Crystalline polyester resin aqueous dispersion, coating composition, coating film, and metal can

Abstract

Provided are: an aqueous crystalline polyester resin dispersion which is free from harmful substances such as bisphenol A and formaldehyde due to the absence of a curing agent, is capable of forming a coating film having excellent processability and retort resistance, and which has excellent solvent solubility, and in which the amount of an organic solvent used in the preparation of an aqueous dispersion is relatively small; and a coating composition, a coating film, and a metal can containing the same. An aqueous crystalline polyester resin dispersion characterized by containing a crystalline polyester resin having a polycarboxylic acid component and a polyol component as copolymerization components, containing 75 mol% or more of terephthalic acid as a polycarboxylic acid component, containing 55 mol% or more of 1, 6-hexanediol as a polyol component, and having an acid value of 180 eq / ton or more, and a crystalline polyester resin having a melting point of 120 to 160 DEG C.

Description

Technical Field

[0001] The present invention relates to an aqueous crystalline polyester resin dispersion. Specifically, it relates to an aqueous crystalline polyester resin dispersion suitable for can coatings, and even more specifically, to an aqueous crystalline polyester resin dispersion suitable for coating cans for beverages or foods (hereinafter collectively referred to as foods and beverages), a coating composition containing the dispersion, a coating film produced from the coating composition, and a metal can having the coating film. Background Art

[0002] Metal cans such as beverage cans and food cans are coated with organic resins to prevent food from corroding the metal (corrosion resistance) and to protect the odor and flavor of the contents (odor). During the process of forming the opening of the bottle or can, the coating film is subjected to high-load processing such as necking and threading. Therefore, the coating film needs to have durability against such subsequent processing (processability). Furthermore, it is also required to have adhesion to the metal material and curability. In addition, depending on the application, it is sometimes subjected to high temperature and high humidity conditions such as retort sterilization. In such cases, it is necessary not only to maintain the adhesion between the coating film and the metal material, but also to prevent the coating film from whitening (retort resistance).

[0003] In the past, epoxy coatings such as epoxy-phenol, epoxy-amino, and epoxy-acrylic coatings, polyester coatings such as polyester-phenolic, polyester-amino, and polyester-isocyanate, and vinyl chloride coatings were widely used as coatings capable of withstanding the aforementioned corrosion resistance, odor, and can molding processes. However, recent studies have reported that bisphenol A, a raw material for epoxy resins, may have estrogenic effects and affect the brains of fetuses and infants. Furthermore, vinyl chloride coatings pose issues with stabilizers and the generation of dioxins during combustion. Formaldehyde, a raw material used in phenolic and amino resins, is known to be carcinogenic and harmful to the human body, and can also adversely affect the odor of the contents. Similarly, isocyanate resins are also known to be carcinogenic and harmful to the human body. Furthermore, there are concerns about environmental pollution and the impact on the working environment caused by the use of organic solvents.

[0004] Due to concerns about these adverse effects on the human body, the market is demanding water-based coatings that avoid these raw materials. However, currently, they lack the performance required for can applications. Furthermore, even water-based coatings require organic solvents during the resin dispersion process. Due to concerns about environmental pollution, their use is required to be kept to a minimum.

[0005] Based on this viewpoint, a water-based coating composition has been proposed as a resin composition for metal containers or metal covers, for example, in which an acrylic-modified polyester resin obtained by graft-polymerizing a polyester resin having ethylenic double bonds at the resin end with a polymerizable unsaturated monomer component and a β-hydroxyalkylamide crosslinking agent are dispersed in an aqueous medium (Patent Document 1).

[0006] On the other hand, Patent Document 2 proposes a method for producing an aqueous dispersion of a crystalline polyester resin for use in producing a toner for developing electrostatic images. Prior art literature Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2020-79393 Patent Document 2: Japanese Patent Application Laid-Open No. 2018-123249 Summary of the Invention Problems to be solved by the invention

[0008] However, when using the high-acid value acrylic-modified polyester resin described in Patent Document 1, sufficient processability is not achieved. Furthermore, unreacted β-hydroxyalkylamide crosslinking agent remains in the coating film, resulting in reduced retort resistance. Furthermore, the crystalline polyester resin described in Patent Document 2 is known to be used in the production of electrostatic image developing toners. However, when used as a resin composition for metal containers or metal covers, it has poor retort resistance due to its low melting point. Furthermore, increasing the melting point of the crystalline polyester resin to improve retort resistance deteriorates its solvent solubility, making it impossible to obtain an aqueous dispersion of the crystalline polyester resin.

[0009] The present invention aims to provide a crystalline polyester resin aqueous dispersion that, because it contains substantially no curing agent, eliminates harmful substances such as bisphenol A and formaldehyde, and, due to its excellent solvent solubility, requires a relatively small amount of organic solvent when producing the aqueous dispersion. Furthermore, it can form a coating film having excellent processability and retort resistance, as well as a coating composition, coating film, and metal can containing the same. In particular, the present invention aims to provide an aqueous dispersion based on a crystalline polyester resin that achieves both excellent retort resistance and solvent solubility, as well as a coating composition, coating film, and metal can containing the same. Means of solving problems

