Temperature-resistant black pigment, a process for its production and its use

A temperature-resistant black pigment which is a mixed oxide of manganese and iron with the structure of hematite and having a manganese content of 3 to 35% by weight, and with absolute color values in colorimetric testing in paint systems, expressed in CIELAB units, in the lightened form (Blend of 10 parts titaniumdioxide and 1 part of black pigment) of 60 to 70 for L*, -0.5 to 1.9 for a* and -4 to -6.5 for b* useful for coloring autoclave-cured building materials and for pigmenting heat-resistant paint systems is obtained by preparing a precursor material of manganese and iron oxides or hydroxides, which contains the phase with the structure of spinel and calcining that precursor material in an oxidizing atmosphere at 500.degree. to 1,000.degree. C.

This invention relates to a temperature-resistant black pigment consisting 
of a mixed oxide of manganese and iron with the crystal structure of 
hematite, to a process for the production of the pigment and to its use. 
BACKGROUND OF THE INVENTION 
Black pigments based on Fe.sub.3 O.sub.4 have been successfully used for 
many applications. On account of their poor heat resistance, however, they 
are unsuitable for applications in autoclave-cured building materials or 
for temperature-resistant paints because they are oxidized to brown or red 
Fe.sub.2 O.sub.3. 
Commercially available black pigments of the Fe.sub.2 O.sub.3 --Mn.sub.2 
O.sub.3 system, as described in U.S. Pat. No. 3,615,810 are highly 
temperature-resistant and are eminently suitable for the applications 
mentioned above. However, pigments such as these have coloring strengths 
of only 40 to 70% compared with commercially available pigments based on 
Fe.sub.3 O.sub.4 with the result that relatively large quantities of the 
pigments mentioned have to be added, for example to building materials, to 
obtain a deep black color impression. 
U.S. Pat. 3,615,810 describes a process for the production of a 
temperature-resistant black pigment of the Fe.sub.2 O.sub.3 --Mn.sub.2 
O.sub.3 system consisting of phases with the structures of hematite and 
the principal constituent of bixbyite. In this process, oxidic or 
oxide-forming starting materials of iron and manganese are mixed, 
mineralizers, such as sodium chloride, are added to the resulting mixture 
and the whole is calcined at temperatures of 800.degree. to 900.degree. C. 
It is not possible by this process to obtain pigments of much higher 
coloring strength than the pigment Bayferrox.RTM. 303T. The pigment 
Bayferrox.RTM. 303T is a commercially available temperature-resistant 
black pigment of the Fe.sub.2 O.sub.3 --Mn.sub.2 O.sub.3 system 
manufactured by Bayer AG. 
DE-A 2 159 364 describes a process for the production of 
temperature-resistant black pigments with the structure of hematite. In 
this process, .beta.-FeOOH is calcined with manganese-containing 
compounds. Due to the chloride structurally present in the .beta.-FeOOH, 
considerable quantities of chlorine gas are released during calcination 
with manganese dioxide, which is a disadvantage for industrial-scale 
operation of the process. Although the pigments obtained by this process 
have very high coloring strength, they are undesirably tinged with a 
yellow-red color undertone. Accordingly, these pigments are unsuitable for 
obtaining a deep black color impression, for example for the production of 
slate-colored building materials or for deep black coil coating 
compositions. 
According to U.S. Pat. No. 4,145,229, heat-stable black pigments are 
produced by calcination of mixtures consisting of iron sulfate and 
manganese-containing compounds. In view of the SO.sub.2 formed during the 
process, the waste gases have to be subjected to expensive 
desulfurization. Black pigments are obtained for manganese contents of 
11.5 to 46%, corresponding to an iron-to-manganese ratio of 5-0.5:1, and 
preferably 14 to 23%. Accordingly, the process follows the teaching of 
U.S. Pat. No. 3,615,810, according to which the phase having the structure 
of bixbyite bears the pigment properties. For manganese contents of less 
than 11. 5%, corresponding to an iron-to-manganese ratio of &gt;5:1, brown 
pigments are obtained. 
According to U.S. Pat. No. 3,655,418, the hydroxides of iron and manganese 
are precipitated from solutions in the presence of flocculating agents and 
subsequently calcined. On account of their quality, the precipitates are 
difficult to filter and wash. The resulting salt contents in the filter 
cake give rise to extensive sintering during the calcination process, so 
that furnace clinkers that are difficult to grind up are obtained. The 
pigments lack coloring strength and are distinctly brownish. In addition, 
the decomposition of the salts during the calcination process means that 
the waste furnace gases have to be expensively purified. 
