EP1694610A1

EP1694610A1 - Dispersion of a metal-oxide powder containing binding agent and layer obtained therewith

Info

Publication number: EP1694610A1
Application number: EP04801183A
Authority: EP
Inventors: Monika Oswald; Yi Deng; Klaus Deller; Rolf Clasen; Georg Fehringer
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2003-12-19
Filing date: 2004-12-01
Publication date: 2006-08-30
Also published as: WO2005058767A1

Abstract

Aqueous dispersion containing metal-oxide powder, a viscosity regulator and a water-soluble binding agent, it being possible for the viscosity regulator and the binding agent to be completely removed by thermal treatment. A metal-oxide layer obtained using this dispersion by means of a single coating operation, with a layer thickness of more than 2.5 μm on a substrate.

Description

Dispersion of a metal-oxide powder containing binding agent and layer obtained therewith

The invention provides an aqueous dispersion that contains a metal-oxide powder, a viscosity regulator and a binding agent . The invention further provides a layer on a substrate, which is obtained by using the dispersion.

It is known to apply metal-oxide layers onto a substrate by means of so-called sol-gel processes. One disadvantage of sol-gel processes is the fact that, by reason of the high , solvent content and the low solids content, severe shrinkage and cracking occurs, even with extremely slow and careful drying. A further disadvantage of this process is the fact that in the case of a single coating operation the layer thicknesses amount, as a rule, to less than 1 μm.

Furthermore, from EP-A-1 321' 444 a process is known in which layers can be produced starting from an aqueous dispersion that contains pyrogenically produced mixed oxide of silicon and titanium, and subsequent sinterinjg. These layers may exhibit distinctly higher layer thicknesses and may, at the same time, be very largely free from cracks. With this process it has proved to be a disadvantage that, even in the case of an optimisation of the process, a certain layer thickness cannot be exceeded when coating substrates if crack-free layers with slight roughness are to be obtained. It has become evident that the aqueous dispersion described in EP-A-1 321 444 is mainly responsible for this.

The object of the invention is to make available a dispersion that permits a substrate to be coated with a thick, crack-free layer of a metal oxide in a single coating step. A thick layer is to be understood to be a layer having a layer thickness of at least 1 μm. The invention provides an aqueous dispersion containing metal-oxiole powder, said dispersion being characterised in that it contains, in addition, a viscosity regulator and a water-soluble binding agent, it being possible for the viscosity regulator and the binding agent to be completely removed by thermal treatment .

The feature that it is possible for the viscosity regulator and the binding agent to be completely removed by thermal treatment is relevant in connection with the application of layers from the dispersion. In this case it has to be ensured tϊiat constituents of the dispersion, with the exception of the metal oxides, can be removed before or during the step of sintering the layer, since only in this way can h±gh-purity metal-oxide layers be obtained.

It has been found that all metal-oxide powders are suitable in principle, with pyrogenically produced metal-oxide powders being preferred. Pyrogenically produced metal- oxide powders are those which can be obtained by flame hydrolysis or flame oxidation of a metal-oxide precursor. In this connection, firstly highly dispersed, non-porous primary particles are formed which in the further course of the reaction are able to coalesce to form aggregates which are able to congregate further to form agglomerates .

Furthermore it may be advantageous if the metal-oxide powder is only slightly structured. This means that the primary particles exhibit only a low degree of intergrowth. For example, in this connection it may be a question of the silicon-dioxide powder Aerosil® OX 50 with a BET surface area of about 50 m²/g, which is produced by Degussa.

In principle, all pyrogenically produced metal-oxide powders are suitable for the dispersion according to the invention . The dispersion according to the invention may preferably contain Si0₂, Al₂0₃, Ti0₂, Zr0₂ and/or physical and/or chemical mixtures of the stated oxides. In particularly preferred manner the dispersion may contain silicon dioxide, which in the sense of the invention is regarded as a metal oxide. '

The BET surface area of these primary particles is, as a rule, between 1 m²/g and 600 m²/g. For the application of layers onto a substrate using the dispersion according to the invention it has proved to be advantageous if the BET surface area is between 5 m²/g and 200 m²/g.

