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

Abstract

Provided is a crystalline polyester resin aqueous dispersion which: does not contain a curing agent, and thus eliminates harmful substances such as bisphenol A and formaldehyde; is excellent in solvent solubility, and thus requires a relatively small amount of an organic solvent for use in water dispersion; and also is capable of forming a coating film excellent in processability and properties of retort resistance. Also provided are: a coating composition; a coating film; and a metal can that include the crystalline polyester resin aqueous dispersion.　The crystalline polyester resin aqueous dispersion is characterized by comprising a crystalline polyester resin which contains: a polycarboxylic acid component and a polyhydric alcohol component as copolymerization components; as the polyvalent carboxylic acid component, a terephthalic acid at 75 mol % or more; and as the polyhydric alcohol component, 1,6-hexanediol at 55 mol % or more. The crystalline polyester resin has an acid value of 180 eq / ton or more, and has a melting point of 120-160°C.

Description

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

[0001] The present invention relates to a crystalline polyester resin aqueous dispersion, specifically a crystalline polyester resin aqueous dispersion suitable for use as a can coating, and even more specifically to a crystalline polyester resin aqueous dispersion suitable for coating cans containing beverages or foods (hereinafter collectively referred to as "food and beverages"), a coating composition containing the same, a coating film made from the coating composition, and a metal can having the coating film.

[0002] Metal cans, such as beverage cans and food cans, are coated with organic resins to prevent corrosion of the metal by food (corrosion resistance) and to preserve the flavor and taste of the contents (flavoring). This coating film undergoes demanding processes, such as necking and threading, during the molding process for the neck of a bottle can. Therefore, the coating film must be durable enough to withstand such post-processing (processability). Furthermore, it must also have adhesion to the metal substrate and hardenability. Depending on the application, the can may be subjected to high-temperature, high-humidity conditions, such as retort sterilization. Even in such cases, the coating film must maintain adhesion to the metal substrate and not whiten (retort resistance).

[0003] Conventionally, epoxy-based paints such as epoxy-phenolic paints, epoxy-amino paints, and epoxy-acrylic paints, polyester-based paints such as polyester-phenolic paints, polyester-amino paints, and polyester-isocyanate paints, and vinyl chloride-based paints have been widely used as paints that offer the aforementioned corrosion resistance, flavor, and durability for can molding. However, recent research has shown that bisphenol A, a raw material for epoxy resins, may have estrogenic effects and may affect the brains of fetuses and infants. Furthermore, vinyl chloride-based paints have issues with stabilizers and the generation of dioxins during incineration. Formaldehyde, which is used as a raw material for phenolic resins and amino resins, and remains in paints, is known to be carcinogenic and harmful to the human body and to adversely affect the flavor of the contents. Similarly, isocyanate resins are known to be carcinogenic and harmful to the human body. Furthermore, there are concerns about the environmental pollution and impact on the working environment caused by the use of organic solvents.

[0004] Due to concerns about these adverse effects on the human body, there is a market demand for water-based paints that do not use these raw materials, but the reality is that they do not provide satisfactory performance for can applications. Furthermore, even water-based paints require organic solvents in the process of dispersing the resin in water, but concerns about environmental pollution, etc., mean that paints that can be used in small amounts are desired.

[0005] From this perspective, as a resin composition for metal containers or metal lids, for example, an aqueous coating composition has been proposed in which an acrylic-modified polyester resin obtained by graft polymerizing a polymerizable unsaturated monomer component onto a polyester resin having an ethylenic double bond at the resin end, 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 used in the production of a toner for developing electrostatic images.

[0007] JP 2020-79393 A JP 2018-123249 A

[0008] However, when the high acid value acrylic-modified polyester resin described in Patent Document 1 was used, sufficient processability could not be obtained. In addition, unreacted β-hydroxyalkylamide crosslinking agent remained in the coating film, resulting in a problem of reduced retort resistance. Furthermore, the crystalline polyester resin described in Patent Document 2 is intended for the production of toner for developing electrostatic images, and it was found that when used as a resin composition for metal containers or metal lids, the low melting point results in poor retort resistance. Furthermore, increasing the melting point of the crystalline polyester resin in order to improve retort resistance results in a deterioration in solvent solubility, making it impossible to obtain a crystalline polyester resin aqueous dispersion.

