Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Invention
Field of the Invention
The present invention relates to a process for
the production of colored protective coatings on
articles of aluminum or aluminum alloys which have been
previously anodized in a special way in order to obtain
products which are particularly suitable for use in
architectural applicati~ns.
Description of the Prior Art
- 10 Much time and attention devoted in the prior artto the production of aluminum articles which are deco-
rative and resistant to abrasion under atmospheric
influence. Early processes have included chemical
coloring of aluminum articles which had previously been
anodically anodized by the treatment of the same with
dyes, such as aniline dyes. As the art is wcll awarej
the thus resulting a~ticles have poor resistance towards
atmospheric influence. Other developments have included
anodic oxidation of aluminum articles, followed by
submersion in chemicals which penetrate into the pores
of the oxide layer, so that when the thus treated
aluminum article is placed in.aqueous solutions of salts
which also penetrate into the pores, combination with
the first used chemical is possible. These processes
have not proven practical for a wide variety of reasons.
It is also known in the prior art to simultaneously
anodize and color aluminum articles. ~owever, the art
is aware that processes of this type result in only a
limited selection o colors and that the processes are
expensive and difficult to carry out and very rigid
requirements are made for the working and heat treatmen~t
of the aluminum articles as the metallic structure
therein is of the utmost importance forthe result
obtained. These simultaneous processes also demand the
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use of large currents and high voltages and long times
and are thus relatively-expensive.
United States Patents 3,669,856; 3,769,180 and
3,849,263 represent recent developments in the field
of coloring aluminum or aluminum alloys. These patents
are, in general, directed towards the coloring of
anodized aluminum by immersing said article in a bath
containing a salt of a particular metal and passing an
alternating current between the previously anodized
article and a counterelectrode.
Although the process of these patents represent a
significant improvement in the field of coloring alumi-
num,nevertheLess, no details are given as to how
the previously formed anodic coating is formed on the
aluminum and, in fact, at least the implication is
present that conventional anodizing techniques are
used.
It is also well known in the art of anodizing alumi-
num that two separate and distinct types of an oxide
layer can be obtained which are generally referred to in
the art as a hard coat or a soft coat. The conventional
anodizing techniques utilized in the art result in the
production of a so-called soft coat. There are processes
known in the art for the production of hard, dense
anodic coatings, but the techniques employed in the art
for the subsequent dyeing of these hard, dense coatings
have involved the conventional immersion in a suitable
dye, as opposed to an electrolytic coloring process.
The explanation for this might be the fact that the
techniques for the production of hard anodized coatings
result in the production of anodic layers which are
significantly colored and thus darker and muddier colors
are only obtainable by the use of organic or
inorganic dyestuffs. The art is also aware that the
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thicker the anodic l~yer is formed that the darker theanodic layer will b~ and, in general, those processes
which produced hard anodic layers had as one of their
criticalities the production of a thick layer. These
S thick layers of anodic film are totally unsuitable for
the novel process of this inve~tion.
As has heretofore been stat~d, there are processes
I known in the art for the production of a hard anodized
layer. Of such known processes, those which have the
anodizing bath at low temperature (around 32F) or
intermediate temperature (around 45F) are unsuited for
the use of the novel process of this invent iOII for
several reasons. In the first place, these processes
are expensive and require substantial energy in order to
operate. Further, these processes produce an anodized
layer which is relatively thick (customarily 1.5 mil or
~ heavier) in order to obtain high uear resistance, and
- which is a darkish, muddied color thereby rendering it
unsuitable for use where light, unmuddied colors are
desired. On the other hand, the known hard coat process
which operates at room temperature (with the anodizing
bath at about 70F) uses a higher current density than
that of the present invention, resulting in the produc-
tion of an anodized layer unsuitable for use in pro-
ducing a wide range of colors.
U.S. Patent 3,524,799 is directed towards a room
- temperature process for anodiæing aluminum in order
to produce hard, dense anodic coatings and the novel
process of the present invention utilizes as one step
thereof a modification of the process disclosed by this
patentee.