According to U.S. Pat. No. 3,276,894, brown or black iron oxides of 
hematite structure are obtained by calcination of precursors produced by 
the aniline process. The black pigments obtained by this known process are 
distinguished by a distinct red tinge and by poor coloring strength. Their 
coloring strength by comparison with Bayferrox.RTM. 303T is only 40 to 
50%. 
Accordingly, the problem addressed by the present invention was to provide 
a heat-stable black pigment of high coloring strength which would not have 
any of the described disadvantages. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to a temperature-resistant black pigment 
which is a mixed oxide of manganese and iron with the structure of 
hematite and having a manganese content of 3 to 35% by weight and with 
absolute color values in colorimetric testing in paint systems, expressed 
in CIEIAB units, in the lightened form (Blend of 10 parts titaniumdioxide 
and 1 part of black pigment) of 60 to 70 for L*, -0.5 to 1.9 for a* and -4 
to -6.5 for b*. The present invention also relates to a process for 
preparing that black pigment which comprises preparing a precursor 
material of Mn and Fe oxides or hydroxides which contains the phase with 
the structure of spinel and then calcining that precursor material in an 
oxidizing atmosphere at 500.degree. to 1,000.degree. C. 
DETAILED DESCRIPTION 
It has now surprisingly been found that deep black pigments of the Fe.sub.2 
O.sub.3 /Mn.sub.2 O.sub.3 system can be obtained by calcination of 
precipitation products consisting of a phase having the structure of 
spinel of the Fe.sub.3 O.sub.4 /Mn.sub.3 O.sub.4 mixed crystal series. 
Using the process according to the invention, black pigments are obtained 
for manganese contents of only 3% by weight wile deep black pigments are 
obtained for manganese contents of about 5 to 10% by weight. 
The pigments of the Fe.sub.2 O.sub.3 /Mn.sub.2 O.sub.3 mixed crystal series 
produced by the process according to the invention consist essentially of 
a phase having the structure of hematite and contain between 3 and 35% 
manganese. The hematite phase makes up at least 60% by weight and 
preferably more than 80% by weight of the pigment. 
Accordingly, the present invention relates to a temperature-resistant black 
pigment consisting of a mixed oxide of manganese and iron with the 
structure of hematite, characterized in that it has a manganese content of 
3 to 35% by weight and preferably 6 to 18% by weight and that, in 
colorimetric testing in paint systems, it shows absolute color values, 
expressed in CIELAB units, in the lightened form (Blend of 10 parts 
titaniumdioxide Bayertitan R-KB-2 and 1 part of black pigment) of 60 to 70 
for L*, -0.5 to 1.9 and preferably 0 to 1.2 for a* and -4 to -6.5 and 
preferably -5.0 to -6.0 for b*. 
The present invention overcomes the prejudice emanating from U.S. Pat. No. 
3,615,810, according to which that phase of black pigments of the Fe.sub.2 
O.sub.3 /Mn.sub.2 O.sub.3 system which has the structure of bixbyite bears 
the properties of the black pigment and the hematite phase is said to show 
an unwanted reddish color undertone and relatively poor coloring strength. 
The black pigment according to the invention preferably has an SiO.sub.2 
content of &lt;1% by weight and, more preferably, &lt;0.3% by weight. 
Accordingly, it differs in this regard from pigments which have been 
obtained by calcination with mineral manganese dioxide. 
The coloring strength of the black pigments according to the invention is 
determined in particular by their manganese content. 
Pigments of particularly high coloring strength are obtained with an 
iron-to-manganese ratio of 10:1 to 4:1, corresponding to a manganese 
content of 6 to 14% by weight. In this range, the pigments develop greater 
coloring strength with increasing manganese content and show fewer red and 
yellow tinges. With an iron-to-manganese ratio of less than about 4:1, a 
phase having the structure of bixbyite is additionally observed in the 
calcined precursors. Accordingly, pigments with an iron-to-manganese ratio 
of less than 4:1 do not develop increasing coloring strength with 
increasing manganese content. 
The coloristic values of the black pigments are also influenced by their 
content of additional elements. Particularly good coloristic values are 
shown by pigments according to the invention which contain one or more of 
elements aluminium, chrominium, titanium, magnesium, calcium and zinc in a 
total quantity of up to 5% by weight in the form of the oxides. 