The dispersion according to the invention may preferably exhibit a content of metal-oxide powder which is between 5 wt.% and 70 wt.%, relative to the total quantity of the dispersion. A dispersion with a content less than 5 wt . % is unsuitable for producing layers with a layer thickness of more than 2.5 μm. By reason of the high viscosity, a dispersion with a content of more than 70 wt.% is unsuitable for forming a layer. In advantageous manner the content of metal-oxide powder in the dispersion is between 10 wt.% and 50 wt.%, the range from 15 wt.% to 40 wt.% being particularly preferred. j

The dispersion according to the invention further contains a viscosity regulator. In this connection it is a question of acid-reacting or alkaline-reacting substances which are present in the dispersion in order to lower the viscosity thereof. Whether an acid-reacting or alkaline-reacting substance is present in the dispersion depends primarily on the metal oxide. In the case of aluminium oxide, an acidic viscosity regulator such as hydrochloric acid is preferred. In the case of silicon dioxide, both alkaline-reacting and acid-reacting viscosity regulators are suitable, such as ammonia, ammonium hydroxide and/or tetramethylammonium hydroxide or hydrochloric acid.

In the case where pyrogenic oxides are employed, it may also be possible to dispense with the addition of a viscosity regulator. As a rule, these oxides contain, to the extent that halogen-containing compounds were employed in the course of their production, residues of halide which are due to the process and which hydrolyse in an aqueous dispersion, forming a viscosity regulator.

The dispersion according to the invention further contains, a water-soluble binding agent. This may preferably be poly(vinyl alcohol) or methylcellulose . In this case it may be particularly preferred if these substances have a short polymer chain length, for example a poly (vinyl alcohol) with a mean molecular weight M_w of less than

20000 , or methylcellulose with a mean molecular weight M_n of less than 20000.

The proportion of the water-soluble binding agent may preferably be between 0.05 wt.% and 5 wt.%, relative to the quantity of metal-oxide powder. The range between 0.5 wt.% and 3 wt.% is particularly preferred. If the metal-oxide powder is aluminium oxide, it has proved advantageous to employ higher proportions, up to 20 wt.%, particularly preferably 10 wt.%, of binding agent.

The dispersion according to the invention may furthermore contain a defoaming agent that, like the viscosity regulator and the water-soluble binding agent, can be completely removed by thermal treatment. This may preferably be a defoaming agent based on fatty acids and hydrocarbons.

The content of the defoaming agent may preferably be between 2 wt.% and 200 wt.%, relative to the quantity of binding agent. A range between 10 wt.% and 100 wt.% may be particularly preferred.

However, it is possible to dispense entirely with a defoaming agent and instead, in case troublesome bubbles are present in the dispersion, to remove them by a vacuum treatment of the dispersion. The temperature at which the viscosity regulator, the binding agent and optionally the defoaming agent can be removed is preferably no more than 600 °C.

Particularly preferred, furthermore, is a dispersion according to the invention that contains by way of pyrogenically produced metal-oxide powder, a silicon-dioxide powder with a BET surface area between 5 m²/g and 200 m²/g, by way of viscosity regulator, tetramethylammonium hydroxide, by way of binding agent, poly (vinyl alcohol) with a mean molecular weight M_w < 20000, and by way of defoaming agent, one based on fatty acids and hydrocarbons .

Moreover, a dispersion according to the invention that contains by way of pyrogenically produced metal-oxidfe powder, a silicon-dioxide powder with a BET surface area between 5 m²/g and 200 m²/g, | - by way of viscosity regulator, tetramethylammonium hydroxide, by way of binding agent, methylcellulose with a molecular weight M_n < 20000, and by way of defoaming agent, one based on fatty acids and hydrocarbons is particularly preferred.

