GB2435192A

GB2435192A - Curable resin composition, clear coating composition, and multilayer coating film forming method using same

Info

Publication number: GB2435192A
Application number: GB0712047A
Authority: GB
Inventors: Hideaki Tsujioka; Junji Shimada; Hisaki Tanabe
Original assignee: Nippon Paint Co Ltd
Current assignee: Nippon Paint Co Ltd
Priority date: 2004-12-22
Filing date: 2007-06-21
Publication date: 2007-08-15
Also published as: GB0712047D0; WO2006068209A1; JP4902115B2; JP2006176632A

Abstract

Disclosed is a curable resin composition containing a hydroxyl group-containing acrylic resin and a polyfunctional isocyanate compound wherein the hydroxyl group-containing acrylic resin is one obtained by using a hydroxyalkyl(meth)acrylate containing a hydroxyalkyl group having 4-9 carbon atoms as at least a part of monomers. This curable resin composition contains a soft segment represented by the following general formula (1): -(CH2)n-(1) (wherein n represents an integer of not less than 4) in an amount of 25-50% by mass relative to the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound. In addition, the curable resin composition is so prepared that the ratio of a soft segment derived from a lactone-containing monomer is not more than 8% by mass relative to the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound.

Description

CURING RESIN COMPOSITION, CLEAR COATING COMPOSITION, AND

METHOD OF FORMING MULTI-LAYER COATING FILM USING THE SAME

Technical Field

The present invention relates to a curing resin composition, a clear coating composition including the above composition as a binder, and a method of forming a multi-layer coating film using the clear coating composition.

Background of the Invention

As a binder used for a top coat material for an automobile, a hydroxyl group-containing polymer and a melamine polymer curing agent are commonly used in combination. However, a cured coating film obtained by using such a melamine polymer as the curing agent generally exhibits poor acid resistance, and is particularly susceptible to degradation due to acid rain which has become a problem in recent years. For this reason, there is a problem that this type of coating film causes defects in an external appearance of the automobile. Since the poor acid resistance of a coating film obtained by using a melamine polymer as the curing agent is attributable to the triazine ring in the melamine polymer, the problem of poor acid resistance is not solved as long as the melamine polymer is used as the curing agent.

To solve the above problem, Japanese Unexamined Patent Application Publication No. Hei 2-45577 and Japanese Unexamined Patent Application Publication No. Hei 3-287650 describe a coating composition in which arnelamine polymer is not used. Because such a coating composition uses, as the cross-linkage point, an ester bond formed by reacting a carboxylic acid group with an epoxy group, the coating composition exhibits good acid resistance, and has a sufficient weather resistance as the top coating film for an automobile.

Furthermore, Japanese Unexamined Patent Application Publication No. 2003-253191 describes a clear coating composition which is composed of: a half-ester acid group-containing acrylic copolymer; an epoxy group-containing acrylic copolymer; and at least one carboxyl group-containing polymer selected from the group consistingof a carboxyl group-containingpolyesterpolymer and a carboxyl group-containing acryl polymer, and which includes a predetermined amount of soft segment part represented by the following formula: -(CH2) r -(where n represents an integer of 4 or more) . Such a clear coating composition can form a coating film having well-balanced physical properties in all properties including resistance against flaw, acid and solvent.

Disclosure of the Invention

It is an object of the present invention to provide a curing resin composition and a clear coating composition which allow the formation of a curing film (for example, a coating film) having further improved flaw resistance (in particular, scratch resistance) and acid resistance (in particular, sulfuric acid resistance), and to provide a method which makes it possible to form a multi-layer coating film with excellent resistance against flaw and acid can be formed.

As a result of extensive studies carried out in order to achieve the above-described object, the inventors of the present invention found out that the above object can be achieved by using the curing resin composition including the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound which is obtained by using hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms as the former component, and by setting the total amount of soft segment part and the content ratio of a soft segment part derived from a lactone-containing monomer to the predetermined amounts.

Thus, the present invention has been completed.

A curing resin composition according to the present invention includes a hydroxyl group-containing acrylic resin and a polyfunctional isocyanate compound. The hydroxyl group-containing acrylic resin is obtained by using hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms as at least a part of its monomers. The curing resin composition is prepared in the following manner. Specifically, the curing resin composition has a soft segment part represented by the following formula: (CH2) r -(1) (where n represents an integer of I or more) . This soft segment part is included by 25 to 50 mass% of the total amount of solid hydroxyl group-containing acrylic resin and the polyfunctional isocyariate compound. Moreover, the content ratio of a soft segment part derived from a lactone-containing monomer, out of the total amount of solid hydroxyle group-containing acrylic resin and the polyfunctional isocyanate compound, is not more than 8 mass%.

In the curing resin composition according to the present invention, hydroxyalkyl(meth)acrylate is preferably 4-hydroxybutyl acrylate.

In the curing resin composition of the present invention, the hydroxyl group-containing acrylic resin is preferably obtained by copolymerizing 30 to 60 mass% hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms, a 30 mass% or less hydroxyl group-containing ethylinic unsaturated monomer (excluding hydroxyalkyl (meth) acrylate) , and a 70 to 40 mass% ethylinic unsaturated monomer (excluding hydroxyalkyl(meth)acrylate and hydroxyl group-containing ethylinic unsaturated monomer) Furthermore, the hydroxyl group-containing acrylic resin according to the present invention preferably satisfies at least one of the following conditions including (i) to (iii).

(i) Thehydroxylgroup-cofltainiflgacryliCresiflhasaflumber average molecular weight of 1000 to 10000, and a hydroxyl value of 50 to 280 mgKOH/g.

(ii) In the hydroxyl group-containing acrylic resin, the content ratio of the soft segment part is 7 to 30 mass% to the total solid content of the hydroxyl group-containing acrylic resin.

(iii) In the hydroxyl group-containing acrylic resin, the content ratio of the lactone part, and the content ratio of a soft segment part derived from a lactone-containing monomer are respectively 25 mass% or less, and 15 mass% or less, to the total solid content of the hydroxyl group-containing acrylic resin.

