Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10891SS ~-
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TREATING AUTODEPOSITED COATINGS WITH Cr COMPOSITION
Field of the Invention
i . . ,, ~
This invention relates to the formation of .~ -
organic coatings on metallic surfaces. More specifical~
- 5 ly, this inverltion relates to the deposition on metallic ;
,~ surfaces of organic coatings by contacting the metallic
surfaces with an acidic aqueous coating solution contain~
ing dispersed particles of an organic coating-~orming ` ~ :
. ~
~ material such as resin particles. . ~ .
;~ 10 A relatively recent development in the coating field is the provision of water-based coating : .~
:~ compositions which are effective, without the aid of ~ -
!~ electricity, in forming on metallic surfaces immersed
.~ therein organic coatings that increase in thickness or -
weight the longer the time the surfaces are immersed in
the compositions. (For convenience, a coating
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1~85~55
formed from such a composition is hereafter referred to as
"an organic coating which grows with time" or as an "autodeposited
coating".) Speaking generally, compositions which are so
effective comprise acidic aqueous coating solutions having
dispersed therein particles of an organic material such as
resin particles. Autodeposited coatings are formed from such
compositions as a result of their ability to attack and dissolve
from the metallic surface metal ions in amounts which cause the
particles to deposit on the surface in a manner such that there
10 is a continuous buildup of organic coating on the surface.
Coatings formed from such compositions are distinctly
different from coatings formed by immersing the metallic sur- '~
faces in conventional latices, that is, compositions compris-
ing solid r?sin paxticles dispersed in water. The weight or
15 thickness of a coating formed by immersing a metallic surface
; in a conventional latex is not influenced by the time the
surface is immersed in the latex. It is in the main influenced
by the amount of resin solids dispersed in the aqueous medium.
. :.
, Coatings formed.from the aforementioned recently
20 developed coating composi~ions are also distinctly'different
from coatings formed from earlier known acidic aqueous coat-
ing solutions containing disper~ed solid resin particles and
relatively high amounts of water soluble corrosion inhibitors,
such as compounds containing hexavalent chromium. The use of
~,~ 25 relatively high amounts of corrosion inhibitors in such sol,u-
tions deters-attack of the metallic surface to an extent such
~ that resinous coatings which grow with time are not obtaine~.
'~ Thus, resinous coatings formed by immersing metallic surfaces
in such compositions are like those formed from immersing the
,30 metallic surfaces in conventional latices in that they do not
grow with time.
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The use of the recently developed coating composi-
tions which produce coatings which grow with time offer a
number of advantages. For example, other factors held constant,
they can be used to apply thicker organic coatinys to the
5 metallic surface in a shorter period of time and in a one-step ~ ~
operation. Also, the coating thickness can be controlled by ~ -
varying the immersion time of the metallic surface in the
coating composition. In general, coatings which have improved
corrosion resistant properties and aesthetic appearance are
10 obtainable. These are but a few of the advantages which flow
. from the use of said compositions.
The aforementioned type of composition is capable
of forming organic coatings of excellent quality. However,
for ~ertain applications it is desired that the corrosion re~
,-.- ,
15 ~istant properties of the coating be better than those possessed ` ~ -
by the coating formed from said composition and/or that the
coating have a different appearance than that of the coating
formed from said composition. -
The present invention relates to improving the -
corrosion resistant properties of autodeposited coatings and
providing coatings which have a matted, dulled or lusterless
appearance rather than a glossy appearance. -~
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Reported Developments
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It is ];~own ~:o improve the corrosion resistant properties
of autodeposited coatings by contacting the wet or unfused coating
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with: (a) an aqueous solution of hexavalent chromium, the
source of which may be a water or acid soluble chromate or
dichromate or CrO3; ~b) an aqueous solution of hexavalent
chromium and formaldehyde-reduced forms of hexavalent chromium;
or an aqueous solution of phosphorous acid. For example, see
U.S. Patent Nos.: 3,585,084; 3,592,699; 3,647,567; and 3,795,546.
The use of an aqueous solution of hexavalent chromium
or of a hexavalent chromium/reduced chromium (formaldehyde-
reduced)solution to dull the glossy surface of an autodeposited
10 coating is disclosed also in U.S. Patent No. 3,795,546.
