JP2006037014A - Coating composition - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating composition for coating metal can for food and beverage, and provide a coating film having excellent processability, adhesiveness, retort resistance, and flavored properties. <P>SOLUTION: The coating composition comprises two kinds of polyester resins (A) and (B), in which the polyester resins (A) is composed of 80-100 mol% aromatic dicarboxylic acid component as the dicarboxylic acid component and 40-90 mol% 2-methyl-1,3-propanediol and 10-60 mol% 1,2-propanediol as the glycol component and has ≥0.45 of limiting viscosity and ≥50°C of glass transition temperature, and the polyester resins (B) is composed of 60-100 mol% aromatic dicarboxylic acid component as the dicarboxylic acid component and 50-100 mol% aliphatic glycol having a side chain as the glycol component and has ≥0.60 of limiting viscosity number and less than 20°C of glass transition temperature, and the mixed ratio of the polyester resins (A) to (B) by mass may be (A)/(B)=90-50/10-50. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a resin composition that can be coated on food and beverage metal cans and can be a coating film excellent in processability, metal adhesion, retort resistance, and flavor properties, and is soluble in general-purpose organic solvents. The present invention relates to a coating composition that can be used as a coating having excellent properties, handling at the time of coating, and excellent storage stability of a solution.

  Conventionally, paints such as polyester resin, epoxy resin, acrylic resin, and vinyl resin have been used when coating steel plates for metal cans. In addition, among them, the paint used for metal cans for food and beverages is used for the purpose of preventing corrosion of the can material due to various foods without damaging the flavor and flavor of the contents, Therefore, it is required to be non-toxic, to withstand heat sterilization, and to have excellent adhesion and workability. However, polyvinyl chloride resins and epoxy-phenol resins are hygienic and environmentally friendly. The current situation is having this problem.

On the other hand, Patent Document 1 and Patent Document 2 propose a polyester-based resin as a resin that is easy to paint and bake, has excellent metal adhesion, and does not generate toxic and corrosive gases during incineration. However, in recent years, food cans and beverage cans have a complicated form, and the metal adhesion is insufficient for such complicated processing, so that it is inferior in workability, and further heated by boiling water or steam. It is also inferior to the retort resistance during processing and the flavor property that the flavor of the contents changes. And as a method for solving these problems, Patent Document 3 and Patent Document 4 propose a polyester containing at least one propylene glycol and 2-methyl-1,3-propanediol. The film performance such as retort resistance and flavor and solubility in general-purpose organic solvents could not be compatible.
JP-B 60-42829 JP-B 61-36548 JP 2001-106969 JP2001-311040

  The present invention solves the above-described problems, has good solubility in general-purpose organic solvents, has excellent metal adhesion, has good workability, retort resistance, and flavor properties, and is a coated metal suitable for cans. An object of the present invention is to provide a coating composition that gives a plate.

  As a result of intensive studies to solve the problems of solvent solubility, adhesion, workability, retort resistance, and flavor properties of the above-described polyester resin, the present inventors have determined that the glass transition temperature is a specific resin composition. By mixing two kinds of polyester resins having different ratios at a specific ratio, a coating composition having high metal adhesion and having both properties of workability and retort resistance has been found, and the present invention has been achieved.

  That is, the present invention is a coating composition containing two types of polyester resins (A) and (B), the polyester resin (A) is composed of 80 to 100 mol% of an aromatic dicarboxylic acid as a dicarboxylic acid component, The glycol component consists of 2-methyl-1,3-propanediol 40 to 90 mol%, 1,2-propanediol 10 to 60 mol%, the intrinsic viscosity is 0.45 or more, and the glass transition temperature is 50 ° C or more, Polyester resin (B) has a dicarboxylic acid component of 60 to 100 mol% of aromatic dicarboxylic acid, a glycol component of 50 to 100 mol% of an aliphatic glycol having a side chain, an intrinsic viscosity of 0.60 or more, and a glass transition temperature. Characterized in that the mixing ratio of the polyester resin (A) and the polyester resin (B) is (A) / (B) = 90-50 / 10-50 by mass ratio. The gist of the present invention is a composition for use.

