Note: Descriptions are shown in the official language in which they were submitted.
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Case No.: P-11,130
PROCESS FOR THE SURFACE TREATMENT OF ALUMINUM
,
The invention relates to a process for the
application of conversion coatings to aluminum surfaces
by means of solutions which contain titanium and/or
zirconium, and also fluoride and phosphate ions, but
which are free from nitrate, nitrite, and chromium ions
and also from organic film-forming polymers and tannin.
Background of the Invention
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For the chemical treatment of metals, for
example as pretreatment for the application of
lacquers, adhesives and plastics, processes are known
in which the metal surface is cleaned in the first
stage, is flushed with water in the second stage, and
is then finally wetted in the third stage with an
aqueous solution which forms chemical conversion
coatings, and the resulting liquid film is then dried.
A thin, non metallic coating is thus formed on the
metal, and can decisively improve the surface quality
when the composition of the treatment liquid and the
reaction conditions are correspondingly selected.
Thus, e.g., coatings of lacquers, adhesives and
plastics, if necessary in the form of foils, can be
distinguished by substantially greater adhesion and a
markedly increased corrosion protection, when they are
applied to metals pretreated in this way.
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Such processes operate, e.g., with an
aqueous solution which contains hexavalent chromium,
trivalent chromium, alkali ions, and silicon dioxide in
given proportions and produces coatings for electrical
insulation, for corrosion protection, and as an
adhesion base for lacquers and the like (DE-AS
1,769,5~2).
Coatings can also be produced on metallic
surfaces by means of coating media which contain a
compound of hexavalent chromium and a polymeric organic
material (so-called primer) and are then drie~ or
stoved (AP-PS 197,164).
Because of the presence of hexavalent
chromium, the disadvantage is common to the
above-mentioned processes that precautionary measures
have to be taken, in particular during the application
of the coating material and the handling of the coated
metal, and that effects on the contents cannot be
excluded when such coated metals are used as container
material for foodstuffs and beverages.
In order to avoid the disadvantages
connected with the use of treatment liquids containing
hexavalent chromium, it is furthermore known to wet the
cleaned metal surfaces, in particular of iron, zinc,
and aluminum, with an acid aqueous solution which
contains chromium-III ions, phosphate ions, and
finely-divided silicic acid, and possibly also acetate,
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maleate, zinc, and/or manganese ions, and to dry the
film of solution (DE-OS 2,711,431). Although this
process has considerable advantages over the
above-mentloned ones, it is disadvantageous that when
the coated metals are used as container material a
certain effect on foodstuffs and beverages due to the
chromium~III content cannot be completely excluded, and
that the treatment liquid tends to be unstable due to
the formation of difficultly soluble chromium
phosphate.
Another category of processes for the
application of conversion coatings, specially for
aluminum surfaces, proposes dip or spray treatment with
compositions which contain given amounts of zirconium
and/or titanium and also phosphate and active fluoride
(U.S. Patent 4,148,670) or polyacrylic acid or ester
and fluozirconic acid, fluotitanic acid or fluosilicic
acid (U.SO Patent 4,191,596) or tannin together with
titanium and fluorine ions (U.S. Patent 4,054,46S)` or
also complex fluorides or boron, titanium, or
zirconium, together with oxidants such as sodium
meta-nitrobenzenesulfonate (DE-AS 1,933,013).
~he last-named processes are disadvantageous
in that because of water-soluble components of the
coating medium a rinse treatment is required, which
often leads to a waste water which is difficult to
treat, or else only a comparatively complicated
monitoring o the bath leads to usable results.
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Tannin-containing coating media sometimes lead to an
undesired discoloration of the treated metal surfaces.
Because of the multiplicity of known tannins, the
coating media are also reproducible only with
difficulty as regards their action. Polymer-containing
coating media tend to ageing of the polymer and to
instability, particularly in the form of concentrates.
In particular~ also, a certain incompatibility with
organic coatings which are then applied can be
foreseenO
The object of the invention is to make
available a process for the application of conversion
coatings to aluminum surfaces which avoids the known
disadvantages, in particular those mentioned above, and
can be carried out without additional cost.
