SK286005B6 - Coating an aluminum alloy substrate - Google Patents

Abstract

An aluminium alloy substrate is pre-treated with an aqueous solution containing an organophosphorus compound, preferably a vinyliphosphonic acid-acrylic acid copolymer, before coating the substrate with a polymer. Passing the substrate through the solution contaminates it with aluminium and other elements. The pre-treatment solution is rejuvenated by removing aluminium 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 returned to the pre-treatment solution.

Description

Technical field

The present invention relates to a process for pretreating an aluminum alloy sheet with a polymer. In particular, the invention relates to a process for pretreating an aluminum alloy substrate with a vinylphosphonic acid-acrylic acid copolymer prior to coating the substrate with a polymer.

BACKGROUND OF THE INVENTION

Although aluminum protects itself against corrosion by forming a natural oxidation coating, protection is not complete. When moisture and electrolytes occur, aluminum alloys corrode much faster than pure aluminum.

Therefore, there is a need to treat aluminum alloy substrates with pretreatment or other chemicals that provide improved corrosion resistance as well as beneficial adhesion to polymers.

Up to now, chemical conversion coatings have been formed on aluminum alloys by converting the metal surface into a sticky coating, some of which consisted of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and improved adhesion of polymer coatings. The chromium-phosphate conversion coating is usually provided by contacting aluminum with an aqueous solution containing hexavalent ions, phosphate ions and fluoride ions. In recent years, concerns have been increasing regarding the pollution of the environment by the effects of chromates and phosphates discharged into watercourses in such processes. Due to the high solubility and strong oxidizing property of hexavalent ions, expensive waste disposal procedures must be used to reduce hexavalent ions to trivalent chromium ions.

Attempts have been made in the art to produce acceptable conversion coatings for chromate-free aluminum. For example, some chromate-free conversion coatings contain zirconium, titanium, hafnium and / or silicon, sometimes combined with fluorides, surfactants and polymers such as polyacrylic acid. Despite extensive efforts in the past, a chromate-free conversion coating or a basis for improving the corrosion-resistant adhesion and resistance of polymers that coat aluminum alloy substrates is not yet satisfactorily solved. Polymer adhesion and corrosion resistance are important properties of the aluminum alloy sheet coatings used to make bodies and end food containers and beverage container ends.

Attempts have also been made in the prior art to treat pre-substrates with various organophosphorus compounds prior to coating them with polymer. As used herein, the term "organophosphorus compounds" includes organophosphorus compounds, organophosphine compounds, organophosphonic compounds, as well as various salts, esters, partial salts, and partial esters of such acids. For example, Dutch patent no. No. 263,668 of April 14, 1961 discloses a process wherein steel sheets are treated with a vinylphosphonic acid-acrylic acid copolymer prior to coating with alkyd resin enamel. Although some organophosphorus pre-treatments may have a similar effect, it is costly to implement them. Thus, there remains a need to provide an efficient and economical process for pretreating an aluminum alloy substrate with an organophosphorus compound prior to applying the polymer coating.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an efficient and economical process for pretreating an aluminum alloy substrate with an organophosphorus compound prior to applying the polymer coating.

In order to satisfy this essential object, the process of the invention provides for measures to remove aluminum and other cations from the pretreatment solutions, thereby avoiding the costly disposal of such solutions.

Other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description.

Overview of the figures in the drawing

A single figure shows a flow diagram of a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a method for coating an aluminum alloy substrate with an organic polymer. The aluminum alloy substrate may be in the form of a sheet, molded or cast, and is preferably sheet-shaped.

For practicing the present invention, a variety of sheet-like aluminum alloys, including AA2000, 3000, 5000, 6000 and 7000 series alloys are preferred. Aluminum and magnesium alloys of the AA5000 series, and in particular AA5042 and AA5182 alloys are preferred. The sheet made of these alloys is useful for shaping into bodies and finishing food containers and beverage container ends.