The present invention relates to a chromium-containing
aqueous solution which is particularly effective in improving
the corrosion resistant properties of an autodeposited coating
and dulling the glossy appearances thereof.
Summary of the Invention
In accordance with this invention a wet or uncured
autodeposited coating is contacted with an aqueous Cr solution
comprlsing~
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`~ (a) at least about 0.3 g/l of Cr+6 in the form of a
hexavalent chromium-containing compound;-and -
.
(b) a Cr+6/reduced Cr mix in an amount such that the ;
total Cr from the mix in the composition is at least
about 0.3 g/1, with about 30 to about 60 wt. % of the -
,
Cr in reduced form. ~
25 The use of the above aqueous Cr solution is effective in imparting ~ ;
a matted appearance to autodeposited coatings which otherwise would ~-
... .
have a glossy appearance and is effective also in improving the
corrosion resistant properties of the autodeposited coatings.
Detailed Description of the Invention
` 3G Coating compositions which are effective in forming
organic coatings which grow with time are known. Examples of
-; such coating compositions are described in U.S. Patent Nos.
~ .
1089155 D-11,759-C
3,585,084, 3,592,699, 3,709,743 and 3,776,848, in
British Patent No. 1,241,991, in Canadian Patent No.
1,032,678 and in Belgian Patent of Addition No. 811,841.
Speaking generally, the acidic aqueous coating
compositions of the aforementioned type function to
attack and dissolve from a metallic surface contacted
therewith metal ions in an amount sufficient to directly
or indirectly cause organic particles in the region of
the metallic surface to deposit thereon in a continuous
fashion, that is, in a manner such that there is a buildup
in the amount of organic material deposited on the surface
the longer the time the surface is in contact with the
composition. This deposition of the organic material on the
metallic surface is achieved through chemical action of
the coating composition on the metallic surface. The use
of electricity which is necessary for the operation of
some coating methods, such as the electrocoating method,
is not required.
¦~ It is believed that the present invention will
¦ 20 be used most widely in connection with coatings formed
1 from compositions that contain solid particles of resin -~
dispersed in the aqueous phase of the compos-ltion~. The
preferred composition has a pH of about 1.6 to about 4 and -
is prepared from water, a ferric-containing compound, most
preferably ferric fluoride, in an amount such that it
contains the equivalent of about 0.5 to about 3.5 g/l
of ferric iron, about 0.2 to about 5 g/l of HF, a pigment ~ -
such as carbon black, and about 50 to about
;~ _ 5 _
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108915S
lO0 ~/1 of resin particles which are all of substantially the
same size and substantially chemically homocJeneous, that is,
each particle is comprised of thq same monomeric constituents
present in substantially the same proportions or resin particles
which are prepared by copolymerizing the following monomer~:
1) ahout 25 to about 70, and preferably about
40 to about 65 wt. ~ of a conjugated diene
having, for example, 4 to about 9 carbon
atoms, such as butadiene or isoprene;
2) about 5 to about 70, and preferably about
30 to about 65 wt. % of CH2=CHR, where;n ~-~
R is an aryl or a cyano group, for example,
styrene or acrylonitrile;
3) about 1 to about 50, and preferably 'a~out 3
to about 15 ~It. ~ of a vinyl halide such as
vinyl chloride or vinylidene chloride; and
4) abou~ 0.5 to about 15, and preferably about
- ~
1 to about 4 wt. % of a monoethylenically
unsaturated ~onomer having a functional group
selected from the class consisting of amide
and carboxylic groups, such as acrylamide,
methacrylamide, octyl acid maleate and mono~
ethylenically unsaturated monocarboxylic and
dicarboxylic acids having about 3 to about
12 carbon atoms, and preferably about 3 to
a~out 5 carbon atoms, such as, for example:
acrylic acid; cinnamic acid; methacrylic acid;
- crotonic acid; itaconic acid; maleic acid;
and fumaric acid.
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101~155
The resin is used most conveniently in the form of a latex
which can be prepared a~cording to availabl~ techniques.
