Water-borne coating composition

Abstract: An aqueous composition comprising an aqueous or water-insoluble resin, and, dispersed in the composition, a graft polymer having a total molecular weight of 5000 to 100,000 and comprising a branch chain with a molecular weight of 1000 to 4000 and a principal chain, wherein one of the branch chain and the principal chain is hydrophobic and has a solubility parameter of 8.0 to 115, and the other is hydrophilic and has a solubility parameter of 10.0 to 13.0, provided that there is a difference between the solubility parameters of at least 1, wherein an aqueous dispersion of the graft polymer has an integrating sphere turbidity of 50 degrees (kaoline) at a concentration of 10 to 70% by weight and a viscosity of 50 to 10,000 cps. There is a difference between the solubility parameters of the aqueous or water-insoluble resin and of a portion of the graft polymer of not more than 2.

Description: Field of the Invention The present invention relates to a water-borne coating composition having good storage stability and coating appearance. Background of the Invention Water-borne compositions have been intensively studied, for coating, because they avoid the fire risk, expense and harmful nature of organic solvents. Japanese Patent Publication (examined) 58101/1986 discloses a water-dispersed coating composition which contains an acrylic graft copolymer. The acrylic graft copolymer has an acid value of 15 to 40 and a glass transition temperature of -10 to 70°C. It also has carboxyl groups, hydroxyl groups and optionally amide groups. The composition, however, has poor coating workability and circulation stability. FR-A-2383971 discloses an aqueous coating composition comprising a graft copolymer containing a hydrophilic portion and a less hydrophilic portion. Summary of the Invention The present invention provides a coating composition which has good coating workability and circulation stability. The coating composition comprises a water-insoluble resin having an acid value of less than 15, and, dispersed in the composition, a graft polymer having a number average molecular weight of 5000 to 100,000 and comprising a branch chain with a molecular weight of 1000 to 4000 and a principal chain; wherein one of the branch chain and the principal chain is hydrophobic and has a solubility parameter of 8.0 to 11.5, and the other is hydrophilic and has a solubility parameter of 10.0 to 13.0, provided that there is a difference between the solubility parameters of at least 1; wherein an aqueous dispersion of the graft polymer has an integrating sphere turbidity according to JIS K 0101-1979 of not more than 50 degrees (kaoline) at a concentration of 10 to 70% by weight and a viscosity of 50 to 10,000 mPas (cps); wherein there is a difference between the solubility parameters of the water-insoluble resin and of the hydrophobic portion of the graft polymer of not more than 2; and wherein each solubility parameter (SP) is determined by SP = ( Vml ·δ ml + Vmh ·δ mh ) / ( Vml + Vmh ) wherein ml represents a low SP solvent, mh represents a high SP solvent, δ represents a solubility parameter and V represents molecular volume at the turbidimetric point. Description of the Invention The graft polymer of the present invention has a hydrophilic portion and a hydrophobic portion. Generally, the hydrophobic portion has relatively a low solubility parameter (SP) value and the hydropnilic portion has relatively a high SP value. It is required that an absolute value of a difference of both solubility parameter (SP) values be at least 1.0. The difference of the solubility parameter (SP) values enhances water dispersibility of the graft polymer. In the present invention, the hydrophobic portion has a solubility parameter (SP) of 8.0 to 11.5 and the hydrophilic portion has a solubility parameter (SP) of 10.0 to 13.0. The hydrophobic or hydrophilic portion may be either a principal chain or a graft chain. In other words, if the principal chain is hydrophilic, the graft chain shall be hydrophobic, and if the principal chain is hydrophobic, then the graft chain shall be hydrophilic. The graft polymer can be prepared by methods known to the art. For example, it may be prepared by adding an unsaturated glycidyl monomer to an acryl polymer having an acid group to form an acryl polymer having an unsaturated double bond, which is then polymerized with other polymerizable monomers. The acid group of the acryl polymer may be introduced by using acrylic acid or methacrylic acid as a monomer. Also, the introduction of acid group may be carried out by a method wherein a polymerization reaction is