Method for preparation of fine particulate titanium dioxide and fine particulate titanium dioxide.

The invention relates to a method for producing fine-particle titanium dioxide which is essentially transparent to visible light and essentially absorbent to UV radiation, and to a fine-particle titanium dioxide which is essentially transparent to visible light and essentially absorbent to UV radiation.

It is known that the scattering power of a pigment, e.g. B. a white pigment, for visible light depends on the particle size. For titanium dioxide pigments, the optimum particle size for the scattering capacity is about half as large as the light wavelength, depending on the surrounding medium. Below this optimal particle size, the scattering capacity decreases with decreasing particle size. As a result of this decrease in the scattering power for visible light, transparent media, into which a finely divided titanium dioxide with a particle size substantially below half the light wavelength is incorporated, largely retain their transparency for visible light due to the very low scattering power of this titanium dioxide. This state of affairs is expressed in a simplified manner in that such a titanium dioxide is referred to as "essentially transparent to visible light". In contrast, the absorption of UV radiation remains practically unaffected by the decreasing particle size of the crystalline titanium dioxide.

This finely divided titanium dioxide, which is essentially transparent to visible light and essentially absorbent to UV radiation, is also referred to below as "subpigmentary titanium dioxide".

There are many areas of application for this product. For example, it is used in plastic films or plastic containers where the wall allows a look inside, but the goods in them are protected by the absorption of the harmful UV component of sunlight by the subpigmentary titanium dioxide in the wall. In paints, e.g. B. wood protection glazes, subpigmentary titanium dioxide is used as a UV absorber. Another area of ​​application is cosmetics, e.g. B. sunscreen cosmetics, and other skin care products in which subpigmentary titanium dioxide is used to absorb the harmful UV radiation. A special area of ​​application is the generation of special optical effects in paints, e.g. B. the so-called "downflop" (see<u style="single">EP-A1-0 270 472</u>) There are various methods for producing subpigmentary titanium dioxide. So after the<u style="single">DE-A1-38 17 909</u> Suitable processes for the production of needle-shaped products are used for this purpose, e.g. B. the hydrolysis of a suitable titanium compound such as titanium tetrachloride or an organic or inorganic titanate or the oxidation of an oxidizable titanium compound, e.g. B. in gaseous state. There, a special process for the production of a needle-shaped subpigmentary titanium dioxide is also described, in which a titanium dioxide hydrate, which has been obtained in the usual way by hydrolysis from a titanyl sulfate solution, is heated in an aqueous slurry and added to a likewise heated aqueous sodium hydroxide solution. The mixture obtained is kept at the boiling point for a while. The suspension now containing sodium titanate is cooled and filtered, and the filter cake is washed. A pH of 2.8 to 3.1 is then set in the mixture containing sodium titanate by adding hydrochloric acid, and the mixture is then peptized by adding further amounts of hydrochloric acid. In this treatment, the sodium titanate is hydrolytically decomposed and the mixture then contains titanium dioxide, which is present in very fine particles and whose small crystallites have a rutile structure. The particles are acicular. The solid contained in the mixture is flocculated by neutralization and then separated from the liquid by filtration and washed. From the<u style="single">CA-A 962 142</u> is also known to anneal a subpigmentary titanium dioxide produced in this way after the addition of K₂O and P₂O₅ at 500 to 800 ° C.

In the known processes, it is difficult to vary the particle size and particle shape of the subpigmentary titanium dioxide in accordance with the respective requirements of the different fields of application.

A new process for the production of finely divided titanium dioxide, which is essentially transparent to visible light and essentially absorbent to UV radiation, has been found, in which these disadvantages of the known processes are avoided. This process is characterized in that in the production of the finely divided titanium dioxide, finely divided tin dioxide, the particle size of which is 1 to 10 nm, preferably 1 to 4 nm, is used in an amount of 0.5 to 10 percent by weight, based on the amount of the titanium dioxide produced becomes.

In addition to the use of the finely divided tin dioxide according to the invention, this process can contain all process steps known per se for the production of subpigmentary titanium dioxide.

