Lustrous pigment and the method of producing the same

The object of the present invention is to provide a lustrous pigment free from the defects of conventional platelike or flaky iron oxide pigments that a mechanical strength is poor, adjustable colors are restricted and that a pigment lustrous is weak even if colors can adjusted variously and a method of producing the same. The present invention relates to a flaky iron oxide lustrous pigment having a hematite structure characterized by containing at least one of the elements selected from Zn, Sb and Sn and Al in solid solution and its platelike crystal having an average thickness of 0.8 .mu.m or more, preferably the content of at least one of the elements selected from Zn, Sb and Sn being in the range of 0.05 to 0.5 weight % as oxides and the content of Al being in the range of 0.5 to 3 weight % as Al.sub.2 O.sub.3.

BACKGROUND OF THE INVENTION 
The present invention relates to a lustrous flaky iron oxide pigment and a 
method of producing the same. The pigment has a broad range of application 
including paints, synthetic resins, cosmetics, ink, synthetic leather and 
wallpaper. 
Conventionally, as lustrous iron oxide pigments, a mica-like or platelike 
iron oxide (hereinafter abbreviated to "MIO") and a flaky red iron oxide 
containing aluminum in solid solution have been known. MIO exhibits a 
blackish-purple color and intense metallic luster (see Japanese Patent 
Publication (KOKOKU) No. 12435/68). However, a problem exist in that its 
platelike crystal structure is broken readily during dispersion into a 
paint or resin because of its poor mechanical strength. As a result, its 
color changes greatly during dispersion and it is hard to control the 
resulting color of a paint or a resin. A flaky red iron oxide containing 
aluminum in solid solution has a Al.sub.x Fe.sub.2-x O.sub.3 hematite 
structure and is characterized in having an intense lustrous and 
opacifying properties. Since the thickness of its crystal is 0.7 .mu.m or 
less, it exhibits a red color (see Japanese Patent Publication (KOKOKU) 
No. 8977/85). The red iron oxide has a high mechanical strength, 
accordingly its platelike crystal is resistant to breakage during 
dispersion into a paint or resin. As a result, any change in color is 
small and it is easy to control the resulting color of a paint or resin. 
However, because of its red color, the range of adjustable colors is 
restricted even if other pigments are incorporated. In addition, the flaky 
red iron oxide containing aluminum in solid solution is characterized by 
having thin crystals and there has never been synthesized a crystal having 
an average thickness greater than 0.8 .mu.m. 
Japanese Patent Public Disclosure (KOKAI) No. 104923/80 discloses a 
platelike iron oxide pigment containing 0.1 to 12 weight % of at least one 
of the oxides of the elements belonging to Group IV, Group V and/or Group 
VI and/or Group B and/or Group lIB according to the periodic table. Said 
pigment is a mixture of an iron oxide particle and a compound of said 
elements and is characterized in that a hematite particle contained in 
said pigment has thin-plate shape of a thickness of 0.6 .mu.m or less and 
its color is red to red yellow. Hence, this pigment also gives rise to the 
problem that the range of adjustable colors are restricted. 
Japanese Patent Publication (KOKOKU) No. 21976/86 discloses an improved 
flaky red iron oxide containing aluminum in solid solution of which 
surface is coated with titanium dioxide hydrate or titanium dioxide. Said 
pigment can be subjected to various color changes upon controlling the 
thickness of a titanium dioxide hydrate layer or a titanium dioxide layer 
on the surface of the flaky red iron oxide containing aluminum in solid 
solution particles. However, it suffers from a defect in that its 
lustrousness, an essential characteristic as a lustrous pigment, is poor 
in comparison with that of a red iron oxide containing aluminum in solid 
solution. 
Japanese Patent Public Disclosure (KOKAI) No. 317559/88 discloses that 
colors of a pigment can be controlled variously from a copper color to a 
black by forming a spinel phase on the surface layer of said pigment with 
a reduction of the surface of a red iron oxide containing aluminum in 
solid solution. However, a pigment obtained by this method gives rise to 
problems in that its lustrous character is weak in comparison with that of 
a red iron oxide containing aluminum in solid solution. Furthermore, the 
change of color during dispersion is large, since its mechanical strength 
is small and its platelike crystal is readily broken during dispersion. 
