JP2014198800A - Coating material composition for precoating, coating film, method for forming coating film, and precoated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coating material composition for precoating exhibiting a low water contact angle since an early stage following coating film formation and capable of providing a coating film excellent in terms of rain drop fouling resistance, weather resistance, and corrosion resistance as well as coating film appearance; a coating film using the same; a method for forming the coating film; and a precoated steel sheet.SOLUTION: The provided coating material composition for precoating includes an organic resin (A), a polyoxyalkylene polymer (B), and an alkyl silicate (C). A coating film formed by using the coating material composition is also provided. A method for forming a coating film wherein the coating material composition is coated onto a steel sheet and then heated and baked until the temperature of the steel sheet comes to be confined to a range of 120 to 300°C is also provided. A precoated steel sheet possessing the coating film is also provided.

Description

  The present invention relates to a coating composition for precoat, a coating film formed using the same, a method for forming the coating film, and a precoated steel sheet.

Conventionally, pre-coated steel plates (coated steel plates) in which a coating film is formed on a steel plate in advance have been widely used for applications such as building materials and automobile parts.
Various performances are required for the precoat paint used for the precoat steel plate. For example, when a coating film formed from a precoat paint is used outdoors, the coating film is required to have excellent raindrop contamination, weather resistance, corrosion resistance, and the like. In order to meet these requirements, various paints have been studied.
For example, Patent Document 1 discloses a coating composition comprising a specific organosilicate and / or a condensate thereof for the purpose of obtaining an organic coating composition having good stain resistance.
Patent Document 2 discloses an emulsifier, an alkyl silicate, and / or an object of gelation by mixing an organosilicate (alkyl silicate) in an aqueous coating composition and a reduction in surface gloss of the resulting coating film. The aqueous | water-based antifouling agent composition containing the partial hydrolysis-condensation product and alcohol and / or a water binder as needed is indicated.

WO1994 / 06870 Japanese Patent Laid-Open No. 10-17850

In the coating compositions described in Patent Documents 1 and 2, alkyl silicate is blended as a hydrophilizing agent. The alkyl silicate on the surface of the coating film is hydrolyzed to form a silanol group (SiOH group), thereby exhibiting hydrophilicity and exhibiting functions such as raindrop resistance and weather resistance.
However, the silanol group (SiOH group) is not sufficiently formed immediately after the coating film is formed, and the hydrophilicity is not sufficiently developed. is there.
The present invention provides a coating composition for a precoat that can provide a coating film having a small water contact angle from the early stage after the coating film formation, excellent raindrop contamination, weather resistance, corrosion resistance, and excellent coating film appearance. It is an object to provide a coated film, a method for forming the coated film, and a precoated steel sheet.

As a result of intensive research, the inventors of the present invention reduce the water contact angle of a coating film earlier than a conventional low-contamination coating film by using an alkyl silicate and a polyoxyalkylene polymer (B) in combination. I found out.
The present invention has been completed based on such findings.

That is, the present invention is as follows.
[1] A coating composition for precoat containing an organic resin (A), a polyoxyalkylene polymer (B), and an alkyl silicate (C).
[2] The coating composition for precoat as described in [1] above, wherein the polyoxyalkylene polymer (B) is a polymer (B1) having a polyoxyalkylene segment and a siloxane segment.

[3] The polymer (B1) has a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), and both ends are independently represented by the following formula ( 3) or represented by the following formula (4), wherein the content of the polymer (B1) is 1% by mass or more and 30% by mass or less based on the total solid content of the coating composition for precoat. [2] The coating composition for precoat as described in [2].

(In the formula (1), R ¹ is an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or more oxyalkylene groups having 1 to 4 carbon atoms. A polyoxyalkylene segment as a structural unit, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group, or an amide group, and R ¹ , R ² , A and B may be the same or different for each structural unit.)

(In Formula (2), R ³ and R ⁴ are each independently an alkyl group having 1 to 2 carbon atoms, and R ³ and R ⁴ may be the same or different for each structural unit.)

(In formula (3), R ^{5 to} R ⁷ are each independently an alkyl group having 1 to 2 carbon atoms, and R ^{5 to} R ⁷ may be the same or different for each structural unit.)

(In the formula (4), R ¹ and R ¹⁰ are each independently an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or 2 carbon atoms having 1 to 4 carbon atoms. A polyoxyalkylene segment having a structural unit of at least a kind of oxyalkylene group, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group or an amide group, and R ¹ , R ² , R ¹⁰ , A and B may be the same or different for each structural unit.

[4] The precoat coating composition according to any one of [1] to [3], wherein the organic resin (A) contains a fluororesin.
[5] The coating composition for precoat as described in [4] above, wherein the fluororesin contains polyvinylidene fluoride.
[6] The coating composition for precoat as described in [4] or [5] above, wherein the organic resin (A) further contains an acrylic resin.
[7] The precoat coating composition according to any one of [1] to [3], wherein the organic resin (A) includes a polyester resin and at least one of a melamine resin and an isocyanate compound.
[8] The SP value (SPa) of the organic resin (A) and the SP value (SPb) of the polyoxyalkylene polymer (B) satisfy the relationship of the following formula (5). [7] The precoat paint composition as described in any one of [7].
SPb-SPa ≧ 0.6 (5)
[9] The precoat coating composition according to any one of [1] to [8], wherein the polyoxyalkylene polymer (B) has an HLB value of 10 to 17.
[10] The coating composition for precoat according to any one of [1] to [9], wherein the polyoxyalkylene polymer (B) has a siloxane segment number average molecular weight of 500 to 1,000.

[11] The coating composition for precoat according to any one of [1] to [10], wherein the polyoxyalkylene segment of the polyoxyalkylene polymer (B) has a number average molecular weight of 1000 to 2000.
[12] The coating composition for precoat as described in any one of [1] to [11] above, wherein the ratio of the polyoxyalkylene segment in the polyoxyalkylene polymer (B) is 50% by mass or more.
[13] At least one functional group selected from the group consisting of a hydroxyl group, a carboxyl group, a carboxymethyl group, an amino group, and an amide group at the terminal of at least a part of the polyoxyalkylene segment in the polyoxyalkylene polymer (B). The coating composition for precoat according to any one of [1] to [12], which has a group.
[14] The coating composition for precoat according to any one of [1] to [13], wherein the polyoxyalkylene polymer (B) has a surface tension of 20 to 25 mN / m.
[15] A coating film formed using the precoat coating composition according to any one of [1] to [14].

[16] After applying the precoat coating composition according to any one of [1] to [14] to a steel sheet, the steel sheet is baked by heating until the temperature of the steel sheet is within a range of 120 to 300 ° C. A method for forming a coating film.
[17] The method for forming a coating film according to [16], wherein the baking heating time is 3 to 90 seconds.
[18] A precoated steel sheet having the coating film according to [15].

