Patent Application
20060078712
The invention concerns a ceramic molded body of oxide-ceramic base material with a surface which is self-cleaning upon spraying or sprinkling with water and a process for the production thereof.
EP 0 590 477 B1 discloses a building material which can be for example an outside wall material or roof material, wherein a thin metal oxide film with a photocatalytic action is applied on the surface of the building material. The metal oxide film is preferably applied by means of sol-gel processes. A titanium dioxide thin film building material is preferably produced using titanium dioxide sol. The thin metal oxide film known from EP 0 590 477 B1 has deodorosing anti-mold properties.
By virtue of its film-like structure the metal oxide film known from EP 0 590 477 B1 is of a small surface area and accordingly has a low level of catalytic activity.
DE 199 11 738 A1 discloses a titanium dioxide photocatalyst which is doped with Fe3+ ions and which has a content of pentavalent ions, which is equimolar or approximately equimolar in relation to the Fe3+ ions. The titanium dioxide photocatalyst known from DE 199 11 738 A1 and doped with Fe3+ ions is produced by way of sol-gel processes.
EP 0 909 747 A1 discloses a process for producing a self-cleaning property of surfaces, in particular the surface of roof tiles, upon being sprayed or sprinkled with water. The surface has hydrophobic raised portions of a height of between 5 and 200 μm in distributed form. To produce those raised portions, a surface is wetted with a dispersion of powder particles of inert material in a siloxane solution and the siloxane is then hardened. The process known from EP 0 909 747 A1 makes it possible to produce a ceramic body having a surface to which particles of dirt can cling poorly. The ceramic body known from EP 0 909 747 A1 does not have any catalytic activity whatsoever.
WO 01/79141 A1 discloses a further process for producing a self-cleaning property of a surface and an article produced with that process. In accordance with that process, a metallorganic compound of titanium oxide is applied to a surface by means of a sol-gel process, the surface is dried and then subjected to heat treatment at elevated temperature. The surface of the titanium oxide layer can be subsequently hydrophobised.
The object of the present invention is to provide a molded body, in particular roof materials, which has an improved self-cleaning capability and improved stability such as for example improved resistance to abrasion.
A further object of the invention is to provide a process for the production of such an improved ceramic molded body.
The object of the invention is attained by a ceramic molded body of oxide-ceramic base material with a surface which is self-cleaning upon spraying or sprinkling with water, wherein the molded body has a porous oxide-ceramic coating, wherein the coating is photocatalytically active and has a specific surface area in a range of between about 25 mg2/g and about 200 m2/g, preferably between about 40 m2/g and about 150 m2/g.
Preferred developments of the ceramic molded body are set forth in appendant claims 2 through 18.
The object of the invention is further attained by a process for the production of a ceramic molded body of oxide-ceramic base material with a surface which is self-cleaning upon spraying or sprinkling with water, wherein the molded body has a photocatalytically active, porous, oxide-ceramic coating with a specific surface area in a range of between about 25 m2/g and about 200 m2/g, preferably about 40 m2/g and about 150 m2/g, wherein the process includes the following steps:
(a) mixing photocatalytically active, oxide-ceramic powder, adjusting agent and/or adhesive and a liquid phase to afford a suspension,
(b) applying the suspension produced in step a) to the oxide-ceramic base material to produce a layer, and
(c) hardening the layer afforded in step (b) to produce a photocatalytically active, porous, oxide-ceramic coating.
Preferred developments of that process are recited in appendant claims 20 through 43.
The ceramic molded body produced using the process according to the invention involves a highly suitable porosity and stability.
Unlike the sol-gel processes preferably used in the state of the art for the production of coatings, in accordance with the invention a suspension of photocatalytically active, oxide-ceramic powder with further components is applied to an oxide-ceramic base material. This therefore does not involve producing a film but a porous structure of large specific surface area.
The structure produced is a highly porous structure, that is to say the specific surface area of the catalytically active, porous, oxide-ceramic coating is in a range of between 25 m2/g and 200 m2/g, further preferably in a range of between about 40 m2/g and about 150 m2/g. More preferably the specific surface area is in a range of between 40 m2/g and about 100 m2/g.
With a specific surface area of about 50 m2/g a highly satisfactory catalytic activity in respect of the applied oxide-ceramic coating is achieved. In that respect the mean layer thickness of the oxide-ceramic coating is preferably in a range of between about 50 nm and about 50 μm, further preferably between about 100 nm and about 10 μm. A highly satisfactory catalytic activity is obtained with a layer thickness of about 1 μm.
