JPH0848939A - Thermosetting coating composition - Google Patents

Abstract

PURPOSE:To obtain a thermosetting coating compsn. which has an improved low-temp. curability and forms a coating film which does not yellow in baking and has excellent appearance, weatherability, and resistance to acid rain. CONSTITUTION:This coating compsn. contains as the essential components a hydroxylated resin and an isocyanate component comprising a blocked isocyanate compd. contg. m-isopropenyl-alpha, alpha-dimethylbenzyl isocyanate (m-TMI) or its copolymer. The m-TMI may be used jointly with another isocyanate compd., e.g. isophorone diisocyanate (IPDI).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermosetting coating composition, and more particularly to a thermosetting coating composition which is excellent in acid rain resistance and is suitable as a top coating composition for automobile bodies. .

[0002]

2. Description of the Related Art Top coat films for automobile bodies are required to have not only weather resistance but also excellent finish appearance such as gloss. Therefore, this type of topcoat paint
Conventionally, thermosetting paints based on hydroxyl group-containing resins such as acrylic resin paints and polyester resin paints have been generally used.

In recent years, however, the damage caused by acid rain such as the death of forests has become a serious social problem, and the top coat film of automobile bodies exposed to the external environment is also deteriorated by the acid rain. Fear is becoming a problem. In particular, the coating film of the thermosetting paint based on the above-mentioned hydroxyl group-containing resin has residual or newly generated hydrophilic hydroxyl groups, so that stains, discoloration, etc. may occur when exposed to acid rain for a long period of time. Or even further, the coating film may be destroyed.

Therefore, recently, for example, Japanese Patent Laid-Open No. 2-2428.
As disclosed in Japanese Patent Publication No. 67, 67, a coating agent containing such a hydroxyl group-containing resin as a base (resin base material), a blocked isocyanate, that is, a blocking agent in which free isocyanate groups of an isocyanate compound are blocked, is a curing agent. A thermosetting coating composition added as the above has been developed.
Then, according to this thermosetting coating composition, the blocking agent is dissociated by the heat during baking when it is baked and cured, and the regenerated isocyanate compound reacts with the hydroxyl group of the resin base to give a hydrophilic property. In addition to reducing or eliminating the hydroxyl groups of the properties, it also crosslinks the molecules of the resin base to form a cured coating film having a high crosslink density. Therefore, the formed coating film is excellent in acid rain resistance and water resistance, as well as in scratch resistance and the like.

Regarding the thermosetting coating composition based on the hydroxyl group-containing resin and having the blocked isocyanate as the curing agent and having excellent acid rain resistance and the like,
In addition, Japanese Patent Application Laid-Open Nos. 2-145666 and 3
-172638, JP-A-4-130168, JP-A-4-170482, and JP-A-4-246.
483, JP-A-5-25431, and JP-A-5
It is disclosed in, for example, JP-A-202335 and JP-A-5-295322.

[0006] In those disclosed in these publications including the above-mentioned JP-A-2-242867, the isocyanate component of the blocked isocyanate is hexamethylene diisocyanate (HDI), trimethylhexamethylene diisocyanate or other aliphatic diisocyanate. And cycloaliphatic diisocyanates such as isophorone diisocyanate (IPDI) and hydrogenated 4.4'-diphenylmethane diisocyanate, and aromatic isocyanates, or burettes, isocyanurates, or adducts thereof are used. Has been done.
Specific examples of the aromatic isocyanate include 4.4'-diphenylmethane diisocyanate, 2,4- or 2,6-tolylene diisocyanate, m- or p-xylylene diisocyanate, m-. Or p-phenylene diisocyanate.

[0007]

As described above, the thermosetting coating composition containing the hydroxyl group-containing resin disclosed in the above publications and the blocked isocyanate as the curing agent as essential components is dissociated. By reacting the above isocyanate with the hydroxyl group in the resin, it was possible to form a coating film excellent in acid rain resistance and the like.

However, the blocked isocyanates used in them all have a relatively high dissociation temperature. Therefore, these conventional thermosetting coating compositions require baking treatment under relatively high temperature conditions. For example, 120 ° C. × 30 minutes, which is generally used for productivity improvement, etc. Under the baking conditions of relatively low temperature and short time, the curing tends to be insufficient, and it was not possible to sufficiently secure the acid rain resistance and the like of the coating film.

