Process for coating the inside surface of a metallic can with a water-base paint

Abstract: A water-base paint for coating the inside surface of a metallic can, said paint comprising fine particles of a polyester resin dispersed in an aqueous medium in the presence of a surface-active agent, wherein (A) the polyester resin comprises units derived from a dicarboxylic acid and units derived from a diol, the units derived from the dicarboxylic acid containing units derived from terephthalic acid and units derived from isophthalic acid in a mole ratio of from 10:90 to 80:20, and at least 30 mole % of the units derived from the diol being derived from ethylene glycol, (B) the surface-active agent is composed of at least one compound selected from alkylaryl sulfonic acid salts, alkylsulfonic acid salts, dialkylsulfosuccinic acid salts, alkylphosphoric acid salts, polyoxyethylene alkylarylether sulfonic acid salts and polyoxyethylene alkylether sulfonic acid salts, and (C) the fine particles of the polyester resin have an average particle diameter of not more than 3 microns.

Description: This invention relates to a process for coating the inside surface of a metallic can with a water-base paint of the aqueous dispersion type which comprises fine particles of a thermoplastic polyester resin very stably dispersed in an aqueous medium. The inside surfaces of metallic cans for holding drinks such as fruit juices, carbonated beverages and alcoholic drinks are usually coated for anti-corrosion or preventing degeneration of such contents owing to the dissolving of the metal. Vinyl chloride/vinyl acetate copolymer and epoxy resins are mainly used as resins for coating the inside surfaces of such metallic cans. Japanese Patent Publication No. 10062/1985 discloses a paint for coating the inside surface of a metallic can, comprising as a main component a thermoplastic polyester resin derived from a dicarboxylic acid component at least 90 mole % of which consists of terephthalic acid and a glycol component composed of 30 to 70 mole % of ethylene glycol, 0.1 to 2.5 mole % of a polyethylene glycol having a molecular weight of 60 to 6,000 and the remainder being diethylene glycol. The above paint comprising the thermoplastic polyester as a main component proves satisfactory for use in coating the inside of a metallic can in respect of adhesion to metal, strength, baking temperature and melting point. However, in the aqueous dispersion type, the dispersed resin particles cannot be made sufficiently small, and the coated film cannot be made sufficiently thin. GB-A-117085 discloses a process for improving the surface characteristics of polyester articles by applying thereto a copolyester at least 85% of the units of which are derived from a water-soluble polyoxyalkylene glycol, terephthalic acid and ethylene glycol. Part of the terephthalic acid can be replaced by, for example, isophthalic acid. US-A-3,563,942 describes solvent-soluble linear copolyesters, for use in coating compositions, which are rendered dispersible in aqueous media by the addition of a metal salt of a sulfonated aromatic compound, such as sodium dimethyl-5-sulfoisophthalate. The linear copolyesters can derive from, for example, ethylene glycol and, as dicarboxylic acids, either (a) terephthalic acid and sebacic acid or (b) terephthalic acid, isophthalic acid and sebacic acid. It has recently become necessary from an economic view-point to reduce the thickness of a coating on the inside surface of a metallic can. With the foregoing background, we have made extensive investigations in order to develop a paint of the aqueous dispersion type comprising a thermoplastic polyester as a main component in which the polyester resin particles can be stably dispersed as very fine particles, and which can be coated in a thin layer on the inside surfaces of a metallic can without there being a problem. According to this invention, there is provided a process for coating the inside surface of a metallic can with a water-based paint, said paint comprising fine particles of a polyester resin dispersed in an aqueous medium in the presence of a surface-active agent, wherein (A) the polyester resin comprises units derived from an aromatic dicarboxylic acid and units derived from a diol, the units derived from the aromatic dicarboxylic acid containing units derived from terephthalic acid and units derived from isophthalic acid in a mole ratio of from 10:90 to 80:20, and at least 30 mole % of the units derived from the diol being derived from ethylene glycol, (B) the surface-active agent is composed of at least one dodecylbenzene sulfonic acid salt, and (C) the fine particles of the polyester resin