The photocatalytically active, porous, oxide-ceramic coating according to the invention provides that mold, fungal and plant growth, for example moss, algae etc, bacterial contamination etc, which are deposited on or in the ceramic molded body are photochemically broken down and removed. The photocatalytic activity of the porous, oxide-ceramic coating is extremely advantageously adequate at ambient temperature to oxidise and thus break down the stated substances and contamination. The oxidised substances have a reduced adhesion capability and are easily flushed off the surface of the molded body according to the invention when subjected to the action of spraying or sprinkling with water.
It is assumed that the photocatalytically active coating can have an oxidative action on the one hand directly on the organic contamination and impurities. On the other hand it is assumed that the oxidative effect of the photocatalytically active coating is effected indirectly by the production of oxygen radicals which subsequently oxidise and accordingly break down the contaminating substances or impurities.
The self-cleaning action of the ceramic molded body according to the invention can be further enhanced if a surface structure with raised portions or depressions is arranged under the photocatalytically active, porous, oxide-ceramic coating and/or if the photocatalytically active, porous, oxide-ceramic coating itself has a surface structure with raised portions and recesses.
It has been found that ceramic surface structures with raised portions, preferably involving a predetermined distribution density, have a surprising self-cleaning property. The raised portions can also be hydrophobised so that the adhesion of hydrophilic soiling substances or contaminants is further greatly reduced.
The raised portions can be formed by the application of particulate material to the ceramic base material. In that respect preferably temperature-resistant crushed material is used as the particulate material, preferably selected from the group which consists of crushed stone, fire clay, clay, minerals, ceramic powder such as SiC, glass, glass chamotte, and mixtures thereof.
It will be appreciated that TiO2, Al2O3, SiO2, and/or Ce2O3 can be used as the particulate material. In that respect particles of a size in a range of up to 1500 nm, preferably between about 50 nm and about 700 nm, have proven to be highly suitable. In addition a particle size range of between about 50 nm and about 200 nm is highly preferred.
It is preferred that the raised portions or recesses are of heights or depths respectively in a range of up to 1500 nm, preferably between about 50 nm and about 700 nm, further preferably between about 50 nm and about 200 nm. In that way the raised portions can also be formed with the aggregation or agglomeration of smaller particles.
In that respect the particulate material can be fixed to the ceramic base material using adhesives. For example the adhesives used can be polysiloxanes which on the one hand fix the particulate material to the surface of the oxide-ceramic base material and on the other hand provide the produced coating with a superhydrophobic surface. The adhesive, for example the polysiloxane, is added in step (a) of the process according to the invention in production of the suspension.
If hydrophobisation of the surface of the coating is to be maintained, in that case the hardening operation in step (c) is not to be effected at a temperature of more than 300° C. If the temperature is increased above 300° C., that can involve thermal decomposition of the polysiloxane and the breakdown of the superhydrophobic surface on the photocatalytically active, porous, oxide-ceramic coating.
It will be appreciated that other adhesives, preferably of organic nature, such as for example carboxymethylcelluloses, can also be used.
When calcining the ceramic molded body which is usually effected in a range of between more than 300° C. and about 1100° C., the particulate material used for producing raised portions is subjected to the action of a temperature which results in superficial softening of the particle surfaces so that a sinter-like join is produced between the particulate material and the oxide-ceramic base material. In that respect it is for example also possible to add fluxing agents which reduce the sintering temperature.
The man skilled in the art is aware from EP 0 909 747, EP 00 115 701 and EP 1 095 023 of various possible ways of fixing particulate material on a ceramic surface. The contents of EP 0 909 747, EP 00 115 701 and EP 1 095 923 are hereby incorporated by reference thereto.
Preferably, the photocatalytically active, porous, oxide-ceramic coating is formed by using photocatalytically active, oxide-ceramic materials selected from the group consisting of TiO2, Al2O3, SiO2, Ce2O3 and mixtures thereof.
In accordance with a further preferred embodiment the above-mentioned photocatalytically active, oxide-ceramic materials may also be contained in the oxide-ceramic base body.
In accordance with a preferred embodiment the photocatalytically active, oxide-ceramic material in the coating and/or in the oxide-ceramic base material includes TiO2 or Al2O3, optionally in combination with further oxide-ceramic materials. For example mixtures of titanium dioxide and silicon dioxide, titanium dioxide and aluminum dioxide, aluminum dioxide and silicon dioxide and also titanium dioxide, aluminum oxide and silicon dioxide have been found to be highly suitable.
In that respect preferably titanium dioxide with an anatase structure is used as the titanium dioxide. The aluminum oxide used is preferably aluminum oxide C which is to be allocated crystallographically to the δ-group and has a strong oxidation-catalytic effect.
A suitable aluminum oxide C can be obtained from Degussa AG, Germany. For example AEROSIL COK 84, a mixture of 84% AEROSIL 200 and 16% aluminum oxide C has proven to be very usable in the present invention.
