JP5751656B2 - Aqueous urethane resin composition - Google Patents

Description

  The present invention relates to a water-based urethane resin composition excellent in stability and a one-component water-based coating composition containing the same.

  Urethane resins are frequently used as binders particularly in the coating field because they can form a film excellent in stretchability and strength. However, the two-component urethane resin binder used by mixing the isocyanate compound and the polyol compound at the time of use may cause fogging due to isocyanate, or the physical properties of the binder may greatly vary if the mixing ratio is different. There was a bad handling. In addition, since the isocyanate compound reacts with water and deactivates, an organic solvent is often used as a diluting solvent, and there is a problem that VOC is generated.

  Therefore, an aqueous coating agent and an aqueous paint using a one-pack type urethane resin binder that can solve these problems have been proposed.

  Patent Document 1 describes a self-crosslinking aqueous coating composition comprising an aqueous dispersion containing at least one polyurethane polymer, and has a hydrazine (or hydrazone) functionality so as to give a self-crosslinking reaction. It has a group and a carbonyl functional group.

  Here, the polyurethane polymer has a hydrazine (or hydrazone) functional group and / or a carbonyl functional group as a chain pendant group. As a synthesis method of such a polyurethane polymer, it is described that the reaction of polyisocyanate and polyol is used, but the polyol exemplified here is polyester, polyesteramide, polyether, polythioether, polycarbonate, polyacetal, polyolefin or poly Siloxane.

  Although diacetone acrylamide is described as the carbonyl compound, an isocyanate-terminated polyurethane prepolymer obtained by reacting with an organic polyol using a carbonyl-functional diamino compound obtained by Michael addition with a diamine as an active hydrogen-containing chain extender; Reacted and incorporated into polyurethane polymer. There is no description of using a vinyl polymer as a polyol, and the vinyl polymer is used as a cross-linked functional group-carrying substance used by mixing with a polyurethane polymer.

Japanese Patent Laid-Open No. 2-6572

  Patent Document 2 discloses an aqueous paint characterized in that a resin main component is a composition obtained by adding a hydrazide compound having at least two hydrazine residues per molecule to a carbonyl group-containing copolymer aqueous dispersion emulsion. It is disclosed. Here, an acrylic emulsion obtained by copolymerizing an unsaturated monomer having a carbonyl functional group and another unsaturated monomer is used, and it is not a urethane resin system.

Japanese Patent Laid-Open No. 5-43821

  Patent Document 3 discloses a self-crosslinking polyurethane-vinyl-hybrid aqueous dispersion. This is due to free radical initiation polymerization of a vinyl group-containing urethane macromonomer in one place with another vinyl monomer, and at that time, using a monomer containing a carbonyl group in at least a part of the other vinyl monomer. Yes. A self-crosslinking aqueous binder is obtained from the carbonyl group-containing urethane-vinyl-hybrid polymer thus obtained and polyhydrazides.

JP 7-233347 A

  Patent Document 4 discloses an organic polyisocyanate compound, a polymer polyol, a neutralized product of an isocyanate group-terminated prepolymer obtained from a compound having an anionic hydrophilic group and at least two active hydrogens in the molecule, as a water-soluble polyamine and hydrazine. An aqueous acrylic-urethane copolymer composition obtained by chain extension in water using at least one chain extender selected from derivatives is disclosed. Here, various (meth) acrylate, (meth) acrylic acid, acrylonitrile, vinyl acetate, vinyl pyridine, vinyl pyrrolidone, methyl croto as monomers to be copolymerized with the monomer having hydroxyl group which is an essential component as acrylic polyol Examples thereof include ethylenic monomers such as narate, maleic anhydride, styrene, and α-methylstyrene.

  Examples of the compound having an anionic hydrophilic group and at least two active hydrogens in the molecule include dimethylolpropionic acid and dimethylolbutanoic acid, and hydrazine is exemplified as an essential chain extender. . The aqueous acrylic-urethane copolymer composition disclosed herein uses a crosslinking agent reactive with a carboxyl group, such as an aqueous oxazoline crosslinking agent, an aqueous (poly) carbodiimide crosslinking agent, an aqueous epoxy resin crosslinking agent, and the like. Therefore, it is stated that it is possible to impart further excellent water resistance.

JP 2000-119361 A

  As described above, many water-based urethane resin compositions are publicly known, but are urethane resins having a carbonyl functional group using an acrylic polyol that is relatively easy to produce, by combining with a hydrazine (or hydrazone) compound. An aqueous resin composition having self-crosslinking property has not been found.

  An aqueous urethane resin composition that is excellent in physical properties such as elongation and flexibility and can be cured at room temperature when combined with a crosslinking agent, and a one-pack type aqueous coating composition containing the composition are obtained.

  As a result of intensive studies, the inventors have found that a urethane resin synthesized from an acrylic polyol obtained by copolymerizing an unsaturated monomer having a carbonyl functional group and another unsaturated monomer and a polyisocyanate compound can solve the problem. The present invention has been completed.

