GB2090617A - Processes and Compositions for Coating Metals - Google Patents

Abstract

A protective coating is formed on an aluminium surface by contacting the surface with a chromium-free aqueous acidic solution containing tannin and an acrylic resin, and generally also phosphoric acid and/or hydrogen-fluoride.

Description

SPECIFICATION Processes and Compositions for Coating Metals It is standard practice to form conversion coatings on metals such as iron, zinc and aluminium in order to provide corrosion resistance and/or improved adhesion to a subsequently applied paint coating, for instance of ink, varnish, lacquer or any other organic coating material. Typical treatments involve cleaning, conversion coating, post-treating, drying and painting, in separate discrete steps separated by water rinses.

The conversion coating step generally employed phosphoric acid to oxidise the surface metal and to form a metal phosphate, often of a metal included in the composition, and the step was given the name "phosphatizing".

Phosphatizing tended to protect the metal from corrosion and to provide an interfacial substrate for various paint applications. Post-treatment of the phosphatized surface with ionic chromium has been employed widely as it tends to augment the corrosion resistance and the adhesion of the subsequently applied paint coatings. Thus the paint base provided by the post-treatment has provided an improved coating that is more resistant to corrosive, humidity, impact and bending problems than non-post-treated surfaces, especially when they are formed of aluminium. However the use of chromium involves environmental problems.

It has now surprisingly been found that it is possible to provide satisfactory coatings on aluminium without the use of chromium and without a conventional phosphatizing treatment. In the invention a protective coating is formed on an aluminium surface by contacting the surface with an aqueous acidic solution that is free of chromium and that comprises tannin and an acrylic resin.

It is possible by the invention to obtain coatings having satisfactory corrosion resistance and paint adhesion properties, and these properties can be obtained without the use of ionic chromium in the coating composition and without the use of a chromium-containing, or indeed any other, posttreatment composition. Thus the invention has the advantage that it avoids the use of chromium and the associated environmental problems that this causes. In particular the invention results in the possibility of using a chromium-free single application composition to form a coating which adheres closely to the surface of aluminium metal to protect the metal from corrosion and discolouration while allowing the unpainted metal to retain substantially the colour and surface characteristics of uncoated aluminium.Additionally the coating provides a conversion coating which obviates the need for posttreatment and provides a paint base for various decorative and protective coatings, for instance of types known in the industry, and provides satisfactory resistance to corrosion, humidity, impact problems and chipping caused by metal flex.

The acrylic resin may be provided in solution or dispersion in the coating composition but is preferably provided as an emulsion. The acrylic resin is preferably a resin that is film forming at ambient temperatures and in any event at the temperatures used during the process. It may be a resin that can cross-link during the process, often with the tannin or with a crosslinking component in the resin. The resin may be derived from monomers such as acrylic acid or methacrylic acid or esters or amides of these.

The alkyl group in the ester moiety may be an optionally substituted alkyl group containing from 1 to 20 carbon atoms.

Suitable unsubstituted alkyl groups include methyl, ethyl, propy, isopropyl, butyl, tert-butyl, pentyl, hexyl, ethyihexyl, octyl, decyl, dodecyl and glycidyl. Suitable examples of hydroxy substituted alkyl groups are hydroxy ethyl, hydroxy propyl, hydroxy butyl, hydroxy pentyl, hydroxy hexyl, hydroxy octyl, hydroxy decyl and hydroxy dodecyl. Suitable examples of amino alkyl groups are amino methyl, amino ethyl, amino propyl, amino butyl, amino pentyl, amino hexyl, amino octyl, amino decyl and amino dodecyl The amino group in the aminoalkyl group may be a primary amino or a secondary amino in which the substituent on the amino may be an alkyl group of from 1 to 6 carbons, preferably 1 to 4 carbon atoms.Suitable examples of mercapto alkyl groups are methyl mercapto, ethyl mercapto, propyl mercapto, butyl mercapto, pentyl mercapto, hexyl mercapto, octyl mercapto, decyl mercapto, dodecyl mercapto and the like. Suitable examples of the amides are acrylamide, methacrylamide, and N-alkyl acrylamide.

The resins may be copolymers formed from one or more acrylic monomers with or without other copolymerisabie monomers such as ethylenically unsaturated materials such as styrene and its derivatives, and glycidyl (meth)acrylate.

The resin may be formed from the chosen monomer or monomers by known polymerisation techniques such as solution, suspension or, preferably, emulsion polymerisation. The polymerisation reaction mechanism may be cationic, anionic or free radical.

A preferred acrylic resin for use in the invention is the material supplied by IMC Chemical Group Inc. under the trade name Aquamac A607.

