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

Abstract

The present invention provides an aqueous dispersion of a crystalline polyester resin which does not contain a curing agent and is 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 an aqueous dispersion, and is capable of forming a coating film which has excellent processability and retort resistance properties; and a coating composition, coating film, and metal can each containing the same. A crystalline polyester resin aqueous dispersion comprising a crystalline polyester resin having copolymerization components of a polycarboxylic acid component and a polyhydric alcohol component, the polycarboxylic acid component containing 75 mol % or more of terephthalic acid, the polyhydric alcohol component containing 55 mol % or more of 1,6-hexanediol, an acid value of 180 eq / ton or more, and a melting point of 120 to 160°C.

Description

[Technical field]

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

[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 (flavor). In the process of forming the mouth of a bottle can, this coating is subjected to high-stress processing such as necking and threading. Therefore, the coating needs to be durable against such post-processing (processability). In addition, adhesion to metal materials and hardening properties are also required. Depending on the application, the can may be subjected to high temperature and high humidity conditions such as retort sterilization, and even in such cases, the coating needs to maintain adhesion to the metal material and not whiten (retort resistance).

[0003] Conventionally, epoxy-based paints such as epoxy-phenol-based paints, epoxy-amino-based paints, and epoxy-acrylic-based paints, polyester-based paints such as polyester-phenol-based paints, polyester-amino-based paints, and polyester-isocyanate-based paints, and vinyl chloride-based paints have been widely used as paints that have the above-mentioned corrosion resistance, flavor, and can-forming processing. However, recent research has reported that bisphenol A, a raw material for epoxy resins, may affect estrogenic activity and the brains of fetuses and infants. In addition, vinyl chloride-based paints have problems 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 the paint, is known to be harmful to the human body, such as carcinogenicity, and to have a negative effect on the flavor of the contents. Similarly, isocyanate resins are known to be harmful to the human body, such as carcinogenicity. Furthermore, there are concerns about the impact of using organic solvents on environmental pollution and the working environment.

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

[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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-79393 A [Patent Document 2] JP 2018-123249 A Summary of the Invention [Problem to be solved by the invention]

[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, there was a problem that unreacted β-hydroxyalkylamide crosslinking agent remained in the coating film, and retort resistance was reduced. In addition, the crystalline polyester resin described in Patent Document 2 is for producing a toner for developing electrostatic images, and it was found that when used as a resin composition for metal containers or metal lids, the retort resistance is poor due to the low melting point. In addition, when the melting point of the crystalline polyester resin is increased in order to improve the retort resistance, the solvent solubility is deteriorated, and there was a problem that a crystalline polyester resin aqueous dispersion could not be obtained.

[0009] The object of the present invention is to provide a crystalline polyester resin aqueous dispersion which is substantially free of a curing agent 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 an aqueous dispersion, and is capable of forming a coating film 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 which achieves both excellent retort resistance and solvent solubility, as well as a coating composition, coating film, and metal can containing the same. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to achieve this object, and have found that by using specific polyvalent carboxylic acid components and polyhydric alcohol components of the crystalline polyester resin, the planarity and symmetry of the molecule can be partially broken while maintaining high crystallinity to a certain extent, and as a result, the coating film obtained by the crystalline polyester resin aqueous dispersion can satisfy both retort resistance and processability, and can also satisfy the solvent solubility of the resin when it is dispersed in water. Furthermore, the acid value and melting point of the crystalline polyester resin are also important, and it has been found that by setting these within specific ranges, retort resistance, processability, and solvent solubility can be achieved at a higher level. Based on these findings, the present inventors have completed the present invention. That is, the present invention is composed of the following configurations. [1] A crystalline polyester resin water 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 [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]. Effect of the Invention

[0011] The crystalline polyester resin aqueous dispersion of the present invention, in which the crystalline polyester resin in the aqueous dispersion is constituted as described above, has excellent solvent solubility when dispersed in water, and can form a coating film having excellent processability and retort resistance properties 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, coating films, and metal cans for use in beverage cans and food cans. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment 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 polyvalent carboxylic 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 polyvalent carboxylic acid component other than terephthalic acid used in the present invention is not particularly limited, but for example, the following polyvalent carboxylic acids or their esters and anhydrides can be used.