Aluminum alloys suitable for container end sheets, for example AA5182, are provided as an ingot or billet or slab by known casting techniques in the art. Prior to processing, the ingot or billet is subjected to homogenization at elevated temperature. The alloy billet is then hot rolled to obtain a calibrated sheet as an intermediate. For example, the material may be hot rolled at a metal inlet temperature of about 371 to 524 ° C to obtain an intermediate having a thickness of about 2.54 to 3.81 mm. This material is cold rolled to obtain a sheet having a thickness ranging from about 1.52 to 3.81 mm. Preferred is an aluminum alloy sheet AA5182 with a H19 temper. The aluminum alloy sheet 5042 at the front ends is preferably of a hardness grade H9.

Aluminum alloys such as AA5042 are provided as an ingot that is homogenized. This is followed by hot rolling to an intermediate caliber of about 2.54 to 3.81 mm. Typically, the intermediate calibrated product is annealed, followed by hot rolling and then cold rolling to obtain a final calibrated product having a thickness of about 1.52 to 3.81 mm. The sheet is coated with polymer and then drawn and re-drawn into the food container body. Preference is given to AA5042 aluminum alloy sheets having a hardness grade of H2x.

The natural oxide coating of the surface of the aluminum alloy sheet is generally sufficient to carry out our invention. The natural oxide coating usually has a thickness of about 3 to 5 nm. For better corrosion protection, the oxide coating can be increased by treatment such as anodic oxidation or hydrothermal treatment in water, water vapor or aqueous solutions.

The aluminum alloy sheet of the present invention is typically cleaned with an alkaline surface cleaner to remove any residual lubricants adhered to the surface and then rinsed with water. However, cleaning can be avoided if the residual lubricant content is negligible.

The cleaned leaf surface is then pretreated in a first tank with a composition comprising an aqueous solution of an organophosphorus compound. The solution preferably contains about 1 to 20 g / l of a vinylphosphonic acid-acrylic acid copolymer (VPAA-AA copolymer). Solutions containing about 4 to 10 g / l of copolymer are preferred. The copolymer usually contains about 5 to 50 mol% vinylphosphonic acid, preferably about 20 to 40 mol%. The VPA-AA copolymer may have a molecular weight of about 20,000 to 100,000, preferably about 50,000 to 80,000. A particularly preferred VPA-AA copolymer comprises about 30 mole% VPA and about 70 mole% AA. The solution has a temperature of about 38 to 93 ° C, more preferably about 49 to 82 ° C. A particularly preferred solution has a temperature of about 77 ° C.

The sheet surface may be immersed in the composition or the composition may be rolled or sprayed onto the sheet surface. A preferred continuous cleaning and pretreatment line is operated at about 158 to 474 m per minute. A contact time of about 6 seconds between the sheet surface and the composition is sufficient when the line is operated at 316m per minute. The VPA-AA copolymer reacts with an oxide or hydroxide coating to form a layer on the sheet surface.

The aluminum alloy sheet passing through the pretreatment solution contaminates the solution with ions of various elements including aluminum, magnesium, iron, chromium and manganese. The pretreatment solution loses effectiveness when the aluminum concentration rises to about 150 to 200 ppm. Thus, a method is provided for removing aluminum and other metal ions from a pretreatment solution.

At least a portion of the pretreatment solution is transferred to a second tank containing the cation exchange resin. Thus, the resin may be in the form of pellets, beads, fibers or particles, and is preferably of the round ball gel type. The resin has a minimum total hydrogen capacity of 1.9 meq / ml moisture. Preferably, the resin has an average particle size of about 650 µm, a specific gravity of about 1.22 to 1.23, and a bulk density of about 2.994 g / cm ³ .

The resin is preferably a gel comprising a copolymer of styrene and divinylbenzene with acidic functional groups, preferably sulfone groups. Alternatively, the copolymer may contain functional acid groups of phosphonic acid and arzonic acid. A particularly preferred cation exchange resin is sold by the Dow Chemical Company of Midland, Michigan under the trademark DOWEX G-26 (H).

