The present invention relates to a method for obtaining glazed ceramic articles.

Ceramic articles, particularly flooring and furring tiles, make a market of primary importance, which course, strictly linked to the building one, is continuously growing up in the whole world.

The need of increasing the productivity of the plants has driven the ceramic industry and the pro- ducers of machines and plants connected thereto to¬ wards the research and the adoption of solutions technologically innovative both as for plant engi¬ neering and ceramic materials for themselves.

In practice, all the steps of the ceramic process for producing tiles have been reviewed and sped up; the glazing steps however still show some opportunity of improvement.

One of the technology for producing flooring and furring tiles is based upon applying an aqueous dispersion of the ceramic materials which are going to form the glaze, to the baked or unbaked support of the tile and on the subsequent baking in roller mono-layer oven (up to 1200°C for 35-60 minutes) thus obtaining the glaze by sintering said ceramic materials.

The ceramic materials which originate the glaze are generally obtained starting from basic oxides and acid oxides subjected to fusion, known in the art as fritting, whereby frit is obtained which is, in fact, the glaze basic materials.

The frit is then milled in an aqueous phase, together with other components, such as pigments charges, milling assistants, etc., in order to ob¬ tain a fine aqueous dispersion suitable to be ap- plied on the support.

The adhesion between the formed glaze and, the support is assured by the conditions of the cy¬ cle (times and temperatures) at which one works and by the composition used; to assure a better adhe- sion, an intermediate layer (having an intermediate composition between the support and the glaze) , called engobe, is used very often, which after the baking, forms and interlayer which has the function to link the support and the true glaze, hindering the reciprocal contamination, and covers the even¬ tual colouring of the support.

Whenever one would or should need a very high surface hardness and abrasion resistance, stoneware tiles, without glazes or engobes, which would re- duce the mechanical characteristics of stoneware

itself, are used. The stoneware tile thus obtained has yet an unrefined aesthetic appearance and can¬ not replace the more refined flooring monobaked tile, even if it is more resistant and appreciated for its low porosity, antifreezing and high break¬ ing load than such tiles. It is however possible to further refine the stoneware tiles mechanically lapping and polishing them and subsequently treat¬ ing them with waxes and varnishes; such operation involving however a large expense.

The target to utilise porcelained stoneware as the tile support material, maintaining the func¬ tional peculiarities of the stoneware (hardness, abrasion, etc.) and improving the aesthetic ones by a glazing step, is however impracticable for at least two reasons, both connected to the fact that the possible glaze should be very hard and there¬ fore inherently endowed with refractory character¬ istics. The first reason is that working in ovens at temperatures and with characteristics unavail¬ able nowadays would be needed for the fritting step. The second reason, even more serious, is that the frit, to be transformed into the glaze, after having been applied to the support, should be baked at very high temperatures, near to the "hemisphere

point", i.e. at the temperatures to which the glaze wets and seeps through the support ensuring the formation of an inter-layer which allows a good ad¬ hesion: in practice the rapidity imposed by the modern baking cycles, the refractory characteris¬ tics of these hard glazes and the fact that the maximum hardness develops itself at the "sphere point" (i.e. before the glaze wets the support) , do not allow a satisfactory technical result. Generally, what just above described repre¬ sents a shortcoming even for other kinds of sup¬ port, whenever high surface hardness, by applying glaze, would be obtained.

The glazing technique, nowadays used by every- body for the production of ceramic tiles and cro- kery, generally show some limit which stimulated the study of alternative solutions.

All the new techniques try to avoid passing through the preparation of the frit, substantially resorting to using the "sol gel" technique.

The sol gel technique consists in using the so called "precursors", containing the chemical ele¬ ments needed for the making of the glaze ceramic material, which, preferably in the presence of or- ganic solvents, carry to a colloidal solution (the

sol) . The latter is applied to the support whereon it changes into gel by evaporating the solvent or by slow polymerisation of the sol. The gel is made of a pre-arranged molecular structure of oxides and hydroxides which, because of the strict contact shows sintering temperatures and/or times decidedly lower than a corresponding ceramic one obtained from crystalline powders or vitreous frits.

The gel is then subjected to a step of thermic treatment which transforms it into ceramic glaze.

