WO2016185416A1

WO2016185416A1 - Method for decoration on ceramic tiles that are pressed but not dried

Info

Publication number: WO2016185416A1
Application number: PCT/IB2016/052935
Authority: WO
Inventors: Mariano Paganelli
Original assignee: Mariano Paganelli
Priority date: 2015-05-21
Filing date: 2016-05-19
Publication date: 2016-11-24
Also published as: ITUB20150733A1

Abstract

Method for decoration on ceramic tiles which provides for the decoration with digital printers on tiles glazed before drying. After the conventional pressing of the tiles using hydraulic or mechanical press (1) provision is made for applying, on the just-pressed tile, the coating layer constituted by engobe and glazes, or only smaltobbio, alternatively using spray, curtain coating or airless spray techniques (2), in a quantity such to not exceed a total amount of water over the piece surface of 0.1 grams per square centimeter, or specific preparations for the InkJet application (3) by digital printing. The method then provides for applying at least one decoration on said just-pressed tile, with multiple inkjet printing heads (3).

Description

TITLE: METHOD FOR DECORATION ON CERAMIC TILES THAT ARE

PRESSED BUT NOT DRIED

DESCRIPTION

FIELD OF APPLICATION OF THE INVENTION

The present finding only applies to the ceramic tiles obtained with the method of dry pressing obtained with the application of a strong specific pressure within a mold .

STATE OF THE ART

The technology of production of decorated ceramic tiles has been in continuous evolution since the dawn of civilization. At the start of the 1970s, it underwent a very significant acceleration with the invention of the quick single-firing method in single-layer roller kilns, according to the teachings of the patent IT1010205. This new technology allowed drastically reducing consumption, accelerating production cycles, hence reducing costs and making the glazed ceramic tile become a product of very wide consumption. The worldwide annual production is today estimated at over 12 billion square meters.

This technology consists of a series of successive processing starting from the preparation of a slightly moist powder of a suitable ceramic mixture. This moist powder can be obtained with various methods, but is characterized in that its humidity is generally comprised between 4% and 10%, depending on whether it was obtained through a dry preparation method or a wet preparation method. This slightly moist powder is then subjected to a forming method, through the application of a strong pressure, within a mold. This forming method is defined "dry forming" since the humidity percentage in the powder is very low. The "wet forming" method instead refers to formulations of ceramic mixtures containing up to 16% water, which are formed in the plastic state, as occurs for roof tiles and for bricks, and in some cases also for the "drawn" tiles such as for clinker and spaltplatten.

As stated above, the present finding is only applied to the ceramic tiles obtained with the method of dry pressing obtained with the application of a strong specific pressure within a mold.

Dry forming methods exist in which the ceramic piece is formed starting from low-moisture powders, by applying specific high pressures between two surfaces lacking lateral containment, i.e. without using a mold, as taught in the patents US7160092 and US7252489.

These technologies are used for making a kind of compacted mixture strip, which is subsequently cut in order to obtain the desired size.

The present finding is not applied to the strip or plate forming systems, for which specific decoration technologies have been developed; in some cases decoration occurs even directly on the just-compacted strip.

Decoration on non-dried ceramic materials is already used in some particular cases: for example, the roof tiles or bricks are often "dirtied" with solutions or suspensions of dyeing metal oxides or salts immediately after the drawing operation. In the drawing operation, the ceramic mixture is in the plastic state since it is very moist, so that it does not have the tendency to absorb the water with which it is wetted, it is not deformed and it does not break. The solution or the suspension, which contains the dying oxides or salts, is poured on the moist piece and produces a decorative effect as described in the patent US2191106.

If such an operation was executed on a non-dried raw tile of 10 mm thickness, dry pressed with 5% humidity, the surface wetted by the water contained in the suspension would readily absorb water, causing a swelling, with consequent bending until breakage of the piece under its own weight.

The application of glazes, engobes or decorations on a non- dried tile can be executed before pressing, by depositing on the surface of the powder mixture that must still be pressed the glaze pigments or granules with the same humidity as the mixture to be pressed. During the pressing, one obtains the penetration of the components of the decoration with the body of the tile. This technology was introduced in the 1980s, but then used only on particular productions and still today is indicated with the technological term "single-pressing", according to the teachings contained in the patents EP0444730 and EP0492733 and subsequent improvements. There was also the attempt to mold the decoration within the mold, such decoration then transferred onto the ceramic manufactured article during the pressing step according to the teaching of the patent US4501714.

