MXPA02004405A - Coating an aluminum alloy substrate - Google Patents

Abstract

An alluminum alloy substrate is pretreated with an aqueous solution containing an organophosphorus compound, preferably a vinylphosphonic acid acrylic acid copolymer, before coating the substrate with a polymer. Passing the substrate through the solution contaminates it with aluminum and other elements. The pretreatment solution is rejuvenated by removing aluminum with a cation exchange resin that preferably contains a styrene divinyl benzene copolymer functionalized with sulfonate groups. Rinsing the substrate contaminates the rinse water with the copolymer. The rinse water is concentrated by reverse osmosis or membrane ultrafiltration and retured to the pretreatment solution.

Description

COATING OF AN ALUMINUM ALLOY SUBSTRATE Field of the Invention The present invention relates to a process for coating an aluminum alloy substrate with a polymer. Particularly, the invention relates to a process for pretreating an aluminum alloy substrate with a copolymer of vinylphosphonic acid-acrylic acid before coating the substrate with a polymer.

BACKGROUND OF THE INVENTION Although aluminum protects itself against corrosion by forming a natural oxide decomposition, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys corrode much faster than pure aluminum. . Accordingly, there is a need to treat the aluminum alloy substrates with pretreatment or other chemicals that provide better corrosion resistance as well as good adhesion of the polymers. In the prior art, the chemical conversion coatings that have been formed on aluminum alloys by "converting" a metal surface into a REF: 138179 strongly adherent coating, part of this consists of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and better adhesion for polymer coatings. A chromium-phosphate conversion coating is typically provided by contacting aluminum with an aqueous solution containing hexavalent chromium ions, phosphate ions, and fluoride ions. In recent years, concerns have been raised about the contaminating effects of chromates and phosphates discharged into waterways by the processes Due to the high solubility and strong oxidizing character of the hexavalent chromium ions, costly waste treatment methods must be employed in order to reduce hexavalent chromium ions to trivalent chromium ions for residual disposal. Attempts have been made in the prior art to produce chromate-free converting coatings for aluminum. For example, some coatings with chromium free conversion; cysts contain zirconium, titanium, hafnium and / or silica, sometimes combined with fluorides, surfactants and polymers such as polyacrylic acid. In spite of the extensive efforts that have been made previously, there is still no coating or first layer with conversion without chromate completely satisfactory for improve the adhesion and corrosion resistance of aluminum-coated substrates coated with polymers. Adhesion of the polymer and corrosion resistance are important features in the aluminum alloy sheet used to make the bodies and ends of food containers and ends of beverage containers. Attempts have also been made in the prior art of pretreating the strands with various organophosphorus compounds before coating them with a polymer. As used herein, the term "organophosphorus compounds" includes organophosphoric acids, organophosphinic acids, organophosphonic acids, as well as different salts, esters, partial salts and partial esters of these acids. For example, German Patent Application No. 263,668, filed on April 14, 1961, reveals a process where steel sheets were treated with a copolymer of vinylphosphonic acid / acrylic acid O before coating with an alkyd resin enamel. Although some pre-treatments with organophosphides can be developed properly, they are very expensive to implement. Therefore, there is still a need for an efficient and economical process to give prior treatment to a substrate With an organic polymer, the aluminum alloy substrate can be provided in the form of a flat sheet, extrusion or casting and preferably is a sheet. Various aluminum alloys available in sheet form are suitable for practicing the present invention., including alloys that are members of the AA2000, 3000, 5000, 6000 and 7000 series. Aluminum-magnesium alloys of the AA5000 series and particularly AA5042 and AA5182 are preferred. The sheet made of these alloys is useful for conforming to the bodies and ends of polymer-coated food containers, and ends of beverage containers. Aluminum alloys suitable for end panels of the AA5182 container are provided as a slug or pack or by means of casting techniques known in the art. Before work, the ingot or package is subjected to a high homogenization temperature. Then the stock of the alloy is rolled with hot rolls to provide a sheet with intermediate gauge. For example, the material can be rolled with hot rollers at a metal inlet temperature of about 371 ° -524 ° C (700 ° -975 ° F) to provide an intermediate product having a thickness of about 2.54xl0 ~ 3 m 3. 81xl0"3 m (0. 100 inch to 0. 150 inches). material is rolled with rod: cold bundles to provide a sheet with a thickness range from approximately 1. 524X10 > - "4 to 3.81X10" m 0.006 to 0.015 inches). The aluminum alloy plate AA5182 with a hardened H19 is preferred. The 5042 aluminum alloy sheet for the end panels is preferably H19 tempered. The alloys of alu inio like AA5042 are provided as an ingot that is hompgenerated. This is followed by rolling with hot rollers to an intermediate caliber of approximately 2.54x10 m a m (0.100 inches to 0.150 inches). Typically, the intermediate gauge product is removed, followed by hot roll rolling and then rolling with cold rolls to a finished gauge product having a thickness of approximately 1.524X10"" 4 3. 81X10"4 m (0.006 to 0.015 in.) The sheet is coated with a polymer and then stretched and stretched again to form the bodies of the food containers.Alum aluminum alloy sheet AA5042 with an annealed H2x is preferred. The natural oxide coating on a surface of the aluminum alloy sheet is generally sufficient to practice our invention. oxide has a thickness of approximately 30-50 Angstroms For better protection against corrosion, the oxide coating can grow with treatments such as anodic oxidation or hydrothermal treatment in water, water vapor or aqueous solutions. The aluminum alloy sheets of the invention are generally cleaned with an alkaline surface cleaner to remove any residual lubricant that adheres to the surface, and then rinsed with water. Cleaning can be avoided if the residual lubricant content is negligible. The surface of the clean sheet is then pretreated in a first container with a composition comprising an aqueous solution of an organophosphorus compound. The solution preferably contains about 1-20 g / L of a copolymer of vinylphosphonic acid-acrylic acid (copolymer VPA-AA, for its acronym in English! Solutions