Process for hydrophilic treatment of aluminum materials and...

Details Process for hydrophilic treatment of aluminum materials and... Process for hydrophilic treatment of aluminum materials and...

2000-02-24

2002-01-15

Cameron, Erma (Department: 1762)

Coating processes

Applying superposed diverse coating or coating a coated base

Synthetic resin coating

C427S419100, C427S419200

Reexamination Certificate

active

06338876

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART STATEMENT
This invention relates to a process for hydrophilic treatment of articles made from aluminum or aluminum alloys (hereinafter referred to as aluminum materials) to bestow hydrophilicity on the surface, to primers useful for said hydrophilic treatment and to hydrophilic coatings.
Aluminum materials are generally characterized by lightweight, good processability, and excellent thermal conductivity and are widely used in air conditioners as fins in the heat-exchanging units, in construction materials such as sashes, and in many other applications. A variety of coatings are applied to the surface of aluminum materials depending upon where and for what purposes they are used.
In particular, bestowment of hydrophilicity on the surface of fins in heat-exchanging units or of construction materials is extremely important as it serves the purpose of preventing moisture condensation and reflector fogging or keeping construction materials from staining (stains adhering to the surface are washed away by rainwater if the surface is highly hydrophilic); furthermore, in the case of fins in heat-exchanging units, raising of thermal efficiency contributes to saving of energy.
However, when a coating designed for bestowing hydrophilicity is applied directly to the surface of an aluminum material, the coating film itself preferentially adsorbs moisture in the air; as a result, the moisture content on the surface increases and the aluminum material tends to corrode or a hydrated oxide layer formed as a result of corrosion tends to grow thicker and besides such oxide is brittle. Hence, the hydrophilic film formed on the surface of the aluminum material peels off relatively easily.
Several procedures for anticorrosive priming have hitherto been proposed to solve the aforementioned problems: for example, (1) to form a film chemically on the surface of aluminum by a chromate-, titanium- or zirconium-based primer; (2) to apply a primer called coat-type chromate to the surface of aluminum and dry by heating to insolubilize the primer film; and (3) to apply a resin-based primer. Incidentally, the films to be obtained by the aforementioned procedures are fundamentally water-repellent and, in case such films are to be formed on the fins in heat exchangers, they must indispensably be treated to render them hydrophilic, for example, by the following procedures: {circle around (1)} to apply water glass [for example, as described in Japan Kokai Tokkyo Koho Sho 58-126989 (1983)]; {circle around (2)} by to apply coatings composed of organic resins and silica or to apply these coatings together with surfactants; {circle around (3)} at to apply coatings composed of organic-inorganic (silica) composite coatings and surfactants [for example, as disclosed in Japan Kokai Tokkyo Koho Sho 59-170170 (1984)]; and {circle around (4)} to apply coatings composed of organic hydrophilic resins [for example, as disclosed in Japan Kokai Tokkyo Koho Sho 64-38481 (1989), Japan Kokai Tokkyo Koho Hei 1-299877 (1989), and Japan Kokai Tokkyo Koho Hei 5-302042 (1993)].
However, these known procedures for anticorrosive priming and hydrophilic treatment had latent problems and some of them have already become reality while others are beginning to attract attention as to their existence.
As for the anticorrosive priming, chromium-based chemical primers and coat-type chromate are still widely used at present as they readily produce films of good corrosion resistance at low cost.
However, these chromium-based primers are suspected of carcinogenicity and, besides, highly toxic Cr(VI) is present not only in the course of treatment but also in the films formed; in consequence, they are beginning to be placed under rigid regulation from the standpoint of both environmental protection and recycling. On the other hand, titanium- or zirconium-based chemical primers are free of such hazards inherent in chromium-based primers, but the growth rate of their films is so low that the productivity needs to be sacrificed to develop sufficient corrosion resistance. In the case of resin-based primers, an increase in film thickness can secure required corrosion resistance, but this lowers the thermal conductivity which is an extremely important characteristic of materials to be used in fabricating fins.
As for the hydrophilic treatment, the films formed from water glass have a merit of low cost, but they have been criticized for abrading the metal mold or emitting a “strongly musty odor” at the startup of an air conditioner in cooling mode. Moreover, rapid loss of their hydrophilicity in a short time by the action of environmental pollutants such as VOC (volatile organic compounds) emitted from newly developed construction materials and from flooring has recently become a tangible problem and there is an incipient move to restrict their usage greatly. The films formed from organic-inorganic composite primers suffer less from the problem of odor or influenced less in hydrophilicity by environmental pollutants than the films of water glass, but they cannot remain unaffected by the problem of abrasion of the metal mold on account of the use of silica in them. By comparison, the organic hydrophilic films have a merit of being affected insignificantly by such problems as abrasion of the metal mold, odor, and degradation of hydrophilicity by environmental pollutants, but it is pointed out that the organic hydrophilic films have shortcomings of tending to lose hydrophilicity under the influence of heat or press oil more easily than the inorganic films.
Regarding the aforementioned effect of heat, the organic films are heated in two ways; {circle around (1)} baking and drying of coatings and {circle around (2)} drying by heating of volatile press oil applied to the hydrophilic film in order to raise the lubricity during press molding. Heat is applied normally at 200 to 300° C. for several seconds to ten and several seconds in the case {circle around (1)} and at 100 to 200° C. for several minutes to several tens of minutes in the case {circle around (1)}.
Why the hydrophilicity deteriorates by heat is not known clearly, but the plausible explanation is that the hydrophilic groups react with other functional groups during heating resulting in the decrease in the number of hydrophilic groups in{circle around (1)} and the hydrophilic groups present on the surface deteriorate by heat or the hydrophilic groups appearing on the surface of the film become unstable energy-wise on account of the hot air itself being hydrophobic and migrate into the more stable inside of the film to cause a decrease in the number of hydrophilic groups on the surface with a concomitant decrease in hydrophilicity in the case {circle around (2)}.
A remedial measure taken for the case {circle around (1)} has been to avoid the simultaneous use of components reactive with the hydrophilic groups or to avoid conducting baking and drying at temperatures favorable for accelerating the reaction of the hydrophilic groups, but under existing circumstances the range of control is too narrow to develop sufficient countermeasures on a commercial scale. As for the case {circle around (2)}, no countermeasure whatsoever is available at the present time.
The effects of press oil are as follows. Press oils currently in practical use are said to be composed mainly of paraffinic hydrocarbons and additives for improving the lubricity. Decrease in hydrophilicity by press oil is ascribed to a part of the components of press oil remaining on the surface of the film. In the case of the films of water glass, the films are alkaline and the residual organic substances are saponified to become soluble in water and the decrease in hydrophilicity posed virtually no problem. However, it is a big problem in the organic films and a variety of proposals have been made to deal with the problem.
For example, Japan Kokai Tokkyo Koho Sho 62-234926 (1987) proposes to apply water-soluble organic polymers of low molecular weight on the hydrophilic films to avo

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