[0016] The polyvalent carboxylic 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, a carboxylic acid group at the 2,6-position, and when the ring is a cyclohexyl ring, a carboxylic acid group at the 1,4-position. The carboxylic acid group of component (a) is 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 alone or in combination.

[0017] The polyvalent carboxylic 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. In addition, among the components (a), a dicarboxylic acid having a naphthalene ring as a ring structure and carboxylic acid groups at the 2- and 6-positions is more preferable because it has less steric hindrance and is particularly easy to maintain the crystallinity.

[0018] When the total amount of the polyvalent carboxylic acid components is taken as 100 mol%, the copolymerization ratio (mol%) of the (a) component is preferably 5 to 20 mol%, more preferably 8 to 18%, and even more preferably 10 to 15 mol%. By making it 5 mol% or more, the solvent solubility of the resulting polyester is good, so that a stable crystalline polyester resin water dispersion can be obtained. In addition, by making it 20 mol% or less, a moderate glass transition temperature is obtained, and the retort resistance and processability of the coating film can be compatible.

[0019] The polyvalent carboxylic acid component other than terephthalic acid used in the present invention includes a dicarboxylic acid (b) having a ring structure other than the component (a) (hereinafter referred to as the component (b)). The ring structure of the component (b) is preferably a benzene ring, a furan ring, a naphthalene ring, or a cyclohexyl ring. The carboxylic acid group of the component (b) is preferably directly bonded to the ring structure. Examples of the 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, 5-sodium 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 (b) component is an optional component, and when the total amount of the polyvalent carboxylic acid components is taken as 100 mol%, the copolymerization ratio of the (b) component 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 be 0 mol%. By making it 20 mol% or less, the retort resistance of the obtained polyester is improved.

[0021] When the (b) component is used, the total copolymerization ratio (mol%) of the (a) component and the (b) component is preferably 5 to 20 mol%, more preferably 8 to 18 mol%, and further preferably 10 to 15 mol%, assuming that the total amount of the polyvalent carboxylic acid components is 100 mol%. By making it 5 mol% or more, the solvent solubility of the resulting polyester is improved. Also, by making it 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, an aromatic polycarboxylic acid, or the like other than component (a) or component (b). For example, the aliphatic polycarboxylic acid may be fumaric acid, adipic acid, sebacic acid, malonic acid, succinic acid, or the like, and the alicyclic polycarboxylic acid or aromatic polycarboxylic acid may be one having three or more functional groups. In addition, one or more of these may be used.

[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 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 polyester obtained is high, and in addition to improving the retort resistance, the processability and solvent solubility are improved. In addition, 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 polyester obtained is improved, and a stable crystalline polyester resin water dispersion can be obtained.

[0025] The polyhydric alcohol component other than 1,6-hexanediol used in the present invention is not particularly limited, but specifically includes 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 branched from a main chain having 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 1 or 2 or more. It 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 kinds.

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

[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, it is 30 mol% or less, and even more preferably, it is 20 mol% or less, and it may be 0 mol%. By making it 20 mol% or less, it is possible to achieve both solvent solubility and retort resistance of the obtained polyester.

[0029] The polyhydric alcohol component other than 1,6-hexanediol used in the present invention includes 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, tricyclodecane dimethanol, hydroquinone, catechol, and resorcinol, and these can be used alone or in combination.

[0030] The 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 the component (e) is preferably 30 mol% or less, more preferably 25 mol% or less, and even more preferably 20 mol% or less, and may be 0 mol%. By making it 30 mol% or less, it is possible to achieve both solvent solubility and retort resistance of the obtained polyester.

[0031] When isosorbide is used as a polyhydric alcohol component other than 1,6-hexanediol used in the present invention, the copolymerization ratio is preferably less than 5 mol%, more preferably less than 1 mol%, and most preferably does not contain isosorbide. By making it less than 5 mol%, the solvent solubility and retort resistance of the obtained polyester can be improved.