Less preferably, the cation exchange resin may comprise ethylene copolymerized with an unsaturated carboxylic acid such as acrylic acid.

Then the pretreatment solution passes through a second container containing a reduced aluminum concentration. The concentration of aluminum in the treatment solution is less than about 75 ppm, more preferably less than 25 ppm and optimally about 10 ppm or less. The treatment solution containing the organophosphorus compound and the reduced aluminum concentration is returned to the first container.

The pretreated sheet may optionally be rinsed with water to remove excess VPA-AA copolymer. The rinse water preferably has a temperature of about 77-82 ° C. The rinse water is concentrated by removing excess water so that the VPA-AA copolymer can be recycled. Some preferred concentration techniques include reverse osmosis and membrane filtration. After concentration, the rinse water may be transferred to the first container to restore the VPA-AA copolymer value.

The prepared sheet is coated with a polymer composition, which preferably comprises an organic polymer dispersed in an organic solvent. Three preferred coating polymers are epoxides, polyvinyl chloride and polyesters. Suitable epoxides include phenol-modified epoxides, polyester-modified epoxides, epoxy-modified polyvinyl chloride, and epoxides with crosslinked bonds. The polymer compositions may be pure or may contain pigment particles. The pigment particles are preferably titanium dioxide, alumina or silica. Titanium dioxide particles with a mean particle size range of 0.5 to 10 microns are preferred.

Alternatively, the prepared sheet may be electrolytically coated, slit coating, extrusion coating, coating coating, spray coating or other permanent coating methods.

The polymer-coated sheet is dried, coiled and finally formed into container bodies or container front plates.

As schematically shown in the figure, there is provided a roll of sheet 10 of AA5182-H19 of aluminum and magnesium alloy having a thickness of about 224 microns. The sheet 10 is cleaned with an alkaline surfactant cleaner in the vat 20 to remove any residual lubricants from the sheet surface. The cleaned sheet is then rinsed in a deionized water bath 30.

The cleaned and rinsed sheet is pretreated in the first tank 40 with a solution containing about 10 g / l VPA-AA copolymer consisting of about 30 mole% VPA groups and about 70 mole% AA dissolved in water. The solution has a temperature of about 77 ° C and initially contains about 10 ppm aluminum. The VPA-AA copolymer reacts with a coating on the surface of an aluminum oxide or aluminum hydroxide sheet to form a layer containing the reaction product of the copolymer and an oxide or hydroxide.

The pretreated sheet is then rinsed with water 50 to remove excess VPA-AA copolymer. The rinsed water 50 preferably has a temperature of about 77-82 ° C.

The rinsed sheet is rolled with a polymer composition 60, which preferably comprises an organic polymer and pigment particles dispersed in an organic solvent. Preferably, the organic polymer is an epoxy resin. Some suitable epoxides include phenol-modified epoxides, polyester-modified epoxides, epoxy-modified polyvinyl chloride, and cross-linked epoxides.

The polymer-coated sheet is dried in a hot air dryer 70 and is then rolled up as a coated sheet product 80.

In order to maintain a low concentration of metal ions in the pretreatment solution, portions of the solution are periodically transferred from the first reservoir 40 to the second reservoir 100 which contains the cation exchange resin. A particularly preferred resin is a solid cation exchange resin sold by Dow Chemical Company of Midland, Michigan under the trademark DOWEX G-26 (H). This solid cation exchange resin is sold as hard spheres of 650 microns (dry mesh). The solid cation exchange resin is a gel comprising a styrene-divinylbenzene copolymer with sulfate functional groups. The resin treatment produces a treated solution having an aluminum concentration that is optimally less than about 10 ppm. The treated solution is returned via line 110 from the second container 100 to the first tank 40.

