MICROPOROUS CERAMIC STRUCTURAL MATERIAL

The present invention concerns a method for the production of microporous ceramic structural material, the ceramic material obtainable by means of said method and uses thereof.

More particularly, the invention refers to the field of ceramic tiles, used to coat floors or walls.

In the present patent application, the term tiles is used to indicate plates cut according to different shapes and dimensions. The term ceramic is used, according to its common meaning, to indicate a product obtained, starting from a mixture of ceramic dusts, mainly based on clays and sand, by means of a production method comprising a formation step (through pressing or extruding) and a subsequent firing step. The formation step task is that of aggregating dusts, having a certaing degree of humidity. The firing step modifies the raw material structure, giving the final product the typical features of ceramic material regarding hardness, mechanical strength, chemical and phisical inertia.

As known, ceramic tiles can be classified according to their capacity of absorbing water. Water absorbtion is a meter of the material porosity; high levels of water absorbtion indicate a porous structure, while low levels of water absorbtion indicate a compact structure. Such a classification allows to directly derive other features of the ceramic material, in particular mechanical strength and modulus of rupture in bending. In fact, generally, the higher the material compatness, the higher its mechanical strength. On the other side, a high porosity generally is to the detriment of the material mechanical strength.

At present, ceramic materials are more and more frequently used not only for domestic application, in which the aesthetic aspect is predominant, but also in public spaces, in which cases the ceramic tiles consitute the ideal solution that ςan satisfy at the same time structural neeeds, such as breaking load, mechanical abrasion resistance and chemical resistance, but also aeshetic needs, since they can have different external appearance with regard to colour, luster, posibility or realising surfaces with particular outlinex.

In particular, when ceramic material tiles are used to realise floors, their surface slippereness is also important, which varies as an inversely proportional amount with respect to the material's coefficient of friction,

i.e., the more the coefficient of friction increases the more the slippereness dicreases. As known, the coefficient of friction depends on the nature of the contact surface, and, as an example, it is as much lower as much the surface is smoother and its tendency to be covered by a thin film of water is higher. This problem is much more important in the case of floors positioned in the open air and subjected to athmospheric precipitations.

At present, the slipping resistance of a material for making floors is improved by increasing its surface roughness, to give an example by realising surface reliefs, which permit the drainage of eventually present liquids. Moreover, in order to assure the floors always maintains the necessary slipping resistance, suitable slopes are realised, in order to make easier the deflux of water (due both to precipitations and washing).

However, the surfaces' features that allow to obtain a good slipping resistance (such as high roughness and presence of reliefs) obstruct the possibility to clean them effectively, since they constitute areas that are able to host and retain the dirt. Moreover, the surface's roughness and the presence of reliefs can be unsuitable for the aesthetic needs typical of the floor in a determined space.

In such context the solution according to the present invention is proposed, whose aim is to provide a microporous ceramic structural material having a high drainage capacity, able to prevent the formation of a film of water or of other liquids on its surface.

In particular, according to the present invention, it is provided a ceramic material that, notwithstanding its high porosity, at the same time presents high mechanical features and appreciable aesthetic featurex. besides, according to the present invention, the provided microporous ceramic material presents a low density opposite to high mechanical capacities and can be used for the construction of ventilated walls having a reduced specific weight. These and other results are obtained, according to the present invention, providing a microporous ceramic structural material, a mixture starting from which said material can be obtained and a particular process for the transformation of the starting mixture to ceramic. The present invention moreover regards floor or wall tiles realised with said material. It is therefore a first specific object of the present invention a method for the production of microporous ceramic structural material, in particular for the production of tiles, comprising the following phases:

- mixing ceramic dusts together with an amount, comprised in the range 1-30% (and preferably 8-15%) of the total weight of the obtained solid mixture, of grains of a formulation based on polyvinyl alcohol (FPVA) in which polyvinyl alcohol having a molecular weight comprised in the range 40000-400000 (preferably 60000-250000) and hydrolysis rate in the range 50-98% (preferably 75-90%), formulated together with an amount comprised in the range 10-25% (preferably 12-20%) by weight of the total polyvinyl alcohol formulation, of linear or branched polyols, including glycols, comprised in the range C ₂ -C ₄₀ (preferably C ₂ -C _2O , more preferably C3-C ₁₀ ), a amount comprised in the range 0-10% (preferably 2-7%) by weight of polyeters having molecular weight comprised in the range 100- 2000 (preferably 150-1000, more preferably 400-750) a amount comprised in the range 0-5% of water and a amount comprised in the range 0-4% (preferably 0,5-3%) by weight of a mixture of process additives, selected from calcium stearate, erucamide, oligosiloxane having a polymerisation ratio comprised in the range 5-50 (preferably 10-30), borax and silica;