[0032] The polyvalent carboxylic acid component and polyhydric alcohol component constituting the crystalline polyester resin (A) in the present invention can be made from raw materials derived from biomass resources. Biomass resources include those in which the solar light energy is converted into starch or cellulose by the photosynthetic action of plants and stored, the bodies of animals that grow by eating plants, and products made by processing plants or animals. Among these, more preferred biomass resources are plant resources, and examples of such resources include wood, rice straw, rice husks, rice bran, old rice, corn, sugar cane, cassava, sago palm, soybean pulp, corn cob, tapioca residue, bagasse, vegetable oil residue, potato, buckwheat, soybean, oils and fats, waste paper, papermaking residue, marine product residue, livestock waste, sewage sludge, and food waste. More preferred are corn, sugar cane, cassava, and sago palm.

[0033] Specific examples of polyvalent carboxylic 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 in combination of two or more.

[0034] Specific examples of polyhydric alcohol raw materials derived from biomass resources include, for example, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, neopentyl glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, etc. These 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. The term "having a branched structure" refers to having a branched structure in the main chain of the polyester, and in order to introduce a branched structure into the polyester, a method of copolymerizing a trifunctional or higher component as a part of a polyvalent carboxylic acid component and / or a polyhydric alcohol component in a polycondensation reaction of the polyester can be mentioned as an example. As the trifunctional or higher polyvalent carboxylic acid component, the following polyvalent carboxylic acids or their esters and polyvalent carboxylic anhydrides can be used. Specific examples include trimellitic acid, pyromellitic acid, and benzophenone tetracarboxylic acid. As the trifunctional or higher polyhydric alcohol component, glycerin, trimethylolethane, trimethylolpropane, mannitol, sorbitol, and pentaerythritol can be mentioned, and one or more of these can be used. When the crystalline polyester resin (A) has a branched structure, the processability of the resulting coating film becomes good.

[0036] The trifunctional or higher polyvalent carboxylic acid component and / or trifunctional or higher polyhydric alcohol component are 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 polyvalent carboxylic acid component and / or the polyhydric alcohol component exceed the above amounts, the crystallinity of the crystalline polyester resin (A) decreases, the retort resistance deteriorates, or gelation may occur during polyester polymerization.

[0037] The crystalline polyester resin (A) in 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 polyvalent carboxylic anhydride group in a molecule at the late stage of polycondensation, a method of making a prepolymer (oligomer) have a high acid value, and then polycondensing the prepolymer to obtain a polyester resin having an acid value, and the like.The former method of adding the compound is preferred because of the ease of operation and the ease of obtaining a 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 the carboxylic acid monoanhydride 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 polyvalent carboxylic acid anhydride groups 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 dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, ethylene glycol bistrimellitate dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, etc., and one or more of these can be selected and used. Among these, ethylene glycol bistrimellitate dianhydride is preferred from the viewpoints 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 180eq / ton or more, preferably 200eq / ton or more, more preferably 220eq / ton or more, and even more preferably 250eq / ton or more. By making it equal to or more than the lower limit, sufficient water 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 400eq / 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, it is 20 eq / ton or less, further preferably, it is 10 eq / ton or less, and particularly preferably, it is 5 eq / ton. By making it less than 50 eq / ton, the hydrophilicity of the obtained 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 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. The reaction time is the time from when the desired reaction temperature is reached to when the subsequent polycondensation reaction is started. 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 reaction temperature for polycondensation 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 long, which is not preferable. 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 octylate. From the viewpoint of reaction activity, an organic titanate compound is 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 heating process, which is measured by heating the polyester resin at -50 to 200°C at 20°C / min 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 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 to 120°C or higher, the crystallinity becomes good and excellent retort resistance can be exhibited. In addition, by setting the melting point to 160°C or lower, excellent processability and solvent solubility can be exhibited.

[0047] In the present invention, the term "crystallinity" refers to the melting point (Tm) exhibited by the polyester resin when measured under the above-mentioned conditions, and "high crystallinity" refers to the polyester resin having 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] <Water dispersion of crystalline polyester resin> The crystalline polyester resin water dispersion of the present invention can be prepared by a method (a) in which the crystalline polyester resin (A) is dissolved in a water-soluble organic solvent in which the crystalline polyester resin (A) is dissolved, and a basic compound and water are added successively as necessary to disperse the crystalline polyester resin (A), or a method (b) in which the crystalline polyester resin (A) and water, a water-soluble organic solvent in which the crystalline polyester resin (A) is dissolved, and a basic compound as necessary are added, and the mixture is heated and dispersed. In addition, if it is desired to reduce the amount of organic solvent or to completely remove the organic solvent to form 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 preferable from the viewpoint of film-forming properties.

