WO1992019795A1 - Process for producing articles comprising anodized films exhibiting areas of different colour and the articles thus produced - Google Patents

Abstract

A process for producing articles comprising anodized films exhibiting areas of different colour and the articles thus produced. The process comprises anodizing a surface (16) of an article made of or coated with aluminum or anodizable aluminum alloys to produce an anodic film (11) having a porous outer layer and a non-porous inner barrier layer (15) between the porous layer and the underlying surface (16) of the metal article. A metal is electrodeposited into the pores (12) of the film to form a semi-reflective layer (18) of metal deposits (17) in the film (11) which generates a visible colour by effects including light interference between light reflected from the semi-reflective layer (18) and light reflected from the underlying metal layer (10). A cathodically biased electrode (20) is brought into contact with or close proximity to limited areas (A) of the film (11) to cause the barrier layer (15) to thicken in those limited areas only. This step in itself may create a pattern of different colours in the contacted and non-contacted areas of the film (11). However, further anodization of the surface is preferably carried out before, after, or both before and after, the contact with the electrode (20), at a voltage which causes the film (11) to thicken below the deposits in the non-contacted areas (B), but not significantly in the contacted areas (A). If desired, the film (11) can be detached from the aluminum article (10) on which it was formed and a very thin layer (23) of metal applied to the exposed surface (22) of the film in order to make a very thin, flexible, planar, patterned article suitable for use as a packaging sheet, or the like.

Description

Process for producing articles comprising anodized films exhibiting areas of different colour and the articles thus produced

TECHNICAL FIELD This invention relates to a process for producing articles comprising anodized films having areas of visibly different colour, hue, colour shading or colour density, etc. (referred to hereinafter generally as different colours) forming a pattern, picture, printing or other marking or indicia, etc. (referred to hereinafter generally as patterns) . More particularly, the invention relates to such a process in which the colours are generated at least in part by optical interference effects. The invention also relates to patterned articles and structures of this kind. BACKGROUND ART

It is well known to provide articles made of aluminum or anodizable aluminum alloys with a protective and/or decorative oxide coating by anodization. This involves electrolyzing the article as an anode in an electrolyte containing a strong acid in order to form a porous oxide layer on the aluminum surface. The oxide layer, or anodic film as it is often called, consists of aluminum oxide having pores open at the exposed outer surface of the film but closed adjacent to the oxide/metal interface by a dense imperforate thin barrier layer of aluminum oxide.

It is also well known that anodized articles produced in this way can be coloured by electrolytically depositing a metal (often referred to as an inorganic pigment) into the pores of the anodic film in order to create scattering and/or absorption of light incident on the article surface. This colouring procedure is often referred to as the ANOLOK (trademark of Alcan Aluminium Limited) process. The standard ANOLOK (trademark) process has been modified in two ways in order to produce a greater range of colours. One such modification is disclosed in US Patent No. 4,066,816 issued on January 3, 1978 to Sheasby et al and assigned to the same assignee as the present application. This modification involves reducing the height of the deposits in the pores so that light interference effects contribute to the observed colour. The interference effects result from the very small distance between the semi-reflective surfaces formed by the outer ends of the deposits and the reflective surface of the underlying oxide/metal interface.

The second modification is disclosed in US Patent 4,310,586 issued on January 12, 1982 to Sheasby et al and also assigned to the same assignee as the present application. This modification is similar to the one described above but involves an additional step of increasing the thickness of the oxide layer between the inner ends of the deposits and the oxide/metal interface by carrying out further anodization. This procedure creates additional colours (which may also be dichroic, i.e. variable with viewing angle) due to the greater spacing between the outer ends of the deposits and the oxide/metal interface, without increasing the lengths of the deposits which would result in increased light absorption and thus "muddying" of the exhibited colours.

US Patent 4,066,516 to Sato issued on January 3, 1978 relates the formation of patterns on anodized articles coloured using an ANOLOK (trademark) type process. In the process disclosed in this patent, the article is anodized in the normal way and then, prior to the deposition of an inorganic pigment, limited areas of the anodized surface are subjected to anodization at high voltage in order to increase the thickness of the barrier layer in such areas. An inorganic pigment is then electro-deposited into the pores in the normal way but, because the thickened barrier layer acts as an electrical insulator in the treated areas, deposition of the pigment takes place only in the untreated areas and a light on dark pattern becomes visible at the surface of the article. While this procedure is capable of producing visible patterns without resorting to the use of masks and multiple treatments, which are difficult to carry out, time consuming and expensive on a commercial scale, it does not make use of the improved colours which can be obtained by the process disclosed in US Patent 4,310,586.

An object of the invention is to provide a process for producing articles having anodized surfaces provided with patterns of different colours, e.g. areas of one colour against a contrasting background.

Another object of the invention is to produce such articles in which both patterned areas and background areas exhibit a colour different from uncoloured anodized surfaces.

Yet another object of the invention is to produce such articles by a technique which can be carried out simply, with relatively few steps, economically and optionally on a continuous basis.