- increasing the humidity ratio of said solid mixture with an amount of water comprised in the range 3-15% (preferably 5-10%) by weight of the solid mixture, supplied as an aqueous solution containing 0,5-30% (preferably 5-25%, more preferably 10-20%) by weight of said formulation based on polyvinyl alcohol (FPVA);

- pressing grains of said solid mixture by means of the application (preferably in dry conditions with isostatic buffers) of a specific pressure comprised in the range 50-350 (preferably 150-300) kg/cm ² ; - inertising the pressed grains, by means of progressive heating (with a gradient preferably comprised in the range 3-10°C/min) up to a maximum temperature comprised in the range 150-800 ⁰ C (preferably 200-600 ⁰ C);

- sinterising the grains, by means of further progressive heating (with a gradient preferably comprised in the range 3-7°C/min) up to a maximum temperature comprised in the range 1000-1250°C (preferably 1120-1220); and

- cooling the sinterised material. \

Preferably, according to the present invention, following the step of increasing the humidity ratio of the solid mixture and before the pressing step, the mixture is allowed to settle for a period of time of about 24 hours.

Moreover, according to the present invention, the dimensions of the ceramic dusts used in said mixing step varies and can reach maximum

values below 1 mm, and said grains of formulation based on polyvinyl alcohol (FPVA) have dimensions preferably below 0,40 mm.

Preferably, according to the invention, grain size of said solid mixture is comprised in a range (defineded in millimetres) selected between 0-0,25; 0-0,30; 0,05-0,30.

According to the present invention, the grain size distribution and relative percentage of the ceramic dusts (PC) and grains of formulation based on polyvinyl alcohol (FPVA) of said solid mixture have the trend summarised in Table 1 : Table 1 - Grain size distribution and relative ercenta e of PC and FPVA

further according to the invention, the grain size distribution of the grains of the solid mixture obtained by mixing together ceramic dusts (PC) and grains of formulation based on polyvinyl alcohol (FPVA) during the initial mixing step has the trend summarised in Table 2:

Table 2 - Grain size distribution of the mixture based on PC and FPVA

According to the present invention, said ceramic dusts contain the following amount of oxydes, defined as a percentage by weight relative to the total of ceramic dusts: SiO ₂ comprised in the range 50-75%, AI ₂ O ₃ comprised in the range 12-25%, CaO comprised in the range 0- 10%, MgO comprised in the range 0-6%, Na ₂ O comprised in the range 0- 5%, K ₂ O comprised in the range 0-4%.

It is moreover a second specific object of the present invention a ceramic material obtainable by means of the method previously defined.

In particular, according to the invention, said ceramic material presents a compressive breaking load comprised in the range 100-400 kg/cm ² and a drainage capacity comprised in the range 20-350 L/(m ² h).

In a first preferred embodiment, said ceramic material presents a compressive breaking load comprised in the range 100-200 kg/cm ² and a drainage capacity comprised in the range 200-350 l_/(m ² h). In a second preferred embodiment, said ceramic material presents a compressive breaking load comprised in the range 250-400 kg/cm ² and a drainage capacity comprised in the range 20-250 L/(m ² h).

In the first case, said ceramic material can be used for the production of wall coating tiles, whereas in the second case it can be used for the production of floor tilex.

More particularly, in the first case, said ceramic dusts (PC) being mixed together with said formulation based on polyvinyl alcohol, in the mixing step of the method previously defined, contain the following amount of oxides, defined in percentage by weight with regard to the whole ceramic dusts: SiO ₂ comprised in the range 50-65%, Al ₂ θ ₃ comprised in the range 12-20%, CaO comprised in the range 3-10%, MgO comprised in the range 2-6%, Na ₂ O comprised in the range 0-1%, K ₂ O comprised in the range 0-1%. In the second case, said ceramic dusts (PC) contain the following amounts of oxides: SiO ₂ comprised in the range 60- 75%, AI ₂ O ₃ comprised in the range 18-25%, CaO comprised in the range 0-1 %, MgO comprised in the range 0-1 %, Na ₂ O comprised in the range 1- 5%, K ₂ O comprised in the range 0\5-4%.

The invention will be described hereinbelow for illustrative, not limitative purposes, with particular reference to some illustrative examples, and to the following tables, wherein Table 3 shows the grain

size distribution of the grains of the formulation based on polyvinyl alcohol (FPVA), in Table 4 the grain size distribution of the ceramic dusts is reported (PC) and Table 5 shows the grain size distribution of the solid mixtures obtained by mixing together ceramic dusts (PC) and grains of the formulation based on polyvinyl alcohol (FPVA), with reference to the different examplex.

Table 3 - FPVA rain size distribution

Table 5 - Grain size distribution of the mixture based on PC

and FPVA

Example 1.

