WO2025164268A1 - Polyester resin, polyester resin composition, coating material composition, coating film, and metal can - Google Patents

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

The present invention relates to a polyester resin. Specifically, it relates to a polyester resin suitable for use in can coatings, and even more specifically to a polyester resin suitable for coating cans that contain beverages or food, as well as a polyester resin composition, coating composition, coating film, and metal can containing the same.

Metal cans such as beverage cans and food cans are coated with organic resins such as polyester to prevent food from corroding the metal (corrosion resistance) and to preserve the flavor and taste of the contents (flavoring). This coating film undergoes high-stress processes such as necking and threading during the molding process for the mouth of a bottle can. Therefore, the coating film must be able to withstand such post-processing (processability). Recently, the diversity of can shape designs and can contents has continued to grow, requiring even greater improvements in corrosion resistance and processability from can paints.

In response to this issue, for example, Patent Document 1 discloses a paint that uses a hydroxyl group-containing polyester resin to provide excellent sterilization stability and flexibility, particularly in acidic media.

Japanese Patent Application Laid-Open No. 2005-42110

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 poses the problem of insufficient corrosion resistance depending on the contents. Furthermore, the molecular weight of the hydroxyl group-containing polyester resin is low, which poses the problem of insufficient processability depending on the part of the can.

The present invention provides a polyester resin that has good reactivity with curing agents and can form coating films with excellent processability and corrosion resistance.

After extensive research into the above, the inventors discovered that by making the polyester resin highly branched and increasing the hydroxyl value of the polyester resin to a predetermined value or greater, a polyester resin with a high glass transition temperature (Tg) and a high weight average molecular weight (Mw) can be obtained, leading to the completion of the present invention. Specifically, the present invention comprises the following features:

[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; (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 constitute the molecular chain of the polyester resin are taken as 100 mol %.
[4] The polyester resin according to [2] or [3], containing 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 100 mol%.
[5] The aromatic dicarboxylic acid includes at least one selected from the group consisting of terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid,
The polyester resin according to any one of [2] to [4], wherein 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 structural units derived from aromatic dicarboxylic acids that constitute the molecular chain of the polyester resin are taken as 100 mol %.
[6] The polyester resin according to any one of [1] to [5], having an acid value of 3 eq/ton or more.
[7] The polyester resin according to any one of [1] to [6], 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 contains a total of 1.0 mol% or more of the structural units derived from a trifunctional or higher polycarboxylic acid and the structural units derived from a trifunctional or higher polyhydric alcohol when all 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 polyester resin contains 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 100 mol%.
[9] The polyester resin according to any one of [1] to [8], wherein the structural units derived from aliphatic dicarboxylic acids and/or alicyclic dicarboxylic acids are 20 mol % or less when the structural units derived from polycarboxylic acids constituting the molecular chain of the polyester resin are 100 mol %.
[10] The polyester resin according to any one of [1] to [9], 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 a polyhydric alcohol constituting the molecular chain of the polyester resin are taken as 100 mol%.
[11] The polyester resin according to [10], wherein the structural units derived from a dihydric alcohol (b) other than the dihydric alcohol (a) account for 50 mol % or less when the structural units derived from a polyhydric alcohol constituting the molecular chain of the polyester resin are taken as 100 mol %.
[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].

The present invention provides a polyester resin capable of forming a coating film with excellent processability and corrosion resistance. Therefore, the polyester resin of the present invention is preferably used in applications such as polyester resin compositions, paint compositions, coating films, and metal cans.

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

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 places a heavy burden on equipment, such as increased torque load during stirring, making it difficult to achieve high molecular weight (particularly high Mw). The inventors' investigations have revealed that highly branched polyester resins and increased hydroxyl values within 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 reaction sites with the curing agent. It also has a high glass transition temperature (Tg), which results in good corrosion resistance of the coating film. Furthermore, because the polyester resin also has a high weight-average molecular weight (Mw), the coating film also has good processability.

In this specification, each of the components listed below may be used alone or in combination of two or more.

In addition, 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 a single structural unit that makes up the molecular chain of a polyester resin. In other words, 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.

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.

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" refer to structural units derived from all copolymerization components constituting the molecular chain of the polyester resin, or structural units derived from A, respectively, 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 proportion of each structural unit constituting the polyester resin is determined, for example, from various analyses such as the amount of copolymerization components charged, ¹ H-NMR analysis, ¹³ C-NMR analysis, etc.