A further object of the invention is to produce such articles without resorting to the use of potentially harmful dies, pigments, masking materials and solvents. A further object of the invention, in preferred forms, is to produce patterned articles consisting of anodic films isolated from any substrate article on which they may have initially been formed. SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a process for producing an article comprising an anodic film exhibiting areas of different colour, comprising: anodizing a surface of a substrate made of or coated with a metal selected from the group consisting of aluminum and anodizable aluminum alloys to produce an anodic film on said surface having a porous outer layer and a non-porous inner barrier layer between said porous outer layer and said surface; electrodepositing a metal into pores in said porous anodic film to form deposits in said pores which generate a colour by effects including interference of light; and bringing a cathodically-biased electrode into contact with or close proximity to limited areas of said porous anodic film to cause said barrier layer to thicken in said limited areas only.

Optionally, either before or after, or alternatively, both before and after, bringing the cathodically biased electrode into contact with or close proximity to the limited areas, further anodizing of the surface may be carried out at a voltage which causes the film to thicken beneath the deposits, at least in areas of the film other than the limited areas contacted by the electrode. However, a further anodizing step of this kind is not absolutely required; the thickening of the barrier layer caused by the cathodically-biased electrode may be sufficient in itself to produce a difference of colour between the limited areas contacted by the electrode and the remaining areas of the film. According to another aspect of the invention there is provided an article having an anodic film exhibiting areas of different colour, comprising: a reflective metal substrate; a porous anodic film overlying a surface of said reflective substrate; and metal deposits in pores of said film having outer ends separated from said metal surface sufficiently to generate a colour by effects including interference of light; wherein said film has different areas in which spacings between inner ends of said deposits and said metal surface are sufficiently different from each other that said different areas exhibit different colours.

It is to be noted that, in the present invention, the anodic film need not be immersed in an electrolyte when the film is contacted by, or brought into close proximity to, the cathodically-biased electrode because liquid carry-over from a previous step (usually a rinse with deionized water) is normally sufficient to act as an electrolyte. This makes it much easier to produce the desired pattern with the electrode. An additional electrolyte may be used, however, if this is desired or if the film is dry.

The electrode used to thicken the film beneath the deposits may be a wand, probe, brush, roller or the like of any suitable thickness at the tip or contacting surface, or a planar electrode such as a sheet or plate, and may be made of a conductive metal or other conductive material (e.g. graphite) . It is unnecessary to pack a hollow electrode with a high viscosity electrolyte as in the Sato process discussed above, which again makes pattern formation much simpler, but an electrode of the type used by Sato may be used, if desired. While the electrode normally is normally brought into actual contact with the surface of the anodic film, a similar barrier layer thickening effect can be obtained if the electrode merely closely approaches the film surface. In such a case, the resulting pattern may have less clear lines of separation due to the formation of an electrical field gradient extending laterally over a substantial distance. A transition zone is then visible between the areas of different colour, but this may be desirable for artistic appeal. In fact, the demarkation between the zones of different colour may not be sharp in some cases even when the electrode is directly contacted with the anodic film. The variables affecting the sharpness of the resulting transition zone include the distance of separation between the electrode and the anodic film, the electrolyte conductivity, the cathode potential of the electrode and the contact duration. These variables can be changed in any particular case in order to produce a required degree of sharpness of colour transition between the areas of different colour. BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1 to 10 are diagrammatic cross-sections of anodic films produced during various steps of several preferred processes according to the present invention, with Figs. 4, 5, 7, 8 and 10 representing final products. Fig. 11 shows preferred equipment used to carry the process of the present invention on a continuous basis. In the drawings, no attempt has been made to show the relative dimensions of various elements and layers to scale.

Throughout the several views, like items are indicated by_% he same reference numerals, where possible. DESCRIPTION OF THE BEST MODES

In the present invention, at least in its preferred forms, an article made of aluminum or an anodizable aluminum alloy (at least at an exposed surface of the article) can be provided with an anodized surface having a visible pattern formed by areas of different colour by a process which involves the following steps.

First of all, the surface of the article to be treated is anodized in an electrolyte which produces a porous anodic film. Suitable electrolytes include, for example, aqueous solutions of strong inorganic acids, such as sulphuric acid or phosphoric acid, or organic acids such as oxalic acid. The anodization step is normally carried out at room temperature at a voltage in the range of 5-25V for sulphuric acid, and preferably at 10-20V, for a time sufficient to produce a porous anodic film having a thickness preferably in the range of.0.1 to 30 x 10^"3 mm (30 microns) . Direct or alternating current conditions can be used in this step. For other acids, these ranges will change but can be determined by simple trial and experimentation, if not already known.

Fig. 1 of the accompanying drawings represents a cross-section of the resulting article in which the substrate metal 10 has an overlying transparent anodic film 11 of aluminum oxide. The film 11 has pores 12 extending inwardly from an outer surface 13 towards the substrate metal 10. However, the pores are closed at their inner ends 14 by an imperforate continuous thin barrier layer 15 of anodic oxide which separates the pores 12 from the underlying metal surface 16.

If necessary or desired, a second step may be carried out to enlarge the cross-sectional areas of the pores 12 at their inner ends. The reason for this is to ensure that, after a metal deposition step to be described below has been carried out, metal deposits in the pores have a sufficient surface area to create strong reflections and consequently strong interference effects. This optional step can be carried out by subjecting the anodized surface to further electrolytic treatment in an electrolyte having a high dissolving power for aluminum oxide, e.g. aqueous phosphoric acid. Direct or alternating current conditions can also be used for this step. Direct current voltages are generally in the range of 8 to 50 volts and alternating current voltages are generally in the range of 5 to 40 volts at temperatures in the range up to 50°C, preferably 10-35^"C, and phosphoric acid concentrations preferably in the range of 10-200 g/1, particularly 50-150 g/1. The upper limit of a dissolution treatment designed to increase pore diameter is set by the point where the film loses strength and becomes powdery or crumbly through reduction of the thickness of oxide lying between adjacent pores.