Therefore, according to the present invention, it is possible to sufficiently cure even when the baking temperature at the time of forming the coating film is relatively low.
An object of the present invention is to provide a thermosetting coating composition that can sufficiently secure acid rain resistance and the like.

[0010]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies mainly focusing on the isocyanate compound itself of a blocked isocyanate as a curing agent, and as a result, have found that m-isopropenyl-α, α-dimethylbenzyl isocyanate [ m-
It has been found that TMI (m-trimethylisocyanate)] is excellent in dissociability, and that a copolymer obtained by copolymerizing this with another vinyl monomer is also excellent in dissociability. Then, it was confirmed that the above problem can be effectively solved by using this m-TMI or its copolymer as at least a part of the isocyanate component of the blocked isocyanate.

That is, the thermosetting coating composition according to claim 1 contains a hydroxyl group-containing resin and m as an isocyanate component.
-A blocked isocyanate containing isopropenyl-α, α-dimethylbenzyl isocyanate is an essential component.

A thermosetting coating composition according to a second aspect of the present invention comprises a hydroxyl group-containing resin and m as an isocyanate component.
-A block isocyanate containing a copolymer of isopropenyl-α, α-dimethylbenzyl isocyanate and a vinyl monomer containing no active hydrogen is an essential component.

[0013]

This m-isopropenyl-α, α-dimethylbenzylisocyanate (m-TMI) is an α, α-dimethylbenzylisocyanate having an isopropenyl group substituted at the meta position as shown by the following chemical structural formula. Is.
Therefore, although some isomers have an isopropenyl group substituted at the ortho position or the para position, this meta position substitution type is general and preferable.

[0014]

Embedded image

According to the thermosetting coating composition of the present invention using the blocked isocyanate containing the m-TMI or the copolymer thereof as an isocyanate component, the composition is coated and then heated and baked. , As in the conventional case, the blocking agent dissociates, and the dissociated m-
The isocyanate group of TMI or its copolymer reacts with the hydroxyl group of the resin base to bond with it. In addition, m-TM
When I is used, the isopropenyl group of m-TMI also reacts at the same time during the baking, so that it polymerizes with other isopropenyl groups. Therefore, the hydroxyl groups contained in the resin base are reduced or eliminated, and the molecules of the resin base are crosslinked, so that a coating film excellent in acid rain resistance, water resistance, and scratch resistance is formed. To be done.

In this case, m-TMI or its copolymer contained as the isocyanate component of the blocked isocyanate has an excellent dissociation action and dissociates at a low temperature of about 120 ° C. or lower. Therefore, even if the baking temperature is relatively low, a sufficiently cured coating film can be formed, and thereby, acid rain resistance and the like can be sufficiently ensured.

It is considered that the excellent dissociation action of m-TMI or its copolymer is based on its molecular structure. In other words, since the isocyanate group is bonded to the dimethylated benzyl group, that is, the tertiary carbon bonded to the benzene nucleus, the blocking agent is structurally more effective than when it is bonded to the alkylene group or the phenylene group. Is likely to be dissociated.

The thermosetting coating composition according to the present invention will be described in more detail below. However, this composition is basically the same as the conventional one except that at least a part of m-TMI or a copolymer thereof is used as the isocyanate component of the blocked isocyanate.

The hydroxyl group-containing resin is a resin containing two or more hydroxyl groups in one molecule, and specifically, acrylic resin, polyester resin, alkyd resin, epoxy resin, fluorine resin, amide resin containing hydroxyl groups. , Urethane resins, polyether resins and the like. Further, a resin that produces a hydroxyl group during bake hardening can be used as well. The resin containing or generating these hydroxyl groups can be used alone or in combination of two or more. However, of these, acrylic resin and polyester resin having a hydroxyl group are preferable.

The hydroxyl group-containing acrylic resin can be generally obtained by copolymerizing an acrylic vinyl monomer having a hydroxyl group with another acrylic or non-acrylic vinyl monomer. And, the number average molecular weight thereof is 1,000 to 15,000 and the hydroxyl value is 50 to 20.
0 and an acid value of about 5 to 50 are generally preferred.