  According to the resin composition for can coating of the present invention, a coating film excellent in retort and flavor properties can be obtained, and the coating film can be peeled off even for advanced processing such as winding and drawing. And without causing cracks, it can be suitably used as an upper lid of a two-piece can such as a food container, a can body, an upper lid of a three-piece can, or a bottom lid.

  Hereinafter, the present invention will be described in detail.

  The coating composition of the present invention contains a polyester resin (A) and a polyester resin (B), and the dicarboxylic acid component of the polyester resin (A) is composed of 80 to 100 mol% of an aromatic dicarboxylic acid. is required. If the aromatic dicarboxylic acid is less than 80 mol%, the glass transition temperature of the resulting resin is lowered, and the resulting coating film is inferior in retort resistance and flavor properties, such being undesirable.

  Further, the glycol component of the polyester resin (A) is composed of 2-methyl-1,3-propanediol 40 to 90 mol% and 1,2-propanediol 10 to 60 mol%.

  In order to improve workability, it is preferable to increase the molecular weight, and in order to improve retort resistance and blocking resistance, a higher glass transition temperature is better. Further, in order to impart solvent solubility, it is preferable to copolymerize a monomer having a side chain. Considering these, it is preferable to use a polyester mainly composed of 1,2-propanediol from the viewpoint of glass transition temperature and solvent solubility. However, when 1,2-propanediol is copolymerized, the polymerizability is lowered due to its poor thermal stability, and it has been difficult to increase the molecular weight to the extent that the processability intended by the present invention can be satisfied. On the other hand, a polyester mainly composed of 2-methyl-1,3-propanediol has improved solvent solubility and can improve workability by increasing molecular weight and imparting flexibility, but has a glass transition temperature of Since it falls, the retort resistance becomes inferior. Therefore, a polyester that can achieve the object of the present invention can be obtained for the first time by appropriately combining 1,2-propanediol and 2-methyl-1,3-propanediol.

  That is, when 2-methyl-1,3-propanediol is less than 40 mol% in the glycol component, the solvent solubility of the resulting polyester resin is lowered, and the processability when formed into a coating film is not preferred. . If it exceeds 90 mol%, the solvent solubility of the polyester resin is lowered, the glass transition temperature is lowered, and the retort resistance, flavor resistance, and blocking resistance are inferior. Further, if the 1,2-propanediol is less than 10 mol%, the solvent solubility of the polyester resin is lowered, the glass transition temperature is lowered, and the retort resistance and flavor properties are inferior, which is not preferable. When it exceeds 60 mol%, the polymerizability of the polyester resin is deteriorated, the productivity is lowered, and the flavor property is inferior.

  Next, the dicarboxylic acid component of the polyester resin (B) needs to be composed of 60 to 100 mol% of an aromatic dicarboxylic acid. If the aromatic dicarboxylic acid content is less than 60 mol%, the flavor of the resin becomes inferior, which is not preferable.

  Further, the glycol component of the polyester resin (B) needs to be composed of 50 to 100 mol% of an aliphatic glycol having a side chain. By copolymerizing an aliphatic glycol having a side chain, the metal adhesion is improved and the hydrophobicity is improved, so that the retort resistance and the solvent solubility are also improved. That is, when the aliphatic glycol having a side chain is less than 50 mol%, the solvent solubility of the resulting resin is inferior, which is not preferable.

  Examples of the aromatic dicarboxylic acid constituting the polyester resins (A) and (B) include terephthalic acid, isophthalic acid, phthalic acid, and naphthalenedicarboxylic acid.

  Further, as a component other than the aromatic dicarboxylic acid constituting the polyester resin (A) or (B), the following carboxylic acid may be copolymerized as long as the effects of the present invention are not impaired. Examples of the carboxylic acid component include aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and hydrogenated dimer acid, and alicyclic rings such as 1,4-cyclohexanedicarboxylic acid. Examples thereof include unsaturated dicarboxylic acids such as aliphatic dicarboxylic acids, maleic acids, fumaric acids, and non-hydrogenated dimer acids, and tricarboxylic acids such as trimellitic acids, among which succinic acid, adipic acid, and sebacic acid are preferable.

  In the present invention, as the glycol component having a side chain constituting the polyester resin (B), 1,2-propanediol, 2-methyl-1,3-propanediol, neopentyl glycol, dipropylene glycol, polypropylene glycol, 2-ethyl-2-butylpropanediol, 3-methyl-1,5-pentanediol, 3-ethyl-1,5-pentanediol, 3-propyl-1,5-pentanediol, 3-methyl-1,6 -Hexanediol, 4-methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, among which 1,2 in terms of glass transition temperature -Propanediol and 2-methyl-1,3-propanediol are preferred.