  According to the present invention, a coating composition for a precoat which can provide a coating film having a small water contact angle from the early stage after the coating film formation, excellent raindrop stain resistance, weather resistance, corrosion resistance, and excellent coating film appearance, The used coating film, a method for forming the coating film, and a precoated steel sheet can be provided.

[Precoat coating composition]
The coating composition for precoats of the present invention contains an organic resin (A), a polyoxyalkylene polymer (B), and an alkyl silicate (C).
According to the precoat coating composition of the present invention, the reason why a coating film with a small water contact angle, excellent raindrop resistance, weather resistance, corrosion resistance and excellent coating film appearance can be obtained from the early stage after the coating film is formed. However, it is presumed that it is as follows.
According to the coating composition for a precoat of the present invention, the alkyl silicate (C) is gradually hydrolyzed to form SiOH groups, thereby expressing hydrophilicity and exhibiting rain dripping resistance and antifouling properties over a long period of time. . Further, according to the coating composition for precoat of the present invention, since the polyoxyalkylene polymer exhibits hydrophilicity from the early stage after the coating film formation, the water contact angle of the coating film becomes small, and from the beginning of the coating film formation. It is considered to exhibit rainproof and antifouling properties. As a result, according to the coating composition for precoat of the present invention, a coating film having a small water contact angle from the early stage after the formation of the coating film, excellent raindrop resistance, weather resistance, corrosion resistance, and excellent coating film appearance. It is thought that it can be obtained.

  The solid content concentration of the coating composition for precoat of the present invention is preferably 20 to 80% by mass, more preferably 50 to 70% by mass, from the viewpoint of handling properties and reduction of the amount of medium (solvent) used.

<Organic resin (A)>
Examples of the organic resin (A) used in the coating composition for precoat of the present invention include known organic resins such as fluororesin, acrylic resin, polyester resin, melamine resin, urethane resin, epoxy resin, alkyd resin, and the like. However, it is not limited to these. These may be used alone or in combination of two or more. Among these, a fluororesin, an acrylic resin, a polyester resin, and a melamine resin are preferable from the viewpoint of cost and high degree of freedom in resin design. Polyvinylidene fluoride, polyvinylidene fluoride, and acrylic resin from the viewpoint of obtaining a coating film having a small water contact angle from the early stage after coating film formation, excellent raindrop contamination, weather resistance, corrosion resistance, and excellent coating film appearance. More preferably, a combination of polyester resin and melamine resin, a combination of polyester resin and isocyanate compound, and a combination of polyester resin, melamine resin and isocyanate compound are more preferable.

  The SP value (SPa) of the organic resin (A) is adjusted by adjusting the solubility with the polyoxyalkylene polymer (B), and the polyoxyalkylene polymer (B) is localized on the coating film surface. From the viewpoint of obtaining a coating film having a small water contact angle from the early stage after formation, excellent raindrop contamination, weather resistance, corrosion resistance, and excellent coating film appearance, it is preferably 11 or less, more preferably 10.5 or less, and further Preferably it is 10 or less, and preferably 8 or more.

Here, the SP value (solubility parameter) is a measure of solubility. The SP value indicates that the larger the value, the higher the polarity, and the smaller the value, the lower the polarity. In addition, SP value in case organic resin (A) is a 2 or more types of mixture makes SP value the weighted average value of the solubility parameter of each component.
The SP value can be measured by the following method [References: SUH, CLARKE, J. et al. P. S. A-1, 5, 1671-1681 (1967)].
Measurement temperature: 20 ° C
Sample: Weigh 0.5 g of resin into a 100 ml beaker and add 10 ml of good solvent.
Add using a whole pipette, with a magnetic stirrer
Dissolve.
Solvent: Good solvent ... dioxane, acetone, etc.
Poor solvent: n-hexane, ion-exchanged water, etc. Muddy point measurement: The poor solvent was dropped using a 50 ml burette, and the point where turbidity occurred
Let it be a dripping amount.
The SP value δ is given by the following equation.

δ = (V _m1 ^1/2 δ _m1 + V _mh ^1/2 δ _mh ) / (V _ml ^1/2 + V _mh ^1/2 )
V _m = V ₁ V ₂ / (φ ₁ V ₂ + φ ₂ V ₁ )
δ _m = φ ₁ δ ₁ + φ ₂ δ ₂

V _i : Molecular volume of the solvent (ml / mol)
φ _i : Volume fraction of each solvent at cloud point δ _i : SP value of solvent ml: Low SP poor solvent mixed system mh: High SP poor solvent mixed system

(Fluorine resin)
The fluororesin may be a thermoplastic fluororesin or a thermosetting fluororesin, such as tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, trifluoroethylene, vinyl fluoride, vinylidene fluoride, etc. All fluorine-containing polymers obtained by homopolymerization of fluorine-containing monomers or copolymerization with other monomers are included. Among these, polyvinyl fluoride and polyvinylidene fluoride are used from the viewpoint of obtaining a coating film having a small water contact angle from the early stage after formation of the coating film, excellent raindrop contamination, weather resistance, corrosion resistance, and excellent coating film appearance. Preferably, polyvinylidene fluoride is more preferable.
Specifically, as a thermoplastic fluororesin, for example, Kyner 500 (polyvinylidene fluoride manufactured by Elf Atchem North America) is used, and as a thermosetting fluororesin, for example, Lumiflon 200 (manufactured by Asahi Glass Co., Ltd.). There is.

  When a fluororesin is used as the organic resin (A) used in the precoat paint composition of the present invention, the content of the fluororesin in the solid content of the precoat paint composition is from the viewpoint of a balance between hydrophilicity and weather resistance. , Preferably 30-50% by mass, more preferably 40-45% by mass.

≪Polyvinylidene fluoride≫
As the polyvinylidene fluoride, those having a melt viscosity of 2000 to 8000 Pa · s at a shear rate of 100 sec ⁻¹ and a temperature of 232 ° C. are preferable. This melt viscosity is more preferably 5000 to 7000 Pa · s, more preferably 5500 to 6200 Pa · s, from the viewpoint of obtaining a low-gloss coating film by reducing the fluidity of polyvinylidene fluoride when the paint is formed and baked. s.
The melt viscosity can be obtained by measuring the viscosity when molten polyvinylidene fluoride maintained at 232 ° C. passes through an orifice having a diameter of 1 mm at a shear rate of 100 sec ⁻¹ using a capillary rheometer.

A typical example of polyvinylidene fluoride used for paints is “KYNAR500 (Kyna-500)” manufactured by Elf Atchem North America, Inc., USA. The melt viscosity of KYNAR500 is about 2900 to 3300 Pa · s.
In addition, examples of the polyvinylidene fluoride having a high melt viscosity include Ausimont USA, Inc. “HYLAR5000 (Hyler 5000)” manufactured by (Augustmont, USA) can be used.