[0009] The object of the present invention is to provide a crystalline polyester resin aqueous dispersion that is substantially free of curing agents and therefore free of harmful substances such as bisphenol A and formaldehyde, has excellent solvent solubility and therefore requires a relatively small amount of organic solvent when preparing the aqueous dispersion, and is capable of forming a coating film that is excellent in processability and retort resistance, as well as a coating composition, coating film, and metal can containing the same.In particular, the object of the present invention is to provide an aqueous dispersion based on a crystalline polyester resin that combines excellent retort resistance and solvent solubility, as well as a coating composition, coating film, and metal can containing the same.

[0010] The present inventors conducted extensive research to achieve this objective and found that by using specific polycarboxylic acid components and specific polyhydric alcohol components of a crystalline polyester resin, it is possible to partially disrupt the molecular planarity and symmetry while maintaining a certain level of crystallinity. As a result, the resulting coating film from the crystalline polyester resin aqueous dispersion can satisfy both retort resistance and processability, and also satisfy the solvent solubility of the resin during aqueous dispersion formation. Furthermore, the inventors found that the acid value and melting point of the crystalline polyester resin are also important, and that by setting these within specific ranges, retort resistance, processability, and solvent solubility can be achieved at even higher levels. Based on these findings, the present inventors have completed the present invention. Specifically, the present invention comprises the following features: [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 polyhydric alcohol component as copolymerization 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 polyhydric alcohol component, having an acid value of 180 eq / ton or more and a melting point of 120 to 160°C. [2] The crystalline polyester resin aqueous dispersion according to claim 1, wherein the concentration of the sulfonic acid metal salt in the crystalline polyester resin (A) is less than 50 eq / ton. [3] A coating composition containing 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 according to [3]. [5] A metal can having the coating film according to [4].

[0011] The crystalline polyester resin aqueous dispersion of the present invention, in which the crystalline polyester resin in the aqueous dispersion is configured as described above, has excellent solvent solubility during aqueous dispersion preparation, can form a coating film excellent in processability and retort resistance even without containing a curing agent, and can eliminate harmful substances such as bisphenol A and formaldehyde. Therefore, the crystalline polyester resin aqueous dispersion of the present invention is suitable for coating compositions and coating films for beverage cans and food cans, and for metal cans.

[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 that can be obtained by polycondensation of a polycarboxylic acid and a polyhydric alcohol, and the polycarboxylic acid and the polyhydric alcohol each consist of one or more selected components.

[0014] When the total amount of the polycarboxylic acid components used in the present invention is taken as 100 mol%, the copolymerization amount of terephthalic acid must be 75 mol% or more, preferably 80 mol% or more, and more preferably 85 mol% or more. By making it 75 mol% or more, the crystallinity and glass transition temperature of the obtained 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, but for example, the following polycarboxylic acids or their esters, and their anhydrides can be used.

[0016] The polycarboxylic acid component other than terephthalic acid used in the present invention can be a dicarboxylic acid (a) (hereinafter referred to as component (a)) having a ring structure and, when the ring is a naphthalene ring, having carboxylic acid groups at the 2- and 6-positions, or when the ring is a cyclohexyl ring, having carboxylic acid groups at the 1- and 4-positions. The carboxylic acid groups in component (a) are preferably directly bonded to the ring structure. Examples of component (a) include 2,6-naphthalenedicarboxylic acid and 1,4-cyclohexanedicarboxylic acid. These can be used singly or in combination.

[0017] The polycarboxylic acid component used in the present invention preferably contains component (a). By containing component (a), the solvent solubility of the polyester resin can be improved without significantly impairing the crystallinity. Furthermore, among component (a), a dicarboxylic acid having a naphthalene ring as a ring structure and carboxylic acid groups at the 2- and 6-positions is more preferred, as it has less steric hindrance and is particularly easy to maintain crystallinity.

[0018] When the total amount of polycarboxylic acid components is taken as 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%. By setting the ratio at 5 mol% or more, the solvent solubility of the resulting polyester is improved, allowing for the production of a stable crystalline polyester resin aqueous dispersion. Furthermore, by setting the ratio at 20 mol% or less, a moderate glass transition temperature is achieved, allowing for the coating film to achieve both retort resistance and processability.

[0019] Examples of polycarboxylic acid components other than terephthalic acid used in the present invention include dicarboxylic acids (b) (hereinafter referred to as component (b)) having a ring structure and excluding component (a). The ring structure of component (b) is preferably a benzene ring, a furan ring, a naphthalene ring, or a cyclohexyl ring. The carboxylic acid group of component (b) is preferably directly bonded to the ring structure. Examples of component (b) include isophthalic 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, 2,7-naphthalenedicarboxylic acid, sodium 5-sulfoisophthalate, 2,5-furandicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and hexahydrophthalic acid. These may be used alone or in combination.

