US3798143A - Electrophoretic deposition of acrylic copolymers - Google Patents

Abstract

ELECTROPHORETIC DEPOSITION OF AN ACRYLIC INTERPOLYMER COMPRISING METHYL METHACRYLATE AND AN ACRYLIC ACID OR RELATED UNSATURATED SHORT-CHAIN CARBOXYLIC ACID ON AN ALUMINUM OR ANODIZED ALUMINUM SUBSTRATE FROM AN AQUEOUS COLLOIDAL DISPERSION.

Description

United States Patent ABSTRACT OF THE DISCLOSURE Electrophoretic deposition of an acrylic interpolymer comprising methyl methacrylate and an acrylic acid or related unsaturated short-chain carboxylic acid on an aluminum or anodized aluminumsubstrate from an aqueous colloidal dispersion.

BACKGROUND OF THE' INVENTION Field of the invention This invention relates to electrophoretic deposition of acrylic copolymers or interpolymers from acrylic dispersions. More particularly, it relates to electrophoretic deposition of acrylic copolymers or interpolymers from aqueous colloidal dispersions on aluminum or anodized aluminum.

DESCRIPTION OF THE PRIOR ART Electrocoating of aluminum and anodized aluminum thus far has been limited mostly to solution polymers dissolved in water. The term aluminum as used herein encompasses both aluminum and aluminum base alloys, especially those containing at least 50% by Weight aluminum. Low molecular weight acrylic, alkyd, or epoxy ester resins solubilized with amines have been successfully electrodeposited on anodized aluminum. Such resins thus electrophoretically deposited generally must be heated to 250 to 450 to thermopolymerize them to a substantially higher molecular weight in order to obtain the hardness and durability required for protective coatings on aluminum and anodized aluminum exterior building components. These high temperatures crack or craze anodic oxide coatings on aluminum because the thermal expansion of these oxide coatings is significantly lower than that of the underlying aluminum base metal. Craze lines extending through the anodic oxide film substantially detract from the protective and esthetic value of the anodic coating. Commercial anodic coatings such as those formed in sulfuric acid or mixed organic-inorganic acid electrolytes are porous and, while substantially hard, do not always provide optimum protection of the base metal against chemical or corrosive components of exterior environments. Such porous anodic coatings are usually sealed in boiling water or aqueous solutions or in steam.

Organic overcoatings can also be used to seal the pores of the anodic coatings and can provide additional protection sometimes superior to aqueous-sealed coatings in resistance to construction damage and in extending the total life of the coating system. Thus, a high quality organic coating that can be applied to anodized aluminum without the crazing associated with thermopolymerization required with solution polymers is of significant commercial value. The elimination of the thermopolymerization step also is a significant economic and technical advantage for organic coatings applied to non-anodized aluminum, especially when combined with the advantages of an electrodeposition method of applying such coatings.

p ice SUMMARY OF THE INVENTION After extended investigation we have found that these advantages may be obtained by electrophoretically coating either aluminum or anodized aluminum with an aqueous colloidal dispersion comprising particles. of an interpolymer of methyl methacrylate and an acid selected from the group consisting of 18% by weight acrylic acid 18% ,by weight methacrylic acid, 4-8% by weight maleic acid and 4-8% by weight itaconic acid dispersed in water, at least about 3 mol percent of said acid being in the form of an acid salt selected from the group consisting of ammonium salts and amine salts, and thereafter coalescing the resulting coating on said aluminum or anodized aluminum. The coalescing may be accomplished according to our invention either by heating the coating at relatively low temperatures not exceeding about 200 F. after application to an aluminum or anodized aluminum substrate, or by treating the electrodeposited coating with a coalescing agent. If a coalescing agent is employed according to the invention, coalescing may be accomplished under atmos pheric conditions, for example, at room temperature, or about 65-90 F. The coalescing agent, when employed, may be applied after application of the dispersion to the aluminum or anodized aluminum substrate by spraying, dipping, or the like.

