Multistage pretreatment of tinplate prior to the coating thereof with lacquer

Abstract

The invention relates to a two-stage method for the anti-corrosive pretreatment of tinplate, in which an anti-corrosive primer coating is applied in a first step, in which the tinplate is anodically polarized in an electrolyte containing at least one inert water-soluble salt and is then in a second step, brought in contact with an acidic aqueous composition containing water-soluble inorganic compounds of the elements Zr, Ti, Hf, and/or Si.

Claims

1. A method for the pretreatment of tinplate prior to coating with an organic topcoat comprising: 1) in a first step, anodically polarizing a tinplate surface in an aqueous electrolyte containing at least one inert, water-soluble salt thereby producing a modified tinplate surface consisting essentially of tin(IV) oxide/hydroxide, wherein no portion of said inert, water-soluble salt is incorporated into said modified tinplate surface and wherein said tinplate surface is not cathodically polarized in said aqueous electrolyte and wherein the aqueous electrolyte further contains at least one organosilane with at least one hydrolyzable substituent, which is split off under hydrolysis as an alcohol having a boiling point of less than 100 C., and at least one non-hydrolyzable substituent; and then, 2) in a second step, passivating the modified tinplate surface produced in the first step, by contacting the modified tinplate surface with an acidic aqueous composition containing water-soluble inorganic compounds of the elements Zr, Ti, Hf and/or Si.

2. The method according to claim 1, wherein the acidic aqueous composition in the second step contains as the water-soluble inorganic compounds of the elements Zr, Ti, Hf and/or Si, a fluoro acid of at least one of said elements and/or a salt of said fluoro acid.

3. The method according to claim 2, wherein the elements are Zr and/or Ti.

4. The method according to claim 3, wherein the acidic aqueous composition in the second step additionally contains phosphate ions in an amount of at least 0.01 wt. %, but no more than 3 wt. %, based on PO.sub.4.

5. The method according to claim 1, wherein the water-soluble inorganic compounds of the elements Zr, Ti, Hf and/or Si in the acidic aqueous composition of the second step are present in an amount in total of at least 0.001 wt. %, but no more than 0.5 wt. %, calculated based on amount of the element Zr, Ti, Hf and/or Si.

6. The method according to claim 1, wherein the acidic aqueous composition in the second step additionally contains water-soluble and/or water-dispersible organic polymers selected from the group consisting of polyacrylates, polyisocyanates, polyepoxides, polyvinylamines, polyalkylene imines and amino-substituted polyvinyl phenol derivatives.

7. The method according to claim 6, wherein the water-soluble and/or water-dispersible organic polymers in the acidic aqueous composition of the second step are present in an amount in total in a range of 0.05 wt. % to 10 wt. %.

8. The method according to claim 1, wherein pH value of the acidic aqueous composition in the second step is in a range of 2.5 to 5.5.

9. The method according to claim 1, wherein in the second step, the passivating of the modified tinplate surface, by contacting the modified tinplate surface with an acidic aqueous composition produces a coating weight of at least 0.3 mg/m.sup.2, but no more than 30 mg/m.sup.2, based on the elements Zr, Ti, Hf and/or Si, on the modified tinplate surface.

10. The method according to claim 1, wherein the contacting the modified tinplate surface with the acidic aqueous composition in the second step is where a wet film of the acidic aqueous composition is applied onto the modified tinplate surface and the wet film is dried immediately after application.

11. The method according to claim 1, wherein the anodic polarization in the first step takes place for at least 0.2 seconds, but in total no longer than 300 seconds.

12. The method according to claim 1, wherein the anodic polarization in the first step takes place at a current density of at least 0.005 A/dm.sup.2, but no more than 6 A/dm.sup.2.

13. The method according to claim 1, wherein the electrolyte in the first step further contains organic dicarboxylic acids and/or salts of the organic dicarboxylic acids, said organic dicarboxylic acids having no more than 6 carbon atoms.

14. The method according to claim 1, wherein the at least one inert water-soluble salt is selected from the group consisting of carbonates, sulfates, nitrates and hydroxides of alkali metals.

15. The method according to claim 1, wherein said at least one non-hydrolyzable substituent has at least some primary amino functions.

16. A method for the pretreatment of tinplate prior to coating with an organic topcoat comprising: 1) in a first step, anodically polarizing a tinplate surface in an aqueous electrolyte containing at least one inert, water-soluble salt thereby producing a modified tinplate surface consisting essentially of tin (IV) oxide/hydroxide, wherein no portion of said inert, water-soluble salt is incorporated into said modified tinplate surface and wherein said tinplate surface is not cathodically polarized in said aqueous electrolyte and wherein the aqueous electrolyte further contains at least one organosilane with at least one hydrolyzable substituent, which is split off under hydrolysis as an alcohol having a boiling point of less than 100 C., and at least one non-hydrolyzable substituent, the aqueous electrolyte further containing at least one water-soluble silicate M.sub.2O.nSiO.sub.2, wherein M is an alkali metal ion or quaternary ammonium ion and n is a natural number between 0.8 and 7; and 2) after the first step, in a second step, passivating the modified tinplate surface produced in the first step, by contacting the modified tinplate surface with an acidic aqueous composition containing water-soluble inorganic compounds of elements Zr, Ti, Hf and/or Si.

