JP2005290025A - Coating composition for precoated metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating composition for precoated metals which forms a coating film excellent in chemical resistance, flexibility, adhesiveness and the like. <P>SOLUTION: In the coating composition for precoated metals comprising a main component and a latent hardener, the latent hardener comprises a prepolymer (a) having an NCO-group terminal obtained by reacting TDI (1) with a polyester polyol (2) having an Mn=500-5,000 and a prepolymer (b) having an NCO-group terminal obtained by reacting TDI (1) with a low molecular weight polyol (3) having an Mn=less than 500. The latent hardener is a blocked isocyanate obtained by encapsulating the NCO group in a polyisocyanate mixture (A) having (a)/(b)=50/50-95/5 (mass ratio) with a blocking agent (B). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a coating composition for a precoat metal. More specifically, the present invention relates to a precoat metal coating composition that forms a coating film excellent in chemical resistance, flex resistance, adhesion and the like.

  Generally, pre-coated metal is pre-painted on galvanized iron plates and other metal plates, then molded into an arbitrary shape for final use. For example, home appliances such as refrigerators, washing machines, electric stoves, automatic sales, etc. Used in metal products such as furniture, office equipment, food display cases, etc. Compared to the post-coating method in which a pre-coating method is applied to form a complicated shape by first molding a metal plate, the painting process is streamlined, the quality is uniform, and the paint is consumed. Its use is expected to expand in the future due to advantages such as saving the amount.

  Since the paint applied to the pre-coated metal is molded into a shape according to the above-mentioned application after the coating film is formed, the coating film has sufficient resistance to withstand molding processes such as bending, roll molding, embossing press, and drawing. It is required to maintain flexibility and adhesion to metal surfaces. On the other hand, products after molding require performance suitable for each end use, such as high weather resistance and chemical resistance including processed parts in the case of building exterior materials. Scratch resistance and contamination resistance are required. In addition to these performances, durability such as gloss, water resistance, chemical resistance and moisture resistance is further required depending on the application. Conventionally, aminoalkyd resins, melamine-cured acrylic polyols or epoxy resins have been used in applications such as the above-mentioned home appliances. However, these resins have a drawback that the coating film is not easily folded and that the coating film cracks when bent at an angle of 90 ° or more.

  For example, a composition comprising a blocked product of bis (isocyanatomethyl) cyclohexane or an adduct thereof and a polyol resin, such as the invention described in Patent Document 1, is applied to a metal plate and heated and cured. A method for producing a pre-coated metal is also known, but the pre-coated metal obtained by this method does not always satisfy all of the above physical properties. Patent Document 2 also discloses a method for producing a pre-coated metal by applying a composition containing a blocked isocyanate compound blocked with ethyleneimine and a polyol resin to a metal plate, followed by heat curing. The pre-coated metal obtained by this method also did not satisfy all of the above physical properties.

JP 56-89548 A

JP 57-10375 A

  The coating film obtained by curing the coating composition for precoat metal of the present invention is excellent in chemical resistance, flex resistance and adhesion. For this reason, the coating composition for pre-coated metal of the present invention can be used for household appliances such as refrigerators, washing machines, electric stoves, furniture including vending machines, office equipment, food display cases, outer panels of automobiles, etc. it can.

  An object of this invention is to provide the coating composition for precoat metals which forms the coating film excellent in chemical-resistance, bending resistance, adhesiveness, etc.

  That is, this invention is shown by the following (1)-(5).

(1) In the coating composition for a precoat metal comprising a main agent and a latent curing agent, the latent curing agent reacts with toluene diisocyanate (a) and a polyester polyol (b) having a number average molecular weight of 500 to 5,000. The isocyanate group-terminated prepolymer (a) obtained, and the isocyanate group-terminated prepolymer (b) obtained by reacting toluene diisocyanate (a) with a low molecular polyol (ha) having a number average molecular weight of less than 500 (a) / (B) = Block isocyanate obtained by sealing an isocyanate group of a polyisocyanate mixture (A) of 50/50 to 95/5 (mass ratio) with a blocking agent (B). Paint composition.

(2) The coating composition for a precoat metal according to (1) above, wherein the acid component of the polyester polyol (b) is a mixed dicarboxylic acid of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid.

