JPS6147143B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a powder clear coating film having long-term weather resistance and applicable to automobile exterior panels and the like. Painting methods using powder paints have been developed since they do not cause pollution due to diffusion of organic solvents and the like and can provide thick coatings with a single application. However, this powder coating method has the drawback that the surface smoothness and sharpness of the resulting coating film are slightly inferior to methods using solvent-based paints. These drawbacks can be alleviated by increasing the coating thickness, for example, by several hundred microns, but this increases the coating cost and reduces productivity, making it difficult to find effective solutions. I can't say that. Furthermore, as another example of a powder coating method, a powder or solvent-based colored base paint is applied to form a paint film, and then a powder clear paint that does not contain a colorant is applied on the base paint film to form a paint film. A method is also known in which both coating films are baked and hardened at the same time after they are formed. Although this method improves the smoothness and image clarity, it is necessary to select a resin with a relatively low molecular weight as the resin for the powder clear paint that can be used in this case, so long-term weather resistance (especially cracks in clear coating)
Therefore, it was not possible to apply this method in consideration of the long-term fastness required for coating automobile exterior panels, etc. Furthermore, it is a well-known method to mix ultraviolet absorbers into the clear coating film itself to prevent cracking, but even with this method, it is difficult to maintain the effect over a long period of several years. It is a well-known place. As an improvement of the powder coating method, JP-A-51-130439
No. 51-87545 is known, but the former is aimed at improving the finish of the metallic coating method for powder paints, does not mention anything about long-term weather resistance, and is based on resins. , there are no restrictions on the coating conditions, and the latter method requires partial curing of the base paint and is not aimed at obtaining a highly weather-resistant coating film. The present invention eliminates the various drawbacks mentioned above and provides a method for forming a powder clear coating film that can provide a high-performance coating film that can withstand long-term outdoor exposure. The method for forming a powder clear coating film according to the present invention is as follows: (1) The base paint contains an amine having a hydroxyl group or a carboxyl group as a functional group and whose boiling point is higher than the melting start temperature of the clear resin. After painting with a water-based paint consisting of a water-soluble or water-dispersible resin neutralized at (2) the temperature at which the neutralizing amine dissociates from the base paint resin , for example, setting at 110°C or lower, (3) After that, after painting with a powder coating consisting of an acrylic resin with a glycidyl group as a functional group as a clear coating and an organic acid and its anhydride as a curing agent, (4) Simultaneously baking and curing at a temperature at which 80% or more of the neutralizing amine in the base paint dissociates, for example, 140°C or higher. It is characterized by this. That is, the present invention uses a water-based paint with a specific neutralizing agent as a base paint and a powder acrylic paint as a clear paint, and by combining these two paints, the weather resistance of the powder clear paint film is improved. This prevents defects such as cracking, cracking, and peeling between the base and clear coating layers. Suitable resins for water-based paints used in the present invention include acrylic and alkyd water-based resins with an acid value of 30 to 120 and having carboxyl groups (-COOH) and hydroxyl groups (-OH) as functional groups, and are neutralized with amines. A ratio of 40 to 120% is preferred. In addition, the neutralizing amine is a tertiary amine such as dimethylethanolamine or triethylamine. Primary or secondary amines have a low dissociation temperature and strong catalytic action, so they can cause pinholes in powder clear coatings. , which is inappropriate as it causes skin irritation. As a hardening agent, methylated methylolmelamine,
For example, hexamethoxymethylmelamine, etc.
The water-based resin/curing agent ratio is between 70:30 and 80:20.
(weight ratio) is preferable. Furthermore, the water-based paint contains metal powder such as aluminum flakes, and if necessary, inorganic/organic pigments within a range that does not impair the design effect of the metal powder. Acrylic resins containing glycidyl groups are used as the resin for powder clear coatings used in the present invention, and in particular powders with an average molecular weight of 4000 to 9500, glycidyl groups as functional groups, and a blocking resistance temperature of 35°C or higher. Body acrylic resin is preferred. The curing agent for the acrylic resin is an organic acid or its anhydride, particularly preferably sebacic acid, phthalic anhydride, trimellitic anhydride, etc., and the ratio of the powder resin/curing agent is 0.5 to 1 per epoxy group.
1.5 (weight ratio = 10:1~2) and the ratio of amine amount in the base paint to amine/curing agent = 0.1~
It is preferable that the content is 1.0 (molar ratio).
