CN111902568B

CN111902568B - Coated galvanized steel sheet

Info

Publication number: CN111902568B
Application number: CN201980021465.6A
Authority: CN
Inventors: 山本哲也; 酒井大辉; 白岩礼士
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2018-03-29
Filing date: 2019-03-12
Publication date: 2022-09-30
Anticipated expiration: 2039-03-12
Also published as: CN111902568A; JP2019171853A; KR20200136013A; JP7166955B2; TWI701358B; TW201943891A; KR102550836B1

Abstract

One aspect of the present invention relates to a coated galvanized steel sheet having a resin coating film containing silica and magnesium hydroxide on a surface of the galvanized steel sheet, wherein a total content of silica and magnesium hydroxide in the resin coating film is 15 to 45 mass%, a content of a resin component in the resin coating film is 55 to 85 mass%, a mass ratio of the magnesium hydroxide to the silica is 0.5 to 2.0, and a thickness of the resin coating film is 0.3 to 1.5 μm.

Description

Coated galvanized steel sheet

Technical Field

The present invention relates to a coated galvanized steel sheet having a coating film containing an inorganic compound in a resin on a surface of the galvanized steel sheet.

Background

The surface-treated steel sheet has excellent corrosion resistance when the surface of the base material is subjected to plating or coating. Among the surface-treated steel sheets, the special chemical conversion-treated steel sheet is a steel sheet having a thin coating film of several micrometers formed on a plating layer, and various properties can be imparted by controlling the composition of the coating film. Therefore, the special chemical conversion treated steel sheet can be applied to various uses.

The composition of the coating film is adjusted according to the use of the particular chemical conversion treated steel sheet. When the composition of the coating film is mainly composed of an organic compound (resin), the balance of various properties required for the coating film, such as corrosion resistance, lubricity, and appearance after processing, can be adjusted relatively easily.

As applications of the special chemical conversion treated steel sheet, for example, there are indoor applications which are mild corrosive environments (mill corrosion environment), and the steel sheet is used for a wide range of products including household electric appliances. When used for these products, the products are sometimes visible to humans depending on the location, and therefore, it is desirable that the appearance is not changed. One cause of the change in appearance is the occurrence of white rust associated with initial corrosion of zinc.

In addition, the present invention is also used for applications (quasi-field applications) in quasi-field corrosive environments (quasi-field applications) which are severer than indoor corrosive environments (semi-field environments) due to excellent corrosion resistance. Examples of quasi-field applications include internal components of outdoor units of air conditioners, doors for houses, and components that are constantly exposed to water.

For example, the internal components of the outdoor unit of the air conditioner are likely to constantly receive fluctuations in the outside air temperature, and although they are disposed in the casing, the casing is usually provided with gaps or through holes, and ultraviolet rays may enter through these gaps or the like. Therefore, in the special chemical conversion treated steel sheet used for the interior parts of the outdoor unit of the air conditioner, the deterioration of the organic compound contained in the coating film is remarkable.

When the organic compound contained in the coating film deteriorates, the protective action of the coating film on the plated steel sheet deteriorates, and the generation of red rust associated with the corrosion of the base steel sheet cannot be sufficiently suppressed. Therefore, for a special chemical conversion treated steel sheet for quasi-field use, measures are taken to suppress the occurrence of red rust, such as increasing the weight per unit area of the plating layer. In addition, in order to suppress the occurrence of red rust, it is also effective to increase the thickness of the coating film.

Magnesium compounds are known to exhibit rust preventive effects on zinc plating. In recent years, a technique for forming a highly corrosion-resistant coating film containing nano-sized magnesium particles has been developed.

As this technique, for example, patent document 1 discloses a coating layer formed from a composition containing nano magnesium hydroxide particles having an average particle diameter of less than 200 nm.

Further, as a technique for repairing and passivating a defective portion of a coating film by self-repairing action to maintain the corrosion resistance of the coating film, patent document 2 discloses a coating film formed using a metal rust inhibitor containing a composite colloid formed of magnesium hydroxide and microsilica.

On the other hand, as a technique for using a magnesium-containing film for a film of a chromium-free organic coated steel sheet, patent document 3 discloses an organic coated steel sheet having a composite oxide film containing oxide particles, phosphoric acid and/or a phosphoric acid compound, and a magnesium compound on the surface of a zinc-plated steel sheet, and having an organic film containing a reaction product of an organic resin and an active hydrogen-containing compound, and a rust-preventing additive component on the composite oxide film.

In the case of a special chemical conversion treated steel sheet, if the thickness of the coating film is increased in order to suppress the occurrence of red rust, the electrical conductivity, which is one of the characteristics of the special chemical conversion treated steel sheet, is lowered, and therefore, the steel sheet cannot be applied to household electric appliances.

In addition, when it is desired to produce a special chemical conversion treated steel sheet by changing the thickness of the coating film depending on the application, it is necessary to introduce an expensive apparatus such as a roll coater which can control the thickness of the coating film with good accuracy. Even if this apparatus is introduced, it is necessary to stop the production line every time the production conditions are changed for individual production, and this inevitably reduces productivity.