A film resulting from this pore widening step is represented in cross-section in Fig. 2. It will be seen that the additional electrolysis has increased the overall thickness of the film 11 and has provided the pores 12 with widened lower ends 12'.

The next step of the procedure is to deposit a metal (sometimes referred to as an inorganic pigment) into the lower ends 12' of the pores 12 in order to create a semi- reflective layer within the film 11. This can be done by electro-deposition in the conventional manner, e.g. as described in US Patents 4,066,816 and 4,310,586 (the disclosures of which are incorporated herein by reference) . The metal chosen for this step should be one which can resist the strongly acidic conditions to which it is exposed in subsequent steps without significant dissolution and the preferred metals for this purpose are noble metals, such as palladium, and acid-resistant alloys, such as Sn-Ni and Cu-Ni alloys. However, less acid-resistant metals, e.g. tin, nickel, cobalt, copper, silver, cadmium, iron, lead, manganese and molybdenum, may be used if they are displaced by or coated with an acid- resistant metal, e.g. a noble metal such as palladium, platinum or gold, after the less acid resistant metals have been deposited in the pores. For example, this can be achieved by immersion of the film containing the less acid resistant metal deposits in a solution of a noble metal salt, or by spraying a solution of the noble metal salt onto the film.

Fig. 3 represents the film following the metal deposition step. The enlarged ends 12' of the pores 12 contain metal deposits 17 which together create a discon- tinuous but generally flat semi-reflective surface 18.

As represented in Fig. 4, limited areas A of the film 11 are then briefly contacted by, or brought into close proximity to, an electrode 20 biased as a cathode whose electrical field potential is greater than that of the general anodizing field potential used for the formation of the film 11. This causes the barrier layer 15 to thicken (from X to X') in the areas A beneath the electrode, but does not affect the thickness (X) of the barrier layer in areas B not contacted by, or brought closely adjacent to, the electrode 20. This thickening effect in areas A comes about because the thickness of the barrier layer 15 is proportional to the applied voltage, whereas the thickness of the porous region of the film is proportional to current passed. When the electrode briefly contacts, or comes close to, the surface 13 of the anodic film 12, localized anodizing takes place and, although the porous region of the film grows only slightly in thickness in the contacted areas, the barrier layer 15 thickens substantially and almost instantaneously. It has been found that a contact time of as little as 0.01 second can produce a visible colour difference between regions A and B in the final product. In general, the contact of electrode 20 is maintained for a period of 0.01 to 10 seconds when the electrode has a cathodic potential in the range of 10 to 50V. This generally produces a thickened barrier film 15' having a thickness in the range of 0.01 to 0.05 microns.

The electrode 20 may be in the form of a wand or the like which is positioned on or stroked across the surface 13 to produce the desired patterning effect, or it may be in the form of a plate or roll having cut-out, contoured or etched regions exhibiting a desired pattern, picture, printing or other indicia, which is brought briefly into contact with the entire surface 13. The wand, plate or roll may be solid or may alternatively be hollow and open at contact points and may contain a high viscosity electrolyte so that it can be used both on wet and dry surfaces. As a further alternative, a thickened electrolyte may be printed (e.g. by silk screening) on the film 11 (when dry) and then overlaid by a continuous metal plate or foil acting as the electrode 20. The barrier layer thickening then takes place only in those regions contacted by the electrolyte since anodization is not possible in the other (dry) regions.

The steps described so far produce areas A and B of different colour if the potential of the electrode 20 is suitably high, in which case no further steps need be carried out. The film 11 appears to be coloured when illuminated by diffuse white light because of (i) selec¬ tive wavelength absorption by the pigment deposits 17 and, to a lesser extent, by the substrate metal 10, (ii) scattering of certain wavelengths by the deposits 17 and (iii) interference effects caused by reflections from the semi-reflective surface 18 and from the oxide/metal interface 16. While the coloration produced by effects (i) and (ii) are essentially the same in both regions A and B of the film 11, the coloration produced by effect (iii) is different in these two regions because of the difference in separation between reflective surfaces 18 and 16 in these two regions which leads to different optical paths taken by reflected light. In this type of film, coloration produced by effect (iii) makes a dominant contribution to the observed colour when the spacing between surfaces 18 and 16 is "optically thin", (less than 3 x 10^"3 mm (3 microns) and preferably less than 1 x 10^"3 mm (1 micron)) . When this is the case, regions A and B have different overall exhibited colours if the distances X and X' are suitably different. Consequently, if the film thickening effect of the electrode 20 is suitably great, the resulting interference effects produce noticeably different colours in the areas A and B and the film 11 then appears to be patterned. However, if necessary or desired, the colour differences between the areas A and B can be enhanced and different colours may be generated by carrying out the following additional steps.

After removal of the electrode 20, further porous anodization of the entire surface may be carried out at a voltage lower than the cathodic voltage of the electrode 20. This produces pore-elongation in the regions B, but it has been unexpectedly found that substantially no pore elongation takes place in regions A having the thickened barrier layer 15'. This may be because anodizing current cannot pass through the thickened barrier layer 15' until sufficient dissolution, and thus thinning, of the barrier layer 15' has taken place.