(3) The polyester polyol (b) is a mixed polyester polyol of a polyester polyol whose acid component is an aromatic dicarboxylic acid and a polyester polyol whose acid component is an aliphatic dicarboxylic acid, ) Coating composition for pre-coated metal.

(4) The molar ratio of the aromatic dicarboxylic acid structure to the aliphatic dicarboxylic acid structure in the polyester polyol (b) is aromatic dicarboxylic acid structure / aliphatic dicarboxylic acid structure = 20/80 to 80/20, The precoat metal coating composition according to any one of (1) to (3).

(5) The coating composition for a precoat metal according to any one of (1) to (4), wherein the main agent is a hydroxyl group-containing epoxy resin.

  The precoat metal coating composition of the present invention comprises a main agent and a latent curing agent. Since the curing agent uses blocked isocyanate, it has a potential, and normally does not exhibit curing performance, and can be supplied in a single liquid state for the painter.

  Examples of the main agent include a resin containing a hydroxyl group, specifically, polyurethane resin, polyamide resin, saturated or unsaturated polyester resin, alkyd resin modified with saturated fatty acid or unsaturated fatty acid, acrylic resin, fluorine resin, epoxy resin, A cellulose resin etc. are mentioned. However, in the pre-coated metal, the object to be coated is a metal plate and has heat resistance, and from the viewpoint of improving the efficiency of the painting work, it is generally performed by baking painting. Furthermore, it is important that the coating film has durability. For these reasons, a hydroxyl group-containing epoxy resin is preferred as the main agent.

  Specific examples of such hydroxyl group-containing epoxy resins include Japan Epoxy Resin “Epicoat” series, Asahi Denka Kogyo “Adeka Resin” series, Dainippon Ink and Chemicals “Epicron” series, and the like. A preferable hydroxyl value of the epoxy resin is 5 to 40 mgKOH / g in terms of solid content. Moreover, a preferable epoxy equivalent is 50-250 g / eq. When the hydroxyl value is too high or the epoxy equivalent is too small, the crosslink density becomes too high and the bending resistance of the coating film tends to decrease. When the hydroxyl value is too low or the epoxy equivalent is too large, the crosslinking density is too small and the chemical resistance of the coating film tends to decrease.

  The latent curing agent used in the present invention is an isocyanate group-terminated prepolymer (a) obtained by reacting toluene diisocyanate (i) with a polyester polyol (b) having a number average molecular weight of 500 to 5,000, and toluene diisocyanate ( (A) / (b) = 50/50 to 95/5 (mass ratio) comprising an isocyanate group-terminated prepolymer (b) obtained by reacting a) with a low molecular polyol (ha) having a number average molecular weight of less than 500 The blocked isocyanate obtained by sealing the isocyanate group of the polyisocyanate mixture (A) with the blocking agent (B). When (b) is too small, the strength and durability of the coating film tend to be insufficient, and when it is too large, the bending resistance of the coating film tends to be insufficient. Here, when hexamethylene diisocyanate polyisocyanate is used for both or one of (a) and (b), the coating strength and durability tend to be insufficient.

  The toluene diisocyanate (a) used in both the isocyanate group-terminated prepolymer (a) and the isocyanate group-terminated prepolymer (b) includes isomers such as 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. However, any ratio of isomer mixtures can be used in the present invention. Considering reactivity and the like, the content of 2,4-toluene diisocyanate is preferably 65% by mass or more, particularly preferably 80% by mass or more.

  The polyester polyol (b) used in the isocyanate group-terminated prepolymer (a) has a number average molecular weight of 500 to 5,000, preferably 1,000 to 3,000. When the number average molecular weight is too small, the coating film becomes too hard and the bending resistance is deteriorated. If it is too large, the coating film hardness and chemical resistance will deteriorate.

