Subpigmentary titanium dioxide with improved photostability.

0001The invention is directed to a process for producing subpigmentary titanium dioxide with increased photostability by doping, a hydrolyzable titanium compound being subjected to a ripening and further treatment in a template, and to a subpigmentary titanium dioxide containing a dopant to improve the photostability.

0002Subpigmentary titanium dioxide (particle size in the range from about 10 to 200 nm) can be used as a transparent UV absorber, for example in paints, glazes, plastics and cosmetic products. In metallic coatings, special optical effects ("down flop", EP 270 472 A1) can be achieved with subpigmentary titanium dioxide. Patent application P 41 05 345.1 describes a production process for a subpigmentary titanium dioxide in which the particle size of the subpigmentary titanium dioxide can be influenced by adding a colloidal tin dioxide produced in a special process.

0003It is known from the use of titanium dioxide as a white pigment (particle size predominantly in the range from 200 to 500 nm) in lacquers, fibers, plastics, etc. that UV radiation triggers undesired reactions which lead to decomposition of the titanium dioxide particle surrounding it Lead medium (HG Völz, G. Kämpf, HG Fitzky, A. Klaeren, ACS Symposium Ser., 151 (1981) 163). A catalytic contribution can be attributed to the titanium dioxide. The reactions on which this decomposition is based mainly take place on the surface of the titanium dioxide particles. The influence of UV radiation on the photostability of a pigment can be described, for example, using a graying test.

0004Since subpigmentary titanium dioxide has an approx. 10 to 20 times larger specific surface area than the pigmentary titanium dioxide due to the smaller particle size, a higher photoactivity or a smaller so-called light stability can be observed on it.

0005It is also known to reduce the photoactivity of titanium dioxide by post-treating the titanium dioxide particles. A coating with oxides of silicon and / or zirconium and / or aluminum is customary (for example EP 393 857 A1, EP 430 424 A2, JP 02-194 063). For many applications, however, such treatment of subpigmentary titanium dioxide does not lead to sufficient photochemical stabilization.

0006In addition, to reduce the photoactivity, it is known that certain substances are added to pigments after the so-called sulfate process before annealing. During a subsequent thermal treatment, the deposited metal ions can partially get into the crystal interior of the titanium dioxide particles by diffusion. With titanium dioxide pigments, this leads to doping with a diffusion concentration profile.

0007In addition, in the case of titanium dioxide particles which are produced by the oxidation of titanium tetrachloride by the addition of dopants to the burner chamber, dopants which increase the light stability or lower the photoactivity can be incorporated directly intracrystalline into the titanium dioxide particles.

0008Intracrystalline incorporation or bulk doping is to be understood as meaning a largely homogeneous distribution of the foreign substances in the crystalline areas of the titanium dioxide particles.

0009In the case of pigments, it is also possible for substances which can act as doping substances to reduce the photostability to be deposited on the titanium dioxide particles and then to be tempered. Since this process is a diffusion-controlled reaction, tempering at high temperatures and sometimes long times must be carried out (DE 25 45 243; 500-1100 ° C, 0.1-10 hours).

0010Such a procedure is not possible for subigmentary titanium dioxide, since under such conditions particle growth of the subpigmentary titanium dioxide already takes place. In addition, such treatment is excluded for economic reasons.

0011With subpigmentary titanium dioxide it has so far not been possible to distribute a dopant intracrystalline in a titanium dioxide particle.

0012The object of the invention is to provide a subpigmentary titanium dioxide with the most homogeneous doping possible, ie intracrystalline distribution of the dopant to improve the photostability, and a process for its production.

0013The object is achieved by a method of the type mentioned at the outset, which is characterized in that a dopant dissolved in acid can be added before the ripening treatment and any remaining dopant residue must be added before the further treatment.

0014The solution to this problem is the addition of the dopant at a suitable point in the process during the production of the subpigmentary titanium dioxide, so that doping of the entire particle (bulk doping) takes place already during the formation of the subpigmentary titanium dioxide and makes thermal diffusion superfluous. A condition for such a procedure is that the addition during the production of the subpigmentary titanium dioxide neither disturbs the formation of the desired crystal structure, nor has a decisive influence on the formation of a certain particle shape or the setting of a desired particle size or particle size distribution.