The film 11 following this further anodization step is represented in Fig. 5. In the regions B, the pores 12 have been elongated by the formation of extended lower ends 12", but substantially no pore elongation (or in some cases a minimal or significantly reduced pore elongation) has taken place in region(s) A. In regions B, the overall thickness of the anodic film 11 has been increased to Y' whereas, in region(s) A, the overall thickness Y remains substantially the same as before. This increases or exaggerates the difference in separation of the surfaces 18 and 16 in the areas A and B caused initially by the electrode 20 and this increased difference of separation causes the regions A and B to exhibit greater differences of colour for the reasons mentioned above.

An alternative procedure for enhancing the differences of colour is shown in Figs. 6 and 7. In this case, the procedure is the same as above until the structure of Fig. 3 is obtained but then the structure is re-anodized without first contacting limited areas of the film 11 with the cathodically-biased electrode 20. The resulting structure is shown in Fig. 6, in which the metal surface 16 is essentially planar and pore extensions 12" are formed in all regions of the film.

The resulting structure is then contacted in area(s) A by the cathodically-biased electrode 20 having a sufficiently high electrode potential to cause thickening of the barrier layer 15 in the regions of the film contacted by the electrode. As a result, as shown in Fig. 7, the separation between the semi-reflective surface 18 and the reflective metal surface 16 is different in the contacted area(s) A and the non-contacted areas B of the film and different colours are exhibited.

Preferably, a further reanodization step is then carried out to produce the structure shown in Fig. 8. The reanodization elongates the pores in the non-contacted areas B to produce pore extensions 12^,/ . In the contacted area(s) A, however, no pore elongation (or in some cases, less pore elongation) takes place. As a result, the optical path between the semi-reflective surface 18 and the metal surface 16 is again different in the contacted and non-contacted areas, which leads to different colours. In such a case, the colours of the contacted area(s) A may be the same in the structures of both Figs. 7 and 8, but the colours in the non-contacted areas B are different. The further reanodization step leading to the structure of Fig. 8 is desirable because, as in the first colour enhancing step described above, it increases the ratio of separation of the semi-reflective layer 18 and the metal surface 16 in the contacted and non-contacted areas A and B.

In the case of all of the above structures produced according to_%the invention, the pores 12 may, if desired, be sealed using conventional procedures for sealing architectural anodized surfaces, e.g. hot water sealing with or without smut inhibitors such as nickel acetate, various cold sealing techniques, the application of overlayers, e.g. clear lacquer, and the like.

Moreover, prior to such pore sealing effects the film may be contacted by a dichromate solution in order to make the deposits 17, if made of non-noble metal, less likely to be leached from the film either during the pore sealing step or later when the article is in use.

If desired, for example to make a patterned packaging sheet or the like, the anodic film 11 may be made detachable from the substrate 10 and the substrate 10 replaced by a very thin sputtered metal film in order to provide the required reflective surface 16. This can be achieved, for example, by the procedure shown in Figs. 9 and 10 which starts from the structure shown in Fig. 5, but could be applied equally well to the structures shown in Fig. 7 or Fig. 8. The procedure employs a pore-branching step as described in European patent application EP 0178831 published on April 23, 1986 and assigned to the same assignee as the present application (the disclosure of which is incorporated herein by reference) . The procedure requires a reduction of voltage in a final anodization step, which causes pore branching 25 at the bottoms of pore extensions 12' ' ' or widened pore sections 12' which collectively introduce weakened strata into the anodic film 11. The starting voltage used for this pore branching step must be the same as, or higher than, the voltage used in the previous anodization step and is preferably between 3 and 200V, more preferably 6-80V. The voltage is then reduced in steps or continuously until it approaches zero.

Following the pore-branching step, an overlayer 21 (preferably a flexible transparent polymer film e.g. of polyester) is adhered by means of an adhesive (not shown) or by heat sealing to the surface 13 of the film to form a structure as shown in Fig. 9.

The overlayer 21 is then used to detach (by pulling or peeling) the film 11 from the substrate 10 and the film acts as a support for the film 11 when detached. The exposed surface 22 of the film is then covered with a thin reflective metal film 23 by a vacuum deposition technique, such as sputtering. The metal used to form the film 23 may be any metal capable of undergoing the deposition technique and need not be aluminum or an aluminum alloy. The resulting structure still exhibits the areas of contrasting colour (visible through the transparent overlayer 21) but is sufficiently thin, flexible and non- porous that it can be used as a packaging sheet or the like.

It should also be mentioned that it is possible to form multiple (more than two) colours in the same anodic film 11 by carrying out the cathodic contact and reanodizing process several times. This results in several areas of different film thicknesses beneath the deposits in different parts of the film. The result is a film having several different colours on a background of uniform colour. One colour is derived from the shortest separation of the surfaces 18 and 16, the second colour from the next largest separation, etc. , with the background colour resulting from the largest separation between the surfaces 18 and 16.

