Patent Description:
Metal cans such as drink cans and food cans have coatings fo5.1rmed of organic resins for preventing foods from causing corrosion of metals (corrosion resistance) and preventing degradation of flavor and relish of contents (flavor preservability). These coatings are required to have processability, corrosion resistance, adhesion to metal materials, curability, and the like. Furthermore, depending on usage, the coatings may be subjected to high-temperature high-humidity conditions for retort-sterilization and the like. Also in such a case, the coating film is required to maintain adhesion to a metal material as a matter of course, and is also required to prevent blushing of the coating film particularly when the coating film is used for outer surface sides of metal lids and the like.

To date, as a coating material having the above described corrosion resistance, flavor preservability, and tolerance for withstanding a can forming process, an epoxy-based coating material such as an epoxy-phenol-based coating material, an epoxy-amino-based coating material, and an epoxy-acryl-based coating material, a polyester-based coating material such as a polyester-phenol-based coating material and a polyester-amino-based coating material, and a vinyl chloride-based coating material have been widely used. However, recent research reports that bisphenol A that is a material of an epoxy resin is likely to have an estrogen action and affect brains of fetuses and infants. Furthermore, a vinyl chloride-based coating material has a problem associated with a stabilizer and a problem that dioxin is generated during incineration. Formaldehyde, which is used as a material of phenol resin, amino resin, and the like and remains in a coating material, is known to be harmful to human bodies due to carcinogenicity and the like and adversely affect flavor preservability of contents. Moreover, there is a concern about influence on environmental pollution and work environment due to use of an organic solvent.

Because of such concerns about various adverse effects on human bodies, an aqueous coating material in which these materials are not used is commercially required. However, satisfactory performance cannot be obtained in practice as usage for cans.

From these viewpoints, as a resin composition for a metal container or a metal lid, for example, an aqueous coating composition in which an acrylic-modified polyester resin obtained by graft polymerization of a polymerizable unsaturated monomer component with polyester resin having an ethylene-based double bond at a resin end portion, and a β-hydroxyalkylamide crosslinking agent are dispersed in an aqueous medium, is suggested (Patent Literature <NUM>).

Patent Literature <NUM> suggests a coating composition formed of polyester polyol and a block polyisocyanate curing agent. Patent Literature <NUM> relates to aqueous resin compositions excellent as a coating material applied on internal surfaces of cans, to coating materials containing the aqueous resin composition, and to coatings provided thereby. Furthermore Patent Literature <NUM> relates to metal plates coated with the coating material. The compositions 1d, 2d and 2i respectively in tables <NUM>, <NUM> and <NUM> disclose compositions comprising <NUM>,<NUM> cyclohexanedimethanol, propylene glycol and ethylene glycol, trimellitic anhydride and terephthalic acid. The acid value of the three compositions is respectively <NUM>, <NUM> and <NUM> eq/ton.

However, in a case where an acrylic-modified polyester resin having a high acid value is used as described in Patent Literature <NUM>, processability cannot be sufficiently obtained. Furthermore, there is a problem that an unreacted product of the β-hydroxyalkylamide crosslinking agent remains in a coating film, and curability and retort blushing property are degraded. The coating film obtained by reaction between polyester and block polyisocyanate as described in Patent Literature <NUM> has a problem that water resistance is poor, and blushing occurs when the contents are sterilized.

An object of the present invention is to provide a polyester resin and a polyester resin composition that can be cured by the polyester resin alone without substantially containing a curing agent, and thus allow reduction of harmful outgas and can form a coating film having excellent characteristics such as curability, retort resistance, and processability.

The inventors of the present invention have found, through various examinations about the foregoing, that a polyester resin composition that contains a polyester resin having a predetermined acid value and that has a predetermined content or more of tetrahydrofuran-insoluble matter when a predetermined heating treatment has been performed, can be cured by the polyester resin alone without substantially using a curing agent. Furthermore, the inventors have found that, when a resin composition is specified and an amount of catalyst is controlled, a polyester resin coating film that has curability and processability in an excellently balanced manner, that allows no harmful outgas to be generated, and that has remarkably enhanced retort resistance can be obtained, and have achieved the present invention.

The present invention can provide a polyester resin composition that does not generate harmful outgas caused by a curing agent, has curability and processability in an excellently balanced manner, and has remarkably enhanced retort resistance, and a coating film thereof.

An embodiment of the present invention will be described below in detail. A polyester resin composition of the present invention contains a polyester resin (A) and satisfies the following requirements (i) to (iii).

The requirement (i) will be described. The polyester resin (A) needs to have an acid value of <NUM> eq/ton or more. The acid value is preferably <NUM> eq/ton or more, more preferably <NUM> eq/ton or more, and even more preferably <NUM> eq/ton or more. If the acid value is smaller than the above-described value, the number of carboxyl groups as crosslinking points is small, and curability may thus be degraded. Furthermore, if the acid value is smaller than the above-described value, a thermal decomposition reaction progresses more preferentially than a curing reaction when heating to <NUM> is performed, and processability may thus be degraded. In a case where the acid value has the above-described lower limit value or more, aqueously dispersing operation can be facilitated. The upper limit value of the acid value is, but is not particularly limited to, preferably <NUM> eq/ton or smaller in order to reduce an amount of an oligomer or an unreacted product of an acid component during an acid addition reaction.

The acid value of the polyester resin (A) of the present invention can be imparted by any method. Examples of the method for imparting the acid value include a method in which addition reaction of polycarboxylic acid anhydride is performed at the late stage of polycondensation, and a method in which a prepolymer (oligomer) is caused to have a high acid value at the stage of the prepolymer, and polycondensation of the prepolymer is subsequently performed to obtain a polyester resin having the acid value. Since an operation is easy and the target acid value can be easily obtained, the former method in which addition reaction is performed is preferable.