0015The invention also relates to a subpigmentary titanium dioxide with bulk doping, ie a titanium dioxide below 100 nm, in which the dopant to reduce the photoactivity is not predominantly deposited on the surface of the particle, but rather the effective foreign atoms are incorporated relatively uniformly in the overall particle.

0016Dopant is understood to mean metal ions, where a dopant can also be a mixture of several types of ions and the metal ions are preferably used in the form of their chlorides or oxides. The following dopants that improve the photostability of titanium dioxide are useful; first the type of ion is mentioned and then the compound in which they are usually added to the hydrolyzed or partially hydrolyzed compound after dissolving them in hydrochloric acid. The amount of solution added is such that the atomic concentration of the dopant, based on titanium, is preferably between 0.1% and 2.0%. In the case of combinations, the atomic concentration of the individual components is to be understood: Fe²⁺ as FeCl₂ Fe³⁺ as FeCl₃ · 4H₂O Sb³⁺ as SbCl₃ Zn²⁺ as ZnCl₂ Mn²⁺ as MnCl₂ · 2H₂O Nb⁵⁺ as NbCl₅ V³⁺ as VCl₃ V⁵⁺ as V₂O₅ Ni²⁺ as NiCl₂ · 6H₂O Cu²⁺ as CuCl₂ Ta⁵⁺ as TaCl₅ V³⁺ / Nb⁵⁺ as VCl₃ / NbCl₅ Mn²⁺ / Nb⁵⁺ as MnCl₂ · 2H₂O / NbCl₅ Fe²⁺ / Fe³⁺ as FeCl₂ / FeCl₃ · 4H₂O Fe²⁺ / Fe³⁺ / Nb⁵⁺ as FeCl₂ / FeCl₃ · 4H₂O / NbCl₅ Ni²⁺ / Sb³⁺ as NiCl₂ · 6H₂O / SbCl₃ Ni²⁺ / Sb⁵⁺ as NiCl₂ · 6H₂O / SbCl₅.

0017At most, a slight change in the photostability of subpigmentary titanium dioxide was observed for the following ions: Al³⁺ as AlCl₃ · 6H₂O In³⁺ as InCl₃ Ce³⁺ as Ce₂ (SO₄) ₃ Ce⁴⁺ as Ce (SO₄) ₂ Sn⁴⁺ as SnCl₄.

0018The hydrolyzable titanium compounds which are suitable as starting substances are inorganic titanates, titanium tetrachloride, organic titanium compounds or a sodium titanate partially hydrolyzed with hydrochloric acid, which is formed by reacting a titanium dioxide hydrate obtained by hydrolysis from titanyl sulfate solution with sodium hydroxide. The reaction of the hydrolyzable titanium compounds takes place in a template; the liquid in the receiver can be water or water mixed with acid or alkalis. The dopant dissolved in acid can be added before the ripening treatment and must be completed before the further treatment. The ripening treatment is usually a heat treatment. The further treatment also includes a heat treatment and in particular the peptization required as a starting substance in the case of inorganic titanates. The peptization causes a largely colloidal distribution of the titanium dioxide. The dopant is dissolved in an acid that is also used in further treatment; This is usually a monobasic acid, such as hydrochloric acid or nitric acid.

0019The doped subpigmentary titanium dioxide can be subjected to tempering and a conventional surface treatment with, for example, inorganic oxides of Si, Zr, Al, Zn etc. In addition, it may be necessary to subject these products to final grinding with the addition of organic compounds, such as silicone oils, polyalcohols, etc.

0020To characterize the light stability, the titanium dioxide is subjected to a graying test.

0021The invention is further described below by way of example. In addition to the production of the subpigment and its bulk doping and in addition to test methods, comparative examples of undoped or only surface-coated subpigment particles are also given.

0022For the graying test, the powdery material is worked into a PVC-U system of the following composition using a laboratory mixing tool from Collin (rolling mill 150): 1000 g suspension PVC (Solvic 271 GC), 1000 g emulsion PVC (Solvic 172 GA), 400 g polymethyl methacrylate (Degalan V26), 60 g of three-base lead sulfate (Naftovin T3), 20 g dibasic lead stearate (Interstab 3150), 20 g glycerol monoleate (Loxiol G 10).