In the patents GB 1 166 991 and GB 1 166 992, there is disclosed the use of the esters of the bo¬ ric and silicic acids combined with metal resi- nates, such as lead, zinc and calcium, in different organic solvents, applied under the shape of a thin film to be subjected to baking to obtain boron- silicic glazes.

In the patents US 4 801 399 and US 4 921 731, there is provided the preparation of colloidal so- lutions (sol) starting from alcoholates and ni¬ trates of aluminium and other metals, by heating in water and various organic solvents.

The colloidal solutions (sol) are applicable to different supports in several ways to attain ceramic articles endowed with better properties.

The patent DE 4 003 627 describes a system of glazing cement supports or the like by using aque¬ ous solutions of aluminium, lead, etc. salts in the presence of a colloidal silica generator (sodium silicate or an organic ester of the silicic acid) . The patent WO 91/08179 describes how vitreous glazes, shining and endowed with an excellent chemical and mechanical strength on a ceramic sub¬ stratum (tiles and crockery) , by applying a sol ob- tained starting from organometallic precursors in organic solvents, can be obtained.

In all these methods, besides to clear advan¬ tages, some drawbacks, which go from the use of ex¬ pensive substances, which manufacture is hard and potentially toxic (resinates, alcoholates, esters of the silicic acid, etc.), to the use of organic solvents, are noticeable.

On one hand, the methods for preparing sol and gel are often complicated, on the other hand, the organic solvents represent an environmental prob¬ lem, because of their handling, both during the preparation of sol and during the step of applying and baking the articles.

Then, they have to be burned off, during the

densifying step, by baking in the making of the glaze, involving clear problems concerning gaseous emissions. Such situation, adversely affects the cost of the whole process, which further shows re- arkable ecologic drawbacks.

Actually, the sol gel technique, widely de¬ scribed in literature too ("Rivestimenti sottili mediante deep coating con metodo sol gel" (Thin coatings by deep coating with the sol gel method) , M. Guglielmi, Rivista della Stazione Sperimentale Vetro, No. 4, 1988, p. 197-199; Sol gel processing of complex oxide films, G. Yi and M. Sayer, Ceramic Bulletin, Vol. 70, No. 7, 1991, p. 1173-1179; Sol gel processing of ceramics, A. Atkinson and J. Se- gal. Ceramic Technology International, April 1993, 187-190) , has been used for special ceramic appli¬ cations only, for instance in the field of elec¬ tronics, optics, as a protecting agent for special glasses, for biomaterials, etc. , but it has not gained a mass utilisation in the glazing of tiles and crockery from an industrial point of view.

In the above cited article by Atkinson, refer¬ ence is made to the possible use, besides alcoho¬ lates, of aqueous sol too, but with the bound that one can form layers having a maximum thickness of 1

micron, as otherwise cracks and damages of the glaze, due to the shrinkage of materials, reveal themselves.

The present invention intends to intervene in the real glazing step, working both on the compo¬ nents necessary to obtain the glaze and on the way to apply them on articles.

One of the purposes of the present invention is to provide a glazing method which does not in- volve the making of the frit, neither milling steps for obtaining glaze dispersions and generally a power and economic saving, besides to the simplify¬ ing of the ceramic process.

Another purpose of the present invention is to provide a method completely carried out in an aque¬ ous environment, easily applicable industrially for the common ceramic applications, such as the glaz¬ ing of tiles and crockery, besides to the other, even particular, ceramic applications such as, for instance, the ones in the field of electronics, op¬ tics, as a protecting agent for glasses, for bio a- terials, etc.

A further purpose of the invention is to allow to obtain glazed ceramic articles having a thick-

ness of the ceramic layer much higher even than l micron.