In the curing resin composition according to the present invention, isocyanurate isocyanate compound preferably constitutes 60 mass% or more of the polyfunctional isocyanate compound.

The content ratio of the soft segment part of the polyfunctional isocyanate compound according to the present invention is preferably 60 mass% or less with respect to the total solid content of the polyfunctional isocyanate compound.

The mixing ratio of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound is preferably such that the number of isocyanate group in the polyfunctional isocyanate compound is 0.5 to 1.5 for every hydroxyl group of the hydroxyl group-containing acrylic resin.

The clear coating composition according to the present invention includes the curing resin composition according to the present invention as a binder.

io The method of forming a multi-layer coating film according to the present invention is a method of forming a multi-layer coating film having a top coat on an object to be coated, and is a method of applying the clear coating composition according to the present invention as the top coat.

In the method of forming a multi-layer coating film according to the present invention, it is preferable that, after an uncured base coating film is obtained by applying a base coating composition onto the object to be coated, the clear coating composition be applied onto the uncured base coating film to obtain an uncured clear coating film, and that the uncured base-coating film and the uncured clear coating film be simultaneously cured by heating.

The present invention makes it possible to provide a curing resin composition and a clear coating composition which allow the formation of a cured film (for example, a coating film) having further improved flaw resistance (in particular, scratch resistance) and further improved acid resistance (in particular, sulfuric acid resistance), and to provide a method which allows the formation of a multi-layer coating film with excellent flaw resistance and excellent acid resistance.

Detailed Description of the Preferred Embodiments

Hereinafter, the present invention will be described in detail by way of its preferred embodiments.

Firstly, the curing resincomposition according to the present invention will be described. Specifically, the curing resin composition according to the present invention includes a hydroxyl group-containing acrylic resin which serves as a resin for forming a cured film, and a polyfunctional isocyanate compound which serves as a curing agent. (i) The hydroxyl group-containing acrylic resin is obtained by using hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms as at least a part of its monomers. (ii) The curing resin composition is prepared in the following manner. Specifically, the curing resin composition has a soft segment part represented by the following formula: -(CH2) r -(1) (where n represents an integer of 4 or more) . This soft segment part is included by 25 to 50 mass% of the total amount of solid hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound. Moreover, the content ratio of a soft segment part derived from a lactone-containing monomer, out of the total amount of solid hydroxyle group-containing acrylic resin and the polyfunctional isocyanate compound, is not more than 8 mass%.

The curing system for the curing resin composition according to the present invention including the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound is as follows. Specifically, the isocyanate group in the polyfunctional isocyanate compound, and the hydroxyl group in the hydroxyl group-containing acrylic resin are reacted with each other by heating to form a cross-linkage point. The curing progresses as the hydroxyl group-containing acrylic resins are cross-linked through the intermediation of the polyfunctional isocyanate compound. Thereby, a high cross-linkage density is achieved. In the cured film formed by heat-curing after a coating film is formed by using the curing resin composition of the present invention, the soft segment part is present in the main chain or in the cross-linkage chain.

The presence of the soft segment part in the main chain or in the cross-linkage chain effectively imparts good flaw resistance to the cured film.

The amount of the soft segment part represented by the above general formula (1) is a value indicating the mass ratio of the soft segment part contained in the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound, which will be described below. The value of n is the above general formula (1) representing the soft segment part needs to be 4 or more.

This is because the soft segment part cannot fully exhibit its property as a soft segment when the value n is 3 or less.

As described above, the above-described value n represents an integer of 4 or more, preferably 4 to 9, and more preferably 4 or 5. When the above-described value n is 4 to 9, a monomer or polymer having the soft segment part represented by the above general formula (1) is easily produced, and is more likely to have excellent handling properties. In addition, two or more kinds of soft segment parts differing in the above-mentioned value of n may be used in combination.

By including the soft segment part at the above ratio, the curing resin composition of the present invention has an effect of further improving the acid resistance thereof while keeping the flaw resistance of the cured film at a high level. When the content ratio of the soft segment part in the total solid content of the curing resin composition is less than 25 mass%, the soft segment part cannot exhibit the sufficient effect of improving the flaw resistance. In contrast, when the content ratio of the soft segment part exceeds 50 mass%, there is a problem that a sufficient acid resistance and solvent resistance cannot be obtained. The lower limit of the content ratio of the soft segment part is preferably 30 mass%. On the other hand, the upper limit of the content ratio of the soft segment part is preferably 45 mass%.

The content ratio of the soft segment part derived from the lactone-containing monomer which can be used in obtaining the hydroxyl group-containing acrylic resin described below needs to be not more than 8 mass% with respect to the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound. When the content ratio of the soft segment part derived from the lactone-containing monomer in the total solid content of the curing resin composition exceeds 8 mass%, there is a problem that a cured film having a sufficient acid resistance cannot be obtained.

Incidentally, the hydroxyl group-containing acrylic resin having the soft segment part can be obtained by mixing a monomer having the soft segment part represented by the above general formula (1) at the time of polymerization reaction thereof. Accordingly, the content of the soft segment part in the hydroxyl group-containing acrylic resin can be determined through the theoretical calculation based on the mixing amount of the monomer composition used for the polymerization and on the amount of soft segment part inthemonomer. Thecontentofthesoftsegmentpartderived from the lactone-containing monomer in the curing resin composition of the present invention can be determined through the theoretical calculation based on the mixing amount of the lactone-containing monomer used for polymerization and on the amount of soft segment part contained therein. Furthermore, the content of the soft segment part derived from the hydroxyl group-containing acrylic resin and the content of the soft segment part derived from the polyfunctional isocyanate compound in the curing resin composition of the present invention can be determined through the theoretical calculation based on the mixing amount thereof and on the amount of the soft segment parts contained therein.

The hydroxyl group-containing acrylic resin used as the cured film forming resin in the curing resin composition of the present invention may be any acrylic resin (acrylic polymer) containing a hydroxyl group, and may include a carboxyl group, an epoxy group or the like. The hydroxyl group-containing acrylic resin according to the present invention needs to be a hydroxyl group-containing acrylic resin obtained by using hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms as at least a part of its monomer. The effect of the present invention, which is the effect of further improving the acid resistance while keeping the flaw resistance at high level, cannot be obtained unless such hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms is used.