In a compound having a polycarboxylic acid anhydride group in a molecule for imparting the acid value to the polyester resin (A) of the present invention, examples of carboxylic acid monoanhydride include monoanhydrides such as phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, citraconic anhydride, and <NUM>-(<NUM>,<NUM>-dioxotetrahydrofurfuryl)-<NUM>-cyclohexene-<NUM>,<NUM>-dicarboxylic anhydride, hexahydrophthalic anhydride, and tetrahydrophthalic anhydride. One or more of them can be selected and used. Among them, trimellitic anhydride is preferable from the viewpoint of versatility and economical efficiency.

In the compound having a polycarboxylic acid anhydride group in a molecule for imparting the acid value to the polyester resin (A) of the present invention, examples of carboxylic acid polyanhydride include pyromellitic anhydride, <NUM>,<NUM>,<NUM>,<NUM>-butanetetracarboxylic acid dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-pentanetetracarboxylic acid dianhydride, <NUM>,<NUM>',<NUM>,<NUM>'-benzophenonetetracarboxylic acid dianhydride, cyclopentanetetracarboxylic acid dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic acid dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic acid dianhydride, ethylene glycol bistrimellitate dianhydride, <NUM>,<NUM>',<NUM>,<NUM>'-diphenyltetracarboxylic acid dianhydride, thiophene-<NUM>,<NUM>,<NUM>,<NUM>-tetracarboxylic acid dianhydride, ethylene tetracarboxylic acid dianhydride, <NUM>,<NUM>'-oxydiphthalic dianhydride, and <NUM>-(<NUM>,<NUM>-dioxotetrahydro-<NUM>-furanyl)-<NUM>-methyl-<NUM>-cyclohexene-<NUM>,<NUM>-dicarboxylic acid anhydride. One or more of them can be selected and used. Among them, ethylene glycol bistrimellitate dianhydride is preferable.

In the compound having a polycarboxylic acid anhydride group in a molecule for imparting the acid value, the carboxylic acid monoanhydride or the carboxylic acid polyanhydride can be used alone, or the carboxylic acid monoanhydride and the carboxylic acid polyanhydride can be used in combination.

Requirement (ii) will be described. The polyester resin composition of the present invention substantially contains no curing agent. The "substantially contains no curing agent" means that "a content of a curing agent is smaller than <NUM> part by mass (in terms of solid content) with respect to <NUM> parts by mass (in terms of solid content) of the polyester resin (A)".

In the description herein, the curing agent represents a known curing agent that reacts with the polyester resin (A) of the present invention to form a crosslinked structure. The crosslinked structure is, for example, in such a form that unsaturated double bonds in polyester resin are caused to react by radical addition reaction, cation addition reaction, anion addition reaction, or the like, to form an intermolecular carbon-carbon bond, or such a form that an intermolecular bond is formed by condensation reaction, polyaddition reaction, transesterification reaction, or the like of polyvalent carboxylic acid groups and polyhydric alcohol groups in the polyester resin. Examples of the curing agent include phenol resin, amino resin, an isocyanate compound, an epoxy compound, a β-hydroxylamide compound, and a resin containing an unsaturated bond.

In the polyester resin composition of the present invention, a content of the curing agent is smaller than <NUM> part by mass with respect to <NUM> parts by mass of the polyester resin (A) (solid content). The content of the curing agent is more preferably smaller than <NUM> parts by mass and even more preferably smaller than <NUM> parts by mass, and the polyester resin composition most preferably contains no curing agent. If the content of the curing agent is larger than a value in the above-described range, economical efficiency may become poor, and, furthermore, processability may be degraded due to self-condensation reaction in the curing agents, a blocking agent is volatilized and harmful outgas such as formaldehyde is generated, and stability for long time storage may be degraded.

Requirement (iii) will be described. A content of tetrahydrofuran-insoluble matter needs to be <NUM> mass% or more when the polyester resin composition of the present invention has been heated at <NUM> for one hour. In a case where the content of tetrahydrofuran (THF)-insoluble matter is <NUM> mass% or more, the polyester resin composition having retort resistance and processability in an excellently balanced manner, and a coating film thereof can be obtained. The content of THF-insoluble matter is preferably <NUM> mass% or more, more preferably <NUM> mass% or more, and even more preferably <NUM> mass%. If the content of the THF-insoluble matter is smaller than <NUM> mass%, curability may become insufficient and the coating film may have insufficient toughness, and thus, retort resistance may be reduced, and it may be impossible to withstand a forming process on a can after coating is formed on a metal plate.

In the description herein, "a content of tetrahydrofuran-insoluble matter is <NUM> mass% or more when the polyester resin composition has been heated at <NUM> for one hour" means that the polyester resin composition is applied onto copper foil such that the thickness is <NUM> after being dried, heat is applied at <NUM> for one hour, and a sample having a longitudinal direction of <NUM> and a transverse direction of <NUM> is obtained, and the mass of the sample before immersion in THF is defined as (X), and the mass of the sample which has been immersed in <NUM> of THF at <NUM> for one hour, and thereafter dried at <NUM> for <NUM> minutes, is defined as a mass (Y) after immersion in THF, and a content of the THF-insoluble matter obtained by the following equation is <NUM> mass% or more.

A content of THF-insoluble matter is preferably smaller than <NUM> mass% when the polyester resin composition of the present invention has been heated at <NUM> for <NUM> minutes. The content of THF-insoluble matter is more preferably smaller than <NUM> mass% and even more preferably smaller than <NUM> mass%, and there is no problem if the content thereof is <NUM> mass%. In a case where the content of THF-insoluble matter is smaller than the above-described value under a heating condition of <NUM>, which is relatively low temperature, generation of aggregate can be inhibited when the polyester resin composition is dissolved in a solvent or formed into an aqueous dispersion.

