Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block – Surface modified glass
Reexamination Certificate
1999-02-16
2001-11-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
Surface modified glass
C065S115000, C065S117000, C264S235000, C264S346000, C219S464100
Reexamination Certificate
active
06319612
ABSTRACT:
BACKGROUND OF THE INVENTION
Mechanical stresses in the form of compression or tensile stresses are produced with uneven heating over the surfaces of flat or curved glass or glass ceramic plates. The level of these stresses depends on the temperature distribution on the plate and on physical material properties, such as, e.g., thermal expansion coefficients, the modulus of elasticity, heat conductivity, etc. If the tensile or compression stresses exceed the maximum permissible limits that are set by the strength of the plate, the plate will break.
Plates of this kind that are made of glass or glass ceramic with unevenly heated areas are used as, e.g., stove cooking surfaces (electric, gas, induction and solid-fuel stoves), grilling surfaces, light covers, heating element covers, etc.
In principle, the glass or glass ceramic plates can have a positive or a negative thermal expansion coefficient. If the thermal expansion coefficient is negative in the temperature range in question, which should be the exception rather than the rule, then tensile stresses will arise in the flat areas of the plate, where the latter is heated. Conversely, in the case of positive thermal expansion coefficients, which occur more frequently, the tensile stress arises in the colder edge areas that adjoin the heated areas, which can make these edge areas vulnerable to breaking. Moreover, because of the mechanical machining of the edge, the edge area of a plate in any case has lesser strength. The risk of breakage being caused by the tensile stresses that arise, especially in the less strong edge areas, means that certain materials are ruled out for some of the above-indicated possible applications or can be used only for a relatively narrow temperature range.
To overcome these drawbacks, in the previous prior art the glass or glass ceramic was subjected to hardening (prestressing), whereby compression stress was produced in the surface layer of the glass product and tensile stress was produced in the interior. In this case, the compression stresses on the surfaces of the glasses increase their strength since, in the presence of tensile stress, these compression stresses on the surface must first be overcome before the formation of tensile pressure peaks ultimately leads to breakage.
For tempering (prestressing) of glass items, basically two processes are available: thermal tempering and chemical tempering.
With thermal tempering, glasses are heated to just below their softening point and are then quickly cooled. Because the interior of the glass cools more slowly than the surface, the surface is placed under compression stress, while the interior is exposed to tensile stress. Since the effects that occur are greater, the greater the thermal expansion of the glass, this process is limited to materials with fairly large expansion coefficients. Another drawback consists in the fact that the increases in hardness are quickly destroyed when the glass is brought to temperatures of its transformation range, so that the process cannot be used on objects that are exposed during use to temperatures that exceed the relaxation temperature of the thermally tempered (prestressed) glass. Moreover, thermal tempering is less efficient in the case of thin-walled objects.
Chemical tempering is based on a compression prestress being produced in the glass surface by altering its chemical composition relative to the glass interior, whereby surface layers with lower thermal expansion coefficients or larger volumes than the interior of the glass are produced. The chemical tempering method consists of an ion exchange. It has the drawback, however, that it is a comparatively time-consuming and expensive process since only layers that are too thin and are under compression stress are produced within economically justifiable periods. Moreover, the method is limited since it can be used only on glasses of specific chemical composition. Another drawback consists in that the compression stress layer has a chemically tempered glass of typically only a thickness of 100 to 200 &mgr;m, such that the process is limited to applications in which large surface damage would penetrate the compression stress layer, which would obviate the protective action.
SUMMARY OF THE INVENTION
An object of the invention was to make available a process for the production of glass and glass ceramic plates that can be used on items without the above-mentioned limits, and with said such plates the property is imparted that, in the case of uneven heating, they can be exposed to considerably higher temperatures without risk of breakage despite the tensile stress that arises.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
According to the process, this object is achieved in that, after cooling, the glass or glass ceramic plates are subjected to zone annealing with a defined temperature-time program, whereby in terms of surface area the plates are divided into two zones in which annealing is done at respectively different temperatures, and the zones with the higher temperature during annealing correspond to the sub-areas of finished glass or glass ceramic plates, in which compression stress is built up according to uneven heating corresponding to the respective specific use of the glass or glass ceramic plates. The glass or glass ceramic plates that are obtained are thus characterized in that they are divided into two sub-areas, whereby one sub-area exhibits structural compression and the other sub-area exhibits compression stress.
In this case, as is common to the specialty in question, annealing is defined as the subsequent heat treatment of a glass product (glass or glass ceramic plate) that is carried out according to a specific temperature-time program in a furnace that operates continuously or periodically for the purpose of modifying the material and/or product parameters.
Structural compression is achieved with the annealing of the sub-areas of the glass or glass ceramic plates that is carried out at elevated temperatures, which causes compression stress to build up in the adjacent areas that are annealed at lower temperatures, and the structural imbalance that is created is maintained permanently in the then finished glass or glass ceramic plates. If during use the area that is provided for heating is heated to the operating temperature, then, e.g., in the case of a material with positive heat expansion, the compression stresses that are produced thermally (and/or mechanically) and that occur in the nonheated areas are completely or partially compensated for by the “frozen compression stresses” there, such that compression stress peaks that result in breakage can no longer occur.
The individual requirements that are imposed on the material that is used for the glass or glass ceramic plate consist in that, on the one hand, the material must be “compatible,” i.e., during the beginning of the cooling process, the material-specific large-volume glass structure that is initially present must be caused to “freeze” by a cooling process, and said glass structure is then further compressed (“compacted”) in later annealing or zone annealing and, on the other hand, it must be possible during the annealing process to achieve a (material-dependent) maximum temperature that exceeds the later operating temperature.
Before the actual zone annealing, the glass or glass ceramic plate must first be toughened and cooled if it has not been already. For toughening and cooling, the plate in question is brought to a temperature that is higher than or equal to transformation temperature T
g
of its material, e.g., up to about 150° C. above T
g
, and then said plate is cooled quickly to a temperature that is below T
g
. The faster the cooling speed (temperature reduction per unit of time), the better the result. For example, cooling can be effected by blowing cold air through air jets onto the material.
During the actual zone annealing, the zones that are to be annealed
Dudek Roland
Nass Peter
Naubik Juergen
Boss Wendy
Jones Deborah
Millen White Zelano & Branigan P.C.
Schott Glas
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