Examples of the hydroxyl group-containing vinyl monomer include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, and other hydroxyalkyl esters of acrylic acid / methacrylic acid having 2 to 24 carbon atoms, Examples thereof include hydroxyalkylamides having 1 to 12 carbon atoms of acrylic acid / methacrylic acid such as N-methylolacrylamide, N-methylolmethacrylamide, N-hydroxyethylacrylamide and N-dihydroxyethylmethacrylamide. These hydroxyl group-containing vinyl monomers may be used alone or in any combination of 2
Any combination of two or more species can be used.

Other vinyl monomers include alkyl acrylates having 1 to 24 carbon atoms such as methyl acrylate, isobutyl acrylate, lauryl acrylate and 2-ethyl butyl acrylate, methyl methacrylate, n-butyl methacrylate and isobutyl. Methacrylic acid, 2-ethylhexyl methacrylate, decyl methacrylate, stearyl methacrylate and other methacrylic acid alkyl esters having 1 to 24 carbon atoms, carboxyl group-containing vinyl monomers such as acrylic acid, methacrylic acid, maleic acid and fumaric acid, glycidyl methacrylate Glycidyl group-containing vinyl monomers such as styrene, vinyltoluene, and the like. These vinyl monomers can also be used alone or in appropriate combination of two or more kinds.

The hydroxyl group-containing polyester resin is an ester reaction product of a polyhydric alcohol and a polybasic acid or an anhydride thereof, and has a residual hydroxyl group in the molecule. And the number average molecular weight thereof is 200 to 1000.
0, hydroxyl value is 50-350, acid value is about 5-50,
Each is generally preferred.

As the polyhydric alcohol for forming the hydroxyl group-containing polyester resin, ethylene glycol, propylene glycol, 1,6-hexanediol, diethylene glycol, glycerin, pentaerythritol or the like may be used alone or in an appropriate combination. Can be used. Further, as the polybasic acid or its anhydride, use phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, succinic acid, maleic anhydride, fumaric acid, trimellitic anhydride, etc. in the same manner. You can In addition, these polyhydric alcohols and polybasic acids or their anhydrides can generally be reacted so that the molar ratio of the hydroxyl groups to the carboxyl groups is 1.2 to 1.8.

The blocked isocyanate used as a curing agent for these hydroxyl group-containing resins is, as described above, m-isopropenyl-α, α-dimethylbenzyl isocyanate (m-TMI) or its copolymer as the isocyanate component. Is included as at least a part of. This m-TMI or its copolymer has an excellent dissociation action, and therefore, by using a blocked isocyanate containing this as an isocyanate component, the low temperature curability of the thermosetting coating composition can be improved. And, in order to sufficiently improve such low temperature curability, this m-TMI or its copolymer can be generally used in a proportion of 10% by weight or more in practical use with respect to the whole isocyanate component. , And more preferably 20
It can be used in a proportion of not less than wt%.

Therefore, although this m-TMI or its copolymer can be used alone as the isocyanate component of the blocked isocyanate, it should be used in combination with one or more other optional isocyanate compounds. You can also Examples of such an isocyanate compound include aliphatic diisocyanates such as hexamethylene diisocyanate (HDI) and trimethylhexamethylene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 2,4- or 2,6-tolylene diisocyanate. Isocyanate (TDI), m- or p-xylylene diisocyanate, m- or p-
Examples thereof include aromatic diisocyanates such as phenylene diisocyanate and naphthylene diisocyanate, isophorone diisocyanate (IPDI), cyclic aliphatic diisocyanates such as hydrogenated MDI and hydrogenated TDI. However, among these, IPDI and the like, which are excellent in non-yellowing at the time of baking, are particularly preferable. In addition, these diisocyanate compounds can be used as a buret body, an isocyanurate body, an adduct body (addition body) with a polyhydric alcohol, or a urethane prepolymer having a relatively low molecular weight.

