Polyester resin, polyester resin composition, coating material composition, coating film, and metal can

Abstract

The present invention provides a polyester resin having good reactivity with a curing agent and capable of forming a coating film having excellent processability and corrosion resistance.　This polyester resin contains a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components, and satisfies the following (1)-(3). (1) The glass transition temperature (Tg) is 60°C or higher; (2) the weight average molecular weight (Mw) is 50,000 or more; and (3) the polyester resin has a hydroxyl value of 182-800 eq / ton.

Description

Polyester resin, polyester resin composition, coating composition, coating film and metal can

[0001] The present invention relates to a polyester resin. More specifically, the present invention relates to a polyester resin suitable for use in can coatings, and even more specifically to a polyester resin suitable for coating cans containing beverages or foods, as well as a polyester resin composition, a coating composition, a coating film, and a metal can containing the same.

[0002] Metal cans, such as beverage cans and food cans, are coated with organic resins such as polyester to prevent corrosion of the metal by food (corrosion resistance) and to preserve the flavor and taste of the contents (flavoring). The coating film is subjected to high-stress processes such as necking and threading during the molding process of the mouth of a bottle can. Therefore, the coating film must be durable enough to withstand such post-processing (processability). Recently, the diversity of can shape designs and can contents has continued to grow, requiring can coatings with even greater corrosion resistance and processability.

[0003] To address this issue, for example, Patent Document 1 discloses a coating material that uses a hydroxyl group-containing polyester resin and thereby has excellent sterilization stability and flexibility, particularly in an acidic medium.

[0004] Japanese Patent Application Laid-Open No. 2005-42110

[0005] However, the hydroxyl group-containing polyester resin described in Patent Document 1 has a low glass transition temperature of -9°C to 40°C, which causes a problem of insufficient corrosion resistance depending on the contents. Furthermore, the hydroxyl group-containing polyester resin has a low molecular weight, which causes a problem of insufficient processability depending on the part of the can.

[0006] The present invention provides a polyester resin that has good reactivity with a curing agent and is capable of forming a coating film that is excellent in processability and corrosion resistance.

[0007] The present inventors have conducted extensive research into the above-mentioned issues and have found that by making a polyester resin highly branched and by increasing the hydroxyl value of the polyester resin to a predetermined value or more, a polyester resin having a high glass transition temperature (Tg) and a high weight average molecular weight (Mw) can be provided, and have thus completed the present invention.

[0008] [1] A polyester resin having a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components, and satisfying the following (1) to (3): (1) a glass transition temperature (Tg) of 60°C or higher, (2) a weight average molecular weight (Mw) of 50,000 or higher, and (3) a hydroxyl value of the polyester resin of 182 to 800 eq / ton. [2] The polyester resin according to [1], which contains a structural unit derived from an aromatic dicarboxylic acid as the polycarboxylic acid component. [3] The polyester resin according to [2], which contains a total of 1 mol% or more of structural units derived from at least one component selected from the group consisting of isophthalic acid, orthophthalic acid, 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, when the structural units derived from aromatic dicarboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol%. [4] The polyester resin according to [2] or [3], which contains 60 mol% or more of structural units derived from aromatic dicarboxylic acids, when the structural units derived from polyvalent carboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol%. [5] The polyester resin according to any one of [2] to [4], wherein the aromatic dicarboxylic acid is at least one selected from the group consisting of terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid, and the polyester resin contains structural units derived from terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid in a total amount of 50 mol % or more, when the total structural units derived from aromatic dicarboxylic acids constituting the molecular chain of the polyester resin are taken as 100 mol %. [6] The polyester resin according to any one of [1] to [5], wherein the polyester resin has an acid value of 3 eq / ton or more. [7] The polyester resin according to any one of [1] to [6], wherein the polyester resin contains structural units derived from a trifunctional or higher polycarboxylic acid and / or structural units derived from a trifunctional or higher polyhydric alcohol, and the polyester resin contains structural units derived from a trifunctional or higher polycarboxylic acid and structural units derived from a trifunctional or higher polyhydric alcohol in a total amount of 1.0 mol % or more, when the total structural units constituting the molecular chain of the polyester resin are taken as 100 mol %.[8] The polyester resin according to any one of [1] to [7], wherein the trifunctional or higher polycarboxylic acid-derived structural units account for 60 mol% or more of the trifunctional or higher polycarboxylic acid-derived structural units, when the total of the trifunctional or higher polycarboxylic acid-derived structural units and the trifunctional or higher polyhydric alcohol-derived structural units is taken as 100 mol%. [9] The polyester resin according to any one of [1] to [8], wherein the aliphatic dicarboxylic acid and / or alicyclic dicarboxylic acid-derived structural units account for 20 mol% or less of the polycarboxylic acid-derived structural units that constitute the molecular chain of the polyester resin, when the polycarboxylic acid-derived structural units that constitute the molecular chain of the polyester resin are taken as 100 mol%.

[10] The polyester resin according to any one of [1] to [9], wherein the dihydric alcohol (a)-derived structural units having one primary hydroxyl group and one secondary hydroxyl group account for 50 mol% or more of the polyhydric alcohol-derived structural units that constitute the molecular chain of the polyester resin, when the polyhydric alcohol-derived structural units that constitute the molecular chain of the polyester resin are taken as 100 mol%.

[11] The polyester resin according to

[10] , wherein, when the structural units derived from polyhydric alcohols constituting the molecular chain of the polyester resin are taken as 100 mol %, the structural units derived from dihydric alcohols (b) other than the dihydric alcohol (a) account for 50 mol % or less.

[12] A polyester resin composition containing the polyester resin according to any one of [1] to

[11] and a curing agent.

[13] A coating composition containing the polyester resin according to any one of [1] to

[11] .

[14] A coating film containing the coating composition according to

[13] .

[15] A metal can containing the coating film according to

[14] .

[0009] According to the present invention, a polyester resin capable of forming a coating film having excellent processability and corrosion resistance is provided. Therefore, the polyester resin of the present invention is preferably used in applications such as polyester resin compositions, coating compositions, coating films, and metal cans.

[0010] <Polyester Resin> 1) Polyester Resin The present invention relates to a polyester resin that contains a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components and satisfies the following (1) to (3): (1) a glass transition temperature (Tg) of 60°C or higher, (2) a weight average molecular weight (Mw) of 50,000 or higher, and (3) a hydroxyl value of the polyester resin of 182 to 800 eq / ton.