Thus, the present invention provides 1. Acrylic obtained by copolymerizing (A) an unsaturated monomer having a carbonyl functional group, an unsaturated monomer having a hydrophilic group, and, if necessary, other unsaturated monomers when the total amount of starting materials is 100 parts by weight a polyol 50-90 parts by weight, and (B) below aliphatic diisocyanates, selected from the group consisting of alicyclic diisocyanates and aromatic diisocyanates, a diisocyanate compound 10-50 parts having no parent-aqueous polar group A urethane resin synthesized by the above reaction is dispersed in an aqueous medium, wherein the acrylic polyol has a molecular weight of 400 or more and 10,000 or less, and an acid value of 10 mgKOH / g or more. 80 mg KOH / g or less, aqueous hydroxyl value of 15 mg KOH / g or more and 180 mg KOH / g or less Fat composition.
Aliphatic diisocyanates: hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, lysine diisocyanate
Alicyclic diisocyanates: isophorone diisocyanate, 4,4′-methylenebis (cyclohexyl isocyanate), methylcyclohexane-2,4- (or-2,6-) diisocyanate, 1,3- (or 1,4-) di ( Isocyanatomethyl) cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate
Aromatic diisocyanates: xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4 ′ -Toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, (m- or p-) phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis (4-Isocyanatophenyl) sulfone, isopropylidenebis (4-phenylisocyanate)
2. Item 2. The aqueous resin composition according to Item 1, wherein the unsaturated monomer having a carbonyl group used in the acrylic polyol is diacetone acrylamide or diacetone methacrylamide.
3. Item 3. The aqueous resin composition according to any one of Items 1 and 2, wherein the urethane resin has an acid value of 5 mgKOH / g or more and 50 mgKOH / g or less.
4). Item 4. The aqueous resin composition, wherein the urethane resin according to any one of Items 1 to 3, the hydrazine compound and / or the hydrazide compound are dispersed in an aqueous medium.
5. Item 5. A one-component aqueous coating composition comprising the aqueous resin composition according to any one of items 1 to 4.
6). Item 6. A coated article in which the one-component aqueous coating composition according to Item 5 is coated on a material that may be previously coated with an undercoat.

  According to the present invention, an aqueous urethane resin composition that is excellent in physical properties such as elongation and flexibility and can be cured at room temperature when combined with a crosslinking agent, and a one-component aqueous coating composition containing the composition are provided. obtain.

  The present invention is an aqueous resin composition characterized in that a urethane resin synthesized from an acrylic polyol and a polyisocyanate compound is dispersed in an aqueous medium. Hereinafter, the acrylic polyol and polyisocyanate compound as raw materials will be described.

Acrylic polyol
The preferred molecular structure of the acrylic polyol used as the raw material for the urethane resin of the present invention is as follows.
(A) There are two or more hydroxyl groups in one molecule of acrylic polyol and a number that is not excessive. When the number of hydroxyl groups is less than 2, even if it reacts with the polyisocyanate compound, the chain extension of the polyurethane resin is insufficient and a stable aqueous dispersion may not be obtained. Further, if there is an excessive amount such as 5 or more per molecule, there is a high possibility that three-dimensional crosslinking occurs when the polyisocyanate compound reacts and the resin is insolubilized by gel formation.

Preferred is an acrylic polyol having 2 to 4 hydroxyl groups in one molecule, and the most preferred molecular structure is acrylic diol.
(B) The position of the hydroxyl group in the acrylic polyol molecule may or may not be at the end of the molecule (for example, near the center of the molecular chain), but is preferably at the end in terms of reactivity with the polyisocyanate. When a hydroxyl group near the center of the molecular chain reacts with an isocyanate group to form a urethane bond, the structure of the acrylic molecular chain extends on both sides of the urethane bond, so the stability of the aqueous dispersion may be insufficient .

  Means for obtaining acrylic polyols having the preferred molecular structures listed in (a) and (b) will be described below.

i. Method of using methacrylate ester dimer having hydroxyl group as chain transfer agent 2,4-disubstituted-4-methyl-1-pentene having a structure represented by the following formula (1) is a dimer of methacrylate ester It is known that it acts as an addition-cleavage chain transfer agent in a radical polymerization reaction system, and an unsaturated monomer (mixture thereof) is mainly composed of a methacrylic ester (70 mol% or more of the total amount). A polymer having a structure represented by the following formula (2) at the molecular terminal is obtained by polymerizing by a solution polymerization method in an organic solvent in the presence of 4-disubstituted-4-methyl-1-pentene and a radical polymerization initiator. It is known that a chiral polymer is obtained as the main product.
Therefore, by using 2,4-disubstituted-4-methyl-1-pentene having a hydroxyl group as the substituent R in the following formula (1), and containing no monomer having a hydroxyl group in the unsaturated monomer mixture to be polymerized, A telechelic acrylic diol of the following formula (2) is obtained as the main product.

[Formula 1]

[Formula 2]

The 2,4-disubstituted-4-methyl-1-pentene used here is a methacrylic acid having a hydroxyl group by a catalytic chain transfer polymerization method (hereinafter sometimes abbreviated as “CCTP method”). It can be efficiently obtained by fractional distillation of an oligomer obtained by copolymerizing an ester.
In the present invention, a methacrylic acid ester having a hydroxyl group as a raw material for 2,4-disubstituted-4-methyl-1-pentene having a hydroxyl group at the substituent R in the formula (1), for example, hydroxyethyl methacrylate, hydroxymethacrylate Propyl, hydroxybutyl methacrylate and the like can be preferably used.
The above-mentioned CCTP method is disclosed in, for example, JP-B-6-23209, JP-B-7-35411, JP-T 9-501457, JP-A-9-176256, Macromolecules 1996, 29, 8083-8089, and the like. This is a method for polymerizing a polymerizable unsaturated monomer in the presence of a metal complex as a catalytic chain transfer agent and a radical polymerization initiator in the absence of a solvent or in an organic solvent.

Examples of the metal complex include a cobalt complex, an iron complex, a nickel complex, a ruthenium complex, a rhodium complex, a palladium complex, a rhenium complex, and an iridium complex. Among these, a cobalt complex is an efficient chain transfer agent. This is preferable because it works.
Examples of the cobalt complex include Japanese Patent Publication No. 6-23209, Japanese Patent Publication No. 7-35411, US Pat. No. 4,526,945, US Pat. No. 4,694,054, US Pat. No. 4,837,326, US Pat. No. 4,886,861, US Pat. No. 5,324,879, WO 95/17435, Those described in JP-T-9-510499 can be used. Specifically, for example, bis (borondifluorodimethyldioxyiminocyclohexane) Co (II), bis (borondifluorodimethylglyoxymate) Co (II), bis (borondifluorodiphenylglyoxymate) Co (II), Cobalt (II) chelate of bicinalimino hydroxyimino compound, cobalt (II) chelate of tetraazatetraalkylcyclotetradecatetraene, N, N'-bis (salicylidene) ethylenediaminocobalt (II) chelate And cobalt (II) chelate of dialkyldiazadioxodialkyldodecadiene, cobalt (II) porphyrin complex, and the like. Of these, bis (borondifluorodiphenylglyoxymate) Co (II), bis (borondifluorodimethylglyoximate) Co (II), and the like are easily available and suitable.

