Ceramic tile and glaze for use thereon

The present invention provides a ceramic tile and a glaze composition for use in forming the tile. The glaze composition comprises a glass component, an expansion modifier and a spodumene crystallization promoter. The spodumene crystallization promoter comprises a material that promotes the formation of beta spodumene during the firing of the glaze composition.

FIELD OF INVENTION
 The present invention concerns a ceramic architectural tile and a glaze
 composition for use in forming such tile.
 BACKGROUND
 The prior art provides various methods for forming ceramic tiles for
 architectural applications, and it also provides various glaze
 compositions which are used to produce the top surface or wear surface of
 the tile.
 Generally, the formation of a ceramic tile involves the steps of forming a
 raw tile or an unfired ceramic body. Such ceramic bodies are formed from
 batches of natural or synthetic raw materials such as clay, kaolin,
 feldspar, wollastonite, talc, calcium carbonate, dolomite, calcined
 kaolin, 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 body.
 Once formed, the bodies are fired to form a biscuit that is hard, tough and
 brittle. In many instances, a glaze composition is applied to the biscuit
 and given a further firing (double firing) in order to vitrify or sinter
 the glaze, depending upon the degree of firing. Upon firing the glaze
 develops a vitreous, transparent or opaque surface that can be glossy or
 dull (matte), or somewhere in between glossy and dull. Glazes generally
 comprise one or more glass frits, fillers, pigments and other additives.
 In addition to double firing, it is well-known in the prior art to produce
 tiles by a single fire approach. In the single fire approach, the glaze is
 applied to the raw ceramic body, and the body and the glaze are then
 subjected to a single firing operation in order to produce the finished
 tile.
 SUMMARY OF THE INVENTION
 The present invention provides a new and improved ceramic architectural
 tile and a new and improved glaze composition for use thereon. The glaze
 composition of the present invention provides a glaze finish that displays
 excellent abrasion resistance. More particularly, the glaze displays a
 degree of abrasion resistance that exceeds all the test criteria of ISO
 10545-7 (1993). The glaze composition of the present invention is also
 capable of producing a smooth finish that is semi-transparent in
 appearance.
 A ceramic tile made in accordance with the present invention comprises a
 fired ceramic body portion and a glaze coating on at least a portion of
 the surface of the ceramic body portion. The glaze coating is formed by
 firing a composition comprising a glass composition, an expansion modifier
 and a spodumene crystallization promotor, In a preferred embodiment the
 glass composition comprises from about 40% to about 82% by weight
 SiO.sub.2, from about 5% to about 32% by weight Al.sub.2 O.sub.3, up to
 about 15% by weight CaO, up to about 20% by weight BaO, up to about 10% by
 weight MgO, up to about 15% by weight ZnO, up to about 4% by weight
 K.sub.2 O, up to about 4% by weight Na.sub.2 O, from about 5% to about 13%
 by weight Li.sub.2 O, up to about 5% ZrO.sub.2, up to about 5% by weight
 TiO.sub.2, up to about 3% by weight P.sub.2 O.sub.5 and up to about 5% by
 weight B.sub.2 O.sub.3. The glass composition may also include up to 10%
 by weight PbO. However, the use of lead oxide is not required.
 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 green or unfired ceramic bodies that form the tiles of the present
 invention may be formed using conventional technologies and methods. Both
 single fire and multiple fire techniques may be employed.
 In the case of the single fire approach, the raw tile is not previously
 fired, the application of the glaze composition being made on the green or
 unfired ceramic body. After application of the glaze composition, the
 coated tile is subjected to a kiln or similar heating device and the body
 and glaze are then simultaneously fired. In the case of the double fire
 approach, the glaze composition is applied to a previously fired body, and
 then the glaze and body are fired, the body thus being subjected to two
 firing operations.
 The glaze composition of the present invention may be applied using any one
 of a variety of conventional application techniques and also in any one of
 various forms. For example, the glaze composition may be applied wet or
 dry. Such application techniques as disk and bell applications, spraying,
 screen printing, brushing and electrostatic application may be employed.
 The glaze composition of the present invention includes a glass
 composition, an expansion modifier and a spodumene crystallization
 material or promoter.
 The glass composition is formed from one or more glass frits using
 conventional glass melting techniques. Set forth below is the composition
 for the glass composition.

