Photoreactive titanium dioxide composition

Various types of photosensitive materials such as carbon black and zinc oxide have been used in photographic and copy paper systems. Although carbon black has excellent conductivity properties, it has little or no photoreactive properties required for use in copier paper systems. Although zinc oxide is used with limited success, it has poor opacity in copying papers.

Titanium dioxide pigments are also known which possess a certain photosensitivity. The disadvantage, however, is that this photosensitivity is insufficient for use in copy paper systems.

From CH-PS 2 87 876 a process for the production of anatase is known, which gives a product with a relatively low sulfur content. This product is obtained by vapor phase oxidation of titanium tetrachloride. Furthermore, it is known from German Offenlegungsschrift No. 10 70 318 that ilmenite is usually digested with sulfuric acid to produce titanium dioxide, ferrous sulfate is precipitated from the digestion solution, the titanium sulfate solution is hydrolyzed in the heat, and the precipitated titanium dioxide hydrate subsequently glows. None of the said processes gives any indication as to the preparation of a photosensitive titanium dioxide which can be used as a photosensitive composite._l0 is suitable in the copying paper industry.

Titanium dioxides from Tappi's monograph 20 (1958), the table on page 2 and 3, as well as "pigments" by Dr. Ing. H. K i 11 e 1,1960, page 246 to 248 are known. r,_s

The object of the invention is to provide a light-sensitive titanium dioxide which, because of its ability to be used or photoreactivity, can be used

It is particularly suitable in the case of copitral paper industry

This object is achieved by a method for producing an photosensitive titanium dioxide having the crystal structure of anatase, wherein titanic acid precipitated from a titanium sulfate solution, the hydrous titanium dioxide bleached, slurried in water, the slurry with an alkaline ammonium compound to a pH of at least 6 , 0 set, washed the hydrous titanium dioxide, heated to a temperature of 250 to 700 ° C and ground to an average particle size of 100 to 500 A, according to patent application P 17 67 783.7-51, which is characterized in that the milled titanium dioxide slurried in water, the slurry with a water-soluble Pb, Bi, Ti, Zn, Si and / or Al salt in an amount sufficient to form from 0.2 to 2.0% by weight of metal oxide in the dry TiO 2₂ On the surface of the titanium dioxide, the corresponding metal oxide is precipitated, the coated titanium dioxide is filtered off, washed, dried and ground again.

According to the above method, the precipitated on the titanium dioxide metal oxide falls first as hydrous oxide.

Compared to the method of the main patent application, the method according to the invention makes it possible to produce a titanium dioxide which, when used in photosensitive systems, gives an improved image quality. Particularly favorable results can be achieved if the titanium dioxide is coated with several metal oxides.

The invention further provides a photosensitive titanium dioxide having the crystal structure of anatase, which has been obtained by the process according to the invention.

The titanium dioxide having anatase crystal structure obtainable according to the present invention is a non-pigmentary titanium dioxide and contains finely divided particles having an average single particle size of 100 to 500 Å, a sulfur content calculated as% sulfur within the range of 0.03 to 0, 3%, a surface area of ​​60 to 250 m²/ g, a moisture content of 2 to 10% and a photosensitivity, measured according to the Mandelsäurei est, corresponding to a change in the reflectance of at least 5 units in 2 minutes under the action of ultraviolet radiation. Compared to the titanium dioxide obtainable according to the main patent application, it is characterized by a metal oxide coating of one or more of the oxides PbO, B12O3, TiO₂, ZnO, SiO₂ and Al 2 O 3, the amount of said coating being in the range of 0.2 to 2.0% metal oxide based on the weight of the titanium dioxide.

By "photosensitive" or "photoreactive" titanium dioxide is meant here a titanium dioxide which reacts in a photosensitive system and in particular in a copier paper system upon exposure to ultraviolet light to form an image.

The titanium dioxide prepared according to the invention was tested for its photosensitivity and found to conform to standard specifications which were chosen to be industrially useful.

These tests and their standard conditions are described below.