When using TiO2 in the oxide-ceramic coating it is preferable that the TiO2 is present at least in part in the anatase structure, preferably in respect of at least 40% by weight, preferably in respect of at least 70% by weight, further preferably in respect of at least 80% by weight, with respect to the total amount of TiO2.
TiO2 which is present in a mixture of about 70-85% by weight anatase and about 30-15% by weight rutile has proven to be highly suitable.
Preferably the TiO2 used in the present invention is obtained by flame hydrolysis of TiCl4 in the form of highly disperse TiO2 which preferably has a particle size of between about 15 nm and 30 nm, preferably 21 nm.
By way of example, it is possible to use for that purpose the titanium dioxide which can be obtained under the name titanium dioxide P25 from Degussa AG, Germany and which comprises a proportion of 70% anatase form and 30% rutile. Extremely advantageously titanium dioxide in the anatase form absorbs UV light at wavelengths of less than 385 nm. Rutile absorbs UV light at a wavelength of less than 415 nm.
In accordance with a preferred development the ceramic molded body according to the invention has a superhydrophobic surface.
It has been found that the self-cleaning property of the surface can be markedly improved if the photocatalytically active, porous, oxide-ceramic coating is provided with a superhydrophobic surface. The oxidised organic soiling substances are still more easily flushed away from the surface by spraying or sprinkling with water.
In accordance with the invention the term superhydrophobic surface is used to denote a surface with an edge angle of at least 140° for water. The edge angle can be determined in conventional manner at a drop of water of a volume of 15 μl, which is put on to a surface.
Preferably the edge angle is at least 150°, further preferably 160°, still further preferably at least 170°.
The photocatalytically active, porous, oxide-ceramic coating can be hydrophobised using Ormoceres, polysiloxane, alkylsilane and/or fluorosilane.
Preferably a mixture of SiO2 and fluorosilane is applied, thereby producing a superhydrophobic surface. That hydrophobising effect or the provision of a superhydrophobic surface is extremely advantageous for the self-cleaning property of the molded body according to the invention.
In accordance with a further preferred embodiment the superhydrophobic surface has raised portions. Those raised portions can be produced when applying the hydrophobising agent by a procedure whereby particulate material is added to the hydrophobising agent and that mixture is subsequently applied to the photocatalytically active, porous, oxide-ceramic coating.
When the surface is hydrophobised with the above-specified hydrophobising agents, the temperature may not be raised above 300° C. as that can then involve thermal decomposition, which has already been mentioned above, of the hydrophobising agents.
Therefore in accordance with the invention hardening is effected by calcining only when no hydrophobic surface has yet been applied to the photocatalytically active, porous, oxide-ceramic coating. If polysiloxane were used as an adhesive and subsequently the molded body were hardened by calcining, the surface usually has to be hydrophobised once again if a hydrophobic surface is to be afforded on the photocatalytically active, porous, oxide-ceramic coating.
Preferably the ceramic molded body is in the form of a roof tile, a tile, a clinker brick or a facade wall.
In the production according to the invention of a ceramic molded body the photocatalytically active, oxide-ceramic powder used in step (a) is preferably in a nano-disperse form. In that respect the particle size range of the oxide-ceramic powder in a range of between 5 nm and about 100 nm, further preferably between about 10 nm and about 50 nm, has proven to be highly suitable.
To produce the ceramic molded body according to the invention a preferably homogenous suspension is produced from oxide-ceramic powder, adjusting agent and/or adhesive and a liquid phase, by mixing. That suspension can be applied in a desired layer thickness to the oxide-ceramic base material.
The suspension may be applied to the oxide-ceramic base material for example by pouring, brushing, spraying, flinging and so forth. It will be appreciated that the oxide-ceramic base material can also be dipped into the suspension.
Preferably the suspension is applied in such a layer thickness that, after the drying and/or calcining operation, the result obtained is a ceramic molded body with a photocatalytically active, porous, oxide-ceramic coating in a thickness of between 50 nm and about 50 μm, preferably between about 100 nm and about 10 μm.
The layer thickness of the undried suspension is usually in a range of between about 0.5 μm and about 100 μm.
The oxide-ceramic base material may be a green body (uncalcined ceramic material) or a pre-calcined or calcined ceramic material.
Preferably organic viscosity regulators, for example carboxymethylcellulose, are used as the adjusting agent. Those viscosity regulators impart a suitable viscosity to the suspension so that it can be reliably applied to the ceramic base material in the desired layer thickness. Extremely advantageously, the organic adjusting agent, preferably the carboxymethylcellulose, burns when the operation of hardening the layer is effected by calcining in a temperature range of between more than 300° C. and about 1100° C. Due to burning of the organic adjusting agent, the formation of a porous structure in the photocatalytically active, porous, oxide-ceramic coating is favored. In that situation the organic adjusting agent preferably burns completely and preferably without residue, forming a porous structure.
