Rust prevention

Details Rust prevention Rust prevention

1999-05-12

2001-05-22

Talbot, Brian K. (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Heating or drying

C427S397700, C427S397800, C427S406000

Reexamination Certificate

active

06235348

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preventing surfaces of a Zn-based metallic coating from rusting, and a rust preventive. Particularly, it relates to a rust preventive agent and a method for preventing surfaces of a base, such as metallic articles and metallic materials including steel plates used as parts or materials in manufacture of automobiles, building and so forth, coated with a Zn-based metal from rusting.
2. Description of the Prior Art
Plated steel plates, which are used as a material in automobiles, building, etc., such as steel plates plated with zinc or zinc-based alloy are used after painting. The plated steel plates are passed through a variety of steps before being painted; therefore they are left unpainted in the course of these steps for a considerably long time. So, while they are left unpainted, rust is generated or various substances are adsorbed or adheres, on the surfaces of the plated steel plates. As a result, for example, the adhesion of a paint applied in the subsequent step may be poor.
Thus, heretofore, the surfaces of plated steel plates have been subjected to chromate treatment as a primary rust prevention treatment. The corrosion resistance obtained by the chromate treatment is about 48 hours in the salt spray test defined in JIS Z-2371. However, the coating film obtained by the chromate treatment contains hexavalent chromium. It is known that the hexavalent chromium causes an allergy (chromate ulcer) when adhering a skin, and also it is pointed out that the hexavalent chromium is suspected to be a carcinogen (carcinoma cutaneum). So, a primary rust prevention treatment not using hexavalent chromium is required.
Meanwhile, as a primary rust prevention treatment other than the chromate treatment there is proposed a method using a coating agent for metal surfaces, which agent is comprised of carboxyl-modified epoxy resin or polyvinyl butyral resin containing a silica. The use of the coating agent results in a corrosion resistance equivalent to or higher than that obtained by the chromate treatment, but the appearance of the resulting coated metal surface is colorless or metallic. However, it is recently required that the metal surface coated by a primary rust prevention treatment has a black or blackish appearance so as to give a feeling of quality.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a rust preventive and a treating method, suitable as a primary rust prevention treatment, which forms a coating film having a corrosion resistance equivalent to that of the coating film obtained by the chromate treatment, but gives no fear of a health problem such as carcinogenesis and provides a black or blackish appearance to the treated metal surface.
The present inventors have earnestly studied in order to satisfy the above requirements, and as a result, have completed the present invention.
More specifically, the present invention provides a rust preventive for a Zn-based metallic coating, comprising (a) a silicic acid compound and (b) an aromatic amine-based condensation product.
The present invention also provides a method for preventing a Zn-based metallic coating formed on a base from rusting, which comprises forming a coating film (hereinafter, referred to as “first coating film” with respect to the “second coating film” mentioned below) comprising (a) a silicic acid compound and (b) an aromatic amine-based condensation product on the surfaces of the Zn-based metallic coating.
In the above method, preferably a second coating film comprising (a) a silicic acid compound is further formed on the first coating film.
According to the present invention, by forming a coating film comprising a silicic acid compound and an aromatic amine-based condensation product on the surfaces of a Zn-based metallic coating formed on a base such as a metal plate, it can provide a treating method, suitable as a primary rust prevention treatment. The resulting coating film exhibits an excellent corrosion resistance, gives no fear of a health problem such as carcinogenesis suspected in the conventional chromate treatment and provides a black or blackish appearance to the surfaces of the base.
Further by forming a second coating film comprising a silicic acid compound on the first coating film, it is possible to provide a higher corrosion resistance of 100 hours or more in the salt spray test defined in JIS Z-2371.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described below in more detail.
Firstly, the essential components used in the rust preventive and the method according to the present invention are described.
(a) Silicic Acid Compound
The silicic acid compound used in the present invention is preferably, for example, at least one compound selected from the group consisting of silicic acid esters, colloidal silica and alkali metal silicates.
The silicic acid esters include, for example, alkoxy silane compounds such as tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, tetra-i-propoxysilane, tetra-n-butoxysilane, tetra-i-butoxysilane, tetra-t-butoxysilane, methyltrimethoxysilane, methyltriethoxysilane, methyltri-n-propoxysilane, methyltri-i-propoxysilane, methyltri-n-butoxysilane and methyltri-t-butoxysilane; and condensates of these alkoxysilane compounds.
The condensates of alkoxysilane compounds can be generally produced by partially hydrolyzing the alkoxysilane compound and then condensing the hydrolysis product. The condensates of alkoxysilane compounds include, for example, ethoxypolysiloxane, which is a condensate of tetraethoxysilane, (trade name: “Ethyl silicate-40”, produced by Tama Kagaku Kogyo K.K.).
The colloidal silica is a colloid containing super fine particles of silicon dioxide (silica) dispersed in a dispersion medium including an aqueous medium or a non-aqueous medium such as methanol, propanol or ethylene glycol. As typical methods for producing a colloidal silica comprised of colloid particles having a particle diameter of generally 5 to 500 nm, for example, a colloidal silica dispersed in an aqueous medium, there is known a method producing a colloidal silica by adding hydrochloric acid to an aqueous solution of sodium silicate. There is also known a method producing a colloidal silica by adding a small amount of water and ethylamine as a catalyst to silicic acid esters such as tetraethoxysilane in an organic solvent such as isopropylalcohol to subject the silicic acid esters to hydrolysis reaction. However, the colloidal silicas produced by any known methods are available in the present invention, and methods for the production thereof are not particularly limited.
The alkali metal silicates include, for example, metasilicates (M
2
SiO
3
), orthosilicates (M
4
SiO
4
), disilicates (M
2
Si
2
O
3
), trisilicates (M
3
Si
3
O
7
) and sesquisilicates (M
4
Si
3
O
10
) (wherein in these formulae, M represents an alkali metal such as lithium, sodium or potassium), of alkali metals; and water glass. Among the silicic acid compounds, preferred is colloidal silica from the viewpoint of obtaining good corrosion resistance.
(b) Aromatic Amine-Based Condensation Products
The aromatic amine-based condensation products used in the present invention are exemplified by:
(A) an aromatic amine compound condensation product,
(B) an alkali-treated product of the aromatic amine compound condensation product (A),
(C) a higher fatty acid-treated product or aromatic carboxylic acid-treated product of the aromatic amine compound condensation product (A), and
(D) an alkylation product, alkenylation product and aralkylation product of the aromatic amine compound condensation product (A). These compounds may be used singly or in combination of two or more.
The aromatic amine compound condensation product (A) used includes preferably one having a weight-average molecular weight in the range of 1,000 to 100,000 in terms of polystyrene when measured by gel permeation chromatography (GPC), and particularly 1,500 or 50,000 fr

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