Anodizing Process for Producing Highly Reflective Aluminum Materials Without Preliminary Brightening Processing

This invention relates to anodizing aluminum. More particularly, this invention relates to improvements in the anodizing to produce highly reflective surfaces on bright rolled aluminum materials which have not been subjected to a preliminary brightening process prior to anodization. 2. Description of the Prior Art

Highly reflective surfaces may be produced on an aluminum material by proper selection of the alloy constituents, bright rolling or mechanical polishing of the aluminum surface and processing of the highly polished or bright rolled surface in a brightening bath which may comprise either electrobrightening or chemical brightening. The highly reflective surface so produced is then protected by anodizing the aluminum to provide a thin, transparent, protective layer of aluminum oxide on the surface as is well known to those skilled in the art. Various attempts at improving the reflectivity of the product have been proposed through the years. One approach is to vary the type of brightener used to treat the aluminum surface prior to anodizing. Typical of such an approach is the aluminum phosphate chemical brightening bath disclosed in U.S. Pat. No. 3,530,048 which uses a combination of aluminum phosphate, nitric acid, phosphoric acid, and copper sulfate. The brightened aluminum surface, according to the patentees, is then anodized in a sulfuric acid bath having a concentration of from 12 to 20 wt. % at a temperature of 20.degree. to 26.6.degree. C. (70.degree. to 80.degree. F.) using a current of about 9.26 to 13.89 amperes per square decimeter (10 to 15 amperes per square foot).

It is also known to vary the alloy constituents to improve the reflectivity of the aluminum surface. U.S. Pat. No. 3,720,508 discloses an aluminum alloy used in the production of a highly reflective aluminum surface which contains from 0.5 to 3% magnesium, from 0.2 to 0.5% silver, from 0.001 to 0.2% iron and from 0.001 to 0.15% silicon.

It is also known to provide additives in the anodizing bath to attempt to improve the bright or reflective surface of aluminum. For example, U.S. Pat. No., 3,671,333 provides for the addition of a natural or synthetic hydrophilic colloid to the reflective aluminum surface during anodizing of the aluminum by adding the colloid to the anodizing bath. Surface coatings produced during the anodization are alleged to be much thinner and apparently more compact than previous anodized aluminum coatings which the patentees allege is believed to be due to the larger molecule of the colloid forming as a colloidate on the reflective surface which apparently compacts the aluminum oxide formed. The thinner coating is then alleged to provide better reflectivity while eliminating the disadvantages of a thin normal anodized coating.

Other attempts at varying the anodization process include the use of AC anodizing using a sulfuric acid bath as shown in British Pat. No. 1,439,933. High current densities of 1 to 10 amperes per square decimeter (about 10 to 90 amperes per square foot) are proposed in U.S. Pat. No. 4,252,620 for use with a highly concentrated sulfuric acid anodizing bath containing 50 to 60% sulfuric and oxalic acid or nickel sulfate to produce a porcelain-like texture although no improvement in reflectance is alleged or apparently desired by the patentee.

U.S. Pat. No. 4,225,399 discloses an anodizing process using a current density greater than 1.5 Amps per square decimeter (13.89 Amps per square foot) in a 20-30 wt. % sulfuric acid anodizing bath at either 30.degree. C..+-.2.degree. C. for soft oxide films or 5.degree. C..+-.2.degree. C. for hard anodizing coatings. Again, the patentee makes no mention of such anodizing procedures having any effect on the reflectance of the resultant product.

It is, therefore, apparent that heretofore little, if any, attempts have been made to improve the reflectivity of an aluminum alloy by altering the anodization parameters to maximize the total reflectance of the anodized aluminum surface of the aluminum material.

In our aforementioned parent patent, U.S. Pat. No. 4,483,750, cross-reference to which is hereby made, we disclosed and claimed a novel anodizing process which produced a surprisingly superior reflectance. In the process claimed therein, the aluminum surface is first subjected to a brightening process and then is anodized by the claimed process. Subsequently, however, we discovered, as described and claimed in our aforementioned parent application Ser. No. 651,912, that the process was so effective, it could be applied to an aluminum surface which had not been previously subjected to a brightening process and still yield an anodized product having a reflectance which, while not quite as good as the results described in our aforementioned parent patent, were still as good or better than prior art reflectances obtained using prior art processes on surfaces which had been previously subjected to brightening processes prior to anodization.

It is, therefore, an object of the invention to provide an improved process capable of producing highly reflective anodized aluminum materials from surfaces which have not been previously subject to brightening processes.

