IMPROVED METHOD FOR APPLYING SILANE-BASED COATINGS ON SOLID SURFACES, IN PARTICULAR ON METAL SURFACES

Abstract

Described herein is an improved method for applying silane-based coatings to solid surfaces, in particular metal surfaces. Also described herein are a silane-containing composition, a solid surface, in particular a metal surface, including a silane-based coating, and a method of using the silane-based coating in the transportation industry or in electrically conductive assembling.

Claims

1. A method for applying a silane-based coating to a solid surface, characterized in that the solid surface is: i) optionally cleaned, etched and/or desmutted, ii) brought into contact with at least one unhydrolyzed silane such that an unhydrolyzed silane layer is formed on the solid surface, iii) brought into contact with water such that the silane layer is at least partially hydrolyzed, iv) at least partially dried such that residues of water and alkanol are at least partially removed from the solid surface, v) optionally heated such that the at least partially hydrolyzed and least partially dried silane layer is cured, and vi) in case that step v) is conducted, optionally painted.

2. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is selected from the group consisting of sulfur-containing silanes.

3. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is not stable in water-based solutions at all and is only stable in organic-solvent-based solutions.

4. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is mixed with at least one corrosion inhibitor and then applied together with the at least one corrosion inhibitor to the solid surface.

5. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is mixed with at least one water-free and water-insoluble electrically conductive powder before applying the at least one unhydrolyzed silane to the solid surface.

6. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is not mixed with organic solvents.

7. The method according to claim 1, characterized in that in step iii) the solid surface is brought into contact with water by immersion of the solid surface into water.

8. The method according to claim 1, characterized in that the contact time in step iii) lies in the range of 8 to 330 seconds.

9. The method according to claim 1, characterized in that step iv) is conducted by air-blowing or by wiping.

10. The method according to claim 9, characterized in that the solid surface is kept for at least 15 seconds to allow water dropping after step iii) and before step iv).

11. The method according to claim 1, characterized in that step v) is conducted by an oven.

12. The method according to claim 1, characterized in that the solid surface is painted not before one week.

13. A silane-containing composition for applying silane-based coatings to solid surfaces, characterized in that it contains a) at least one unhydrolyzed silane and b) at least one corrosion inhibitor and/or at least one water-free and water-insoluble powder, wherein the composition does not contain water.

14. A solid surface with a silane-based coating characterized in that it is obtained by the method according to claim 1, wherein the silane-based coating exhibits an average thickness of at least 100 nanometers.

15. A method of using the solid surface according to claim 14, the method comprising using the solid surface in the transportation industry or in electrically conductive assembling.

16. The method according to claim 1, wherein the solid surface is a metal surface.

17. The method according to claim 1, wherein the solid surface is an anodized or conversion-coated metal surface.

18. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is selected from the group consisting of polysulfane silanes and mercapto silanes.

19. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is mixed with benzotriazole, and then applied together with the benzotriazole to the solid surface.

20. The method according to claim 1, characterized in that the at least one unhydrolyzed silane is mixed with at least one water-free and water-insoluble electrically conductive powder containing graphite, graphene, zirconium oxide, titanium oxide, silicon oxide, silicon carbide and/or aluminum oxide, before applying the at least one unhydrolyzed silane to the solid surface.

Description

EXAMPLES

[0088] Oxsilan® MG-0611, which is a mixture of unhydrolyzed bi-silanes available from Chemetall GmbH (Germany), was used in the examples.

Stability of Specific Silanes in Water-Based Solution

[0089] Comparative Solution no. 1 was prepared in accordance with the manufacturer instruction: 50 ml of Oxsilan® MG-0611 were mixed with 50 ml of deionized water and stirred for four hours. Then 900 ml of a 1:1 mixture of Dowanol® PM and Dowanol® PnB glycol ether solvents (Dow, USA) were added to the solution of hydrolyzed silanes and mixed.

[0090] Comparative Solution no. 2 was prepared by addition of 50 ml of Oxsilan® MG-0611 to 950 ml of deionized water during mechanical stirring.

[0091] The stability of both solutions was visually checked. Comparative Solution no. 1 was still clear without any evidence of silane condensation after 6 months, whereas, Comparative Solution no. 2 became completely milky already after 10 minutes due to full condensation of the contained silanes.

Preparation of Inventive Solution

[0092] The Inventive Solution was prepared by addition of 5 gram of Irgamet® BTZ corrosion inhibitor (BASF, Germany) to one liter of Oxsilan® MG-0611. The resulting mixture was stirred until full dissolution of the inhibitor.

Blank Corrosion Resistance

[0093] Standard AA2024-T3 bare aluminum panels (available from Constellium company, The Netherlands) were cleaned in Ardrox® 6490, alkaline etched in Oakite® 160 and desmutted in Ardrox® 295 GD (all solutions available from Chemetall GmbH, Germany). Subsequently, the panels were immersed into the Inventive Solution for 5 minutes. The resulting silane layer was then hydrolyzed by immersion of the panels into deionized water in accordance with the following Tab. 1. Three panels were treated in every batch.

TABLE-US-00001 TABLE 1 Batch no. 1 2 3 4 5 6 7 8 9 Contact time 10 30 60 90 120 150 180 240 300 (sec)

[0094] Batch no. 10 was immersed into Comparative Solution no. 1 containing pre-hydrolyzed silanes (see above) for 5 minutes. A subsequent immersion into deionized water, i.e. an additional hydrolysis, was not conducted.