[0011] The polyester resin of the present invention is characterized by a high glass transition temperature (Tg) and a high weight-average molecular weight (Mw) (requirements (1) and (2)). Generally, resins with a high glass transition temperature (Tg) often have limited molecular motion and tend to require higher temperatures to relax molecular motion. This results in high melt viscosity during polymerization, which increases the torque load during stirring, resulting in a large equipment load, making it difficult to achieve high molecular weight (particularly a high Mw). The inventors' investigations have revealed that highly branched polyester resins and increased hydroxyl values ​​in the polyester resins are effective in providing polyester resins with high glass transition temperatures (Tg) and high weight-average molecular weights (Mw) (requirement (3)). This is because highly branched polyester resins reduce entanglement of molecular chains and reduce melt viscosity. This reduces torque load and equipment load, which is expected to result in improved polyester resin yields. Furthermore, the polyester resin of the present invention has a high hydroxyl value, which provides many reactive sites for curing agents, and a high glass transition temperature (Tg), which results in good corrosion resistance of the coating film.Furthermore, the polyester resin also has a high weight average molecular weight (Mw), which results in good processability of the coating film.

[0012] In this specification, each of the components shown below can be used alone or in combination of two or more.

[0013] In this specification, the term "branched structure" refers to a branched structure in a polymer chain, and specifically refers to a structure in which three or more branches (molecular chains) extend from one structural unit that constitutes the molecular chain of a polyester resin. That is, when a polyester resin has a branched structure, it means that the polymer molecular chain of the polyester resin has, for example, a triester structure, a tetraester structure, or a pentaester structure.

[0014] 2) Polycarboxylic Acid Component / Polyhydric Alcohol Component Polyester resin has a chemical structure that can be obtained by polycondensation of a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components.

[0015] In this specification, "all structural units constituting the molecular chain of the polyester resin" and "structural units derived from A constituting the molecular chain of the polyester resin" respectively refer to structural units derived from all copolymerization components constituting the molecular chain of the polyester resin, or structural units derived from A, and do not include structural units derived from a compound having a polycarboxylic acid anhydride group in the molecule that is introduced to the end of the polyester resin after completion of the polycondensation reaction in order to adjust the acid value of the polyester resin. The ratio of each structural unit constituting the polyester resin can be determined, for example, by the amount of copolymerization components charged, １ H-NMR analysis, １3 It is identified by various analyses such as C-NMR analysis.

[0016] In this specification, examples of polycarboxylic acids include tri- or higher functional polycarboxylic acids and dicarboxylic acids. Tri- or higher functional polycarboxylic acids specifically refer to polycarboxylic acids having three or more carboxy groups, and the number of functional groups is preferably tri- to penta-functional, more preferably tri- to tetra-functional. Dicarboxylic acids specifically refer to polycarboxylic acids having two carboxy groups.

[0017] Examples of trifunctional or higher polyvalent carboxylic acids include trimellitic acid, pyromellitic acid, trimesic acid, benzophenone tetracarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenone tetracarboxylic anhydride, ethylene glycol bis(anhydrotrimellitate), cyclopentane tetracarboxylic dianhydride, 2,3,6,7-naphthalene tetracarboxylic dianhydride, 1,2,5,6-naphthalene tetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, and 1,2,3,4-butane tetracarboxylic acid and polyvalent carboxylic acids and anhydrides thereof. Among these, trimellitic acid, pyromellitic acid, trimesic acid, benzophenone tetracarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenone tetracarboxylic anhydride, ethylene glycol bis(anhydrotrimellitate), and 1,2,3,4-butane tetracarboxylic acid are preferred, with trimellitic acid being more preferred. As the trifunctional or higher polycarboxylic acid, one having an aromatic ring such as a benzene ring or a naphthalene ring in the molecule is preferred. By using a polycarboxylic acid having an aromatic ring, a rigid skeleton can be introduced into the molecule, which inhibits hydrolysis and facilitates the formation of a coating film with excellent corrosion resistance.

[0018] Examples of dicarboxylic acids include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, and alicyclic dicarboxylic acids. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-furandicarboxylic acid, 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, as well as their anhydrides. Examples of aliphatic dicarboxylic acids (preferably acyclic) include succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, dimer acid, fumaric acid, maleic acid, itaconic acid, and citraconic acid, as well as their anhydrides. Examples of the alicyclic dicarboxylic acid include 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, 1,2-cyclohexenedicarboxylic acid, 2,5-norbornanedicarboxylic acid, and anhydrides thereof.

[0019] In this specification, examples of polyhydric alcohols include tri- or higher functional polyhydric alcohols and dihydric alcohols. Tri- or higher functional polyhydric alcohols specifically refer to polyhydric alcohols having three or more hydroxy groups, and the number of functional groups is preferably tri- to penta-functional, more preferably tri- to tetra-functional. Dihydric alcohols specifically refer to polyhydric alcohols having two hydroxy groups.

[0020] Examples of trifunctional or higher polyhydric alcohols include glycerin, trimethylolethane, trimethylolpropane, mannitol, sorbitol, pentaerythritol, etc. Among these, from the viewpoint of the heat resistance of the monomer itself, glycerin, trimethylolethane, trimethylolpropane, and pentaerythritol are preferred, and trimethylolethane is more preferred.

[0021] Examples of the dihydric alcohol include a dihydric alcohol (a) having one primary hydroxyl group and one secondary hydroxyl group; and a dihydric alcohol (b) other than the dihydric alcohol (a). Examples of the dihydric alcohol (a) include 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, and 1,2-hexanediol. Among these, from the viewpoint of improving corrosion resistance, 1,2-propanediol and 1,2-butanediol are preferred, and 1,2-propanediol is more preferred. Examples of the dihydric alcohol (b) include aliphatic glycols such as ethylene glycol, 1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 1,4-butanediol, 2,4-diethyl-1,5-pentanediol, 1,6-hexanediol, 2-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4-methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 1,9-nonanediol, and dimer diol; polyether glycols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; and polyhydric alcohols having a ring skeleton such as 1,4-cyclohexanedimethanol, tricyclodecane dimethanol, hydroquinone, catechol, and resorcinol.

[0022] The polycarboxylic acid component and polyhydric alcohol component that make up the molecular chain of the polyester resin can be derived from biomass resources. Biomass resources include the stored materials obtained by converting solar energy into starch, cellulose, etc. through photosynthesis in plants, the bodies of animals that grow by eating plants, and products made by processing plants or animals. Among these, plant resources are more preferred, including wood, rice straw, rice husks, rice bran, used rice, corn, sugarcane, cassava, sago palm, soybean pulp, corn cob, tapioca dregs, bagasse, vegetable oil cake, potato, buckwheat, soybeans, oils and fats, waste paper, papermaking residues, seafood residues, livestock excrement, sewage sludge, and food waste. Corn, sugarcane, cassava, and sago palm are even more preferred.

[0023] Specific examples of polycarboxylic acid raw materials derived from biomass resources include adipic acid, sebacic acid, fumaric acid, itaconic acid, terephthalic acid, and 2,5-furandicarboxylic acid.

