Cooking surface for cooking food having a glass ceramic...

Electric heating – Heating devices – Combined with container – enclosure – or support for material...

Reexamination Certificate

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C501S021000

Reexamination Certificate

active

06525300

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cooking surface for cooking food having a glass ceramic surface with a glass coating thereon.
2. Background Information
Conventional glass-ceramics contain high-quartz and/or keatite mixed crystals as the main crystal phase; these are responsible for the low coefficient of thermal expansion. Depending on the crystal phase and size, these glass-ceramics can be transparent, translucent or opaque. Coloring via pigments is carried out depending on the desired application, for example increasing the hiding power or in order to achieve certain color impressions. However, the desired color impression can also be achieved by using coloring oxides, which dissolve in the glass and thus produce a colored glaze. The principal area of application of such glass-ceramics of low thermal expansion is in thermal shock-resistant laboratory equipment, cooking utensils, fire-protection glasses, chimney inspection windows and especially also treatable plates, for example cooking surfaces. Decorative coating compositions are generally categorized under “glazes” or “enamels”. The glazes generally comprise a clear or colored glass (glass flux), while enamels are coating compositions containing coloring, non-transparent materials, such as pigments. The pigments used can be colored, inorganic compounds. In this application, the pigments must usually only permit slight attack by the glass flux, or none at all. Glazes and enamels are also used for coating and finishing of glass-ceramics. Large-area coatings frequently serve the purpose of protection, for example against chemical or physical attack, covering or in order to achieve a desired appearance. Decorations are used for inscriptions, for achieving a desired design or also in order to support certain technical functions, for example in the case of display windows or for marking cooking zones. The glaze or enamel is fired at temperatures below the softening point of the article to be coated, during which the respective glass composition of the glaze or enamel melts and forms a stable bond to the surface of the article. The firing temperatures are generally below the softening point of the article to be coated in order that uncontrolled deformation cannot occur. The firing also serves to volatize organic auxiliaries used, for example, as suspension medium for application of the glaze or enamel. During coating of glass-ceramics or glasses having a coefficient of thermal expansion in the region of about 4×10
−6
/K or greater, it is possible to find glazes or enamels having a matched coefficient of thermal expansion. According to the known art, the aim is for the decorative coatings to have a slightly lower coefficient of thermal expansion than the article to be coated. This is intended to ensure that the glaze or enamel is subjected to compressive stress during cooling after firing and thus does not exert any negative, in particular strength-reducing action on the properties of the substrate. If the coefficient of thermal expansion is not matched, stresses form between the decorative coating and substrate during cooling, which can result in crazing or cracking, possibly down into the substrate material. The mismatch means that the stresses caused impair the adhesion. A greater mismatch can result in the decorative coatings flaking off the substrate immediately or in impair the adhesion. A greater mismatch can result in the decorative coatings flaking off tile substrate immediately or in the course of time during use.
Problems have hitherto arisen in glaze or enamel decoration of glass-ceramics of low thermal expansion based on high-quartz or keatite mixed crystals produced by thermal treatment, known as ceramicization, of a suitable starting glass. Such glass-ceramics are distinguished by a coefficient of thermal expansion of less than 2×10
−6/
K (per degree Kelvin) in the temperature range between 20 and 700° C. (i.e., between 20 degrees Celsius and 700 degrees Celsius). Taking into account the softening point and the thermal stability of these glass-ceramics, the decoration is usually carried out at temperatures below 1200° C. In the case of glass-ceramics, the firing of the glaze or enamel is preferably carried out during the ceramicization process, i.e. the decorative coatings are applied to the green glass and fired during ceramicization (primary firing). In the case of a second decoration or for technical reasons associated. with production, for example in the case of small batch sizes, it may be desirable for the firing of the decorations to take place in a second heat treatment (secondary firing) on the pre-ceramicized glass-ceramic after the ceramicization. It would be economically advantageous if the desired properties could be established with an enamel, glaze or decorative composition both in primary and in secondary firing. For such glass-ceramics having a low coefficient of thermal expansion, decorative coatings with a matched coefficient of thermal expansion which are economical to produce have not hitherto been available. Attempts have been made in various ways to solve this problem of mismatch in order to avoid the occurrence of serious disadvantages in the desired properties.
In particular in the case of full-area coatings or dense decorations, a reduction in the tensile strength in bending occurs as a severe disadvantage The reduction in the tensile strength in bending is due first to the unavoidable formation of stresses between the decoration and the substrate as a result of the mismatch and second to the fact that adhesion of the decoration to the substrate requires a certain dissolution of the substrate by the decoration and formation of a reaction layer. It is possible to overcome this problem of reduction in the tensile strength in bending through a very sparse decoration, but this excludes full-area coatings in order to achieve protection or relatively dense decorations as a design feature. For adequate tensile strength in bending during handling, installation and later use of the decorated glass-ceramics, a mean tensile strength in bending of greater than 30 MPa is regarded as necessary.
Application of decorative coatings having a low layer thickness, for example, allows the resultant stresses to he reduced even in the case of mismatches in the coefficients of thermal expansion. However, this also means that the color effect (hiding power, color impression) and the protective action may be considerably restricted.
Compared with dense or full-area decoration, the abrasion susceptibility of sparse decorations is problematic, since sparse decoration areas are stressed to a relatively greater extent in the case of loads than are dense versions. This thus greatly restricts design freedom, for example in the case of discreet decorations with thin lines and dots.
The glazes and enamels used hitherto for coating and/or decoration of glass-ceramics having a low coefficient of thermal expansion frequently contain lead and sometimes also cadmium. Besides its favorable action in reducing the firing temperature, the use of lead and cadmium also allows decorative coatings with good adhesion to be achieved, although the coefficient of thermal expansion is in the order of from 5×10
−6
/K up to as much as 10×10
−6
/K. The reason why these mismatches between the decorative coatings and glass-ceramic substrates can be tolerated. without damage is ascribed to the plasticity of lead-containing fluxes. In addition, lead and cadmium additives are favorable for the strength of decorated glass-ceramics and give the glass fluxes and enamels comparatively good chemical resistance to weak acids and bases, as used in the household and also in industry as cleaners or which occur in foods.
In spite of these favorable properties of lead-containing coatings, glazes and enamels are nowadays no longer allowed to contain lead and cadmium owing to the unfavorable toxicological properties of these elements. For this reason, vario

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