The number average molecular weight of the polyvinylidene fluoride is preferably 1 from the viewpoint of obtaining a coating film having a small water contact angle from the early stage after forming the coating film, excellent raindrop resistance, weather resistance, corrosion resistance, and excellent coating film appearance. It is 1,000-200,000, More preferably, it is 5,000-100,000, More preferably, it is 10,000-50,000, More preferably, it is 12,000-20,000.
In addition, in this invention, a number average molecular weight is the value which converted the measured value by the gel permeation chromatography (GPC) method by the polystyrene standard.

  For outdoor use, a low-gloss coating is generally preferred, and an inorganic matting agent such as silica powder may be blended to achieve low gloss. However, durability such as weather resistance of the coating film tends to decrease as the blending amount of the inorganic matting agent increases. Therefore, by using the above-mentioned polyvinylidene fluoride having a high melt viscosity, it is possible to achieve a low gloss without adding an inorganic matting agent. Thereby, the blending of the inorganic matting agent is unnecessary, or even if blended, it is sufficient to add a small amount for fine adjustment of the gloss, so that the durability can be improved.

(acrylic resin)
There is no restriction | limiting in particular in the acrylic resin which is organic resin (A) in the coating composition for precoats of this invention.
Examples of the monomer component constituting the acrylic resin include aromatic vinyl monomers such as styrene, vinyltoluene, 2-methylstyrene, t-butylstyrene, and chlorostyrene; methyl acrylate, ethyl acrylate, and n-propyl acrylate. , Isopropyl acrylate, n-, i- or t-butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, decyl acrylate, lauryl acrylate, cyclohexyl acrylate, methyl methacrylate, methacryl Ethyl acetate, n-propyl methacrylate, isopropyl methacrylate, n-, i- or t-butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, decyl methacrylate, lauryl methacrylate C1-C18 alkyl ester or cycloalkyl ester of acrylic acid or methacrylic acid such as cyclohexyl methacrylate; 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, hydroxybutyl acrylate, 2-hydroxyethyl C2-C8 hydroxyalkyl esters of acrylic acid or methacrylic acid such as methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, hydroxybutyl methacrylate; N-methylolacrylamide, N-butoxymethylacrylamide, N-methoxymethylacrylamide, N -N-substituted acrylamides such as methylol methacrylamide and N-butoxymethyl methacrylamide N- substituted methacrylamide monomers, acrylic acid, methacrylic acid, glycidyl methacrylate, can include one or a mixture of two or more of such glycidyl acrylate.

  An acrylic resin can be produced by polymerizing the monomer components by a conventional method such as solution polymerization or bulk polymerization.

  The number average molecular weight of the acrylic resin is not particularly limited, but the water contact angle is small from the early stage after the formation of the coating film, it is excellent in raindrop resistance, weather resistance, corrosion resistance, and coating film appearance. Is preferably 1,000 to 200,000, more preferably 5,000 to 100,000, still more preferably 10,000 to 50,000, and still more preferably 12,000 to 20,000. ,

(Combination of polyvinylidene fluoride and acrylic resin)
The organic resin (A) in the composition of the present invention is preferably a combination of polyvinylidene fluoride and an acrylic resin.
In that case, the acrylic resin acts as a binder between the polyvinylidene fluoride particles when a paint is applied to form a coating film, and has good heat resistance against the baking conditions of the coating film. It is easy to blend in with the melted polyvinylidene fluoride and shows a good coating film appearance.
In the coating composition of the present invention, the content of polyvinylidene fluoride based on the total of 100% by mass of the polyvinylidene fluoride and acrylic resin is preferably 50 to 90% by mass, more preferably 60 to 80% by mass, from the same viewpoint. is there. From the same viewpoint, the content of the acrylic resin with respect to 100% by mass of the total of the polyvinylidene fluoride and the acrylic resin is preferably 10 to 50% by mass, more preferably 20 to 40% by mass.

(Polyester resin)
The polyester resin which is the organic resin (A) in the precoat coating composition of the present invention is preferably a resin obtained by an ester reaction between a polybasic acid and a polyhydric alcohol.
Polybasic acids include dibasic acids such as terephthalic acid, isophthalic acid, succinic acid, adipic acid, sebacic acid, phthalic acid, maleic anhydride, tetrahydromaleic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, methylhydroexene A tribasic or higher polybasic acid such as tricarboxylic acid or pyromellitic anhydride is used, and these may be used in combination.

  Polyhydric alcohols include aliphatic or alicyclic divalents such as ethylene glycol, propylene glycol, diethylene glycol, butanediol, neopentyl glycol, 3-methylpentadiol, 1,4-hexanediol, 1,6-hexanediol, etc. Alcohol is mainly used, and a trihydric or higher polyhydric alcohol such as glycerin, trimethylolethane, trimethylolpropane, pentaerythrole may be used in combination as necessary. Esterification of polybasic acid and polyhydric alcohol can be performed by a known method.

  The number average molecular weight of the polyester resin is preferably in the range of 1,000 to 30,000. When the number average molecular weight is 1,000 or more, the processability is good, and when it is 30,000 or less, the compatibility with the silicate compound is good. From this viewpoint, the number average molecular weight of the polyester resin is more preferably 2,000 to 20,000, still more preferably 2,000 to 10,000, and still more preferably 2,000 to 5,000.

  When a polyester resin is used as the organic resin (A) used in the precoat paint composition of the present invention, the polyester resin content in the solid content of the precoat paint composition is from the viewpoint of a balance between hydrophilicity and weather resistance. , Preferably it is 10-70 mass%, More preferably, it is 20-65 mass%.

(Combination of polyester resin and curing agent)
As an organic resin (A) in the coating composition for precoats of this invention, you may use together at least 1 sort (s) of a melamine resin and an isocyanate compound as a hardening | curing agent with a polyester resin.

≪Melamine resin≫
The melamine resin is not particularly limited, and a methylated melamine resin, a butylated melamine resin, or a methyl / butyl mixed melamine resin can be used. Examples thereof include “Cymel-303” and “Cymel 254” manufactured by Nippon Cytec Co., Ltd., “Uban 20N60” and “Uban 128” manufactured by Mitsui Chemicals, and “Sumimar Series” manufactured by Sumitomo Chemical.

  The amount of the melamine resin used is preferably 10 to 50 parts by mass with respect to 100 parts by mass of the solid content of the polyester resin. If the amount used is 10 parts by mass or more, the curability is sufficient, and if it is 50 parts by mass or less, the cured film is prevented from becoming too hard, and the chipping property is improved when a coating film is formed. From the said viewpoint, the usage-amount of the said melamine resin is 15-40 mass parts more preferably with respect to 100 mass parts of solid content of a polyester resin, More preferably, it is 20-30 mass parts.

  The content of the melamine resin in the solid content of the coating composition for precoat is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and further preferably 3 to 3% from the viewpoint of the balance between hydrophilicity and weather resistance. It is 12 mass%, More preferably, it is 5-10 mass%.