[0020] The component (b) is an optional component, and when the total amount of the polycarboxylic acid components is taken as 100 mol%, the copolymerization ratio of the component (b) is preferably 20 mol% or less, more preferably 15 mol% or less, even more preferably 10 mol% or less, and may even be 0 mol%. By making it 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 %, assuming the total amount of polycarboxylic acid components to be 100 mol %. By adjusting the ratio to 5 mol % or more, the solvent solubility of the resulting polyester is improved. Furthermore, by adjusting the 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 be an aliphatic polycarboxylic acid, an alicyclic polycarboxylic acid, or an aromatic polycarboxylic acid other than component (a) or (b). For example, the aliphatic polycarboxylic acid may be fumaric acid, adipic acid, sebacic acid, malonic acid, or succinic acid, while the alicyclic polycarboxylic acid or aromatic polycarboxylic acid may be one having a functionality of three or more. These may be used singly or in combination.

[0023] The polyhydric alcohol 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 polyhydric alcohol components is taken as 100 mol%, the copolymerization ratio of 1,6-hexanediol must be 55 mol% or more, more preferably 60 mol% or more, even more preferably 65 mol% or more, and particularly preferably 70 mol% or more. By making it 55 mol% or more, the crystallinity of the resulting polyester is increased, and in addition to improving retort resistance, processability and solvent solubility are also improved. Furthermore, it is preferably 90 mol% or less, more preferably 80 mol% or less. By making it 90 mol% or less, the solvent solubility of the resulting polyester is improved, and a stable crystalline polyester resin aqueous dispersion can be obtained.

[0025] The polyhydric alcohol component other than 1,6-hexanediol used in the present invention is not particularly limited, but specific examples include a polyhydric alcohol (c) having a side chain (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 main chain that has a hydrocarbon group (carbon chain) connecting two hydroxyl groups. The side chain in component (c) is preferably an alkyl group. The number of carbon atoms in the alkyl group is preferably 1 to 50, more preferably 2 to 40, and even more preferably 3 to 35. The number of side chains may be one or two or more. The number is preferably 5 or less, more preferably 4 or less, and even more preferably 3 or less. The alkyl group may be linear or may have a side chain. Examples of component (c) include 1,2-propylene glycol, 1,2-butanediol, 1,3-butanediol, 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 diol, polypropylene glycol, and pentaerythritol. The component (c) may be used alone or in combination of two or more.

[0026] The component (c) is an optional component, and when the total amount of the polyhydric alcohol components is taken as 100 mol%, the copolymerization ratio of the component (c) is preferably 20 mol% or less. More preferably, it is 15 mol% or less, and even more preferably, it is 10 mol% or less, and it may even be 0 mol%. By making it 20 mol% or less, it is possible to achieve both solvent solubility and retort resistance of the resulting polyester.

[0027] The polyhydric alcohol component other than 1,6-hexanediol used in the present invention includes a linear polyhydric alcohol (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 these can be used alone or in combination.

[0028] The component (d) is an optional component, and when the total amount of the polyhydric alcohol components is taken as 100 mol%, the copolymerization ratio of the component (d) is preferably 40 mol% or less, more preferably 30 mol% or less, even more preferably 20 mol% or less, and may even be 0 mol%. By setting it to 20 mol% or less, the solvent solubility and retort resistance of the resulting polyester can be both achieved.

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

[0030] Component (e) is an optional component, and when the total amount of the polyhydric alcohol components is taken as 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 may even be 0 mol%. By setting it to 30 mol% or less, the resulting polyester can achieve both solvent solubility and retort resistance.

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

[0032] The polycarboxylic acid component and polyhydric alcohol component constituting the crystalline polyester resin (A) of the present invention can be derived from biomass resources. Biomass resources include the stored products of plants that convert solar energy into starch, cellulose, etc. 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.

[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. These may be used alone or in combination.

[0034] Specific examples of polyhydric alcohol 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, which may be used alone or in combination of two or more.

