GB1558744A

GB1558744A - Colouring of aluminium or aluminiumbased alloys

Info

Publication number: GB1558744A
Application number: GB4355/78A
Authority: GB
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1977-02-03
Filing date: 1978-02-03
Publication date: 1980-01-09
Also published as: FR2379620A1; DE2714394A1; DE2714394B2; DE2714394C3; SE7801257L; FR2379620B1; AT358351B; NL7800636A; ATA71578A; BE863561A

Description

(54) COLOURING OF ALUMINIUM OR ALUMINIUM-BASED ALLOYS (71) We, SwIss ALUMINIUM LTD., a company organised under the laws of Switzerland, of Chippis (Canton of Valais), Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the folIowing statement: The invention concerns objects made of aluminium or aluminium based alloy in a form such as sheet, foil, containers and the like and having an anodically oxidised surface layer, and colouring that layer by means of heat transfer printing.

Aluminium and aluminium-based alloys in the form of finished or semi-finished products, which are expected to exhibit good corrosion resistance and wear resistance as well as having an attractive, decorative appearance, are usually given an anodic oxidation treatment.

An oxide layer is formed in electrolyte which are generally made up of dilute sulphuric acid, sometimes with additions of oxalic acid, less often of dilute oxalic acid alone, or of dilute phosphoric or chromic acid and by applying an electrical current, usually in the form of direct current, less often in the form of alternating current or by superimposing or switching alternating and direct current, the items or semifabricated product being made the anode in the circuit.

These oxide layers are in general made up of a very thin, almost pore-free dielectric base layer. the so called barrier layer, and on top of this a top layer which has many fine pores in it. The barrier layer is self-regenerating, being formed by the conversion of aluminium to aluminium oxide, at the same rate as the top layer is formed from it during anodic oxidation.

The top layer is made up of bundles of fibres which lie essentially perpendicular to the surface of the metal and in general are transparent and colourless when produced using dilute sulphuric acid as the electrolyte and direct current.

There are many processes which can be used to produce colour effects in the anodic oxide layer on aluminium. These processes can be divided into four groups according to the way they work: 1. Colour can be introduced by using special electrolytes e.g. aqueous solutions of carbonic acid or sulphonic acid.

2. Deposition of metals in the pores in the fibre bundles of the top layer of a trans parent, colourless oxide layer, by means of an alternating current applied in an aqueous metal salt solution. 3. Deposition of inorganic pigments or organic colouring agents in the pores in the fibre bundle of the top layer of a transparent, colourless anodic layer by means of immersion in a warm solution containing the colouring substance.

4. Deposition of organic colouring agents in the pores of the fibre bundle making up the top layer of a transparent, colour less oxide layer by bringing the oxide into direct contact with a hydrolysis resistant, colouring agent which can- sub limate and which is printed on a sub strate, e.g. a paper substrate, with the result that the anodic oxide layer sucks up the colouring agent into the pores in the fibre bundle under the influence of heat. This is known as a "heat transfer process".The colouring agents which are suitable for this process are dispersion colouring substances with anthraquinone as the basis, with at least one of the positions 1, 4, 5 or 8 occupied by either H, OH-, amino or amido groups and at least one active hydrogen: or zero colouring agents with an OH- group in the ortho position of the azo- group; or colouring agents with a 1,3 indane dione group.

After the colouring substance has been deposited in the oxide, the pores in the anodic oxide layer containing the colouring substance are closed or sealed by a treatment in hot deionised water. As a result of the hot water treatment, at least a part of the Al203 of the newly produced oxide layer is converted to ALOOF, so called pseudo-boehmite.

On looking at the four different processes for colouring anodic oxide layers on aluminium it is clear that anodic oxide layers wllicll are multi-coloured, patterned or carrying a picture can be produced commerically in a particularly favourable manner by the process listed under point 4) viz. using the transfer of colouring material which con be sublimated from a paper substrate under the influence of contact pressure and heat by so called heat transfer printing.