Summary of the Invention
The problem is solved in that the procèss of
the kind mentioned at the beginning is designed
according to the invention such that the aluminum
surface is brought in contact with a solution which has
a pH of at most 3.5 and contains fluoride ions, at
least 1 g/l zirconium ions and/or at least 0.5 g/l
titanium ions and also at least 1.5 g/l phosphate ions,
in which the molar ratio of phosphate ions to zirconium
and/or titanium ions is at least 0.5 and of fluoride
ions to zirconium and/or titanium ions is at least 5,
such that when the liquid film is subsequently dried a
film weight of 10 - 300 mg/m2 results.
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The solution can be brought into contact
with the aluminum surface in any conventional way, such
as dipping and subsequently allowing to drip off,
pouring over and centrifuging off, brushing, spraying
with use of one- or two-material nozzles, spraying or
rolling on with flat or configured rollers running in
the same or opposite directions. Rolling-on processes,
because of the comparatively precise metering of the
amounts of solution, are particularly suitable.
The application of an amount of solution of
about 3 - 20 ml/m2 of aluminum surface is
particularly advantageous. Because of the small amount
of water to be evaporated off, the amounts of solution
should be as far as possible in the lower part of the
range .
The drying of the solution film applied to
the aluminum surface can basically take place already
at room temperature. For better formation of the
conversion coating, however, higher temperatures~
preferably of 50 - 120~C - given as temperature of the
article - are recommended.
Detailed Description of the Invention
According to a preferred improvement of the
invention, the aluminum surface is brought into contact
with a solution in which the molar ratio of phosphate
ions to ~irconium and/or titanium ions is at mos~ 5,
In this way a satisfactory film formation is ensured on
the one hand, and on the other hand the precipitation
of difficultly soluble zirconium phosphate is
prevented.
Furthermore, it is advantageous to bring the
aluminum surface into contact with a solution in which
the molar ratio of fluoride to zirconium and/or
titanium ions is at most 15. The increased supply of
fluoride accelerates the reaction taking place on the
aluminum surface. In addition, a concentration of
phosphate which is advantageous ~or the above-mentioned
satisfactory film formation can be faultlessly
stabilized.
As regards a subsequent treatment by
application of an organic coating, optimum results are
achieved when~ in a further advantageous embodiment of
the invention, the aluminum surface is brought into
contact with a solution such that a film weight of 20 -
200 mg/m2 results.
Before the process according to the
invention is carried out, thè aluminum surfaces are to
be thoroughly cleaned. The cleaning can take place
with acid cleaners based on phosphoric acid or sulfuric
acid, which should also contain surfactant, in
particular of the nonionic type, and possibly also
fluoride, or also with strongly alkaline cleaners,
e.g., with a content of sodium hydroxide, condensed
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phosphate, and surfactant. Rinsing thoroughly with
water should then follow, most suitably with fully
desalted water in the last stage. According to the
nature of the contamination, degreasing with, or
supplementarily with, halogenated hydrocarbons can also
be carried out.
The invention is explained, by way of
example and in more detail, with reference to the
Examples.
Examples
Aluminum sheets of dimension 100 x 200 mm
were cleaned at 65C for 15 sec by dipping in a
strongly alkaline cleaner based on caustic soda,
condensed phosphate, and non-ionic surfactant, rinsed
thoroughly with water, and freed from excess rinse
water by squeeze rolling. The cleaned aluminum sheets
were then dipped for about 1 sec in the treatment`
solution at reaction temperature and guided between
fluted rollers or smooth rollers such that 6 or 3 ml
solution film per m2 Of aluminum surface remained.
The drying of the solution took place at
80C article temperature.
The solutions used, which all had a pH in
the range 1~5 - 2.0 had the constitution summarized in
the following table~ The data on amounts of chemicals
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used or on the titanium and zirconium content are in
g/l.