The thickness of the coating layer disclosed in patent document 1 is 2.5 to 75 μm, and a plated steel sheet having the coating layer cannot obtain sufficient conductivity. In addition, in patent document 1, no consideration is given to the compound added to the coating layer together with magnesium hydroxide, and sufficient rust-preventing effect is not exhibited when only magnesium hydroxide is added and the thickness of the coating layer is several micrometers or less.

In the coating disclosed in patent document 2, it is necessary to use a metal rust inhibitor containing a composite colloid composed of magnesium hydroxide and microsilica in forming the coating. However, this complex colloid is unstable due to reaction with the treatment liquid component, and is likely to cause a problem in the coating step of gelling the complex colloid. The coating disclosed in patent document 2 contains a water-soluble component, and therefore has insufficient water resistance, and is likely to be discolored due to condensation, wetting with water during transportation, or the like.

In the organic coated steel sheet disclosed in patent document 3, since the magnesium compound is added as water-soluble ions or molecules when the composite oxide film is formed, the stability of the treatment solution is lowered when the amount of the magnesium compound added is increased. Therefore, there is a limit to improving the corrosion-inhibiting effect of the composite oxide film by increasing the amount of the magnesium component. Further, the organic coated steel sheet disclosed in patent document 3 has a problem of low productivity and high production cost because it is necessary to form an organic coating after forming the composite oxide coating.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a coated galvanized steel sheet having electrical conductivity and excellent white rust resistance and red rust resistance.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2016-104574

Patent document 2: japanese laid-open patent publication No. 2002-322569

Patent document 3: japanese laid-open patent publication No. 2002-053979

Disclosure of Invention

The above objects, features and advantages of the present invention will become more apparent from the following detailed description and the accompanying drawings.

Detailed Description

The present inventors have made various studies to achieve the above object. As a result, they found that: the above object can be achieved by appropriately adjusting the total content of silica and magnesium hydroxide in the resin film, the mass ratio of magnesium hydroxide to silica, and the thickness of the resin film, and the present invention has been completed. Hereinafter, "white rust" refers to zinc oxide or zinc hydroxide generated by initial corrosion of zinc contained in a plating layer, and "red rust" refers to iron oxide generated by corrosion of a base steel sheet of a coated galvanized steel sheet. Further, "white rust resistance" refers to corrosion resistance against white rust, and "red rust resistance" refers to corrosion resistance against red rust.

A coated galvanized steel sheet according to an embodiment of the present invention has a resin film containing silica and magnesium hydroxide on the surface of the galvanized steel sheet. The total content of silica and magnesium hydroxide in the resin film is 15 to 45 mass%. The resin component content of the resin coating is 55-85 mass%. The mass ratio of the magnesium hydroxide to the silica is 0.5 to 2.0. The thickness of the resin coating is 0.3 to 1.5 μm.

According to the present invention, a coated galvanized steel sheet having electrical conductivity and excellent white rust resistance and red rust resistance can be provided.

The present embodiment will be described more specifically below, but the present invention is not limited to these embodiments.

[ total content of silica and magnesium hydroxide: 15 to 45 mass% ]

In the present embodiment, Silica (SiO) in the resin coating film ₂ ) And magnesium hydroxide (Mg (OH) ₂ ) The total content of (B) is 15 to 45 mass%. The resin component in the resin coating film has a barrier effect against corrosion factors from the outside with respect to the galvanized steel sheet. However, the rust preventive effect accompanied by chemical reaction is not generally exhibited on the zinc plating. On the other hand, silica and magnesium hydroxide in the resin film have an anticorrosive effect associated with a chemical reaction in zinc plating. However, if the total content of silica and magnesium hydroxide in the resin film is less than 15 mass%, the rust-preventing effect cannot be sufficiently obtained, and both the white rust resistance and the red rust resistance become insufficient. The total content of silica and magnesium hydroxide in the resin film is excellentIs selected to be 20 mass% or more. On the other hand, if the total content of silica and magnesium hydroxide in the resin film exceeds 45 mass%, the interface between the silica particles and magnesium hydroxide particles and the resin component becomes too large, the barrier property of the resin component is lowered, and the corrosion resistance as a coated galvanized steel sheet cannot be ensured. The total content of silica and magnesium hydroxide in the resin film is preferably 30% by mass or less. When the total content of silica and magnesium hydroxide in the resin film is 20 to 30% by mass, the balance between the resin component and other film properties such as workability of the coated galvanized steel sheet is good as long as the resin component is a general-purpose resin.

The silica used in the present embodiment is desirably colloidal silica having excellent compatibility with an aqueous resin described later. In addition, if the average particle diameter of silica is too large, the density of the coating may decrease or coating defects may occur, and therefore, the average particle diameter D may be too large ₅₀ Preferably 500nm or less, more preferably 450nm or less. The average particle diameter D of silica ₅₀ The average particle diameter is defined as the average particle diameter at which the value obtained by integrating silica (integrated value) is 50 mass%.

