Heat-Stable Iron Oxide Black Pigments, Processes for Their Production and Their Use

This invention relates to new heat-stable iron oxide black pigments, to processes for their production and to their use.

Pigments based on magnetite (Fe.sub.3 O.sub.4) having particle sizes in the range from 0.1 to 2.0 .mu.m are used for a wide range of applications. As black pigments, they are used for coloring building materials and lacquers. Their magnetic properties are utilized in one-component toners for photocopiers. Their heat resistance is generally judged to be adequate. However, where they are used in building materials which harden under hydrothermal conditions, they undergo a change in color to red or brown.

Pigments having the narrowest possible particle size distribution are generally required to obtain optimal properties.

Accordingly, the production of such pigments is an important development target. The general processes by which pigments of the type in question can be produced have been adequately described.

Thus, DE-A 900 257 describes the so-called precipitation process in which iron(II) salts are reacted with atmospheric oxygen in the presence of alkalis to form Fe.sub.3 O.sub.4 pigments.

Another variant is the two-step precipitation process. In this process, which is described in U.S. Pat. No. 2,631,085, an iron(III) oxide or iron(III) hydroxide is reacted with iron(II) salts with addition of alkalis to form magnetite pigments.

However, the heat resistance of these pigments is inadequate, so that hitherto it has only been possible to use expensive mixed-phase pigments, such as for example metal (II)-containing ferrites (for example copper-manganese ferrites).

Fe.sub.3 O.sub.4 pigments having very good color properties can be produced by the two-step process. Pigments such as these are also distinguished by a narrow particle size distribution.

Magnetite (Fe.sub.3 O.sub.4) crystallizes in the cubic system with a spinel structure in the structure type O.sub.h 7. Development of the crystal faces (100) leads to cubic crystals and, of the (111) crystal faces, to the octahedral forms which have hitherto been observed among the magnetite pigments. With irregular shaped pigments particles, there is no development of a defined crystal face.

The object of the present invention is to provide iron oxide black pigments which do not undergo any change of color in the production of steam-hardened building materials such as for example, lime-sand bricks or fiber-reinforced cement.

It has now surprisingly been found that, using Na.sub.2 CO.sub.3 as the precipitant and precipitation temperatures of from 80.degree. C. to the boiling point or NaOH/Na.sub.2 CO.sub.3 mixtures as precipitant, it is possible by the two-step precipitation process to obtain iron oxide black pigments which, in the absence of further measures, withstand the necessary autoclave treatment without changing color.

The present invention relates to heat-stable iron oxide black pigments and the process for production of such pigments.

These pigments show the desired black color tinged with blue and have a low specific surface of approximately 0.5 to 5 m.sup.2 /g combined with good dispersibility.

Accordingly, the present invention relates to heat-stable iron oxide black pigments which are resistant to hydrothermal treatment in air at temperatures above 180.degree. C. and which have a specific BET surface, as determined by the nitrogen method (DIN 66 131), of 0.5 to 5 m.sup.2 /g.

The invention also relates to a process for the production of the iron oxide black pigments according to the invention.

Na.sub.2 CO.sub.3 solution or mixtures of Na.sub.2 CO.sub.3 and NaOH are added to a suspension of .alpha.-FeOOH in an iron salt solution until precipitation of the iron salts is complete, followed by stirring at the reaction temperature of 70.degree. C. to the boiling point until the reaction is complete. The pH value is then adjusted to values above 8.5 by addition of more sodium hydroxide, followed by stirring for a time.

After working up in the usual way (filtration, washing and drying), the pigments obtained are incorporated in fiber-reinforced cement which is then treated in an autoclave for 8 hours at 180.degree. C./8 bar or for 4 hours at 204.degree. C./16 bar. The pigmenting level is 5%. The formulations are described in H. Gundlach, "Dampfgehartete Baustoffe (Steam-Hardened Building Materials)", Wiesbaden-Berlin, 1973, pages 229 et seq. The hardened fiber-reinforced cement blocks were visually assessed.

39 l of a suspension of .alpha.-FeOOH (55 g/l) in an aqueous FeSO.sub.4 solution (53 g/l), Fe.sup.III :Fe.sup.II ratio 1.8, are heated with stirring (800 r.p.m.) to 90.degree. C. under inert conditions (N.sub.2) in a 60 liter stirred vessel, followed by the addition over a period of 10 minutes of the quantity of Na.sub.2 CO.sub.3 required for precipitation (6.9 l; 210 g/l). After stirring for 5 hours, the pH value is adjusted to pH 9.5 with sodium hydroxide (400 g/l), followed by stirring for another 4 hours. The consumption of NaOH is 1.1 l. The vessel is then opened and its contents stirred for 1 hour in the absence of nitrogen.

After filtration and washing, the product is dried at 40.degree. C.

The magnetite obtained is cube-shaped with chamfered edges.

Particle size: 1.5 .mu.m.

Specific surface (BET): 1.2 m.sup.2 /g.

40 l of a suspension of .alpha.-FeOOH (53 g/l) in an aqueous FeSO.sub.4 solution (50 g/l), Fe.sup.III :Fe.sup.II ratio 1.8, are heated with stirring (800 r.p.m.) to 70.degree. C. under inert conditions (N.sub.2) in a 30 liter stirred vessel, followed by the addition over a period of 8 minutes of the quantity of a mixture of Na.sub.2 CO.sub.3 and NaOH in the form of an aqueous solution required for precipitation (3400 ml; 207 g/l: 2600 ml; 206 g/l).

The mixture is then heated at 90.degree. C. in 60 minutes and stirred for 4 hours.

The apparatus is then opened and its contents stirred for 1 hour in the absence of nitrogen.

After filtration and washing, the product is dried at 40.degree. C.

The magnetite obtained is transitional between cubic and rhombododecahedral in shape.

Particle size: 0.5 .mu.m.

Specific surface (BET): 3.0 m.sup.2 /g.

Fiber-reinforced cement blocks were produced using the magnetite pigment of Example 1 and hardened for 8 h at 180.degree. C./8 bar.

Fiber-reinforced cement blocks were produced using the magnetite pigment of Example 2 and hardened for 8 h at 204.degree. C./16 bar.

Fiber-reinforced cement blocks were produced using a commercially available blue-tinged iron oxide black pigment (Bayferrox.RTM. 306, a product of Bayer AG) and hardened for 8 h at 80.degree. C./8 bar.

Fiber-reinforced cement blocks produced using the same pigment as in Comparison Example 1a were hardened for 4 h at 204.degree. C./16 bar.