In a helical mixer a mixture was prepared being constituted by atomised ceramic dusts and a granulometric formulation based on polyvinyl alcohol (FPVA) having a grain size distribution of the kind reported in Table 3. The FPVA \formulation was constituted by 75% by weight of polyvinyl alcohol (PVA) having a molecular weight of 90000 and a hydrolysis rate of 88%, pentaerythritol (12% by weight), a mixture of oxy- ethylene glycols (mono-, di- and tri-oxy-ethylene glycol, 8% by weight), water (2% by weight), process additives (calcium stearate, oligosiloxane, borax and silica, 3% by weight). From the aforementioned combination a fluid solid mixture is obtained having a granulometric structure (with grain size distribution of the kind shown in Table 4) having the following composition percentage (defined by weight): white ceramic mixture (45%), black ceramic mixture (20%), blue ceramic mixture (25%), FPVA (10%).

An aqueous solution of FPVA (20%) was added to the solid mixture, up to a humidity of the mixture of 10%.

The mixture was subsequently put in a press with isostatic buffers, operated by applying a specific pressure of 200 kg/cm ² , in order to obtain articles with dimensions of 150 x 200 x 10 mm.

The formed articles subsequently underwent a quick firing inside a roller kiln, wherein in a first step the temperature was gradually increased, with a temperature gradient of 10°C/min, up to a maximum temperature of 700 ⁰ C, obtaining the complete FPVA removal after 90 minutes.

Subsequently, the temperature inside the kiln was increased, with a temperature gradient of 30°C/min, up to the maximum temperature of 121O ⁰ C, obtaining the complete sintering of the article after 60 minutes.

Tiles were obtained having the given dimensions, hourly drainage capacity of 200 L/m ² and mechanical strength (compressive breaking load) of 190 kg/cm ² .

Obtained tiles can be conveniently used for coating floors. Example 2

In a helical mixer a mixture was prepared which was constituted by atomised ceramic dusts and a granulometric formulation based on polyvinyl alcohol (FPVA) with dimensional distribution of the kind shown in

Table 3. The FPVA formulation was constituted by 80% by weight of PVA having molecular weight of 13.0000, hydrolysis rate of 78%, polyols mixture [tri-methylol propane (10% by weight), beta-cyclodextrine (6% by weight) and glycerol (2% by weight)] and process additives (calcium stearate, oligosiloxane, borax and silica, 2% by weight). From the aforesaid combination a fluid solid mixture is obtained having a granulometric structure (with grain size distribution of the kind shown in

Table 4) with the following composition percentage (defined by weight): white ceramic mixture (45%), black ceramic mixture (20%), dark blue ceramic mixture (23%), FPVA (12%).

An aqueous solution of FPVA 15% was added to the solid mixture, until the humidity of the mixture reached a value of 15%.

The mixture was subsequently put in a press with isostatic buffers, operated by applying a specific pressure of 180 kg/cm ² , in order to obtain articles with dimensions of 150 x 200 x 12 mm.

The formed articles subsequently underwent a quick firing inside a roller kiln, wherein in a first step .the temperature was gradually increased up to 800 ⁰ C, with a temperature gradient of 7°C/min, obtaining the complete FPVA removal after 110 minutes.

The following sintering was obtained in 60 minutes, by increasing the temperature inside the kiln up to the maximum temperature of 1200 ⁰ C, with a temperature gradient of 30°C/min.

Tiles were obtained having the given dimensions, hourly drainage capacity of 250 L/m ² and mechanical strength (compressive breaking load) of 160 kg/cm ²

Obtained tiles are particularly recommended for coating floors.

Example 3

In a helical mixer a mixture was prepared which was constituted by atomized ceramic dusts and a granulometric formulation based on polyvinyl alcohol (FPVA) having dimensional distribution of the kind shown in Table 3. The FPVA formulation was constituted by 70% by weight of

PVA having molecular weight of 60000, hydrolysis rate of 98%, 10% of

PVA having hydrolysis rate of 79% and molecular weight of 100000, polyols mixture [pentaerythritol (7% by weight), glycerol (3% by weight),

PEG 200 (7% by weight)] and process additives (calcium stearate, oligosiloxane, borax, erucamide and silica, 3% by weight). From the aforementioned combination a fluid solid mixture fluida is obtained having a granulometric structure (grain size distribution of the kind shown in Table

4) with the following composition percentage (defined by weight): white ceramic mixture (45%), black ceramic mixture (20%), blue ceramic mixture (25%), FPVA (10%).

An aqueous solution of FPVA (20%) was added to the solid mixture, until the humidity of the mixture reached a value of 10%.

