AU2004252269A1

AU2004252269A1 - Composition for forming conversion coatings on aluminium surfaces

Info

Publication number: AU2004252269A1
Application number: AU2004252269A
Authority: AU
Inventors: Richard Franklyn Foster; Alison Jane Hartopp
Original assignee: NATECH Ltd
Current assignee: NATECH Ltd
Priority date: 2003-06-26
Filing date: 2004-06-24
Publication date: 2005-01-06
Also published as: BRPI0411926A; GB0314901D0; JP2007527465A; EP1639151A1; WO2005001158A1

Description

WO 2005/001158 PCT/GB2004/002720 1 COMPOSITION FOR FORMING CONVERSION COATINGS ON ALUMINIUM SURFACES This invention concerns a coating material, a coating material concentrate, and a method of coating, and particularly but not exclusively a 5 material or method for coating surfaces of aluminium or aluminium alloys. It is known to coat aluminium surfaces with aqueous coating solutions that are effective in forming thereon coatings which are corrosion resistant and thereby protect the surface from degradation due to attack by corrosive 10 materials. In general, the coatings formed from such coating solutions should also have properties such that overlying coatings which are applied thereto adhere tightly and strongly. Such overlying coatings are decorative or functional in nature and are formed from materials such as paints, lacquers, inks, etc. 15 An example of an aluminium coating operation is the coating of aluminium cans. In general, the corrosion resistant and adherent coatings which are applied to aluminium cans should be uniformly clear and colourless so that the coated cans have the bright shiny natural appearance of the 20 underlying aluminium. This bright shiny natural appearance is desired in the final product even though portions of the can may be covered with coloured paints or inks. Another specific property that coated aluminium cans should have is 25 the ability to resist discoloration when the coated can is subjected to moderately hot water, for example, water having a temperature within the range of about 60 to 80'C. In certain applications aluminium cans are treated in this way to achieve "pasteurisation" of the cans. This treatment has a tendency to cause an uncoated or even a coated aluminium surface to 30 blacken or otherwise discolour thereby leaving the can with an unattractive appearance. The most widely used coating solutions for aluminium cans contain a source of zirconium, free fluoride ions and a source of vegetable tannin.

WO 2005/001158 PCT/GB2004/002720 2 While such a coating solution is capable of forming coatings of the type desired, their use creates waste disposal and workplace hazard problems because of the presence therein of the highly corrosive and toxic fluoride ions. 5 Additional considerations of importance for the treatment bath which is applied to the aluminium and which provides the coating are: (1) the energy utilised by the method of application of the conversion coating, including the temperature at which the coating is applied; (2) the quality of the resulting coating; and (3) the control necessary to achieve the desired coating. A high 10 quality coating with short application times and relatively low process temperatures is most desired. Coatings which produce satisfactory results with respect to corrosion, adhesion and colour, must do so within permissible treatment times which, 15 under current manufacturing conditions are short, generally about 10 to 20 seconds. This has usually been achieved by utilising free fluoride ions, which present corrosion and toxic hazards to users and are detrimental to the environment. 20 According to the present invention there is provided a coating material, the material comprising a coating solution usable on aluminium surfaces, the solution being acidic with a pH of between 2 and 4.5, and including: 0.01 - 0.1 5g per litre zirconium ions; 25 0.05 - 0.5g per litre polysaccharide; 0.01 - 1Og per litre phosphate; and 0.01 - 1 Og per litre of an alkali salt of carboxylic acid, or of carboxylic acid. 30 The solution is preferably aqueous. The coating material is preferably such as to not require the presence of free fluoride ions for its effectiveness.

WO 2005/001158 PCT/GB2004/002720 3 The material is preferably substantially free of vegetable tannins. The material preferably provides a substantially transparent coating when applied. 5 The zirconium ions may be obtained from fluorozirconic acid, but may be obtained from alkali metal or ammonium fluorozirconates, or zirconium fluoride, nitrates or carbonates. 10 The solution preferably includes nitric acid, the nitric acid concentration being preferably between 0.05g per litre and 0.2g per litre. The solution may include a complex fluoride. The complex fluoride may be fluorosilicic acid, fluoroboric acid, or a similar acid. The complex 15 fluoride concentration is preferably between 0.05 and 0.2g per litre. The polysaccharide may be polyglucoside. The polyglucoside is preferably derived from any of starch, pectins, alginates and gums from plants, trees and seaweed by hydrolysis using acid or enzymes, either 20 synthetic or those produced by yeasts and bacteria. The polysaccharide is preferably alkylated, ethoxylated or propoxylated. 25 The invention also provides a concentrate for making a coating solution usable on aluminium surfaces, the concentrate including: 2 - 1Og per litre zirconium ions; 5 - 20g per litre polysaccharide; 30 2.5 - 20g per litre phosphate; and 2 - 50g per litre of an alkali salt of carboxylic acid, or of carboxylic acid.

