Process for depositing latex films on metal surfaces

Process for depositing latexes having a net electrical charge on metal surfaces without the application of electrical current, preferably by the generation of the metal cation at the surface of the metal in a bath containing hydrogen peroxide and an organic acid which forms negatively-charged or neutral complexes with that particular metal ion.

FIELD OF THE INVENTION 
This invention relates to an improved process for chemiphoretic coating of 
metals from aqueous media. 
BACKGROUND OF THE INVENTION 
In the past, various processes have been devised for coating metal surfaces 
with latex by inserting the metal part to be coated into an aqueous bath 
containing a fine dispersion of latex particles. By using the metal part 
as an electrode and by causing an electrical current to flow through the 
bath, the charged latex particles will be attracted to and will adhere to 
the surface of the metal part. Electrical processes of this type, known as 
electrophoretic processes, are relatively complex and costly. 
Similar processes for depositing coating on metals from aqueous media in 
the absence of applied electrical currents are known as chemiphoretic 
processes. 
One chemiphoretic process which has been used to form latex coatings 
employs an aqueous bath containing both hydrofluoric acid and hydrogen 
peroxide (See German Offenlegungsschrift No. 22 11 490 dated Sept. 21, 
1972 and German Offenlegungsschrift No. 24 09 876 dated Sept. 26, 1974). 
In this process, an electrical current is not required in order to form a 
latex film; however hydrofluoric acid is a dangerous chemical and 
excessive metal is dissolved during the coating process. Furthermore, a 
relatively long time, on the order of 2 minutes, is required to form a 
1-mil film; and the build-up of salts in the bath due to the highly 
corrosive hydrofluoric acid rapidly reduces its effectiveness. 
A similar chemiphoretic process is described in U.S. Pat. No. 3,585,084 and 
in Prieve et al, Ind. Eng. Chem. Prod. Res. Dev., 17, Number 1, 1978, 
Pages 32-36. 
Another chemiphoretic process for coating metal articles with latex is 
described in German Offenlegungsschrift No. 26 23 895 dated Dec. 9, 1976. 
In this process, a metal object, preferably a steel object, is brought 
into contact with an aqueous cationic dispersion of latex particles, the 
aqueous dispersion containing a carboxylic acid radical such as the 
carboxylic radical of mandelic acid or citric acid. The application is 
primarily concerned with the production of cationic latexes by emulsion 
polymerization of monomers such as vinyl esters of fatty acids with 1 to 
18 carbon atoms, unsaturated acids such as acrylic acid, esters of 
unsaturated acids with alcohol, glycols or epoxides with 1 to 18 carbon 
atoms, acrylonitrile or styrene, 1,3-butadiene. The German application 
implies that special cationically stabilized latexes are required in order 
to carry out the coating process. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a new and improved autodeposition 
process is provided for applying latex coatings on metallic surfaces to 
achieve a satisfactory coating. The latex particles, in aqueous solution, 
must carry a net positive electrical charge, or must be stabilized with a 
positively charged surfactant. 
In carrying out the process, a metallic surface is dipped into an aqueous 
solution containing 2 to 30% by weight of latex particles which have a net 
positive electrical charge, this aqueous solution additionally containing 
about 10.times.10.sup.-3 to 300.times.10.sup.-3 mol per liter of hydrogen 
peroxide and about 10.times.10.sup.-3 to 300.times.10.sup.-3 mol per liter 
of an organic acid. The metallic article to be coated is maintained within 
the bath until a latex coating of a predetermined thickness has formed 
thereon. Acids which have been used successfully in the coating process of 
the invention are oxalic, malonic, malic, mandelic, citric and tartaric 
acids. However, any other acid capable of forming stable charged metal 
complexes with a particular metal ion which has been released from the 
surface of the substrate by the acid can be utilized. 
In the preferred embodiment of the invention, the peroxide concentration 
should not exceed about 250.times.10.sup.-3 mol per liter; and the ratio 
of the acid to the peroxide should be greater than 0.5 and less than 10. 
Metals on which coatings have been formed, depending upon the acid 
employed, include aluminum, zinc and iron or plain carbon steel. The 
application employs hydrogen peroxide to generate the latex which is used 
in the process but does not employ hydrogen peroxide in any coating 
process. 
The aqueous solution should be substantially free of inorganic 
electrolytes. The presence of an inorganic electrolyte, such as fluoride 
ion, tends to interfere with the coating process. In appreciable 
quantities, the electrolytes will terminate the coating process.