A particularly preferred latex contains particles
of resin prepared from the aforementioned monomers, which
particles are chemically and physically homogeneous. The
resin particles of the-preferred latex are prepared from sty-
rene, butadiene, vinylidene chloride and methacrylic acid.
In addition, the emulsifier content of the preferred latex ;
is about 1 to about 4% based on the resin solids and comprises ;
10 at least 90 wt. %, most preferably 100 wt. ~ of an anionic
emulsifier such as a sulfonate, for example, sodium dodecyl-
; benzene sulfonate, or ~ sulfosuccinate, for example, sodium `-
oleoyl isopropanolamide sulfosuccinate, or a mixture thereof. -~
Although the coating compGsition can be contacted
15 with the metallic surface in a variety of ways, it is believed ~`
that the most widely-used method-of contact will comprise im~
mersing the metallic surface in the coating composition at room
temperature. As mentioned above, the longer the metallic sur- ;~
; face is immersed in the coating composition, the greater the build~
20 up in coating thickness. It is believed that for most appli~
, ~ .
cations, desired coating thicknesses can be obtained by immers~
~-~ ing the metallic surface in the composition for a period of
time within the range of about 30 seconds to about 3 minutes. .
However, it should be understood that longer or shorter periods
25 of time can be used.
Agitating the composition aids in maintaining it
uniform. Also, agitation of the composition is effective in
improving the uniformity of the coatings formed.
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101~5~15S
Water rinsing the coated surface after it has been
withdrawn from the composition, and before significant drying
takes place is effective in removing therefrom residuals such
as acid and other ingredients of the bath that adhere to the
coated surface. If such residuals are allowed to remain on
the coated surface, they may change or adversely affect the
quality of the coating. For a specific application, a deter~
mination can be made as to whether the residuals cause adverse
effects which are not tolerable. If they do, they should be
removed, for example, by water rinsing with tap or deionized
water. If they do not, this step of removing them can be avoided.
Follo~ing any water rinse step that might be employed
or after the coated surface is withdrawn from the compositio~
it is subjected to the aqueous chromium composition of this ~ ~
15 invention (described in detail below) and then dried. Fusion ~ ~ -
of the resinous coating renders it continuous, thereby improv-
ing its resistance to corrosion and adherence to the underlying
metallic surface. ~`
The conditions under which the drying and/or fusion
operation is carried out depend somewhat upon the type of resin
employed. In general, heat will be required to fuse the resin.
The corrosion resistant properties of coatings fused at elevated
temperature have been observed to be better than coatings which
have been air dried. However, there are applications where
air dried coatings can be used satisfactorily. The fusion of
the coating should be carried out below temperatures which
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cause the resinous coating to degrade. Exemplary conditions
used in fusing coatings produced according to the present in-
vention are temperatures within the range of about 100 to
about 200C for periods of time within the range of about 10
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IolB5~l5s
to about 30 minutes, dependincJ on the mass of the coated part.
Baking the co~ting for a period of time until the metallic sur- :
face has reached the temperature of the heated environment has
been used effectively.
For applications in which it is desired that the coated
surface have particularly good corrosion resistant properties and
....... a matted appearance, as opposed to a glossy appearance, the co~ted
surface is subjected to the aqueous chromium composition of the
present invention prior to the curing or fusing of the coating. .
The Cr composi.tion of the present invention is prepared by mix-
ing in water a soluble hexa~iale.~lt chrolmium compound t~i.th he~-
avalent chromium/reduced chromium (Cr+6/reduced Cr) compounds. ~.
Examples of the soluble hexavalent chromium compound ;
that can be used are ammonium dichromate and alkali metal di-
.15 chromates, for example, lithium dichromate, sodium dichromate
and potasslum dichromate.
With respect to the Cr+6/reduced ~r constituent, it is
i . ~
obtained by reacting a hexavalent chromium compound or mixture ~::
of such.compounds with a material that is effective in reducing
.~ 20 some of the hexavalent chromium to a reduced form. Although var~
:: ious materials can be used to accomplish this, as is known in
: . the art, it is preferred to use formaldehyde as the reducing .
. agent as disclosed in U.S. Patent No. 3,063,~77. For example, an :
aqueous concentrate of the composition is prepared by treating an
: 25 aqueous solution of a hexavalent chromium compound, for example,
an aqueous solution of chromic acid, with formaldehyde to reduce
~; a portion of the hexavalent chromium.