These and other purposes which will be under¬ stood by the comprehension of the present disclo- sure, are attained by a method for obtaining glazed ceramic articles which comprises applying to a ce¬ ramic support at least an aqueous solution or dis¬ persion containing at least a precursor of the de¬ sired glaze wherein said at least a precursor is: an inorganic compound selected from an anhy¬ dride of an element of the III, IV or V group of the Periodic Table, or the corresponding oxyacid, or a salt thereof, or a salt of a hydrohalide acid or an organic compound selected from the salts of the C1-C4 carboxylic acids, having a branched or linear chain, with a metal or a transition metal, and at least a subsequent thermic treatment. In the present invention, as a precursor, that compound or mixture of organic or inorganic com¬ pounds, soluble or dispersible in water, able to originate the glaze at the conditions utilized, is meant. According to the invention, the glaze precur

sors are in a form which allows to obtain the for¬ mation of a good interlayer (thus avoiding the use of an engobe) and, more generally, to lower times and temperatures at which the transformations of the compounds, which make the precursors, to origi¬ nate the glaze, occur.

It is therefore possible, by carrying out the present invention, to obtain very hard glazes even by fast baking tiles in monolayer roller ovens in monobaking.

According to the invention, the compounds which form said precursor, contained in said aque¬ ous solution or dispersion, can be organic or inor¬ ganic. Advantageously, such inorganic compounds are selected from the compounds of boron, silicon, ni¬ trogen and chlorine.

The preferred inorganic compounds are the salts having a very good water solubility, for in- stance with the boric and silicic acids, for the alkali metals, and with volatile acids, such as the hydrochloric and the nitric acids, for the other metals and transition metals.

The preferred organic compounds are the salts, endowed with a very good water solubility, of the

acetic acid, the formic acid and the glycolic acid, with the alkali and earth-alkaline metals and the transition metals.

Further, boron as boric acid and silicon as silica, can be present. The latter can be of the kind obtained by precipitation (by acidification of aqueous solutions of sodium silicate) or by piroly- sis of silicon tetrachloride under oxygen.

The preferred metals and transition metals are sodium, calcium, magnesium, aluminium, zinc, tita¬ nium, yttrium and zirconium.

Examples of the preferred precursors to carry out the present invention are aluminium nitrate, zinc acetate, magnesium acetate, zirconium oxychlo- ride, calcium nitrate, yttrium acetate, calcium acetate and colloidal silica.

The chemical composition can vary within broad ranges; the compounds which make the precursors of the desired glaze must be preferably water soluble; alternatively, some of them can be disposable in a solid, extremely divided, form, like for the col¬ loidal silica, eventually pre-dispersed in water.

The formulations necessary to carry out glaz¬ ing according to the present invention are obtained simply by dissolving or dispersing the compounds in

water, with or without heating and under stirring.

The content of dry material of these formula¬ tions can be varied within broad ranges, generally between 15 and 70% by weight, preferably at the highest concentration compatible with the solubil¬ ity and applicability to the support.

The precursors are mixed in the ratios re¬ quired by the final composition of the glaze, which is a function of the aesthetic and functional char- acteristics according to well known and consoli¬ dated art (L. Contoli, A. Brusa, Ceramica Informaz- ione, May 1991, N. 302, p. 273-282: Smalti ce- ramici, S. Stefanov, S. Batschwarov, Faenza Edi- trice 1991) . The formulations can further contain various additives necessary to stabilising the formulation itself (to avoid the separation of the phases, for example) or to control the reology in the applica¬ tion step or to adjust the water release or the penetrating speed after the application on the sup¬ port. Said compounds can be modified, natural or synthetic, polymers or surfactants, or pH adjusting agents.

It is further possible that the formulation contains other materials, for instance pigments,

which give the article special effects.

Now, it has been surprisingly found that it is sufficient to apply these aqueous formulations to the ceramic support and proceed to baking according a time-temperature cycle suitable to the desired physico-chemical characteristics (30-120 minutes at 500-1300°C, conveniently, in the case of monobak- ing, 35-60 minutes at 1200°C at most, to obtain glazed articles) . Said articles result endowed with very good aesthetic and functional characteristics.

The advantage obtained in the case of the por- celained stoneware is both technical/aesthetic and economic because the polishing step is avoided. In the case of the tiles obtained by glost firing, the advantage is mainly economic because the tem¬ perature of the formation of the glaze layer is comprised between 600° and 950°C, i.e. noticeably lower than the traditional technique (up to 1200°C) . The formulations can be applied following techniques known in the art, for instance by air- brushing, or distributed on the support while mov¬ ing or, further, by dipping paint screen process¬ ing. The amount of the formulation deposited on the

support can vary within broad limits, depending upon the desired effect and the applying technique, generally between 2 and 800 g/m ² , preferably be¬ tween 20 and 200 g/m ² of precursors, as dry mate- rial, corresponding to an amount comprised between 1 and 100 g/m ² and between 10 and 50 g/m ² as dry oxides and anhydrides, respectively.