This process which has been known for some time now has not been able to develop into a usuable technology as it suffered from the serious disadvantage that on transferring the hydrolysis-resistant, sublimable organic colouring substance from the substrate to the absorbent 5 to 20 Lm thick anodic oxide layer bv heating to the temperature of 120 to 220"C necessary for that process. fine hair-line cracks occurred and these were disturbing to the eye especially when viewed at acute angles of incident light.

In a method according to the present invention of colouring by the heat transfer process an oxide layer which has been produced by anodic oxidation on aluminium or- on an aluminium-based alloy. the oxide layer used to receive the colouring is 5 to 25 x.m thick, and exhibits a crack-free elongation of at least 0.650/cho in the noncoloured, non-sealed condition, and the ratio of the crack-free elongation of the oxide layer in the non-coloured, non-sealed condition to the crack-free elongation in the coloured and non-sealed condition lies between 1:1.2 to 1: 5.5.

The crack-free elongation can be determined by a method according to Zurbriigg, in which a test strip of predetermined thickness is bent round a scroll-shaped former at increasing curvature until cracks are seen to be forming.

The thickness of the anodic oxide layer is preferably 10 to 22 ,am, the crack-free elongation in non-coloured, non-sealed condition between 0.7 and 40/cho and the ratio of the crack-free elongation in the noncoloured, non-sealed condition to the coloured, non-sealed condition between 1:1.7 to 1:5.0.

Extensive trials have shown that, when the normal colouring substances are used e.g. anthroachinon based substances, azocolouring substances, or substances with an indandion group, the crack-free elongation of the coloured, non-sealed oxide layer is principally determined by the anodising conditions and the alloy employed i.e. it is to a large extent independent of the kind of colouring substance. It was also found that the crack-free elongation of the coloured, non-sealed oxide layer lies between 3.4 and 6 /oo and can not be represented as a function of the crack-free elongation of the oxide layer in the non-coloured condition.

It is thus a characteristic property of oxide layers which have been produced by anodic oxidation.

In the final coloured and sealed condition, anodic oxide layers exhibit a crack-free elongation of 3.3 to 50/cho. It was evident from this that the elongation of the layers was in most cases reduced by an amount up to 0.60/cho by the sealing operation. The wear resistance and the hardness, determined by the abrasimeter test according to Haueisen, were not related to the oxide layer properties required in accordance with the invention. In the case of oxide layers produced in the same electrolyte by the same kind of electrical current (direct current, alternating current etc.) no difference could be found in wear resistance, independent of whether hair-line cracks were produced in the oxide layer by heat-transfer printing or not.

The properties of the absorbent oxide layer required by the process of the invention are obtained by controlled interaction of the following parameters: (a) - Alloy cornposition and condition of the product or semi-finished product to be anodixed, in particular sheet and extruded section.

(b) Composition and concentration of the electrolyte.

(c) Electrolyte temperature.

(d) Current density.

In practice, numerous objects of identical composition and condition are produced from a batch of alloy. The first few of these objects are subjected to individual experi ments involving production of an oxide layer with differing values of (b), (c), and (d), followed by testing of the crack-free elongation, before and after colouring by the heat transfer process. (Variation of (b), (c) and (d) is a known technique for varying other properties of the oxide layer).

These experiments will reveal a combination of values of (b), (c) and (d) which produces an oxide layer having crack-free elongations within the ranges required by this invention. Such a combination of values can then be used in the production of oxide layers on the remaining objects from the batch of allov in question. As between one batch of alloy and another, there are likely to be differences in the comnosition and condition (parameter (a)). Therefore for each batch one similarly makes experiments, and then produces oxide layers on the remaining objects of the batch.

Oxide layers which have been found to be particularly suitable are those on Almg alloys containing 0.5 to 4% magnesium, preferably 1 to 3% magnesium. These alloys are used preferably in the half-hard condition, as specified by the German specification DIN 17007 sheet 4, in the rolled or recovered condition.