[0050] In this case, the temperature for dissolving the crystalline polyester resin (A) 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. 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 released, while if the temperature exceeds 160° C., there is an increased risk of deterioration of the crystalline polyester resin (A). Examples of organic solvents in which the crystalline polyester resin (A) can be dissolved 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. Among 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 drying or baking during coating film formation, and ammonia and / or an organic amine compound having a boiling point of 250° C. or less are preferably used. For example, 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, N-ethylmorpholine, etc. are preferably used. These basic compounds are required to be at least partially neutralized with respect to the carboxyl group of the crystalline polyester resin (A), and specifically, it is desirable to add 0.5 to 1.5 equivalents with respect to the carboxyl group equivalent.

[0053] The average particle size of the crystalline polyester resin water dispersion according to the present invention is very important since it greatly affects the coating appearance and storage stability, and is preferably 30 to 300 nm. More preferably, it is 50 to 250 nm, and particularly preferably 70 to 200 nm. If the average particle size exceeds 300 nm, not only the dispersion stability is greatly reduced, but also the film-forming property is reduced, so that the appearance of the obtained film is deteriorated. On the other hand, if it is less than 30 nm, the film-forming property tends to be significantly improved, but therefore, fusion or aggregation between the dispersed particles is likely to occur, which is undesirable since it increases the possibility of thickening or poor dispersion as a result.

[0054] The crystalline polyester resin aqueous dispersion of the present invention is preferably prepared with a resin solid content concentration of 10 to 45% by mass. More preferably, it is 15 to 40% by mass, and even more preferably, it is in the range of 20 to 35% by mass. If the resin solid content concentration exceeds 45% by mass, the viscosity of the aqueous dispersion increases and aggregation between the resin particles is likely to occur, so that the dispersion stability is significantly reduced. If the resin solid content concentration is less than 10% by mass, it is difficult to say that it is practical from the viewpoints of both production and use.

[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 component with the highest content (mass ratio) among the solid components (non-volatile components excluding volatile substances such as water and organic solvents) that form the coating film in the coating composition is defined as the main component.

[0056] The coating composition of the present invention can form a coating film using the crystalline polyester resin aqueous dispersion alone without adding a curing agent. Therefore, it is preferable that the coating composition of the present invention does not substantially contain a curing agent, that is, the content of the curing agent is preferably less than 1 part by mass (solid content equivalent) per 100 parts by mass (solid content equivalent) 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 the curing agent is not contained. If the content of the curing agent is more than the above range, not only is it inferior in economic efficiency, but there is also a risk of a decrease in processability due to a 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 examples of the form of the crosslinked structure include a reaction in which an unsaturated double bond in a polyester resin is reacted by a radical addition reaction, a cationic addition reaction, an anionic addition reaction, or the like to generate an intermolecular carbon-carbon bond, and a condensation reaction, a polyaddition reaction, or an ester exchange reaction with a polyvalent carboxylic acid group or a polyhydric alcohol group in the polyester resin to form an intermolecular bond. Examples of the curing agent include a phenol resin, an amino resin, an isocyanate compound, an epoxy compound, a β-hydroxylamide compound, an unsaturated bond-containing resin, and the like.

[0059] The coating composition of the present invention is most suitable for coatings for food and beverage cans. In order to make it into a coating for food and beverage cans, various additives may be blended according to the purpose. Plasticizers for improving solubility in organic solvents, leveling agents and surfactants for improving applicability, smoothness of the coating film, and appearance, lubricants for preventing scratches on the coating film, and further coloring pigments and, in some cases, polyester resins other than the crystalline polyester resin of the present invention, resins other than polyester resins, such as acrylic resin emulsions and polyurethane resin emulsions, can be blended within a range that does not impair the purpose 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 modifying the coating film by 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 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, by a gravure roll coater, a comma coater, or a spray method. There are no particular limitations on the thickness of the coating, but the dry thickness is usually 3 to 18 μm, and more preferably 5 to 15 μm. The baking conditions for the coating are usually in the range of about 180 to 260° C. and about 20 seconds to 1 hour, and more preferably in the range of about 200 to 240° C. and about 30 seconds to 10 minutes.