WO 2005/001158 PCT/GB2004/002720 4 The invention further provides a method of coating an aluminium or aluminium alloy surface, the method comprising using a material according to any of the preceding eleven paragraphs. 5 The coating may be formed from a concentrate as previously defined, and nitric acid may be added to the concentrate. The solution may be sprayed onto the surface. The coating may take place at a temperature of between 20 and 400C, and desirably between 25 and 350C. The coating may take place for between 10 and 60 seconds. 10 Embodiments of the present invention will now be described by way of example only. Five compositions were prepared from concentrates, with the first two 15 being according to the invention and the others included for reference. These were applied by spraying onto previously cleaned, drawn and ironed aluminium cans. Following spraying the cans were rinsed with tap water and then cold deionised water and dried for 2 minutes at 1800C. The spraying was carried out at 380C for 40 seconds with a spray pressure of 1 bar. The 20 contents of the compositions are given in Table 1 below where the rows are for the composition and the columns are respectively: A) The zirconium ion content in grams per litre; B) The polysaccharide content in grams per litre; 25 C) The carboxylic acid content in grams per litre; and D) The phosphate content in grams per litre. Table 1 Composition A - B C D 1 0.05 0.1 0.05 0.11 2 0.05 0.1 0.05 0.5 WO 2005/001158 PCT/GB2004/002720 5 3 0.05 0.1 0 0.11 40.05 0 0.05 0.11 5 0.05 0 0 0.11 Four tests were carried out on the coated cans as indicated below. Muffle Test 5 After cleaning, treating and drying, the cans were placed in a muffle furnace at around 5250C for 5 minutes. The discolouration of the aluminium surface was observed. A golden coloured can being desirable. A light muffle indicates inadequate treatment, a dark muffle indicates over treatment. 10 Dome Stain Test After cleaning, treating and drying, the cut off domes of the can were placed in a 0.3g per litre solution of Borax at 800C, for 20 minutes. Any 15 surface discolouration of the aluminium surface was observed. A discoloured dome being a failure. Tape Adhesion Test 20 The tape adhesion test was performed to measure the adhesion between a cleaned surface and an organic finish or overcoating. White ink from BASF was applied, using a rubber brayer. Water-borne, wet-ink varnish from PPG Company, was roll-coated with a #1 0 draw-down bar to achieve a coating thickness of 3gm 2 . The coated surface was cured in a forced-air oven 25 for 90 seconds at about 177'C. The finished (i.e. painted) surface, after being cured, was immersed in boiling tap water or a 1% detergent (such as "JOY", a commercially available PROCTOR & GAMBLE product) solution for 15 minutes, rinsed in tap water, and dried. The treated surface was then cross hatched, and SCOTCH brand transparent tape (#61 0) (commercially 30 available from 3M) was applied to the cross-hatched area. The amount of WO 2005/001158 PCT/GB2004/002720 6 paint removed by the tape was observed, and the results were rated as follows: 10: excellent adhesion 5 8-9: very slight removal 5-8: obvious removal 1-5: very obvious removal 0 complete removal. 10 Lacquer Adhesion Test This check measures lacquer integrity. Results are given in milliamps, with 2 milliamps being the maximum reading allowed for a single can. Tests were carried out on 50 cans taken at random through the test run. The result 15 is the number of cans that gave a reading above 2 milliamps. The results of these tests are provided in Table 2 below. Table 2 20 Muffle Test Dome Stain Tape Adhesion Lacquer Adhesion 1 Golden brown Pass 10 0 2 Dark brown Pass 9 0 3 Light brown Slight Stain 10 1 4 Very light brown Fail 9 3 5 No brown colour Fail 8 2 The invention therefore provides coatings with good properties which for instance avoid discolouration if pasteurisation is carried out. The materials according to the invention are free of fluoride ions which prevent corrosion WO 2005/001158 PCT/GB2004/002720 7 and toxic hazards generally encountered when fluoride ions are present. Furthermore, waste disposal problems are substantially avoided. Furthermore, coatings according -to the present invention can be 5 provided at relatively low temperatures and with short process times. With coatings according to the invention it is possible to utilise high levels of phosphate to give good coatings which are very resistant to corrosion especially to darkening for example during pasteurisation. This is enabled by the polysaccharide, which prevents high levels of phosphate from causing a 10 hydrophobic effect on the conversion coating layer and hence inhibiting the process. Moreover, the treating bath that produces the coating is not as sensitive to "cleaning solution drag", the proclivity of a cleaning solution to 15 undesirably alter the pH of the treating bath for the conversion coating. For example "drag-in" from an acidic cleaning solution used to clean cans of lubricant and fines in a can production line will lower the pH of a subsequent treating bath for a conversion coating, below the commercially operable limits of the bath. 20 A further advantage provided is the inhibition of dissolution of aluminium from the can during conversion coating. This ensures that the total amount of aluminium released into the solution is much less than with previous arrangements. This in turn means that there does not have to be an 25 excess of free fluoride in the solution to sequester aluminium which would otherwise' react with the source of zirconium and inhibit the conversion coating process. As already mentioned, the polysaccharide inhibits dissolution of 30 aluminium from the can surface during conversion coating. The small amounts of aluminium that are released are sequestered by the carboxylic acid or its derivative, which avoids precipitation of aluminium from the bath. This also means that very bright and shiny cans are produced by the invention as there is no fluoride to etch the can. The polysaccharide improves the WO 2005/001158 PCT/GB2004/002720 8 conversion coating, giving improved crystal structure and a more even coating. This allows optimum adhesion for subsequent applied coatings. Various modifications may be made without departing from the scope 5 of the invention. For instance, coating materials according to the invention could be used with solvent-based systems in addition to aqueous systems. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it 10 should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A coating material, the material comprising a coating solution usable on 5 aluminium surfaces, the solution being acidic with a pH of between 2 and 4.5, and including: 0.01 - 0.1 5g per litre zirconium ions; 0.05 - 0.5g per litre polysaccharide; 10 0.01 - 1 Og per litre phosphate; and 0.01 - 10g per litre of an alkali salt of carboxylic acid, or of carboxylic acid.

