JPH10323615A - Coating for coated steel plate, coated steel plate and manufacturing of the steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coating which brings about the same effects as when a protecting film is applied which reduces the degree of a packing-mark leaving by pressing a packing material when a coated steel plate is conveyed or stored, a scratch by handling or a flow due to an impact by a tool during a processing time as coating mars, and a steel plate to which the coating is applied and a method for manufacturing the coated steel plate. SOLUTION: This coating for finishing coat is of polyester resin, urethane resin or acrylic resin and contains 10-50 pts.wt. of polyolefin resin particle with a particle diameter of 100 μm or less for 10 pts.wt. of heating residue of the coating. In addition, the top layer part of the coated steel plate is coated with the described coating and therefore, shows outstanding anti-marring and resistance to packing mark leaving. The coated steel plate is manufactured by applying the coating and baking it as the final application of a surface coating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated steel sheet having a surface coated with a paint in a state of a flat plate which is used for home appliances, building materials, automobiles and the like at the factory shipment.

[0002]

2. Description of the Related Art When thin steel sheets or galvanized steel sheets are applied to building materials such as outer walls and roofs, home appliances such as washing machines, refrigerators and coolers, or automobile parts, the final products are rust prevention and design. Due to the above requirements, the surface is often coated with a paint made of an organic resin. In contrast to the method of painting the surface after molding or assembling into the final shape, if a steel sheet previously coated with organic resin paint is used,
Problems such as poor working environment due to painting, reduced efficiency for sufficient painting of individual parts, waste of paint, waste gas and waste liquid treatment can be avoided. The coated steel sheet or the precoated steel sheet developed for such a purpose has been rapidly increasing in use in recent years.

[0003] Since the surface of a coated steel plate is coated under conditions that are sufficiently controlled in advance, there is an advantage that a coated product can be manufactured only by processing and assembling steps on the user side. However, in order to make full use of it, it is necessary that the coating film has sufficient workability like steel sheet, corrosion prevention of cut end face, weldability, etc., during the process from steel sheet to product. There are still many problems to be solved, such as scratches on the coating film.

[0004] The scratches on the coating film may be caused by the steel plates being rubbed against each other in a pressurized state during handling, by being strongly pressed against packing materials during packing or transportation, or by contact with tools during processing. Some of them occur, and in particular, the pressing of packing materials is sometimes called a pressure mark. Such scratches are inevitable to some extent due to the relatively heavy and hard steel plate with a soft coating, but this will greatly reduce the commercial value of the product. It must be deterred as much as possible. Increasing the molecular weight of the resin, which is the main component of the film-forming element, increases the hardness of the resin, which reduces these flaws. There is a limit because the film breaks or peels off.

On the other hand, a mineral powder having a particle size of 2 to 10 μm and a Mohs hardness of 5 or more is mixed into the coating material, or an organic resin powder having a particle size of 2 to 50 μm which is incompatible with the organic resin forming the coating film is mixed. The invention which suppresses the pressure mark is disclosed in JP-A-8-53645 or JP-A-8-53646. By this, pressure marks will be less likely to occur, but the incorporation of mineral powder may reduce the coating effect of the coating, and the incorporation of resin powder has the disadvantage of deteriorating the gloss of the coating. It is presumed.

There is also a method of attaching a polymer film of vinyl chloride or the like for the purpose of protecting the surface in order to prevent scratches on the painted surface. This is a process in which the protective film is attached to the final part, and finally the film is peeled off. In addition to the pressure mark, scratches that occur easily during handling can be suppressed, so it is extremely effective in protecting the coating film However, a significant increase in the cost for attaching the film is inevitable. Further, as for the protective film of vinyl chloride resin, the film itself is inexpensive, but there is also a problem that toxic gas is generated when the film is incinerated after peeling.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a coating film that is left behind by pressing a packing material when transporting or storing a coated steel sheet, scratches in handling, scratches on a tool during processing, etc. Paint that has the same effect as pasting the protective film, a steel plate coated with the paint,
And a method of manufacturing the steel sheet.

[0008]

In the above-described measures for preventing damage to a coating film, methods such as hardening the coating film and adding other ones to the coating film eventually result in the performance required for the coating film. I have to sacrifice to some extent. Although the use of a protective film is considered to be ideal for preventing the coating film from being damaged, it is unavoidable that the cost increases due to an increase in steps. Therefore, the present inventors have studied the formation of a protective film by adding a component capable of forming a film to a paint.