have an average particle diameter of not more than 2 µm, the process comprising (a) preparing the paint by mixing a solution of the polyester resin in an organic solvent selected from halogenated hydrocarbons and anisole with an aqueous medium containing the surface-active agent with stirring at a high speed to disperse the polyester resin, and then removing the organic solvent, and (b) coating the inside surface of a metallic can with the resulting paint. The paint used in accordance with this invention for coating the inside surface of a metallic can will now be described in greater detail. Polyster resin The polyester resin used in the process of this invention basically contains units (to be referred to as the "aromatic dicarboxylic acid component") derived from an aromatic dicarboxylic acid and units (to be referred to as the "diol component") derived from a diol. The aromatic dicarboxylic acid component consists essentially of units derived from terephthalic acid and units derived from isophthalic acid, and may contain third structural units as required. The mole ratio of the units derived from terephthalic acid and to the units derived from isophthalic acid is from 10:90 to 80:20, preferably 30:70 to 10:90, especially preferably from 40:60 to 60:40. Examples of other aromatic dicarboxylic acids that can form the third structural units which may be included as desired are 2,6-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid and phthalic acid. These other aromatic dicarboxylic acids may be used singly or in combination with each other. The content of the units derived from these dicarboxylic acids is not more than 60 mole %, preferably not more than 30 mole %, and especially preferably 20 to 0 mole %, based on the total amount of the aromatic dicarboxylic acid component. At least 30 mole %, preferably at least 35 mole %, especially preferably at least 40 mole %, of the diol component constituting the polyester resin consists of units derived from ethylene glycol. The diol component may be substantially composed of only the units derived from ethylene glycol, but according to the properties desired of the polyester resin, it may contain units derived from another diol and/or a polyhydric alcohols. Examples of the other diol are diethylene glycol and polyethylene glycols having a molecular weight of 600 to 6,000. They may be used singly or in combination with each other. The content of the units derived from the other diol is at most 70 mole %, preferably at most 75 mole %, especially preferably 50 to 0 mole %, of the total amount of the diol component. Examples of the polyhydric alcohol containing at least 3 hydroxyl groups are trimethylmethane, trimethylolethane and trimethylolpropane. If the content of the units derived from the polyhydric alcohols is too high, the resulting polyester resin is susceptible to gellation. Desirably, therefore, it is limited to at most 10 mole %, preferably not more than 5 mole %, especially preferably not more than 3 mole %. The above polyester resin composed of the aromatic dicarboxylic acid component and the diol component can be produced by esterification reaction of the aromatic dicarboxylic acids and the diols in accordance with a known method. It may also be produced by mixing the aromatic dicarboxylic acid ester and the diol, performing ester-interchange reaction of the aromatic dicarboxylic acid ester, and further subjecting the materials to polycondensation reaction. The esterification reaction or the polycondensation reaction may be carried out in the presence of a catalyst such as germanium oxide, antimony trioxide, tetrabutyltitanate, tetrabutoxy titanate, manganese acetate, calcium acetate, zinc acetate, lead acetate or p-toluenesulfonic acid. Additives normally used in the field of paints, for example, antioxidants, flow regulating agents and blocking preventing agents can be added. The polyester resin used in the process of this invention is film-forming. Usually, the polyester resin has an intrinsic viscosity, measured in ortho-fluoro-phenol at 25°C, of 0.4 to 1.5 dl/g, preferably 0.6 to 1.2 dl/g. Surface-active agent In the preparation of the water-base paint at least one dodecylbenzene sulfonic acid salt is used as a dispersing agent for dispersing the polyester resin in an aqueous medium. The use of this specific anionic surface-active agent enables the polyester resin to be dispersed stably in an aqueous medium as fine particles with an average particle diameter of not more than 3 µm. The amount of the surface-active agent used is not critical and may be varied depending upon the type of the polyester resin and the surface-active agent. Generally, it is 0.2 