JPH0237374B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a can coating composition used as a repair coating for can body joints.

Conventionally, metal can bodies have generally been joined together by lock seaming and soldering. In recent years, a method in which both cut ends of can body materials are overlapped and joined by welding (welded can) has become popular. With such welded cans, the cut end surface remains unpainted.
Since it is exposed on the inner surface of the metal can, it has the disadvantage of being susceptible to corrosion. For this reason, it is common practice to repaint the cut end surfaces after joining. However, it is difficult to completely cover the edges of the cut end surface, and many proposals have been made regarding repair methods and coating compositions.

JP-A-54-138039 discloses repair coating of cut end surfaces using a dispersion liquid in which polyamide powder is dispersed in a poor solvent having a specific gravity substantially equal to that of the polyamide.
According to the research conducted by the present inventors, since the dispersion medium is limited to a poor solvent having substantially the same specific gravity, it is difficult to control the drying rate, and there is no drying time, which has the disadvantage of increasing can manufacturing costs. In addition, polyamide resin has poor adhesion to metals, so it has the disadvantage that it easily peels off when subjected to severe sterilization treatment such as boiling. In addition, in JP-A-56-13350, a thermoplastic resin was mixed in a thermosetting resin solution with a ratio of 95:5 to 25:
A dispersion liquid dispersed at a volume ratio of 75% is used for repair painting. When the can body joint of a welded can is repaired using this method, a repair coating with excellent shielding properties and corrosion resistance can be obtained on the cut end surface. If necessary, there is a drawback that foaming tends to occur during the drying process.

The inventors of the present invention conducted intensive research on repair coating of the can body joints of welded cans, and as a result, discovered a new paint composition for cans.

That is, the present invention comprises 1 to 70 parts by weight of a carboxyl group-containing polyolefin resin and 30 to 99 parts by weight of a polyamide resin in a liquid medium in which a soluble resin is dissolved as necessary in an organic solvent containing at least an aromatic solvent. The object of the present invention is to provide a can body joint repair paint made by dispersing in a solid state.

An important feature of the present invention is that a carboxyl group-containing polyolefin resin is used as a component of the film-forming material, and that the majority of the film-forming material is dispersed in a liquid medium. The carboxyl group-containing polyolefin resin itself has particularly excellent adhesion to metals, and when used as part of a coating film-forming substance, it has the effect of significantly improving the adhesion between the coating film and the metal. The repair coating for the joint of the can body of a welded can varies depending on the thickness of the metal plate used, but usually a coating thickness of about 30 to 200 microns is required to completely cover the cut end surface. Furthermore, since the repair coating film undergoes processing steps such as flanging and seaming, it is necessary to use a polyamide resin with excellent workability as the coating film forming material. However, these thermoplastic resins have poor adhesion to metals, and when metal cans are filled with contents and subjected to harsh treatments such as boiling, they peel off, causing corrosion. . The present invention improves adhesion to metal by using carboxyl group-containing polyolefin resin as a component of the paint, which allows it to be sterilized not only by boiling but also by steam sterilization at 120 to 130°C. This made it possible.

In addition to improving adhesion, the carboxyl group-containing polyolefin resin has the effect of preventing the polyamide resin from settling in a liquid medium. Most organic solvents, including aromatic solvents, that are normally used as liquid media have a specific gravity of 1 or less, but the specific gravity of polyamide resin that is dispersed in a solid state is greater than 1, so a special dispersant is required. Polyamide resins typically settle unless used. Although it is possible to coat a dispersion liquid that tends to settle, any method used significantly reduces workability. Carboxyl group-containing polyolefin resin particles are dispersed in a considerably swollen state in a liquid medium containing an aromatic solvent compared to polyamide resin particles, making the system thixotropic. Therefore, it is extremely effective in preventing sedimentation of polyamide resin particles. The aforementioned Japanese Patent Application Publication No. 1973-
138039 uses a solvent with substantially equal specific gravity for the purpose of preventing sedimentation, but according to the present invention, there is no need to limit the solvent to a solvent with equal specific gravity, and a solvent with good workability for painting and drying can be freely used. You can choose.