[0035] The crystalline polyester resin (A) in the present invention preferably has a branched structure. Having a branched structure refers to the presence of a branched structure in the polyester main chain. To introduce a branched structure into a polyester, for example, a method of copolymerizing a trifunctional or higher functional component as part of a polycarboxylic acid component and / or a polyhydric alcohol component in a polyester polycondensation reaction can be mentioned. Examples of trifunctional or higher functional polycarboxylic acid components include the following polycarboxylic acids or their esters, and polycarboxylic acid anhydrides. Specific examples include trimellitic acid, pyromellitic acid, and benzophenone tetracarboxylic acid. Examples of trifunctional or higher functional polyhydric alcohol components include glycerin, trimethylolethane, trimethylolpropane, mannitol, sorbitol, and pentaerythritol, and these can be used alone or in combination. The branched structure of the crystalline polyester resin (A) improves the processability of the resulting coating film.

[0036] The trifunctional or higher polycarboxylic acid component and / or trifunctional or higher polyhydric alcohol component is preferably 0.3 mol% or more, more preferably 0.4 mol% or more, and even more preferably 0.5 mol% or more, when the entire crystalline polyester resin (A) is taken as 100 mol%. Also, 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 polycarboxylic acid component and / or polyhydric alcohol component exceed the above range, the crystallinity of the crystalline polyester resin (A) will decrease, the retort resistance will deteriorate, or gelation will occur during polyester polymerization.

[0037] The crystalline polyester resin (A) of the present invention can be given an acid value by any method.The method of giving an acid value includes a method of adding a compound having a polycarboxylic anhydride group in the molecule at the late stage of polycondensation, a method of making it high in acid value at the stage of prepolymer (oligomer), then polycondensing it to obtain a polyester resin having an acid value, etc., but the former method of adding reaction is preferred because it is easy to operate and can easily obtain the target acid value.

[0038] Among the compounds having a polyvalent carboxylic acid anhydride group in the molecule for imparting 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, citraconic anhydride, etc., and one or more of these can be selected and used. Among these, trimellitic anhydride is preferred from the viewpoints of versatility and economy.

[0039] Among the compounds having a polyvalent carboxylic acid anhydride group in the molecule for imparting 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 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, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, etc., and one or more of these can be selected and used. Among these, ethylene glycol bistrimellitate dianhydride is preferred from the standpoint of versatility and economy.

[0040] The compound having a polyvalent carboxylic acid anhydride group in the molecule for imparting the acid value may be a carboxylic acid monoanhydride or a carboxylic acid polyanhydride, which may be used alone or in combination.

[0041] The acid value of the crystalline polyester resin (A) in the present invention must be 180 eq / ton or more, preferably 200 eq / ton or more, more preferably 220 eq / ton or more, and even more preferably 250 eq / ton or more. By setting it to the lower limit or more, sufficient aqueous dispersion stability can be ensured. There is no particular upper limit for the acid value, but in order to reduce the amount of unreacted acid components and oligomers during the acid addition reaction, it is preferably 400 eq / ton or less.

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

[0043] Next, a 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 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 130 Pa or less. Insufficient vacuum tends to prolong the polycondensation time, which is undesirable. The time required for reducing the pressure from atmospheric pressure to 130 Pa or less is preferably 30 to 180 minutes.

[0044] In the esterification / exchange reaction and polycondensation reaction, polymerization is carried out as necessary using an organic titanate compound such as tetrabutyl titanate, or an organic tin compound such as germanium dioxide, antimony oxide, or tin octoate. From the viewpoint of reactivity, organic titanate compounds are preferred, and from the viewpoint of 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, and is preferably 40° C. or lower, more preferably 35° C. or lower, from the viewpoint of processability.

[0046] The melting point (Tm) of the crystalline polyester resin (A) in the present invention is the temperature at the apex of the endothermic peak with the largest heat of fusion during the temperature rise process, measured using a differential scanning calorimeter (DSC) after aging the polyester resin at 100°C for 30 hours and then raising the temperature from -50 to 200°C at 20°C / min. The melting point (Tm) of the crystalline polyester resin (A) in the present invention is in the range of 120 to 160°C, preferably 125 to 155°C, more preferably 130 to 150°C, and even more preferably 135 to 145°C. By setting the melting point at 120°C or higher, good crystallinity can be achieved and excellent retort resistance can be exhibited. Furthermore, by setting the melting point at 160°C or lower, excellent processability and solvent solubility can be exhibited.

[0047] In the present invention, the term "crystalline" refers to the melting point (Tm) exhibited by the polyester resin when measured under the above-mentioned conditions. Furthermore, "high crystallinity" means that the polyester resin has a high melting point.