The magnesium hydroxide used in the present embodiment is not particularly limited as long as the aqueous dispersion is stable, and the powder of the magnesium hydroxide and the method of dispersion are not particularly limited. Average particle diameter D of magnesium hydroxide dispersed in water ₅₀ I.e., the average particle diameter D of magnesium hydroxide in the aqueous magnesium hydroxide dispersion ₅₀ Considering the thickness of the resin film, it is preferably 0.7 μm or less, and more preferably 0.1 μm or more, for example. The average particle diameter D of magnesium hydroxide ₅₀ The average particle diameter is an average particle diameter at which a value obtained by integrating magnesium hydroxide (integrated value) is 50 mass%.

When the aqueous magnesium hydroxide dispersion is prepared, a polymer dispersant (for example, a water-soluble acrylic resin, a water-soluble styrene acrylic resin, or a nonionic surfactant) having little adverse effect on corrosion resistance when it is formed into a resin film can be used.

[ content of resin component in resin coating film: 55 to 85 mass% ]

In the present embodiment, the content of the resin component in the resin film is 55 to 85 mass%. If the resin component in the resin film is insufficient, the film has many defects, and the corrosion resistance is deteriorated. From this viewpoint, the content of the resin component in the resin film is 55 mass% or more, and preferably 70 mass% or more. However, if the content of the resin component in the resin film is too large, the resin film may be softened and the generation of film residue during press molding may increase in addition to the deterioration of corrosion resistance due to the decrease in the degree of denseness of the resin film. From this viewpoint, the content of the resin component in the resin film is 85 mass% or less, preferably 80 mass% or less.

[ mass ratio of magnesium hydroxide to silica: 0.5 to 2.0]

In the present embodiment, the mass ratio of magnesium hydroxide to silica is 0.5 to 2.0. Magnesium hydroxide and silica are known as rust preventives for zinc plating. The present inventors have found that: by blending magnesium hydroxide and silica in a specific mass ratio to the resin film, excellent corrosion resistance can be obtained even if the film has a thickness that can ensure the conductivity of the coated galvanized steel sheet. When the mass ratio of magnesium hydroxide to silica [ Mg (OH) ] ₂ /SiO ₂ ]When the content is in the range of 0.5 to 2.0, excellent corrosion resistance is exhibited. This mass ratio also affects either the white rust resistance or the red rust resistance of the coated galvanized steel sheet, and the red rust resistance is more affected. The mass ratio is preferably 0.6 or more, and preferably 1.0 or less.

The mechanism of the improvement of the corrosion resistance by adjusting the above-mentioned mass ratio to a suitable range is not clear, and it is considered that the following is possible. Namely, it is considered that: magnesium ions eluted from magnesium hydroxide stabilize corrosion products (hereinafter, sometimes simply referred to as "corrosion products") generated from silica and having a high protective effect against galvanization, and the barrier effect of the stabilized corrosion products is improved. The above-mentioned protective effect against galvanization means barrier property against corrosion factors such as water or oxygen. When the mass ratio is less than 0.5, the stabilization of corrosion products by magnesium ions becomes insufficient, and the corrosion resistance is lowered. If the mass ratio exceeds 2.0, corrosion products are insufficient, and in this case, corrosion resistance is also reduced.

Further, by using the particulate magnesium hydroxide, the ratio of the magnesium component added to the resin film can be increased without impairing the stability of the treatment liquid. As a result, it is estimated that: the above mechanism continues for a long time to exhibit excellent corrosion resistance.

[ thickness of resin coating film: 0.3 to 1.5 μm ]

Generally, the greater the thickness of the resin coating film of a coated galvanized steel sheet, the higher the corrosion resistance, i.e., the white rust resistance and the red rust resistance. However, when the thickness of the resin film is less than 0.3 μm, the corrosion resistance is insufficient. In the present embodiment, the lower limit of the thickness of the resin film is set to 0.3 μm or more, preferably 0.4 μm or more, in order to obtain sufficient corrosion resistance. If the thickness of the resin film exceeds 1.5 μm, sufficient conductivity cannot be obtained in the coated galvanized steel sheet. Therefore, in the present embodiment, the upper limit of the thickness of the resin film is 1.5 μm or less, preferably 0.8 μm or less. Further, when the thickness of the resin film is 0.4 to 0.8. mu.m, the balance between the corrosion resistance and the conductivity is excellent.

[ kind of resin ]

The type of resin used in the present embodiment is not particularly limited, and both an aqueous resin and a nonaqueous resin can be used. In the case of using an aqueous dispersion of magnesium hydroxide or colloidal silica, an aqueous resin is preferably used. The aqueous resin is also not particularly limited, and may be preferably mixed with an aqueous dispersion of magnesium hydroxide and colloidal silica. The aqueous resin in the present embodiment refers to a resin that forms an aqueous dispersion or a water-soluble resin.

The aqueous resin is preferably a polyolefin resin, a polyurethane resin, or a polyester resin, and of these, a polyolefin resin and a polyurethane resin are more preferred. Hereinafter, the polyolefin-based resin and the polyurethane-based resin will be specifically described.

[ polyolefin resin ]

The polyolefin resin is preferably an ethylene-unsaturated carboxylic acid copolymer. As the ethylene-unsaturated carboxylic acid copolymer, for example, the ethylene-unsaturated carboxylic acid copolymers described in Japanese patent laid-open publication No. 2005-246953 and Japanese patent laid-open publication No. 2006-43913 can be used.