The invention further provides a process for producing the dispersion according to the invention, said process being characterised in that - in a first step a predispersion is generated, by the metal-oxide powder being charged, continuously or all at once, under dispersing conditions into a receiving flask that contains water, and by the viscosity regulator being added, during or after the addition of the metal-oxide powder, in such a quantity until the desired viscosity arises, in a second step there are added to the predispersion an aqueous solution of the binding agent in such a quantity that the proportion of the binding agent is between 0.05 wt.% and 5 wt.%, relative to the quantity of metal-oxide powder, and optionally an aqueous solution of the defoaming agent, and - optionally small quantities of viscosity regulator for the purpose of adjusting the original pH value in order to obtain the dispersion, production of the predispersion and of the dispersion being undertaken with like or different energy input.

Suitable for the purpose of predispersal and dispersal are, for example, dissolvers, toothed discs, rotor-stator machines, such as Ultra Turrax (manufactured by IKA) or those manufactured by Ystral, furthermore ball mills, agitator ball mills. Higher energy inputs are possible with a planetary kneader/mixer . The effectiveness of this system, however, is associated with a sufficiently high viscosity of the processed mixture in order to introduce the requisite high shear energies for the purpose of dividing- the particles. The input of very high energies is possible with high-pressure homogenisers .

With these devices, two predispersed streams of suspension under high pressure are decompressed through a nozzle. The two jets of dispersion come into exact collision, and the particles grind themselves.

In another embodiment the predispersion is likewise placed under high pressure, but the collision of the particles is effected! against armour-plated wall regions. The operation may be repeated arbitrarily often, in order to obtain smaller particle sizes. The invention further provides an individual metal-oxide layer that is obtained by using the dispersion according to the invention, having a layer thickness of at least 2.5 μm on a substrate. In preferred manner the layer thickness may amount to between 3 μm and 15 μm. The layer may be glass-like or ceramic.

Suitable substrates may be metal substrates and alloy substrates, materials having very low coefficients of thermal expansion (ultra-low-expansion materials) , borosilicate glass, silica glass or glass ceramic, with glass substrates being particularly preferred.

Layers having a surface roughness that is less than 30 nra may be preferred.

The invention further provides a process for producing the layer which is applied in a single coating step, said process being characterised in that an individual layer is applied by dip coating onto a substrate by means of the dispersion according to the invention, forming a green body, - the green body is subsequently dried, firstly at temperatures less than 100 °C, is subsequently heated to temperatures at which the binding agent and optionally the defoaming agent are partially or completely removed from the green body, ancl is subsequently sintered.

In advantageous manner the process according to the invention may be carried out in such a way that the drawing—rate at which the substrate is drawn out of the dispersion lies between 1 mm/s and 100 mm/s. A range between 2 mm/s and 40 mm/s is particularly preferred.

The sintering of the layer may be carried out in accordance with processes known to a person skilled in the art, for example by flame sintering or laser sintering. In the process according to the invention, laser sintering is particularly preferred. '

In this connection it may be advantageous to preheat the substrate to temperatures of about 300 °C .

In preferred manner the rate at which the green body is moved through under the laser beam lies between 1 mm/s and 10 mm/s.

The invention further provides the use of the metal-oxide ' layer according to the invention on a substrate for materials having very low coefficients of expansion (ultra- low-expansion materials, ULE materials) , for photocatalytic applications, as a coating for self-cleaning mirrors (superhydrophilic constituent) , for optical items such as lenses, as a seal for gases and liquids, as a mechanical protective layer, as use in composite materials., I The present invention describes an aqueous dispersion with which it is possible to coat substrates with a layer of a metal oxide having a layer thickness of more than 2.5 μm by dip coating in a single coating step. The layer that is obtained is free from cracks and exhibits a low surface roughness .

Examples A 30 weight-percent dispersion of pyrogenically produced silicon-dioxide powder (Aerosil® OX 50, Degussa AG) in bidistilled water is produced by means of a dissolver (Nl- SIP, VMA-Getzmann) .