In a case where the polyester resin composition of the present invention contains the polyester resin (A) and satisfies the above-described requirements (i) to (iii), the polyester resin composition can be cured by the polyester resin alone without substantially using a curing agent. Therefore, the polyester resin composition of the present invention may be formed merely of the polyester resin (A) without containing a component other than the polyester resin (A).

Next, the polyester resin (A) of the present invention will be described. The polyester resin (A) of the present invention is mainly formed of a polycarboxylic acid component and a polyol component.

The polyester resin (A) of the present invention preferably includes, as a polyol component, a diol (a) (hereinafter, may be referred to as component (a)) having two primary hydroxy groups and having no alicyclic structure. Examples of the diol (a) having two primary hydroxy groups and having no alicyclic structure in the polyester resin (A) include aliphatic glycols such as ethylene glycol, <NUM>,<NUM>-propanediol, <NUM>-butyl-<NUM>-ethyl-<NUM>,<NUM>-propanediol, <NUM>-methyl-<NUM>,<NUM>-propanediol, neopentyl glycol, <NUM>-methyl-<NUM>,<NUM>-pentanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-diethyl-<NUM>,<NUM>-pentanediol, <NUM>,<NUM>-hexanediol, <NUM>-methyl-<NUM>,<NUM>-octanediol, <NUM>-methyl-<NUM>,<NUM>-hexanediol, <NUM>-methyl-<NUM>,<NUM>-heptanediol, <NUM>-methyl-<NUM>,<NUM>-octanediol, and <NUM>,<NUM>-nonanediol, and polyether glycols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. One or more of them can be selected and used. Among them, ethylene glycol, <NUM>-methyl-<NUM>,<NUM>-propanediol, and <NUM>,<NUM>-hexanediol are preferably used.

A copolymerization ratio of the diol (a) having two primary hydroxy groups and having no alicyclic structure in the polyester resin (A) is preferably <NUM> to <NUM> mol%, more preferably <NUM> to <NUM> mol%, and even more preferably <NUM> to <NUM> mol%, with respect to the entire polyol component. Within the above-described range, curability and retort resistance are preferable.

The polyester resin (A) of the present invention preferably includes, as a polyol component, a diol (b) having an alicyclic structure. In a case where the diol (b) (hereinafter, may be referred to as component (b)) having an alicyclic structure is contained, both processability and retort resistance can be easily achieved. Examples of the diol (b) that constitutes the polyester resin (A) of the present invention and has an alicyclic structure include <NUM>,<NUM>-cyclohexanedimethanol, <NUM>,<NUM>-cyclohexanedimethanol, <NUM>,<NUM>-cyclohexanedimethanol, tricyclodecane glycols, and hydrogenated bisphenols. One or more of them can be selected and used. Among them, <NUM>,<NUM>-cyclohexanedimethanol is preferably used from the viewpoint of curability, processability, and retort resistance.

A copolymerization ratio of the diol (b) that constitutes the polyester resin (A) of the present invention and has an alicyclic structure is preferably <NUM> to <NUM> mol%, more preferably <NUM> to <NUM> mol%, and even more preferably <NUM> to <NUM> mol%, with respect to the entire polyol component. Within the above-described range, processability is preferable.

The polyester resin (A) of the present invention preferably includes, as a polyol component, a diol (c) (hereinafter, may be referred to as component (c)) having one primary hydroxy group and one secondary hydroxy group and having no alicyclic structure. Examples of the diol (c) having one primary hydroxy group and one secondary hydroxy group and having no alicyclic structure in the polyester resin (A) of the present invention include <NUM>,<NUM>-propylene glycol and <NUM>,<NUM>-butanediol. One or more of them can be selected and used. Among them, <NUM>,<NUM>-propylene glycol is preferably used.

A copolymerization ratio of the diol (c) having one primary hydroxy group and one secondary hydroxy group and having no alicyclic structure in the polyester resin (A) of the present invention is preferably <NUM> to <NUM> mol%, more preferably <NUM> to <NUM> mol%, and even more preferably <NUM> to <NUM> mol%, with respect to the entire polyol component. Within the above-described range, curability and retort resistance are preferable.

The polyester resin (A) of the present invention preferably includes, as a polyol component, the component (a), and one or both of the component (b) and the component (c). The component (a) allows an ester bond to be easily formed whereas the component (b) and the component (c) allow an ester bond to be cleaved more easily than the component (a). Therefore, in a case where the component (a), and the component (b) and/or the component (c) are contained, rearrangement and re-bonding of ester bonds are promoted during heating treatment, a content of THF-insoluble matter is increased, and a coating film having both curability and flexibility, i.e., processability, can be estimated to be formed.

Examples of the polycarboxylic acid component constituting the polyester resin (A) of the present invention include aromatic dicarboxylic acid components such as terephthalic acid, isophthalic acid, orthophthalic acid, <NUM>,<NUM>-naphthalenedicarboxylic acid, <NUM>, <NUM>-naphthalenedicarboxylic acid, and <NUM>,<NUM>-naphthalenedicarboxylic acid. One of them may be used alone, or two or more of them may be used in combination. Among them, terephthalic acid and <NUM>,<NUM>-naphthalenedicarboxylic acid are preferable.

Examples of another polycarboxylic acid component constituting the polyester resin (A) of the present invention include aliphatic dicarboxylic acid components and alicyclic dicarboxylic acid components. Examples of the aliphatic dicarboxylic acid component include succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and dimer acid. Examples of the alicyclic dicarboxylic acid component include <NUM>,<NUM>-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, and <NUM>,<NUM>-cyclohexene dicarboxylic acid. One or more of them may be selected and used. Among them, adipic acid and <NUM>,<NUM>-cyclohexanedicarboxylic acid are preferable from the viewpoint of reactivity and economical efficiency.