A special embodiment of the method according to the invention is characterized in that<ul id="ul0001" list-style="none"><li>a) a titanium dioxide hydrate with anatase structure produced by hydrolysis of a titanyl sulfate solution is slurried with water to form a suspension with a concentration of 20 to 26 percent by weight of TiO 2,</li><li>b) the titanium dioxide hydrate suspension obtained is heated to 60 to 70 ° C,</li><li>c) the heated titanium dioxide hydrate suspension is added to an amount of an aqueous sodium hydroxide solution heated to 90 to 100 ° C. with stirring, that after its addition the weight ratio of NaOH to TiO₂ in a mixture is 1.25 to 1.65,</li><li>d) the mixture is heated to boiling and kept at the boiling point for 120 to 140 minutes,</li><li>e) the mixture is then cooled to about 50 to 60 ° C. and filtered,</li><li>f) the filter cake obtained is washed until the SO₄²⁻ content in the wash filtrate is less than 0.05 g / l,</li><li>g) the washed filter cake is pasted with water up to a TiO 2 content of 10 to 25 percent by weight and the resulting mixture is mixed with a hydrochloric acid containing 20 to 25 percent by weight HCl up to a pH of 2.8 to 3.1,</li><li>h) the mixture is heated to 55 to 65 ° C. and left at this temperature for 30 to 45 minutes, the pH range from 2.8 to 3.1 being maintained,</li><li>i) a colloidal tin dioxide solution is added to the mixture, in which the tin dioxide has a particle size of 1 to 10 nm, preferably 1 to 4 nm,</li><li>j) the mixture containing the colloidal tin dioxide solution is peptized with as much 20 to 25 percent by weight HCl hydrochloric acid at 55 to 65 ° C that a weight ratio of hydrochloric acid added in this stage calculated as HCl to TiO₂ from 0.15: 1 to 0 , 25: 1 is set,</li><li>k) the suspension obtained is heated to boiling over the course of 30 to 40 minutes and kept at the boiling point for 60 to 90 minutes, and</li><li>l) the suspension is neutralized to a pH range of 4 to 11, preferably 6 to 8.</li></ul>

Another suitable embodiment of the process according to the invention is characterized in that the finely divided titanium dioxide is produced by hydrolysis of a compound of the tetravalent titanium which hydrolyzes under the relevant hydrolysis conditions with the formation of rutile in the presence of tin dioxide, the particle size of which is 1 to 10 nm, preferably 1 to 4 nm .

A particularly suitable embodiment of this process is characterized in that titanium tetrachloride or a compound which is formed by the reaction of titanium tetrachloride with water is used as the compound of the tetravalent titanium.

The hydrolysis can e.g. B. with heating, optionally no ch diluting the solution with water or a suitable aqueous solution, and / or by adding an alkaline substance, e.g. B. sodium hydroxide.

It is essential that the tin dioxide is used in a particle size which is predominantly 1 to 10 nm, preferably 1 to 4 nm. Since there is a statistical particle size distribution in the case of tin dioxide, the claimed particle size ranges are understood in the usual way in such a way that the tin dioxide can to a lesser extent also contain particles which are outside the stated limits. As a rule, the particles with a size of 1 to 4 nm should be particularly strongly represented in the claimed particle size range from 1 to 10 nm, since particles of this size are particularly effective for the process according to the invention. The use of tin dioxide with a particle size above 10 nm is unsuitable because fewer tin dioxide particles are then available when the same amount of tin dioxide is used.

It has been found that it is possible to produce and use tin dioxide with the claimed particle size ranges by a certain procedure. A particularly suitable embodiment of the process according to the invention is characterized in that the tin dioxide is used in the form of a colloidal solution which is added to distilled water by slowly adding tin tetrachloride at a temperature of at most 22 ° C. to a concentration of 180 to 220 g / l SnO₂, dilute this solution with distilled water to 10 g / l SnO₂, the temperature not exceeding 22 ° C, and maturing the diluted solution at 20 to 22 ° C for one hour.

Other procedures which likewise lead to a tin dioxide with a particle size of 1 to 10 nm, preferably 1 to 4 nm, are not to be excluded thereby.

The finely divided titanium dioxide produced according to the invention can be in the form of a suspension, and this suspension can be processed further without the finely divided titanium dioxide being separated from it.

According to a particular embodiment of the method according to the invention, the titanium dioxide produced is flocculated from the suspension and separated from the liquid phase by filtration. The separated titanium dioxide can be washed.