Examples of the unsaturated carboxylic acid include (meth) acrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconic acid, and the like, and an ethylene-unsaturated carboxylic acid copolymer can be obtained by polymerizing ethylene with 1 or more of these by a known high-temperature high-pressure polymerization method or the like.

The copolymerization ratio of the unsaturated carboxylic acid to ethylene is preferably 10% by mass or more, more preferably 15% by mass or more, and on the other hand, preferably 40% by mass or less, more preferably 25% by mass or less, when the total monomer amount is 100% by mass. If the unsaturated carboxylic acid content is less than 10% by mass, the carboxyl group which becomes the origin of intermolecular association by the ion cluster is small, the film strength effect is not exhibited, and the emulsion stability of the coating liquid (emulsion composition) described later is poor. On the other hand, if the unsaturated carboxylic acid content exceeds 40 mass%, the resin film may have poor corrosion resistance and water resistance.

The ethylene-unsaturated carboxylic acid copolymer has a carboxyl group, and therefore, neutralization with an organic base or a metal ion allows emulsion liquefaction (water dispersion) of the coating liquid.

As the organic base, an amine having a boiling point of 100 ℃ or lower under atmospheric pressure is preferable from the viewpoint of not excessively lowering the corrosion resistance of the resin film. Specific examples thereof include tertiary amines such as triethylamine; secondary amines such as diethylamine; primary amines such as propylamine and the like may be used alone or in combination of two or more thereof. Of these, tertiary amines are preferred, and triethylamine is most preferred. In addition, from the viewpoint of improving solvent resistance and film hardness, it is preferable to use a metal ion having a valence of 1 together with the amine.

From the viewpoint of ensuring corrosion resistance, the amine is preferably 0.2 mol or more and 0.8 mol or less based on 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. Further, it is more preferably 0.3 mol or more, and on the other hand, it is more preferably 0.6 mol or less.

From the viewpoint of ensuring emulsion stability of the coating liquid, the amount of the metal ion having a valence of 1 is preferably 0.02 mol or more, and more preferably 0.03 mol or more, based on 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. On the other hand, from the viewpoint of ensuring corrosion resistance, the amount is preferably 0.4 mol or less, and more preferably 0.3 mol or less, based on 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. The metal compound for providing a 1-valent metal ion is preferably NaOH, KOH, LiOH, or the like, and NaOH is preferred because of its best performance.

The ethylene-unsaturated carboxylic acid copolymer may be emulsified (emulsified) by, for example, high-speed stirring for 1 to 6 hours in a vessel capable of performing a reaction at a high temperature (about 150 ℃) and a high pressure (about 5 atm) in the presence of a carboxylic acid polymer described later, if necessary. In the emulsification, a compound having a surfactant function such as tall oil fatty acid may be added in an appropriate amount. In addition, a small amount of a hydrophilic organic solvent, for example, a lower alcohol having about 1 to 5 carbon atoms, may be added to the water.

The lower limit of the mass average molecular weight (Mw) of the ethylene-unsaturated carboxylic acid copolymer is preferably 1,000, more preferably 3,000, and still more preferably 5,000 in terms of polystyrene. The upper limit of the Mw of the ethylene-unsaturated carboxylic acid copolymer is preferably 10 ten thousand, more preferably 7 ten thousand, and still more preferably 3 ten thousand. The Mw can be determined by Gel Permeation Chromatography (GPC) using polystyrene as a standard.

As the resin component, a carboxylic acid polymer may also be used. As the carboxylic acid polymer, any polymer having, as a constituent unit, an unsaturated carboxylic acid exemplified as an example that can be used for synthesizing the above-mentioned ethylene-unsaturated carboxylic acid copolymer can be used. Among these, acrylic acid and maleic acid are preferable, and maleic acid is more preferable. The carboxylic acid polymer may contain a constituent unit derived from a monomer other than the unsaturated carboxylic acid, but the amount of the constituent unit derived from another monomer is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably a carboxylic acid polymer composed only of the unsaturated carboxylic acid. Preferred examples of the carboxylic acid polymer include polyacrylic acid, polymethacrylic acid, acrylic acid-maleic acid copolymer, and polymaleic acid. Among these, polymaleic acid is more preferable from the viewpoint of adhesion of the resin film and corrosion resistance. The exact mechanism for improving corrosion resistance and the like by using polymaleic acid is not known, but it is considered that: since the amount of carboxyl groups is large, the adhesion between the resin coating and the galvanized steel sheet is improved, and the corrosion resistance is improved accordingly. However, the present invention is not limited to this presumption.

The lower limit of the mass average molecular weight (Mw) of the carboxylic acid polymer used in the present embodiment is preferably 500, more preferably 800, still more preferably 900, and most preferably 1,000 in terms of polystyrene. The upper limit of Mw of the carboxylic acid polymer is preferably 3 ten thousand, more preferably 1 ten thousand, further preferably 3,000, and most preferably 2,000. The Mw can be determined by GPC using polystyrene as a standard.

