Polyester resin composition, aqueous dispersion, paint composition, and coating film

Abstract

[Problem] To provide a polyester resin and a polyester resin composition that are capable of suppressing toxic outgassing and forming a coating film which exhibits superior curing properties, retort processing durability, machining properties, and the like. [Solution] A polyester resin composition characterized by containing a polyester resin (A) and conforming to the following requirements (i)-(iii). (i) The acid value of the polyester resin (A) is 100 eq / ton or higher. (ii) There is substantially no curing agent. (iii) The tetrahydrofuran insoluble fraction when the polyester resin composition is subjected to a heat treatment at 240°C for one hour is at least 10 mass%.

Description

Technical Field

[0001] The present invention relates to a polyester resin composition. More specifically, it relates to a polyester resin composition suitable for can coatings. Further more specifically, it relates to a polyester resin composition suitable for coating the inner surface of cans containing beverages and foods (hereinafter collectively referred to as food and beverages), an aqueous dispersion containing the same, a coating composition, a coating film, and a metal can having the coating film.

Background Art

[0002] Metal cans such as beverage cans and food cans are coated with an organic resin to prevent corrosion of the metal by food (corrosion resistance) and avoid impairing the flavor and taste of the contents (flavor retention). These coatings are required to have processability, corrosion resistance, adhesion to metal materials, curability, and other properties. Depending on the application, the coating may be exposed to high-temperature and high-humidity conditions such as retort sterilization. In such cases, it is required that the coating maintains its adhesion to the metal material, and especially when applied to the outer surface of metal lids and the like, the coating is also required not to cause whitening.

[0003] Conventionally, as coatings capable of withstanding the above-described corrosion resistance, flavor retention, and can forming process, epoxy-based coatings such as epoxy-phenolic coatings, epoxy-amino coatings, and epoxy-acrylic coatings, polyester-based coatings such as polyester-phenolic coatings and polyester-amino coatings, and vinyl chloride-based coatings have been widely used. However, recent studies have reported that bisphenol A, which is a raw material for epoxy resins, may have estrogenic effects and may affect the brain of fetuses and infants. In addition, vinyl chloride-based coatings have problems related to stabilizers and the generation of dioxins during incineration. Formaldehyde, which is used as a raw material for phenolic resins, amino resins, and the like and remains in coatings, is known to be carcinogenic and harmful to the human body, and also adversely affects the flavor retention of the contents. Furthermore, there are also concerns about environmental pollution and impacts on the working environment caused by the use of organic solvents.

[0004] Due to concerns about various 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 satisfactory performance for can applications cannot be achieved.

[0005] From this viewpoint, 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 polymerization of a polymerizable unsaturated monomer component onto a polyester resin having ethylenic double bonds at the resin ends, and a β-hydroxyalkylamide crosslinking agent are dispersed in an aqueous medium (Patent Document 1). Furthermore, Patent Document 2 proposes a coating composition comprising a polyester polyol and a blocked polyisocyanate curing agent. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2001-81160 [Patent Document 2] Japanese Patent Publication No. 2006-169535 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, when using the high acid value acrylic-modified polyester resin described in Patent Document 1, sufficient processability could not be obtained. In addition, unreacted β-hydroxyalkylamide crosslinking agents remained in the coating film, resulting in problems with reduced curability and retort whitening resistance. The coating film obtained by reacting polyester with blocked polyisocyanate described in Patent Document 2 has poor water resistance and suffers from the problem of whitening occurring when sterilizing the contents.

[0008] The object of the present invention is to provide a polyester resin and a polyester resin composition that can be cured by polyester resin alone without substantially containing a curing agent, thereby suppressing harmful outgassing and forming a coating film with excellent properties such as curability, retort resistance, and processability. [Means for solving the problem]

[0009] The inventors of the present invention conducted various studies on the above and found that a polyester resin composition containing a predetermined amount of acid value and having a certain amount or more of tetrahydrofuran insoluble material when subjected to a predetermined heat treatment can be cured by the polyester resin alone, without substantially using a curing agent. Furthermore, by specifying the resin composition and controlling the amount of catalyst, it was found that a polyester resin coating film with an excellent balance of curability and processability, no harmful outgassing, and significantly improved retort resistance can be obtained, leading to the present invention. That is, the present invention consists of the following configuration.

[0010] [1] A polyester resin composition comprising a polyester resin (A) and characterized by satisfying the following requirements (i) to (iii). (i) The acid value of the polyester resin (A) is 100 eq / ton or more. (ii) Substantially contains no hardening agent. (iii) The tetrahydrofuran insoluble content of the polyester resin composition after heat treatment at 240°C for 1 hour is 10% by mass or more. [2] The polyester resin composition according to [1], wherein the polyester resin (A) has a branched structure. [3] The polyester resin composition according to [1] or [2], wherein the polyester resin (A) comprises, as a polyol component constituting the polyester resin (A), a diol (a) having two primary hydroxyl groups and not having an alicyclic structure, and further comprising either or both of a diol (b) having an alicyclic structure and a diol (c) having one primary hydroxyl group and one secondary hydroxyl group and not having an alicyclic structure. [4] A polyester resin composition according to any one of [1] to [3] above, wherein the polyester resin (A) has at least one selected from the group consisting of adipic acid, 2,6-naphthalenedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid as a polycarboxylic acid component constituting the polyester resin (A). [5] The polyester resin composition according to any one of [1] to [4], wherein the polyester resin (A) has an unsaturated dicarboxylic acid (d) as a polycarboxylic acid component constituting the polyester resin (A). [6] The polyester resin composition according to [1] to [5] above, wherein the tetrahydrofuran insoluble content after heat treatment at 150°C for 30 minutes is less than 10% by mass, and further comprises one or more catalysts (B). [7] The polyester resin composition according to [6], wherein the content of catalyst (B) is 0.01 to 0.5 parts by mass per 100 parts by mass of polyester resin (A). [8] A polyester resin aqueous dispersion comprising the polyester resin composition described in any of [1] to [7] above and an aqueous medium. [9] A paint composition containing either the polyester resin composition described in any of [1] to [7] above or the polyester resin aqueous dispersion described in [8] above.