[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. If the reduced viscosity is 0.2 dl / g or less, the toughness of the coating film may decrease, and the processability may be insufficient. On the other hand, if the reduced viscosity is 0.8 dl / g or more, the solvent solubility may decrease.

[0049] <Crystalline polyester resin aqueous dispersion> The crystalline polyester resin aqueous dispersion of the present invention can be prepared by, for example, (a) dissolving the crystalline polyester resin (A) in a water-soluble organic solvent capable of dissolving the crystalline polyester resin (A), and then adding a basic compound and water as necessary to disperse the crystalline polyester resin (A), or (b) adding the crystalline polyester resin (A) and water, a water-soluble organic solvent capable of dissolving the crystalline polyester resin (A), and a basic compound as necessary, and then heating and dispersing the mixture. Furthermore, if it is desired to reduce the amount of organic solvent or to completely remove the organic solvent to obtain an aqueous dispersion, it is also possible to disperse the mixture using an organic solvent having a boiling point of 100°C or less, and then remove the solvent by heating or under reduced pressure. In the case of a crystalline polyester resin, the former method (a) is preferred from the viewpoint of 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., even more preferably 60 to 130° C., and most preferably 70 to 100° C. If the temperature is less than 40° C., the crystalline polyester resin (A) may not be sufficiently dissolved, and the entanglement of molecular chains may not be sufficiently disentangled, while if the temperature exceeds 160° C., there is an increased risk of deterioration of the crystalline polyester resin (A). Examples of organic solvents that can dissolve the crystalline polyester resin (A) by heating in the 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, triethylene glycol monobutyl ether, etc. Of these, methyl ethyl ketone, butyl cellosolve, propylene glycol monopropyl ether, propylene glycol monobutyl ether, 2-propanol, etc. are preferred.

[0051] When the crystalline polyester resin (A) is dissolved at 100°C or higher, it is necessary to cool the temperature of 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 cause phase inversion, thereby obtaining an aqueous dispersion.

[0052] The basic compound used when dispersing the crystalline polyester resin (A) of the present invention in water is preferably a compound that volatilizes during the drying or baking process during coating film formation, and ammonia and / or an organic amine compound having a boiling point of 250°C or less are preferably used. Examples of preferred basic compounds include triethylamine, N,N-diethylethanolamine, N,N-dimethylethanolamine, aminoethanolamine, N-methyl-N,N-diethanolamine, isopropylamine, iminobispropylamine, ethylamine, diethylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, sec-butylamine, propylamine, methylaminopropylamine, dimethylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, N-methylmorpholine, and N-ethylmorpholine. These basic compounds must be present in an amount sufficient to at least partially neutralize the carboxyl groups of the crystalline polyester resin (A); specifically, it is desirable to add 0.5 to 1.5 equivalents per carboxyl group equivalent.

[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 coating appearance and storage stability, and is preferably 30 to 300 nm. It is more preferably 50 to 250 nm, and particularly preferably 70 to 200 nm. If the average particle size exceeds 300 nm, not only is dispersion stability significantly reduced, but film-forming properties are also reduced, resulting in a poor appearance of the resulting coating. Conversely, if the average particle size is less than 30 nm, film-forming properties tend to be significantly improved, but this is undesirable because fusion and aggregation between dispersed particles are more likely to occur, resulting in increased viscosity and poor dispersion.

[0054] The crystalline polyester resin aqueous dispersion of the present invention is preferably prepared with a resin solids concentration of 10 to 45% by mass. A resin solids concentration of 15 to 40% by mass is more preferable, and a resin solids concentration of 20 to 35% by mass is even more preferable. If the resin solids concentration exceeds 45% by mass, the viscosity of the aqueous dispersion increases and aggregation between resin particles is more likely to occur, resulting in a significant decrease in dispersion stability. Furthermore, if the resin solids concentration is less than 10% by mass, it is difficult to say that the dispersion is practical from the standpoint of both production and application.

[0055] The coating composition of the present invention contains at least the above-mentioned crystalline polyester resin aqueous dispersion. The crystalline polyester (A) in the aqueous dispersion is contained as a main component. In the coating composition, the main component is defined as 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.

