JP4534528B2 - Environment-friendly pre-coated steel sheet with excellent corrosion resistance, moisture resistance, workability, and coating peel resistance - Google Patents

Description

  The present invention is a chromium-free pre-coated steel sheet that does not contain chromium in the coating film or coating film, and has excellent coating film peeling resistance with extremely little coating film damage in forming processing, etc., and the basic performance of the pre-coated steel sheet The present invention relates to an environmentally friendly pre-coated steel sheet having excellent corrosion resistance, bending workability, and moisture resistance. This pre-coated steel sheet is suitable, for example, for home appliances and building materials, and can also be used for automobiles.

  In contrast to the post-coating system in which a steel sheet is coated after forming, the pre-coating system is a system in which a pre-coated steel sheet is formed, and the coated steel sheet used for this is called a pre-coated steel sheet. Pre-coated steel sheets are required to have high formability and corrosion resistance that do not cause scratches or peeling of the coating film during the forming process. In order to ensure corrosion resistance and adhesion, the pre-coated steel plates currently used are subjected to chemical conversion treatment containing chromium, and many of them contain a chromium-based rust preventive pigment in the undercoat film (for example, Patent Document 1). However, recently, it is desired to regulate the use of chromium, which is an environmentally regulated substance, and for this reason, development of a technology that does not use chromium in a chemical conversion treatment film or a coating film is also desired for pre-coated steel sheets.

Conventionally, various proposals have been made in response to the demand for the chromium-free precoated steel sheet. For example, with respect to corrosion resistance, Patent Document 2 states that a non-chromium precoated steel sheet having excellent corrosion resistance can be obtained by including a basic phosphite-based rust-preventing pigment in the undercoat coating film. Moreover, in patent document 3, it is supposed that the non-chromium precoat steel plate excellent in corrosion resistance will be obtained by containing an isocyanate compound and a phosphate type | system | group antirust pigment in undercoat. Moreover, in patent document 4 and patent document 5, the coating composition excellent in corrosion resistance which does not contain heavy metals, such as lead and chromium, is obtained by containing the mixture of a phosphate type | system | group antirust pigment and calcium ion exchange silica. It is said. Further, Patent Document 6 states that both chromium-free and corrosion resistance can be achieved by a combination of a specific resin raw material and a non-chromium rust preventive additive.
Regarding the improvement of the adhesion of the coating film, it is provided in Patent Document 7 that a specific base treatment layer made of an aqueous resin, tannic acid, fine particle silica, etc. is provided, and in Patent Document 8, tannic acid, a silane coupling agent, It is possible to provide a base treatment layer made of fine silica or the like, and in Patent Document 9, a base treatment layer made of resin components such as urethane resin, epoxy resin, acrylic resin, silane coupling agent, and fine particles having an average particle size of 1 μm or less is provided. Providing each is disclosed.

JP 7-316497 A JP-A-8-319437 JP-A-8-11257 Japanese Patent Laid-Open No. 9-12931 Japanese Patent No. 3389191 JP 2000-198963 A JP 2000-167482 A JP 2001-89868 A JP 2001-81392 A

Most of the conventional chromium-free precoated steel sheets focus on corrosion resistance, and the obtained corrosion resistance, particularly the corrosion resistance of the cut end face portion, is inferior to that using a chromium-based pigment. In addition, the conventional technology that focuses on coating film adhesion is merely a measure that improves the coating film adhesion at the flat part and the bent part, and is an advanced technique required under severe processing conditions in actual molding. It does not satisfy the coating film adhesion.
Therefore, the object of the present invention is to solve such problems of the prior art, and is a chromium-free pre-coated steel sheet that does not contain chromium in the film or paint film, and does not cause damage such as paint film peeling in the actual forming process. Environmentally friendly pre-coated steel sheet that has excellent formability (especially sliding part coating film peelability during forming process) and excellent basic properties of pre-coated steel sheets such as corrosion resistance, bending workability, and moisture resistance Is to provide.

  The present inventors have solved the above-mentioned problems of the prior art and repeatedly studied the coating composition of the precoated steel sheet having the excellent performance as described above. As a result, the surface of the zinc-based plated steel sheet contains a chemical compound containing dry silica. In addition to forming a treated film, a magnesium silicate compound anti-rust pigment and a polyphosphate anti-corrosion pigment are added to an organic resin that is a mixture or reaction product of a specific polyester resin and epoxy resin. In addition, by forming the coating composition with the pigment content and Mg concentration regulated within a specific range, a precoated steel sheet having excellent coating film peel resistance, corrosion resistance, moisture resistance and bending workability is obtained. I found out that

The present invention has been made based on such findings, and the features thereof are as follows.
[1] A pre-coated steel sheet having a chemical conversion treatment film, an undercoat film and a topcoat film from the lower layer side on the surface of the zinc-based plated steel sheet, and containing no chromium in the chemical conversion film and the coating film,
The chemical conversion coating contains dry silica,
The undercoating film is a coating composition containing the following organic resin (b) or / and (b) and a rust-preventing pigment, wherein the magnesium silicate compound-based rust-preventive pigment ( A) 1.5 to 14% by mass, polyphosphate rust preventive pigment (B) 25 to 50% by mass, and the total content of all pigments including these rust preventive pigments (A) and (B) Corrosion resistance, moisture resistance, processability, and 65% by mass or less, characterized in that the total content (C) of Mg contained as part of the pigment is 4.0 to 10% by mass. An environmentally conscious pre-coated steel sheet with excellent peel resistance.
(A) A polyester resin (D) having a number average molecular weight of 12,000 to 26000 and a glass transition temperature of 0 to 30 ° C., an epoxy resin (E), and a curing agent (F) as main components, and the polyester resin (D) Organic resin having a ratio D / E (molar ratio) to the epoxy resin (E) of 1.0 / 1.0 to 1.0 / 4.0
(B) Modified polyester resin (G) obtained by reaction of polyester resin (D) having a number average molecular weight of 12,000 to 26000 and glass transition temperature of 0 to 30 ° C. and epoxy resin (E), and curing agent (F) An organic resin having a ratio D / E (molar ratio) between the polyester resin (D) and the epoxy resin (E) of 1.0 / 1.2 to 1.0 / 4.0.
[2] In the precoated steel sheet according to [1], the content of dry silica in the chemical conversion film is 20 to 70% by mass, and the silica adhesion is 10 to 100 mg / m ² , An environmentally conscious pre-coated steel sheet with excellent moisture resistance, workability, and film peel resistance.