  This polyester polyol (b) is obtained by condensation from a dicarboxylic acid (or a derivative thereof) and a low molecular polyol having a number average molecular weight of less than 500. Here, as dicarboxylic acid, aromatic dicarboxylic acid such as phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, tartaric acid, oxalic acid, malonic acid, glutaric acid, pimelic acid, suberic acid, kurtaconic acid, azelaic acid, Examples thereof include aliphatic dicarboxylic acids such as sebacic acid, succinic acid, adipic acid, maleic acid and fumaric acid, and alicyclic dicarboxylic acids such as 1,4-cyclohexyl dicarboxylic acid. Low molecular polyols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol. 1,6-hexanediol, 2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, 1,8-octanediol, 1,9-nonanediol, decamethylene Glycol, diethylene glycol, dipropylene glycol, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2,4-trimethyl-1,3- Pentanediol, 2-ethyl-1,3-hexanediol, 2-n-hexadecane-1,2-ethylene Recall, 2-n-eicosane-1,2-ethylene glycol, 2-n-octacosane-1,2-ethylene glycol, cyclohexane-1,4-diol, cyclohexane-1,4-dimethanol, 3-hydroxy-2 , 2-dimethylpropyl-3-hydroxy-2,2-dimethylpropionate, dimer acid diol, hydrogenated bisphenol A, bisphenol A ethylene oxide or propylene oxide adduct, hydrogenated bisphenol A ethylene oxide or propylene oxide addition Products, trimethylolpropane, glycerin, hexanetriol, quadrol, pentaerythritol, sorbitol and the like. Further, it can be obtained by ring-opening polymerization of cyclic ester (so-called lactone) monomers such as ε-caprolactone, alkyl-substituted ε-caprolactone, δ-valerolactone, alkyl-substituted δ-valerolactone, etc., using the above-mentioned low molecular polyol as an initiator. Examples include lactone polyester polyols. Furthermore, it replaces with a part of low molecular polyol, and the polyamide ester polyol obtained using low molecular polyamines, such as hexamethylenediamine, isophoronediamine, and monoethanolamine, and low molecular amino alcohol.

  The polyester polyol (b) preferably uses an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid in combination in consideration of adhesion to a metal, flexibility of a coating film, and the like. As a form of combined use of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid, (1) the acid component is a mixed dicarboxylic acid of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid, and (2) the acid component is an aromatic dicarboxylic acid. Two forms of a mixed polyester polyol of a certain polyester polyol and a polyester polyol whose acid component is an aliphatic dicarboxylic acid are preferred. In any case, the molar ratio of the aromatic dicarboxylic acid structure to the aliphatic dicarboxylic acid structure in the polyester polyol (b) is aromatic dicarboxylic acid structure / aliphatic dicarboxylic acid structure = 20/80 to 80/20. Is preferred. When the aromatic ring content is too large, the bending resistance of the coating film tends to be insufficient, and when it is too small, the coating film strength and durability tend to be insufficient.

  Preferable aromatic dicarboxylic acids constituting the polyester polyol (b) are phthalic acid, isophthalic acid, and terephthalic acid, and isophthalic acid is particularly preferable. With phthalic acid, the molecular skeleton is insufficiently rigid, and the chemical resistance and heat resistance of the coating film may be insufficient. On the contrary, terephthalic acid is too rigid, and the bending resistance of the coating film may be insufficient. On the other hand, preferred aliphatic dicarboxylic acids are aliphatic dicarboxylic acids having 4 to 20 carbon atoms, and adipic acid, azelaic acid, and sebacic acid are particularly preferred. An aliphatic dicarboxylic acid having too few carbon atoms is likely to be thermally decomposed, so that the target polyester polyol itself may not be obtained. Moreover, what has too many carbon number may produce a tack in a coating film.

  The low molecular weight polyol constituting the polyester polyol (b) is preferably an aliphatic glycol having 2 to 10 carbon atoms, and in particular, 2-methyl-1,3-propanediol, neopentyl glycol, 2-n-butyl-2- A low molecular polyol containing 40 mol% or more of a side chain-containing aliphatic glycol selected from ethyl-1,3-propanediol is preferable because compatibility with the main agent, adhesion to a steel sheet, and the like are improved. In addition, when using a low molecular glycol that is easily sublimated, such as neopentyl glycol, when a low molecular glycol having a relatively low boiling point such as ethylene glycol is used, the polyester polyol is adhered to the inner wall surface of the reactor. Since the low molecular polyol is steam-washed and returned to the reaction system, it becomes efficient.

  The low molecular polyol (c) used in the isocyanate group-terminated prepolymer (b) has a number average molecular weight of less than 500. When the number average molecular weight is too large, the strength and durability of the coating film tend to be insufficient.