conducted in the presence of a chain transfer agent, such as thioglycol or by a method wherein a polymerization reaction is conducted using a polymerization initiator having a terminal acid group. Typical examples of the monomers which are used both for the preparation of the acryl polymer and for grafting, provided that the term "(meth)acryl" herein includes both acryl and methacryl, are (meth)acryl esters, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, methoxybutyl (meth)acrylate, methoxyethyl (meth)acrylate, ethoxybutyl (meth)acrylate, allyl (meth)acrylate, hyroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, diethylaminoethyl (meth)acrylate, methylaminoethyl (meth)acrylate and allyloxyethyl (meth)acrylate; (meth)acrylamide, such as N-methylol (meth)acrylamide and N-alkoxymethylol (meth)acrylamide (e.g. N-isobutoxymethylol and (meth)acrylamide); vinyl aromatic compounds, such as styrene, alpha-methylstyrene, vinyltoluene, gamma-chlorostyrene and vinylpyridine; polyolefin compounds, such as butadiene, isoprene and chloroprene; others, such as acrylonitrile, methacrylonitrile and methyl isopropenyl ketone; A typical example of the unsaturated glycidyl monomer is glycidyl (meth)acrylate. According to the selection of the monomers, each portion of the graft polymer is made hydrophobic or hydrophilic. The graft polymer has a number average molecular weight of 5,000 to 100,000, preferably 5,000 to 50,000, in which the graft chain has a number average molecular weight of 1,000 to 4,000. The molecular weight is determined by gel permeation chromatography (GPC). The solubility parameter (SP) is determined by the following formula of K.SUH, J.M. CORBETT; Journal of Applied Polymer Science, 12 , 2359('68): SP = ( Vml ·δml + Vmh ·δmh) / ( Vml + Vmh ) wherein ml represents a low SP solvent, mh represents a high SP solvent, δ represents a solubility parameter and V represents molecular volume at the turbidimetric point. The graft polymer, if necessary, may be made water dispersible by neutralizing acid groups in the polymer with basic materials. Typical examples of the basic materials are monomethylamine, dimethylamine, trimethylamine, monoethylamine, triethylamine, monoisopropylamine, diisopropylamine, diethylenetriamine, triethylenetetramine, monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, dimethylethanolamine, morpholine, piperadine, ammonia, sodium hydroxide, potassium hydroxide and lithium hydroxide. According to the present invention, when the graft polymer is dispersed in an aqueous medium to form a water dispersion having a concentration of 10 to 70 % by weight and a viscosity of 50 to 10,000 mPas (cps), the dispersion has an integrating sphere turbidity of not more than 50 degree (kaoline). The integrating sphere turbidity is determined according to JIS K 0101-1979 and expressed as "degree (kaoline)". In the present invention, the graft polymer is dispersed in an aqueous medium and then a water-insoluble resin is dispersed therein. The water-insoluble resin has the function of reducing the hot melt viscosity of the graft polymer. Preferably, it has a number average molecular weight of 3,000 to 10,000. If the defined difference in solubility parameter of the resin with respect to the graft polymer is more than 2, the obtained coating composition becomes turbid, reducing brightness and transparency. If the water-insoluble resin has an acid value more than 15, dispersion stability and circulation stability are poor. The amount of the water-insoluble resin is preferably 5 to 100, more preferably 5 to 50, parts by weight per 100 parts by weight of the graft polymer. Amounts of more than 100 parts by weight reduce dispersion stability, causing precipitation problems or poor appearance of the cured film, especially poor transparency. The resin can be formulated into the water-borne dispersion by a mixer. The water-borne coating composition of the present invention may further contain an amino resin to form a thermosetting water-borne coating composition. Examples of the amino resins are an alkoxylized melamine-formaldehyde resin, i.e. an alkoxylized condensate of melamine with formaldehyde or paraformaldehyde, such as methoxylated methylolmelamine, isobutoxylated methylolmelamine or n-butoxylated methylolmelamine. The water-borne coating composition may further contain other solvents, color pigments, dyes and extenders , if necessary. Metal powder may also be formulated therein. Other additives, such as defoaming agents, leveling agents, UV absorbers and antioxidants may further be added in the coating composition. The water-borne coating composition of the present invention