The invention concerns a method for producing a nanoscale rutile or a nanoscale oxide with a primary particle size of less than 40 nm, which redisperses to primary particle size in all conventional solvents, preferably water and alcohols. The method is also suited to produce other nanoscale oxides such as anatases, ZrO2, ZnO, SnO2, ATO (SnO2 doped with Sb), In2O3, BaO, CaO, MgO, CeO2 and BaTiO3.
In nature, titanium dioxide exists in three crystal modifications: rutile, brookite and anatase, wherein brookite has the least technical importance. The anatase and rutile modifications can absorb UV light due to their electronic structure, wherein the absorption edge for anatase is at 3.23 eV and for rutile at 3.05 eV. With this absorption, reactive electrons/hole pairs are formed in the TiO2 particle which migrate to the surface of the TiO2 particle where they are available for chemical reactions, whether this is desired or not. This property known as photocatalytic activity is more pronounced in anatase than in rutile.
The high photocatalytic activity of anatase can be utilized e.g. for cleaning sewage. Nanoscale anatase conventionally removes heavy metal ions from aqueous solutions by reduction. The heavy metal reduced through the catalysis precipitates at the surface of the anatase. The reducing agents are organic impurities which are also destroyed in this process [Photocatalytic Purification and Treatment of Water and Air, Editors Ollis D. F. and Al-Ekabi H., 253ff (1993)]. The photocatalytic oxidation of organic impurities in water using anatase has been frequently described [Pelizzeti E. et. Al., Euro Courses: Environ. Menage., 2 101ff (194) and Hidaka H. et. Al., Journal of Photochemistry and Photobiology, A: Chemistry, 47 103ff (1989)].
Rutile is somewhat less photocatalytically active. The recombination of the reactive electrons/hole pairs is larger than its reducing effect. To completely suppress the remaining photocatalytic activity, rutile (sub-μm rutile is usually used) is lattice-doped with an inorganic coating of Al2O3, SiO2 and ZrO2 and/or with trivalent ions such as Al3+, Fe3+, V3+, Ru3+, Os3+, or Rh3+. The coating effects spatial separation of the matrix surrounding the rutile from the reactive intermediates (electron/hole pairs) [Furlong D. N. et. Al., J. of Colloid and Interace science, 69 (3), 409ff (1979)]. The lattice doping provides recombination possibilities for the generated charge carriers. Only this measure permits use of rutile e.g. as white pigment in paints or as UV protection in polymers. The rutile usually has a primary particle size of not less than 200 nm.
In comparison with organic UV absorbers, rutile has the advantage that it is not destroyed by the UV radiation and cannot be extracted from a matrix (e.g. a polymer). [Heller H., European Polymer Journal-Supplement, Pergamon Press. England, 105ff (1969) and Valet A., Farbe und Lack 96, 185ff (3/1990)]. Only the size of the coated n-rutile is disadvantageous, since a sub-μm rutile refracts the light, thereby precluding transparent applications.
In view of the above, a nanoscale rutile of particle sizes of less than 40 nm would provide completely new perspectives, since particle sizes which are considerably smaller than the wavelength of the visible light permit combination of effective UV absorption with transparency. Transparent polymers or transparent sun blockers with one inorganic UV absorber would thereby be possible. The reduction of the particle size results in an increase in the specific powder surface and photocatalytic activity, such that the nanoscale rutile should either be lattice-doped and/or be provided with an inorganic coating.
The production of nanoscale rutile is described in prior art using aqueous and alcoholic solutions. Hydro-thermal post-crystallization is also known.
Pigmentary sub-Mm TiO2 is a useful white pigment which is technically obtained on a large scale from Ilmenite FeTiO3 in accordance with the “sulfate” and the “chloride method” [Hollemann Wiberg, ISBN 3-11-007511-3, (1985)]. Both methods use water as a solvent. The resulting rutile particles have the shape of a needle and an average primary particle size of 110 nm. The production of ultrafine rutile is also described in Kutty et al. with the same result [Materials Chemistry and Physics, 19, 533ff (1988)] and in Cheng et al. [Chem. Mater. 7 663ff (1995)].
Since the particle size of the rutile is excessively large for the above-described applications, Cheng et al. [Chem. Mater. 7 663ff (1995)] and Elfenthal et al. [DE 4105345A (1991)] have tried to reduce the particle size before precipitation of the fine rutile through addition of mineralizers such as SnCl4, NH4Cl or NaCl. This actually reduces the primary particle size of rutile, but not the particle size of rutile which forms during redispersion of rutile in the solvent, since the products produced are highly aggregated. The primary particle size of the n-rutile is e.g. reduced to 35 nm, through the addition of 4 weight % of SnO2 and to 15 nm through the addition of 10 weight % of SnO2 [DE4105345A (1991)]. Attempts to redisperse these rutile particles always produce milky products having a particle size distribution which corresponds to that of the larger pin-shaped rutile. Elfenthal et al. [DE4105345A (1991)] describe the difficulty of transparent redispersion of the subpigmentary titanium dioxide, and have produced rutile having a particle size of between 20 and 100 nm and incorporated it into polyethylene. In accordance with Elfenthal et al. there is incomplete transparency in the visible spectral range, even with the finest products. This means that that the particle size is always much larger than 40 nm.
