1. Field of the Invention
The invention relates alumino-silicate glasses having high thermal stability and low processing temperature.
2. Description of the Related Art
Soda-lime glass which is also known as alkaline-earth alkaline-silicate glass belongs to the oldest known glass type and represents the most widely used (“normal glass”). Soda-lime glass finds its application mostly in the form of flat glass, for example as minor or window glass, and as packaging glass, for example for bottles, food packaging, drinking glasses, etc. A disadvantage of soda-lime glass is that it has only a relatively low thermal stability, for example with a transformation temperature (Tg) in the range of approximately 490° to 530° C. This clearly limits application of the hitherto known soda lime glass.
There is a constant requirement to vary and to modify glass compositions in order to change and improve the characteristics and to better adapt them to desired applications. However, in this context it is always a problem that reducing or increasing one share of a component can already trigger a multitude of effects which will influence the glass characteristics differently. The reactions and consequences when exchanging or modifying several components in a glass composition are even more complex and often difficult or even impossible to predict. It is therefore relatively difficult to provide tailor-made glass compositions for particular applications.
There are many publications available in the current state of the art which address generic glass.
JP 07-101748 A describes alkaline glasses with low aluminum content for plasma-display panels, whereby the glass composition consists of 0.3-2.5 weight % Li2O, 7.0-12 weight % Na2O, 1.5-4.5 weight % K2O, 0-5.0 weight % MgO, 6.0-9.0 weight % CaO, 0-5 weight % SrO, 3.5-15.0 weight % BaO, 2.0-4.5 weight % Al2O3, 57.0-68.0 weight % SiO2, 0-5.0 weight % ZrO2 and 0-0.5 weight % CeO2, whereby the sum Li2O+Na2O+K2O is 9.0-16.0 weight %.
U.S. Pat. No. 5,858,897 describes a glass composition for a substrate which is especially suitable for a flat display, preferably a plasma display (PDP, plasma display panel). The glass composition consists essentially of: 59-72 weight % SiO2, 1-15 weight % Al2O3, 0.5-9 weight % MgO, 0.5-11 weight % CaO, 0-6 weight % SrO, 0-5 weight % BaO, 4-19 weight % MgO+CaO+SrO+BaO, 0-9 weight % Na2O, 4-21 weight % K2O, 10-22 weight % Na2O+K2O, 0.5-10.5 weight % ZrO2, whereby the difference between SiO2-content and Al2O3-content is 50 to 71 weight % and the relative density less than 2.6.
Moreover, EP 0 769 481 A1 discloses a glass composition for a substrate, in particular for plasma displays, whereby the glass composition comprises: 52-62 weight % SiO2, 5-12 weight % Al2O3, 0-4 weight % MgO, 3-5.5 weight % CaO, 6-9 weight % SrO, 0-13 weight % BaO, 17-27 weight % MgO+CaO+SrO+BaO, 7-14 weight % Li2O+Na2O+K2O, 0.2-6 weight % ZrO2 and 0-0.6 weight % SO3. Such a high SrO-content in the glass composition has however greater disadvantages. SrO is a relatively expensive material so that the production of the glass becomes clearly more expensive. The advantages claimed in EP 0 769 481 A1 that the transformation temperature clearly increases and the thermal heat expansion coefficient rises could not be substantiated according to the invention. On the contrary, an increased SrO content provided no positive influence upon the characteristics and effects for the inventive fields of application; according to the invention SrO is therefore not present in the here described high amounts.
Moreover, EP 0 879 800 A1 describes solarization stable alumino-silicate glass which is suitable for application in display technology, in particular for plasma display panels and which have the following composition:
SiO245-68weight %Al2O3>5-18weight %Na2O0-5weight %K2O>9-15weight % withNa2O + K2O≧10weight %CaO0-10weight %SrO0.5-18weight %BaO0-10weight % withCaO + SrO + BaO8-<17weight %ZrO21-6weight %TiO20.2-5weight %.
Finally, US 2005/0003136 A1 discloses a glass composition which possesses excellent thermal stability as well as very good ion-exchange capacity. An increased mechanical strength of the glass is achieved by an additional chemical treatment through ion-exchange. The glass composition comprises: 59 to 68 weight % SiO2, 9.5 to 15 weight % Al2O3, 0 to 1 weight % Li2O, 3 to 18 weight % Na2O, 0 to 3.5 weight % K2O, 0 to 15 weight % MgO, 1 to 15 weight % CaO, 0 to 4.5 weight % SrO, 0 to 1 weight % BaO, 0 to 2 weight % TiO2 and 1 to 10 weight % ZrO2. This glass composition is to be used preferably as glass substrate for magnetic recording media. A disadvantage of this glass composition is however the insufficient crystallization stability. This means that during cooling of the glass during the manufacturing process, for example during tube-drawing, devitrification crystals form which inhibit shaping of the glass on the glass surface. Besides, the actual glass character is lost through a crystallization.
A requirement exists therefore to improve known glasses in regard to their characteristics.
It is therefore the aim of the current invention, and what is needed in the art is, to avoid the disadvantages of the current state of the art and to provide an alternative to soda lime glasses which have a similar thermal expansion of approximately 8 to 10×10−6/K and a higher thermal resistance (Tg) and at the same time have similar or only slightly higher processing temperatures (VA) compared to soda lime glasses. Furthermore, the boron content should be as low as possible because of toxicological as well as economic considerations.