[0024] Specific examples of polyhydric alcohol raw materials derived from biomass resources include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol.

[0025] 3) Trifunctional or Higher Functional Components The polyester resin preferably contains structural units derived from a trifunctional or higher polycarboxylic acid and / or structural units derived from a trifunctional or higher polyhydric alcohol. In this case, when the total structural units constituting the molecular chain of the polyester resin is taken as 100 mol%, the structural units derived from a trifunctional or higher polycarboxylic acid and the structural units derived from a trifunctional or higher polyhydric alcohol may be present in a total amount of, for example, 1.0 mol% or more, preferably 1.0 to 6.0 mol%, preferably 1.2 to 6.0 mol%, more preferably 1.5 to 5.0 mol%, and even more preferably 2.0 to 4.0 mol%. The trifunctional or higher polycarboxylic acid and the trifunctional or higher polyhydric alcohol function as branching components in the polyester resin, facilitating the formation of a branched structure. By ensuring the content within the above range, sufficient reaction sites with the curing agent can be secured, resulting in a coating film with excellent corrosion resistance. Furthermore, the polymerization reaction becomes easier to control, resulting in a polyester resin with a higher molecular weight.

[0026] In the present invention, both trifunctional or higher polycarboxylic acids and trifunctional or higher polyhydric alcohols can be used as branching components, but it is preferable that the branching components are mainly derived from trifunctional or higher polycarboxylic acids. When the total of the trifunctional or higher polycarboxylic acid-derived structural units and the trifunctional or higher polyhydric alcohol-derived structural units is taken as 100 mol %, the trifunctional or higher polycarboxylic acid-derived structural units preferably account for 60 to 100 mol %, more preferably 75 to 99 mol %, and even more preferably 95 to 98 mol %.

[0027] 4) Dicarboxylic Acid Component The polyester resin preferably contains structural units derived from aromatic dicarboxylic acids. The inclusion of structural units derived from aromatic dicarboxylic acids increases the glass transition temperature and improves the corrosion resistance of the coating film. In this case, the structural units derived from aromatic dicarboxylic acids are preferably contained in an amount of 60 to 100 mol%, more preferably 65 to 99 mol%, and even more preferably 70 to 98 mol%, based on 100 mol% of the structural units derived from polycarboxylic acids constituting the molecular chain of the polyester resin. By ensuring that the amount is equal to or greater than the lower limit, the glass transition temperature increases and the corrosion resistance of the coating film improves. The upper limit is not particularly limited, but is preferably 100 mol% or less, more preferably 99 mol% or less, and even more preferably 98 mol% or less. From an industrial perspective, a content below 100 mol% is acceptable.

[0028] Furthermore, from the viewpoint of improving corrosion resistance, the polyester resin preferably contains, as the aromatic dicarboxylic acid, at least one selected from the group consisting of terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid. In this case, when the structural units derived from aromatic dicarboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol %, the structural units derived from terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid preferably account for a total of 50 to 100 mol %, more preferably 60 to 95 mol %, and even more preferably 70 to 90 mol %. By keeping the structural units within this range, the corrosion resistance of the coating film is improved.

[0029] Furthermore, from the viewpoint of effectively suppressing gelation during polymerization, the polyester resin preferably contains, as the aromatic dicarboxylic acid, at least one selected from the group consisting of isophthalic acid, orthophthalic acid, 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, more preferably isophthalic acid and / or orthophthalic acid, and even more preferably orthophthalic acid. In this case, when the structural units derived from aromatic dicarboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol %, the structural units derived from at least one component selected from the group consisting of isophthalic acid, orthophthalic acid, 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid preferably account for 1 to 50 mol %, more preferably 5 to 40 mol %, and even more preferably 10 to 30 mol % in total. By keeping the amount within this range, it is possible to effectively suppress gelation during polymerization.

[0030] The polyester resin may contain structural units derived from aliphatic dicarboxylic acids and / or alicyclic dicarboxylic acids. In this case, when the structural units derived from polycarboxylic acids constituting the molecular chain of the polyester resin are taken as 100 mol%, the structural units derived from aliphatic dicarboxylic acids and / or alicyclic dicarboxylic acids preferably account for a total of 20 mol% or less, more preferably 19 mol% or less, even more preferably 18 mol% or less, even more preferably 15 mol% or less, and particularly preferably 10 mol% or less. By keeping the content below the upper limit, the corrosion resistance of the coating film can be maintained and the processability of the coating film can be improved.

[0031] 5) Dihydric alcohol component The polyester resin preferably contains a structural unit derived from a dihydric alcohol (a) having one primary hydroxyl group and one secondary hydroxyl group. In this case, when the structural unit derived from the polyhydric alcohol constituting the molecular chain of the polyester resin is taken as 100 mol%, the structural unit derived from the dihydric alcohol (a) may be, for example, 50 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more. The upper limit is not particularly limited, but is preferably 100 mol% or less, and may be, for example, 98 mol% or less, 95 mol% or less, 90 mol% or less, or 80 mol% or less. That is, when the polyhydric alcohol-derived structural units constituting the molecular chain of the polyester resin are taken as 100 mol%, the dihydric alcohol (a)-derived structural units preferably comprise 50 to 100 mol%, 50 to 98 mol%, 50 to 95 mol%, 50 to 90 mol%, 50 to 80 mol%, 60 to 100 mol%, 70 to 100 mol%, 80 to 100 mol%, or 90 to 100 mol%. If a large amount of branched components is used to increase the hydroxyl value of the polyester resin and make the polyester resin highly branched, gelation proceeds rapidly during polymerization, making it difficult to achieve high molecular weight (particularly a high Mw). While the mechanism of action is not limited to the following, it is believed that copolymerizing a predetermined amount of a specific dihydric alcohol (a) with relatively low reactivity allows the polycondensation reaction to proceed slowly, thereby suppressing gelation despite the presence of a large number of branched components. Therefore, by setting the content at or above the lower limit, the reaction rate during polyester resin polymerization decreases, making it easier to control the reaction and suppressing gelation.

[0032] The polyester resin may also contain an optional component, a structural unit derived from a dihydric alcohol (b) other than the dihydric alcohol (a). When the structural units derived from polyhydric alcohols constituting the molecular chain of the polyester resin are taken as 100 mol%, the structural units derived from the dihydric alcohol (b) may preferably be 50 mol% or less, more preferably 40 mol% or less, and even more preferably 30 mol% or less, or even 0 mol%. By keeping the structural unit content below the upper limit, the reaction rate during polymerization of the polyester resin decreases, making it easier to control the reaction and suppressing gelation.