[0010] The present inventors conducted intensive research to achieve the above-mentioned objectives and discovered that by using specific substances as the polycarboxylic acid component and polyol component of a crystalline polyester resin, it is possible to partially disrupt the planarity and symmetry of the molecules while maintaining a certain degree of crystallinity. As a result, the use of a crystalline polyester resin aqueous dispersion allows the resulting coating film to satisfy both retort resistance and processability, and also to satisfy solvent solubility requirements when the resin is dispersed in water. Furthermore, they discovered that the acid value and melting point of the crystalline polyester resin are also important, and by setting these within specific ranges, it is possible to achieve a further high level of balance between retort resistance, processability, and solvent solubility. Based on these findings, the present inventors have completed the present invention. Specifically, the present invention comprises the following. [1] A crystalline polyester resin aqueous dispersion comprising the following crystalline polyester resin (A). Crystalline polyester resin (A): A crystalline polyester resin having a polycarboxylic acid component and a polyol component as copolymer components, containing 75 mol% or more of terephthalic acid as the polycarboxylic acid component and 55 mol% or more of 1,6-hexanediol as the polyol component, an acid value of 180 eq / ton or more, and a melting point of 120-160°C. [2] The crystalline polyester resin aqueous dispersion according to [1], wherein the concentration of the sulfonic acid metal salt of the crystalline polyester resin (A) is less than 50 eq / ton. [3] A coating composition comprising the crystalline polyester resin aqueous dispersion according to [1] or [2], wherein the curing agent content is less than 1 part by mass per 100 parts by mass of the crystalline polyester resin (A) (solid content) in the aqueous dispersion. [4] A coating film obtained from the coating composition described in [3]. [5] A metal can having the coating film described in [4]. Effects of the Invention

[0011] The crystalline polyester resin aqueous dispersion of the present invention has the aforementioned composition. Therefore, it exhibits excellent solvent solubility during aqueous dispersion, can form a coating film with excellent processability and retort resistance even without the presence of a curing agent, and can exclude harmful substances such as bisphenol A and formaldehyde. Therefore, the crystalline polyester resin aqueous dispersion of the present invention is suitable for use in coating compositions, coating films, and metal cans for beverage and food cans. DETAILED DESCRIPTION

[0012] Hereinafter, embodiments of the present invention will be described in detail.

[0013] <Crystalline Polyester Resin (A)> The crystalline polyester resin (A) of the present invention has a chemical structure obtainable by a polycondensate of a polycarboxylic acid and a polyol, and is composed of one or more components selected from polycarboxylic acids and polyols.

[0014] When the total amount of the polycarboxylic acid component used in the present invention is 100 mol%, the copolymerization amount of terephthalic acid needs to be 75 mol% or more, preferably 80 mol% or more, and more preferably 85 mol% or more. By setting the copolymerization amount to 75 mol% or more, the crystallinity and glass transition temperature of the resulting polyester are increased, and the retort resistance of the coating film is improved.

[0015] The polycarboxylic acid component other than terephthalic acid used in the present invention is not particularly limited, and for example, the following polycarboxylic acids, esters thereof, and anhydrides thereof can be used.

[0016] The polycarboxylic acid component other than terephthalic acid used in the present invention may be a dicarboxylic acid (a) having a ring structure, which has carboxylic acid groups at the 2 and 6 positions when the ring is a naphthalene ring, or at the 1 and 4 positions when the ring is a cyclohexyl ring (hereinafter referred to as component (a)). The carboxylic acid group of component (a) is preferably directly bonded to the ring structure. Examples of component (a) include 2,6-naphthalene dicarboxylic acid and 1,4-cyclohexane dicarboxylic acid. One or more of these may be used.

[0017] The polycarboxylic acid component used in the present invention preferably contains component (a). The inclusion of component (a) improves solvent solubility without significantly impairing the crystallinity of the polyester resin. Furthermore, among component (a), dicarboxylic acids having a naphthalene ring structure and carboxylic acid groups at the 2- and 6-positions are more preferred because they have minimal steric hindrance and are particularly effective in maintaining crystallinity.

[0018] When the total amount of the polycarboxylic acid component is 100 mol%, the copolymerization ratio (mol%) of component (a) is preferably 5 to 20 mol%, more preferably 8 to 18%, and even more preferably 10 to 15 mol%. A ratio of 5 mol% or greater improves the solvent solubility of the resulting polyester, thereby providing a stable crystalline polyester resin aqueous dispersion. Furthermore, a ratio of 20 mol% or less achieves a moderate glass transition temperature, achieving a balance between retort resistance and processability of the coating film.

[0019] As the polycarboxylic acid component other than the terephthalic acid used in the present invention, dicarboxylic acid (b) (hereinafter referred to as (b) component) having a ring structure and other than the (a) component can be listed. The ring structure of the (b) component is preferably a benzene ring, a furan ring, a naphthalene ring, or a cyclohexyl ring. The carboxylic acid group of the (b) component is preferably directly bonded to the ring structure. As the (b) component, for example, isophthalic acid, 1,2-naphthalene dicarboxylic acid, 1,3-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, 1,6-naphthalene dicarboxylic acid, 1,7-naphthalene dicarboxylic acid, 1,8-naphthalene dicarboxylic acid, 2,3-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 5-sodium sulfoisophthalate, 2,5-furan dicarboxylic acid, 1,2-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid, hexahydrophthalic acid, etc. can be listed. In addition, these can use one or more kinds.

[0020] Component (b) is an optional component. When the total amount of the polycarboxylic acid component is 100 mol%, the copolymerization ratio of component (b) is preferably 20 mol% or less. It is more preferably 15 mol% or less, further preferably 10 mol% or less, and even 0 mol% is acceptable. By setting the ratio to 20 mol% or less, the retort resistance of the resulting polyester is improved.

[0021] When component (b) is used, the total copolymerization ratio (mol %) of components (a) and (b) is preferably 5 to 20 mol %, more preferably 8 to 18 mol %, and even more preferably 10 to 15 mol %, based on the total amount of the polycarboxylic acid component being 100 mol %. By setting the total copolymerization ratio to 5 mol % or greater, the solvent solubility of the resulting polyester is improved. Furthermore, by setting the total copolymerization ratio to 20 mol % or less, the retort resistance of the resulting polyester is improved.