0023100 g of this basic mixture are placed on the warmed up laboratory mixing tool (roller temperature 175 ± 5 ° C, peripheral speed front roller 15 min Wal¹, rear roller 18 min⁻¹). Falling material is continuously returned to the roller until a coherent film has formed. 3 g of titanium dioxide are sprinkled on and incorporated while the film is constantly being folded over. After a mixing time of 10 minutes, the film is removed and cut into pieces measuring approximately 9 x 9 cm². The film sections are cleaned with ethanol, placed in 3 layers in a stencil frame and pressed between 2 polished chrome steel plates in an oil-hydraulic top pressure press (temperature 150 ° C., pressure 4 N / mm², pressing time 4 min, cooling phase 8 min). The plates obtained are cut into test specimens measuring 2.5 × 2.5 cm², on which the exposure test is carried out. To protect against uncontrolled exposure, the samples are stored in the dark.

0024The brightness L₀ * is measured before exposure. Then the test specimens (maximum 20) are pushed under the clamping rail of the SUNTEST device CPS from Heraeus, flooded with distilled water and exposed to the xenon lamp for 30 minutes. Immediately after the end of the exposure, the brightness L 1 * is measured again with the three-area color measuring device HUNTERLAB (measuring geometry 45 ° / 0 °, standard illuminant C, size of the measuring opening 1.2 cm diameter). The difference in brightness<maths id="math0001" num=""><math display="inline"><mrow><msub><mrow><mtext>ΔL</mtext></mrow><mrow><mtext>*</mtext></mrow></msub><mtext> = L₀ * - L₁ *</mtext></mrow></math><img file="EP0579062A2_D0001.tif" /></maths> serves as a measure of graying and is used to assess the photocatalytic activity of the titanium dioxide.

Example 1 (undoped subpigmentary titanium dioxide, comparative example)

0025220 g of an aqueous suspension of a washed titanium dioxide hydrate of the anatase form, calculated as TiO₂, which has been obtained by hydrolysis from a titanyl sulfate solution obtained by digestion of ilmenite in sulfuric acid, separation of iron (II) sulfate heptahydrate, clarification and evaporation, are obtained with distilled water diluted to a suspension with a TiO₂ content of 26 wt .-% and this suspension is heated to 60 ° C.

0026The heated suspension is added over the course of 30 minutes with stirring to 550 g of an aqueous solution containing 50% by weight sodium hydroxide and heated to 90 ° C. The mixture obtained is then held at the boiling point (about 108 ° C.) for 2 hours, then cooled to about 60 ° C. and filtered, and the filter cake is washed with distilled water until the sulfate content in the wash filtrate is less than 0.05 g / l is (SO₄ detection by BaCl₂ test).

0027An amount of the washed filter cake, corresponding to 100 g TiO₂, is made into a suspension with distilled water with a TiO₂ content of 220 g / l. Then 25% hydrochloric acid is added until a pH of 2.8 to 3.1 is reached in the mixture. The suspension is stirred for 10 minutes, the pH is checked and corrected if necessary.

0028The suspension is heated to 60 ° C. and left at this temperature for 30 minutes. The mixture is mixed with as much 25% hydrochloric acid until the weight ratio of the hydrochloric acid added at this point, calculated as HCl to TiO₂, is 0.15: 1 to 0.25: 1. The mixture is then heated to boiling in about 30 minutes (boiling point about 108 ° C.) and kept at the boiling point for 90 minutes.

0029The titanium dioxide in the suspension is flocculated by neutralization with a sodium hydroxide solution up to a pH of 6 to 7 and then separated by filtration and chloride-free with distilled water and sodium sulfate solution (2.5 g / l, 60-80 ° C.) washed. In the still moist filter paste, an emulsion of dimethylpolysiloxane is mixed, corresponding to 2 wt .-% siloxane (based on TiO₂) and then dried at 120 ° C for 10 to 16 hours. The dried product is subjected to grinding on a laboratory mill.