[0033] 6) Other Components The polyester resin can also contain structural units derived from components other than those described above. The polyester resin can also contain structural units derived from components having phenolic hydroxyl groups, such as diphenolic acid, p-hydroxybenzoic acid, p-hydroxyphenylacetic acid, p-hydroxyphenylpropionic acid, p-hydroxyphenethyl alcohol, and 5-hydroxyisophthalic acid. However, because phenolic hydroxyl groups do not contribute to the esterification reaction, using such components would likely result in the terminal capping of the polyester resin, making it difficult to adjust the weight-average molecular weight (Mw). Therefore, when the total structural units constituting the molecular chain of the polyester resin is taken as 100 mol%, the structural units derived from components having phenolic hydroxyl groups are preferably 5 mol% or less, more preferably 3 mol% or less, even more preferably 1 mol% or less, and most preferably 0 mol%. By keeping the structural units below the upper limit, the weight-average molecular weight (Mw) can be kept within a preferred range, improving the processability of the coating film.

[0034] The polyester resin may also contain, for example, structural units derived from components with a molecular weight of 500 or more. Specific examples of components with a molecular weight of 500 or more include dimer acid, dimer diol, polytetramethylene glycol, polyethylene glycol, polypropylene glycol, hydroxyl-terminated polybutadiene, hydroxyl-terminated polyisoprene, and hydroxyl-terminated polyolefin. In the present invention, a smaller amount of copolymerization is preferable, and when all structural units constituting the molecular chain of the polyester resin are taken as 100 mol%, the structural units derived from components with a molecular weight of 500 or more are preferably 5 mol% or less, more preferably 3 mol% or less, even more preferably 1 mol% or less, and most preferably 0 mol%. By keeping the amount below the upper limit, the branched structures (reaction points) in the polymer molecular chain of the polyester resin are located close to each other, resulting in a resin with a higher crosslink density.

[0035] 7) Polyester Characteristics The weight-average molecular weight (Mw) of the polyester resin is 50,000 or more, desirably 50,000 to 400,000, preferably 51,000 to 400,000, more preferably 55,000 to 350,000, even more preferably 60,000 to 300,000, and still more preferably 70,000 to 280,000, as determined by gel permeation chromatography (GPC) analysis using a polystyrene standard sample. By keeping the Mw within this range, reactivity with the curing agent is improved, and a coating film with excellent corrosion resistance and processability can be obtained.

[0036] The number average molecular weight (Mn) of the polyester resin, as determined by gel permeation chromatography (GPC) analysis using a polystyrene standard sample, is preferably 8,000 to 25,000, more preferably 9,000 to 20,000, and even more preferably 10,000 to 18,000. By keeping the Mn within this range, reactivity with the curing agent is improved, and a coating film having excellent corrosion resistance and processability can be obtained.

[0037] The molecular weight distribution (Mw / Mn) of the polyester resin, as determined by gel permeation chromatography (GPC) analysis using a polystyrene standard sample, is preferably 5.0 to 30.0, more preferably 5.5 to 25.0, and even more preferably 6.5 to 20.0. By ensuring that the molecular weight distribution is within this range, reactivity with the curing agent is improved, and a coating film with excellent corrosion resistance and processability can be obtained. Furthermore, since the polyester resin of the present invention has a particularly high weight-average molecular weight (Mw), the molecular weight distribution (Mw / Mn) tends to be large.

[0038] The hydroxyl value of the polyester resin is 182 to 800 eq / ton, more preferably 190 to 600 eq / ton, and even more preferably 200 to 400 eq / ton. By keeping the hydroxyl value within this range, reactivity with the curing agent is improved, and a coating film with excellent corrosion resistance and processability can be obtained. In particular, in the present invention, by increasing the branched structure in the molecular chain of the polyester resin to make the polyester resin highly branched, entanglement of molecular chains is reduced and melt viscosity is reduced. A highly branched polyester resin relatively increases the number of polymer molecular chain ends in the polyester resin, which also increases the hydroxyl value and is preferable from the standpoint of corrosion resistance. Making the polyester resin highly branched also contributes to improving the weight average molecular weight (Mw).

[0039] The acid value of the polyester resin is, for example, 3 eq / ton or more, desirably 3 to 600 eq / ton, preferably 70 to 600 eq / ton, more preferably 80 to 500 eq / ton, even more preferably 90 to 400 eq / ton, and even more preferably 100 to 350 eq / ton. In particular, by making the acid value 70 eq / ton or more, more reaction sites with the curing agent can be introduced, and the crosslinking density can be increased. By making the acid value equal to or less than the upper limit, it becomes easier to control the reaction during polymerization, and a polyester with a higher molecular weight can be obtained.

[0040] The polyester resin can be given an acid value by any method. Methods for giving an acid value include a method of adding a compound having a polycarboxylic anhydride group in the molecule in the later stage of polycondensation, and a method of increasing the acid value at the prepolymer (oligomer) stage and then polycondensing this to obtain a polyester resin having an acid value. The former method of adding a compound is preferred because of ease of operation and the ease of obtaining a target acid value.

[0041] Among compounds having a polycarboxylic acid anhydride group in the molecule for imparting an acid value to a polyester resin, examples of carboxylic acid monoanhydrides include phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, and citraconic anhydride, with trimellitic anhydride being preferred from the standpoints of versatility and economy. Among compounds having a polycarboxylic acid anhydride group in the molecule for imparting an acid value to a polyester resin, examples of carboxylic acid polyanhydrides include pyromellitic anhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, ethylene glycol bistrimellitate dianhydride, and 2,2',3,3'-biphenyltetracarboxylic dianhydride, with ethylene glycol bistrimellitate dianhydride being preferred from the standpoints of versatility and economy. The carboxylic acid monoanhydrides and carboxylic acid polyanhydrides may each be used alone or in combination of two or more.

[0042] The glass transition temperature (Tg) of the polyester resin is 60°C or higher, desirably 60 to 100°C, preferably 62 to 100°C, more preferably 65 to 95°C, and even more preferably 70 to 90°C. By setting the Tg at or above the lower limit, the corrosion resistance of the coating film can be improved. By setting the Tg at or below the upper limit, the processability of the coating film can be improved. The polyester resin may be either a crystalline resin or an amorphous resin, but is preferably an amorphous resin.

[0043] The reduced viscosity of the polyester resin is preferably 0.30 to 0.70 dl / g, more preferably 0.35 to 0.65 dl / g, and even more preferably 0.4 to 0.60 dl / g. By setting it to the upper limit or less, the toughness of the coating film is improved and processability is improved. By setting it to the lower limit or more, gelation during polymerization can be suppressed.

[0044] The polyester resin can also be used as a polyester resin aqueous dispersion by dispersing it in an aqueous medium using a known dispersion method, such as a dispersion method using an emulsifier.