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The specification and claims of this patent are
directed to the formation of hard, dense anodic coatings
on aluminum or aluminum alloys by anodizing the aluminum
in an aqueous electrolyte containing a mineral acid,
such as sulfuric acid, a polyhydric alcohol of 3 to 6
carbon atoms, an organic carboxylic acid containing at
least one reactive group in the alpha-position to the
carboxylic acid group, such as lactic acid or glycine,
and an alkali salt of a titanic acid complex of a
hydroxyaliphatic carboxylic acid containing from 2 to 8
carbon atoms, such as, for example, titanium dilactate
ammonium salt.
We have now discovered that the use of such anodizing
techniques without the al};ali salt of a titanic acid
complex provide extremely dense and hard anodic coatings
1~ optimally suited to architectural applications and that
such anodized layers when color-ed using the tcchniques
described in U.S. Patent No. 3,669,~56 provide aluminum
and aluminum alloy surface of very pleasing, architec-
turally pure colors of exceptional uniformity. Addition-
ally, the use of the combination of these prior arttechniques apparently provides exceptional throwing
power in the coloring operation. Throwing power is a
term of art defining the ability of a coloring bath and
process to provide color uniformly to all surfaces of
the surface of a workpiece undergoing coloring. Thus, a
process and bath which demonstrates high throwing power
provides uniform .color to small creases, cracks, nooks,
detents, etc., as well as the larger uniform surfaces of
an aluminum or aluminum alloy workpiece being colored.
High throwing power also permits the introduction into
the coloring bath of a mix of workpieces in terms of
their alloy composition and overall physical configur-
ation to obtain uniform color of all such workpieces. In
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prior art coloring techniques it was often difficult,
if not impossible to obtain uniform coloring of work-
pieces of different alloys or shapes in a single coloring
bath at the same time. Furthermore, as is well recog-
nized by the skilled artisan in the aluminum coloring
field, spacing of the various workpieces in the coloring
bath has been a critical factor in successfully uniformly
coloring aluminum extrusions, particularly for archi-
tectural purposes. Such spacing restraints often
required leaving sufficient distances between the
undivided pieces being colored that substantial portions
of the working volume of a given coloring tank were left
empty during a coloring operation resulting in very
inefficient use of coloring tank capacity. The excep-
tional throwing power of the technique of the instant
invention permits minimal spacing of the workpieces in
the coloring ba*h and thus optimum usage of the coloring
capacity of a coloring tank. This results not only in a
more optimum efficiency in terms of use of tank capacity,
bu~ reduces substantially the chemical and power require-
ments of the electrolytic coloring process.
According to the present invention, a novel process
is disclosed for the production of colored coatings on
articles of aluminum or aluminum alloys which are
particularly adapted to be employed for architectural
uses which involves first forming a hard, dense anodic
coating on aluminum and aluminum base alloys by anodizing
the aluminum in a specific electrolyte comprising
sulfuric acid, a polyhydric alcohol of 3 to 6 carbon
atoms and an organic carboxylic acid containing at least
one reactive group in the alpha-position in order to ob-
tain a material having a film thickness of between about
0.2 to about 1.1 mils and thereafter electrolytically
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coloring said anodized aluminum by passing alternating
current between said anodized aluminum and a counter-
electrode immersed in an acidic aqueous bath and a metal
salt, such as tin, while modulating as to amplitude or
frequency the alternating voltage externally of said
electrode system so as to apply voltage of controlled
asymmetry of said electrodes.
In order to obtain the architecturally acceptable
and desirable hard anodic coatings of pure clean color
as described above, it is absolutely critical that
the anodic layer be between about .2 and about 1.1 mils
in thickness, as opposed to the 1-5 mils set forth at
column 3, line 26 of said U.S. Patent 3,S24,799.
As already pointed out, the electrolyte used to
anodize the aluminum must be of the type described in
U.S. Patent No. 3,524,799 without any alkali salt of
titanic acid complex.
Apparently, as described in U.S. Patent No. 3,524,799,
the combination is an anodizing bath of a polyhydric
alcohol containing from 3 to 6 carbon atoms, and an
organic carboxylic acid containing a reactive group in
alpha-position to the carboxylic acid group will react
with the hot reaction products formed during anodizing
with or adjacent to the surface of the pore base, and
thereby suppress the attack or dissolution of the
forming oxide film by these products.