The invention will now be described in greater detail with the help of two examples: EXAMPLE 1 A 19 rm thick oxide layer which was produced on a half-hard rolled 0.8 mm thick sheet of the alloy AlMg1 .5 (anodising grade) exhibited a crack-free elongation of 0.80/cho in the non-coloured, non-sealed condition.

A substrate made of paper suitable for low pressure heat transfer printing containing various hydrolysis-resistant, sublimable dispersion colouring substances, such as are used in the low pressure printing process, in the form of a mirror image pattern, was then laid on the anodic oxide layer of the AlMg sheet and held for 1 minute under a pressure of 0.1 kplcm2 at a temperature of 180"C, during which time the coloured image was transferred to the anodic oxide layer which then bore the coloured image in the correct, reversed manner. There were no hair-line cracks in the anodic oxide layer which exhibited a crack-free elongation of 3.7 /oo in this coloured, non-sealed condition. The ratio of crack-free elongation in the non-coloured condition to that in the coloured condition was then 1:4.6.

Next, the coloured anodic oxide layer was sealed by immersion for 45 min in a bath of deionised, boiling water containing additions of commercially available sealing salts i.e. the pores of the anodic oxide layer which now contained the hydrolysisresistant, sublimable colouring substance at their base were closed by forming aluminium hydrates.

After this sealing treatment the anodic oxide layer exhibited a crack-free elongation of 3.5 /oo and a Haueison abrasion hardness of 8.3 sec per m of oxide layer thickness.

EXAMPLE 2 In contrast to Example 1 the behaviour of an oxide layer which is unsuitable for heat-transfer printing will be illustrated here.

A 10 ym thick oxide layer was produced on a sheet of the same type as used in Example 1 and in the non-sealed, noncoloured condition exhibited a crack-free elongation of 0.63oleo.

A- substrate made of a paper suitable for low pressure heat transfer printing containing various hydrolysis-resistant, sublimable dispersion colouring substances, such as are used in the low pressure printing process, in the form of a mirror image pattern, was then laid on the anodic oxide layer and held for 1 minute under a pressure of 0.1 kp/cm2 at a temperature of 180 C during which time the coloured image was transferred to the anodic oxide layer which then bore the coloured image in the correct, reversed manner.

The anodic oxide layer exhibit several fine hair-line cracks which were very disturbing to the eye when the image is viewed with the naked eye at an acute angle to the horizontal.

The crack-free elongation of this oxide layer (i.e. of the areas of the layer between the hair-line cracks mentioned above) in the coloured, non-sealed condition was 3.7 /oo and the ratio of crack-free elongation in the non-coloured, non-sealed condition to the coloured, non-sealed condition was 1:5.9.

WHAT WE CLAIM IS:- 1. A method of colouring by the heat transfer process an oxide layer which has been produced by anodic oxidation on aluminium or on an aluminium-based alloy, in which the oxide layer used to receive the colouring is 5 to 25 m thick and exhibits a crack-free elongation of at least 0.650/cho in the non-coloured, non-sealed condition, and the ratio of the crack-free elongation of the oxide layer in the non-coloured, nonsealed condition to the crack-free elongation in the coloured and non-sealed condition lies between 1:1.2 and 1:5.5.

2. A method according to claim 1, in which the thickness of the oxide layer is 10 to 22am.

3. A method according to claim 1 or claim 2, in which the crack-free elongation of the oxide layer in the non-coloured, nonsealed condition lies between 0.7 and 40/cho.

4. A method according to any of claims 1 to 3, in which the ratio of the crack-free elongation of the oxide layer in the noncoloured, non-sealed condition to that of the coloured, non-sealed condition lies between 1:1.7. and 1:5.0.