Thermoplastic and thermosetting acrylic resins are available for use. However, although we do not wish to be held to any one theory, it is believed that thermosetting compositions are formed. This belief is based on the presence of the polyphenolic tannins which should reasonably be expected to undergo cross-linking reactions with a number of acrylic resins. Cross-linking may also be effected by more traditional means by providing acrylic monomers with functional groups generally considered suitable for cross-linking reactions such as amide, carboxyl, hydroxyl and epoxy. With traditional groups, such as the hydroxy containing acrylics, cross-linking may be effected using melamine, urea, or epoxy crosslinking agents and the like to form a thermoset acrylic.

The resin:tannin weight ratio may vary from 1:5 to 8:1 but is generally from 1:5 to 5:1,with 1:4 to 5:1 often being preferred. Best results are sometimes obtained with a weight ratio 1:4 to 5:7 or 1:5-7 but generally ratios in the range 1:2 to 4:1 are preferred. Both the resin and the tannin should be present in effective corrosion inhibiting amounts. Generally, anywhere from 0.01 to 100 g/l of resin will be satisfactory but 1 to 20 g/l is preferred. As for the tannin, 0.005 or 0.01 to 50 g/l is acceptable but 1 to 10 g/l is preferred.

The tannin is added because of its corrosion inhibiting properties and its tendency to provide a more adhesive coating. The tannins themselves are widely known for their ability to tan skins by combining with collagen and other protein matter to form leather. Although tannins are known to be gallic acid derivatives, their chemistry is not completely understood. These polyphenolic compounds have molecular weights ranging from 400 to 3000 and may be classified either as hydrolyzable, condensed or mixed. The hydrolyzable tannins are solubilised by boiling in mineral acids, while the condensed tannins are insoluble under the same conditions.Many of these tannins are found in nature in bark such as wattle, mangrove, oak, eucalyptus, hemlock, pine, larch, and willow; wood such as quebracho, chestnut, oak and urunday, cutch and turkish; fruits, such as myrobalans, valonia, divi-divi, tera and algarrovilla; leaves, such as sumac and gambier; and roots, such as canaigre and palmetto.

The tannines may be further classified as "vegetable tannins" or "mineral tannins". The vegetable tannins are the non-mineral containing organic tannins described,above, in contrast to mineral tannins which contain inorganic metals, such as chromium and zirconium. Preferably, the post treatment composition of this invention will utilise the vegetable tannins: hydrolyzable, condensed or mixed.

The compositions used in the invention must be acidic and generally have a pH of from 2 to 6.0, most preferably 2.5 to 4.5. The acidity is required since acid conditions tend to foster polymerisation of the acrylic resin and to condition the aluminium surface so that a satisfactory base coating is obtained.

Enough acid should therefore be provided to keep the composition acidic. While various acids may be used for this purpose it is preferred to provide the acidity by incorporating an inorganic acid, most preferably phosphoric acid or hydrofluoric acid. Although the mechanism is not known it is believed that any phosphate and fluoride ions introduced in the acid tend to aid in the formation of a coating that is tightly bound to the aluminium surface. Although mixtures of phosphoric acid and hydrofluoric acid may be used it is generally preferred to use one acid only. The amount of acid that is included in the composition is generally at least 0.01 g/l, most preferably 1 to 5 g/l.

Other ingredients well known in the art, such as accelerators, surfactants, chelating agents, colouring agents, stabilising agents, and the commonly employed metal cations, such as zinc, manganese, cobalt, nickel, and iron, may be added to the composition as long as they do not interfere with the corrosion resistance and adhesion fostering properties of the composition itself.

The compositions may be made by combining the desired amounts of individual ingredients with water or by diluting with water a more concentrated composition.

The aluminium surface to which the composition is applied should be clean, and thus may previously have been cleaned in conventional manner and rinsed. The composition may be applied by known methods such as dipping, rolling on and spraying, although the roll on method is preferred.

When the roll on method is used, it is unnecessary deliberately to preheat the metal substrate.

Although the metal may be at ambient temperature it will usualiy be above ambient because of heat transfer occasioned by pretreatment processes, such as the cleaning and rinsing steps. Similarly, it is unnecessary deliberately to increase the temperature of the coating composition above ambient but the temperatures generated by the processing machinery generally increases the temperature from ambient up to approximately 600 C. In general temperatures between ambient and about 950C can be used. Although it is unnecessary to aim deliberately at the use of temperatures above ambient they do have the property of providing shorter drying times when, as is generally preferred, the solution is dried onto the surface, without any water rinse.

