Coating processes – Removable protective coating applied
Reexamination Certificate
2000-01-31
2002-04-30
Beck, Shrive P. (Department: 1762)
Coating processes
Removable protective coating applied
C427S165000, C427S168000, C427S289000, C427S293000, C427S368000, C427S065000, C427S060000, C427S061000
Reexamination Certificate
active
06379746
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method for temporarily protecting glass articles, in particular for protecting the surfaces of glass articles such as Liquid Crystal Display (LCD) glass. The method is useful, for example, for protecting glass articles from ambient contaminants and for preventing glass chip adhesions during cutting or grinding of the glass article.
BACKGROUND OF THE INVENTION
Many uses of glass, including LCD glass, require a very clean glass surface that is substantially free of dust and organic contaminants. When exposed to the environment, glass can quickly become contaminated with organic contaminants, with contamination being observed within a few minutes. Cleaning processes currently used for cleaning LCD glass often involve several steps and require a variety of chemicals. There is a need, therefore, for a method of protecting a glass surface from ambient dirt during manufacture, shipping and storage to minimize or even eliminate the need for chemicals to provide a clean glass surface.
Current procedures used to cut and grind glass surfaces and edges often generate small glass chips (e.g., chips having a size greater than 1 micron and less than about 100 microns). Some of these particles irreversibly adhere to the clean glass surface, rendering the glass useless for most applications. This is particularly a serious problem in the case of LCD glass surfaces.
LCD glass can be made by a fusion draw process, which yields flat, smooth glass surfaces which can be cut or ground to the desired size. Some of the glass chips generated from the cutting process originate from the surface of the glass. When the flat surface of these chips comes into contact with the surface of the glass plate, there can be a large contact area between the chips and the glass surface which promotes strong adhesion. If a water film condenses between these two surfaces, permanent chemical bonding may occur, in which case the adhesion of the glass chips to the surface becomes irreversible. This may make the glass useless for LCD applications.
One known method of protecting glass sheets, specifically, sheets of LCD glass, is to apply a polymer film on both major surfaces of the glass to protect the glass during the scoring, breaking and beveling processes. In a typical method, one major surface has a polymer film attached with an adhesive, and the other major surface has a film attached by static charge. The first film is removed after the edge finishing (cutting or grinding) of the sheet is completed, while the second is removed prior to the finishing process. Although the adhesive-backed film protects the surface from scratching by the handling equipment, it causes other problems. For example, the polymer may entrap glass chips produced during the finishing process, leading to a build up of glass chips and scratching of the glass surface, particularly near the edges of the surface. Another problem with this film is that it may leave an adhesive residue on the glass surface. There is a need, therefore, for a method of protecting a glass surface from chip adhesions that does not leave any residual coating on the glass surface, and for a method of temporarily protecting glass surfaces, whereby a glass article with a clean, coating-free surface can be readily obtained for further use.
Organic coatings have been used to protect glass surfaces for many years. See, for example, Smay, G. L.
Glass Technology
1985, 26, 46-59. Often oleic acid or stearic acid solutions are applied to create a lubricious coating allowing glass bottles to slide among each other without generating flaws or cracks in the manufacturing process. Oleic and stearic acids have long aliphatic chains which render them relatively insoluble in water. Therefore, an alcohol/water solution is required if, for example, the lubricating layer is to be applied by spraying. In terms of the manufacturing of glass and, in particular, LCD glass, the alcohol in the alcohol/water solution is a serious problem since when sprayed on a hot glass surface the alcohol will rapidly evaporate and can produce a flammable and potentially explosive mixture of air and alcohol vapor.
Oleic and stearic acid coatings are also unsuitable for use in protecting LCD glass because their chain-chain interactions are very strong. As a result, the coatings are difficult to remove even in a basic, detergent wash.
A critical aspect of any coating used to temporarily protect LCD glass is removability. Manufacturers of liquid crystal displays use LCD glass as the starting point for complex manufacturing processes which typically involve forming semiconductor devices, e.g., thin film transistors, on the glass substrate. To not adversely affect such processes, any coating used to protect LCD glass must be readily removable prior to the beginning of the LCD production process.
Some materials, such as silanes and siloxanes, may be able to satisfy the removability criterion in a qualitative sense, i.e., under suitable conditions they can be substantially completely removed from a glass surface, and yet the materials are still unacceptable for use with LCD glass based on this very same criterion. This is because manufacturers of liquid crystal displays consider these materials so potentially disruptive of the LCD production process that even the most minimal residual levels of the materials are not considered acceptable. Accordingly, although basically removable, these materials do not satisfy the removability criterion as applied in practice (i.e., the level of removability which a protective coating must achieve is a function of the composition of the coating).
Another class of materials which needs to be avoided in connection with glasses to be used to make liquid crystal displays are those that contain alkalis. This is because alkalis, even in small amounts, are known to poison thin film transistors. Similarly, metals, especially heavy metals, are undesirable since they can change the electrical properties of the glass surface.
Like the oleic and stearic acid coatings discussed above, anionic and cationic surfactants have been applied to glass surfaces. While most anionic surfactants are soluble in water, they do not form good stable coatings on glass in the presence of water. As shown by the data presented below, these surfactants were found unsuitable for use in protecting LCD glass.
The adsorption of cationic surfactants onto silica in solutions has been studied by a number of investigators in connection with understanding the dispersion of colloidal silica. See Goloub, T. P., Koopal, L. K., Bijsterbosch, B. H.
Langmuir
1986, 12, 3188-3194; Goloub, T. P., Koopal, L. K.,
Langmuir
1997, 13, 673-681; Zajac, J., Tompette, J. L., Partyka, S.,
Langmuir
1996, 12, 1357-1367; Rosen,
M. J. Surfactants and Interfacial Phenomena
, J. Wiley & Sons, New York, 1989, Chapter 2; and Harell, J. H., Scamehorn, J. F. “Adsorption from Mixed Surfactant Systems”, in
Mixed Surfactant Systems
, Surfactant Series Vol. 46, Ogino, K, and Abe, M, Ed.; Marcel Dekker, Inc. New York, 1992, pp. 263-281. At neutral pH, silicate surfaces are usually negatively charged allowing the ready adsorption of cationic species. The application of a cationic surfactant to glass at a temperature below the boiling point of water has been disclosed in Evans, U.S. Pat. No. 4,544,395.
As described in detail below, in accordance with the present invention, it has been discovered that cationic surfactants of various types (as well as non-ionic surfactants and betaines) are able to rapidly organize on hot glass (i.e., on glass having a temperature above 175° C.) and form a coating which is sufficiently hydrophobic (i.e., has a sessile drop contact angle of at least 40°) to substantially reduce adhesion of glass chips to the glass (e.g., to reduce adhesion of glass chips of a size greater than 1 micron by at least 80%). The prior work involving cationic surfactants and glass do not disclose or suggest this important result.
To summarize, there has been a need in the art for a met
Birch William
Bookbinder Dana C.
Carre Alain R. E.
Tennent David L.
Beck Shrive P.
Corning Incorporated
Crockford Kirsten A.
Klee Maurice M.
Nwaneri Angela N.
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