Ceramic tiles obtained with the method of dry pressing within a mold represent nearly the entire worldwide production, and the universally adopted technological protocol provides that, immediately after the operation of forming in a mold and the subsequent extraction from the mold, the just-pressed tile is subjected to a drying operation. This protocol has never been the object of a patent, but lies within the body of knowledge acquired over the course of millennia of ceramic production.

The drying operation, within suitable drying systems, is necessary for increasing the mechanical strength of the just-formed piece, in a manner so as to render it strong enough to pass through the subsequent processing steps without being damaged . The dried tile attains a mechanical strength that is several times higher than that of the non-dried piece, and exits from the dryer at temperatures generally greater than 100°C. The piece is therefore very hot and rather strong in order to be able to tolerate the subsequent processing step.

The subsequent processing step consists of applying one or more coating layers which service to make the tile impermeable, easy to clean and above all attractive and decorative. There can also be multiple coating layers, defined with a multitude of specific technology names: engobe, glazes, smaltobbio, flaming, blasting, etc. Most of these preparations are applied in aqueous base, in the form of suspensions of very fine particles dispersed in water. The heat contained in the tile that just exited from the dryer allows evaporating the water contained in the suspensions.

If the tiles were cold, they would be soaked with water and they would break under their own weight.

As soon as the glaze layers are dried, one proceeds with the operation of decoration. For many years, decorations were applied with the technologies of serigraphy, intaglio or tampography, which provide for the contact between the mold means and the surface to be decorated. Over the last fifteen years, however, the digital decoration technique has been affirmed, which uses heads of "ink- Jet" type, due to which the print means does not come into contact with the surface to be decorated . We mention, with deference, the first patent on a ceramic ink for digital printing : US5407474; and the first patent of a digital printer for ceramic tiles: EP1038689, which were followed by many other patents.

The inkjet digital decoration technique has undergone an incredible development, which has brought it to supplant most of the preceding techniques, but it has not resolved one of the most difficult problems that affect tiles decoration: that of stability of the decoration tonality. Due to digital printers, it is today possible to accurately reproduce any image directly on the surface of the tile, but it is still difficult to reproduce the same image each time with the same color tones. Sometimes the tonality changes from one production batch to another and even during the production of the same batch. One of the main reasons for the tone variation in digital printing is the variation of the temperature and surface humidity of the tiles during the decoration step. The solution employed today is to insert cooling systems before printing which, in addition to being an extremely inefficient solution from the energy standpoint, sometimes represents a further element of variability.

Digital printers have considerably improved the clearness of the decoration, due to the development of print heads that attain exceptional resolutions, but they are negatively affected by surface temperature and humidity variations, since the size of the drops applied by a digital printer is a few dozen picoliters. Even small variations of the surface temperature and humidity lead to the variation of the color output. The definition of the image remains excellent but the tone changes. The human eye has the capacity to discriminate even tiny tone variations and this obliges tiles producers to select the production batches such that they have as uniform a tone as possible. The tone variations lead to the fragmentation of the production batches in the warehouse, with the consequent increase of the need to store and reduction of the productive flexibility.

The organic solvents

According to the state of the art, using the single firing production protocol, the dried tiles, after having been decorated, enter into the firing kiln. According to the currently used protocol, the step directly following the decoration is in fact the simultaneous firing step of the ceramic support and the layers that constitute the glaze, the decorations and the surface finishes. At this point, the organic liquids that constituted the liquid phase of the inks, and which have been absorbed by the dried raw tile, are exposed to a sudden increase of temperature that can cause radical transformations in their composition and chemical structure.

The firing kilns for ceramic tiles are, in most cases, fed with gaseous fuels (natural gas or LPG) or liquid fuels (naphtha), which must be mixed with great quantities of air (one part methane, 10 parts air). Thus, enormous volumes of exhaust fumes are generated, which must be evacuated . The hot fumes coming from the firing zones are used for pre-heating the entering pieces, but it is not possible to further lower the temperature at the inlet to the kilns, since at lower temperatures the formation of acidic condensates in the ventilators commences. These acidic condensates will then be removed by the suitable fume filters.

Actually, at the inlet of the firing kilns, the decorated tiles are hit by hot fumes, at over 300°C temperature. This high temperature does not cause any problem for the materials glazed and decorated with water-based suspensions, when the residual humidity in the raw tiles is less than 1%. The residual water quickly evaporates while the temperature of the material increases very quickly.