The application can be made in only one or in more steps depending upon the thickness or the de- sired effect. The article thus treated is then sub¬ jected to baking which can be carried out, for in¬ stance, in monolayer roller ovens and with fast cy¬ cles of 35-120 minutes and generally at tempera¬ tures lower than the ones required in the known art for the development of the desired properties.

In fact, it is possible, for instance, to ob¬ tain very hard and shining glazes at temperatures of only 1200°C.

The final characteristics of the articles have a very good quality.

It is possible to obtain a very high abrasion resistance, from a functional point of view, evalu¬ ated according to the PEI method of the loss of ap¬ pearance to abrasion. The articles obtained by the method of the

present invention show a strength till values higher than IV PEI and a hardness otherwise unap¬ proachable (1200 at the Vickers microhardometer) . The formation of an interlayer completely pene- trated in the support assures a perfect adhesion.

It is possible to obtain, from an aesthetic point of view, a various range of surfaces: from the ones extremely shining (bright) and transpar¬ ent, to those ones dull and glazed, endowed or not with chromatic effects. It is further possible to obtain particular effects subsequently applying, before baking, two or more formulations, having a different composition, superimposed or geometri¬ cally combined, depending on the desired decora- tions.

To obtain chromatic effects, it is sufficient to add, to the liquid formulation, the suitable amount of a ceramic pigment (in a pre-milled form) generally 5-10% on the dry content of the liquid formulation, and to apply the new composition.

The application can be sprayed, for instance to obtain a uniform colour or applied by paint screen processing, when one wishes to reproduce a decoration, eventually with subsequent steps and different colours.

In the specific case of the porcelained stone¬

ware, the advantage of a colour obtained applying

only a thin surface layer is evident in comparison

with the traditional massive colouring of tiles.

The following examples illustrate the inven¬ tion without limiting it.

The Examples 1-6 have been reported as a table and compared with the traditional glazes.

Many of the applying examples of the present

invention, because of the perspectives of the

growth of this market, the importance gained by the porcelained stoneware and the difficulties which

its glazing entails, are referred to this support. It is possible, in any case, to replace such sup-

port with other ones, maintaining analogous advan¬ tages in the new glazing process.

Support: oxide composition (% by weight)

Si0 66.95

A1 ₂ 0 ₃ 23.51

Ti0 ₂ 0.15

Fe 0 ₃ 0.49

CaO 0.55

MgO 0.29

K ₂ 0 0.55

Na ₂ 0 3.35

Firing loss 4.16 The powders have been obtained by wet milling, atomising drying till a 5% residual humidity, and subsequent pressing using a press having isostatic plugs at 450 kg/cm ² . Subsequent drying with an out¬ let temperature of 80°C and a humidity of 1.5%. Crude dimension: 32.4 x 32.4 cm, 6 mm (thickness) (shrinkage: about 8%) .

Three kinds of compositions which respec- tively, after baking, originate a glaze having amorphous characteristics, i.e. glass, another one having crystalline characteristics, i.e. a ceramic material and a mixed vitreous-ceramic system, have been selected (Table 1, Examples 1-6) . The formulations have been prepared by adding precursor compounds to water and by heating under stirring at 65-70°C for 30 minutes and subsequent cooling at room temperature. The additives have been added at the end in the form of concentrated aqueous solutions (10-20%) .

The liquid formulations have been manually sprayed, by airbrushing, in more steps, maintaining the tiles at a distance of 40 cm, in order to de¬ posit the amount required for the glazing, as shown in Table 1.

5 minutes after the application of the formu¬ lations, the tiles have been dried at 140°C for 2 h in a hot air oven.

The dried tiles have been baked in a mono- layer roller oven with a baking cycle of 57 minutes and a maximum temperature of 1200°C.

After baking and cooling, the tiles have been examined as to the aesthetic and functional charac¬ teristics, as it is reported in Table 2. Standardised methods have been used when pos¬ sible, and reported, in any case, below Table 2.