Dispersion Dl: The dispersion produced above is subsequently adjusted to a solids content of 20 wt.% and a pH value of 9.7 by addition of further bidistilled water and tetramethylammonium-hydroxide solution. Dispersion D2 r The 30 weight-percent dispersion is diluted further by addition of a solution that contains methylcellulose with short polymer chain length (M_n of about 14000) a.s binding agent, of a defoaming agent (contraspum KWE, Zschimmer & Schwarz) and tetramethylammonium-hydroxide solution in such a way as to result in a dispersion having a content of silicon-dioxide powder of 20 wt.%. The proportion of methylcellulose in this dispersion amounts to 2 wt.%, relative to the silicon- dioxide powder; the proportion of defoaming agent amounts to 100 wt.%, relative to the binding agent. The pH value of the dispersion amounts to 9.2.

Dip coating

A borosilicate-glass disc (100 mm x 60 mm x 3 mm) is coated by means of a dip-coating device at drawing-rates of 2 mm/s and 40 mm/s by once-only dipping in dispersions Dl and D2 and is subsequently dried in contact with the air.

Subsequently the binding agent and the defoamer are partially removed from the disc coated with D2 at temperatures of about 250 °C over a period of 2 h. The disc coated with Dl without binding agent is treated analogously.

The green bodies obtained are subsequently sintered by means of a laser (C0₂ laser, Rofin-Sinar, RS700 SM) with a maximum power of 700 W and with a wavelength of 10.6 μm. To this end, the laser beam is moved over the surface of the coated material by means of movable mirrors . The speed of the mirror is adjusted in such a way that the laser beam is guided over the sample at a frequency of 200 Hz. In order to avoid stresses on the surface, it is advantageous to preheat the sample to 300 °C. The layers are sintered with a power density of 5.5 W/mm². Subsequently the layer thicknesses obtained are determined. Dispersion D3 is produced in a. manner analogous to that for D2, but with poly (vinyl alcohol) with short polymer chain length (Mw 9000 to 10000) by way of binding agent instead of methylcellulose. The dip coating is effected in a manner analogous to that for D2.

The Table shows clearly that the coating with dispersion containing binding■ agent results in distinctly higher layer thicknesses. At drawing-rates of 40 mm/s it was possible to apply layer thicknesses of more than 3 μm with a single coating step. After the thermal treatment for the purpose, of removing the binding agent, and after sintering, the layers obtained with D2 or D3 are free from cracks and, in comparison with the layer obtained from Dl, exhibit a lower surface roughness. The layers applied display a very good adhesion on the substrate after sintering.

Table: Maximum layer thickness of the layers obtained with Dl to D3 in μm j

* ^vdra = drawing-rate

By variation of the metal-oxide powder, of the BET surface area of the metal-oxide powder, of the solids content in the dispersion, of the aggregate size of the metal-oxide particles in the dispersion, of the type of the binding agent , of the proportion of binding agent , - of the drawing-rate in the course of dip coating higher layer thicknesses may also be obtained.

The sintered samples are examined with regard to their ^• mechanical and chemical properties .

In order to compare the fracture strength of the uncoated glasses with those of the coated glasses , double-ring bend tests are carried out in a manner following the model of DIN 52 292 . The measurements of the sample amount in this case to 60 mm x 60 mm. The evaluation was undertaken by the Weibull method.

The double-ring bend tests show that the fracture strength of glass discs can be improved by means of a layer of 0X50. The fracture strengths of the samples coated with dispersion D2 and D3 according to the invention are about 20 % to 30 % above that of the uncoated glass disc . In addition, the Weibull evaluation shows that the probability of failure, expressed by the higher gradient of the straight lines , of the coated samples is better . The reason lies in the fact that areas of surface damage to the substrates, which serve as crack initiators , are covered by the layer .

The chemical resistance is measured, by the layers produced with dispersions D2 and D3 according to the invention and an uncoated substrate being etched with a 40 percent hydrofluoric acid for 20 min. In order to be able to assess the excavation qualitatively in the course of etching, the depth of etching is determined with the aid of a profilomete . In this connection it becomes evident that that the depth of etching in the case of the uncoated glass substrate amounts to 24 μm, whereas in the case of the coated glass substrates^' the depth of etching amounts to 15 μm. This means that the chemical resistance of glass substrates is improved by the coating.

