Note: Descriptions are shown in the official language in which they were submitted.
P-l 04 4 8 ~1~SS3~2~
BACKGROUND OF THE INVENTION
This invention relates to the art of chemically
treating an aluminum surface. More specifically, it relates
to the art of treating an aluminum surface to improve both
the corrosion resistance of the bare surface and the adhesion
of an organic finish subsequently applied to the treated
surface. This invention also concerns a process for accom~
plishing the foregoing results with an aqueous solution which -
has a less detrimental effect upon the environment than ;
conventional treating solutions because it does not require
the presence of chromium or phosphate in the solution. ~ `~
This invention provides an even further advantage
in the manufacture and preparation of aluminum cans. The
concentration and processing conditions can be adjusted sb
that the aluminum sur~ace is transformed without the ~ormation
of any measurable coating on the surface. Typically, no
i organic finish is applied to the bottom of aluminum cans. The
absence of any substantial coating permits the manufacturer
to improve the corrosion resistance of those unfinished
portions without changing the appearance of the surface of
the aluminum.
~ In the processing of aluminum cans after forming,
I the following procedure is typical:
1) wash with warm water;
,25 2) clean,usually with an acid-type cleaner;
3) water rinse;
4) apply treatment chemical;
5) water rinse;
6) deionized water rinse;
7) dry;
8) apply decorative organic finish to the
can exterior;
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9) cure organic finish at elevated
temperature;
10) apply interior sanitary lacquer; and
11) cure interior sanitary lacquer.
For step 4), standard practice is to employ an
aqueous solution containing from one-half to one weight percent
of a mixture of hexavalent chromium, phosphoric acid, and
fluoride. Such treating solutions have produced satisfactory
quality in terms of both corrosion resistance and paint
adhesion. However, the chromium and phosphate components are
environmentally objectionable, and their use therefore entails
additional recovery expense. It would,therefore,be highly
desirable to be able to use a treating solution which would
produce acceptable results which did not at the same time
create the environmental problem of solutions containing
chromium and phosphate.
The use of tannins in connection with metal treating
has been suggested by the prior art. U. S. 2,502,441 discloses
an alkali metal phosphatizing solution containing a two-
component accelerator which may be used for the treatment of
iron and steel surfaces and also possibly for other metals
j such as aluminum. The accelerator portion of the composition ~`
contains either a molybdenum or tungsten compound and a
phenolic substance such as a tannin. The patentee notes,
however, that if the alkali metal plus tannin is used without
th~ molybdenum compound, deposition of a coating seems to be
completely inhibited. U. S. 2,854,368 teaches the use of a
phosphoric acid solution containing a tannin ~or the treatment
of iron or steel and also possibly for other metals such as
~30 aluminum. The most dilute solution suggested by the patentee
i5 one containing one mole of phosphoric acid and one weight
percent tannin. When this solution was substituted for that of
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the present invention, completely unsatisfactory adhesions were
obtained.
SUMMARY OF THE INVENTION
I~ has now been discovered that an aqueous solution
containing at least 0.000025 weight percent of a vegetable
tannin, when adjusted to a pH of from 3 to 9, will trans~orm
an aluminum surface to enhance its corrosion resistance and
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its receptivity to an organic finish. If coating weights not
in excess of 1 mg/ft2 are desired, they may be obtained by
using relatively low concentrations and contact times. The
term "organic finish" includes, for example, base coat, ink, -
paint, over-varnish and sanitary lacquer.
DETAILED DESCRIPTION OF THE INVENTION -
The chemistry of tanning agents is not completely
understood. They include a large group of water soluble,
complex organic compounds widely distributed throughout the
vegetable kingdom. All have the common property of precipi-
tating gelatin from solutions and of combining with collagen
and other protein matter in hides to form leather. All tannin
extracts examined contain mixtures of polyphenolic substances
and normally have associated with them certain sugars. (It is
not known whether these sugars are an integral part of the
structure). For a discussion of tannins, see Encyclopedia of
Chemical Technoloqy, 2nd edition, Kirk-Othmer, XII (1967) pp.
303-341 and The Chemistry and Technoloqy of Leather, Reinhold
Publishing Corporation, New York, pp. 98-220 (1958). ;
Tannins are generally characterized as polyphenolic ~ -
substances having molecular weights of from about 400 to about
3000. They may be classified as "hydrolyzable" or "condensed"
depending upon whether the product of hydrolysis in boiling
mineral acid is soluble or insoluble, respectively. Often
extracts are mixed and contain both hydrolyzable and condensed
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forms. No two tannin extracts are exactly alike. Principal
sources of tannln extracts include bark such a-; wattle, mangrove,
oak, eucalyptus, hemlock, pine, larch, and willow; woods such
as quebracho, chesnut, oak and urunday, cutch and turkish;
fruits such as myrobalans, valonia, divi-divi, tera, and
algarrobilla; leaves such as sumac and gamb:ier; and roots
such as canaigre and palmetto.