Examples of such hydroxyalkyl(meth)acrylate having hydroxyalkyl group with 4 to 9 carbon atoms include 4-hydroxybutyl (meth) acrylate, 5-hydroxypentyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, 7-hydroxyheptyl (meth) acrylate, 8-hydroxyoctyl (meth) acrylate, 7-methyl-8-hydroxyoctyl (meth) acrylate, 2-methyl-8-hydroxyoctyl(meth)acrylate, and 9-hydroxynonyl(rneth)acrylate. Among these, 4-hydroxybutyl (meth) acrylate and 6-hydroxyhexyl (meth) acrylate are particularly preferred in view of the fact that they are readily available, and that they have excellent handling properties.

The hydroxyl group-containing acrylic resin according to the present invention is obtained by copolyrnerizing hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms and, as appropriate, other ethylinic unsaturated monomer (hydroxyl group-containing ethylinic unsaturated monomer and/or other ethylinic unsaturated monomer other than the above-mentioned hydroxyalkyl (meth) acrylate) Specific examples of a hydroxyl group-containing ethylinic unsaturated monomer other than the hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl(meth)acrylate, allyl alcohol, methacryl alcohol, and adducts of these with lactones (-propiolactone, dimethylpropiolactone, butyl lactone, y-valerolactone, -capro1actone, y-caprolactone, y-caprylo actone, crotonolactone, -valerol actone, ö-caprolactone and the like) . Incidentally, a monomer having the soft segment part may be used as at least a part of such a hydroxyl-group containing ethylinic unsaturated monomer. In particular, the adduct of 2-hydroxyethyl (meth) acrylate with c-caprolactone is preferably used. Such a hydroxyl-group containing ethylinic unsaturated monomer can be used by itself or in combination of two or more kinds thereof.

The other ethylinic unsaturated monomer is not specifically limited, and includes an ethylinic unsaturated monomer having a carboxyl group can be primarily cited.

Examples thereof include a (meth)acrylate derivative (for example, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, acrylate dirner, and a-hydro-w-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-l, 6-hexanediol) ) or the like obtained by adding c-caprolactone to acrylic acid) ; and unsaturated dibasic acid, and the half-ester thereof, half-amide and the half-thioester thereof (for example, maleic acid, fumaric acid, itaconic acid, thehalf-ester thereof, half-amideandhalf-thioester thereof, or the like) Moreover, examples of ethylinic unsaturated monomers other than those having a carboxyl group include (meth)acrylate ester monomer (for example, methyl(meth)acrylate, ethyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethyihexyl (meth) acrylate, lauryl methacrylate, phenyl acrylate, isobornyl(meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl(meth)acrylate, dicyclopentadienyl (meth) acrylate, dihydrodicyclopentadienyl(meth)acrylate, or the like), polymerizing aromatic compounds (for example, styrene, a-methyl styrene, vinylketone, t-butyl styrene, parachrolostyrene, vinylnaphthalene, or the like), polymerizing nitrile (for example, acrylonitrile, methacrylonitrile, or the like), a-olefin (for example, ethylene, propylene, orthe like), vinyl ester (for example, vinyl acetate, vinyl propionate, or the like), diene (for example, butadiene, isoprene, or the like), and, as appropriate, an isocyanate group-containing monomer or the like. Such other ethylinic unsaturated monomer can be used by itself or in combination of two or more kinds thereof.

The hydroxyl group-containing acrylic resin suitable for use in the present invention can be obtained by copolymerizing hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms and, as appropriate, another ethylinic unsaturated monomer. The polymerization method is not particularly limited, and any common method described in publicly-known documents, such as solution radical polymerization, can be used. n example of the polymerization method includes a method of stirring an appropriate radical polymerization initiator and a monomer mixture solution while dripping them into an appropriate solvent atapolymerization temperatureof 60 C to 160 C over the period of 2 hours to 10 hours. The radical polymerization initiator used herein is not particularly limited as long as it is the one commonly used for polymerization. Examples of the radical polymerization initiator include an azo compound (for example, dimethyl-2, and 2'-azobisisobutyrate), a peroxide (for example, t-butylperoxy-2-ethylhexanoate), and the like. The amount of such used initiator is generally 0.1 to 15 mass%, preferably 0.5 to 12 mass% with respect to the total amount of unsaturated monomers. The solvent which can be used herein is not particularly limited as long as it does not adverselyaffect the reaction. The examples include ketone, a hydrocarbon solvent (for example, propylene glycol monomethylether acetate or xylene), or the like. To control the molecular weight, mercaptan such as lauryl mercaptan, or a chain transfer agent such as ct-methylstyrene dimer can be used as appropriate.

The hydroxyl group-containing acrylic resin according to the present invention preferably has a number average molecular weight (Mn) of 1000 to 10000, more preferably 1100 to 8000. A number average molecular weight less than the lower limit described above tends to reduce the coating workability and the layer-mixing property with the cured film. On the other hand, a number average molecular weight more than the above-described upper limit tends to reduce a nonvolatile content during application of the hydroxyl group-containing acrylic resin. As a result, workability tends to decrease. The hydroxyl group-containing acrylic resin preferably has a number average molecular weight within the range of 1200 to 7000 in view of the external appearance of the cured film. The hydroxyl group-containing acrylic resin according to the present invention preferably has a hydroxyl value of 50 mgKOH/g to 280 mgKOH/g, and more preferably 70 mgKOH/g to 260 mgKOH/g.

A hydroxyl value more than the above-described upper limit tends to reduce the water resistance of the cured film. On the other hand, a hydroxyl value below the above-described lower limit tends to reduce the curability of the cured film.

The mixing ratio of the hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms for obtaining the hydroxyl group-containing acrylic resin according to the present invention is preferably 30 to 60 mass% with respect to the total amount of monomer used for producing the hydroxyl group-containing acrylic resin.

