Epoxy Resin Composition Containing Polycarboxylic Acid Compound

1. FIELD OF THE INVENTION

The present invention relates to a resin composition for powder coatings.

2. DESCRIPTION OF THE PRIOR ART

With the recent development of the technique of electrostatic powder coating of objects, powder coating compositions have been noted as being valuable. Powder coatings have various advantages. For example, unlike solvent-type paints, powder coatings do not contain volatile solvents at all and hence, they do not cause environmental pollution. The coated articles can be used immediately after coating and a thick coating can be obtained by one coating operation. Because of these advantages, the use of powder coatings has been rapidly increasing in recent years.

As resins for powder coatings, there have heretofore been used thermoplastic resins such as polyvinyl chloride, nylon and polypropylene, and thermosetting resins such as epoxy resins, thermosetting polyester resins and thermosetting acrylic resins. In general, thermoplastic resins are disadvantageous because the use of a primer is indispensable and the properties of the resulting coatings are insufficient. Accordingly, investigations have been made vigorously to develop powder coatings comprising thermosetting resins.

Epoxy resins having thermosetting characteristics provide coatings which are excellent in corrosion resistance, water resistance, adhesion to metals and mechanical strength, but coatings made from a powder coating composition comprising an epoxy resin are inferior in weatherability.As a thermosetting resin capable of overcoming this disadvantage, there can be mentioned a thermosetting acrylic resin, and it provides coatings possessing excellent weatherability and surface smoothness. Thermosetting acrylic resins are used in powder coatings for painting automobiles and household electric appliances. However, the thermosetting acrylic resin is inferior to the epoxy resin in corrosion resistance and adhesion to metals. Most of the thermosetting polyester resins have properties intermediate between the properties of epoxy and thermosetting acrylic resins, but because a good balance of the various required properties of coatings cannot be attained with thermosetting polyester resins, the utilization thereof has lagged behind the use of other resins. Thus, various powder coatings have advantages and disadvantages and powder coating compositions having fully satisfactory properties have not been developed.

We have discovered a novel thermosetting resin composition for powder coatings which provides a powder coating composition having satisfactory properties, especially excellent smoothness, adhesion and mechanical properties (such as impact resistance and hardness) of the coatings made therefrom.

More specifically, in accordance with the present invention, there is provided a resin composition for powder coating compositions comprising a polyepoxy compound and a polycarboxylic acid obtained by reacting a polyoxyalkylene ether of a dihydric phenol having the formula: ##STR1## wherein A is alkylene having 1 to 4 carbon atoms or carbonyl, R is alkylene having 2 to 4 carbon atoms, and each of m and n is a number of at least 1 and the sum of m and n is in the range of from 2 to 14,

with trimellitic acid or trimellitic anhydride, wherein the amount of trimellitic acid or trimellitic anhydride is from 1 to 4 moles per mole of said polyoxyalkylene ether of dihydric phenol of the above-mentioned formula.

The polyoxyalkylene ether of the dihydric phenol having the above formula, that is used in the present invention, can easily be prepared by reacting the corresponding dihydric phenol ##STR2## with an alkylene oxide having 2 to 4 carbon atoms, in the presence of a catalyst such as an alkali metal catalyst.

As the starting dihydric phenol used to make the polyoxyalkylene ether of the dihydric phenol, there can be used, for example 2,2-di-(4-hydroxyphenyl)propane, di-(4-hydroxyphenyl)methane, 2,2-di-(4-hydroxyphenyl)butane and 4,4-dihydroxyphenylketone.

As specific examples of the alkylene oxide that is used as the starting material for preparing the polyoxyalkylene ether of the dihydric phenol, there can be mentioned ethylene oxide, propylene oxide and butylene oxide, and halohydrins thereof.