to 20 parts by weight, preferably 0.25 to 10 parts by weight, especially preferably 0.5 to 5.0 parts by weight, per 100 parts by weight of the polyester resin. Preparation of the water-base paint The water-base paint is prepared by dissolving the polyester resin in the organic solvent, dispersing the resulting organic solvent solution of the polyester in an aqueous medium containing the specific surface-active agent and then removing the organic solvent. The organic solvent for the polyester is one which has a great dissolving power for the polyester resin and can be easily removed from the dispersion obtained by dispersing the organic solvent solution of the polyester resin in the aqueous medium. Examples of the halogenated hydrocarbons which can be used are chloroform, methylene chloride, carbon tetrachloride, 1,1-dichloroethane, trichloroethane, tetrachloroethane, chlorobenzene, 1,3-dichloro-2-methylpropane and o-chlorotoluene. Anisole may also be used. These organic solvents may be used singly or in combination with each other. Among the above organic solvents those having a boiling point of 30 to 160°C are preferred. Methylene chloride and anisole are especially preferred. The concentration of the polyester resin in the solution in the organic solvent is not particularly limited. In view of handlability in the coating operation and dispersibility in the dispersing operation, the concentration of the polyester resin is preferably 1 to 20 % by weight in general, and 5 to 15 % by weight in particular. The surface-active agent is added to an aqueous medium such as water to prepare a uniform aqueous solution, and then the aqueous solution is mixed with the organic solvent solution of the polyester resin with stirring at a high speed to disperse the polyester resin (including the organic solvent) in the aqueous solution. The concentration of the surface-active agent in the aqueous solution is not critical. Conveniently, it is generally 0.01 to 10 % by weight, preferably 0.1 to 3 % by weight. Mixing of the organic solvent solution of the polyester resin and the aqueous solution of the surface-active agent is carried out by using a high-speed stirring device such as a homomixer. The stirring conditions are set so that the average particle size of the dispersed phase (the organic solvent solution of the polyester resin) will become not more than 3 µm. Specifically, the stirring speed is usually 3,000 to 15,000 rpm, and the stirring time is usually 2 to 120 minutes. After the polyester resin containing the organic solvent is dispersed in the aqueous medium, the organic solvent is removed by, for example, heating the dispersion of the polyester resin containing the organic solvent under reduced pressure to a temperature near the boiling point of the organic solvent, for example by using an evaporator. As a result of removing the organic solvent, an aqueous dispersion is obtained in which fine particles of the polyester resin having an average particle diameter of not more than 3 µm, preferably not more than 2 µm, especially preferably not more than 1 µm, are dispersed in the aqueous medium stably by the aid of the surface-active agent. The content of the fine polyester resin particles in the aqueous dispersion is not strictly limited, and can be varied over a wide range according to, for example, the method of coating the paint of this invention. Generally, the suitable content of the fine polyester resin particles is 10 to 70 % by weight, preferably 20 to 60 % by weight, especially preferably 25 to 50 % by weight, based on the weight of the aqueous dispersion. The aqueous dispersion so prepared has very good dispersion stability since the fine polyester resin particles are stably dispersed in the aqueous medium by the specific surface-active agent. The adjoining fine particles do not agglomerate, and even with the lapse of time, the fine particles hardly agglomerate and change in particle diameter. Since the paint used in the process of this invention is an aqueous dispersion of the polyester resin particles, it does not contaminate the working environment by organic solvent as is the case with an organic solvent dispersion, nor does it require a large-sized apparatus as in the case of using a powder paint. In addition, the coated layer of the paint on the inside of a metallic can does not give off a malodor as in the case of using an epoxy resin, for example, because its main component is the polyester resin. The water-base paint can be coated on the inside surface of a metallic container for holding drinks or the like by methods known per se , for example spray coating, roll coating or flow coating. In a preferred embodiment, the