In this specification, examples of polycarboxylic acids include tri- or higher functional polycarboxylic acids and dicarboxylic acids. A tri- or higher functional polycarboxylic acid specifically refers to a polycarboxylic acid having three or more carboxy groups, and the number of functional groups is preferably tri- to penta-functional, and more preferably tri- to tetra-functional. A dicarboxylic acid specifically refers to a polycarboxylic acid having two carboxy groups.

Examples of tri- or higher functional polycarboxylic 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, 1,2,3,4-butane tetracarboxylic acid, and other polycarboxylic acids and their anhydrides. 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 trifunctional or higher polycarboxylic acids, those having an aromatic ring such as a benzene ring or a naphthalene ring within the molecule are preferred. Using polycarboxylic acids having an aromatic ring allows for the introduction of a rigid skeleton within the molecule, which inhibits hydrolysis and facilitates the formation of a coating film with excellent corrosion resistance.

Examples of the dicarboxylic acid 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, 2,7-naphthalenedicarboxylic acid, and anhydrides thereof.
Examples of the aliphatic dicarboxylic acid (preferably acyclic) include succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, dimer acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, and anhydrides thereof.
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.

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 3 to 5, and more preferably 3 to 4. Dihydric alcohols specifically refer to polyhydric alcohols having two hydroxy groups.

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

Examples of the dihydric alcohol include (a) a dihydric alcohol having one primary hydroxyl group and one secondary hydroxyl group; and (b) a dihydric alcohol other than (a).
Examples of the dihydric alcohol (a) include 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 1,2-hexanediol, etc. 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.

The polycarboxylic acid components and polyhydric alcohol components that make up the molecular chains of polyester resins can be derived from biomass resources. Biomass resources include stored materials such as starch and cellulose, which are formed by converting solar energy into plants through photosynthesis, 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 cakes, potatoes, 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.

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.

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.

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.

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

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.

Furthermore, from the perspective of improving corrosion resistance, it is preferable that the polyester resin contains, as an 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.

Furthermore, from the viewpoint of effectively suppressing gelation during polymerization, it is preferable that the polyester resin contains, as an 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, with isophthalic acid and/or orthophthalic acid being more preferable, and orthophthalic acid being even more preferable. 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 a total of 1 to 50 mol %, more preferably 5 to 40 mol %, and even more preferably 10 to 30 mol %. By keeping the amount within this range, gelation during polymerization can be effectively suppressed.

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 that make up 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 amount below the upper limit, the corrosion resistance of the coating film can be maintained and the processability of the coating film is improved.

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.

The polyester resin can also contain, as an optional component, structural units derived from a dihydric alcohol (b) other than the dihydric alcohol (a). When the structural units derived from polyhydric alcohols that make up the molecular chain of the polyester resin are taken as 100 mol %, the structural units derived from the dihydric alcohol (b) should 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 units below the upper limit, the reaction rate during polymerization of the polyester resin decreases, making it easier to control the reaction and suppressing gelation.

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.

The polyester resin may also contain, for example, structural units derived from components with a molecular weight of 500 or greater. Specific examples of components with a molecular weight of 500 or greater 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 lower copolymerization amount 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 greater 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 closer to each other, resulting in a resin with a higher crosslink density.

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.

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 it within this range, reactivity with the curing agent is improved, and a coating film with excellent corrosion resistance and processability can be obtained.

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 keeping it within this range, reactivity with the curing agent is improved, resulting in a coating film with excellent corrosion resistance and processability. Furthermore, because 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.

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 it 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 and making the polyester resin highly branched, entanglement of molecular chains is reduced and melt viscosity is lowered. 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 an improvement in the weight average molecular weight (Mw).

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 it 70 eq/ton or more, more reaction points with the curing agent can be introduced, increasing the crosslink density. By making it below the upper limit, it becomes easier to control the polymerization reaction, and a polyester with a higher molecular weight can be obtained.

Polyester resins can be given an acid value by any method. Methods for giving an acid value include an addition reaction of a compound having a polycarboxylic anhydride group in the molecule in the later stages of polycondensation, or a method of giving a high acid value to a prepolymer (oligomer) at the prepolymer stage and then polycondensing this to obtain a polyester resin with an acid value. However, the former addition reaction method is preferred due to its ease of operation and the ease with which the target acid value can be obtained.