  Further, for example, a complex described in Japanese Patent Publication No. 8-19172 in which a group capable of direct radical cleavage is bonded to a metal can be used instead of the above metal complex. Examples of the group capable of radical cleavage include an alkyl group, an aryl group, and a heterocyclic group. In addition, substituted derivatives that can be homolytically dissociated from metal ions by irradiation with visible light or ultraviolet light or heating; halides bonded to chelate metal ions; substituted with carbon atoms bonded to other anions, nitriles, esters, metal ions An aromatic or substituted aromatic group etc. are mentioned. In a system using a complex in which a group capable of direct radical cleavage is bonded to these metals, a radical polymerization initiator may not be used in combination.

ii. Method using a thiol compound having a hydroxyl group as a chain transfer agent A thiol compound (general formula: R—S—H, R represents a hydrocarbon group and may have various functional groups) is a chain in a radical polymerization reaction system. It is a compound that is widely used industrially as a transfer agent. An unsaturated monomer is polymerized by a solution polymerization method in an organic solvent in the presence of a thiol compound and a radical polymerization initiator. A polymer having a structure of —S—R (wherein P represents a polymer terminal of a polymerizable unsaturated monomer and R represents a hydrocarbon group derived from a thiol compound) is obtained as a main product. Are known. Therefore, by using a thiol compound having a hydroxyl group as the substituent R, a polymer having a hydroxyl group at one end can be obtained as a main product. By adding a monomer having a hydroxyl group to the unsaturated monomer (mixture) to be polymerized at this time, a hydroxyl group can be introduced in addition to the end of the molecular chain (for example, near the center of the molecular chain). By adjusting the blending amount of the monomer having a hydroxyl group, the total amount of hydroxyl groups in one molecule of acrylic polyol can be adjusted from 2 to 4.

  As the thiol compound used in the present method, any thiol compound having a hydroxyl group can be suitably used without limitation. Among them, 2-mercaptoethanol is most easily available and easy to use.

  Typically, the acrylic polyol of the present invention synthesized by the above method i or ii uses an unsaturated monomer having a carbonyl functional group, an unsaturated monomer having a hydrophilic group, and other unsaturated monomers as required.

  Examples of unsaturated monomers having a carbonyl group include acrolein, diacetone acrylamide, diacetone methacrylamide, formyl styrene, vinyl alkyl ketone having 4 to 7 carbon atoms (for example, vinyl methyl ketone, vinyl ethyl ketone, vinyl butyl ketone), etc. Is mentioned. Particularly preferred are diacetone acrylamide and diacetone methacrylamide.

As unsaturated monomers having a hydrophilic group, monomers having an anionic hydrophilic group such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, β-acryloyloxypropionic acid, β-carboxyethyl acrylate, vinyl sulfonic acid , Styrenesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid Such as maleic anhydride, fumaric anhydride, itaconic anhydride and citraconic anhydride, which are monomers having such a potential anionic hydrophilic group, which are regenerated by half-esterification with a hydroxyl group-containing compound,
Monomers having nonionic hydrophilic groups such as acrylamide, methacrylamide, N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, methoxypolyethylene glycol (meth) acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine, etc.
N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylamino which are monomers having a cationic hydrophilic group Propyl (meth) acrylamide and quaternary salts thereof can be combined as necessary.

Other monomers include hydroxyl-containing monomers such as 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, and hydroxybutyl (meth) acrylate. _C 2 -C ₈ hydroxyalkyl (meth) acrylate acid, allyl alcohol, having a hydroxyl group such as ε- caprolactone modified products of the above _C 2 -C ₈ hydroxyalkyl (meth) acrylate (meth) acrylate; molecular end with hydroxyl groups (Meth) acrylate having a certain polyoxyethylene chain and other monomers, for example, methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n- Til (meth) acrylate, i-butyl (meth) acrylate, tert-butyl (meth) acrylate, n-hexyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, nonyl (meth) acrylate, Tridecyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, isostearyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, cyclododecyl ( Alkyl) such as meth) acrylate or cycloalkyl (meth) acrylate; polymerizable unsaturated compound having an isobornyl group such as isobornyl (meth) acrylate; adamantyl (meth) Polymerizable unsaturated compounds having an adamantyl group such as acrylate; vinyl aromatic compounds such as styrene, α-methylstyrene and vinyl toluene; perfluoro such as perfluorobutylethyl (meth) acrylate and perfluorooctylethyl (meth) acrylate Alkyl (meth) acrylate; polymerizable unsaturated compound having a fluorinated alkyl group such as fluoroolefin; vinyl compound such as ethylene, butadiene, chloroprene, vinyl propionate, vinyl acetate; glycidyl (meth) acrylate, β-methylglycidyl ( (Meth) acrylate, 3,4-epoxycyclohexylmethyl (meth) acrylate, 3,4-epoxycyclohexylethyl (meth) acrylate, 3,4-epoxycyclohexylpropyl (meth) acrylate An epoxy group-containing polymerizable unsaturated monomers such as allyl glycidyl ether, may be used.

  These monomers are appropriately combined in order to adjust the resin physical properties according to the intended use of the composition of the present invention.