The term "vegetable tannins" is ernployed to distinguish
organic tannins such as those listed in the previous paragraph
from the mineral tanning materials such as those containing
chromium, zirconium and the like. Experimental work has
shown that hydrolyzable, condensed, and mixed varieties of
vegetable tannins may all be suitably used in the present ;
invention. Quebracho has been found to be a very effective
condensed tannin and myrobalan, an effective hydrolyzable
tannin.
Very small concentrations of the tannin extract
have been found effective for improving the corrosion resistance
and organic finish adhesion of an aluminum surface. The
concentration to be used depends upon the particular tannin
employed, the processing conditions selected and the quality
and thickness of the resulting coating. If all conditions
are properly adjusted, concentrations as low as 0.000025
weight percènt are effecti~e. Generally, the tannin concentra-
tion will be between this lower limit and 25 weight percent and,
i about
under the usual conditions, betwee_/0.002 and .25 weight
percent. Most preferably, the concentration will be about
0.025 weight percent. Lower concentrations do not produce
an appreciable improvement in characteristics, and higher
concentrations result in an increased dragout of valuable
l chemicals on the workpieces. The pH of the aqueous solution
i must be adjusted to a value of at least 3 and is preferably
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P-10448
less than about 9 and most pre~erably between 4 and 8. A pH
somewhat on the acid s:Lde (as low as abou~ 3) is typically
obtained when a natural extract is dissolved in water.
pH values below 3 do not produce the desired improvement in
properties, and there is generally no reason to adjust to a
pH above 9. The pH may be adjusted with any compatible acid
or base typically used for that purpose such as, hydrochloric,
sulfuric, phosphoric, hydrofluoric, chromic or acetic acids
and the alkali metal hydroxides, carbonates or silicates.
Only very small amounts are necessar~. It is important to
note that, while it is permissible to employ small quantities
of phosphoric acid to adjust the solution pH, the presence of
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phosphate ions is totally inessential to the operability of
the invention.
The processing conditions of temperature, contact
time, and contact method are interdependent; spray, immersion,
and roll-on techniques may ke employed. In the case of can
manufacture, application of the chemicals is conventionally
by the spray technique and, considering normal plant operations,
the temperature of the solution will normally be from 90 to
125F (preferably 100~105F) and the contact time will be
between 5 and 30 seconds. Of course, with suitable adjustment
of the solution or processing condition, values could be
outside the above normal ranges.
Aside from the obvious environmental advantages of
using the solution of the present invention, a particular
advantage in connection with can manufacture is that the
treatment conditions can be adjusted to give improved results
even though there is formed substantially no measurable coating.
By no measurable coating, we mean that the coatin~ weight
detectable on the processed can is less than one milligram
per square foot. The conventional chromium-phosphate treatment
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for aluminum cans results in a coating weight of between five
and fifteen milligrams per square foot.
In practice, the severest problems of discoloration
of an unfinished surface will be encountered during pasteurizing.
s Typically, no organic finish is applied to the can bottom
to protect it from corrosion. If left untreated, it will
discolor during pasteurization, turning brownish.
The adhesion of the organic finish to the surface
normally meets its severest test when the cans are subjected
to a hot detergent solution to sanitize the cans before ;
filling.
Accordingly, tests have been developed to measure the
bare surface corrosion resistance and finish adhesion imparted
to a can by a particular treating procedure. In the cor~osion
test, the can is immersed in tap water and subjected to
temperatures of 140 to 160F for 45 minutes. Then the
unpainted portion is observed for discoloration. Since
pasteurization normally takes about 15 minutes, this test is
somewha~ more severe than would be encountered in practice. In ~-
the adhesion test, the can is subjected to a boiling one
percent detergent solution, rinsed in tap water, cross-hatched
with a knife~ dried, and tape-pulled. The amount of paint
removed from the surface and adhering to the transparent tape
is then measured.
In addition to the foregoing tests, cans treated
according to this invention were filled with a food product
and then aged. The contents were then tested for flavor and -
found acceptable. Aside from the mentioned pH adjusters,
additional ccmpatible components may optionally be included
in the solution such as accelerators, surfactants, and chelating
agents.
The following examples demonstrate the proces~ of
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the invention using varying concen-trations and p~ values for
different types of vegetable tannins.
EX~MPLE 1 CONDENSED TANNIN
_
Solutions containing .001, .002, .012, .025, .25
and .5 weight percent quebracho powder extract at pH = 7.7 -
7.9 were used to treat aluminum cans. The following process
sequence was used:
(1) 15 sec. hot water rinse;
(2) 30 sec. spray cleaning using a sulfuric
acid-based cleaner,
(3) 15 sec. hot wa-ter rinse,
(4) 20 sec. spray treatment at lO0 - 105F
with the quebracho solution,
(5) 15 sec. cold water rinse,
(6) 3 min. oven dry at 350 F.
