1. Field of the Invention
The present invention relates to a glass composition having a high strain point, a low expansion coefficient, and good chemical resistance. The glass composition can be used for producing pharmaceutical bottles and feeding bottles, and for producing sheets which, when cut to the desired size and optionally polished or treated, can be used as elements for producing fire-proof glazings or substrates on which electrically active layers are deposited. Such layer-coated substrates are generally referred to as slabs in the electronics industry and may be used for forming display screens such as liquid crystal, plasma or light-emitting screens.
2. Discussion of the Background
Glass compositions possessing high physical stability during a temperature rise, and good chemical resistance are known. However, they generally belong to the large family of borosilicate glasses, and more specifically to aluminoborosilicate glasses incorporating alkaline earth oxides.
Glass compositions which are transformed into sheets and then cut to serve as substrates are selected based upon their physicochemical properties. For example, a glass substrate upon which electrically active layers are to be deposited will be subjected to a number of operations during which the glass will achieve relatively high temperatures. The glass must not deform during these operations and must therefore have the highest possible strain point. Moreover, the thermal expansion coefficient of the glass forming the substrate must be compatible with that of the layers deposited thereon. Finally, the glass must not contain elements liable to migrate into the layers formed and lead to a deterioration of their properties, which is the case with alkalis.
The glass composition must have appropriate viscosity and devitrification characteristics consistent with the process chosen for obtaining a glass sheet to ensure that the thickness of the glass sheet is as uniform as possible and the surface of the glass sheet is as regular as possible. The glass composition must also have good chemical resistance to acid media, including hydrofluoric acid-based solutions.
A process which can be used for producing glass sheets consists of introducing a molten glass composition into an apparatus in which the top of the side walls, which converge downward, serve as a spout. The glass flows along the side walls, thus forming two streams which join at the top or tip of the apparatus, before being drawn vertically from top to bottom in the form of a planar sheet. Such a process requires that the glass has, at the liquidus temperature, a viscosity of at least 2 to 3.times.10.sup.5 poise. Such a glass composition is described in U.S. Pat. No. 4,824,808.
Another process consists of spraying molten glass onto a metallic tin bath in accordance with the float glass process. Glass compositions which can be floated and which are to serve as substrates for forming flat screens are described in WO 89/02877.
However, these glass compositions belong to the family of aluminoborosilicates having a high alkaline earth oxide content. Although these glass compositions have good physicochemical characteristics, they contain high percentages of onerous oxides such as B.sub.2 O.sub.3, SrO and BaO. In addition, they are relatively viscous for the float glass process compared with the glass compositions used in the examples illustrated in this application. Thus, the temperature corresponding to log .eta.=4 is equal to or higher than 1150.degree. C. or even 1200.degree. C. The temperature corresponding to the same viscosity for an ordinary float glass (soda-lime-silica glass) is between 1000.degree. and 1050.degree. C.
A glass composition which can be formed by the float glass process is described in French patent application 91.08201, filed on Jul. 2, 1991. However, this glass composition is characterized by a low viscosity at a high temperature. The temperature of the glass corresponding to log .eta.=3.5 is generally below 1180.degree. C. In addition, this glass composition has a very low maximum devitrification rate. The low devitrification rate makes it possible to avoid the formation of crystals as the glass is produced despite the fact that the liquidus temperature may reach 1230.degree. C.