[10] A coating film containing the polyester resin composition described in any of [1] to [7] above.

[11] A metal can having the coating described in

[10] above. [Effects of the Invention]

[0011] The present invention provides a polyester resin composition and its coating film that does not generate harmful outgassing caused by the curing agent, has an excellent balance of curability and processability, and exhibits significantly improved retort resistance. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described in detail below. The polyester resin composition of the present invention is a polyester resin composition comprising a polyester resin (A) and characterized by satisfying the following requirements (i) to (iii).

[0013] <Requirement (i)> Requirement (i) is explained below. The acid value of the polyester resin (A) must be 100 eq / ton or higher. Preferably it is 200 eq / ton or higher, more preferably 250 eq / ton or higher, and even more preferably 300 eq / ton or higher. If it is lower than the above, there are fewer carboxyl groups that can serve as crosslinking sites, which may reduce the curability. Furthermore, if the acid value is lower than the above, when heated to 240°C, the thermal decomposition reaction may proceed more favorably than the curing reaction, which may reduce the processability. Also, having an acid value above the lower limit makes aqueous dispersion easier. There is no particular upper limit for the acid value, but to reduce the amount of unreacted acid components and oligomers during the acid addition reaction, it is preferable to have an acid value of 1200 eq / ton or lower.

[0014] The acid value of the polyester resin (A) in the present invention can be determined by any method. Methods for determining the acid value include adding a polycarboxylic acid anhydride in the later stages of polycondensation, and making the prepolymer (oligomer) highly acidic, then polycondensing it to obtain a polyester resin with an acid value. However, the former method of adding a polycarboxylic acid anhydride is preferred due to its ease of operation and ease of obtaining the target acid value.

[0015] Among the compounds having polycarboxylic acid anhydride groups in the molecule for imparting an acid value to the 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, monoanhydrides such as 5-(2,5-dioxotetrahydrofurfuryl)-3-cyclohexen-1,2-dicarboxylic acid anhydride, hexahydrophthalic anhydride, and tetrahydrophthalic anhydride. One or more of these can be selected and used. Among these, trimellitic anhydride is preferred in terms of versatility and economic efficiency.

[0016] Among the compounds having a polycarboxylic anhydride group in the molecule for imparting an acid value to the polyester resin (A) of the present invention, examples of the carboxylic polyanhydride include pyromellitic anhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-pentanetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, ethylene glycol bis(trimellitate) dianhydride, 2,2',3,3'-diphenyltetracarboxylic dianhydride, thiophene-2,3,4,5-tetracarboxylic dianhydride, ethylenetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, and 5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride. One or two or more selected from these can be used. Among these, ethylene glycol bis(trimellitate) dianhydride is preferred.

[0017] The compound having a polycarboxylic anhydride group in the molecule for imparting the aforementioned acid value may be used with a carboxylic monoanhydride and a carboxylic polyanhydride used alone respectively, or may be used in combination.

[0018] <Requirement (ii)> Requirement (ii) will be described below. The polyester resin composition of the present invention substantially contains no curing agent. The expression "substantially contains no curing agent" means "the content of the curing agent is less than 1 part by mass (in terms of solid content) relative to 100 parts by mass of the polyester resin (A) (in terms of solid content)". As used herein, the curing agent refers to a known curing agent that reacts with the polyester resin (A) of the present invention to form a crosslinked structure. Examples of the form of the crosslinked structure include a reaction of forming intermolecular carbon-carbon bonds by reacting unsaturated double bonds in the polyester resin through radical addition reaction, cationic addition reaction, anion addition reaction or the like, and formation of intermolecular bonds through condensation reaction, polyaddition reaction, transesterification reaction or the like between polycarboxylic acid groups or polyhydric alcohol groups in the polyester resin. Examples of the curing agent include phenolic resins, amino resins, isocyanate compounds, epoxy compounds, β-hydroxylamide compounds, unsaturated bond-containing resins, and the like.

[0019] In the polyester resin composition of the present invention, the content of the curing agent is less than 1 part by mass relative to 100 parts by mass of the solid content of the polyester resin (A). A content of less than 0.5 parts by mass is more preferred, a content of less than 0.1 parts by mass is even more preferred, and no curing agent is most preferred. When the content of the curing agent is higher than the above range, it is not only economically disadvantageous, but also may cause reduced processability due to self-condensation reaction between curing agents, volatilization of blocking agents, generation of harmful outgases such as formaldehyde, and poor long-term storage stability.

[0020] <Requirement (iii)> Requirement (iii) is described below. The polyester resin composition of the present invention is required to have a tetrahydrofuran insoluble content of 10% by mass or more when heat-treated at 240° C. for 1 hour. When the tetrahydrofuran (THF) insoluble content is 10% by mass or more, a polyester resin composition and a coating film thereof having an excellent balance between retort resistance and processability can be obtained. The THF insoluble content is preferably 30% by mass or more, more preferably 50% by mass or more, and still more preferably 70% by mass. If the content is less than 10% by mass, the curability will be insufficient, the toughness of the coating film will be insufficient, which may lead to decreased retort resistance and failure to withstand molding processing into cans after coating on metal plates. Here, "the tetrahydrofuran insoluble content of the polyester resin composition after heat treatment at 240°C for 1 hour is 10% by mass or more" means that the THF insoluble content calculated using the following formula is 10% by mass or more, when the polyester resin composition is applied to copper foil to a thickness of 10 μm after drying, heated at 240°C for 1 hour, and the sample is made to a size of 10 cm in length and 2.5 cm in width. Let (X) be the mass of the sample before THF immersion, and (Y) be the mass of the sample after immersion in 60 ml of THF at 25°C for 1 hour and then dried at 100°C for 10 minutes. THF insoluble content (mass%) = [{(Y)-mass of copper foil} / {(X)-mass of copper foil}]×100

[0021] Furthermore, it is preferable that the polyester resin composition of the present invention has a THF insoluble content of less than 10% by mass when heat-treated at 150°C for 30 minutes. More preferably, it is less than 5% by mass, even more preferably less than 1% by mass, and it may even be 0% by mass. Under relatively low heating conditions of about 150°C, having a THF insoluble content below the above value can suppress the generation of aggregates when dissolved in a solvent or when dispersed in water.