[0095] After one minute for water dropping, the panels were air-blown to reduce residues of water or—in case of batch no. 10—of treatment solution and dried in an oven at 120° C. for 30 minutes.

[0096] Subsequently, the panels were cooled down to room temperature, stored for one week and then tested in a Neutral Salt Spray (NSS) test in accordance with ASTM B117 standard. The test results were evaluated in accordance with MIL-DTL-5541E standard.

[0097] Batches no. 1, 2, 6, 7, 8, 9 showed more than 5 pits after 168 hours. However, the corrosion was only in form of small isolated pits. The reference panels (batch no. 10) were significantly corroded already after 48 hours and, by far, showed the worst result in the test: 100% of the surface was corroded after 168 hours. In contrast to that, batches no. 3, 4 and 5 showed less than 5 small isolated pits after 168 hours, i.e. no or only minor corrosion.

[0098] Thus, when applying an inventive solution containing unhydrolyzed silanes with the inventive method, significantly improved blank corrosion resistance was achieved in comparison with applying a comparative solution containing pre-hydrolyzed silanes.

[0099] Moreover, there was an optimal window for the contact time of the silane layer with the deionized water achieving a maximum of protection. It has surprisingly been found that the further exposure to deionized water seems to at least partially remove the silane layer.

Investigation of “in Place” Hydrolyzed Silane Layer

[0100] Uncoated panels and the panels of batch no. 3 exhibiting a hydrolyzed and cured silane-based coating (obtained as described above) were investigated using Infrared Reflection Absorption Spectroscopy (IRRAS). As for the panels of batch no. 3, the spectra showed clear evidence of the Si—O—Si (siloxane) compound spectrum at approximately 1060 cm.sup.−1 and approximately 1130 cm.sup.−1, which is not the case for the uncoated panels.

[0101] The Inventive Solution and the panels of batch no. 3 were investigated using Attenuated Total Reflection (ATR). The coated panels of batch no. 3 showed clear evidence of —OH group spectrum at approximately 3300 cm.sup.−1 which is not observed for the Inventive Solution. This result proves the hydrolysis “in place” as well as the presence of active silanol molecules in the polysiloxane layer when the method of the invention is used.

Use of Inventive Solution for Sealing of Anodic Layers

[0102] The Inventive Solution was prepared in accordance with the procedure described in the example “Preparation of Inventive Solution”. 5 panels (for each alloy) of AA2024-T3 and AA7075-T6 were anodized in a tartaric sulfuric anodizing process in accordance with an aerospace specification, rinsed and then immersed into the Inventive Solution for 5 minutes. Then, the panels were immersed into deionized water for 1 minute to hydrolyze the silane layer. After 1 minute for water dropping, the panels were air-blown to reduce residues of water and dried in an oven at 120° C. for 30 minutes. The panels were tested in the salt spray chamber in accordance with ASTM B117 standard for 1008 hours to evaluate anticorrosion performance.

[0103] After 1008 hours in the test, the AA2024-T3 panels only showed a minor amount of very small pits, whereas, the AA7075-T6 panels did not show any corrosion.

[0104] These results are significantly better than the 336 hours with a maximum of 5 isolated pits each having a diameter of not more than 0.031 inch required by the aerospace specification and/or MIL-A-8625 standard for sealed anodic layers.

Anticorrosion Performance and Electrical Resistance Obtained with Graphene Additive

[0105] The Inventive Solution was prepared in accordance with the procedure described in the example “Preparation of Inventive Solution. Then, 25 gram of graphene powder (available from Talga) were added to 1 liter of the Solution and properly mixed, wherein the Solution changed the color from yellow to black. The stability of the such prepared Inventive Solution was checked by the naked eye after 2 weeks: The Solution remained black without any visible precipitation.

[0106] 2 standard panels (for each alloy) of AA2024-T3 and AA7075-T6 were cleaned in Ardrox® 6490, alkaline etched in Oakite® 160 and desmutted in Ardrox® 295 GD (all solutions available from Chemetall GmbH, Germany). Then, the panels were immersed into the Inventive Solution containing graphene for 5 minutes. The resulting silane layer was then hydrolyzed by immersion of the panels into deionized water for 1 minute. After 1 minute for water dropping, the panels were air-blown to reduce residues of water and dried in an oven at 120° C. for 30 minutes. The panels were cooled down to room temperature, stored for 1 week and then tested in a Neutral Salt Spray (NSS) test in accordance with ASTM B117 standard for 168 hours. Both sets of panels did not show any evidence of corrosion after the test.

[0107] The test results showed, that the graphene additive improved anticorrosion protection for AA2024-T3 alloy: Without graphene, there were small isolated pits—mainly close to the panel edges—already after 120 hours. In case of AA7075-T6 alloy, the samples passed 168 hours in the test also without graphene.

[0108] Then, the same panels were tested for electrical resistance by means of the special test device MRP 29 manufactured by Schuetz GmbH, Germany. All tested samples showed an average electrical resistance of less than 1000 μΩ. Thus, the results are significantly better than required by the MIL-DTL-5541E standard for electrically conductive coatings being 10.000 μΩ maximum after 168 hours of Salt Spray test.