  In order to impart water dispersibility to the urethane resin, it is necessary to include a monomer having a hydrophilic group. Among the monomers having a hydrophilic group, it is preferable to include a monomer having an anionic hydrophilic group, particularly a monomer having a carboxyl group, and acrylic acid and methacrylic acid. As the amount thereof, the acid value of the acrylic polyol is 100 mgKOH / g or less. It is preferable to use it as described above, and more preferably in the range of 10 to 80 mgKOH / g. When the acid value of the acrylic polyol is higher than 100 mgKOH / g, when the urethane resin aqueous dispersion is used, the dispersed particles may swell and the stability may be deteriorated.

  It is convenient to dilute the acrylic polyol in an appropriate amount of solvent to obtain a varnish in order to facilitate the synthesis reaction in the subsequent synthesis of the urethane resin. As the diluting solvent used at this time, a polar solvent is particularly preferable in order to be inactive with respect to the isocyanate and to uniformly disperse in water without causing any fouling during subsequent water dispersion. Examples of such solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, N-methylpyrrolidone, ethyl acetate, tetrahydrofuran, dimethylformamide, dioxane and the like.

Polyisocyanate compound Examples of the polyisocyanate compound used in the present invention include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; and burette type adducts of these diisocyanates, isocyanurate rings. Adducts: isophorone diisocyanate, 4,4′-methylenebis (cyclohexyl isocyanate), methylcyclohexane-2,4- (or-2,6-) diisocyanate, 1,3- (or 1,4-) di (isocyanatomethyl) Cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate, etc. Cyanates; and bullet type adducts, isocyanurate cycloadducts of these diisocyanates; xylylene diisocyanate, metaxylylene diisocyanate, tetramethyl xylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4′-toluidine diisocyanate, 4,4′-diphenyl ether diisocyanate, (m- or p-) phenylene diisocyanate, 4,4′-biphenylene Aromatic dii such as diisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate, bis (4-isocyanatophenyl) sulfone, isopropylidenebis (4-phenylisocyanate) Cyanate compounds; and bullet type adducts of these diisocyanates, isocyanurate cycloadducts; triphenylmethane-4,4 ′, 4 ″ -triisocyanate, 1,3,5-triisocyanatobenzene, Polyisocyanates having three or more isocyanate groups in one molecule such as 2,4,6-triisocyanatotoluene, 4,4′-dimethyldiphenylmethane-2,2 ′, 5,5′-tetraisocyanate; and Burette type adducts, isocyanurate cycloadducts of these polyisocyanates; hydroxyl groups of polyols such as ethylene glycol, propylene glycol, 1,4-butylene glycol, dimethylolpropionic acid, polyalkylene glycol, trimethylolpropane and hexanetriol Isocyanate group Urethane adduct obtained by reacting a polyisocyanate compound in a ratio comprising an excess; and biuret type adducts of these polyisocyanates, isocyanurate ring adducts; and the like, generally known compounds that can be used.

  Among them, aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; isophorone diisocyanate, 4,4′- because it does not cause gelation due to three-dimensional crosslinking in the urethanization reaction with acrylic polyol. Methylene bis (cyclohexyl isocyanate), methylcyclohexane-2,4- (or-2,6-) diisocyanate, 1,3- (or 1,4-) di (isocyanatomethyl) cyclohexane, 1,4-cyclohexane diisocyanate, 1, Alicyclic diisocyanates such as 3-cyclopentane diisocyanate and 1,2-cyclohexane diisocyanate; xylylene diisocyanate, metaxylylene diisocyanate , Tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4′-toluidine diisocyanate, 4,4 '-Diphenyl ether diisocyanate, (m- or p-) phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis (4-isocyanatophenyl) sulfone, isopropylidene It is preferable to use diisocyanate compounds such as aromatic diisocyanates such as bis (4-phenylisocyanate).

Synthesis of urethane resin by acrylic polyol and polyisocyanate The synthesis of polyurethane resin is not particularly limited, and a conventionally known method can be adopted. For example, acrylic polyol and polyisocyanate compound may be reacted at once, You may make it react. The synthesis of the polyurethane resin is conveniently performed in an organic solvent inert to the isocyanate group. The ratio of the acrylic polyol and polyisocyanate compound used can be variously changed, but the equivalent ratio of the hydroxyl group and isocyanate group of the acrylic polyol is generally 0.3: 1 to 3.3: 1, preferably 0.4: 1. It is desirable that the ratio be ˜1.6: 1.

  If the amount of hydroxyl group is less than 0.3: 1, the isocyanate at the end becomes excessive, and an amine is produced while generating carbon dioxide gas by reaction with water during the production of an aqueous dispersion in the latter stage. Generation of carbon dioxide gas impairs manufacturability during water dispersion due to foaming. Moreover, the resin terminal amine works competitively with the basic substance added as a neutralizing agent in neutralizing the anionic hydrophilic group preferably contained in the acrylic polyol during the water dispersion operation. Thereby, a water dispersion state may become unstable.

If the amount of hydroxyl group is larger than 3.3: 1, the excess hydroxyl component that does not react increases, which may reduce the stability during the production of the aqueous resin dispersion. Moreover, the molecular weight of the urethane resin being low may also cause the water dispersion state to become unstable.
The synthesis reaction of the polyurethane resin is usually performed at a temperature of 40 to 180 ° C, preferably 60 to 130 ° C. In order to promote this reaction, amine-based catalysts such as triethylamine, N-ethylmorpholine, and triethylenediamine used in ordinary urethanization reactions and tin-based catalysts such as dibutyltin dilaurate and dioctyltin dilaurate are used as necessary. It may be used.

  The polyurethane resin thus produced preferably has an acid value of 5 mgKOH / g or more and 50 mgKOH / g or less, preferably 10 mgKOH / g or more and 40 mgKOH / g or less from the viewpoint of water dispersibility. When the acid value is less than 5 mgKOH / g, water dispersibility may be insufficient, and when it exceeds 50 mgKOH / g, water resistance may be insufficient when a coating is formed.

Dispersion of polyurethane resin The polyurethane resin is dispersed in an aqueous medium. The aqueous medium is a medium containing water as a main component, and may contain an ionic water-soluble component such as a polar solvent or a low molecular weight amine as a secondary component of approximately 25% by weight or less based on the total amount of the medium.