A special embodiment of the invention is characterized in that the titanium dioxide separated from the suspension from the liquid phase is dried. This dried titanium dioxide can be ground.

It is often advisable to add a substance after neutralization, but in the case of drying the titanium dioxide before it dries, which prevents the formation of hard agglomerates; suitable for this are e.g. B. alkanolamines or polyols.

The product obtained usually consists of needle-shaped particles with a rutile structure.

According to a particular embodiment of the process according to the invention, the dried titanium dioxide is at 300 to 800 ° C; preferably annealed 400 to 600 ° C.

If the subpigmentary titanium dioxide according to the invention consists of needle-shaped particles, rounding of these particles can be achieved by tempering. In contrast to the known methods, the addition of K₂O and P₂O₅ as well as other substances is not necessary. If a temperature between 400 and 600 ° C. is maintained during the tempering, the particles will noticeably round off without these particles experiencing a significant increase in size. At temperatures below 400 ° C there is still a slight rounding effect, at temperatures above 600 ° C there is often undesirable coarsening of the particles.

The subpigmentary titanium dioxide can be used without further after-treatment. However, it is also often expedient to post-treat the titanium dioxide with one or more inorganic and / or organic substances in accordance with a particular embodiment of the invention.

In this aftertreatment, the subpigmentary titanium dioxide is coated with inorganic and / or organic substances. The aftertreatment can already take place in the suspension in which the subpigmentary titanium dioxide produced is initially present, this aftertreatment also being able to be carried out in the course of the final neutralization step. However, it can also be carried out on the flocculated titanium dioxide before or after its separation from the suspension and in the latter case before or after drying and optionally tempering. The aftertreatment can be preceded by grinding. It may be necessary to add a dispersant to the subpigmentary titanium dioxide before grinding. Methods known per se, such as those described in CA-A-962 142, can be used for the aftertreatment.

The invention also relates to a finely divided titanium dioxide which is essentially transparent to visible light and essentially absorbent to UV radiation, consisting of needle-shaped particles with a rutile structure with a length of 10 to 100 nm and a length: diameter ratio of 8: 1 to 3: 1, these particles containing 0.5 to 10 weight percent SnO₂.

The invention also relates to an aqueous suspension of such a finely divided titanium dioxide.

Another object of the invention is a titanium dioxide which is essentially transparent to visible light and essentially absorbent for UV radiation, consisting of rounded particles with a rutile structure with a diameter of 10 to 100 nm, these particles being 0.5 to 10 percent by weight Contain SnO₂.

According to a particular embodiment of the invention, this finely divided titanium dioxide, which consists of needle-shaped or rounded particles, contains a coating of at least one inorganic and / or organic substance applied by post-treatment.

The invention also relates to an aqueous suspension of a finely divided titanium dioxide containing such a coating.

It has been found that the particle size of the subpigmentary titanium dioxide produced in the range from 10 to 100 nm for the longest axis of the acicular particles with a small size distribution range can be influenced in a simple manner by the amount of tin dioxide added.

The higher the addition of SnO₂, the smaller the particle size of the titanium dioxide obtained. The amount of tin dioxide added in the preparation of the subpigmentary titanium dioxide is preferably between 1 and 10 percent by weight, based on TiO₂. The size of the particles obtained can be done by electron microscopy or by determining the surface size according to BET.

Essential for the effectiveness of the added colloidal tin dioxide solution is the fact that this colloidal tin dioxide solution is produced in such a way that not only the critical particle size range is set during the production of the tin dioxide, but also that it is ensured that the tin dioxide formed after it has been produced and before it Use in the production of the subpigmentary titanium dioxide does not experience particle coarsening.

For this purpose, the procedure can preferably be as follows: In a template with distilled water, tin tetrachloride is added dropwise to a concentration of 180 to 220 g / l of SnO₂ with cooling at a temperature of at most 22 °. The stock solution thus obtained is stable in storage for a long time. To prepare the desired colloidal tin dioxide solution, this stock solution is diluted with distilled water to 10 g / l SnO₂. The temperature must not exceed 22 ° C. The mixture is then kept at 20 to 22 ° C for one hour and ripened. The solution is then practically clear and must be used immediately.