[0022] The polycarboxylic acid component other than terephthalic acid used in the present invention may also include aliphatic polycarboxylic acids, alicyclic polycarboxylic acids, aromatic polycarboxylic acids, and the like other than component (a) or component (b). Examples of aliphatic polycarboxylic acids include fumaric acid, adipic acid, sebacic acid, malonic acid, and succinic acid. Examples of alicyclic polycarboxylic acids and aromatic polycarboxylic acids include trifunctional or higher functional ones. These may be used alone or in combination.

[0023] The polyol component used in the present invention contains 1,6-hexanediol. The linear structure and alkyl chain length of 1,6-hexanediol contribute to high crystallinity that satisfies solvent solubility, retort resistance, and processability.

[0024] When the total amount of the polyol component is 100 mol%, the copolymerization ratio of 1,6-hexanediol needs to be 55 mol% or higher, more preferably 60 mol% or higher, even more preferably 65 mol% or higher, and particularly preferably 70 mol% or higher. A ratio of 55 mol% or higher improves the crystallinity of the resulting polyester, improving not only retort resistance but also processability and solvent solubility. Furthermore, a ratio of 90 mol% or lower is preferably 90 mol% or lower, and even more preferably 80 mol% or lower. A ratio of 90 mol% or lower improves the solvent solubility of the resulting polyester, enabling the production of a stable crystalline polyester resin aqueous dispersion.

[0025] The polyol components other than 1,6-hexanediol used in the present invention are not particularly limited, and specifically include polyols (c) having side chains (hereinafter referred to as component (c)). Component (c) is preferably a diol having a side chain. The side chain in component (c) refers to an atom or atomic group branching from a hydrocarbon group (carbon chain) connecting two hydroxyl groups as a main chain. The side chain in component (c) is preferably an alkyl group. The number of carbon atoms of the above-mentioned alkyl group is preferably 1 to 50, more preferably 2 to 40, and further preferably 3 to 35. The number of side chains may be 1 or more. It is preferably 5 or less, more preferably 4 or less, and further preferably 3 or less. The above-mentioned alkyl group may be a straight chain or may have a side chain. Examples of the component (c) include 1,2-propylene glycol, 1,2-butylene glycol, 1,3-butylene glycol, 2-methyl-1,3-propanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 2-methyl-1,3-hexanediol, 2-methyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-ethyl-2-n-propyl-1,3-propanediol, 2,2-di-n-propyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, mannitol, sorbitol, dimer glycol, polypropylene glycol, and pentaerythritol. The component (c) may be used alone or in combination.

[0026] Component (c) is an optional component. When the total amount of the polyol component is 100 mol%, the copolymerization ratio of component (c) is preferably 20 mol% or less. It is more preferably 15 mol% or less, further preferably 10 mol% or less, and even 0 mol% is acceptable. By setting the ratio to 20 mol% or less, both solvent solubility and retort resistance of the resulting polyester can be achieved.

[0027] Examples of polyol components other than 1,6-hexanediol used in the present invention include linear polyols (d) (hereinafter referred to as component (d)). Examples of component (d) include ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,5-pentanediol, and 1,8-octanediol, and one or more of these can be used.

[0028] Component (d) is an optional component. When the total amount of the polyol component is 100 mol%, the copolymerization ratio of component (d) is preferably 40 mol% or less. It is more preferably 30 mol% or less, further preferably 20 mol% or less, and even 0 mol% is acceptable. By setting the ratio to 20 mol% or less, both solvent solubility and retort resistance of the resulting polyester can be achieved.

[0029] Examples of the polyol component other than 1,6-hexanediol used in the present invention include polyols (e) having an aromatic ring skeleton or an alicyclic skeleton (hereinafter referred to as component (e)). Component (e) includes 1,4-cyclohexanedimethanol, tricyclodecanedimethanol, hydroquinone, catechol, and resorcinol, and one or more of these can be used.

[0030] Component (e) is an optional component. When the total amount of the polyol component is 100 mol%, the copolymerization ratio of component (e) is preferably 30 mol% or less, more preferably 25 mol% or less, even more preferably 20 mol% or less, and even 0 mol% is acceptable. By setting the ratio to 30 mol% or less, both solvent solubility and retort resistance of the resulting polyester can be achieved.

[0031] When isosorbide is used as a polyol component other than 1,6-hexanediol in the present invention, its copolymerization ratio is preferably less than 5 mol%, more preferably less than 1 mol%, and most preferably no isosorbide is contained. By setting the ratio to less than 5 mol%, the solvent solubility and retort resistance of the resulting polyester can be improved.

[0032] The polycarboxylic acid component and the polyol component constituting the crystalline polyester resin (A) of the present invention can be made of raw materials derived from biomass resources. Biomass resources refer to resources that are stored by converting the sun's light energy into the form of starch, cellulose, etc. through the photosynthesis of plants, including animals that grow and develop by eating plants or products made from plants and animals. Among them, more preferred biomass resources are plant resources, for example, wood, straw, rice husks, rice bran, old rice, corn, sugarcane, cassava, sago, bean dregs, corn cobs, cassava residues, sugarcane bagasse, vegetable oil residues, taro, buckwheat, soybeans, oils and fats, waste paper, papermaking residues, aquatic product residues, livestock excrement, sewage sludge, food waste, etc. More preferably, corn, sugarcane, cassava, and sago are used.

[0033] 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, etc. These may be used alone or as a mixture of two or more.

[0034] Specific examples of polyol raw materials derived from biomass resources include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, neopentyl glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol, etc. These may be used alone or as a mixture of two or more.