  The chromium-free pre-coated steel sheet of the present invention has excellent coating film peeling resistance with extremely little coating film damage in forming processing and the like, and is excellent in the basic properties of the pre-coated steel sheet, such as corrosion resistance, bending workability, and moisture resistance. Yes. For this reason, it is extremely useful as a pre-coated steel sheet used for parts requiring high environmental harmony, forming processability, and corrosion resistance in applications such as home appliances, building materials, and automobiles.

Hereinafter, the details of the precoated steel sheet of the present invention and the reasons for limitation will be described.
Examples of the zinc-based plated steel sheet used as the base steel sheet include a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, an electrogalvanized steel sheet, a hot-dip Zn-Al alloy-plated steel sheet (for example, hot-dip Zn-5% Al alloy-plated steel sheet, hot-melt Zn). Various zinc-based plated steel sheets such as those represented by -55% Al alloy-plated steel sheets can be used. Moreover, in order to improve adhesiveness with a chemical conversion treatment film, what performed the surface adjustment process after zinc type plating can be used as a zinc type plated steel plate. This surface conditioning treatment may use either an acidic treatment liquid or an alkaline treatment liquid, and the treatment can be performed by dipping or spraying.

  The chemical conversion treatment film formed on the surface of the galvanized steel sheet is a non-chromium (containing no chromium) film containing dry silica. The inventors of the present invention have found that by including dry silica (dry type silica fine particles) in the chemical conversion coating film, the coating film peel resistance at the sliding portion during molding or the like is greatly improved. Examples of the dry-type silica fine particles include “Aerosil # 200” and “Aerosil # 300” (both trade names, manufactured by Nippon Aerosil Co., Ltd.). The addition of dry silica to the chemical conversion coating improves the anti-peeling resistance of the sliding part because the silica fine particles are very fine and the secondary agglomeration is less likely to occur. It is considered that the anchor effect of the formed undercoat film is appropriately generated and the adhesion of the film is improved.

In addition to the above-mentioned dry silica, the chemical conversion treatment film includes, as the binder, for example, one or more selected from water-soluble or water-dispersible organic polymers and oxyacid salts (excluding chromate). Can be contained. Examples of the water-soluble or water-dispersible organic polymer include poly (meth) acrylic acid and polyvinyl alcohol. Examples of the oxyacid salt include phosphate, molybdate, tungstate, and vanadate. By blending such a binder in the chemical conversion film, the bonding between the silica fine particles and the adhesion of the silica fine particles to the plating surface can be improved.
In addition, one or more of Zr compounds, Ti compounds, and Hf compounds (for example, fluoro complex salts) are added as additives to the treatment liquid for forming the chemical conversion coating, and these are subjected to chemical conversion coating. It can be contained. Furthermore, a stabilizer may be added in order to improve the stability of the treatment liquid. As this stabilizer, 1 or more types, such as carboxylic acid, amines, aminopolycarboxylic acid, can be used.

The content of dry silica in the chemical conversion coating is 20 to 70% by mass, preferably 30 to 60% by mass in terms of the solid content in the coating. When the content of dry silica is less than 20% by mass, the anchor effect of the undercoat film by silica is poor. On the other hand, when the content exceeds 70% by mass, the chemical conversion coating itself becomes brittle and the adhesion to the plated surface is reduced. Also in this case, the sliding part coating film peeling resistance tends to decrease.
The amount of dry silica deposited is 10 to 100 mg / m ² , preferably 30 to 70 mg / m ² . If the adhesion amount is less than 10 mg / m ² , the adhesion to the base metal and the corrosion resistance are not sufficient. On the other hand, if it exceeds 100 mg / m ² , the adhesion with the undercoat film of the chemical conversion coating upper layer tends to decrease. .

Moreover, when the chemical conversion treatment film contains a binder, the blending ratio of the dry silica and the binder is a solid content mass ratio, and the dry silica / binder is about 1 / 0.01 to 1/10. From the viewpoint of cohesive fracture resistance of the film, adhesion with the undercoat film, and the like.
Although there is no restriction | limiting in particular in the processing method for forming a chemical conversion treatment film, Generally, a chemical conversion liquid is applied with a roll coater, and it is made to dry after that. In this drying, the film is usually dried at a reaching plate temperature of about 50 to 150 ° C. by heating means such as hot air heating, infrared heating, induction heating and the like.

