Process For Forming Resinous Films On Anodized Aluminum Substrates

Abstract

A process is disclosed for producing a smooth, level and firmly adhered resinous film on an aluminum or aluminum alloy surface. The process is carried out at certain temperatures to reduce the viscosity of water-soluble resin coatings.

Description

This invention relates to a process for producing on an aluminum surface a protective, ornamental film and more specifically to an improved process which permits the formation on an aluminum or aluminum alloy surface of a smooth, level and strong resinous film by immersion-deposit of a coating at certain elevated temperatures.

This invention is directed to improvements in the process disclosed in our copending U.S. Pat. application Ser. No. 27,424 filed on Apr. 10, 1970, wherein anodized porous aluminum or aluminum alloy surface is treated and covered with a resinous protective film while the surface is wet and moist subsequent to rinsing with water.

A number of processes have hitherto been proposed for treating and finishing aluminum-bearing substrates so that product aluminum articles are resistant to environmental attack, i.e., corrosion by acidic or alkaline atmosphere and present a continuous, smooth, level surface appearance.

One of the prior-art processes is an immersion-deposit process wherein pretreated aluminum-bearing substrates may be immersed in an immersion bath containing a thermosetting resin coating, pulled out, disposed to set and heated to harden. However, highly viscous resin coatings are difficult by such conventional processes to form a smooth, level and thick resinous film on an aluminum surface because the coatings tend to gather locally at the lower part of the surface and because of relatively high surface tension of the coating tending to retard the wetting of the aluminum surface. It is known that the viscosity and surface tension of thermosetting resin coatings decrease with increasing temperature.

Various coating processes employing heated coating compositions are known, including, a hot spray coating, a hot airless spray coating and a hot electrostatic coating. Since the coating according to these processes is heated immediately before applying, there are encountered no critical problems associated with preservation of the coating and evaporation of solvents.

However, in the immersion-deposit process conducted at elevated temperature a resin coating tends to polymerize resulting in increased viscosity and less stability for preservation. Furthermore, organic solvents incorporated in the coating tend themselves to quick vaporization with the result that the film obtained is not smooth or level. It has therefore been difficult to carry out the immersion-deposit process with success at an elevated temperature.

Whereas, it is an object of the present invention to provide a new, useful process which will eliminate or alleviate the foregoing disadvantages encountered with the prior-art processes for the production of coated films on aluminum or aluminum alloy substrates.

It is a further object of this invention to provide an economical process for forming on an aluminum or aluminum alloy surface a smooth, level, thick and firmly adhered film.

It is still another object of this invention to provide an improved process for forming a level and thick film on an elongate workpiece of aluminum or aluminum alloy whereby the coating is prevented from being locally collected on the workpiece.

These objects and other features of this invention will be apparent from the following description taken in connection with certain embodiments of the invention.

As a result of extensive reserch of the relationship between the aluminum surface characteristics and the coating compositions to be applied thereon, said relationship having a bearing upon the quality of the film, it has been found that the above-mentioned objects of the invention may be achieved by the use of certain water-soluble, thermosetting resin coating compositions and of certain process steps that appear hereinafter.

Briefly stated, the inventive concept of this invention resides in a process wherein a pretreated aluminum or aluminum alloy surface, which may be electrically anodized, is immersed without prior filling of cavities formed in the surface by said anodization in a coating composition comprising 20 - 35 percent of one or more of water-soluble thermosetting resins selected from the group consisting of high temperature reaction type acrylic resins, alkyd resins, acrylalkyd resins, melamine resins, phenolic resins, urea resins and aminoalkyd resins and 0.1 - 10 percent of high boiling, organic amine (stabilizer) such as dimethylethanolamine, monoethanolamine, mono normal propylamine, triethanolamine, tetraethylenepentamine and ethylene diamine, heated at 40.degree. - 70.degree.C, and allowed to set, dry and harden at 150.degree. - 200.degree.C over a period of 15 - 30 minutes.

Acrylic resins employed herein may be prepared by the process in which one or more of acrylic acid, methacrylic acid and esters thereof having a hydroxy group or other derivatives of acrylic acid are polymerized at 80.degree. - 120.degree.C in a high boiling water-soluble solvent such as ethylcellosolve, isopropylcellosolve, butylecellosolve, butylcarbinol in the presence of an initiator such as benzoylperoxide, and to the resulting polymers are added alcohol amines of high boiling point thereby obtaining water soluble amine salts.

It has been found that the water-soluble thermosetting resins when added in amounts of less than 20 percent of the coating composition will fail to form a desirably thick film on an aluminum surface, but when added in amounts exceeding 35 percent will produce irregularities in the surface finish.

Importantly, therefore the immersion-deposit process according to the present invention should be carried out at temperatures ranging between 40.degree. - 70.degree.C. Repeated experiments have indicated that temperatures lower than 40.degree.C tend to increase objectionably the viscosity of the coating, while temperatures higher than 70.degree.C provide inadequate viscosity.

The immersion operations need not be repeated any definite number of times, but it is sufficient to dip the aluminum substrate in the immersion bath only until its temperature grows substantially equal to the temperature of the coating.

The speed of pulling the substrate out of the immersion bath is also not imperative, but a preferred range of this speed is between 20 and 100 mm/sec. Satisfactory results have been obtained by setting the substrate for 10 - 15 minutes in the air heated at 30.degree. - 50.degree.C. The substrate should preferably be dried by means of an infrared drying apparatus or a hot air drying apparatus. It has been also found that a firmly adhered film as desired may be obtained by applying to anodized porous aluminum surface a coating which has been heated at a predetermined temperature in accordance with the invention. The coating when heated will readily penetrate into the pores or pits in anodized aluminum surface believably due to increased momenta of the coating particles.