0030Half of the product is annealed at 500 ° C for 2 hours. The loss of organic substance determined by a carbon analysis is supplemented by adding an appropriate amount of dimethylpolysiloxane emulsion and dried and ground again.

0031The obtained subpimgent titanium dioxide consists of needle-like particles after drying, and after the thermal treatment at 500 ° C of more spherical particles, the crystalline areas of which have a rutile structure. The average particle size of the acicular product is 100 nm (longest axis), that of the spherical products is approximately 90 nm.

0032In the graying test described above, this undoped subpigmentary titanium dioxide has a ΔL * of 41.1 after a temperature treatment of 120 ° C and a ΔL * of 23.5 after a temperature treatment of 500 ° C (see Table 1) .

Example 2 (Subpigmentary TiO₂ doped with antimony)

0033Starting material and pretreatment are until they are pasted with distilled water and 25% hydrochloric acid is added until a pH of 2.8 to 3.1 is reached and the 10-minute waiting time is as in Example 1.

00340.1 atom% Sb³⁺ (2a), 1 atom% Sb³⁺ (2b) and 2 atom% Sb³⁺ (2c) (as SbCl₃) are each added in 5 ml 25% HCl (2a-c) Room temperature completely dissolved and half of this volume of the suspension was added dropwise with stirring within 5 min. The suspension is heated to 60 ° C. and left at this temperature for 30 minutes.

0035To the remaining hydrochloric acid containing the dopant, enough hydrochloric acid is added until the weight ratio (calculated as HCl to TiO₂) of the hydrochloric acid added at this point, minus the hydrochloric acid added in the first doping step, is 0.15 to 1. Now proceed to heating to boiling temperature in 30 min as in Example 1. Depending on the previous thermal treatment at 120 or 500 ° C the graying decreases considerably after 30 min exposure with increasing doping concentration (see Table 1). The thermal treatment causes a decrease in the water content and better rutilization or an improved rutile mixed crystal formation.

Example 3 (doping with nickel)

0036The doping is carried out as in Example 2. The solution of NiCl₂ · 6H₂O is carried out at 0.1 atom% in 5 ml of 25% HCl, at 1 atom% and 2 atom% in 20 ml of HCl. The results are summarized in the table under 3a to 3c. Compared to doping with Sb, the photostability is improved.

Example 4 (doping with antimony and nickel)

0037A mixture of antimony³⁺ and nickel²⁺ is used as the dopant. For this purpose, the respective amount is dissolved in 20 ml of 25% HCl when doping with 0.1 atom% Sb³⁺ and 0.1 atom% Ni²⁺, when doping with 2 atom% Sb³⁺ with 2 atoms -% Ni²⁺ the respective amounts in the total volume necessary for peptization brought hydrochloric acid into solution. The graying test shows a further improvement in photostability (see Tables 1, 4a and b).

Example 5 (comparative example, striking instead of doping)

0038Subpigmentary titanium dioxide is produced according to the procedure of Example 1. After the cooking time over 90 min, the suspension is cooled and 1 atom% (5a) or 2 atom% (5b) Sb³⁺ as antimony chloride, based in each case on titanium, is added dropwise with stirring. This is followed by the dropwise addition of 30% by weight sodium hydroxide solution to pH 7.7 to 8.3. It is then filtered, washed free of chloride and an emulsion of dimethylpolysiloxane is mixed into the still moist filter paste (corresponding to 2% by weight of siloxane based on TiO₂), and then dried at 120 ° C. for 10 to 16 hours. The dried product is ground with a laboratory mill.

0039As can be seen in Table 1, the graying values ​​5a and 5b are significantly worse in comparison to the intracrystalline doping of equal amounts of the doping substance (Examples 2b and 2c).

