JPH06262139A - Fluororesin coated steel sheet with excellent workability - Google Patents

Abstract

(57) [Summary] [Purpose] To prevent workability deterioration of coating films, which is a problem for fluorocarbon resin coated steel sheets. [Structure] The average particle diameter of vinylidene fluoride (PVdF) contained as a fluororesin in the coating film is 5 μm or less. P
The crystallization of VdF is suppressed, and the deterioration of the workability of the coating film over time is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluororesin-coated steel sheet suitable for exterior building materials, which are required to have excellent weather resistance, workability and economy, particularly curtain walls for buildings, roofing materials for gymnasiums and the like.

[0002]

2. Description of the Related Art In addition to having excellent weather resistance, exterior building materials such as building curtain walls and gym roofing materials,
Excellent workability and economy are required.

Conventionally, resin-coated steel sheets or vinyl chloride steel sheets having a galvanized steel sheet as a base have been mainly used for such applications. The former resin-coated steel sheet is a hot-dip galvanized steel sheet after suitable chemical conversion treatment and primer coating.
It is a steel sheet coated with polyester resin and is called a colored galvanized steel sheet. The latter PVC steel sheet is
It is a steel sheet that is coated or laminated with vinyl chloride after chemical conversion treatment on a hot dip galvanized steel sheet.

These materials were once said to have sufficient durability. However, in environments with severe corrosiveness such as coastal areas and industrial areas, lack of durability begins to be pointed out, and in recent circumstances where market demands for guarantees are becoming strict, performance is usually high even for residential areas. It is becoming inadequate.

In consideration of such a situation, a product in which the base material is changed to a galvanized steel plate, a zinc-aluminum plated steel plate, a stainless steel plate, etc., which are more excellent in corrosion resistance than the hot dip galvanized steel plate, is developed, and at the same time, the durability of the coating film is improved. Therefore, steel sheets coated with fluorocarbon resin are now on the market.

Fluorine resin is generally considered to be difficult to form a coating film, but vinylidene fluoride resin (hereinafter referred to as PVdF resin) can be formed into a coating film by adding an acrylic resin. Therefore, a coated steel sheet using a fluororesin whose main component is PVdF resin is commercialized together with a laminated steel sheet.

[0007]

A steel sheet coated with a fluororesin is remarkably excellent in weather resistance, reflecting the excellent durability of the coating. In particular, a coated steel sheet has a coating formed by a coating method such as roll coating, so that it has good productivity, can easily cope with hue changes, and is economical.

However, since the PVdF resin in the coating film crystallizes, the coating film cannot withstand severe processing. Since this crystallization phenomenon is an aging phenomenon, in general, the longer it is stored, the more remarkable the deterioration of workability becomes.

As described above, the fluororesin-coated steel sheet has a drawback in that it is economical, but is poor in workability. This drawback becomes a big problem in exterior building materials that require high workability.

On the other hand, since the laminated steel sheet of fluororesin is formed by laminating prefabricated films,
Excellent workability. However, the manufacturing cost is high due to the large number of manufacturing steps and the frequent hue change, so that the economical efficiency is worse than that of the coated steel sheet.

With respect to the drawbacks of such laminated steel sheets,
The present inventors have proposed one measure (corresponding to color change by adhesive) in Japanese Patent Laid-Open No. 4-175158, but as long as it is a laminated steel plate, it is possible to improve the economical efficiency to the same level as the coated steel plate. It is impossible due to the process.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a fluororesin-coated steel sheet having both workability and economic efficiency by solving the workability problem pointed out in the coated steel sheet. To aim.

[0013]

In order to provide a steel sheet coated with a fluororesin at low cost, cost-effective coating is indispensable. In that case, it is necessary to improve the workability of the PVdF resin coating film. .

Regarding the coating of the PVdF resin, as described above, a coating film is generally formed by adding an acrylic resin, and it is said that the cause of the deterioration of the workability of the coating film is the crystallization of the PVdF resin. . In fact, if the crystallization of the PVdF resin is suppressed by means such as rapid water cooling after baking the coating, it is possible to considerably suppress the deterioration of the workability.

Here, acrylic resin (for example, PMMA)
There are four main reasons for using the following.