In a case where the polyester resin (A) has the aliphatic dicarboxylic acid component and the alicyclic dicarboxylic acid component as structural units, the copolymerization ratio thereof is preferably <NUM> to <NUM> mol% with respect to the polycarboxylic acid component. The copolymerization ratio is more preferably <NUM> to <NUM> mol%. Outside the above-described range, a glass transition temperature of the polyester resin (A) is significantly lowered, and retort resistance may be reduced.

As the polycarboxylic acid component constituting the polyester resin (A) of the present invention, an unsaturated dicarboxylic acid (d) is also preferably contained as a structural unit. In a case where the unsaturated dicarboxylic acid (d) is contained, curability can be enhanced through reaction of generating intermolecular carbon-carbon bond by cleavage of an unsaturated bond during heating treatment. Examples of the unsaturated dicarboxylic acid (d) include fumaric acid, maleic acid, itaconic acid, citraconic acid, <NUM>,<NUM>-norbornane dicarboxylic acid, tetrahydrophthalic acid, and acid anhydrides thereof. One or more of them can be used.

In a case where the polyester resin (A) has the unsaturated dicarboxylic acid (d) as a structural unit, a copolymerization ratio of the unsaturated dicarboxylic acid (d) is preferably <NUM> to <NUM> mol% with respect to the polycarboxylic acid component. The copolymerization ratio is more preferably <NUM> to <NUM> mol%. Within the above-described range, both processability and curability can be achieved.

The polyester resin (A) of the present invention preferably has a branched structure. Having a branched structure means that a branched structure is contained in a main chain of polyester. Examples of a method for introducing a branched structure into the polyester resin (A) include a method in which a tri- or higher functional component is copolymerized as a part of the polycarboxylic acid component and/or the polyol component in polycondensation of polyester. Examples of the tri- or higher functional polycarboxylic acid component include trimellitic acid, pyromellitic acid, and benzophenone tetracarboxylic acid. Examples of the tri- or higher functional polyol include glycerin, trimethylolethane, trimethylolpropane, mannitol, sorbitol, pentaerythritol, and α-methyl glucoside. In a case where the polyester resin (A) has a branched structure, a crosslink density is enhanced when rearrangement and re-bonding of ester bonds occur during heating treatment, and thus, the content of THF-insoluble matter is increased, and curability and processability can be enhanced.

A copolymerization ratio of the tri- or higher functional polycarboxylic acid component is preferably <NUM> mol% or more, more preferably <NUM> mol% or more, and even more preferably <NUM> mol% or more when the entire polycarboxylic acid component is <NUM> mol%. The copolymerization ratio thereof is preferably <NUM> mol% or smaller, more preferably <NUM> mol% or smaller, even more preferably <NUM> mol% or smaller, and particularly preferably <NUM> mol% or smaller. The copolymerization ratio of the tri- or higher functional polyol component is preferably <NUM> mol% or more, more preferably <NUM> mol% or more, and even more preferably <NUM> mol% or more when the entire polyol component is <NUM> mol%. The copolymerization ratio thereof is preferably <NUM> mol% or smaller, more preferably <NUM> mol% or smaller, even more preferably <NUM> mol% or smaller, and particularly preferably <NUM> mol% or smaller. If the copolymerization ratio of each of the polycarboxylic acid component and the polyol component is larger than the above-described values, flexibility of the polyester resin is lost, and processability may be degraded or gelation may occur during polymerization of polyester.

For the polycarboxylic acid component and the polyol component constituting the polyester resin (A) of the present invention, a raw material derived from biomass resource can be used. The biomass resource includes, for example, resources stored by converting sunlight energy into the form of starch, cellulose, or the like through photosynthesis of plants, and products obtained by processing bodies of animals that eat plant bodies and grow or processing plant bodies or animal bodies. Among them, the biomass resource is more preferably plant resource, and examples thereof include wood, rice straw, rice hull, rice bran, old rice, maize, sugar cane, cassava, sago palm, soy pulp, corn cob, tapioca meal, bagasse, vegetable oil meal, tubers, buckwheat, soy, fat, waste paper, paper residues, aquatic product residues, livestock excrement, sewage sludge, and food wastes. The biomass resource is even more preferably maize, sugar cane, cassava, or sago palm.

Next, a method for producing the polyester resin (A) of the present invention will be described. In esterification/transesterification reaction, the entire monomer component and/or a low polymer thereof are heated and melted to react with each other. The temperature of the esterification/transesterification reaction is preferably <NUM> to <NUM> and more preferably <NUM> to <NUM>. The reaction time is preferably <NUM> to <NUM> hours and more preferably <NUM> hours to <NUM> hours. The reaction time is a time from a time when a desired reaction temperature is reached to the succeeding polycondensation reaction. In polycondensation reaction, a polyol component is evaporated from the esterified product obtained by the esterification reaction at a temperature of <NUM> to <NUM> under a reduced pressure, and the polycondensation reaction is progressed until a desired molecular weight is achieved. The reaction temperature for the polycondensation is preferably <NUM> to <NUM> and more preferably <NUM> to <NUM>. The degree of pressure reduction is preferably <NUM> Pa or lower. If the degree of pressure reduction is insufficient, the polycondensation time tends to be long, and it is not preferable. A time of pressure reduction from atmospheric pressure to <NUM> Pa or lower is preferably <NUM> to <NUM> minutes during which the pressure is gradually reduced.

In the esterification/transesterification reaction and the polycondensation reaction, polymerization is performed with use of an organotitanate compound such as tetrabutyl titanate, germanium dioxide, antimony oxide, and an organotin compound such as tin octylate as necessary. The organotitanate compound is preferable from the viewpoint of reaction activity, and germanium dioxide is preferable from the viewpoint of coloring of resin.

The glass transition temperature of the polyester resin (A) of the present invention is preferably <NUM> or higher and more preferably <NUM> or higher from the viewpoint of water resistance, in particular, retort resistance of the coating film. The upper limit of the glass transition temperature is, but is not particularly limited to, <NUM> or lower in general.

