JPH04176367A - Dull finish coated steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To obtain a dull finish coated steel sheet excellent in workability and stain resistance by using, as a flat coating, a coating material containing a linear polyester, a branched polyester, a melamine curing resin, and a curing accelerator as principal film forming components in specific proportions. CONSTITUTION:A top coat flat coating containing a linear polyester, a branched polyester, a melamine curing resin, and a curing accelerator as film forming components is applied to a steel sheet after ordinary pretreatment. It is preferable to regulate the mixing ratio between the linear polyester and the branched polyester to about 70:30 to 40:60. The amounts of the melamine curing resin and the curing accelerator used are regulated to 20-40 pts.wt. and 3-8 pts.wt., respectively, based on 100 pts.wt. of the sum of both polyester resins mentioned above. As the curing accelerator, paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, methanesulphonic acid, etc., can be used.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention not only improves the workability and hardness, which are the basic performances of pre-coated steel sheets, but also improves the design effect by adding innovations to the film-forming components. This relates to matte coated steel sheets.

<Prior art> Conventionally, methods for reducing the gloss of the coating film of matte-coated steel sheets include increasing the pigment concentration in the paint, adding matting agents such as fine powder, and even adding 2 fils. A method of triblending powder coatings, and a method of obtaining a resin composition for matte powder coatings by blending a specific polyester resin (for example, see Japanese Patent Laid-Open No. 154771/1983) are known.

<Problems to be Solved by the Invention> However, in the above method, the viscosity of the paint increases and the fluidity deteriorates, so it is difficult not only to improve the workability of painting but also to make it into a paint. Even if low gloss is achieved, the paint film becomes too hard, resulting in poor workability. In addition, powder coatings have problems such as not being able to form a uniform surface when formed into a coating, and therefore not being applicable to pre-coated steel sheets.

An object of the present invention is to solve the problems of the prior art and provide a matte coated steel sheet with excellent workability, hardness and stain resistance.

Means for Solving the Problems〉 The present inventors have conducted intensive research to solve the problems of the conventional technology as described above, and as a result of the results, it has been found that baking is the main element for forming a hardening type film. It has been discovered that the desired purpose can be advantageously achieved by using a combination of linear polyester and branched polyester, and adding a melamine-based curing resin and a curing accelerator to this as a vehicle. The present invention has now been completed.

That is, according to the present invention, in order to achieve the above object,
The main elements for forming a film are linear and branched polyester (A), melamine-based hardening resin (B), and hardening accelerator (C) on a chemically treated steel plate, and the above (A) and (B) are
) and component (C) in a weight ratio of 100:20.
A matte coated steel sheet with excellent workability is provided by coating and baking a vehicle with a ratio of ~40:3 to 8.

The present invention will be explained in more detail below.

The polyester resin, which is one of the film-forming components of the present invention, is composed of a linear polyester and a branched polyester. Examples of the aromatic dicarboxylic acid component of this linear polyester include terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and their lower alkyl esters and acid anhydrides, and one or more of these can be used. In addition, examples of aliphatic dicarboxylic acid components include adipic acid, sebacic acid, azelaic acid, succinic acid, fumaric acid, maleic acid, and hemic acid, and their lower alkyl esters and acid anhydrides may also be used. One or more of these can be used.

In addition, as dialcohol components, ethylene glycol, 1.2-propanediol, 1.3-butanediol, 1,4-butanediol, 1.5-bentanediol, 1.6-hexanediol, diethylene glycol,
Neopentyl glycol, 1,4-cyclohexane dimetatool, 3-methylpentane-1,5-diol, 1
．． One or more aliphatic or aromatic dialcohols such as 4-dicyclohexane dimetatool, xylene glycol, and hydrogenated bisphenol A can be used.

As the dicarboxylic acid component of the branched polyester, the same dicarboxylic acids as those used for the linear polyester can be used. In addition, as the trivalent or higher polyvalent carboxylic acid component, there are polyvalent carboxylic acids such as trimellitic acid, trimesic acid, pyromellitic acid, etc., and their lower alkyl esters, acid anhydrides, etc. may also be used. One or more of these can be used.

Moreover, as the dialcohol component, the same dialcohol as that used for the linear polyester can be used. Further, examples of the trivalent or higher polyhydric alcohol component include trimethylolethane, trimethylolpropane, glycerin, pentaerythritol, etc., and one or more of these can be used.

The number average molecular weight of linear polyester resin is 10,000
The above is preferable. When the number average molecular weight is less than 10,000, processability, coating hardness, and stain resistance are poor. Also,
The number average molecular weight of the branched polyester resin is preferably 2000 or more. If the number average molecular weight is less than 2,000, coating film defects such as pinholes are likely to occur during coating, and coating stability also decreases.

The mixing ratio of the linear polyester and branched polyester is preferably in the range of 70:30 to 40:60. If the mixing ratio is outside this range, excessive branching will increase the hardness of the coating film and significantly reduce processability. . If the linear type is used in excess, a sufficient matting effect cannot be obtained.

Isocyanate compounds and melamine-based curing resins are usually used as curing agents in polyester resins, but when isocyanate compounds are used, the coating film is easily damaged when handling or processing the steel plate after painting. Therefore, the pre-coated steel sheet targeted by the present invention is not accessible. Therefore, the curing agent for ensuring the workability of the coating film necessary for the pre-coated steel sheet is limited to melamine-based curing resins.

