Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block – Surface modified glass
Patent
1995-02-23
1998-09-08
Speer, Timothy M.
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
Surface modified glass
428426, 65 3014, 65 31, 65 601, B32B 1700
Patent
active
058043179
DESCRIPTION:
BRIEF SUMMARY
The invention relates to procedures for increasing the mechanical strength of glass objects, particularly flat or curved glass plates. It in particular applies to aircraft glazings or more generally to all cases where a high mechanical strength must be ensured.
It is known that the mechanical strength of a glass object can be increased by a tempering operation consisting of placing the surface of the object under compression beforehand either thermally or chemically by an ion exchange process, which leads to the surface replacement of part of the sodium ions by larger ions such as e.g. potassium ions, which thus place the surface of the object under compression.
The final mechanical strength obtained is obviously dependent on the type of tempering treatment performed, but also on the quality of the surface prior to the treatment of the object. In the case of a glass plate, the most important surface defects are essentially encountered close to edges which have undergone a cutting treatment. It is certainly possible to minimize this problem by a high-quality polishing of the edges, e.g. using a so-called polished, full round edge method, but this treatment is relatively complicated and there is always a risk of part of the edges being incompletely polished. Moreover, the defects are not solely located on the edges, even when the glass is obtained by a process not normally requiring any making good by machining and in particular by the presently most widely used process, namely the float process, or other standard processes such as the fusion draw process, the Fourcault process or rolling processes with or without marks.
Thus, on measuring the mechanical strength of a large number of tempered plates, there is found to be relatively wide dispersion of the effective tempering levels and that even if the mean value is significantly higher than the mean value of the mechanical strength of an untempered plate, certain plates still have mechanical strength values substantially identical to the initial mechanical strength. Moreover, the breaking stress levels can sometimes, particularly in the case of very large plates, such as e.g. those intended for aircraft windscreens, prove inadequate for the envisaged application.
It is in particular known from FR-A-2 138 710 or its equivalent U.S. Pat. No. 3,843,472 to reinforce a glass article by smoothing by an abrasion process part of the surface and/or edge, which then undergoes, before and/or after the chemical tempering of the glass, to a so-called grinding treatment by means of an acid agent such as an aqueous solution of hydrofluoric acid and optionally sulphuric acid. The shock resistance obtained by the combination of the abrasion treatment and the grinding treatment is superior to that obtained when only one of the treatments is performed prior to the thermal tempering and well above that obtained when the glass only undergoes a grinding using hydrofluoric acid.
Thus, hydrofluoric acid grinding alone does not make it possible to eliminate major surface defects because the etching is not selective. The concave parts of the defects are reached in the same way and the convex parts, so that essentially there is only a reproduction by translation of the geometry of the glass surface and consequently that of the defects. However, a slight, but not usually significant gain is obtained, because the treatment makes it possible to slightly widen the defects and therefore the mechanical strength is slightly increased.
In order to be effective, the known process of the aforementioned patent requires a careful abrasion with a treatment of the edges by a succession of flexible abrasive belts, which are oriented differently and which differ by the nature and size of the grains used. Such a treatment takes a long time and must be carried out by particularly skilled workers, without it being possible to check the quality of the treatment by any tests which are not destructive and this can obviously not be performed in a systematic manner.
Moreover, although it is true that most de
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Saint-Gobain Vitrage International
Speer Timothy M.
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