Method for forming an abrasion-resistant white oxidized surface film of high opacity on an aluminium substrate

Abstract

 A method is proposed for forming an abrasion-resistant anodized oxide film on an aluminium article, which is coloured in white with high opacity. The method comprises a step of a pre-treatment of the anodized aluminium article by dipping in a specific liquid such as an extract of rice bran prior to whitening by dipping in an aqueous hydrochloric acid solution. The whitened aluminium article is further coated with a urethane-acrylate resin using a resin solution of a specified concentration diluted with butyl acetate.

Description

[0001] The present invention relates to a method for forming an abrasion-resistant white oxidized surface film of high opacity on a substrate of aluminium or an aluminium-based alloy.

[0002] It is an established industrial process that most of articles made from aluminium or an alloy mainly composed of aluminium are subjected to an anodization treatment in order to be provided on the surface with a protective coating film of oxidized aluminium. As a modification of this well established electrolytic technology, various proposals and attempts have been made when the coating film thus formed should be coloured to comply with various aesthetic requirements. For example, coloration of the anodized surface film in amber, dark grey, dark red, olive, dark brown and other colours can be obtained by conducting the anodizing electrolysis in an acidic electrolyte bath containing various kinds of metal ions such as nickel, tin and the like. These methods are widely employed in the manufacture of various kinds of aluminium-made articles used in architectural works and elsewhere such as window sashes, garden fences and the like where the articles are required to be highly durable as well as aesthetic. When such a specific colouring means is not undertaken, the coating film on the anodixed aluminium article usually exhibits a semi-transparent appearance in the colour of white or light yellow with dullness or haziness due to the numberless micropores in the coating film.

[0003] The above described method is not very versatile in respect of the kind of the colours in which the anodized surface film on an aluminium article is coloured. For example, it is sometimes desirable that an anodized aluminium article is coloured in white with high opacity and the above mentioned method is not applicable here. Several methods, of course, have been proposed in the prior art for the formation of a white-coloured anodized coating film on an aluminium substrate. For example, Japanese Patent Publication 35-14519 teaches a two-step electrolytic method by using a sulphuric acid-based and chromic acid-based electrolytic baths. Further, Japanese Patent Publication 41-1523 teaches a method in which the anodization treatment is performed following a pre-treatment step of surface etching by an AC-DC electrolytic immersion treatment. These prior art methods have problems that the coating film obtained thereby has no bright whiteness but rather greyish shade with low opacity and is relatively poor in respect of the abrasion resistance.

[0004] Alternatively, Japanese Patent Publication 48-5088 teaches a method for forming an abrasion-resistant and highly opaque white-coloured coating film on an anodized aluminium article, according to which the aluminium article is first anodized in a conventional manner and then immersed in an acidic or alkaline aqueous solution so as to partly dissolve away the oxidized coating film followed by washing with water and drying so that the surface can be imparted with white appearance. The article is then washed again with water and subjected to a treatment of electrodeposition of a resin for electrodeposition such as acrylic and alkyd-based ones within the micropores of the coating layer. This method, however, is industrially not advantageous in respect of the many steps to be successively undertaken including the rather troublesome electrodeposition of a specific resin.

[0005] As a general matter, the oxide coating film formed by anodization on the surface of an article made from aluminium or an aluminium-based alloy is highly porous and water-absorptive so that, even when a white coating film could be obtained by a prior art method, the white appearance is lost when the surface is wet with water or subjected to a hydration sealing treatment. In addition, the white coating film formed by anodization in the prior art method is usually poor in respect of the resistance against abrasion.

[0006] Accordingly, the inventors have conducted extensive investigations with an object to develop a novel and efficient method for the formation of a highly abrasion-resistant white oxidized coating film of high opacity on the surface of a substrate article made from aluminium or an aluminium-based alloy without the above mentioned problems and disadvantages in the prior art methods.

[0007] Thus, the method of the present invention for forming an abrasion-resistant white oxidized coating film on the surface of a substrate made from aluminium or an aluminium-based alloy comprises the steps of:

(a) anodizing the surface of a substrate article made from aluminium or an aluminium-based alloy to form an oxidized coating film;

(b) immersing the anodized substrate article in an aqueous pre-treatment liquid which is an extract of a fermentative enzymatic material having a pH in the range from 4 to 10;

(c) immersing the substrate article after step (b) in an aqueous hydrochloric acid solution having a pH in the range from 0.3 to 2 to effect whitening of the surface;

(d) coating the thus whitened surface of the substrate article with an ultraviolet-curable acrylate resin solution of 8 to 30 times dilution with butyl acetate as a diluent; and

(e) drying and curing the coating layer of the resin.