When the tiles are decorated or even glazed with suspensions with organic compound base, the situation radically changes. Even if the starting organic compounds are absolutely innocuous, when they are subjected to a quick heating, these molecules do not limit themselves to evaporating, but rather they crack and reform, giving rise to new uncontrollable organic compounds, which can be quite odorous, also strongly irritating and even toxic.

One of the carrier fluids most used in ceramic inks for digital printers is propylene glycol, which is universally recognized as absolutely non-toxic and safe, such that it is used in the preparation of injectable pharmaceutical products (Diazepam™), cosmetic products (Nivea Visage™) and as food additive (E1520). Nevertheless, it is known that, following processes of thermal degradation greater than its boiling temperature, it leads to the formation of propionaldehyde (propanal) which is a corrosive and irritating chemical compound [1] . If, instead, propylene glycol remains below the temperature of 188°C, it evaporates without being decomposed .

Another typical example of anti-settling liquid present in an ink for digital printers is ethylene glycol ether. This organic compound evaporates without being decomposed at the temperature of 135°C. But when these compounds are exposed to higher temperatures, they undergo a pyrolysis process, their molecules are cracked and reformed, giving rise to dimers and oligomers with unpredictable properties.

A typical example is DGBE (diethylene glycol butyl ether), which at room temperature is considered absolutely safe but over 170°C is decomposed into an uncontrollable mixture that contains aldehydes, ketones and organic acids. [2]

According to the teachings of the patent US20150037501, a ceramic ink for digital printers can be prepared with ethylene glycol. Ethylene glycol is per se toxic, and if ingested it can be fatal - even if only 30 milliliters are ingested . Ethylene glycol vapors are still very dangerous and it is absolutely necessary to avoid contact with skin and eyes. From a document available online by DOW Chemical [3] which provides the product, it is inferred that the thermal degradation of ethylene glycol leads to the formation of oxalic acid and formic acid, both extremely toxic. The boiling point however is 197°C, so that below this temperature it evaporates without being degraded. The vapors however remain irritating and, if in high concentrations, even toxic.

According to the teachings of the patent US7803221, a ceramic ink for digital printers can be prepared using as solvent PGMEA (PolyGlycolMonoMethylEtherAcetate). From the safety card (MSDS) of PGMEA [4], it is inferred that PGMEA emits toxic fumes in case of fire, i.e. in case of quick overheating, but that it evaporates at 140°C. The vapors of this chemical compound are irritating but not toxic.

This situation is generating considerable concern in the bodies set to control the environment (ARPA; EPA), which are evaluating the need to place a limit on the use of these substances when they are exposed to high temperatures, until a solution has been developed that allows definitively controlling this emission type.

The state of the art for the elimination of the dangerous organic substances from the emissions of industrial plants consists of a step of post-combustion of the fumes of the kilns: raising the temperature of the fumes over 800°C causes the complete combustion of all organic residues originally present in the gaseous flows, reducing the carbon dioxide emission and water. Nevertheless, this solution is very difficult to implement, since it involves a high cost in terms of energy consumption. A current average-size kiln for firing tiles emits into the atmosphere tens of thousands of cubic meters of exhaust gas each hour. The temperature of these gases, after the removal operations necessary for eliminating the powders and the acidic condensates, has lowered to about 120°C. Once again heating these volumes of gas to 800°C, signifies increasing the energy consumption tied to the firing step by at least 50-70%.

EXPOSITION AND ADVANTAGES OF THE FINDING

One object of the present invention is to provide the art with a method for decoration on ceramic tiles that are pressed but not dried, in the scope of a solution that improves the performances thereof.

Another object of the finding is the reduction of the organic compounds in the gaseous flows of the firing kilns of the ceramic tiles decorated with organic base inks.

It follows that, with the present method, there is an improvement of the stability of the tones in the production of ceramic tiles.

These and other objects are achieved due to the characteristics of the invention reported in the independent claim 1. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.

Said objects and advantages are all achieved by the method for decoration on ceramic tiles that are pressed but not dried, object of the present finding, which is characterized for that provided in the below-reported claims.

BRIEF DESCRIPTION OF THE FIGURES

This and other characteristics will be more evident from the following description of several embodiments that are illustrated merely by way of a non-limiting example in the enclosed drawing tables.

