Decorated glass-ceramic plate

Stock material or miscellaneous articles – Composite – Of quartz or glass

Reexamination Certificate

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C428S427000, C428S688000, C427S374100

Reexamination Certificate

active

06517943

ABSTRACT:

FIELD OF THE INVENTION
A cooking unit comprising a glass-ceramic sheet adapted to use as a cooktop and having a coating to provide a decorative effect.
BACKGROUND OF THE INVENTION
The present invention relates to a decorated glass-ceramic plate as well as to a method of obtaining such a plate, i.e. a method of decorating a glass-ceramic plate. Such plates are especially used as cooktops.
Another object of the present invention is an enamel, useful in particular in the implementation of said decoration method.
The cooking surface defined by a glass-ceramic plate, when it is used as a cooktop, must withstand not only the elevated temperatures employed for cooking without softening or distorting, but also it must withstand large differences in temperature which result from the fact that only one or more parts of the plate are heated. This has led to the use, in this field, of glass-ceramic plates having a coefficient of thermal expansion of zero or near-zero, i.e. generally equal to 0±3×10
−7
K
−1
, and preferably zero.
Glass-ceramics having these characteristics are well known and are widely described in the literature. U.S. Pat. No. 5,070,045 (Comte et al.) describes such glass-ceramics the predominant crystal phase of which is a beta-quartz crystal phase. According to the present invention, decorative materials have been developed which are intended to be used especially, but not exclusively, with such glass-ceramics. It is as such that they can also be used with other low expansion glass-ceramics such as those described in the French patent application No. 97 09912.
Glass-ceramic plates have proved to be extremely satisfactory from the functional point of view as cooktops, but for aesthetic reasons, it was found that it was desirable to decorate the upper surface of said plates.
Enamelling is the decoration method which has proved to be the easiest to implement and which, consequently, is the most generally used. The main points of this technique, which are familiar to the person skilled in the art, have been summarised in the introductory part of the French patent application FR-A-2 701 473.
The initial problem which arises is that of the compatibility of the decorations with the glass-ceramics. In fact, there generally exists a difference in the thermal expansion between the glass-ceramic and the decoration. This difference in expansion of course lowers the mechanical strength of said plate comprising said decoration.
Glass-ceramics have a more than satisfactory inherent mechanical strength, the modulus of rupture (MOR) of the materials actually used being about 180 MPa. However, while the mechanical strength of the actual decorated plates is still adequate, it is significantly reduced and it has revealed to be desirable to obtain higher values, particularly it has revealed to be desirable to obtain decorated glass-ceramic plates having a modulus of rupture (MOR) of at least 120 MPa, preferably of at least about 130 MPa.
Apart from the aesthetic aspect and the problem of the mechanical strength of the decorated plate, a cooktop must also resist staining due to food, it must be easy to clean and must have a smooth surface in order to prevent marks due to contact with metal utensils. These requirements severely limit the potential decoration materials.
For example, a logical candidate was a glass frit that crystallises to produce a low expansion crystal phase, especially a beta-quartz crystal phase. The coatings constituted by these crystallised frits have a very satisfactory modulus of rupture (MOR) but their surface properties are mediocre since the crystals give rise to a surface roughness which is unacceptable, as regards marks left by metallic utensils, and possibilities of easy cleaning.
The U.S. Pat. No. 5,326,728 (Boury et al.), which corresponds to the French patent application FR-A- 2 701 473, describes enamels used in the production of decorative materials. Although these materials have proved to be satisfactory, efforts have been made to produce further improved decorative materials. In particular, these efforts have been directed at obtaining a coefficient of thermal expansion of the decoration which is more compatible with that of the plate this being in order to obtain a greater mechanical strength of the decorated article, and this is indicated by a higher modulus of rupture (MOR).
Thus, an object of the present invention is to provide a glass-ceramic plate which comprises an improved decoration and which has a modulus of rupture (MOR) of greater than 120 MPa whilst at the same time meets the various other requirements of such an article used as a cooktop (requirements recalled above), which has a great flexibility in decoration, which has an improved decoration comprising a mixture of a glass frit with pigments capable of providing a desired decorative effect; as well as to provide a decorative material (an enamel) which can be fired to produce an adherent decoration, especially during the firing of a glass plate to convert the glass into a glass-ceramic; as well as to provide a method of decorating such a glass-ceramic plate.
SUMMARY OF THE INVENTION
The present invention does in fact relate to a decorated glass-ceramic plate comprising a glass-ceramic plate having a low coefficient of thermal expansion between ±15×10
−7
K
−1
(20-700° C.), preferably near-zero (i.e. equal to 0±3×10
−7
K
−1
(20-700° C.)), and a decoration on the surface of said plate, said decoration comprising a vitrified glass frit containing 10 to 35% by weight of pigments (for 90 to 65% by weight of frit), and said decorated glass-ceramic plate having a modulus of rupture (MOR) of at least 120 MPa.
Preferably, the decorated glass-ceramic plate is constituted by a glass-ceramic the predominant crystal phase of which is a solid solution of beta-quartz and which advantageously has a coefficient of thermal expansion of 0±3×10
−7
K
−1
(20-700° C.).
The present invention also relates to a method of decorating a glass-ceramic plate, especially for generating a decorated cooktop for a cooking appliance, said glass-ceramic plate having a low coefficient of thermal expansion (see above), preferably near-zero (see above), said method comprising:
producing a mixture of 65 to 90% by weight of a glass frit and of 10 to 35% by weight of pigments; said glass frit having a coefficient of thermal expansion of 30-40×10
−7
K
−1
(0-300° C.) (preferably of 30-35×10
−7
K
−1
(10-300° C.)) and a softening point of at least 750° C. (preferably of at least 775° C.);
applying said glass frit/pigments mixture onto the surface of a glass-ceramic plate or onto the surface of a glass-ceramic precursor glass plate;
firing said plate coated with said glass frit/pigments mixture, to optionally transform said glass plate into a glass-ceramic plate, to vitrify the glass frit in the frit/pigments mixture, to obtain an adherence of the generated decoration with the glass-ceramic; and
cooling said fired plate in order to obtain a decorated glass-ceramic plate having a modulus of rupture of at least 120 MPa.
DESCRIPTION OF THE INVENTION
The present invention is derived from research carried out to produce an improved decorative material for a glass-ceramic plate. In particular, a material was sought which was suitable for glass-ceramic plates the predominant crystal phase of which is a beta-quartz crystal phase and which has a coefficient of thermal expansion of near to zero, preferably of 0±3×10
−7
K
−1
(20-700° C.). The decorative material according to the invention meets this requirement but is not limited to such an application and can be used effectively on other glass-ceramics having a low coefficient of thermal expansion, i.e. situated in the range of 0±15×10
−7
K
−1
(20-700° C.).
The decorative materials used according to the prior art generally comprise pigments dispersed in a glass frit. Said pigments confer to said mate

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