JPH04256469A - Precoated steel sheet having superior workability and contamination resistance - Google Patents

Abstract

PURPOSE:To provide a precoated steel sheet having satisfactory workability and superior contamination resistance. CONSTITUTION:High-molecular straight-chain polyester resin as the principal film forming element of bake hardening coating and melamine type curable resin are used as film forming components, a curing accelerator is added to the resins and the resulting resin compsn. is used as a vehicle. The polyester resin having hydroxyl groups has >=10,000 number average mol.wt. and 15-35 deg.C glass transition temp.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a pre-painted steel sheet that has improved workability and hardness, which are the basic properties of pre-painted steel sheets, and has a coating with excellent stain resistance and chemical resistance required for home appliances, etc. It is.

0002

[Prior Art] Generally, prepainted steel sheets are steel sheets or other metal plates that are coated with paint in advance and then molded into an arbitrary shape for final use.For example, they are used in refrigerators, refrigerators, etc.
It is used in home appliances such as washing machines and electric stoves, as well as metal products such as vending machines, office equipment, and food display cases.

[0003] In recent years, when forming and processing durable consumer goods using thin steel sheets for home appliances, etc., measures have been taken to rationalize the painting line, improve productivity, prevent pollution, improve the working environment, etc. As a result, the post-coat painting method, in which a steel plate is formed, processed, and then painted, is being replaced by a pre-coat painting method, in which a flat steel plate is painted, then formed, and processed.

[0004] Such a pre-coated steel sheet is processed into a complicated shape depending on the intended use after being painted, and therefore requires high formability. Furthermore, products after molding are required to have performance that is compatible with each use. For example, a refrigerator
Washing machines and the like are required to have performances such as scratch resistance related to coating hardness, stain resistance, and chemical resistance. In addition to these properties, other properties such as weather resistance, corrosion resistance, gloss, water resistance, and moisture resistance are required depending on the application.

[0005] In order to improve moldability, it is possible to coat a steel plate with a paint that has a soft coating film and is highly processable, but this paint has a soft coating film that is easily scratched, that is, it has poor scratch resistance. In addition, the stain resistance is extremely poor.On the other hand, in order to improve the stain resistance, it is possible to coat the steel plate with a paint that becomes hard after baking and has excellent stain resistance, but on the other hand,
They have contradictory drawbacks, such as extremely poor workability, and pre-coated steel sheets coated with either paint cannot achieve the required performance.

[0006] In this way, among the performances required of paints, processability, high hardness, and stain resistance are contradictory performances, and these performances are dependent on the elongation of the coating film and the glass transition temperature (Tg).
)caused by. The glass transition temperature depends on the crosslinking density of the coating film,
It is related to the molecular structure of the resin. That is, when the glass transition temperature is high, the hardness and stain resistance are improved, and when the glass transition temperature is low, the elongation of the coating film is increased and the processability is improved.

Conventionally, polyester paints, for example, have been used as top coat paints for pre-coated steel sheets with high workability.

[0008] For example, JP-A-57-73057 discloses a coating material whose main component is a linear polyester coating material and which contains a spherical acrylonitrile polymer and a polyfluoroacrylate polymer in predetermined parts by weight. There is. For example, acid value 5 to 5 obtained by reacting a polyester precondensate, a polyepoxy compound, and a polycarboxylic anhydride.
A coating resin composition containing a high molecular weight polyester resin No. 30 as a main component is disclosed in JP-A-59-217768. Furthermore, for example, copolymerized polyester,
An undercoat paint composition for metals containing a bisphenol A type epoxy resin, a butyl etherified melamine resin and a polyoxyalkylene polyol was disclosed in JP-A-62-274.
It is disclosed in Publication No. 70.

[0009] However, all of the proposed polyester resins have the drawback that the processability, which is the performance required for paints, is not well balanced with stain resistance and coating hardness. For this reason, current pre-painted steel sheets use pre-painted steel sheets that have been pre-coated with a highly workable paint at the expense of some degree of stain resistance.

