Can pretreatment for improved coat adhesion

Abstract

The invention relates to use of an aqueous composition for pretreating can body stock, whereby an inorganic-organic conversion layer is formed, which effects outstanding sliding behavior of the shaped can body stock and in addition offers an outstanding holding primer for subsequent coating. The invention comprises a process in which can body stock which is deep-drawn to form a half-open can cylinder, before further shaping processes, is contacted with an acidic aqueous composition which contains water-soluble inorganic compounds of Zr, Ti, Si, Hf or Ce, water-soluble polymers having carboxyl groups or hydroxyl groups and also a dispersed wax. Both outer and inner surfaces of metallic can cylinders can be pretreated in the process. The invention also relates to an acidic aqueous composition for use in the pretreatment process, which contains water-soluble polymers selected from condensation products of glycoluril and aldehydes.

Claims

1. A process for surface-treating a can cylinder which is deep-drawn from a metal sheet and is open at one end, in which process: 1.) at least an outer circumferential surface of the can cylinder is contacted with an acidic aqueous composition which contains: a) at least one water-soluble compound of elements Zr, Ti, Si, Hf, and/or Ce; b) at least one water-soluble organic resin, which comprises at least carboxyl groups or hydroxyl groups, having an acid value and/or a hydroxyl value wherein a sum of the acid value and/or hydroxyl value of the at least one water-soluble organic resin is at least 50 mg KOH/g, wherein b) comprises condensation products of glycoluril with primary aldehydes, the condensation products having a degree of alkylation below 20%; and c) at least one dispersed wax; and 2.) wherein at least the outer circumferential surface of the can cylinder that is open at one end is provided with a protective coating after being brought into contact with the acidic aqueous composition; and, optionally, after a directly subsequent flushing step.

2. The process according to claim 1, wherein the water-soluble compounds of component a) of the acidic aqueous composition are selected from fluoro complexes of the elements Zr, Ti, and/or Si.

3. The process according to claim 1, wherein content of water-soluble compounds of the elements Zr, Ti, Si, Hf, and/or Ce of component a) in the acidic aqueous composition is in a range of 0.01 to 1 g/L with respect to a total content of said elements.

4. The process according to claim 1, wherein the water-soluble organic resin of component b) of the acidic aqueous composition is selected from polymers or copolymers based on vinyl ethers, vinyl alcohols, (meth)acrylic acid, maleic acid, or fumaric acid, from hydroxyl group-containing polyesters, and from condensation products of glycoluril or melamine with aldehydes.

5. The process according to claim 1, wherein content of water-soluble organic resins of component b) in the acidic aqueous composition is in range of 0.1 to 50 g/L.

6. The process according to claim 1, wherein the wax in the acidic aqueous composition is selected from synthetic waxes.

7. The process according to claim 1, wherein content of dispersed waxes in the acidic aqueous composition is in a range of 0.1 to 10 g/L.

8. The process according to claim 1, wherein the acidic aqueous composition additionally contains phosphate ions in an amount of at least 0.1 g/L, but not more than 10 g/L of phosphate ions.

9. The process according to claim 1, wherein the acidic aqueous composition has a pH value of not less than 2, and not greater than 6.

10. The process according to claim 1, wherein after bringing the can cylinder that is open at one end into contact with the acidic aqueous composition and before application of the protective coat onto the outer circumferential surface of the can cylinder, there are no further wet-chemical pretreatment steps that are not flushing steps.

11. The process according to claim 1, wherein the can cylinder is not shaped in a rim region that is open at one end.

12. The process according to claim 1, wherein the can cylinder is deep-drawn from tinplate, steel sheet, or aluminum sheet.

13. A production process for can cylinders, wherein: A. a circular blank of a metal sheet is deep-drawn to form a can cylinder that is open at one end; B. the can cylinder that is open at one end is pretreated and coated in a process according to claim 1; and C. the pretreated and coated can cylinder is drawn-in in a rim region that is open at one end in order to taper a diameter of the can cylinder and/or is shaped to form a flange in the rim region that is open at one end.