[0045] 8) Manufacturing Method The manufacturing method of the polyester resin will be described. In the esterification / exchange reaction, all monomer components and / or their oligomers are heated, melted, and reacted. The esterification / exchange reaction temperature is preferably 180 to 250°C, more preferably 200 to 250°C. The reaction time is preferably 1.5 to 10 hours, more preferably 3 to 6 hours. The reaction time is the time from when the desired reaction temperature is reached until the subsequent polycondensation reaction begins. In the polycondensation reaction, the polyhydric alcohol component is distilled off from the esterified product obtained in the esterification reaction under reduced pressure at a temperature of 220 to 280°C, and the polycondensation reaction is continued until the desired molecular weight is reached. The polycondensation reaction temperature is preferably 220 to 280°C, more preferably 240 to 275°C. The degree of vacuum is preferably 150 Pa or less. An insufficient degree of vacuum tends to prolong the polycondensation time, which is undesirable. The time required for reducing the pressure from atmospheric pressure to 150 Pa or less is preferably 30 to 180 minutes.

[0046] In the esterification / exchange reaction and polycondensation reaction, polymerization is carried out, as necessary, using organic titanate compounds such as tetrabutyl titanate, tetraisopropyl titanate, and titanium oxyacetylcetonate; germanium compounds such as germanium dioxide and tetra-n-butoxygermanium; antimony compounds such as antimony oxide and tributoxyantimony; organic tin compounds such as tin octoate; metal acetates such as magnesium, iron, zinc, manganese, cobalt, and aluminum; etc. In terms of reactivity, organic titanate compounds are preferred, and in terms of resin coloration, germanium dioxide is preferred.

[0047] <Polyester Resin Composition> The polyester resin composition of the present invention contains at least the polyester resin and a curing agent. By blending the curing agent, the polyester resin composition can be used as an adhesive, paint, coating agent, etc., and a cured coating film with excellent processability and corrosion resistance can be obtained.

[0048] Here, the term "curing agent" refers to a known curing agent that reacts with a polyester resin to form a crosslinked structure. Examples of the crosslinked structure include a reaction in which unsaturated double bonds in the polyester resin are reacted via a radical addition reaction, a cation addition reaction, an anion addition reaction, or the like to form an intermolecular carbon-carbon bond, or a condensation reaction, a polyaddition reaction, or an ester exchange reaction with a polycarboxylic acid group or a polyhydric alcohol group in the polyester resin to form an intermolecular bond. Examples of curing agents include polyols such as phenolic resins, amino resins, epoxy compounds, isocyanate compounds, and β-hydroxylamide compounds, polycarboxylic acids, and unsaturated bond-containing resins. Of these, phenolic resins, amino resins, epoxy compounds, and isocyanate compounds are preferred because they are soluble in both aqueous and organic solvent systems and easily produce cured coating films with high crosslink density.

[0049] Examples of phenolic resins include those synthesized from trifunctional phenolic compounds such as carbolic acid, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol, or bifunctional phenolic compounds such as p-cresol, o-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, 2,5-xylenol, and m-methoxyphenol, and formaldehyde in the presence of an alkali catalyst, and those in which some or all of the methylol groups have been etherified with a lower alcohol.

[0050] Examples of amino resins include formalin adducts of urea, melamine, and benzoguanamine, and those etherified by reacting these with lower alcohols.

[0051] The epoxy compound is not particularly limited as long as it has two or more epoxy groups in one molecule. Specific examples include glycidyl ether of bisphenol-A and its oligomer, orthophthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, p-hydroxybenzoic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, succinic acid diglycidyl ester, adipic acid diglycidyl ester, sebacic acid diglycidyl ester, ethylene glycol diglycidyl ester, propylene glycol diglycidyl ester, and 1,4-butanediol diglycidyl ester. Examples of suitable curing catalysts include 1,6-hexanediol diglycidyl ester, polyalkylene glycol diglycidyl esters, trimellitic acid triglycidyl ester, triglycidyl isocyanurate, 1,4-diglycidyloxybenzene, diglycidyl propylene urea, glycerol triglycidyl ether, trimethylolethane triglycidyl ether, trimethylolpropane triglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol tetraglycidyl ether, and triglycidyl ethers of glycerol alkylene oxide adducts. These may be used alone or in combination of two or more. Among these, adducts of glycidyl groups to sorbitol, glycerin, and pentaerythritol are preferred because they are soluble in both aqueous and organic solvent systems and easily produce cured coatings with high crosslink density. Various amine-based catalysts are effective as curing catalysts.

[0052] Examples of the isocyanate compound include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylene-1,4-diisocyanate, xylene-1,3-diisocyanate, tetramethylxylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate, 2-nitrodiphenyl-4,4'-diisocyanate, 2,2'-diphenylpropane-4,4'-diisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 4,4'-diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4 diisocyanate, naphthylene-1,5-diisocyanate, 3,3'-dimethoxydiphenyl- Examples of the diisocyanates include aromatic diisocyanates such as 4,4'-diisocyanate, aromatic polyisocyanates such as polymethylene polyisocyanate and crude tolylene diisocyanate, aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (HDI), decamethylene diisocyanate and lysine diisocyanate, alicyclic diisocyanates such as isophorone diisocyanate (IPDI), hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate and hydrogenated diphenylmethane diisocyanate, and biuret, uretdione-modified, carbodiimide-modified, isocyanurate-modified, uretonimine-modified, adducts with polyols, and mixed modified products thereof, and these may be used alone or in combination of two or more. Furthermore, it may also be used in the form of a urethane precursor such as a prepolymer, modified product, derivative, or mixture, which is made of an isocyanate compound and an active hydrogen-containing compound such as a polyol or polyamine.

[0053] From the viewpoint of stability after blending the curing agent, it is particularly preferable to use a blocked isocyanate compound obtained by blocking the terminal NCO group of an isocyanate compound as the curing agent. Examples of blocking agents include phenolic compounds such as phenol, cresol, ethylphenol, and butylphenol, alcoholic compounds such as 2-hydroxypyridine, butyl cellosolve, propylene glycol monomethyl ether, benzyl alcohol, methanol, ethanol, n-butanol, isobutanol, and 2-ethylhexanol, active methylene compounds such as dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate, and acetylaceton, mercaptan compounds such as butyl mercaptan and dodecyl mercaptan, and acetanilide. Examples of suitable amine compounds include acid amide compounds such as amide and acetic acid amide, lactam compounds such as ε-caprolactam, δ-valerolactam, and γ-butyrolactam, imidazole compounds such as imidazole and 2-methylimidazole, urea compounds such as urea, thiourea, and ethyleneurea, oxime compounds such as formamide oxime, acetaldoxime, acetone oxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, and cyclohexanone oxime, and amine compounds such as diphenylaniline, aniline, carbazole, ethyleneimine, and polyethyleneimine. These may be used alone or in combination of two or more.