  Further, as the glycol constituting the polyester resins (A) and (B), the following glycols may be copolymerized as long as the effects of the present invention are not impaired. That is, aliphatic glycols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1, Examples include alicyclic glycols such as 4-cyclohexanedimethanol, polyhydric alcohols such as trimethylolpropane, trimethylolethane, and pentaerythritol. Among these, ethylene glycol, 1,4-butanediol, and diethylene glycol are preferable.

  The intrinsic viscosity of the polyester resin (A) needs to be 0.45 or more, preferably 0.48 or more. The intrinsic viscosity of the polyester resin (B) needs to be 0.60 or more, preferably 0.65 or more. If the intrinsic viscosity of each polyester resin is less than the desired range, the resulting coating film becomes fragile and inferior in workability, such being undesirable.

  Further, the glass transition temperature of the polyester resin (A) needs to be 50 ° C. or higher, preferably 60 ° C. or higher. The glass transition temperature of the polyester resin (B) needs to be less than 20 ° C, preferably less than 15 ° C. If the glass transition temperature of the polyester resin (A) is less than 60 ° C., the retort resistance tends to decrease, which is not preferable. On the other hand, when the glass transition temperature of the polyester resin (B) is 20 ° C. or higher, the effect of imparting flexibility by the obtained coating film is lowered, so that workability is lowered, which is not preferable.

  The acid value of the polyester resins (A) and (B) is preferably less than 3 mgKOH / g, more preferably less than 2 mgKOH / g. When the acid value of the polyester resin is 3 mgKOH / g or more, the hydrolysis resistance of the polyester resin is lowered, so that the viscosity is lowered during the retort treatment.

  The polyester resin (A) and the polyester resin (B) must be mixed in a mass ratio of (A) / (B) = 90-50 / 10-50. When the ratio of the polyester resin (A) exceeds 90% by mass, the molecular weight of the coating composition is lowered, and the stretchability of the coated metal plate is lowered, which is not preferable. In addition, when the ratio of the polyester resin (A) is less than 50% by mass, the glass transition temperature when the coating film is formed becomes low, and not only the retort resistance and flavor property are deteriorated, but also the coated metal plates are stacked or wound. It is not preferable because it is likely to cause blocking when stored in a wet state.

  A resin having a low glass transition temperature has good processability in stretching, and a method of improving processability by mixing with a resin having a high glass transition temperature is common. However, since the polyester resin (B) described above is excellent in metal adhesion itself, mixing with the polyester resin (A) allows processing without impairing the high metal adhesion of the polyester resin (A). Thus, it has become possible to obtain a coating film having sufficient stretch followability for applications that require high stretch workability such as deep drawing.

  The method for producing the polyester resin used in the coating composition of the present invention is not particularly limited, and the conventional method using melt polycondensation such as direct esterification or transesterification using the above-described dicarboxylic acid component and glycol component. It can be manufactured by a known manufacturing method.

  For example, a dicarboxylic acid component, a glycol component, and a polycondensation catalyst are charged all at once into the reactor, the air in the system is discharged, and nitrogen substitution is performed. Thereafter, the temperature is raised to the esterification temperature (usually 200 to 260 ° C.), and the reaction is carried out for 3 to 5 hours with stirring. After completion of the esterification reaction, the temperature is raised to a polycondensation temperature (usually 220 to 260 ° C.), and the system is depressurized to carry out a polycondensation reaction under high vacuum (5 hPa or less). Although reaction time changes with kinds of polyester resin to manufacture, it is 4 to 6 hours normally. After completion of the polycondensation reaction, nitrogen may be enclosed in the system to release the reduced pressure, and then the obtained resin may be discharged.

Moreover, as a polycondensation catalyst, metal compounds, such as tin, titanium, antimony, germanium, cobalt, etc. generally used conventionally are suitable.
The coating composition of the present invention comprises a curing agent and an organic solvent in addition to the above-described polyester resin of the present invention.