14. A process for surface-treating a can cylinder which is deep-drawn from a metal sheet and is open at one end, in which process: 1.) at least an outer circumferential surface of the can cylinder is contacted with an, acidic aqueous composition which comprises: a) 0.005 to 0.5 wt. % of water-soluble fluoro complexes of elements Zr, Ti, and/or Si; b) 0.05 to 3 wt. % of water-soluble resins selected from condensation products of glycoluril with aldehydes, said resins having an acid value and/or hydroxyl value wherein a sum of the acid value and/or hydroxyl value of the resins is at least 50 mg KOH/g; and c) 0.1 to 5 wt. % of oxidized polyalkylene waxes, said composition having a pH value in a range of 2 to 5; and 2.) wherein at least the outer circumferential surface of the can cylinder that is open at one end is provided with a protective coating after being brought into contact with the acidic aqueous composition; and, optionally, after a directly subsequent flushing step.

Description

EMBODIMENTS

(1) An acidic aqueous treatment solution having the following composition was used as a base formulation for the pretreatment of aluminum cans (EN AW-3104): 50 ppm Zr from H.sub.2ZrF.sub.6 40 ppm B from boric acid 80 ppm PO.sub.4 from phosphoric acid 300 ppm NO.sub.3 from nitric acid 25 ppm free fluoride (measured with an ion-selective electrode) pH value 3.2

(2) Compositions for pretreatment according to the invention also contained an organic polymer and an emulsified, non-ionogenic, oxidized polyethylene wax. The following organic polymers were used: Org1: tetramethylol glycoluril resin (hydroxyl value 450 to 480 mg KOH/g) Org2: maleic acid-methyl vinyl ether copolymer (acid value 220 to 280 mg KOH/g) Org3: polyacrylic acid

(3) The outer surfaces of the aluminum cans were sprayed with the treatment solutions set forth in table 1, then flushed with deionized water (<1 Scm.sup.1) and dried at a 60 C. object temperature.

(4) TABLE-US-00001 TABLE 1 Formulations for surface treatment of aluminum cans Wax Coating Example Polymer in g/L in layer.sup.1 of Zr no. Org1 Org2 Org3 g/L mg/m.sup.2 V1 17 V2 7 18 V2 6 17 E1 6 7 20 E2 6 7 18 E3 6 7 21 .sup.1measured by X-ray fluorescence analysis (XRF)

(5) The properties of the conversion coatings that were obtained after the pretreatments with acidic compositions according to table 1 are reproduced in table 2 as regards the sliding behavior and coat adhesion.

(6) The sliding behavior was determined by stacking three cans in the form of a triangle, wherein the lower two cans that formed the base were raised at one end vertically to the longitudinal direction of the can. In the one-sided raising of the can stack, the angle between the can axis and the horizontal is indicated as the slip angle at which the upper can began to slip. This test was repeated five times with different but similarly pretreated cans, and the mean value of each of the determined slip angles was established.

(7) After the executed outer coating of the pretreated cans with a commercially available can coat (coat base: acrylate-modified polyester from DSM, Uradil SZ250; layer thickness about 15 m) and after the tapering or compression of the open can rim (necking) and 90 flanging of the can rim, the coat adhesion was determined. The assessment was carried out visually in the shaping region of the can, in five-can increments, according to the following criteria: 1: No visible cracks or chipping of the coat 2: Cracks and slight chipping of the coat 3: Severe chipping of the coat

(8) TABLE-US-00002 TABLE 2 Sliding behavior and coat adhesion of the outer can surfaces pretreated according to table 1 Example no. Slip angle Coat adhesion V1 35 3 V2 35 3 V2 39 2 E1 22 1 E2 25 1-2 E3 30 1-2

(9) The results show that with compositions for pretreatment according to the invention, especially with compositions containing the glycoluril resin, the lowest slip angles and best coat adhesion are obtained (E1-E3).