The content ratio of the ethylene-unsaturated carboxylic acid copolymer to the carboxylic acid polymer was 1,000: 1-10: 1. the lower limit of the content ratio is preferably 200: 1, the upper limit is preferably 20: 1. the reason for this is that: if the content ratio of the carboxylic acid polymer is too low, the effect of combining the olefin-acid copolymer and the carboxylic acid polymer cannot be sufficiently exhibited. The reason for this is also that: on the other hand, if the content ratio of the carboxylic acid polymer is too high, the olefin-acid copolymer and the carboxylic acid polymer may be phase-separated in the coating liquid for forming the first layer, and a uniform resin film may not be formed.

[ polyurethane resin ]

The polyurethane resin is preferably a carboxyl group-containing polyurethane resin. As the carboxyl group-containing polyurethane resin, for example, a polyurethane resin described in Japanese patent laid-open publication No. 2006-43913 can be used.

The carboxyl group-containing polyurethane resin is preferably a carboxyl group-containing polyurethane resin obtained by chain-extending a urethane prepolymer with a chain extender. The urethane prepolymer is obtained by, for example, reacting a polyisocyanate component and a polyol component.

As the polyisocyanate component, at least 1 polyisocyanate selected from the group consisting of Toluene Diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and dicyclohexylmethane diisocyanate (hydrogenated MDI) is preferably used from the viewpoint of obtaining a resin film excellent in corrosion resistance and stability in reaction control. In addition to the above polyisocyanates, other polyisocyanates may be used within a range not to lower the corrosion resistance and stability of reaction control. However, from the viewpoint of ensuring the corrosion resistance of the resin film and the stability of the reaction control, the content of the polyisocyanate is preferably 70% by mass or more of the total polyisocyanate component. Examples of the polyisocyanate other than the polyisocyanate component include tetramethylene diisocyanate, hexamethylene diisocyanate, dodecanemethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, and 1 or more of these can be used.

As the polyol component, 3 kinds of polyols, 1, 4-cyclohexanedimethanol, polyether polyol, and polyol having a carboxyl group, are preferably used from the viewpoint of obtaining a resin film excellent in corrosion resistance and sliding properties. Further, as the polyol component, 3 kinds of diols, 1, 4-cyclohexanedimethanol, polyether diol, and diol having a carboxyl group, are more preferably used. It should be noted that: by using 1, 4-cyclohexanedimethanol as the polyol component, the rust preventive effect of the obtained polyurethane resin can be improved.

The polyether polyol is not particularly limited as long as it has at least 2 or more hydroxyl groups in the molecular chain and has a main skeleton composed of oxyalkylene units. Specific examples thereof include polyoxyethylene glycol, polyoxypropylene glycol, and polyoxytetramethylene glycol, and polyoxypropylene glycol or polytetramethylene ether glycol is preferably used. The number of functional groups of the polyether polyol is not particularly limited as long as it is at least 2, and it may be, for example, 3-functional or 4-functional or more. From the viewpoint of obtaining a resin film having an appropriate hardness, the average molecular weight of the polyether polyol is preferably about 400 to 4000. The average molecular weight can be determined by measuring the OH value (hydroxyl value).

From the viewpoint of further improving the rust-preventive effect of the resin film, among the polyol components, 1, 4-cyclohexanedimethanol: polyether polyol 1: 1-1: 19. the polyol having a carboxyl group is not particularly limited as long as it has at least 1 or more carboxyl groups and at least 2 or more hydroxyl groups. Specific examples thereof include dimethylolpropionic acid, dimethylolbutyric acid, dihydroxypropionic acid, and dihydroxysuccinic acid.

In the above polyol component, other polyols may be used in addition to the above 3 polyols within the range in which the corrosion resistance is not lowered. However, from the viewpoint of ensuring the corrosion resistance of the resin film, the content of the 3 kinds of polyols is preferably 70 mass% or more of the total polyol components. The polyols other than the 3 kinds of polyols are not particularly limited as long as they have a plurality of hydroxyl groups. Examples thereof include low molecular weight polyols and high molecular weight polyols. The low molecular weight polyol is a polyol having an average molecular weight of about 500 or less. Specific examples thereof include diols such as ethylene glycol, diethylene glycol, triethylene glycol, 1, 3-butanediol, 1, 4-butanediol, neopentyl glycol, and 1, 6-hexanediol; triols such as glycerin, trimethylolpropane, hexanetriol, etc. The high molecular weight polyol is a polyol having an average molecular weight of more than about 500. Specific examples thereof include condensation polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone-based polyester polyols such as poly-epsilon-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols and the like.

The chain extender is not particularly limited, and examples thereof include polyamines, low molecular weight polyols, and alkanolamines. Examples of the polyamine include aliphatic polyamines such as ethylenediamine, propylenediamine, and hexamethylenediamine; aromatic polyamines such as tolylenediamine, xylylenediamine, and diaminodiphenylmethane; alicyclic polyamines such as diaminocyclohexylmethane, piperazine, and isophoronediamine; hydrazines such as hydrazine, succinic dihydrazide, adipic dihydrazide and phthalic dihydrazide, and the like. Of these, ethylene diamine and/or hydrazine is preferably used as the chain extender component. Examples of the alkanolamine include diethanolamine and monoethanolamine.