Figure 1 : illustrates the progression of the tile bending, with determination of the bending speed as a function of the time and temperature,

Figure 2: illustrates a plant scheme achieving the method that is the object of the invention.

DESCRIPTION OF THE FINDING

The present finding consists of a new technological decoration method which provides for the decoration of tiles formed by means of dry pressing within a mold, directly on the tiles just extracted from the mold, while they are still cold, and hence provided with very low mechanical strength and very high ability to absorb water.

At first sight, this could appear to be an application of already-known techniques, simply applied in a different order. It could appear to be a solution that is obvious for the man skilled in the art. In reality, however, this solution is so new and unprecedented that, if subjected to a man skilled in the art, he/she would consider it to be foolish : everyone knows that the tiles at the outlet of the press, before being dried, cannot be glazed and decorated, since they are too weak and since they would absorb too much water, and be bent and broken.

We surprisingly found that the just-pressed tile 1 can be subjected to an operation of application of a coating glaze using conventional application methods (curtain coating, spray 2) and it can also be decorated using the digital printing technique 3.

We surprisingly found that the water contained in the preparations of the ceramic coating glazes is absorbed by the raw tile just pressed with a very precise kinetics. Using the absolute optical deflectometer, of the type described in the patent application ITPR2013A000064, it was possible to follow the progression of the tile bending and determine the bending speed as a function of the time and of the temperature. See Fig. l . The experiment was conducted by applying 0.1 grams of water per square centimeter on a just-pressed raw tile with 10 mm thickness. The bending after 1 minute of wetting is equal to 0.5% of the sample length. This signifies that a piece with size equal to 60 cm x 60 cm will be bent 3 mm in 60 seconds. After these studies, we reached the conclusion that the initial wetting caused by the application of the glaze leads to a bending of the piece which is more than proportional to the quantity of water applied . If the quantity of water is very little, the bending progressively slows until it is stopped. There is a brief time interval during which the bending is manifested in a very limited way. If the piece is heated during this time interval, the bending is reabsorbed without any damage. The time interval allowed for executing the decoration operations can vary in accordance with the characteristics of the mixture and hence should be determined each time. Quite generally, it can be established that such time interval is less than 60 seconds.

The teaching of the present finding therefore consists of this working protocol :

Conventional pressing of the tiles using hydraulic or mechanical press 1 using powders, presses and molds that are already known

Application on the just-pressed tile of the coating layer constituted by engobe and glazes, or only smaltobbio, using conventional spray, curtain coating or airless spray techniques 2, in a quantity such to not exceed a total amount of water over the piece surface of 0.1 grams per square centimeter

Alternatively, the coating layer can be applied using specific preparations for the InkJet application 3 by digital printing

Application of the decoration with multiple inkjet printing heads 3

Application of the finishing layer with an airless spray application system 4 or with specific preparations for digital inkjet printing 3. The amount of water of said finishing layer must also be included in the calculation of the total water, which is less than 0.1 g/cm². If the latter is not possible, the application of the final finishing layer can be executed after the drying of the piece.

The maximum time for executing these operations is a function of the behavior of the just-pressed ceramic mixture, and must be determined for each mixture composition. As a general indication, it can be indicated that the time interval between the application of the first coating layer and the transition to the following drying step 5 is on the order of 60 seconds.

Unexpectedly, therefore, the technique of decoration directly on the just-pressed pieces, object of the present finding, provides surprising results with productive, economical and environmental implications that go well beyond the initial objectives.

The aesthetic outcome is identical to that of the tiles decorated after drying, but the stability of the decoration tone is enormously improved, since the piece is cold and the decoration is not affected by the surface temperature variations. The decoration with digital printers on tiles glazed before being dried does not negatively affect the resolution level of the digital printing, but it decidedly improves the stability thereof.

More in detail, the teaching of the present finding consists of the fact that the just-pressed tiles 1, glazed with a "smaltobbio", or with conventional engobe and glazes 2 and subsequently decorated using digital inks 3, do not directly enter the firing kiln, but pass through a drying step 4, necessary for removing the water contained in the just-pressed mixture. This heating step does not represent an additional thermal treatment, since according to the single firing technological protocol, the tiles are still dried 5 before entering the firing kiln 9. Hence there is no increase of the energy consumption. The drying can occur in a conventional dryer for raw tiles. The difference, with respect to the conventional single firing, consists of the fact that this drying step is executed after the decoration. These dryers normally function with maximum temperatures around 150-180°C.