The mixing ratio of the hydroxyalkyl(meth)acrylate less than the above-described lower limit tends to reduce the flaw resistance of the obtained cured film, whereas the mixing ratio more than the above-described upper limit tends to reduce the acid resistance and the water resistance of the obtained cured film.

The mixing ratio of the hydroxyl group-containing ethylinic unsaturated monomer (the total amount of the above-described hydroxyalkyl(meth)acrylate and the other hydroxyl group-containing ethylinic unsaturated monomer) is preferably 5 to 60 mass% for obtaining the hydroxyl group-containing acrylic resin according to the present invention, more preferably 30 to 60 mass%. The mixing ratio of the hydroxyl group-containing ethylinic unsaturated monomer other than hydroxyalkyl(meth)acrylate is preferably 30mass% or less. Furthermore, themixing ratio of another ethylinic unsaturated monomer (excluding above-described hydroxyalky]. (meth) crylate and other hydroxyl group-containing ethylinic unsaturated monomers) is preferably 95 to 40 mass%, and more preferably, 70 to mass%. The mixing ratio of the hydroxyl group-containing ethylinic unsaturated monomer less than the above-described lower limit tends to reduce production stability, whereas the mixing ratio more than the above-described upper limit tends to reduce the water resistance of the obtained cured film.

Furthermore, the content ratio of all of the soft segment parts in the hydroxyl group-containing acrylic resin according to the present invention is preferably 7 to 30 mass% with respect to the total solid content thereof.

The content ratio of all of the soft segment parts less than the above-described lower limit tends to make it difficult to obtain a cured film having a sufficiently-improved flaw resistance, whereas the content ratio more than the above-described upper limit tends to make it difficult to obtain a cured film having sufficient acid resistance. In the hydroxyl group-containing acrylic resin according to the present invention, the content ratio of the lactone part (the lactone portion in a case where a lactone-containing monomer is used) is preferably not higher than 25 mass% with respect to the total solid content of the hydroxyl group-containing acrylic resin, and the content ratio of the soft segment part derived from the lactone-containing monomer is preferably not higher than 15 rnass% with respect to the total solid content of the hydroxyl group-containing acrylic resin. The content ratio of the soft segment part derived from the lactone-containing monomer more than the above-described upper limit tends to make it difficult to obtain a cured film having sufficient acid resistance.

Note that such a hydroxyl group-containing acrylic resin may be used by itself or in combination of two or more kinds thereof.

The polyfunctional isocyanate compound to be used as the curing agent in the curing resin composition of the present invention can include an aliphatic, alicyclic, aromatic group-containing aliphatic or aromatic polyfunctiorial isocyanatecompound. Thesuitableexamples include diisocyanate, dimer of diisocyanate, trimer of di isocyanate (preferably, isocyanurate-type isocyanate (so-called isocyanurate) ) . Such polyfunctional isocyanate compound may be of so-called asymmetry type.

As the above diisocyanate, the one including, in general, 5 to 24 carbon atoms, preferably 6 to 18 carbon atoms can be used. Examples of such dilsocyanate include trimethylene diisocyanate, tetrarnethylene di isocyanate, hexamethylene diisocyanate (MDI), 2, 2, -trimethylhexane diisocyanate, undecane diisocyanate-(l, 11), lysine ester diisocyanate, cyclohexane-l, 3-and 1, 4-diisocyanate, 1 -isocyanato-3-isocyanatomethyl-3, 5, 5-trirnethylcyclohexane(isophoronediisocyanate: IPDI), 4, 4'-diisocyanatodicyclomethane, o, o'-dipropylether diisocyanate, thiodipropyl diisocyanate, cyclohexyl-1, 4-diisocyanate, dicyclohexylmethane-4, 4'-diisocyanate, 1, 5-dirnethyl-2, 4-bis(isocyanatomethyl)benzene, 1, 3,5-trirnethyl-2, 4-bis(o-isocyanatoethyl)-benzene, 1, 3, 5-trirnethyl-2, 4-bis(isocyanatomethyl)benzene, 1, 3, 5-triethyl-2, 4-bis(isocyanatomethyl)benzene, d.icyclohexyldimethylmethane-4, 4'-diisocyanate, and the like. Also can be used is aromatic diisocyanate such as 2, 4-diisocyanatotoluene and/or 2, 6-diisocyanatotoluene, 4, 4'-diisocyanatodipl-ienylmethane, or 1, 4-diisocyanatoisopropylbenzene The above isocyanurate-type isocynate includes the above-mentioned trimer of diisocyanate. Such polyfunctional isocyanate compound may be used by itself or in combination of two or more kinds thereof.

It is preferable that the polyfunctional isocyanate compound used in the present invention at least partly include isocyanurate-type isocyanate compound.

Specifically, in the present invention, the above isocyanurate-type isocyanate compound may be used as a mixture produced by combining the isocyanurate-type isocyanate compound with an aliphatic, alicyclic, aromatic group-containing aliphatic, or aromatic polyfunctional isocyanate compound (favorably, the above diisocyanate) In this case, the content ratio of the isocyanurate-type isocyanate compound in the total amount of the polyfunctional isocyanate compound is preferably 60 mass% or more. A content ratio less than the above lower limit tends to make it difficult to obtain a cured film having sufficient acid resistance.

Note that the above soft segment part may be present in the above polyfunctional isocyanate compound. The content ratio of the above soft segment part in the polyfunctional isocyanatecompoundaccordingtothepresent invention is preferably 60 mass% or less with respect to the total solid content thereof.

The curing resin composition according to the present invention includes a hydroxyl group-containing acrylic resin as the cured-film forming resin, and a polyfunctional isocyanate compound as the curing agent. The mixing ratio of the hydroxyl group-containing acrylic resin to the polyfunctional isocyanate compound is preferably such that the number of isocyanate group in the polyfunctionai. isocyanate compound is within 0.5 to 1.5 for every hydroxyl group of

the hydroxyl group-containing acrylic resin. A content ratio of the polyfunctional isocyariate compound less than the above lower limit tends to make it impossible to obtain sufficient curing. On the other hand, a content ratio more than the above upper limit tends to cause unreacted isocyanate group to react with the moisture in the air. As a result, the Tm of cured film increases, and this leads to deterioration in flaw resistance.

