METHOD FOR SEALING OXIDE PROTECTIVE LAYERS ON METAL SUBSTRATES

Abstract

A method for sealing oxide protective layers on metal substrates using aqueous compositions containing a copolymer or a copolymer mixture of at least one aliphatic and acyclic alkene with at least one ,-unsaturated carboxylic acid in a water-dispersed and/or water-dissolved form with a copolymer or copolymer mixture acid number of at least 20 mg KOH/g, but not more than 200 mg KOH/g, the invention also relates to the use of such copolymers or such a copolymer mixture for sealing protective layers based on oxides and/or hydroxides of the elements Si, Ti and/or Zr on an aluminum substrate where the protective layer has a thickness of at least 2 microns.

Claims

1. A method for sealing oxide protective layers on metal substrates, wherein the metal substrate provided with the oxide protective layer is subjected to the following steps: (i) contacting with an aqueous composition for sealing, comprising: a) at least 1 wt. % of a copolymer or of a copolymer mixture of at least one aliphatic and acyclic alkene with at least one ,-unsaturated carboxylic acid in water-dispersed and/or water-dissolved form, based on the aqueous composition, wherein of the copolymer or the copolymer mixture has an acid number that is at least 20 mg KOH/g, but no more than 200 mg KOH/g; and b) a cross-linking agent; and subsequently (ii) drying and/or curing takes place while supplying thermal energy to thereby form a cured seal; wherein the aqueous composition for sealing in step (i) has a flow time of no more than 50 seconds, measured by way of a 4 mm DIN flow cup.

2. The method according to claim 1, wherein the copolymer or the copolymer mixture is present in an amount of at least 5 wt. %, but no more than 30 wt. %, based on the aqueous composition.

3. The method according to claim 1, wherein the cross-linking agent b) is selected from water-soluble inorganic compounds of the elements Zr and/or Ti, from water-soluble and/or water-dispersible aminoplasts and/or carboimides.

4. The method according to claim 1, wherein the at least one aliphatic and acyclic alkene of the copolymer or the copolymer mixture of the aqueous composition for sealing in step (i) is selected from ethene, propene, 1-butene, 2-butene, isobutene, 1,3-butadiene and/or 2-methylbuta-1,3-diene.

5. The method according to claim 1, wherein the at least one aliphatic and acyclic alkene of the copolymer or the copolymer mixture of the aqueous composition for sealing in step (i) is selected from ethene and/or propene.

6. The method according to claim 1, wherein the ,-unsaturated carboxylic acid of the copolymer or of the copolymer mixture of the aqueous composition for sealing in step (i) is selected from cinnamic acid, crotonic acid, fumaric acid, itaconic acid, maleic acid, acrylic acid and/or methacrylic acid.

7. The method according to claim 1, wherein the ,-unsaturated carboxylic acid of the copolymer or of the copolymer mixture of the aqueous composition for sealing in step (i) is selected from acrylic acid and/or methacrylic acid.

8. The method according to claim 1, wherein at least 20%, but no more than 60% of the acid groups of the copolymer or of the copolymer mixture in the aqueous composition for sealing is present in neutralized form.

9. The method according to claim 3, wherein at least 30%, but no more than 50% of the acid groups of the copolymer or of the copolymer mixture in the aqueous composition for sealing is present in neutralized form.

10. The method according to claim 4, wherein at least 20%, but no more than 50% of the acid groups of the copolymer or of the copolymer mixture in the aqueous composition for sealing is present in neutralized form.

11. The method according to claim 1, wherein the contacting step (i) takes place by applying a wet film of the aqueous composition for sealing by a spraying or by a dipping process.

12. The method according to claim 1, wherein the contacting step (i) takes place by applying a wet film of the aqueous composition for sealing by a dipping process, wherein, after termination of the dipping process, excess wet film is removed before the drying and/or curing step (ii) is carried out, such that a wet film adhering in a homogeneous film thickness is formed.

13. The method according to claim 1, wherein the metal substrate is selected from aluminum, magnesium and/or titanium.

14. The method according to claim 1, wherein the oxide protective layer is substantially made of oxides and/or hydroxides of the elements Si, Ti, Zr, Nb, Ta and/or Sn.

15. The method according to claim 1, wherein the metal substrate is selected from, magnesium and/or aluminum and the oxide protective layer is substantially made of oxides and/or hydroxides of the elements Si, Ti and/or Zr.

16. The method according to claim 1, wherein the supplying of thermal energy in step (ii) results in a peak temperature of the metal substrate of at least 120 C., but less than 200 C. (PMT, so-called peak metal temperature).

17. The method according to claim 1, wherein, after step ii), an amount of organic solid components of the aqueous composition for sealing remains on the oxide protective layer such that, with complete pyrolysis, the cured seal releases an amount of at least 1 g CO.sub.2, but no more than 20 g CO.sub.2.

18. The method according to claim 8, wherein the metal substrate is an aluminum substrate and the protective layer is based on oxides and/or hydroxides of the elements Si, Ti and/or Zr and has a thickness of at least 2 m.

19. The method according to claim 18, wherein the protective layer is based on oxides and/or hydroxides of the element Ti and has a density of less than 3.5 g/cm.sup.3.

Description

EXEMPLARY EMBODIMENTS

[0080] The formulations in Table I were used to seal aluminum sheets (AA6014; test sheets from Chemetall) coated by way of plasma electrolysis.

[0081] For this purpose, initially cleaned and degreased aluminum sheets were potentiostatically coated for 3 minutes in an electrolyte (pH value 2.5) comprising 4.5 g/L phosphoric acid and 12 g/L hexafluorotitanic acid at a voltage of 435 V. The resulting titanium oxide- and titanium hydroxide-based protective layer had a layer thickness of 10 to 12 m, measured by way of an eddy current probe in accordance with DIN-ISO 2360 (DUALSCOPE@ MP40E-S with measuring probe ED10; Fischer).

[0082] The sheets thus coated were then dipped for 30, 60 or 120 seconds into formulations A to C and subsequently hung for one minute for dripping. The wet film remaining on the metal sheets after dripping was then cured in the furnace at 230 C. for 2 minutes. The layer thickness of the cured seal was approximately 8 m after measurement according to the aforementioned eddy current method.

TABLE-US-00001 TABLE 1 Aqueous formulations A to C for sealing, comprising an ethylene-acrylic acid copolymer A B C Ethylene acrylic acid copolymer 5.8 5.8 5.6 (18 to 20 wt. % acrylic acid) Dimethylethanolamine 1.4 1.4 Ammonia (15 wt. %) 0.7 Bacote 20 .sup.1 (Mel 2.0 Chemicals) Cymel 327 .sup.2 (Allnex) 4.0 4.0 Polyacrylate .sup.3 0.5 0.5 0.5 4 mm DIN flow time in seconds 22 22 22 .sup.1 Basic ammonium zirconium carbonate solution (20 wt. % Zr) .sup.2 Melamine resin (drying over 20 minutes at 230 C.) .sup.3 Thickener having a pH value of 2.5; 30 to 70 mPas according to ISO 2555

[0083] The sealed plasma electrolytically coated aluminum sheets were subjected to a CASS test (240 hours) in accordance with DIN EN ISO 9227. After the loading time, all sealed metal sheets had a degree of blistering (0 (S0) to 5 (S5)) in accordance with DIN EN ISO 4628-1 of 0 (S0), and a degree of rusting (0 to 5) in accordance with DIN EN ISO 4628-3 of maximally 1. The unsealed plasma electrolytically coated aluminum sheet exhibited a degree of rusting of 5.