The comparison has been effected registering the data of the evaluations effected on tiles ob¬ tained by the examples of the present invention and comparing them to those ones of the traditional glazed surfaces of shining onoporous (comparison 1) and mat monobaked (comparison 2) tiles, respec¬ tively.

The results of the comparison checks effected on unglazed porcelained stoneware (the same support and the same baking cycle used in the examples 1-6, both as such and after polishing, have been further reported.

(1) Carboxymethylated starch Lamberti - dray mate¬

rial 20%

(2) Hydroxymethylcellulose Hercules (Natrosol HHR)

(3) Purified carboxymethylcellulose Lamberti

(Carbocel MM 30)

(4) Glaze obtained by milling in a ball mill till

obtainment of a residual lower than on 40 micron

TABLE 2 follows

(1) Monolayer roller oven

(2) Specular; shining; semimat; mat

(3) Smooth, slightly irregular, orange skin, rough

(4) Transparent; veiled; dull

(5) Uniform, regularly distributed unevenness from

4 to 0 (the worst)

(6) Vickers microhardness, method ASTM E 384 (Test

method for microhardness of materials)

(7) Mohs hardness, gauge EN 101

(8) Loss of appearance to abrasion PEI, gauge EN

154

(9) Determined by detection with scanning electron

microscoping (SEM/EDX) of a section of a baked tile

(10) X analysis by punctiform energy dispersion

(EDS) at various depths (micron) starting from the

surface, in a section of a baked tile (the values

represent only the course of the variation and not

the real concentration, the data having been obtai¬

ned by punctiform analysis)

(11) Non-glazed support used for the examples 1-6

baked in the same conditions

(12) Support as (11), mechanically lapped after ba¬

king to obtain a specular or shining surface

(13) Thermic sudden change strength, gauge EN 104 and EN 105 (for the crack strength)

(14) Chemical attack strength, gauge EN 122/EN 106 (the chemical strength grows as the letters nuinber grows AA>A>B while the stainability grows as the number grows 1>2.

The Examples 1 and 2 show how it is possible to attain glazed tiles having a surface with an ap¬ pearance bright and shining (transparent coating) endowed with a sufficiently high degree of hardness and abrasion resistance.

The advantages attained, being able to use a porcelained stoneware-like support (comparison 3, deficiency: low hardness) , obtaining an appearance very close to polished porcelained stoneware (comparison 4) without resorting yet to the expen¬ sive step of polishing, are evident.

The Examples 3 and 4 illustrate how it is pos¬ sible to obtain glazes having a dull appearance and mats endowed with a very high mechanical resistance both in comparison with crude porcelained stoneware (comparison 3) and the dull monolayer traditionally glazed article (comparison 2) with the possibility to obtain aesthetic effects extremely appreciated associated with a high chemical resistance.

The X-ray analysis shows the appearance of tetragonal zircon which inherently possess charac¬ teristics of great hardness.

The Example 4 and 5 show a system of glaze precursors very versatile which permits to obtain both dull covering glazes having an exceptional hardness (example 5) and glazes having a medium hardness but very shining and bright (Example 6) .

The addition of nucleating agents (NaF or Zrθ2) helps the crystallisation of very hard com¬ pounds dispersed in a vitreous matrix, as it has been verified by the X-ray analysis which evidenced the formation of anorthite (pseudowollastonite) and magnesium spinels. The addition of borax helps instead the forma¬ tion of the vitreous phase improving brightness but decreasing hardness.

The glaze obtained, for all the examples (1- 6) , is endowed with a very good adhesion to the ma- trix of the support, as it can be noticed from the thickness of the interface, which is clearly seenable by scanning electron microscoping effected on sections of tiles normal to glazing. Also the punctiform X-ray analysis by dispersing power carried out on the same sections shows that

characteristic elements of the glaze (zinc oxide in the Example 6 or the ratio Al/Si in the Example 1) decrease as one enters the matrix, which means a deep penetration of the phases. The optical microscoping and the scanning electron microscoping effected on the glaze sur¬ faces show in any case a full coating with zones of imperfection very small and regularly distributed. EXAMPLE 7 Tests of coloured glazing and decoration have been carried out using the same support and manner described in the Examples 1-6.