The mixture was subsequently put in a press with isostatic buffers, operated by applying a specific pressure of of 200 kg/cm ² , in order to obtain articles with dimensions of 150 x 200 x 10 mm.

The formed articles subsequently underwent a slow firing inside an electric muffle kiln, with such a temperature gradient as to reach 250 ⁰ C in 60 minutes, then 45O ⁰ C in 60 minutes more, 800 ⁰ C after 120 minutes more and 1200 ⁰ C after further 120 minutes. Then, the temperature was maintained constant at 1200 ⁰ C for 10 minutes more. The complete FPVA removal was obtained after 110 minutes.

The following sintering was obtained in 60 minutes, increasing the temperature inside the kiln up to the maximum temperature of 1200 ⁰ C, with a temperature gradient of 30°C/min. Tiles were obtained having the given dimensions, hourly drainage capacity of 150 L/m ² and mechanical strength (compressive breaking load) of 250 kg/cm ² .

Obtained tiles are particularly recommended for coating floors. Example 4 In a helical mixer a mixture was prepared which was constituted by atomized ceramic dusts and a granulometric formulation based on polyvinyl alcohol (FPVA) having dimensional distribution of the kind shown

in Table 3. The FPVA formulation was constituted by 60% by weight of PVA having molecular weight of;60000, hydrolysis rate of 60%, 20% PVA having hydrolysis rate of 79% and molecular weight of 100000, polyols mixture [pentaerythritol (6% by weight), glycerol (4% by weight), PEG 400 (7% by weight)] and process additives (calcium stearate, erucamide, oligosiloxane, borax and silica 3% by weight). From the aforementioned combination a fluid solid mixture is obtained having a granulometric structure (grain size distribution of the kind shown in Table 4) and the following composition percentage (defined by weight): white ceramic mixture (45%), superwhite ceramic mixture (25%), dark blue ceramic mixture (22%), FPVA (8%).

An aqueous solution of FPVA (10%) was added to the solid mixture, until the humidity of the mixture reached a value of 12%.

The mixture was subsequently put in a press with isostatic buffers, operated by applying a specific pressure of 220 kg/cm ² , in order to obtain articles with dimensions of 150 x 200 x 11 mm.

The formed articles subsequently underwent a quick firing inside a roller kiln, wherein in a first step, by increasing the temperature with a temperature gradient of 15°C7min, up to a maximum temperature of 750 ⁰ C, the complete removal of FPVA was obtained in 80 minutes.

Subsequently, the temperature inside the kiln was increased, at a temperature gradient of 30°C/min, up to the maximum temperature of 121O ⁰ C, thus obtaining the complete sintering of the article after 60 minutes. Tiles were obtained having the given dimensions, hourly drainage capacity of 95 L/m ² and mechanical strength (compressive breaking load) of 300 kg/cm ² .

Obtained tiles can be used for coating floors. Example 5 In a helical mixer a mixture was prepared which was constituted by atomized ceramic dusts and a granulometric formulation based on polyvinyl alcohol (FPVA) having.dimensional distribution of the kind shown in Table 3. The FPVA formulation was constituted by 80% by weight di PVA having molecular weight of 205000, hydrolysis rate of 88%, pentaerythritol (12% by weight), PEG 400 (5% by weight)] and process additives (calcium stearate, oligosiloxane, borax and silica 3% by weight). From the aforementioned combination a fluid solid mixture is obtained

having a granulometric structure (grain size distribution of the kind shown in Table 4) with the following composition percentage (defined by weight): white ceramic mixture (40%), black ceramic mixture (23%), dark blue ceramic mixture (22%), FPVA (15%). An aqueous solution of FPVA (10%) was added to the solid mixture, until the humidity of the mixture reached a value of 10%.

A specific pressure of 160 kg/cm ² was applied to the mixture by means of a press with isostatic buffers, in order to obtain articles with dimensions of 150 x 200 x 8 mm. The formed articles subsequently underwent a quick firing inside a roller kiln, wherein in a first step, by increasing the temperature with a temperature gradient of 6°C7min, up to the maximum temperature of 75O ⁰ C, the complete removal of FPVA was obtained in 120 minutes.

In the second step, the temperature inside the kiln was increased, with a temperature gradient of 18°C/min, up to the maximum temperature of 113O ⁰ C, obtaining the complete sintering of the article after 60 minutes.

Tiles were obtained having the given dimensions, hourly drainage capacity of 300 L/m ² and mechanical strength (compressive breaking load) of 130 kg/cm ² . Obtained tiles can be used for coating walls.

The present invention was described for illustrative, non limitative purposes, according to some prefered embodiments thereof, but it is to be understood that any change and/or modification can be made by the persons skilled in the art without consequently escaping the relevant protection scope, as defined by the enclosed claims.