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WO2008068062A1 - Dispersion containing titanium dioxide - Google Patents

Dispersion containing titanium dioxide Download PDF

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Publication number
WO2008068062A1
WO2008068062A1 PCT/EP2007/058930 EP2007058930W WO2008068062A1 WO 2008068062 A1 WO2008068062 A1 WO 2008068062A1 EP 2007058930 W EP2007058930 W EP 2007058930W WO 2008068062 A1 WO2008068062 A1 WO 2008068062A1
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WIPO (PCT)
Prior art keywords
titanium dioxide
dispersion
surface area
dispersion according
aminoalcohol
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/EP2007/058930
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French (fr)
Inventor
Wolfgang Lortz
Gabriele Perlet
Uwe Diener
Sascha Reitz
Reinhard Vormberg
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Evonik Operations GmbH
Original Assignee
Evonik Degussa GmbH
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Filing date
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Publication of WO2008068062A1 publication Critical patent/WO2008068062A1/en
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
    • C04B41/81—Coating or impregnation
    • C04B41/85—Coating or impregnation with inorganic materials
    • C04B41/87—Ceramics
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/22—Surface treatment of glass, not in the form of fibres or filaments, by coating with other inorganic material
    • C03C17/23—Oxides
    • C03C17/25—Oxides by deposition from the liquid phase
    • C03C17/256—Coating containing TiO2
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
    • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
    • C04B41/5025—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials with ceramic materials
    • C04B41/5041—Titanium oxide or titanates
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D17/00—Pigment pastes, e.g. for mixing in paints
    • C09D17/004—Pigment pastes, e.g. for mixing in paints containing an inorganic pigment
    • C09D17/007—Metal oxide
    • C09D17/008—Titanium dioxide
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D7/00—Features of coating compositions, not provided for in group C09D5/00; Processes for incorporating ingredients in coating compositions
    • C09D7/40—Additives
    • C09D7/60—Additives non-macromolecular
    • C09D7/61—Additives non-macromolecular inorganic
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D7/00—Features of coating compositions, not provided for in group C09D5/00; Processes for incorporating ingredients in coating compositions
    • C09D7/40—Additives
    • C09D7/66—Additives characterised by particle size
    • C09D7/67—Particle size smaller than 100 nm
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/063—Titanium; Oxides or hydroxides thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/39—Photocatalytic properties
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/22—Rheological behaviour as dispersion, e.g. viscosity, sedimentation stability
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C2217/00—Coatings on glass
    • C03C2217/20—Materials for coating a single layer on glass
    • C03C2217/21—Oxides
    • C03C2217/212—TiO2
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C2217/00—Coatings on glass
    • C03C2217/70—Properties of coatings
    • C03C2217/71—Photocatalytic coatings
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
    • C04B2111/0081—Uses not provided for elsewhere in C04B2111/00 as catalysts or catalyst carriers
    • C04B2111/00827—Photocatalysts
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/80—Optical properties, e.g. transparency or reflexibility
    • C04B2111/805—Transparent material
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/18—Oxygen-containing compounds, e.g. metal carbonyls
    • C08K3/20—Oxides; Hydroxides
    • C08K3/22—Oxides; Hydroxides of metals

Definitions

  • the invention relates to an aqueous dispersion which contains titanium dioxide powder, an alcoholamine and a carboxylic acid.
  • Titanium dioxide is a light-sensitive semiconductor, and absorbs electromagnetic radiation in the near UV region.
  • the energy difference between the valence and the conductivity bands in the solid state is 3.05 eV for rutile and 3.29 eV for anatase, corresponding to an absorption band at ⁇ 415 nm for rutile and ⁇ 385 nm for anatase.
  • the anatase form requires higher light energy than the rutile form, but shows a stronger photoactivity. This can be explained with the longer lifetime of the excited state in anatase and the better adsorption of oxygen in anionic form at the anatase surface.
  • organic compounds can be fully mineralized to the following end products: organic molecules -> CO2 + H 2 O, organic N-compounds -> HNO3 + CO 2 + H 2 O; organic S-compounds -> H 2 SO 4 + CO 2 + H 2 O; organic Cl-compounds -> HCl + CO 2 + H 2 O.
  • the radicals formed on the titanium dioxide surface when it is irradiated with UV light can also attack the cells of microorganisms, so that nanoscale titanium dioxide can effectively be used to inhibit the growth of bacteria, viruses, algae, yeast, mould, and other microorganisms on surfaces or in liquids.
  • UV excitation produces electron-hole pairs which can oxidize bridging 0 2 ⁇ species to oxygen thus creating "oxygen vacancies”.
  • a hydroxylation takes place and the surface properties change to a considerably more hydrophilic behaviour. Water contact angles of less than 5° can be measured and such surfaces are considered as being superhydrophilic . The process is reversed in the dark.
  • Examples for the photocatalytic action of titania comprise depolluting cement, self-cleaning paint, air and water purification, deodorization, antimicrobial surfaces e.g. roofings and tiles, self-cleaning tiles and glasses and antifogging mirrors.
  • the titanium dioxide can be applied, for example, by sol- gel processes, as described in EP-A-590477.
  • DE-A-10324519 describes a process in which a dispersion of a photocatalytically active metal oxide powder having a specific surface area of 25 m 2 /g to 200 m 2 /g is applied to an oxide ceramic base material with formation of a layer, and the layer is subsequently hardened with formation of a photocatalytically active, porous oxide ceramic coating.
  • the photocatalytically active metal oxide powder used is preferably titanium dioxide which is obtained by flame hydrolysis of TiCl 4 .
  • the primary particles of such powders usually have a size of about 15 nm to about 30 nm.
  • titanium dioxide P25 from Degussa can be used.
  • DE-A-10324519 does not reveal the nature of the metal oxide dispersion in order for said dispersion to be suitable as coating material. All that is stated is that it must contain standardizing agents and/or adhesives.
  • Preferably used standardizing agents are organic viscosity regulators, for example carboxymethylcellulose . These viscosity regulators are necessary for imparting a suitable viscosity to the suspension so that the latter can be reliably applied to the ceramic base material in the desired layer thickness.
