The present invention provides a method of restrengthening flat brittle oxide substrates (e.g. window glass) that has been weakened by surface flaws such as when glass is cut by scoring and breaking. Coatings have been known for repairing surface flaws in glass and thereby restrengthening the glass towards the strength of unflawed glass. Particularly useful strengthening compositions are aqueous solutions containing silane-based compositions especially polymerized cross-linked siloxane. The present invention relates to a method of strengthening or restoring strength to cut flat glass and the resulting restrengthened flat glass article.
Articles made from brittle materials, such as glass window panes, generally have substantially lower tensile strength than predicted. This weakening can be the result of such factors as imperfections in the article, or small amounts of impurities in either the body or the surface of the article. Historically many types of surface treatments of brittle material may be used to protect the surface from abrasion, damage and to provide a small measure of support to brittle articles.
Glass is intrinsically one of the strongest materials known to man. Theoretically, standard silicate glasses should be able to support stresses as high as 14 to 20 gigapascals (2 to 3 million pounds per square inch (psi)). In practice, however, the strengths typically obtained are on the order of 70 megapascals (MPa), about 10,000 psi.
The explanation of the discrepancy between predicted and measured values is the existence of surface flaws or cracks. These flaws essentially fracture the siloxane network (Si--O--Si), which is the backbone of the glass. This damaged point in the glass becomes the focal point of forces on the glass and act to concentrate the force and cause catastrophic failure of the glass article, typically at much lower stresses than otherwise expected.
Researchers have long sought a means to alleviate the problems with glass strength. Many modifications to the forming and handling process of glass articles have led to unsatisfactory increases in the strength because these modified handling procedures still cause some flaws in the glass articles. For this reason, it has been a goal of researchers to reduce the effect of flaws after they are inevitably formed on the object.
Some approaches to improving the strength of glass include Aratani et al., U.S. Pat. No. 4,859,636, wherein metal ions in the glass are exchanged with ions of a larger radius to develop a surface compressive stress. Poole et al., U.S. Pat. No. 3,743,491, also relates to a surface protective treatment. Hashimoto et al., U.S. Pat. No. 4,891,241 relates to strengthening glass surfaces with the application and cure of silane coupling agents in conjunction with acryloyl and methacrylol compounds. The combination is essential.
While the patents described above each provide some improvement to the strength of the glass so treated, they are not without shortcomings. Some of these treatments require longer times than available during manufacturing, necessitating off-line processing. There are also concerns related to worker safety and health. In particular, the use and handling of organic solvents, as well as the acrylate and methacrylate compounds, are a safety and health concern to the manufacturer.
Flat glass is produced commercially by a "float" process, that produces a wide continuous sheet of glass. The flat glass is often cut into more useful sizes. The cutting process introduces flaws into the glass. Cut flat glass pieces are often heat treated to soften and repair such flaws. Heat treatment, or annealing is an expensive process.
Compositions for strengthening of brittle oxide substrates, particularly glass are disclosed in U.S. Pat. No. 5,567,235 by Carson, et al., entitled "A Method of Strengthening a Brittle Oxide Substrate Silane-Based Compositions and a Polymerized Cross-Linked Silane Coated Brittle Oxide Substrate", and U.S. Pat. No. 5,486,693 entitled Method of Strengthening Glass, inventor Bryan Ellis, et al; and in Master Thesis, Alfred University, New York, "Glass Strengthening Via Silane Coupling-Agent Coatings", R. Bennett (February 1989) and in the references cited in the aforementioned references. However, those references do not teach the selective application of the strengthening composition to the edge of flat articles to achieve as good or better strength improvement as applying the strengthening composition to both the edges and the major flat surfaces of the article.