The carboxyl group-containing polyurethane resin can be emulsified (lactonized) by a known method, and for example, the following method is available. That is, a method of neutralizing the carboxyl group of a carboxyl group-containing urethane prepolymer with a base, emulsifying and dispersing the neutralized carboxyl group in an aqueous medium, and carrying out a chain extension reaction; a method for carrying out a chain extension reaction by emulsifying and dispersing a carboxyl group-containing polyurethane resin with high shear force in the presence of an emulsifier.

The acid value of the carboxyl group-containing polyurethane resin is preferably not less than 10mgKOH/g from the viewpoint of ensuring the stability of the coating liquid, and is preferably not more than 60mgKOH/g from the viewpoint of ensuring the corrosion resistance of the resin film. The acid value was measured in accordance with JIS-K0070 (1992).

[ additives in coating liquids ]

In the present embodiment, the resin film may be formed as follows: the coating liquid is applied to the surface of the galvanized steel sheet by a known coating method, that is, a roll coater method, a bar coater method, a spray method, a curtain flow method, or the like, and dried by heating. The coating liquid contains specified amounts of silica, magnesium hydroxide and the resin. The resin solid content in the coating liquid is preferably about 15 to 25 mass%. The coating liquid may contain various additives in a range not to inhibit the effect of the present invention, to improve the coating performance. Examples of the additives include silane coupling agents, dissolution inhibitors, rust inhibitors, waxes, crosslinking agents, diluents, antiskinning agents, surfactants, emulsifiers, dispersants, leveling agents, antifoaming agents, penetrating agents, film forming aids, dyes, pigments, thickeners, and lubricants.

For example, when a silane coupling agent is used as an additive, the resin film is densified to improve corrosion resistance. In addition, the adhesion between the galvanized steel sheet and the resin coating film is also improved, thereby improving the corrosion resistance. And has the effect of improving the binding force between the resin component and the colloidal silica, and improving the toughness of the coating. Among them, the glycidoxy-based silane coupling agent has high reactivity and a large effect of improving corrosion resistance. Examples of the glycidyl group-containing silane coupling agent include gamma-glycidoxypropylmethyldiethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-glycidoxymethyldimethoxysilane, and beta- (3, 4-epoxycyclohexyl) ethyltrimethoxysilane.

In the resin film of the present embodiment, the amount of the silane coupling agent is preferably 0.1 part by mass or more, more preferably 3 parts by mass or more, and further preferably 5 parts by mass or more, per 100 parts by mass of the total of the inorganic compound (silica and magnesium hydroxide) and the resin component in the film. The reason for this is that: if the amount of the silane coupling agent is less than 0.1 part by mass, the adhesion between the galvanized steel sheet and the resin coating and/or the bonding force between the resin component and the colloidal silica may be insufficient, and the toughness and/or corrosion resistance of the coating may be insufficient. On the other hand, the amount of the silane coupling agent is preferably 10 parts by mass or less, more preferably 9 parts by mass or less, and still more preferably 7 parts by mass or less, based on 100 parts by mass of the total of the inorganic compound and the resin component in the coating film. The reason for this is that: even if the amount of the silane coupling agent exceeds 10 parts by mass, not only the effect of improving the adhesion between the metal plate and the resin film is saturated, but also the functional group in the resin is reduced, and the coatability may be lowered. The reason for this is that: the silane coupling agents are hydrolyzed and condensed with each other, and the stability of the coating liquid is lowered, which may cause gelation or precipitation of colloidal silica.

Further, for example, when metavanadate as an elution inhibitor is used as an additive, elution of metavanadate suppresses dissolution or elution of a galvanized steel sheet, thereby improving corrosion resistance. Metavanadate particularly has the effect of improving bare corrosion resistance of the alloyed hot-dip galvanized steel sheet. Examples of the metavanadate include sodium metavanadate (NaVO) ₃ ) Ammonium metavanadate (NH) ₄ VO ₃ ) Potassium metavanadate (KVO) ₃ ) And the like. These may be used in 1 kind or two or more kinds.

The amount of the metavanadate is preferably 0.5 parts by mass or more, more preferably 0.7 parts by mass or more, and further preferably 1.0 parts by mass or more, per 100 parts by mass of the total of the inorganic compound (silica and magnesium hydroxide) and the resin component in the film. The reason for this is that: if the amount of metavanadate is less than 0.5 parts by mass, the effect of improving bare corrosion resistance (bare corrosion resistance) becomes insufficient. On the other hand, the amount of the metavanadate is preferably 5.5 parts by mass or less, more preferably 5.0 parts by mass or less, and further preferably 3.0 parts by mass or less, based on 100 parts by mass of the total of the inorganic compound and the resin component in the film. The reason for this is that: when the amount of metavanadate exceeds 5.5 parts by mass, not only the bare corrosion resistance tends to be slightly lowered, but also the film adhesion tends to be remarkably lowered. The amount of the metavanadate is preferably calculated as the amount of the element V.