has excellent circulation stability and coating workability. It is particularly preferred for electrocoating. EXAMPLES The present invention is illustrated by the following examples which are not construed as limiting the present invention to their details. Production Example 1 Preparation of a graft polymer (i) A one liter reaction vessel equipped with a stirrer, a temperature controlling device and a condenser was charged with 400 parts by weight of xylene and heated to 120 °C with stirring. A mixture of 73.6 parts by weight of styrene, 151.6 parts by weight of methyl methacrylate, 100.8 parts by weight of ethylhexyl acrylate, 74.4 parts by weight of 2-hydroxyethyl methacrylate, 40 parts by weight of an initiator (Kaya Ester O available from Kayaku Nuly Company) and 17.2 parts by weight of methacrylic acid was added dropwise for 3 hours and continued to mix for another 2 hours. Next, 0.1 part by weight of hydroquinone and 17.0 parts by weight of glycidyl methacrylate were added to the obtained resin solution and mixed at 140 °C for 3 hours to obtain a hydrophobic oligomer having an acid value of 3.0, a number average molecular weight of 2,500, an SP value of 10.3 and a nonvolatile content of 50 %. A same reaction vessel as used above was charged with 45 parts by weight of butyl diglycol and heated to 120 °C with stirring. To the reaction vessel, a mixture of 400 parts by weight of the above hydrophobic oligomer, 52.6 parts by weight of styrene, 63.8 parts by weight of methyl methacrylate, 35.0 parts by weight of ethylhexyl acrylate, 30.2 parts by weight of hydroxyethyl acrylate, 18.4 parts by weight of methacrylic acid and 14 parts by weight of Kaya Ester O was added dropwise for 3 hours to polymerize, and continued to mix. Xylene was removed by distillation under a reduced pressure to obtain a graft polymer solution having a nonvolatile content 87 %. The graft polymer had a number average molecular weight of 8,000, an acid value of 30, a hydroxyl value of 74, a rate of hydrophobic portion / hydrophilic potion of 50/50 and a hydrophilic portion SP value of 11.4. The graft polymer was neutralized 100 % with dimethylethanolamine and diluted with water up to a nonvolatile content of 20 %. The obtained aqueous solution has a viscosity of 500 mPas (cps) and a turbidity of 3 degree (kaoline). Production Example 2 Preparation of a graft polymer (ii) A same reaction vessel as Production Example 1 was charged with 400 parts by weight of xylene and heated to 120 °C with stirring. A mixture cf 121.6 parts by weight of ethylhexyl methacrylate, 110.0 parts by weight of lauryl methacrylate, 94.0 parts by weight of t-butyl methacrylate, 74.4 parts by weight of 2-hydroxyethyl methacrylate, 40 parts by weight of Kaya Ester O and 17.2 parts by weight of methacrylic acid was added dropwise for 3 hours and continued to mix for another 2 hours. Next, 0.1 part by weight of hydroquinone and 17.0 parts by weight of glycidyl methacrylate were added to the obtained resin solution and mixed at 140 °C for 3 hours to obtain a hydrophobic oligomer having an acid value of 2.5, a number average molecular weight of 2,600, an SP value of 9.5 and a nonvolatile content of 50 %. A same reaction vessel as used above was charged with 45 parts by weight of butyl diglycol and heated to 120 °C with stirring. To the reaction vessel, a mixture of 400 parts by weight of the hydrophobic oligomer, 52.6 parts by weight of styrene, 63.8 parts by weight of methyl methacrylate, 35.0 parts by weight of ethylhexyl acrylate, 30.2 parts by weight of hydroxyethyl acrylate, 18.4 parts by weight of methacrylic acid and 14 parts by weight of Kaya Ester O was added dropwise for 3 hours to polymerize, and continued to mix. Xylene was removed by distillation under a reduced pressure to obtain a graft polymer solution having a nonvolatile content 86 %. The graft polymer had a number average molecular weight of 8,000, an acid value of 30, a hydroxyl value of 74, a rate of hydrophobic portion / hydrophilic portion of 50/50 and a hydrophilic portion SP value of 11.4. The graft polymer was neutralized 100 % with dimethylethanolamine and diluted with water up to a nonvolatile content of 20 %. The obtained aqueous solution has a viscosity of 300 mPas (cps) and a turbidity of 8 degree (kaoline). Production Example 3 A reaction vessel as described in Production Example 1 was charged with 400 parts by weight of xylene and heated to 120 °C with stirring. A mixture of 20 parts by weight of styrene, 35.2 parts by weight of methyl methacrylate, 25.0 parts by weight of ethylhexyl acrylate, 18.6 parts by weight of 2-hydroxyethyl methacrylate, 5 parts by weight