[0056] The coating composition of the present invention is capable of forming a coating film using the crystalline polyester resin aqueous dispersion alone without blending a curing agent. Therefore, it is preferable that the coating composition of the present invention contains substantially no curing agent, that is, the content of the curing agent is preferably less than 1 part by mass (solids content) per 100 parts by mass (solids 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). Less than 0.5 parts by mass is more preferable, less than 0.1 parts by mass is even more preferable, and it is most preferable that no curing agent is contained. If the content of the curing agent is higher than the above range, not only is it inferior in economic efficiency, but there is also a risk of reduced processability due to self-condensation reaction between the curing agents, volatilization of the blocking agent, generation of harmful outgassing such as formaldehyde, and poor stability during long-term storage.

[0058] Here, the curing agent refers to a known curing agent that reacts with a polyester resin to form a crosslinked structure, and the form of the crosslinked structure can be, for example, a reaction in which unsaturated double bonds in the polyester resin are reacted by a radical addition reaction, a cation addition reaction, an anion addition reaction, etc. to form an intermolecular carbon-carbon bond, or the formation of an intermolecular bond by a condensation reaction with a polycarboxylic acid group or a polyhydric alcohol group in the polyester resin, a polyaddition reaction, an ester exchange reaction, etc. Examples of the curing agent include phenolic resins, amino resins, isocyanate compounds, epoxy compounds, β-hydroxylamide compounds, unsaturated bond-containing resins, etc.

[0059] The coating composition of the present invention is ideal for coating food and beverage cans. To produce coatings for food and beverage cans, various additives may be blended depending on the purpose. Plasticizers for improving solubility in organic solvents, leveling agents and surfactants for improving coatability, coating film smoothness, and appearance, lubricants for preventing scratches on the coating film, coloring pigments, and, in some cases, polyester resins other than the crystalline polyester resin of the present invention, and resins other than polyester resins, such as acrylic resin emulsions and polyurethane resin emulsions, may be blended within a range that does not impair the object of the present invention, such as food hygiene and flavor properties.

[0060] The coating composition of the present invention can be blended with other resins for the purpose of improving the properties of the coating film, such as imparting flexibility, adhesion, etc. Examples of other resins include amorphous polyesters, crystalline polyesters, ethylene-polymerizable unsaturated carboxylic acid copolymers, and ethylene-polymerizable carboxylic acid copolymer ionomers, and blending at least one resin selected from these may sometimes impart flexibility and / or adhesion 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 spray method. There are no particular limitations on the coating thickness, but a dry film thickness of 3 to 18 μm is generally preferred, with a range of 5 to 15 μm being preferred. The coating is typically baked at a temperature in the range of about 180 to 260°C for about 20 seconds to 1 hour, preferably at a temperature in the range of about 200 to 240°C for about 30 seconds to 10 minutes.

[0062] The coating film made of the crystalline polyester resin aqueous dispersion of the present invention is preferably baked within the above range and then subjected to an aging treatment, which further promotes crystallization in the coating film and improves retort resistance.

[0063] The coating film of the present invention is a substrate coated with the crystalline polyester resin water dispersion (two layers: substrate / crystalline polyester resin water dispersion). It may also be configured such that a layer of another resin is superimposed on either the top or bottom of the crystalline polyester resin water dispersion layer. In this case, the layer of another resin is referred to as a coating layer. The coating film of the present invention can be obtained by laminating the crystalline polyester resin water dispersion of the present invention on various substrates according to a conventional method, and then laminating another resin layer thereon.

[0064] The metal can of the present invention has the coating film. Metal cans can be obtained by coating one or both sides, and if necessary, 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.

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Various properties were evaluated according to the following methods. Parts simply indicate parts by mass, and % indicates % by mass.

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

[0067] (2) Measurement of reduced viscosity (unit: dl / g) 0.1 g of a crystalline 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 at 30°C using an Ubbelohde viscometer.

[0068] (3) Measurement of Melting Point (Tm) and Glass Transition Temperature (Tg) Measurements were made using a differential scanning calorimeter (DSC) DSC-220 manufactured by Seiko Instruments Inc. Aging treatment of the crystalline polyester resin was performed under aging conditions of 100°C x 30 hours, and 5 mg of the treated crystalline polyester resin sample was placed in an aluminum clamp-lid container and sealed. Nitrogen gas was flowed at 30 ml / min to create a nitrogen atmosphere. The sample was then cooled to -50°C using liquid nitrogen and then heated to 200°C at 20°C / min. The temperature at the apex of the maximum peak of the heat of fusion obtained during this process was determined as the melting point (Tm, unit: °C). Using the same measuring device, the sample was heated to 200°C under the same conditions, then rapidly cooled to -50°C, and again heated to 200°C at 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 line to the endothermic peak was taken as the glass transition temperature (Tg, unit: ° C.).

