JPH0463108B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method of painting using a two-coat, one-bake (2C/1B) method using a topcoat paint that provides a coating film with excellent gloss that is suitable for use as a topcoat film for automobiles, etc. . Conventional technology Laminated paint films, typified by automotive paint films, usually include one coat of rust-preventing undercoat, one or two intermediate coats that assist the functions of the base coat and top coat, and one or two top coats. Finished with two coats of paint. In this type of coating system, in order to shorten the process and save baking energy, the intermediate coat and top coat are not baked each time, but are applied using a so-called wet-on-wet two-coat, one-bake method. is partially or partially adopted. However, although automotive top coat paints need to have an excellent appearance, when paints of different hues are applied using the wet-on-wet method, it is difficult to maintain the desired appearance and gloss due to the effects of blending and mist of paints of different hues. It was difficult to do so. In this way, even when the top coat is applied using the 2C/1B method, it is possible to provide a film that has the various performances required as a top coat and the expected performance such as adhesion with the base coat. Only then can it be put to practical use. Under these circumstances, conventional coating systems using the 2C/1B method mainly focused on improving coating film performance such as adhesion and abrasion resistance and finish quality. On the other hand, even if the same resin is used for the first and second layer paints in the 2C/1B coating system described above and paints with different pigment compositions are applied, the compatibility between the two paints is too good. It is well known that the paint film often lacks gloss. By the way, when painting with the 2C/1B method, the paint film with the high level of gloss that is the object of the present invention can be obtained even if different resins are used for the first and second layer paints in terms of finish, etc. Conventionally, it has been impossible to obtain a coating film with the desired full gloss because matte fading occurs easily. This means that even when there is no compatibility between the first and second layer paints, the appearance of the resulting paint film cannot necessarily be said to be good. This is because the surface tension of the wet coating film or the curing speed of the coating film differs due to the difference in pigment composition, etc. in the coating material. Furthermore, a paint with improved dispersibility of the paint composition itself has been proposed as a paint with a good finish. It was not possible to obtain an excellent finish as a film. In this way, parts such as the front panel, back panel, and inner panel of automobiles are painted using the 2C/1B method, and paints containing alkyd/melamine resins are often used as paints. As a paint whose main component is such an alkyd resin/melamine resin, for example, Japanese Patent Publication No. 57-43102
No. 4,808,301 discloses a coating material containing 80 to 60% by weight of alkyd resin/20 to 40% by weight of melamine resin. The composition of this paint was selected because if the melamine resin ratio exceeds 50%, the coating film becomes too hard, and conversely, if it is too low, the coating film strength is insufficient. However, when this paint is applied using a two-coat, one-bake method, there is a problem that the compatibility between the paints is too good and the paint lacks gloss. Purpose of the Invention The present invention eliminates the problems of the prior art described above, and provides a 2C/1B coating system with excellent dispersion, with a 60° specular reflectance of 90 or more and a 20° specular reflectance of 85 or more. The purpose is to provide a full gloss paint system. Structure of the Invention The coating method according to the present invention is such that when applying a top coat paint using a two-coat one-bake method, () the resin solid content ratio of alkyd resin/melamine resin is 80 to 60% by weight/20 to 40% by weight; A first layer coating film is applied using a paint containing 1 to 45% by weight of a coloring pigment, and then a coloring pigment is applied at a resin solid content ratio of alkyd resin/melamine resin of 70 to 50% by weight/30 to 50% by weight. The second layer coating film is coated using a paint containing 1 to 45% by weight of melamine resin and having a melamine resin content of 10 to 20% more by weight than the paint for the first layer. As the alkyd resin to be mixed into the coating material used in the method of the present invention, any resin obtained by reacting an ordinary alcohol component or a natural oil-modified fatty acid with an acid component can be used. On the other hand, as the melamine resin, any melamine resin conventionally used as a coating resin can be used, such as ordinary butyl etherified melamine resin and methyl etherified melamine resin. The melamine resin blended into the paint used in the method of the present invention acts as a curing agent for the alkyd resin, and the resin solid content is 20 to 30% by weight of the alkyd resin in the first layer paint. The amount
