DESCRIPTION

Technical Field

The present invention concerns the ceramic and glass-ceramic sectors and specifically the techniques used in such sectors to improve the mechanical characteristics of the glass- ceramic articles produced by pressing glass powders.

State of the Art

In the present status of the art, glass-ceramic materials share many properties both with glasses and with ceramics. Glass-ceramics are characterized by the presence of an amorphous phase and of one or several crystal phases; the latter are produced by a controlled crystallization which is different from spontaneous crystallization, which usually is searched in glass manufacture.

In general, glass-ceramics feature a crystallinity ranging from 30% to 90% and materials are obtained therefrom featuring interesting thermo-mechanical properties

Most glass-ceramics are produced in two steps: first glass is formed and is subsequently cooled down and heated up in a second operating step.

In this hot treatment, glass partially crystallizes. In most cases formation agents are added to the basic composition of the glass-ceramic which facilitate and control the crystallization process.

Considering that usually there are no pressure and sintering, glass-ceramics do not have holes, unlike sintered ceramics. In the glass sector a number of processes are known for long time which are carried out to enhance the mechanical characteristics of glass sheets.

It is known that glass features a high compressive strength, but a very poor resistance to tensile stresses; consequently, in the case of stresses that induce flexural, the outer layer that is subjected to traction has a very poor strength, and a fragile breakage phenomenon can easily be primed from that stretched surface.

For this reason a number of processes are aimed at obtaining a sheet of glass whose outer layers are compressed with respect to the inner layer; in this way, in the case of a flexural, the stressed outer layer is in a co-action status, given the pre-compression imparted thereto during the production step, which shall run out before the applied stress induces an actual tension status.

The above described effect is obtainable by using thermal or chemical hardening techniques, or by realizing rolled glasses or layered glasses.

A thermal hardening process consists of an abrupt cooling of glass, previously heated up to temperatures higher than the creep limit: in this way, at the start of a cooling down cycle, the outer layers of glass feature a temperature lower than the inner layers, which will consequently tend to shrink to a greater extent in the true cooling down step, thus inducing the above described internal tension status. This type of production technique requires the use of dedicated furnaces, capable of reaching very high temperatures and provided with advanced auxiliary equipment management systems .

In the case of a thermal hardening, glass is dipped into a bath of melted potassium salts at a temperature greater than 380 °C, i.e. below the glass transition temperature. In this way an ion exchange takes place between the potassium ions present in the bath and the sodium ions present on the surface of glass. The potassium ions have dimensions considerably bigger than the sodium ions, consequently the replacement of the latter by the former ones results in the occurrence of compression tensions onto the outer surfaces of glass, which are compensated for by tensile tensions inside the glass.

In order to perform a thermal hardening, a specific, very complex machinery is necessary, which is used to melt the mix of potassium salts; also, it is worth emphasizing that the introduction of glass sheets into said bath shall take place, the sheets being set in a vertical position in order not to bend them, consequently the machinery shall necessarily have big dimensions and shall be provided with handling robots suitable for operating in a high temperature environment. Therefore the complete process is complex and consequently expensive .

Rolled glasses are obtained by coupling sheets of melted glass and then rolling them by means of rollers. In this case it is necessary to have two smelting furnaces and a special feeding system which allows to feed the rollers with the three layers, the innermost one having a coefficient of thermal expansion greater than the outer ones. In the rolling process, the glasses to be coupled together shall feature similar compositions and shall feature the same workability range and the same viscosity, in order to be able to go through the feeding system at the same speed; said requirements contrast with the opposing requirement of having a different coefficient of thermal expansion for the innermost layer as compared to the outer ones in order to get the desired technical effect.

In the rolling process, it necessary to preset two different machining lines, instead of one only, to be able to produce sheets featuring different coefficients of thermal expansion simultaneously, starting from the raw materials up to the rolling plants.