The mineral acid component of the electrolyte is sul-
furic acid. The anodizing bath concentration of sul-
furic acid is generally maintained between about 12% and
about 20% by weight, preferably about 15%.
Polyhydric alcohols containing from 3 to 6 carbon
atoms which may be employed in the practice of the
invention, singly or in admixture, include glycerol,
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butane-diol-1,4, pentanediol-l,S, mannitol and sorbitol.
The total arnount of polyhydric alcohol employed ran~es
from about 1~ to about 4% by volume of the anodizing
electrolyte. The preferred polyhydric alcohol is
S glycerol.
The organic carboxylic acids containing a reactive
group in alpha-position to the carboxylic acid group
include acids in which the reactice group is hydroxy,
amino, keto, or carboxyl. Examples of such acids
include glycolic (hydroxyacetic), lactic (hydroxypro-
pionic), malic (hydroxysuccinic), oxalic, pyruvic, and
aminoacetic acids. Acyclic carbo~ylic acids such as
lactic, malic, and amino-acetic (glycine) acids are
preferred. A mixture of two or more of these acids may
be employed in combination with the mineral acid and the
polyhydric alcohol. The amount of- carboxylic acid
included in the electrolyte is preferably between about
lg and about 4% by volume of the bath.
Glycolic acid is specifically preferred as the
carboxylic acid.
- In order to achieve the results described above, the
temperature at which anodizing is carried out must range
from about 65 to about 85F with room temperature con-
dition, i.e., 68-75F being preferred.
In order to achieve the exceptionally hard and
readily colored anodic coatings, it is also necessary
- that the current density which is used in the anodizin~
operation should be in the range of from about 24
to about 36 amperesfsq. ft.
The time required to achieve the desired film
thic~ness of between about 0.2 and 1.1 mils will vary
with the other parameters of temperature, current
! density, chemical composition of the bath, etc., but
generally anodizing times on the order of from about
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8 to about 30 minutes produce acceptable results.
Following the special anodizing treatment, above-
described, the aluminum article is thereafter colored
electrolytically by passing alternating current between
said article and a counterelectrode in an aqueous acidic
solution containing a water soluble metal salt.
~ he particular manner of electrolytically coloring
the anodized aluminum in accordance with the novel
process of this invention is set forth in United States
3,669,856. It has been found that if the
aluminum is anodized in the manner above-described and
thereafter electrolytically colored in accordance with
the teachings of U.S. 3,669,856 many significant advan-
tages will be obtained as opposed to the use of conven-
tional anodizing techniques. In the first place, the
article which is obtained has a hard coating which makes
it particularly adapted to be used in architectural
applications. Additionally, the process of this in-
vention permits the simultaneous electrolytic coloring of
articles of varying sizes and shapes. This has been
difficult if not impossible, to achieve in prior art
processes due to the fact that uneven color was obtained
when articles of different sizes and shapes were simul-
taneously electrolytically colored. Another advantage
of the novel process of this invention resides in the
act that the aluminum article to be colored need only
have electric contact at one edge thereof, as opposed to
both edges. This results in a sïgnificant manpower
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savings. Another advantage of the novel process of this
invention resides in the fact that it is possible to
correct for too dark a color electrolytically which has
heretofore been impossible with processes utilizing dyes
or with processes involving simultaneous anodizing and
coloring. According to this technique, after application
of an excess of color the polarity of the coloring
system is reversed and color can be subtracted from the
anodized layer.