Coating films formed from carboxyl group-containing polyolefin resins and polyamide resins generally have insufficient corrosion resistance compared to epoxy/phenol paints and epoxy/amino paints. This may be mainly due to the adhesion to metals, or may be due to the content resistance of the coating itself. Problems caused by adhesion can be improved by using a carboxyl group-containing polyolefin resin, but it is difficult to improve the coating film's resistance to contents with just the thermoplastic resin mentioned above. It is. In the present invention, the liquid medium is one or more thermosetting resins selected from epoxy resins, phenolic resins, amino resins, thermosetting acrylic resins, and thermosetting polyester resins (including alkyd resins and oil-free alkyd resins). It is preferable to use a solution of The aforementioned Japanese Patent Application Publication No. 1983-
13350 discloses a technology that uses a thermosetting resin and a thermoplastic resin in combination, creating a coating structure in which the thermosetting resin is the continuous phase and the thermoplastic resin is the dispersed phase. The present invention is a method of forming a continuous coating film by heating and melting the thermoplastic resin, so there is no need to limit the thermosetting resin to 25% by volume or more in the coating film, and a smaller amount is sufficient. It can be used.

As detailed above, the present invention prevents sedimentation of a dispersion of polyamide resin particles and significantly improves adhesion to metal as a coating film by partially using a carboxyl group-containing polyolefin resin. The present invention provides a can coating composition that is used for repair painting of can body joints and has improved corrosion resistance.

The constituent elements used in the present invention will be explained below.

The carboxyl group-containing polyolefin resin used in the present invention includes olefinic monomers such as ethylene, propylene, or butene, and ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, and maleic anhydride. It is a product obtained by copolymerizing an acid, or a product obtained by graft polymerizing an ethylenically unsaturated carboxylic acid or its acid anhydride onto a polyolefin such as polyethylene, polypropylene, or polybutene. Alternatively, it may be copolymerized or graft polymerized using an ester of ethylenically unsaturated carboxylic acid, and then the ester group is saponified to convert it into a carboxyl group. When the can body is subjected to high-temperature steam sterilization, high-density polyethylene, polypropylene, or the like is preferable as the polyolefin. The amount of ethylenically unsaturated carboxylic acid is the olefin component
It is preferably 0.01 to 50 parts by weight per 100 parts by weight.
If it is less than 0.01 part by weight, the effect of adhesion and prevention of sedimentation is weak, and if it is more than 50 parts by weight, the effect of improving adhesion is not observed and processability is reduced. Any known method can be used to disperse such carboxyl group-containing polyolefin resin in a solid state in a liquid medium. For example, a method of obtaining a dispersion liquid by emulsion polymerization or suspension polymerization of constituent monomers, a method of dispersing a resin powder obtained by crushing a resin under cooling in a liquid medium, a method of dispersing a resin powder at a high temperature, and a method of dispersing a resin powder at a high temperature. A method in which a dispersion liquid is obtained by cooling and precipitation, a method in which a resin solution is brought into contact with a non-solvent to precipitate and a dispersion liquid is obtained, a method in which a resin solution is sprayed into an air stream and dried to obtain a resin powder, and the resin powder is obtained and dispersed in a liquid medium. method etc. can be taken. The average particle size of the carboxyl group-containing polyolefin resin in the liquid medium is preferably 100 microns or less. 100
If it exceeds a micron, adhesion properties are not affected, but the effect of preventing sedimentation is weak and painting work is reduced. Further, in terms of preventing sedimentation, a particle size of 20 microns or less is preferable, but the average particle size does not need to be 20 microns or less, and it is effective as long as microparticles of 20 microns or less are present in the particles.