U.S. Pat. No. 4,826,537 discloses a pigment based on an iron oxide of the 
general formula Mn.sub.x Al.sub.y Fe.sub.2-(x+y) O.sub.3. The color of the 
pigment changes depending on the diameter of the particle. It is described 
that the pigment has a reddish yellow color at 10 .mu.m in diameter and 
which shifts toward violet with increasing diameter. However, the pigment 
shows violet color when its diameter becomes larger than 50 .mu.m. Such a 
large pigment is readily broken during dispersion and it is hard to 
control the resulting color of a paint or a resin. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a lustrous pigment having 
no defects of conventional platelike or flaky iron oxide pigments and a 
method of producing the same. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a lustrous flaky iron oxide pigment having a 
hematite structure characterized by containing aluminum and at least one 
of the elements selected from Zn, Sb and Sn in solid solution and thicker 
than 0.8 .mu.m, preferably than 1.0 .mu.m, in an average thickness. 
Preferably, the pigment contains 0.05 to 0.5 weight % of said element as 
oxides thereof and contains 0.5 to 3 weight % of Al as Al.sub.2 O.sub.3. 
Furthermore, the present invention provides a process for preparing said 
pigment comprising steps of dispersing an iron oxyhydroxide and at least 
one of salts or oxides of an element selected from Zn, Sb and Sn into an 
aqueous aluminate solution and subjecting the resultant solution to a 
hydrothermal treatment at above 250.degree. C. or steps of dispersing a 
ferric colloidal precipitate obtained by adding an alkali into an aqueous 
solution of ferric salt and at least one of the salts or oxides of an 
element selected from Zn, Sb and Sn into an aqueous aluminate solution and 
subjecting the resultant solution to a hydrothermal treatment at above 
150.degree. C. 
Aluminum and said elements selected from Zn, Sb and Sn mainly exist on 
crystal lattices. 
The lustrous pigment of the present invention has advantageous effects in 
that a color of the lustrous pigment can be adjusted optionally from red 
to black and an intense metallic luster is obtained. Furthermore, since 
the pigment has good mechanical strength, thee change of a color during 
dispersion is small and hence it is easy to control the resulting colors 
of paints and resins. 
When an average diameter is less than 10 .mu.m, a lustrous becomes weak. 
And when an average diameter is larger than 20 .mu.m, the change of color 
during dispersion becomes greater. 
A prior flaky iron oxide having an average thickness larger than 0.8 .mu.m 
contains a large amount of penetration twin and is readily broken during 
dispersion. On the other hand, the flaky iron oxide of the present 
invention does not contain penetration twin substantially, accordingly it 
is resistant to breakage during dispersion. 
It is preferable to keep the average thickness of the flaky iron oxide less 
than 3 .mu.m, because thick particles are apt to contain penetration twin. 
The lustrous pigment of the present invention can be produced by the 
process comprising steps of dispersing iron oxyhydroxide and at least one 
salts or oxides of an element selected from Zn, Sb and Sn into an aqueous 
solution of aluminate, subjecting the resultant solution to a hydrothermal 
treatment at a temperature of more than 250.degree. C. The Lustrous 
pigment of the present invention can be produced also by the process 
comprising a step of subjecting a ferric colloidal precipitate obtained by 
adding alkali into an aqueous solution of ferric salt and at least one of 
salts or oxides of an element selected from Zn, Sb and Sn to a 
hydrothermal treatment in an aqueous solution of aluminate at a 
temperature of more than 150.degree. C. Specifically, for example, 
.alpha.-iron oxyhydroxide (.alpha.-FeOOH) and an oxide of an element 
selected from Zn, Sb and Sn are dispersed into an aqueous solution of 
sodium aluminate and the resultant product is put into an autoclave and 
heated to a temperature of 250.degree. C. or more; thereby the whole oxide 
of an element selected from Zn, Sb and Sn is dissolved in the aqueous 
solution of aluminate during the heating. When the temperature of the 
solution becomes above 250.degree. C., .alpha.-iron oxyhydroxide is 
decomposed as 260-FeOOH.fwdarw..alpha.-Fe.sub.2 O.sub.3 +H.sub.2 O to 
crystallize out MIO (.alpha.-Fe.sub.2 O.sub.3). During the 
crystallization, an ion of said added metal and an aluminum ion in the 
solution are taken into the crystal lattices thereby a flaky iron oxide 
pigment having a hematite structure characterized in that it contains at 
least one of the elements selected from Zn, Sb and Sn and Al in solid 
solution and its platelike crystal has an average thickness of 0.8 .mu.m 
or more is formed. 