It should further be noted that, in the structures of the invention, the thickness of the part of anodic film 11 above the metal deposits 17 may be either "optically thin" (less than about 3 x 10^"3 mm (3 microns) ) or "optically thick" (about 3 x 10^"3 mm (3 microns) or more) . This thickness is controlled in part by the duration of the initial anodization step since the total thickness of the film 11 increases as the duration of the anodization step increases, whereas the inner ends 14 of the pores 12 remain in approximately the same place relative to the metal substrate 10, being separated therefrom by the barrier layer 15. The thickness is also controlled by the duration of the metal deposition step which governs the height of the metal deposits 17. If the part of the film above the deposits 17 is "optically thin", interference may take place between light reflected from the upper surfaces 18 of the deposits and the outer surface 13 of the film 11, as well as between surfaces 18 and 16 as described above. This additional interference effect makes an additional contribution to the overall colour of the film, but this colour contribution is not affected by differences in distances X and Y in parts A and B of the film. This type of structure is referred to as oxide-metal-oxide-metal (OMOM) .

If the part of the film 11 above the deposits is "optically thick", there is no interference between light reflected from surfaces 13 and 18 and such reflections do not contribute to the overall colour of the film. Such structures are referred to as "buried" metal-oxide-metal (MOM) structures.

The present invention encompasses both types of film structures, i.e. those in which the upper film part above the deposits is "optically thick" and those in which the upper film part is "optically thin."

The structures of the present invention can be used, for example, as architectural finishes, signs, indoor decorative materials for stores, picture frames, decorative packaging films, foils and laminates. The process of the invention can be carried out batchwise on individual metal plates or foils 10, or may be carried out continuously on an elongated metal strip. Fig. 11 shows an apparatus comprising a series of tanks and power supplies suitable for a continuous process.

In the illustrated apparatus, a continuous strip 30 of flexible aluminum (sheet or foil) passes through tanks 32-44 in the direction of arrow A. Rollers 45 control the path of the strip 30 and ensure that the strip is fully immersed in each tank.

The aluminum foil strip 30 first passes over the first three rollers 45 which direct it through a liquid contact tank 32. Alternatively, an optional electrical contact roll would obviate the need for this tank since its primary function is only to complete the electrical circuits for the down-stream process steps. The strip 30 then passes in succession through a primary anodizing tank 33, a rinse tank 34, a pore modification tank 35, a rinse tank 36, a metal deposition tank 37, a rinse tank 38, a metal displacement tank 39, a rinse tank 40, an optional reanodizing tank 41, an optional rinse tank 42, over a patterning roller 50, through a second optional reanodizing tank 43, and a final rinse tank 44. Equipment for sealing the anodic film (not shown) would normally be included for carrying out a final step of the continuous process.

In the illustrated apparatus, the primary anodizing tank 33 and the pore modification tank 35 are conveniently (but not necessarily) connected to a common power supply 55. The voltage drop across each of these process stages is preferably similar, and depends primarily upon the nature of the anodizing electrolyte, which for the preferred sulphuric acid can range from 5-25 volts. It can be ac, dc or a mixture thereof. The residence time and the current density in the primary anodizing tank 33 determines whether the final structure has an optically thin oxide layer above the metal deposits or an optically thick layer over a buried metal-oxide-metal (MOM) structure.

Following pore modification in tank 35, a semi- transparent metal layer (e.g. nickel, cobalt, tin, or copper) is electrodeposited in the pores of the film in tank 37. Electrolytes for this process step are generally based on a solubilized salt of the metal with appropriate buffering agents and salts to improve deposition uniformity. An ac power source 56 is coupled to an optional capacitance 57 which is used as a means of suppressing unwanted dc current flow through the metal deposition tank 37. The metal deposited is then stabilized by conversion to a more corrosion resistant metal (e.g. platinum group metals) by simple immersion in tank 39 which contains a solution of a soluble salt of the more noble metal.

The thickness of the oxide layer beneath the metal deposits is optionally increased in the first reanodizing tank 41. Power, which is supplied by power source 58, can be either ac, dc, or a combination of both.

The anodized aluminum strip 30 then passes over a patterning roller 50, connected to power source 59 and is pressed into contact with the roller surface by an insulated pinch roller 51. It is important that the voltage drop across the anodic film during patterning be greater than the voltage drop across the anodic film during preceding anodizing or metal deposition steps.

The physical arrangement provided for the patterning may require direct contact between the patterning electrode and the metal strip in order to maximise the pattern definition. In such cases a brush, or roller arrangement as is illustrated, may be used. However, if a lack of definition were desired, then the patterning electrode could be positioned at a distance away from the metal strip and a solid electrode could be effectively used. If this latter method were adopted, it would require that the strip and electrode be immersed in a suitable electrolyte, in order to insure electrical contact between the two. A preferred patterning roller 50 comprises a flexible, conductive rubber sheet (e.g. COUNSIL-SC™ catalogue #86-10198 supplied by TECKNIT, 129-T Dermody St., Cranford New Jersey, USA) which has been silk screen printed with a non-conductive, vulcanizing, silicone rubber compound (e.g. DOW CORNING RTV Sealant 732™). After curing the rubber is wrapped around a metal roller which is connected to power source 59. After patterning, i.e. modifying the barrier layer, further reanodizing optionally takes place in tank 43 which is connected to power source 58. This step need not take place if the modified areas of the barrier layer have been substantially thickened so as to produce a desired variation in interference colour to its surrounding interference colour.

The invention is illustrated further by the following non-limiting Examples 1 to 11.