For optimum properties it is desirable to use an interpolymer having an average molecular weight of from 10,000 to 150,000 and preferably from 50,000 to 150,000. The particles in the aqueous dispersion should range in size from about 0.01 to 0.1 micron in diameter, and at least about by number, of the particles should be within this size range. A more detailed description of a copolymer or interpolymer such as that useful according to the invention is to be found in Canadian Pat. 815,089 or the equivalent British Pat. 1,114,133.

The resin particles in the aqueous colloidal dispersion are negatively charged. Hence, under the influence of an applied (D.C.) potential, they migrate to and are deposited on the aluminum or anodized aluminum anode. Stainless steel is a suitable cathode material for use in this electrodeposition process. Because of the colloidal size of the dispersed resin particles, little or no agitation is required to keep them in suspension. The temperature of the aqueous colloidal dispersion is held in the range from about 90 to about F. during electrodeposition. Below about 90 F. little deposition takes place and above about 110 F. excessive amounts of gas are generated at the anode, thus disrupting the deposited film.

The quantity, and hence thickness, of resin film deposited from the colloidal suspension is a function of the quantity (coulombs) of electricity employed during deposition. At a constant voltage, the current flowing during deposition is relatively high but decreases with time because of the increasing film resistance as the film is deposited and grows. To avoid initial surges of large amounts of current, it is convenient to use a rectifier that can be programmed for a limited current and voltage. Certain minimum voltages are required to eifect deposition, and above certain maximum voltages excessive gas generated at the anode disrupts the deposited film. Suitable ranges for current density and voltage are:

The electrical conditions required for electrocoating anodized aluminum with the dispersion according to the invention vary with the type, thickness and condition sults are obtained if the surfaces are wet when introduced into the electrocoating bath. Electrodeposition of the resin coatings according to the invention may be facilitated by first rinsing the anodized aluminum in a 1:1 (by volume) mixture of concentrated nitric acid and water for 30-60 seconds and then rinsing in deionized or distilled water and placing in the aqueous dispersion electrocoating bath while still wet. An induction period may be employed when forming anodic coatings on aluminum that are'to -'-"be subsequently electrocoated from a resin dispersion according to this invention. The induction period may be for several minutes, preferably at the beginning of the anodizing process. During this period an initial very low current density is'applied to the aluminum article being anodized before applying the generally higher current density employed in the anodizing process.

When the aluminum or anodized aluminum treated according to the previously described electrodeposition process is removed from the electrodeposition bath, it is preferably rinsed in deionized or distilled water and dried under ambient conditions or in flowing clean dry air. The

- deposited coating now consists of a film of colloidal resin particles which adhere to the aluminum or anodized aluminum surface. The film appears opaque and white. The particles may then be coalesced to form a continuous adherent film by gentle heating in air for several minutes at temperatures up to 200 F. (150 to 180 F. preferred). This may be accomplished over a low flame, in an electric oven, or with infrared radiation. Immersion in boiling water may also be used to coalesce the particles, but this sometimes results in a less attractive surface. After coalescing, the adhering resin film is transparent and has a satin-gloss texture very pleasing to the eye and touch.

Instead of being heated, the uncoalesced resin film may be treated with a coalescing agent, such as xylol or butyl Cellosolve, for example, by spraying, dipping, or the like to coalesce it at ambient temperatures of about 6590 The temperatures required to coalesce the resin films electrodeposited according to this invention are below those at which extensive crazing of anodic coatings occurs. Accelerated tests have shown that the electrodeposited and coalesced resin films produced according to the invention have excellent adhesion, protection and durability characteristics and thus enhance the usefulness of both anodized and non-anodized aluminum, particularly in aggressive outdoor environments.

The acrylic dispersions used according to the invention a may be pigmented with conventional, substantially waterinsoluble color pigments by dispersing said pigments in water using a suitable wetting agent, and blending said mixture with the resulting arcylic colloidal resinuous dispersion. Suitable, but not required, pigmentation may be obtained by use of titanium dioxide, phthalocyanine blue, green iron oxides or like pigments and dyes.

DESCRIPTION on THE PREFERRED EMBODIMENTS ,For a better understanding of our invention, reference will now be made tocertain examples which areillustrative of the practice thereof.