Component Range (Wt %) Preferred Range (Wt %)
 SiO.sub.2 40-82 57-82
 Al.sub.2 O.sub.3 5-32 5-24
 CaO 0-15 0-10
 BaO 0-20 0-18
 MgO 0-10 0-5
 ZnO 0-15 0-12
 K.sub.2 O 0-4 0-2
 Na.sub.2 O 0-4 0-1
 Li.sub.2 O 5-13 6-13
 ZrO.sub.2 0-5 0-4
 TiO.sub.2 0-5 0-4
 P.sub.2 O.sub.5 0-3 0-2
 B.sub.2 O.sub.3 0-5 0-4
 PbO 0-20 0-10
 The glass frits of the present invention can be melted in a conventional
 manner including through the use of a continuous melter, discontinuous
 melters such as rotary melters and refractory crucibles at temperatures of
 from about 1400.degree.-1600.degree. C. The glass melt may be handled by
 suitable conventional means though typically it is quenched by steel rolls
 into flake or frit, using techniques well-known to those skilled in the
 art.
 Various expansion modifiers may be included in the glaze composition in
 order to help ensure a good expansion match as between the resultant glaze
 coating and the tile body. Examples of such modifiers include quartz,
 zirconia, spinels and mixtures thereof Additionally, it will be
 appreciated that depending upon the specific application, an intermediate
 or engobe layer may be employed between the tile body and the layer of
 glaze.
 Upon firing, the glaze composition displays a beta spodumene crystal
 structure. Preferably, the glaze displays at least about 30% by volume
 beta spodumene. Preferably, the beta spodumene does not exceed about 70%
 by volume of the structure of the fired glaze. It will be appreciated that
 in addition to beta spodumene, other phases such as virgilite may form. In
 order to ensure such beta spodumene structure, the glaze composition
 includes a material that promotes the formation of beta spodumene during
 firing. Examples of spodumene crystallization promoters suitable for use
 in the present invention include materials such as, for example,
 spodumene, eucryptite, alumina, amblygonite, calcium phosphate, petalite,
 and mixtures of the foregoing.
 The glass components, the expansion modifiers and the spodumene
 crystallization promoters define or form the solids portion of the glaze
 composition. In addition to the "solids portion," it will be appreciated
 that the glaze composition may include other additives and fillers such as
 vehicles, flocculants, antifoaming agents, deflocculants, binders, wetting
 agents, antifoaming agents, dyes, pigments, tin oxide, zirconium oxide,
 silicates to opacify the glaze, zinc oxide, wollastonite, feldspar,
 dolomite and magnesium carbonate may be employed.
 Generally, the solids portion of the glaze composition comprises from about
 70% to about 99% by weight glass component, from about 0% to about 40% by
 weight expansion modifier, and from about 0.5% to about 50% by weight
 spodumene crystallization promoter.
 The glaze composition is made according to the following technique. The
 glass frit(s), in flake or granular form, expansion modifier(s), spodumene
 promoter(s) and other additives are ball milled with water (finely milled
 to an average size of about 10-15 microns). The resulting slip, which
 comprises generally from about 30% to about 50% water, is then in
 condition for application upon a green ceramic body. Of course, it will be
 appreciated that the amount of water utilized in a slip can be varied to
 best accommodate the specific application technique being employed. Also,
 instead of water, other conventional vehicles may be employed.
 Alternatively, if the glaze composition is intended for dry or
 electrostatic application, the glaze can be dry milled or granulated.
 Other additives, such as, for example, glues, binder, organopolysiloxanes
 may be employed in the dry system. It will be appreciated that the glaze
 compositions of the present invention may be applied utilizing various
 application techniques, for example, screen printing, spraying, brushing,
 hand printing or painting, electrostatic application or using other
 methods that are known in the art.
 Firing times greatly depend on such factors as the previous firing history
 of the ceramic body, furnace conditions, and the size of the charge or
 load placed in the furnace or kiln. However, generally, the coated ceramic
 bodies are fired for a period of from about 15 minutes to about 8 hours.
 Generally, the glaze compositions of the present invention mature at a
 temperature of from about 900.degree. to about 1250.degree..
 A glaze made in accordance with the principles of the present invention
 displays excellent abrasion resistance. Generally, such glaze displays an
 abrasion resistance which displays nearly one-half the weight loss due to
 abrasion as compared to a conventional floor tile. More particularly, such
 glazes are capable of passing the criteria for Class 5 of ISO 10545-7
 (1993). Class 5 is the most stringent level of abrasion resistance defined
 by such standard. This standard is also known as EN154 in Europe.
 By way of illustration and not by any limitation, the following examples
 will describe specific compositions within the scope of the present
 invention. Unless otherwise indicated, all parts and percentages are by
 weight, and all temperatures in degrees Centigrade (.degree. C.). The frit
 and glaze compositions utilized in Examples 1-4 are specified below. The
 frit compositions were prepared utilizing conventional melting techniques.
 The tile bodies employed in the Examples were conventional in nature,
 composed principally of clay with silica, feldspar and other conventional
 additives included.