1,6 parts by weight of a pre-milled pigment (95% lower than 1 micron) were added to 100 parts by weight of the liquid formulation of the Example 2 (formulation A 8 Na) :

7a) coral pink (Zr, Fe, Si) - CK 11138 (produced by Ferro Corporation, Cleveland, Ohio, USA) 7b) praseodymium yellow (Zr, Pr, Si) - CK 10032 (produced by Ferro Corporation, Cleveland, Ohio, USA) ;

7c) Blue (Zr, V, Si) - CK 15116 (produced by Ferro Corporation, Cleveland, Ohio, USA) 7d) cobalt green (Co, Cr, Zn) - CK 14126 (produced by Ferro Corporation, Cleveland, Ohio, USA) .

The so obtained mixture have been sprayed (300 g/m ² ) obtaining , after baking at the conditions of the Examples 1-6, surfaces shining and coloured, having a perfect hiding power and colour uniform- ity.

The same mixtures have been applied by paint- screen processing obtaining, after baking, an accu¬ rate reproduction of the decoration and character¬ istics of the decoration comparable with those ones of the common technique.

The example illustrates how it is possible to obtain coloured or decorated glazed surfaces, com¬ pletely comparable with those ones obtained by the expensive process of mass colouring. EXAMPLE 8

Glazing in glost firing.

The formulation of the Example 2 (A 8 Na) was sprayed (300 g/m ² ) on the surface of a pre-baked tile (biscuit) obtained starting from a common fur- ring malm (red clays) .

The piece was then subjected to glost firing in monolayer roller oven using: 8a) 60 minutes and maximum at 830°c 8b) 30 minutes and maximum at 1070°C.

A shining vitreous layer endowed with very good aesthetic characteristics was obtained in both cases.

The example illustrates how it is possible to obtain glazed surfaces endowed with aesthetic char¬ acteristics comparable with common glost firing (baking time 60 minutes and temperature 1050- 1070°C) , using the formulations of the present in¬ vention and baking temperature and time extremely lower.

EXAMPLE 9

Glazing of monobaking supports different from stoneware.

Glazed tiles, using the supports of the com- parative examples 1 (monoporous) and 2 (monobaking) , by spraying the liquid formulations described in the Examples 2 and 3, respectively, with the same weights in grams and methods of treatment therein described, were obtained. The tiles are endowed with aesthetic charac¬ teristics as much satisfactory as those ones of the comparative examples, having yet a hardness sur¬ face, abrasion resistance and chemical strength higher, and near to those ones of the Examples 2 and 3.

EXAMPLES 10

Comparison with the sol gel technique. Start¬ ing from tetraethyl silicate (10,4 g) and triethyl- borate (29 g) dissolved in ethyl alcohol (40 g) , by treatment, under stirring, with 1 g of 0,01M hydro¬ chloric acid and 1 g of water, a sol was prepared following the teachings of the WO 91-08179 patent. A solution of aluminium isopropoxide (2,1 g) in isopropanol (12,68 g) and ethylacetoacetate (2,5 g) was then added dropwise within 40 minutes. The mix¬ ture was subjected to sonicating for 15 minutes and subsequently zinc acethylacetonate hydrate (1,32 g) was added. The resulting mixture was maintained in a closed vessel for 4 days at room temperature. The oxide content of such mixture is equal to that one of the Example 2, as it results from the following table

The mixture was sprayed on a support for por¬ celained stoneware (Example 1-6) at a weight in grams of 300 g/m ² .

The support was then treated in oven at the same conditions of the Example 2. The surface was very irregular showing a great flaw which compro¬ mises the aesthetic appearance and the stainability with methylene blue and permanganate.

It has to be noticed that the process for pre¬ paring the sol is a complicated operation and that the mixture is based on easily inflammable organic solvents (about 8°C) , which is cause of danger both during the preparation and above all, in the apply-

ing phase (in ceramics, a process based on the use of the solvent would be unacceptable) .

Besides, the difficulty to obtain uniform ce¬ ramic layers, having a thickness greater than 1 i- cron, by sol-gel technique, was pointed out.