The polyamide resin used in the present invention together with the carboxyl group-containing polyolefin resin is the main component of the coating film, and has excellent processability and flexibility.
Even if the contents are food, there is no problem with hygiene. Polyamide resins are, for example, 6-, 6.6-, 6.10-,
11-, 12-polyamide resins or copolymers and mixtures thereof can be used. In addition, the particle size in the polyamide resin dispersion is expressed as the average particle diameter.
It is preferably 100 microns or less, and a smoother coated surface can be obtained if it is 60 microns or less.

The liquid medium used in the present invention has drying properties,
It is necessary to use aromatic solvents such as toluene and xylene, which have excellent coating suitability, but it is also possible to use a mixed solvent of aromatic organic solvents and other solvents such as ester solvents. . When strong corrosion resistance is required for repair coatings, thermosetting resins such as epoxy resins, phenolic resins, amino resins, thermosetting acrylic resins, and thermosetting polyester resins can be dissolved in the liquid medium. can. As these thermosetting resins, use of epoxy-phenol, epoxy-amino, acrylic-amino, and polyester-amino resins significantly improves corrosion resistance. In addition, in order to improve the adhesion between the inner coating film applied to the inner surface of the can (other than the repaired area) and the repair coating film, in addition to the thermosetting resins mentioned above, polyvinyl chloride resin, vinyl chloride copolymer, Thermoplastic resins such as plastic acrylic resin, thermoplastic polyester resin, and polybutadiene resin can also be dissolved. These thermosetting resins or thermoplastic resins provide an auxiliary function to the continuous film of carboxyl group-containing polyolefin resin and polyamide resin that are dispersed in a solid state in the dispersion liquid, and the amount is particularly limited. It can be used without any restrictions and is sufficiently effective even at 25% by volume or less in the coating film-forming substance.

The solid content of the thus obtained dispersion liquid can coating composition may be within a range that provides appropriate workability and suitability for thick coating, preferably 5 to 50% by weight. Materials for cans to which the can coating composition of the present invention is applied include tinplate, galvanized steel sheets, chrome-plated steel sheets, steel sheets chemically treated with chromic acid, phosphoric acid, etc., chemically treated steel sheets, and untreated steel sheets. Treated steel plates and even light metal plates such as aluminum are used. It can be applied to the can body joint, regardless of the purpose or shape of the can.

The coating composition for cans according to the present invention can be applied to the can body joint by any method such as roller coating, spray coating, flow coating, brush coating, and dip coating. After coating, a continuous coating film is created by heating and melting the carboxyl group-containing polyolefin resin and polyamide resin above their softening points. It is preferable to go through the process. Heating methods include direct heat, hot oven,
Induction heating, resistance heating, infrared heating, etc. can be used, and appropriate conditions can be selected from the range of 1 second to 10 minutes at 150° C. to 400° C.

The can coating composition according to the present invention is particularly useful for repair painting of can body joints of welded cans;
It can also be used to repair and paint the joints of solder can bodies.

Hereinafter, the present invention will be specifically explained using examples. The middle part of the example shows parts by weight.

Example 1 10 parts of maleic acid grafted polypropylene (Toa Fuel Industries Co., Ltd. trade name, C-902X-MA) and 40 parts of xylene were heated to 120°C with stirring, and then stirred into 50 parts of cold xylene. The mixture was added dropwise to obtain a white dispersion with an average particle size of 15 microns. 100 parts of the obtained dispersion, 210 parts of toluene, and 90 parts of polyamide-11 powder (average particle size 38 microns) manufactured by Nippon Rilsan Co., Ltd. were thoroughly stirred to obtain a dispersion. The dispersion liquid showed a uniform dispersion state and no change was observed even after one week.

The obtained dispersion was sprayed onto the can body joint of a welded can (chromium-treated plate 18 cans) with a spray gun (nozzle diameter 0.55 mm).
Then, we applied a repair coat to a width of 15 mm and a thickness of about 100 microns, and immediately blown it with 250℃ hot air from the back side for 60 seconds to dry and melt it.
It was used as a repair coating. I scratched the repair coating with a knife and peeled it off, but no peeling occurred. In addition, the repaired sections were immersed in soy sauce and stored at 50°C for 3 months, but no changes were observed.