[ kinds of galvanized Steel sheets ]

The kind of the galvanized steel sheet used in the present embodiment is not particularly limited, and any of a galvanized steel sheet (EG), a hot-dip galvanized steel sheet (GI), and a galvannealed steel sheet (GA) (hereinafter, these may be referred to as "base sheet") may be used. The kind of the galvanized layer is not particularly limited, and as long as the galvanized layer is a plating mainly containing zinc, the plating layer may contain an alloy element such as aluminum or magnesium. The galvanized layer may be coated on one side or both sides of the base steel sheet, and the resin coating may be coated on one side or both sides of the galvanized steel sheet.

As described above, one aspect of the present invention relates to a coated galvanized steel sheet having a resin coating film containing silica and magnesium hydroxide on a surface of the galvanized steel sheet, wherein a total content of silica and magnesium hydroxide in the resin coating film is 15 to 45 mass%, a content of a resin component in the resin coating film is 55 to 85 mass%, a mass ratio of the magnesium hydroxide to the silica is 0.5 to 2.0, and a thickness of the resin coating film is 0.3 to 1.5 μm.

According to this configuration, a coated galvanized steel sheet having conductivity and excellent white rust resistance and red rust resistance can be realized.

Examples

The present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples, and can be carried out with modifications within a range that can meet the gist of the invention described above and below, and all of them are included in the technical scope of the present invention.

(preparation of magnesium hydroxide Dispersion)

An aqueous dispersion (solid resin content: about 30% by mass, average particle diameter D) of magnesium hydroxide particles (KISUMA 5Q-S, product of Kyowa chemical Co., Ltd.) was prepared by dispersing the magnesium hydroxide particles with water as a dispersant and a polymer dispersant ₅₀ ：0.69μm)。

Regarding the average particle diameter D of magnesium hydroxide in the dispersion ₅₀ The resulting solution was diluted with a 0.2 mass% aqueous solution of sodium hexametaphosphate and then measured with a laser diffraction scattering particle size distribution measuring instrument (product name: Microtrac MT3300EXII, product of Mickelbilk Co., Ltd.).

(resin)

As a resin for forming the resin film, a polyethylene resin manufactured by tokyo chemical co and a urethane resin manufactured by tokyo chemical industry are used.

A polyethylene resin manufactured by Toho chemical Co., Ltd and an aqueous dispersion thereof were prepared in the following manner.

An autoclave equipped with an emulsifying device having a stirrer, a thermometer, and a temperature controller was charged with 200.0 parts by mass of an ethylene-acrylic acid copolymer (product name: PRIMACOR 5990I, product name: 20% by mass of a constituent unit derived from acrylic acid, mass average molecular weight (Mw): 20,000, melt index: 1300, acid value: 150), an aqueous polymaleic acid solution (product name: PRIMACO-50W, Mw: about 1100 (in terms of polystyrene), 50% by mass product, manufactured by Nikko Co., Ltd.), 8.0 parts by mass of triethylamine 35.5 parts by mass (0.63 equivalent to the carboxyl group of the ethylene-acrylic acid copolymer), 6.9 parts by mass of a 48% aqueous NaOH solution (0.15 equivalent to the carboxyl group of the ethylene-acrylic acid copolymer), 3.5 parts by mass of tall oil fatty acid (product name: Hartall FA3, manufactured by Tall), and 792.6 parts by mass of ion exchange water, and sealed, stirred at high speed for 3 hours at 150 ℃ and 5 atmospheres, and then cooled to 30 ℃.

Subsequently, 10.4 parts by mass of gamma-glycidoxypropyltrimethoxysilane (trade name: TSL8350, manufactured by Momont & ltd & gt., Ltd.), 31.2 parts by mass of polycarbodiimide (trade name: CARBODILITE SV-02, Mw: 2, 700, solid content: 40% by mass, manufactured by Nisshinbo Co., Ltd.) and 72.8 parts by mass of ion-exchanged water were added thereto, and the mixture was stirred for 10 minutes to emulsify the ethylene-acrylic acid copolymer, thereby obtaining an aqueous polyethylene resin dispersion (resin solid content: 20.3% by mass, measured in accordance with JIS K6833) mixed with the respective components.

Urethane resin produced by Toho chemical Co., Ltd and an aqueous dispersion thereof were prepared by the following methods.

60g of polytetramethylene ether glycol (average molecular weight: 1,000, manufactured by Fugu chemical Co., Ltd.) and 14g of 1, 4-cyclohexanedimethanol and 20g of dimethylolpropionic acid were charged into a synthesis apparatus having an internal volume of 0.8L and equipped with a stirrer, a thermometer and a temperature controller, and 30.0g of N-methylpyrrolidone was further charged. Then, 104g of toluene diisocyanate was added, and the temperature was raised from 80 ℃ to 85 ℃ to react for 5 hours. The NCO content of the resulting prepolymer was 8.9%. Further, 16g of triethylamine was added to neutralize the reaction mixture, and then a mixed aqueous solution of 16g of ethylenediamine and 480g of water was added thereto, followed by emulsification at 50 ℃ for 4 hours to conduct a chain extension reaction, thereby obtaining an aqueous urethane resin dispersion (29.1% of nonvolatile resin component and acid value of 41.4).