The present invention also relates to a process for the production of the 
black pigments according to the invention, characterized in that a 
precursor consisting of Mn and Fe oxides or hydroxides, which contains the 
phase with the structure of spinel, is initially prepared and is 
subsequently calcined in an oxidizing atmosphere at temperatures in the 
range from 500.degree. to 1,000.degree. C. and preferably at temperatures 
in the range from 600.degree. to 800.degree. C. 
Precursors essentially consisting of a phase with the structure of spinel 
having the approximate composition Fe.sub.3 --.sub.x Mn.sub.x O.sub.4 with 
0&lt;x &lt;3 are particularly suitable. 
The precursors are preferably prepared by a precipitation process in which 
salts of iron and manganese are oxidized in the presence of alkalis. 
A process for their production is known, for example, from U.S. Pat. No. 
3,822,210. 
Precursors having specific surface area of 2 to 20 m.sup.2 /g and, 
preferably, 4 to 10 m.sup.2 /g, as determined by the nitrogen adsorption 
method (DIN 66 131), are particularly suitable for the production of the 
pigments according to the invention. The calcination of relatively 
fine-particle precursors gives pigments which show an unwanted reddish or 
yellowish color undertone whereas the calcination of very coarse-particle 
precursors gives pigments of poor coloring strength. Accordingly, 
precursors having specific surfaces of 4 to 10 m.sup.2 /g are particularly 
suitable because pigments of high coloring strength with no unwanted color 
undertone are obtained in their case. 
The precipitated precursors are calcined at temperatures of 500.degree. to 
1,000.degree. C. and preferably at temperatures of 600.degree. to 
800.degree. C., typically over periods (residence tines) of 15 to 180 
minutes. Residence times of about 5 minutes at 800.degree. C. are 
sufficient to obtain a complete reaction to a phase of hematite structure. 
The calcinations are preferably carried out at temperatures of 650.degree. 
to 750.degree. C. over residence times of 1 to 180 minutes and 30 to 120 
minutes because, under these conditions, no distinct sintering occurs so 
that there is no need for laborious grinding. Moist filter cakes or dried 
precursor may be used for calcination. The salt content of the precursor 
is less than 1% and preferably less than 0.5%. with increasing salt 
content, distinct sintering is observed during calcination. In that case, 
pigments of high coloring strength can only be obtained after laborious 
grinding. 
The calcinations are carried out with direct or indirect firing in an 
oxygen-containing atmosphere with typical oxygen contents of 10 to 20% by 
volume. 
In a general embodiment of the process, salts of divalvent iron and 
manganese in molar ratios of 20:1 to 2:1 are precipitated with alkaline 
preciptants, the quantity of precipitant used being at least twice the 
molar quantity of the metal salts used. The alkaline precipitation is 
accompanied by aeration in a stirred tank for 3 to 8 hours at temperatures 
of 60.degree. C. to the boiling point. The suspension obtained is filtered 
on nutsches and washed to salt contents of less than 1% by weight. The 
washed filter cake is calcined in moist form or after drying. 
The calcination is carried out in an oxidizing atmosphere at temperatures 
of 500.degree. to 900.degree. C. either in a muffle furnace or in a rotary 
kiln. The residence times may range from a few minutes to a few hours. 
After grinding in the mills normally used for pigment production, for 
example in pinned disk mills, vibrating ball mills or jet mills, tests are 
carried out in the binder F 48 or L 64 in accordance with DIN 6174/ISO 
7724, 1-3 drafts, for determining coloring strength and color tinge and 
the CIELAB (C/2 degree) values are shown in the Examples. The black 
pigment Bayferrox.RTM. 303T (a product of Bayer AG) is used as reference. 
The present invention also relates to the use of the black pigment 
according to the invention for coloring building materials and 
heat-resistant paint systems.

The following Examples are intended to illustrate the invention without 
limiting it in any way. 
COMISON EXAMPLE 1 
Example 4 of DE-A 02 159 364 was copied. According to diffractometric 
analysis, the washed, dried precursor consists essentially of 
.beta.-FeOOH. In addition to a hematite phase, the calcined pigment 
contains other phases. The pigment was ground in a Dismembrator.RTM. (3 
mins., steel ball). The pigment has a coloring strength of 350% for da* 
=0.9 and db*=4.7. The pigment shows a distinct brown color undertone and 
is not suitable for deep black coloring. 
