JPH0428540B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a precoated metal having a coating film appearance with good image clarity in addition to coating film physical properties such as workability and adhesion. Conventional technology From the viewpoint of labor saving and pollution prevention, the use of pre-coated metal plates used in home appliances and various interior materials has been progressing. There is an increasing demand for pre-coated metals that have a coating film appearance with good image clarity. Pre-coated metal is made by applying paint twice and baking it onto a metal plate that has been pre-treated as necessary.
A two-coat, two-bake method is common, but it has been difficult to obtain good image sharpness with this type of coating. Conventional approaches to achieving high definition include (1) methods of lowering the surface roughness of metal plates for painting (Japanese Patent Application Laid-Open Nos. 59-1636 and 60-75368); (2) methods of reducing the surface roughness of the paint film when forming it; (3) A method of applying a clear coat on top of the two-layer coating formed by the two-coat, two-bake method (Japanese Patent Application Laid-open No.
61-189931, 62-61677), etc. Problems to be Solved by the Invention However, in the method (1) of lowering the surface roughness, the effect differs depending on the paint used, and this method alone does not necessarily provide high image clarity. The disadvantage of roughening is that it increases costs. In addition, with method (2), it was difficult to achieve both the sharpness of the coating film and the physical properties such as workability and adhesion required for pre-coated metal. The method (3) had drawbacks such as requiring 3-coat, 3-bake painting equipment and increasing costs due to the additional steps. The present invention was made to solve the above-mentioned problems, and provides a pre-coated metal with excellent coating film properties and image clarity by examining the resin system, molecular weight, and composition of the crosslinking agent of the coating material. The purpose is to Means for Solving the Problems That is, the present invention provides the following: 1. A chemical conversion treatment is applied directly or as necessary to a metal plate, and then a number average molecular weight coating is applied as an undercoat.
A coating film is formed using a paint containing an amino resin mixed with an epoxy-modified polyester resin having a molecular weight of 5,000 to 70,000, and then a top coat with a number average molecular weight of 2,000 to 70,000 is used.
1000 polyester resins with different crosslinking speeds 2
Pre-coated metal with excellent image clarity, characterized by a coating film formed with a paint containing more than one type of amino resin. 2 The surface roughness of the metal plate is 0.3 in terms of center line average roughness.
The pre-coated metal according to claim 1, characterized in that the pre-coated metal has a thickness of 0.7μ. It provides: The present invention will be explained in detail below. Effect Since the sharpness of the image depends on the specular reflection of the light,
In the case of painted steel plates containing pre-coated metal, the sharpness of the image is significantly influenced by the smoothness of the coating film. In the past, there have been cases in which the surface roughness of the metal plate for painting has been lowered to improve the smoothness of the paint film, but the final factor affecting image clarity is the smoothness of the paint film. It is thought that it is important to consider the paint not only in terms of physical properties but also in terms of image clarity. From this perspective, we considered the resin system, molecular weight, and crosslinking agent composition of each paint for the top coat and undercoat to be the main factors in pre-coated metal of the 2-coat, 2-bake format, and investigated the combination effect of the paint composition and the top coat/undercoat film. As a result, they discovered that a pre-coated metal with excellent image clarity and physical properties can be obtained by combining a top coat formulated with emphasis on image clarity with an undercoat that emphasizes physical properties, leading to the completion of the present invention. Each of the top coat and undercoat paints will be explained below. (1) Top coats Acrylic resins and polyester resins are the main types of top coat paints for pre-coated metals used for home appliances and various interior materials. As a result of examining the relationship between the molecular weight and image clarity of these resins, it was found that the lower the molecular weight, the better the leveling ability of the coating film and the better the image clarity. However, the processability of the coating film decreases as the molecular weight decreases, and this tendency is particularly strong in acrylic resins. Therefore, in order to achieve both image clarity and processability in polyester resins, we investigated the use of crosslinking agents, and found that among the various resins, amino resins are the best in terms of crosslinking agents. A great effect can be obtained by blending two or more types of amino resins that have considerable selectivity with respect to polyester resins, have good compatibility with polyester resins, and have different crosslinking rates at 10% by weight or more of the total amount of amino resins. I found out. Here, the crosslinking rate can be evaluated by the number of methylol groups in one molecule of amino resin, and it is known that the larger the value, the faster the crosslinking rate. It was found that a large effect can be obtained by blending types in which the average number of methylol groups differs by 0.3 or more. This is thought to be because the use of two or more types of crosslinking agents with different crosslinking rates caused the crosslinking reaction to occur sequentially, increasing the leveling effect during gelation progress. Further, as a result of examining the molecular weight of the polyester resin in the above formulation, it was found that when the number average molecular weight is less than 2000, the physical properties, especially processability, are insufficient, and when the number average molecular weight exceeds 1000, the image clarity is insufficient.