The substrates used in Examples 1-11 were pre- processed in an identical fashion as follows. Aluminum alloy AA5252 sheet was cut into 2.5 cm by 20 cm panels, etched in 5% NaOH at 65^"C for 5 minutes, and anodized in 21°C 1.5M H₂S0₄ at 16 volts DC for a period of 30 minutes to create a porous anodic film measuring 12 microns in thickness. The panels were subsequently re-anodized in 21^βC H₃PO₄ at 15 volts DC for 2 minutes, rinsed well, and transferred to a room temperature solution containing 25g/l NiS0₄.7H₂0, 20g/l MgS0₄.7H₂0, 25g/l H₃B0₄, and 15g/l (NH₄)₂S0₄ at pH 5.5. A voltage of 11 volts peak AC was applied between the anodized panel and a graphite electrode for a period of 20 seconds. The deposit was then stabilized by immersing for 2 minutes in a 350 ppm Pd (as PdS0₄.2H₂0) solution of pH 2. At this stage in the process the panel appeared medium bronze in colour. EXAMPLE 1

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) the panel was then rinsed and with it still being connected to positive a 20 volt DC cathodically biased graphite brush wetted in the pH 2 rinse water was brought into contact with the surface of the panel and stroked on localized areas as an artist would apply paint to canvas. The panel was then re- immersed in the anodizing electrolyte and anodized for an additional 30 seconds at 15 volts DC (the background colour was blue with yellow brush stroked areas) . The barrier modification procedure was repeated and the panel was anodized as before for 20 more seconds. The final result was a pink panel with a yellow and blue brush stroked pattern. Violet fringes to the yellow areas and green fringes to the blue areas were the result of a delayed current recovery where the barrier layer had not been thickened to the same extent as those areas where direct contact was made and residence time was sufficient. Processing was completed by sealing the anodic film in boiling water for 30 minutes. EXAMPLE 2

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 1 second with a 20 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm diameter yellow spot. Around the perimeter of the spot one could see a very thin pink line. EXAMPLE 3

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 0.1 second with a 20 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm crescent shaped pink spot. It was obvious that either the pulse duration or the 5 volt differential between the pulse and normal anodizing voltages was not sufficient to preclude a delayed current recovery hence a pink rather than yellow spot of odd shape. EXAMPLE 4

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 0.1 second with a 25 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm diameter yellow spot. Around the perimeter of the spot one could see a very thin pink line. EXAMPLE 5

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 0.01 second (10 milliseconds) with a 25 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm crescent shaped pink spot. It was obvious that either the pulse duration or the 10 volt differential between the pulse and normal anodizing voltages was not sufficient to preclude a delayed current recovery hence a pink rather than yellow spot of odd shape. EXAMPLE 6

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 0.01 second with a 30 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm diameter yellow spot. Around the perimeter of the spot one could see a very thin pink line. EXAMPLE 7

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 1 second with a 30 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 3mm diameter yellow spot. Around the perimeter of the spot one could see a pink line measuring 0.5-1 mm in width. EXAMPLE 8 Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . While still immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought within about 2mm of the panel surface. The wire was then pulsed for a period of 1 second with a 30 volt cathodically biased DC voltage. The probe was removed and normal 15 volt DC anodizing resumed for an additional 30 seconds. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a 10mm diameter yellow spot. Around the perimeter of the spot was a pink zone measuring 5mm in width. EXAMPLE 9

Following deposit stabilization, the panel was immersed in the sulphuric acid solution. While immersed, a 3mm diameter aluminum wire (electrically shielded with tape everywhere except its tip) was brought into contact with the panel. The wire was then pulsed for a period of 1 second with a 25 volt cathodically biased DC voltage. The probe was removed and 40 seconds of normal 15 volt DC anodizing was initiated. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a yellow panel with a 3mm diameter bronze spot. EXAMPLE 10

Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) . It was then removed and rinsed in water at pH 2. A fine stainless steel mesh grid was wrapped around a 3cm diameter rubber roller and connected via a graphite brush to a DC power supply. 20 Volts was applied to the cathodically biased roll grid and the anodically biased panel (still wet with rinse water) was passed under the roller at a speed of about lOm/min. The panel was re-immersed in the anodizing solution. 30 seconds of normal 15 volt DC anodizing was initiated. The panel was then rinsed and sealed in boiling water for 30 minutes. The result was a blue panel with a distinctive yellow grid pattern. EXAMPLE 11

An aluminum foil/polyester laminate was cut to 6cm by 15cm and anodized in 21"C 1.5M H₂S0₄ at 15 volts DC for a period of 1 minute to create a porous anodic film measuring less than 0.5 microns in thickness. The laminate was subsequently re-anodized in 30°C H₃P0₄ at 15 volts DC for 2 minutes, rinsed well, and transferred to a room temperature solution containing 25g/l NiS0₄.7H₂0, 20g/l MgS0₄.7H₂0, 25g/l H₃B0₄, and 15g/l (NH₄)₂S0₄ at pH 5.5. 11 volts peak AC was applied between the anodized panel and a graphite electrode for a period of 20 seconds. The deposit was then stabilized by immersing for 30 seconds in a 350 ppm Pd (as PdS0₄.2H₂0) , pH 2. Following deposit stabilization, the panel was immersed in the sulphuric acid solution and once again connected to the positive terminal of a DC power supply. 15 V was applied for a period of 40 seconds (the panel colour was yellow) the laminate was then rinsed and with it still being connected to positive a 20 volt DC cathodically biased graphite brush wetted in the pH 2 rinse water was brought into contact with the surface of the panel and stroked on localized areas as an artist would apply paint to canvas. The panel was then re-immersed in the anodizing electrolyte and anodized for an additional 30 seconds at 15 volts DC (background colour was blue with yellow brush stroked areas) . The laminate was then transferred to the phosphoric acid anodizing bath and anodized at 20 volts DC for 30 seconds. The voltage was subsequently step-wise reduced at a constant rate until after 200 seconds, 0 volts was being applied. The laminate was allowed to soak undisturbed for 60 seconds. It was then withdrawn, rinsed, and air dried. The surface appeared green with pink brush strokes. The pattern was no longer yellow because the applied 20 volts in the final anodizing bath caused the film to grow uniformly throughout, then thin uniformly as the voltage was being reduced. The patterned surface of the laminate was then heat seal laminated to plastic and this plastic layer was delaminated from the surface pulling with it the porous oxide film. At this point the pattern was lost since the aluminum reflecting layer was removed. This reflecting layer was replaced with 0.1 microns of sputter deposited gold, and with it the pattern and colours returned. The result was a coloured pattern transferred to plastic. EXAMPLE 12