Short contact times are generally sufficient, typically between 0.1 and 40 seconds. For roll on processes contact times of from 1 to 30 seconds are usuaily adequate and indeed times of 1 to 3 seconds are generally satisfactory.

The coating weights may vary from 1 to 30 or even up to 100 milligrams per square foot.

Normally, 5 to 30 milligrams per square foot may be obtained, while typically the coating weight of 5 to 1 5 milligrams/square foot will be found. Wet coating thicknesses of 1 to 3 mils and dry-coating thicknesses of 0.0025 to 0.075 mils are generally suitable.

The following tests have been employed in the examples to evaluate the quality of the treated surface: Salt Spray Corrosion Resistance Salt spray corrosion resistance was measured in accordance with the procedure of ASTM B 1 761. The panels were rated in terms of the amount of paint loss from a scribe in 1/16" (1 .6mm) increments: N for no loss of paint at any point; F for few blisters; VF for very few blisters. The panels may be further rated in terms of the size of the blisters ranging from 9 for very small size to 1 for very large, with 10 representing no blisters, so that F9 indicates that the blisters are few in number and small in size, as in the humidity corrosion resistance test.The principal numbers represent the general range of the creepage from the scribe along its length whereas the superscripts represent spot or nonrepresentative creepage at the point of maximum creepage along the length of the scribe. Thus, O15 means representative creepage varied from 0/16 to 1/16" with a maximum of 1/16" atone or two spots.

Acetic Acid Salt Spray Resistance Acetic acid salt spray resistance was measured in accordance with ASTM B287. Conditions are similar to ordinary salt spray testing except the salt solution is adjusted to pH 3.2 with acetic acid and the chamber is maintained at 350 C. Ratings are given as in the Salt Spray Test.

Humidity Corrosion Resistance Humidity corrosion resistance was measured in accordance with the procedure ASTM 2247- 64T. The panels were rated in terms of the number and size of the blisters, F for few, M for medium, D for dense, VF for very few, and from 9 for very small size to 1 for very large. 10 represents no blisters.

Where the rating is preceded by a G or C, the panel gave a 10 rating except for blisters due to handling (G) or concentration effects (C) such as those which would result from solution run down.

Impact This test is designed to show the effect upon paint adhesion of an impact deformation. A 5/8 inch diameter tool is impacted on the unpainted side of a panel. The force of the impact is approximately 2000 times the panel thickness (e.g. 50 inch-lbs. for a panel 0.025 inch thick). The standard impact test is performed shortly after the paint is cured and at ambient temperatures. A "Cold Impact" is performed on a painted panel which has been refrigerated to a temperature of 00C or less. A "Delayed Cold Impact" is performed on a panel at least 5 days after painting. In any impact test, adhesion is measured by the application and removal of Scotch-brand transparent tape to the deformed surface and the proportion of paint remaining on the surface is rated from 10 (100% adhesion) to O (0% adhesion).

Bend Adhesion The test for paint adhesion is the 1 800 O--T bend test. In this test the painted panel is bent 1800. The radius of the bend may be controlled by bending the test panel around a mandrel of predetermined thickness, usually one or more panels of the same thickness as the test panel. The most severe test is where no mandrel is employed and the panel is bent so that the untreated surfaces are touching. This is the so-called O-T (zero mandrel thickness) bend. Bending around one panel thickness would be a 1-T bend, etc.After bending, the panel is tested for paint adhesion by the application and removal of a standard transparent tape (Scotch No. 1 70). The extent of paint removed by the tape is rated 10 for essentially no removal to 0 to complete removal. Values of 9 through 1 are assigned for intermediate adherence ratings in proportion to % paint adherence to the substrate.

In the following examples all parts and percentages are by weight.

Example A (comparative) Aluminium panels are cleaned with detergent and rinsed with water and then conversion coated.

Conversion coating is effected by rolling onto the panels a working bath consisting of 0.34% CrO3, 0.36% Aerosil 200, 0.23% of 75% H3PO4, with the remainder of the working bath being water, followed by drying in an oven. The panels are then painted.

Example B (comparative) Aluminium panels are cleaned with detergent, rinsed with water and then conversion coated.

Conversion coating is effected by spraying the panels with a working bath consisting of 0.23% CrO3, 0.44% Na2MoO4, 0.04% of 68% HNO3 and 0.095% of 70% HF, with the remainder of the working bath being water, rinsing the panels with 0.05% aqueous CrO3 and then drying them in an oven. The panels are then painted.