[0008] In particular, the above mentioned fermentative enzymatic material can be an intermediary product in a brewing process of an alcoholic beverage, e.g., beer, Japanese sake, wine and the like, or an extract or washing of a cereal or pulse material such as rice, rice bran, soybeans, soybean-curd lees and the like.

[0009] Further, the ultraviolet-curable acrylate resin is preferably a urethane-acrylate resin or an epoxy-acrylate resin.

[0010] The substrate article, on which a white oxidized coating film is formed according to the inventive method, is made from high-purity aluminium or an aluminium-based alloy such as an alloy of aluminium containing 10% by weight or less of magnesium within the range to form a solid solution. It is of course not particularly detrimental that the aluminium or the alloy contains a small amount of impurity elements or other metallic elements such as chromium, manganese and the like provided that the clarity of the coating film of aluminium oxide as formed is not substantially decreased thereby.

[0011] The procedure for the anodization in step (a) of the inventive method is not particularly limitative and can be conventional, for example, using an aqueous sulphuric acid solution as the electrolyte bath.

[0012] The pre-treatment in step (b) is important in order to achieve full whitening of the anodized coating film. This pre-treatment is performed by immersing the anodized substrate article in a pre-treatment liquid or solution containing a fermentative enzymatic material which can be an intermediary product in brewing of an alcoholic beverage such as beer, Japanese sake, wine and the like or an extract or washing of a cereal or pulse material such as rice, rice bran, soybeans, soybean-curd lees and the like. The liquid should have a pH adjusted to 4 to 10. Although a sufficient effect as desired can be obtained with an immersion time of 1 to 6 hours, the exact length of the immersion time should be selected after a preliminary test taking various factors into consideration.

[0013] In step (c) of the inventive method, the aluminium article after the above mentioned pre-treatment is immersed in an aqueous hydrochloric acid solution having a pH in the range from 0.3 to 2. It has been an unexpected discovery that, while an anodized oxide film on the substrate surface without the pre-treatment in step (b) is attacked by and readily dissolved away in a hydrochloric acid solution as a result of the rapid and instantaneous permeation of the acid even into the finest micropores in the oxide layer reaching the substrate surface, the oxide film after an adequate pre-treatment in step (b) is no longer attacked by a hydrochloric acid solution presumably as a consequence of the formation of a barrier against permeation of the acid solution into the oxide film. The conditions in steps (b) and (c) are the factors determinant of the nature of the resulting white oxide coating film such as the thickness, hardness, whiteness, opacity and the like. In this regard, the nature of the pre-treatment liquid used in step (b) is very important to ensure full permeability to any finest micropores and high protecting effect against the attack of the oxide layer by the acid solution. The performance of the pre-treatment liquid used in step (b) can be improved by the optional addition of various kinds of certain chemicals having acidic auxochromes, e.g., hydroxy groups, such as tannic acid, tin hydroxide, lactates, formates and the like. Acetic acid, glucose and the like can also be used in the same purpose.

[0014] When the immersion time in the hydrochloric acid solution is unduly extended, for example, even the anodized oxide film after the pre-treatment in step (b) is attacked by the acid and finally dissolved away not to give a whitened coating film. When the concentration of the acid solution is too high, the whitened coating film may have a somewhat decreased hardness. When the concentration of the acid solution is too low, on the other hand, no sufficient effect of whitening can naturally be obtained. Thus, the concentration of the acid solution and the length of the immersion time therein are the important parameters which determine the whitening effect. At any rate, it should not be intended to obtain a whitened oxide coating film of unnecessarily high whiteness or opacity since otherwise the whitened oxide coating film may eventually have a powdery appearance with decreased adhesion to the substrate surface so as to readily fall off by rubbing.

