Crystallizing glaze system

Details Crystallizing glaze system Crystallizing glaze system

2000-06-23

2002-02-19

Dunn, Tom (Department: 1725)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S072000, C501S021000

Reexamination Certificate

active

06348425

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns a glass-ceramic glaze for use on tile. More particularly, the present invention concerns a diopside crystallizing glass-ceramic glaze for use in forming a smooth matte finish on architectural tile that is both chemically durable and abrasion resistant.
BACKGROUND
The prior art provides various methods for forming ceramic tiles and various glaze compositions which can be used to produce a protective surface on such tiles. Generally speaking, ceramic tiles are formed from batches of natural or synthetic raw materials such as clay, kaolin, quartz, feldspar, wollastonite, talc, calcium carbonate, dolomite, calcined kaolin, hard clay chamotte-grog, oxides such as alumina, silica, corundum and mixtures of the foregoing. Binders and other additives may also be employed to increase the raw mechanical strength of the raw tiles. Once formed, the raw ceramic tiles must be fired to form biscuits that are hard, tough, and brittle.
In many instances, a glaze composition, which usually comprises one or more glass frits, fillers, pigments, and other additives, is applied to the raw tiles prior to the firing operation to impart a protective surface to the ceramic tiles. Upon firing, the raw tile hardens and the glaze composition forms a vitreous, transparent to opaque surface layer that can be glossy to dull (matte) or somewhere in between. It addition to the single fire approach, it is also well-known in the art to produce glazed ceramic tiles by a double fire approach. In the double fire approach, the glaze composition is applied to a once-fired biscuit and the glaze composition-coated biscuit is subjected to a second firing operation in order to produce the finished ceramic tile.
For certain applications, such as in the production of architectural floor tiles for example, it is highly desirable for the glaze composition to form a smooth semi-transparent to translucent matte finish on the ceramic tile that is both chemically durable and abrasion resistant. Some crystallizing glaze compositions are known to be useful for this purpose, but they are usually formed from relatively expensive raw materials. Attempts have been made in the past to develop glazes based upon the crystallization of diopside (chemical structure: CaMgSi
2
O
6
), which can be formed from virtually any silica containing glass frit composition that contains an appropriate quantity of MgO and CaO, a low quantity of other alkali-earth oxides, and a low quantity of fluxing oxides. However, such prior attempts have been unsuccessful because diopside energetically crystallizes in single-fast-fire (SFF) and gres porcellanato ceramic tile firing cycles, which results in a glaze which is over-crystallized and exhibits an unpleasant appearing very rough, hard and dry surface.
SUMMARY OF THE INVENTION
The present invention provides a novel glass-ceramic glaze composition for use on ceramic tile. The glaze composition comprises a glass composition from which diopside crystals can be formed, a diopside crystallization rate controller, a surface texture controller, and other optional fillers, pigments and additives. Preferably, the glass composition comprises one or more glass frits containing precursors from which diopside crystals can be formed. The diopside crystallization rate controller preferably comprises mill added sodium feldspar, potassium feldspar, nepheline, quartz, and/or spodumene, with sodium feldspar being most preferred. The surface texture controller preferably comprises a combination of zinc oxide and calcined alumina.
Using the glaze composition according to the present invention, it is possible to produce a semi-transparent to translucent, smooth, matte finish on ceramic tile that displays excellent chemical durability and abrasion resistance. The glaze composition is preferably applied to raw tiles which are then fired using single fast firing (SFF) or gres porcellanato firing cycles. The diopside crystallization rate controller moderates the speed and quantity of diopside crystallization during firing, and prevents the glaze from over-crystallizing. The surface texture controller helps the glaze form a smooth matte finish that is aesthetically pleasing. A ceramic tile made in accordance with the present invention comprises a fired ceramic body portion having the novel glaze composition fired on at least a portion of the surface thereof.
The novel glaze composition of the present invention is particularly well-suited for application on ceramic architectural tiles for it produces a semi-transparent to translucent, smooth, matte surface that exhibits excellent chemical durability and abrasion resistance.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
DETAILED DESCRIPTION
The solids portion of the glaze composition of the present invention comprises by weight from about 35% to about 60% of a glass composition from which diopside crystals can be formed upon firing, from about 25% to about 60% of a diopside crystallization rate controller, and from about 1% to about 20% of a surface texture controller. The solids portion of the glaze composition can further comprise up to about 30% of other optional fillers, pigments, and additives.
It is important that the glass composition contain precursors from which diop side crystals can be formed upon firing. Preferably, the glass composition comprises one or more glass frits. Glass frits preferably including appropriate quantities of SiO
2
, CaO, MgO, a low quantity of other alkali-earth oxides, and a low quantity of fluxing oxides will form diopside as the primary crystalline phase during a SFF or gres porcellanato firing cycle. It will be appreciated that if too much MgO is present, the primary crystalline phase formed will be forsterite (chemical structure: Mg
2
SiO
4
), with some diopside being present as a lesser phase. Similarly, if too much CaO is present, akermanite (chemical structure: Ca
2
MgSi
2
O
7
) or wollastonite (chemical structure: CaSiO
3
) will predominate over diopside. If too much alumina is present, anorthite (chemical structure: CaAl
2
Si
2
O
8
) will be favored. And, if too much SiO
2
is present, quartz or cristobalite will form and phase separation will likely occur.
Generally speaking, the glass composition for use in the invention will preferably comprise one or more glass frits providing a total oxide composition as follows:
Component
Range (Wt %)
Preferred Range (Wt %)
SiO
2
38-66
48-60
CaO
18-30
19-25
MgO
 6-15
 9-14
Al
2
O
3
 0-20
 5-10
K
2
O
0-3
2-3
Na
2
O
0-3
1-2
ZnO
 0-10
0
ZrO
2
0-9
0
B
2
O
3
0-6
0
TiO
2
0-5
0
P
2
O
5
0-5
0
In the most preferred embodiment, the glass composition comprises a glass frit which is close to the eutectic area between CaO, MgO, and SiO
2
, and comprises in weight percent about 51-57% SiO
2
, about 20-24% CaO, about 10-12%MgO, about 6-8% Al
2
O
3
, about 3% K
2
O, and about 2% Na
2
O. Such a frit can be obtained by smelting appropriate amounts of mixed feldspar (Na and K) and dolomite, which are readily available.
The glass frit or frits that comprise the glass component may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fased silica, or platinum crucible may be used to prepare the glass frit. Typically, selected oxides are smelted at temperatures of from about 1,200° C. to about 1,440° C. for 30 minutes. The molten glass formed in the crucible is then converted to glass frit using water-cooled steel rollers or a water-quenching technique and milling equipment. It will be appreciated that the step of producing the glass frit is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.
The glass compositions described above, and p

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