Next, the undercoat coating film formed on the surface of the chemical conversion coating will be described.
This undercoat coating film is a coating composition containing a specific organic resin and a rust preventive pigment, and is composed of a magnesium silicate compound rust preventive pigment (A) and a polyphosphate rust preventive pigment ( B) and a paint composition having a pigment content and a Mg content defined in a specific range, and a specific effect is exhibited by such a specific composition. This undercoat coating film does not contain a chromium component such as a chromium-based anticorrosive pigment.
As mentioned earlier, the conventional technology of adding calcium ion-exchanged silica and polyphosphate anticorrosive pigments in paint compositions is known, but the anti-corrosion effect is improved by the combined addition of both antirust components. Suppresses elution of calcium ions by polyphosphate ions forming ion bonds such as chelate bonds to the silica particles of calcium ion-exchanged silica, and corrosive ions such as hydrogen ions by the pH buffering action of polyphosphate. It is believed that this is due to the weakening of acidification caused by the decrease in calcium ion elution.

  In contrast to such a conventional technique, the inventors of the present invention have a remarkable effect of the magnesium silicate compound-based rust preventive pigment due to the composite addition of the polyphosphate rust preventive pigment as compared with calcium ion-exchanged silica. I found out. The reason for this is that polyphosphate ions generated from the polyphosphate anticorrosive pigment chelate-bond to the zinc of the base plating to form a polyphosphate passive film, but (1) the pH of calcium ion exchanged silica Is about 8 to 9, whereas the pH of the magnesium silicate compound anticorrosive pigment is about 9.5, and this pH is a more stable pH region of zinc contained in the base plating or the base plating. (2) The polyphosphate passive film is further stabilized for that purpose, and (3) The corrosion product incorporating Mg of the magnesium silicate compound anticorrosive pigment is more stable than the Ca system, (4) In addition, the corrosion products are kept in a more stable environment because the pH of the corrosion products is close to the neutral range compared to the Ca system. It is estimated that.

Here, various forms of magnesium silicate (compound) can be used as the magnesium silicate compound-based anticorrosive pigment (A), among which amorphous hydrous magnesium silicate compounds, especially Mg / Si atoms. An amorphous hydrous magnesium silicate compound powder having a ratio of 0.025 to 1.0, particularly preferably 0.025 to 0.8 is preferred. A rust preventive pigment made of powder of this amorphous hydrous magnesium silicate compound is, for example, a rust preventive pigment disclosed in Japanese Patent Application Laid-Open No. 2000-273363, and is synthesized by using an alkali metal silicate and a water-soluble magnesium salt as starting materials. Can be manufactured.
For example, a precipitate formed by mixing and stirring an aqueous solution of an alkali metal silicate such as potassium silicate or sodium silicate (water glass) and an aqueous solution of a water-soluble magnesium salt such as magnesium sulfate, magnesium chloride or magnesium nitrate. The product can be produced by separation and purification (for example, washing with water → drying → pulverization).

  The magnesium silicate compound rust preventive pigment (A) used in the present invention may be a composite with other pigment, for example, a form that covers the surface of other pigment particles. Examples of composites with other pigments as described above include “TC720” (trade name, manufactured by Teica). In addition, as a mixture with other pigments, for example, “TC710” (trade name, manufactured by Teika) can be mentioned. Even when using the magnesium silicate compound rust preventive pigment (A) combined or mixed with other pigments as described above, the content is defined by the amount of the net magnesium silicate compound excluding the other pigments. Is done.

The type of the polyphosphate rust preventive pigment (B) is not particularly limited, but pyrophosphates such as titanium pyrophosphate, metaphosphates such as aluminum metaphosphate, and tripolyphosphates such as aluminum dihydrogen triphosphate are excellent. Since it shows rust prevention property, it is preferable to use one or more of these. Of these, aluminum dihydrogen tripolyphosphate rust preventive pigments are particularly suitable.
The polyphosphate rust preventive pigment (B) includes, in addition to the above-mentioned untreated polyphosphate such as aluminum dihydrogen tripolyphosphate, Mg compound (for example, MgO), Ca compound (for example, CaO), Zn Those treated (modified) with a compound (for example, ZnO) may be used. That is, this type of polyphosphate rust preventive pigment is usually obtained by a method such as wet mixing of a polyphosphate such as aluminum dihydrogen tripolyphosphate and the above metal compound. A polyphosphate treated with Mg or Zn (modified) is obtained by mixing a metal compound such as MgO or ZnO with an aqueous slurry, performing wet treatment, and then drying and pulverizing. Examples of such a polyphosphate rust preventive pigment include, for example, Mg-treated trihydrogenphosphate aluminum dihydrogen: “K-WHITE G105” (trade name, manufactured by Teika), Zn-treated tripolyphosphate aluminum dihydrogen: “K-WHITE K140W”. "(Trade name, manufactured by Teica), Ca-treated aluminum dihydrogen tripolyphosphate:" K-WHITE Ca605 "(trade name, manufactured by Teica), and the like.

In order to obtain the above-described effects by the combined addition of the magnesium silicate compound anticorrosive pigment (A) and the polyphosphate anticorrosive pigment (B), the content thereof in the coating composition is optimal. It is necessary to make it. That is, the content of the magnesium silicate compound-based rust preventive pigment (A) is 1.5 to 14% by mass, preferably 2.5 to 10% by mass, based on the solid content of the coating composition. The content of the rust preventive pigment (B) is 25 to 50% by mass, preferably 30 to 45% by mass.
When the content of the magnesium silicate compound anticorrosive pigment (A) is less than 1.5% by mass, the synergistic effect due to the combined addition with the polyphosphate anticorrosive pigment (B) is small, and in particular, the corrosion resistance of the crosscut portion is low. It becomes insufficient. On the other hand, when the content exceeds 14% by mass, the magnesium silicate compound-based rust preventive pigment (A) absorbs moisture more than necessary, and thus moisture resistance is lowered. In addition, when the content of the polyphosphate rust preventive pigment (B) is less than 25% by mass, chelate bonding to zinc is not sufficient, and particularly, the corrosion resistance at the cut end is insufficient. On the other hand, when the content exceeds 50% by mass, the polyphosphate rust preventive pigment (B) absorbs water more than necessary and is eluted, so that the moisture resistance decreases.