polyester resin is dispersed in the aqueous medium as completely spherical particles. The following Examples illustrate the present invention more specifically. The average particle diameter of the polyester resin, as used in the present specification and the appended claims, is measured by a microtrack particle size analyzer (supplied by Leeds & Northrup Instruments Nikkisou Co., Ltd.). EXAMPLE 1 A slurry composed of 50 parts by weight of terephthalic acid, 50 parts by weight of isophthalic acid and 100 parts by weight of ethylene glycol was fed into a continuous esterification reactor, and esterified at 250 °C under atmospheric pressure to give a reaction product having an esterification degree of 95 %. Germanium dioxide (0.016 part by weight) as a catalyst was added per 124 parts by weight of the esterification product, and polycondensation was carried out at 280 °C under a reduced pressure of 0.5 mmHg. The polycondensation reaction product was extruded as a strand from a nozzle, cooled and cut to obtain a prepolymer. The prepolymer was subjected to a preliminary crystallization treatment in a vacuum dryer at 160 °C for 2 hours. Subsequently, 400 g of the treated product was introduced into a vacuum tumbler-type solid-phase polymerization device and subjected to solid phase polymerization. The resulting polyester resin (terephthalic acid/isophthalic acid/ethylene glycol=50 moles/50 moles/100 moles) had an intrinsic viscosity, measured in orthochlorophenol at 25 °C, of 0.8 dl/g. Twenty grams of the polyester resin was dissolved in 180 g of methylene chloride to obtain a 10 % by weight solution of the polyester resin (solution A). Separately, 4 g of sodium dodecylbenzenesulfonate (NEOPELEX F-25, a surface-active agent produced by Kao Co., Ltd.; concentration 25 % by weight) was dissolved in 96 g of distilled water to give an aqueous solution (B) of the surface-active agent in a concentration of 1 % by weight. Then, 200 g of the solution (A) and 100 g of the aqueous solution (B) were put in a 0.5-liter homo-mixer, and stirred at 10,000 rpm and 15 °C for 1.0 hour to disperse the polyester resin in water. The resulting aqueous dispersion was heated at 35 to 45 °C under atmospheric pressure for 3 hours by using an evaporator to evaporate the methylene chloride substantially. The polyester resin was dispersed as fine particles having an average particle diameter of 0.6 µm in the resulting aqueous dispersion. The dispersed state of the particles was very good and stable with the lapse of time. EXAMPLE 2 Example 1 was repeated except that anisole was used as the organic solvent instead of methylene chloride. The polyester resin particles in the resulting aqueous dispersion had an average particle diameter of 0.9 µm. COMPARATIVE EXAMPLE 1 Example 1 was repeated except that a sorbitan-type nonionic surface-active agent having an HLB of 8.6 (RHEODOL SP-L10, a tradename for a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be dispersed well. COMPARATIVE EXAMPLE 2 Example 1 was repeated except that a sorbitan-type nonionic surface active agent having an HLB value of 4.3 (RHEODOL SP-110, a tradename for a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 3 Example 1 was repeated except that monoglyceride of medium purity having an HLB of 2.8 (EXEL 300, a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 4 Example 1 was repeated except that a sorbitan-type nonionic surface active agent having an HLB value of 1.8 (RHEODOL SP-030, a tradename for a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 5 Example 1 was repeated except that POE type nonionic surface-active agent having an HLB of 10.5 (EMULGEN 705, a tradename for a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 6 Example 1 was repeated except that POE type nonionic surface-active agent (EMULGEN 931, HLB 17.2, a tradename for a product of Kao Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 7 Example 1 was repeated except that potassium oleate was used instead of sodium dodecylbenzenesulfonate. The polyester resin could not be well dispersed in water. COMPARATIVE EXAMPLE 8 Example 1 was repeated except that a saponification product of polyvinyl acetate (PVA GH-17, a tradename for a product of Nihon Gosei Co., Ltd.) was used instead of sodium dodecylbenzenesulfonate. The resulting polyester resin particles in the aqueous dispersion had an average particle diameter of 4.9 µm. The dispersed state of the particles was good immediately after dispersion. But with time, it became unstable. As the time elapsed, the particles agglomerated and within a short period of time, the polyester particles sedimented and could not be redispersed. EXAMPLE 3 By using a