[0022] The polyester resin composition of the present invention comprises polyester resin (A) and satisfies the requirements of (i) to (iii) above, so that it can be cured by the polyester resin alone without substantially using a curing agent. For this reason, the polyester resin composition of the present invention may contain no components other than polyester resin (A) and may be in the form of polyester resin (A) alone.

[0023] <Polyester resin (A)> Next, the polyester resin (A) in the present invention will be described. The polyester resin (A) in the present invention is mainly composed of a polycarboxylic acid component and a polyol component.

[0024] In the present invention, the polyester resin (A) preferably has a diol (a) (hereinafter sometimes referred to as component (a)) as a polyol component, which has two primary hydroxyl groups and does not have an alicyclic structure. Examples of diols (a) in polyester resin (A) that have two primary hydroxyl groups and do not have an alicyclic structure include aliphatic glycols such as ethylene glycol, 1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 3-methyl-1,5-pentanediol, 1,4-butanediol, 2,4-diethyl-1,5-pentanediol, 1,6-hexanediol, 2-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4-methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, and 1,9-nonanediol, as well as polyether glycols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. One or more of these can be selected and used. Among these, ethylene glycol, 2-methyl-1,3-propanediol, and 1,6-hexanediol are preferred.

[0025] In polyester resin (A), the copolymerization ratio of diol (a), which has two primary hydroxyl groups and does not have an alicyclic structure, is preferably 20 to 80 mol%, more preferably 20 to 60 mol%, and even more preferably 20 to 40 mol% of the total polyol components. Within this range, good curability and retort resistance are obtained.

[0026] In the present invention, the polyester resin (A) preferably contains a diol (b) having an alicyclic structure as a polyol component. Having a diol (b) having an alicyclic structure (hereinafter sometimes referred to as component (b)) makes it easy to achieve both processability and retort resistance. Examples of the diol (b) having an alicyclic structure that constitutes the polyester resin (A) in the present invention include 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, tricyclodecane glycols, and water-added bisphenols, and one or more of these can be selected and used. Among these, 1,4-cyclohexanedimethanol is preferred in terms of curability, processability, and retort resistance.

[0027] In the present invention, the copolymerization ratio of the alicyclic diol (b) constituting the polyester resin (A) is preferably 5 to 50 mol%, more preferably 10 to 40 mol%, and even more preferably 20 to 30 mol% of the total polyol components. Good processability is achieved within this range.

[0028] In the present invention, the polyester resin (A) preferably has a diol (c) (hereinafter sometimes referred to as component (c)) as a polyol component, which has one primary hydroxyl group and one secondary hydroxyl group and does not have an alicyclic structure. Examples of the diol (c) in the polyester resin (A) of the present invention that has one primary hydroxyl group and one secondary hydroxyl group and does not have an alicyclic structure include 1,2-propylene glycol and 1,2-butanediol, and one or more of these can be selected and used. Among these, 1,2-propylene glycol is preferred.

[0029] In the present invention, the copolymerization ratio of the diol (c) in the polyester resin (A), which has one primary hydroxyl group and one secondary hydroxyl group and does not have an alicyclic structure, is preferably 5 to 75 mol%, more preferably 10 to 65 mol%, and even more preferably 15 to 50 mol% of the total polyol components. Within this range, good curability and retort resistance are obtained.

[0030] In the present invention, the polyester resin (A) preferably contains component (a) and one or both of components (b) and (c) as polyol components. Since component (a) readily forms ester bonds, while the ester bonds of components (b) and (c) are more easily cleaved than those of component (a), it is presumed that by having component (a) and components (b) and (c), rearrangement and recombination of ester bonds are promoted during heat treatment, increasing the THF insoluble content, and forming a coating film that combines curability and flexibility, i.e., processability.

[0031] Examples of polycarboxylic acid components constituting the polyester resin (A) in the present invention include aromatic dicarboxylic acid components such as terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 1,8-naphthalenedicarboxylic acid. These can be used individually or in combination of two or more. Among these, terephthalic acid and 2,6-naphthalenedicarboxylic acid are preferred.

[0032] Other polycarboxylic acid components constituting the polyester resin (A) in the present invention include, for example, aliphatic dicarboxylic acid components and alicyclic dicarboxylic acid components. Examples of aliphatic dicarboxylic acid components include succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedionic acid, and dimer acid. Examples of alicyclic dicarboxylic acid components include 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, and 1,2-cyclohexenedicarboxylic acid. One or more of these can be selected and used. Among these, adipic acid and 1,4-cyclohexanedicarboxylic acid are preferred in terms of reactivity and economic efficiency.

[0033] When polyester resin (A) contains aliphatic dicarboxylic acid components and alicyclic dicarboxylic acid components as constituent units, the copolymerization ratio of these components is preferably 5 to 40 mol% of the polycarboxylic acid component. More preferably, it is 10 to 30 mol%. If the ratio deviates from this range, the glass transition temperature of polyester resin (A) may decrease significantly, and its retort resistance may decrease.