Since the PVdF resin alone does not sufficiently adhere to the primer, the acrylic resin improves the adhesiveness. The acrylic resin solution has good pigment dispersibility and enhances color development. Since PVdF resin alone is costly, the acrylic resin improves the economy. The thermosetting process of the acrylic resin heat-mixes the PVdF resin and the acrylic resin, hinders the regular orientation of the PVdF resin, and suppresses the crystallization thereof.

It should be noted that acrylic resin is P
It has the effect of suppressing crystallization of the VdF resin. Therefore, it is considered that the workability of the coating film does not deteriorate if the effect is sufficient. However, in reality, the workability is deteriorated.

By the way, the PVdF resin is dispersed as fine particles in the paint or coating film.

The inventors of the present invention consider that the reason why the workability of the PVdF resin coating film is lowered is that the crystallization suppressing effect of the acrylic resin is not sufficiently functioning, and the reason is that the acrylic resin at the time of baking is It was hypothesized that the PVdF resin compatibility was insufficient.

In this case, as a countermeasure, it is possible to select an acrylic resin having a high compatibility with the PVdF resin. In addition, it is considered that measures for lowering the molecular weight of the PVdF resin to lower the melt viscosity of the resin are also effective for improving the processability of the coating film. However, both measures are likely to adversely affect the essential properties of the coating film such as weather resistance.

The present inventors have focused on reducing the particle size of the PVdF resin as a method of increasing the compatibility between the acrylic resin and the PVdF resin without such a secondary effect, and repeated various experiments. As a result, if the average particle size of the PVdF resin is 5 μm or less, the compatibilization of the acrylic resin and the PVdF resin is promoted during baking, and the crystallization of the PVdF resin is suppressed. As a result, the workability of the PVdF resin coating film is improved. It was confirmed that it was remarkably improved, and that it did not cause a problematic side effect such as deterioration in weather resistance.

The present invention has been made on the basis of the above findings, and a chromic acid-based and / or phosphoric acid-based undercoating film is formed on the surface of a metal plate, and a PVdF resin is formed on the undercoating film through a primer coating film. Is characterized in that a fluororesin baked coating film having an average particle size of 5 μm or less is formed.

Incidentally, a PVdF resin having a particle size of about 10 μm has been conventionally used.

The PVdF resin can be atomized by freeze pulverization or selection of a dispersant, and the particle size can be directly measured by microscopic observation.

[0025]

The constituent features of the present invention will be explained below, and the respective actions will be clarified.

The metal plate serving as the metal plate base is, for example, a steel plate, a plated steel plate, a stainless steel plate, an aluminum plate or the like, and is not particularly limited. Examples of the galvanized steel sheet include hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, aluminum-plated steel sheet, zinc-nickel alloy-plated steel sheet, zinc-aluminum alloy hot-dipped steel sheet and the like.

Undercoating film In order to secure the adhesion between the base metal plate and the coating film, the surface of the base metal plate is subjected to a chromic acid-based phosphoric acid-based or both of them. For example, when the base material is a hot-dip galvanized steel sheet, after the zinc phosphate treatment, coating chromate treatment may be further performed. Known treatments can be applied to the chromic acid treatment and phosphoric acid treatment.

Primer In order to improve the adhesion between the coating film base and the coating film and to improve the rust preventive power, a primer treatment is further applied to the undercoating film by a conventional method. A paint containing an epoxy resin as a main component is suitable as the primer. An extender pigment such as silica or calcium carbonate, an anticorrosive pigment such as strontium or chromate, and a crosslinking agent such as amine or melamine can be contained in the primer.

Fluorine Resin Baking Coating In order to secure excellent weather resistance, a fluorine coating containing a PVdF resin is baked on a primer coating as a top coat. Since PVdF resin is difficult to form into a coating film by itself, it is combined with a coating formation promoting resin such as an acrylic resin. As the acrylic resin, for example, methyl methacrylate (MMA) such as polymethacrylate (PMMA) and poly-n-butyl methacrylate, ethyl methacrylate (EMA), butyl methacrylate, methyl acrylate, ethyl acrylate and other monomers are used. The resin used as the raw material is used.

In this fluororesin paint, PVdF
The particle size of the resin is important and is limited to 5 μm or less on average. If the particle size of the PVdF resin is large, the PVdF resin and the acrylic resin are not sufficiently compatibilized with each other at the general baking temperature and baking time in the coating baking process. Therefore, economical crystallization is likely to occur later, and as a result, the workability of the coating film deteriorates.