It is another object of the invention to provide an improved process capable of producing highly reflective anodized aluminum materials from surfaces which have not been previously subject to brightening processes by optimizing anodization parameters used to provide the anodized finish on the reflective aluminum surface.

It is yet another object of the invention to provide an improved process capable of producing highly reflective anodized aluminum materials from surfaces which have not been previously subject to brightening processes by providing an improved range of sulfuric acid concentration, current density and temperature range to be utilized during the anodization process.

These and other objects of the invention will be apparent from the description of the preferred embodiment and the accompanying flowsheet.

In accordance with the invention, an improved process for the production of aluminum reflective material having a higher total reflectance value is provided which comprises controlling the anodizing conditions of an aluminum alloy which preferably has been subjected to bright rolling or other mechanical brightening by immersing the alloy in a DC anodizing bath containing at least 26 wt. % sulfuric acid and anodizing the sheet at a current density of at least 1.94 amperes per square decimeter (18 amperes per square foot) at a temperature of at least 15.56.degree. C. (60.degree. F.) for a time period sufficient to form a coating thickness of from about 0.06 to about 0.22 mils.

FIG. 1 is a flow sheet illustrating the process of the invention.

FIG. 2 is a series of contour curves illustrating the interrelationship between the anodizing parameters.

In accordance with the invention, an improved anodizing process is provided for the production of highly reflective aluminum material from an aluminum alloy. The alloy is either bright rolled at the plant or else is first mechanically finished or polished to provide a smooth surface. The material may then be treated in a brightening bath which may comprise a chemical brightener or an electrobrightener. However, in the practice of the present invention, the use of a brightening bath is not necessary when the surface is anodized in accordance with the invention. The bright rolled or polished aluminum surface is then anodized in accordance with the invention to provide the desired highly reflective surface.

Although the process of the invention may be successfully utilized using any of the conventional aluminum alloys normally used in the production of reflectorized aluminum materials, such as aluminum reflector sheet or the like, preferably the aluminum alloy comprises 0 to 2.5 wt. % Mg, 0 to 1 wt. % Fe, 0 to 0.2 wt. % Cu and 0 to 0.2 wt. % Mn.

The aluminum alloy material used to form the highly reflective product may comprise as-rolled or bright rolled sheet or may be subjected to any conventional mechanical polishing techniques as are well known to those skilled in the art. When the aluminum material is subjected to a conventional brightening step, it may, for example, comprise a treatment with a chemical brightener, such as the Alcoa 5 chemical brightening treatment. This treatment comprises the use of a hot mixture of 85% phosphoric acid and 70% nitric acid which is initially mixed in a 19:1 volumetric ratio, although this ratio will change during use due to accumulation of aluminum phosphate in the solution.

The brightened aluminum surface is then anodized to provide a protective layer of aluminum oxide over the brightened aluminum surface. In accordance with the invention, a sulfuric acid bath is used having a concentration of from 26 to 32 wt. % sulfuric acid, preferably 28 to 32 wt. % sulfuric acid. The temperature of the bath during anodizing is maintained, in accordance with the invention, at from about 15.56.degree. to 27.78.degree. C. (60.degree. to 82.degree. F.), preferably 19.44.degree. to 27.78.degree. C. (67.degree. to 82.degree. F.), more preferably 21.11.degree. to 26.67.degree. C. (70.degree. to 80.degree. F.), and most preferably about 22.78.degree. to 23.89.degree. C. (73.degree. to 75.degree. F.).

The reflective aluminum material is subjected to DC anodizing, i.e., anodizing using direct current with the reflective aluminum material serving as the anode, while maintaining a current density of at least 1.94 amperes per square decimeter (18 amperes per square foot), preferably from 2.92 to PG,9 7.78 amperes per square decimeter (27 to 72 amperes per square foot), and most preferably, from 3.24 to 4.86 amperes per square decimeter (30 to 45 amperes per square foot) during the time of anodizing.

The anodizing is carried out for a time period sufficient to provide an anodized coating thickness of from about 0.06 to about 0.22 mils. The necessary time to provide this coating thickness will vary with the current density. Generally, however, the time will range from about 0.5 minutes (to obtain a 0.06 mil coating at a current density of 72 Amps per square foot) to about 10 minutes (to obtain a 0.22 mil coating at 18 Amps per square foot). It is important, however, that the coating be at least about 0.06 mils to provide the minimum protection to the aluminum surface and yet not exceed about 0.22 mils to avoid undue attenuation of the reflectance of the surface.