As has heretofore been stated, the electrolytic
coloring process is carried out by passing an alternating
current between the anodized article of aluminum or
aluminum alloy and counterelectrode, which has been carried
out in the manner above-described, and a counterelectrode
immersed in an acid aqueous bath containing metal salts
having coloring cations, wherein the colored tones of the
coatings can be controlled in a simple manner by modulating
the shape of the curve of the applied alternating voltage in such
a manner that during the coloring process the alternating
voltage will provide a suitable ratio between the two
current directions to achieve an advantageous transport of
material and course of reaction with regard to said
anodized aluminum article. The alternating voltage
supplied is modulated with regard to its amplitude and/or -
frequency so as to make them asymmetrical, thereby to control
the color tone of the aluminum article. As is known in
the art, the modulation of the alternating voltage can
I be carried out in several ways, such as simultaneously
¦ supplying two or more different alternating voltages or
¦ 30 a superimposed direct voltage or by generating an
! alternating voltage having the desired frequency and
curve shape.
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The material for the counterelectrodes can be
stainless steel, titanium, copper, nickel, but prefer-
ably tin because they lead to advantageously low
- energy consumption.
The strength of the alternating voltage in
S the modulation of the amplitude and/or fre~uency
thereof according to the present process is from 5-50
volts, depending upon the composition of the electro-
lyte and the properties of the oxide layer previously
formed. Preferably there is used a current density of
rom 0.1 to O.S A/dm2, dependent on the electrolyte
employed and a low treatment period of from 1 to 10
minutes
As is ~nown in the art, various soluble metallic
salts can be employed. The preferred salts are
those of tin, althouyh salts of nickel, cobalt,
copper, silicomolybdic acid and silicotungstic acid
can also be employed. The electrolytic coloring bath
also contains a strong acid which is desirably either
sulfuric or hydrochloric.
As is well known in the art, the metallic salts,
e.g., sulfates, chlorides, acetates, etc. desired
to provide the particular color can be~utilized at a
concentration of from 0.5 to 20~ by weight, preferably
about 2~ by weight based on the electrolyte. The pH
of the electrolyte may vary considerably within the
acid range, but pHs of about 1.5.have been found to be
useful.
A particularly preferred embodiment resides in
having present in the electrolyte a certain amount of
aluminum. In this connection, the aluminum can be
provided by the addition of suitable aluminum compounds,
such as aluminum sulfate or a certain part of a
previously used electrolytic bath can also be used.
The amount of aluminum which is present in the electro-
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lyte can range from 0-12 grams/liter, and more desir-
ably, from 4-8 grams/liter.
As has heretofore been pointed out, the novel
process of this invention is applicable to color
articles made from aluminum, as well as from aluminum
base alloys of all kinds.
It is particularly preferred initially to supply
a symmetrical alternating current and then add asymmetrical
alternating current.
In addition, the coloring takes place faster, more
efficiently if the alternating current is regulated
relatively slowly and in particular of the order of a few
seconds, from zero voltage to the voltage which is desired
for the coloring. This relates both to the alternating
current at the start up of the coloring and to a later
supply of alternating current at the voltage desired for
coloring.
Further improvement in the throwing power of
the coloring solution of the instant invention can be
achieved by incorporation of material which serves as
a complexing or sequestering agent for the coloring
metal ion. Although the mechanism for this further
improvement is not fully understood, it has been found
that the addition of small amounts on the order of
5x10-5 to 5xlO 3 grams/liter of, for example a
combination of ~-naphthol and gelatin in a ratio of
-about 2:1 naphthol to gelatin or 4,4 di(dimethyl-amino)
diphenyl methane to the coloring bath provides even
more improved coloring baths.
The following examples will illustrate the novel
- 30 process of this invention.
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EXAMPLE 1
An aluminum article was anodized in accordance
with normal anodizing techniques utilizing a current
density of 24 amperes/sq. ft. and an electrolytic bath
comprising 20 weight percent sulfuric acid, and 8
grams/liter of oxalic acid. The temperature utilized
ranged from 18-21C, and the resulting aluminum
article had an anodized layer of 25 microns. The
resulting product was not suitable for coloring due to
the fact that it was darkish in color.
As is obvious from the above experiment, the
anodizing solution used was other than that of the
instant invention.
EXAMPLE 2
An aluminum article was anodized using a solution
compxising 18 weight percent sulfuric acid, 1% glycolic
acid and 1% glycerol. The anodizing was carried out
at a current density of 36 amperes/sq. ft. at a tem-
perature of about 19.5C. After 13 minutes an anodized
layer of appro~imately 0.83 mils was obtained.