[0054] The reaction between such a blocking agent and an isocyanate curing agent component can be carried out, for example, at 20 to 200° C., using a known inert solvent or catalyst, if necessary. The blocking agent is preferably used in an amount of 0.7 to 1.5 times by mole relative to the terminal isocyanate groups.

[0055] The amount of curing agent in the polyester resin composition is preferably 1 to 50 parts by mass, more preferably 5 to 40 parts by mass, and even more preferably 8 to 30 parts by mass, per 100 parts by mass of the polyester resin. By setting the amount within this range, a coating film having excellent corrosion resistance and processability can be obtained.

[0056] The reactivity of the polyester resin with the curing agent can be evaluated by the gel (solvent-insoluble component) fraction. The gel fraction measured by the method described in the Examples is preferably 50 to 100%, more preferably 60 to 100%, even more preferably 70 to 98%, and even more preferably 80 to 98%, with the upper limit being 100%, although it may be 98% or less. By keeping the gel fraction within this range, the reactivity with the curing agent is improved, and a coating film with excellent corrosion resistance and processability can be obtained.

[0057] <Coating composition> The coating composition contains at least the polyester resin of the present invention and may further contain an organic solvent. In the coating composition, the polyester resin is contained as a main component. In the coating composition, the component with the highest content (mass ratio) among the solid components (non-volatile components excluding volatile substances such as water and organic solvents) that form the coating film in the coating composition is defined as the main component.

[0058] Examples of organic solvents 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, Solvesso, etc. These may be used alone or in combination of two or more, taking into consideration solubility, evaporation rate, etc.

[0059] The coating composition can contain known additives such as known inorganic pigments such as titanium oxide and silica, phosphoric acid and its esters, surface smoothing agents, antifoaming agents, dispersants, lubricants, crystal nucleating agents, and plasticizers, depending on the required properties. Lubricants are particularly important for imparting the lubricity of the coating film required when molding DI cans, DR (or DRD) cans, etc. Suitable examples of lubricants include fatty acid ester waxes, which are esters of polyol compounds and fatty acids, silicone waxes, fluorine-based waxes, polyolefin waxes such as polyethylene, lanolin waxes, montan waxes, and microcrystalline waxes. Lubricants can be used alone or in combination of two or more.

[0060] The coating composition may contain other resins for the purpose of improving the coating film's flexibility, adhesion, etc. Examples of other resins include amorphous polyesters, crystalline polyesters, ethylene-polymerizable unsaturated carboxylic acid copolymers, and ethylene-polymerizable carboxylic acid copolymer ionomers, and by incorporating at least one resin selected from these, it may be possible to impart flexibility and / or adhesion to the coating film.

[0061] The coating composition can be applied to a metal plate by a known coating method such as roll coating or spray coating. The coating thickness is not particularly limited, but a dry thickness of 3 to 18 μm, preferably 5 to 15 μm, is preferred. The coating is typically baked at a temperature of about 120 to 260°C for about 5 seconds to 30 minutes, preferably at a temperature of about 140 to 240°C for about 10 seconds to 20 minutes.

[0062] <Coating Film> The coating film of the present invention contains at least the coating composition. Specifically, the coating film refers to a polyester resin layer formed by coating a substrate with the coating composition of the present invention. The coating film may have a configuration in which a coating layer made of a resin other than the polyester resin of the present invention is superimposed on either the top or bottom of the polyester resin layer. That is, the present invention encompasses laminate structures such as substrate / polyester resin layer, substrate / coating layer / polyester resin layer, substrate / polyester resin layer / coating layer, and substrate / coating layer / polyester resin layer / coating layer.

[0063] <Metal Can> The metal can of the present invention contains at least the coating film. Metal cans can be obtained by forming a coating film on one or both sides, and if necessary, on the end faces, of a metal plate made of a metal material that can be used for, for example, beverage cans, canned food cans, their lids, caps, etc. Examples of the metal material include tinplate, tin-free steel, and aluminum. Metal plates made of these metal materials may be previously subjected to a corrosion prevention treatment using a phosphate treatment, a chromate chromate treatment, a chromate phosphate treatment, or other rust prevention agent, or a surface treatment for improving the adhesion of the coating film.

[0064] The polyester resin of the present invention can be made into a powder coating material by a known pulverization method. Examples of known pulverization methods include a pulverization method. In the pulverization method, a mixture of the polyester resin composition of the present invention, optionally a rust-preventive pigment, and additives, is dry-mixed in a mixer such as a tumbler mixer or a Henschel mixer, and then melt-kneaded in a kneader. Examples of kneaders that can be used include common kneaders such as a single- or twin-screw extruder, a three-roll mill, and a lab blast mill. The kneaded mixture is cooled and solidified, and the solidified material is coarsely and finely pulverized to obtain a pulverized material. Examples of pulverizers include a jet pulverizer that uses a supersonic jet stream to pulverize the material, and an impact pulverizer that introduces and pulverizes the solidified material into the space formed between a rotor and a stator (liner) rotating at high speed. If necessary, additives may be added to the pulverized material. The pulverized material is classified to adjust the powder to a desired particle size and particle size distribution, thereby obtaining a powder coating composition. For classification, a known classifier capable of removing over-pulverized toner base particles by classification using centrifugal force and wind force can be used, for example, a rotary wind classifier (rotary wind classifier) ​​or the like.

[0065] This application claims the benefit of priority based on Japanese Patent Application No. 2024-013102, filed on January 31, 2024. The entire contents of the specification of Japanese Patent Application No. 2024-013102, filed on January 31, 2024, are incorporated herein by reference.

[0066] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples, and can of course be practiced with appropriate modifications within the scope of the above and below-described aims, all of which are included within the technical scope of the present invention. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

[0067] (1) Measurement of Resin Composition A polyester resin sample was dissolved in deuterated chloroform and analyzed using a nuclear magnetic resonance (NMR) device AVANCE-NEO600 manufactured by BRUKER. １ H-NMR analysis or １3C-NMR analysis was carried out, and the molar ratio was calculated from the ratio of the integral values.

[0068] (2) Measurement of reduced viscosity (unit: dl / g) 0.1 g of a polyester resin sample was dissolved in 25 cc of a mixed solvent of phenol / tetrachloroethane (mass ratio 6 / 4), and the reduced viscosity was measured using an Ubbelohde viscometer at 30° C. In the evaluation of (7) described later, this measured value is referred to as (X).