  The number average molecular weight of the melamine resin is preferably 500 from the viewpoint of obtaining a coating film having a small water contact angle from the early stage after the coating film formation, excellent raindrop resistance, weather resistance, corrosion resistance, and excellent coating film appearance. ~ 200,000, more preferably 800 to 10,000, still more preferably 1,000 to 5,000, still more preferably 1,100 to 2,000,

≪Isocyanate compound≫
The isocyanate compound is not particularly limited as long as it is a compound having at least two isocyanate groups in one molecule. For example, aliphatic diisocyanates such as hexamethylene diisocyanate (HMDI) and trimethylhexamethylene diisocyanate (TMDI); Cycloaliphatic diisocyanates such as isophorone diisocyanate (IPDI); aromatic-aliphatic diisocyanates such as xylylene diisocyanate (XDI); aromatic diisocyanates such as tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI) Hydrogenated diisocyanates such as dimer acid diisocyanate (DDI), hydrogenated TDI (HTDI), hydrogenated XDI (H6XDI), hydrogenated MDI (H12MDI) Dimers, trimers, tetramers or higher polyisocyanates; adducts of these with polyhydric alcohols such as trimethylolpropane, water, or low molecular weight polyester resins. be able to.

  The isocyanate compound is usually used as a blocked isocyanate compound in which a reactive group is blocked with an appropriate blocking agent in order to enhance the stability of the resulting coating composition. The blocking agent is not particularly limited, and examples thereof include oxime blocking agents such as methyl ethyl ketoxime, acetoxime, cyclohexanone oxime, acetophenone oxime, and benzophenone oxime; phenol blocking agents such as m-cresol and xylenol; methanol, ethanol Alcohol-based blocking agents such as butanol, 2-ethylhexanol, cyclohexanol, ethylene glycol monoethyl ether; lactam-based blocking agents such as ε-caprolactam; diketone-based blocking agents such as diethyl malonate and acetoacetate; Examples include mercaptan blocking agents such as thiophenol; urea blocking agents such as thiourea; imidazole blocking agents; carbamic acid blocking agents. Of these, lactam blocking agents, oxime blocking agents, and diketone blocking agents are preferred.

The blocked isocyanate compound is obtained by reacting the isocyanate compound and the blocking agent by a conventional method until free isocyanate groups disappear. Some of these are commercially available. For example, Death Module series (manufactured by Sumitomo Bayer Urethane Co., Ltd.), Barnock D series (manufactured by Dainippon Ink & Chemicals, Inc.), Takenate B series (manufactured by Takeda Pharmaceutical Co., Ltd.), Coronate 2500 series (Nippon Polyurethane Industry Co., Ltd.) etc. can be used.
When using the said block isocyanate compound as said hardening | curing agent, if the compounding quantity of the said block isocyanate compound is the quantity which can provide an isocyanate group more than an equivalent with respect to the hydroxyl value of the said polyol resin (a1). It is usually 0.8 to 1.5 times the equivalent. If the amount is too small, the curability is lowered and only a soft coating film can be obtained. Not only the hardness but also the acid resistance, alkali resistance, and stain resistance are also lowered. In addition to the addition of a large amount of isocyanate compound or blocked isocyanate compound, physical properties such as the strength, hardness, and workability of the coating film designed based on the physical properties of the polyol resin (a1) are reduced. In addition, acid resistance and alkali resistance are also lowered. In addition, the yellowing and weather resistance of the coating film are likely to decrease. More preferably, it is 1.0 to 1.2 times the equivalent.
When the isocyanate compound or the blocked isocyanate compound is used as the curing agent (a2), a known tin compound such as dibutyltin laurate is usually used as the catalyst (D) for promoting the reaction with the polyol resin (a1). It is preferable to use it.

  The amount of the isocyanate compound used is preferably 10 to 50 parts by mass with respect to 100 parts by mass of the solid content of the polyester resin. If the amount used is 10 parts by mass or more, the curability is sufficient, and if it is 50 parts by mass or less, the cured film is prevented from becoming too hard, and the chipping property is improved when a coating film is formed. From the said viewpoint, the usage-amount of the said isocyanate compound is more preferably 15-40 mass parts with respect to 100 mass parts of solid content of a polyester resin, More preferably, it is 20-30 mass parts.

  The content of the isocyanate compound in the solid content of the coating composition for precoat is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and further preferably 3 to 3% from the viewpoint of the balance between hydrophilicity and weather resistance. It is 12 mass%, More preferably, it is 2-10 mass%.

<Polyoxyalkylene polymer (B)>
The polyoxyalkylene polymer (B) used in the precoat coating composition of the present invention is a compound having a polyoxyalkylene chain structure (hereinafter also referred to as “polyoxyalkylene segment”).
The polyoxyalkylene chain structure is a linear glycol such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol. The derived unit is a repeating unit. Of these, only a unit derived from one type of linear glycol may be a repeating unit, and a unit derived from a plurality of types of linear glycols may be a repeating unit.

  Examples of the polyoxyalkylene polymer used in the present invention include polyethylene glycol, a block copolymer or a random copolymer of propylene oxide and ethylene oxide, a block copolymer or a random copolymer of tetrahydrofuran and ethylene oxide, and the like.

  It has at least one functional group selected from the group consisting of a hydroxyl group, a carboxyl group, a carboxymethyl group, an amino group and an amide group at the terminal of at least a part of the polyoxyalkylene segment in the polyoxyalkylene polymer (B). Is preferred. By reacting this terminal functional group with the organic resin (A), it becomes possible for the polyoxyalkylene polymer (B) to be stably present in the coating film. In addition, hydrophilicity is imparted.

The surface tension of the polyoxyalkylene polymer (B) is preferably 20 to 25 mN / m. As a result, since the polyoxyalkylene polymer (B) can be localized on the surface of the coating without impairing the stability of the coating, the water contact angle is small from the early stage after the coating is formed, and it is resistant to raindrops. A coating film having excellent weather resistance and corrosion resistance and excellent coating film appearance can be obtained. From this viewpoint, it is more preferably 20 to 24 mN / m, still more preferably 20 to 23 mN / m.
This surface tension is measured using a DYNOMETER manufactured by BYK by the platinum ring pulling method.

  The content of the polyoxyalkylene polymer (B) is preferably 1% by mass or more and 30% by mass or less with respect to the total solid content of the coating composition for precoat. Within this range, since the polyoxyalkylene polymer (B) can be localized on the surface of the coating without impairing the stability of the coating, the water contact angle is small from the early stage after the coating is formed, It is possible to obtain a coating film having excellent raindrop contamination, weather resistance, and corrosion resistance and excellent coating film appearance. From this viewpoint, the proportion is more preferably 3% by mass or more, still more preferably 5% by mass or more, and preferably 20% by mass or less, more preferably 15% by mass or less.

The HLB (hydrophilic-lipophilic balance) value of the polyoxyalkylene polymer (B) imparts hydrophilicity to the coating film, and the water contact angle is small from the early stage after the coating film is formed. From the viewpoint of obtaining a coating film having excellent properties and corrosion resistance and excellent coating film appearance, it is preferably 10-17, more preferably 10-15, still more preferably 10.5-13, and even more preferably 11-12. .
The HLB value is a value calculated based on the Griffin equation by “HLB = 20 × (sum of formula weight of hydrophilic portion) / (molecular weight)” [(Reference) “Surfactant” by Takeuchi Takeshi, Yoneda Publishing (1999)].