containing about 4-10 g / L of the copolymer are referred to. The copolymer usually comprises about 5-50 mol% vinylphosphonic acid, preferably about 20-40 mol% The copolymer rPA-AA can have a molecular weight of about 20,000 to 100,000, preferably about 50,000 80,000 A copolymer of VPA- Particularly preferred AA contains about 30% mol of VPA and about 70% mol of AA. The solution has a temperature of about 38 ° -93 ° C, 100 ° -200 ° F), preferably about 49 ° -82 ° C (120 ° -180 ° F). A particularly preferred solution has a temperature of about 77 ° C (170 ° F) The surface of the sheet can be bathed in the composition or the composition can be roller coated or sprayed onto the surface of the sheet.Preferably a continuous cleaning line is operated and pre-treatment at approximately 2.54-7.62 meters per second (500-1500 feet per minute) The contact time is approximately 6 seconds between the surface of the sheet and the composition is sufficient when operating the line at 5.08 meters per second (1000 feet per minute) The VPA-AA copolymer reacts with the oxide or hydroxide coating to form a layer on the surface of the sheet.The aluminum alloy sheet passing through the previous treatment solution D contaminates the solution with ions of different the elements, including aluminum, magnesium, iron, chromium and manganese.The solution of the previous treatment loses efectiv ad when the aluminum concentration increases to approximately 150-200 ppm. Accordingly, a process is provided for Remove the aluminum ions and other metals from the pre-treatment solution. At least a portion of the pretreatment solution is transferred to a second container containing a cation exchange resin. The resin can be provided as pellets, beads, fibers or particles and preferably is a hard, spherical, gel-like bead. The resin has a minimum total layer in hydrogenated, wet form, of 1.9 meq / m.L. A preferred resin has an average particle size of about 6.5x10 ~ 4 m (650 microns), a specific gravity of about 1.22-1.23, and a mass density of about 799 kg / m3 (49.9 lb / ft. Preferred resin: Te is a gel comprising a functional styrene-divinylbenzene copolymer with acid groups, preferably sulfonate groups Alternatively, the copolymer may be functional with the phosphonic acid or arsenic acid groups A particularly preferred cation exchange resin is sold by The Dow Chemical Company Copany of Midlarid, Michigan under the trademark DOWEX G-26 (H) Less preferred, the cation exchange resin may comprise copolymerized ethylene with an unsaturated carboxylic acid such as acrylic acid. i. *.? - -Ai After the pretreatment solution passes through the second container, it contains a reduced concentration of aluminum. the concentration of aluminum in the treated solution is less than about 75 ppm, preferably less than about 25 ppm, and optimally about 10 ppm or less. The treated solution, containing the organophosphorous compound and a reduced concentration of ajluminium, is returned to the first container. Optionally, the previously treated sheet can be rinsed with water to remove excess VPA-AA copolymer. The rinse water preferably has a temperature of about 77 ° -82 ° C (170 ° -180 ° F). The rinse water is concentrated by removing the excess water so that the VPA-AA copolymer can be recycled. Preferred concentration techniques include reverse osmosis and membrane filtration. After concentration, the rinse water can be transferred to the first container to recover the VPA-AA copolymer values. The sheet with a primer layer is coated with a polymer composition which preferably includes an organic polymer dispersed in an organic solvent. Three preferred coating polymers are epoxy resin, vinyl chloride and polyesters. The Suitable epoxy resins include phenol-modified epoxy resins, polyester-modified epoxy resins, epoxy resin modified livinyl chloride, and crosslinkable epoxy resins. The polymer composition may be transparent or may contain pigment particles. The pigment particles are preferably titanium dioxide, alumina or silica. Particulate titanium dioxide particles with a range of average particle size from 5 × 10 ~ 7 to 10 × 10 -5 m (0.5 to 10 microns) are preferred. Alternatively, the sheet with a primer layer may be coated by electrodeposition coating, extrusion die coating, extrusion coating, flow coating, spray coating, or other continuous coating processes. The polymer-coated sheet was dried, rolled up, and finally shaped into the container bodies or panels at the ends of the container. As shown schematically in the Figure, a roll of 1. sheet 10 of aluminum-magnesium alloy AA5182-H 9 having a thickness of approximately 224 micrfnes (8.8 mils) is provided. The sheet 10 was cleaned with an alkaline cleaner of surfaces in a tub 20 to remove any residual lubricant on the surface. the surface of the sheet The clean sheet was then rinsed in a bath of deionized water 30. The clean sheet is treated with p.revolution and rinsed in a first container 40 with a solution comprising approx. VPA-AA copolymer containing about 30 mole percent VPA and about 70 mole percent of the AA units, dissolved in water. The solution has a temperature of about 77 ° C (170 F) and initially contains about 10 ppm of aluminum. The VPA-AA copolymer reacts with an aluminum oxide or hydroxide coating on the surface of the sheet to form a layer comprising a reaction product of the copolymer and the oxide or hydroxide. The treated sheet was then rinsed with water to remove the VPA-AA copolymer ex-ploy. The rinse water 50 preferably has a temperature of about 77-82 ° C (L70-180 ° F). The rinsed sheet is roller coated with a polymer composition 60 which preferably includes an organic polymer and pigment particles dispersed in an organic solvent. The organic polymer is preferably an epoxy resin. Some epoxy resins include phenolically modified epoxy resins, epoxy resins modified with polyester, polyvinyl modified with epoxy resin, and epoxy resins that can be crosslinked. The polymer-coated sheet is dried in a hot air dryer 70 and then rolled again as a product of the coated sheet 80. To maintain a tight concentration of metal ions in the pretreatment solutionportions of the solution are periodically transferred from the first container 40 to a second container 100 having a cation exchange resin. A particularly preferred resin is sold by the Dow Chemical Company of Midland, Mich. Under the trademark DOWEX G-26 (H) strong cationic exchange resin. The strong cation exchange resin is sold as hard spherical beads with a dry mesh size of 6. 5x10? - "4 m (650 micronsi) The strong cation exchange resin is a gel comprised of a functional styrene-divinylbenzene copolymer with the sulfonate groups.The treatment with the resin produces a treated solution having an aluminum concentration which is optimally less than about 10 ppm The treated solution is returned through a pipe 110 from the second container 100 to the first container 40.