of Kaya Ester O and 1.23 parts by weight of methacrylic acid was added dropwise for 3 hours to obtain an acryl resin having an acid value of 8.0, an SP value of 10.4, a hydroxy value of 80 and a number average molecular weight of 4,000. Production Example 4 A reaction vessel as described in Production Example 1 was charged with 400 parts by weight of xylene and heated to 120 °C with stirring. A mixture of 20 parts by weight of styrene, 2.6 parts by weight of lauryl methacrylate, 58.1 parts by weight of ethylhexyl acrylate, 18.6 parts by weight of 2-hydroxyethyl methacrylate, 10 parts by weight of Kaya Ester O and 0.8 parts by weight of methacrylic acid was added dropwise for 3 hours to obtain an acryl resin having an acid value of 5.0, an SP value of 9.5, a hydroxy value of 80 and a number average molecular weight of 2,500. Example 1 A reaction vessel was charged with 115 parts by weight of the graft polymer (i) and 4 parts by weight of dimethylethanolamine and diluted with 167 parts by weight of deionized water to obtain a water borne varnish having a nonvolatile content of 35 %. To the water borne varnish, a mixture of 30 parts by weight of the water insoluble polymer of Production Example 3 and 56 parts by weight of Cymel 303 was dispersed. Dispersing was carried out by slowly adding the mixture to the water borne varnish with stirring by a labo mixer for one hour. The dispersion is stable and has a particle size of 0.1 µm. The obtained dispersion was diluted with deionized water to adjust to a No. 4 Ford Cup viscosity of at least 30 seconds. The paint was spray-coated on a steel panel at 25 °C and a relative humidity of 80 % and allowed to stand for 5 minutes. The panel was baked at 150 °C for 15 minutes. Workability of the paint and brightness of the coating were evaluated and the results are shown in Table 1. Example 2 A vessel was charged with 115 parts by weight of the graft polymer (ii) and 4 parts by weight of dimethylethanolamine and diluted with 167 parts by weight of deionized water to obtain a water borne varnish having a nonvolatile content of 35 %. A mixture of 30 parts by weight of the water insoluble polymer of Production Example 5 and 56 parts by weight of Cymel 303 was dispersed into the water borne varnish. Dispersing was carried out as generally described in Example 1. The water borne dispersion had a particle size of 0.1 µm. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 1. Example 3 The graft polymer (i) of Production Example 1 was diluted with water to form a water-borne resin solution having a nonvolatile content of 20 %. Forty parts by weight of the water-borne resin solution and 40 parts by weight of a white pigment (Taipake R-930 available from Ishihara Sangyo Company) were preliminary mixed and further ground using glass beads at room temperature for 40 minutes in a paint conditioner to obtain a pigment paste. The pigment paste was mixed with the water borne dispersion of Example 1 in a weight ratio of 40/50 (dispersion/paste) to obtain a white paint. The same evaluation was carried out as generally described in Example 1. and the results are shown in Table 1. Example 4 A white paint was prepared as generally described in Example 3, with the exception that the graft polymer (i) of Production Example 1 was changed to the graft polymer (ii) of Production Example 2 and the water borne dispersion of Example 1 is changed to that of Example 2. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 1. Comparative Example The graft polymer of Production Example 1 was neutralized with dimethylethanolamine and diluted with water, to which Cymel 303 was added in a solid ratio of 70/30 to form a clear paint. The same evaluation was carried out as generally I described in Example 1 and the results are shown in Table 1. Production Example 5 A reaction vessel as described in Production Example 1 was charged with 35 parts by weight of butyl diglycol and heated to 120 °C with stirring. A mixture of 20 parts by weight of styrene, 25.7 parts by weight of methyl methacrylate, 28.1 parts by weight of ethylhexyl acrylate, 18.5 parts by weight of 2-hydroxyethyl methacrylate, 7 parts by weight of Kaya Ester O and 7.67 parts by weight of methacrylic acid was added dropwise for 3 hours to obtain an acryl resin (nonvolatile content = 73 %) having an acid value of 50, an SP value of 11.0, a hydroxy value of 80 and a number average molecular weight of 4,000. Production Example 6 A reaction vessel as described in Production Example 1 was charged with 35 parts by weight of butyl diglycol and heated to 120 °C with stirring. A mixture of 10 parts by weight of styrene, 40.6 parts by weight