[0069] (4) Measurement of Acid Value 0.2 g of a crystalline polyester resin sample was dissolved in 40 ml of chloroform, and titrated with a 0.01 N potassium hydroxide ethanol solution to obtain 10% of the polyester resin. ６ The equivalent weight per gram (eq / ton) was calculated. Phenolphthalein was used as an indicator.

[0070] (5) Evaluation of Solvent Solubility 2 g of crystalline polyester resin was dissolved in 8 g of cyclohexanone, and the resulting solution was heated (left to stand) at 100°C for 3 hours. The state of dissolution of the crystalline polyester resin was visually evaluated as follows: (Evaluation) ◎: Almost completely dissolved (80% or more dissolved) ○: A small amount of undissolved matter remained (70% or more dissolved) △: A small amount of undissolved matter remained (60% or more dissolved) ×: Undissolved matter remained (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 (Beckman Coulter Counter LS13 320). A particle distribution was calculated on a volume basis using this analyzer, and the average particle size was used as the average particle size.

[0072] (7) Evaluation of storage stability After leaving the polyester resin aqueous dispersion at 25°C for 24 hours, the dispersion state was visually evaluated as follows: (Evaluation) ⊚: No change in appearance ◯: Sediment present ×: Solidified

[0073] <Preparation of Test Piece> A crystalline polyester resin aqueous dispersion 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 × 30 seconds, followed by aging treatment at 80°C × 1 hour. This was used as a test piece (hereinafter referred to as test piece).

[0074] (8) Evaluation of Coating Appearance The coating appearance of the obtained test pieces was visually judged as follows: (Judgment) ⊚: No cracks or aggregates. ◯: Cracks or aggregates are observed. Δ: Coating is partially peeled off. ×: Coating is entirely peeled off.

[0075] (9) Evaluation of Workability The obtained test specimen was bent 180° so that the coating film was facing outward, and the cracks in the coating film occurring at the bend were evaluated by measuring the current value. The bending was performed without any interposition (so-called 0T). An aluminum plate electrode (width 20 mm, depth 50 mm, thickness 0.5 mm) was placed on top of a sponge (width 20 mm, depth 50 mm, thickness 10 mm) soaked in 1% NaCl aqueous solution, and the test specimen was placed near the center of the bent portion of the test specimen in contact with the sponge so that it was parallel to the 20 mm side of the sponge. A DC voltage of 5.0 V was applied between the aluminum plate electrode and the uncoated portion on the back of the test specimen, and the current value was measured. A smaller current value indicates better bending properties. (Judgment) ◎: Less than 0.5 mA ○: 0.5 mA or more but less than 1.0 mA △: 1.0 mA or more but less than 2.0 mA ×: 2.0 mA or more

[0076] (10) Evaluation of retort resistance A test specimen was placed upright in a stainless steel cup, and ion-exchanged water was poured into it up to half the height of the test specimen. This was then placed in the pressure cooker of a retort tester (ES-315, manufactured by Tomy Kogyo Co., Ltd.) and subjected to retort treatment at 125°C for 30 minutes. Evaluation after treatment was carried out at the steam contact point, which is generally thought to be the most severe condition for a coating film, and the whitening and blistering of the cured film were visually judged as follows: (Judgment) ◎: Good (no whitening or blisters) ○: Slight whitening but no blisters △: Slight whitening and / or slight blisters ×: Significant whitening and / or significant blisters

[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 (0.03 mol% relative to the total acid components) of tetra-n-butyl titanate (hereinafter sometimes abbreviated as TBT) as a catalyst were charged into a 3 L four-neck flask, and the temperature was gradually raised to 240 ° C. over 3 hours, while an esterification reaction was carried out. After the esterification reaction was completed, the pressure in the system was gradually reduced, and polymerization was carried out under reduced pressure to 10 mmHg over 1 hour, while the temperature was raised to 250 ° C., and further post-polymerization was carried out for 90 minutes under a vacuum of 1 mmHg or less. After the polycondensation reaction was completed, the mixture was cooled to 210 ° C. under a nitrogen atmosphere, and then a predetermined amount of trimellitic anhydride was added. The reaction was carried out by continuing stirring under a nitrogen atmosphere at 220 ° C. for 30 minutes. After the reaction was completed, the mixture was removed and crystalline polyester resin (a) was obtained. The resulting 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) In the same manner as in the synthesis of crystalline polyester resin (a), crystalline polyester resins (b) to (f), (n) to (u), and (x) were produced by a direct esterification method, and crystalline polyester resins (g) to (m), and (v) to (w) were produced by a transesterification method, except that the charged compositions were changed to have the resin compositions shown in Table 1.