A further extraordinary and unexpected advantage that is obtained is that at these temperatures, the organic solvents evaporate without undergoing cracking and reforming phenomena and exit from the stack of the dryer 6 without having been transformed into dangerous compounds.

If the organic liquids do not perse have dangerous or odorous characteristics, such as propylene glycol, the fumes emitted from the stack of the dryer 6 are not dangerous, since these organic components of the digital inks have not reached the temperatures which cause the decomposition into toxic, corrosive or simply irritating components (as they have unpleasant odor) .

The application of the present finding allows obtaining further advantages, due to the application of another teaching already known and already in use in other industrial sectors: the recovery of the solvent, already known according to the teachings of the patents US4788776A, EP0839569 and many others.

The hot air volumes emitted by a dryer are considerably less than those emitted by a firing kiln and lack acidic condensates. This allows employing devices, such as for example the insertion of a condenser 7 after the stack of the dryer 6 for capturing and condensing the solvents, as has already become common use in other industrial sectors. The condensation and the recovery of the solvent, instead of representing an industrial cost, can be transformed into a recovery of resources with the consequent reduction of the overall industrial cost while respecting the environment. A drying system can be specifically designed for optimizing the recovery of the solvents. In this case, it is suitable to use radiant heating systems, so as to drastically reduce the volumes of heated gases that must be cooled in order to condense the volatile compounds. Radiant heating is more costly than convection heating, but the economical benefits of the recovery of the solvents quickly make up for the expense.

The possibility of recovering the solvents leads to the possibility of using the digital technique even for the application of the coating glaze {"smaitobbio") and of the final finishing layer ("transparent" or "matte-like glaze" or other), using the digital technique 3. The possibility of precisely controlling the step of application of the coating layer allows obtaining a further improvement of the tone stability.

The preparations for digital printing contain about 50% solvents. By applying 300 grams on dry basis per m² of components for the digital decoration, the quantity of solvents used equals the dry fraction by weight. A typical modern production line produces 2.5 million m² per year. Considering 300 grams per square meter of solvents, the potential recovery can reach the impressive number of 750000 Kg per year, for an equivalent value at least equal to the same number in Euro. Even in the case that the quality of the recovered solvent is not sufficient for allowing its direct reuse in the preparation of the same inks, one would still obtain the considerable result of avoiding the introduction into the atmosphere of 750 tons of organic molecules, which in some cases are certainly not dangerous but could undergo transformations such to make them dangerous later. One need only recall the case of the fluorinated solvents, apparently innocuous for man, but which have demonstrated their danger for the entire planet several decades after their inconsiderate, widespread use around the world, due to a series of chemical reactions that were at the time completely unknown.

EXAMPLE No. 1

Production of tiles in 30x60 cm format, 10 mm thickness, classifiable according to ISO10545 standard in Bla class, following the production profile known as "glazed porcelain". The pressed pieces 1 are extracted from the cavities of the molds and sent, by means of suitable conveyance systems, to a decoration line with a belt conveyor; during said passage, a heating operation is provided for; with regard to said decoration line, this is constituted by an application of "smaltobbio" in aqueous suspension by means of a spray applicator 2, according to the known technique of the "airless" application, equal to 300 grams per square meter, followed by a belt conveyor with 8 bars of high-resolution ink-jet heads 3 fed with ceramic pigments suspended in an organic liquid with polyol and esterified polyol base.

The aforesaid surface heating operation is executed in order to lightly heat the just-pressed piece surface before applying the smaltobbio layer. Said operation is executed by making the pieces that just exited from the press pass under a radiant panel (electrical or gas).

The operation of depositing the coating layer and the decoration layer requires less than 60 seconds. Immediately after the decoration step, the piece not-yet-dried but already completely decorated is transferred onto a roller conveyor which conveys it into a heated multi-level, single-layer roller dryer 5, with direct introduction of hot air generated by a series of gas burners. The temperature at the roller dryer inlet is 120°C, the maximum temperature reaches 180°C and the stay time is 30 minutes. The hot and moist fumes exiting from the stack of the dryer 6 contain the vapors of the organic components that constituted the liquid phase of the glaze and digital inks. These organic compounds are still in their original molecular form, since they have not undergone molecular degradation processes that occur at higher temperatures. Since these are materials generally used in the cosmetic, pharmaceutical and food industry, they are classified as non-harmful, non-irritating and non-allergenic materials. Therefore, also their vapors, extremely diluted in great volumes of moist fumes, remain non-harmful.