In addition, the present inventors include a magnesium silicate compound-based rust preventive pigment (A), and further a polyphosphate-based rust preventive additive (B) and other pigments in the coating composition. It has been found that the content of Mg to be highly correlated with corrosion resistance and moisture resistance. Specifically, the total content (C) of Mg contained as a part of the pigment is 4.0 to 10% by mass, preferably 5.0 to 8.0% by mass in the solid content of the coating composition. % Particularly good corrosion resistance and moisture resistance are obtained. Therefore, in the coating composition of the present invention, the total content (C) of Mg contained as a part of the pigment is 4.0 to 10% by mass, preferably 5.0 to 8.0% by mass in the solid content. %. If the Mg content (C) is less than 4% by mass, the corrosion product has insufficient stability, so that the corrosion resistance at the cutting edge is inferior. On the other hand, if it exceeds 10% by mass, the Mg elution amount becomes too large. In particular, the moisture resistance is poor.
Here, as Mg prescribed | regulated by Mg content (C), it treats with Mg compounds, such as magnesium oxide, as a polyphosphate type | system | group antirust pigment (B) besides a magnesium silicate type anticorrosion pigment (A). When using (modified) or when using a magnesium phosphate rust preventive pigment described later, Mg contained therein is also included.

Further, the total of all the pigments in the coating composition containing the magnesium silicate compound-based rust preventive pigment (A) and the polyphosphate rust preventive pigment (B) is 65% by mass or less, preferably in terms of the solid content. 55 mass% or less. When the total pigment exceeds 65% by mass, the resin component is reduced, so that the film tends to be brittle, particularly the workability is lowered, and cracks are easily generated during bending. In addition, the paint is unbalanced and the paintability is adversely affected.
In the undercoating film, calcium ion-exchanged silica, zinc phosphate-based rust-preventing pigments as rust-preventing pigments other than the above-mentioned magnesium silicate compound-based rust-preventing pigment (A) and polyphosphate-based rust-preventing pigment (B) , Magnesium phosphate rust preventive pigment, zinc phosphite rust preventive pigment, etc. may be added. Other pigments include calcium carbonate, kaolin, clay, titanium oxide, talc, barium sulfate. One or more of mica, bengara, manganese blue, carbon black, aluminum powder, pearl mica and the like may be added.

As the organic resin constituting the coating composition for the undercoat coating film, the following (A) or / and (B) organic resin is used in order to obtain particularly excellent sliding part coating film peeling resistance.
(A) A polyester resin (D) having a number average molecular weight of 12,000 to 26000 and a glass transition temperature of 0 to 30 ° C., an epoxy resin (E), and a curing agent (F) as main components, and the polyester resin (D) Organic resin (b) having a ratio D / E (molar ratio) to the epoxy resin (E) of 1.0 / 1.0 to 1.0 / 4.0 Number average molecular weight 12000 to 26000, glass transition temperature 0 The polyester resin (D) and the epoxy are mainly composed of a modified polyester resin (G) obtained by a reaction between a polyester resin (D) and an epoxy resin (E) at -30 ° C, and a curing agent (F). Organic resin having ratio D / E (molar ratio) to resin (E) of 1.0 / 1.2 to 1.0 / 4.0

First, a resin mainly composed of the polyester resin (D), the epoxy resin (E), and the curing agent (F) will be described.
The polyester resin (D) is mainly an ester compound of a polybasic acid and a polyhydric alcohol. As polybasic acids, dibasic acids such as terephthalic acid, isophthalic acid, succinic acid, adipic acid, sebacic acid, fumaric acid and maleic anhydride, tribasic or higher polybasic acids such as trimellitic anhydride and pyromellitic anhydride An acid or the like is used, and two or more of these polybasic acid components can be used in combination.
Examples of the polyhydric alcohol include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 3-methylpentadiol, neopentylene glycol, 1,4-butanediol, 1,5-pentanediol, Aliphatic or alicyclic dihydric alcohols such as 1,4-hexanediol, 1,6-hexanediol and 1,4-cyclohexanedimethanol are mainly used, and if necessary, glycerin, trimethylolethane Trivalent or higher polyhydric alcohols such as trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol and dipentaerythritol can also be used in combination.

  The polyester resin (D) needs to have a number average molecular weight of 12000 to 26000 and a glass transition temperature of 0 to 30 ° C. Moreover, the more preferable number average molecular weights of a polyester resin (D) are 14000-21000, and a glass transition temperature is 5-25 degreeC. If the number average molecular weight of the polyester resin (D) is less than 12000, the elongation of the coating film becomes insufficient and the bending workability is lowered. On the other hand, if the number average molecular weight exceeds 26000, the toughness of the coating film decreases, so that even if used in combination with an epoxy resin, the effect of improving the coating film peel resistance of the sliding portion becomes insufficient. Furthermore, since the coating composition has a high viscosity, an excessive dilution solvent is required, which is not preferable from the viewpoint of coating workability and environmental harmony. On the other hand, when the glass transition temperature of the polyester resin (D) exceeds 30 ° C., the coating film is insufficiently stretched and bending workability is lowered. On the other hand, if the glass transition temperature is less than 0 ° C., the toughness of the coating film decreases, and sufficient peel resistance cannot be obtained.