The aqueous aluminate solution used as the mother liquid for the 
hydrothermal treatment in the above process can be prepared by dissolving 
an aluminate salt in water or aqueous alkaline solution, however the 
aqueous aluminate solution may also be prepared from other aluminum 
compounds. Specifically, a) those aluminum compounds such as aluminum 
chloride, aluminum sulfate and aluminum nitrate, whose aqueous solutions 
exhibit acidic nature, may be dispersed first in water and then used after 
adjusting the pH value of the aqueous solution to greater than 10 by the 
addition of an alkaline agent. Since, aluminum is considered to be present 
as aluminate ions aqueous solution of pH value greater than 10 
("Qualitative Analytical Chemistry II", written by G. Charlot, translated 
by Kozo Sekine and Genji Tanaka, published by Kyoritsu Shuppan K.K. in 
1974), the aqueous aluminate solution is prepared by the above procedures. 
b) In the case of using metallic aluminum, aluminum trioxide and the like, 
these are dissolved in strong acid or strong alkali. They are applied with 
the same procedures as shown in a) above when dissolved into the strong 
acid, or used as they are when dissolved into the strong alkali. In each 
case, the solution is adjusted to an appropriate concentration. 
A ratio of a concentration of Al.sub.2 O.sub.3 and an alkali concentration 
in an aqueous aluminate solution is important. When the alkali 
concentration is too high relative to the concentration of Al.sub.2 
O.sub.3, a ratio of aluminum dissolved into a MIO crystal decreases and 
hence the product becomes similar to MIO, accordingly the mechanical 
strength becomes low. On the other hand, when the alkali concentration is 
too low relative to the concentration of Al.sub.2 O.sub.3, it is hard to 
obtain a platelike product having more than 0.8 .mu.m in thickness. In 
case of using sodium aluminate as aluminate, a preferable ratio of a 
concentration of NaOH (g/liter) to a concentration of Al.sub.2 O.sub.3 
(g/liter) is 2 to 5. 
A preferable concentration of Al.sub.2 O.sub.3 in an aqueous aluminate 
solution is in the range of 5 to 70 g/liter. That is, in case of a 
concentration of less than 5 g/liter, an average particle diameter of a 
product becomes less than 5 .mu.m and its luster becomes poor. Besides, in 
case that said concentration of Al.sub.2 O.sub.3 is larger than 70 
g/liter, the aluminum content in a product becomes more than 3% and hence 
it becomes hard to obtain a platelike product having a thickness of more 
than 0.8 .mu.m. 
When a salt or an oxide of an element selected from Zn, Sb and Sn is 
dispersed into an aqueous solution of alkali metal aluminate and heated in 
an autoclave, Zn, Sb and Sn are dissolved as a zincic acid ion, an 
antimonic acid ion and a stannic acid ion, respectively. The amounts of 
these metals to be added vary according to the concentration, composition 
and kind of alkali metal aluminate. When sodium aluminate of the above 
composition and concentration range is used, it is preferable to use the 
metals as oxides thereof from 1 to 15 weight % on the basis of the weight 
of Al.sub.2 O.sub.3 which is obtainable from an aluminum contained in an 
aqueous aluminate solution. 
Zn, Sb and Sn can be used in any forms of oxides, hydroxides, sulfates, 
carbonates, chlorides, nitrates and alkali salts. It is important that 
these elements exist as ions thereof in an aqueous aluminate solution when 
the lustrous pigment of the present invention having a hematite structure 
is formed in a hot aqueous solution. That is, when said elements exist as 
a solid, the thickness of iron oxide crystals to be formed becomes thin 
and a product having a thickness of more than 0.8 .mu.m is hard to obtain. 
The existence of zinc in iron oxyhydroxide as an impurity is undesirable, 
because such a zinc inhibits growth of the flaky iron oxide and makes it 
difficult to grow it to greater than 10 .mu.m in average diameter. A 
preferable content of zinc as ZnO is less than 0.24. 
In case of iron oxyhydroxide is used as an iron source, the concentrations 
of a slurry at the time of a hydrothermal treatment vary according to the 
kinds and particle size of iron oxyhydroxide. However, in many cases, 200 
g/liter or less is preferable. 150 g/liter or less is preferable to obtain 
a product having a large and uniform particle size. 
The lustrous pigment of the present invention is an iron oxide pigment 
having a hematite structure. Hence, a streak color is red. The colors can 
be changed from red to black in accordance with the synthesis conditions. 
A pulverized powder exhibits colors of red to dark red.

Hereunder, the present invention will be described in more detail according 
to Examples. The following Examples are mentioned only for 
exemplification, and the present invention is not restricted by them in 
any way. 
EXAMPLE 1 
110 g of a commercially available yellow iron oxide (.alpha.-FeOOH: TAROX 
LL-XLO) and 2 g of zinc oxide were dispersed into 1000 ml of an aqueous 
solution of sodium aluminate containing Al.sub.2 O.sub.3 of 35 g/liter and 
NaOH of 120 g/liter, and the resultant solution was put into a 
nickel-lined autoclave, heated to 300.degree. C. by 3 hours under stirring 
at 350 rpm and kept for 20 minutes. 