That is, the number average molecular weight range for achieving both image clarity and processability is 2,000 to 10,000, particularly preferably 3,000 to 5,000. The amount of the total amino resin blended here is preferably 10 to 50 parts by weight per 100 parts by weight of the polyester resin in terms of solid content, and resins such as crosslinking agents other than amino resins are based on the total amount of crosslinking agent in terms of solid content. They may be used together if the amount is up to 30%. Further, there is no need to particularly limit the pigments and solvents, and those commonly used as paints can be used. Topcoats formulated based on the above considerations include, for example, the HC series manufactured by Kawakami Paint Co., Ltd., such as Coil Coat 410HC, Coil Coat 570HC,
Coil coat 710HC etc. By applying this topcoat film to a normal undercoat film, it was possible to realize a precoat metal that was excellent in both image clarity and physical properties. However, since these properties are expected to have a strong synergistic effect with the undercoat film, we next investigated the undercoat paint in order to create a pre-coated metal with even better image clarity and physical properties. (2) Undercoat paint Undercoat paints are generally epoxy resins, polyester resins, epoxy-modified polyester resins with or without amino resins, and have excellent corrosion resistance, hiding power, workability, and adhesion. Resin systems are selected taking into consideration the following factors. As a result of examining the relationship between molecular weight and image clarity after coating film formation with the above-mentioned top coat in these resin systems, an inverse relationship was found for epoxy resins and polyester resins, regardless of the type or composition of the crosslinking agent. was observed, and it was necessary to reduce the molecular weight of the resin in order to obtain high definition images, and it was therefore difficult to obtain good physical properties. However, it has been discovered that the above-mentioned inverse correlation is not necessarily observed in epoxy-modified polyester resins, and there are cases in which the image clarity of the topcoat film does not deteriorate even if the molecular weight becomes considerably high.
When considering physical properties, higher molecular weight is advantageous, so we discovered the possibility of creating a pre-coated metal with excellent image clarity and physical properties by considering epoxy-modified polyester resin as the undercoat. As a result of investigating the crosslinking agent, we found that by incorporating an amino resin with good compatibility with epoxy-modified polyester as a crosslinking agent, the image clarity of the topcoat film would not deteriorate even if the molecular weight was high. On the other hand, when examining physical properties such as processability, secondary adhesion, and corrosion resistance, the number average molecular weight of epoxy-modified polyester needs to be 5,000 or more, and if the number average molecular weight exceeds 70,000, the sharpness after forming the topcoat film will deteriorate. It was found that there was a marked decrease in the coating properties, and that the painting workability was also worsened. In this way, the number average molecular weight is 5000~
By using an undercoat paint containing 70,000 epoxy-modified polyester resin and an amino resin that is highly compatible with the resin, it is possible to obtain good physical properties without impairing image clarity after forming the above-mentioned topcoat film. did it. Particularly preferably, the number average molecular weight range of the epoxy-modified polyester resin is 20,000 to 60,000. Here, the epoxy-modified polyester resin is a mixture of a resin having at least one epoxy group and a polyester resin. The amount of amino resin to be blended is arbitrary with respect to the epoxy-modified polyester resin, and resins such as crosslinking agents other than amino resin should be added in an amount of 30% of the total amount of crosslinking agent.