Finally, layered glasses are produced by coupling sheets of glass with sheets of a plastic material, for instance polycarbonate or polyvynilbutyral , suitable for absorbing mechanical energy while increasing toughness and/or for retaining splinters should glass break; this solution is also extremely expensive in that, the production quantities being equal, costs increase because of the greater depreciations as well as of the greater management costs, including labor, energy, maintenance, etc..

In the glass-ceramic sector processes have been developed based on the physical principle of the different coefficient of thermal expansion in superimposed layers of glass-ceramic obtained by a multi-stage process, said processes are completely different indeed in the different sectors of application, the latter ranging from electronics to the production of big-size sheets for the building industry and the like.

In particular, patent document US 6699605 B2 describes a process, suitable for electronic cards, which comprises the following steps: preparing a mix of mineral components; melting this mix at a temperature from 1400 to 1500°C; rolling the melt material between cooled down rollers, so as to form thin sheets of glass; milling glass in a humid atmosphere to obtain fine particles; preparing different mixes each having a specific coefficient of thermal expansion, by adding a binder (polybutylmethacrylate ) , a platicizer (butylbencilphtalate ) and a solvent (toluene) up to forming a mixture; laying the different mixes by means of a doctor to obtain a plurality of green sheets, forming a pile of min. three to max. seven superimposed green sheets; heating and baking the pile.

In TW 219354 B a process is disclosed, also dedicated to the electronic industry, which comprises the previous preparation . of three sheets featuring different coefficients of thermal expansion, obtained by casting, thanks to the use of appropriate chemical agents to control rheology, and subsequently laying them on a surface by means of a doctor, to subsequently join them by rolling them together, high pressure and temperature being held for at least ten minutes. Both above mentioned technologies are suitable for producing extremely thin articles only, typical of the electronic industry, since they are based on the use of a preparation technique in a humid environment and a mixture laying mode based on the use a doctor. In these cases the thicknesses of the layers are in the order of magnitude of ten millimeters, because, in the case of thicker layers, the liquid part tends to infiltrate the lower layer thus deforming and jeopardizing it.

In particular, the process illustrated in US 6699605 B2 supports the formation of three superimposed soft layers, a process very difficult to implement because the liquid in the physical form of a sol shall become semi-solid before being able to lay a second layer, which is obtained by making the physical status be transformed from sol to gel immediately after laying the layer or via an intermediate drying, as in the case under examination.

This implies a very accurate control of rheology in the sol to gel transition, and consequently of the parameters that affect it, including for instance an extreme accuracy of the composition, of the water content in the mix, of the ambient temperature, etc.; this control could not be economically implemented in the production of big-size sheets.

The second method, the partial drying one, also implies a dimensional shrinkage of the laid film, with a consequent big difficulty in controlling the phenomenon in the second layer, without affecting the first one; such difficulty increases even more for the subsequent layers, furthermore the passage in the draying zone makes costs and production times increase .

Patent TW 219354 B focused on rheology. The different layers, after being formed, are dried and finally rolled together at 70°C and 2000 psi, through a process that is very difficult to implement.

No applications are known of the physical principle of the different coefficients of thermal expansion of superimposed layers of powders for the production of greater-size glass- ceramic articles, like those requested by the building industry.

Whereas glass-ceramic production processes exist which specify a distribution of several layers of powders in one die, such methods warranty dimensional tolerances below few percentages only in the case that every layer is individually submitted to pressing before laying the next one, as in the process described in WO 9846540 Al . However, such production sequence is complicated to implement because of the times and costs resulting from the plurality of successive pressing operations that are necessary.

In the ceramic industry, press feeding apparatuses are used with superimposed layers of powder featuring good accuracies. For instance, patent application EP 2318189 A2 discloses a particularly complete feeding and loading apparatus which allows to obtain numerous appearance effects thanks to the laying of several layers.

Objects and summary of the invention

An object of the present invention is to provide an innovative process to produce typically sheet-like glass- ceramic articles, characterized by a high strength to mechanical stresses, in particular by high toughness and ultimate flexural strength values.