The reduced viscosity of the polyester resin (A) of the present invention is preferably <NUM> to <NUM> dl/g and more preferably <NUM> to <NUM> dl/g. If the reduced viscosity is <NUM> dl/g or smaller, curability may become insufficient and the coating film may have insufficient toughness, and thus, it may be impossible to withstand a forming process on a can after coating is formed on a metal plate. Meanwhile, if the reduced viscosity is <NUM> dl/g or larger, a melt viscosity or a solution viscosity may become high and operability may be reduced, and, furthermore, the number of hydroxy group ends is reduced and thus, an acid value may not be sufficiently imparted.

Preferably, the polyester resin composition of the present invention further contains a catalyst (B). In a case where the catalyst (B) is contained, self-crosslinkability of the polyester resin (A) during heating treatment thereof is promoted, the content of THF-insoluble matter can be increased, and curability can be enhanced. Examples of the catalyst include acid catalysts such as sulfuric acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, naphthalenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid, camphorsulfonic acid, and phosphoric acid, products obtained by blocking them by amine (partially neutralized by adding amine), organotin compounds such as dibutyltin dilaurate, titanium compounds such as titanium tetrabutoxide, zinc compounds such as zinc acetate, hafnium compounds such as hafnium chloride·THF complex, and rare earth compounds such as scandium triflate. One of them may be used, or two or more of them may be used in combination. Among them, dodecylbenzenesulfonic acid and a neutralized product thereof are preferable from the viewpoint of compatibility with the polyester resin (A) and from the viewpoint of hygiene.

A blending ratio between the polyester resin (A) and the catalyst (B) in the polyester resin composition of the present invention preferably satisfies (A)/(B)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio), more preferably satisfies (A)/(B)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio), and most preferably satisfies (A)/(B)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio). Within the above-described range, curability of the polyester resin composition can be enhanced.

In the polyester resin composition of the present invention, the catalyst (B) may be contained in the polyester resin (A), or may be added later. From the viewpoint of avoiding gelation during polymerization of the polyester resin (A), the catalyst (B) is preferably added after production of the polyester resin (A).

A radical polymerization inhibitor (C) may be added to the polyester resin composition of the present invention. Although the radical polymerization inhibitor (C) is used mainly for preventing gelation due to cleavage of an unsaturated bond during polymerization of the polyester resin (A), the radical polymerization inhibitor (C) may be added after the polymerization in order to enhance storage stability of the polyester resin (A). Examples of the radical polymerization inhibitor (C) include known ones such as a phenol-based antioxidant, a phosphorus-based antioxidant, an amine-based antioxidant, a sulfur-based antioxidant, and an inorganic compound-based antioxidant.

Examples of the phenol-based antioxidant include <NUM>,<NUM>-di-t-butylhydroquinone, <NUM>,<NUM>'-butylidenebis(<NUM>-methyl-<NUM>-t-butylphenol), <NUM>,<NUM>,<NUM>-tris(<NUM>-methyl-<NUM>-hydroxy-<NUM>-t-butylphenyl)butane, <NUM>,<NUM>,<NUM>-tris-methyl-<NUM>,<NUM>,<NUM>-tris(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxybenzyl)benzene, tris(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)isocyanurate, and derivatives thereof.

Examples of the phosphorus-based antioxidant include tri(nonylphenyl) phosphite, triphenyl phosphite, diphenyl isodecyl phosphite, trioctadecyl phosphite, tridecyl phosphite, diphenyldecyl phosphite, <NUM>,<NUM>'-butylidene-bis(<NUM>-methyl-<NUM>-t-butylphenylditridecyl phosphite), distearyl-pentaerythritol diphosphite, trilauryl trithio phosphite, and derivatives thereof.

Examples of the amine-based antioxidant include phenyl-beta-naphthylamine, phenothiazine, N,N'-diphenyl-p-phenylenediamine, N,N'-di-betanaphthyl-p-phenylenediamine, N-cyclohexyl-N'-phenyl-p-phenylenediamine, aldol-alpha-naphthylamine, <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-dihydroquinoline polymer, and derivatives thereof.

Examples of the sulfur-based antioxidant include thiobis(N-phenyl-beta-naphthylamine, <NUM>-mercaptobenzothiazole, <NUM>-mercaptobenzimidazole, tetramethylthiuram disulfide, nickel isopropyl xanthate, and derivatives thereof.

Examples of a nitro compound-based antioxidant include <NUM>,<NUM>,<NUM>-trinitrobenzene, p-nitrosodiphenylamine, p-nitrosodimethylaniline, <NUM>-chloro-<NUM>-nitrobenzene, o-dinitrobenzene, m-dinitrobenzene, p-dinitrobenzene, p-nitrobenzoic acid, nitrobenzene, <NUM>-nitro-<NUM>-cyanothiophene, and derivatives thereof.

Examples of the inorganic compound-based antioxidant include FeCl3, Fe(CN)<NUM>, CuCl2, CoCl3, Co(ClO4)<NUM>, Co(NO3)<NUM>, and Co2(SO4)<NUM>.

As the radical polymerization inhibitor (C) used in the present invention, the phenol-based antioxidant and the amine-based antioxidant are preferable among the above-described antioxidants from the viewpoint of thermal stability, a radical polymerization inhibitor having a melting point of <NUM> or higher and a molecular weight of <NUM> or higher is more preferable, and a radical polymerization inhibitor having a melting point of <NUM> or higher is even more preferable. Specifically, for example, phenothiazine and <NUM>,<NUM>'-butylidenebis(<NUM>-methyl-<NUM>-t-butylphenol) are preferable.

A blending ratio between the polyester resin (A) and the radical polymerization inhibitor (C) in the polyester resin composition of the present invention preferably satisfies (A)/(C)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio), even more preferably satisfies (A)/(C)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio), and most preferably satisfies (A)/(C)=<NUM>/<NUM> to <NUM>/<NUM> (mass ratio). Within the above-described range, gelation can be inhibited during production of the polyester resin (A).