Example 1 0.0681 part of sodium lauryl .made up of about 0.519 part sodiumlauryl sulfate and sulfate surfactant solution 16.1 parts of water. Agitation is begun in a vessel heated at 84 C., after which a previously prepared solution of 0.205 part ammonium persulfate in 5 parts of water is added. Continuous additionv of the following monomer and surfactant solutions is then begun at rates such that complete addition of both solutions is achieved in about 50 minutes, during which time the reaction temperature remains at approximately C. The following monomer solution is used in preparing the aforementioned interpolymer of this example.

Parts Methyl methacrylate 65.1 Ethyl acrylate 23.6 Methacrylic acid 4.93 n-Butyl mercaptan 0.365

A solution of 0.0968 part of ammonium persulfate in 1.42 parts water is then added and the temperaturemaintained at 8084 C. for 30 minutes. Upon cooling, the product is a white dispersion containing only a trace of coagulated polymer. The dispersed polymer particles have diameters in the range of 0.09'0.22 micron. The dispersion contains 42.7% polymer solids in which the 'copolymerized methacrylic acid content is 5.02%.

One hundred parts of the dispersion product is charged to a vessel containing an efficient agitator, and heated to 70 C. with mild agitation. A solution containing 4.30 parts diethylene glycol monoethyl ether, 7.95 parts isopropyl alcohol and 7.10 parts water is added and agitation continued for 10 minutes. A solution of 1.08 parts of concentrated (28%) aqueous ammonium hydroxide in 2.73 parts of water is then added over a 5-minute period. Agitator speed is increased and vigorous agitation continued for an hour. The cooled product is a' semi-transparent dispersion with a faint yellow color when viewed with transmitted light. It contains 34.2% solids and has a viscosity of 370 cps. (25 C.). The dispersed particles have diameters of 0.03-0.06 micron. Portions of this colloidal dispersion are applied electrophoretically to a sample of unanodized aluminum and to a separate sample of anodized aluminum. A substantially dry adherent film is formed upon leaving the samples at 75 F. for about 15 minutes. The coating electrophoretically deposited on the aluminum and anodized aluminum samples is obtained by employing a maximum current density of about'4 a.s.f. and a voltage of 25 'volts for 45 seconds followed by 'SO volts for 45 seconds before Withdrawing the samples from the colloidal aqueous dispersion.

Example 2 A 10% solids electrodeposition bath of an acrylic interpolymer such as that of Example 1 was prepared by dilution with deionized water. Pieces of aluminum alloy '1 (Aluminum Association designation) sheet were prepared by etching in caustic and removing the etching smut in ntiric acid. The electrodeposition bath was maintained in the'95 to 100 F. temperature range, and-an aluminum piece was connected as the anode. Stainless steel was used as a cathode. Anode and cathode were connected to 'a programmable rectifier which was programmed to limit the direct current to a maximum of 4 amperes per square foot (a.s.f.) of aluminumanode and the voltage to a maximum of 50 volts. Current was then applied for 60 seconds. The aluminum sample was then disconnected, withdrawn from the bath, rinsed with deionized water, and dried with a stream of clean dry air.

A white opaque film of colloidal acrylic interpolym'er particles adhered to the aluminum. This film of colloidal particles was coalesced into a continuous film by heating in an electric oven at F. for 15 minutes. The coalesced film had a satin-gloss texture, was approximately 0.0001-inch thick and was very hard but not brittle. Its hardness was found to be superior to resin coatings electrodeposited from solution polymer baths and subsequently heated to 350 F. or above to fully thermopolymerize them. No colloidal acrylic particles were deposited when the aluminum was made cathode or when no current was applied. When the aluminum was made anode no deposition occurred when the bath temperature was lower than 90 F. or when the voltage was lower than 25 volts (95100 F. bath temperature). When the voltage was higher than 50 volts, gas generated at the aluminum anode surface hindered electrodeposition. A duplicate resin-coated sample was dipped in xylol'and drained after electrodeposition, rinsing in water and drying. This application of xylol resulted in coalescence of the film at ambient temperature conditions (approximately 75 F.) into a hard film that was somewhat more glossy than the first-described sample.

