Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for preparing colored aluminum
powders, and more particularly to a process for preparing a colored aluminum
pcwder to be added to a coating ccmposition as a pigment to give the com-
position a metallic color.
The term "aluminum" as used herein and in the claims includes
pure aluminum, cammercial aluminum containing small amol~nts of impurities
and aluminum alloys in which aluminum predominates.
It is known to add finely divided metal to a coating composition
as a pigment to prepare a coating composition having a metallic color. As
such metal powder pigment, alumlnum pcwder is used to obtain a silver color,
or brass powder is used to give a gold color. Use of brass powder involves
problems in that it is expensive, unusable for articles related to beverages
and foods because it is poisonous, prone to discoloration depending on the
environment, liable to delustering and subject to color change to gray at
a -temperature of 300 to 500C.
Attempts have also been made to add colored aluminum pawders to
coating compositions to prepare compositions having varying metallic colors.
For this purpose, various studies have been made on methods for coloring
alumlnum particles which mainly include two methods: one in which an oxide
film formed on the surface of aluminum particles is colored wi-th an organic
dye, and the other in which a colored synthetic resin film is coated with
aluminum by vacuum evaporation and the coated film is then ccmminuted. How-
ever, these methods have the disadvantages that the aluminum particles
colored by the former method are not fully resistant to weather, while the
latter method requires a very expensive apparatus for vacuum evaporation
coating.
This invention provides for a process for preparing a colored alun-
inum powder comprising the steps of immersing finely divided aluminum in a weak
alkali solution having a pH of 8 to 13 containing a salt of a metal selected
from the group consisting of metals having coordination numbers of 4 or 6 and
an organic compound haviny a chelating ability, and separat m g the resulting
aluminum particles from the solution to obtain colored aluminum particles.
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In a second embodiment, this invention provides for a process for
preparing a colored alum mum powder comprising the steps of immersing finely
divided alur,linum in an aqueous solution having a pH of 4 to 12 to form a
b oe hmite film on the surface of the aluminum particles, separating the
resulting aluminum particles from the solution, irnnersing the separated
aluminum particles in a weak alkali solution having a pH of from 8 to 13
containing a salt of a metal selected from the group consisting of metals
having coordination nu~bers of 4 to 6 and an organic co~pound having a
chelating ability, and separating the resulting aluminum particles from
the solution.
The present process gives gold and various other colors. ~ne gold-
colored aluminum ~owder prepared by the present process is superior to brass
powder in that it can be prepared from a less expensive material; has one
third the specific gravity of brass powder and is therefore serviceable in
amounts correspondingly smaller in weight; and is usable for any article,
because aluminum is not poisonous, and has higher resistance to weather,
heat and corrosion. As compared with the foregoing to methods of variously
coloring aluminum powderf the present process is superior to one of them in
giving products of higher weather resistance and is economically advantageous
over the other in that it does not requlre an expensi~e apparatus.
EX 9 1es of the metal salts useful in this invention are salts of
zinc, copp~r and like metals having a coordination number of 4 and salts of
iron, nickel, cobalt and chromium and like metals ha~ing a coordination
number of 6.
Examples of the organic compounds having a chelating ability are
0-coordination chelating agents including dicarboxylic acids such as oxalic
acid, malonic acid, maleic acid, succinic acid, etcO and derivatives thereo~,
hydrox~fcarboxylic acids such a malic acid, tartaric acid, citric acid,
lactic acid, etc. and derivatives thereof, aromatic dicarbo~flic acid such
as phthalic acid, terephathalic acid, etc. and derivatives thereof, polyhydric
alcohols such as glycerin, etc., N-coordination chelating agents including
aliphatic amines such as methylamine, dimeth~flanune, ethylamine, etc. and
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derivatives thereof; and chelatiny agents having both N-coordination group
and 0-coordination group and including aliphatic amines having OH group(s)
such as manoethanolamine, diethanolamine, triethanolamune, etc. and
derivatives thereof, and amino acids and amides such as aspartic acid,
ylutamic acid, fonm~nide, etc~ and derivatives thereof. These chelating agents
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be used singly, or at lèast two of them are usable in admixture.
The organic compound having a chelating ability prevents the metal
salt from precipitating from the weak alkali solution in the form of a
hydrated oxide, chelating the salt to maintain the metal component in the
form of ions~ The kind of the organic compound to be used as the chelating
agent differs with the kind of the metal salt to be used in combination
therewith. For exampleg when finely divided aluminum is colored with iron
ions, it is preferable to use triethanolamine signly or in admixture with
oxalic acid. The amount of the organic compound to be used also varies with
the kind and concentration of the metal salt, the degree to which the
aluminum particles are to be colored and the amount of the film to be formed
on the surface of the aluminum particles. ~hen iron ions are used for
coloration, a preferable example of the solution contains from 2 x 10 5 to
0.1 mol/litre ferric nitrate, 0.005 to 0.5 mol/litre of triethanolamine and
2 x 10 5 to 0.1 mol/litre of oxalic acid per litre of the solution~ If the
amounts of triethanolamine and oxalic acid exceed the above-mentioned upper
limits, the iron chelate becomes too stabili~ed, inhibiting formation of
hydrated iron oxide film on the surface of the aluminum particles. Con-
versely if the amounts are less than the lower limits, molecules of water
will be coordinated to some of the iron ions, impairing the stability of
the chelate.
The term "weak alkali solution" refers to a solution having a pH
of 8 to 13~ When the solution containing only the metal salt and the
organic compound having a chelating ability has a pH value outside the
foregoing range, an alkali such as aIkali hydroxide, ammonia, amine, alkali
carbonate or alkali aluminate can be added to the solution to adjust the
pH to the above-mentioned range. If the pH value is lower than 8, the
resulting chelate has poor stability, whereas if it is above 13, the
aluminum becomes excessively etched with the alkali, making it difficult
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for the metal ions to effectively form a colored hydrated oxide film.