  • DE-A-10229761 discloses metal oxide dispersions which contain phosphates or polyphosphates. Such dispersions are unsuitable as coating material of ceramic substrates since the (poly) phosphates, in contrast to organic additives, are not removed during heating of the layer .
  • EP-A-981584 describes a process for the preparation of a dispersion which has a solids content of at least 78% by weight of titanium dioxide pigment and of at least 3% by weight of aluminium oxide.
  • the dispersion is as a rule diluted for transport and further milled before further use in order to reduce the size of the titanium dioxide particles.
  • EP-A-850203 describes a dispersion which contains monodisperse, porous titanium dioxide particles in organic solvents, which dispersion is used for coating substrates.
  • the preparation of this dispersion is complicated.
  • the titanium dioxide particles are produced by hydrolysis of an organotitanium compound in the presence of carboxylates or phosphates in an aqueous medium, separated off by filtration and then redispersed in an organic solvent.
  • the titanium dioxide content of the organic dispersion may be up to 300 g/1.
  • US-6509841 discloses a process for photocatalytic removal of organic substances from waste waters, in which the photocatalyst used is made of granules based on pyrogenically prepared titanium dioxide.
  • the dispersion in water for preparing the granules can exhibit a concentration of titanium dioxide from 3 to 25 wt . %.
  • Organic auxiliary substances may be added to the dispersion in order to enhance the stability of the dispersion and to improve the particle morphology after spray drying.
  • US 6992042 discloses a photocatalyst comprising pyrogenically prepared titanium dioxide doped with an aerosol and containing, as a doping component, an oxide selected from the group consisting of zinc oxide, platinum oxide, magnesium oxide, and aluminium oxide.
  • the photocatalyst has either a) a BET surface area of 65 m 2 /g to 80 m 2 /g and a doping component concentration of 40 ppm to 800 ppm or b) a BET surface area of 35 m 2 /g to 60 m 2 /g and a doping component concentration of more than 1000 ppm.
  • the photocatalytic activity in optionally acidified aqueous suspension can be increased or reduced by doping with oxides of metals/noble metals or metalloids.
  • the aqueous dispersion disclosed contains 1 g/1 of the doped particles.
  • US5698177 discloses a process for preparing titanium dioxide powder comprising the steps of mixing vapour phase TiCl 4 and O2 in a reaction area, externally heating said mixture in said reaction area and collecting the titanium dioxide powder formed.
  • the titanium dioxide powder can be used as a photocatalyst .
  • US6777374 discloses a photocatalyst for partial oxidation reactions comprising a titanium dioxide that has been deposited from a coating precursor onto a substrate by flame aerosol deposition to form a nanostructured film.
  • US 6,884,753 discloses a method for producing a dispersion, by heating a mixture of a titania, a dispersant and a solvent at a temperature of about 70 0 C or higher without substantially letting the solvent out of the reaction system.
  • a 2 wt% dispersion in titania was prepared.
  • the titiana shows anatase and rutile type crystal phase and had an average particle diameter of 152 nm.
  • the amount of the dispersant, i.e., the oxalic acid and ammonium oxalate, in the titanium oxide dispersion composition was 0.1 mol per 1 mole of titanium oxide.
  • the object of the invention is to provide a dispersion comprising titania, which has a high solids content, has a low viscosity and does not contain any further inorganic constituents other than the photocatalytically active metal oxide component.
  • the dispersion should be pourable at room temperature and should be stable to sedimentation and thickening for at least one month. Coatings based on the dispersion should provide mostly transparent, homogeneous coatings.
  • a further object of the invention is to provide a process for the preparation of the dispersion.
  • the invention relates to an aqueous dispersion which contains titanium dioxide, water, at least one aminoalcohol and at least one hydroxycarboxylic acid, the titanium dioxide being present in the form of aggregated primary particles having a mean, volume-related aggregate diameter of 70 to 100 nm and the proportion in the dispersion of
  • the proportion of aminoalcohol in the dispersion is pprreeffeerraabbllyy 33 ttoo 66 ⁇ mol/m 2 of specific surface area of titanium dioxide.
  • the proportion of hydroxycarboxylic acid in the dispersion is 1.5 to 2.5 ⁇ mol/m 2 of specific surface area of titanium dioxide.
  • a ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, of 1.9 to 2.6 may be preferred.
  • a dispersion according to the invention in which the aminoalcohol is present in the dispersion in a proportion of 3 to 6 ⁇ mol/m 2 of specific surface area of titanium dioxide and the hydroxycarboxylic acid is present in the dispersion in a proportion of 1.5 to 2.5 ⁇ mol/m 2 of specific surface area of titanium dioxide and the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, is 1.9 to 2.6 may be particularly preferred.
  • the proportion of water in the dispersion according to the invention can preferably be 48 to 73% by weight.
  • a dispersion whose proportion of titanium dioxide, water, aminoalcohol and hydroxycarboxylic acid is at least 98% by weight in total is particularly preferred.
  • the titanium dioxide present in the dispersion can be obtained by a precipitation process, a sol-gel process or a pyrogenic process.
  • a pyrogenically prepared titanium dioxide can preferably be used. Pyrogenic is to be understood as meaning a powder obtainable by flame hydrolysis or flame oxidation.
  • the powders thus prepared consist of aggregates of primary particles which are sintered together and are first formed during the reaction. A plurality of aggregates can subsequently form agglomerates. Because of the reaction conditions, pyrogenically prepared powders have only very low surface porosity and up to 5 OH/nm 2 of hydroxyl groups on the surface.
  • the titanium dioxide powders present in the dispersion according to the invention may be present in the rutile or the anatase form or as a mixture of the two forms. With the use of pyrogenically prepared titanium dioxide powders, as a rule rutile and anatase modifications are present.
  • the anatase/rutile ratio may be in the range of 2:98 to 98:2. The range of 70:30 to 95:5 may be particularly preferred. Anatase has lower hardness than rutile. Rutile on the other hand has a higher refractive index and better weather stability.