[0034] In the present invention, it is also preferable that the polyester resin (A) contains an unsaturated dicarboxylic acid (d) as a constituent unit. The presence of an unsaturated dicarboxylic acid (d) improves curability by generating intermolecular carbon-carbon bonds through the cleavage of unsaturated bonds during heat treatment. Examples of the unsaturated dicarboxylic acid (d) include fumaric acid, maleic acid, itaconic acid, citraconic acid, 2,5-norbornanedicarboxylic acid, and tetrahydrophthalic acid, as well as their acid anhydrides, and one or more of these can be used.

[0035] When the polyester resin (A) contains an unsaturated dicarboxylic acid (d) as a constituent unit, the copolymerization ratio of the unsaturated dicarboxylic acid (d) is preferably 5 to 20 mol% of the polycarboxylic acid component. More preferably, it is 10 to 15 mol%. Being within this range allows for a balance between processability and curability.

[0036] In the present invention, the polyester resin (A) preferably has a branched structure. Having a branched structure means that the polyester main chain has a branched structure. An example of introducing a branched structure in polyester resin (A) is by copolymerizing a trifunctional or higher component as part of the polycarboxylic acid component and / or polyol component in the polycondensation reaction of polyester. Examples of trifunctional or higher polycarboxylic acid components include trimellitic acid, pyromellitic acid, and benzophenonetetracarboxylic acid, and examples of trifunctional or higher polyols include glycerin, trimethylolethane, trimethylolpropane, mannitol, sorbitol, pentaerythritol, and α-methylglucoside. Because the polyester resin (A) has a branched structure, the crosslinking density increases when ester bond rearrangement and recombination occur during heat treatment, which increases the THF insoluble content and improves curability and processability.

[0037] The copolymerization ratio of the trifunctional or higher polycarboxylic acid component is preferably 0.1 mol% or more, more preferably 0.5 mol% or more, and even more preferably 1 mol% or more, when the total polycarboxylic acid component is considered to be 100 mol%. It is also preferably 7 mol% or less, more preferably 6 mol% or less, even more preferably 5 mol% or less, and particularly preferably 4 mol% or less. The copolymerization ratio of the trifunctional or higher polyol component is preferably 0.1 mol% or more, more preferably 0.5 mol% or more, and even more preferably 1 mol% or more, when the total polyol component is considered to be 100 mol%. It is also preferably 5 mol% or less, more preferably 3 mol% or less, even more preferably 2 mol% or less, and particularly preferably 1 mol% or less. If the polycarboxylic acid component and polyol component each exceed the above values, the flexibility of the polyester resin will be lost and its processability will decrease, or gelation may occur during the polymerization of the polyester.

[0038] In the present invention, the polycarboxylic acid and polyol components constituting the polyester resin (A) can be made from raw materials derived from biomass resources. Biomass resources include materials stored when sunlight energy is converted into starch, cellulose, etc., through the photosynthesis of plants, the bodies of animals that grow by eating plants, and products made by processing plant or animal bodies. Among these, plant resources are more preferred as biomass resources, and examples include wood, rice straw, rice husks, rice bran, old rice, corn, sugarcane, cassava, sago palm, okara, corn cob, tapioca residue, bagasse, vegetable oil residue, potatoes, buckwheat, soybeans, oils and fats, waste paper, papermaking residue, fishery residue, livestock excrement, sewage sludge, food waste, etc. More preferably are corn, sugarcane, cassava, and sago palm.

[0039] Next, the method for producing polyester resin (A) in the present invention will be described. In the esterification / exchange reaction, all monomer components and / or their low polymers are heated, melted, and reacted. The esterification / exchange reaction temperature is preferably 180 to 250°C, and more preferably 200 to 250°C. The reaction time is preferably 1.5 to 10 hours, and more preferably 3 to 6 hours. The reaction time is defined as the time from when the desired reaction temperature is reached until the subsequent polycondensation reaction. In the polycondensation reaction, under reduced pressure, at a temperature of 220 to 280°C, the polyol component is distilled off from the esterified product obtained in the esterification reaction, and the polycondensation reaction is carried out until the desired molecular weight is reached. The reaction temperature for polycondensation is preferably 220 to 280°C, and 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 longer, which is undesirable. It is preferable to gradually reduce the pressure from atmospheric pressure to below 130 Pa over a period of 30 to 180 minutes.

[0040] During esterification / exchange and polycondensation reactions, polymerization is carried out using organotitanic acid compounds such as tetrabutyl titanate, or organotin compounds such as germanium dioxide, antimony oxide, or tin octoate, as needed. Organotitanic acid compounds are preferred in terms of reaction activity, while germanium dioxide is preferred in terms of resin coloring.

[0041] In the present invention, the glass transition temperature of the polyester resin (A) is preferably 40°C or higher, and more preferably 60°C or higher, from the viewpoint of water resistance, and especially the retort resistance of the coating film. There is no particular upper limit to the glass transition temperature, but it is usually 130°C or lower.

[0042] In the present invention, the reduced viscosity of the polyester resin (A) is preferably 0.2 to 0.6 dl / g, and more preferably 0.3 to 0.5 dl / g. If the reduced viscosity is 0.2 dl / g or less, the curability will be insufficient, and the toughness of the coating film will be insufficient, which may prevent it from withstanding molding into a can after coating a metal plate. On the other hand, if the reduced viscosity is 0.6 dl / g or more, the melt viscosity and solution viscosity will increase, which not only reduces workability but may also prevent sufficient acid value from being imparted due to a decrease in the number of hydroxyl group terminals.