of methyl methacrylate, 21.63 parts by weight of ethylhexyl acrylate, 18.56 parts by weight of 2-hydroxyethyl methacrylate, 7 parts by weight of Kaya Ester O and 9.21 parts by weight of methacrylic acid was added dropwise in a short time to obtain an acryl resin (nonvolatile content = 73) having an acid value of 60, an SP value of 11.5, a hydroxy value of 80 and a number average molecular weight of 4,000. Example 5 A reaction vessel was charged with 115 parts by weight of the graft polymer (i) and 4 parts by weight of dimethylethanolamine and diluted with 167 parts by weight of deionized water to obtain a water borne varnish having a nonvolatile content of 35 %. To the water borne varnish, a mixture of 41 parts by weight of the water insoluble polymer of Production Example 5 and 40 parts by weight of Cymel 303 was dispersed. Dispersing was carried out at 60 °C by slowly adding the mixture to the water borne varnish with stirring by a labo mixer for one hour. The dispersion is stable and has a particle size of 0.1 micron. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 2. Example 6 A vessel was charged with 115 parts by weight of the water insoluble graft polymer (ii) and 4 parts by weight of dimethylethanolamine and diluted with 167 parts by weight of deionized water to obtain a water borne varnish having a nonvolatile content of 35 %. A mixture of 41 parts by weight of the water insoluble polymer of Production Example 6 and 40 parts by weight of Cymel 303 was dispersed into the water borne varnish. Dispersing was carried out as generally described in Example 5. The water borne dispersion had a particle size of 0.1 µm. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 2. Example 7 The graft polymer (i) of Production Example 1 was diluted with water to form a water-borne resin solution having a nonvolatile content of 20 %. Forty parts by weight of the water-borne resin solution and 40 parts by weight of a white pigment (Taipake R-930 available from Ishihara Sangyo Company) were preliminary mixed and further ground using glass beads at room temperature for 40 minutes in a paint conditioner to obtain a pigment paste. The pigment paste was mixed with the water borne dispersion of Example 5 in a weight ratio of 50/40 (dispersion/paste) to obtain a white paint. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 2. Example 8 A white paint was prepared as generally described in Example 7, with the exception that the graft polymer (i) of Production Example 1 was changed to the graft polymer (ii) of Production Example 2 and the water borne dispersion of Example 5 is changed to that of Example 6. The same evaluation was carried out as generally described in Example 1 and the results are shown in Table 2.

Claims: An aqueous composition comprising a water-insoluble resin having an acid value of less than 15, and, dispersed in the composition, a graft polymer having a number average molecular weight of 5000 to 100,000 and comprising a branch chain with a molecular weight of 1000 to 4000 and a principal chain; wherein one of the branch chain and the principal chain is hydrophobic and has a solubility parameter of 8.0 to 11.5, and the other is hydrophilic and has a solubility parameter of 10.0 to 13.0, provided that there is a difference between the solubility parameters of at least 1; wherein an aqueous dispersion of the graft polymer has an integrating sphere turbidity according to JIS K 0101-1979 of not more than 50 degrees (kaoline) at a concentration of 10 to 70% by weight and a viscosity of 50 to 10,000 mPas (cps); wherein there is a difference between the solubility parameters of the water-insoluble resin and of the hydrophobic portion of the graft polymer of not more than 2; and wherein each solubility parameter (SP) is determined by SP = ( Vml ·δ ml + Vmh ·δ mh ) / ( Vml + Vmh ) wherein ml represents a low SP solvent, mh represents a high SP solvent, δ represents a solubility parameter and V represents molecular volume at the turbidimetric point. A composition according to claim 1, wherein the water-insoluble resin has a number average molecular weight of 3,000 to 10,000. A composition according to claim 1 or claim 2, which comprises 5 to 100 parts by weight of the water-insoluble 5 resin per 100 parts by weight of the graft polymer. A composition according to any of claims 1 to 3, further comprising an amino-resin as a curing agent.

Inventors: KUWAJIMA TERUAKI, NODA HIDEYOSHI, YOSHIKAWA MOTOYOSHI, KUWAJIMA, TERUAKI, NODA, HIDEYOSHI, YOSHIKAWA, MOTOYOSHI
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1989
Main classification: C08L51
IPC codes: C09D151, C08L51, C09D153, C09D5