In some cases, an ultraviolet absorber may be added in addition to the curing agent to improve the weather resistance of the clear coating. The coating film thickness of the base paint resin and clear paint resin according to the present invention does not need to be very thick; in the case of the former, the film thickness after formation should be about 10 to 20 μm, and in the case of the latter, the film thickness after formation should be about 50 to 80 μm. is preferred. In addition, the compatibility of the base paint resin and the clear paint resin (excluding the curing agent) of the present invention was determined by dissolving and mixing both resins in an organic solvent to form a film, and then determining the transmittance of the film by visible light. Set so that the transmittance is approximately 40 to 80% when measured. To explain the present invention in comparison with the conventional method, in the conventional powder clear coating method, in order to achieve smoothness, a resin with an average molecular weight of 10,000 or less is used as the base coating resin due to its melt film forming behavior. As a result, blocking resistance is adversely affected. (Also, it is necessary to use a slow-drying curing agent in consideration of the blocking resistance of the powder.)
Also, it is not very desirable to mix a reaction catalyst from the beginning due to storage stability, so if a low-reactivity catalyst is used, the baking temperature must be as high as 200 to 220 degrees Celsius in order to completely cure it. However, if such high temperatures are applied to automobile painting, it will cause thermal distortion of the body and have an adverse effect on the sealer, etc., so it is not preferable to apply it. In addition, in order to achieve smoothness and sharpness, it is better that there is no compatibility between the base resin and clear resin during curing.
On the other hand, from the standpoint of long-term weather resistance, the lack of compatibility causes the adhesion between the base and clear layer to deteriorate, resulting in peeling. Therefore, even if the various coating conditions described above are combined, all of the coating conditions will be 1 to 1.
Within 1.5 years, cracking of the clear paint film and peeling occurred between the base paint film and the clear paint film in a water resistance test, so it cannot be said to be the best method for topcoating automobile exterior panels. In the present invention, the neutralizing amine, such as dimethylethanolamine, in the base paint is baked and cured in an aqueous acrylic resin at various temperatures for 20 minutes, and the amount of residual amine in the formed coating film is measured by N analysis. It decreases according to the curve shown in the figure. The amine dissociated during high-temperature baking first reacts at the base/clear coating boundary as follows: This promotes the adhesion between the two layers after curing and effectively prevents peeling of the area. In addition, the dissociated amine causes a curing reaction of the clear coating film itself: The clear coating film can also be cured by baking at a low temperature of, for example, 180°C or lower. Therefore, in the present invention, by curing and baking a wet water-based base paint and a powder clear paint at the same time, the dissociated amine in the water-based base paint during baking accelerates the reaction of the base paint itself, and contributes to the mutual reaction between the resin in the clear paint and the resin in the base paint, which are different reactive systems, and also improves interlayer adhesion and crack resistance through a synergistic effect such as increasing the reactivity of the clear paint itself. sex,
It achieves low temperature curability. The clear powder coating used in the present invention is an acid-curing type, excluding an amine-curing type. This is because the reactivity of the latter is inherently higher than that of the former, which causes problems in the smoothness and blocking properties of the coating film during painting in the summer (30 to 40°C), making it unsuitable for automotive coating. As a result, it was possible to ensure the long-term weather resistance quality of the powder clear coating, and no cracking or peeling was observed even after at least three years of outdoor exposure. Furthermore, the present invention has a wide range of applications, as it is applicable not only to automobile painting, but also to areas exposed to sunlight outdoors and which require long-term durability for several years. In order to explain the coating film forming method according to the present invention, Examples and Comparative Examples are summarized in the following table. 【table】

[Brief explanation of the drawing]

The figure is a graph showing the residual amine amount (%) in the coating film measured by N analysis.

Claims (1)

[Claims] 1. An uncured base coating film is formed on the object to be coated using a first thermosetting colored paint (base paint), and then a second colored pigment is not included on the base coating film. In a two-coat one-bake coating method in which a thermosetting transparent paint (clear paint) is applied to form a paint film and both paint films are baked and cured simultaneously, the base paint contains a hydroxyl group, a carboxyl group as a functional group, and a carboxyl group as a functional group. A water-based resin consisting of a water-soluble or water-dispersible resin in which some or all of the groups are neutralized with an amine whose boiling point is higher than the melting start temperature of the clear paint resin, a methylated methylolmelamine resin as a curing agent, and a colored pigment. After applying the colored base paint, setting is performed at a temperature below the temperature at which the neutralizing amine dissociates from the resin of the base paint, and then, as a clear paint, an acrylic resin with a glycidyl group as a functional group and a hardened resin are used. After coating with a powder coating composed of an organic acid and its anhydride as an agent, both coatings are simultaneously baked and cured at a temperature above which 80% or more of the neutralizing amine in the base coating dissociates. A method for forming a highly weather resistant powder clear coating film.