They may be used together if the amount is up to %. Further, there is no need to particularly limit the pigments and solvents, and those commonly used for paints can be used. Examples of undercoat paints formulated based on the above considerations include Coil Coat HC series paints manufactured by Kawakami Paint Co., Ltd., such as Coil Coat 31HC and Coil Coat 75HC. Next, the metal plate for painting of the present invention may be any metal plate that can be used as a metal plate for pre-coated metal. For example, cold-rolled steel sheets, galvanized steel sheets, alloyed galvanized steel sheets, tin-plated steel sheets, chrome-plated steel sheets, aluminum-plated steel sheets, lead-plated steel sheets, nickel-plated steel sheets, aluminum plates, titanium plates, stainless steel plates, etc.
After cleaning the surface with alkaline degreasing or the like if necessary, they can be used directly, or they can be painted after being subjected to a conventional chemical conversion treatment. This chemical conversion treatment may be any treatment used as a pre-treatment for pre-coated metal, and includes chromate chemical conversion treatment, phosphate chemical treatment, composite oxide film treatment, etc. Chromate chemical conversion treatment includes:
Electrolytic chromate, coating-type chromate, and reactive chromate treatments are applicable to phosphate chemical conversion treatment, zinc phosphate treatment, iron phosphate treatment, and treatment containing nickel and cobalt to composite oxide film treatment. Furthermore, the surface roughness of the metal plate is preferably such that the central average roughness Ra is 0.7μ or less in order to improve image clarity.
On the other hand, if Ra is less than 0.3μ, the adhesion of the paint film will decrease, so if adhesion is required in addition to image clarity, the center line average roughness Ra should range from 0.3 to
Preferably, it is 0.7μ. Surface roughness is determined by measuring the center line average roughness according to JIS using the stylus method. The undercoat and topcoat paints are sprayed and
The coating is applied using an appropriate coating method such as roll coating or flow coating, dried to form a coating film, and then baked and dried at 150 to 250°C. The coating film thickness (dry film thickness) is about 10 to 50 μm in total for the undercoat and topcoat, and it is preferable for the topcoat to be thicker than the undercoat in terms of both image clarity and physical properties. Examples The present invention will be explained below based on Examples and Comparative Examples. Examples 1 to 12 As examples, samples were prepared with the configurations shown in Examples 1 to 12 in Table 1, and the image clarity and physical properties of the coating film were evaluated. From the evaluation results shown in Table 1, it can be seen that the precoated metal produced by the combination of the top coat and undercoat according to the present invention has good image clarity and physical properties. In addition,
Looking at the influence of the composition of the pre-coated metal on the evaluation results, we find that the smaller the roughness of the original, the better the image sharpness, and reducing the roughness of the original is effective for achieving even higher sharpness. Example 11 (Center line average roughness Ra
It is recognized that extremely low roughness such as 0.1 μ) causes a slight decrease in adhesion. On the other hand, in the case of Example 12 (Ra=0.9μ), compared to Comparative Example 13 (Ra=0.9μ) in the same table, which is not the coating system of the present invention,
Image clarity is excellent, but at a low level. Therefore, when the requirements for image clarity and adhesion are particularly strong, it is desirable that the centerline average roughness (Ra) of the surface of the original plate be within the range of 0.3 to 0.7μ. As for the molecular weight of the top coat resin, when compared with the same color system, the smaller the molecular weight, the lower the processability, but the better the hardness and sharpness. Regarding the film thickness, when the total film thickness is the same, the thicker the overcoat film, the better the image clarity. Comparative Examples 1 to 2 An example in which the number average molecular weight of the top coat paint is less than 2000 is shown in Comparative Example 1 in Table 1, and an example in which it is over 1000 is shown in Comparative Example 2. Comparative Example 1 has good image clarity but extremely poor processability, while Comparative Example 2 has good processability but extremely