The invention is explained in more detail by the following examples:

example 1

220 g of an aqueous suspension of a titanium dioxide hydrate of the anatase form, calculated as TiO₂, which had been obtained and washed by the usual hydrolysis from a titanyl sulfate solution obtained by digestion of ilmenite in sulfuric acid, separation of iron (II) sulfate heptahydrate, clarification and evaporation diluted with distilled water to a suspension with a TiO₂ content of 26 percent by weight, and this suspension was heated to about 60 ° C.

The heated suspension was added over 30 minutes with stirring to 550 g of an aqueous solution containing 50% by weight of NaOH and heated to 90 ° C. The resulting mixture was then held at the boiling point (about 108 ° C) for 2 hours, then cooled to about 60 ° C and filtered, and the filter cake was washed with distilled water until the SO₄²⁻ content in the wash filtrate was less than 0. Was 05 g / l (SO₄²⁻ detection by the BaCl₂ test).

The washed filter cake was pasted with distilled water to a suspension with a TiO₂ content of 220 g / l. Then 25% hydrochloric acid was added until a pH of 2.8 to 3.1 was set in the mixture. The suspension was then heated to 60 ° C. and left at this temperature for 30 minutes. The specified pH range could not be left. For this purpose, the pH was checked and adjusted if necessary.

A colloidal tin dioxide solution was then added, which was prepared as follows: To prepare a stock solution, tin tetrachloride (pa, 99% SnCl₄) was added dropwise to a concentration of 200 g / l SnO₂ in a template from distilled water with cooling. The amount of the stock solution required for the amount of SnO₂ required for the production of the subpigmentary titanium dioxide was diluted to 10 g / l SnO₂ with distilled water. Care was taken to ensure that the temperature did not rise above 20 ° C. The mixture was then kept at 20 ° C. for one hour and ripened in the process. The solution was now practically clear and had to be used immediately afterwards.

After the colloidal tin dioxide solution had been added, the suspension was treated with hydrochloric acid containing 25% by weight of HCl so that the weight ratio of the amount of hydrochloric acid added here, calculated as HCl to TiO₂, was 0.15: 1. The suspension was peptized. The mixture was heated to boiling in about 30 minutes (boiling point about 108 ° C) and held at the boiling point for 90 minutes. The mixture now had, depending on the volume of the colloidal tin dioxide solution added, a TiO 2 content of about 100 to 160 g / l.

The titanium dioxide in the suspension was flocculated by neutralization with a sodium hydroxide solution up to a pH of 6 to 7 and then separated by filtration and washed. The subpigmentary titanium dioxide obtained consisted of acicular particles, the crystalline regions of which had a rutile structure.

In the preparation of the subpigmentary titanium dioxide, the amount of colloidal tin dioxide solution added was varied between 1 and 10 percent by weight, calculated as SnO₂ and based on TiO₂. The products obtained were examined by electron microscopy and their surface was determined according to BET in accordance with DIN 66132. The results are shown in Table 1. For comparison, a product is also listed in Table 1, which was produced in the same way as the products according to the invention, but without the addition of colloidal tin dioxide solution. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</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="col1" align="center">SnO₂ addition weight percent based on TiO₂</entry><entry namest="col2" nameend="col2" align="center">BET m² / g</entry><entry namest="col3" nameend="col3" align="center">Particle size, measured on the largest axis nm</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="char" char=",">0</entry><entry namest="col2" nameend="col2" align="right">98-116</entry><entry namest="col3" nameend="col3" align="char" char=",">110 ± 10</entry></row><row><entry namest="col1" nameend="col1" align="char" char=",">1</entry><entry namest="col2" nameend="col2" align="right">110</entry><entry namest="col3" nameend="col3" align="char" char=",">80 ± 10</entry></row><row><entry namest="col1" nameend="col1" align="char" char=",">2</entry><entry namest="col2" nameend="col2" align="right">135</entry><entry namest="col3" nameend="col3" align="char" char=",">42 ± 7</entry></row><row><entry namest="col1" nameend="col1" align="char" char=",">4</entry><entry namest="col2" nameend="col2" align="right">145</entry><entry namest="col3" nameend="col3" align="char" char=",">35 ± 10</entry></row><row><entry namest="col1" nameend="col1" align="char" char=",">5,5</entry><entry namest="col2" nameend="col2" align="right">155</entry><entry namest="col3" nameend="col3" align="char" char=",">25 ± 10</entry></row><row><entry namest="col1" nameend="col1" align="char" char=",">7,5</entry><entry namest="col2" nameend="col2" align="right">155</entry><entry namest="col3" nameend="col3" align="char" char=",">20 ± 10</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="char" char=",">10</entry><entry namest="col2" nameend="col2" align="right">165</entry><entry namest="col3" nameend="col3" align="char" char=",">15 ± 5</entry></row></tbody></tgroup></table></tables>