Here, this m-TMI can be used as it is as an isocyanate component of a blocked isocyanate and blocked with a blocking agent. However, this monoisocyanate compound, m-TM
I tends to have a relatively high saturated vapor pressure at room temperature. Therefore, from the viewpoint of better safety, this m-TMI is preferably derivatized to have a high molecular weight to eliminate its volatility or reduce its vapor pressure before use. However, since m-TMI is a monoisocyanate compound, it is not appropriate to use it as a buret body or an adduct body as in the case of an ordinary isocyanate compound. Although it can be used as a dimer or trimer (isocyanurate), in this case, the reactivity is lowered and a relatively high temperature is required for the reaction with the hydroxyl group. Therefore, one preferable method for increasing the molecular weight of m-TMI is to utilize the polymerization reactivity of the isopropenyl group to thermally polymerize m-TMI to obtain a polymer composed of its dimer or trimer. Is.

However, it is more preferable to polymerize m-TMI with other vinyl monomers to form a copolymer. Thereby, m-TMI can be polymerized to an arbitrary molecular weight, and the isocyanate group equivalent can also be adjusted arbitrarily. Then, such a vinyl monomer needs to be a compound that does not contain active hydrogen such as an amino group or a hydroxyl group that reacts with an isocyanate group, but is a liquid at room temperature and has m-TMI. It is preferable that the above-mentioned polymerization operation is easy, and that it is easily polymerized with the isopropenyl group of m-TMI. Specifically, styrene,
Examples thereof include vinyltoluene, acrylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, cyclohexyl acrylate, and benzyl acrylate, or similar methacrylic acid esters. In particular, the m-TMI copolymer using these active hydrogen-free acrylic monomers is preferable because it has good compatibility with the above-mentioned acrylic resin as the hydroxyl group-containing resin. The copolymer of m-TMI is generally 500 to 500 as long as good compatibility with the composition and dispersibility can be maintained.
It can be used as a molecular weight of about 3000.

The isocyanate component containing m-TMI or its copolymer is used as a blocked isocyanate in which the free isocyanate groups are partially or completely blocked by a blocking agent. Here, examples of the blocking agent include oximes such as methyl ethyl ketone oxime (MEKO), acetone oxime, cyclohexane oxime, acetophenone oxime, and benzophenone oxime, alcohols, phenols, lactams, and the like. Among these, MEKO is the most typical one and can be used particularly preferably.

The blocked isocyanate containing m-TMI blocked with a blocking agent as described above is generally used as a curing agent for a hydroxyl group-containing resin in an amount of 50 to 25% thereof.
It may be used in an amount of 0% by weight, or in an equivalent ratio (molar ratio) of isocyanate groups and hydroxyl groups of generally 0.5 to 2.0, preferably 0.8 to 1.2. it can.

In the present invention, the blocked isocyanate as described above is an essential component as a curing agent. However, as disclosed in the above-mentioned publication, an amino resin such as an alkyl etherified melamine resin is used as the curing agent. It can also be used together. Further, a curing catalyst may be used in order to accelerate the action of the blocked isocyanate as a curing agent, and it is also preferable. Although any curing catalyst can be used, it is particularly preferable to use an organotin compound which is a salt of organotin and a carboxylic acid. Here, as the organic tin, alkyl tin such as methyl, butyl and octyl is preferable, and further dialkyl tin is preferable. As the carboxylic acid, an aliphatic monocarboxylic acid such as lauric acid, maleic acid, phthalic acid,
Etc. are preferred. And as a particularly preferable organic tin compound, for example, di-n-butyltin dilaurate and the like can be mentioned.

Further, in the thermosetting coating composition of the present invention, as the solvent, any conventional solvent can be used. For example, alcohol solvents such as butanol and propanol, ether solvents such as ethylene glycol monobutyl ether, hydrocarbon solvents such as Solvesso 100, ester solvents such as ethyl acetate, ketone solvents such as methyl ethyl ketone, xylene and toluene. Aromatic solvents and the like. These solvents may be used alone or in admixture of two or more.

Further, the thermosetting coating composition of the present invention comprises
An organic pigment or an inorganic pigment, an ultraviolet absorber, an antioxidant and the like can be appropriately blended according to the specific application and the like.

Examples of organic pigments include azo pigments and phthalocyanine pigments or dyes, and examples of inorganic pigments include general colored pigments and extender pigments, as well as metallic powders and rust preventive pigments. . These pigments can be optionally used as needed.