poor image clarity. Comparative Examples 3 to 7 Comparative Examples 3 to 7 in Table 1 are examples in which only one type of amino resin is used as a crosslinking agent in the top coat and two types of amino resins with approximately the same reaction rate are used. show. Both formulations have lower image clarity than the coating film obtained by the method of the present invention. Comparative Examples 8 to 9 An example in which the number average molecular weight of the undercoat paint is less than 5,000 is shown in Comparative Example 8 in Table 1, and an example in which it is over 7,000 is shown in Comparative Example 9. Comparative Example 8 has good image clarity but poor processability and secondary adhesion, while Comparative Example 9 has good coating film properties but poor image clarity. Comparative Examples 10 to 12 Comparative Examples 10 to 12 in Table 1 show examples in which paint systems other than the present invention were used as undercoat paints. Both are inferior to the product of the present invention when considering the balance of image clarity and physical properties of the coating film. As is clear from the results of these Examples and Comparative Examples, the precoated metal obtained by the method of the present invention has the advantage of good image clarity and excellent coating film properties. The test conditions shown in Table 1 are as follows. (1) Original sheet type: Electrogalvanized steel sheet (2) Pretreatment: Electrolytic chromate (3) Coating conditions: (Coating form) 2 coats, 2 bakes (Coating method) Bar coater (Baking plate temperature) Undercoat 180℃, 60 seconds Topcoat 230°C, 60 seconds (4) Top coat type: Polyester type (5) Crosslinking agent: Shown in Tables 2 and 3. The blending amount is 25 parts by weight per 100 parts by weight of polyester resin, and the combined amount when blending two types of crosslinking agents is 12.5 parts by weight.
12.5 (parts by weight). Moreover, the evaluation method of coating film performance is as shown below. Image sharpness: (1) Evaluation value (Gd value) of a portable sharpness gloss meter "PGD" (manufactured by Tokyo Kohden Co., Ltd.) and (2) NGIC value of an NSIC meter manufactured by Suga Test Instruments Co., Ltd. were used. Workability: In an atmosphere of 20℃, several plates of the same thickness are sandwiched between a sample that has been bent by 180゜U and crushed using a press. Observe this processed part with a 30x magnifying glass, and use the limit of the number of plates (T value) when no cracks occur as the evaluation value. Pencil hardness: The evaluation value is the maximum pencil hardness that does not scratch the paint film using a Mitsubishi Uni-pencil. Adhesion: Create 100 grid lines on the sample surface using an NT cutter, extrude 7mm using an Erichsen tester, and then evaluate taping. The evaluation method is as follows. ◎: No peeling was observed. ○: Minute peeling occurs around cutter scratches. ×: 10 or more pieces out of 100 pieces are peeled off. Secondary adhesion: After immersing the sample in boiling water for 2 hours,
Perform an adhesion test. The evaluation method is the same as the adhesion test. Corrosion resistance: A cross cut was made with an NT cutter on the flat surface of the sample with the edges sealed, and a salt spray test was conducted at 500°C.
Do time. The evaluation method is as follows. ◎: The maximum bulge width on one side of the cross cut part is 1.5
mm or less. ○: The maximum bulge width on one side of the cross cut part is 1.5
More than mm and less than 3 mm. ×: The maximum bulge width on one side of the cross cut part is 3 mm
Something that exceeds.

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[Table] Effects of the Invention As explained above, the precoated metal of the present invention has excellent workability, adhesion, image clarity, etc.

Claims (1)

[Scope of Claims] 1. A chemical conversion treatment is applied directly or as necessary to the metal plate, and then an undercoat with a number average molecular weight of 5000 is applied.
~70,000 epoxy-modified polyester resin mixed with an amino resin to form a coating film, and then as a top coat a polyester resin with a number average molecular weight of 2,000 to 10,000 mixed with two or more types of amino resins with different crosslinking speeds. A pre-coated metal with excellent image clarity, characterized by a coating film formed by 2 The surface roughness of the metal plate is 0.3 in terms of center line average roughness.
The pre-coated metal according to claim 1, characterized in that the pre-coated metal has a thickness of 0.7μ.