Example 6 (control of the part size with tin dioxide and doping with nickel)

0040In addition to the Ni² Ni doping, this example describes the control of the particle size of the subpigmentary titanium dioxide. The dough is added dropwise to the colloidal tin dioxide solution after doping with the first portion of the dopant and 30 minutes of scratching at 60 ° C. 4% by weight (based on TiO₂). The colloidal tin dioxide solution is prepared as follows: in a template from distilled water, tin tetrachloride (pa, 99% SnCl₄) is added dropwise to a concentration of 200 g / l SnO₂. The amount of tin dioxide required for the amount of tin dioxide required to produce a certain particle size in the preparation of the subpigmentary titanium dioxide is diluted to 10 g / l SnO₂ with distilled water. The temperature must not rise above 20 ° C. This mixture is kept at 20 ° C. for one hour for ripening. The solution must now be used immediately.

0041The subpigmentary titanium dioxide obtained has a rutile structure, is needle-shaped and has an average particle size of 30 nm (longest axis).

0042The results of the graying measurement are slightly worse than the measured values ​​of the products in Example 3. The reason for the increased photoactivity is the increase in the specific surface area of ​​the subpigmentary titanium dioxide due to a smaller particle size.

Example 7 (comparative example to example 6 without dopant)

0043Very poor graying values ​​are again obtained from undoped subpigmentary titanium dioxide produced with tin dioxide nuclei (see Table 1).

Example 8 (Differentiation of a Subpigment with an Sb Doping from a Subpigment with an Sb Precipitation)

0044<ul id="ul0001" list-style="none"><li>a) Leach attempt Subpigmentary titanium dioxide doped with antimony and titanium dioxide coated with antimony as in Example 5b, prepared according to the invention as in Example 2c, each after a thermal treatment at 500 ° C., is mixed with 98% sulfuric acid at 100, 140 and 180 ° C. for 2 hours Brought reaction. The amount of antimony released in the solution during this leaching test is analyzed with the help of atomic emission spectroscopy with inductively coupled plasma (ICP-AES). In Table 2 it can be seen that, as expected, this antimony is more readily dissolved at a lower temperature than is the case with the doped subpigment. With this product, the antimony present in the lattice only gradually dissolves with the progressive destruction of the titanium dioxide lattice.</li><li>b) X-ray structure analysis It is known that when rutile mixed-phase compounds of antimony and nickel are formed, an X-ray reflex is shifted in the X-ray structure analysis. This reflex is expected in the pure rutile at an angle 2ϑ = 54.0 to 54.5 ° (HB Krause, HW Reamer, JL Martin, Mat. Res. Bul., Vol. 3 (1968) 233-240 and HB Krause, same journal, 241-252).</li></ul>

0045In the case of undoped subpigmentary titanium dioxide (as in Example 1), an angle of 54.4 ° is measured. Subpigmentary titanium dioxide with a 2 atom% antimony precipitation as in Example 5b shows an X-ray reflex unchanged at 2ϑ = 54.4 °. If, on the other hand, the antimony is incorporated intracrystalline into the subpigmentary titanium dioxide particle as in Example 2c, the angle changes significantly to 2ϑ = 54.3 °. According to HB Krause et al. be correlated with the formation of a rutile mixed phase compound.<tables id="tabl0001" num="0001"><img file="EP0579062A2_D0002.tif" /></tables><tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Leach tests (Sb concentration 2 atom%)</entry></row><row><entry namest="col1" nameend="col1" align="center">Temperature in ° C</entry><entry namest="col2" nameend="col2" align="center">dissolved Sb / TiO₂ in% with doped Sb</entry><entry namest="col3" nameend="col3" align="center">dissolved Sb / TiO₂ in% with Sb</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">100</entry><entry namest="col2" nameend="col2" align="char" char=",">0,4</entry><entry namest="col3" nameend="col3" align="char" char=",">4,2</entry></row><row><entry namest="col1" nameend="col1" align="right">140</entry><entry namest="col2" nameend="col2" align="char" char=",">1,3</entry><entry namest="col3" nameend="col3" align="char" char=",">3,5</entry></row><row><entry namest="col1" nameend="col1" align="right">180</entry><entry namest="col2" nameend="col2" align="char" char=",">3,1</entry><entry namest="col3" nameend="col3" align="char" char=",">3,1</entry></row><row rowsep="1"><entry namest="col1" nameend="col3" align="justify">(thermal treatment of the samples each at 500 ° C)</entry></row></tbody></tgroup></table></tables>