Aqueous Acrylic-Carboxylated Polyether Coating Compositions

The invention will be further described in connection with several examples which follow. These examples are given as illustrative of the invention and are not to be construed as limiting it to their details. All parts and percentages in the examples, as well as throughout the specification, are by weight unless otherwise indicated.

A carboxylated polyether formed from reacting and then hydrolyzing a polyepoxide having an epoxide equivalency greater than one was prepared from the following charge:

The para-hydroxy methyl benzoate, Bisphenol A and EPON 829 were charged to a suitable reaction vessel and heated to 177.degree. C. and held at this temperature for one hour. The ethylene glycol monobutyl ether was then added and the reaction mixture cooled to 99.degree. C. The KOH pellets were dissolved in the first portion of deionized water and this solution added to saponify the methyl ester. Phosphoric acid was then added to neutralize the potassium salt, and the reaction mixture filtered to remove the potassium phosphate. The filtrate was then solubilized with the dimethyl ethanolamine and thinned with the second portion of deionized water to form a 32.5 percent solids solution having a Gardner-Holdt viscosity (measured at 23.degree. C.) of Z-. The product had an acid value of 6.6 and a pH of 8.5.

A high molecular weight acrylic latex was prepared from the following charge:

The acrylic acid was charged to a reaction vessel and heated to 82.degree. C. with agitation. The deionized water was added slowly followed by the addition of the sodium laural sulfate. The 2-ethyl hexyl acrylate was added slowly, with the temperature of the reaction vessel being maintained between 77.degree.-82.degree. C. The addition of the 2-ethyl hexyl acrylate was extended over a period of five hours. After 2-1/2 hours into the addition, the styrene addition was started and completed over a period of 2-1/2 hours. After the addition of the 2-ethyl hexyl acrylate and the styrene, the methyl methacrylate, sodium bicarbonate and ammonium persulfate were charged to the reaction vessel, and the reaction mixture held for one hour at 77.degree.-82.degree. C. The reaction mixture was then neutralized with dimethyl ethanolamine to adjust the pH to 8.5 to 9.0. The latex had a solids content of 38.9 percent, a viscosity of 600 centipoises measured at 23.degree. C., 20 revolutions per minute using a No. 3 spindle. The acid value of the latex was 6.96. The molecular weight of the acrylic polymer was about 1,000,000 on a weight average basis as determined by Gel Permeation Chromatography.

A pigment grind was formed from the following charge:

The charge was ground in a sand mill to a Hegman 7.5. 251.67 parts by weight of the pigment grind was then thinned with 29.96 parts of deionized water to form a paste.

A coating formulation was prepared from the following charge:

The carboxylated polyether, the acrylic latex and the deionized water were charged to a reaction vessel and the pigment paste was added slowly under agitation. The resulting composition had a total resin solids content of 40.1 percent. When applied to a galvanized steel surface and baked at 121.degree.-135.degree. C. for 60 seconds, the composition formed an excellent moisture resistant primer coating. When topcoated with a high molecular weight acrylic latex sold commercially by Rohm and Haas Company under the trade name AC 658 (butyl acrylate-acrylic acid-methyl methacrylate copolymer), excellent adhesion was developed between the topcoat and the primer coating when the topcoat was cured at about 260.degree. C. for 80 seconds.

For purposes of comparison, an aqueous composition of the present invention containing a high molecular weight, water-insoluble acrylic polymer and a carboxylated polyether was compared with two other aqueous coating compositions, one of which contained a low molecular weight water-dispersible acrylic in combination with a carboxylated polyether and the second of which contained 10 percent by weight of a melamine-formaldehyde crosslinker in addition to the low molecular weight acrylic and carboxylated polyether.

The aqueous coating composition of the invention, designated as Composition E in the table below, contained about 37 percent resin solids and was prepared from the following charge:

The first comparative aqueous coating composition designated as Composition F in the table below contained about 31 percent resin solids and was prepared from the following charge:

The second comparative aqueous coating composition designated as Composition G in the table below contained about 31 percent resin solids and was prepared from the following charge:

BONDERITE.RTM.1303 pretreated, hot dipped, galvanized steel panels (25 mils in thickness) were coated with the compositions disclosed above by drawing down with wire-wrapped draw bars and baked for 30 seconds at 500.degree. F. (260.degree. C.) to form primer coatings of about 0.3 mil in thickness.

The primer coated panels were then topcoated with a high molecular weight acrylic latex which was a blend in a 2:1 ratio (based on solids) of two high molecular weight acrylic latices commercially available from Rohm and Haas under the trade names AC-658 and AC-604, respectively. Both materials were butyl acrylate-acrylic acid-methyl methacrylate copolymers. The topcoat was cured at 500.degree. F. (260.degree. C.) for about 75 seconds to form topcoats of about 0.7 mil thickness.

The topcoated panels were then evaluated for flexibility, humidity, salt spray and solvent resistance. The results of the testing are reported in Table I below.