The above-described curing resin composition may have an organic tin compound curing catalyst. The organic tin compound curing catalyst is not particularly limited. The examples thereof include dibutyltin dilaurate, dibutyltin diacetate, dibutyltin dioctate and the like. The mixing amount of the curing catalyst is preferably between the lower limit of 0.005 mass parts and the upper limit of 0.05 mass parts with respect to 100 mass parts of the total solid content of the polymer in the curing resin composition.

Various kinds of cured films such as a coating film, a coating material, a resin film and the like having excellent flaw resistance (particularly, scratch resistance) and excellent acid resistance (particularly, sulfuric acid resistance) can be obtained by curing the above-described curing resin composition of the present invention. A method of curing the curing resin composition of the present invention and the conditions thereof are not particularly limited, and the normal method is employed as appropriate. For example, a method of curing the composition by heat at 100 C to 180 C is favorably employed.

It is preferable that the curing resin composition of the present invention be prepared in a way that the crosslink density of the obtained cured film is not less than 0.8 lO3mol/cc (more preferably, 1.0 x to 2.5 x lO3mol/cc) Jhen the crosslink density of the obtained cured film is below the above lower limit, improvement in the stain resistance against water stain and the like tends to be insufficient, and the maintenance properties of flaw resistance tend to be reduced.

Such crosslink density is determined by the following method. Specifically, the dynamic elastic modulus (E') at the time when a temperature rises is determined by using a forced stretching vibration-type of viscoelasticity measurement equipment (Vibron available from Orientec Co., Ltd.) . Subsequently, the crosslink density is calculated from the temperature, at which the value E' is at the minimum, and this minimum value of E, by using the following equation.

The measurement frequency is set at 11 Hz. E' = 3nRT (n: crosslink density, R: gas constant, T: absolute temperature) The clear coatingcompositionof thepresent invention will be described below. The clear coating composition of the present invention is characterized by including the above-described curing resin composition of the present invention as a binder component. In addition to the above binder component, an ultraviolet absorber, ahindered amine light stabilizer, an anti-oxidant, cross-linking resin particles, a surface regulator or the like may be mixed into the above clear coating composition of the present invention.

In a case where the above cross-linking resin particles is used, the mixing ratio thereof is preferably between the lower limit of 0.01 mass% and the upper limit of 10 mass% with respect to the resin solid content of the clear coating composition of the present invention. The above lower limit is more preferably 0.1 mass%, and the above upper limit is more preferably 5 mass%. The amount of added cross-linking resin particles exceeding 10 mass% tends to deteriorate the external appearance of the obtained coating film. On the other hand, if the amount of added cross-linking resin particles is below 0.01 mass%, then an obtained rheology-control effect tends to be insufficient.

The clear coating composition of the present invent ion can be applied by spray coating, brush coating, dip coating, roll coating, flow coating or the like. The clear coating composition of the present invention can be used advantageously on any kinds of bases, for example, wood, metal, glass, cloth, plastic, foamor the like, particularly on plastic and metal (for example, steel, aluminum and an alloy thereof) surfaces. In particular, the clear coating composition of the present invention can be favorably used as a clear coating material for an automobile.

The method of forming a multi-layer coating film according to the present invention will be described below.

Specifically, the method of forming a multi-layer coating film according to the present invention is a method of forming a multi-layer coating film having a top coat on an object to be coated. The method is characterized in that the above-described clear coating composition according to the present invention is applied as a top coat.

Various kinds of bases, for example, a metal molding, a plastic molding, a foam and the like can be used as the above object to be coated. The object to be coated for an automobile, on which a multi-layer coating film is formed, includes a metal molding such as iron, aluminum and the alloy thereof, a plastic molding, or the like. The coating is preferably formed on a cationic electrodeposition-coatable metal molding. The surface of the above object to be coated has preferably been treated with chemical conversion.

Furthermore, an electrodeposited coating film may be formed on the object to be coated. A cationic-or anionic-type electrodeposition coating material can be used as the above electrodeposition coating material. However, a cationic electrodeposition material is preferable in view of corrosion resistance.

An intermediate coating film may be further formed as necessary. An intermediate coating material is used to form the intermediate coating film. The intermediate coating material is not particularly limited, and includes water-based or organic solvent-type intermediate coating material which is well known to those skilled in the art.

In the method of forming a multi-layer coating film according to the present invention, the following processes are preferable. Specifically, after an uncured base coating film is obtained by applying a base coating compo5ition onto the object to be coated, the clear coating composition of the present invention is applied onto the uncured base coating film to obtain an uncured clear coating film. Subsequently, the uncured base coating film and the uncured clear coating film are simultaneously heated and cured. It is also possible to apply the clear coating composition according to the present invention as a second clear coating on the cured film including base and clear coating films, and then to heat and cure the applied clear coating composition.

The above base coating is not particularly limited, and may include, for example, a coating film-forming resin, a curing agent, coloring pigment such as an organic, inorganic or luster material, and extender pigment. The form of the base-coating material is not particularly limited, and includes water-based or organic solvent-type base-coating material.

The method of applying the base-coating material onto the object to be coated is not particularly limited, and includes spray coating, rotary-spray coating or the like.

In view of an improved external appearance, multi-stage coating using these methods, or a coating method in which these methods are combined is preferable.

In the method of forming a multi-layer coating film according to the present invention, the thickness of the coating film formed of the above base-coating material is preferably between the lower limit of 10 pm and the upper limit of 20 pm as a dry-film thickness. In the method of forming a multi-layer coating film according to the present invention, in a case where the above base-coating material is water-based, it is desirable that the uncured base-coating film be previously heated at 40 C to 100 C for 2 to 10 minutes prior to the application of the above clear coating composition to obtain a well-finished coating film.

In the method of forming a multi-layer coating film according to the present invention, specific examples of a method of applying the clear coating composition of the present invention include a coating method by use of an electrostatic coaterof a rotary-spraycoating type, called Micro-Micro Bell or Micro Bell.