The method according to the present patent application considerably differs from the processes used in the glass industry as well as from the processes called above. As a matter of fact, it does not require any rolling processes nor laying systems operating in a humid environment, but rather the proposed process is based on forming layers with the use of powders and machineries already used at the industrial level .

The articles produced according to the present process are particularly suitable for being used in the civil coating and paving sectors, especially in the urban paving one.

Generally, but not exclusively, by using the process according to the present invention paving sheets are produced having thicknesses in excess of 15 millimeters, values for ultimate flexural strength greater than 100 MPa, and toughness values greater than those which characterize stony materials .

In the innovative process according to the present invention said articles are obtained by pressing and subsequently baking glass powders resulting from milling raw materials of vitreous natures.

Very conveniently, before the compression step, are these powders subdivided into different stacks which are properly prepared by adding one or several additiving substances thereto suitable for modifying the coefficient of thermal expansion of the material, whereby every stack of powders has its own coefficient of thermal expansion.

By using a common press of a type used in the ceramic sector for compressing powders, the die of said press is loaded with three superimposed layers of glass powders properly additived as described above, an intermediate layer of powder being interposed between two outer layers, featuring a coefficient of thermal expansion greater than that of the outer layers. Following the compression step, an article of three-layer compacted article is obtained, wherein the intermediate layer features a thermal behavior different from that of the upper and lower layers.

In the cooling down step following the baking step, the latter following in turn the powder compression step, the intermediate layer should theoretically undergo a linear deformation (shrinkage) greater than the upper and lower layers, just because of the different thermal behaviors of the three layers.

However, such condition is not possible because said three layers are not bound to each other and the deformations should be consistent.

The final dimensional configuration of the article will thus correspond to a condition intermediate between that which would be got if the article were made from powders of the intermediate layer only and that which conversely would be obtained in the case of an article made from powders of the upper and lower layers only.

The finished product obtained according to the present process thus features, internally, a heterogeneous tensile^ status, the upper and lower layers being compressed whereas the intermediate layer is subjected to traction.

Such internal tensile status imparts to the article a greater flexural strength, thus countering the tensile breakage of the outer layers which would otherwise take place in the case of high flexural stresses.

In this way, in the case of flexural stresses, before the applied stress induces a real tensile status in the stressed outer layer, it is necessary to cancel the pre-compression status imparted to said outer layer upon producing it.

As compared to the techniques currently known in the status of the art, the present process makes it possible to obtain a sheet-like glass-ceramic article with superior mechanical characteristics and featuring a high commercial value, very resistant to flexural stresses and also usable for urban pavings, by using glass powders.

This object of the present invention and others are obtained by using a low cost production process which does not require the use of particularly expensive plants and machineries, nor the use of particularly advanced or accurate production cycle control systems, nor the use of big quantities of labor.

Machines already exist on the market which feed the press used in the ceramic industry with different layers of powders, to get special appearance effects; by using and slightly retrofitting such machines it is possible to feed the press, specified in the process according to the present patent application, with three layers of glass powders, wherein the intermediate layer is formed of a glass powder which reaches a coefficient of thermal expansion higher in the course of the baking and cooling-down process that transforms glass powder into glass-ceramic, the difference between the layers ranging from 0.2 x 10 ^~6 °C ^_1 to 2.5 x 10 ^-6 "C ^"1 .

Said precision feeding apparatuses of the presses of the ceramic industry transfer into the die the powders coming from one or several hoppers and are provided with special devices to cater for a homogeneous spread of the product on a surface. In the case of dry lyings, such devices concern, for instance, a constant reciprocal movement between the hopper and the laying surface, the shape of the runner and of the interception device, and the presence of elements interposed between the hopper and the laying surface, for instance grids, to foster material spreading. Some apparatuses possibly also comprise a brush to equalize the upper surface of the layer once the material is deposited.

