Composition and process for obtaining crystal structures of russellite (bismuth wolframate) on ceramic parts.

This invention relates to a base composition of oxides and the products derived from said composition, characterised in that following its application on a ceramic substrate and its subsequent firing, crystal formations of Russellite are obtained on the surface of the ceramic part that provide it with an appearance that is rich in chromatic hues, iridescent, metallic and shiny depending on the amount and size of the crystals present.

This invention lies within the sector of the ceramic industry, specifically that of materials for flooring and ceramic wall lining obtained by single or double firing, structural ceramics, roof tiles, and sanitary and artistic ceramics.

BACKGROUND OF THE STATE OF THE ART

In the ceramic industry, the surface coating of ceramic supports provides the parts with decoration or technical features that allow differentiation and diversification of the products, also providing them with great added value. These surface coatings are compositions that contain vitreous materials obtained from the fusion of inorganic substances (frit) and/or raw materials that after being subjected to the firing processes of the ceramic industry produce a vitreous matrix in which the different crystalline phases are dispersed.

The crystallisation consists in the formation of crystalline structures within a glaze. For the crystallisation to occur, determined by the number of crystallisation centres formed and by the growth rate of crystals, the factors to be considered are: cooling conditions, molten ceramic viscosity, surface tension and the chemical composition of the glaze or enamel.

It is known that long cooling cycles and therefore low cooling speeds favour the production of crystallised glazes, that low viscosities and surface tensions favour crystallisation capacity, with the chemical composition of the glaze logically influencing said capacity, since there are oxides that favour or inhibit the formation of crystalline structures (amongst the oxides favouring the formation of crystalline structures are zirconium, titanium, zinc, wolfram, molybdenum, etc.) and that for the formation of crystalline phases within a glaze to produce the largest possible crystals, the molten glaze has to be as fluid as possible for the components necessary for the formation of the crystalline grid to have sufficient mobility and low resistance to rapid crystal growth. This is facilitated with high contents of powerful fluxes such as lead, lithium, sodium and potassium, lead being the most used traditionally, but which is no longer used due to its toxicity. On the other hand, for high firing temperatures zinc oxide and titanium oxide, classified as crystal formers, also act as fluxes.

Currently, the ceramic industry knows and commonly uses coatings that contain crystalline phases. As examples we can mention crystallisations of willemite, celsian, anorthite, cordierite, mullite, pyroxene, zircon, gahnite, titanite, etc. Some of these crystalline phases are responsible for the matt, glossy or white appearance of ceramic coatings, specifically in the coatings of ceramic tiles, whereas others are used to increase or improve their mechanical and chemical properties.

There are numerous patents and articles that relate to obtaining and studying these crystalline phases on ceramic parts, as an example: - F. Lucas, A. Belda, F. J. Torres, J. Alarcon. "Estudio y caracterizacion de vidriados vitroceramicos basados en piroxeno" (Study and characterisation of pyroxene-based vitroceramic glazes). Bol. Soc. Esp. Ceram. V., 43 (5) 849- 854 (2004).

- J. Ma. Rincon. "Principles of Nucleation and Controlled Crystallization of Glasses". Poym.-Plast. Technol. Eng., 31 (3-4) 309-357 (1992).

I. Vicente-Mingarro, P. Callejas, J. Ma. Rincon. "Materiales Vitroceramicos: El proceso Vitroceramico" (Vitroceramic materials: The vitroceramic process). Bol. Soc. Esp. Ceram. V., 32 (3) 157-167 (1993).

- H. S. Kim, R. D. Rawling, P.S. Rogers. "Sintering and Crystallization Phenomena in Silceram Glass", J. Mater. Sci., 23,2622-2630 (1988).

- J. Ma. Rincon, M. Romero, J. Marco, V. Caballer. "Some aspects of crystallization microstructure on new glass-ceramic glazes". Mat. Res. Bull., 33 (8) 1159-1164 (1998)

G. Baldi, E. Generalli, C. Leonelli, T. Manfredini, G. C. Pellacani, C.Siligardi. "Effects of nucleating agents on diopside crystallization in new glass- ceramics for tile-glaze application", J. Mater, Sci., 30, 3251-3255 (1995).

- JP 2008266048 "Method for manufacturing crystallized glass product".