The cation exchange resin eventually becomes saturated with metal salts. The resin is regenerated by washing with a strong solution of acid 120, such as 6 to 10% HCl or 6 to 12% sulfuric acid in water by volume. The metal salts 130 washed out of the second container 100 are discarded.

The rinse water used from the water rinse 50 is also recycled to recover VPA-AA copolymer values. The rinsing water used is first introduced into a concentration device 140 where water is removed, for example by reverse osmosis and membrane ultrafiltration. The concentrated rinse water is then returned to the first tank 40.

The cation exchange process of the present invention maintains the aluminum concentration at acceptable levels in the pretreatment solution. A 200 ml aliquot of the 60 ° C pre-treatment solution containing 10 g / l VPA-AA copolymer, 350 ppm aluminum and other metals was placed in a 250 ml Erlenmeyer flask containing 40 ml wet volume of DOWEX G-26 (H) resin in hydrogen cycle. The flask was placed in a water bath and kept at 60 ° C for 16-20 hours. The resin is prepared by washing with 400-600 ml 6% HCl by volume, followed by rinsing with 600-800 ml deionized water.

After a contact time of 16 to 20 hours, the pretreatment solution is filtered and the resin is rinsed with 25 ml of deionized water. The solution is analyzed and the results are shown in the following table. All concentrations are corrected for comparison with a 200 ml volume.

Table - analysis of pretreatment solution

substance Initial concentration (ppm) After G-26 (H) (ppm) Al 350 5.3 On the 14 0.5 Are u 11 12 fe 22 18 ca 4.1 0.8 mg 100 0.3 Mn 1.5 .DELTA.n.d Ni 0.6 .DELTA.n.d Zn 0.3 .DELTA.n.d Cr 11 10 The 1.8 0.5 P 820 777

n.d undetectable

One skilled in the art will appreciate that, with reference to some of the presently preferred embodiments, the described invention has the understanding that the present invention may be practiced otherwise without departing from the spirit and scope of the appended claims.

Claims (10)

PATENT CLAIMS A method of pretreating an aluminum alloy sheet having a portion of a surface to improve the adhesion of a polymer coating to that portion of a surface, comprising: (a) in a first tank, a pre-treatment of an aluminum alloy sheet having a portion of the surface containing alumina or aluminum hydroxide with a solution consisting essentially of water and an organophosphorus compound, thereby forming a layer containing the reaction product of said compound and oxide or hydroxide; . b) transferring at least a portion of this solution to a second tank containing a polymer-containing cation exchange resin and thereby adsorbing aluminum ions to the resin, thereby producing a treated solution containing the compound having a reduced concentration of aluminum ions; and c) returning the so treated solution to the first tank. The method of claim 1, wherein the sheet comprises an aluminum alloy of the AA2000, 3000, 5000, 6000 or 7000 series and further comprises: d) coating said layer with a coating composition comprising a polymer selected from the group consisting of polyvinyl chloride, epoxides and polyesters. The process according to claim 1, wherein the resin comprises a copolymer of styrene and divinylbenzene with sulfonate functional groups. The process of claim 1, wherein the treated solution contains less than about 75 ppm aluminum ions. G) returning at least a portion of the rinse water treated in step (f) to the first tank. 5. The method of claim 1 wherein the treated solution contains less than about 25 ppm aluminum ions. 6. The method of claim 1, further comprising: e) after step (a), rinsing the substrate with water to produce a rinse water comprising an organophosphorus compound and aluminum ions; (f) concentrating the rinsing water by removing water; and The method of claim 1, wherein the organophosphoric compound comprises a vinylphosphonic acid / acrylic acid copolymer. The process of claim 7, wherein the copolymer has a molecular weight of about 20,000 to 100,000. 10 9. The process of claim 7, wherein the pretreatment solution contains about 1 to 20 g / l of copolymer. The process of claim 7, wherein the pretreatment solution has a temperature of about 49 to 93 ° C in step (a).