[0043] <Catalyst (B)> The polyester resin composition of the present invention preferably further contains catalyst (B). By including catalyst (B), the self-crosslinking properties of the polyester resin (A) during heat treatment can be promoted, the THF-insoluble content can be increased, and the curability can be improved. Examples of catalysts include acid catalysts such as sulfuric acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, naphthalenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid, camphor sulfonic acid, and phosphoric acid, as well as amine blocks (partially neutralized by adding an amine) of these, organotin compounds such as dibutyltin dilaurylate, titanium compounds such as titanium tetrabutoxide, zinc compounds such as zinc acetate, hafnium compounds such as hafnium chloride-THF complex, and rare earth compounds such as scandium triflate. One or more of these can be used in combination. Among these, dodecylbenzenesulfonic acid and its neutralized product are preferred in terms of compatibility with polyester resin (A) and hygiene.

[0044] The mixing ratio of polyester resin (A) and catalyst (B) in the polyester resin composition of the present invention is preferably (A) / (B) = 100 / 0.01~0.5 (mass ratio), more preferably 100 / 0.05~0.4 (mass ratio), and most preferably 100 / 0.1~0.3 (mass ratio). Within this range, the curability of the polyester resin composition can be improved.

[0045] In the polyester resin composition of the present invention, catalyst (B) may be included in the polyester resin (A) or added later. From the viewpoint of avoiding gelation during the polymerization of polyester resin (A), it is preferable to add catalyst (B) after the production of polyester resin (A).

[0046] The polyester resin composition of the present invention may contain a radical polymerization inhibitor (C). While primarily used to prevent gelation due to unsaturated bond cleavage during the polymerization of the polyester resin (A), it may also be added after polymerization to enhance the storage stability of the polyester resin (A). Examples of known radical polymerization inhibitors (C) include phenolic antioxidants, phosphorus-based antioxidants, amine-based antioxidants, sulfur-based antioxidants, and inorganic compound-based antioxidants.

[0047] Examples of phenolic antioxidants include 2,5-di-t-butylhydroquinone, 4,4'-butyldenbis(3-methyl-6-t-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,3,5-tris-methyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, tris(3,5-di-t-butyl-4-hydroxyphenyl)isocyanurate, or derivatives thereof.

[0048] Examples of phosphorus-based antioxidants include tri(nonylphenyl) phosphite, triphenyl phosphite, diphenylisodecyl phosphite, trioctadecyl phosphite, tridecyl phosphite, diphenyldecyl phosphite, 4,4'-butylidene-bis(3-methyl-6-t-butylphenyl ditridecyl phosphite), distearyl-pentaerythritol diphosphite, trilauryl trithiophosphite, or their derivatives.

[0049] Examples of amine-based antioxidants include phenyl-beta-naphthylamine, phenothiazine, N,N'-diphenyl-p-phenylenediamine, N,N'-di-beta-naphthyl-p-phenylenediamine, N-cyclohexyl-N'-phenyl-p-phenylenediamine, aldol-alpha-naphthylamine, 2,2,4-trimethyl-1,2-dihydroquinoline polymer, or derivatives thereof.

[0050] Examples of sulfur-based antioxidants include thiobis(N-phenyl-beta-naphthylamine, 2-mercaptobentiazole, 2-mercaptobenzimidazole, tetramethylthiuram disulfide, nickel isopropyl xanthate, or derivatives thereof.

[0051] Examples of nitro compound-based antioxidants include 1,3,5-trinitrobenzene, p-nitrosodiphenylamine, p-nitrosodimethylaniline, 1-chloro-3-nitrobenzene, o-dinitrobenzene, m-dinitrobenzene, p-dinitrobenzene, p-nitrobenzoic acid, nitrobenzene, 2-nitro-5-cyanothiophene, or derivatives thereof.

[0052] Examples of inorganic compound antioxidants include FeCl3, Fe(CN)3, CuCl2, CoCl3, Co(ClO4)3, Co(NO3)3, and Co2(SO4)3.

[0053] As the radical polymerization inhibitor (C) used in the present invention, among the above antioxidants, phenolic antioxidants and amine-based antioxidants are preferred in terms of thermal stability, more preferably those with a melting point of 120°C or higher and a molecular weight of 200 or higher, and even more preferably those with a melting point of 170°C or higher. Specifically, these include phenothiazine and 4,4'-butyldenbis(3-methyl-6-t-butylphenol).

[0054] The blending ratio of the polyester resin (A) and the radical polymerization inhibitor (C) in the polyester resin composition of the present invention is preferably (A) / (C) = 100 / 0.001 to 0.5 (mass ratio), more preferably 100 / 0.01 to 0.1 (mass ratio), and most preferably 100 / 0.02 to 0.08 (mass ratio). Being within this range suppresses gelation during the production of the polyester resin (A).

[0055] The polyester resin composition of the present invention may be blended with known inorganic pigments such as titanium dioxide and silica, known additives such as phosphoric acid and its esters, surface smoothers, defoamers, dispersants, and lubricants, according to the required properties. In particular, lubricants are important for providing the lubricity of the coating film required during molding of DI cans and DR (or DRD) cans, etc. Suitable lubricants include fatty acid ester waxes, which are esters of polyol compounds and fatty acids, silicone waxes, fluorine waxes, polyolefin waxes such as polyethylene, lanolin waxes, montan waxes, and microcrystalline waxes. One or more lubricants can be used in combination.

[0056] The polyester resin composition of the present invention can be made into a paint when dissolved in a known organic solvent. Examples of organic solvents used for paint formation include toluene, xylene, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, methyl cellosolve, butyl cellosolve, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, ethylene glycol monoacetate, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and Solvesso. One or more of these can be selected and used, taking into consideration solubility, evaporation rate, etc.