  Dispersion in an aqueous medium can be carried out by a conventionally known method without any particular limitation. For example, it is carried out by neutralizing an acidic group contained in the polyurethane resin with a basic compound, and then gradually adding water with vigorous stirring to convert it into an aqueous dispersion. At this time, a forced emulsification method can also be used. For example, the polyurethane resin solution and deionized water that have been neutralized in advance are premixed to a predetermined concentration, and then an ultra-high pressure homogenizer (for example, “Artemizer System” manufactured by Sugino Machine Co., Ltd.) is used. In this method, forced emulsification is performed under high pressure conditions to obtain an emulsion. In addition, the chain extension reaction with an aqueous medium may be partially allowed to proceed simultaneously with the dispersion, and then the chain extension reaction may be further performed with a chain extender to increase the molecular weight. Examples of the chain extender include ethylenediamine, 1,2-propanediamine, 1,6-hexamethylenediamine, piperazine, 2,5-dimethylpiperazine, isophoronediamine, 4,4′-dicyclohexylmethanediamine, and 3,3′-. Diamines such as dimethyl-4,4′-dicyclohexylmethanediamine, 1,4-cyclohexanediamine, polyoxyethylenediamine, polyoxypropylenediamine, amine-terminated polyoxyethylene-polyoxypropylene copolymer; diethylenetriamine, dipropylenetriamine, tri Polyamines such as ethylenetetramine and tetraethylenepentamine; hydroxyethylhydrazine, hydroxyethyldiethylenetriamine, 2-[(2-aminoethyl) amino] ethanol, 3-aminopropane Compounds having an amino group and a hydroxyl group such as ol; hydrazines, acid hydrazides, and the like. These may be used singly or in combination.

  Examples of the basic compound used for neutralizing the polyurethane resin include monomethylamine, dimethylamine, trimethylamine, monoethylamine, triethylamine, monoisopropylamine, diisopropylamine, diethylenetriamine, triethylenetetramine, monoethanolamine, and diethanolamine. Organic amine compounds such as triethanolamine, monoisopropanolamine, diisopropanolamine, dimethylethanolamine, 2-aminomethylpropanol, morpholine, methylmorpholine, piperazine, ammonia, sodium hydroxide, potassium hydroxide and lithium hydroxide. Can be mentioned. Among them, it is preferable to use an organic amine compound. The neutralization equivalent is preferably in the range of 0.4 to 1.2. Conversion to a stable aqueous dispersion may not be possible under acid excess conditions where the neutralization equivalent is less than 0.4. Further, when the basic compound is excessively larger than 1.2, there may be a problem of odor caused by organic amine compound or ammonia or skin fog caused by inorganic alkali.

  Moreover, in order to reduce VOC components, such as an organic solvent, in the obtained aqueous resin composition, it is possible to appropriately reduce the VOC component by distillation under reduced pressure.

  The aqueous resin composition obtained in this way forms a film with excellent cohesive properties and excellent physical properties due to urethane resin when the solvent where water is the main component is volatilized. It can be suitably used as a binder for paints.

Aqueous resin composition having room temperature curability The aqueous resin composition of the present invention may further comprise a hydrazine compound and / or a hydrazide compound in order to impart room temperature curability.

Typical examples of the hydrazine compound include hydrazine, hydrazine hydrate (NH ₂ NH ₂ .H ₂ O), monomethyl hydrazine, monoethyl hydrazine, and the like. Among them, hydrazine and hydrazine hydrate are preferable.

  Specific examples of the hydrazide compound include saturated dicarboxylic acid dihydrazides having 2 to 18 carbon atoms such as oxalic acid dihydrazide, malonic acid dihydrazide, glutaric acid dihydrazide, succinic acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, and the like. Monoolefinic unsaturated dicarboxylic acid dihydrazide such as maleic acid dihydrazide, fumaric acid dihydrazide, itaconic acid dihydrazide; phthalic acid dihydrazide, terephthalic acid dihydrazide or isophthalic acid dihydrazide; pyromellitic acid dihydrazide, trihydrazide or tetrahydrazide; nitrilotrihydride Citric acid trihydrazide, 1,2,4-benzenetrihydrazide, ethylenediaminetetraacetic acid tetrahydrazide, 1,4,5,8-naphtho Acid tetrahydrazide; polyhydrazide obtained by reacting a low polymer having a carboxylic acid lower alkyl ester group with hydrazine or hydrazine hydrate (hydrazide group-containing compound such as carbonic dihydrazide; bissemicarbazide; hexamethylene diisocyanate or isophorone diisocyanate A polyfunctional semicarbazide (for example, “Hardener SC”: Asahi Kasei Co., Ltd.) obtained by excessively reacting a N, N-substituted hydrazine such as N, N-dimethylhydrazine or the above-mentioned hydrazide with a diisocyanate such as Chemicals Co., Ltd., aqueous polyfunctional semicarbazide 50% aqueous solution), the polyisocyanate compound, polyether, polyols, polyethylene glycol monoalkyl ethers, etc. An aqueous polyfunctional semicarbazide obtained by excessively reacting the above-exemplified dihydrazide with an isocyanate group in a reaction product with an active hydrogen compound containing a hydrophilic group; semicarbazide such as a mixture of the polyfunctional semicarbazide and an aqueous polyfunctional semicarbazide A compound having a group; a compound having a hydrazone group such as bisacetyldihydrazone.

  The above-described aqueous resin composition of the present invention is used as it is, and generally used antifoaming agents, thickeners, film-forming aids, antiseptics, fungicides, antifreezing agents, pH adjusters, flash last inhibitors, aldehydes Low-contamination agents such as scavengers, layered minerals, surfactants, surface conditioners, plasticizers, anti-settling agents, antistatic agents, antibacterial agents, fragrances, UV absorbers, UV stabilizers, alkylene glycol-modified alkyl silicates , Etc. can be blended to form a transparent or translucent one-pack type aqueous coating composition.