5Solution H~TiF~H~ZrF~ ~ NH4F
1 5.30 - 4.15 3.60
2 7.85 - 3.18 1.77
3 10.20 - 4.14 2.30
4 - 6.03 3.68 3.23
- 8.46 2.72 1.51
6 - 6.66 2.14 1.89
Molar Ratios
15Solution Ti Zr PO4 : Ti or ZrF : Ti or Zr
l ~.54 - 1.35 9.0
2 2.28 - 0.70 7.0
3 2.96 - 0.70 7.0
4 - 2.65 1.33 g.0
- 3.72 0.70 7.0
6 - 2.93 0.70 7.6
Amount of Solution Film Weight
25SolutionApplied in ml _ Produced in mg/m2
1 6 78
2 6 77
3 6 100
4 3 39
6 76
6 3 32
A 6 60 -
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Sheets were used for comparison which had
been treated with a solution containing 5 g/l of
chromic acid and 5 g/l of silicic acid (in the form of
Aerosil) (denoted Solution A), or sheets were taken
which had only been degreased and rinsed with fully
desalted water (denoted Solution s below).
After application of the conversion coating,
the aluminum sheets were subjected to several tests.
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Within a first test series, the aluminum
sheets provided with the conversion coating were
lacquered with an epoxy lacquer (~obil SHX 2636) and
the lacquer adhesion was tested by the T-bend test.
A second series of aluminum sheets was
provided with a gold lacquer and after the boiling test
was subjected to a test of blister formation,
discoloration, and lacquer shedding.
In a third series of aluminum sheets, a hard
PVC sheet was sealed on by means of a PVC-based
heat-seal lacquer, and the adhesion of the sheet was
determined by the peel test.
The test procedure is explained below:
In the T-bend test, the lacquered sheet was
bent 180. The radius of the bend results ~rom the
selection of the cushion acting as a mandrel. ~sually
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one or more sheets are used of the same thickness as
the test sheet. For the severest stressing, operation
is without a cushion and the sheet is bent until the
untreated sides touch (so-called T-O-bending). Bending
over one sheet is denoted as T-1-bending, over two
sheets as T-2-bending. After bending, the lacquer
adhesion is determined by application and removal of
adhes ve tape in the region of the place stressed by
bending.
In the tables given below, the test results
are given for loosening of lacquer for the T-bending
test in percent -- with respect to the point of
bending.
The boiling test consists of first boiling
the lacquered aluminum sheets for 3 hours in completely
desalted water and then judging blister formation and
discoloration. Here one part of the sheets is
subjected before the boiling treatment to a mechanical
stressing corresponding to the above-mentioned
T-bending test or to impact drawing. In the stressing
corresponding to impact drawing, a drop body is allowed
to strike the unlacquered side of the sheet from a
given heightr shortly after hardening of the lacquer
and at room temperature, with a ball-shaped impact part
(ball diameter 16.9 mm).
The peeling test is characterized in that
the sheet metal strips, or strips provided with a
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conversion coating, 200 mm long and 15 mm wide, are
provide on one side with a sealing lacquer and then
provided with a PVC foil by means of heated press jaws
at a press pressure oE 300 Newton/cm2, a duration of
4 sec and a temperature of the preheated press jaws of
180C. After storage in water for three days, the
adhesion loss in comparison with a sheet not stored in
water is determined by pulling the foil off, and is
given in percent. The speed at which the foil is
pulled off is here 30 mm/min.
The results of the above-mentionecl tests are
collected below in a table. It should be added that in
the boiling test of the metal sheets provided with a
conversion coating, a minimal, hardly detectable
discoloration could be observed. In the sheets
mechanically stressed as in impact drawing, a slight
loosening of lacquer was just detectable. The metal
sheets which were only degreated and rinsed with
completely desalted water showed complete loosening of
lacquer.
Test Results
Boiling Test
T2-Bending T1-Bending Peeling
Solution Test Blisters Test Test
1 5 0 5 26
2 10 0 5 16
3 5 0 5 19
4 0 0 5 15
0 0 0 20
6 5 0 5 21
A 0 0 5 17
B 80 100 100 100
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