Using equipment of the type shown in Fig. 11, two different patterned structures were prepared on a continuous basis.

The operating conditions for the two processes are shown in Tables 1 and 2 below. The process conditions shown in Table 1 results in a patterned structure having an optically thin top oxide layer, so the final product has an OMOM layered structure in which the lower oxide layer may be below the thickness required for optical interference colours in the wear modified by the patterning roller 50, and of a thickness which contributes to the apparent colour in the unmodified areas. The process condition of Table 2 produces a patterned 3-layer MOM structure buried under an optically thick upper oxide layer. In this case, the central oxide layer determines the interference colour observed, and is of different thickness in modified and unmodified areas. TABLE 1 PATTERNED THIN FILM INTERFERENCE COLOURS

Web Width: 15 cm

Web Speed: i.25 m/min

Liquid Contact 165 g/L sulphuric acid, 20°C Immersion 22 seconds Primary Anodize 165 g/L sulphuric acid, - 30°C Immersion 67 seconds 15.2 V (DC) , 21 A

Pore Modification 120 g/L phosphoric acid, 35°C Immersion 67 seconds 15.2 V (DC) , 6.0 A

Nickel Deposition ANOLOK 51X electrolyte, 25°C (NiS0₄, MgS0₄, H₃B0₃) Immersion 26 seconds - 10 V (AC) , 2.5 A

Palladium Immersion 400 ppm PdS0₄, 5 g/L H₂S0₄

Immersion 52 seconds

First Re-Anodize 165 g/L sulphuric acid, 25°C Immersion 19 seconds 15-17 V (DC), 0.7-1.5 A

Patterning 20 second fresh water immersion rinse before contact

29-44 V (DC) ,

Contact time ~ 1.1 seconds

Second Re-Anodize 165 g/L sulphuric acid, 25°C Immersion 22 seconds 15-17 V (DC), 2.5-4.0 A

Results:

Before re-anodizing, a bright yellow colour was produced across the strip. At low re-anodizing currents, a yellow pattern (where barrier layer thickening had occurred) on a red background was the final result. At successively higher re-anodizing currents, a red pattern on a grey- green background, then a green pattern on a pink background were produced.

At patterning voltages below - 35 V (DC) , the pattern was not well defined, or incomplete. TABLE 2 PATTERNED EXTENDED RANGE INTERFERENCE COLOURS

Web Width: Ϊ5 cm

Web Speed: 0.70 m/min

Liquid Contact 165 g/L sulphuric acid, 20°C Immersion 65 seconds Primary Anodize 165 g/L sulphuric acid, 38°C Immersion 121 seconds 15.2 V (DC) , 38 A

Pore Modification 120 g/L phosphoric acid, 35^"C Immersion 121 seconds 15.2 V (DC) , 5.5 A

Nickel Deposition ANOLOK 51X electrolyte, 25°C (NiS0₄, MgS0₄, H₃B0₃) Immersion 47 seconds ~ 10.0 V (AC) , 2.2 A

Palladium Immersion 400 ppm PdS0₄, 5 g/L H₂S0₄

Immersion 93 seconds

First Re-Anodize 165 g/L sulphuric acid, 25°C Immersion 37 seconds 16 V (DC) , 1.3 A

Patterning 60 second fresh water immersion rinse before contact 35-39 V (DC) , 0.2 A Contact time - 1.9 seconds

Second Re-Anodize 165 g/L sulphuric acid, 25°C Immersion 41 seconds 15.9 V (DC) , 3.2 A

Results:

At the re-anodizing currents given in this example, a blue pattern (where barrier layer thickening had occurred) on a red background was the final result. With no re-anodize before patterning, a range of coloured patterns on a light bronze background were created.

At patterning voltages below ~ 35 V (DC) , the pattern was not well defined, or incomplete. INDUSTRIAL APPLICABILITY

The present invention can be used for producing coloured patterns on various metal articles of manufacture or for producing a flexible film having areas of different colours.