- W09916727 "Manufacture of ceramic tiles from industrial waste"

- US6348425 "Crystallizing glaze system"

The objective of this invention is to obtain a new crystalline formation that is currently unknown to the industry of ceramic coatings by compositions comprising a mixture of essential oxides, which are wolfram oxide (also called tungsten oxide) and bismuth oxide, together with other optional oxides as well as the method for obtaining them.

Although there are numerous patents on the use of bismuth in glazes, frits and enamels, (such as for example patents US6207285, US4892847, EP370683, EP0598199, US6255239, US6936556, EP1331207, ES2212409), the purpose pursued in these patents is to achieve products for decorating or coating glass, especially glass for automobiles and to provide the products with high durability, corrosion- resistance or good chemical resistance. However, none of these patents includes wolfram oxide as an essential oxide in the compositions of the products nor intends to obtain this new crystalline phase (russellite) in ceramic coatings.

Patent EP1970357 (Materials for coating ceramic bodies, preparation and use thereof and ceramic bodies comprising them), perfectly shows the state of the art and relates to a series of materials for coating ceramic bodies comprising a mixture of oxides from groups IB, VIB and VII of the periodic table and in particular wolfram oxide, cobalt oxide and copper oxide, in percentages by weight from 20 to 70% for wolfram oxide, from 2 to 30% for cobalt oxide and from 5 to 30% for copper oxide and other oxides in a percentage between 0 and 20%.

After applying the materials on ceramic bodies and firing them, we specifically obtain crystals of cobalt wolframate (CoWo4) belonging to the crystalline phase Krasnoselskite (ICSD 15851), which surface may also present crystals of copper oxide in the matrix around the main crystalline phase.

However, this patent does not include bismuth oxide in the composition of the materials nor does it refer at any time to obtaining crystalline phases of bismuth wolframate (russellite).

Patent US3480566 (Low Melting glass and Compositions containing the same) discloses a composition used in the manufacture of electric circuits that requires the addition of noble metal powder and a liquid vehicle to a glass of the PbO-Bi203- B203-Si02 system in contrast to this invention, where it is not essential to use noble metals or lead oxide in the compositions.

The field of application of patent US3480566 is that of the manufacture of electric circuits. The firing temperatures and the application techniques differ from those of this invention, and nowhere does patent US3480566 refer to or intend to obtain crystals of bismuth wolframate (russellite).

Patent US2663658 (Ornamental Crystalline glaze) describes an ornamental or decorative glaze that contains crystals that are big enough to be visible.

Specifically, crystals of one or more oxides of the third, fourth or fifth group of the periodic table. The decorative or ornamental crystalline glaze is formed by oversaturation with an oxide or some of the oxides of the following elements: V, Ge, W, Ti, Zr, Ce, Cr, Se, Al, Te, in a matrix of glass-formers of the group consisting of PbO, Na20, K20, MgO, CaO, BaO, B203, ZnO, SrO and Bi203, except for silicon oxide. According to this patent, the silicon oxide has an inhibiting effect for the formation of these crystals.

In the specific case of wolfram (tungsten) glazes, it refers to a composition in which the proportion between the wolfram oxide and the glass-forming oxides is more than 1:1 and less than 19:1.

As an example it cites a composition formed by 48.5 to 65.8% of lead oxide and between 51.5 to 34.2% of wolfram oxide, which after firing produces a matt crystalline glaze.

The addition of colorants to this composition provides different colorations of the crystals.

These compositions are applied on ceramic bodies but nowhere does it mention obtaining russellite crystals, and also the range of percent by weight in the compositions of Patent US2663658 differs from the working range of the composition presented in this invention.

Patent US3607320 (Phototropic glass-ceramic articles containing bismuth doped calcium tungstate crystals) discloses a phototropic ceramic glass consisting in a vitreous matrix with 10 to 90% by weight of calcium wolframate crystals as a phototropic agent

Said crystals are doped with bismuth and the vitreous matrix contains as forming elements boric oxide and silicon oxide.

Throughout the patent it is stated that the crystalline phase present in this ceramic glass is calcium wolframate (Scheelite) and not the crystalline phase russellite, as described in this invention.

On the other hand, bismuth oxide is present in the compositions at very low percentages, compared to this invention.