Table 1 clearly shows the effectiveness of the addition of SnO₂ on the particle size of the subpigmentary titanium dioxide obtained; Likewise, it can be seen that the desired particle size could be easily controlled by the amount of SnO₂ addition chosen.

Example 2

A subpigmentary titanium dioxide was produced as in Example 1, with various amounts of the colloidal tin dioxide solution being added. The products obtained were each annealed at different temperatures for 2 hours. For the annealed products, the longest axis and the axial ratio (longest axis: shortest axis) of the particles were determined by electron microscopy and the surface using the BET method in accordance with DIN 66132. The results are shown in Table 2. For comparison, non-tempered products and a product that was produced without the addition of SnO₂ are listed.<tables id="tabl0002" num="0002"><img file="EP0499863B1_D0001.tif" /></tables>

From Table 2 it can be seen that when the subpigmentary titanium dioxide was tempered, the particles became significantly rounded. This rounding was accompanied by a certain reduction in the surface of the products produced with SnO₂ addition at tempering temperatures of 400 to 600 ° C, but this is essentially due to the rounding itself. At an annealing temperature of 800 ° C., the surface area was significantly reduced further, which was due to an increase in the size of the particles. Table 2 also shows that the annealed products with SnO₂ have a much larger surface area than the annealed product without the addition of SnO₂, which is due to a much smaller particle size of the first-mentioned products.

Example 3

In a template from 671.2 ml of distilled water, 260 ml of a colloidal tin dioxide solution with a SnO₂ content of 10 g / l, which was prepared as in Example 1, were added at 20 to 22 ° C. with stirring within 2 minutes. Immediately after the addition of the colloidal tin dioxide solution, 118.7 g of titanium tetrachloride were added dropwise to this mixture with cooling to 20 to 22 ° C. over the course of 140 minutes, and the mixture was stirred at this temperature for a further 5 minutes after the addition of titanium tetrachloride had ended.

The resulting slightly cloudy solution was heated to the boiling temperature (about 107 ° C.) with stirring at a rate of about 2 to 3 ° C./minute and left at this temperature for 120 minutes. During the heating, the solution initially became clear, but then became cloudy again at about 90 ° C. because titanium dioxide precipitated out. The suspension was then cooled to 40 ° C. and the titanium dioxide was separated off by filtration. The amount of SnO₂ added was 5.2 percent by weight, based on TiO₂ produced.

The subpigmentary titanium dioxide obtained consisted of acicular particles with a rutile structure, which had a size of 40 ± 10 nm, measured on the longest axis, and an axis ratio (longest axis: shortest axis) of about 4.

The invention relates to a method for producing finely divided titanium dioxide and finely divided titanium dioxide. The titanium dioxide is essentially transparent to visible light and essentially absorbent to UV radiation. In its manufacture, finely divided tin dioxide, the particle size of which is 1 to 10 nm, preferably 1 to 4 nm, is used in an amount of 0.5 to 10 percent by weight, based on TiO₂. The particle size of the titanium dioxide is specifically adjusted by the addition of tin dioxide. By heating at 300 to 800 ° C, preferably 400 to 600 ° C, needle-shaped particles are rounded. The particles can be coated with inorganic and / or organic substances by post-treatment. The finely divided titanium dioxide can be prepared over sodium titanate by decomposition with hydrochloric acid or by suitable hydrolysis of a compound of the tetravalent titanium which leads to the formation of rutile. The tin dioxide is preferably used as a colloidal solution which is formed by reacting tin tetrachloride with water at a maximum of 22 ° C.