0010

[Problems to be Solved by the Invention] However, many of the high-molecular polyester resins used as highly processable paints have a low glass transition temperature of 10°C or less and are linear high-molecular polyester resins, so it is difficult to cure them. When a resin is added and cured, the glass transition temperature of the cured coating film is low and the crosslinking density is low, resulting in high coating film elongation and good workability. Poor contamination.

[0011] An object of the present invention is to improve the above-mentioned disadvantages and to provide a pre-coated steel sheet that has good workability and excellent stain resistance.

0012

[Means for Solving the Problems] As a result of extensive research in order to solve the problems of the prior art as described above, the present inventors have discovered that the main film-forming element of a bake-curable paint is a film-forming component. A resin composition containing a certain polymeric linear polyester resin, a melamine-based cured resin, and a curing accelerator was used as a vehicle, and the molecular weight and glass transition temperature (Tg) range of the polymeric linear polyester resin were determined. By stipulating this, we have completed an invention that advantageously achieves the intended purpose.

That is, according to the present invention, as a component forming a film on a steel sheet, a component having a number average molecular weight of 10 and having a hydroxyl group is used.
000 or higher and a glass transition temperature of 15 to 35°C, 80 to 60 parts by weight of a polymer linear polyester resin, 20 to 40 parts by weight of a melamine cured resin, and a total of 100 parts by weight, and a sulfonic acid cured resin. To provide a precoated steel sheet having excellent workability and stain resistance, which is formed by coating and baking a resin composition containing a bake-hardening vehicle containing 1 to 8 parts by weight of an accelerator.

[0014] Preferably, the glass transition temperature is 15 to
The 35° C. linear polymeric polyester resin is preferably a resin made by blending two or more linear polymeric polyester resins having a glass transition temperature in the range of 25±20° C.

The present invention will be explained in detail below.

The polyester resin used in the present invention contains an aromatic dicarboxylic acid component, an aliphatic dicarboxylic acid component, and a dialcohol.

[0017] Examples of the aromatic dicarboxylic acid component of the polyester resin include terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, etc., or lower alkyl esters and acid anhydrides thereof, and one or more of these may be used in combination. Can be done. In addition, as the aliphatic dicarboxylic acid component of the polyester resin composition, adipic acid,
Examples include sebacic acid, azelaic acid, succinic acid, fumaric acid, maleic acid, hymic acid, etc., and lower alkyl esters and acid anhydrides thereof may be used, or one or more of these may be used in combination.

Furthermore, as the dialcohol of the polyester resin composition, ethylene glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6 -hexanediol, diethylene glycol, neopentyl glycol, 1,4-dicyclohexanedimethanol,
One or more aliphatic or aromatic dialcohols such as xylene glycol and hydrogenated bisphenol A can be used in combination.

Further, as an example of the combination of the three monomer components in the above polyester resin, 2.5 mo of isophthalic acid
l% terephthalic acid 4
mol% ethylene glycol 6.
5mol% neopentyl glycol 3
Examples include mol%. The polyester resin used in the present invention is a polymeric linear polyester resin having hydroxyl groups only at the ends.

The above is the composition of the polymer linear polyester resin having hydroxyl groups. The number average molecular weight of the linear polyester resin is 10,000 or more, preferably 1
0000 to 30000. Number average molecular weight is 1000
If it is less than 0, workability will be poor, and if it exceeds 30,000, paintability will be poor.

The glass transition temperature of the linear polyester resin is 15 to 35°C. Glass transition temperature is 15℃
If it is less than 3, the hardness will be low and the scratch resistance will be low.
If it exceeds 5°C, the hardness will increase and the workability will decrease significantly.