In carrying out the invention, an aqueous solution is prepared containing 2 
to 30% by weight of cationically stabilized latex particles, with an 
excess of the cationic surfactant present in the solution of the bath 
(0.1-3%). The latex particles preferably have an average diameter in the 
range of about 0.15 to 2 micron. In the case of a non-ionic stabilized 
latex, the addition of a cation surfactant is necessary in the amount of 
about 0.2 to 3.5% by weight of the solution. Added to the aqueous solution 
is 10 to 25% of a water solution of an acid selected from the group 
hereinafter given as well as a 30% solution of hydrogen peroxide. The 
aqueous bath solution should contain about 10.times.10.sup.-3 to 
300.times.10.sup.-3 mol per liter of hydrogen peroxide and about 
10.times.10.sup.-3 to 300.times.10.sup.-3 mol per liter of an organic 
acid, the ratio of the acid to hydrogen peroxide being in the range of 
about 0.25 to 10. The bath is maintained substantially free of dissolved 
inorganic electrolyte. A metal substrate selected from the group 
hereinafter given is then dipped into the aforesaid aqueous solution for a 
period of about 1 minute and less than 1.5 minutes, depending upon the 
thickness of the film desired. 
The following is an example of a process according to the invention: A 
concentrated solution of cationically stabilized latexes was weighed in a 
plastic container such that the total solid content of the resulting 
aqueous solution was 4.2%.+-.0.1% by weight. 118.37.times.10.sup.-3 mol 
per liter of oxalic acid was then added to the solution and the solution 
stirred for 3 minutes at a pH of 1-3. Thereafter, 118.37.times.10.sup.-3 
mol per liter of hydrogen peroxide was added to the solution and stirred 
for 5 minutes, the addition of the acid and the peroxide being in a water 
solution of 10 to 30%. The solution was then ready for a coating 
operation. 
In the example given, a 4".times.6" carbon steel panel was dipped into the 
aqueous solution for 60 seconds and then removed. The result was the 
deposition of a white coating film of latex with very good adhesion. 93% 
of the weight of this film was water. Thereafter, the panel was washed 
with water for a few seconds and then dried in an oven at 60.degree. to 
80.degree. C. for 30 minutes. The resulting coating was green colored with 
a uniform thickness of 1 mil and exhibited very good adhesion. This 
thickness, however, can be increased by maintaining the panel in the bath 
for a longer period of time, up to 90 seconds, by increasing the total 
solid content of the latex in the bath from 4.2% to 10%, for example, or 
by adjusting the molar ratio of the acid to the peroxide in the bath. 
The expected reactions at the surface of the metal plate can be understood 
from the equations given below as well as by reference to FIG. 1 wherein 
the numeral 10 designates the steel plate being coated and the numeral 12 
designates the latex coating: 
##EQU1## 
As can be seen from the foregoing equations, metal is dissolved by the acid 
at the surface, producing Fe.sup.+2 at a low pH of about 1 to 3. Gaseous 
hydrogen is formed in the reaction which bubbles to the surface; while 
Fe.sup.+2 is converted to Fe.sup.+3 ions and water in a free radical 
reaction due to the presence of H.sub.2 O.sub.2. As a side reaction, the 
presence of Fe.sup.+2 and Fe.sup.+3 ions at the surface of the metal is 
believed to decarboxylate the organic acid to give CO, CO.sub.2, H.sub.2 
and CH.sub.4, for example. Fe.sup.+3 with the negative ligand of the acid 
forms negative complexes which are absorbed on the surface of the cationic 
latex, causing the latex particles to fluctuate at the surface of the 
steel panel. Possibly, some of the acid ligand adsorbs on the surface of 
the latex particles, neutralizing some of its charge. 
As shown by the following Tables I and II, not all organic acids can be 
used in carrying out the invention; and not all metals can be coated with 
this particular embodiment of the invention. 
TABLE I 
__________________________________________________________________________ 
FORMATION 
OF NEGATIVE 
COMPLEXES 
K.sub.a1 
K.sub.a2 
K.sub.a3 
WITH 
NAME OF ACID CHEMICAL STRUCTURE OF THE ACID 
@ 25.degree. C. 
@ 25.degree. C. 
@ 25.degree. C. 