For the purposes of this invention, about 30 t:o abc,ut 60
wt~ % o the hexavalen~ chromi.uln should be :reduced. Suitable amounts
of reducing ag~nt can ~e used to reduce the desired amount of
1(389~5S
hexavalent chromium. In preparing the aqueous concentra-te of -~ ,
Cr+6/reduced chromium, it may be found that the concentrate
tends to gel on standing. The tendency toward gel formation
may be substantially eliminated by the addition of small
amounts of phosphoric acid either before or after the con-
centrate is diluted. As little as about 0.05 ml of 75~ ~
H3PO4 solution (about 0.06 g of H3PO4) per gram of the total '''
chromium (both Cr+6 and reduced Cr) is usually sufficient to
prevent such gelling. Higher amounts of H3PO4 can be used. ,~
The aqueous chromium solution for treating the coated `~
surface should comprise at least about 0.3 g/1 of Cr+6 added '~
,as a soluble Cr+6 compound and a Cr+6~reduced Cr mixture in '`
an amount such that the total, chrom~Dm in the composition from
the mixture is at least abovt 0.3 g/l with about 30 to about
60 wt. ~ of the Cr in reduced form and preferably about 50 wt.
% of,the Cr in reduced form. Although the aforementioned in-
: .
gredients can be used in amounts up to their solubility limits, ',~ '
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satisfactory results can generally,be attained by using no ~ ~'
more than about 3 g/1 of Cr+6 in the form of the soluble Cr+6
compound and a Cr+6/reduced Cr mix in an amount such that the
~' total Cr concentration from the mix in the composition is no , ;~
more than about 3 g~1. ' "~'
l'he pH of the chromium solution is preferably within ,~ '',
the range of about 2.5 to about 6.
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108~1SS
The dilute aqueous Cr composition can be pre-
pared by diluting with an appropriate amount of water an
aqueous concentrate containing about 25 to about 400 g/l
of Cr~6, added as the soluble Cr+6 compound, and a Cr+6/
reduced Cr mix in an amount such that the total Cr concentra-
tion from the mix in the composition is about 25 to about
400 g/l with about 30 to about 60 wt. % of the Cr in re-
duced form. ~;
After treating the coated substrate with the
1~ Cr composition of the present invention, the coating of thesurface can be fused as de~cribed above or it may be rinsed
with water prior to the fusion of the coating.
Various types of metallic surfaces can be coated
and treated in accordance with the present invention. Ferrifer~
1~ ous surface such as, for example, hot rolled and cold rolled ;~
; steel, and other metallic surfaces, for example, aluminum and
zinc, can be used.
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~xamples below are illustrative of the practice of
20~the present invention.
The basic coating compositions used in all of the
examples were prepared by combining
Ingredients Amounts
latex containing about 54% solids 180 g . -
25 fer.~ic fluoride 3 g `~ `
hydro~luoric acid . . 2.3 g .-
black pigment dispersion ~ S g
- w~ter to make 1 liter.
~ ~ .
~10895155
.
` The resin of the latex used in the above composi-
tion compl-ised ~bou~ 62~ styrene, about 30~ bu~adiene, about
5~ vinylidcne chlo~ide and about 3~ methacrylic acid. A
. film formed from the resin is soluble in refluxing chloro-
benzene to the extent of about 13~. That the resin is cross-
linked is indicated by its insolubility in Soxhlet extraction
with chlorobenzene. The water soluble content of the latex
is about 2~ based on the weight of dried resin, with the
water soluble content comprising about lOg sodium phosphate,
about 13% sodium oleoyl isopropanolamide sulfosuccinate and
about 75% sodium dodecylbenzene sulfonate, the first mentioned
ingredient being a buferlng agent used ln preparing tne
latex, and the last ~ mentioned ingredients being emulsifierS.
The pH of the latex was about 7.8 and the surface tension
thereof about 45-50 dynes/cm. The average par,ticle size of
the resin was about 2,000 A.