COMISON EXAMPLE 2 
Example 3 of DE-A 02 159 364 was copied. The pigment consists essentially 
of a phase with the structure of hematite and is brown-black in color. It 
has a coloring strength of 355% for da*=4.9 and db*=12. 
COMISON EXAMPLE 3 
Example 1 of U.S. Pat. No. 4,145,229 was copied. The pigment was calcined 
for 3 hours at 850.degree. C. and ground in a Dismembrator.RTM. (3 mins. , 
steel ball) . By comparison with Bayferrox.RTM. 303T, the pigment has a 
coloring strength of 105% for da*=0.1 and db*=-0.1. 
According to diffractometric analysis, the pigment contains phases having 
the structures of bixbyite and hematite. 
COMISON EXAMPLE 4 
Example 4 of U.S. Pat. No. 3,276,894 was copied. A pigment brown-black in 
color was obtained by calcination at temperatures of 600.RTM. to 
800.degree. C. This pigment is not suitable for deep black coloring. 
According to diffractometric analysis, the pigment contains phases with 
the structures of bixbyite and hematite. 
EXAMPLE 1 
In a 30 liter stirred tank equipped with an aerating stirrer, 10.1 l 
FeSO.sub.4 solution containing 200 g FeSO.sub.4 l and 0.9 1 MnSO.sub.4 
solution containing 600 g/l MnSO.sub.4.H.sub.2 O were combined and purged 
with nitrogen. 2.3 liters sodium hydroxide having a concentration of 656 
g/l NaOH were then added over a period of 15 minutes, followed by heating 
with stirring to 85.degree. C. 300 l/h air was then passed through 5.5 
hours at 85.degree. C. The black suspension was washed with about 20 l 
water on nutsches and dried at 80.degree. C. in a recirculating air drying 
cabinet. According to analysis, the dried precursor contains 0.2% soluble 
salts. The specific surface of the precursor measures 5.3 m.sup.2 /g. 
According to diffractometric analysis, the precursor consists of a phase 
with the structure of spinel. 
300 g of the precursor were calcined for 4 hours at 700.degree. C. in a 
rotary kiln while 500 l air/h was passed through. According to analysis, 
the pigment contains 12.5% Mn. An X-ray diffractogram of the pigment shows 
only the diffraction lines of hematite. The pigment was ground in 
different ways in a vibrating ball mill of the Dismembrator.RTM. type: 
a) 1 minute, agate ball 
b) 3 minutes, steel ball 
c) steam jet grinding. 
Colorimetric testing in L 64, F 48 and in spar revealed coloring strengths 
of 200 to 260%. The pigment is deep black and does not show any unwanted 
color tinge. 
______________________________________ 
Grinding Testing in 
Coloring strength 
da* db* 
______________________________________ 
1 min. agate 
L 64 201 -0.2 -0.3 
3 mins. steel 
L 64 260 0 0.1 
Steam jet 
L 64 260 0.2 0.1 
Steam jet 
F 48 252 0.4 0 
Steam jet 
Spar 214 0.2 -0.9 
______________________________________ 
EXAMPLE 2 
The procedure was as in Example 1, except that 11.85 l FeSO.sub.4 (200 g/l 
), 0.28 l MnSO.sub.4 solution (600 g/l MnSO.sub.4.H.sub.2 O) and 2.32 l 
sodium hydroxide (656 g/l ) were used to prepare the precursor. The 
specific surface of the precursor measures 3.8 m.sup.2 /g. Its salt 
content is 0.3% by weight. According to analysis, the precursor consists 
of a phase with the structure of spinel. The precursor was dried and 
calcined for 4 hours at 700.degree. C. in a rotary kiln through which 500 
l air/h was passed. After grinding in a Dismembrator.RTM. (1 minute, agate 
ball), the pigment has a coloring strength of 164% for da*=1.4 and 
db*=0.4. 
According to analysis, the pigment contains 3.9% by weight Mn and consists 
solely of a phase with the structure of hematite. 
EXAMPLE 3 
The procedure was as in Example 1, except that 11.39 l FeSO.sub.4 (200 g/l 
) and 0.47 l MnSO.sub.4 (600 g/l MnSO.sub.4.H.sub.2 O) and 2.32 l NaOH 
(656 g/l ) were used to produce the precursor. According to analysis, the 
precursor consists of a phase with the structure of spinel. The specific 
surface measures 3.6 m.sup.2 /g. 