[0057] The polyester resin composition of the present invention can be made into a powder coating by known grinding methods. Known grinding methods include, for example, grinding methods. In the grinding method, a mixture of the polyester resin composition of the present invention, optionally a rust-preventive pigment and additives, is dry-mixed in a mixer such as a tumbler mixer or a Henschel mixer, and then melt-kneaded in a kneader. As a kneader, for example, a general kneader such as a single-screw or double-screw extruder, a three-roll kneader, or a lab blast mill can be used. The kneaded mixture is cooled and solidified, and the solidified material is coarsely and finely ground to obtain a powder. Examples of grinders include a jet-type grinder that uses a supersonic jet stream for grinding, and an impact-type grinder that introduces the solidified material into the space formed between a rotor and stator that rotate at high speed for grinding. Additives may also be added to the powder if necessary. The powder can be classified to adjust the powder to a desired particle size and particle size distribution to obtain a powder coating composition. For classification, known classifiers capable of removing over-pulverized toner mother particles by centrifugal force and wind force can be used, such as a rotary wind classifier.

[0058] The polyester resin composition of the present invention may be blended with other resins for the purpose of modifying the coating film, such as by imparting flexibility and adhesion. Examples of other resins include amorphous polyester, crystalline polyester, ethylene-polymerizable unsaturated carboxylic acid copolymer, and ethylene-polymerizable carboxylic acid copolymer ionomer. By blending at least one resin selected from these, flexibility and / or adhesion of the coating film may be imparted.

[0059] The polyester resin composition of the present invention can be applied to one or both sides, and if necessary, to the edges, of a metal sheet made of a metal material that can be used for beverage cans, canned food cans, their lids, caps, etc. Examples of the metal material include tinplate, tin-free steel, and aluminum. The metal sheet made of these metal materials may be used after being pre-treated with phosphate treatment, chromate treatment, phosphate treatment, or other rust-preventive treatments, or surface treatments aimed at improving the adhesion of the coating film.

[0060] The polyester resin composition of the present invention can be applied to a metal plate and cured by known coating methods such as roll coating and spray coating. The coating film thickness is not particularly limited, but a dry film thickness of 3 to 18 μm, and more preferably 5 to 15 μm, is preferred. The curing conditions for the coating film are usually in the range of approximately 180 to 260°C for about 10 minutes to 2 hours, and more preferably in the range of approximately 200 to 240°C for about 5 minutes to 1 hour.

[0061] <Polyester resin aqueous dispersion> The polyester resin composition of the present invention can also be dispersed in an aqueous medium and used as a polyester resin aqueous dispersion. Methods for forming an aqueous dispersion of the polyester resin (A) used in the present invention include (a) dissolving the polyester resin (A) in a water-soluble organic solvent in which the polyester resin (A) dissolves, and then sequentially adding a basic compound and water as needed to disperse it; and (b) adding the polyester resin (A), water, a water-soluble organic solvent that dissolves the polyester resin (A), and a basic compound as needed, and then heating and dispersing it. Furthermore, if it is desired to reduce the amount of organic solvent, or to completely remove it and form an aqueous dispersion, it is also possible to disperse it using an organic solvent having a boiling point of 100°C or lower, and then remove the solvent by heating or under reduced pressure. In the case of polyester resin (A), the former method (a) is preferred from the viewpoint of film-forming properties.

[0062] In this case, the temperature at which the polyester resin (A) is dissolved is preferably 40 to 160°C, more preferably 50 to 140°C, even more preferably 60 to 120°C, and most preferably 70 to 100°C. Below 40°C, the dissolution of the polyester resin (A) may be insufficient, making it difficult to sufficiently untangle the molecular chains. Above 160°C, the risk of degradation of the polyester resin (A) increases. Examples of organic solvents that can dissolve the polyester resin (A) when heated in the temperature range of 40 to 160°C include methyl ethyl ketone, 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 monopropyl ether, propylene glycol monobutyl ether, and triethylene glycol monobutyl ether. Of these, methyl ethyl ketone, butyl cellosolve, propylene glycol monopropyl ether, and propylene glycol monobutyl ether are preferred.

[0063] When polyester resin (A) is dissolved at a temperature of 100°C or higher, it is necessary to cool the temperature of the polyester resin solution to 100°C or lower, and then, while stirring the resin solution, sequentially add water and, if necessary, a basic compound to perform a phase change and obtain an aqueous dispersion.

[0064] The basic compound used when forming the polyester resin (A) of the present invention into an aqueous dispersion is preferably a compound that volatilizes during the drying and baking processes during film formation, such as ammonia and / or organic amine compounds with a boiling point of 250°C or lower. Preferably, examples 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, N-ethylmorpholine, etc. The basic compound must be in an amount that can at least partially neutralize the carboxyl groups of the polyester resin (A), and specifically, it is desirable to add 0.5 to 1.5 equivalents relative to the carboxyl group equivalents.

[0065] The average particle size of the polyester resin aqueous dispersion according to the present invention is extremely important as it greatly affects the appearance and storage stability of the coating film, and is preferably 30 to 250 nm. More preferably 50 to 200 nm, and particularly preferably 70 to 150 nm. If the average particle size exceeds 250 nm, not only does the dispersion stability decrease significantly, but the film-forming ability also decreases, resulting in a deterioration of the appearance of the resulting film. Conversely, if the particle size is less than 30 nm, the film-forming ability tends to improve significantly, but this is undesirable because it makes fusion and aggregation between dispersed particles more likely, resulting in increased viscosity and poor dispersion.

[0066] The polyester resin aqueous dispersion of the present invention is preferably prepared with a resin solids content concentration of 10 to 45% by mass. More preferably, it is in the range of 15 to 40% by mass, and even more preferably, 20 to 35% by mass. If the resin solids content concentration exceeds 45% by mass, the viscosity of the aqueous dispersion increases, and aggregation between resin particles becomes more likely, resulting in a significant decrease in dispersion stability. Furthermore, concentrations below 10% by mass are not practical from both a manufacturing and application standpoint.