In the method of forming a multi-layer coating film according to the present invention, the thickness of the coating film formed of the above clear coating material is preferably between the lower limit of 30 pm and the upper limit of 45 pm as a dry-film thickness. It is preferable that the uncured base-coating film and the uncured clear coating film, which are formedby the above-describedmethod, be simultaneously heated for curing. Thereby, the multi-layer coating film is formed. The above heating is preferably performed within a range of the lower limit of 100 C and the upper limit of 180 C. The above lower limit and the upper limit are more preferably 120 C and 160 C, respectively. A heat-curing time is varied depending on a curing temperature. A heat-curing time of 10 to 30 minutes is suitable when the above heat curing temperature is used.

The film thickness of the multi-layer coating film thus obtained is preferably within a range of the lower limit of 40 pm and the upper limit of 65 pm. The multi-layer coating film obtained by the above method of forming a multi-layer coating film of the present invention has excellent flaw resistance (particularly, scratch resistance) and a very excellent acid resistance (particularly, sulfuric acid resistance).

Examples

Specific explanation will be given below of the present invention on the basis of examples and comparative examples. The present invention is not limited to the examples described below. A simple description of ht%U refers to "mass% in the present example.

(Synthesis Example 1) Synthesis of hydroxyl group-containing acrylic resin "a" 358.0 g of propylene glycol monomethylether acetate and 90.0 g of xylene were put into a glass container (2 liter) equippedwithastirringblade, anitrogen introducing tube, a cooling condenser, and a dropping funnel. Then, the mixture was heated at 130 C under a nitrogen atmosphere.

Using the dropping funnel, 100.0 g of propylene glycol monomethylether acetate, 100.0 g of tert-butyl peroxy-2-ethylhexanoate, 68.9 g of styrene, 312.0 g of n-butyl acrylate, 182. 1 g of 2-ethyihexyl methacrylate, and 437.0 g of 4-hydroxybutyl acrylate were dripped into the above container at a constant rate over three hours.

Thereafter, the mixture was maintained at 130 C for 0.5 hours. 10.0 g of tert-butyl peroxy-2-ethylhexanoate dissolved in 50.0 g of propylene glycol monomethylether acetate was dripped into the above mixture at a constant rate over the period of 30 minutes. Furthermore, the mixture was continuously heated at 130 C for 1.0 hour.

Thereby, the targeted hydroxyl group-containing acrylic resin "a" was obtained.

The value of molecular weight of the thus synthesized hydroxyl group-containing acrylic resin "a" equivalent to that of the standard polystyrene obtained by using GPC were Mn= 3900, andMw= 9700. The hydroxyl value was 170 mgKOH/g.

The calculation Tg was -35 C. The resin solid content was 62.8%. The lactone part content ratio was 0%. Moreover, the content ratio of the above soft segment part in the hydroxyl group-containing acrylic resin "a" was 16.93% with respect to the total solid content. The content ratio of the soft segment part derived from a lactone-containing monomer was 0%. The composition and the physical properties of the above-described hydroxyl group-containing acrylic resin "a" are shown in Table 1.

(Synthesis Examples 2 through 6) Synthesis of hydroxyl group-containing acrylic resins b through f Hydroxyl group-containing acrylic resins b through f were each synthesized in the same manner as that in Synthesis Example 1 except that the monomer components, solvents, and polymerization initiators shown in Table 1 were used, and that the mixing amounts thereof where set to be those shown in Table 1. It should be noted that a 1:2 adduct (available from DAICEL CHEMICAL INDUSTRIES, LTD.) of 2-hydroxyethyl methacrylate and E-capro lactone was used as Placcel FM-2.

A 1:5 adduct (available from DAICEL CHEMICAL INDUSTRIES, LTD.) of 2-hydroxyethyl methacrylate and -capro lactone was used as Placcel F'M-5.

The molecular weight (Mn) , the molecular weight (Mw) the hydroxyl value, the calculation Tg, the resin solid content, the lactone part content ratio, the content ratio of all of the soft segment parts in the resin solid content, the content ratio of soft segment part derived from a lactone-containing monomer, and the content ratio of the lactone-free monomer-derived soft segment part, of the synthesized hydroxyl group-containing acrylic resins b through f were as shown in Table 1.

[Table 1]

Hydroxyl-group-containing acrylic resin [*1] a b} c J d J e Propylene glycol monomethylether acetate (solvent) (g) 508.0 508.0 508.0 508.0 508.0 598.0 Xylene (solvent) (g) 90.0 90.0 90.0 90.0 90.0 tert- butyl peroxy-2-ethylhexanoate (polymerization initiator) (g) 110 110 110 110 110 110 Styrene (lactone-free monomer) (g) 68.9 200.0 160.0 150.0 140.0 20.0 n-butyl acrylate (lactone-free monomer) (g) 312.0 204.3 97.6 91.5 85.4 n-butyl methacrylate (lactone-free monomer) (g) 152.8 143.3 133.7 2-ethylhexyl acrylate (lactone-free monomer) (g) 95.0 2-ethyihexyl methacrytate (lactone-free monomer) (g) 182.1 74.5 40 37.5 35.0 2-hydroxyethyl acrylate (lactone-free monomer) (g) 100.0 4-hydroxybutyl acrylate (tactone-free monomer) (g) 437.0 349.6 327.8 305.9 435.0 6-hydroxyhexyl acrylate (lactone-free monomer) (g) 521.2 Placcel FM-2 (lactone-containing monomer) (g) 200.0 249.9 300 70.0 Placcel FM-5 (lactone-containing monomer) (g) 280.0 Total of monomer (g) 1000.0 1000.0 1000.0 1000.0 1000.0 1000.0 Calculation Tg ( C) -35 --16 -16 -17 -40 Hydroxyl value (mgKOH/g) 170 170 167 167 166 251 Molecular weight (Mn) 3900 4500 4300 4300 4500 4700 Molecular weight (Mw) 9700 12800 9900 9900 10800 11300 Resin solid content (mass%) 62.8 62.5 62.1 62.2 63.2 63.3 Content ratio of lactone part (mass%) 0 0.0 12.7 15.9 19.1 27.3 Content ratio of soft Content ratio or soft segment part derived from 0.00 0.00 7 82 9.77 11 73 16.74 lactone-containing monomer (mass%) segment part in solid Content ratio of soft segment part derived from 16.93 25 44 13 60 12.75 11.90 16 91 resin content lactone-free monomer (mass%) (mass%) ::tt0 or total soft segment parts 16.93 25.44 21.42 22.52 23.63 33.65 *1: Blank columns represent 0.