Mandelic Acid Test of Photosensitivity of Titanium Dioxide Materials

10 Titanium dioxide is mixed with a sufficient amount of a 0.5 M aqueous solution of mandelic acid to form a soft paste. The paste is placed on a microscope glass slide (127 mm χ 127 mm χ 1 mm), covered with a thick glass plate (127 mm χ 127 mm double thick window glass) and pressed to spread the paste over an area of ​​about 76 mm in diameter, using microscope slides (25 mm χ 76 mm χ 1 mm) as spacers between the plates. The plate edges are bonded together to prevent evaporation with a 25 mm tape. The initial reflectance of the paste through the microscope slide glass plate is measured with a green color differential colorimeter. The plates are placed with the microscope slide glass plate side up on a rotating turntable under 3 quartz lamps (275 watts) mounted in an equilateral triangle 33 cm from center to chest, about 33 cm above the turntable. Before each measurement, the ultraviolet intensity of the lamp is measured and weak lamps are replaced. The reflectance measurements of the pastes are made at appropriate intervals, and the change in reflectivity with time is determined. This number is a measure of the photoreactivity of the titanium dioxide material. A minimum reflectivity change of at least 5 units in 10 minutes is required to provide useful images in paper coating systems, photographic emulsions, and similar engineering applications.

The average particle size of titanium dioxide is determined by X-ray diffraction analysis.

The moisture content of the product was determined by heating a 1 g sample for one hour at 750 ° C and determining the weight loss.

The following examples illustrate the invention.

example 1

400 g powdered titanium dioxide, prepared as in Example 1 according to patent application P 17 67 783.7-51, were slurried in water to a solids content of 23%. To the slurry was added a lead acetate solution in such an amount that 0.2% PbO based on TiO₂, were received. After thoroughly stirring the mixture, the slurry was added with such an amount of NH₄OH treated to obtain a slurry having a pH of 7.5 and precipitated the lead as a hydrous oxide. The pH of the slurry was then maintained at 7.5 for one hour. The slurry was then dehydrated and the filter cake was washed thoroughly with water to remove the soluble salts. The washed cake was then dried and ground. To demonstrate the effectiveness of the titanium dioxide coating, images were prepared using the uncoated product and the coated product on a substrate

The images were created by mixing the titanium dioxide with water and a polyacrylate and coating a substrate with the mixture. Of the

The coating on the substrate was then subjected to a redox reaction to develop the image. The details of the preparation of the image are similar to those described in French Patent 14 82 724.

Using the method described by the American Standards Association in a report entitled American Standard Sensitometry of Photographic Papers, March 17, 1966, image properties were determined by maximum optical density and brightness.

Using the uncoated material, the resulting image had a maximum optical density of 0.8 at e^;brightness 16. When producing an image using the coated product, the maximum optical density increased by about 25% without lowering the brightness.

Examples 2 to 7

The procedure of Example 1 was repeated using various agents to prepare the metal oxide coatings. The chlorides of bismuth, titanium, aluminum, silicon and zinc were added to form the respective metal oxide coatings on the titanium dioxide. In Example 5, a mixture of bismuth and titanium chloride was used, while in Example 6 sodium metasilicate as SiO₂-Quel-Ie was added. The agents used and the quantities applied are given in the following table:

<p><tgroup cols="3"><tbody><row><entry>Used salt</entry><entry>got</entry><entry>Oxydmenge</entry></row><row><entry></entry><entry>metal oxide</entry><entry> in % </entry></row><row><entry>BiCl₃</entry><entry>Bi₂O₃</entry><entry>0,2</entry></row><row><entry>TiCl₄</entry><entry>TiO₂</entry><entry>0,2</entry></row><row><entry>ZnCl₂</entry><entry>ZnO</entry><entry>0,2</entry></row><row><entry>BiCl₃ + TiCl₄</entry><entry>Bi₂O₃ + TiO₂</entry><entry>0,2 + 0,2</entry></row><row><entry>N / A₂SiO₃</entry><entry>SiO₂</entry><entry>0,2</entry></row><row><entry>AlCl₃</entry><entry>al₂O₃</entry><entry>0,2</entry></row></tbody></tgroup></p>

When producing images using these coated materials, the image intensity values ​​were increased by 10 to 25% over that obtained using the uncoated product, without the brightness decreasing.