On the other hand, regarding the iridescent, metallic and glossy appearance with great chromatic richness that this new crystalline formation produces on the ceramic part, although metallic enamels have been used for years for decoration and obtaining metal highlights and iridescence on tiles, there were many limitations and disadvantages in the existing compositions and methodologies for obtaining them.

Amongst the different mechanisms that are widely known to obtain this type of effects are lead glazes with the addition of heavy metals (e.g.: GB-369534) with high contents in lead and oxides such as nickel, manganese and copper, with limitations due to problems of toxicity, as well as the use of noble metals together with organic substances (US-3313632, US-6077570, amongst others) with drawbacks of toxicity, high costs, low technical features of the parts obtained, requirements of firing the parts at lower temperatures (technique known as "third firing"), etc.

These limitations are widely discussed in patents ES2310139 (Metallic enamel with metallic lustre, method for obtaining it and application), ES2161193 and EP1306355 by the applicant and ES2246166, among others.

In turn, Patent ES2310139 mentions obtaining a metallic finish using a vapour phase deposition technology (e.g. according to Patent EP1498402 Al) and states its limitations.

In recent years, the ceramic industry has seen the appearance of a series of alternative formulations and methodologies to those mentioned above, all of which intend to eliminate the limitations existing to date. Amongst these we can highlight:

• ES2161193 and EP1306355 by the applicant, which disclose a composition and method for obtaining metallic effects in ceramic tiles and their applications. The composition disclosed comprises a mixture of essential oxides such as Si02, AI203, Fe203 and P205, together with other optional oxides. Although the applicant has not found problems for the use and industrial application of the compositions disclosed in said patent, they find limitations regarding the possibility of coloration of the products obtained due to the high content in Fe203 in said formulations, the objective of this invention being that of opening a new line of work from different formulations that avoid such limitation.

• Document ES2301364 (Formulation for producing metallic effects) is characterised in that it comprises a first oxide chosen from AI203, P205, Si02, PbO and combinations thereof and Fe203 introduced as magnetite into the composition. Although said patent mentions the advantage of introducing Fe203 as magnetite to minimise the environmental contamination of the waste generated in the application processes of the formulations, due to the greater particle size of the material used and its greater density, it makes no mention of the problem of using PbO in its formulations, which is very limited due to problems of toxicity. We must indicate that the composition proposed in this patent also produces limited colorations in the products obtained.

Document ES2246166 (base composition of oxides for obtaining calcined pigments, method of synthesis of the pigments and use) discloses a base composition of oxides, characterised in that it comprises at least P205 in an amount comprised between 40-70% by weight, Fe203 in an amount comprised between 10-50% by weight and Document EP2000443 Al (Metallic glaze composition), which discloses the same composition as above but broadening its essential components with the introduction of Li20 between 0-30% by weight. These patents propose obtaining a calcined pigment that is subsequently added or mixed to give rise to an enamel slip. Again we find formulations of P205 and Fe203 as essential components, and although these patents mention as an advantage that when obtaining the calcined pigment they do not have the problem of intense red iron oxide and therefore its corresponding problems of cleaning and contamination on other ceramic products due to the strong pigmentation of iron oxide, they do not mention that the waste of the enamel slip obtained with said pigments may contaminate other ceramic products, slip preparation mills and enamel lines, which after firing the parts in the ceramic furnaces lead to contamination with metallised spots. On the other hand, we find once again the same limitations regarding the colouring of the products or tiles obtained using these formulations.

ES2310139 Al (Ceramic enamel with metallic gloss, method for obtaining it and application) proposes a formulation based on the use of particles of micro/nanometric size of different metals, metal oxides and alloys, mixed homogenously with ceramic frits, coal powder and other additives. This patent mentions obtaining different enamels with a metallic gloss with different hues. To perform the invention of this patent it is necessary to generate a local and temporary reduction atmosphere inside the furnace during the firing process of the ceramic tile, achieved according to the patent by adding coal powder, for example. On the other hand, creating a reduction atmosphere inside the furnace during the industrial firing of ceramic tiles (oxidizing firing) has limitations regarding the stability of the hues obtained. As we have seen, therefore, the use of mixtures of different oxides to obtain enamels that produce metallic highlights and iridescence on ceramic parts after firing is indeed known, but none of these patents mention or intend to obtain a metallic appearance on ceramic parts from bismuth oxide and wolfram oxide, nor do they obtain the crystalline phase we mention in this invention.