A paint is known which separates into two layers, a surface layer and an adhered surface, by drying after coating to form a strong protective film on the surface layer side. In this case, however, the protective film is usually formed. It is used in the state where it was. Here, if a protective film is formed on the surface by coating and drying to separate it into two layers, and after processing, etc. to make the final shape, if there is a paint that can peel off the film, use this as a protective film Thus, a coated steel sheet with a protective film can be manufactured without increasing the number of steps. From such a viewpoint, various studies have been made on a method of adding a component capable of forming a protective film in the overcoat paint of a steel sheet, and as a result, the present invention has been achieved. That is, the gist of the present invention is as follows.

(1) Polyolefin resin particles having a particle diameter of 100 μm or less are mixed with polyester resin, urethane resin, or acrylic resin paint, and 10 to 50 parts by weight of the heating residue of the paint. A paint for top coating of a coated steel sheet characterized by containing by weight.

(2) A coated steel sheet excellent in scratch resistance and packaging trace resistance, wherein the uppermost layer is coated with the paint described in (1) above.

(3) The method for producing a coated steel sheet according to (2), wherein the paint described in (1) above is applied to the final coating of the surface coating film and baked.

The top coat applied to the uppermost layer of the coated steel sheet is classified according to the resin which is the main component of the coating film forming element, and includes polyester resin, urethane resin, acrylic resin, vinyl chloride resin, and fluoride. A vinylidene resin system or the like is generally used. Among them, polyester resin, urethane resin and acrylic resin are thermosetting paints, and vinyl chloride resin and vinylidene fluoride resin are thermoplastic paints.

There are various types of resins that can form a protective film, but from the viewpoints of low cost, appropriate workability, non-polarity, and no generation of toxic gas during incineration, etc. And a polyolefin resin. The polyolefin resin means a linear polymer resin such as polyethylene and polypropylene. Among these, it was thought that non-polarity was not particularly good in adhesion to the coating film, but on the other hand, it was expected that the surface layer could be easily peeled off after separation into two layers after coating and drying. .

[0015] Polyolefin resin particles having various molecular weights and particle diameters are added to the above-mentioned various top coatings, and after heat drying or baking, it is determined whether or not the formed film on the surface can be a target protective film. A survey was conducted. As a result, when the coating material was mainly composed of a thermoplastic resin, the polyolefin resin film formed on the surface layer was in close contact with the coating material and could not be peeled off. On the other hand, it has been found that, in the case of a thermosetting resin coating, the surface layer can be peeled off after baking. This is due to mutual fusion with the polyolefin resin in the case of a thermoplastic resin during the heating of dry baking, but in the case of a thermosetting resin, during the mutual curing reaction between the molecules of the coating film forming resin, It is considered that the polyolefin resin separated and did not fuse.

Therefore, the conditions under which the peelable protective film can be formed more easily are studied, and a clear reaction between the curing reaction starting temperature of the film forming element resin of the thermosetting paint and the softening point of the polyolefin resin is considered. The result is that the difference is important. That is, the curing reaction start temperature of the paint is
Since the performance of the obtained coating film cannot be changed greatly, it is necessary to select a polyolefin resin having a softening point sufficiently lower than that.

The reason why such a protective film is formed after baking and drying with a paint containing polyolefin resin particles is considered as follows. When the temperature for baking rises after the application, first, the polyolefin resin particles soften, melt, and begin to fuse with each other. Polyolefin resin is non-polar, so it does not adhere well to the undercoat or undercoat, and tends to move away from the coated surface of the steel sheet. It is separated in the form of being unevenly distributed on the steel plate side.
As the temperature rises, intermolecular bonding of the coating film forming resin, that is, a curing reaction starts, and the polyolefin resin is further excluded from the formed coating film. As a result, the coating film of the paint adheres to the surface to be coated of the steel sheet, and is cooled in such a form that the polyolefin film covers the coating film. Therefore, if the curing reaction start temperature is equal to or lower than the softening point of the polyolefin resin, the coating is cured before the polyolefin resin separates and melts, concentrates on the surface, and a sufficient film is formed, An unhealthy coating film incorporating resin particles and a protective film having poor releasability are formed.