  As the curing agent used in the coating composition of the present invention, at least one selected from a phenol resin, an aminoplast resin, a melamine resin, a polyfunctional isocyanate compound, a blocked isocyanate compound, and a polyfunctional aziridine compound is used. The blending amount of the curing agent is preferably 0.9 to 1.2 times the hydroxyl value of the polyester resin. When the blending amount of the curing agent is less than 0.9 times, the cross-linking reaction becomes insufficient, and the problem of retort whitening easily occurs particularly when a can is used. On the other hand, if the blending amount of the curing agent exceeds 1.2 times, the curing agent becomes excessive, so that it tends to dissolve in the beverage and deteriorate the flavor properties.

  As the organic solvent that can be used in the coating composition of the present invention, any organic solvent may be used as long as it dissolves both the polyester resin and the curing agent.

  Specific examples of usable organic solvents include aromatic solvents such as benzene, toluene and xylene, methylene chloride, chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane. Chlorine solvents such as chlorobenzene, o-dichlorobenzene, m-dichlorobenzene, p-dichlorobenzene, ester solvents such as ethyl acetate and γ-butyrolactone, isophorone, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, etc. Ketone solvents, diethyl ether, butyl cellosolve, ethyl cellosolve, ether solvents such as tetrahydrofuran, 1,4-dioxane, alcohol solvents such as methanol, ethanol, propanol, isopropanol, butanol, butane, pentane, Heki Down, cyclohexane, heptane, octane, nonane, Solvesso 100, and a solvent of aliphatic hydrocarbon such as Solvesso 150. These can be used alone or in combination of two or more. Among these, those suitably used include cyclohexanone, a mixed solvent of cyclohexanone and Solvesso 100, a mixed solvent of toluene and methyl ethyl ketone, ethyl acetate, etc., and can be appropriately selected and used in consideration of solubility and evaporation rate. Good.

  In addition, the composition for coatings includes resins other than polyester resins and resins used as curing agents, such as alkyd resins, urethane resins, epoxy resins that do not contain a bisphenol structure, and acrylic resin-modified materials, as long as the characteristics of the present invention are not impaired. Olefin resins, cellulose derivatives and the like can be used in combination. Furthermore, if necessary, a reaction catalyst that accelerates the curing reaction, a repellency inhibitor, a leveling agent, an antifoaming agent, an anti-fogging agent, a rheology control agent, a pigment dispersant, a lubricant, a release agent, and the like can be used in combination.

  The solid concentration in the coating composition is preferably 10% by mass or more, and more preferably in the range of 15 to 50% by mass. When the solid content concentration is less than 10% by mass, not only is it difficult to form a thick coating film, but the ratio of the organic solvent in the coating is increased, and the solvent is distilled off when forming the coating film. Takes time and causes a problem that productivity decreases.

  The coating composition of the present invention is uniformly applied to a metal plate by dip coating, brush coating, roll coating, spray coating, gravure coating, curtain flow coating, various printing methods, and the like.

  Next, the coated metal plate of the present invention will be described.

  As a metal plate which is a base material for application | coating, the sheet-like or strip | belt-shaped steel plate, the aluminum plate, or what performed various plating processing and chemical conversion treatment to those surfaces is mentioned. Among them, those having a chromium hydrated oxide film are preferable, and tin-free steel (TFS) having a two-layered film of a lower layer of metallic chromium and an upper layer of chromium hydrated oxide is preferable. In addition, one or two or more types of multilayer plating or alloy plating such as tin, nickel, zinc, and aluminum are applied to the surface of the steel plate, and a film having the above two-layer structure or a chromium hydrated oxide film is formed on the upper layer. It is also possible to use a material obtained by subjecting aluminum to electrolytic chromic acid treatment or immersion chromic acid treatment or the like to form a hydrated chromium oxide film on the surface layer.

  And a can inner surface coat layer can be formed by baking after applying a paint on a metal plate. The thickness of the coating film is preferably 0.2 to 100 μm, and more preferably 1 to 10 μm. If the thickness is less than 0.2 μm, the coating film is damaged (peeled or cracked) in the can molding process, and only cans with poor corrosion resistance and flavor properties can be obtained. On the other hand, when the thickness exceeds 100 μm, there is a possibility that the organic solvent used in the paint may remain, or even if the organic solvent can be completely removed, there is a problem that the solvent distillation process takes time and productivity is lowered.