In the polyester resin composition of the present invention, according to required characteristics, a known inorganic pigment like titanium oxide and silica, and a known additive such as phosphoric acid and esterified products thereof, a surface smoother, a defoamer, a dispersant, and a lubricant can be blended. Particularly, a lubricant is important in order to impart lubricity to a coating film which is required when DI cans, DR (or DRD) cans, and the like are formed. Preferable examples of the lubricant include fatty acid ester wax which is an esterified product of a polyol compound and fatty acid, silicon-based wax, fluorine-based wax, polyolefin wax such as polyethylene, lanoline-based wax, montan wax, and microcrystalline wax. One kind of the lubricant can be used, or two or more kinds of the lubricants can be mixed and used.

The polyester resin composition of the present invention can be formed into a coating material in a state of being dissolved in a known organic solvent. Examples of the organic solvent used for forming the coating material include toluene, xylene, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, methyl cellosolve, butyl cellosolve, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, ethylene glycol monoacetate, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and SOLVESSO. One or more of them can be selected and used in consideration of solubility, an evaporation rate, and the like.

The polyester resin composition of the present invention can be formed into a powder coating material by a known pulverizing method. Examples of the known pulverizing method include a grinding method. In the grinding method, the polyester resin composition of the present invention, and, as necessary, a mixture of an anticorrosive pigment and an additive are dry-mixed by a mixer such as a tumbler mixer and a Henschel mixer, and are melt-kneaded by a kneader. Examples of the kneader include standard kneaders such as single-screw or twinscrew extruders, three-rolls, and Labo Plastomill. The kneaded product is cooled and solidified, and the solidified product is roughly ground and finely pulverized, thereby obtaining a pulverized product. Examples of the pulverizing machine include a jet-type pulverizing machine which performs pulverization with use of supersonic jet stream, and an impact-type pulverizing machine which introduces the solidified product into a space formed between a stator (liner) and a rotator (rotor) that rotates at a high speed and performs pulverization. An additive may be further added to the pulverized product as necessary. The pulverized product is subjected to classification to adjust a powder to desired particle size and desired particle size distribution, whereby a powder coating composition can be obtained. For the classification, a known classifier which can remove overpulverized toner base particles through classification by centrifugal force and wind power, can be used. For example, a rotating-type wind power classifier (rotary wind power classifier) can be used.

Another resin can be blended in the polyester resin composition of the present invention for the purpose of modification for, for example, imparting adhesion and flexibility of the coating film. Examples of the other resin include amorphous polyester, crystalline polyester, ethylene-polymerizable unsaturated carboxylic acid copolymer, and ethylene - polymerizable carboxylic acid copolymer ionomer. In a case where at least one kind of resin selected from them is blended, flexibility and/or adhesion can be imparted to the coating film in some cases.

The polyester resin composition of the present invention can be applied to one surface or both surfaces, or, as necessary, end surfaces of any metal plate formed of a metal material that can be used for, for example, drink cans, cans for canned foods, lids thereof, and caps. Examples of the metal material include a tin plate, tin-free steel, and aluminium. The metal plates formed of theses metal materials may be subjected beforehand to phosphoric acid treatment, chromic acid chromate treatment, phosphoric acid chromate treatment, other anticorrosion treatment by an anti-corrosive agent, or surface treatment for enhancing adhesion of the coating film.

The polyester resin composition of the present invention can be applied to the metal plate by a known coating method such as roll coater coating and spray coating, and cured. Although the thickness of the coating film is not particularly limited, the thickness of the film in a dry state is preferably <NUM> to <NUM> and more preferably <NUM> to <NUM>. In general, the baking condition for the coating film is such that the baking is performed preferably at a temperature in a range from about <NUM> to <NUM> for about <NUM> minutes to about two hours, and more preferably at a temperature in a range from about <NUM> to <NUM> for about five minutes to about one hour.

The polyester resin composition of the present invention can be dispersed in an aqueous medium and can also be used as a polyester resin aqueous dispersion.

As a method for forming the polyester resin (A) used in the present invention as an aqueous dispersion, for example, a method (a) in which the polyester resin (A) is dispersed by dissolving the polyester resin (A) in a water-soluble organic solvent in which the polyester resin (A) is to be dissolved, and sequentially adding a basic compound and water as necessary, or a method (b) in which the polyester resin (A), water, and a water-soluble organic solvent in which the polyester resin (A) is to be dissolved, and, in addition thereto, a basic compound as necessary, are heated to disperse the polyester resin (A), is used. Furthermore, in a case where an amount of the organic solvent is to be reduced, or in a case where an aqueous dispersion is to be formed by fully removing the organic solvent, the organic solvent having a boiling point of <NUM> or lower is used to disperse the polyester resin (A), and the solvent can be thereafter removed through heating or under a reduced pressure. In the case of the polyester resin (A), the former method (a) is preferably used from the viewpoint of film formability.

In this case, the temperature at which the polyester resin (A) is dissolved is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, even more preferably <NUM> to <NUM>, and most preferably <NUM> to <NUM>. If the temperature is lower than <NUM>, the polyester resin (A) may be insufficiently dissolved, and thus, disentanglement among the molecular chains cannot be sufficiently performed. Meanwhile, if the temperature is higher than <NUM>, the polyester resin (A) is highly likely to deteriorate. Examples of an organic solvent in which the polyester resin (A) can be dissolved by being heated at a temperature in a range from <NUM> to <NUM> include methyl ethyl ketone, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, <NUM>,<NUM>-dioxane, <NUM>,<NUM>-dioxane, <NUM>,<NUM>-dioxolane, <NUM>,<NUM>-hexanediol, methyl cellosolve, butyl cellosolve, ethyl carbitol, butyl carbitol, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and triethylene glycol monobutyl ether. Among them, for example, methyl ethyl ketone, butyl cellosolve, propylene glycol monopropyl ether, and propylene glycol monobutyl ether are preferable.