After being allowed to cool, the contents were removed from the autoclave, 
washed with water until the electrical conductance of the filtrates 
decreased below 100 .mu.S/cm or less and dried. 
This product had a black color with an intense luster. The average diameter 
in a plate direction was 15 .mu.m. The average thickness of the crystals 
determined by ultra-microtomy was about 3 .mu.m. 
The chemical analysis gave 0.84 Al.sub.2 O.sub.3 and 0.24 of ZnO. Analysis 
with an analytical electron microscope and a scanning electron microscope 
confirmed that Al and Zn were distributed uniformly in particles and the 
surface of platelike particles was smooth and no particle existed other 
than hematite. Furthermore, x-ray powder diffraction confirmed that there 
was no phase except that of the compound having a hematite structure. 
COMATIVE EXAMPLE 1 
110 g of the same commercially available yellow iron oxide as used in 
Example 1 were dispersed into 1000 ml of an aqueous solution of sodium 
aluminate containing Al.sub.2 O.sub.3 of 35 g/liter and NaOH of 120 
g/liter, and the resultant solution was put into an nickel-lined 
autoclave, heated to 300.degree. C. by 3 hours under stirring at 350 rpm 
and kept for 20 minutes. After being allowed to cool, the contents were 
removed from the autoclave, washed with water until the electrical 
conductance of the filtrate decreased below 100 .mu.S/cm and dried. 
This product had a red color and an intense luster. The average diameter in 
a plate direction was 13 .mu.m. The average thickness of the crystals 
determined by ultra-microtomy was about 0.4 .mu.m. 
X-ray powder diffraction confirmed that there was no phase except that of 
the compound having a hematite structure and the chemical analysis gave 
0.9% Al.sub.2 O.sub.3. 
EXAMPLE 2 
The same procedure as in Example 1 was repeated except that an aqueous 
solution of sodium aluminate had an Al.sub.2 O.sub.3 concentration of 45 
g/liter and an NaOH concentration of 130 g/liter and that the amount of 
zinc oxide to be added was varied variously in the range of 0.1 to 7 g. 
The results of the experiment are shown in Table 1. 
TABLE 1 
______________________________________ 
Zinc Amount Amount 
Oxide of ZnO of Al.sub.2 O.sub.3 
Average Average 
Amount in prod- in prod- particle 
thick- 
added ucts ucts diameter 
ness 
(g) (%) (%) (.mu.m) (.mu.m) 
______________________________________ 
0.1 &lt;0.00 1.1 12 0.4 
0.5 0.02 1.1 12 0.7 
0.7 0.05 1.0 13 1.2 
1.0 0.09 0.9 14 2.1 
2.0 0.25 0.8 14 2.8 
3.0 0.50 0.8 11 0.8 
6.0 1.08 1.3 7 0.3 
7.0 1.25 1.5 5 0.2 
______________________________________ 
EXAMPLE 3 
70 g of a commercially available yellow iron oxide (.alpha.-FeOOH: TAROX 
LL-XLO) and 4 g of antimony oxide were dispersed into 1000 ml of an 
aqueous solution of sodium aluminate containing Al.sub.2 O.sub.3 of 40 
g/liter and NaOH of 120 g/liter, and the resultant solution was put into a 
nickel-lined autoclave, heated to 280.degree. C. by 1 hours under stirring 
at 350 rpm and kept for 30 minutes. After being allowed to cool, the 
contents were removed from the autoclave, washed with water until the 
electrical conductance of the filtrates decreased to below 100 .mu.S/cm or 
less and dried. 
This product had a blackish-purple color with an intense luster. The 
average diameter in a plate direction was 13 .mu.m. The average thickness 
of the crystals determined by ultra-microtomy was about 1.5 .mu.m. 
The chemical analysis gave 1.2% and Al.sub.2 O.sub.3 and 0.3% of Sb.sub.2 
O.sub.3. Analysis with an analytical electron microscope and a scanning 
electron microscope confirmed that Al and Sb were distributed uniformly in 
particles and the surface of platelike particles was smooth and no 
particles existed other than hematite. Furthermore, x-ray powder 
diffraction confirmed that there was no phase except that of the compound 
having a hematite structure. 