Among compounds having a polyvalent carboxylic 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. Among these, trimellitic anhydride is preferred from the viewpoints of versatility and economy.
Among compounds having a polyvalent carboxylic 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 acid dianhydride, 1,2,3,4-cyclopentanetetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, ethylene glycol bistrimellitate dianhydride, and 2,2',3,3'-biphenyltetracarboxylic acid dianhydride. Of these, ethylene glycol bistrimellitate dianhydride is preferred from the standpoint of versatility and economy.
The carboxylic acid monoanhydrides and carboxylic acid polyanhydrides may each be used alone or in combination of two or more.

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. Furthermore, the polyester resin may be either a crystalline resin or an amorphous resin, but is preferably an amorphous resin.

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 below the upper limit, the toughness of the coating film is improved and processability is improved. By setting it above the lower limit, gelation during polymerization can be suppressed.

Polyester resin can also be dispersed in an aqueous medium using a known dispersion method and used as a polyester resin aqueous dispersion. Known methods include dispersion methods using an emulsifier.

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.

During the esterification/exchange reaction and polycondensation reaction, polymerization is carried out using, as necessary, 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; and metal acetates such as magnesium, iron, zinc, manganese, cobalt, and aluminum. Organic titanate compounds are preferred in terms of reactivity, while germanium dioxide is preferred in terms of resin coloration.

<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 excellent in processability and corrosion resistance can be obtained.

Here, the term "curing agent" refers to a known curing agent that reacts with polyester resin to form a crosslinked structure. Examples of the crosslinked structure include reactions in which unsaturated double bonds in the polyester resin react through radical addition reactions, cationic addition reactions, or anionic addition reactions to form intermolecular carbon-carbon bonds, or intermolecular bonds formed through condensation reactions, polyaddition reactions, or transesterification reactions with polycarboxylic acid groups or polyhydric alcohol groups in the polyester resin. 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 coatings with high crosslink density.

Phenol 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 alkaline catalyst, or those in which some or all of the methylol groups have been etherified with a lower alcohol.

Amino resins include formalin adducts of urea, melamine, and benzoguanamine, as well as those etherified by reacting these with lower alcohols.

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 oligomers, 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 can be used alone or in combination of two or more. Of 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.

Examples of isocyanate compounds 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, 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 can be used alone or in combination of two or more. They can also be used in the form of urethane precursors, such as prepolymers, modified products, derivatives, and mixtures, made from an isocyanate compound and an active hydrogen-containing compound such as a polyol or polyamine.

From the viewpoint of stability after blending with the curing agent, it is particularly preferable to use a blocked isocyanate compound in which the terminal NCO group of an isocyanate compound has been blocked. 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 can be used alone or in combination of two or more types.

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 as needed. It is preferable to use 0.7 to 1.5 times the molar amount of the blocking agent relative to the terminal isocyanate groups.

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 polyester resin. By keeping the amount within this range, a coating film with excellent corrosion resistance and processability can be obtained.

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

<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.

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, and Solvesso. Taking into consideration solubility, evaporation rate, etc., these can be used alone or in combination of two or more.

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 wax, which is an ester of a polyol compound and a fatty acid, silicone wax, fluorine-based wax, polyolefin wax such as polyethylene, lanolin wax, montan wax, and microcrystalline wax. Lubricants can be used alone or in combination of two or more types.

Other resins can be blended into the coating composition to improve the coating film's flexibility, adhesion, etc. Examples of other resins include amorphous polyester, crystalline polyester, ethylene-polymerizable unsaturated carboxylic acid copolymer, and ethylene-polymerizable carboxylic acid copolymer ionomer. Blending at least one resin selected from these may impart flexibility and/or adhesion to the coating film.

The coating composition can be applied to a metal plate using known coating methods such as roll coating or spray coating. There are no particular restrictions on the coating thickness, but a dry thickness of 3 to 18 μm, and preferably 5 to 15 μm, is preferred. The coating is typically baked at temperatures ranging from approximately 120 to 260°C for approximately 5 seconds to 30 minutes, preferably from approximately 140 to 240°C for approximately 10 seconds to 20 minutes.

<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.