[0067] The polyester resin aqueous dispersion of the present invention is ideal for use as an inner coating for food and beverage cans. Depending on the purpose, various additives may be incorporated to make it suitable for use as an inner coating for food and beverage cans. These may include leveling agents and surfactants to improve applicability, film smoothness, and appearance; lubricants to prevent scratches on the film; and coloring pigments. In some cases, other polyester resins besides polyester resin (A), or other resins such as acrylic resin emulsions or polyurethane resin emulsions, can be added, provided they do not impair the objectives of the present invention, such as food hygiene and flavor.

[0068] The coating using the polyester resin aqueous dispersion of the present invention can be applied to metal substrates for cans, such as aluminum, stainless steel, and tinplate, by gravure roll coaters, comma coaters, spray methods, etc. There are no particular restrictions on the film thickness, but it is usually preferably in the range of 3 to 18 μm, and more preferably in the range of 5 to 15 μm, with a dry film thickness of approximately 3 to 18 μm. The curing conditions for the coating film are usually in the range of approximately 180 to 260°C for about 10 minutes to 2 hours, and more preferably in the range of approximately 200 to 240°C for about 5 minutes to 1 hour. [Examples]

[0069] 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" refers to parts by mass, and "%" refers to mass percent.

[0070] <Polyester resin (A)> (1) Measurement of resin composition A sample of polyester resin (A) was dissolved in deuterated chloroform and subjected to 1H-NMR analysis using a VARIAN 400-MR nuclear magnetic resonance (NMR) spectrometer. The molar ratio was determined from the ratio of the integrated values.

[0071] (2) Measurement of reduced viscosity (unit: dl / g) A 0.1 g sample of polyester resin (A) was dissolved in 25 cc of a phenol / tetrachloroethane mixed solvent (mass ratio 6 / 4) and measured at 30°C.

[0072] (3) Measurement of glass transition temperature (Tg) The temperature was measured using a differential scanning calorimeter (SII, DSC-200). A 5 mg sample of polyester resin (A) was placed in an aluminum container with a retaining lid and sealed. It was cooled to -50°C using liquid nitrogen, and then heated to 150°C at a rate of 20°C / min. In the endothermic curve obtained during this process, the temperature at the intersection of the baseline before the endothermic peak and the tangent line toward the endothermic peak was defined as the glass transition temperature (Tg, in °C).

[0073] (4) Measurement of acid value 0.2 g of polyester resin (A) sample was dissolved in 40 ml of chloroform and titrated with a 0.01 N potassium hydroxide ethanol solution, resulting in polyester resin 10 6 The equivalent weight per gram (eq / ton) was determined. Phenolphthalein was used as the indicator.

[0074] <Preparation of polyester resin composition> 100 parts by mass (solids) of polyester resin (A) was dissolved in cyclohexanone / Solvesso-150 at a ratio of 1 / 1 (mass ratio) to obtain a viscosity suitable for coating. A polyester resin composition (solids content approximately 35% by mass) was obtained according to the formulations shown in Tables 2 and 3.

[0075] (5) Measurement of THF-insoluble content The THF-insoluble portion after heat treatment at 240°C for 1 hour was calculated by applying a polyester resin composition to copper foil to a thickness of 10 μm after drying, heating it at 240°C for 1 hour, and measuring 10 cm in length and 2.5 cm in width. The mass of the sample before THF immersion was (X), and the mass of the sample after immersion in 60 ml of THF at 25°C for 1 hour, followed by drying at 100°C for 10 minutes was (Y), and the mass was calculated using the following formula. THF insoluble content (mass%) = [{(Y)-mass of copper foil} / {(X)-mass of copper foil}]×100 Furthermore, the THF-insoluble content after heat treatment at 150°C for 30 minutes was determined in the same manner as above, except that the temperature was set to 150°C and the heating time was 30 minutes.

[0076] <Preparation of test specimens> A polyester resin composition was applied to one side of a tinplate (JIS G 3303 (2008) SPTE, 70 mm × 150 mm × 0.3 mm) using a bar coater to achieve a dry film thickness of 10 ± 2 μm. Curing was then performed at 240°C (PMT: maximum temperature achievable by the substrate) for 1 hour, and this was used as a test specimen (hereinafter referred to as the test specimen).

[0077] (6) Evaluation of processability The obtained test specimens were bent 180° in the direction where the hardened film faces outward, and the cracking of the hardened film at the bend was evaluated by measuring the current flow. The bending was performed without any material being inserted between the specimens (so-called 0T). An aluminum plate electrode (20mm wide, 50mm deep, 0.5mm thick) was placed on top of a sponge (20mm wide, 50mm deep, 10mm thick) soaked in a 1% NaCl aqueous solution, and the central part of the bend of the test specimen was placed in contact with the sponge so as to be parallel to the 20mm side of the sponge. A DC voltage of 5.0V was applied between the aluminum plate electrode and the unpainted area on the back of the test plate, and the current flow was measured. A smaller current flow indicates 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

[0078] (7) Evaluation of curing properties A gauze felt soaked in methyl ethyl ketone was pressed onto the hardened film surface of the test specimen so that it was in contact with a 1 cm² area, and a rubbing test was performed by applying a load of 500 g. The number of rubbing cycles required for the hardened film to peel off (one cycle counted as one back-and-forth) was evaluated according to the following criteria. (judgement) ◎: The coating did not peel off even after more than 50 applications, and no changes were observed in the hardened film. ○: After 25-49 cycles, the hardened film peeled off, exposing the tin plate. △: The hardened film peeled off after 16-24 applications, exposing the tin plate. ×: The hardened film peeled off after 15 or fewer applications, exposing the tin plate.