  Specific examples of the low molecular polyol (c) include low molecular polyols that constitute the polyester polyol (b). In the present invention, in view of the mechanical properties and durability of the coating film, the followability of the coating film necessary for bending the steel sheet after coating, etc., the combined use of glycol and triol is preferred, especially ethylene glycol, propylene glycol ( Glycerin, trimethylolpropane, hexanetriol (regardless of the type of isomer), butanediol (regardless of the type of isomer), glycol selected from diethylene glycol and dipropylene glycol, and glycerin, trimethylolpropane, hexanetriol A combination of triols selected from is preferred.

  The isocyanate group-terminated prepolymers (a) and (b) are generally in an atmosphere in which the isocyanate group is in excess of the hydroxyl group, preferably the equivalent ratio of isocyanate group / hydroxyl group is 1.5 to 50, usually 40 to 140 ° C., preferably Can be obtained by removing the unreacted tolylene diisocyanate by a thin film distillation method or an extraction method, etc., if necessary after performing a urethanization reaction at 70 to 100 ° C. In the urethanization reaction, an amine-based catalyst or a tin-based, lead-based, zinc-based, or iron-based organometallic catalyst may be used.

  In the reaction, an organic solvent can be used. This organic solvent is a solvent having no active hydrogen group used in the urethane industry (eg, aromatic solvents such as benzene, toluene and xylene, petroleum solvents such as Solvesso-100 and Solvesso-200, ethyl acetate and butyl acetate). Ester solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, ketone solvents such as cyclohexanone, ether solvents such as tetrahydrofuran, and the like.

  The mixing ratio of the isocyanate group-terminated prepolymers (a) and (b) is (a) / (b) = 50/50 to 95/5, preferably (a) / (b) = 50/50 to mass ratio. 90/10. When (b) is too much, the flexibility and followability of the coating film tends to be insufficient, and when it is too small, the coating film strength and durability are insufficient.

  In the present invention, the preferred isocyanate content (in terms of solid content) of the isocyanate group-terminated prepolymer (a) is 5 to 10% by mass, and the preferred isocyanate content (in terms of solid content) of the isocyanate group-terminated prepolymer (b) is 10 to 20%. % By mass. Moreover, the preferable isocyanate content (solid content conversion) of a polyisocyanate mixture (A) is 5-15 mass%.

  By blocking the isocyanate group of the polyisocyanate mixture (A) obtained as described above with a blocking agent (B), a blocked isocyanate as a latent curing agent used in the present invention is obtained.

  Examples of the blocking agent (B) include phenol, lactam, active methylene, alcohol, mercaptan, acid amide, imide, amine, imidazole, urea, carbamate, imine, and oxime. System, sulfite system and the like. In particular, phenol, oxime, lactam and imine are advantageously used. Specific examples of the blocking agent (B) include phenol, cresol, xylenol, nitrophenol, chlorophenol, ethylphenol, p-hydroxydiphenyl, t-butylphenol, o-isopropylphenol, o-sec-butylphenol, p-nonylphenol, phenolic blocking agents such as pt-octylphenol, hydroxybenzoic acid, hydroxybenzoic acid ester, etc., lactam blocking agents such as ε-caprolactam, δ-valerolactam, γ-butyrolactam, β-propiolactam, diethyl malonate, Active methylene blocking agents such as dimethyl malonate, ethyl acetoacetate, methyl acetoacetate, acetylacetone, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol , Isobutyl alcohol, t-butyl alcohol, n-amyl alcohol, t-amyl alcohol, lauryl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol Glycolic acid esters such as monomethyl ether, benzyl alcohol, methoxymethanol, glycolic acid, methyl glycolate, ethyl glycolate, butyl glycolate, lactic acid esters such as lactic acid, methyl lactate, ethyl lactate, butyl lactate, methylol urea, methylol melamine, Diacetone alcohol, ethylene chlorohydrin, ethylene bromohydrin, 1,3-dichloro- Alcohol-based blocking agents such as 2-propanol, ω-hydroperfluoroalcohol, and acetocyanhydrin, butyl mercaptan, hexyl mercaptan, t-butyl mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol, methylthiophenol, Mercaptan block agents such as ethylthiophenol, acetanilide, acetanisidide, acetolide, acid amide block agents such as acrylamide, methacrylamide, acetic acid amide, stearamide, benzamide, succinimide, phthalimide, maleic imide, etc. Imide blocking agent, diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butyraldehyde Amine block agents such as styrene, dibutylamine and butylphenylamine, imidazole block agents such as imidazole and 2-ethylimidazole, urea blocks such as urea, thiourea, ethyleneurea, ethylenethiourea and 1,3-diphenylurea Agents, carbamate block agents such as phenyl N-phenylcarbamate and 2-oxazolidone, imine block agents such as ethyleneimine and propyleneimine, formamidoxime, acetaldoxime, acetoxime, methylethylketoxime, diacetylmonooxime, benzophenone Examples thereof include oxime blocking agents such as oxime and cyclohexanone oxime, and sulfite blocking agents such as sodium bisulfite and potassium bisulfite.