The obtained protective film must have a sufficient protective effect on the coating film, and must be in close contact with the coating film and must be resistant to processing. In addition, the paint must be easily applied to a steel plate, and a protective film having a required thickness must be obtained. Consider each of the above conditions,
The present invention was completed by clarifying the limits for obtaining the optimum result.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A conventional paint for a coated steel sheet comprises a film-forming element, a solvent, a paint pigment, and auxiliary materials.
After being applied to the surface of the steel sheet, the solvent is evaporated by heating, and the heating residue is melt-solidified or reaction-hardened to form a coating film. In the paint of the present invention, the resin that is the main component of the coating film forming element is a thermosetting paint such as a polyester resin, a urethane resin, and an acrylic resin. This is because in the case of a thermoplastic resin that is melted and solidified, a polyolefin resin film formed as an upper layer of the coating film is fused.

The paint of the present invention contains 10 to 50 parts by weight of a polyolefin resin with respect to 10 parts by weight of the heating residue. The polyolefin resin has a molecular weight of about 30,000 to 100,000. The particle size of the resin to be added should be 100 μm or less.

The protective film for preventing the coating film of the coated steel plate from being damaged is a polyolefin resin film having a thickness of 30 μm.
The above thickness was required. This is because not only a sufficient protective effect is ensured, but also when the formed protective film is finally removed, the protective film can be peeled off without being appropriately broken. However, as the thickness increases, not only does it take time to evaporate the solvent and separate into two layers, but also it is easy for defects such as air bubbles to appear in the coating film, which increases the cost.
To the extent seemed to be the limit. That is, the thickness of the protective film is desirably about 30 to 150 μm.

On the other hand, the thickness of the coating film, which is the residue of heating after baking of the top coat of the coated steel sheet, needs to be 15 μm in order to sufficiently hide the color of the base. The density of this coating is roughly
1.5 to 2.0 g / cm ³ , and the density of the polyolefin resin is
Since it is 0.85~0.90g / cm ^3, in order to obtain a protective film of about. 30 to 150 [mu] m, compared heating residue 10 parts by weight, contained in the coating material for coating a 10 to 50 parts by weight of the polyolefin resin to the steel sheet I have to do it.

After the coating, it is separated into two layers by drying and baking,
And finally, in order to be a film that can be peeled,
The melting point or softening point of the polyolefin resin must be lower than the onset temperature of the curing reaction of the resin of the film forming element. Since the onset temperature of the curing reaction of a thermosetting overcoat which is usually used is considered to be 150 ° C. or higher, the melting point or softening point of the polyolefin resin is preferably about 80 to 120 ° C. Further, the performance of the protective film formed on the surface needs to have a sufficient protective effect, sufficient workability, and a strength that facilitates the peeling operation. To satisfy these requirements, the molecular weight of the polyolefin resin is preferably about 30,000 to 100,000. In this case, if the molecular weight is less than 30,000, the strength is insufficient, the effect as a protective film is insufficient, and peeling is difficult. Also, if it exceeds 100,000, the melting point will increase,
A sufficiently uniform protective film cannot be obtained at the time of drying and baking, and the protective film becomes too hard, so that the workability as the protective film and the pressure reducing effect of pressing the packing material or the like are lost.

Next, when the particle diameter of the polyolefin resin added to the coating exceeds 100 μm, when the coating is performed using a roll coater, the particles cannot pass through the nip portion of the coating roll.
Not only can it not be applied to the steel sheet, but in some cases it also results in coating defects such as streaking. Therefore, the particle size must be less than 100 μm. A smaller diameter is preferred in terms of uniformity, but is naturally limited because of the cost of grinding. The paint of the present invention is
Since such a large amount of polyolefin resin particles are added to the paint of the conventional composition, it is necessary to appropriately control the amount of the solvent and to add auxiliary materials for the paint to make the painting work easier. desirable.

The base material of the steel sheet to which the present invention is applied includes hot-dip galvanized steel sheet, electro-galvanized steel sheet, various zinc alloy-based coated steel sheets, aluminized steel sheet, stainless steel sheet, and cold-rolled steel sheet and hot-rolled steel sheet. It is. When coating on these steel sheets, usually a zinc phosphate treatment, an iron phosphate treatment, a chromate treatment or the like is applied to the steel sheet as a base treatment, and a paint is applied. If necessary, an undercoat or an intermediate coat is performed. In any case, the paint of the present invention is applied to the uppermost layer of the steel sheet. Further, the coating of the paint of the present invention on a steel plate may be performed by using a roll coater, a curtain flow coater, a nozzle coater, or the like which has been conventionally used for coating a coated steel plate, followed by drying and baking.

In the baking after the application of the paint, the coating is heated to a maximum heating temperature of about 230 ° C. in about 40 to 80 seconds. The heating temperature is
When the temperature is lower than 170 ° C., the cured film is insufficiently cured, and a sound protective film is hardly formed. If the temperature exceeds 260 ° C,
Not only is the adhesion of the formed protective film and the coating film too strong, it becomes difficult to peel off, but the protective film is discolored and the appearance is deteriorated.