Example 3 A 10% solids dispersion was prepared as in Example 2, heated and maintained in the 92-95" F. temperature range. The anode to be electrocoated was a piece of aluminum alloy 1100 sheet previously anodized in a conventional sulfuric acid electrolyte to provide on its surface an anodic oxide coating of about 0.0003 inch in thickness. Using a stainless steel cathode and a limiting maximum' current density of 4 a.s.f., a voltage of 25 volts was impressed on the electrolytic cell for 45 seconds, and this was immediately followed by a 45-second period at 50 volts. The electrodeposited coating was then rinsed and dried as in Example 2 and coalesced by subjecting it to the heat from an infra-red lamp for 20 minutes. During the coalescing period the surface of the sample reached approximately 150 F. The resulting resin coating had a satin-gloss texture and was very resistant to scratching and wetting by water. Its electrical impedance was more than five times higher than a conventional aqueous-sealed anodic coating of equal thickness, indicating the high effectiveness of the pore-sealing treatment.

Example 4 A group of anodized aluminum samples were prepared using a conventional sulfuric acid electrolyte to provide a nominal anodic coating thickness of 0.001 inch. An induction period of 3 minutes at 4 a.s.f. was employed before anodizing in the sulfuric acid at 24 a.s.f. Separate samples dry, rinsed with water and wet, and rinsed with nitric acid (1:1 by volume, concentrated nitric acid: water) followed by rinsing with water were introduced into a 10% solids acrylic interpolymer dispersion electrodeposition bath prepared as in Example 2. The bath was at 95-l00 F. Limiting maximum values for electrodeposition current density and voltages were 1.5 a.s.f. and 100 volts, respectively. Electrodeposition time was 90 seconds. After electrodeposition, the samples were- Sample entry Relative electrical condition: impedance, kiloholms Dry 830 Wet 1280 Nitric acid rinsed and wet 1800 Blank-conventionally sealed in boiling water 410 These impedance values indicate the superior poresealing of the electrodeposited acrylic colloid over conventional boiling water sealing of anodic coatings.

Example 5 Anodized aluminum samples were prepared as in Example 4 and electrophoretically coatedusing the acrylic interpolymer dispersion electrodeposition bath and electrodeposition and coalescing conditions of Example 4. The resulting resin coating thickness was 0.0005 inch. These samples and samples anodized in the same way but sealed in boiling water instead of being electrophoretically coated were tested in a controlled environment test chamber. In this chamber, the samples were subjected to alternate condensations of the test vapor on their surfaces and drying. Condensation and drying periods were approximately /2 hour each, and the vapor contained 3% hydrochloric acid and 3.5% nitric acid. The pH of the condensate on the sample surfaces was 3.6. After 70 test cycles, the electrophoretically treated samples were substantially better looking than the boiling water sealed samples and showed much less surface whitening and staining in this exaggeratedly severe test environment. After the test, Scotch tape was applied to the resin coating and was pulled off in accordance with a commonly used test for adhesion of organic coatings. No resin coating lifted from the surfaces when the tape was pulled off. This indicates that the adhesion of the electrophoretically applied resin was excellent and that it had not deteriorated during the accelerated environmental testing.

Example 6 A piece of 6061 alloy (Aluminum Association designation) sheet, 4 x 9 x 0.064 inches, was anodized in a mixed organic-inorganic acid electrolyte at 75 The sample was anodized at 24 a.s.f. until the voltage reached 75 volts. Then anodizing was continued at 14 a.s.f. until a total time of 45 minutes elapsed. After anodizing, the sample was rinsed first in a mixture of concentrated nitric acid and water (equal volumes) and then in Water and was introduced into the electrocoating bath while still wet. The electrocoating bath contained 10% solids, as in Examples 2-5 above, and was maintained in the temperature range from to F. The sample was electrophoretically coated for 90 seconds at 1.5 a.s.f. with a limiting maximum voltage of 100 volts. The sample was then withdrawn, rinsed with water, dried and coalesced by heating in an electric oven at F. for 20 minutes. This samplerwas exposed outdoors (at an angle of 45 and facing south) to the natural atmosphere at New Kensington, Pa. for one year. After this exposure, the sample had an excellent appearance indicative of good durability. Very little soil was retained on the sample surface, and this slight amount of soil was easily rinsed off with running water. The sample surface appeared non-wettable. No substantial surface staining or coating breakdowns were observed, nor was there any coating adhesion failure. Thus, the electrophoretic treatment resulted in a protective film of good durability and high resistance to appearance change.