[0022] Furthermore, when mixing two or more types of linear polyester resins having a number average molecular weight of 10,000 or more,
The above-mentioned polyester resins are blended so that the glass transition temperature of the mixed resin is 15 to 35 °C by mixing two or more types of polymeric linear polyester resins having a glass transition temperature of 25 ± 20 °C. preferable. Glass transition temperature is 25
This is because if the above-mentioned polyester resins outside the range of ±20°C are mixed, the desired processability and stain resistance cannot be obtained. Resin compositions have high hardness and poor processability; conversely, those containing a high proportion of resins with a glass transition temperature of less than 5°C have low hardness and poor stain resistance.

[0023] Mixing two or more types of polyester resins provides better performance (chemical resistance, weather resistance, etc.) than using one type of polyester resin.
This is because it has the advantage that paint design is easy when considering
(shown in Comparative Example).

Examples of the melamine-based cured resin used in the present invention include fully methylated melamine, fully butylated melamine, mixed alkylated melamine, imino group-containing alkylated melamine, and methylol group-containing melamine resin.

The blending amounts of this melamine-based cured resin and the polyester resin are 80 to 60 parts by weight for the polyester resin and 20 to 40 parts by weight for the cured resin, and the total amount is 100 to 60 parts by weight.
Make sure it is by weight. If the proportion of the melamine-based cured resin in the polyester resin is less than 20 parts by weight, the hardness will be low and the stain resistance will be reduced, and if it exceeds 40 parts by weight, the hardness will be high and the processability will be significantly deteriorated.

Examples of the curing accelerator used in the present invention include paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenesulfonic acid, and methanesulfonic acid. The blending amount of this curing accelerator is preferably 1 to 8 parts by weight based on 100 parts by weight of the total resin. If it is less than 1 part by weight, the hardness will be low and the stain resistance will be poor. If the amount exceeds 8 parts by weight, processability will decrease and appearance such as gloss will deteriorate. In addition to this, other components can be added to the extent that the effects of the present invention are not impaired to impart various properties.

The above constitutes the vehicle in the present invention. In addition to the vehicle, pigments and the like are added to the coating composition. That is, pigments are added to paints consisting only of vehicles in order to impart color and other properties such as anticorrosion and antifouling properties.

[0028] The coloring pigment that colors the paint is not particularly limited and may be selected according to various uses, but examples include titanium oxide, titanium yellow,
Examples include titanium black, chromic acid, and phthalocyanine blue.

[0029] The amount of the coloring pigment to be blended is preferably 50 to 150 phr based on the resin in view of performance such as hiding power and processability.

[0030] As described above, the above-mentioned main film-forming elements are mixed with the above-mentioned various additives in the above-mentioned preferred ranges, and the coating for pre-coated steel sheets, which is a film-forming element diluted with a solvent as necessary, is directly applied. Or after normal pre-treatment,
It is applied to the surface of the base steel plate and baked to form a coating film. If necessary, the resin composition of the present invention can be applied after a primer coating is applied and dried, or without drying. For example, pre-painting treatments include chromate chemical conversion treatment, phosphate chemical conversion treatment, and composite oxide film treatment. Chromate treatment includes electrolytic chromate, applied chromate, and reactive chromate. For example, phosphate chemical treatment includes zinc phosphate treatment and iron phosphate treatment. For example, composite oxide coating treatment includes treatment containing nickel and cobalt. As a primer paint,
Commonly used materials such as epoxy resins and high-molecular polyesters can be used.

[0031] As a method for applying the paint, any conventionally known method can be used, such as a roll coater method, a curtain flow coater method, and a spray method.

[0032]Although the baking treatment varies depending on the composition of the resin, etc., the temperature range of 180 to 300°C is 0.5°C.
It is preferable to carry out the reaction for about 3 minutes. As the base steel plate, not only general cold-rolled steel plates but also those subjected to chemical conversion treatment, plating treatment, or even primer treatment can be suitably used. The coating thickness after baking is 15-40μ
It is desirable to set it to about m.