[0069] (3) Measurement of Acid Value 0.2 g of a polyester resin sample was dissolved in 40 ml of chloroform, and titrated with a 0.01 N potassium hydroxide ethanol solution to obtain 10% of the polyester resin. ６ The equivalent weight per gram (eq / ton) was calculated. Phenolphthalein was used as an indicator.

[0070] (4) Measurement of hydroxyl value The polyester resin was crushed and dried under reduced pressure on Teflon (registered trademark) at 50°C for at least 24 hours. Next, approximately 0.5 g of the sample was weighed out, and 10 ml of an acetylating agent (0.5 mol% / L acetic anhydride pyridine solution) was added. The sample was then immersed in a water bath at 95°C or higher for 1.5 hours. After that, 10 ml of pure water was added and the sample was allowed to cool to room temperature. Subsequently, titration was performed with N / 5-NaOH using phenolphthalein as an indicator. The same procedure was performed on a blank sample without the sample, and the hydroxyl value (eq / ton) was calculated according to the following formula: hydroxyl value = {(B - A) x 0.2 x f x 1000 / W} + acid value (A = titration constant (ml), B = titration constant of blank (ml), f = N / 5-NaOH factor, W = sample weight (g)).

[0071] (5) Measurement of Glass Transition Temperature (Tg) The glass transition temperature (Tg) was measured using a differential scanning calorimeter (DSC) DSC-220 manufactured by Seiko Instruments Inc. 5 mg of a polyester resin sample was placed in an aluminum clamp-lid container and sealed, cooled to −50° C. using liquid nitrogen, and then heated to 200° C. at a rate of 20° C. / min. In the endothermic curve obtained in this process, the temperature at the intersection of the baseline before the endothermic peak and the tangent to the endothermic peak was taken as the glass transition temperature (Tg, unit: ° C.).

[0072] (6) Measurement of number average molecular weight (Mn), weight average molecular weight (Mw), and molecular weight distribution (Mw / Mn) A polyester resin sample was dissolved and / or diluted with tetrahydrofuran to a resin concentration of approximately 0.5 wt%, and filtered through a polytetrafluoroethylene membrane filter with a pore size of 0.5 μm to prepare a measurement sample. The molecular weight was measured by gel permeation chromatography (GPC) using tetrahydrofuran as the mobile phase and a differential refractometer as a detector. The flow rate was 1 mL / min, and the column temperature was 30°C. Showa Denko KF-802, 804L, and 806L columns were used. Monodisperse polystyrene was used as the molecular weight standard, and the molecular weight was calculated as a standard polystyrene equivalent value, excluding the portion corresponding to a molecular weight of less than 1,000.

[0073] (7) Evaluation of Polymerizability A 10 g sample of polyester resin was heat-treated on Teflon (registered trademark) in a nitrogen atmosphere at 230°C for 1 hour. Next, the reduced viscosity (Y) of the polyester resin after heat treatment was measured using the same procedure as the above-mentioned measurement method. Using the obtained reduced viscosity (X) before heat treatment and reduced viscosity (Y) after heat treatment, the following evaluation was made. (Evaluation) ◎: (X) - (Y) is 0.1 or more (reduced viscosity has decreased). ○: (X) - (Y) is 0.05 or more but less than 0.1 (reduced viscosity has slightly decreased). △: (X) - (Y) is 0 or more but less than 0.05 (reduced viscosity remains almost unchanged). ×: (X) - (Y) is less than 0 (reduced viscosity increases over time and is prone to gelation during polymerization).

[0074] (8) Measurement of gel fraction The gel fraction was used as an evaluation index for curability. The coating composition was applied to a copper foil so that the thickness after drying would be 10 μm, and the sample was heated at 200° C. for 10 minutes to a size of 10 cm long and 2.5 cm wide. The mass of the sample before immersion in tetrahydrofuran (THF) was (X), and the mass of the sample after immersion in THF after immersion in 60 ml of THF at 25° C. for 1 hour and drying at 100° C. for 10 minutes was (Y). The gel fraction (mass %) was calculated using the following formula: Gel fraction (mass %) = [{(Y) - copper foil mass} / {(X) - copper foil mass}] × 100

[0075] (9) Evaluation of Workability The obtained test specimen was bent 180° so that the coating film was facing outward, and the cracks in the coating film at the bend were evaluated by measuring the current value. The bending was performed without any interposition (so-called 0T). An aluminum plate electrode (width 20 mm, depth 50 mm, thickness 0.5 mm) was placed on top of a sponge (width 20 mm, depth 50 mm, thickness 10 mm) soaked in 1% NaCl aqueous solution. The test specimen was placed in contact with the sponge near the center of the bent portion so that it was parallel to the 20 mm side of the sponge. A DC voltage of 5.0 V was applied between the aluminum plate electrode and the uncoated portion on the back of the test specimen, and the current value was measured. A smaller current value indicates better bending properties. (Judgment) ⊚: Less than 0.5 mA ◯: 0.5 mA or more but less than 2.0 mA ×: 2.0 mA or more

[0076] (10) Evaluation of corrosion resistance The obtained test piece was placed upright in a stainless steel cup, and an aqueous solution containing 1% by weight of salt and 5% by weight of acetic acid was poured into it until it reached half the height of the test piece. This was then placed in the pressure cooker of a retort tester (ES-315, manufactured by Tomy Kogyo Co., Ltd.) and subjected to retort treatment at 125°C for 90 minutes. Evaluation after treatment was carried out at the steam contact point, which is generally thought to be exposed to more severe conditions for a coating film, and the whitening and blistering state of the cured film was visually judged as follows: (Judgment) ◎: Good (no whitening or blisters) ◯: Very slight whitening and / or blisters ×: Significant whitening and / or significant blisters

[0077] <Examples 1 to 16, Comparative Examples 1 and 2> <Preparation of Polyester Resin> Synthesis Example (a) 570 parts of dimethyl terephthalate, 14 parts of trimellitic anhydride, 330 parts of 1,2-propanediol, 300 parts of neopentyl glycol, and 0.4 parts (0.03 mol% relative to the total acid components) of tetra-n-butyl titanate (hereinafter sometimes abbreviated as TBT) as a catalyst were charged into a 3 L four-neck flask, and a transesterification reaction was carried out while gradually increasing the temperature to 240 ° C. over 3 hours. Next, the temperature was lowered to 160 ° C., and 100 parts of orthophthalic acid was added, and an esterification reaction was carried out while gradually increasing the temperature to 240 ° C. over 3 hours. After completion of the esterification reaction, the pressure in the system was gradually reduced, and reduced-pressure polymerization was carried out to 10 mmHg over 1 hour, and the temperature was raised to 240 ° C., and post-polymerization was further carried out for 90 minutes under a vacuum of 1 mmHg or less. When the target molecular weight was reached, the resin was taken out to obtain a polyester resin (Synthesis Example (a)).