  The baking step is preferably performed at a temperature of 180 to 250 ° C. for 10 to 60 minutes, more preferably at a temperature of 200 to 240 ° C. for 20 to 40 minutes. When baked at a temperature of less than 180 ° C., removal of the organic solvent is incomplete or the curing reaction does not proceed sufficiently, so that only cans with poor corrosion resistance can be obtained. On the other hand, when baked at a temperature exceeding 250 ° C., the reaction with the curing agent proceeds sufficiently, but the polyester resin may be thermally decomposed. When the baking time is less than 10 minutes, the removal of the organic solvent is incomplete, the curing reaction does not proceed sufficiently, and only a can with poor corrosion resistance can be obtained. On the other hand, when the baking time exceeds 60 minutes, productivity decreases.

  By using the metal plate on which the resin film obtained in this way is formed, it has excellent heat resistance and can be subjected to high temperature processing such as retort processing, and pinholes, microcracks, etc. even after severe processing Thus, a metal can body that is excellent in corrosion resistance and impact resistance can be produced.

  The metal plate having the coating film obtained as described above can be used as a member that requires workability, such as a can body and upper lid of a two-piece can, an upper lid of a three-piece can, and a bottom lid material.

  The reason why the coating composition of the present invention exhibits excellent processability is that 1,2-propylene glycol is copolymerized in the polyester resin, so that the adhesion with the metal as the base material is improved, and 2 Since the flexibility and stretchability of the polyester resin are improved by the copolymerization of -methyl-1,3-propanediol, the followability to metal deformation is very high. Furthermore, the processability is further improved by combining a glycol having a side chain with a resin having a low glass transition temperature and excellent metal adhesion.

Next, the present invention will be described more specifically with reference to examples.
Each measurement and evaluation item was based on the following method.
(1) Measurement of resin composition It measured with the 1H-NMR spectrometer JNM-LA400 apparatus by JEOL.
(2) Measurement of intrinsic viscosity ([η]) Measurement was carried out at a temperature of 20 ° C. using an equal mass mixture of phenol and ethane tetrachloride as a solvent.
(3) Measurement of glass transition temperature (Tg) The glass transition temperature (Tg) was measured at a rate of temperature increase of 10 ° C./min using a differential scanning calorimeter SSC5200 manufactured by Seiko Denshi Kogyo.
(4) Solvent Solubility The obtained polyester resin was dissolved in cyclohexanone to make a 30% by mass solution, and the solubility after standing at 25 ° C. for 1 week was evaluated as solubility.
○: Good x: White turbidity, solidified or insoluble (5) Preparation of coating composition A phenol resin (Regitop PL-4523, manufactured by Gunei Chemical Co., Ltd.) as a curing agent is blended in an amount equal to the terminal hydroxyl value of the polyester resin. A coating composition was prepared by dissolving in a mixed solvent of cyclohexanone / solvesso 100 (volume ratio 1/1) so as to be 30% by mass.
(6) Preparation of coated metal plate The coating composition prepared in (5) above is applied to TFS (Tin Free Steel, 150mm x 150mm x 0.3mm) with a bar coater # 38 so that the film thickness becomes 10-15μm. This was coated and subsequently dried and baked for 30 minutes in a hot air circulating oven adjusted to 200 ° C. to prepare a coated metal plate.
(7) Workability The coated metal plate created in (6) above is punched into a circle and thin-walled deep drawing is performed to create a cup with a diameter of 100mm, a depth of 120mm, and a drawing ratio (ratio of depth to diameter) of 1.2. did. A 1% by mass saline solution was placed in the cup and heated at 80 ° C. for 24 hours, and the amount and state of rust produced in the cup were visually determined.
○: Rust is hardly seen.
X: Rust is seen.
(8) Retort resistance After the coated metal plate was treated under steam at 130 ° C. for 30 minutes, the whitening state of the coating film was visually determined.
○: Good x: Whitening occurred (9) Flavor property A coating resin composition was applied on a fluororesin sheet to a film thickness of 50 μm, dried and cured at 230 ° C. for 60 seconds, and then the film was peeled off to obtain a film sample. It was. The obtained film was cut into 150 mm × 450 mm, dipped in a 95% aqueous solution of D-limonene, and dipped at 40 ° C. for 10 days. Thereafter, the film was taken out, heat-treated at 80 ° C. for 30 minutes, and the amount of D-limonene adsorbed per 1 g of the film (mg / g) was measured by gas chromatography. In addition, D-limonene adsorption amount was set to 10 mg / g or less as the pass.
A: D-limonene adsorption amount is 10 mg / g or less.
X: D-limonene adsorption amount exceeds 10 mg / g.