The treating solution may have a suitable temperature within the
range of from room temperature to the boiling point of the solution. The
finely divided aluminum material may be immersed in the solution for 3 to
90 minutes.
When coloring the aluminum material with iron ions, it is most
preferable to use a weak alkali solution containing 0.0001 to 0.02 mol/litre
of ferric nitrate, 0.005 to 0.05 mol/litre of triethanolamine and 0-0001 to
0~02 mol/litre of oxalic acid per liter of the solution and having a pH of
about 9 to about 11. This solution ensures the uniformity of the color of
the hydrated iron oxide film formed on the surface of the aluminum particles.
When the finely divided aluminum material is immersed in the
solution, a colored hydrated iron oxide film is formed on the surface of
the aluminum particles, presumabl~ for the following reason. In the
solution, two molecules of triethanolamine and one molecule of oxalic acid
are coordinated to the iron ion in the form of a chelate. When al~inum
particles are immersed in the solution maintained in this state at room
temperature, e.g. at about 20 C, the iron chelate is reduced on the sur-
face of the aluminum particles, releasing iron ions, which react wlth the
alkali to give hydrated iron oxide. Because the hydrated iron oxide is
extremely active and highly cohesive~ a uniformly colored film of hydrated
iron oxide is formed on the surface of the aluminum particles.
Further when the finely divided aluminum material is immersed in
the above solution as maintained at about 60 C to the boiling point of the
solution, the surface of the aluminum particles is formed with a hydrated
aluminum oxide film and a hydrated iron oxide film in a composite state,
presumably by virtue of the following mechanism. The above-mentioned
treatment first forms a hydrated aluminum oxide film on the surface of the
aluminum particles. The electrons produced at this time are accepted by
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the trivalent iron ions contained in the solution, whereby the iron ions are
reduced to bivalent iron ions, which in turn react with the alkali to form
Fe¦OH)2. Since the ferrous hydroxide is active, it is converted to Fe(OH)3,
which is so cohesive that it combines uniformly with the hydrated aluminum
oxide film, consequently forming a uniform colored hydrated oxide film on
the surface of the aluminum particles. Because the colored film on the sur-
face of the aluminum particles includes the hydrated aluminum oxide film, it
has very high corrosion resistance.
According to this invention, the color given by the hydrated oxide
film on the surface of the aluminum particles has higher resistance to
weather and corrosion than the color conventionally obtained by dying an
oxide film formed on the surface of aluminum particles with an organic dye.
Since the color is given by the metal oxide according to this invention, it
has increased thermal stability, i.e. improved heat resistance.
Preferably the finely divided alu~inum material may be immersed
in an aqueous solution of pH 4 to 12 to form a boehmite film thereon before
being subjected to the coloring process, whereby the aluminum particles can
be formed on the surface thereof with a colored film having a color of im-
proved uniformity and increased resistance to ¢orrosion and weather.
The colored aluminum particles are separated from the treating
solution generally by filtration. The particles are of course separable by
centrifugation.
When the treating solution has become degraded, the solution can
be regenerated by acidifying the solution with an acid and adding the
specified metal salt and alkali to the solution.
By suitably selecting the combination of the metal salt and the
organic compound having a chelating ability, varying colors such as gold,
blackish brown, grayish white, etc. can be given to the film on the surface
of the aluminum particles. Further by altering the composition of the weak
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alkali solution, the lightness, density and saturation of the color are also
controllable.
A magnetic aluminum powder can be prepared by using a salt of iron,
nickel or cobalt as the metal salt under selected treating conditions. The
magnetic eolored aluminum powder will afford new applications to aluminum.
.
Ten gram portions of finely divided aluminum were immersed in
various treating solutions under varying conditions, and the treated
aluminum particles were separated from the solution by filtration. The
eompositions of the solution, treating conditions and the colors obtained
are listed below.
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Example 2
For pre~reatment and hydration, 10 g of finely divided aluminum
was immersed for 15 minutes in an aqueous solution containing 0.01 mol/litre
of triethanolamine and 0.01 mol/litre of sodium hydroxide and having a pH of
10.3 and a temperature of 90 C and was there after separated from the
solution by filtration.
The aluminum particles having a boehmite film formed on their
surface by this treatment were immersed for 15 minutes in 1 litre of solution
con~aining 0.01 mol/litre of ferric nitrate, 0.04 mol/litre of sodium
oxala~e and 0.01 mol/litre of triethanolamine and having a pH of 10.5 and
a temperature of 90 C and were thereafter separated from the solution by
filtration. The colored aluminum particles obtained had a uniform gold
color.
Example 3
To the coloring solution used in Example 2 and recovered by
filtration were added 0.01 mol/litre of ferric nitrate and 0.03 mol/litre of
sodium hydroxide to prepare a regenerated treating solution having a pH of
10.5. A 10 g quantity of aluminum particles having a boehmite film formed
on the surface thereof in the same manner as in E~ample 2 were immersed in
the regenerated solution at 90 C for 15 minutes and thereafter separated
from the solution by filtration. The resulting aluminum particles were
found to have been uniformly colored gold.
This invention may be otherwise embodied without departing from
the spirit and basic features of the invention. Accordingly it is to be
understood that the examples herein disclosed are given solely for illus-
trative purposes and are not limitative, and that the scope of this invention
is defined by the appended claims rather than by the specification. Thus
other changes and modifications may be made within the scope of the claims.