The following examples are provided to further illustrate the process of the present invention, but these are not to be regarded as limiting.

EXAMPLES I - VIII

An aluminum test piece 50 .times. 50 .times. 1,000 mm in size was dipped in a 10% caustic soda at 70.degree.C to dissolve oxides and other foreign particles thereon, washed with water and subsequently with 10% nitric acid solution at room temperature and again with water. The pretreated test piece was subjected to anodization using a 15% H.sub.2 SO.sub.4 electrolyte operated with a DC 16 volts at a current density of 1 A/dm.sup.2 at a bath temperature of 20.degree.C. It was thereafter washed with water and immersed in a manner that the test piece was hung down longitudinally by a hoist cable and thus dipped for 2 minutes in an immersion bath containing the coating compositions (shown in Table 1) which have been diluted with water of 100 parts or 130 parts and heated at 40.degree.C or 60.degree.C. The test piece was taken out of the bath of a speed of 50 mm/sec. and disposed to set over a period of 10 minutes at 50.degree.C. The coated aluminum surface was then heated in an electrical furnace at a temperature of 180.degree.C for 20 minutes, until it was dried and hardened. A smooth and level resinous film was formed on the aluminum test pieces as shown in Table 2. The finished aluminum test piece was subjected to X-ray microanalyzer to reveal the presence of carbon about 2 microns deep in pores of the aluminum surface, indicating a firm deposit of the resin coating on the aluminum surface.

TABLE 1

Coat A B Water-soluble acrylic resins 40 40 High temperature reaction type melamine resins 10 10 Diethylene glycol monobutylether -- 38.5 Ethylene glycol monobutylether 38.5 -- Dimethylethanolamine 5.6 5.6 Water 11.5 11.5 Total 105.6 105.6 Note: The figures appearing in the foregoing Table are parts by weight.

TABLE 2

Level- Thickness of the film (.mu.) ing of Temp. upper lower dif. the Ex. Coat (.degree.C) end end film 9.7 12.0 2.3 9.4 12.5 3.1 well I B 40 10.5 12.5 2.0 (100) 9.9 12.3 2.4 8.3 11.0 2.7 9.1 12.1 3.0 Do. II B 40 8.4 11.4 3.0 (130) 8.6 11.5 2.9 10.3 14.2 3.9 11.6 14.9 3.3 Do. III A 40 11.3 14.8 3.5 (100) 11.1 14.6 3.5 10.5 12.3 1.8 10.6 11.4 0.8 Do. IV A 40 10.0 12.2 2.2 (130) 10.3 12.0 1.7 10.6 12.8 2.2 9.4 12.5 3.1 Do. V B 60 10.7 13.7 3.0 (100) 10.2 13.0 2.8 10.2 13.4 3.2 9.0 12.3 3.3 Do. VI B 60 10.1 11.9 1.8 (130) 9.8 12.5 2.7 10.2 14.0 3.8 11.0 14.9 3.9 Do. VII A 60 11.3 14.7 3.4 (100) 10.8 14.5 3.7 10.9 12.1 1.2 10.9 13.5 2.6 Do. VIII A 60 9.9 12.6 2.7 (130) 10.6 12.7 2.1 Note: Bracketed figures in the above Table indicate parts of added water to the coating.

The aluminum test piece coated in accordance with Example I was subjected to various performance tests as shown in Table 3.

TABLE 3

Test Results of Test Conditions of Test Gloss above 120 Reflection ratio of 60.degree. mirror surface Pencil hardness above 3H Mitsubishi UNI pencil Cross cut test 100/100 Erichsen test above 4.5 mm Erichsen tester Bending test below 3mm Bending tester Impact test .phi.1/2inch.times.500g.times. Dupont impact tester 50 cm Peeling test 23 kg/cm.sup.2 Riken Peeling tester Wear resistance 15.4 mg/1000 r.p.m. Taper wear tester (CS17500g) Yellowing resistance no change Sterilizing lamp(15W 20 cm) 500 hours Weathering test: by weather-ometer no change 500 hours by dew-cycle do. 100 hours in the open air do. 1 year Alkali resistance no change 5% NaOH 10 hours Acid resistance do. 5% H.sub.2 SO.sub.4 10 hours Humidity resistance do. 40.degree.C, 100% RH 400 hours Mortar resistance no change Portland cement 100 parts Sand 300 parts Water 100 parts 5 days Solvent resistance: xylol no change Immersion for 30 minutes at 20.degree.C methanol do. Immersion for 30 minutes at 20.degree.C Stain resistance rouge no change Wipe off after three days dry ink do. Wipe off with thinner after three days ink do. Water-wash after three days tobacco do. Wipe off with thinner after three days sauce do. Water-wash after three days Cass test rating No. 10 JIS method 80 hours Salt spraying test rating No. 10 JIS method 200 hours

Although the present invention has been illustrated by reference to specific examples, it will be understood that such various changes and modifications thereof will be apparent to those skilled in the art as fall within the scope of the appended claims.

Claims

What is claimed is:

1. A process for coating an aluminum or aluminum alloy surface which comprises: degreasing, cleaning and water-rinsing the surface; etching the water-rinsed surface with an alkaline solution; neutralizing and water-washing the etched surface; anodizing the surface; water-rinsing the anodized surface; immersing the surface in a resin coating composition comprising 20 - 35 percents of one or mone water-soluble thermosetting resins selected from the group consisting of high temperature reaction type acrylic resins, alkyd resins, melamine resins, phenolic resins, urea resins and aminoalkyd resins and 0.1-10 per cent of a high boiling organic amine, said composition being heated at 40.degree. - 70.degree.C; allowing the resins to set; and drying and hardening the coated surface at a temperature of 150.degree. - 200.degree.C for 15 - 30 minutes.