Resin production example A-1
38.8 kg of terephthalic acid, 14.7 kg of 2-methyl-1,3-propanediol, and 16.0 kg of 1,2-propanediol were charged into an esterification reactor and subjected to esterification at a pressure of 0.5 MPa and a temperature of 230 ° C. for 4 hours. . The obtained polyester oligomer was transferred to a polymerization reactor, and after 39.1 g of hydroxybutyltin oxide was added, the pressure in the reaction system was gradually reduced to 0.4 hPa over 60 minutes, and then at a temperature of 230 ° C. for 4 hours. A polycondensation reaction was performed to obtain a polyester resin. The characteristic values of the obtained polyester resin are shown in Table 1.

Resin production examples A-2 to 8, B-1 to 5
A polyester resin was obtained in the same manner as in Resin Production Example 1, except that the raw material charging conditions were changed and the composition and intrinsic viscosity of the polyester resin were changed as shown in Table 1. The characteristic values of the obtained polyester resin are shown in Table 1.

Example 1
Using the resin obtained in Resin Production Example A-1 as the polyester resin (A) and the resin obtained in Resin Production Example B-1 as the polyester resin (B), the mass ratio (A) / (B) = 80 After mixing so as to be / 20, a coating composition and a coated metal plate were obtained by the method described above. Table 2 shows the evaluation results of workability and retort resistance of the obtained coated metal plate.

Examples 2-5, Comparative Examples 1-7
A coating composition and a coated metal plate were obtained in the same manner as in Example 1 except that the polyester resins (A) and (B) and the mixing ratio were changed as shown in Table 2. Table 2 shows the evaluation results of workability and retort resistance of the obtained coated metal plate.

As is apparent from Tables 1 and 2, the coating films obtained from the polyester resins of Examples 1 to 5 satisfied all processability, retortability, and flavor properties. On the other hand, in Comparative Example 1, since 2-methyl-1,3-propanediol was not contained in the polyester resin (A), it was poor in solvent solubility and could not be made into a paint. In Comparative Example 2, since 1,2-propanediol was not contained in the polyester resin (A), the solvent solubility was inferior and a paint could not be formed. In Comparative Example 3, since the polyester resin (A) had a large amount of 2-methyl-1,3-propanediol, the glass transition temperature was low and the retort property and flavor property were inferior. In Comparative Example 4, the amount of sebacic acid copolymerized with the polyester resin (B) was large, so that the flavor property was inferior. In Comparative Example 5, since the amount of the aliphatic glycol having a side chain of the polyester resin (B) is small, the solvent solubility is inferior, and a paint cannot be formed. In Comparative Example 6, since the mixing ratio of the polyester resin (A) was low, the glass transition temperature of the coating film was lowered, and the retort resistance and flavor properties were lowered. In Comparative Example 7, since the intrinsic viscosity of the polyester resins (A) and (B) was low, the processability was poor.

Claims (2)

A coating composition containing two types of polyester resins (A) and (B), wherein the polyester resin (A) comprises a dicarboxylic acid component of 80 to 100 mol% of an aromatic dicarboxylic acid, and a glycol component of 2- Polyester resin (B) comprising 40 to 90 mol% of methyl-1,3-propanediol and 10 to 60 mol% of 1,2-propanediol, having an intrinsic viscosity of 0.45 or more and a glass transition temperature of 50 ° C. or more. The polycarboxylic acid component is composed of 60 to 100 mol% aromatic dicarboxylic acid, the glycol component is composed of 50 to 100 mol% aliphatic glycol having a side chain, the intrinsic viscosity is 0.60 or more, and the glass transition temperature is less than 20 ° C. A coating composition, wherein the mixing ratio of the polyester resin (A) and the polyester resin (B) is (A) / (B) = 90-50 / 10-50 by mass ratio. A coated metal plate, wherein the coating composition according to claim 1 is applied to at least one surface of a metal plate.