  Further, conventionally known color pigments, extender pigments, and rust preventive pigments can be blended as pigment components, and pigment dispersants, dispersants, powdered or particulate activated carbon, and photocatalytic titanium oxide are appropriately used as necessary. It can also be selected and combined to form a one-component water-based colored coating composition.

  The coated surface on which the one-component aqueous coating composition of the present invention can be applied is not particularly limited, and is a metal such as iron or aluminum; an organic base material such as plastic; a concrete block, wood, or stone. Examples thereof include a base material such as an inorganic base material, and a coating film on the base material. Examples of the coating film include acrylic resin-based, acrylic urethane resin-based, polyurethane resin-based, fluororesin-based, silicon acrylic resin-based, vinyl acetate resin-based, epoxy resin-based, alkyd resin, and the like. Further, these coated surfaces may be subjected to chemical conversion treatment, undercoating, intermediate coating, or the like. It is also possible to apply a known paint after applying the coating composition of the present invention.

  The coating of the one-component aqueous coating composition of the present invention can be performed using a conventionally known method such as air spray, airless spray, lysine gun, universal gun, roller, brush, electrostatic coating or the like. Moreover, as a drying method, any of normal temperature drying, forced drying, and heat drying may be sufficient, and it can adjust suitably according to a composition. In this specification, the drying condition of less than 40 ° C. is normal temperature drying, the drying condition of 40 ° C. or more and less than 80 ° C. is forced drying, and the drying condition of 80 ° C. or more is heat drying.

Hereinafter, the present invention will be described in more detail with reference to synthesis examples, examples and comparative examples. However, the present invention is not limited by these. “Part” and “%” are both based on mass.

Synthesis Example 1 Synthesis of acrylic polyol without using a chain transfer agent.
A flask was charged with 40 parts of N-methylpyrrolidone, stirred while blowing nitrogen at 150 ° C., and 15 parts of styrene, 13 parts of isobutyl methacrylate, 30 parts of 2-ethylhexyl acrylate, 20 parts of diacetone acrylamide, and 2-hydroxyethyl methacrylate 17 Part, acrylic acid 5 parts, and perhexyl D (manufactured by NOF Corporation, peroxide radical polymerization initiator) 6 parts of the mixture was added dropwise over 4 hours, and after completion of the addition, left at 150 ° C. for 1 hour, A mixed solution of 0.5 part of 2,2′-azobisisobutyronitrile and 3 parts of N-methylpyrrolidone was added dropwise over 30 minutes. After completion of the addition, the mixture was allowed to stand at 150 ° C. for 1 hour, and an acrylic having a solid content of 70%. A polyol solution (AP-1) was obtained. The number average molecular weight (Note 1) of the obtained acrylic polyol was 1,730, the acid value was 39 mgKOH / g, and the hydroxyl value was 73 mgKOH / g. The calculated number of hydroxyl groups in one molecule is 2.3 on average.
Note 1: The molecular weight described below was measured by GPC method according to JIS K0124-83 at a flow rate of 1.0 ml / min at 40 ° C. and using GPC tetrahydrofuran as an eluent, and calculated from a standard polystyrene calibration curve. Value. As the GPC apparatus, “HLC8120GPC” (trade name, manufactured by Tosoh Corporation) was used. Columns are “TSKgel G-4000H × L”, “TSKgel G-3000H × L”, “TSKgel G-2500H × L”, “TSKgel G-2000H × L” (both manufactured by Tosoh Corporation, trade names) ) Were used in combination.

Synthesis example 2
An acrylic polyol solution (AP-7) having a solid content of 70% was obtained in the same manner as in Synthesis Example 1 except that the composition shown in Table 1 was used and the reaction temperature was 130 ° C. Table 2 shows the number average molecular weight, acid value, hydroxyl value, and calculated number of hydroxyl groups in one molecule (calculated from the number average molecular weight and hydroxyl value) of the resulting acrylic polyol.

Synthesis Example 3 Synthesis of methacrylate ester dimer having a hydroxyl group.
The methacrylic acid ester and the solvent used were degassed (deoxygenated) by passing nitrogen gas through them for at least 1 hour before use.
A reaction vessel equipped with a thermometer, thermostat, stirrer, reflux condenser, nitrogen gas inlet tube and dropping device was charged with 30 parts of xylene and 25 parts of ethyl acetate, and heated to 105 ° C while passing nitrogen gas through the liquid. 100 parts of hydroxypropyl methacrylate, 0.05 parts of bis (borondifluorodimethylglyoxymate) Co (II) as a metal complex, 2,2′-azobis (2-methylbutyronitrile) 1 as a radical initiator A mixture of 15 parts of ethyl acetate as an additional solvent was added dropwise over 3 hours. After completion of dropping, the mixture was allowed to stand at 105 ° C. for 1 hour, and then 0.5 part of 2,2′-azobis (2-methylbutyronitrile) and 12 parts of xylene were added dropwise over 1 hour. The polymer solution was allowed to stand to obtain an oligomer solution of hydroxypropyl methacrylate (solid content 55%).
The resulting oligomer solution was depressurized to remove xylene and ethyl acetate to a solid content of 98% or more, and then distilled under reduced pressure to obtain hydroxypropyl methacrylate dimer (indicated as HPMA dimer in Table 1). (90% yield based on hydroxypropyl methacrylate).