  • rutile-rich dispersions can be preferably used where the stability to UV light is important.
  • Anatase-rich dispersions can be used where low abrasion is important.
  • a pyrogenically prepared titanium dioxide powder which has a narrow primary particle distribution may be present in the dispersion according to the invention.
  • the preparation of the powder is described in the German patent application DE-A- 102004055165.
  • pyrogenically prepared titanium dioxide powders also comprise doped titanium dioxide powders or metal oxide/titanium dioxide mixed oxide powders in which in each case at least a part of the doping component or the metal oxide component is present on the surface.
  • the oxides of aluminium, silicon, cerium, iron, copper or zirconium are suitable as doping components and metal oxide components.
  • the proportion of doping component or metal oxide component, based on the powder, may preferably be between 10 ppm and 20% by weight.
  • the dispersion according to the invention may also contain pyrogenically prepared metal oxide powders which were subsequently surrounded with a titanium dioxide covering.
  • powders which comprise titanium dioxide as the only component are preferred. These may be, for example, Aeroxide P25 (Degussa) having a BET surface area of about 50 m 2 /g and Aeroxide P90 having a BET surface area of about 90 m 2 /g (Degussa) .
  • a dispersion according to the invention in which the titanium dioxide powder has a specific surface area of 50 ⁇ 15 m 2 /g or 90 ⁇ 15 m 2 /g is particularly preferred.
  • the titanium dioxide content is preferably 40 ⁇ 5% by weight.
  • the titanium dioxide content is preferably 30 ⁇ 3% by weight.
  • the dispersion according to the invention has a monomodal distribution of the aggregate diameters, which means that only one signal results in the analysis of the aggregate diameter distribution .
  • no particles of more than 200 nm are detectable in the dispersion according to the invention by the customary methods of light scattering for determining particle size distributions in dispersions, such as, for example, dynamic (e.g. Malvern Zetasizer) or static light scattering (e.g. Horiba LA-910).
  • dynamic e.g. Malvern Zetasizer
  • static light scattering e.g. Horiba LA-910
  • the aminoalcohols used preferably have 3 to 5 carbon atoms. They can preferably be selected from the group consisting of monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanol- amine, N, N-dimethylisopropanolamine, 3-amino-l-propanol, l-amino-2-propanol and/or 2-amino-2-methyl-l-propanol,
  • the hydroxycarboxylic acids used preferably have 4 to 6 carbon atoms. They can preferably be selected from the group consisting of malic acid, tartaric acid and/or citric acid, citric acid being particularly preferred.
  • the dispersion according to the invention may optionally contain at least one preservative.
  • Suitable preservatives may be: aqueous formulations of 2-methylisothiazolin-3-one (MIT) and benzoisothiazolinone (BIT) , MIT/BIT and 2-bromo- 2-nitropropane-l , 3-diol, 3 (2H) -5-chloro-2-methyliso- thiazolone (CIT) /MIT; formaldehyde donors based on dimethylol- or trimethylolurea, formamidemethylol, paraformaldehyde; Bronopol, nitrilodibromopropionamide, 1,3- di (hydroxymethyl) -5, 5-dimethylhydantoin or hexahydro- triazines .
  • MIT 2-methylisothiazolin-3-one
  • BIT benzoisothiazolinone
  • CIT 2-bromo-2-methyliso- thi
  • the preservative is usually present in an amount of 0.5-5% by weight, based on the total amount of the formulation.
  • 0.05-0.4% by weight of the formulation based on the total amount of the dispersion, may be present.
  • Preservatives from the food sector such as, for example, sorbic acid/alkali metal sorbates, propionic acid, benzoic acid/alkali metal benzoates, PHB esters and alkali metal sulphites, may also be present in the dispersion according to the invention, usually in a proportion of 0.1-0.5% by weight, based on the total amount of the dispersion.
  • a particularly preferred dispersion according to the invention is distinguished in that the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 50 ⁇ 5 m 2 /g, the aminoalcohol is 2-amino-2-methyl-l- propanol and the hydroxycarboxylic acid is citric acid and the proportion of
  • - 2-amino-2-methyl-l-propanol is 3 to 3.5 ⁇ mol/m 2 of specific surface area of titanium dioxide
  • - citric acid is 1.6 to 1.8 ⁇ mol/m 2 of specific surface area
  • a further preferred dispersion according to the invention is one in which the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 90 ⁇ 5 m 2 /g, the aminoalcohol is 2-amino-2-methyl-l-propanol and the hyrdoxycarboxylic acid is citric acid and the proportion of
  • the invention furthermore relates to a process for the preparation of the dispersion according to the invention, in which a mixture of
  • a predispersion is produced by introducing less than 1000 kJ/m 3 of energy, - the predispersion is divided into at least two part- streams, these part-streams are subjected to a pressure of at least 500 bar in a high-energy mill, let down via a nozzle and allowed to come into contact with one another in a gas- or liquid-filled reaction space and optionally a preservative is added.
  • Suitable dispersing apparatuses for the preparation of the predispersion are, for example, rotor/stator machines or toothed discs.
  • the pressure is at least
  • the invention furthermore relates to the use of the dispersion according to the invention for the coating, in particular for the transparent coating, of glass and ceramic and metal surfaces.
  • the titanium dioxide powder used in Examples 1 and 5-8 is Aeroxide ® Ti ⁇ 2 P25, and that used in Example 2 is Aeroxide ® Ti0 2 P90, both from Degussa AG.
  • the titanium dioxide powders used in Examples 3 and 4 are prepared as follows:
  • Titanium dioxide powder used in Example 3 160 kg/h of TiCl 4 are vaporized in an evaporator at 140 0 C.
  • the vapours are transferred to a mixing chamber by means of 15 m 3 (S.T.P.)/h of nitrogen as carrier gas having a carrier gas humidity of 15 g/m 3 of carrier gas.
  • 52 m 3 (S.T.P.)/h of hydrogen and 525 m 3 (S.T.P.)/h of primary air are introduced into the mixing chamber.
  • the reaction mixture is fed to a burner and ignited. The flame burns in a water-cooled flame tube.
