, and it is intended to cover in the appended claims all such modifications and equivalents as fall within the scope of this invention.

Claims: A method of forming a glass sheet with a ceramic paint thereon, the method comprising applying an oilbase ceramic paint to said glass sheet, said oil-base ceramic paint including finely divided zinc metal powder as a constitutent thereof, said oil-base ceramic paint containing no stannous oxide, heating said glass sheet to drive off the volatiles contained in said oil-base ceramic paint and to bond said remaining portion of said oil-base ceramic paint to said glass sheet, engaging said glass sheet and said oil-base ceramic paint thereon with a fiberglass covered forming die to form said heated glass sheet to a desired shape, removing said forming die from engagement with said glass sheet and, cooling said glass sheet to obtain a formed glass sheet with a ceramic paint thereon. A method as claimed in Claim 1 in which said zinc metal powder is present in said oil-base ceramic paint in an amount from 5% to 60% by weight of said oil-base ceramic paint.

Inventors: BOAZ, PREMAKARAN TUCKER
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1987
Main classification: C03C17
IPC codes: C03B23, C03C17