<Metal cans>
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 such metal materials include tinplate, tin-free steel, and aluminum. Metal plates made of these metal materials may be previously subjected to a phosphate treatment, a chromate chromate treatment, a chromate phosphate treatment, or other corrosion prevention treatment using a rust prevention agent, or a surface treatment for improving the adhesion of the coating film.

The polyester resin of the present invention can be made into a powder coating by a known pulverization method. Examples of known pulverization methods include pulverization. In pulverization, a mixture of the polyester resin composition of the present invention, optionally including an anti-rust pigment and additives, is dry-mixed in a mixer such as a tumbler mixer or 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, or a lab blast mill. The kneaded mixture is cooled and solidified, and the solidified product is coarsely and finely pulverized to obtain a pulverized product. Examples of pulverizers include jet pulverizers that pulverize using a supersonic jet stream, and impact pulverizers that introduce and pulverize the solidified product into the space formed between a rotor and liner rotating at high speed. If necessary, additives may be added to the pulverized product. The pulverized product is classified to adjust the powder to the desired particle size and particle size distribution, thereby obtaining a powder coating composition. For classification, a known classifier that can remove over-pulverized toner base particles by classification using centrifugal force and wind force can be used, such as a rotary wind classifier.

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

The present invention will be explained in more detail below using examples, but the present invention is not limited to the examples below, and it is of course possible to make appropriate modifications within the scope of the intent described above and below, all of which are within the technical scope of the present invention. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

(1) Measurement of Resin Composition A polyester resin sample was dissolved in deuterated chloroform and subjected to ¹ H-NMR analysis or ¹³ C-NMR analysis using a nuclear magnetic resonance (NMR) apparatus AVANCE-NEO600 manufactured by BRUKER Co., Ltd. The molar ratio was determined from the ratio of the integral values.

(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).

(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 determine the equivalent weight (eq/ton) per 10 ⁶ g of polyester resin. Phenolphthalein was used as an indicator.

(4) Measurement of hydroxyl value The polyester resin was pulverized and dried under reduced pressure on Teflon (registered trademark) at 50°C for at least 24 hours. Next, approximately 0.5 g of sample was precisely weighed, 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 which 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 repeated for a blank 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))

(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.).

(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.

(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 the heat treatment was measured using the same procedure as the above-mentioned measurement method. Using the obtained reduced viscosity (X) before the heat treatment and the reduced viscosity (Y) after the heat treatment, the following evaluation was made.
(judgement)
◎: (X) - (Y) is 0.1 or more (reduced viscosity is decreased)
○: (X) - (Y) is 0.05 or more and less than 0.1 (reduced viscosity is slightly decreased)
△: (X) - (Y) is 0 or more and less than 0.05 (reduced viscosity remains almost unchanged)
×: (X) - (Y) is less than 0 (reduced viscosity increases over time and gelation is likely to occur during polymerization).

(8) Measurement of gel fraction The gel fraction was used as an evaluation index of curability. The coating composition was applied to a copper foil so that the thickness after drying was 10 μm, and heated at 200° C. for 10 minutes to obtain a sample measuring 10 cm in length and 2.5 cm in width. The mass of the sample before immersion in tetrahydrofuran (THF) was (X), and the mass of the sample after immersion in THF (Y) was determined by the following formula.
Gel fraction (mass%) = [{(Y) - mass of copper foil} / {(X) - mass of copper foil}] x 100

(9) Evaluation of Workability The obtained test specimen was bent 180° with the coating facing outward, and the cracks in the coating film occurring at the bend were evaluated by measuring the energization 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 prepared, on which a sponge (width 20 mm, depth 50 mm, thickness 10 mm) soaked in 1% NaCl aqueous solution was placed. 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 energization value was measured. A smaller energization value indicates better bending properties.
(judgement)
◎: Less than 0.5 mA ○: 0.5 mA or more and less than 2.0 mA ×: 2.0 mA or more

(10) Evaluation of corrosion resistance The obtained test piece was placed upright in a stainless steel cup, and an aqueous solution containing 1 wt % salt and 5 wt % acetic acid was poured into it to reach half the height of the test piece, which 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 thought to be generally exposed to more severe conditions for a coating film, and the whitening and blistering of the cured film were visually evaluated as follows:
(judgement)
◎: Good (no whitening or blisters)
◯: Very slight whitening and/or blistering is present. ×: Significant whitening and/or significant blistering is present.