5. A method according to any of claims I to 4, in which the oxide layer has been produced on an AlMg alloy in which the magnesium content of the alloy lies between 0.5 and 4%.

6. A method according to claim 5, in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and then produces oxide layers on the remaining objects of the batch. Oxide layers which have been found to be particularly suitable are those on Almg alloys containing 0.5 to 4% magnesium, preferably 1 to 3% magnesium. These alloys are used preferably in the half-hard condition, as specified by the German specification DIN 17007 sheet 4, in the rolled or recovered condition. The invention will now be described in greater detail with the help of two examples: EXAMPLE 1 A 19 rm thick oxide layer which was produced on a half-hard rolled 0.8 mm thick sheet of the alloy AlMg1 .5 (anodising grade) exhibited a crack-free elongation of 0.80/cho in the non-coloured, non-sealed condition. A substrate made of paper suitable for low pressure heat transfer printing containing various hydrolysis-resistant, sublimable dispersion colouring substances, such as are used in the low pressure printing process, in the form of a mirror image pattern, was then laid on the anodic oxide layer of the AlMg sheet and held for 1 minute under a pressure of 0.1 kplcm2 at a temperature of 180"C, during which time the coloured image was transferred to the anodic oxide layer which then bore the coloured image in the correct, reversed manner. There were no hair-line cracks in the anodic oxide layer which exhibited a crack-free elongation of 3.7 /oo in this coloured, non-sealed condition. The ratio of crack-free elongation in the non-coloured condition to that in the coloured condition was then 1:4.6. Next, the coloured anodic oxide layer was sealed by immersion for 45 min in a bath of deionised, boiling water containing additions of commercially available sealing salts i.e. the pores of the anodic oxide layer which now contained the hydrolysisresistant, sublimable colouring substance at their base were closed by forming aluminium hydrates. After this sealing treatment the anodic oxide layer exhibited a crack-free elongation of 3.5 /oo and a Haueison abrasion hardness of 8.3 sec per m of oxide layer thickness. EXAMPLE 2 In contrast to Example 1 the behaviour of an oxide layer which is unsuitable for heat-transfer printing will be illustrated here. A 10 ym thick oxide layer was produced on a sheet of the same type as used in Example 1 and in the non-sealed, noncoloured condition exhibited a crack-free elongation of 0.63oleo. A- substrate made of a paper suitable for low pressure heat transfer printing containing various hydrolysis-resistant, sublimable dispersion colouring substances, such as are used in the low pressure printing process, in the form of a mirror image pattern, was then laid on the anodic oxide layer and held for 1 minute under a pressure of 0.1 kp/cm2 at a temperature of 180 C during which time the coloured image was transferred to the anodic oxide layer which then bore the coloured image in the correct, reversed manner. The anodic oxide layer exhibit several fine hair-line cracks which were very disturbing to the eye when the image is viewed with the naked eye at an acute angle to the horizontal. The crack-free elongation of this oxide layer (i.e. of the areas of the layer between the hair-line cracks mentioned above) in the coloured, non-sealed condition was 3.7 /oo and the ratio of crack-free elongation in the non-coloured, non-sealed condition to the coloured, non-sealed condition was 1:5.9. WHAT WE CLAIM IS:-

1. A method of colouring by the heat transfer process an oxide layer which has been produced by anodic oxidation on aluminium or on an aluminium-based alloy, in which the oxide layer used to receive the colouring is 5 to 25 m thick and exhibits a crack-free elongation of at least 0.650/cho in the non-coloured, non-sealed condition, and the ratio of the crack-free elongation of the oxide layer in the non-coloured, nonsealed condition to the crack-free elongation in the coloured and non-sealed condition lies between 1:1.2 and 1:5.5.

6. A method according to claim 5, in

which the magnesium content lies between 1 and 3%.

7. A method according to claim 5 or claim 6, in which the AlMg alloy is in the half-hard condition as defined by the German specification DIN 17007 sheet 4.