[0033]

EXAMPLES The present invention will be specifically explained below. Plate thickness 0.
After applying zinc phosphate treatment to a 5 mm electrogalvanized steel sheet (coating amount: 20 g/m2 on one side), an epoxy-modified polyester resin paint (manufactured by Nippon Paint Co., Ltd.) was applied as an undercoat, and the dried film was thick. Apply to a thickness of 5 to 7 μm. Next, after baking under the conditions that the maximum plate temperature reached was 210±10℃ and the baking time was 60 seconds, Tables 1 and 2
A top coat of the composition shown in is applied to a dry film thickness of 15 to 20 μm, and then the maximum board temperature is 230 ± 10
A coated steel plate for testing was prepared by baking under the conditions of ℃ and baking time of 60 seconds. Table 1 shows the compositions of Examples of the present invention, and Table 2 shows the compositions of Comparative Examples.

In Tables 1 and 2, hexamethoxymethylolmelamine was used as the melamine resin, dodecylbenzenesulfonic acid was used as the curing accelerator, and titanium oxide was used as the coloring pigment.

The paint film performance of the test coated steel sheets thus obtained was examined in terms of pencil hardness, workability, and stain resistance, and the results are shown in Tables 1 and 2. However, Table 1 shows the coating film performance tested for Examples 1 to 12, and Table 2 shows the coating film performance tested for Comparative Examples 1 to 10, respectively.

(Test method) Pencil hardness: Scratch mark method (JI
SK 5400), the coating film was drawn using a Mitsubishi Uni, and the hardness with which it would cause scratches was examined and evaluated accordingly. - Workability: Evaluation was performed by bending 180°. Specifically, a test piece was sandwiched between steel plates of the same thickness, and a pressure of 50 kg/cm2 was applied using a hand press to bend the test piece at 180 degrees, and cracks in the test piece at the bent portion were observed using a 15x magnifying glass. It is expressed as the minimum number of iron plates (T) that can sandwich the test piece without entering. - Stain resistance: Red marker ink was applied to the painted surface of a painted test piece, and after being left for 24 hours, it was wiped off with ethanol, and evaluated based on the degree of residual marker ink at that time. Evaluation ◎: Very good, ○: Good, △: Slightly poor, ×: Very poor

[0037]

[Table 1]

[0038]

[Table 2]

As can be seen from Table 1, the pre-coated steel sheet of the present invention has extremely improved stain resistance while maintaining the required hardness and high workability. It can be seen that both performances are satisfied at the same time. In Comparative Examples 1 to 10 shown in Table 2, workability and hardness,
Furthermore, the stain resistance was not balanced, and the performance of any of the comparative examples was inferior.

[0040]

As is clear from the above description, according to the present invention, the film-forming component has a relatively high number average molecular weight and a glass transition temperature in an appropriate range, which is the main element for forming a bake-curable film. A predetermined amount of a melamine-based curing resin is added to a polymer linear polyester resin having a hydroxyl group, and a coating composition containing a vehicle in which a predetermined amount of a sulfonic acid-based curing accelerator is added to these resins is applied to a steel plate, By baking, a precoated steel sheet with a high balance of workability, hardness, and stain resistance can be obtained.

Claims (2)

[Claims] 1. Components for forming a film on a steel plate include 80 to 60 parts by weight of a linear polymeric polyester resin having a hydroxyl group, a number average molecular weight of 10,000 or more, and a glass transition temperature of 15 to 35°C, and a melamine-based polyester resin. Cured resin 20-40
Workability and stain resistance characterized by coating and baking a resin composition containing a vehicle containing 100 parts by weight and 1 to 8 parts by weight of a sulfonic acid curing accelerator. Pre-painted steel plate with excellent properties. 2. The linear polymeric polyester resin having a glass transition temperature of 15 to 35°C comprises two or more linear polymeric polyester resins having a glass transition temperature of 25±20°C. 2. The pre-coated steel sheet with excellent workability and stain resistance according to claim 1, which is a resin blended with the above-mentioned resin.