As specific examples of the polyoxyalkylene ether of the dihydric phenol having the above formula, there can be mentioned polyoxypropylene (2)-2,2-di-(4-hydroxyphenyl)propane, polyoxypropylene (6)-2,2-di-(4-hydroxyphenyl)propane, polyoxypropylene (14)-2,2-di-(4-hydroxyphenyl)propane, polyoxypropylene (3)-di-(4-hydroxyphenyl)methane, polyoxybutylene (4)-2,2-di-(4-hydroxyphenyl)butane, polyoxyethylene (3)-2,2-di-(4-hydroxyphenyl)propane, polyoxyethylene (10)-2,2-di-(4-hydroxyphenyl)methane, polyoxyethylene(12)-4,4-dihydroxydiphenylketone and polyoxypropylene (2)-4,4-dihydroxydiphenylketone.

The polycarboxylic acid formed by reacting the polyoxyalkylene ether of the dihydric phenol with trimellitic acid or trimellitic anhydride in an amount of 1 to 4 moles of the latter, per one mole of said polyoxyalkylene ether, is used as the first component of the resin composition according to the invention, and a polyepoxy compound is used as the second component.

The preferred mixing ratio of the polycarboxylic acid and the polyepoxy compound is such that the carboxyl groups of the polycarboxylic acid are present in an amount of from 0.5 to 2.0 equivalents, especially 0.8 to 1.2 equivalents, per 1.0 equivalent of the epoxy groups of the polyepoxy compound. It is preferred that both the compounds be melt-mixed at the above mixing ratio.

The resulting mixture can easily be pulverized and it is stable at normal ambient temperatures. It can be cured by heating, irradiation of light, ionizing radiation and the like.

As the polyepoxy compound, there can be used organic compounds and resins containing an average of more than one oxirane group per molecule, for example, diglycidyl ether of 2,2-di(4-hydroxyphenyl)propane, triglycidyl isocyanurate, vinylcylohexene dioxide, diglycidyl dimethylhydantoin, polyglycidyl ethers of a phenol-novolak resin, diglycidyl ester of tetrahydrophthalic acid, diglycidyl ester of hexahydrophthalic acid and polyepoxy group-containing vinyl compounds.

As coating methods for using the powder coating compositions, there can be mentioned the electrostatic coating method, the fluidized-bed dipping method, the flame spray coating method and the like. Powder coatings comprising the resin composition of the present invention can be coated by any of these known coating methods in the conventional manner. A coated object is prepared by coating the object with a powder paint composition followed by fusing the powder paint composition to form a coating film and curing the resin.

The resin composition of the present invention provides a powder coating composition which is especially excellent in such properties as smoothness, mechanical properties and adhesion of the resulting coating. This characteristic is believed to be due to the bis-phenol structure introduced into the main chain of the polycarboxylic acid and which in turn is reacted with the polyepoxy compound. In other words, it is believed that the bulky and relatively immobile structure of the bis-phenol is manifested as excellent properties of the cured coating.

For further description of the present invention, illustrative examples will now be described. These examples do not limit the scope of the present invention.

At 120.degree. C 2 moles (384 g) of trimellitic anhydride was reacted with 1 mole of (580 g) of polyoxypropylene (6)-2,2-di-(4-hydroxyphenyl)propane for 3 hours to synthesize a polycarboxylic acid having a softening point of 125.degree. C. Then, one g-equivalent (240 g) of the thus-obtained polycarboxylic acid was mixed with 1 g-equivalent (950 g) of a diglycidyl ether of 2,2-di-(4-hydroxyphenyl)propane (Epikote 1004 manufactured by Shell Chemical Co.) and with 476 g of rutile type titanium oxide (Tipake R-550 manufactured by Ishihara Sangyo Kabushiki Kaisha) as a pigment. The mixture was kneaded at 120.degree. C for 5 minutes by a roll, cooled, pulverized by a mixer, and passed through a 200 mesh sieve. The powder that passed through the sieve was coated on a plate of mild steel G-3141 (JIS) which has been treated with zinc phosphate, according to the electrostatic spray coating method using an electrostatic coating machine (Model 720 manufactured by Gema Co.). Subsequently, the curing reaction was conducted at 210.degree. C for 20 minutes.