The literature widely describes the production of nanoscale TiO2 via the sol-gel method or via non-aqueous synthesis methods. However, only amorphous TiO2 or anatases are produced in this way. To synthesize rutile in this fashion, the obtained products must be thermally or hydro-thermally post-treated at temperatures of usually between 550 and 800° C. This thermal treatment causes the particles to grow and intergrow among each other which means that synthesizing of a product which can be redispersed below 100 nm is not possible. Koebrugge et al. produced an amorphous TiO2 [J. Mater. Chem., 3 (11), 1095ff (1993)] which converts into rutile only above 800° C. and with a dwell time of 8 hours. The primary particle size thereby increases from 7.7 nm (200° C.) to 13.2 nm (450° C.) and to 89 nm (800° C.). During temperature treatment, the nanoscale particles grow into aggregates of a size from sub-μm to μm. Bao et al. [Wuji Cailiao Xuebao, (1996), 11 (3), 453ff], Zhou et al. [Hwahak Konghak, 33 (5), (1995) 544ff] have made similar experiences. Liu et al. [Yingyong H. (1994), 11 (5), 36ff] were able to reduce the conversion temperature to 400° C. through the addition of diethyl ether, but the quality of the rutile was not changed.
In summary, rutile needles (of a length between 70 and 100 nm) which have good redispersion properties and doped rutile particles which are difficult to redisperse can be generated using aqueous synthesis methods. Neither of these cases produces a product which can be transparently worked into an organic matrix, since particles having sizes in excess of 70 nm refract light. There are several hundred literature citations concerning the sol/gel and non-aqueous synthesis methods which produce either amorphous TiO2 or anatase. In contrast to the aqueous method, rutile is not produced directly thereby. Many of these attempts merely report that rutile is formed during thermal post-treatment usually at more than 550° C. These products cannot be redispersed to primary particle size in consequence of these high temperatures.
Since redispersion of the nanocrystalline titanium dioxide to a particle size of <40 nm, as described above, is required to transparently redisperse the titanium dioxide as a UV absorber in organic polymers, in particular plastic materials, glazes, cosmetic products or paints, none of the nanoscale titanium dioxide powders mentioned in literature are suited in the present case.
The finest rutile available today is currently used in sun blockers to increase the UV factor [EP0518773, 1992]. It is usually pin-shaped, has a length of at least 70 nm and a width of up to 10 nm. The fact that the manufacturers of n-rutile, e.g. Sachtleben, discuss a particle size for primary crystals in the region of 10-15 nm does not change this, since they themselves say that these primary crystals accumulate in the products into pin-shape objects, i.e. form aggregates, thereby attaining lengths of 100 nm. Manufacturers of coated rutile still face the problem that there is no conventional method to uniformly coat defined, individual particles having a size of considerably less than 100 nm. Coating usually produces agglomeration of the particles, and the generated agglomerates (which look like “grapes”) are coated thereby producing “growth towards each other”. The size of the rutile and the associated refraction of light produce a white color when the cream is applied to the skin, which is, of course, less the thinner the layer. An absolutely colorless cream would obviously be more acceptable, but rutile which can be distributed to a particle size of <40 nm is presently not available, as has been described in detail above. The scattering losses absolutely preclude use of the known rutile in transparent polymers. A new field of application for fine-particle titanium oxides has only been obtained in the area of decorative automotive painting. Combined with current aluminum pigments, one obtains an interesting iridescent effect in two-layer metal decorative painting. The visual impression changes in dependence on the angle of observation [Farbe und Lacke 98, 93ff (2/1992)].
To be able to use n-rutile as UV absorber in paints, polymers, creams etc. its catalytic activity must be minimized either through precise doping or through application of inorganic layers of Al2O3, SiO2 and ZrO2. These layers are usually applied through dispersion of TiO2 in water and precipitation of a hydroxide of aluminum, zirconium and silicon via a pH value change. Iller is based on sodium silicate [U.S. Pat. No. 2,885,366, 1959], Werner A. H J. on sodium silicate and sodium aluminate [U.S. Pat. No. 3,437,502, 1969], Jacobson H. W. et al. on sodium aluminate [U.S. Pat. No. 4,461,810], Luginsland H. H. et al. on zirconyl chloride and zirconyl sulfate [DE 2223524, 1972], Sayer et al. [EP 059992, 1993] on zinc chloride and Hyun S. H. et al. on aluminum sulfate and aluminum nitrate [J. of Korean Ceram. Soc. 28(4), 338ff (1991)]. A variant is described by Dardaris et al. [U.S. Pat. No. 5,256,728, 1993] who coat TiO2 with polysiloxanes. This US patent describes the production of pigmented polycarbonate. Another possibility to reduce the catalytic activity of TiO2 is to dope the titanium dioxide lattice with trivalent or pentavalent elements which can intercept the generated reactive intermediate stages. Doping with Fe, Sb, Al, In, Ce, Nb and V is thereby possible [DE 4222905, 1992].
Inorganic coating of nanoscale rutile having a particle size of less than 40 nm is very difficult since the conventional pH value change of the suspension during the coating method produces agglomeration of the particles such that the produced agglomerates are coated rather than the primary particles. Prior art provides no solution.