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

[0080] Examples 1 to 14 and 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]

[0082]

[0083] Preparation of Crystalline Polyester Resin Water Dispersion (B-1) The crystalline polyester resin (a) was dispersed in water according to the following procedure. 20 parts by mass of the crystalline polyester resin (a) and 10 parts by mass of cyclohexanone were charged into a reaction vessel equipped with a stirrer, a condenser, and a thermometer, and the resin was dissolved at 130°C for 2 hours. After dissolution, the temperature was lowered to 100°C, and 5 parts by mass of 2-propanol and 0.5 parts by mass of dimethylaminoethanol were added and stirred for 30 minutes. Next, 65 parts of warm water was added and stirred for 1 hour. The stirring speed was set to 200 rpm. The internal temperature was then lowered to room temperature to obtain a crystalline polyester resin water dispersion (B-1).

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

[0085] The resulting crystalline polyester resin aqueous dispersions (B-1) to (B-14) and (B-15) to (B-24) were used to evaluate various properties. The evaluation results are shown in Tables 4 and 5.

[0086]

[0087]

[0088] As is clear from the property evaluations in the above table, the solvent solubility of the crystalline polyester resins of Examples 1 to 14 was excellent or good. Furthermore, the crystalline polyester resin aqueous dispersions of Examples 15 to 28, which used the crystalline polyester resins of Examples 1 to 14, respectively, were excellent or good in terms of storage stability, coating film appearance when formed into a coating film, processability, and retort resistance.

[0089] On the other hand, the crystalline polyester resin of Comparative Example 1 had a high melting point, resulting in poor solvent solubility, and it could not be used to prepare an aqueous dispersion in Comparative Example 11. In Comparative Example 2, the melting point of the crystalline polyester resin was lowered by using 1,7-heptanediol instead of 1,6-hexanediol. An aqueous dispersion was prepared using this resin in Comparative Example 12, but the retort resistance of the coating film was significantly poor. In Comparative Example 3, the melting point of the crystalline polyester resin was increased by using 1,4-butanediol instead of 1,6-hexanediol, resulting in poor solvent solubility. It could not be used to prepare an aqueous dispersion in Comparative Example 13. In Comparative Example 4, isophthalic acid was used instead of terephthalic acid, which probably caused the main chain of the crystalline polyester resin to bend more than necessary. The processability of the coating film of the aqueous dispersion using this resin was significantly poor in Comparative Example 14. Because the melting point of the crystalline polyester resin in Comparative Example 5 was low, the retort resistance of the coating film of the aqueous dispersion using this resin in Comparative Example 15 was significantly poor. In Comparative Examples 6 and 7, the copolymerization ratio of 1,6-hexanediol in the polyhydric alcohol component was low, resulting in poor solvent solubility, and in Comparative Examples 16 and 17, aqueous dispersions could not be prepared using these. In Comparative Example 8, the acid value of the crystalline polyester resin was low, and in Comparative Example 18, aqueous dispersions could not be prepared using these. In Comparative Example 9, the terephthalic acid content of the crystalline polyester resin was low, resulting in a high glass transition temperature, and therefore the processability of the coating film of the aqueous dispersion using this resin was significantly poor in Comparative Example 19. In Comparative Example 10, the terephthalic acid content of the crystalline polyester resin was low, resulting in a low glass transition temperature, and therefore the retort resistance of the coating film of the aqueous dispersion using this resin was significantly poor in Comparative Example 20.

[0090] The crystalline polyester resin of the present invention has excellent solvent solubility, and when formed into a coating film, an aqueous dispersion using the resin has excellent coating film appearance, processability, and retort resistance, making it suitable as a base agent for paints to be applied to metal cans for food and beverages, etc.

Claims

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 polyhydric alcohol component as copolymerization 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 polyhydric alcohol component, having an acid value of 180 eq / ton or more, and having a melting point of 120 to 160°C.

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

3. A coating composition comprising the crystalline polyester resin aqueous dispersion according to claim 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 according to claim 3.

5. A metal can having the coating film according to claim 4.

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