[0027]

[Example] A hot-dip galvanized steel sheet with a thickness of 0.8 mm and an adhesion amount of 60 g / m ² was used as a base material, and a 75 mm wide and 150 mm long specimen was cut out therefrom.
After applying 40 mg / m ^{2 and} drying, apply an epoxy resin paint (NP250 made by Nippon Paint Co., Ltd.) as a primer paint with a bar coat aiming at a dry film thickness of 5 μm. Dry baking for 40 s. Various top coats having different types of resin, polyolefin resin, and polyolefin as coating film forming elements were applied thereon using a curtain flow coater, and dried and baked. In this case, by controlling the coating thickness of the paint, the coating thickness without the protective film formed on the surface after drying and baking, in any case, 15μm
I tried to be. Dry baking has a maximum temperature of 235 ° C
The processing was performed for about 80 seconds.

Table 1 shows the type of resin of the top coating material used and the type and content ratio of the added polyolefin resin.

[0029]

[Table 1]

After the coating was completed, the test pieces were visually observed to evaluate the appearance of the coating. When the film formed on the surface could be peeled, the surface condition after peeling was further confirmed. The peel strength was determined by a peeling test. The test temperature was 23 ° C., the tensile speed was 50 mm / min, and the maximum peel strength per 25 mm width was measured. The resistance to packing marks is as follows: At 30 ° C, apply a load of 1 kg /
After pressing for 1 day at cm ² , the paint surface from which the film on the surface had been removed was visually observed.

Table 1 also shows the test results. Exam number 1
Nos. 19 to 19 are paints in which various amounts of polyethylene particles having a molecular weight of 50,000 and a softening point of 100 ° C. and a particle size of 60 μm are added to acrylic resin, urethane resin and polyester resin based paint film forming elements. Things. As described above, the coating thickness was 15 μm in each case. As can be seen from these comparisons, the content ratio of the polyethylene resin is within the range defined by the present invention in any type of paint, that is, 10 parts by weight based on 10 parts by weight of the heating residue of the paint mainly containing the resin of the coating film forming element. If it is 50 parts by weight, a steel plate having good appearance, easy peeling and excellent resistance to packing marks can be obtained. However, when the content ratio of the polyethylene resin is too small, the protective film becomes difficult to peel off and the anti-packing trace resistance is not sufficient. When the content ratio is too large, the appearance deteriorates.

When the results of Test Nos. 13 to 19 and Test Nos. 20 to 30 are combined and compared, the effect of the addition of various polyolefin resins to the polyester resin-based coating film forming element coating can be understood. First, even if the polyolefin resin is polyethylene or polypropylene, its content and particle size are within the ranges defined by the present invention, and if the molecular weight and softening point are within the desired ranges, the appearance, peelability, and packing trace resistance are as follows. All are good. On the other hand, in Test Nos. 25 to 26 in which polyethylene having a high molecular weight and a high softening point was added, the protective film could not be sufficiently formed to make peeling difficult, and the diameter of the added polyethylene particles was determined by the present invention.
In Test Nos. 28 to 30 exceeding 00 μm or less, the coating film after removal of the protective film has many irregularities, and the appearance is not good.

[0033]

When the paint of the present invention is used for overcoating of a coated steel sheet, a coated steel sheet having a protective film adhered on the coating film can be easily obtained. Since the coated surface of this steel sheet is covered with the protective film, the coated film surface can be protected from remaining packing marks due to pressing of packing materials during transportation and storage, and abrasion during handling. In addition, since the steel sheet using this paint can be manufactured in a steel sheet coating line using a roll coater or the like without requiring a separate process, the use of the coated steel sheet is greatly expanded.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C09D 133/00 C09D 133/00 167/00 167/00 175/04 175/04

Claims (3)

[Claims] 1. A polyester resin-based, urethane resin-based, or acrylic resin-based paint, wherein 10 to 50 parts by weight of a polyolefin-based resin particle having a particle size of 100 μm or less is added to 10 parts by weight of the heating residue of the paint. A top coating for coated steel sheets, characterized in that it contains part by weight. 2. A coated steel sheet excellent in scratch resistance and packaging trace resistance, wherein the coating of the paint according to claim 1 is applied to the uppermost layer. 3. The method for producing a coated steel sheet according to claim 2, wherein the paint of claim 1 is applied to the final coating of the surface coating film and baked.