[0015] The whitened oxide coating film on the substrate surface formed in the steps (a) to (c) described above is then coated with an ultraviolet-curable acrylate coating resin so as to be imparted with further enhanced resistance against abrasion and scratches by virtue of the protective coating layer of the resin. Examples of suitable ultraviolet-curable acrylate resins include urethane-acrylate resins and epoxy-acrylate resins though not particularly limitative thereto. Commercial products of these types available on the market include, for example, Unidic V-4205 and Unidic V-5500 each manufactured by Dai-Nippon Ink Chemical Co.

[0016] As is described above, the inventive method comprises a very unique step of a preliminary immersion treatment of an anodized aluminium article in a specific liquid which is an extract of a fermentative enzymatic material. This preliminary treatment is effective to have the effect of the subsequent whitening treatment by dipping in a hydrochloric acid solution reaching the very depth of the porous anodized oxide layer to exhibit a colour of pure white. Further, the thus whitened coating film is overcoated with a specific resin in a specific concentration in a specific diluent solvent. As a consequence, the thus finished aluminium article has excellent weatherability, insusceptibility to stain and very high resistance against chemicals in addition to the greatly improved abrasion resistance. Accordingly, the method of the invention can be widely applied to the manufacture of white-coloured ornamental aluminium articles, display panels, materials for fine arts and the like although the applicability of the inventive method is not limited to purely white articles but the whitened coating layer can be imparted with a tint of light yellow or pale red when the aluminium of the substrate is alloyed with a small amount of chromium or a small amount of manganese, gold and the like, respectively.

[0017] In the following, examples are given to illustrate the method of the invention in more detail but not to limit the scope of the invention in any way. Example 1.

[0018] An aluminium plate of 99.99% purity was, after a degreasing treatment and washing with water, anodized in an aqueous solution of sulphuric acid in a concentration of 180 g H₂SO₄/litre as an electrolyte bath at 15 °C for 80 minutes at a current density of 1.5 A/dm² so as to form a transparent oxide film having a thickness of about 44 µm on the surface. The thus anodized aluminium plate was immersed and kept for 48 hours in an aqueous extract solution of rice bran having a pH adjusted to 6 under agitation as a preliminary treatment and then immersed for about 2 hours in an aqueous hydrochloric acid solution having a pH of about 0.7 so that conversion of the transparent oxide film took place into white with beautifulness.

[0019] The aluminium plate provided with the whitened oxide layer thereon was further coated with a butyl acetate solution of a urethane-acrylate resin (Unidic 17-806, a product by Dai-Nippon Ink Chemical Co.) in varied concentration of the master resin as purchased by dilution up to 32 times and dried to form a protecting surface film. When the coating liquid was the master resin as such, the coating film had a transparent appearance while the film was fully white allover the surface when the coating liquid was prepared by 8 times to 32 times dilution of the master resin. When a coating liquid of 2 times or 4 times dilution was used, the appearance of the coating film was not uniform.

[0020] The thus obtained coated surfaces were subjected to the test of abrasion resistance by using a testing machine specified in ISO 8252:1987. The abrasive powder used there was a silicon carbide powder C#100 specified in JIS R 6111 freely falling at a rate of 20 to 30 g/minute and blown at the surface from a 2 mm diameter nozzle under a jet pressure of 116 mmHg (15 kPa). The results were recorded by the length of time in seconds until the coating film was worn out by the abrasive powder determined by the appearance of electric conduction. The results were as shown below. Incidentally, conventional anodized aluminium plate has an abrasion resistance of about 13 seconds as determined in the same manner.

Claims

1. A method for forming an abrasion-resistant white oxidized coating film on the surface of a substrate made from aluminium or an aluminium-based alloy which comprises the steps of:

(a) anodizing the surface of a substrate article made from aluminium or an aluminium-based alloy to form an oxidized coating film;

(b) immersing the anodized substrate article in an aqueous pre-treatment liquid which is an extract of a fermentative enzymatic material having a pH in the range from 4 to 10;

(c) immersing the substrate article after step (b) in an aqueous hydrochloric acid solution having a pH in the range from 0.3 to 2 to effect whitening of the surface;

(d) coating the thus whitened surface of the substrate article with an ultraviolet-curable acrylate resin solution of 8 to 30 times dilution with butyl acetate as a diluent; and

(e) drying and curing the coating layer of the resin.

2. The method according to claim 1 wherein the fermentative enzymatic material is rice bran.

3. The method according to claim 1 wherein the acrylate resin is a urethane-acrylate resin.