The anodized aluminum article was then electro-
lytically colored by immersing the same into a bath
comprising 25 grams/liter sulfuric acid, 22 grams/liter
sulfonic acid, 25 grams/liter tin sulfate, 5 grams/liter
aluminum sulfate, 0.2 grams/liter of ~-naphthol and
0.4 grams of gelatin per liter. The electrolytic
coloring was carried out by applying alternating
current through the electrolyte at a voltage of 8
volts for three minutes. Three minutes of alternating
current of half-wave was then applied.
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An aluminum article having a blackish color
was obtained.
EXAMPL~ 3
An aluminum article was anodized utilizing
the electrolyte solution of E~ample 2 at a current
density of 40 amperes/sq. ft. at a temperature
of 20C.
The anodized article which was obtained was
thereafter electrolytically colored in accordance
with the techniques of United States 3,669,856.
This resulted in an article having poor color.
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EXAMPLE ~
An aluminum article was anodized utilizing
the anodixing solution-set forth in ~xample 2 at
a temperature of 20C and at a current density of 48
amperes/sq. ft. The anodizing was carried out until
an anodized layer was obtained which had a thic~ness
of about 1.65 mils. Subsequent color anodizing
of this material in accordance with the techniques of
this invention resulted in spalling on the anodic
film.
It is apparent that the thickness of the anodized
layer obtained during the anodizing step was simply
too great to produce the satisfactory product desired
by the techniques of this invention.
EXAMPLE 5
An aluminum article was anodized utilizing the
electrolyte solution of Example 2 at a temperature
of 21C until an anodized layer having a thickness
of about .8 mils was obtained.
This material was then electrolytically colored
utilizing the techniques of United States 3,669,856
; ~ and the color solution of Example 2. Alternating
~ ; current was applied for 1 1/2 minutes and thereafter
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a half-wave alternating current was applied for a
hal~-minute. The resulking material was colored
satisfactorily and was capable for use as an archi-
tectural material.
.
EXAMPLE 6
An aluminum article was anodized using the
electrolytic solution of Example 2 at a temperature
of 20C for six minutes in order to obtain an article
which had a thickness of approximately 0.4 mils.
This material was then electrolytically colored
utilizing the solution of Example 2 by passing normal
AC current between the aluminum article and a cour-ter-
electrode for two minutes, thereafter an alternating
current having a minus half-wave which was asymmetrical
was applied for one minute.
A very acceptable black color was obLained. It
is to be noted that normal anodizing techniques
could not have obtained color in an anodic film this
thin.
EXAMPLE 7
An aluminum article was anodized utilizing the
solution of Example 2 at a temperature of 20C, a
current density of 36 amperes/sq. ft. in-ord~r to
obtain a material which had a thickness of 1.1 mils.
The material was thereafter color anodized utilizing
the tin solution set forth in Example 2 and the tech-
nique of United States 3,669,856. Alternating current
was applied for 1 1/2 minutes followed by half-wave at
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one minute. A perfectly acceptable colored article
was obta1ned.
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EX~MPLE 8
The process of ~xa~ple 5 is repeated with the
exception that ater the product was run to a bronze
color, it was immersed in an oxidizing acid, preferably
20-30 volume % nitric acid at room temperature, which
resulted in a uniform champagne color. This color is
virtually impossible to produce in a uniform manner by
any other known process.
The desirable hardness of coatings made in
accordance with the invention is evidenced by high
coating density. For example, four samples were
prepared using 6063 alloy and the process of Example 5
except that the current times ir. the coloring bath
were varied as follows, with the following results:
Alternating V2 Wave Coating Coating
Sample Color Current Current Thickness ~Yeight
-Minutes -Minutes -Mil ~in2
A Light 2 0 .84 ;68
Bronze
8 ~dium 3 O .75 70
Bronze
C - ~ark 3 1 1/2 .83 90
Bronze
D Black 3 4 .86 138
,~It will be readily understood that the description
'~ herein is ~or the purpose of illustration and that the
¦scope of the invention is limited only by the appended
clai~s.
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