[0062] It is preferable that the coating film made of the crystalline polyester resin aqueous dispersion of the present invention is 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). A layer made of other resin may be laminated on either the top or bottom of the crystalline polyester resin water dispersion layer. In this case, the layer made of other resin is 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 further laminating another resin layer.

[0064] The metal can of the present invention has the coating film. The metal can 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, cans for canned foods, and their lids and caps. Examples of the metal material include tinplate, tin-free steel, and aluminum. Metal plates made of these metal materials may be previously subjected to a phosphoric acid treatment, a chromate chromate treatment, a phosphoric acid chromate treatment, or other anticorrosive treatment with an anticorrosive agent, or a surface treatment for improving the adhesion of the coating film. EXAMPLES

[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 sample of the crystalline polyester resin was dissolved in deuterated chloroform and subjected to 1H-NMR analysis using a 400-MR nuclear magnetic resonance (NMR) device manufactured by VARIAN Co., Ltd. The molar ratio was calculated from the integral value ratio.

[0067] (2) Measurement of reduced viscosity (unit: dl / g) 0.1 g of a sample of the 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) Measurement 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 Inc. The aging treatment of the crystalline polyester resin was performed under the aging treatment conditions of 100°C x 30 hours, and 5 mg of the sample of the crystalline polyester resin after the treatment was placed in an aluminum clamp-lid type container and sealed, and nitrogen gas was flowed at 30 ml / min to create a nitrogen atmosphere, and then the container was cooled to -50°C using liquid nitrogen, and then heated to 200°C at 20°C / min. The temperature of the apex of the maximum peak of the heat of fusion obtained in this 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 heated again to 200°C at 20°C / min. In the endothermic curve obtained in this process, the temperature of the intersection between the baseline before the endothermic peak and the tangent line toward the endothermic peak was determined as the glass transition temperature (Tg, unit: °C).

[0069] (4) Acid value measurement 0.2 g of a crystalline polyester resin sample was dissolved in 40 ml of chloroform and titrated with 0.01 N potassium hydroxide ethanol solution to obtain 10% polyester resin. 6 The equivalent weight per gram (eq / ton) was calculated. 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 solution state of the crystalline polyester resin after heating (standing) at 100° C. for 3 hours was visually judged as follows. (judgement) ◎: Almost completely dissolved (more than 80% dissolved) ○: Slightly undissolved (70% or more dissolved) △: A small amount remains undissolved (more than 60% dissolved) ×: Some dissolution remains (less than 50%)

[0071] (6) Average particle size The average particle size of the polyester resin water dispersion was measured. A laser diffraction / scattering particle size distribution analyzer (Beckman Coulter Counter LS13 320) was used for the measurement. The particle distribution was prepared on a volume basis using this device, and the average diameter value was taken as the average particle size.

[0072] (7) Evaluation of storage stability The polyester resin aqueous dispersion was allowed to stand at 25° C. for 24 hours, and then the dispersion state was visually evaluated as follows. (judgement) ◎: No change in appearance ○: Sediment present ×: Solidified

[0073] <Preparation of test specimen> A crystalline polyester resin water dispersion was applied to one side of a tinplate (JIS G 3303 (2008) SPTE, 70 mm x 150 mm x 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 x 30 seconds, and then aging treatment was performed at 80 ° C x 1 hour. This was used as the test piece (hereinafter referred to as the test piece).