Calcination of the layer produced in step (b) can be effected on the one hand by calcining the molded body in a calcining furnace or in a calcining chamber at a temperature of more than 300° C. to about 1100° C. In addition the calcining operation is preferably effected in a temperature range of between about 700° C. and about 1100° C.
The drying operation is effected at a substantially lower temperature than the calcining operation. Drying is usually effected in a temperature range of between 50° C. and 300° C., preferably between 80° C. and 100° C. In that temperature range an applied superhydrophobic coating is not broken down or destroyed.
When using adhesive there is preferably added to the suspension polysiloxane which promotes adhesion of the oxide-ceramic powder to the oxide-ceramic base material. Besides its adhesive effect polysiloxane also results in hydrophobisation of the structure. In addition addition of adhesive such as for example polysiloxane also produces an increase in the viscosity of the suspension produced in step (a) of the process according to the invention. Accordingly an adjusting agent does not necessarily have to be added when adding adhesive to the suspension in step (a). The viscosity which is adjusted using adhesive can be sufficient so that in step (b) the suspension can be applied to the oxide-ceramic base material, to form a layer.
The liquid phase used is preferably water.
In a further configuration of the process particulate material can also be added to the suspension produced in step (a). In this alternative configuration of the process, the raised portions which are advantageous in regard to the self-cleaning effect of the surface and also the catalytically active, porous, oxide-ceramic coating are produced in one step.
In a ceramic molded body produced in accordance with this alternative configuration of the process, there is then not a separate layer structure consisting of a layer with raised portions and, arranged thereover, catalytically active, porous, oxide-ceramic coating. Rather, the raised portions produced using particulate material and the photocatalytically active, oxide-ceramic components are present in substantially mutually juxtaposed relationship or intimately mixed with each other.
Optionally a hydrophobising agent can then also be added to that suspension so that superhydrophobisation of the oxide-ceramic surface is effected in the same step in the process. In this alternative form of the process the hardening operation can then be effected only by drying so that no thermal decomposition of the superhydrophobic surface occurs.
It will be appreciated that it is also possible firstly for the above-mentioned particulate material to be applied to the oxide-ceramic base material to produce raised portions and for it to be fixed to the surface of the ceramic base material by means of adhesive and/or sintering, for that surface which is prepared in that way and which has raised portions to be provided with a photocatalytically active, porous, oxide-ceramic coating using the process according to the invention, and for a superhydrophobic surface optionally to be subsequently produced on the photocatalytically active coating.
The hydrophobising agents used are preferably inorganic-organic hybrid molecules such as for example siloxanes, in particular polysiloxanes. In addition Ormoceres, alkylsilanes and/or fluorosilanes have proven to be suitable as the hydrophobising agents.
The hydrophobising agents can be applied by a suitable process, for example spraying, pouring, flinging, sprinkling etc. For example, firstly a hydrophobising solution or suspension can be produced using a preferably aqueous liquid phase. Optionally particulate materials can also be added to that hydrophobising solution or suspension if raised portions are to be produced in the superhydrophobic surface. That hydrophobising solution or suspension can then be applied in the above-described conventional manner.
The term superhydrophobic surface is used in accordance with the invention to denote a superhydrophobic layer, wherein the edge angle for water is at least 140°, preferably 160°, further preferably 170°.
In addition, a pre-drying step can also be carried out after application of the suspension produced in step (a) to the oxide-ceramic base material, prior to the calcining operation. In that pre-drying step the liquid phase, preferably water, can be removed by evaporation. That can be effected for example by heating, for example in a circulating air furnace or a radiant furnace. It will be appreciated that it is also possible to use other drying processes, for example using microwave technology.
The pre-drying step has proven to be advantageous in order to avoid cracking or tearing of the layer produced from the suspension, in the calcining operation.
After the calcining operation a superhydrophobic surface can then be applied in the above-described manner.
In a preferred embodiment, after the calcining step and the optionally implemented hydrophobising operation, it is possible to carry out a post-treatment of the photocatalytically active, porous, oxide-ceramic coating produced. The post-treatment is effected by irradiation with laser light, or NIR or UV light. That post-treatment can improve the adhesion between the photocatalytically active coating and the oxide-ceramic base material.
It has been found that the ceramic molded body according to the invention, besides an improved self-cleaning property, also has improved mechanical stability. There is the very great advantage that the catalytically active, porous, oxide-ceramic coating with a possibly superhydrophobic surface adheres very firmly and reliably to the ceramic base material. Thus when that coating is applied for example to roof tiles it is not destroyed or abraded when a person walks on the roof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 24 110.4 | May 2002 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE03/01743 | 5/28/2003 | WO | 9/12/2005 |
