CA1071510A

CA1071510A - Process for preparing colored aluminum powder

Info

Publication number: CA1071510A
Application number: CA255,228A
Authority: CA
Inventors: Toshimitsu Uchiyama; Minoru Hasegawa; Tatsuo Ootsuka; Hiroshi Matsumoto
Original assignee: Showa Aluminum Corp
Current assignee: Showa Aluminum Can Corp
Priority date: 1975-06-19
Filing date: 1976-06-18
Publication date: 1980-02-12
Also published as: AT346438B; DE2627428C3; JPS534004B2; JPS51150532A; CH603816A5; GB1546426A; ATA444876A; DE2627428A1; DE2627428B2

Abstract

Abstract of the Disclosure Colored aluminum powder is prepared by immersing finely divided aluminum in a weak alkali solution containing a metal salt and an organic compound having a chelating ability and separating the aluminum from the solution. Such powders find use as a pigment in coating compositions.

Description

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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 ~ -2-~1[317~5~(~
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 ~ay ~2a-~L~7~5~0 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 ~7~L5~

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 _ q, _ ~L~7~5~0 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 7~5~

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.

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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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a colored aluminum 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 consist-ing of metals having coordination numbers of 4 or 6 and annorganic compound having a chelating ability, and separating the resulting aluminum particles from the solution to obtain colored aluminum particles.

2. A process as defined in claim 1 wherein the organic compound is at least one chelating agent selected from the group consisting of O-co-ordination chelating agents, N-coordination chelating agents and chelating agents containing both O-coordination group and N-coordination group.

3. A process as defined in claim 1 wherein the weak alkali solution further contains an alkali selected from the group consisting of alkali hydroxides, ammonia, amines, alkali carbonates and alkali aluminates and has its pH value adjusted to 8 to 13 with the alkali.

4. A process as defined in claim 1 wherein the weak alkali solution has room temperature.

5. A process as defined in claim 1 wherein the weak alkali solution has a temperature of 60 C to the boiling point of the solution.

6. A process as defined in claim 1 wherein the metal having a co-ordination number of 4 is iron.

7. A process for preparing a colored aluminum powder comprising the steps of immersing finely divided aluminum in an aqueous solution having a pH of 4 to 12 to form a boehmite film on the surface of the aluminum particles, separating the resulting aluminum particles from the solution, immersing 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 numbers of 4 to 6 and an organic com-pound having a chelating ability, and separating the resulting aluminum par-ticles from the solution.