[0035] The crystalline polyester resin (A) in the present invention preferably has a branched structure. Having a branched structure means that the main chain of the polyester has a branched structure. To introduce a branched structure into the polyester, an example of a method in which a trifunctional or higher component is copolymerized as a part of a polycarboxylic acid component and / or a polyol component in the polycondensation reaction of the polyester can be cited. As a trifunctional or higher polycarboxylic acid component, polycarboxylic acids or their esters as shown below, and polycarboxylic anhydrides can be used. Specifically, for example, trimellitic acid, pyromellitic acid, benzophenonetetracarboxylic acid, etc. can be cited. As a trifunctional or higher polyol component, glycerol, trimethylolethane, trimethylolpropane, mannitol, sorbitol, pentaerythritol, etc. can be cited, and one or more of these can be used. By making the crystalline polyester resin (A) have a branched structure, the processability of the resulting coating film becomes good.

[0036] When the total crystalline polyester resin (A) is taken as 100 mol%, the content of the trifunctional or higher polycarboxylic acid component and / or the trifunctional or higher polyol component is preferably 0.3 mol% or more, more preferably 0.4 mol% or more, and even more preferably 0.5 mol% or more. Furthermore, it is preferably 2 mol% or less, more preferably 1.5 mol% or less, even more preferably 1 mol% or less, and particularly preferably 0.8 mol% or less. If the content of the polycarboxylic acid component and / or the polyol component exceeds the above-mentioned content, the crystallinity of the crystalline polyester resin (A) may decrease, the retort resistance may deteriorate, or gelation may occur during polyester polymerization.

[0037] The crystalline polyester resin (A) of the present invention can be given an acid value by any method. Examples of methods for imparting an acid value include a method of subjecting a compound having a polycarboxylic acid anhydride group in the molecule to an addition reaction in the late stage of polycondensation, and a method of obtaining a polyester resin having an acid value by setting the prepolymer (oligomer) to a high acid value and then polycondensing the prepolymer. However, the former method of subjecting the prepolymer to an addition reaction is preferred due to ease of operation and the ease of obtaining the target acid value.

[0038] Among the compounds having a polycarboxylic acid anhydride group in the molecule used to impart an acid value to the crystalline polyester resin (A) in the present invention, examples of carboxylic acid monoanhydrides include phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, and citraconic anhydride. One or more selected from these can be used. Among these, trimellitic anhydride is preferred due to its versatility and cost-effectiveness.

[0039] Among the compounds having a polycarboxylic acid anhydride group in the molecule used to impart an acid value to the crystalline polyester resin (A) in the present invention, 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 trimellitic anhydride, and 2,2',3,3'-biphenyltetracarboxylic dianhydride. One or more selected from these can be used. Among these, ethylene glycol trimellitic anhydride is preferred due to its versatility and economic efficiency.

[0040] As the compound having a polycarboxylic acid anhydride group in the molecule for imparting an acid value, carboxylic acid monoanhydride and carboxylic acid polyanhydride may be used alone or in combination.

[0041] The acid value of the crystalline polyester resin (A) in the present invention needs to be 180 eq / ton or higher, preferably 200 eq / ton or higher, more preferably 220 eq / ton or higher, and even more preferably 250 eq / ton or higher. By setting the acid value above this lower limit, sufficient water dispersion stability can be ensured. The upper limit of the acid value is not particularly limited, but is preferably 400 eq / ton or lower to reduce the amount of unreacted acid components and oligomers during the acid addition reaction.

[0042] The concentration of the metal sulfonate salt in the crystalline polyester resin (A) of the present invention is preferably less than 50 eq / ton. It is more preferably 20 eq / ton or less, further preferably 10 eq / ton or less, and particularly preferably 5 eq / ton. By setting the concentration to less than 50 eq / ton, the hydrophilicity of the resulting polyester is reduced, and retort resistance is improved.

[0043] Next, the method for producing the crystalline polyester resin (A) of the present invention will be described. In the esterification / exchange reaction, all monomer components and / or oligomers thereof are heated and melted to react. 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. In addition, the reaction time is the time from the time the desired reaction temperature is reached until the subsequent polycondensation reaction. In the polycondensation reaction, the polyol component is distilled off from the esterified product obtained by the esterification reaction under reduced pressure at a temperature of 220 to 280°C, and the polycondensation reaction is carried out until the desired molecular weight is reached. The reaction temperature of the polycondensation reaction is preferably 220 to 280°C, more preferably 240 to 275°C. The degree of reduced pressure is preferably 130 Pa or less. If the degree of reduced pressure is insufficient, the polycondensation time tends to be prolonged, which is not preferred. As the time for reducing pressure from atmospheric pressure to 130 Pa or less, it is preferred to reduce pressure slowly over 30 to 180 minutes.

[0044] During the esterification / exchange reaction and polycondensation reaction, polymerization is carried out using, as needed, organotitanate compounds such as tetrabutyl titanate, germanium dioxide, antimony oxide, and organotin compounds such as tin octoate. For reactivity, organotitanate compounds are preferred, while for resin coloration, germanium dioxide is preferred.

[0045] The glass transition temperature of the crystalline polyester resin (A) in the present invention is preferably 10°C or higher, more preferably 15°C or higher, from the viewpoint of water resistance, particularly retort resistance of the coating film. Furthermore, from the viewpoint of processability, it is preferably 40°C or lower, more preferably 35°C or lower.