Examples of the ultraviolet absorber include benzotriazole compounds such as 2- (2'-hydroxy-5'-methyl-phenyl) benzotriazole, benzophenone compounds such as 2,4-dihydroxybenzophenone, and acrylates such as diphenyl acrylate. Compounds, salicylate compounds such as p-octyl-phenyl salicylate, and other compounds such as 2-hydroxy-naphthophenone and nickel-bisoctylphenyl sulfide. These may be used alone or in combination of two or more. You can

This ultraviolet absorber can be used alone, but preferably it can be used in combination with an antioxidant. Examples of antioxidants include hindered amine-based compounds such as bis (1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, phenol-based compounds, and sulfide-based compounds.

Further, surface modifiers such as silicone and other organic polymers, anti-sagging agents such as silica powder, thickeners and the like can be appropriately used as required. Furthermore, if necessary, a resin containing no hydroxyl group can be used as a vehicle component in addition to the resin containing hydroxyl group.

[0038]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

[Preparation of clear coating: Examples 1 to 4 and Comparative Examples 1 and 2] The thermosetting coating composition of the present invention was applied as a clear top coating, and the composition (parts by weight) shown in FIG. 1-4 clear paints were prepared. For comparison, the clear paints of Comparative Examples 1 and 2 were combined and prepared.

Here, these clear paints are obtained by mixing an acrylic resin as a hydroxyl group-containing resin, curing agents A, B and C made of blocked isocyanate, and a curing catalyst. The acrylic resin, the curing agent, and The curing catalyst has the following compositions or components.

Acrylic resin is styrene 20 parts by weight / n
-Butyl methacrylate 37 parts by weight / butyl acrylate 18 parts by weight / hydroxyethyl methacrylate 23
2 parts by weight of acrylic acid and 2 parts by weight of azoisobutyronitrile (AIBN), mixed solvent n-butanol /
It was obtained by dropping in xylene (3/7) at 120 ° C. for 4 hours to react, and then adding 1 part by weight of AIBN evenly every 5 minutes, holding for 2 hours, and further holding for 1 hour. is there. And this acrylic resin (varnish)
Has a weight average molecular weight of 12,000, a hydroxyl value of 100, an acid value of 15, and a solid content of 60%.

The curing agent A is m-isopropenyl-α, α-dimethylbenzyl isocyanate (m-TM).
A block isocyanate (“Takenate XB-78-M1” manufactured by Takeda Pharmaceutical Co., Ltd.) in which the copolymer of I) with an acrylic ester is blocked using methyl ethyl ketone oxime (MEKO) as a blocking agent.

The curing agent B is a blocked isocyanate obtained by blocking an isocyanurate of isophorone diisocyanate (IPDI) with MEKO as a blocking agent.

Further, the curing agent C is a blocked isocyanate obtained by similarly blocking an isocyanurate body of hexamethylene diisocyanate (HDI) with MEKO.

The curing catalyst consisted of a 50% solution of di-n-butyltin fatty acid salt (SCA manufactured by Sankyo Machine Gosei Co., Ltd.).
T-2L).

As shown in FIG. 1, in Examples 1 to 4 and Comparative Examples 1 and 2, the type of the curing agent and the type of the curing agent were variously changed while keeping the amount of the acrylic resin compounded substantially constant. That is, in Example 1, only 59 parts by weight of the curing agent A (m-TMI) was used. Further, in Examples 2 to 4, the curing agent A was used.
(M-TMI) and curing agent B (IPDI) are used in combination, and in Example 2, curing agent A: 46 parts by weight / curing agent B: 8 parts by weight, and in Example 3, curing agent A: 36 parts by weight / curing Agent B: 1
4 parts by weight, in Example 4, the curing agent A: 9 parts by weight / the curing agent B: 30 parts by weight, which were mixed in different proportions. On the other hand, in Comparative Example 1, curing agent B (IPD
33 parts by weight of only I) and 29 parts by weight of curing agent C (HDI) alone in Comparative Example 2.

The curing catalyst was compounded in an amount of 1 part by weight in both the examples and comparative examples.