  The number average molecular weight of the polyoxyalkylene segment of the polyoxyalkylene polymer (B) is preferably 1000 to 2000. As a result, since the polyoxyalkylene polymer (B) can be localized on the surface of the coating without impairing the stability of the coating, the water contact angle is small from the early stage after the coating is formed, and it is resistant to raindrops. A coating film having excellent weather resistance and corrosion resistance and excellent coating film appearance can be obtained. Here, the number average molecular weight of the polyoxyalkylene segment means the number average molecular weight of the polyoxyalkylene that is a component derived from the polyoxyalkylene segment. The number average molecular weight is measured by a gel permeation chromatography (GPC) method using tetrahydrofuran as a solvent, using polystyrene as a standard substance.

  The proportion of the polyoxyalkylene segment (not including the terminal hydrogen or alkyl group) in the total mass of the polyoxyalkylene polymer (B) is preferably 50% by mass or more. Since it can be localized on the surface of the coating film from which the polymer (B1) can be obtained without impairing the stability of the coating if it is 50% by mass or more, the water contact angle is small from the early stage after the coating is formed, It is possible to obtain a coating film having excellent raindrop contamination, weather resistance, and corrosion resistance and excellent coating film appearance. From this viewpoint, the proportion is more preferably 60% by mass or more, still more preferably 70% by mass or more, still more preferably 75% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass. Hereinafter, it is more preferably 85% by mass or less.

The SP value (SPa) of the organic resin (A) and the SP value (SPb) of the polyoxyalkylene polymer (B) preferably satisfy the relationship of the following formula (5). As a result, the polyoxyalkylene polymer (B) is likely to be localized in the vicinity of the coating film surface, so that the water contact angle is small from the early stage after the coating film is formed, and it is excellent in raindrop contamination, weather resistance and corrosion resistance. A coating film having excellent film appearance can be obtained.
SPb-SPa ≧ 0.6 (5)
From this viewpoint, [SPb-SPa] is more preferably 0.65 or more, still more preferably 0.70 or more, and preferably 2.4 or less, more preferably 2.3 or less, and still more preferably 2 .2 or less.
From the same viewpoint, the SP value (SPa) of the polyoxyalkylene polymer (B) is preferably 11.2 or more, more preferably 11.4 or more, and further preferably 11.5 or more. From the viewpoint of stability, it is preferably 12.3 or less, more preferably 12.1 or less, and still more preferably 12.0 or less.

  Further, the polyoxyalkylene polymer used in the present invention is preferably a compound having a polyoxyalkylene chain as part of its structure, and the polymer (B1) having a polyoxyalkylene segment and a siloxane segment is More preferred. Next, the polymer (B1) will be described.

(Polymer (B1))
In the present invention, the polymer (B1) means a polymer having a polyoxyalkylene segment and a siloxane segment.
This polymer (B1) may be a branched polymer (B1-A) having a polyoxyalkylene segment in at least a portion other than the terminal of the siloxane segment, and the polyoxyalkylene segment is only at the terminal of the siloxane segment. Although it may be a linear polymer (B1-B), a branched polymer (B1-A) is preferred because of excellent compatibility with the organic resin (A). As a result, since the polyoxyalkylene polymer (B) can be localized on the surface of the coating without impairing the stability of the coating, the water contact angle is small from the early stage after the coating is formed, and it is resistant to raindrops. A coating film having excellent weather resistance and corrosion resistance and excellent coating film appearance can be obtained.

  The number average molecular weight of the siloxane segment of the polymer (B1) has a small water contact angle from the early stage after the formation of the coating film, excellent raindrop resistance, weather resistance, corrosion resistance, and a viewpoint of obtaining a coating film having excellent coating film appearance. From 500 to 1000.

  This polymer (B1) is represented by the structural unit represented by the following formula (1) and the following formula (2) from the viewpoint of reducing the water contact angle of the resulting coating film and improving the raindrop resistance. It is preferable that both ends are each independently a branched polymer (B1-A) represented by the following formula (3) or the following formula (4).

(In the formula (1), R ¹ is an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or more oxyalkylene groups having 1 to 4 carbon atoms. A polyoxyalkylene segment as a structural unit, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group, or an amide group, and R ¹ , R ² , A and B may be the same or different for each structural unit.)
R ¹ is more preferably a methyl group from the above viewpoint.
From the above viewpoint, R ² is more preferably a straight-chain alkyl composed of 3 carbons.
From the above viewpoint, A is a structure in which ethylene oxide and propylene oxide are randomly bonded.
B is more preferably an alkyl group having 1 to 4 carbon atoms substituted with at least one functional group selected from the group consisting of a hydroxyl group, a carboxyl group, a carboxymethyl group, an amino group, and an amide group. By reacting this terminal functional group with the organic resin (A), the polyoxyalkylene polymer (B) can be stably present in the coating film. As well as being expressed, the hydrophilicity becomes better.

(In Formula (2), R ³ and R ⁴ are each independently an alkyl group having 1 to 2 carbon atoms, and R ³ and R ⁴ may be the same or different for each structural unit.)
R ³ is more preferably a methyl group from the above viewpoint.
R ⁴ is more preferably a methyl group from the above viewpoint.

(In formula (3), R ^{5 to} R ⁷ are each independently an alkyl group having 1 to 2 carbon atoms, and R ^{5 to} R ⁷ may be the same or different for each structural unit.)
R ⁵ , R ⁶ and R ⁷ are more preferably all methyl groups from the above viewpoint.

(In the formula (4), R ¹ and R ¹⁰ are each independently an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or 2 carbon atoms having 1 to 4 carbon atoms. A polyoxyalkylene segment having a structural unit of at least a kind of oxyalkylene group, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group or an amide group, and R ¹ , R ² , R ¹⁰ , A and B may be the same or different for each structural unit.
R ¹ and R ¹⁰ are more preferably a methyl group from the above viewpoint.
From the above viewpoint, R ² is more preferably a straight-chain alkyl composed of 3 carbons.
From the above viewpoint, A is a structure in which ethylene oxide and propylene oxide are randomly bonded.
From the above viewpoint, B is more preferably a hydroxyl group.

  A commercially available product may be used as the polymer (B1). Examples of the branched polymer (B1-A) include polyether-modified silicone (“8616ADDITIVE” manufactured by Toray Dow Co., Ltd.). Examples of the linear polymer (B1-B) include polyether-modified silicone (“X22-4952” manufactured by Shin-Etsu Silicone Co., Ltd.).