After anodizing, the reflector material is rinsed in deionized water and the anodized coating is sealed by immersion in hot (95.degree. C.) water or a nickel acetate solution for 5 minutes and then removed and dried. Other, more involved, sealing techniques may be used, but may not be necessary.

The total reflectance of the anodized reflector may then be measured using an integrating sphere type total reflectometer, such as Dianos TRI Reflectometer which was used to produce the data in the examples below. Reflective materials, anodized in accordance with the invention, have total reflectance values usually over 80%, and in some instances, over 85% when measures using this technique.

The following examples will serve to illustrate the invention.

a number of sheet samples of 5005 type alloy were DC anodized in a sulfuric acid bath following chemical brightening in a hot mixture of 85% phosphoric acid and 70% nitric acid in a 19:1 ratio. Various combinations of acid concentrations, current densities, and bath temperatures were used. The results are shown in Table I.

The above Table I shows the descending order of total reflectance values of the as-processed samples, corrected coating weight, and corrected coating thickness values correlated with the processing parameters. Since variations in anodizing parameters cause differences in coating weight or thickness that have a known effect on reflectance, it was necessary to correct the data to a constant coating weight or thickness to eliminate this variable.

It will be seen that, in every instance, where all three parameters were in the range of the invention a total reflectance (uncorrected) of at least 79% was obtained. Furthermore, it will be noted that where one of the parameters is at the low end of the range, this may be compensated for by adjustment of one or both of the other parameters. It will be further noted that when all of the parameters fell within the preferred ranges, the total reflectance was 81.8%.

To further illustrate the process of the invention, a number of samples similar to those used in Example 1 were brightened as in Example 1 and then DC anodized in a 32 wt. % sulfuric acid bath at various temperatures and current densities. As shown in Table II, at this acid concentration, every sample had a total reflectance of at least 81.3%.

Based on the data produced in Examples 1 and 2, a series of contour curves were developed as shown in FIG. 2 to show the relationship between the three parameters of current density, sulfuric acid concentration, and bath temperature to achieve the desired total reflectivity. The results indicate that the improved process of the invention provides for the production of highly reflective aluminum whereby control and adjustment of the anodizing parameters can be made to maximize the total reflectance of the product.

To illustrate the process of the invention even further, various aluminum alloy compositions were buffed, chemically brightened and anodized using, respectively, conventional anodizing practices, i.e., 15% sulfuric acid, 21.1.degree. C. (70.degree. F.), 1.30 amperes per square decimeter (12 amperes per square foot) for 10 minutes and one embodiments of the improved process of the invention, i.e., 28% sulfuric acid, 23.33.degree. C. (74.degree. F.) and 4.54 amperes per square decimeter (42 amperes per square foot) for 3 minutes. Alloys with and without magnesium having various Fe/Si impurity levels were chosen for the test to illustrate the applicability of the invention to a wide range of alloy compositions that might be considered for aluminum reflectors. As shown in Table III, the process of the invention improved the total reflectance on all of the alloy combinations tested. The table further shows that the amount of improvement increases as the purity of the aluminum is decreased.

To illustrate another aspect of the invention, samples of AA3002 and AA5005 type alloys were treated as provided in Table IV and percent total reflectance measured. Samples of AA3002 and AA5005 type alloys, with and without being subject to a brightening process, were anodized using conventional and the improved anodizing. The results in Table IV show that bright rolled samples not subject to a chemical brightening process but anodized in accordance with the improved process in accordance with the present invention had a high level of reflectance.

To further illustrate the invention, anodizing times were calculated for samples anodized in accordance with the invention at several current densities to illustrate the times needed to obtain the minimum, maximum, and optimal coating thicknesses at various current densities within the range of the process of the invention.

Comparative tests were also run to illustrate the difference in times between the process of the invention and the prior art to achieve a coating thickness of 0.10 mil as well as the reflectivity achieved using both the prior art process and the process of the invention on a 5005 type alloy. The results are shown in Table VI below.

These results show that the process of the invention not only yields a superior reflectance at the same coating thickness, but permits this superior coating to be formed in a much shorter time as well.

Thus, the invention provides an improved process for the production of highly reflective aluminum whereby control and adjustment of the anodizing parameters can be made to maximize the total reflectance of the product and achieve a superior reflectance over that of prior art processes even when a preliminary brightening process is not used.