In a case where the polyester resin (A) has been dissolved at <NUM> or higher, the aqueous dispersion needs to be obtained in a manner in which the temperature of the polyester resin solution is cooled to be <NUM> or lower, and thereafter, water and, as necessary, a basic compound are sequentially added while the resin solution is stirred, and phase transition is performed.

As the basic compound used for forming the polyester resin (A) of the present invention as the aqueous dispersion, a compound which volatilizes in a drying or baking process step during formation of the coating film is preferable, and, for example, ammonia and/or an organoamine compound having a boiling point of <NUM> or lower are/is used. Preferable examples of the basic compound include triethylamine, N,N-diethylethanolamine, N,N-dimethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, isopropylamine, iminobispropylamine, ethylamine, diethylamine, <NUM>-ethoxypropylamine, <NUM>-diethylaminopropylamine, sec-butylamine, propylamine, methylaminopropylamine, dimethylaminopropylamine, methyliminobispropylamine, <NUM>-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. An amount of the basic compound is required to be an amount that allows carboxyl groups of the polyester resin (A) to be at least partially neutralized. Specifically, the amount of the basic compound to be added is desirably <NUM> to <NUM> equivalent with respect to the carboxyl group equivalent.

The average particle diameter of the polyester resin aqueous dispersion of the present invention exerts a large influence on outer appearance of the coating film and storage stability, and thus, is very important. The average particle diameter is preferably <NUM> to <NUM>, even more preferably <NUM> to <NUM>, and particularly preferably <NUM> to <NUM>. If the average particle diameter is larger than <NUM>, not only dispersion stability is substantially reduced but also the film formability is also degraded, and therefore, the outer appearance of the obtained coating is degraded. Meanwhile, the average particle diameter of smaller than <NUM> is not preferable because, although the film formability tends to be significantly enhanced, this is likely to cause fusion or aggregation of dispersed particles, so that increase of viscosity or poor dispersion is highly likely to be caused.

The polyester resin aqueous dispersion of the present invention is preferably produced at a resin solid content concentration of <NUM> to <NUM> mass%, more preferably <NUM> to <NUM> mass%, and even more preferably <NUM> to <NUM> mass%. If the resin solid content concentration is larger than <NUM> mass%, the viscosity of the aqueous dispersion becomes high and resin particles are likely to aggregate, so that the dispersion stability is significantly reduced. Meanwhile, if the resin solid content concentration is smaller than <NUM> mass%, the polyester resin aqueous dispersion cannot be said to be practical from the viewpoint of both production and usage.

The polyester resin aqueous dispersion of the present invention is optimal as a coating material for the inner surfaces of food cans and drink cans. In order to form a coating material for the inner surfaces of food cans and drink cans, various additives may be blended depending on the purpose. A leveling agent or surfactant for enhancing coatability, and smoothness and outer appearance of the coating film, and a lubricant for preventing flaws in the coating film can be blended, and, furthermore, a coloring pigment can be blended, and, depending on cases, polyester resin other than the polyester resin (A), and resin other than polyester resin such as acrylic resin emulsion and polyurethane resin emulsion can be blended to the extent that the purposes of the present invention such as food sanitation and flavor preservability are not degraded.

The coating material using the polyester resin aqueous dispersion of the present invention can be applied to a can metal base material such as aluminium, stainless steel, or tin plate, by a gravure roll coater, a comma coater, a spray method, or the like. Although the thickness of the film is not particularly limited, the thickness of the film in a dry state is preferably in a range from <NUM> to <NUM> and more preferably in a range from <NUM> to <NUM> in general. In general, the baking condition for the coating film is such that the baking is performed preferably at a temperature in a range from about <NUM> to <NUM> for about ten minutes to about two hours and more preferably at a temperature in a range from about <NUM> to <NUM> for about five minutes to about one hour.

Hereinafter, the present invention will be described in more detail by means of examples. However, the present invention is not limited to these examples. Various characteristics are evaluated according to the following methods. "Parts" indicates parts by mass, and "%" indicates mass %.

A sample of the polyester resin (A) was dissolved in deuterated chloroform, and <NUM>-NMR analysis was performed by using a nuclear magnetic resonance (NMR) device <NUM>-MR manufactured by VARIAN. The molar ratio was obtained from the obtained integration value ratio.

In <NUM> cc of a mixture solvent of phenol/tetrachloroethane (mass ratio of <NUM>/<NUM>), <NUM> of the sample of the polyester resin (A) was dissolved and measurement was performed at <NUM>.

The glass transition temperature was measured by a differential scanning calorimeter (manufactured by SII, DSC-<NUM>). In a snap-in-lid-type aluminium container, <NUM> of the sample of the polyester resin (A) was placed and sealed, and cooled to -<NUM> by using liquid nitrogen. Subsequently, the temperature was increased to <NUM> at <NUM>/minute. In the endothermic curve obtained in this process, the temperature at the intersection of a baseline before appearance of an endothermic peak and a tangent toward the endothermic peak was determined as the glass transition temperature (Tg, unit:°C).

In <NUM> of chloroform, <NUM> of the sample of the polyester resin (A) was dissolved, and the obtained product was subjected to titration by using <NUM> N potassium hydroxide ethanol solution, and an equivalent (eq/ton) per <NUM><NUM> g of the polyester resin was obtained. Phenolphthalein was used as an indicator.

At cyclohexanone/SOLVESSO-<NUM>=<NUM>/<NUM> (mass ratio), <NUM> parts by mass (solid content) of the polyester resin (A) was dissolved so as to obtain a viscosity suitable for coating. The polyester resin composition (solid content of about <NUM> mass%) was obtained according to blending indicated in Table <NUM> and Table <NUM>.