In view of all these difficulties, the objective of the present invention is to disclose a new composition as well as a method to obtain articles decorated with exceptional features, moreover, with advantages that exceed the state of the art mentioned above, such as: it has no limitations due to problems of toxicity such as with lead glazes with the addition of metals such as nickel, manganese and copper.

No noble metals are used diluted in organic substances, which reduces the cost of these products and their environmental impact.

- There is no need for specific firing or firing at lower temperatures (technique known as "third firing").

- This invention does not require any kind of modification in the usual production, application and firing systems used in the industry of coating ceramic bodies, with the extra savings this entails.

- This invention is not limited with respect to the coloration of the products obtained, as in the patents discussed above, with high content in iron oxide or pigments and the chromatic possibilities of the parts obtained are unlimited.

DESCRIPTION OF THE INVENTION

This invention relates to obtaining a new crystalline formation unknown to date in the industry of coatings for ceramic bodies, the compositions for obtaining said crystalline phase, the process for obtaining the new crystalline phase and the ceramic products obtained.

More specifically, this invention relates to compositions comprising a mixture of oxides, which after being applied on ceramic bodies and fired, produce said crystalline phase, providing an appearance that is rich in chromatic hues, iridescent, metallic and glossy depending on the amount and size of the crystals present.

It is an objective of this invention to provide said composition with the required stability and reproducibility for its use in the decoration of ceramic bodies obtained in the usual firing cycles of the ceramic industry.

The composition is applicable in general as a coating for any ceramic body, and specifically as a material for coating and decorating ceramic flooring and wall coating tiles obtained by single or double firing, structural ceramics, roof tiles and sanitary and artistic ceramics.

The crystalline phase obtained with the composition object of the present invention is the bismuth wolframate known as russellite, with chemical formula Bi2W06.

For general information on this crystalline phase we shall refer to publications:

- Russellite, Mineral Data Publishing, version 1.

- Palache, C, H. Berman and C.Frondel (1944) Dana's system of mineralogy, (7th edition), v. I, 604-605.

- Hodge, L.C. (1970) Russellite: a second occurrence. Mineral. Mag., 37, 705- 707

- Knight, K.S. (1992) The crystal structure of russellite; a re-determination using neutron powder diffraction of synthetic Bi2W06. Mineral. Mag., 56, 399-409. The composition object of the invention comprises a mixture of oxides. Of these oxides, two are considered "basic or substantial" for obtaining the product, and the rest can be considered "excipients" or "non-substantial" for obtaining the product, since although they provide other features to the product, they are not essential to obtain the intended crystalline phase.

The "basic or substantial" oxides are wolfram oxide (also called tungsten oxide) and bismuth oxide.

Regarding the remaining optional oxides present in the composition, these shall be chosen from amongst those commonly used in the industry of coatings for ceramic bodies, the function of which shall be to provide the composition with the necessary melting and flowability, favour the occurrence of the crystalline structure, crystal growth, modifying the viscosity and surface tension of the melt, formation of glass, etc.

Amongst these oxides, and as a non-limiting example, we can mention: silicon oxide, boric oxide, lead oxide, sodium oxide, potassium oxide, lithium oxide, zinc oxide, barium oxide, aluminium oxide, calcium oxide, magnesium oxide, zirconium oxide, cerium oxide, tin oxide, titanium oxide, phosphorous oxide, etc. On the other hand, the compositions object of the invention may contain all kinds of chromophore oxides, the purpose of which is to modify and enrich the colour of the final product obtained.

As an example of these oxides we shall mention, without limitation, manganese oxide, copper oxide, cobalt oxide, chromium oxide, nickel oxide, iron oxide, vanadium oxide, etc.

Preferably, according to the invention, the ratio of the composition of the mixture of substantial oxides that chemically form the product, expressed in percent with respect to the total weight of the material, are:

Bismuth oxide 27-97%

Wolfram oxide 5-20%

Regarding the remaining oxides, if they are present in the composition, they will preferably be within the range of between 0 and 70%.