[0046] The melting point (Tm) of the crystalline polyester resin (A) in the present invention refers to the temperature at the top of the endothermic peak with the maximum heat of fusion, measured by heating the polyester resin from -50°C to 200°C at 20°C / minute using a differential scanning calorimeter (DSC) after aging the polyester resin at 100°C for 30 hours. The melting point (Tm) of the crystalline polyester resin (A) in the present invention is in the range of 120-160°C, preferably 125-155°C, more preferably 130-150°C, and even more preferably 135-145°C. A melting point of 120°C or higher improves crystallinity and exhibits excellent retort resistance. Furthermore, a melting point of 160°C or lower exhibits excellent processability and solvent solubility.

[0047] The crystallinity in the present invention refers to the property of the polyester resin showing a melting point (Tm) when measured under the above conditions. A high crystallinity means a high melting point of the polyester resin.

[0048] The reduced viscosity of the crystalline polyester resin (A) of the present invention is preferably 0.2 to 0.8 dl / g, more preferably 0.4 to 0.8 dl / g, and even more preferably 0.6 to 0.8 dl / g. A reduced viscosity of 0.2 dl / g or less may result in reduced toughness and workability of the coating film. On the other hand, a reduced viscosity of 0.8 dl / g or greater may result in reduced solvent solubility.

[0049] <Crystalline Polyester Resin Aqueous Dispersion> The crystalline polyester resin aqueous dispersion of the present invention can be produced by the following methods: method (a) of dissolving the crystalline polyester resin (A) in a water-soluble organic solvent capable of dissolving the crystalline polyester resin (A), and then sequentially adding an alkaline compound and water as needed to disperse the resulting mixture; or method (b) of adding the crystalline polyester resin (A), water, a water-soluble organic solvent capable of dissolving the crystalline polyester resin (A), and, if necessary, an alkaline compound, and then heating and dispersing the resulting mixture. Furthermore, to reduce the amount of organic solvent or completely remove the organic solvent to form an aqueous dispersion, dispersion can be performed using an organic solvent with a boiling point of 100°C or lower, followed by extraction of the solvent by heating or reduced pressure. In the case of crystalline polyester resins, the former method (a) is preferred for film-forming properties.

[0050] In this case, the temperature at which the crystalline polyester resin (A) is dissolved is preferably 40 to 160° C., more preferably 50 to 140° C., further preferably 60 to 130° C., and most preferably 70 to 100° C. This is because if the temperature is lower than 40° C., the dissolution of the crystalline polyester resin (A) may become insufficient, thereby failing to fully disentangle the molecular chains. Furthermore, if the temperature is higher than 160° C., there is a high possibility that the crystalline polyester resin (A) may deteriorate. Examples of organic solvents capable of dissolving the crystalline polyester resin (A) upon heating within a temperature range of 40 to 160° C. include methyl ethyl ketone, cyclohexanone, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, 1,4-dioxane, 1,3-dioxane, 1,3-dioxolane, 1,2-hexanediol, methyl cellosolve, butyl cellosolve, ethyl carbitol, butyl carbitol, propylene glycol monomethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and triethylene glycol monobutyl ether. Preferred among these are methyl ethyl ketone, butyl cellosolve, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and 2-propanol.

[0051] When the crystalline polyester resin (A) is dissolved at 100° C. or higher, it is necessary to cool the crystalline polyester resin solution to 100° C. or lower, and then gradually add water and, if necessary, a basic compound while stirring the resin solution to perform phase transfer to obtain an aqueous dispersion.

[0052] As the basic compound used when dispersing the crystalline polyester resin (A) of the present invention in water, compounds that volatilize during the drying and calcining steps during film formation are preferably used, such as ammonia and / or organic amine compounds with a boiling point of 250°C or lower. Preferred examples include triethylamine, N,N-diethylethanolamine, N,N-dimethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, isopropylamine, diaminodipropylamine, ethylamine, diethylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, sec-butylamine, propylamine, methylaminopropylamine, dimethylaminopropylamine, methyldiaminodipropylamine, 3-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. These basic compounds are preferably added in an amount sufficient to neutralize at least a portion of the carboxyl groups in the crystalline polyester resin (A). Specifically, the amount is preferably 0.5 to 1.5 equivalents per equivalent of the carboxyl groups.

[0053] The average particle size of the crystalline polyester resin aqueous dispersion according to the present invention is very important because it significantly affects the appearance and storage stability of the coating film. It is preferably 30 to 300 nm. It is more preferably 50 to 250 nm, and particularly preferably 70 to 200 nm. When the average particle size exceeds 300 nm, not only is the dispersion stability significantly reduced, but the film-forming properties are also impaired, resulting in a deteriorated appearance of the resulting film. Conversely, when the average particle size is less than 30 nm, the film-forming properties tend to be significantly improved. However, this also increases the likelihood of fusion and aggregation between the dispersed particles, resulting in an increased likelihood of thickening and poor dispersion, making it undesirable.

[0054] The crystalline polyester resin aqueous dispersion of the present invention is preferably prepared at a resin solids concentration of 10 to 45% by mass. It is more preferably in the range of 15 to 40% by mass, and even more preferably in the range of 20 to 35% by mass. A resin solids concentration exceeding 45% by mass increases the viscosity of the aqueous dispersion and significantly reduces dispersion stability due to increased coagulation between resin particles. Furthermore, a concentration below 10% by mass is difficult to be considered practical for both manufacturing and application reasons.

[0055] The coating composition of the present invention contains at least the aforementioned crystalline polyester resin aqueous dispersion. Furthermore, it contains the crystalline polyester (A) in the aqueous dispersion as a main agent. In the coating composition, the component with the highest content (by mass ratio) among the solid components (non-volatile components excluding volatile substances such as water and organic solvents) forming the coating film in the coating composition is defined as the main agent.