At the outlet of the dryer, the pieces are completely dried and lack the organic component of the liquid phase that was contained in the products for digital printing . At the outlet of the dryer, the pieces, still very hot at temperatures of over 100°C, are conveyed onto a belt conveyor line and subjected to a further decoration step, with the application of a thick layer of coating glaze in aqueous suspension 8, which allows the final step of polishing (lapping) that is executed after the firing. The water contained in the coating glaze quickly evaporates, as normally occurs in glazing processes on the dried material, typical of single firing .

The pieces are then sent to subsequent processing steps, following the conventional protocol : firing 9, possible grinding, possible lapping, possible brushing, selection, building and palletizing.

Following the complete evaporation of the organic component during the first drying step at low temperature before insertion in the kiln, the fumes exiting from the stack of the firing kiln 10 do not contain odorous or harmful organic compounds deriving from the thermal degradation process of the organic compounds contained in the inks for digital printing.

EXAMPLE No. 2

Production of coating tiles with 25x75 cm size and with 8 mm thickness, classifiable according to standard ISO10545 in class Bill, with porous mixture following the production protocol known with the technological term "monoporous".

The just-pressed tiles 1, immediately after extraction from the mold, are conveyed onto a conveyor belt. The conveyor belt passes under a machine for digital printing 3 equipped with six inkjet printing bars provided with ink-jet heads with nozzles with high ejection volume. These heads are fed with a suspension of coating glaze termed "engobe". The anti-settling fluid is constituted by a mixture of esters and glycols. Subsequently, the material passes under a series of 6 printing bars provided with high- resolution ink-jet heads 3 for color printing. The pigments are of inorganic nature according to the state of the art. The anti-settling fluids are constituted by a mixture of esters and glycols.

The operation of deposition of the coating layer and the decoration layer requires less than 45 seconds. Immediately after the decoration step, the piece not-yet-dried but already completely decorated is transferred onto a roller conveyor that conveys it into a suspended tray dryer 5 heated with a gas burner and supplemented with hot air coming from a heat exchanger installed in the cooling zone of the firing kiln. The temperature at the suspended tray dryer inlet is 80°C, the maximum temperature reaches 170°C and the stay time is 45 minutes. The hot and moist fumes exiting from the stack 6 of the dryer contain the vapors of the organic components that constituted the liquid phase of the glaze and digital inks. These organic compounds are still in their original molecular form, since they have not undergone thermal degradation processes that occur at higher temperatures. Since these are materials generally used in the cosmetic, pharmaceutical and food industry, they are classified as non-harmful, non-irritating and non-allergenic materials. Therefore, also their vapors, extremely diluted in great volumes of moist fumes, remain non- harmful and do not cause disturbance, since they do not contain odorous substances.

The next step of processing, after exiting the dryer, consists of applying a coating layer of a shiny and transparent glaze ground in aqueous suspension (crystalline). The dried tiles exit from the dryer at the temperature of 120°C. The first step of application consists of spraying finely nebulized water on the previously- decorated surface. This application of water 8 is indispensable for improving the adhesion of the subsequent glaze layer, which, in this manner, encounters a surface that is already wet. As soon as the vapor development has terminated (which was caused by the application of water), the coating glaze is applied by using a spray application device 8. This coating glaze reaches complete melting in the subsequent thermal treatment that occurs in the firing kiln 9 and confers a shiny and brilliant appearance to the product, which enhances the underlying decoration and confers depth to the design.

By operating according to the teachings of this finding, the fumes emitted by the stack of the firing kiln 10 do not contain organic compounds coming from the thermal degradation of the organic products used as anti-settling agents in the inks and glazes for digital printing . The fumes coming from the kiln are subjected to the treatments of depulverization and removal of the acidic inorganic components (HF, HCI, H2S04) provided by the current environmental laws, but completely lack organic compounds with uncontrollable composition, which could derive from the thermal degradation of the inks for digital printing.

EXAMPLE 3

Production of floor tiles made of red paste, classifiable according to standard ISO10545 in class BII, with the technique known as single fired floor with dry grinding .