  The reaction conditions for the blocking are as follows: the equivalent ratio of isocyanate groups to the blocking agent is isocyanate groups / active hydrogen groups = 0.8 to 1.2, preferably 0.95 to 1.05, and the reaction temperature is 40. It is -140 degreeC, Preferably it is 70-100 degreeC.

  Thereafter, additives can be used as necessary. Examples of the additive include catalysts, pigments, dyes, ultraviolet absorbers, antioxidants, fillers, flame retardants, plasticizers, antibacterial / antifungal agents, and dispersants. The curing agent thus obtained has latent reactivity and does not react at room temperature, but the blocking agent dissociates from the isocyanate group sealed by heating to regenerate the isocyanate group. An isocyanate group reacts with a hydroxyl group or an epoxy group in the main agent to give a cured coating film.

  In the precoat metal to which the coating composition for precoat metal of the present invention is applied, the metal plate used is not particularly limited as long as it is usually used as a metal for precoat metal. For example, cold-rolled steel plate, galvanized steel plate, alloy Examples thereof include zinc-plated steel sheets, tin-plated steel sheets, chromium-plated steel sheets, aluminum-plated steel sheets, lead-plated steel sheets, nickel-plated steel sheets, aluminum plates, titanium plates, and stainless steel plates. The shape of the metal plate may be any of flat plate shape, cylindrical shape, and the like.

  As a coating method using the coating composition for a precoat metal of the present invention, the above-mentioned metal plate is applied directly or after being pretreated, and then heated and cured. The coating amount is not limited and can be determined freely. However, an amount that provides a dry film thickness of 10 to 30 μm is preferable. Examples of the application means include a spray gun, a roll coater, and a flow coater. Although heating temperature changes with kinds etc. of a blocking agent, it is about 150-350 degreeC, and the time is about about 20-120 second. By this heating operation, the blocking agent contained in the blocked isocyanate is dissociated to regenerate the isocyanate group, and this regenerated isocyanate group reacts with a hydroxyl group or an epoxy group in the main agent to cause crosslinking, resulting in a tough coating film. Become. Examples of the pretreatment of the metal plate include primer coating, chromate chemical conversion treatment, phosphate chemical conversion treatment, and composite oxide film treatment. As the primer, those usually used such as epoxy resin and high molecular polyester can be used. The chromate conversion treatment includes electrolytic chromate, coating chromate, and reactive chromate treatment, the phosphate chemical conversion treatment includes zinc phosphate treatment and iron phosphate treatment, and the composite oxide film treatment includes treatment containing nickel and cobalt. is there.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to these. In Examples and Comparative Examples, unless otherwise specified, the ratio is a mass ratio, and “%” is “mass%”.

[Production of polyester polyol]
Synthesis example 1
Capacity with stirrer, thermometer, nitrogen seal tube, and condenser: 3 L reactor was charged with 60.8 g of ethylene glycol, 917.8 g of neopentyl glycol, 438.5 g of isophthalic acid, 899.9 g of adipic acid, and under normal pressure And after reaction at 200 ° C. When the condensed water stops distilling, 0.02 g of tetrabutyl titanate is added, the internal temperature is increased to 220 ° C., and the internal pressure is gradually reduced to 0.7 kPa to further advance the reaction, thereby further increasing the polyester polyol PES. -1 was obtained. The hydroxyl value of PES-1 was 55.6 mgKOH / g, and the acid value was 0.5 mgKOH / g.