EXAMPLE 1
 Floor tile was made with single fast firing technology. First, glaze
 composition 1 was prepared by milling in 60 parts of water for every 100
 parts of dry charge. The milling residue on a 40 microns sieve being
 0.1-0.2% for 100 parts of dry charge. The glaze was applied on a dried
 green tile body with a suitable engobe (an engobe available from Ferro
 (Italia) SRL of Casinalbo, Italy, under the trade designation FE.ENG 304).
 The glaze was applied at a rate of 1.25-1.5 kg/m.sup.2. The tile was then
 subjected to a firing cycle of 50 minutes in length with maximum
 temperature of 1180.degree. C. The resultant tile had a smooth surface
 with 27% of reflected light (measured with 60.degree. angle). The glaze
 also exhibited a sufficient and good transparency to allow underglaze
 decoration and abrasion resistance--the loss in weight after 12000
 revolutions MCC (according to EN154 std but longer run) being about 0.22
 grams. The surface is not attackable by acids and bases (class AA
 according EN122 std).
 EXAMPLE 2
 Wall-floor tile was made using single fast firing technology
 ("monoporosa"). First, glaze composition 2 was prepared as in Example 1.
 The glaze was then applied on a dried green tile body with suitable engobe
 (available from Ferro (Italia) SRL of Casinalbo, Italy, under the trade
 designation FE.ENG1039. The glaze was applied at a rate of 1-1.3
 kg/m.sup.2. The tile was then subjected to a firing cycle of 45 minutes in
 length with a maximum temperature of 1140.degree. C. The resultant tile
 had a smooth surface with 18% of reflected light (measured with 60.degree.
 angle). The glaze also exhibited sufficient transparency to allow
 underglaze decoration. The surface is not attackable by the acids and
 bases (class AA according EN122 std.).
 EXAMPLE 3
 Wall tile was made with double fast firing technology. First, glaze
 composition 3 was prepared as in Example 1. The glaze was then applied on
 a fired tile body over suitable engobe (available from Ferro (Italia) SRL
 of Casinalbo, Italy, under the trade designation FE.IT1726). The glaze was
 applied at a rate of 1-1.3 kg/m.sup.2. The tile was then subjected to a
 firing cycle of 30 minutes in length with a maximum temperature of
 1030.degree. C. The resultant tile had a smooth surface with 12% of
 reflected light (measured with 60.degree. angle). The glaze also exhibited
 sufficient transparency to allow underglaze decoration. The surface is not
 attackable by the acids and bases (class AA according to EN122 std.).
 EXAMPLE 4
 Wall tile was made with traditional double firing technology. First, glaze
 composition 4 was prepared as in Example 1. The glaze was then applied on
 a fired tile body over suitable engobe (available from Ferro (Italia) SRL
 of Casinalbo, Italy, under the trade designation FE.IT1726). The glaze was
 applied at a rate of 1-1.3 kg/m.sup.2.
 The tile was then subjected to a firing cycle of 8 hours in length with a
 maximum temperature of 1030.degree. C. The tile had a smooth surface with
 8% reflected light (measured with 60.degree. angle). The tile also
 exhibited good transparency. The surface is not attackable by the acids
 and bases (class AA according EN122 std.).
 Glaze Composition
 (Parts By Weight)

Glass Frit Compositions
 (% By Weight)
 Frit A Frit B Frit C
 SiO.sub.2 70 57.7 82
 Al.sub.2 O.sub.3 11.2 23.7 5.1
 CaO 10.9 7.8 0.1
 MgO 0 4.1 0
 K.sub.2 O 1.2 0.1 0.1
 Na.sub.2 O 0.2 0.2 0
 Li.sub.2 O 6.5 6.6 12.7
 In summary, the present invention affords a glazed tile wherein the glaze
 displays high mechanical resistance such as high abrasion resistance.
 Furthermore, the glaze is capable of producing a smooth finish that may be
 glossy or dull (matte), semimatte, or semitransparent in appearance if
 desired. Of course, it will be appreciated that by using specific
 additives, the glaze can be utilized to produce surfaces that are opaque,
 semiopaque or semitransparent. Additionally, the glaze of the present
 invention may be applied at significant thicknesses to facilitate
 mechanical polishing of the surface. Finally, it will be appreciated that
 in addition to tile, the glaze composition of the present invention may be
 utilized in conjunction with other ceramic bodies such as, for example,
 chinaware, dinnerware, fine bone china and sanitary ware.
 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.