The amount of the melamine-based cured resin (B) needs to be an amount of functional groups that approximately corresponds to the amount of hydroxyl groups in the polyester resin, and is in the range of 20 to 40 parts by weight based on 100 parts by weight of the polyester resin. . The type of melamine-based cured resin is not particularly limited, and 3 to 8 parts by weight of a curing accelerator (C) must be added to ensure sufficient curing. If the amount added is less than 3 parts by weight, the self-condensation reaction of the melamine-based cured resin will be insufficient,
Poor coating hardness and stain resistance. If the amount exceeds 8 parts by weight, the color tone of the appearance will deteriorate. Examples of the curing accelerator include, but are not limited to, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid,
Known acids such as methanesulfonic acid can be used.

In order to mix each of the above-mentioned materials as the main elements for film formation in the above-mentioned preferred ranges and use the vehicle for coating and baking, the solubility and baking of linear polyester and branched polyester are a matter of time, so the solvent is Tsurupetz #. 100, Tsurpetz #150, methyl isobutyl ketone, and cyclohexanone are preferably used. In terms of paintability, Nv = 40 ~
It is necessary to adjust it to 80%. Apply pre-coated steel plate paint diluted with solvent as necessary to the surface of the raw steel plate,
A coating film is formed by baking, and any conventionally known method such as a roll coater method, curtain flow coater method, or bar coater method can be used for the application. It is preferable to carry out the reaction for about 0.5 to 3 minutes.

In addition, as the raw steel sheet, not only general cold-rolled steel sheets but also those subjected to chemical conversion treatment, plating treatment, and even brimer treatment can be suitably used.

The coating thickness after baking is preferably about 15 to 40 μm.

According to the present invention, by using a mixture of polymeric linear polyester and branched polyester as the film-forming component, which is hardened during baking, a mixture of polymeric linear polyester and branched polyester is used as the film-forming component. The reason why a coating film with the performance characteristics of workability, high hardness, and good stain resistance, weather resistance, and corrosion resistance can be obtained is not clearly elucidated, but it is thought to be as follows.

It is believed that by mixing a branched polyester with a polymeric linear polyester at a predetermined ratio, the crosslinking density increases, preventing the penetration and diffusion of contaminants, and increasing the hardness of the coating film. Furthermore, it is thought that good processability is maintained due to the high molecular weight linear polyester.

<Example> The present invention will be specifically explained below based on Examples.

(Examples 1 to 4) Electrogalvanized plate with a thickness of 0.5 mm! After applying normal zinc phosphate treatment to a steel plate (plating amount: 20 g/m'' on both sides), an epoxy-modified polyester resin paint is applied as an undercoat to a dry film thickness of 5 to 7 μm, and then After painting under the conditions that the maximum board temperature is 210±10℃ and the baking time is 60 seconds, the top coat is coated with 50 parts of linear polyester (a), 50 parts of branched polyester (b), and methylated as the main film forming elements. A polyester resin paint containing 20 parts of melamine resin, 3 parts of curing accelerator, and 100 parts of titanium dioxide was applied to give a dry film thickness of 15 to 20 μm, and then the maximum board temperature was 230±10°C. A coated steel plate for testing was prepared by baking for 60 seconds (Example 1).

Table 1 shows the results of an investigation of the matte coated steel sheet thus obtained.

As is clear from the results shown in Table 1, by using a mixture of linear polyester and branched polyester in a specific ratio as the main film forming element according to the present invention, the processability can be improved without deteriorating processability. A matte steel sheet with excellent hardness, stain resistance, and corrosion resistance was obtained.

For Examples 2 to 4, the mixing ratio of linear polyester and branched polyester, which are the main elements for film formation, was 40.
A matte steel plate was obtained in the same manner as in Example 1 except that the ratio was changed to: 60.80: 40.70:30.

Table 1 shows the results of coating film evaluation.

(Comparative Example 1) A matte steel plate was prepared in the same manner as in Example 1 using a mixture of linear polyester and branched polyester, which are the main elements for film formation, at a mixing ratio of 30:70 and other additives. Obtained.

As the proportion of branched polyester increases, processability deteriorates.

Table 1 shows the results of coating film evaluation.

(Comparative Example 2) A matte steel plate was prepared in the same manner as in Example 1 using a mixture of linear polyester and branched polyester, which are the main elements for film formation, at a mixing ratio of 80:20 and other additives. Obtained.

As the proportion of linear polyester increases, the hardness deteriorates.

Table 1 shows the results of coating film evaluation.

Table 1 (Test method) Gloss: Displayed as 60 degree specular reflectance (%)/gloss Pencil hardness: Measured using Mitsubishi Uni.

Workability: Bending was performed at 180°, and evaluation was made based on cracks at the bent portion.

Stain resistance: After 24 hours, the marker ink marks on the test piece
Wipe off with ethanol.

Evaluation: ◎Very good, O good, △Slightly poor, ×Significantly poor <Effects of the invention> Since the present invention is configured as explained above, the present invention provides a matting effect on conventional pre-coated steel sheets. It is possible to obtain a matte coated steel sheet with improved workability, hardness, and stain resistance, which were concerns in the case of the method.

Claims (1)

[Claims] (1) Linear and branched polyester (A), melamine-based hardening resin (B), and hardening accelerator (C) are used as main film forming elements on a chemically treated steel plate, and the above (A), A matte coated steel sheet with excellent workability, which is obtained by coating and baking a vehicle in which the weight ratio of components (B) and (C) is 100:20 to 40:3 to 8.