2. A coating material according to claim 1, characterised in that the 15 coating solution is aqueous.

3. A coating material according to claims 1 or 2, characterised in that the coating material is such as to not require the presence of free fluoride ions for its effectiveness. 20

4. A coating material according to any of the preceding claims, characterised in that the coating material is substantially free of vegetable tannins, 25

5. A coating material according to any of the preceding claims, characterised in that the coating material provides a substantially transparent coating when applied.

6. A coating material according to any of the preceding claims, 30 characterised in that the zirconium ions are obtained from fluorozirconic acid.

7. A coating material according to any of claims 1 to 5, characterised in that the zirconium ions are obtained from alkali metal or ammonium fluorozirconates, or zirconium fluoride, nitrates or carbonates. WO 2005/001158 PCT/GB2004/002720 10

8. A coating material according to any of the preceding claims, in which the solution includes nitric acid. 5

9. A coating material according to claim 8, characterised in that the nitric acid concentration is between 0.05g per litre and 0.2g per litre.

10. A coating material according to any of the preceding claims, characterised in that the coating solution includes a complex fluoride. 10

11. A coating material according to claim 10, characterised in that the complex fluoride is fluorosilicic acid, fluoroboric acid, or a similar acid.

12. A coating material according to claims 10 or 11, characterised in that 15 the complex fluoride concentration is between 0.05 and 0.2g per litre.

13. A coating material according to any of the preceding claims, characterised in that the polysaccharide is polyglucoside. 20

14. A coating material according to claim 13, characterised in that the polyglucoside is derived from any of starch, pectins, alginates and gums from plants, trees and seaweed by hydrolysis using acid or enzymes, either synthetic or those produced by yeasts and bacteria. 25

15. A coating material according to any of the preceding claims, characterised in that the polysaccharide is alkylated, ethoxylated or propoxylated.

16. A concentrate for making a coating solution usable on aluminium 30 surfaces, the concentrate including: 2 - 1 Og per litre zirconium ions; 5 - 20g per litre polysaccharide; 2.5 - 20g per litre phosphate; and WO 2005/001158 PCT/GB2004/002720 11 2 - 50g per litre of an alkali salt of carboxylic acid, or of carboxylic acid.

17. A method of coating an aluminium or aluminium alloy surface, characterised in that the method comprises using a material according to any 5 of claims 1 to 15.

18. A method according to claim 17, characterised in that the coating material is formed from a concentrate according to claim 16. 10

19. A method according to claim 18, characterised in that nitric acid is added to the concentrate.

20. A method according to any of claims 17 to 19, characterised in that the coating solution is sprayed onto the surface. 15

21. A method according to any of claims 17 to 20, characterised in that the coating takes place at a temperature of between 20 and 400C.

22. A method according to claim 21, characterised in that the coating takes 20 place at a temperature of between 25 and 350C.

23. A method according to any of claims 17 to 22, characterised in that the coating takes place for between 10 and 60 seconds. 25

24. A coating material usable on aluminium surfaces, the material being substantially as hereinbefore described.

25. A concentrate for making a coating solution usable on aluminium surfaces, the concentrate being substantially as hereinbefore described. 30

26. A method of coating an aluminium or aluminium alloy surface, the method being substantially as hereinbefore described. WO 2005/001158 PCT/GB2004/002720 12

27. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.