<Alkyl silicate (C)>
The precoat coating composition of the present invention contains an alkyl silicate (C). When this alkyl silicate (C) produces | generates a silanol group by hydrolysis, the rain-stain resistance of a coating film improves with the hydrophilic property of the silanol group.
Alkyl silicate (C) is a silane compound having one or more alkoxy groups in one molecule or a condensate thereof, and is preferably a silane compound having two or more alkoxy groups in one molecule or a condensate thereof. . Further, an alkyl group may be contained in one molecule in addition to the alkoxy group. These alkoxy groups and alkyl groups may be partially substituted with other functional groups. The other functional group is preferably at least one of an amino group, a glycidoxy group, an epoxy group, a chloro group, and a mercapto group.

Examples of the alkyl silicate (C) include tetramethoxy silicate, tetraethoxy silicate, tetrabutoxy silicate, tetra-2-methoxyethyl silicate, methyl trimethoxy silane, methyl triethoxy silane, methyl triisopropoxy silane, methyl tributoxy silane, methyl Triphenoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, N- (2-aminoethyl) 3-aminopropylmethyldimethoxysilane, N- (2-aminoethyl) 3-aminopropyltrimethoxysilane, 3-aminopropyl Triethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 2- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, 3- Rollo propyl methyl dimethoxy silane, 3-chloropropyl trimethoxy silane, alkoxysilanes such as 3-mercaptopropyltrimethoxysilane, further include condensates thereof.
Among these, from the above viewpoint, at least one of tetramethoxysilicate, tetraethoxysilicate, and tetrabutoxysilicate is preferable.

  The content of the alkyl silicate (C) in the solid content of the coating composition for precoat is preferably 1 to 6% by mass, more preferably 3 to 6% by mass, and still more preferably, from the viewpoint of the balance between hydrophilicity and weather resistance. Is 4-5 mass%.

<Pigment>
The precoat paint composition of the present invention may be an enamel paint composition containing a pigment or a clear paint composition containing no pigment. Addition of the pigment imparts a base hiding property to the coating film.
As the pigment, those conventionally known for use in paints can be used. For example, azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanines Pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, metal complex pigments, and other organic pigments, yellow lead, yellow iron oxide, bengara, carbon black Inorganic pigments such as titanium dioxide and various fired pigments. These may be used alone or in combination of two or more.
Content of a pigment is 10-50 mass% normally in solid content of a coating composition, Preferably it is 15-50 mass%, More preferably, it is 20-45 mass%. When the pigment content is 10% by mass or more, a sufficient base hiding property can be imparted. When the pigment content is 50% by mass or less, the coating film becomes strong, and the follow-up of the coating film during pre-coated steel sheet processing. Good properties.

<Medium>
The medium in the precoat coating composition of the present invention is preferably an organic solvent. As the organic solvent, cyclohexanone, isophorone, n-butanol, isobutanol, toluene, xylene, methyl ethyl ketone, acetone, Solvesso 100, Solvesso 150, Solvesso 200, and the like are preferable. These may be used alone or in combination of two or more. Among these, at least one of cyclohexanone, isophorone and butanol is preferable.

<Other additives>
In addition to the above, the precoat coating composition of the present invention can be formulated with any of those generally used for coatings. For example, fillers other than pigments as described above, pH adjusters, leveling agents, thickeners, water repellents, antifoaming agents, plasticizers, antioxidants, UV absorbers, anti-repellent agents, anti-skinning agents Various known and commonly used materials such as (anti-burr agent), a dispersant, or resins can be appropriately added and used.

[Coating film, its production method and pre-coated steel sheet]
The coating film of this invention is formed using the above-mentioned coating composition for precoats. Since the coating film of the present invention is thus formed using the above-mentioned coating composition for precoat, the water contact angle is small from the early stage after the coating film is formed, and it has excellent raindrop resistance, weather resistance, and corrosion resistance. Excellent coating film appearance.

This coating film is preferably formed on a steel plate. As the steel sheet, for example, a galvanized steel sheet, an alloyed galvanized steel sheet, a hot dip zinc-aluminum alloy plated steel sheet, an aluminum alloy plated steel sheet, a hot dip zinc-aluminum-magnesium alloy plated steel sheet, a stainless steel sheet, a cold rolled steel sheet, or the like may be used. it can.
The steel sheet may be subjected to surface treatment such as zinc phosphate treatment, surf coat EC2200 manufactured by Nippon Paint Co., Ltd., coating type chromate, etc., and particularly when corrosion resistance is required. A primer layer such as an epoxy resin primer, a polyurethane-modified epoxy resin primer, or a polyester resin primer may be further provided on the surface treatment layer.

The film thickness of the coating film of the present invention is not particularly limited, but is preferably 2 μm or more, more preferably 5 μm or more, and further preferably 10 μm or more from the same viewpoint as described above. Further, from the viewpoint of the followability of the coating film during pre-coated steel sheet processing, the film thickness is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less.
The film thickness of the coating film is calculated from an average value obtained by measuring 10 arbitrarily selected locations using an electromagnetic film thickness meter (Fischer).

The coating film of the present invention can be produced by applying the precoating coating composition described above to a steel sheet, followed by heating and baking.
The coating method is not particularly limited, and a conventionally known method such as a roll coater, airless spray, electrostatic spray, curtain flow coater, etc. can be employed.

The baking temperature (that is, the temperature of the steel sheet) in the baking treatment is preferably 120 to 300 ° C., more preferably 150 to 280 ° C., and still more preferably from the viewpoint of preventing decomposition of the resin component and obtaining a uniform coating film. It is 180-260 degreeC, More preferably, it is 200-250 degreeC.
The baking time in the baking treatment is not particularly limited, and is preferably 3 to 120 seconds, more preferably 10 to 100 seconds, and further preferably 30 to 60 seconds from the same viewpoint as described above.

  Hereinafter, the present invention will be described in more detail with reference to examples. In the examples, “part” means “part by mass” unless otherwise specified, and “%” means “% by mass” unless otherwise specified.

[Raw materials]
In the following examples and comparative examples, the following raw materials were used.
(1) Polyester resin “Byron GK-57CS” manufactured by Toyobo Co., Ltd., glass transition temperature 13 ° C.
Number average molecular weight: 4,500, SP value: 11,
(2) Melamine resin
“Super Becamine L-155-70” manufactured by DIC,
Number average molecular weight: 1,600, SP value: 10.1
(3) Polyvinylidene fluoride (PVdF)
`` HYLAR5000 '' manufactured by Augmont, USA,
Number average molecular weight: 15,000, SP value: 9.6,
(4) Isocyanate compound “Sunplex BL-160” manufactured by Sanyo Chemical Industries
Number average molecular weight 1,100, SP value: 12
(5) Acrylic resin “Paraloid B44” manufactured by Rohm and Haas,
Number average molecular weight: 15,000, SP value: 9.4
(6) Polytrifluoroethylene vinyl ether “Lumiflon Series” manufactured by Asahi Glass Co., Ltd.
Number average molecular weight 15,000, SP value: 10.0