Comparative Example 1 200 parts of toluene and 90 parts of polyamide 11 powder (average particle size 38 microns) manufactured by Nippon Rilsan Co., Ltd. were sufficiently stirred to form an unstable dispersion in which the particles settled within a few minutes. Ta. While thoroughly stirring the dispersion liquid, welded a can (made of chromium-treated steel plate 18 mm) in the same manner as in Example 1.
Repair painting and dry melting were performed on the can body joints of cans. When the repaired paint film was scratched with a knife and peeled off, the paint film was easily peeled off. Furthermore, when the repaired section was immersed in soy sauce and stored at 50°C for 3 months, rust was observed at the gas-liquid interface and in the gas phase.

Example 2 8 parts of bisphenol A epoxy resin (trade name, Epicoat 1007, manufactured by Yuka Ciel Epoxy Co., Ltd.) and 4 parts of urea resin (trade name, Betsukamine P-138, manufactured by Dainippon Ink Chemical Co., Ltd.) were mixed with 9 parts of toluene. 1 part, 30 parts of an epoxy/urea resin solution dissolved in 9 parts of ethylene glycol monomethyl ether, and a maleic acid grafted polypropylene dispersion prepared in the same manner as in Example 1.
100 parts of toluene, 180 parts of toluene, and 90 parts of polyamide-11 powder (average particle size 38 microns) manufactured by Nippon Rilsan Co., Ltd. were thoroughly stirred to obtain a dispersion. The dispersion liquid showed a uniform dispersion state, and no change was observed even after one week. The resulting dispersion was applied to the joints of the can bodies of welded cans (18 chromium-treated steel plates) for repair in the same manner as in Example 1, and then dried and melted. The resulting repaired coating film was scratched with a knife and peeled off, but no peeling occurred. In addition, the repaired section was immersed in soy sauce and heated to 50°C for 3 days.
Although it was stored for several months, no change was observed. In addition, the repaired sections obtained in Example 1 and Comparative Example 2 were immersed in an aqueous solution containing 2% sodium chloride and citric acid, respectively, and stored at 50°C for 3 months. The section showed considerable corrosion, and peeling of the paint film was also observed. Although no peeling was observed in the sample of Example 1, some corrosion was observed at the gas-liquid interface. In contrast, no change was observed in the sections of this example.

Comparative Example 2 100 parts of polypropylene (MI value 10), 20 parts of maleic anhydride and 375 parts of xylene were placed in a flask, heated and dissolved at 130°C, and 0.1 part of benzoyl peroxide was added and reacted by heating for 60 minutes. Cooled to room temperature over several minutes. After filtering the resulting suspension, unreacted maleic anhydride was removed with methyl ethyl ketone.
Washed twice and dried. The obtained polypropylene-maleic anhydride copolymer was a powder with an average particle size of 34 microns. 100 parts of this powder was sufficiently stirred in 200 parts of ethyl alcohol to obtain a dispersion. The dispersion liquid exhibited a uniform dispersion state and was stable.

Using the obtained dispersion liquid, repair coating was applied to the joint portion of the can body of a welded can in the same manner as in Example 1, followed by drying and melting. When the resulting repaired coating was scratched with a knife, cracks occurred around the scratches. In addition, cracks appeared in the 180° bending test, and the flexibility was insufficient to be used as a repair coating for joints.

Claims (1)

[Scope of Claims] 1. A liquid medium containing 1 to 70 parts by weight of a carboxyl group-containing polyolefin resin and 30 to 99 parts by weight of a polyamide resin, in which a soluble resin is dissolved as necessary in an organic solvent containing at least an aromatic solvent. Can body joint repair paint that is dispersed in a solid state. 2 Soluble resin is epoxy resin, phenolic resin, amino resin, acrylic resin, vinyl chloride resin, vinyl chloride copolymer, polyester resin,
The can body joint repair paint according to claim 1, which is one or more of the group consisting of polybutadiene resins.