Color Intensive Iron Oxide Black Pigments Obtained by the Nitrobenzene Reduction Process and a Process for Its Preparation

FIG. 1 is a photo micrograph of a conventional iron oxide black pigments obtained from the nitrobenzene reduction process.

FIG. 2 is a photo micrograph of pigments according to the present invention .

The present invention relates to iron oxide black pigments obtained by the nitrobenzene reduction process, having a crystallite size of less than 80 nm and a relative color intensity of more than 150%, preferably 200 to 250%, with a blue tinge of .DELTA.b*=.+-.0 to -2.0 units, based on .RTM.Bayferrox 320.

The iron oxide black pigments according to the invention are readily dispersible and attain their final color intensity even without additional grinding. The .RTM.Bayferrox 320 used for comparison is an iron oxide black pigment produced by the nitrobenzene reduction process and having an iron content of 92 to 95%, calculated as Fe.sub.2 O.sub.3. .RTM.Bayferrox 320 is a Trade product of Bayer AG. The iron oxide black pigments normally produced by the nitrobenzene reduction process can be recognized under electron microscopic magnification by their irregular, clefted surface as shown in FIG. 1. The products according to the present invention, on the other hand, have smooth pigment surfaces as shown in FIG. 2. Due to the starting materials used in nitrobenzene reduction process (SiO.sub.2 -containing scrap or iron turnings), all the pigments have an SiO.sub.2 content of more than 1%.

It is surprisingly found that the iron oxide black pigments according to this invention may be obtained quite simply by subjecting finely divided iron oxide black pigments produced by the nitrobenzene reduction process to a heat treatment at an elevated temperature.

The present invention thus relates to a process for the preparation of iron oxide black pigments by the reaction of aromatic nitro compounds with metallic iron, optionally with the addition of control chemicals, to produce finely divided pigments which are predominantly Fe.sub.3 O.sub.4, characterised in that the finely divided pigments thus prepared are subjected to a temperature treatment at 200.degree. C. to 800.degree. C. in a non-oxidizing atmosphere.

The temperature treatment is most preferably carried out at 400.degree. C. to 750.degree. C. Especially good results are obtained if the non-oxidizing atmosphere contains steam. It has been found that the dwell time for the temperature treatment is advantageously 2 minutes for 5 hours.

The process according to the invention may advantegeously be carried out on any iron oxide black pigments which have been obtained by the nitrobenzene reduction process. An exceptionally great improvement in the intensity of the color is obtained, however, when the process according to the invention is carried out on a product containing predominantly Fe.sub.3 O.sub.4 which is not ordinarily suitable as black pigment due to its brown tinge.

The non-oxidizing atmosphere used in the present invention can be any gas which will not oxidize the iron oxide pigment. Suitable gases include steam or water vapor, nitrogen, argon, and soforth.

It has been found particularly advantageous to carry out the process according to the invention continously in a rotary tubular furnace.

The process according to the invention is generally carried out as follows:

A finely divided, brown tinged Fe.sub.3 O .sub.4 pigment is introduced either ground or unground and either dried or as a moist filter cake into a rotary convertor furnace and heated to over 400.degree. C. under a protective furnace gas such as N.sub.2, Ar, or H.sub.2 O, and then cooled to below 100.degree. C. This process may be followed by the usual methods for stabilizing iron oxide black pigments as described, for example, in U.S. Pat. No. 4,082,905 or U.S. Pat. No. 4,491,619.

The pigments should be cooled under the protective gas to temperatures below 100.degree. C. after the temperature treatment in order to prevent any deterioration in their optical properties due to oxidation.

All Fe.sub.3 O.sub.4 pigments produced by the nitrobenzene reduction process are suitable for the process according to the invention and the quality of their color is improved by the process. The pigments may be put into the process either as pigment pastes (solids content 60-70%) or as dried, unground pigments or as dried, ground pigments.

The invention will now be illustrated by Examples but is not limited by them.

500 g of each of the iron oxide pigments known under the Trade names of .RTM.Bayferrox 306, 316, 318 and 320 obtained by the nitrobenzene process (according to DE-C 551 255) were heated to 500.degree. C. in a laboratory rotary convertor furnace in an atmosphere of 250 l of N.sub.2 /h which had been saturated with water vapor at room temperature, and the pigment was kept at 500.degree. C. for 2 hours. After cooling to a temperature below 100.degree. C., the color values were determined on a white blend (mixture of the black pigment with a commercial TiO.sub.2 white pigment in proportions by weight of 1:5) according to ISO 7224-2 and the results were converted to CIELAB 2/d values according to ISO 7724-3 and compared with that of the Trade product of Bayer AG.RTM. Bayferrox 320 in .RTM.Alkydal F 48 lacquer (Trade product of Bayer AG). Negative .DELTA.b* values denote a blue tinge.

All pigments were ground in a vibratory ball mill (model Mikrodismembrator II or B. Braun) for 30 seconds unless otherwise indicated.

The changes in color values of commercial types of .RTM.Bayferrox are summarized in Table 1.

Exceptionally finely divided black pigments with a pronounced brown tinge were tempered at 700.degree. C. for 30 minutes as described in Examples 1 to 4. The increase in relative color intensity and the increase in blue tinge are summarized in Table 2 together with other pigment properties. .RTM.Bayferrox 320 is used as reference.

The specific surface area was determined by the BET method using nitrogen. The mean crystallite size was determined radiographically from the integral line width of a diffraction reflex.

The effect of dwell time is shown in Table 3.

Table 4 shows the influence of temperature.

Table 5 shows the excellent dispersion properties of the tempered samples.

It is both surprising and advantageous to find that almost identical color values are obtained in Examples 11 b and 12 c even though the samples were not ground after tempering.