(7) Branched polymer (B1-A)
Polyether-modified silicone "8616ADDITIVE" manufactured by Toray Dow,
Number average molecular weight of siloxane compound constituting siloxane segment: 500,
Number average molecular weight of polyoxyalkylene oxide composed of a random copolymer of ethylene oxide and propylene oxide: 2,000,
Ratio of polyoxyalkylene segment: 80% by mass,
Functional group at the terminal of the polyoxyalkylene segment: OH group,
SP value: 11.6, HLB value: 11.5, surface tension: 21 mN / m

(8) Linear polymer (B1-B)
Polyether-modified silicone "X-22-4952" made by Shin-Etsu Silicone,
Number average molecular weight of the siloxane compound constituting the siloxane segment: 600,
Number average molecular weight of polyoxyalkylene oxide constituting the polyoxyalkylene segment: 1,400,
Ratio of polyoxyalkylene segment: 80% by mass,
Functional group at the terminal of the polyoxyalkylene segment: OH group,
SP value: 12, HLB value: 12.5, surface tension: 23 mN / m
(9) PEG2000
Polyethylene glycol “PEG-2000” manufactured by Sanyo Chemical Co., Ltd.
Number average molecular weight: 2,000,
SP value: 17.7

(10) Alkyl silicate (compound having hydrolyzable silyl group)
“MKC silicate MS57” manufactured by Mitsubishi Chemical Corporation, compound name: condensate of tetramethoxysilicate

[Example 1]
20 parts of cyclohexanone and 40 parts of isophorone were added, and an acrylic resin ("Paralloid B44" manufactured by Rohm and Haas) was gradually added and dissolved to obtain an acrylic resin solution (paraloid B-44 solution) having a solid content of 40%. .
Further, 15 parts of this acrylic resin solution (Paralloid B-44 solution), 35 parts of isophorone, and 50 parts of polyvinylidene fluoride resin (manufactured by Augmont, USA, “HYLAR5000”) were charged to prepare a fluororesin dispersion base.
50 parts of a titanium oxide pigment and 10 parts of isophorone were mixed with 40 parts of an acrylic resin solution (Paralloid B-44 solution) to obtain a white pigment paste. Next, 85.8 parts of a fluororesin dispersion base and 5 parts of isophorone were added to 57.2 parts of the white pigment paste obtained as described above and mixed uniformly to prepare a white paint. To this white paint, 4.3 parts of a compound having a hydrolyzable silyl group (“MKC silicate MS57” manufactured by Mitsubishi Chemical Corporation) was added to prepare a stain-resistant paint. 10 parts of polyethylene glycol (“PEG-2000” manufactured by Sanyo Kasei Co., Ltd.) as a solid content concentration was added to the paint to prepare a paint composition of Example 1.

[Examples 2 to 9]
In Examples 2 to 9, a branched polymer (B1-A) or a linear polymer (B1-B) is used in place of polyethylene glycol, and in Example 8, polyvinylidene fluoride is further added as a fluororesin to polytrifluoroethylene. Instead of vinyl ether, a coating composition was prepared in the same manner as in Example 1, except that the blending amounts shown in Table 1 were used.
[Comparative Examples 1-2]
In Comparative Example 1, the polyoxyalkylene polymer (B) was not added, and in Comparative Example 2, the alkyl silicate (C) was not added. The coating compositions of Comparative Examples 1 and 2 were prepared respectively.

[Example 10]
Polyester resin ("Byron GK-57CS", manufactured by Toyobo Co., Ltd.) 34.9 parts (solid content), titanium oxide pigment 41.8 parts, cyclohexanone 10 parts, butanol 25 parts are mixed, bead milled, and after the meat 3. As a curing agent, melamine resin (8.4 parts (solid content) manufactured by DIC “Super Becamine L-155-70”, compound having hydrolyzable silyl group (“MKC silicate MS57” manufactured by Mitsubishi Chemical Corporation)) 9 parts were added to prepare a polyester / melamine paint as a base.
The coating composition of Example 10 was prepared by adding 10 parts of polyethylene glycol (“PEG-2000” manufactured by Sanyo Chemical Co., Ltd.) as a solid content concentration to the coating material.

[Examples 11 to 18 and Comparative Examples 3 to 4]
In Examples 11 to 18, a branched polymer (B1-A) or a linear polymer (B1-B) is used instead of polyethylene glycol, and in Example 17, the melamine resin is further replaced with an isocyanate compound. Coating compositions of Examples 11 to 18 were prepared in the same manner as in Example 10 except that they were prepared with the blending amounts shown in Table 2.
In Comparative Example 3, the polyoxyalkylene polymer (B) was not added, and in Comparative Example 4, the alkyl silicate (C) was not added. The coating compositions of Comparative Examples 3 to 4 were prepared respectively.

[Preparation of coated steel sheet for evaluation]
Using the prepared coating composition, a dry film thickness of 20 μm is applied on a 0.4 mm thick galvanized steel sheet to which an epoxy primer (a primer “Fine Tough C JT-25” manufactured by Nippon Fine Coatings Co., Ltd.) has been applied in advance. Was baked for 45 seconds so that the maximum temperature reached 230 ° C. to obtain a coated steel sheet.
The following evaluation was performed about each coated steel plate obtained in this way. The results are shown in Tables 1 and 2.

[Evaluation]
(1) Water contact angle (evaluation of hydrophilicity)
Each coated steel plate immediately after coating was held in an atmosphere of 50 ° C. and 95% RH for 3 hours for humidification promotion treatment, and then subjected to room temperature air drying for 1 hour was used as a sample.
2 μL of ion-exchanged water was dropped on the surface of the coating film of the sample, and the water contact angle after 30 seconds was measured. For the measurement, a portable contact angle meter “PCA-1” manufactured by Kyowa Interface Science Co., Ltd. was used. Those having a water contact angle of 50 ° or less were judged to have a low water contact angle and good hydrophilicity from an early stage after the formation of the coating film.

(2) Raindrop contamination Each coated steel plate was placed vertically so that rainwater fell under the corrugated sheet, exposed for 6 months, and the extent of raindrop contamination was evaluated according to the following criteria.
A + (excellent): no dirt A (excellent): thin dirt adheres to the whole A- (slightly good): thin rain stripes adhere B (good): thin rain stripes adhere to several places C (bad) : Conspicuous rain streaks adhere

(3) Appearance of coating film Each coated steel sheet immediately after coating was confirmed with the naked eye, and it was evaluated according to the following criteria whether the surface was smooth or smooth.
A + (excellent): smooth with no flecks or repellency A (good): smooth but slight repellency appearance B (poor): problem with smoothness

(4) Weather resistance <Super UV test>
UV light (wavelength: 295 to 450 nm, intensity: 100 mW / cm or less) was irradiated in an atmosphere of a temperature of 35 ° C. and a humidity of 70% RH for 24 hours with an Isuper UV tester F2 type manufactured by Iwasaki Electric Co., Ltd. Then, it put into the moisture-proof test machine designated by JISK-5600-7-9, and hold | maintained for 24 hours on the conditions of temperature 50 degreeC and humidity 98RH%. This was regarded as one cycle, and a total of eight cycles were tested.
The gloss value of the coated steel sheet with a specular reflection of 60 degrees before and after this 8-cycle test was measured using a gloss meter “UVG-6P” manufactured by Suga Test Instruments Co., Ltd. to evaluate the gloss retention.
The gloss retention is a percentage of the coating film gloss value of the coated plate after the test with respect to the coating film gloss value of the coated plate before the test. The gloss retention of 90% or more for the fluororesin system and the gloss retention of 60% or more for the polyester system were acceptable. The gloss retention is more preferably 95% or more for the fluororesin system and 70% or more for the polyester system.