Synthesis Examples 2-5, 7
The raw materials shown in Table 1 were charged in the same reactor as in Synthesis Example 1, and PES-2 to 5 and 7 were obtained in the same procedure as in Synthesis Example 1.

Synthesis Example 6
The same reactor as in Synthesis Example 1 was charged with 62.4 g of ethylene glycol, 941.4 g of neopentyl glycol, and 1322.4 g of adipic acid, and reacted at 180 ° C. under normal pressure. When the condensed water stops distilling, 0.02 g of tetrabutyl titanate is added, the internal temperature is increased to 200 ° C., and the internal pressure is gradually reduced to 1.0 kPa to further advance the reaction to further increase the polyester polyol PES. -6 was obtained. PES-6 had a hydroxyl value of 55.8 mgKOH / g and an acid value of 0.3 mgKOH / g.

In Table 1, EG: ethylene glycol NPG: neopentyl glycol HD: 1,6-hexanediol iPA: isophthalic acid AA: adipic acid TBT: tetrabutyl titanate

[Production of low molecular polyol-modified prepolymer]
Synthesis example 8
Capacity with stirrer, thermometer, nitrogen seal tube and condenser: 2L reactor, TDI-1 (2,4-tolylene diisocyanate / 2,6-tolylene diisocyanate = 80/20 (mass ratio) 900 g, 45 g of trimethylolpropane, 16 g of glycerin, 10 g of 1,3-butanediol, and 29 g of dipropylene glycol were reacted at 70 ° C. for 3 hours. Thereafter, thin film distillation was performed under conditions of 140 ° C. and 1 Pa to remove unreacted raw material diisocyanate, and then the solid content was diluted to 75% with ethyl acetate to obtain a low molecular polyol-modified polyisocyanate NCO-1. The isocyanate content of NCO-1 was 13.5%.

Synthesis Example 9
Capacity with stirrer, thermometer, nitrogen seal tube and condenser: 2 L reactor was charged with 900 g of HDI and 8.5 g of 1,3-butanediol and reacted at 80 ° C. for 2 hours. Thereafter, 0.2 g of potassium caprate and 1 g of phenol as a co-catalyst were added, and an isocyanuration reaction was performed at 60 ° C. for 6 hours. Then, 0.13 g of phosphoric acid was added and stirred at the reaction temperature for 1 hour, and isocyanurate. The reaction was stopped. Thereafter, thin film distillation was performed under the conditions of 120 ° C. and 1 Pa to remove unreacted raw material diisocyanate to obtain low molecular polyol-modified polyisocyanate NCO-2. The isocyanate content of NCO-2 was 21.2%.

[Production of blocked isocyanate]
Example 1
Capacity with stirrer, thermometer, nitrogen seal tube, and cooler: 2L reactor was charged with 89.3g of TDI-1 and adjusted to 50 ° C, then 293.4g of PES-1 was charged to 80 ° C. For 3 hours to obtain a polyester-modified prepolymer PR-1. The isocyanate content of PR-1 was 8.0%. Next, 340.2 g of NCO-1 was charged and stirred uniformly. The isocyanate content of this mixed polyisocyanate was 10.6%. Next, 75 g of xylene was charged and the internal temperature was adjusted to 70 ° C., then 162.1 g of methyl ethyl ketoxime was gradually added dropwise into the reactor so that no sudden heat generation occurred. After completion of the dropwise addition, the mixture was further reacted at 60 ° C. for 1 hour, and then charged with 40 g of cyclohexanone and stirred uniformly to obtain blocked isocyanate BI-1. The solid content of BI-1 was 80.0%, and the effective isocyanate content was 7.7%.

Examples 2-12, Comparative Examples 1-5
Raw materials shown in Tables 2 to 4 were charged into the same reactor as in Example 1, and BI-2 to 17 were obtained in the same procedure as in Example 1.