  • 200 m 3 (S.T.P.)/h of secondary air are introduced into the reaction space.
  • the powder formed is separated off in a downstream filter and then treated countercurrently with air and steam at 520 0 C.
  • the titanium dioxide powder has the following physicochemical properties: BET surface area 48 m 2 /g, full width at half height of primary particles 11.0 nm, proportion of anatase 89%.
  • Titanium dioxide powder used in Example 4 40 kg/h of TiCl 4 are vaporized in an evaporator at 140 0 C.
  • the vapours are transferred to a mixing chamber by means of 15 m 3 (S.T.P.)/h of nitrogen as carrier gas having a carrier gas humidity of 6 g/m 3 of carrier gas.
  • 67 m 3 (S.T.P.)/h of hydrogen and 550 m 3 (S.T.P.)/h of primary air are introduced into the mixing chamber.
  • the reaction mixture is fed to a burner and ignited. The flame burns in a water-cooled flame tube.
  • 200 m 3 (S.T.P.)/h of secondary air are introduced into the reaction space.
  • the powder formed is separated off in a downstream filter and then treated countercurrently with air and steam at 520 0 C.
  • the titanium dioxide powder has the following physicochemical properties: BET surface area 91 m 2 /g, full width at half height of primary particles 4.8 nm, proportion of anatase 90%.
  • citric acid and water are initially introduced.
  • the aminoalcohol is added in proportion to the addition of the amount of powder, in order to obtain a flowable predispersion .
  • the titanium dioxide powder is drawn in via the suction pipe of an Ystral Conti-TDS 3 under shear conditions and, at the end of the induction, shearing is continued for a further 15 min at 3000 rpm.
  • This predispersion is fed in two passes through a high- energy Sugino Ultimaizer HJP-25050 at a pressure of 2500 bar and diamond nozzles of 0.3 mm in diameter.
  • Table 1 shows the starting materials and the amount thereof for the examples carried out according to the general method. Furthermore, Table 1 contains the physicochemical data of the dispersions obtained.
  • the mean, volume-related aggregate diameter of the particles from Example 1 is 75 nm. No coarser particles can be detected over and above this.
  • Examples 5 and 6 show that aminoalcohol and carboxylic acid are necessary for the preparation of the dispersion according to the invention. If one component is omitted, the result is a highly viscous, inhomogeneous predispersion which is not suitable for further milling.
  • Examples 7 and 8 show that the amount of aminoalcohol and carboxylic acid is critical for obtaining a dispersion according to the invention. In these examples, the amount of one component each is outside the claimed range. The resulting viscosities of the predispersion make further processing in a high-energy mill impossible.
  • the high-energy milling is essential for obtaining the dispersion according to the invention. If the starting materials are chosen as described in Exmaple 1, but low-energy milling is carried out, a highly viscous dispersion having low stability and a mean aggregate size of more than 150 nm is obtained.
  • the dispersions according to the invention of Examples 1 to 4 show extremely low viscosity values in combination with excellent stability.
  • ⁇ mean volume-related aggregate diameter; determined with Horiba LA 910;

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Abstract

Aqueous dispersion which contains titanium dioxide, water, at least one aminoalcohol and at least one hydroxycarboxylic acid, the titanium dioxide being present in the form of aggregated primary particles having a mean, volume-related aggregate diameter of 70 to 100 nm and the proportion in the dispersion of titanium dioxide being 25 to 50% by weight, aminoalcohol being 2.5 to 6.5 µmol/m2 of specific surface area of titanium dioxide and hydroxycarboxylic acid being 1 to 3 µmol/m2 of specific surface area and the ratio of aminoalcohol/hydroxy-carboxylic acid, in mol/mol, being 2 to 3.

Description

Dispersion containing titanium dioxide
The invention relates to an aqueous dispersion which contains titanium dioxide powder, an alcoholamine and a carboxylic acid.
In order to understand semiconductor photochemistry three modes of action are discussed in literature, which are photomineralization, photosterilization and photoinduced superhydrophilicity .
Titanium dioxide is a light-sensitive semiconductor, and absorbs electromagnetic radiation in the near UV region. The energy difference between the valence and the conductivity bands in the solid state is 3.05 eV for rutile and 3.29 eV for anatase, corresponding to an absorption band at < 415 nm for rutile and < 385 nm for anatase.
Absorption of light energy causes an electron to be promoted from the valence band to the conductivity band. This electron and the newly created positive "electron hole" can move on the surface of the solid where it can take part in redox reactions. Water molecules can be oxidized, which are commonly adsorbed onto the titanium dioxide surface, generating OH radicals. These are a by far stronger oxidizing agent than either ozone or chlorine. On the other hand, reduction of oxygen forming superoxide anions O2" and in a second reduction step peroxide anions O22~ can occur. These anions bear intermediate oxidizing power. All these oxidizing species can cause complete oxidation of organic compounds to carbon dioxide and water.
The anatase form requires higher light energy than the rutile form, but shows a stronger photoactivity. This can be explained with the longer lifetime of the excited state in anatase and the better adsorption of oxygen in anionic form at the anatase surface.
Depending on the reaction conditions organic compounds can be fully mineralized to the following end products: organic molecules -> CO2 + H2O, organic N-compounds -> HNO3 + CO2 + H2O; organic S-compounds -> H2SO4 + CO2 + H2O; organic Cl-compounds -> HCl + CO2 + H2O.
Because the reactions take place at the surface of a solid, diffusion to the catalyst surface is the rate- determining step. In liquid phase reactions, the generation of various intermediate decomposition products occurs. In some cases, these intermediate products inac- tivate the catalyst surface.
The radicals formed on the titanium dioxide surface when it is irradiated with UV light can also attack the cells of microorganisms, so that nanoscale titanium dioxide can effectively be used to inhibit the growth of bacteria, viruses, algae, yeast, mould, and other microorganisms on surfaces or in liquids.