Claims

CLAIMS :

1. A process for producing an article having a coating film exhibiting areas of different colour, in which a surface (16) of a substrate (10) made of or coated with aluminum or an anodizable aluminum alloy is anodized to produce an anodic film (11) on said surface (16) having a porous outer layer and a non-porous inner barrier layer (15) between said porous outer layer and said surface (16) , and a metal is electrodeposited into pores (12) in said anodic film (11) to generate a colour by effects including interference of light reflected from deposits (17) of said metal in said pores (12) and said surface (16) ; characterized in that a cathodically-biased electrode (20) is brought into contact with or close proximity to limited areas (A) of said porous anodic film (11) to cause said barrier layer (15) to thicken in said limited areas (A) only, thereby changing said colour in said limited areas (A) .

2. A process according to claim 1 characterized in that, prior to bringing said electrode (20) into contact with or close proximity to said film (11) having said deposits (17) therein, an additional anodization step is carried out to thicken said film between said deposits (17) and said surface (16) in order to change the colour exhibited by the film.

3. A process according to claim 1 characterized in that, after bringing said electrode (20) into contact with or close proximity to said film (11) , an additional anodization step is carried out to thicken said film between said deposits (17) and said surface (16) only in areas (B) of said film not contacted by, or brought into close proximity to, said electrode (20) .

4. A process according to claim 1 characterized in that, prior to bringing said electrode (20) into contact with or close proximity to said film (11) having said deposits (17) therein, a first additional anodization step is carried out to thicken said film (11) between said deposits (17) and said surface (16) in order to change the colour exhibited by the film, and in that, after bringing said electrode into contact with or close proximity to said film (11) , a second additional anodization step is carried out to thicken said film (11) between said deposits (17) and said surface (16) only in areas (B) of said film not contacted by, or brought into close proximity to, said electrode (20) .

5. A process according to claim 3 characterized in that said steps of bringing said cathodically-biased electrode

(20) into contact with or close proximity to limited areas (A) of said film (11) and said further anodization step are each repeated at least once, said limited areas (A) being made at least partially different from each other each time said cathodically-biased electrode (20) is contacted with or brought into close proximity to said film (11) .

6. A process according to claim 4 characterized in that said steps of bringing said cathodically-biased electrode (20) into contact with or close proximity to limited areas (A) of said film (11) and said second further anodization step are each repeated at least once, said limited areas (A) being made at least partially different from each other each time said cathodically-biased electrode (20) is brought into contact with or close proximity to said film

(11) •

7. A process according to claim 1, claim 2, claim 3, claim 4, claim 5 or claim 6 further characterized by carrying out a pore-branching electrolytic step in order to introduce weakened strata into said film (11) , adhering a transparent flexible member (21) to an outer surface (13) of said film (11) , detaching said film (11) from said metal substrate (10) and applying a layer (23) of reflective material to a thereby exposed surface (22) of said film (11) .

8. A process according to claim 1 characterized in that an electrolyte containing an acid is used for said anodizing step and in that, prior to said electrodeposition step, pores (12) in said porous region are increased in diameter, at least at inner ends (12') thereof, by subjecting said surface (16) to further electrolysis₄ in an electrolyte containing an acid having a greater solubilizing effect on said film (11) than said acid used in said anodizing step.

9. A process according to claim 1 characterized in that said cathodically-biased electrode (20) is a solid electrically conductive object and in that said electrode is brought into contact with said limited areas (A) of said film in the absence of added electrolyte when said film (11) is wet.

10. A process according to claim 1 characterized in that said metal electrodeposited into said pores (12) is an acid-resistant metal.

11. A process according to claim 1 characterized in that said metal electrodeposited into said pores (12) is at least partially replaced by or coated with an acid- resistant metal.

12. A process according to claim 1, characterized in that said process is carried out on a continuous basis on a flexible aluminum strip as said substrate.

13. A process according to claim 12, characterized in that said electrode (20) used in said process is a roller (50) having a flexible electrically conductive rubber surface over-printed in limited areas with an electrically non-conductive rubber compound.

14. A process according to claim 1, characterized in that said anodization step and said metal deposition step are carried out for such periods of time that a part of said film lying between said deposits and the outer surface (13) of the film (11) is optically thick.

15. A process according to claim 1, characterized in that said anodization step and said metal deposition step are carried out for such periods of time that a part of said film lying between said deposits and the outer surface (13) of the film (11) is optically thin.

16. An article having an anodic film exhibiting areas of different colour, having a reflective metal substrate (10) , a porous anodic film (11) overlying a surface (16) of said reflective metal substrate (10) , and metal deposits (17) in pores (12) of said film (11) , said deposits (17) having outer ends (18) separated from said surface (16) sufficiently to generate a colour by effects including light interference, characterized in that said film (11) has different areas (A,B) in which spacing

(X,X') between inner ends of said deposits (17) and said surface (16) are different from each other so that said different areas (A,B) exhibit different colours.

17. An article according to claim 16, characterized in that said spacings (X,X') have been made different by bringing a cathodically-biased electrode (20) into contact with or close proximity to said film (11) following deposition of said deposits (17) into said pores (12) .

18. An article according to claim 16 or 17 characterized in that said reflective metal substrate (10) comprises aluminum or an anodizable aluminum alloy.

19. An article according to claim 16 or 17 characterized in that said reflective metal substrate (10) comprises a vacuum deposited metal layer (23) .

20. An article according to claim 19 further characterized by a transparent overlayer (21) adhered to said anodic film (11) on a surface opposite to said metal layer (23) .

21. An article according to claim 16 or 17 further characterized by more than two of said different areas (A,B), and in that said spacings (X,X^**) are different in each of said different areas (A,B) so that said areas exhibit at least three mutually different colours.