17. The method according to claim 16, wherein said at least one non-hydrolyzable substituent has at least some primary amino functions.

18. The method according to claim 16, wherein the at least one inert water-soluble salt is selected from the group consisting of carbonates, sulfates, nitrates and hydroxides of alkali metals.

19. The method according to claim 16, wherein the electrolyte in the first step further contains organic dicarboxylic acids and/or salts of the organic dicarboxylic acids, said organic dicarboxylic acids having no more than 6 carbon atoms.

20. The method according to claim 16, wherein the water-soluble inorganic compounds of the elements Zr, Ti, Hf and/or Si in the acidic aqueous composition of the second step are present in an amount in total of at least 0.001 wt. %, but no more than 0.5 wt. %, calculated based on amount of the element Zr, Ti, Hf and/or Si.

21. The method according to claim 20, wherein the elements are Zr and/or Ti.

22. The method according to claim 20, wherein the acidic aqueous composition in the second step additionally contains phosphate ions in an amount of at least 0.01 wt. %, but no more than 3 wt. %, based on PO.sub.4.

23. The method according to claim 16, wherein the acidic aqueous composition in the second step contains as the water-soluble inorganic compounds of the elements Zr, Ti, Hf and/or Si, a fluoro acid of at least one of said elements and/or a salt of said fluoro acid.

24. The method according to claim 16, wherein the acidic aqueous composition in the second step additionally contains water-soluble and/or water-dispersible organic polymers selected from the group consisting of polyacrylates, polyisocyanates, polyepoxides, polyvinylamines, polyalkylene imines and amino-substituted polyvinyl phenol derivatives present in an amount in total in a range of 0.05 wt. % to 10 wt. %.

25. The method according to claim 16, wherein in the second step, the passivating of the modified tinplate surface, by contacting the modified tinplate surface with an acidic aqueous composition produces a coating weight of at least 0.3 mg/m.sup.2, but no more than 30 mg/m.sup.2, based on the elements Zr, Ti, Hf and/or Si, on the modified tinplate surface.

26. The method according to claim 16, wherein the anodic polarization in the first step takes place for at least 0.2 seconds, but in total no longer than 300 seconds at a current density of at least 0.005 A/dm.sup.2, but no more than 6 A/dm.sup.2.

Description

EXEMPLARY EMBODIMENTS

(1) To illustrate the method according to the invention, cleaned tinplate (tin coating 2.8 g/m.sup.2) was first pretreated electrolytically and then rinsed with distilled water, after which a wet film of a passivating agent was applied using a Chemcoater and dried at 50 C. for 1 min. The corresponding series of tests are listed in Table 1.

(2) The tinplates treated in this way, without a topcoat, were half immersed in a potassium sulfide solution (5 g/l K.sub.2S+5 g/l NaOH in water) for 1 min at 90 C., rinsed with water and dried.

(3) The blackening of the tinplates was evaluated optically according to the following scale:

(4) 0: no discoloration; metallic sheen

(5) 1: individual black discolorations; <10% of the surface

(6) 2: speckled black discolorations; <30% of the surface

(7) 3: speckled black discolorations; <50% of the surface

(8) 4: speckled black discolorations>50% and almost complete loss of metallic sheen

(9) 5: speckled black discolorations>50% and complete loss of metallic sheen

(10) The results in terms of black discolorations after contact of the sheets with the potassium sulfide solution (sulfide test) are listed in Table 2.

(11) TABLE-US-00001 TABLE 2 Results of the sulfide test Test number Sulfide test/scale 0-5 E1 1 E2 0 CE1 3 CE2 2

(12) It can be seen from Table 2 that, in a direct comparison, the method according to the invention which the anodic polarization followed by passivation with an acidic composition containing water-soluble compounds of Zr and Ti gives a significantly better result in terms of resistance of the tinplate surface to black discolorations than a sequence of methods known in the prior art consisting of anodic polarization and subsequent chromating (cf. E1-CE1). In addition, it is clear that anodic polarization in electrolytes containing water glass is particularly advantageous and, in the method according to the invention, produces tinplate surfaces which are completely inert in the sulfide test and display an unchanged metallic sheen.

(13) TABLE-US-00002 TABLE 1 Test series for the anodic polarization of tinplate (tin coating weight 2.8 g/m.sup.2) and subsequent passivation Anodic polarization Test Electrolyte Current Time/ Post number (75 g/l) density/Adm.sup.2 seconds passivation E1 Na.sub.2CO.sub.3 3.5 60 Ti, Zr E2 Sodium water glass 3.5 60 Ti, Zr 37/40 CE1 Na.sub.2CO.sub.3 3.5 60 Cr(VI) CE2 Sodium water glass 3.5 60 Cr(VI) 37/40 Ti, Zr: Passivation with 7 wt. % Granodine 1456 (Henkel) corresponding to 770 ppm Ti and 500 ppm Zr; Coating weight of titanium: 3 mg/m.sup.2 measured by X-ray fluorescence analysis (Axio Advanced, Panalytical), corresponding additionally to approx. 2 mg/m.sup.2 coating weight of zirconium Cr(VI): Chromating (0.12 wt. % CrO.sub.3); Coating weight of chromium: 3 mg/m.sup.2 measured by X-ray fluorescence analysis (Axio Advanced, Panalytical)