(Examples 1 through 5 and Comparative Example 1) The clear coating compositions of Examples 1 through and of Comparative Example 1 were obtained by mixing components according to the mixing ratio shown in Table 2, and by stirring the mixture with a disper. The above clear coating compositions were each diluted with thinner composed of propylene glycol monomethylether acetate / 3-ethyl ethoxypropionate = 1/2 (the mass ratio) by using a I'Jo. 4 ford cup so that a condition of 25 seconds/20 C was obtained. Incidentally, Sumi-Jour N-75 available from Sumika Bayer Urethane Co., Ltd. was used as the burette-type isocyanate curing agent.

Subsequently, a dull steel plate having an area of 150 x 300 x 0.8 mm which had been zinc-phosphate-treated was electrodeposition-coated with Power Top U-50 (a cationic electrodeposition coating material available from Nippon Paint Co.., Ltd.) so that a dry-film thickness of 20 was obtained, and was then baked at 160 C for 30 minutes to obtain a coated plate. Thereafter, the above coated plate was air-spray coated, in two stages with ORGA P-2 (a melamine-curing polyester resin-based gray intermediate coating material available from Nippon Paint Co., Ltd.) so that a dry-film thickness of 35.im was obtained. Here, ORGA P-2 had been pre-diluted with a No. 4 ford cup so that a condition of 25 seconds/20 C was obtained. Subsequently, the coated plate was baked at 14 0 C for 30 minutes, and then was cooled. Thereby, an intermediate coatingsubstratewas obtained. The above intermediate coating substrate was spray-coated with Super Rack M-260 Black (a water base coating material available from Nippon Paint Co., Ltd.) at room temperature of 25 C, and of humidity of 85% so that a dry-film thickness of 15 jim was obtained. Thereafter, the substrate was pre-heated at 80 C for 3 minutes. After the pre-heating, the above intermediate coating plate was cooled down to room temperature. Super Rack 0-170 (acrylic melarnine curing-type clear coating material available from Nippon Paint Co., Ltd.) was applied thereon as a first clear coating so that a dry-film thickness of 30 pm was formed.

The formed coating was heat-cured at 140 C for 25 minutes.

Then, the above first clear substrate was polished with #2000 wet sand paper, and thereafter each of the above diluted clear coating compositions was spray-coated so that a dry-film thickness of 35 pin was obtained. Subsequently, the coating was heated at 14 0 C for 25 minutes in a drying furnace, and thereby a multi-layer coating film was formed on the substrate.

Sown in Table 2 are the content ratios of the respective various kinds of soft segment parts (the content ratio of the soft segment part derived from a lactone-containing monomer, the content ratio of the soft segment part derived from a lactone-free monomer, the content ratio of all of the soft segment parts derived from a hydroxyl group-containing acrylic resin, the content ratio of the soft segment part derived from an isocyanate curing agent, and the content ratio of all of the soft segment parts (rnass%) ) in the total solid content of the clear coating compositions obtained in Examples 1 through and in Comparative Example 1.

<Acid resistance test> With regard to the multi-layer coating films obtained by using the clear coating compositions of Examples 1 through 5 and Comparative Example 1, the acid resistance thereof against a 40% sulfuric acid aqueous solution was evaluated by using the following evaluation method.

Specifically, first, the 40% sulfuric acid aqueous solution was prepared by using ion-exchanged water and special grade sulfuric acid. Subsequently, 0.6 ml of the above sulfuric acid aqueous solution was dripped onto each of the multi-layer coating films obtained by using the clear coating compositions of Examples 1 through 5 and Comparative Example 1. The resultant multi-layer coating film was held at 80 C for 30 minutes in a heating oven, and then was washed with water. Thereafter, spot marks on the multi-layer coating film were visually observed, and were evaluated on the basis of the following criteria. The obtained results are shown in Table 2.

A: The peeling of the coating film is not observed in the spot portion.

B: The partial peeling of the coating film is observed in the spot portion.

C: The coating film is completely peeled off of the spot portion.

<Scratch resistance test> The scratch resistance of the multi-layer coating f jim obtained by using the clear coating compositions of Examples 1 through 5 and Comparative Example 1, was evaluated by using the following evaluation method.

Specifically, first, a test dust compositioncomposed of 15 g of a test dust (a mixture of seven kinds of dust with a particle size of 27 to 31 pm) and 100 g of water was spread on the platform of a miniature car washing machine.

Subsequently, the miniature car washing machine was rotated (at 45 rpm) and reciprocally moved once without running water, and thereby the dust was caused to adhere onto the car wash brush. Thereafter, the coated plate (70 mm x 150 mm) was fixed onto the platform, and then about 5 g of molding sand was sprinkled onto the coated plate. Subsequently, the miniature car washing machine was rotated (at 96 rpm) and then was reciprocally moved three times without running water. After the test, the coated plate was washed with water, and then was dried. The 20 gloss of the coated plate was measured to calculate a gloss retention rate (initial 20 GR (%)) which is a ratio of the measured 20 gloss to the 20 gloss before the test. The obtained results are

shown in Table 2.