The cation exchange resin eventually becomes saturated with the metal salts. The resin is regenerated by washing it with a strongly acid solution 120, such as 6-10% vol. HCl or 6-12% vol. of sulfuric acid in water. The washed metal salts 130 are discharged from the second container 100. The rinse water used from the rinse with water 50 is also recycled to recover the VPA-AA copolymer values. The rinse water used is first sent to a concentrator 14C where the water is removed, for example by osmosis in \ ersa or ultrafiltration with membrane. The recycled rinse water is returned to the first container 40. The cation exchange process of the invention maintains the aluminum concentrations at acceptable levels in the pretreatment solution. An aliquot of 200 mL of the pre-treatment step at 60 ° C (140 ° F) containing 10 g / L of the VPA-AA copolymer, 350 ppm of aluminum and other metals, was placed in an Ehrlenmeyer flask, which .has 40 mL of wet volume of DOWEX G-26 resin (H) in hydrogenated form. The flask was placed in a water bath and kept at 60 ° C (140 ° F) for 16-20 hours. The resin was prepared by washing it with 400-600 mL of 6% vol. of HCl, followed by rinsing with 600-800 mL of deionized water. After 16 to 20 hours of contact time, the pretreatment solution was filtered and the resin was rinsed with 25 mL of deionized water. The solution was analyzed and the results are presented immediately in the Table. All concentrations were corrected to reflect a volume of 200 mL, for comparison practice said invention is that which is clear from the present invention.

Claims (10)

1. (c) returning the treated solution to the first container.
2. 2. The process of claim 1, characterized in that the sheet comprises an aluminum alloy of the series AA2000, 3000, 5000, 6000 or 7000, and further comprises: d) coating the fall with a coating composition comprising a polymer selected from the group consisting of polyvinyl chloride, epoxy resins and polyesters.
3. 3. The process of claim 1, characterized in that the resin comprises a functional styrene-divinylbenzene copolymer with the sulfonate groups.
4. 4. The process of claim 1, characterized in that the treated solution contains less than about 75 ppm of aluminum ions.
5. 5. The process of claim 1, characterized in that the treated solution contains less than about 25 ppm of io is aluminum, 6.
6. The process of claim 1, characterized in that it further comprises: (e) after step (a) ), rinse the substrate with water, by means of which rinse water containing the organ compound is produced: osphorous and aluminum ions, (f) concentrating the rinse water by removing the water, and g) returning at least a portion of the rinse water to the first container: treated in step (f).
7. The process of claim 1, characterized in that the organophosphorus compound comprises a copolymer of vinylphosphonic acid-acrylic acid.
8. 8. The process of claim 7, characterized in that the copolymer has a molecular weight of about 20,000 to 10C, 000.
9. 9. The process of claim 7, characterized in that the pre-treatment solution comprises about 1-20 g / L of the copolymer.
10. 10. The process of claim 7, characterized in that the pretreatment solution has a temperature of approximately 49-93 ° C (120-170 ° F) in step .. »• >