[0078] Synthesis Examples (b) to (r) Synthesis Examples (b) to (e), (g) to (i), (k) to (m), and (q) were prepared by transesterification and esterification in the same manner as Synthesis Example (a), except that the starting composition was changed to produce polyester resins with the resin compositions shown in the table. Synthesis Example (f) was also prepared by transesterification and esterification in the same manner as Synthesis Example (a), except that the post-polymerization time was set to 50 minutes to produce a polyester resin. Synthesis Examples (n) to (o) were polymerized in the same manner as Synthesis Example (a), and after the polycondensation reaction was completed, the mixture was cooled to 220°C under a nitrogen atmosphere. A predetermined amount of trimellitic anhydride was then added, and the mixture was stirred at 220°C for 30 minutes under a nitrogen atmosphere. After the reaction was completed, the mixture was removed to obtain a polyester resin. Synthesis Example (j) was prepared by a direct polymerization method (omitting the transesterification step in Synthesis Example (a)), producing a polyester resin with the resin composition shown in the table. In Synthesis Example (p), only the transesterification reaction was carried out, but gelation occurred when the final polymerization time was 90 minutes, so the final polymerization time was set to 50 minutes to produce a polyester resin. In Synthesis Example (r), the transesterification reaction and the esterification reaction were carried out in the same manner as in Synthesis Example (a), but the final polymerization time was set to 50 minutes to produce a polyester resin.

[0079] <Preparation of Coating Composition> 100 parts (solids) of the resulting polyester resin was dissolved in cyclohexanone to obtain a polyester resin solution (solids content approximately 40%). Next, 212.5 parts of the polyester resin solution were mixed with 25 parts of IPDI-based blocked isocyanate (manufactured by Covestro, DESMODUR VP LS 2078 / 2, solids content 60 wt%) as a curing agent and 0.1 parts of DBTL (dibutyltin dilaurate) as a catalyst, and then diluted with cyclohexanone to a viscosity suitable for coating to obtain a coating composition. The resulting coating composition was used to evaluate gel fraction, processability, and corrosion resistance.

[0080] <Preparation of test piece (coating film)> The obtained coating composition was applied to one side of a tin plate (JIS G 3303 (2008) SPTE, 70 mm × 150 mm × 0.3 mm) using a bar coater so that the film thickness after drying would be 10 ± 2 μm, and the coating was baked under baking conditions of 200°C × 10 minutes to prepare a test piece (coating film).

[0081]

[0082]

[0083] In Examples 1 to 16, excellent results were obtained in both the processability and corrosion resistance of the coating films containing polyester resin.

[0084] On the other hand, in Comparative Example 1, although the glass transition temperature (Tg) was high, the hydroxyl value was low and the polyester resin did not have a sufficient branched structure, making it difficult to increase the weight average molecular weight (Mw). In Comparative Example 1, the weight average molecular weight (Mw) of the polyester resin was low, resulting in poor processability of the coating film. Furthermore, the low hydroxyl value meant that there were insufficient reaction sites with the curing agent, and the resulting polyester resin had poor reactivity with the curing agent, resulting in poor corrosion resistance of the coating film. In Comparative Example 2, the weight average molecular weight (Mw) of the polyester resin was low, resulting in poor processability of the coating film.

[0085] The present invention relates to a polyester resin that has good reactivity with a curing agent and can form a coating film that is excellent in processability and corrosion resistance, and a polyester resin composition containing the polyester resin of the present invention is applicable to various uses. In particular, the polyester resin of the present invention can be effectively used in the form of a paint composition, an adhesive composition, a coating composition, etc., and is preferably used as a base agent for a paint for coating metal cans that contain beverages or foods.

Claims

1. A polyester resin containing a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components, which satisfies the following (1) to (3): (1) a glass transition temperature (Tg) of 60°C or higher, (2) a weight average molecular weight (Mw) of 50,000 or higher, and (3) a hydroxyl value of the polyester resin of 182 to 800 eq / ton.

2. The polyester resin according to claim 1, wherein the polycarboxylic acid component contains a structural unit derived from an aromatic dicarboxylic acid.

3. The polyester resin according to claim 2, which contains a total of 1 mol % or more of structural units derived from at least one component selected from the group consisting of isophthalic acid, orthophthalic acid, 1,2-naphthalenedicarboxylic acid, 1,3-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, when the structural units derived from aromatic dicarboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol %.

4. A polyester resin according to claim 2, which contains 60 mol % or more of structural units derived from aromatic dicarboxylic acids when the structural units derived from polycarboxylic acids constituting the molecular chain of the polyester resin are taken as 100 mol %.

5. The polyester resin according to claim 2, wherein the aromatic dicarboxylic acid is at least one selected from the group consisting of terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid, and the polyester resin contains a total of 50 mol % or more of structural units derived from terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid, when the structural units derived from aromatic dicarboxylic acids that make up the molecular chain of the polyester resin are taken as 100 mol %.

6. The polyester resin according to claim 1, which has an acid value of 3 eq / ton or more.

7. A polyester resin according to claim 1, which contains structural units derived from a trifunctional or higher polycarboxylic acid and / or structural units derived from a trifunctional or higher polyhydric alcohol, and which contains a total of 1.0 mol% or more of structural units derived from a trifunctional or higher polycarboxylic acid and structural units derived from a trifunctional or higher polyhydric alcohol when the total structural units constituting the molecular chain of the polyester resin are taken as 100 mol%.

8. A polyester resin according to claim 7, which contains 60 mol% or more of the structural units derived from the tri- or higher functional polycarboxylic acid when the total of the structural units derived from the tri- or higher functional polyhydric alcohol is 100 mol%.

9. A polyester resin according to claim 1, in which the structural units derived from aliphatic dicarboxylic acids and / or alicyclic dicarboxylic acids account for 20 mol % or less when the structural units derived from polycarboxylic acids constituting the molecular chain of the polyester resin are taken as 100 mol %.

10. A polyester resin according to claim 1, which contains 50 mol % or more of structural units derived from a dihydric alcohol (a) having one primary hydroxyl group and one secondary hydroxyl group, when the structural units derived from polyhydric alcohols constituting the molecular chain of the polyester resin are taken as 100 mol %.

11. A polyester resin according to claim 10, in which the proportion of structural units derived from dihydric alcohols (b) other than the dihydric alcohol (a) is 50 mol % or less when the proportion of structural units derived from polyhydric alcohols constituting the molecular chain of the polyester resin is 100 mol %.

12. A polyester resin composition containing the polyester resin according to any one of claims 1 to 11 and a curing agent.

13. A coating composition containing the polyester resin according to any one of claims 1 to 11.

14. A coating film containing the coating composition according to claim 13.

15. A metal can containing the coating film of claim 14.

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