[Table 2J

Content ratio, Comparative Content of soft Examples (solid content loading example 1 Content ratio of segment parts Content ratio of Hydroxyl group-containing segment parts derived amount) (mass%) [*1] (solid content lactone parts derived from total soft segment acrylic resin from lactone-Iree ______ ______ ______ ______ ______ loading (mass%) lactone-containing parts (mass%) monomer (mass%) monomer (mass%) 1 2 3 4 amount) ___________________ ______________________ ___________________ ______ ______ ______ ______ ______ (mass%) Hydroxyl group-containing o 0 16.93 16.9 63.2 acrylic resin a Hydroxyl group-containing 0.0 0.0 25.4 25.4 64.4 acrylic resin b Hydroxyl group-containing 12.7 7.8 13.6 21.4 63.6 acrylic resin c Hydroxyl group-containing 15.9 98 12.8 22.5 63.6 acrylic resin d Hydroxyl group-containing 19.1 11.7 11.9 23.5 63.7 acrylic resin e L.J _________ ____________ _____________ ____________ ____ ____ Hydroxyl group-containing 27.3 16.7 16.9 33.7 53.8 acrylic resin Burette-type isocyanate 0 0 0 52.7 36.8 35.6 36.4 36.4 36.3 46.2 curing agent Content ratio of lactone parts in the total solid content of clear coating composition (mass%) 0 0.0 8.1 10.1 12.2 14.7 Content ratio of soft segment parts derived from lactone-containing monomer (mass%) 0 0.0 5.0 6.2 7.5 9.0 Content ratio of soft Content ratio of soft segment parts derived from lactone-free monomer (mass%) 10.7 16.4 8.6 8.1 7.6 9.1 segment parts in the Content ratio of total soft segment parts derived from hydroxyl group-containing acrylic total solid content of 10.7 16.4 13.6 14.3 15.1 18.1 resin (mass%) ______ clear coating composition (mass !0) Content ratio of soft segment parts derived from isocyanate curing agent (mass%) 19.4 18.8 19.2 19.2 19.1 24.3 Total 30.1 35.1 32.8 33.5 34.2 42.5 Acid resistance A B B B B C Car wash scratch resistance 96 97 95 96 97 98 *j Blank columns represent 0.

As apparent from the results shown in Table 2, the multi-layer coating film obtained by using the clear coating composition of the present invention has excellent flaw resistance (scratch resistance) and excellent acid resistance (sulfuric acid resistance), while the multi-layer coating film obtained by using the clear coating compositions of Comparative Example has a deteriorated acid resistance.

Industrial Applicability

Asdescribedabove, according to thepresent invention, it is possible to obtain a curing resin composition which allows the formation of a cured film (for example, a coating film) having a flaw resistance (particularly scratch resistance) and an acid resistance (particularly sulfuric acid resistance) which are further improved. Accordingly, the clear coating composition of the present invention including the curing resin composition of the present invention as a binder makes it possible to form a clear coating filrnhaving excellent flaw resistance and excellent acid resistance. Moreover, according to the method of forming a multi-layer coating film of the present invention usingtheclearcoatingcompositionofthepresent invention, it is possible to efficiently and surely form a multi-layer coating film for an automobile having excellent flaw resistance and excellent acid resistance.

Claims

C I I4 S 1. A curing resin composition, comprising a hydroxyl

group-containing acrylic resin and a polyfunctional isocyanate compound, wherein the hydroxy]. group-containing acrylic resin is a hydroxyl group-containing acrylic resin obtained by using hydroxyalkyi(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms as at least a part of its monomer, the curing resin composition includes 25 to 50-mass% soft segment part with respect to the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound, the soft segment part being represented by the following general formula (1): -(CH2) r -(1) (where n represents an integer of 4 or more), and the curing resin composition is prepared so that the content ratio of a soft segment part derived from a lactone-containing monomer is not more than 8 rnass% with respect to the total solid content of the hydroxyl group-containing acrylic resin and the polyfunctional isocyanate compound.

2. The curing resin composition according to claim 1, wherein the hydroxyalkyl(meth)acrylate is 4-hydroxybutyl acrylate.

3. The curing resin composition according to claim 1, wherein the hydroxyl group-containing acrylic resin is obtained by copolyrnerizing 30 to 60 mass% of hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 4 to 9 carbon atoms, 30 mass% or less of hydroxyl group-containing ethylinic unsaturated monomers (excluding the hydroxyalkyl(rneth)acrylate) and 70 to 40 mass% of ethylinic unsaturated monomer (excluding the hydroxyalkyl(meth)acrylate and the hydroxyl group-containing ethylinic unsaturated monomer).

4. The curing resin composition according to claim 1, wherein the number average molecular weight of the hydroxyl group-containing acrylic resin is 1000 to 10000, and the hydroxyl value of the hydroxyl group-containing acrylic resin is 50 to 280 mgKOH/g.

5. The curing resin composition according to claim 1, wherein the content ratio of the soft segment part in the hydroxyl group-containing acrylic resin is 7 to 30 mass% with respect to the total solid content of the hydroxyl group-containing acrylic resin.

6. The curing resin composition according to claim 1, wherein the content ratio of the lactone part in the hydroxyl group-containing acrylic resin, and the content ratio of the soft segment part derived from the lactone-containing monomer are respectively 25 mass% or less and 15 mass% or less, with respect to the total solid content of the hydroxyl group-containing acrylic resin.

7. The curing resin composition according to claim 1, wherein an isocyanurate-type isocyanate compound constitutes 60 mass% or more o the polyfunctional isocyanate compound.

8. The curing resin composition according to claim 1, wherein the content ratio of the soft segment part in the polyfunctional isocyanate compound is 60mass% or less with respect to the total solid content of the polyfunctional isocyanate compound.

9. The curing resin composition according to claim 1, wherein the mixing ratio of the hydroxyl group-containing acrylic resin to the polyfunctional isocyanate compound is such that the number of isocyanate group in the polyfunctional isocyanate compound is within 0.5 to 1.5 for every hydroxyl group of the hydroxyl group-containing acrylic resin.

10. Aclearcoatingcompositionincludingthecuringresiri composition according to claim 1 as a binder.

11. A method of forming a multi-layer coating film having a top coat on an object to be coated, wherein the clear coating composition according to claim 10 is applied as the top coat.

12. The method of forming a multi-layer coating film according to claim 11, wherein a base coating composition is applied onto the object to be coated to obtain an uncured base coating film, then the clear coating composition is applied onto the uncured base coating film to obtain an uncured clear coating film, and then the uncured base coating film and the uncured clear coating film are simultaneously cured by heating.