[0056] The coating composition of the present invention can form a coating film solely from the crystalline polyester resin aqueous dispersion, even without adding a curing agent. Therefore, the coating composition of the present invention preferably contains substantially no curing agent, that is, the curing agent content is preferably less than 1 part by mass (based on solid content) per 100 parts by mass (based on solid content) of the crystalline polyester resin (A) in the aqueous dispersion.

[0057] In the coating composition of the present invention, the content of the curing agent is preferably less than 1 part by mass per 100 parts by mass of the crystalline polyester resin (A) (solid content). It is more preferably less than 0.5 parts by mass, further preferably less than 0.1 parts by mass, and most preferably contains no curing agent. If the content of the curing agent exceeds the above range, not only will economic efficiency decline, but it may also cause self-condensation reactions between the curing agents, resulting in reduced processability, volatilization of the anti-blocking agent, generation of harmful gases such as formaldehyde, and poor long-term storage stability.

[0058] The curing agent herein refers to a known curing agent that reacts with the polyester resin to form a crosslinked structure. Examples of the crosslinked structure include reactions that react unsaturated double bonds in the polyester resin through free radical addition, cationic addition, or anionic addition reactions to form intermolecular carbon-carbon bonds; and reactions that form intermolecular bonds through condensation, addition polymerization, or transesterification with polycarboxylic acid groups or polyol groups in the polyester resin. Examples of curing agents include phenolic resins, amino resins, isocyanate compounds, epoxy compounds, β-hydroxyamide compounds, and resins containing unsaturated bonds.

[0059] The coating composition of the present invention is most suitable for coating food and beverage cans. When preparing coatings for food and beverage cans, various additives may be added depending on the intended purpose. Within the scope of the present invention, such as those that do not impair food hygiene and odor resistance, a plasticizer to improve solubility in organic solvents, a leveling agent or surfactant to improve coating properties and the smoothness and appearance of the coating film, a lubricant to prevent damage to the coating film, a coloring pigment, and, if appropriate, a polyester resin other than the crystalline polyester resin of the present invention, or a resin other than a polyester resin, such as an acrylic resin emulsion or a polyurethane resin emulsion, may be added.

[0060] The coating composition of the present invention may be blended with other resins for the purpose of improving the coating film, such as imparting flexibility and adhesion. Examples of other resins include amorphous polyesters, crystalline polyesters, ethylene-polymerizable unsaturated carboxylic acid copolymers, and ethylene-polymerizable carboxylic acid copolymer ionomers. By blending at least one resin selected from these, flexibility and / or adhesion may be imparted to the coating film.

[0061] The coating composition of the present invention can be applied to metal substrates for cans, such as aluminum, stainless steel, and tinplate, using a gravure roll coater, comma coater, or spraying. The film thickness is not particularly limited, but is typically 3 to 18 μm, more preferably 5 to 15 μm, as a dry film thickness. The coating film is typically sintered at a temperature of approximately 180 to 260°C for approximately 20 seconds to 1 hour, more preferably at a temperature of approximately 200 to 240°C for approximately 30 seconds to 10 minutes.

[0062] The coating film formed from the crystalline polyester resin aqueous dispersion of the present invention is preferably sintered within the above range and then subjected to an aging treatment. The aging treatment further advances crystallization in the coating film, improving retort resistance.

[0063] The coating film of the present invention refers to a film obtained by coating a substrate with a crystalline polyester resin aqueous dispersion (two layers: substrate / crystalline polyester resin aqueous dispersion). Alternatively, a structure may be employed in which a layer composed of another resin overlaps with either the upper or lower layer of the crystalline polyester resin aqueous dispersion layer. In this case, the layer composed of another resin refers to a coating layer. The coating film of the present invention can be obtained by laminating the crystalline polyester resin aqueous dispersion of the present invention on various substrates using conventional methods, and further laminating another resin layer.

[0064] The metal can of the present invention has the above-mentioned coating. The metal can is obtained, for example, by coating one or both sides of a metal plate formed from a metal material such as cans for beverages, canned goods, their lids, bottle caps, and the like, and, if necessary, the end faces. Examples of the above-mentioned metal material include tinplate, tin-free steel, and aluminum. Metal plates made from these metal materials that have been previously surface-treated with phosphoric acid treatment, chromate treatment, chromate-phosphate treatment, or other rust-preventive treatments for corrosion protection or to improve the adhesion of the coating can also be used. Example

[0065] The present invention is further described in detail below by way of examples, but the present invention is not limited to these examples. Various properties were evaluated according to the following methods. Individual "parts" represent parts by mass, and % represents mass %.

[0066] <Crystalline Polyester Resin (A)> (1) Determination of resin composition A sample of the crystalline polyester resin was dissolved in deuterated chloroform and analyzed by 1H-NMR using a nuclear magnetic resonance (NMR) apparatus 400-MR manufactured by VARIAN. The molar ratio was determined from the ratio of the integrated values.

[0067] (2) Determination of reduced viscosity (unit: dl / g) A 0.1 g sample of a crystalline polyester resin was dissolved in 25 cc of a mixed solvent of phenol / tetrachloroethane (mass ratio 6 / 4), and the viscosity was measured at 30°C using an Ubbelohde viscometer.