Fe.sup.+3 
__________________________________________________________________________ 
FORMIC 
##STR1## 1.74.times.10.sup.-4 
-- -- Not Reported 
ACETIC 
##STR2## 1.75.times.10.sup.-5 
-- -- Not Reported 
MONOCHLORACETIC 
##STR3## 1.51.times.10.sup.-3 
-- -- Not Reported 
LACTIC 
##STR4## 1.38.times.10.sup.-4 
-- -- Not Reported 
OXALIC 
##STR5## 6.2.times.10.sup.-2 
6.1.times.10.sup.-5 
-- Yes 
MALONIC 
##STR6## 1.58.times.10.sup.-3 
8.0.times.10.sup.-7 
-- Yes 
MALIC 
##STR7## 4.0.times.10.sup.-4 
8.9.times.10.sup.-6 
-- Yes 
TARTARIC 
##STR8## 9.4.times.10.sup.-4 
2.9.times.10.sup.-5 
-- Yes 
CITRIC 
##STR9## 7.4.times.10.sup.-4 
1.74.times.10.sup.-5 
4.0.times.10.sup.-7 
Yes 
MANDELIC 
##STR10## 4.3.times.10.sup.-4 
-- -- Yes 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
EFFECT ON 
EFFECT EFFECT 
GAL- EFFECT ON pH 
ON EFFECT ON 
ON VANIZED STAINLESS 
EFFECT 
CHANGE 
COPPER 
ALUMINUM 
ZINC STEEL STEEL STEEL DURING 
NAME OF ACID PANELS 
PANELS PANELS 
PANELS PANELS PANELS PROCESS 
__________________________________________________________________________ 
FORMIC (-) (-) (-) (-) (-) (-) 0.00 
(0) (0) (0) 
ACETIC (-) (-) (-) (-) (-) (-) +.03 
(0) (0) 
Partially 
MONOCHLORACETIC 
(-) (-) (-) (-) (-) (-) +.03 
(0) (0) 
LACTIC (-) (-) (-) (-) (-) (-) 0.00 
(0) (0) (0) 
OXALIC (-) (+) ** (+) (+) (-) (+) 0.00 
(0) Longer than Thin layer 
5 min. 
MALONIC (-) (-) (+) (-) (-) (+) 0.00 
(0) Thin layer 
MALIC (-) (-) (+) (-) (-) (+) 0.00 
(0) Thin layer 
TARTARIC (-) (-) (+) (-) (-) (+) 0.00 
(0) Thin layer 
CITRIC (-) (-) (+) (-) (-) (+) 0.00 
(0) Thin layer 
MANDELIC (-) (-) (+) (-) (-) (+) 0.00 
(0) Thin layer 
__________________________________________________________________________ 
(+) Coating of the panels has taken place 
(-) No coating has taken place 
(0) Metal has been dissolved but no coating has taken place 
** When the aluminum panel remains in contact with a copper panel for a 
period longer than 5 min., the coating takes place 
Table II showns that the organic acids which can be used include oxalic 
acid, malonic acid, malic acid, tartaric acid, citric acid and mandelic 
acid. All of these acids will produce films on plain carbon steels but 
will not produce films on stainless steels. Likewise, they will produce 
latex films on zinc. Only oxalic acid can be utilized to produce films on 
aluminum and zinc. None of the acids are effective in producing coatings 
on copper at these conditions. Formic acetic, monochloracetic and lactic 
acids are all ineffective in producing the coatings on any metals tested. 
FIG. 2 illustrates the effect of hydrogen peroxide additions. When no 
hydrogen peroxide is added (curve E), no coating occurs. At a constant 
acid concentration of about 79.times.10.sup.-3 mol per liter, the greatest 
amount or thickness of coating is achieved for a given time when the 
peroxide concentration is about 238.times.10.sup.-3 mol per liter. Above 
this level, and at about 317.times.10.sup.-3 mol per liter, for example, 
the mass of dry coating deposited drops drastically as illustrated by 
curve A in FIG 2. 
FIG. 3 shows the effect of peroxide addition on the mass of metal dissolved 
at the surface. As would be expected, increasing amounts of peroxide cause 
increasing amounts of metal to be dissolved at the surface. 
FIGS. 4 and 5 illustrate the effect of acid concentration on the mass of 
dry coating achieved for a given time and the mass of metal dissolved at 
the surface during the same time. As can be seen, increasing the acid 
concentration correspondingly increases the mass of dry coating as well as 
the mass of metal dissolved. 
FIG. 6 is a plot of mass of dry coating deposited versus the mass of metal 
dissolved for various types of acids. It will be noted that citric acid 
dissolves a much greater amount of metal for a given mass of coating 
(curve 1) than malic acid, for example (curve 8). On the other hand, and 
as shown in FIG. 7, less dipping time for a given mass of coating 
deposited is required for citric acid than for malonic acid. FIG. 8 shows 
that the water content in the wet coating film produced in accordance with 
the process of the invention decreases as the water percent in the bath 
correspondingly increases; while FIG. 9 shows that, as expected, the dry 
film thickness increases as a function of dipping time. 
FIG. 10 is a plot of time of dipping versus a mass of dry coating deposited 
for various latex concentrations in the bath. Again, as expected, as the 
latex concentration increases so also does the mass of dry coating 
deposited for a given dipping time. Finally, FIG. 11 shows the effect of 
temperature on the process. As temperature increases, so does the coating 
mass for a given dipping time; although it is not a particularly critical 
factor. 