[0079] (8) Evaluation of retort resistance The test specimen was placed upright in a stainless steel cup, and deionized water was poured in until it reached half the height of the specimen. This was then placed in the pressure vessel of a retort testing machine (ES-315, manufactured by Tommy Industries Co., Ltd.) and subjected to retort treatment at 125°C for 30 minutes. Post-treatment evaluation was performed on the steam contact area, which is generally expected to be subjected to more severe conditions for the cured film, and the whitening and blistering of the cured film were visually judged as follows. (judgement) ◎: Good (no whitening or blistering) ○: There is slight whitening, but no blistering. △: Slight whitening and / or slight blistering present. ×: Significant whitening and / or significant blistering present.

[0080] Example of synthesis of polyester resin (A) (a) 740 parts by mass of dimethyl terephthalate, 100 parts by mass of dimethyl 2,6-naphthalenedicarboxylate, 8 parts by mass of trimellitic anhydride, 190 parts by mass of ethylene glycol, 330 parts by mass of 1,2-propanediol, 190 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% of the total acid components) were placed in a 3 L four-necked flask, and the esterification reaction was carried out while gradually raising the temperature to 230°C over 3 hours. Next, the pressure in the system was gradually reduced, and polymerization was carried out under reduced pressure to 10 mmHg over 1 hour, while raising the temperature to 250°C. Further polymerization was carried out for 50 minutes under a vacuum of 1 mmHg or less. Once the target molecular weight was reached, it was cooled to 210°C under a nitrogen atmosphere. Next, 26 parts by mass of trimellitic anhydride were added, and stirring was continued for 30 minutes at 200-230°C under a nitrogen atmosphere. This was extracted to obtain a polyester resin (synthesis example (a)). The reduced viscosity of the obtained polyester resin was 0.33 dl / g, the glass transition temperature (Tg) was 65°C, and the acid value was 300 eq / t.

[0081] Synthesis examples (b)~(t) Polyester resins (synthesis examples (b) to (t)) with resin compositions shown in Table 1 were produced by direct polymerization, similar to synthesis example (a), but with a change in the starting composition.

[0082] [Table 1]

[0083] Polyester resin compositions were prepared using the obtained polyester resin, and their processability, curability, and retort resistance were evaluated. The formulations of the polyester resin compositions and the evaluation results are shown in Tables 2 and 3.

[0084] [Table 2]

[0085] [Table 3]

[0086] As is clear from Table 2, the cured film (coating) obtained from the polyester resin composition using the polyester resin (A) of the present invention exhibits excellent processability, curability, and retort resistance. On the other hand, as shown in Table 3, in Comparative Examples 1 and 2, the acid value of the polyester resin was low, resulting in insufficient curability and poor processability and retort resistance. In Comparative Example 3, the inclusion of a curing agent resulted in poor processability. [Industrial applicability]

[0087] The present invention provides a polyester resin composition and a polyester resin aqueous dispersion with excellent processability, curability, and retort resistance, as well as paints and coatings containing the same, and is suitable as a main component for paints applied to metal cans for food and beverages, etc.

Claims

1. A polyester resin composition comprising a polyester resin (A) and a catalyst (B), characterized in that it satisfies the following requirements (i) to (vii). (i) The acid value of the polyester resin (A) is 100 eq / ton or more and 320 eq / ton or less. (ii) The hardening agent content is less than 1 part by mass per 100 parts by mass of polyester resin (A) on a solids basis. (iii) The polyester resin composition is applied to copper foil to a thickness of 10 μm after drying, and heated at 240°C for 1 hour. The mass of the sample, which measures 10 cm in length and 2.5 cm in width, before immersion in tetrahydrofuran is (X), and the mass of the sample after immersion in 60 ml of tetrahydrofuran at 25°C for 1 hour, followed by drying at 100°C for 10 minutes, is (Y). The tetrahydrofuran insoluble content calculated by the following formula is 10% by mass or more. Tetrahydrofuran insoluble content (mass%) = [{(Y) - mass of copper foil} / {(X) - mass of copper foil}] x 100 (iv) The content of catalyst (B) is 0.01 to 0.5 parts by mass per 100 parts by mass of polyester resin (A). (v) The glass transition temperature of the polyester resin (A) is 40°C or higher. (vi) The polyester resin (A) contains a polycarboxylic acid component with three or more functionalities and / or a polyol component with three or more functionalities as constituent units. (vii) Catalyst (B) is at least one selected from the group consisting of acid catalysts and amine block products of acid catalysts.

2. The polyester resin composition according to claim 1, wherein the polyester resin (A) comprises, as a polyol component constituting the polyester resin (A), a diol (a) having two primary hydroxyl groups and not having an alicyclic structure, and further comprising either or both of a diol (b) having an alicyclic structure and a diol (c) having one primary hydroxyl group and one secondary hydroxyl group and not having an alicyclic structure.

3. The polyester resin composition according to claim 1 or 2, wherein the polyester resin (A) has at least one selected from the group consisting of adipic acid, 2,6-naphthalenedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid as a polycarboxylic acid component constituting the polyester resin (A).

4. The polyester resin composition according to any one of claims 1 to 3, wherein the polyester resin (A) has an unsaturated dicarboxylic acid (d) as a polycarboxylic acid component constituting the polyester resin (A).

5. A polyester resin composition according to any one of claims 1 to 4, wherein the tetrahydrofuran insoluble content after heat treatment at 150°C for 30 minutes is less than 10% by mass.

6. A polyester resin aqueous dispersion comprising the polyester resin composition according to any one of claims 1 to 5 and an aqueous medium.

7. A paint composition containing either the polyester resin composition described in any one of claims 1 to 5 or the polyester resin aqueous dispersion described in claim 6.

8. A coating film containing the polyester resin composition according to any one of claims 1 to 5.

9. A metal can having the coating film described in claim 8.

Images