The epoxy resin (E) is blended in order to improve adhesion to the base. Examples of the epoxy resin suitable for use in the present invention include bisphenols such as bisphenol A, bisphenol F, and bisphenol S. And an epoxy compound obtained by a reaction of epihalohydrin or β-methyl epihalohydrin, or a bisphenol type epoxy resin that is a copolymer thereof.
Moreover, it is preferable to use a thing with a number average molecular weight of 500-5000 as an epoxy resin (E). When the number average molecular weight of the epoxy resin is less than 500, the adhesiveness is lowered. On the other hand, when the number average molecular weight exceeds 5,000, the resin viscosity is increased, so that the compatibility with the polyester resin (D) is lowered.

Epoxy resin (E) provides strong adhesion by forming a cross-linked structure by reaction with polyester resin (D) and curing agent (F) when the undercoat is applied to an object and baked and cured. To do.
The compounding ratio (molar ratio) of the polyester resin (D) and the epoxy resin (E) is D / E = 1.0 / 1.0 to 1.0 / 4.0. Polyester resin (D): When the ratio of the epoxy resin (E) is less than 1.0 with respect to 1.0, the adhesive strength of the resin is insufficient, and the effect of improving the coating film peeling resistance becomes insufficient. If it exceeds 4.0, the flexibility of the resin is insufficient, so that the bending workability is lowered.

As the curing agent (F), an amino resin and / or a polyisocyanate compound can be used.
The amino resin is a resin obtained by alkylating a part or all of a product obtained by condensation reaction of urea, benzoguanamine, melamine or the like with formaldehyde with an alcohol such as methanol, ethanol, or butanol. Specific examples include methylated urea resins, n-butylated benzoguanamine resins, methylated melamine resins, n-butylated melamine resins, iso-butylated melamine resins, and the like. Can be used.
As the polyisocyanate compound, an isocyanate compound obtained by a general production method can be used. Among them, a polyisocyanate compound blocked with a blocking agent such as phenol, cresol, aromatic secondary amine, tertiary alcohol, lactam, oxime and the like, which can be used as a one-pack type paint, is particularly preferable. By using this blocked polyisocyanate compound, it is possible to store in one liquid, and the use as a paint becomes easy.

  More preferred polyisocyanate compounds include non-yellowing hexamethylene diisocyanate and derivatives thereof, tolylene diisocyanate and derivatives thereof, 4,4′-diphenylmethane diisocyanate and derivatives thereof, xylylene diisocyanate and derivatives thereof, isophorone diisocyanate and derivatives thereof, And trimethylhexamethylene diisocyanate and derivatives thereof, hydrogenated tolylene diisocyanate and derivatives thereof, hydrogenated 4,4′-diphenylmethane diisocyanate and derivatives thereof, hydrogenated xylylene diisocyanate and derivatives thereof, and the like, Two or more kinds can be used.

  The blending ratio of the polyester resin (D) and the epoxy resin (E) and the curing agent (F) is [polyester resin (D) + epoxy resin (E)] / curing agent (F) = 90/10 in terms of solid mass ratio. It is preferably ~ 65/35. When the ratio of [polyester resin (D) + epoxy resin (E)] / curing agent (F) is more than 90/10, sufficient curability cannot be obtained, so that scratch resistance and solvent resistance are reduced. If it is less than 65/35, a side reaction between excess curing agents occurs, so that the workability and the adhesion of the processed part are deteriorated.

Next, the resin mainly composed of the modified polyester resin (G) obtained by the reaction between the polyester resin (D) and the epoxy resin (E) and the curing agent (F) will be described.
For the polyester resin (D) and the epoxy resin (E) for obtaining the modified polyester resin (G), it is necessary to use a resin having a number average molecular weight and a glass transition temperature as described above for the resin (a). The polyester resin (D) is an ester compound of a polybasic acid and a polyhydric alcohol, and the epoxy resin (E) is a reaction of bisphenols such as bisphenol A, bisphenol F, and bisphenol S with epihalohydrin or β-methyl epihalohydrin. The bisphenol type epoxy resin etc. which are the epoxy compounds obtained by this, or these copolymers can be used.

As a method of modifying the polyester resin (D) with the epoxy resin (E), there are a method of incorporating an epoxy resin in the synthesis of the polyester resin, a method of reacting the polyester resin and the epoxy resin in the presence of an amine catalyst, and the like. Among these modification methods, from the viewpoint of obtaining a high degree of adhesion without impairing the flexibility of the coating film, a method of reacting with an epoxy resin in the presence of an amine catalyst after the synthesis of the polyester resin is preferable.
Moreover, the compounding ratio (molar ratio) of the polyester resin (D) and the epoxy resin (E) for obtaining the modified polyester resin (G) is A / B = 1.0 / 1.2 to 1.0 / 4. 0. Polyester resin (D): If the ratio of the epoxy resin (E) is less than 1.2 with respect to 1.0, the effect of improving the peel resistance of the coating film becomes insufficient due to insufficient adhesion of the resin, If it exceeds 4.0, the flexibility of the resin is insufficient, so that the bending workability is lowered.
As said hardening | curing agent (F), the 1 type (s) or 2 or more types chosen from the amino resin as mentioned above and a polyisocyanate compound can be used.

The blending ratio of the modified polyester resin (G) and the curing agent (F) is preferably modified polyester resin (G) / curing agent (F) = 90/10 to 65/35 in terms of solid content mass ratio. When the ratio of the modified polyester resin (G) exceeds 90/10, sufficient curability cannot be obtained, so that the scratch resistance and solvent resistance are lowered. On the other hand, when it is less than 65/35, excessive curing agents or curing are performed. Since a side reaction between the agent and the modified polyester resin (G) occurs, processability and processed part adhesion are deteriorated.
Any one of the organic resins (a) and (b) described above may be used alone, or a mixture of both may be used.