The high mechanical performance glass ceramics obtained through the use of superimposed layers of powders featuring different coefficients of thermal expansion require dimensional tolerances, both in the individual layers and in the overall thickness, in the order of 1% or in no case greater than 2%, to prevent the articles from being broken or degraded. Such strict tolerances warranty homogeneous behaviors in the finished articles, besides preventing internal creep tensions exceeding the accepted ones from being generated upon cooling-down.

A preferred embodiment, which guarantees the necessary accuracies, specifies that the apparatus used to control the accuracy of the thicknesses of the powder layers be derived from a machine used to load and deposit the presses of the ceramic industry, said machine comprising a hopper, generally referred to as "tramoggino" (small hopper) , provided with a shutter .

In this embodiment, the exit of the powders from the hopper, such as to get an accurate distribution of the layers, is warranted by the presence of a inclined shutter, i.e. a shutter whose stroke occurs on a plane inclined with respect to the horizontal one, preferably by 45°.

Advantageously is the edge of said shutter ground and its movement controlled by a high accuracy, even micrometric, device .

Conveniently can the hopper be properly shaped in the area where powders go through, in order to prevent or limit the presence of points of stagnation.

In a preferred embodiment, the surfaces in contact with powders are coated by Teflon.

Another advantage offered by the process according to the present patent application consists in that the powders used to produce the glass-ceramic article can be obtained from a transformation of solid, even hazardous wastes. Brief description of the drawings

Fig. 1 schematically shows the theoretical shrinkage of three layers featuring different coefficients of thermal expansion in the free status.

Fig. 2 schematically shows the internal tensile status of the three layers shown in figure 1, formed because of the consistency of the deformations of the individual layers. Fig. 3 shows a diagram of an embodiment of the process according to the present patent application which comprises the following steps:

f . internally to a die of a ceramic press, using a precision feeding apparatus to place at least three layers of glass powder one over the other, each characterized by its own coefficient of thermal expansion, the intermediate layer being formed of a glass powder featuring the highest coefficient of thermal expansion among those used;

g. compressing said pile of glass powder layers to obtain a compact glass powder article;

h. heating said compact powder article until the glass powders are fully ceramized;

i. letting said article cool down.

Fig. 4 shows a diagram of an embodiment of the process according to the present patent application which comprises, before the steps described in the diagram in figure 3, the following steps:

d. milling a mix of vitrified material so as to obtain glass powder;

e. mixing it with one or several additiving substances, the latter being suitable for modifying, i.e. either increasing or decreasing, the coefficient of thermal expansion of the glass powder.

Fig. 5 shows a diagram of an embodiment of the process according to the present patent application which comprises, before the steps described in the diagram in figure 4, the following steps:

a. preparing a mix of loose materials substantially formed of non-metal inorganic substances;

b. milling and/or mixing and melting said mix up to the formation of melted glass;

c. letting glass solidify so as to obtain a mix of vitrified material.

Fig. 6 shows a diagram of an embodiment similar to that shown in figure 5, but which also comprises a step (dl) whereby a binder is added to and mixed with said glass powder in order to improve the cohesion capabilities.

Fig. 7 shows a diagram of an embodiment similar to that shown in figure 6, but which also comprises a step (d2) whereby preparations are added to get colors or to obtain special surface effects.

Detailed description of an embodiment of the invention

The following detailed description, which is made for purely explanatory non limitative purposes, with reference to the attached drawings, highlight the characteristics that are integral parts of the subject invention and the advantages deriving therefrom.

In its most essential form, the process according to the present patent application comprises the following steps: f. internally to a die of a ceramic press, using a precision feeding apparatus to place at least three layers of glass powder one over the other, each characterized by its own coefficient of thermal expansion, the intermediate layer being formed of a glass powder featuring the highest coefficient of thermal expansion among those used;

g. compressing said pile of glass powder layers to obtain a compact glass powder article;

h. heating said compact powder article until the glass powders are fully ceramized;

i. letting said article cool down.

According to a particularly practical solution, both glass powders used to make-up the layer adjacent to said intermediate layer feature the same coefficient of thermal expansion; in general, layers symmetrical with respect to the intermediate layer feature the same coefficient of thermal expansion.