If it is less than 20% by weight, the strength of the coating film will be insufficient, which is undesirable, and if it exceeds 30% by weight, the storage stability of the paint will deteriorate, which is not preferred. The preferred amount of melamine resin is 35 to 45% by weight. On the other hand, in the paint for the second layer, melamine resin is
If it is blended in an amount of 50% by weight, less than 30% by weight is undesirable because the coating film strength will be insufficient, and if it exceeds 50% by weight, the storage stability of the paint will be deteriorated, which is not preferred. The preferred amount of melamine resin in the second layer coating is 35 to 45% by weight. In the method of the present invention, the amount of melamine resin contained in the paint for the second layer is increased by 10 to 20% by weight than that of the melamine resin contained in the paint for the first layer. By adjusting the curing speed of the coating film of the layer, it became possible to achieve the above-mentioned object of the present invention. Coatings applied according to the invention are heat cured in the 2C/1B system. There are no particular limitations on the heat curing conditions, but baking is generally performed at 130 to 150°C for 20 to 40 minutes.
In addition, when the coating film is thermally cured, the energy transferred to the first layer increases due to heat transfer from the underlying iron plate, which is the object to be coated, and the curing of the first layer appears to be accelerated. The pigment to be mixed in the paint used in the present invention may be any organic or inorganic pigment for ordinary paints, and is blended in an amount of 1 to 45% by weight. Specific examples of such pigments include carbon black, titanium oxide, aluminum powder, phthalocyanine, iron oxide, azo pigments, quinacridone pigments, and perylene pigments. In addition to the above-mentioned essential components, the paint used in the method of the present invention contains optional additives such as silicone, polymeric acrylic, amine salts as ordinary surface conditioners, and polyamide-based and polyethylene-based anti-settling agents. can do. In carrying out the coating method according to the present invention, a conventional air spray, electrostatic coating machine, etc. can be used as appropriate, and coating can be carried out by a wet-on-wet method. Examples Examples of the present invention will be described below, but it goes without saying that the technical scope of the present invention is not limited to these examples. (1) Paint used Paint A: Polyester resin paint (manufactured by Nippon Paint Co., Ltd., Olga S-90 Sealer) Alkyd resin/melamine resin = 72/28 (pigment content in the paint)
40% by weight) Paint B: Polyester resin paint (Nippon Paint Co., Ltd. Olga NS black) Alkyd resin/melamine resin = 60/40 (pigment content in the paint 3% by weight) Paint C: Polyester resin paint (Nippon Paint Co., Ltd.) (Manufactured by Olga S-90, exclusive top coating for internal use) alkyd/melamine resin = 60/40 (pigment in the paint: 16% by weight) (2) Painting method SPC-1 steel plate is subjected to chemical conversion treatment, and electrodeposition paint (cationic After undercoating with type electrodeposition paint (Power Top U-100 manufactured by Nippon Paint Co., Ltd.), wet-on the first and second layers as shown in the table below.
Painted with wet paint, 140 after flashing off
A cured coating film was obtained by baking at °C for 30 minutes. The results obtained were as follows.

[Table] Effects of the invention As is clear from the above results, according to the present invention,
In addition to the conventional performance, the 2C/1B method makes it possible to establish a full-gloss coating system with excellent specular reflectance at 60° and 20°.

Claims (1)

[Claims] 1. When applying a top coat paint using a two-coat one-bake method, () the resin solid content ratio of alkyd resin/melamine resin is 80 to 60% by weight/20 to 40% by weight, and the coloring pigment is 1% by weight. A first layer coating is applied using a paint containing ~45% by weight, followed by () a resin solids ratio of 70~50% by weight/30~50% by weight of an alkyd resin/melamine resin and a coloring pigment of 1~45% by weight. A method for applying a top coat, which comprises applying a second layer coating film using a coating material containing 10 to 20% by weight more melamine resin than the coating material for the first layer.