In addition to the organic resins, rust preventive pigments, and other pigments described above, various additives such as curing catalysts, antifoaming agents, and flow inhibitors are required for coating compositions for undercoat films. It can be added depending on.
The curing catalyst is used as necessary to accelerate the curing reaction of the resin components (main resin and curing agent), and the usable curing catalyst includes an acid or a neutralized product thereof. Typical examples include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid and neutralized amines thereof, tin octoate, dibutyltin dilaurate, lead 2-ethylhexoate, etc. It is mentioned as a typical thing.

When the coating composition is actually used to form the undercoat film, it is dissolved in an organic solvent. Examples of organic solvents to be used include xylene, toluene, methyl ethyl ketone, methyl isobutyl ketone, methyl cellosolve, butyl cellosolve, cellosolve acetate, butyl cellosolve acetate, carbitol, ethyl carbitol, butyl carbitol, butyl acetate, n-butyl acetate, acetic acid Ethyl, methanol, ethanol, cyclohexanone, petroleum-based solvents such as petroleum ether and petroleum naphtha, mineral spirits, Solvesso 100 (trade name, manufactured by Exxon Chemical), Solvesso 150 (trade name, manufactured by Exxon Chemical), Solvesso 200 (trade name, manufactured by Exxon Chemical Co., Ltd.) and the like, and one or more of these organic solvents can be used depending on the resin type to be applied.
In preparing the coating composition, an ordinary disperser or kneader such as a sand grind mill, a ball mill, or a blender can be selected and used, and each component can be blended.

Although there are no particular restrictions on the method of applying the undercoat film, the coating composition is preferably applied by a method such as roll coater coating or curtain flow coating. After applying the coating composition for the undercoat film on the surface of the zinc-based plated steel sheet on which the above-mentioned chemical conversion treatment film is formed, it usually reaches about 180 to 260 ° C. by heating means such as hot air heating, infrared heating, induction heating, etc. A baking process is performed at a plate temperature for about 30 seconds to 1 minute.
The thickness of the undercoat coating film is desirably 2 to 20 μm, preferably 4 to 15 μm in terms of dry film thickness. If the film thickness is less than 2 μm, sufficient corrosion resistance cannot be obtained, while if it exceeds 20 μm, the bending workability is insufficient.

Next, the top coat film formed on the upper layer of the undercoat film will be described.
There is no special restriction | limiting about the structure of top coat film, As a resin, 1 type (s) or 2 or more types, such as a polyester resin, an acrylic resin, a fluororesin, and a urethane resin, can be used. These resins may be used in combination with a crosslinking agent of an amino resin or an isocyanate compound. This top coating film also does not contain chromium components such as chromium-based rust preventive pigments.
Further, an appropriate amount of wax can be blended in the top coat film depending on the purpose and application. As this wax, natural wax or synthetic wax can be used.
In addition, the coating composition for the top coat film includes a curing catalyst such as p-toluenesulfonic acid, tin octoate, dibutyltin dilaurate, and 2-ethylhexoate lead; calcium carbonate, kaolin, Clay, titanium oxide, talc, barium sulfate, mica, petal, manganese blue, carbon black, aluminum powder, pigments such as pearl mica; and other additives such as antifoaming agents and flow inhibitors may be added as appropriate. it can.

In actually using the coating composition for forming the top coat film, these are dissolved in an organic solvent and used. Examples of the organic solvent to be used include methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, Solvesso 100 (trade name, manufactured by Exxon Chemical), Solvesso 150 (trade name, manufactured by Exxon Chemical), and Solvesso 200 (trade name, Exxon Chemical). Manufactured), toluene, xylene, methyl cellosolve, butyl cellosolve, cellosolve acetate, butyl cellosolve acetate, carbitol, ethyl garbitol, butyl carbitol, ethyl acetate, butyl acetate, petroleum ether, petroleum naphtha, etc. Two or more kinds can be used.
In preparing the coating composition for the top coat film, an ordinary dispersing machine or kneader such as a sand grind mill, a ball mill, or a blender is selected and used, and each component can be blended.

Although there is no restriction | limiting in particular in the coating method of the coating composition for top coat films, Preferably it is good to apply | coat a coating composition by methods, such as roll coater coating and curtain flow coating.
After coating the coating composition for the top coating film on the undercoating film, the coating film is baked by heating means such as hot air heating, infrared heating, induction heating, etc., and the resin is crosslinked to obtain a cured coating film. The baking treatment for heat-curing the top coat film is usually performed at a reached plate temperature of about 180 ° C. to 260 ° C., and baking is performed for about 30 seconds to 3 minutes in this temperature range.
The film thickness of the top coat film is preferably 10 to 20 μm. If the film thickness is less than 10 μm, there is a risk that the overall coating performance as a top coat film may not be sufficiently obtained. On the other hand, if the film thickness exceeds 20 μm, foaming and side walls are not preferable.
In addition, the precoat steel plate of this invention may form a coating film (for example, clear coating film) further on top coat film, and may be used by 3 coat * 3 baking.