Preferably, the materials according to this invention have the following composition:

Bismuth oxide 35-97%

Wolfram oxide 5-20%

Other oxides 0- 60 % The mixture of oxides that make up the chemical composition may come from different raw materials, and the percentages of the components of the formula will be adjusted in each case in order to achieve the desired percentages of oxides.

Therefore, for example, bismuth oxide can be introduced as an oxide, a nitrate, a carbonate, an aluminate of bismuth, wolfram oxide as an oxide, as calcium wolframate, as metal wolfram, etc.

The starting materials or raw materials making up the chemical composition will be mixed and/or ground and/or shall be subjected to a melting or fritting process until obtaining a homogeneous product with a suitable particle size according to the chosen application methodology or technique.

The particle size of the materials object of the invention will be determined by how they are presented as well as by the application technique used on the product.

The materials object of the invention can come in different forms, all of which are intended to coat ceramic bodies and are widely used in the ceramic industry, such as for example: as a powder, as a suspension in an organic or inorganic medium (enamels, screen printing inks), as a frit, granulate, pelletised, sinterised, sprayed, and in any other form.

When the composition of the invention is used as a frit, the mixture of the starting materials is performed by melting them at high temperatures and then allowing them to cool.

Then, the granules of frit obtained are ground using suitable techniques that are known in the ceramic industry until achieving the desired particle size. This milling can be performed both as dry milling (producing fine grain, pellets, etc.) or by wet milling together with suitable vehicles and admixtures, producing a suspension or enamel that will be applied onto the ceramic support using any of the techniques used in the ceramic industry, which support will finally be fired.

In the case of fine grain and pellets, these are directly applied onto the ceramic support as granules and the whole is then subjected to the thermal cycle or firing.

The composition object of the invention can also be achieved at ambient temperature, without requiring fritting, or introducing part of the components of the composition as frit.

In this case, suspensions or enamels will be obtained by mixing, homogenising and grinding the starting materials in a suspension medium and will then be applied onto the ceramic support using any of the techniques mentioned above, which will then finally be fired.

In the case of suspensions or enamels, the vehicle or dispersing medium is usually water, and the admixtures are those commonly used to guarantee their good application, suspension agents, plasticisers, glues, kaolin, bentonite, etc.

The suspensions or enamels can be sprayed, producing sprayed granules that will then be applied on the ceramic supports before, during or after the pressing step of the ceramic body.

Once applied the ceramic body or support will then be fired.

In turn, the suspensions or enamels can be dried, micronized and mixed with suitable vehicles to provide screen printing inks.

In this case the vehicles or dispersion media are usually organic vehicles, in order to provide the screen printing inks with suitable properties of density, viscosity, wettability, drying, etc.

These screen printing inks will be applied onto the ceramic supports using planar templates, rotary machines, flexography, gravure or injection, and the assembly will then be subjected to the firing step. We therefore conclude that the methods for application of the product of the invention on the ceramic bodies will be those commonly used in the industry of ceramic coatings, which as an example, and with a non-limiting nature, may be:

Enamelling by disks, hood, aerograph, spraying, immersion, screen printing applications using flat screens, rotary machines, injection machines, dry applications before, during or after the pressing step.

The firing step will depend on the ceramic body to be coated, without any specific adjustments required either in the temperatures or firing times of the cycles usually used.

After the firing step a coating will be obtained in which there will be russellite crystals, providing the ceramic part with an appearance that is rich in chromatic hues, iridescent, metallic and glossy, depending on the amount and size of crystals present.

The materials of this invention can also be applied in a single or in several layers, combining different proportions or colours or overlaying different applications on the same ceramic part, producing variations in the appearance, colour and texture of the part and allowing a great variety and diversity of aesthetic results.

In turn, these materials can be applied and combined with other materials intended to coat ceramic bodies, slips, enamels, granulates and/or other serigraphy of common use in the industry.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

For a greater understanding of this invention, we describe an embodiment thereof as Example 1, also including other examples with other embodiments. Please note that these should not be considered as a limitation or restriction of the scope of this invention.

Example 1:

Bismuth oxide, clay, wolfram oxide and CMC glue were mixed together. The mixture was introduced into a ball mill with enough water to obtain a suspension with a density of more than 2200 g/l and enough time to obtain a residue of less than 3% in 45 micron mesh.

