JPS6146188B2 - - 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 the diffusion of organic solvents and can provide thick coatings with a single application. However, this powder coating method has a disadvantage in that the surface smoothness and sharpness of the resulting coating film are slightly inferior to methods using solvent-based coatings. 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, so it is not an effective method. I can not say. Furthermore, as another example of powder coating method,
A method in which a powder or solvent-based colored base paint is applied to form a paint film, a powder clear paint that does not contain a colorant is applied onto the base paint film, and after the paint film is formed, both paint films are baked and cured at the same time. is also known. 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 It was impossible to obtain clear coating film (cracking), and therefore, it was not applicable in view of the long-term waxing properties that are essential for painting 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, Japanese Patent Application Laid-Open No. 51-117733
No. 1, and Japanese Patent Application Laid-Open No. 1983-69446 are known, but both of these are aimed at improving the finish of metallic coating methods using powder coatings, and do not mention anything about long-term weather resistance. Also resin,
There are no limitations on coating conditions, and this method differs from the method of the present invention in which an amine is included in the solvent as described below. 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 includes:
A thermosetting resin having hydroxyl and carboxyl groups as functional groups is used as the base paint resin, and a part of the solvent has a boiling point higher than the melting start temperature of the powder clear paint resin. On the other hand, a solvent-based colored paint made of amines that have a curing accelerating effect is used, and after the colored base paint is applied, it is set at a temperature below the boiling point of the amine, and then the glycidyl group is functionalized as a clear paint. After coating with a powder coating consisting of an acrylic resin as a base and an organic acid and its anhydride as a curing agent, the boiling point of the amine in the base coating, e.g. 150
It is characterized by simultaneously baking and hardening both coatings at temperatures of ~180°C or higher. That is, the present invention uses a solvent-based paint containing a specific amine as a part of the solvent that has a curing accelerating effect as a base paint, and uses a powder acrylic paint as a clear paint, and by combining these two paints, powder The weather resistance of the clear coating film is improved to prevent defects such as cracking and peeling between the base and clear coating layers. The base coating resin used in the present invention has an acid value of 30 to 120 and an OH value of 40 to 80, and has a carboxyl group (-COOH) and a hydroxy group (-
Suitable materials include solvent-based thermosetting acrylic resins, acrylic/melamine resins, etc. that have OH), and non-aqueous dispersion paints (NADs) are also included as base paints. In addition, the amine used as a part of the solvent promotes the reaction between the hydroxyl group and carboxyl group of the base paint resin and the glycidyl group of the clear paint resin, as well as the reaction between the glycidyl group of the clear paint resin and the curing agent. It is a primary, secondary or tertiary amine with a boiling point of 100° C. or higher, such as dimethylethanolamine. The curing agent for the resin for the base coating is a butylated melamine resin, etc., and the resin/curing agent ratio for the solvent-based coating is preferably 70:30 to 80:20 (weight ratio). Furthermore, the base 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, they have an average molecular weight of 4000 to 9500, have glycidyl groups as functional groups, and have a blocking resistance temperature of
Powdered acrylic resin having a temperature of 35°C or higher is preferred. As curing agents for acrylic resin, organic acids and their anhydrides,
Particularly preferred are sebacic acid, phthalic anhydride, trimellitic anhydride, etc., and the ratio of the powder resin/curing agent is 0.5 to 1.5 per epoxy group (weight ratio = 10:1 to 2) and base The ratio of the amount of amine in the paint is amine/curing agent = 0.1 to 1.0 (molar ratio)
It is preferable that the content be in the proportion of . 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 resin for base paint and the resin for clear paint according to the present invention does not need to be very thick; in the case of the former, the film thickness after formation is about 10 to 20μ, and in the case of the latter, the film thickness after formation is about 50 to 80μ. It is preferable to do so. In addition, the compatibility of the resin for base paint and the resin for clear paint (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 measuring the compatibility with visible light of the film. Set the transmittance so that it 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 molten coating behavior. As a result, blocking resistance is adversely affected. Also, it is not very desirable to mix a reaction catalyst from the beginning in terms of storage stability, so if a low-reactivity catalyst is used, the baking temperature must be raised to a high temperature of about 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 adversely affect the sealer, etc., so it is not preferable to do so. In addition, in order to achieve smoothness and sharpness, it is better to have no compatibility between the resin for the base paint and the resin for the clear paint during curing, but conversely, from the perspective of long-term weather resistance, the compatibility is lacking. Therefore, there is a problem in that the adhesion between the base and the clear layer deteriorates, causing peeling. Therefore, even if the above various coating conditions are combined, the clear coating will crack within 1 to 1.5 years, and peeling will occur between the base coating and the clear coating in the water resistance test, and the car will be damaged. It cannot be said to be the best method for topcoating exterior panels. In the present invention, solvent amines such as dimethylethanolamine in the base paint can be determined by baking and curing the solvent-based acrylic resin at various temperatures for 20 minutes and measuring the amount of residual amine in the formed coating film by N analysis. It decreases according to the curve shown.
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 solvent-based colored base paint and a powder clear paint at the same time, the dissociated amine in the base paint during baking accelerates the reaction of the base paint itself, and inherently Contributes to the mutual reaction between the resin in the clear paint and the resin in the base paint, which are different reactive systems,
Interlayer adhesion, crack resistance, and low-temperature curing properties are achieved through mutual effects such as increasing the reactivity of the clear paint itself. The clear powder coating used in the present invention is an acid-curing type, excluding an amine-curing type. This is because the latter has higher reactivity than the former, which causes problems with the smoothness and blocking properties of the paint film during painting in the summer (30-40℃).
This is because it is unsuitable for automotive painting. 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 that are 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】

【table】 [Brief explanation of the drawing]

The figure is a graph showing the amount (%) of residual amine 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 2-coat, 1-bake type painting method in which a thermosetting transparent paint (clear paint) is applied to form a paint film and both paint films are simultaneously baked and cured, the resin for the base paint has hydroxyl groups and carboxyl groups as functional groups. A solvent-based colored paint that uses a thermosetting resin and contains, as a part of the solvent, amines that have a boiling point higher than the melting start temperature of the resin for powder clear paints and that act to accelerate the curing of the base paint and clear paint. After applying the colored base paint, it is set at a temperature below the boiling point of the amine, and then an acrylic resin with a glycidyl group as a functional group is used as the resin for the clear paint, and an organic acid and its curing agent are used. A method for forming a highly weather-resistant powder clear coating film, which comprises coating with a powder clear coating material using an anhydride, and then simultaneously baking and curing both coating films at a temperature equal to or higher than the boiling point of the amine.