It is common to add an ultraviolet absorber and an antioxidant to this type of clear topcoat paint, but these are not blended here. When these are blended, for example, "Tinuvin 900" (manufactured by Ciba-Geigy), which is a benzotriazole-based ultraviolet absorber, and "Tinuvin 292 (manufactured by Ciba-Geigy)", which is a hindered amine antioxidant, should be used. And each of them is mixed at a ratio of about 0.6% by weight. It is also preferable to add a surface conditioner, for example, "BYK-300" which is a silicone-based leveling agent.
(Manufactured by BYK) can be used in an amount of about 0.2% by weight.

Although m-TMI was used as the copolymer here, when it is used as it is, a radical polymerization accelerator should be further added in order to accelerate the polymerization reaction of its isopropenyl group. Is preferred.

[Preparation of Painted Plate] Using the clear paints of Examples 1 to 4 and Comparative Examples 1 and 2 described above, a paint plate for test was prepared by the same coating method as in the case of coating the outer plate of an automobile.

First, an epoxy-based cationic electrodeposition coating was applied to a 0.8 mm thick dull steel sheet that had been subjected to zinc phosphate chemical conversion treatment, an intermediate coating composition was applied to this, and dry baking was performed, followed by water polishing and degreasing. Then, preparations for topcoat painting were made.

Then, a metallic base coating material was applied to the prepared coated plate so that the thickness of the dried coating film was 15 μm, and then the clear coating materials of Examples 1 to 4 and Comparative Example were dried by wet-on-wet. Apply each so that the thickness of the coating film is 30 μm, then after setting,
The coating film was baked and dried under two heating conditions of 120 ° C. × 30 minutes and 140 ° C. × 30 minutes.

The metallic base paint has the following composition. 4.5 parts by weight of acrylic resin (“Acridic A-332” manufactured by Dainippon Ink and Machinery Co., Ltd.), melamine resin (“Super Beckamine L-117-
60 "manufactured by Dainippon Ink and Machinery Co., Ltd. 27.3 parts by weight, aluminum flakes (" Alpaste 7160N "manufactured by Toyo Aluminum Co., Ltd.) 5.7 parts by weight, thinner {acetic ester / toluene / Solvesso 100 (40 / 40/20)} 6
2.5 parts by weight.

[Evaluation test of coating film] Regarding the coating films of the clear paints of Examples 1 to 4 and Comparative Examples 1 and 2 formed as described above, their gloss, pencil hardness at each baking temperature, solvent resistance, and water resistance. The properties, the yellowing due to baking, the weather resistance and the acid resistance were tested and evaluated. The result is also shown in FIG.

The test methods and evaluation criteria for each are as follows. Further, the gloss, baking yellowing, weather resistance, and acid resistance were applied to the coating film under baking conditions of 120 ° C. × 30 minutes.

Gloss: The specular reflectance was measured at 20 °. Pencil hardness: The pencil hardness was measured according to JIS-K5400-8.4.2.

Solvent resistance: A xylene rubbing test was carried out, the surface of the coating film was rubbed 10 times with a gauze impregnated with xylene, and the surface of the coating film was observed and evaluated according to the following criteria. ◯: No abnormality □: Slightly scratched Δ: Discolored ×: The coating film dissolves.

Water resistance: 240 coated plates in warm water at 40 ° C
After immersion for a period of time, the coating film surface was observed and evaluated according to the following criteria. ◯: No abnormality □: Micro blister occurs Δ: Blister occurs or discolors ×: Whitening occurs.

Baking yellowing: The surface of the coating film after baking was observed and evaluated according to the following criteria. ◯: No abnormality □: Slight discoloration is recognized Δ: Yellowing is recognized ×: Significant yellowing is recognized.

Weather resistance: An accelerated weather resistance test was performed for 2000 hours using SWOM, and the surface of the coating film was observed and evaluated according to the following criteria. ◯: No abnormality X: Cracks, remarkable discoloration, and low gloss.

Acid resistance: A 40% aqueous sulfuric acid solution was applied to the coating film in an amount of 0.
The solution was dropped into 5 ml spots, left at 50 ° C. for 30 minutes, washed with water, and the surface of the coating film was observed and evaluated according to the following criteria. ◯: No abnormality □: No discoloration, etc., but slight dents occur in the spot part.