The raw materials forming the mixture were introduced in the proportions indicated (percentage by weight with respect to the total mixture):

Bismuth oxide 86.96%

Clay 4.35 %

Wolfram oxide 8.69% 50 g of the resulting enamel were applied onto a 33x33 cm ceramic tile of porcelain stoneware using the hood application technique.

The tile was allowed to dry and once dry it was subjected to a firing cycle in an industrial furnace at 1200°C for 65 minutes.

The resulting ceramic part had an iridescent coating.

The crystalline phases present in said coating were identified by x-ray diffraction using a Bruker D8-Discover unit. Interpretation of the diffractogram indicates the presence of a very oriented crystalline phase, concluding that it is russellite, syn with composition v-Bi2W06 and ICDD card (International Centre for Diffraction Data) 39-0256.

Example 2:

The following raw materials were mixed with water in the percentage by weight indicated with respect to total weight:

Bismuth oxide 86.96%

Clay 4.34 %

Wolfram oxide 6.09 %

Cobalt oxide 2.61%

The aqueous suspension obtained was homogenised by grinding until obtaining a residue over a 45 micron mesh of less than 0.5%. The mixture was then dried at 115°C in a laboratory oven.

Once dried the mixture was micronised.

100 g of micronised mixture were weighed and 35 g were added of a screen printing vehicle formed by a mixture of synthetic polymers.

After homogenising the mixture, it was applied onto a white paste porous ceramic floor tile with the aid of a 21-thread serigraph screen.

The printed floor tile was allowed to dry and was subjected to a firing cycle in an industrial furnace at 1135°C for 50 minutes.

The resulting ceramic part had a metallised and iridescent coating with blue and greenish hues.

Identification of the crystalline phases present by x-ray diffraction indicated the presence of russellite as the crystalline phase causing the effect obtained.

Example 3:

The following raw materials were mixed in the percentage by weight indicated with respect to total weight:

Bismuth oxide 63.7%

Wolfram oxide 9.1 % Quartz 12.4%

Alumina 10.8%

Potassium feldspar 4.0%

This mixture was introduced into a melting furnace for 45 minutes at 14005C.

The resulting frit was then cooled in water and dried using a standard technique, using a laboratory oven, for example, at 100-120 ⁹ C, or using a drying lamp.

The dry mixture was ground and a granulometric selection was performed using sieves with different mesh sizes.

The pellets or fine grain obtained was applied onto a part of porcelain stonewear using fine grain glue as an adhesive agent.

The floor tile was then dried and subjected to a firing cycle in an industrial furnace at 1200?C for 80 minutes.

The resulting ceramic part had an iridescent and glossy coating.

Identification of the crystalline phases present by x-ray diffraction once again indicated the presence of russellite as the crystalline phase causing the effect obtained.

Example 4:

The following raw materials were mixed in the percentage by weight indicated with respect to total weight:

Bismuth subnitrate 43.3%

Bismuth subcarbonate 48.0 %

Quartz 3.9%

Alumina 3.5%

Potassium feldspar 1.3%

This mixture was introduced into a melting furnace for 45 minutes at 10002C.

The resulting frit was then cooled in water and dried using a standard technique, using a laboratory oven, for example, at 100-120 ⁹ C, or using a drying lamp.

Once dry, this frit was mixed with other raw materials in the percentages indicated below with respect to the total weight of the mixture:

Frit 70%

Wolfram oxide 10 %

Quartz 1.2% Alumina 0.8%

Nepheline 2.1%

Iron oxide 5.6%

Zirconium silicate 2.3%

Clay 8%

The mixture was introduced into a ball mill with enough water to obtain a suspension with a density of more than 2200 g/l and enough time to obtain a residue of less than 1.5% in a 45 micron mesh.

The mixture was then atomised in a laboratory atomiser.

The atomised granules obtained were then subjected to granulometric selection using sieves with different mesh sizes.

These granules were applied onto a support of a ceramic porcelain stoneware tile during the pressing stage, which tile was then dried and subjected to a firing cycle in an industrial furnace at 1200°C for 60 minutes.

The resulting ceramic floor tile had a coating with a metallic and iridescent appearance in a red-brown colour. Identification of the crystalline phases present by x-ray diffraction indicated the presence of russellite as the crystalline phase causing the effect obtained.