An advantage of our invention is that in the electrophoretic application of the colloidal acrylic interpolymer to anodized aluminum, the anodizing process and the electrophoretic deposition process may be either separate or combined processes. For example, the two processes may be combined into a continuous process, for example, by anodizing coiled aluminum stock continuously and without interruption, conducting the anodized stock directly into the colloidal acrylic interpolymer electrophoretic deposition bath, electrophoretically depositing the acrylic coating, coalescing the coating and either recoiling the finished anodized and resin coated stock or mechanically cutting and forming it into desired useful articles. Likewise, the electrophoretic application of the colloidal acrylic interpolymer to the surface of non-anodized aluminum may be either a stepwise or a continuous process.

While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described our invention and certain embodiments thereof, we claim:

1. A process for coating aluminum which comprises treating aluminum electrophoretically with an aqueous colloidal dispersion comprising particles of an interpolymer of methyl methacrylate and an acid selected from the group consisting of 18% by weight acrylic acid,

18% by weight methacrylic acid, 48% by weight maleic acid, and 48% by weight itaconic acid dispersed in water, at least about 3 mol percent of said acid being in the form of an acid salt selected from the group consisting of ammonium salts and amine salts and at least about 95% by number of said particles being from 0.03-

0.06 micron in diameter, and thereafter coalescing the resulting coating on said aluminum under atmospheric conditions with a coalescing agent selected from the group consisting of xylol and butyl Cellosolve.

" 2. The process of claim 1 wherein the aluminum treated electrophoretically is anodized prior to the electrophoretic treatment.

foot and at a voltage of from about 25 to about 100 volts. 6. The process of claim 1 wherein the electrophoretic 7 treatment of the aluminum is accomplished by introducing the aluminum into a bath of the dispersion and treating it therein, and the aluminum is wet with water when introduced into the bath.

7. The process of claim 1 wherein the temperature of the aqueous colloidal dispersion is maintained at from about 90 to about 110 F. during the electrophoretic treatment.

8. The process of claim 1 wherein the atmoshperic conditions comprise ambient temperatures.

9. The process of claim 1 wherein the atmospheric conditions comprise temperatures of from about F. to about F.

References Cited UNITED STATES PATENTS 3,540,990 11/1970 Onishi et a1. 204-1 81 3,645,872 2/1972 Weigel 204-481 3,544,440 12/1970 Weigel 204181 FOREIGN PATENTS 1,156,671 7/1969 Great Britain 2041'81 1,114,133 5/1968 Great Britain 204181 OTHER REFERENCES Industrial Finishing, Braun, January 1967, pp. 22, 23,

24, 26 and 30.

HOWARD S. WILLIAMS, Primary Examiner v UNITED STATES PATENT OFFICE CERTIFICATE O CORRECTION PetentNo. 3, ?8 1 3 I f a d M M 'H1 .7%s...

Inventofls) Rplles and Paul E. Wallace It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4-, line 18 Change "part" to "parts".

Claim 8, line 1 Change "atmoshperic" to --etmospheric--.

Signed and sealed this 2nd day of Juiy- 1974.

(SEAL Mite-51::

EDWARD M. FLETCHER,JR. C.MARSHALL DANN Attescing Officer Commissioner of Patents 5 FORM PQ-IOSO (10-69) pswmwm wand," I U.S. GOVEINMINY PIIIIHNG OFFICQ SI OTvJC-JJC.