[0068] (3) Determination of melting point (Tm) and glass transition temperature (Tg) The measurement was performed using a differential scanning calorimeter (DSC) DSC-220 manufactured by Seiko Instruments. The aging treatment of the crystalline polyester resin was carried out under 100°C × 30 hours as the aging treatment condition. A 5 mg sample of the treated crystalline polyester resin was placed in a sealed container made of aluminum with a pressure cap. After nitrogen gas was introduced at 30 ml / min to form a nitrogen atmosphere, the container was cooled to -50°C using liquid nitrogen and then heated to 200°C at 20°C / min. The temperature of the top of the maximum peak of the heat of fusion obtained in the process was determined as the melting point (Tm, unit: °C). In addition, using the above-mentioned measuring device, the temperature was raised to 200°C under the same conditions, then rapidly cooled to -50°C, and then heated to 200°C again at 20°C / min. In the endothermic curve obtained in the process, the temperature of the intersection of the baseline before the endothermic peak appears and the tangent line toward the endothermic peak is set as the glass transition temperature (Tg, unit: °C).

[0069] (4) Determination of acid value A 0.2 g sample of crystalline polyester resin was dissolved in 40 ml of chloroform and titrated with 0.01 N potassium hydroxide ethanol solution. The yield per 10 6 Equivalent weight (eq / ton) corresponding to g polyester resin. Phenolphthalein was used as the indicator.

[0070] (5) Evaluation of solvent solubility 2 g of the crystalline polyester resin was dissolved in 8 g of cyclohexanone, and the dissolved state of the crystalline polyester resin when heated at 100° C. for 3 hours (and allowed to stand) was visually determined as follows. (determination) ◎: Almost completely dissolved (more than 80% dissolved) ○: There is a small amount of dissolved residue (more than 70% dissolved) △: There is a small amount of dissolved residue (more than 60% dissolved) ×: Dissolved residue (less than 50%)

[0071] (6) Average particle size The average particle size of the polyester resin aqueous dispersion was measured using a laser diffraction / scattering particle size distribution analyzer (Coulter Counter LS13320, manufactured by Beckman Coulter). The particle size distribution was then generated using this device on a volume basis, and the average diameter was defined as the average particle size.

[0072] (7) Evaluation of storage stability The dispersion state of the polyester resin aqueous dispersion after being allowed to stand at 25° C. for 24 hours was visually evaluated as follows. (determination) ◎: No appearance change ○: There is sediment ×: Curing occurs

[0073] <Preparation of test pieces> A crystalline polyester resin aqueous dispersion was applied to one side of a tinplate sheet (JIS G 3303 (2008) SPTE, 70 mm × 150 mm × 0.3 mm) using a bar coater to a film thickness of 10 ± 2 μm after drying. The sheet was sintered at 200°C for 30 seconds and then aged at 80°C for 1 hour. This was used as a test piece (hereinafter referred to as a test piece).

[0074] (8) Evaluation of coating appearance The coating film appearance of the obtained test piece was visually evaluated as follows. (determination) ◎: No cracks or aggregates were generated. ○: Cracks or aggregates were generated. △: The coating film partially peeled off. ×: The entire coating film peeled off.

[0075] (9) Evaluation of processability The obtained test piece is subjected to a 180° bending process with the coating facing outward, and the rupture of the coating produced in the bent portion is evaluated by measuring the energization value. In addition, during the bending process, the bending is performed without sandwiching any objects in the middle (so-called 0T). A sponge (20 mm wide, 50 mm deep, 10 mm thick) immersed in a 1% NaCl aqueous solution is placed on an aluminum plate electrode (20 mm wide, 50 mm deep, 0.5 mm thick), so that the center of the bent portion of the test piece contacts the sponge in a manner parallel to the 20 mm edge of the sponge. A DC voltage of 5.0 V is applied between the aluminum plate electrode and the non-coated portion of the inner surface of the test plate, and the energization value is measured. The smaller the energization value, the better the bending characteristics. (determination) ◎: less than 0.5mA ○: 0.5mA or more, less than 1.0mA △: 1.0mA or more, less than 2.0mA ×: 2.0mA or more

[0076] (10) Evaluation of boiling resistance The test piece was placed upright in a stainless steel cup. Ion-exchanged water was poured into the cup to half the height of the test piece. The cup was then placed in an autoclave using a retort tester (ES-315 manufactured by TOMY KOGYO CO., LTD.) and retorted at 125°C for 30 minutes. Post-treatment evaluation was performed on the steam-exposed portion, which is generally considered to be exposed to more severe conditions for the coating film. The cured film was visually evaluated for whitening and blistering as described below. (determination) ◎: Good (no whitening or blistering) ○: Slight whitening, but no blistering △: There is some whitening and / or some blistering ×: Significant whitening and / or significant blistering

[0077] Synthesis of crystalline polyester resin (a) 650 parts by mass of terephthalic acid, 870 parts by mass of 1,6-hexanediol, 50 parts by mass of 1,4-cyclohexanedimethanol, and 0.4 parts by mass of tetrabutyl titanate (hereinafter, sometimes referred to as TBT) as a catalyst (0.03 mol% relative to the total acid components) were added to a 3L four-necked flask, and the temperature was slowly raised to 240°C over 3 hours to carry out an esterification reaction. After the esterification reaction, the system was slowly decompressed, and the reduced pressure polymerization was carried out to 10 mmHg over 1 hour, while the temperature was raised to 250°C, and further post-polymerization was carried out in a vacuum of less than 1 mmHg for 90 minutes. After the polycondensation reaction was completed, it was cooled to 210°C in a nitrogen atmosphere, and then a specified amount of trimellitic anhydride was added, and the synthesis was carried out by continuing stirring at 220°C in a nitrogen atmosphere for 30 minutes. After the reaction was completed, it was taken out to obtain a crystalline polyester resin (a). The obtained crystalline polyester resin (a) had a reduced viscosity of 0.60 dl / g, a glass transition temperature (Tg) of 15° C., a crystalline melting point (Tm) of 155° C., and an acid value of 220 eq / ton.