The polyester resin composition was applied onto copper foil such that the thickness was <NUM> after being dried, heat was applied at <NUM> for one hour, and a sample having a longitudinal direction of <NUM> and a transverse direction of <NUM> was obtained, and the mass of the sample before immersion in THF was defined as (X), and the mass of the sample which was immersed in <NUM> of THF at <NUM> for one hour, and thereafter dried at <NUM> for <NUM> minutes, was defined as a mass (Y) after immersion in THF, and a content of THF-insoluble matter obtained when the polyester resin composition was heated at <NUM> for one hour was calculated by the following equation.

Furthermore, a content of THF-insoluble matter in the case of the polyester resin composition being heated at <NUM> for <NUM> minutes was obtained in the same manner as described above except that heating was performed at a temperature of <NUM> for <NUM> minutes.

The polyester resin composition was applied to one surface of a tin plate (JIS G <NUM>(<NUM>) SPTE, <NUM>×<NUM>×<NUM>) with a bar coater such that the thickness of the film was <NUM>±<NUM> after drying, and curing by baking was performed under a baking condition of <NUM> (PMT: peak base material temperature)× <NUM> hour, and the obtained product was used as a test piece (hereinafter, referred to as test piece).

The thus-obtained test piece was bent by <NUM>° in such a direction that the cured film was on the outer side, and the electrical current value was measured at a crack generated at the bent portion in the cured film, thereby evaluating processability. The test piece was bent by interposing no object (so-called 0T). Sponge (width of <NUM>, depth of <NUM>, thickness of <NUM>) immersed in <NUM>% NaCl aqueous solution was prepared so as to be placed on an aluminium plate electrode (width of <NUM>, depth of <NUM>, thickness of <NUM>), and the bent portion of the test piece was brought into contact with the sponge near the center portion of the bent portion so as to be parallel to the <NUM> side of the sponge. DC voltage of <NUM> V was applied across the aluminium plate electrode and a non-coated portion of the back surface of the test plate, and the electrical current value was measured. The smaller electrical current value indicates better bending characteristic.

Gauze felt immersed in methyl ethyl ketone was pressed against the cured film face of the test piece such that the contact area was <NUM> cm2, and a <NUM> load was applied to perform a rubbing test. The number of times (one reciprocation was one time) before the cured film was peeled off was evaluated according to the following criteria.

The test piece was placed so as to be stood in a stainless steel cup, ion-exchanged water was poured into the cup until the height of the water reached half the height of the test piece, and the obtained product was set in an autoclave of a retort testing machine (manufactured by TOMY KOGYO CO. , ES-<NUM>), and was subjected to retort treatment at <NUM>×<NUM> minutes. Evaluation after the treatment was performed at a portion with which vapor came into contact and at which the cured film was likely to be subjected to more severe conditions in general, and blushing and blister of the cured film were visually determined as follows.

In a <NUM> four-necked flask, <NUM> parts by mass of dimethyl terephthalate, <NUM> parts by mass of dimethyl <NUM>,<NUM>-naphthalenedicarboxylate, <NUM> parts by mass of trimellitic anhydride, <NUM> parts by mass of ethylene glycol, <NUM> parts by mass of <NUM>,<NUM>-propanediol, <NUM> parts by mass of <NUM>,<NUM>-cyclohexanedimethanol, and <NUM> parts by mass of tetra-n-butyl titanate (hereinafter, may be abbreviated as TBT) as a catalyst (<NUM> mol% with respect to the entire acid component) were put, and the temperature was gradually increased to <NUM> over three hours, to perform esterification. Subsequently, the pressure in the system was gradually reduced, polymerization under the reduced pressure was performed over one hour to <NUM> mmHg, the temperature was increased to <NUM>, and late-stage polymerization was further performed under vacuum of <NUM> mmHg or lower for <NUM> minutes. When the target molecular weight was reached, the obtained product was cooled to <NUM> in a nitrogen atmosphere. Subsequently, <NUM> parts by mass of trimellitic anhydride was put, and the obtained product was continuously stirred at <NUM> to <NUM> in a nitrogen atmosphere for <NUM> minutes. The obtained product was taken out and the polyester resin (synthetic example (a)) was obtained. The obtained polyester resin had a reduced viscosity of <NUM> dl/g, a glass transition temperature (Tg) of <NUM>, and an acid value of <NUM> eq/t.

Polyester resins (synthetic examples (b) to (t)) having resin compositions as indicated in Table <NUM> were each produced by a direct polymerization method similar to that of the synthetic example (a), except that the composition to be put was changed.

A polyester resin composition was produced by using the obtained polyester resin, and processability, curability, and retort resistance were evaluated. The blending for the polyester resin compositions, and the evaluated results are indicated in Table <NUM> and Table <NUM>.

As is apparent from Table <NUM>, the cured film (coating film) obtained from the polyester resin composition using the polyester resin (A) of the present invention was excellent in all of processability, curability, and retort resistance. Meanwhile, as shown in Table <NUM>, in Comparative Examples <NUM> and <NUM>, since the acid value of the polyester resin is low, curability was insufficient, and processability and retort resistance were also inferior. In Comparative Example <NUM>, curing agent was blended, resulting in inferior processability.

Claim 1:
A polyester resin composition comprising polyester resin (A), wherein the polyester resin composition satisfies the following requirements (i) to (iii),
(i) the polyester resin (A) has an acid value of <NUM> eq/ton or more,
(ii) the polyester resin composition substantially contains no curing agent, "substantially" meaning that a content of a curing agent is smaller than <NUM> part by mass (in terms of solid content) with respect to <NUM> parts by mass (in terms of solid content) of the polyester resin (A), and
(iii) a content of tetrahydrofuran-insoluble matter is <NUM> mass% or more when the polyester resin composition has been heated at <NUM> for one hour, wherein the tetrahydrofuran-insoluble matter is determined as indicated in the specification.