[Test Results] As shown in the test results of FIG. 1, the compositions of Examples 1 to 4 contained curing agent A, namely m-isopropenyl-α, α-dimethylbenzyl isocyanate (m-TMI). At least a part of the curing agent is a block isocyanate having a copolymer as an isocyanate component. According to these, the formed coating film has sufficient gloss and has excellent appearance. Also, it is excellent in non-yellowing and weather resistance during baking. Also,
Regarding the acid resistance, that is, the resistance to acid rain, a comparative example corresponding to a conventional technique using a curing agent B containing isophorone diisocyanate (IPDI) as an isocyanate component and a curing agent C containing hexamethylene diisocyanate (HDI) as an isocyanate component, respectively. Even when compared with 1 and 2, they are sufficiently excellent.

However, the effects of Examples 1 to 4 are particularly manifested in the low temperature curability. That is, in the pencil hardness, the solvent resistance, and the water resistance related to the curability of the coating film, Comparative Example 1 has almost sufficient curability under the baking condition of 140 ° C.
Under the baking condition of 20 ° C., the solvent resistance and water resistance are particularly insufficient, and in Comparative Example 2, the solvent resistance is particularly insufficient even under the baking condition of 140 ° C. On the other hand, Examples 1 to 4
In Example 4, although a slightly insufficient tendency was observed in Example 4 in which the amount of the curing agent A was relatively small, a good result was obtained under the baking condition of 120 ° C.

Thus, from this test result, it is possible to improve the low temperature curability of the thermosetting coating composition by using a blocked isocyanate having m-TMI as an isocyanate component as a curing agent, and It can be seen that a coating film excellent in appearance, non-yellowing upon baking, weather resistance, and acid rain resistance can be provided.

In addition, an example in which the thermosetting coating composition of the present invention was applied as a clear top coating for automobiles was explained, but the thermosetting coating composition of the present invention is not limited to this application. For example, it can be used as a colored paint for coating various articles. Further, the thermosetting coating composition of the present invention is particularly suitable as a top coat paint for automobiles and outdoor structures exposed to wind and rain, but can also be applied as an intermediate coat paint or an undercoat paint,
Furthermore, it can be applied as a paint for indoor articles, if necessary.

[0066]

As described above, the thermosetting coating composition according to claim 1 contains a hydroxyl group-containing resin and m-isopropenyl-α, α-dimethylbenzyl isocyanate (m-TMI) as an isocyanate component. The blocked isocyanate is an essential component.

The thermosetting coating composition according to claim 2 is a composition obtained by polymerizing the above-mentioned m-TMI into a copolymer and using it. The resin containing a hydroxyl group and m-isopropenyl as an isocyanate component. A blocked isocyanate containing a copolymer of -α, α-dimethylbenzyl isocyanate and a vinyl monomer containing no active hydrogen is an essential component.

Therefore, m is added to at least a part of the isocyanate component of the blocked isocyanate as the curing agent.
Since TMI or a copolymer thereof is used, low temperature curability can be improved. For example, a baking temperature of about 120 ° C. forms a sufficiently cured coating film excellent in water resistance, solvent resistance and the like. be able to. At the same time, a coating film having good appearance, non-yellowing at the time of baking, weather resistance and excellent acid rain resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a table showing the composition of clear paints of Examples of the present invention and Comparative Examples and the results of evaluation tests of coating films thereof.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Satoshi Owada Satoshi Owada 1141, Okawagahara, Iino, Fujioka-machi, Nishikamo-gun, Aichi Prefecture Aisin Kako Co., Ltd. (72) Minor Nagai Fujioka-cho, Nishikamo-gun, Aichi Iino 1141 Okawagahara 1 Aisin Kako Co., Ltd. (72) Inventor Yoshinori Narita 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Masataka Muramatsu 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Within the corporation

Claims (2)

[Claims] 1. A hydroxyl group-containing resin, and as an isocyanate component, m-isopropenyl-α,
A thermosetting coating composition comprising a blocked isocyanate containing α-dimethylbenzyl isocyanate as an essential component. 2. A hydroxyl group-containing resin, and as an isocyanate component, m-isopropenyl-α,
A thermosetting coating composition, which comprises, as an essential component, a blocked isocyanate containing a copolymer of α-dimethylbenzyl isocyanate and a vinyl monomer containing no active hydrogen.