According to the following (1.1) and (1.2), resin compositions for undercoat paints shown in Tables 1 and 2 were prepared.
(1.1) Preparation of polyester resin (D) used for resin composition for undercoating and polyester resin synthesis example 1
Terephthalic acid 215.8 parts by mass (1.3 mol), isophthalic acid 182.6 parts by mass (1.1 mol), adipic acid 189.8 parts by mass (1.3 mol), ethylene glycol 124 parts by mass (2. 0 mol), 166.4 parts by mass of neopentyl glycol (1.6 mol), 30.4 parts by mass of “Epiclon 850” (trade name, manufactured by Dainippon Ink Co., Ltd.), and 0.1 parts by mass of dioctyltin oxide The esterification reaction was performed at 240 ° C. for 2 hours in a nitrogen stream. Thereafter, the pressure is reduced to 1 mmHg over 1 hour, and further reacted at 260 ° C. for 1 hour, and dissolved in Solvesso 150 to obtain a polyester resin (D1) having a nonvolatile content of 35%, an average molecular weight of 20000, and a glass transition temperature of 10 ° C. It was.

・ Polyester resin synthesis example 2
Terephthalic acid 215.8 parts by mass (1.3 mol), isophthalic acid 182.6 parts by mass (1.1 mol), adipic acid 189.8 parts by mass (1.3 mol), ethylene glycol 124 parts by mass (2. 0 mol), 166.4 parts by mass of neopentyl glycol (1.6 mol), 30.4 parts by mass of “Epiclon 850” (trade name, manufactured by Dainippon Ink Co., Ltd.), and 0.1 parts by mass of dioctyltin oxide The esterification reaction was performed at 240 ° C. for 2 hours in a nitrogen stream. Thereafter, the pressure is reduced to 1 mmHg over 1 hour, and the reaction is further carried out at 260 ° C. for 20 minutes and dissolved in Solvesso 150 to obtain a polyester resin (D2) having a nonvolatile content of 35%, an average molecular weight of 16000, and a glass transition temperature of 15 ° C. It was.

(1.2) Preparation of resin composition for undercoat paint For the resin compositions (R1) to (R6) and (R8) to (R11) in Table 1, the polyester resins (D1) and (D2), commercially available polyester resins Table 1 shows commercially available epoxy resins (E1) or (E2), similarly curing agents (F1) or (F2), and also curing catalysts (f1) or (f2) in any one of (D3) and (D4). It was prepared by blending at the ratio shown in FIG. The resin composition (R7) was prepared by blending the polyester resin (D1) with the curing agent (F1) and the curing catalyst (f1) in the ratio shown in Table 1 without blending the epoxy resin.
For the resin compositions (R12) to (R22) in Table 2, any one of the polyester resins (D1), (D2), (D3), and (D4) includes an epoxy resin (E1) or (E2). Are added at a ratio shown in Table 2, 0.5 parts by mass of triethylamine is added, and the reaction is performed in a nitrogen stream at 140 ° C. for 2 hours to synthesize a modified polyester resin, and the curing agent (F1) or ( F2), the curing catalyst (f1) or (f2) was blended at the ratio shown in Table 2 and prepared.

In Tables 1 and 2, * 1 to * 8 indicate the following contents.
(* 1): D1, D2 = polyester resin (D1), (D2), D3 = polyester resin “Byron GK780” (trade name, manufactured by Toyobo Co., Ltd.) , D4 = Polyester resin “Byron 550” (trade name, manufactured by Toyobo)
(* 2): Number average molecular weight (* 3): Glass transition temperature (° C)
(* 4): parts by mass (solid content)
(* 5): E1 = Epoxy resin “Epicron 4050” (trade name, manufactured by Dainippon Ink and Chemicals), E2 = Epoxy resin “Epicron 9050” (trade name, manufactured by Dainippon Ink and Chemicals)
(* 6): Compounding ratio (molar ratio) D / E of polyester resin (D) and epoxy resin (E)
(* 7): F1 = melamine resin “Super Becamine L117” (trade name, manufactured by Dainippon Ink and Chemicals, nonvolatile content 60%), F2 = polyisocyanate compound “Bernock D550” (trade name, Dainippon Ink and Chemicals, Inc.) Manufactured, non-volatile content 55%)
(* 8): f1 = dodecylbenzenesulfonic acid, f2 = dibutyltin dilaurate

For the undercoat paint resin compositions shown in Tables 1 and 2, after blending various pigments at the blending ratios shown in Tables 4 to 17, they were dispersed with a sand mill until the particle size became 10 μm or less. The coating composition was prepared. The coating composition was adjusted to a nonvolatile content of 40% by mass using a solvent.
After degreasing a hot-dip galvanized steel sheet with a thickness of 0.5 mm as the base steel sheet (plating adhesion amount per side: 30 g / m ² ), a chemical conversion treatment solution was applied and dried at a final plate temperature of 100 ° C. The chemical conversion treatment film shown in FIG. On top of that, a coating composition for an undercoat film adjusted to the composition shown in Tables 4 to 17 was applied so as to have a predetermined dry film thickness, followed by a baking temperature (attainment temperature) of 215 ° C. and a baking time of 60 seconds. The undercoat coating film was formed by performing the baking process. Furthermore, after applying a polyester top coat “SRF05” (trade name, manufactured by Nippon Fine Coatings) so that the dry film thickness is 15 μm, a baking temperature (attainment temperature) of 230 ° C. and a baking time of 60 seconds is applied. Baking treatment was performed to form a top coat film, and precoated steel sheets of the present invention and comparative examples were obtained. The results of examining various performances of these precoated steel sheets are shown in Tables 4 to 17.
In addition, the addition amount of each pigment shown to Table 4-Table 17 is the ratio (mass%) in solid content of a coating composition.