The thickness of the resulting coating was 61 .mu.. The properties of the coating were tested according to coating test methods specified by the Japanese Paint Inspection Association. The results obtained are set forth below.

Gloss (60.degree. mirror surface reflection): 82%

Square cut adhesion test: 100/100

Du Pont impact test (500 g, 6 mm diameter): 50 cm

Smoothness: excellent

Erichsen test: 9.8 mm

Pencil hardness: 2H

Acetone tackiness test: no change

At 120.degree. C 2 moles of trimellitic acid was reacted with 1 mole of polyoxypropylene(6.7)-2,2 di(4-hydroxyphenyl)propane for 3 hours to synthesize a polycarboxylic acid having a softening point of 120.degree. C. Then, 1 g-equivalent (250 g) of the thus-obtained polycarboxylic acid was mixed with 1 g-equivalent (950 g) of a diglycidyl ether of 2,2-di(4-hydroxyphenyl)propane (Epikote 1004 manufactured by Shell Chemical Co.) and with 480 g of rutile type titanium oxide (Tipake R-550 manufactured by Ishihara Sangyo Kabushiki Kaisha) as a pigment. The mixture was kneaded at 120.degree. C for 5 minutes by a roll, cooled, pulverized by a mixer, and passed through a 200-mesh sieve (Tyler). The powder that passed through the sieve was coated on a plate of mild steel G-3141 (JIS) which had been treated with zinc phosphate, according to the electrostatic spray coating method using an electrostatic coating machine (Model 720 manufactured by Gema Co.). Subsequently, the curing reaction was conducted at 210.degree. C for 20 minutes.

The thickness of the resulting coating was 85 .mu.. The properties of the coating were tested. The results set forth below were obtained.

Gloss (60.degree. mirror surface reflection): 97%

Square cut adhesion test: 100/100

Du Pont impact test (500 g, 6 mm diameter): 45 cm

Smoothness: excellent

Erichsen test: 6.8 mm

Pencil hardness: 2H

Acetone tackiness test: no change

To 1 g-equivalent (64 g) of trimellitic anhydride there were added 1 g-equivalent (950 g) of Epikote 1004 and 215 g of rutile type titanium oxide, and the mixture was kneaded at 110.degree. C for 5 minutes by a roll. In the same manner as described in Example 1, the mixture was pulverized and coated on a mild steel plate according to the electrostatic spray coating method and the coating was cured.

The thickness of the resulting coating was 72 .mu.. The properties of the coating were tested. The results set forth below were obtained.

Gloss (60.degree. mirror surface reflection): 64%

Square cut adhesion test: 100/100

Du Pont impact test (500 g, 6 mm diameter): 30 cm

Smoothness: good

Erichsen test: 6.6 mm

Pencil hardness: H

Acetone tackiness test: no change

At 120.degree. C 2 moles trimellitic anhydride was reacted with 1 mole of polyoxyethylene(12)-di-(4-hydroxyphenyl)methane for 3 hours to synthesize a polycarboxylic acid. Then, 3 moles of the thus-obtained polycarboxylic acid was mixed with 4 moles of triglycidyl isocyanurate and 940 g of rutile type titanium oxide. The mixture was kneaded at 110.degree. C for 5 minutes by a roll. In the same manner as described in Example 1, the mixture was pulverized and coated on a mild steel plate according to the electrostatic spray coating method and the coating was cured.

The thickness of the resulting coating was 65 .mu.. The properties of the coating were tested. The results set forth below were obtained.

Gloss (60.degree. mirror surface reflection): 85%

Square cut adhesion test: 100/100

Du Pont impact test (500 g, 6 mm diameter): 40 cm

Smoothness: excellent

Erichsen test: 7.6 mm

Pencil hardness: 2H

Acetone tackiness test: no change