Synthesis Example 4 Synthesis of acrylic polyol using methacrylate ester dimer having a hydroxyl group as a chain transfer agent
A flask is charged with 30 parts of N-methylpyrrolidone and 15 parts of the hydroxypropyl methacrylate dimer obtained in Synthesis Example 3 above, heated to 150 ° C. while blowing nitrogen into the gas phase, and stirred, A mixture of 15 parts 2-ethylhexyl methacrylate, 43 parts n-butyl methacrylate, 2 parts 2-hydroxyethyl methacrylate, 19 parts diacetone acrylamide, 6 parts methacrylic acid, 13 parts N-methylpyrrolidone and 5 parts perhexyl D for 4 hours It was dripped over. After stirring as it was for 2 hours, it was cooled to obtain an acrylic polyol solution (AP-2) having a solid content of 70%. The number average molecular weight of the obtained acrylic polyol was 1,920, the acid value was 39 mgKOH / g, and the hydroxyl value was 67 mgKOH / g. The calculated number of hydroxyl groups in one molecule is 2.3 on average.

Synthesis example 5
An acrylic polyol solution (AP-8) having a solid content of 70% was obtained in the same manner as in Synthesis Example 4 except that the formulation shown in Table 1 was used. Table 2 shows the number average molecular weight, acid value, hydroxyl value, and calculated hydroxyl number in one molecule of the resulting acrylic polyol.

Synthesis Example 6 Synthesis of acrylic polyol using a thiol compound having a hydroxyl group as a chain transfer agent.
A flask is charged with 40 parts of N-methylpyrrolidone and stirred while blowing nitrogen at 130 ° C., and 15 parts of styrene, 16 parts of isobutyl methacrylate, 30 parts of 2-ethylhexyl acrylate, 20 parts of diacetone acrylamide, 2-hydroxyethyl methacrylate 10 parts, 5 parts of acrylic acid, 2-mercaptoethanol 4 parts of 2,2'-azobisisobutyronitrile 4 parts mixture was added dropwise over 4 hours, allowed to stand for 1 hour after completion of the dropwise addition 130 ° C., further 2 , 2'-azobisisobutyronitrile and a mixed solution of 0.5 part of N-methylpyrrolidone are added dropwise over 1 hour. After completion of the dropwise addition, the mixture is allowed to stand at 130 ° C for 1 hour to obtain an acrylic polyol having a solid content of 70%. A solution (AP-3) was obtained. The number average molecular weight of the obtained acrylic polyol was 1,950, the acid value was 39 mgKOH / g, and the hydroxyl value was 72 mgKOH / g. The calculated number of hydroxyl groups in one molecule is 2.5 on average.

Synthesis Examples 7-15
Acrylic polyol solutions (AP-4) to (AP-6) and (AP-9) to (AP-14) having a solid content of 70% were obtained in the same manner as in Synthesis Example 4 except that the formulation shown in Table 1 was used. It was. Table 2 shows the number average molecular weight, acid value, hydroxyl value, and calculated hydroxyl number in one molecule of the resulting acrylic polyol.

Synthesis Example 16
In a reaction vessel equipped with a thermometer, thermostat, stirring device, reflux condenser and dropping device, 60 parts of deionized water, New Coal 707-SF (trade name, manufactured by Nippon Emulsifier Co., Ltd., polyoxyethylene polycyclic phenyl ether sulfate salt) , Active ingredient 30%) 1.7 parts was charged, stirred and mixed in a nitrogen stream, and heated to 80 ° C. Next, 1% of the total amount of the following monomer emulsion and 5.3 parts of a 6% aqueous ammonium persulfate solution were introduced into the reaction vessel, and maintained at 80 ° C. for 15 minutes. Thereafter, the remaining monomer emulsion was dropped into a reaction vessel maintained at the same temperature over 3 hours, and aged for 1 hour after the completion of the dropping. Thereafter, a polymerization initiator solution consisting of 14 parts of deionized water and 0.03 part of ammonium persulfate was added dropwise over 1 hour. After completion of the addition, the mixture was stirred and aged for 1 hour to obtain a copolymer emulsion (AP- 15) was obtained. The resulting copolymer has an acid value of 7 mgKOH / g and a hydroxyl value of 43 mgKOH / g.
Monomer emulsion: 80 parts deionized water, 3.3 parts Newcol 707-SF, 5 parts diacetone acrylamide, 10 parts 2-hydroxyethyl methacrylate, 1 part methacrylic acid, 15 parts styrene, 34 parts isobutyl methacrylate, 2-ethylhexyl 20 parts of acrylate and 15 parts of 2-ethylhexyl methacrylate were mixed and stirred to obtain a monomer emulsion.

Example 1 An aqueous resin composition in which a urethane resin is dispersed in an aqueous medium.
While blowing nitrogen into the gas phase of the flask, 114 parts of 70% acrylic polyol solution (AP-1) (80 parts of acrylic polyol solid content, 34 parts of N-methylpyrrolidone) and 20 parts of dicyclohexylmethane diisocyanate were charged at about 90 ° C. After stirring for 6 hours, it was cooled to 50 ° C. Subsequently, 4 parts of dimethylethanolamine (neutralization equivalent to acidic group is 0.81) was added and stirred for 10 minutes, and then 140 parts of deionized water was added dropwise over 15 minutes. The obtained aqueous dispersion was kept at 40 ° C., and then a mixture of 1.2 parts of ethylenediamine and 10 parts of deionized water was added dropwise over 15 minutes as a chain extender, and then stirred at 40 ° C. for 1 hour. An aqueous resin composition (ACU-1, solid content 35%, acid value 31 mgKOH / g) was obtained as a milky white dispersion.

Examples 2-9, Comparative Examples 1-10
Aqueous resin compositions (ACU-2) to (ACU-19) having a solid content of 35% were prepared in the same manner as in Example 1 except that the composition and reaction time shown in Table 3 or Table 4 were used. The acid value and visual state of the obtained aqueous resin composition are shown together in Table 3.

Evaluation of the state of the aqueous resin composition
Although the visual evaluation result of the state of the aqueous resin composition adjusted in Examples 1-9 and Comparative Examples 1-10 was shown in Table 3 and Table 4, Examples 1-9 and Comparative Examples 1, 2, 5, 8 In the other comparative examples, the aqueous resin composition was able to obtain a stable milky white dispersion, but in other comparative examples, the resin was insufficient in water dispersibility or gelation at the stage of urethanization. It became a lump gel and a dispersion could not be obtained.