[0078] Synthesis of crystalline polyester resins (b) to (x) Similar to the synthesis of crystalline polyester resin (a), the raw material composition was changed according to the resin composition shown in Table 1, and crystalline polyester resins (b) to (f), (n) to (u), and (x) were produced by direct esterification, and crystalline polyester resins (g) to (m), (v) to (w) were produced by transesterification.

[0079] The composition and properties of each crystalline polyester resin are shown in Table 1. [Table 1]

[0080] Examples 1 to 14, Comparative Examples 1 to 10 The solvent solubility of the obtained crystalline polyester resins (a) to (n) and (o) to (x) was evaluated. The evaluation results are shown in Tables 2 and 3.

[0081] [Table 2]

[0082] [Table 3]

[0083] Preparation of crystalline polyester resin aqueous dispersion (B-1) Crystalline polyester resin (a) was dispersed in water using the following procedure. Add 20 parts by mass of crystalline polyester resin (a) and 10 parts by mass of cyclohexanone to a reaction vessel equipped with a stirrer, condenser, and thermometer, and dissolve the resin at 130°C for 2 hours. After dissolution, cool the mixture to 100°C, add 5 parts by mass of 2-propanol and 0.5 parts by mass of dimethylaminoethanol, and stir for 30 minutes. Then, add 65 parts of warm water and stir for 1 hour. Stir at 200 rpm. Then, cool the mixture to room temperature to obtain a crystalline polyester resin aqueous dispersion (B-1).

[0084] Production of crystalline polyester resin aqueous dispersions (B-2) to (B-14), (B-15) to (B-24) In the same manner as in the preparation of the crystalline polyester resin aqueous dispersion (B-1), crystalline polyester resins (b) to (n) and (o) to (x) were used, respectively, to obtain crystalline polyester resin aqueous dispersions (B-2) to (B-14) and (B-15) to (B-24).

[0085] Examples 15 to 28, Comparative Examples 11 to 20 Various property evaluations were performed using the obtained crystalline polyester resin aqueous dispersions (B-1) to (B-14) and (B-15) to (B-24). The evaluation results are shown in Tables 4 and 5.

[0086] [Table 4]

[0087] [Table 5]

[0088] The property evaluations in the above table demonstrate that the crystalline polyester resins of Examples 1 to 14 exhibit excellent or good solvent solubility. Furthermore, the crystalline polyester resin aqueous dispersions of Examples 15 to 28, each using the crystalline polyester resins of Examples 1 to 14, exhibit excellent or good storage stability, coating film appearance, processability, and retort resistance.

[0089] On the other hand, the crystalline polyester resin in Comparative Example 1 had a high melting point, resulting in poor solvent solubility, making it impossible to form an aqueous dispersion using it in Comparative Example 11. In Comparative Example 2, the use of 1,7-heptanediol instead of 1,6-hexanediol lowered the melting point of the crystalline polyester resin. Although an aqueous dispersion was produced using it in Comparative Example 12, the retort resistance of the resulting coating was significantly poor. In Comparative Example 3, the use of 1,4-butanediol instead of 1,6-hexanediol increased the melting point of the crystalline polyester resin, resulting in poor solvent solubility, making it impossible to form an aqueous dispersion using it in Comparative Example 13. In Comparative Example 4, the use of isophthalic acid instead of terephthalic acid may have caused unnecessary bending in the main chain of the crystalline polyester resin, resulting in significantly poor processability of the coating film produced using this aqueous dispersion in Comparative Example 14. In Comparative Example 5, the low melting point of the crystalline polyester resin resulted in significantly poor retort resistance of the coating film produced using this aqueous dispersion in Comparative Example 15. In Comparative Examples 6 and 7, the copolymerization ratio of 1,6-hexanediol in the polyol component was low, resulting in poor solvent solubility. Therefore, aqueous dispersions could not be formed using these in Comparative Examples 16 and 17. In Comparative Example 8, due to the low acid value of the crystalline polyester resin, aqueous dispersions could not be formed using this in Comparative Example 18. In Comparative Example 9, due to the low terephthalic acid content and high glass transition temperature of the crystalline polyester resin, the processability of the coating film formed using this aqueous dispersion in Comparative Example 19 was significantly poor. In Comparative Example 10, due to the low terephthalic acid content and low glass transition temperature of the crystalline polyester resin, the retort resistance of the coating film formed using this aqueous dispersion in Comparative Example 20 was significantly poor. Industrial applicability

[0090] The crystalline polyester resin of the present invention has excellent solvent solubility. When formed into a coating film using its aqueous dispersion, the coating film has excellent appearance, processability, and retort resistance, and is suitable as a main agent for coatings applied to metal cans for food and beverages.

Claims

1. A crystalline polyester resin aqueous dispersion, characterized in that Containing the following crystalline polyester resin (A), Crystalline polyester resin (A): A crystalline polyester resin having a polycarboxylic acid component and a polyol component as copolymer components, containing 75 mol% or more of terephthalic acid as the polycarboxylic acid component and 55 mol% or more of 1,6-hexanediol as the polyol component, an acid value of 180 eq / ton or more, and a melting point of 120-160°C.

2. The crystalline polyester resin aqueous dispersion according to claim 1, wherein The concentration of the sulfonic acid metal salt of the crystalline polyester resin (A) is less than 50 eq / ton.

3. A coating composition, characterized in that The crystalline polyester resin aqueous dispersion according to claim 1 or 2, wherein the curing agent content is less than 1 part by mass based on 100 parts by mass of the solid content of the crystalline polyester resin (A) in the aqueous dispersion. A coating film obtained from the coating composition according to claim 3. A metal can comprising the coating film according to claim 4 .

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