Another mechanism is discussed for the so-called photoinduced superhydrophilicity observed on surfaces coated with thin TiO2 films. UV excitation produces electron-hole pairs which can oxidize bridging 02~ species to oxygen thus creating "oxygen vacancies". Following adsorption of water a hydroxylation takes place and the surface properties change to a considerably more hydrophilic behaviour. Water contact angles of less than 5° can be measured and such surfaces are considered as being superhydrophilic . The process is reversed in the dark.
The self-cleaning and defogging action of such surfaces arises from the fact that dirt and grime that collect on the surfaces are readily washed away by water.
Examples for the photocatalytic action of titania comprise depolluting cement, self-cleaning paint, air and water purification, deodorization, antimicrobial surfaces e.g. roofings and tiles, self-cleaning tiles and glasses and antifogging mirrors.
The titanium dioxide can be applied, for example, by sol- gel processes, as described in EP-A-590477.
DE-A-10324519 describes a process in which a dispersion of a photocatalytically active metal oxide powder having a specific surface area of 25 m2/g to 200 m2/g is applied to an oxide ceramic base material with formation of a layer, and the layer is subsequently hardened with formation of a photocatalytically active, porous oxide ceramic coating. The photocatalytically active metal oxide powder used is preferably titanium dioxide which is obtained by flame hydrolysis of TiCl4. The primary particles of such powders usually have a size of about 15 nm to about 30 nm. For example, titanium dioxide P25 from Degussa can be used.
DE-A-10324519 does not reveal the nature of the metal oxide dispersion in order for said dispersion to be suitable as coating material. All that is stated is that it must contain standardizing agents and/or adhesives. Preferably used standardizing agents are organic viscosity regulators, for example carboxymethylcellulose . These viscosity regulators are necessary for imparting a suitable viscosity to the suspension so that the latter can be reliably applied to the ceramic base material in the desired layer thickness.
Furthermore, DE-A-10229761 discloses metal oxide dispersions which contain phosphates or polyphosphates. Such dispersions are unsuitable as coating material of ceramic substrates since the (poly) phosphates, in contrast to organic additives, are not removed during heating of the layer .
EP-A-981584 describes a process for the preparation of a dispersion which has a solids content of at least 78% by weight of titanium dioxide pigment and of at least 3% by weight of aluminium oxide. The dispersion is as a rule diluted for transport and further milled before further use in order to reduce the size of the titanium dioxide particles.
EP-A-850203 describes a dispersion which contains monodisperse, porous titanium dioxide particles in organic solvents, which dispersion is used for coating substrates. The preparation of this dispersion is complicated. First, the titanium dioxide particles are produced by hydrolysis of an organotitanium compound in the presence of carboxylates or phosphates in an aqueous medium, separated off by filtration and then redispersed in an organic solvent. The titanium dioxide content of the organic dispersion may be up to 300 g/1.
US-6509841 discloses a process for photocatalytic removal of organic substances from waste waters, in which the photocatalyst used is made of granules based on pyrogenically prepared titanium dioxide. The dispersion in water for preparing the granules can exhibit a concentration of titanium dioxide from 3 to 25 wt . %. Organic auxiliary substances may be added to the dispersion in order to enhance the stability of the dispersion and to improve the particle morphology after spray drying.
US 6992042 discloses a photocatalyst comprising pyrogenically prepared titanium dioxide doped with an aerosol and containing, as a doping component, an oxide selected from the group consisting of zinc oxide, platinum oxide, magnesium oxide, and aluminium oxide. The photocatalyst has either a) a BET surface area of 65 m2/g to 80 m2/g and a doping component concentration of 40 ppm to 800 ppm or b) a BET surface area of 35 m2/g to 60 m2/g and a doping component concentration of more than 1000 ppm. The photocatalytic activity in optionally acidified aqueous suspension can be increased or reduced by doping with oxides of metals/noble metals or metalloids. The aqueous dispersion disclosed contains 1 g/1 of the doped particles.
US5698177 discloses a process for preparing titanium dioxide powder comprising the steps of mixing vapour phase TiCl4 and O2 in a reaction area, externally heating said mixture in said reaction area and collecting the titanium dioxide powder formed. The titanium dioxide powder can be used as a photocatalyst .
US6777374 discloses a photocatalyst for partial oxidation reactions comprising a titanium dioxide that has been deposited from a coating precursor onto a substrate by flame aerosol deposition to form a nanostructured film.
US 6,884,753 discloses a method for producing a dispersion, by heating a mixture of a titania, a dispersant and a solvent at a temperature of about 700C or higher without substantially letting the solvent out of the reaction system. In an Example, a 2 wt% dispersion in titania was prepared. The titiana shows anatase and rutile type crystal phase and had an average particle diameter of 152 nm. The amount of the dispersant, i.e., the oxalic acid and ammonium oxalate, in the titanium oxide dispersion composition, was 0.1 mol per 1 mole of titanium oxide.
Although there are numerous documents of the prior art on the photocatalytic activity of titania, the method of providing the titania still needs to be improved.
Thus the object of the invention is to provide a dispersion comprising titania, which has a high solids content, has a low viscosity and does not contain any further inorganic constituents other than the photocatalytically active metal oxide component. Moreover, the dispersion should be pourable at room temperature and should be stable to sedimentation and thickening for at least one month. Coatings based on the dispersion should provide mostly transparent, homogeneous coatings.
A further object of the invention is to provide a process for the preparation of the dispersion.
The invention relates to an aqueous dispersion which contains titanium dioxide, water, at least one aminoalcohol and at least one hydroxycarboxylic acid, the titanium dioxide being present in the form of aggregated primary particles having a mean, volume-related aggregate diameter of 70 to 100 nm and the proportion in the dispersion of
- titanium dioxide being 25 to 50% by weight,
- aammiinnooaallccoohhooll bbeeiinngg 22..55 ttoo 66..55 μμmmcol/m2 of specific surface area of titanium dioxide,
- hydroxycarboxylic acid being from 1 to 3 μmol/m2 of specific surface area and
- the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, being 2 to 3.
The proportion of aminoalcohol in the dispersion is pprreeffeerraabbllyy 33 ttoo 66 μmol/m2 of specific surface area of titanium dioxide.