Below, it shows about the test method and evaluation method of the performance test of a precoat steel plate.
(1) Corrosion resistance at the cut end After the salt spray test (SST) described in JIS Z 2371 was performed for 240 hours on the test piece with the cut end exposed, the blister width at the cut end was measured. Evaluated by criteria.
◎: Maximum blister width 1.5 mm or less ○: Maximum blister width 1.5 mm or more, 2.5 mm or less Δ: Maximum blister width 2.5 mm or more, 5.0 mm or less ×: Maximum blister width 5.0 mm or more (2) Cross Corrosion resistance of cut part A cross cut reaching the base plating was put into the test piece, and after performing a salt spray test (SST) described in JIS Z 2371 for 240 hours, the blister width of the cross cut part was measured and evaluated according to the following criteria.
◎: Maximum blister width 1.5 mm or less ○: Maximum blister width 1.5 mm or more, 2.5 mm or less Δ: Maximum blister width 2.5 mm or more, 5.0 mm or less ×: Maximum blister width 5.0 mm or less

(3) Moisture resistance The specimen is exposed for 240 hours in a constant temperature and humidity tester with a relative humidity of 98% or higher and a test temperature of 50 ° C, and the appearance of the blisters on the coating surface after the test is observed based on JIS K 5600-8-2. And evaluated according to the following criteria.
○: Blow size class 1 or less and density class 1 or less are satisfied at the same time. Δ: Blow size class 2 or less and density class 2 or less are satisfied at the same time. Peelability A bead pull-out test was conducted in a state where a bead with a tip of 1 mmR was pressed against a test piece with a width of 30 mm with a load of 200 kgf.
◎: No peeling of coating film ○: 5% or less of coating film peeling rate (no problem in practical use)
Δ: Coating film peeling rate over 5%, 50% or less (practical problem)
×: Coating film peeling rate of more than 50% (practical problem)
(5) Bending workability 3T bending is performed on the test piece kept at 20 ° C., and the bending part is observed at two places with a diameter of 2 mmφ using a 30-fold magnifier, and the number of cracks generated in the observation part. Was measured and evaluated according to the following criteria based on the total number.
◎: No crack ○: Number of cracks 1 to 3 △: Number of cracks 4 to 8 ×: Number of cracks 9 or more

As the various pigments shown in Tables 4 to 17, the following were used.
(1) Magnesium silicate compound ("Mg content" in the column of magnesium silicate compound in the table is the Mg content in the magnesium silicate compound)
・ Amorphous hydrous magnesium silicate compound with Mg content of 5.2 mass% ・ Amorphous hydrous magnesium silicate compound with Mg content of 3.8 mass%
(2) Dihydrogen tripolyphosphate Al
Untreated trihydrogen phosphate Al: “K-WHITE # 82” (trade name, manufactured by Teika)
-Mg treated (modified) dihydrogen tripolyphosphate Al: "K-WHITE G105" (trade name, manufactured by Teika)
・ Zn-treated (modified) dihydrogen tripolyphosphate Al: “K-WHITE K140W” (trade name, manufactured by Teika)
-Ca-treated (modified) dihydrogen tripolyphosphate Al: “K-WHITE Ca605” (trade name, manufactured by Teika)
(3) Calcium ion exchanged silica: “Sealdex C330” (trade name, manufactured by Grace)
(4) Zinc phosphate pigment (= silicate-modified zinc phosphate): “LF Bowsei D1” (trade name, manufactured by Kikuchi Color)
(5) Magnesium phosphate pigment (= magnesium phosphate-zinc phosphite mixed crystal type rust preventive pigment): “LE Bowsei MZP-500” (trade name, manufactured by Kikuchi Color)
(6) Zinc phosphite rust preventive pigment: “EXPERT NP-1500” (trade name, manufactured by Toho Pigment Industries)
(7) Strontium chromate (manufactured by Kikuchi Color)
(8) Titanium oxide: “Taipeke R820” (trade name, manufactured by Ishihara Sangyo)

Claims (2)

On the surface of the galvanized steel sheet, it has a chemical conversion treatment film, an undercoat film and a topcoat film from the lower layer side, and is a precoat steel sheet not containing chromium in the chemical conversion treatment film and the coating film,
The chemical conversion coating contains dry silica,
The undercoating film is a coating composition containing the following organic resin (b) or / and (b) and a rust-preventing pigment, wherein the magnesium silicate compound-based rust-preventive pigment ( A) 1.5 to 14% by mass, polyphosphate rust preventive pigment (B) 25 to 50% by mass, and the total content of all pigments including these rust preventive pigments (A) and (B) Corrosion resistance, moisture resistance, processability, and 65% by mass or less, characterized in that the total content (C) of Mg contained as part of the pigment is 4.0 to 10% by mass. An environmentally conscious pre-coated steel sheet with excellent peel resistance.
(A) A polyester resin (D) having a number average molecular weight of 12,000 to 26000 and a glass transition temperature of 0 to 30 ° C., an epoxy resin (E), and a curing agent (F) as main components, and the polyester resin (D) Organic resin having a ratio D / E (molar ratio) to the epoxy resin (E) of 1.0 / 1.0 to 1.0 / 4.0
(B) Modified polyester resin (G) obtained by reaction of polyester resin (D) having a number average molecular weight of 12,000 to 26000 and glass transition temperature of 0 to 30 ° C. and epoxy resin (E), and curing agent (F) An organic resin having a ratio D / E (molar ratio) between the polyester resin (D) and the epoxy resin (E) of 1.0 / 1.2 to 1.0 / 4.0. The content of dry silica in the chemical conversion coating is 20 to 70% by mass, and the silica adhesion amount is 10 to 100 mg / m ^2. The corrosion resistance, moisture resistance, workability and An environmentally conscious pre-coated steel sheet with excellent peel resistance.