Since such atomization of the PVdF resin does not change the chemical characteristics of the resin, the processability of the coating film is improved without impairing the excellent weather resistance of PVdF resin and other inherent characteristics. There are advantages.

The fluororesin paint obtained by atomizing the PVdF resin may be applied as a single layer on the primer coating film, or may be a multi-layer coating of two or more layers in which the content of the acrylic resin is changed stepwise. In any case, it is desirable that the film thickness obtained by one baking coating is approximately 10 to 25 μm. If it is less than that, coloring is insufficient due to the problem of hiding and when the film thickness is excessively hot. Is more likely to cause armpits.

The lower limit of the particle size of the PVdF resin is not particularly limited, but it is industrially disadvantageous because crushing increases the cost to obtain fine particles.

The amount of PVdF resin in the coating film is generally 7
Many of those having a content of 0% or more are commercially available, but are not particularly limited.

[0035]

EXAMPLES Examples and comparative examples of the present invention will be described below.

As the base material, a stainless steel plate (SUS304, 2B finish) having a plate thickness of 0.5 mm and a hot-dip galvanized steel plate (adhesion amount: 60 g / m ² ) were used.

As a base treatment, a stainless steel sheet was subjected to coating type chromate treatment (30 mg / m ² ) and a hot-dip galvanized steel sheet was subjected to zinc phosphate treatment (0.8 g / m ² ).

On the surface of the stainless steel plate and the hot-dip galvanized steel plate that have been subjected to the base treatment, as a primer treatment, a Nippon resin primer coating is applied to a dry film thickness of 5 μm and a baking temperature of 2
Baking was performed under the condition of 10 ° C.

Fluorine resin paint was dried to a thickness of 15 μm on the stainless steel plate and the galvanized steel plate after the primer treatment.
m, and the baking temperature was 230 ° C. Fluororesin paint is PMMA (molecular weight 5000) 20 in terms of solid content
Parts, PVdF (molecular weight 20000) 80 parts, and titanium dioxide 80 parts, and the average particle size of the PVdF resin was set to three types of 1 to 2 μm and 3 to 5 μm, which are smaller than the normal level, and 10 to 15 μm of the normal level.

The initial workability, aging workability and weather resistance of the obtained fluororesin-coated steel sheet were investigated by the following methods.

Initial workability is as follows:
When the steel sheet was bent at 23 ° C. to 180 ° C. with the coating film removed, evaluation was made by the minimum number of plate sandwiches that did not cause cracks in the coating film.

The workability over time is evaluated by the minimum number of plate scissors which do not cause cracks in the coating film when the coating film is bent to 23 ° C. and 180 ° C. one week after the steel plate is manufactured. did.

The weather resistance was evaluated by measuring the color change after 2000 hours of exposure with a sunshine weather ohmmeter based on JIS K5500.

[0044]

[Table 1]

The results are shown in Table 1. The fluororesin-coated steel sheet of the present invention using the PVdF resin having an average particle size smaller than the normal level is compared with the fluororesin-coated steel sheet for comparison using the normal level of PVdF. The initial workability was good, and the difference in the workability was further expanded by the workability with time, and it was possible to prevent the workability from decreasing with time. Further, it showed weather resistance comparable to that of the steel sheet for comparison, and no adverse effect was observed due to the smaller PVdF particle size.

[0046]

EFFECT OF THE INVENTION Since the fluororesin coated steel sheet of the present invention is a coating type, it has good productivity, can easily cope with hue changes, and is excellent in economic efficiency. Even though it is a coating type, it has excellent workability by preventing the workability of the film thickness from being lowered due to crystallization of the PVdF resin. Therefore, an exterior building material having excellent weather resistance and workability can be provided at a low cost, and a great effect can be exerted in reducing the cost of the building material.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location B32B 15/08 102 B 7148-4F (72) Inventor Akihiro Yauchi 4 Kitahama, Chuo-ku, Osaka-shi, Osaka 5-53, Sumitomo Metal Industries, Ltd. (72) Inventor, Shimada Yozo, 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. A chromic acid-based and / or phosphoric acid-based undercoating film is formed on the surface of a metal plate, and a vinylidene fluoride resin particle size is 5 μm or less on average through a primer coating film. A fluororesin-coated steel sheet with excellent workability, characterized in that a fluororesin-baked coating film adjusted to 1. is formed.