The color of the resulting dry film after drying as described above, is 
dependent upon the kind of acid and the metal substrate used as shown in 
the following Table III: 
TABLE III 
______________________________________ 
Acid Substrate Film Color 
______________________________________ 
Oxalic Steel Green 
Oxalic Aluminum Colorless 
Oxalic Zinc White 
Mandelic Steel Black 
Citric Steel Gold 
Tartaric Steel Colorless 
Malic Steel Gold 
______________________________________ 
The properties and color of the foregoing films can be changed, however, by 
simply adding mixtures of two or more of the described acids to the bath, 
rather than one. It is believed that the resultant variations in color are 
due to different complexes present in the dry coating film due to the 
different acids. 
Atomic adsorption determination of iron dissolved in carrying out the 
invention shows that 87% of the iron is in droplets which drop from the 
panel or other article when it is removed from the bath for drying; 9.5% 
of the iron is in the dry coating film itself; and only 3.5% of the iron 
remains in the coating solution. 
The molar ratio of the acid to the peroxide is very important in that it 
affects the adhesion of the wet coating film on the metal surface and the 
development of undesirable small gas bubbles at the surface of the metal 
during the coating process. Additionally, the ratio affects the thickness 
of the coating film. A dipping time longer than 15 minutes will cause a 
heavy coating which, in turn, reduces the adhesion of the coating to the 
surface. Proper operating conditions necessary to achieve a very well 
adhered film, with almost no bubbles in it, involve a dipping time of less 
than 90 seconds and an acid-peroxide ratio in the range of 0.25 to 10. The 
criticality of the ratio is illustrated in FIG. 2 where the coating 
deposited decreases (curve A) at an acid-peroxide ratio of about 0.25 but 
produces optimum results (curve B) at a ratio of about 0.33. Below a ratio 
of about 0.25, too much metal is dissolved and above a ratio of 10, little 
latex is deposited. 
Increasing the total mol acid and peroxide added to the whole bath will 
affect the appearance of the dry film by darkening the color and will 
increase the metal dissolved which plays a large role in shortening the 
lifetime of the bath solution. Additionally, increasing total mol acid and 
peroxide added will increase the thickness of the dry coating. For best 
results, the peroxide added should not exceed about 300.times.10.sup.-3 
mol per liter using the aforesaid acid-peroxide ratio of 0.25 to 10. By 
adding about 140.times.10.sup.-3 mol per liter of potassium dichromate in 
solution form (i.e., 0.02%), the adhesion of the film is improved. 
As was mentioned above, any latex which carries a net positive electrical 
charge can be utilized in the process of the invention. For example, a 
non-ionic latex comprising 60% butylacrylate and 40% styrene can be 
utilized with the addition of about 1 to 3% of a cationic surfactant such 
as arquard (16) which has the following formula: 
EQU (CH.sub.3).sub.3 N.sup..sym. --CH.sub.2 --(CH.sub.2).sub.14 --CH.sub.3 
Alternatively, a non-ionic latex comprising 40% butylacrylate and 60% 
styrene can be employed with the addition of about 1 to 3% of a cationic 
surfactant such as arquard (18) which has the following formula: 
EQU (CH.sub.3).sub.3 N.sup..sym. --CH.sub.2 --(CH.sub.2).sub.16 --CH.sub.3 
As another example, a latex which can be used comprises 80% ethylacrylate 
and 20% methylmethacrylate. Commercially available latexes such as E-1037 
Rohm and Haas cationically stabilized lates, W. R. Grace 3462-D latex and 
E-1050 Rohm and Haas cationically stabliized latex have been used 
successfully in practicing the invention. 
A red pigment, for example, can be added to the coating by grinding 
together red iron oxide and talc and mixing these materials with a 
cationic surfactant which is added to the aqueous coating bath. 
Pigmentation is possible with any pigment which is stable at a pH of 1 to 
3. Coating can be achieved by dipping, spraying or by brush. 
In a specific example of pigmented coatings, 50 grams of Fe.sub.2 O.sub.3 
were mixed with 50 grams of talc and 100 grams of water and ground for 16 
hours to produce a red paste. Three grams of this red paste were mixed 
with 3 grams of a cationic surfactant (arquard) and stirred for 5 minutes. 
This was added to an aqueous solution containing 5% by weight of latex 
particles, 110.times.10.sup.-3 mol per liter of H.sub.2 O.sub.2 and 
118.times.10.sup.-3 mol per liter of citric acid. Coating was carried out 
as above to give a good red latex film. White films can be produced 
utilizing TiO.sub.2 rather than Fe.sub.2 O.sub.3. 
Although the invention has been shown in connection with certain specific 
embodiments, it will be readily apparent to those skilled in the art that 
various changes in form and arrangement of parts may be made to suit 
requirements without departing from the spirit and scope of the invention.