Examples 1,2 and 3 Clean aluminium panels, for instance obtained by cleaning with a detergent followed by water rinsing, are conversion coated. The conversion coating is formed by application by rolling on a composition according to the invention followed by drying the panels. The panels are then painted without intermediate rinsing.

The three compositions used in these examples are made by mixing the following ingredients, together with water, to make a 500 ml bath of working solution.

Mls Mls Mls Mls Example Aquamac 10% 10% 20% No. A607 Tannic HF H3P04 1 13.8 25 10 0 2 13.8 25 15 0 3 13.8 25 0 20 Aquamac A607 is a fast drying acrylic emulsion supplied by IMC Corporation having a 32% solids content.

The five sets of panels produced in the preceding examples were painted with three different paints and the results are shown in Tables 1, 2 and 3 below.

In Table 1 the paint was an alkyd paint provided by DuPont called "Dulux 704" (Dulux is a trade mark). In Table 2 the paint was a silicated acrylic paint provided by PPG Industries, Inc. called PPG 1 - LW-1 0294. In Table 3 the paint was a straight acrylic paint provided by PPG Industries, Inc, called PPG-i 403-2840-19.

Table 1 504 hrs Acetic Room Example 1008 hrs Acid 1008 hrs Cold Temp. 0/T 2/T No. Salt Spray Salt Spray Humidity Impact Impact Bend Bend 1 0-2 0-1 F9.5 0 9 0 8 2 015 0-1 F9.5 0 7 0 8 3 1-1 0-1 c9 0 7 5 7 VF+9 A N N 10 0 8 0 7 B N N G9 0 4 0 5 VF9 Table 2 504hrs Acetic Room Example 1008 hrs Acid 1008 hrs Cold Temp. 0/T 2/T No. Salt Spray Salt Spray Humidity Impact Impact Bend Bend 1 N N 10 8 6 5 9 2 015 0-1 10 7 7 3 7 3 0--1S 0--1S 10 7 7 2 6 A N N 10 8 8 3 8 B N N 10 7 6 2 7 Table 3 504 hrs Acetic Room Example 1008 hrs Acid 1008 hrs Cold Temp. 0/T 2/T No. Salt Spray Salt Spray Humidity Impact Impact Bend Bend F9 N 10 8 7 5 9 2 VF+S 015 10 10 8 6 9 3 N N 10 7 5 2 3 A VF9 N 10 8 8 8.5 7 B N N 10 9 9 8 9

Claims (18)

Claims

1. A process in which a protective coating is formed on an aluminium surface by contacting the surface with an aqueous acidic solution that is free of chromium and that comprises tannin and an acrylic resin.

2. A process according to claim 1 in which the solution contains at least 0.01 9/l acrylic resin and at least 0.005 g/l tannin and has a pH of from 2 to 6.

3. A process according to claim 1 or claim 2 in which the solution contains from 1 to 20 g/l acrylic resin and from 1 to 10 9/1 tannin.

4. A process according to any preceding claim in which the solution has a pH of from 2.5 to 4.5

5. A process according to any preceding claim in which the ratio by weight resin:tannin is from 1:5two8:1.

6. A process according to claim 5 in which the ratio by weight resin:tannin is from 1:2 to 4:1.

7. A process according to any preceding claim in which the acidity of the solution is provided by including in the solution an acid selected from hydrogen-fluoride and phosphoric acid and mixtures thereof.

8. A process according to claim 7 in which the amount of the acid is from 1 to 5 9/l.

9. A process according to any preceding claim in which paint is applied to the protective coating without application of any intermediate post-treatment.

10. A process according to claim 9 in which the solution is dried onto the surface and paint is applied to the surface without any intermediate rinse or post-treatment.

11. A composition suitable for use in forming a protective coating on aluminium and comprising an aqueous acidic solution that is free of chromium and that comprises tannin and an acrylic resin.

12. A composition according to claim 11 and which contains at least 0.01 g/l acrylic resin and at least 0.005 9/I tannin and has a pH of from 2 to 6.

1 3. A composition according to claim 11 or claim 1 2 and that contains from 1 to 20 9/I acrylic resin and from 1 to 10 g/l tannin.

14. A composition according to any of claims 11 to 13 having a pH of from 2.5 to 4.5.

1 5. A composition according to any preceding claim in which the ratio by weight resin:tannin is from 1:5to8:1.

16. A composition according to claim 1 5 in which the ratio is 1:2 to 4:1.

1 7. A composition according to any of claims ii to 1 6 in which the acidity is provided by acid selected from hydrogenfluoride and phosphoric acid and mixtures thereof.

18. A composition according to claim 1 7 in which the amount of the acid is from 1 to 5 9/l.