Coating processes – Removable protective coating applied
Reexamination Certificate
2000-05-05
2004-02-10
Nakarani, D. S. (Department: 1773)
Coating processes
Removable protective coating applied
C428S332000, C428S442000
Reexamination Certificate
active
06689414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of applying at least one layer of a releasable polymer coating on a glass substrate, especially a display glass and a hard disk blank, as a protective layer. The invention relates to electronic devices having display glass and to the display glass including the protective layer. The invention also relates to hard disk blanks made from glass or glass-ceramics.
2. Prior Art
The term “glass substrate”, for the purposes of the present disclosure, means a substrate made from conventional glass or from glass-ceramics. There is a substantial yield problem in the manufacture of glass substrates, especially for display glasses and hard disk blanks, since the glass substrates have defects in the form of scratches and particles because of processing and transport. These scratches or cracks can also cause glass breakage, which is self-destructive when it occurs during processing, since the process must be interrupted in order to clean the machines of any splinters, which would otherwise cause further cracking or scratching.
Glass particles are released during cutting to length and breaking off glass pieces, which are deposited on the glass surfaces as abrasive particles and lead to scratches on the surface during further transport and working. Furthermore edge working after detaching the substrate leads to loading the glass surface with glass particles, which also act as abrasive bodies. Moreover a high pressure water stream is directed on the edges during edge processing to cool and to remove grinding solution.
Methods in which a polymer foil is laminated on both sides are known. These methods have several disadvantages. The protective foil can first be applied after cutting of the glass, which can thus be protected only during transport and edge working. During edge working the protective foil on the edges is partially lifted off the substrate and the grinding slurry collects there. As a result, the glass is damaged. Furthermore in thin glass particularly glass breakage increases because of the required drawing force for removal of the protective foil.
Adhesives are required for laminating the polymer protective layer on the substrate, which cannot always be removed without leaving a residue. The glass surfaces must also be subsequently polished, in order to obtain the required quality. The subsequent polishing is primarily mechanical and of course removes the adhesive residue, but does not produce a microscopically planar surface with optimum surface properties. This latter type of surface is desired especially for display glass and optical special glasses or also hard disk blank, since the polishing method produces minute cracks. The large amount of waste caused by glass breakage during polishing is a grave disadvantage.
A method is disclosed in German Published Patent Application DE 36 15 277 A1, in which flat glass is provided with a crack-resistant splinter-protecting coating. Immediately after manufacture, plastic powder is applied during cooling of the glass, which melts on the glass piece. A base layer and a crack-resistant covering layer are applied to increase the mechanical stability. The splinter-protecting layer made in this way may not be removed again. Furthermore the resulting surface does not have sufficiently good properties for use in displays. Furthermore it is not possible to use this product for hard disk blanks because of the plastic layer.
Japanese Published Patent Application JP-H10226537 describes a method, in which an aqueous protective film is applied, which does not react with the glass (and also contains no alkali salts) and is resistant to moisture. Polyvinyl alcohol solutions in water are used for the protective film. These polyvinyl alcohol solutions contain polyvinyl alcohols, which have an average molecular weight of at most 51,600 g/mol, preferably 25,800 g/mol, and which are partially hydrolyzed, preferably between 55% and 85%. The reasons for the upper limits are as follows: with higher average molecular weight and with hydrolysis degrees at about 95%, the water solubility greatly reduces. At small hydrolysis degree, in contrast, the water solubility increases with increasing hydrolysis degree. A protective film made from polyvinyl alcohol with an average molecular weight less than 50,000 g/mol and a hydrolysis degree between 55% and 85% already is very soluble in water at 30° C. This sort of polyvinyl protective film is removed or dissolved in a very short time during edge working.
An additional method based on a water-soluble polymer-protective film is disclosed in the Patent BE 714,347. In this method a mixture of hydroxyethyl cellulose and dialdehyde in water or a mixture of polyvinyl alcohol and dialdehyde in water is prepared. The dialdeyde acts as a cross-linking agent, which makes the protective film capable of swelling and the adherence of the protective layer on glass is thus reduced. The water solubility of the polymer protective layer is adjusted by mixing the dialdehydes into it. No statement is made regarding the polymerization degree of the preferred polyvinyl alcohol. The hydrolysis degree should be at 85% or higher. It has been shown however that this protective layer is removed when the edges are worked.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method, with which the surfaces of glass substrates are protected from destructive cracking and abrasive bodies during manufacture until the last stage of the processing, especially during wet edge working, and in all intervening transport steps, which has as few method steps as possible, which keeps waste to a minimum and guarantees as high as possible surface quality of the end product.
It is also an object of the present invention to provide a display glass, which has excellent surface quality with small manufacturing costs, and to provide an electronic device, which has display glass of this type.
It is another object of the present invention to provide a hard disk blank, which has excellent surface quality with reduced manufacturing costs, and applications for this type of hard disk blank.
According to the protective method of the invention, the glass substrates, especially display glasses or hard disk blanks, are coated with at least one removable protective layer of a polyvinyl alcohol with a molecular weight of greater than or equal to (i.e. not less than) 55,000 g/mol and a hydrolysis degree of greater than or equal to (i.e. not less than) 95% to form a removable protective layer. The at least one dissolvable or removable layer is only poorly soluble in cold water and withstands the wet edge working process. The glass substrate may be coated on only one side or on both sides as required.
The coating may be removed by washing with water at a temperature of 50° C. or higher, especially 60° C. and higher, preferably 70° C. and higher, and most preferably 80° C. and higher. The higher the water temperature, the faster the coating will be removed. The pH value is adjustable arbitrarily within the bounds of the glass resistance to acidic and basic conditions. The coating is rapidly removed in both acidic and basic media.
The original surface quality of the glass substrate surface is preserved by the at least one polyvinyl alcohol layer throughout the entire working process and during transport. The removal of the at least one polyvinyl alcohol layer by washing with water is, on the one hand, performed carefully, so that little glass breakage occurs, even with very thin glass, and on the other hand very thorough, since the at least one polymer layer is completely removed.
The surface quality of the glass would be impaired by microscopic cracks due to additional after-polishing and thus the waste due to broken glass would be increased. The after-polishing is therefore dispensed with for the glass made by the method according to the invention.
It has proven advantageous to employ a polyvinyl alcohol with an average molecular weight greater than or equal to 10
Berndt Andreas
Buerkle Roland
Henze Inka
Mauch Reiner
Schlatterbeck Dirk
Nakarani D. S.
Schott Glas
Striker & Sttenby
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