Preferably, the proportion of hydroxycarboxylic acid in the dispersion is 1.5 to 2.5 μmol/m2 of specific surface area of titanium dioxide.
Furthermore, a ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, of 1.9 to 2.6 may be preferred.
A dispersion according to the invention in which the aminoalcohol is present in the dispersion in a proportion of 3 to 6 μmol/m2 of specific surface area of titanium dioxide and the hydroxycarboxylic acid is present in the dispersion in a proportion of 1.5 to 2.5 μmol/m2 of specific surface area of titanium dioxide and the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, is 1.9 to 2.6 may be particularly preferred.
The proportion of water in the dispersion according to the invention can preferably be 48 to 73% by weight.
A dispersion whose proportion of titanium dioxide, water, aminoalcohol and hydroxycarboxylic acid is at least 98% by weight in total is particularly preferred.
The titanium dioxide present in the dispersion can be obtained by a precipitation process, a sol-gel process or a pyrogenic process. A pyrogenically prepared titanium dioxide can preferably be used. Pyrogenic is to be understood as meaning a powder obtainable by flame hydrolysis or flame oxidation. The powders thus prepared consist of aggregates of primary particles which are sintered together and are first formed during the reaction. A plurality of aggregates can subsequently form agglomerates. Because of the reaction conditions, pyrogenically prepared powders have only very low surface porosity and up to 5 OH/nm2 of hydroxyl groups on the surface.
The titanium dioxide powders present in the dispersion according to the invention may be present in the rutile or the anatase form or as a mixture of the two forms. With the use of pyrogenically prepared titanium dioxide powders, as a rule rutile and anatase modifications are present. The anatase/rutile ratio may be in the range of 2:98 to 98:2. The range of 70:30 to 95:5 may be particularly preferred. Anatase has lower hardness than rutile. Rutile on the other hand has a higher refractive index and better weather stability.
Owing to the different properties of rutile and anatase, it is possible, according to the invention, to prepare dispersions for specific applications. Thus, rutile-rich dispersions can be preferably used where the stability to UV light is important. Anatase-rich dispersions can be used where low abrasion is important.
Furthermore, a pyrogenically prepared titanium dioxide powder which has a narrow primary particle distribution may be present in the dispersion according to the invention. Such a powder is characterized by a BET surface area of 20 to 200 m2/g; a full width at half height FWHH, in nanometres, of the primary particle distribution with values according to the formula FWHH = a X BET where a=670xl09 m3/g and -1.3 < f < -1.0; a proportion of particles having a diameter of more than 45 μm in the range of 0.0001 to 0.05% by weight. The preparation of the powder is described in the German patent application DE-A- 102004055165.
For the purposes of the invention, pyrogenically prepared titanium dioxide powders also comprise doped titanium dioxide powders or metal oxide/titanium dioxide mixed oxide powders in which in each case at least a part of the doping component or the metal oxide component is present on the surface. In particular, the oxides of aluminium, silicon, cerium, iron, copper or zirconium are suitable as doping components and metal oxide components. The proportion of doping component or metal oxide component, based on the powder, may preferably be between 10 ppm and 20% by weight.
Furthermore, the dispersion according to the invention may also contain pyrogenically prepared metal oxide powders which were subsequently surrounded with a titanium dioxide covering.
For the purposes of the invention, however, powders which comprise titanium dioxide as the only component are preferred. These may be, for example, Aeroxide P25 (Degussa) having a BET surface area of about 50 m2/g and Aeroxide P90 having a BET surface area of about 90 m2/g (Degussa) .
A dispersion according to the invention in which the titanium dioxide powder has a specific surface area of 50 ± 15 m2/g or 90 ± 15 m2/g is particularly preferred.
In the case of the dispersion containing the titanium dioxide powder having a specific surface area of 50 ± 15 m2/g, the titanium dioxide content is preferably 40 ± 5% by weight.
In the case of the dispersion containing the titanium dioxide powder having a specific surface area of 90 ± 15 m2/g, the titanium dioxide content is preferably 30 ± 3% by weight.
Furthermore, it may be advantageous if the dispersion according to the invention has a monomodal distribution of the aggregate diameters, which means that only one signal results in the analysis of the aggregate diameter distribution .
Furthermore, it may be advantageous if no particles of more than 200 nm are detectable in the dispersion according to the invention by the customary methods of light scattering for determining particle size distributions in dispersions, such as, for example, dynamic (e.g. Malvern Zetasizer) or static light scattering (e.g. Horiba LA-910).
The aminoalcohols used preferably have 3 to 5 carbon atoms. They can preferably be selected from the group consisting of monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanol- amine, N, N-dimethylisopropanolamine, 3-amino-l-propanol, l-amino-2-propanol and/or 2-amino-2-methyl-l-propanol,
2-amino-2-methyl-l-propanol being particularly preferred. The hydroxycarboxylic acids used preferably have 4 to 6 carbon atoms. They can preferably be selected from the group consisting of malic acid, tartaric acid and/or citric acid, citric acid being particularly preferred.
The dispersion according to the invention may optionally contain at least one preservative. Suitable preservatives may be: aqueous formulations of 2-methylisothiazolin-3-one (MIT) and benzoisothiazolinone (BIT) , MIT/BIT and 2-bromo- 2-nitropropane-l , 3-diol, 3 (2H) -5-chloro-2-methyliso- thiazolone (CIT) /MIT; formaldehyde donors based on dimethylol- or trimethylolurea, formamidemethylol, paraformaldehyde; Bronopol, nitrilodibromopropionamide, 1,3- di (hydroxymethyl) -5, 5-dimethylhydantoin or hexahydro- triazines .
The preservative is usually present in an amount of 0.5-5% by weight, based on the total amount of the formulation. In the dispersion according to the invention, 0.05-0.4% by weight of the formulation, based on the total amount of the dispersion, may be present.
Preservatives from the food sector, such as, for example, sorbic acid/alkali metal sorbates, propionic acid, benzoic acid/alkali metal benzoates, PHB esters and alkali metal sulphites, may also be present in the dispersion according to the invention, usually in a proportion of 0.1-0.5% by weight, based on the total amount of the dispersion.