Dispersion D4 Alu C dispersion

30 g Aluminiuirioxid C (Degussa AG) is stirred into 243 g bidistilled water by means of a dissolver. Then 30 g methylcellulose solution (10 wt.% methylcellulose, Sigma M7140, in water) are added. The dispersion is put into an' ultrasonic disintegrator (Sonifier II, W-450) and dispersed therein for 3 min at about 450 W. As a result of the ultrasonic treatment, the introduction of bubbles in the course of dispersal is reduced. Subsequently the dispersion is subjected to aftertreatment at about 100 mbar for 10 min. As a result, residual bubbles are eliminated.

Aluminium sheets (AlMg₃; pretreatment : initial ettching with NaOH and/or sandblasting) and glass substrates are coated with this dispersion in a dip-coating apparatus at a rate of 40 mm/s.

The thicknesses of the layers, which are determined with a profilometer, lie within the range from about 5 μm to about 20 μm, independently of the substrate. The layers are free from cracks .

The roughness values of the aluminium-oxide layers on Al substrate lie within the range from 200 nm to 1000 nm for the mean roughness, and within the range from 1000 nm to 10000 nm for the peak-to-valley values.

The roughness values of the aluminium-oxide layers on borosiliate glass lie within the range from 7 nm to 40 nm for the mean roughness, and within the range from 100 nm to 500 nm for the peak-to-valley value. The layers are sintered with the aid of a C0₂ laser at a power density of about 5 W/mm², the substrate being moved through under the laser beam at about 5 mm/s. Layer thicknesses .of about 2 μm to 3 μm are obtained. In order to avoid stress cracks, the coated glass substrates are stored in a furnace at 500 °C for 6 h and are then cooled to room temperature within 8 h.

The fracture strength of the coated glasses is determined with the aid of double-ring bend tests (DIN 52 292) .

The abrasion behaviour of coated aluminium sheets is determined with the aid of the Taber Abraser process and compared with the abrasion of uncoated aluminium sheets.

After sintering, the layer thicknesses of the aluminium- oxide layers lie within the range amounting to about 50 % of the green thickness .

By virtue of. the aluminium-oxide layer the mechaijiical strength of the glass substrates is enhanced by 20 % - 50%. I

The abrasion resistance of the Al substrates can be enhanced by virtue of the aluminium-oxide layers (characterisation by "Taber Abraser test").

The aluminium-oxide layers enhance the chemical resistance of the aluminium substrates and glass substrates to lyes (NaOH) and acids (HF, HCl) .

Claims

Claims :

1. An aqueous dispersion containing metal-oxide powder, characterised in that it contains a viscosity regulator and a water-soluble binding agent, it being possible for the viscosity regulator and the binding agent to be completely removed by thermal treatment .

2. Aqueous dispersion according to Claim 1, characterised in that the metal-oxide powders are pyrogenically produced metal-oxide powders from the group comprising Si0₂, A1₂0₃, Ti0₂ , Zr0 and/or physical and/or chemical mixtures of the stated oxides .

3. Aqueous dispersion according to Claim 1 or 2 , characterised in that the content of metal-oxide powder is between 5 w . % and 70 wt.%.

4. Aqueous dispersion according to Claims 1 to 3 , characterised in that the viscosity regulator is selected from the group comprising NH₃, NHOH and/or NMe₄0H or hydrochloric acid.

5. Aqueous dispersion according to Claims 1 to 4, characterised in that the water-soluble binding agent is poly (vinyl alcohol) or methylcellulose.

6. Aqueous dispersion according to Claims 1 to 5, characterised in that the proportion of the water- soluble binding agent is between 0.05 wt.% and 5 wt.%, relative to the quantity of metal-oxide powder.

7. Aqueous dispersion according to Claims 1 to 6, characterised in that it contains a defoaming agent that can be completely removed by thermal treatment.