slurry composed of 50 parts by weight of terephthalic acid, 50 parts of isophthalic acid and 50 parts by weight of ethylene glycol, a prepolymer and a polymer were synthesized in the same way as in Example 1. The polymer was then treated in the same way as in Example 1 to give an aqueous dispersion of the polyester resin. The fine polyester resin particles in the aqueous dispersion had an average particle diameter of 1.0 µm. EXAMPLE 4 By using a slurry composed of 70 parts by weight of terephthalic acid, 30 parts of isophthalic acid and 100 parts by weight of ethylene glycol, a prepolymer and a polymer were synthesized in the same way as in Example 1. The polymer was then treated in the same way as in Example 1 to give an aqueous dispersion of the polyester resin. The fine polyester resin particles in the aqueous dispersion had an average particle diameter of 1.0 µm. EXAMPLE 5 By using a slurry composed of 40 parts by weight of terephthalic acid, 60 parts of isophthalic acid and 100 parts by weight of ethylene glycol, a prepolymer and a polymer were synthesized in the same way as in Example 1. The polymer was then treated in the same way as in Example 1 to give an aqueous dispersion of the polyester resin. The fine polyester resin particles in the aqueous dispersion had an average particle diameter of 1.1 µms. COMPARATIVE EXAMPLE 9 By using a slurry composed of 100 parts by weight of terephthalic acid and 100 parts by weight of ethylene glycol, a prepolymer and a polymer were synthesized in the same way as in Example 1. The polymer was then treated in the same way as in Example 1 to give an aqueous dispersion of the polyester resin. The fine polyester resin particles in the aqueous dispersion had an average particle diameter of 9.8 µms. Each of the aqueous dispersions obtained in Examples 1 to 5 was coated on an aluminum or iron plate, and the coating was subjected to a crosscut tape test. The result was 100/100 in all runs, and the coated films showed good adhesion. When the coated iron plates were left to stand for one month, no rust occurred. Separately, the paint prepared in Comparative Example 9 was coated on an aluminum or iron plate and the coated plate was subjected to a crosscut tape test. The result was 100/100. But when the coated iron plate was left to stand for one month, rust occurred.

Claims: A process for coating the inside surface of a metallic can with a water-based paint, said paint comprising fine particles of a polyester resin dispersed in an aqueous medium in the presence of a surface-active agent, wherein (A) the polyester resin comprises units derived from an aromatic dicarboxylic acid and units derived from a diol, the units derived from the aromatic dicarboxylic acid containing units derived from terephthalic acid and units derived from isophthalic acid in a mole ratio of from 10:90 to 80:20, and at least 30 mole % of the units derived from the diol being derived from ethylene glycol, (B) the surface-active agent is composed of at least one dodecylbenzene sulfonic acid salt, and (C) the fine particles of the polyester resin have an average particle diameter of not more than 2 µm, the process comprising (a) preparing the paint by mixing a solution of the polyester resin in an organic solvent selected from halogenated hydrocarbons and anisole with an aqueous medium containing the surface-active agent with stirring at a high speed to disperse the polyester resin, and then removing the organic solvent, and (b) coating the inside surface of a metallic can with the resulting paint. A process according to claim 1 wherein the polyester resin is one in which the mole ratio of the units derived from terephthalic acid to the units derived from isophthalic acid is from 30:70 to 70:30. A process according to claim 1 or 2 wherein the polyester resin is one in which the units derived from a diol contain at least 35 mole % of units derived from ethylene glycol. A process according to claim 1, 2 or 3 in which the polyester resin has an intrinsic viscosity, measured in o-fluorophenol at 25°C, of 0.4 to 1.5 dl/g. A process according to any one of the preceding claims in which the surface-active agent is used in an amount of 0.2 to 20 parts by weight per 100 parts by weight of the polyester resin. A process according to claim 5 in which the surface-active agent is used in an amount of 0.25 to 10 parts by weight per 100 parts by weight of the polyester resin.

Inventors: KIGAMI TAKEO C O MITSUI PETROC, MORINAGA AKIO C O MITSUI PETRO, KIGAMI, TAKEO C/O MITSUI PETROCHEMICAL IND. LTD., MORINAGA, AKIO C/O MITSUI PETROCHEMICAL IND. LTD.
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C09D167
IPC codes: C09D5, C09D167