In Examples 1 to 12, Comparative Examples 1 to 5, and Tables 2 to 4, PES-1 to 7:
See Synthesis Examples 1-7 above TDI-1:
Mixture of 2,4-tolylene diisocyanate / 2,6-tolylene diisocyanate = 80/20 (mass ratio) TDI-2:
Mixture HDI of 2,4-tolylene diisocyanate / 2,6-tolylene diisocyanate = 99/1 (mass ratio):
Hexamethylene diisocyanate TMP:
Trimethylolpropane NCO-1 and 2:
See Synthesis Examples 8-9 above MEKO:
Methyl ethyl ketoxime low temperature storage stability:
The produced blocked isocyanate is put into a 100 ml glass bottle, sealed and placed in an atmosphere at 0 ° C. for 1 week, and then evaluated by the appearance.
○: No change in appearance △: Turbidity is observed ×: Crystals and suspended matter are observed

  From Examples 1 to 12, Comparative Examples 1 to 5, and Tables 2 to 4, the blocked isocyanates of the examples had good low-temperature storage stability, but in the comparative examples, those other than Comparative Example 2 were not so good. . It was found that when the mixing ratio of the polyester prepolymer (a) and the low molecular polyol prepolymer was outside the appropriate range, the low-temperature storage stability deteriorated.

[Coating film evaluation]
Examples 13-24, Comparative Examples 5-10
A baking paint was prepared with the following composition. This paint was applied to a steel sheet (Partec, SPCC-SB, PF-1077 treatment) with a coating thickness of 50 μm (wet) and baked at 200 ° C. for 15 minutes to obtain a coating sample. The results are shown in Tables 5-6.
Baking paint formulation Resin for main agent:
Epicron P-439 (Epoxy resin manufactured by Dainippon Ink and Chemicals)
Solid content = 40%
Hydroxyl value = 8.7 mgKOH / g (Applicant analysis value)
Pigment:
Titanium White Diluent:
Propylene glycol monomethyl ether acetate (PMA)
Main agent / curing agent combination:
Formulated so that the hydroxyl groups in the base resin and the isocyanate groups in the curing agent are equivalent.
Resin (main agent + curing agent, solid content) / pigment / solvent (additional component + main component resin component + curing agent component)
= 24/16/60 (mass ratio)

Acid resistance in Tables 5-6:
Drip one drop of 10% hydrochloric acid on the painted surface, and then cover with a cover glass. In this state, the coated surface after being allowed to stand for 24 hours in a room temperature atmosphere was observed and evaluated.
○: No change in appearance △: “Roughness” is observed on the painted surface ×: Bending resistance allowing the substrate to be confirmed:
With the painted surface facing up, the painted plate is folded 180 degrees, and the bent portion is tightened with a vise. Thereafter, the coated plate was stretched (open the bent portion), and the bent portion was observed and evaluated.
○: no change in appearance ×: “cracking” of the coating film can be confirmed

From Table 6, the low molecular weight polyol-modified prepolymer (b) having a large amount of poor flex resistance, the one not using (b), the one using HDI in (a) or (b) has poor acid resistance. It turns out that it is enough.

Claims (5)

  Isocyanate obtained by reacting toluene diisocyanate (a) and polyester polyol (b) having a number average molecular weight of 500 to 5,000 in the coating composition for precoat metal comprising a main agent and a latent curing agent. (A) / (b) comprising a group-terminated prepolymer (a) and an isocyanate group-terminated prepolymer (b) obtained by reacting toluene diisocyanate (a) with a low molecular polyol (ha) having a number average molecular weight of less than 500. = Block isocyanate obtained by sealing an isocyanate group of a polyisocyanate mixture (A) of 50/50 to 95/5 (mass ratio) with a blocking agent (B), a coating composition for precoat metal .   The coating composition for precoated metals according to claim 1, wherein the acid component of the polyester polyol (b) is a mixed dicarboxylic acid of an aromatic dicarboxylic acid and an aliphatic dicarboxylic acid.   The polyester polyol (b) is a mixed polyester polyol of a polyester polyol whose acid component is an aromatic dicarboxylic acid and a polyester polyol whose acid component is an aliphatic dicarboxylic acid. Pre-coating metal coating composition.   The molar ratio of the aromatic dicarboxylic acid structure to the aliphatic dicarboxylic acid structure in the polyester polyol (b) is aromatic dicarboxylic acid structure / aliphatic dicarboxylic acid structure = 20/80 to 80/20, Item 4. The precoat metal coating composition according to any one of Items 1 to 3. The coating composition for precoated metals according to any one of claims 1 to 4, wherein the main agent is a hydroxyl group-containing epoxy resin.