An organic finish was then applied to the exterior
side walls of the thus-treated aluminum cans as follows:
Coke red ink (Acme Ink Co. alkyd-based) was applied
using rubber rolls. A clear overvarnish (Clement Coverall
Co., Code No. P-550-G alkyd polyester) was then applied over
the wet ink using a No. 5 draw down bar. The cans were then
baked 5 min. at 350~F followed by 3 min. at 410F to cure.
Adhesion was then evaluated after subjecting the
can to the following; Immerse for 30 minutes in a 1% by
volume Joy*(m~nufactured by Proctor & Gamble) solution at
boiling, rinse in cold water, cross hatch with a knife, dry
` and tape pull. The peel results are summarized in Table I.
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P-1044
TABLE I
~eight Percent Exterior
Quebracho Adhesion
.001 Poor 90 - 100~ Peel
.002 Excellent 0% Peel
.012 Excellent 0~ Peel
.025 Excellent 0~ Peel
.25 Excellent 0~ Peel
.5 Excellent 0~ Peel
For comparison, aluminum cans were also treated with
a commercially accepted CrO3 - H3PO4-HF solution of the type
described above to obtain about 6 mg/ft2 coating weight.
When finished and tested as above, excellent paint adhesion
with 0% peel was obtained. Aluminum cans were also prepared
without either the tannin or chromium based treatment tcleaned
and water rinsed only). When finished and tested as above,
poor adhesion with 100~ peel resulted.
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The bottoms of the treated aluminum cans were
immersed in water for 45 min. @ 150F. No discoloration was
observed with the commercially accepted CrO3-H3PO4-HF treatment.
A commercially acceptable very sli~ht discoloration was
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observed on the can bottoms treated with the quebracho
solutions. The untreated aluminum can bottoms showed a
greater and commercially unacceptable degree of discoloration.
EXAMPLE 2 HYDROLYZABLE TANNIN
A solution was prepared by dissolving 6 gm of 'ITannic
i Acid" (nutgall extract~ in 6 liters of water which gave a pH
of 6.20. The Tannic Acid was supplied by Merck ~ Co. Inc.
and labeled N. F. Fluffy. The pH o-E the Tannic Acid solution
was contxolled by using very small amounts of H3PO~ and/or
~0 NaOH. Aluminum cans were processed at pH's of 8.65, 6.20, 4.40
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and 2.70 using the follo~ing process sequence:
(l) 60 sec. spray cleaning using a sulfuric
acid-based cleaner;
(2) 30 sec. hot water rinse; -~
(3) 20 sec. spray treatment at 120F with
the Tannic Acid solution;
(4) 30 sec. cold water rinse; ;
(5) 3 min. oven dry at 250F.
The cans were then finished and tested as in Example l. Results
were as shown in Table IIo ~`
TABLE II
Tannic Acid Exterior
Solution pH Adhesion
8.65 Excellent 0~ Peel
6.20 ExcelIent 0~ Peel
4.40 Excellent 0% Peel
2.70 Poor 100% Peel
EXAMPLE 3 CONDENSED TANNIN
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A solution consisting of .025~ quebracho at pH
~alues of 8.60, 5.06 and 2.73 was used to treat aluminum cans.
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The pH o~ the quebracho solution was adjusted to the desired
value using small amounts of H3PO4 and/or NaOH. The following
process sequence was used to treat the aluminum cans.
(l) 60 sec. spray cleaning using a sulfuric
acld-based cleaner;
(2) 30 sec. hot water rLnse;
~ (3) 20 sec. spray treatment at 100F with
`~ the quebracho solution;
(4) 30 sec. cold water rinse;
(5) 3 min. oven dry at 250F.
The cans were finished as in Example 1 and then a sanitary
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lacquer (Mobi~e Paint Co. ~ S-6839009 thermosetting vinyl) was
applied to the can interior with a ~20 draw down bar and
cured for 3 minutes at 410F to give the results of Table III.
TABLE III
~uebracho Exterior
Solution pH Ad_esion
8.60 Excellent 0% Peel
5.06 Excellent 0~ Peel
2.73 Poor 100% Peel
The interior surface of the can having only the clear, sanitary
lacquer applied to the treated surface was also tested for
adhesion as above and 0% peel was observed, Untreated cans
exhibited peels as high as 25%.
EXAMPLE 4 HYDROLYZABLE T~NNIN
A solution was prepared by dissolving 6 gm of
myrobalan extract in 6 liters of water which gave a pH of
3.7. The myrobalan was manufactured by Tannins and Chemicals,
Inc. and labeled "Spray Dried Myrobalan Powder 60%". An `
aluminum can was processed in this solution us'ng the following
process sequence:
2~ (1) 60 sec. spray cleaning using a sulfuric acid-
based cleaner;
(2) 30 sec. hot water rinse;
(3) 20 sec. spray treatment at 120F with the ~ `
myrobalan solution;
(4) 30 sec. cold water rinse;
(5) 3 min. oven dry at 350F.
The same procedure as in Example 1 was used to finish and test
the can. Excellent adhesion with 0% peel was obtained.
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