JPS6136548B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel coating agent for the inner surface of metal cans. Coating agents for the inner surface of metal cans are used for the purpose of not impairing the taste and flavor of the contents and preventing corrosion of the can material, and first of all, it must be non-toxic and resistant to heat sterilization treatment. It is required to have a small amount of eluted substances, and also to have good adhesion and processability. Conventionally, vinyl chloride polymers have been used as coating agents for the inner surfaces of metal cans, but they have the following serious problems. First, it has been pointed out that the vinyl chloride monomer remaining in vinyl chloride-based polymers is a substance that poses serious hygiene problems such as carcinogenicity. When the discarded cans are then incinerated, highly toxic and corrosive chlorine and hydrogen chloride gases are generated from the vinyl chloride polymer, leading to environmental pollution and corrosion of the incinerator. Furthermore, vinyl chloride polymers do not have sufficient adhesion to metal, which is the material for the can, and must be coated after being treated with an epoxy resin. In order to solve these problems with polyvinyl chloride coating agents, attempts have been made to use various polymers, but satisfactory results have not yet been obtained. Polyester, which has excellent adhesion to metals and does not emit toxic or corrosive gas when incinerated, has been considered as a coating agent for the inside of metal cans, but it has not been possible to obtain a material with sufficient blister resistance and whitening resistance during sterilization treatment. , improvements were requested. An example of using polyester as a coating agent for the inner surface of metal cans is JP-A-52-77142.
Publication No. 1978-78587, Japanese Patent Application Laid-open No. 1987-78587
103433, when aliphatic dicarboxylic acids are used, the blister resistance of the coating film decreases, and when bisphenol A is used, ordinary direct polymerization method or transesterification method is used. Polyester with a high degree of polymerization is difficult to obtain through polymerization. Furthermore, polyester resins containing neopentyl glycol and ethylene glycol as main components have insufficient whitening resistance. As a result of intensive research into polyester that has excellent blister resistance and whitening resistance even during sterilization treatment, the present inventors found that it contains terephthalic acid and an aromatic dicarboxylic acid other than terephthalic acid as a dicarboxylic acid component. Propylene glycol and 1,4-butanediol as glycol components
and propylene glycol if necessary, 1.4
- Polyester containing a glycol with 2 to 8 carbon atoms other than butanediol and having a reduced viscosity of 0.3 or higher not only has excellent blister resistance and whitening resistance, but also has excellent adhesion to metals, which is even more surprising. The present invention was achieved by discovering that the amount of eluted substances is extremely small. That is, in the present invention, the dicarboxylic acid component consists of 75 to 15 mol% of terephthalic acid and 25 to 85 mol% of an aromatic dicarboxylic acid other than terephthalic acid, and the glycol component consists of 65 to 20 mol% of propylene glycol (a),
1,4-butanediol (b) 5 to 80 mol% and
This is a coating agent for the inner surface of metal cans, which is mainly composed of polyester having a reduced viscosity of 0.3 or more and consisting of 30 to 0 mol% of a glycol having 2 to 8 carbon atoms other than those in (a) and (b). In the polyester of the present invention, the dicarboxylic acid component consists of 75 to 15 mol% of terephthalic acid and 25 to 85 mol% of aromatic dicarboxylic acids other than terephthalic acid,
Examples of aromatic dicarboxylic acid components other than terephthalic acid include isophthalic acid, orthophthalic acid, 2,6-naphthalene dicarboxylic acid, and 2,7-naphthalene dicarboxylic acid, but isophthalic acid is preferred in terms of flexibility and blister resistance. desirable. In the polyester of the present invention, when the terephthalic acid component is more than 75 mol% and the aromatic dicarboxylic acid component other than terephthalic acid is less than 25 mol%, and when the terephthalic acid component is less than 15 mol%, the aromatic dicarboxylic acid component other than terephthalic acid is When the content of the component exceeds 85 mol%, the whitening resistance decreases and the desired coating agent cannot be obtained. When an aliphatic dicarboxylic acid component is used as the dicarboxylic acid component, not only the whitening resistance and blister resistance are significantly reduced, but also the amount of eluted substances is significantly increased. On the other hand, the polyester of the present invention has a glycol component of 65 to 20 mol% of propylene glycol (a) and 1.4% of propylene glycol (a).
- Consisting of 5 to 80 mol% of butanediol (b) and 30 to 0 mol% of a glycol having 2 to 8 carbon atoms other than (a) and (b), but having 2 to 8 carbon atoms other than (a) and (b) Glycols of 8 include ethylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexadiol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,4- Examples include cyclohexanedimethanol and p-xylylene glycol. If propylene glycol exceeds 65% of the glycol component in the polyester of the present invention, only a low molecular weight polyester will be obtained, resulting in decreased flexibility and blister resistance. Furthermore, if the propylene glycol content is less than 20 mol%, the solubility in general-purpose solvents will be poor, making painting workability difficult. On the other hand, 1,4-butanediol is a glycol component.
If it exceeds 80 mol%, the whitening resistance and solvent solubility will decrease, and if it is less than 5 mol%, the flexibility will decrease. A particularly desirable range of 1,4-butanediol is 20 to 60 mole percent. If necessary, glycols having 2 to 8 carbon atoms other than propylene glycol and 1,4-butanediol can be used in a range of 0 to 30 mol%, but if the amount exceeds 30 mol%, As a result, the solvent solubility, flexibility, whitening resistance, and blister resistance are reduced, and the desired coating agent cannot be obtained. The polyester of the present invention needs to have a reduced viscosity of 0.3 or more. When the reduced viscosity is less than 0.3, not only the blister resistance and adhesiveness are significantly reduced, but also the amount of eluted substances increases, making it impossible to obtain the desired coating agent. The polyester of the present invention is produced by a conventional method for producing high molecular weight polyesters, such as a transesterification method or a direct esterification method. However, when considering food applications, the use of heavy metals and compounds that pose hygiene problems as catalysts and additives should be avoided. The polyester of the present invention has excellent blister resistance, whitening resistance, and adhesion, has an extremely small amount of leached substances, and has no hygienic problems, so it can be used not only for foods, water-based soft drinks, and alcoholic beverages, but also for metal containers for non-food applications. Ideal as a coating agent for internal surfaces. EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to the Examples. In the examples, parts simply indicate parts by weight. The mole % of each component in the polyester was determined by NMR analysis (solvent: CDCl ₃ ). Reduced viscosity is determined using phenol/tetrachloroethane (6/4
(weight ratio) at a concentration of 0.100 g/25 ml at 30°C. Blistering resistance and whitening resistance tests were conducted as follows. Dissolve 100 parts by weight of polyester in a mixed solvent of 200 parts by weight of methyl ethyl ketone and 200 parts by weight of toluene, and dry this solution to form a film with a thickness of 10 μm.
A 150mm x 70mm x 0.8mm mild steel plate (JIS
G3141) using a bar coater, air-dried for 30 minutes, and then dried at 150°C for 4 minutes to obtain a test piece. This test piece was immersed in boiling distilled water for 1 hour,
The occurrence of blisters and whitening were visually determined. A case where there was no abnormality in the coating film was judged as good, and a case where blistering or whitening occurred was judged as poor. The adhesion test was conducted in the following manner to also evaluate processability. 100 parts by weight of polyester is dissolved in a mixed solvent of 200 parts by weight of methyl ethyl ketone and 200 parts by weight of toluene, and the film thickness after drying of this solution is
Apply it to a 150mm x 70mm x 0.3mm tin plate (JIS G3303) using a bar coater to give a thickness of 10μ.
After air drying for 30 minutes, the sample was dried at 150° C. for 4 minutes to obtain a test piece. Using this test piece, a bending test (JIS
K5400: Mandrel diameter: 2 mm, auxiliary plate thickness: 4 mm).If no cracks occurred at the bent part, it was judged as good, and if cracks occurred, it was judged as bad. The amount of extracted substances was measured by the following method according to FDA No. 121.2514. Dissolve 100 parts by weight of polyester in a mixed solvent of 200 parts by weight of methyl ethyl ketone and 200 parts by weight of toluene, and apply a bar coater to both sides of aluminum foil measuring 300 mm x 200 mm x 0.05 mm so that the film thickness after drying is 10 μm. A test piece was obtained by applying the coating using a paint, air-drying it for 30 minutes, and then drying it at 150°C for 4 minutes. This test piece was mixed with water (250〓), n-heptane (150〓) and 8% ethanol (150〓).
After soaking for 2 hours, the soaking liquid was concentrated and dried, and the amount of extracted material per square inch was measured. Example 1 In a stainless steel autoclave equipped with a stirrer, a thermometer, and a partial reflux condenser, 194 parts of dimethyl terephthalate, 194 parts of dimethyl isophthalate, 200 parts of propylene glycol, 1.
120 parts of 4-butanediol, ethylene glycol
27 parts and 0.27 parts of tetra-n-butyl titanate were added, the temperature was gradually raised, and the reaction was continued for 4 hours until the reaction temperature reached 220°C. Next, the temperature was raised to 265°C, the pressure of the reaction yarn was gradually reduced, and the reaction was carried out for 1 hour and 30 minutes under a reduced pressure of 0.2 mm or more to obtain polyester (A). The obtained polyester (A) was pale yellow and transparent with a reduced viscosity of 0.680. The composition measured by NMR was as follows. Dicarboxylic acid component Terephthalic acid 50 mol% Isophthalic acid 50 mol% Glycol component Propylene glycol 52 mol% 1,4-butanediol
40 mol% Ethylene glycol 8 mol% Various polyesters (B to G) shown in Table 1 were produced in the same manner. Table 1 shows the reduced viscosity of each polyester and the results of compositional analysis measured by NMR. In the table, each component indicates the number of moles. Table 2 shows the test results of coating films obtained using each polyester.