A particularly preferred dispersion according to the invention is distinguished in that the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 50 ± 5 m2/g, the aminoalcohol is 2-amino-2-methyl-l- propanol and the hydroxycarboxylic acid is citric acid and the proportion of
- titanium dioxide is 40 ± 5% by weight,
- 2-amino-2-methyl-l-propanol is 3 to 3.5 μmol/m2 of specific surface area of titanium dioxide, - citric acid is 1.6 to 1.8 μmol/m2 of specific surface area and
- water is 55-59 wt.-%,
- the ratio of 2-amino-2-methyl-l-propanol/citric acid, in mol/mol, being 1.9 to 2.1.
A further preferred dispersion according to the invention is one in which the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 90 ± 5 m2/g, the aminoalcohol is 2-amino-2-methyl-l-propanol and the hyrdoxycarboxylic acid is citric acid and the proportion of
- titanium dioxide is 30 ± 3% by weight,
- 2-amino-2-methyl-l-propanol is 5 to 5.5 μmol/m2 of specific surface area of titanium dioxide,
- citric acid is 1.95 to 2.15 μmol/m2 of specific surface area and
- water is 65 to 68 wt.-%,
- the ratio of 2-amino-2-methyl-l-propanol/citric acid, in mol/mol, being 2.4 to 2.6.
The invention furthermore relates to a process for the preparation of the dispersion according to the invention, in which a mixture of
- 25 to 50% by weight of titanium dioxide powder,
2.5 to 6.5 μmol of aminoalcohol/m2 of specific surface area of titanium dioxide, - 1.5 to 3 μmol of hydroxycarboxylic acid/m2 of specific surface area of titanium dioxide, the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, being 1.5 to 3, and
- water is intially introduced, - a predispersion is produced by introducing less than 1000 kJ/m3 of energy, - the predispersion is divided into at least two part- streams, these part-streams are subjected to a pressure of at least 500 bar in a high-energy mill, let down via a nozzle and allowed to come into contact with one another in a gas- or liquid-filled reaction space and optionally a preservative is added.
Suitable dispersing apparatuses for the preparation of the predispersion are, for example, rotor/stator machines or toothed discs.
In a preferred embodiment, the pressure is at least
2000 bar. Furthermore, it may be advantageous to subject the dispersion to the high-energy milling process several times .
The invention furthermore relates to the use of the dispersion according to the invention for the coating, in particular for the transparent coating, of glass and ceramic and metal surfaces.
Examples
Starting materials:
The titanium dioxide powder used in Examples 1 and 5-8 is Aeroxide®Tiθ2 P25, and that used in Example 2 is Aeroxide®Ti02 P90, both from Degussa AG.
The titanium dioxide powders used in Examples 3 and 4 are prepared as follows:
Titanium dioxide powder used in Example 3: 160 kg/h of TiCl4 are vaporized in an evaporator at 1400C. The vapours are transferred to a mixing chamber by means of 15 m3 (S.T.P.)/h of nitrogen as carrier gas having a carrier gas humidity of 15 g/m3 of carrier gas. Separately therefrom, 52 m3 (S.T.P.)/h of hydrogen and 525 m3 (S.T.P.)/h of primary air are introduced into the mixing chamber. In a central tube, the reaction mixture is fed to a burner and ignited. The flame burns in a water-cooled flame tube. In addition, 200 m3 (S.T.P.)/h of secondary air are introduced into the reaction space. The powder formed is separated off in a downstream filter and then treated countercurrently with air and steam at 5200C.
The titanium dioxide powder has the following physicochemical properties: BET surface area 48 m2/g, full width at half height of primary particles 11.0 nm, proportion of anatase 89%.
Titanium dioxide powder used in Example 4 : 40 kg/h of TiCl4 are vaporized in an evaporator at 1400C. The vapours are transferred to a mixing chamber by means of 15 m3 (S.T.P.)/h of nitrogen as carrier gas having a carrier gas humidity of 6 g/m3 of carrier gas. Separately therefrom, 67 m3 (S.T.P.)/h of hydrogen and 550 m3 (S.T.P.)/h of primary air are introduced into the mixing chamber. In a central tube, the reaction mixture is fed to a burner and ignited. The flame burns in a water-cooled flame tube. In addition, 200 m3 (S.T.P.)/h of secondary air are introduced into the reaction space. The powder formed is separated off in a downstream filter and then treated countercurrently with air and steam at 5200C.
The titanium dioxide powder has the following physicochemical properties: BET surface area 91 m2/g, full width at half height of primary particles 4.8 nm, proportion of anatase 90%.
General method for the preparation of the dispersion according to the invention: citric acid and water are initially introduced. The aminoalcohol is added in proportion to the addition of the amount of powder, in order to obtain a flowable predispersion . For this purpose, the titanium dioxide powder is drawn in via the suction pipe of an Ystral Conti-TDS 3 under shear conditions and, at the end of the induction, shearing is continued for a further 15 min at 3000 rpm.
This predispersion is fed in two passes through a high- energy Sugino Ultimaizer HJP-25050 at a pressure of 2500 bar and diamond nozzles of 0.3 mm in diameter.
Table 1 shows the starting materials and the amount thereof for the examples carried out according to the general method. Furthermore, Table 1 contains the physicochemical data of the dispersions obtained.
The mean, volume-related aggregate diameter of the particles from Example 1 is 75 nm. No coarser particles can be detected over and above this.
Examples 5 and 6 show that aminoalcohol and carboxylic acid are necessary for the preparation of the dispersion according to the invention. If one component is omitted, the result is a highly viscous, inhomogeneous predispersion which is not suitable for further milling.
Examples 7 and 8 show that the amount of aminoalcohol and carboxylic acid is critical for obtaining a dispersion according to the invention. In these examples, the amount of one component each is outside the claimed range. The resulting viscosities of the predispersion make further processing in a high-energy mill impossible.