8. Aqueous dispersion according to Claims 1 to 7 , characterised in that the content of defoaming agent is between 2 wt.% and 200 wt.%, relative to the quantity of binding agent. I

9. Aqueous dispersion according to Claims 1 to 8, characterised in that it contains - by way of metal-oxide powder, a silicon-dioxide powder with a BET surface area between 5 m²/g and 200 m²/g, - by way of viscosity regulator, tetramethylammonium hydroxide or hydrochloric acid, by way of binding agent, poly (vinyl alcohol) with a mean molecular weight M_w < 20000, and by way of defoaming agent, one based on fatty acids and hydrocarbons .

10. Aqueous dispersion according to Claims 1 to 8, characterised in that it contains - by way of metal-oxide powder, a silicon-dioxide powder with a BET surface area between 5 m²/g and 200 m²/g, by way of viscosity regulator, tetramethylammonium hydroxide or hydrochloric acid, by way of binding agent, methylcellulose with a molecular weight M_n < 20000, and by way of defoaming agent, one based on fatty acids and hydrocarbons .

11. Aqueous dispersion according to Claim 9 or 10, characterised in that the proportion - of silicon-dioxide powder with a BET surface area between 5 m²/g and 200 m²/g amounts to 15 wt.% to 25 wt.%, relative to the dispersion, of tetramethylammonium hydroxide is chosen in such a way that the dispersion exhibits a pH value from 8.5 to 10, or of hydrochloric acid is chosen in such a way that the dispersion exhibits a pH value of 1.5-3.5, of poly (vinyl alcohol) or methylcellulose amounts to 0.5 wt.% to 1 wt.%, relative to the quantity of silicon-dioxide powder, and of defoaming agent amounts to 20 wt.% to 67 wt.%, relative to the quantity of poly (vinyl alcohol) or methylcellulose .

12. A process for producing the aqueous dispersion according to Claims 1 to 11, characterised in that in a first step a predispersion is generated, by the metal-oxide powder being charged, continuously or all at once, under dispersing conditions into a receiving flask that contains water, and by the viscosity regulator being added, during or after the addition of the metal-oxide powder, in such a quantity until the desired viscosity arises, in a second step there are added to the predispersion an aqueous solution of the binding agent in a quantity such that the proportion of the binding agent is between 0.05 wt.% and 5 wt.%, relative to the quantity of metal-oxide powder, and optionally an aqueous solution of the defoaming agent and optionally small quantities of viscosity regulator for the purpose of adjusting the original pH value in order to obtain the dispersion, production of the predispersion and of the dispersion being undertaken with like or different energy input.

13. An individual metal-oxide layer, obtained by using the dispersion according to Claims 1 to 11, with a layer thickness of at least 2.5 μm on a substrate.

14. Metal-oxide layer according to Claim 13, characterised in that the surface roughness of the layer is less than 30 nm.

15. A process for producing the metal-oxide layer on a substrate according to Claim 13 or 14, characterised in that an individual layer is applied by dip coating onto a substrate by means of the dispersion according to Claims 1 to 12, forming a green body, the green body is subsequently dried, firstly at temperatures less than 100 °C, is subsequently heated to temperatures at which the binding agent and optionally the defoaming agent are partially ^0:t: completely removed from the green body, and - is subsequently sintered.

16. Process according to Claim 15, characterised in that the drawing-rate at which the substrate is drawn out of the dispersion lies between 1 mm/s and 100 mm/s.

17. Process according to Claim 15 or 16, characterised in that the sintering operation is effected by means of a laser.

18. Process according to Claims 15 to 17, characterised in that in the case of sintering by means of a l^ser the substrate is preheated to temperatures of about 300 °C.

19. Use of the metal-oxide layer according to Claim 13 or 14 for materials having very low coefficients of expansion (ultra-low-expansion materials, ULE materials) , for photocatalytic applications, as a coating for self-cleaning mirrors (superhydrophilic constituent) , for optical items such as- lenses, as a seal for gases and liquids, as a mechanical protective layer, as use in composite materials.