The just-pressed tiles, starting from dry ground mixture powders wetted with the addition of 6% water, immediately after the extraction from the mold 1 are conveyed onto a conveyor belt. The conveyor belt passes under a digital printing machine 3 equipped with six printing inkjet bars provided with ink-jet heads with high injection volume nozzles. These heads are fed with a suspension of coating glaze termed "smaltobbio". The anti-settling fluid is constituted by a mixture of polyesters, according to the teachings of the patent WO2012116878. Subsequently, the material passes under a series of 6 printing bars provided with high- resolution inkjet heads for color printing . The pigments are of inorganic nature according to the state of the art. The anti-settling fluids are constituted by a mixture of polyesters and other chemical additives. Immediately after, the material passes under 2 bars equipped with digital printing heads with high discharge volume for the application of the final finishing layer. This complete glazing and decoration operation lasts less than about 30 seconds.

As soon as the decoration application step has terminated (which comprises applying all the decoration layers), the tile is sent to a drying system 5 provided with infrared heating and vapor cooling systems 7. The vapors that contain the aqueous component coming from the drying of the just-pressed tiles, and the organic solvents, coming from the inks used for the digital printing, are made to pass into a plate exchanger 7, which causes the condensation of the liquid components.

Considering a production of 8000 m² per day, with application of dry 300 grams of inks that contain 50% organic solvents, the system of condensation of the fumes of the dryer is potentially able to recover up to 2400 Kg per day of organic solvents. The efficiency of the recovery via condensation is strongly affected by the chemical affinity between the molecules of the solvents and water. In any case, the organic solvents are not introduced into the atmosphere.

Cited documents

[1] Mechanisms of Propylene Glycol and Triacetin Pyrolysis Teodoro Laino,*,† Christian Tuma,† Philippe Moor,* Elyette Martin,* Steffen Stolz,*,§

and Alessandro Curioni†

†IBM Research-Zurich, Saumerstrasse 4, 8803 Ruschlikon, Switzerland

iPhilip Morris International R&D, Quai Jeanrenaud 5, 2000 Neucha el, Switzerland

§University of Twente, Faculty EEMCS, P.O. Box 217, 7500 AE Enschede, The Netherlands]

[2] The DOW Chemical Company - Form No. 233-00262-MM- 1214X - Product Safety Assessment: Diethylene Glycol Butyl Ether

Revised : December 23, 2014

[3]

http ://dowac.custhelp.com/app/answers/detail/a_id/11979

[4] HoecstCelanese AZ1500 thinner MSDS

Claims

C LA I M S

1. Method for decoration on ceramic tiles characterized in that it provides for the decoration being realized with digital printers on dry-pressed tiles glazed before drying.

2. Method according to claim 1, characterized in that the pressed tiles 1, glazed 2 and decorated using digital inks 3 do not directly enter the firing kiln but pass through a drying step 4, necessary for removing the water contained in the just-pressed mixture; said drying step being executed after the decoration, with maximum temperatures not exceeding 200°C.

3. Method according to claim 1, characterized in that the tiles decoration occurs directly on the tiles extracted from the mold and while they are still cold .

4. Method according to claim 1, characterized in that it provides for the execution of:

a. A conventional dry pressing of the tiles using hydraulic or mechanical press (1)

b. The application, on the tile just pressed, of the coating layer constituted by engobe and glazes, or only smaltobbio, using

i. spray, curtain coating or airless spray techniques (2), in a quantity such to not exceed a total amount of water over the piece surface of 0.1 grams per square centimeter ii. specific preparations for InkJet application (3) by digital printing

c. The application of at least one decoration on said just- pressed tile, with multiple inkjet printing heads (3)

5. Method according to claim 4, characterized in that it further provides for the application of at least one finishing layer with an airless spray application system (4) or with specific preparations for digital inkjet printing (3); and wherein the amount of water of said finishing layer must be included in the calculation of the total water, which is less than 0.1 g/cm2; if the latter is not possible, the application of the final finishing layer is executed after the drying of the piece.

6. Method according to claim 4, characterized in that the time interval between the application of the first coating layer and the transition to the following drying step 5 is on the order of 60 seconds.

7. Method according to claim 4, characterized in that the maximum time for executing the operations is a function of the behavior of the just-pressed ceramic mixture and is determined for each mixture composition.

8. Method according to claim 4, characterized in that it further provides for the condensation and the recovery of the solvent which evaporates during said drying step.

9. Method according to claim 4, characterized in that said drying step uses radiant heating systems, reducing the volumes of heated gases that must be cooled in order to condense the volatile compounds.