However, it is worth noting that said coefficients might even be different if the effects of said difference are compensated for by different thicknesses of the layers.

According to a more complete sequence, each of said glass powders mentioned above can be realized according to the following step:

d. milling a mix of vitrified material so as to obtain glass powder;

e. mixing it with one or several additiving substances, the latter being suitable for modifying, i.e. either increasing or decreasing, the coefficient of thermal expansion of the glass powder.

In particular, said one or several additiving substances possibly belong to the group of the minerals and, very conveniently, are in a powdery form.

Note that said additiving substances usually include granite and/or alumina to increase the coefficient of thermal expansion or graphite and/or porcelain to decrease it.

Just as an example, remember that granite features a coefficient of thermal expansion of 9xl0 ^-6 °C ^_1 , whereas alumina and quartz feature a coefficient in the order of 8.5xl0" ⁶ °C ^_1 and variable according to orientation. Adding such finely milled components to the glass powder makes the coefficient of thermal expansion of the final glass-ceramic increase .

According to a particularly complete working cycle, steps (d) whereby a mix of even raw, vitrified material is milled, is preceded by the following steps:

a. preparing a mix of loose materials substantially formed of non-metal inorganic substances;

b. milling and/or mixing and melting said mix up to the formation of melted glass;

c. letting glass solidify so as to obtain a mix of vitrified material.

In particular, the mix prepared in step (a) could be made from wastes and/or hazardous wastes.

In this event, the exact computation of the coefficient of thermal expansion shall take account of the fact that quartz and granite can react with the remaining silicates that are formed during the transformation of glass into glass-ceramic, and consequently the final coefficient of thermal expansion of the mix strongly depends on the type and composition of the wastes used. Since the coefficient of thermal expansion of glass-ceramics made from hazardous waste can range from 6xl0 ^-6 °C ^_1 to 7.5xl0" ⁶ °C ^_1 , a mineral powder can be added to the glass powder in every formulation; in order to select the most appropriate mineral powder, it is necessary to take account of the initial formulation of the hazardous mineral wastes, including ashes from energy-from-waste-facilities , industrial muds, reclamation soils, etc., by calculating to what percentages will it react with the remaining components of the initial formulation.

To get better results, it is recommended that in step (d) the mix of vitrified material be milled in such a way as to get a glass powder featuring particle sizes smaller than 30 microns. The glass powder is then preferably submitted to micronization .

Immediately after said milling step (d) and before mixing step (e) a further step (dl) might be conveniently inserted, during which a binder is added to and mixed with said glass powder to improve its cohesion capabilities.

Said binder added in step (dl) belongs to the group of the clays, or of the thermoplastic materials, or of the synthetic waxes, or of the liquid thermosetting plastic materials.

In particular, if said binder belongs to the group of the clays, it includes kaolin in a percentage ranging from 3% to 10% and bentonite in a percentage ranging from 1% to 5%; conversely, if said binder belongs to the group of the synthetic waxes, it includes emulsifiable waxes in a percentage ranging from 0.2% to 1%; said binder might also include carbometylcellulose in a percentage ranging from 0.1% to 0.5% or polysaccharides, like starch, in a percentage ranging from 0.2% to 1%; said binder might also include gum arabic in a percentage ranging from 0.1% to 0.5%.

It is worth noting that, immediately after said step (dl), it is also possible to insert a further step (d2) in which preparations are added to get colors or to obtain special surface effects.

From the point of view of the features necessary to implement the process, remember that they are widely used, as a matter of fact in step (f) and in the subsequent step (g) use is made of a simple die in which the glass powders are loaded and of a press for the manufacture of sheet-like articles. In the process according to the present patent application step (h) comprises a heating at approximately 900 °C followed by a sequence of heating, stay, and cooling-down cycles at temperatures ranging from 900°C to 1100°C.

Furthermore, in step (h) baking is made in such a way as to obtain both sintering and crystallization of the compacted glass powders.