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

[0075] (9) Evaluation of processability The obtained test piece was bent 180° in the direction in which the coating film was on the outside, and the cracks in the coating film that occurred at the bent part were evaluated by measuring the electric current value. The bending process was performed without sandwiching anything between the test piece and the aluminum plate electrode (width 20 mm, depth 50 mm, thickness 0.5 mm), and a sponge (width 20 mm, depth 50 mm, thickness 10 mm) soaked in 1% NaCl aqueous solution was placed on the aluminum plate electrode (width 20 mm, depth 50 mm, thickness 0.5 mm). The sponge was placed near the center of the bent part of the test piece so that it was parallel to the 20 mm side of the sponge. A direct current voltage of 5.0 V was applied between the aluminum plate electrode and the uncoated part on the back side of the test plate, and the electric current value was measured. A smaller electric current value means better bending characteristics. (judgement) ◎: Less than 0.5mA ○: 0.5mA or more and less than 1.0mA △: 1.0mA or more and less than 2.0mA ×: 2.0mA or more

[0076] (10) Evaluation of retort resistance The test piece was placed upright in a stainless steel cup, ion-exchanged water was poured into it until it reached half the height of the test piece, and this was then placed in the pressure cooker of a retort tester (ES-315, manufactured by Tommy Kogyo Co., Ltd.) and subjected to retort treatment at 125°C for 30 minutes. Evaluation after treatment was performed on the steam contact part, which is generally considered to be exposed to more severe conditions for the coating film, and the whitening and blistering state of the cured film were visually judged as follows. (judgement) ◎: Good (no whitening or blisters) ○: Slight whitening but no blisters △: Slight whitening and / or slight 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 tetra-n-butyl titanate (hereinafter sometimes abbreviated as TBT) as a catalyst (0.03 mol% relative to the total acid component) were charged into a 3L four-neck flask, and the temperature was gradually raised to 240 ° C. over 3 hours to carry out an esterification reaction. After the esterification reaction was completed, the pressure in the system was gradually reduced, and 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 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, and the mixture was stirred for 30 minutes at 220 ° C. under a nitrogen atmosphere. After the reaction was completed, the mixture was taken out to obtain a crystalline polyester resin (a). 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) Similarly to the synthesis of crystalline polyester resin (a), crystalline polyester resins (b) to (f), (n) to (u), and (x) were produced by the direct esterification method, and crystalline polyester resins (g) to (m), and (v) to (w) were produced by the ester exchange method, except that the feed 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. [Table 1]

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

[0081] [Table 2]

[0082] [Table 3]

[0083] Production of crystalline polyester resin aqueous 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 hot water were added and stirred for 1 hour. The stirring speed was 200 rpm. The internal temperature was then lowered to room temperature to obtain a crystalline polyester resin water dispersion (B-1).

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

[0085] Examples 15 to 28, Comparative Examples 11 to 20 The obtained crystalline polyester resin water dispersions (B-1) to (B-14) and (B-15) to (B-24) were used to carry out various property evaluations. The evaluation results are shown in Tables 4 and 5.

[0086] [Table 4]

[0087] [Table 5]

[0088] As is clear from the property evaluation in the above table, the solvent solubility of the crystalline polyester resins of Examples 1 to 14 was excellent or good. In addition, 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 storage stability, coating film appearance when made 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, and therefore was poor in solvent solubility, and could not be used to make 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, and an aqueous dispersion was prepared using the 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 higher because 1,4-butanediol was used instead of 1,6-hexanediol, and therefore the solvent solubility was poor, and an aqueous dispersion could not be prepared using the resin 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, and the processability of the coating film of the aqueous dispersion using the resin in Comparative Example 14 was significantly poor. In Comparative Example 5, the melting point of the crystalline polyester resin was low, and therefore the retort resistance of the coating film of the aqueous dispersion using the 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, it was not possible to use them to make an aqueous dispersion. In Comparative Example 8, the acid value of the crystalline polyester resin was low, so in Comparative Example 18, it was not possible to use them to make an aqueous dispersion. In Comparative Example 9, the content of terephthalic acid in the crystalline polyester resin was low, resulting in a high glass transition temperature, so in Comparative Example 19, the processability of the coating film of the aqueous dispersion using it was significantly poor. In Comparative Example 10, the content of terephthalic acid in the crystalline polyester resin was low, resulting in a low glass transition temperature, so in Comparative Example 20, the retort resistance of the coating film of the aqueous dispersion using it was significantly poor. [Industrial Applicability]

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

Claims

1. A crystalline polyester resin aqueous dispersion comprising the following crystalline polyester resin (A) in a resin solids concentration of 10 to 45 mass %: 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. 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.