(5) Cross cut test (water resistance to boiling)
Each coated steel sheet cut to a size of 5 cm × 10 cm was immersed in boiling water at about 100 ° C. for 5 hours, and then pulled up to evaluate the appearance of the coating film on the surface side, and evaluated by performing a cross-cut tape adhesion test.
In the cross-cut tape adhesion test, according to the cross-cut tape method of JIS K-5600-5-6, the gap between the cuts is set to 1 mm, 100 cross-cuts are made, and cellophane adhesive tape is adhered to the surface. The remaining state of the coating film after peeling off was classified. A coating state of classification 0 to 1 was regarded as acceptable. As the state of the coating film, classification 0 is more preferable.

As is clear from Table 1, the coating compositions of Examples 1 to 9 were excellent in all of the results of water contact angle (hydrophilicity), raindrop contamination, coating film appearance, and weather resistance cross cut test.
Of Examples 1 to 9, Examples 2 to 6, 8 and 9 using the polymer (B1-A) (branched type) as the polyoxyalkylene polymer (B) are polymers (B1-B). ) Compared with Example 7 using (linear type) and Example 1 using polyethylene glycol (PEG-2000), the water contact angle was small, the hydrophilicity was excellent, and the raindrop contamination was also excellent.
Furthermore, among Examples 2-6, 8 and 9 using the polymer (B1-A) (branched type), Examples 3-5 whose content is 5-15% by mass are more contaminated with raindrops. The coating film appearance was excellent.

Moreover, as is clear from Table 2, the coating compositions of Examples 10 to 18 are excellent in all of the results of the water contact angle (hydrophilicity), raindrop contamination, coating film appearance, and weather resistance cross-cut test. It was.
Of Examples 10 to 18, Examples 11 to 15, 17 and 18 using the polymer (B1-A) (branched type) as the polyoxyalkylene polymer (B) are polymers (B1-B). ) Compared with Example 16 using (linear type) and Example 10 using polyethylene glycol (PEG2000), the water contact angle was small, the hydrophilicity was excellent, and the raindrop contamination was also excellent.
Furthermore, of Examples 11 to 15, 17 and 18 using the polymer (B1-A) (branched type), Examples 12 to 14 having a content of 5 to 15% by mass are more contaminated with raindrops. The coating film appearance was excellent.

Claims (18)

  A coating composition for precoat containing an organic resin (A), a polyoxyalkylene polymer (B), and an alkyl silicate (C).   The coating composition for precoat according to claim 1, wherein the polyoxyalkylene polymer (B) is a polymer (B1) having a polyoxyalkylene segment and a siloxane segment. The polymer (B1) has a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), and both ends are independently represented by the following formula (3) or It is represented by the following formula (4),
The coating composition for precoats of Claim 2 whose content of the said polymer (B1) is 1 to 30 mass% with respect to the solid content whole quantity of the coating composition for precoats.

(In the formula (1), R ¹ is an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or more oxyalkylene groups having 1 to 4 carbon atoms. A polyoxyalkylene segment as a structural unit, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group, or an amide group, and R ¹ , R ² , A and B may be the same or different for each structural unit.)

(In Formula (2), R ³ and R ⁴ are each independently an alkyl group having 1 to 2 carbon atoms, and R ³ and R ⁴ may be the same or different for each structural unit.)

(In formula (3), R ^{5 to} R ⁷ are each independently an alkyl group having 1 to 2 carbon atoms, and R ^{5 to} R ⁷ may be the same or different for each structural unit.)

(In the formula (4), R ¹ and R ¹⁰ are each independently an alkyl group having 1 to ² carbon atoms, R ² is an alkylene group having 1 to 4 carbon atoms, and A is one or 2 carbon atoms having 1 to 4 carbon atoms. A polyoxyalkylene segment having a structural unit of at least a kind of oxyalkylene group, B is hydrogen, an alkyl group having 1 to 4 carbon atoms which may be partially substituted with a functional group, an amino group or an amide group, and R ¹ , R ² , R ¹⁰ , A and B may be the same or different for each structural unit.   The coating composition for precoat in any one of Claims 1-3 whose said organic resin (A) contains a fluororesin.   The coating composition for precoat according to claim 4, wherein the fluororesin contains polyvinylidene fluoride.   The coating composition for precoat according to claim 4 or 5, wherein the organic resin (A) further contains an acrylic resin.   The coating composition for precoat in any one of Claims 1-3 in which the said organic resin (A) contains a polyester resin and at least 1 type of a melamine resin and an isocyanate compound. The SP value (SPa) of the organic resin (A) and the SP value (SPb) of the polyoxyalkylene polymer (B) satisfy the relationship of the following formula (5). The coating composition for precoats described in 1.
SPb-SPa ≧ 0.6 (5)   The coating composition for precoat in any one of Claims 1-8 whose HLB value of the said polyoxyalkylene polymer (B) is 10-17.   The coating composition for precoat in any one of Claims 1-9 whose number average molecular weights of the siloxane segment of the said polyoxyalkylene polymer (B) are 500-1000.   The coating composition for precoat in any one of Claims 1-10 whose number average molecular weights of the polyoxyalkylene segment of the said polyoxyalkylene polymer (B) are 1000-2000.   The coating composition for precoat in any one of Claims 1-11 whose ratio of the polyoxyalkylene segment in the said polyoxyalkylene polymer (B) is 50 mass% or more.   At least a part of the polyoxyalkylene segment in the polyoxyalkylene polymer (B) has at least one functional group selected from the group consisting of a hydroxyl group, a carboxyl group, a carboxymethyl group, an amino group, and an amide group. The coating composition for precoat in any one of Claims 1-12.   The coating composition for precoat in any one of Claims 1-13 whose surface tension of the said polyoxyalkylene polymer (B) is 20-25 mN / m.   The coating film formed using the coating composition for precoats in any one of Claims 1-14.   Formation of the coating film which performs baking after apply | coating the coating composition for precoats in any one of Claims 1-14 to a steel plate until the temperature of the said steel plate becomes in the range of 120-300 degreeC. Method.   The method for forming a coating film according to claim 16, wherein the baking heating time is 3 to 90 seconds.   A precoated steel sheet having the coating film according to claim 15.