Glass manufacturing – Processes – With chemically reactive treatment of glass preform
Reexamination Certificate
2002-04-24
2004-11-02
Group, Karl (Department: 1755)
Glass manufacturing
Processes
With chemically reactive treatment of glass preform
C065S030100, C065S030140, C501S072000, C501S070000
Reexamination Certificate
active
06810688
ABSTRACT:
The invention relates to a process for treating glass substrates, and more specifically glass sheets, which are intended to be subsequently coated with layers or other coatings for the production of display screens.
The display screens to which the invention relates are especially plasma screens of all types, FED (Field Emission Display) screens, LED (Light-Emitting Display) screens, LCD (Liquid Crystal Display) screens and more generally all display screens whose glass substrates have to undergo heat treatments during production of the screen.
The invention will be more particularly described with reference to the production of a plasma screen, which is essentially composed of two glass sheets. On at least one of these glass sheets are deposited one or more arrays of electrodes, a layer of dielectric material and layers consisting of phosphors corresponding, for example, to the green, red and blue colours. Before being joined together, the glass sheets also receive barriers and spacers whose functions consist in forming a multitude of cells and keeping the two glass sheets spaced apart.
All of the operations for producing these electrodes, layers or even spacers are accompanied by heat treatments.
The glass compositions normally used for this type of substrate are of the silica-soda-lime type which, if they are used as they are, undergo dimensional changes during the abovementioned heat treatments because of the temperatures reached, which are greater than the strain point temperatures of the said compositions.
These dimensional changes which occur are negligible for other applications. However, in the case of the aforementioned display screens, great dimensional precision is required, especially so as to allow production of individual cells where a plasma forms. This is because the precision in producing these cells or more precisely the precision in depositing the various abovementioned layers has a direct influence on the operating quality of the screen. The precision in aligning the electrodes and in the various deposition steps makes it possible to improve screen resolution and image quality.
A first requirement relating to these glass sheets is therefore dimensional stability during the various heat treatments that the glass sheets undergo during production of display screens.
Solutions have already been suggested for improving this dimensional stability.
A first solution already proposed consists in making the glass sheet undergo a “precompaction” treatment; such a treatment consists of a heat treatment with a heat cycle tailored to the glass composition and to the heat treatments that the glass sheets will undergo during production of the screens. Such a treatment is, for example, carried out on sheets made from a glass composition of the silica-soda-lime type, before the heat treatments corresponding to the manufacture of the display screen are carried out.
Another solution that has also been proposed consists in producing glass sheets with particular compositions having high strain point values. The heat treatments undergone by such glass sheets result in smaller dimensional changes than in glass compositions of the more usual silica-soda-lime type.
A final solution, especially described in documents WO 99/13471 and WO 99/15472, consists in chemically toughening the glass sheets of the usual silica-soda-lime type by dipping them in an alkaline bath.
Apart from the better dimensional stability obtained according to these documents, this solution also meets a second requirement relating to the application of display screens since the chemical toughening carried out on the glass sheets imposes on them compressive surface stresses which increase their mechanical strength. This is because such sheets undergo an enormous number of handling operations until the end of the display screen manufacturing cycle, especially because of the many deposition steps that have to be carried out. Furthermore, the glass sheets are stressed during the heat treatments associated with the deposition of layers during the processes for manufacturing display screens. These thermally induced is stresses give rise to the risk of fracture during manufacture, this risk becoming more acute when the manufacturer seeks to accelerate the manufacturing cycles and increase the production rates. Improving the mechanical properties thus makes it possible to limit as far as possible the risk of the said glass substrates fracturing. Moreover, it seems that the improvement produced by the chemical toughening disappears after the various heat treatments carried out for depositing the various layers.
At the present time, there is a trend towards a new requirement regarding glass substrates for the manufacture of display screens since the desire in the display screen industry is to provide screens, the glass panes of which has already been assembled, and therefore all the heat treatments have been carried out thereon, which have sufficient mechanical strength. The screens thus formed must also undergo various handling operations in order to complete the manufacture and, furthermore, they are also liable during their use to be subjected to tensile forces either of an accidental nature or simply because of their use; for example, FED-type screens are subjected to tensile forces created by the atmospheric pressure which is exerted on the surface of the glass. Another example relates to plasma screens which are subjected to thermal stresses due especially to overheating at the centre of the screen with respect to the edges.
The inventors were thus tasked with producing glass substrates or sheets which meet, on the one hand, the first requirements mentioned, namely satisfactory dimensional stability during the heat treatments associated with the deposition of layers and mechanical reinforcement created before the said treatments, and which have, on the other hand, after assembling the glass sheets, and therefore after all of the said heat treatments, satisfactory mechanical strength.
This objective has been achieved according to the invention by a process for treating a glass sheet which consists of a glass composition having a strain point (the temperature corresponding to a viscosity of 10
14.6
poise) above 540° C. and is intended for producing display screens, the said process including at least one ion-exchange treatment on at least part of the surface of the glass sheet and a precompaction treatment.
The precompaction treatment is a treatment carried out at a temperature below the dilatometric softening point. This treatment is used to relax the structure so that it becomes more stable during the subsequent heat cycles, such as those corresponding to the various layer deposition operations.
Advantageously, this treatment is therefore carried out at a glass viscosity of greater than 10
12
poise.
Preferably, the glass sheet has a resistivity &rgr; (expressed in ohm.cm at 250° C.) such that log
10
&rgr;>7.5. Such resistivities are particularly advantageous in the case of display screens because of the high electrical voltages used to make them operate.
Advantageously, and especially for obtaining products that can be produced on an industrial scale, the treatment process is designed to be applied to glass sheets obtained according to the float process.
According to a first method of implementing the invention, the precompaction treatment is combined with an ion-exchange treatment and is thus carried out simultaneously, the ion-exchange thermal cycle being adapted for carrying out a precompaction treatment.
According to a second method of implementing the invention, the treatment is carried out in two successive steps, the first consisting of a precontraction step and the second consisting of ion exchange on at least part of the surface of the glass sheet.
The inventors have in fact demonstrated that the treatment according to the invention, applied to glasses having a strain point above 540° C., makes it possible, on the one hand, to meet the dimensional stability constraints and, on the other hand, confers
Duisit Géraldine
Gaume Olivier
Gy Rene
Bolden Elizabeth A.
Group Karl
Saint-Gobain Glass France
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