【table】

【table】

[Table] Comparative Example 1 Various polyesters (H to P) shown in Table 3 were produced using the same reaction apparatus as in Example 1 and in the same manner as in Example 1. Table 3 shows the reduced viscosity of each polyester and the results of compositional analysis measured by NMR. In the table, each component indicates the number of moles. Table 4 shows the test results for coatings obtained using each polyester.

【table】

【table】

[Table] Comparative Example 2 A thermoplastic polyester resin with a reduced viscosity of 0.550 and a softening temperature of 135°C, Toyobo Vylon 200 (manufactured by Toyobo Co., Ltd.), or a thermoplastic polyester resin with a softening temperature of 90°C and a glass transition temperature of 4°C. A coating film test was conducted in the same manner as in Example 1 using a plastic polyester resin, Toyobo Vylon 300 (manufactured by Toyobo Co., Ltd.), and a thermoplastic polyester resin, Dynapol L-411 (manufactured by Dynamite Nobel Co., Ltd.). The results are shown in Table 5.

【table】

【table】

Claims (1)

[Claims] 1 The dicarboxylic acid component is 75 to 15 mol% of terephthalic acid and aromatic dicarboxylic acids other than terephthalic acid.
It consists of 25 to 85 mol%, and the glycol components are propylene glycol (a) 65 to 20 mol%, 1,4-butanediol (b) 5 to 80 mol%, and carbon atoms other than (a) and (b) 2 to 2. A coating agent for the inner surface of metal cans, characterized in that the coating agent is mainly composed of polyester having a reduced viscosity of 0.3 or more and comprising 30 to 0 mol% of glycol No. 8.