Furthermore, the high-energy milling is essential for obtaining the dispersion according to the invention. If the starting materials are chosen as described in Exmaple 1, but low-energy milling is carried out, a highly viscous dispersion having low stability and a mean aggregate size of more than 150 nm is obtained. The dispersions according to the invention of Examples 1 to 4 show extremely low viscosity values in combination with excellent stability.
Table 1 : Starting materials/amounts used and physicochemical data of the dispersions
Figure imgf000017_0001
§ = mean volume-related aggregate diameter; determined with Horiba LA 910;

Claims

Patent claims :
1. Aqueous dispersion, characterized in that it contains titanium dioxide, water, at least one aminoalcohol and at least one hydroxycarboxylic acid, the titanium dioxide being present in the form of aggregated primary particles having a mean, volume-related aggregate diameter of 70 to 100 nm and the proportion in the dispersion of
- titanium dioxide being 25 to 50% by weight, - aammiinnooaallccoohhooll bbeeiinngg 22..55 ttoo 66..55 μμmmool/m2 of specific surface area of titanium dioxide,
- hydroxycarboxylic acid being 1 to 3 μmol/m2 of specific surface area and the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, being 2 to 3.
2. Dispersion according to Claim 1, characterized in that the aminoalcohol is present in the dispersion in a proportion of 3 to 6 μmol/m2 of specific surface area of titanium dioxide.
3. Dispersion according to Claim 1 or 2, characterized in that the hydroxycarboxylic acid is present in the dispersion in a proportion of 1.5 to 2.5 μmol/m2 of specific surface area of titanium dioxide.
4. Dispersion according to any of Claims 1 to 3, characterized in that the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, is 1.9 to 2.6.
5. Dispersion according to Claim 1, characterized in that the aminoalcohol is present in the dispersion in a proportion of 3 to 6 μmol/m2 of specific surface area of titanium dioxide and the hydroxycarboxylic acid is present in the dispersion in a proportion of 1.5 to
2.5 μmol/m2 of specific surface area of titanium dioxide and the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, is 1.9 to 2.6.
6. Dispersion according to any of Claims 1 to 5, characterized in that water is present in the dispersion in a proportion of 48 to 73% by weight.
7. Dispersion according to any of Claims 1 to 6, characterized in that the proportion of titanium dioxide, water, aminoalcohol and hydroxycarboxylic acid is at least 98% by weight.
8. Dispersion according to any of Claims 1 to 7, characterized in that the titanium dioxide is a pyrogenically prepared titanium dioxide.
9. Dispersion according to any of Claims 1 to 8, characterized in that the titanium dioxide has an anatase/rutile ratio of 70:30 to 90:10.
10. Dispersion according to any of Claims 1 to 9, characterized in that the BET surface area of the titanium dioxide is 50 ± 5 m2/g.
11. Dispersion according to Claim 10, characterized in that the titanium dioxide content in the dispersion is 40 ± 5% by weight.
12. Dispersion according to any of Claims 1 to 9, characterized in that the BET surface area of the titanium dioxide is 90 ± 10 m2/g.
13. Dispersion according to Claim 12, characterized in that the proportion of titanium dioxide in the dispersion is 30 ± 3% by weight.
14. Dispersion according to any of Claims 1 to 13, characterized in that the aminoalcohol has 3 to 5 carbon atoms .
15. Dispersion according to any of Claims 1 to 14, characterized in that the hydroxycarboxylic acid has 4 to 6 carbon atoms .
16. Dispersion according to any of Claims 1 to 15, characterized in that water is present in the dispersion in a proportion of 48 to 73% by weight.
17. Dispersion according to any of Claims 1 to 16, characterized in that the proportion of titanium dioxide, water, aminoalcohol and hydroxycarboxylic acid is at least 98% by weight.
18. Dispersion according to any of Claims 1 to 17, characterized in that the dispersion contains at least one preservative.
19. Aqueous dispersion according to Claim 1, characterized in that the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 50 ± 5 m2/g, the aminoalcohol is 2-amino-2-methyl-l-propanol and the hydroxycarboxylic acid is citric acid and the proportion of - titanium dioxide is 40 ± 5% by weight,
- 2-amino-2-methyl-l-propanol is 3 to 3.5 μmol/m2 of specific surface area of titanium dioxide,
- citric acid is 1.6 to 1.8 μmol/m2 of specific surface area and - water is 55 to 59 wt.-%,
- the ratio of 2-amino-2-methyl-l-propanol/citric acid, in mol/mol, being 1.9 to 2.1.
20. Aqueous dispersion according to Claim 1, characterized in that the titanium dioxide is a pyrogenic titanium dioxide having a BET surface area of 90 ± 5 m2/g, the aminoalcohol is 2-amino-2-methyl-l-propanol and the hydroxycarboxylic acid is citric acid and the proportion of
- titanium dioxide is 30 ± 3% by weight,
2-amino-2-methyl-l-propanol is 5 to 5.5 μmol/m2 of specific surface area of titanium dioxide,
- citric acid is 1.95 to 2.15 μmol/m2 of specific surface area and
- water is 65 to 68 wt.-%, the ratio of 2-amino-2-methyl-l-propanol/citric acid, in mol/mol, being 2.4 to 2.6.
21. Process for the preparation of the dispersion according to any of Claims 1 to 20, characterized in that a mixture of
- 25 to 50% by weight of titanium dioxide powder, - 2.5 to 6.5 μmol of aminoalcohol/m2 of specific surface area of titanium dioxide,
- 1.5 to 3 μmol of hydroxycarboxylic acid/m2 of specific surface area of titanium dioxide, the ratio of aminoalcohol/hydroxycarboxylic acid, in mol/mol, being 1.5 to 3, and
- water is intially introduced,
- a predispersion is produced by introducing less than 1000 kJ/m3 of energy,
- the predispersion is divided into at least two part- streams, these part-streams are subjected to a pressure of at least 500 bar in a high-energy mill, let down via a nozzle and allowed to come into contact with one another in a gas- or liquid-filled reaction space and optionally a preservative is added.
22. Use of the dispersion according to any of Claims 1 to 20 for the coating of glass, ceramic and metal surfaces.
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