<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 of tetra-n-butyl titanate (hereinafter sometimes abbreviated as TBT) as a catalyst (0.03 mol% relative to the total acid components) were charged into a 3 L four-neck flask, and the temperature was gradually raised to 240 ° C. over 3 hours to carry out a transesterification reaction. Next, the temperature was lowered to 160 ° C., and 100 parts of orthophthalic acid was added. The temperature was gradually raised to 240 ° C. over 3 hours to carry out an esterification reaction. 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., followed by post-polymerization for 90 minutes under a vacuum of 1 mmHg or less. When the target molecular weight was reached, the mixture was removed to obtain a polyester resin (Synthesis Example (a)).

Synthesis examples (b) to (r)
In Synthesis Examples (b) to (e), (g) to (i), (k) to (m), and (q), polyester resins having the resin compositions shown in the table were produced by transesterification and esterification in the same manner as Synthesis Example (a), except that the charged compositions were changed.
In Synthesis Example (f), a polyester resin was produced by carrying out the transesterification reaction and esterification reaction in the same manner as in Synthesis Example (a), except that the post-polymerization time was set to 50 minutes.
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, and then a predetermined amount of trimellitic anhydride was added and stirred for 30 minutes at 220°C under a nitrogen atmosphere. After the reaction was completed, the mixture was taken out to obtain a polyester resin.
In Synthesis Example (j), a polyester resin having the resin composition shown in the table was produced by a direct polymerization method (omitting the transesterification reaction step in Synthesis Example (a)).
In Synthesis Example (p), only the transesterification reaction was carried out, but since gelation occurred when the final polymerization time was 90 minutes, the final polymerization time was set to 50 minutes to produce a polyester resin.
In Synthesis Example (r), a polyester resin was produced by carrying out the transesterification reaction and esterification reaction in the same manner as in Synthesis Example (a), except that the post-polymerization time was set to 50 minutes.

<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 blended with 25 parts of an IPDI-based blocked isocyanate (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.

<Preparation of test piece (coating film)>
The obtained coating composition was applied to one side of a tinplate (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).

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

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, because the hydroxyl value was low, 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, and as a result, the coating film had poor processability.

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 (15)

A polyester resin containing a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components, which satisfies the following (1) to (3):
(1) The glass transition temperature (Tg) is 60°C or higher. (2) The weight average molecular weight (Mw) is 50,000 or higher. (3) The hydroxyl value of the polyester resin is 182 to 800 eq/ton. The polyester resin according to claim 1, wherein the polycarboxylic acid component contains structural units derived from an aromatic dicarboxylic acid. The polyester resin according to claim 2, wherein the polyester resin 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 %. The 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 that make up the molecular chain of the polyester resin are taken as 100 mol %. The aromatic dicarboxylic acid includes at least one selected from the group consisting of terephthalic acid, 2,5-furandicarboxylic acid, and 2,6-naphthalenedicarboxylic acid,
The polyester resin according to claim 2, wherein 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 structural units derived from aromatic dicarboxylic acids that constitute the molecular chain of the polyester resin are taken as 100 mol %. The polyester resin according to claim 1, having an acid value of 3 eq/ton or more. The 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 all structural units constituting the molecular chain of the polyester resin are taken as 100 mol%. The polyester resin according to claim 7, wherein the structural units derived from the tri- or higher functional polycarboxylic acid are 60 mol % or more, when the total of the structural units derived from the tri- or higher functional polycarboxylic acid and the structural units derived from the tri- or higher functional polyhydric alcohol is 100 mol %. The polyester resin according to claim 1, wherein the structural units derived from aliphatic dicarboxylic acids and/or alicyclic dicarboxylic acids account for 20 mol % or less of the structural units derived from polycarboxylic acids that make up the molecular chain of the polyester resin, taken as 100 mol %. The 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 a polyhydric alcohol that make up the molecular chain of the polyester resin are taken as 100 mol %. The polyester resin according to claim 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 alcohol (b) other than the dihydric alcohol (a) account for 50 mol % or less. A polyester resin composition containing the polyester resin according to any one of claims 1 to 11 and a curing agent. A coating composition containing the polyester resin described in any one of claims 1 to 11. A coating film containing the coating composition described in claim 13. A metal can containing the coating film described in claim 14.