(preparation of coating liquid)

The aqueous magnesium hydroxide dispersion, the aqueous polyethylene resin dispersion or the aqueous urethane resin dispersion, and colloidal silica (trade name: SNOWTEX-XS, manufactured by Nissan chemical industries, Ltd.) were mixed to prepare a coating liquid containing about 5 mass% of a resin solid content.

(kind of original plate)

Galvanized steel sheet (EG): plate thickness 0.8mm, zinc basis weight: surface 18g/m ² 18g/m of back surface ²

(pretreatment of galvanized Steel sheet)

Degreasing: alkali degreasing (tradename: FINE CLEANER, manufactured by JAPONICA Rice-flour noodles Co., Ltd.)

And (3) drying: hot air drying to evaporate water.

(coating method)

The method comprises the following steps: bar coating

Thickness of resin film: the bar number was selected so as to obtain a specified film thickness.

(drying method)

Time: 1 minute

Conditions are as follows: the maximum temperature reached of the coated plate was 80 deg.C (confirmed by thermal-label)

(evaluation items)

Within the above range, various coated galvanized steel sheets (test nos. 1 to 19) were produced by changing the conditions as shown in table 1 below, and the corrosion resistance of the obtained coated galvanized steel sheets was evaluated by the following method. For the evaluation of corrosion resistance, the flat plate portion (portion other than the end surface portion) of the galvanized steel sheet was coated.

(Corrosion resistance)

The obtained coated galvanized steel sheet (sample) was subjected to a salt spray test for 216 hours in accordance with JIS Z2371 (2015), and the white rust generation rate (100 × area where white rust is generated/total area of the resin-coated metal sheet) on the surface of the sample was calculated. Then, based on the following criteria, o was evaluated as pass, and x was evaluated as fail.

Evaluation criteria

O: the white rust generation rate is 5 area% or less

X: the white rust generation rate exceeds 5% by area

The obtained coated galvanized steel sheet (sample) was subjected to a salt spray test for 480 hours in accordance with JIS Z2371 (2015), and the presence or absence of red rust on the sample surface was examined by visual observation. Then, based on the following criteria, o was evaluated as pass, and x was evaluated as fail.

Evaluation criteria

O: no red rust generation

X: red rust generation

(conductivity)

The obtained coated galvanized steel sheet (sample) was measured for the resistance value by using a simulation tester (model number: CX-270N, made by CUSTOM, K.K.) and sliding the terminal on the surface of the sample. Then, based on the following criteria, o was evaluated as pass, and x was evaluated as fail.

Evaluation criteria

O: the resistance value of the pointer of the analog tester is below 1000 omega

X: the resistance value shown by the pointer of the analog tester exceeds 1000 omega

The results were compared with the conditions for producing each coated galvanized steel sheet (type of resin, composition ratio of resin coating, mass ratio of magnesium hydroxide to silica [ Mg (OH)) ₂ /SiO ₂ ]The thickness of the resin film) are shown in table 1 below.

From the results, it is found that: in the case where the content of the resin is less than 55% by mass (Nos. 11 and 12), the white rust resistance is deteriorated because the content of the resin is too small. In the case where the resin content is 90 mass% (nos. 13 and 14), the resin content is too large, and therefore the degree of denseness of the resin film is reduced, which results in deterioration of both white rust resistance and red rust resistance. Mass ratio [ Mg (OH) ₂ /SiO ₂ ]Examples outside the range of 0.5 to 2.0 (test Nos. 14, 17, 18, and 19) are inferior in at least red rust resistance. In the case where the thickness of the resin film was 0.2 μm (test No.15), the white rust resistance was deteriorated because the resin film was too thin. In the case where the thickness of the resin film was 1.7 μm (test No.16), the conductivity was deteriorated because the resin film was too thick.

On the other hand, the total content and mass ratio of silica and magnesium hydroxide [ Mg (OH) ] ₂ /SiO ₂ ]And the invention in which the thickness of the resin film is adjusted to an appropriate levelThe coated galvanized steel sheets (test Nos. 1 to 10) showed excellent corrosion resistance and conductivity.

The application is based on Japanese patent application special application 2018-064757 applied on 29/3/2018 and Japanese patent application special application 2019-030546 applied on 22/2/2019, and the contents of the application are included in the application.

In order to describe the present invention, the present invention has been appropriately and sufficiently described in the above embodiments with reference to specific examples, but it should be understood that a person skilled in the art can easily change and/or improve the above embodiments.

Therefore, the modified embodiments or modified embodiments that can be implemented by those skilled in the art are intended to be included in the scope of the claims as long as they do not depart from the scope of the claims set forth in the claims.

Industrial applicability

The present invention has wide industrial applicability in the technical field of galvanized steel sheets.

Claims

1. A coated galvanized steel sheet characterized in that the surface of the galvanized steel sheet is provided with a resin coating film containing silica and magnesium hydroxide,

the total content of silica and magnesium hydroxide in the resin film is 15-45 mass%, and the content of the resin component in the resin film is 55-85 mass%,

the mass ratio of the magnesium hydroxide to the silica is 0.5 to 2.0, the thickness of the resin coating is 0.3 to 1.2 μm,

the average particle diameter D of the magnesium hydroxide ₅₀ Is 0.1 to 0.7 μm in thickness.