The~ black pigment dispersion used in the above com-
position is an aqueous dispersion having a total solids conte
of about 36%. Carbon black comprises about 30~ of the dis-
persion. It has a pH of about 10-11.5 and a specific gravity
of about 1.17. The di$persion contains a nonionic dispersing
agent for the solids, and is sold under the trademark Aquablak
; 115.
Unless stated otherwise, the metallic surfaces coated ~ ~
25 in the examples below are unpolished cold rolled steel panels -
(Q-panels) 3" x 4". All metallic surfaces were cleaned with ` -~ -
a conventional alkali cleaner and rinsed with water prior to ` ~--
being coated.
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1~}89155
The salt spray test used in the examples was
ASTM D-117, with the coating being scribed. At the co~pletion
of the salt spray tests, the panels were rated on a numerical
scale ranging from 10 to 0, the number '!10" representing no
failure, that is, the appearance of the coated panel before ~-
and af.er the salt spray test was basically the same, and the
number "0" representing a coating failure of 1" or more from
the scribe.
Example No. 1
One liter of aqueous Cr solution was prepared by ; -~
combining 3.6 g of ammonium dichromate with a formaldehyde~
reduced aqueous CrO3 solution in an amount such that the total -`~
Cr concentration from this solution was about 1.5 g, one-half
o whicn was in ~e~uced form, and about 0.3 g Gf ~3PO~
A steel panel was immersed in the above described
coating composition fox 2 minutes. The coated panel was then
partially dried in air for 1 minute, immersed in the afore-
i~ mentioned aqueous Cr solution for 30 seconds and thereafter
y~
baked for 10 minutes at 170C. The baked coating had a
matted appearance. After 168 hours of salt spray testing,
the rating of the coating was 9 and after 336 hours and 480 -
hours of testing the rating was 7. -~ `~
Example Nos. 2 to 5
The aqueous chromium solutions set forth in Table 1
:.
~ 25 below were prepared. ~ ~
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~O~91~S
TABLE 1
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Formaldehyde-Reduced Cr Solution
Ex. g/l of Cr+6 from
No. Cr+6 g/lreduced Cr, g/l (NH4)2 Cr27
2 0.13 0.13 0.28
3 0.26 0.26 0.56
4 0.51 0.51 1.12
0.77 0.77 1.67
The compositions of the above examples contained also 0.125,
0.25, 0.5 and 0.75 g/l respectively of 75% H3PO4. Steel head-
light mounting rings were immersed in the above described coat- ~;~
ing compos;tion for 75 seconds. The coated rings were then
partially dried in air for 1 minute, immersed in water for 30
seconds, immersed in the aqueous chromium solutions of Example ~ ~
.....
Nos. 2 to 5 above and then baked for 10 minutes at 220F and
for 10 minutes at 315F. The baked coatings had a matted ~ -~
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appearance. The coated parts were then subjected to salt
spray testing for 168 hours. The salt spray ratings are set ` ; ~
forth below. ~ ;
Cr Solution Rating
Ex. No. 2 5 (coating blistered)
~ 3 7
7 ~ ~
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A mounting ring treated in the same way, but without the Cr
. ~ . ~ . .
treatment, had a glossy coating and a rating of 0.
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1089:~55
Example No. 6
An aqueous chromium concentrate containing 150 g/l
of Na2O7.2H2O and a mixture of Cr+6/formaldehyde-reduced Cr in
an amount such that the total Cr concentration from the mix-
ture in said concentrate was S0 g/l with about 50 wt. % of theCr in reduced form, and 25 g/l of 75 wt. % H3PO4 was prepared.
The concentrate was diluted with water to provide a Cr treat-
ment solution comprising about 3 wt. % of the concentrate.
A hot rolled steel car frame was immersed in the
coating composition described above for 90 seconds. The coated
frame was dried partially in air for 60 seconds, immersed in -~
water for 30 seconds, immersed in the above described Cr solu-
tion for 30 seconds and then baked for 15 minutes at 315F.
The black coating on the frame had a matted appearance and the ;
salt spray rating after 504 hours of testing was 9. A frame
ubiected to the same process steps, except for the Cr treat-
ment step, has thereon a glossy coating and a ra~ting of 0 after
504 hours of salt spray testing.
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