COMATIVE EXAMPLE 2 
70 g of a commercially available yellow iron oxide (.alpha.-FeOOH: TAROX 
LL-XLO) were dispersed into 1000 ml of an aqueous solution of sodium 
aluminate containing Al.sub.2 O.sub.3 of 40 g/liter and NaOH of 120 
g/liter, and the resultant solution was put into an nickel-lined 
autoclave, heated to 280.degree. C. by 1 hour under stirring at 350 rpm 
and kept for 30 minutes. After being allowed to cool, the contents were 
removed from the autoclave, washed with water until the electrical 
conductance of the filtrate decreased to below 100 .mu.S/cm and dried. 
This product had a red color and an intense luster. The average diameter in 
a plate direction was 17 .mu.m. The average thickness of the crystals 
determined by ultra-microtomy was about 0.4 .mu.m. 
X-ray powder diffraction confirmed that there is no phase except that of 
the compound having a hematite structure. The chemical analysis gave 1.14 
Al.sub.2 O.sub.3. 
EXAMPLE 4 
70 g of yellow iron oxide obtained by a conventional method, that is 
air-oxiding an aqueous solution of ferrous sulfate while adding an alkali, 
filtering the obtained 21 m.sup.2 /g Of yellow iron oxide, washing and 
drying at 110.degree. C., and 3 g of tin oxide were dispersed into 1000 ml 
of an aqueous solution of sodium aluminate containing Al.sub.2 O.sub.3 of 
20 g/liter and NaOH of 60 g/liter, and the resultant solution was put into 
nickel-lined autoclave, heated to 330.degree. C. by 2 hours under stirring 
at 250 rpm and kept for 10 minutes. After being allowed to cool, the 
contents were removed from the autoclave, washed with water until the 
electrical conductance of the filtrates decreased to below 100 .mu.S/cm or 
less and dried. 
This product had a dark brown color with an intense luster. The average 
diameter in a plate direction was 18 .mu.m. The average thickness of the 
crystals determined by ultra-microtomy was about 2 .mu.m. 
The chemical analysis gave 1.3% and Al.sub.2 O.sub.3 and 0.3% of SnO.sub.2. 
Analysis with an analytical electron microscope and a scanning electron 
microscope confirmed that Al and Sn were distributed uniformly in 
particles and the surface of platelike particles was smooth and no 
existence of particles other than hematite. Furthermore, x-ray powder 
diffraction confirmed that there was no phase except that of the compound 
having a hematite structure. 
COMATIVE EXAMPLE 3 
70 g of yellow iron oxide used in Example 4 were dispersed into 1000 ml of 
an aqueous solution of sodium aluminate containing Al.sub.2 O.sub.3 of 20 
g/liter and NaOH of 60 g/liter, and the resultant solution was put into 
nickel-lined autoclave, heated to 330.degree. C. by 2 hours under stirring 
at 250 rpm and kept for 10 minutes. After being allowed to cool, the 
contents were removed from the autoclave, washed with water until the 
electrical conductance of the filtrates decreased to below 100 .mu.S/cm or 
less and dried. 
This product had a red color with an intense luster. The average diameter 
in a plate direction was 20 .mu.m. The average thickness of the crystals 
determined by ultra-microtomy was about 0.5 .mu.m. 
The chemical analysis gave 1.34 Al.sub.2 O.sub.3. X-ray powder diffraction 
confirmed that there was no phase except that of the compound having a 
hematite structure. 
EXAMPLE 5 
Into 250 ml of an aqueous solution of ferric sulfate containing Fe.sub.2 
(SO.sub.4).sub.3 of 375 g/liter was added an aqueous solution of sodium 
hydroxide of 350 g/liter until the pH became 13, after that 3 g of zinc 
oxide and 1 g of antimony oxide were added thereinto, and further 240 ml 
of an aqueous solution of sodium aluminate containing Al.sub.2 O.sub.3 of 
70 g/liter and NaOH of 320 g/liter were added into the solution, and the 
resultant solution was put into nickel-lined autoclave, heated to 
270.degree. C. by 1 hour under stirring at 260 rpm and kept for 2 hours. 
After being allowed to cool, the contents were removed from the autoclave, 
washed with water until the electrical conductance of the filtrates 
decreased to below 100 .mu.S/cm or less and dried. 
This product had a dark green color with an intense luster. The average 
diameter in a plate direction was 8 .mu.m. The average thickness of the 
crystals determined by ultra-microtomy was about 1.5 .mu.m. 
The chemical analysis gave 1.5% Al.sub.2 O.sub.3, 0.2% ZnO and 0.14 
Sb.sub.2 O.sub.3. Analysis with an analytical electron microscope and a 
scanning electron microscope confirmed that A1, Zn and Sb were distributed 
uniformly in particles and the surface of platelike particles was smooth 
and no particles existed other than hematite. Furthermore, x-ray powder 
diffraction confirmed that there was no phase except that of the compound 
having a hematite structure.