The precursor was calcined for 4 hours at 700.degree. C. and then ground in 
a steam jet mill with a ratio of steam-to-product of 3:1. According to 
colorimetric evaluation in L 64, the pigment has a coloring strength by 
comparison with 303T of 208% for da*=0.6 and db*=0.6. According to 
analysis, the pigment contains 6.4% Mn and consists of a phase with the 
structure of hematite. 
EXAMPLE 4 
The precursor was produced in the same way as described in Example 1, 
except that 10.1 l FeSO.sub.4 (200 g/l ) and 0.94 1 MnSO.sub.4 (600 g/l 
MnSO.sub.4) and 2.32 l NaOH (656 g/l ) were used. 
The specific surface of the precursor measures 5.2 m.sup.2 /g. According to 
analysis, the precursor consists of a phase with the structure of spinel. 
The precursor was calcined for 4 hours at 550.degree. C. and was ground for 
1 minute with an agate ball in a Dismembrator.RTM.. The pigment has a 
coloring strength in L 64 of 222% for da*=-0.1 and db*=0. It has a 
manganese content of 13.8%. According to phase analysis, the pigment 
consists of a phase with the structure of hematite. 
EXAMPLE 5 
The precursor of Example 4, accommodated in a preheated porcelain dish, was 
calcined for various tines at 700.degree. C. in a muffle furnace and then 
ground in a Dismembrator.RTM. (1 minute agate ball) 
______________________________________ 
Calcination 
Coloring 
time strength da* db* 
______________________________________ 
10 mins. 197 -0.5 0.4 
30 mins. 192 -0.6 0.5 
60 mins. 188 -0.7 0.6 
2 mins. 225 0.9 1.3 
4 mins. 224 0 0.1 
______________________________________ 
EXAMPLE 6 
The procedure was as in Example 1, except that the addition of the sodium 
hydroxide was controlled in such a way that a constant pH value of 10.5 
was maintained during the reaction. The precursor has a specific surface 
of 10 m.sup.2 /g. The precursor was calcined for 2 hours at 700 .degree. 
C. and ground for 3 minutes with a steel ball in a Dismembrator.RTM.. The 
pigment has a coloring strength in L 64 of 330% for da* =1.3 and db*=1.7. 
TABLE 1 
______________________________________ 
Absolute color values for Examples 1 to 6 and Comparison 
Examples 1 to 3. Lightening in L 64: Blend of 10 parts 
titaniumdioxide Bayertitan R-KB-2 and 1 part of black pigment. 
L* a* b* C* 
______________________________________ 
Example 
1a 64.6 0.7 -6.1 6.1 
1b 64.3 0.8 -5.8 5.9 
1c 64.4 0.8 -5.9 5.9 
2 69.5 1.9 -5.1 5.4 
3 66.7 1.2 -5.2 5.3 
4 66.6 0.5 -6.0 6.0 
5a 64.6 0.7 -5.7 5.7 
b 65.1 0.5 -5.8 5.8 
c 64.7 0.5 -6.0 6.0 
d 66.1 1.5 -4.5 4.7 
e 66.1 0.6 -5.7 5.7 
6 61.3 2.0 -4.1 4.6 
7 67.1 -0.2 -5.9 5.9 
Comparison 
Example 
1 61.5 1.6 -1.6 2.3 
2 61.4 5.6 -5.8 8.1 
3 73.3 0.5 -5.0 5.0 
303T 73.9 0.4 -5.0 5.0 
______________________________________ 
EXAMPLE 7 
The procedure was as in Example 1, except that 10.84 l FeSO.sub.4 (200 g/l 
), 1.99 l MnSO.sub.4 (600 g/l MnSO.sub.4.H.sub.2 O) and 5.25 l NaOH (350 
g/l ) were used. 
According to analysis, the precursor consists of a phase with the structure 
of spinel. The specific surface measures 6.4 m.sup.2/ g. The precursor was 
calcined for 4 hours at 750.degree. C. and ground in a dismembrator.RTM. 
(1 minute, agate ball). The pigment has a coloring strength of 208% for 
da* =-0.6, db*=0.1 and dc*=-0.1. The pigment contains 23% Mn. 
According to phase analysis, the pigment contains phases with the 
structures of hematite and bixbyite. The hematite phase makes up 
approximately 85%.