Example 10
286 parts of aqueous resin composition (ACU-1) obtained in Example 1 (100 parts of solid content), 6.6 parts of adipic acid dihydrazide and 12 parts of deionized water were uniformly mixed to form an aqueous coating material having a solid content of 35%. A composition (ST-1) was obtained.

Examples 11-20, Comparative Examples 11-14, 16
Except for the formulations shown in Table 5 or Table 6, aqueous coating compositions (ST-2) to (ST-15) and (ST-17) having a solid content of 35% were obtained in the same manner as in Example 10.

Comparative Example 15
Soap-free water dispersibility that can be easily dispersed in water by adding a nonionic hydrophilic group to the polyisocyanate compound as the B liquid, copolymer emulsion AP-15: 263 parts obtained in Synthesis Example 16 as the A liquid A two-component aqueous coating composition (ST-16) comprising a hardener aqueous solution in which 18.6 parts of Takenate WD-220 (product of Mitsui Chemicals Polyurethane Co., Ltd.), a polyisocyanate, is diluted with 57.3 parts of deionized water. Obtained.

Performance evaluation of aqueous coating composition
Test plates coated with the aqueous coating compositions obtained in Examples 10 to 20 and Comparative Examples 11 to 16 were prepared.

  Solid aluminum plate (150 x 70 x 0.5 mm) surface is roughened to the extent that it is no longer glossy using # 400 sandpaper, and then degreased with xylene. Was applied with a brush so that the application amount was 100 g / m 2. Further, after the first drying, the second coating was performed in the same manner as the first coating, and each test coating plate was prepared by drying for 7 days under the conditions of an air temperature of 20 ° C. and a relative humidity of 60%. In Comparative Example 15, two liquids were mixed 15 minutes before coating.

Next, performance evaluation was performed on the following items.
1. Rubbing test: Approximately 2 ml of methyl ethyl ketone was placed on the surface of the coated plate, and two layers of gauze (FC gauze manufactured by White Cross Co., Ltd.) were folded in four, and rubbing was performed 20 times with a stroke of 5 cm while pressing with the middle finger. The state of the subsequent coated plate was evaluated.
○: No changes such as scratches or gloss on the painted surface.
Δ: Fine scratches occurred after rubbing, and gloss was clearly reduced.
X: The coating film was destroyed and peeled off and was not left on the coated plate.

2. Bending test: The test plate was cut into a rectangular shape having a width of 20 mm, a round bar having a diameter of 10 mm was applied to the uncoated back side, and the test plate was bent 180 degrees along the round bar. The state of the painted surface of the bent part after 5 minutes was evaluated.
○: No changes such as scratches or gloss on the painted surface.
(Triangle | delta): The gloss fell clearly or the faint whitening was seen by the bending part.
X: The coating film of the bending part has broken.

  3. Pencil hardness: According to JIS K 5600-5-4 (1999), a pencil lead is applied at an angle of about 45 degrees with respect to the test coating plate surface, and it is evenly pressed forward against the test coating plate surface to the extent that the core does not break. It moved about 10 mm at a high speed. The hardness symbol of the hardest pencil when this operation was repeated 5 times while changing the test location and the coating film was not torn was defined as pencil hardness.

4). Water resistance test: The test plate was immersed in warm water at 50 ° C. for 60 minutes, and then water droplets on the pulled surface were wiped off.
○: There is no change in the paint surface.
(Triangle | delta): Although whitening was seen on the coating surface, when it was left to stand for 24 hours, it returned to the original transparent coating surface.
X: The coating film was markedly whitened and peeled off simply by rubbing lightly.

  The results are shown in Table 7.

Claims (6)

Acrylic obtained by copolymerizing (A) an unsaturated monomer having a carbonyl functional group, an unsaturated monomer having a hydrophilic group, and, if necessary, other unsaturated monomers when the total amount of starting materials is 100 parts by weight a polyol 50-90 parts by weight, and (B) below aliphatic diisocyanates, selected from the group consisting of alicyclic diisocyanates and aromatic diisocyanates, a diisocyanate compound 10-50 parts having no parent-aqueous polar group A urethane resin synthesized by the above reaction is dispersed in an aqueous medium, wherein the acrylic polyol has a molecular weight of 400 or more and 10,000 or less, and an acid value of 10 mgKOH / g or more. 80 mg KOH / g or less, aqueous hydroxyl value of 15 mg KOH / g or more and 180 mg KOH / g or less Fat composition.
Aliphatic diisocyanates: hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, lysine diisocyanate
Alicyclic diisocyanates: isophorone diisocyanate, 4,4′-methylenebis (cyclohexyl isocyanate), methylcyclohexane-2,4- (or-2,6-) diisocyanate, 1,3- (or 1,4-) di ( Isocyanatomethyl) cyclohexane, 1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate
Aromatic diisocyanates: xylylene diisocyanate, metaxylylene diisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalene diisocyanate, 4,4 ′ -Toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, (m- or p-) phenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, bis (4-Isocyanatophenyl) sulfone, isopropylidenebis (4-phenylisocyanate) The aqueous resin composition according to claim 1, wherein the unsaturated monomer having a carbonyl group used in the acrylic polyol is diacetone acrylamide or diacetone methacrylamide. The aqueous resin composition according to claim 1 or 2, wherein the acid value of the urethane resin is 5 mgKOH / g or more and 50 mgKOH / g or less. The urethane resin of any one of Claims 1-3, the hydrazine compound, and / or the hydrazide compound are disperse | distributed in the aqueous medium, The aqueous resin composition characterized by the above-mentioned. A one-pack type aqueous coating composition comprising the aqueous resin composition according to any one of claims 1 to 4. A coated article in which the one-component water-based paint composition according to claim 5 is coated on a material that may be previously coated with an undercoat.