Epoxy Graft Acrylic Water-Based Primer Surfaces

This invention relates to epoxy ester polymers having acrylic portions grafted thereto. More particularly, it relates to coating compositions containing such polymers.

U.S. Pat. No. 4,302,373--Steinmetz (Nov. 24, 1981) describes water-borne coating compositions of epoxy resin, polymeric acid and tertiary amine, wherein the epoxy functionality is partially capped with a carboxylic acid polymer to form a hydrogel. Coating compositions made therefrom are taught for use in can coatings and for automotive and paper coatings.

An acetic acid-neutralized aqueous polyaminoamido resin made using dimeric fatty acids is disclosed in European Patent Application Publication No. 60581 of Sept. 22, 1982--Guenter et al (Akzo).

Other aqueous epoxy coating compositions also outside the present invention are disclosed in U.S. Pat. Nos. 4,446,258 of May 1, 1984--Chu et al (Mobil Oil), 4,446,260 of May 1, 1984--Woods et al (International Paint), and 4,444,806 of Apr. 24, 1984--Morgan et al (W. R. Grace).

The present invention provides a dispersion in water, in precentages by weight based on the total dispersion, of:

(a) 10-60% of an epoxy ester graft acrylic polymer formed by reacting 5-30% of an epoxy ester polymer formed by reaction of n moles of a difunctional epoxy resin with terminal oxirane groups and a number average molecular weight (Mn) of 300-5000, n-1 moles of a diacid, and 2 moles of a monofunctional acid to form a reaction product, onto which is grafted 5-30% of an acrylic monomer by using a peroxide or azo initiator, to produce said epoxy ester acrylic polymer with a Mn=1000-50,000 and an acid number of 30-150,

(b) amines being added to an extent equivalent to neutralizing 50-200% of the moles of acid functionality,

(c) organic solvents,

such dispersion having not more than 10% volatile organic solvents plus amines, and

(d) balance water.

The epoxy ester graft acrylic polymers of (a) above, and methods of making them, are different aspects of the invention.

The present invention is made possible by a new method of synthesis of an epoxy ester graft acrylic polymer. The process involves the synthesis of a liner epoxy ester formed by reaction at 80.degree.-190.degree. C. of an oxirane terminated bisphenol A epoxy resin with aliphatic diacids and monoacids in approximately stoichiometric proportions so that all the oxirane groups and acid groups have reacted. The reaction is done in a minimum amount of an organic solvent, preferably aromatic for ease of stripping, preferably using a small percentage of a tertiary amine as catalyst. In a second stage an acid functional acrylic is grafted onto the epoxy ester using a peroxide or azo initiator. Grafting onto the epoxy ester is believed to go via hydrogen abstraction of activited carbon-hydrogen bonds. In a third stage the acid functionality on the epoxy ester is neutralized with an amine (50-200% neutralization), the polymer is dispersed in deionized water the excess organic solvents are distilled off.

These epoxy ester-acrylic dispersions then contain a minimum of organic solvents, so as to be able to formulate primer compositions, preferably with no more than 5% organic volatiles.

When combined with water soluble or dispersible crosslinkers, like hexamethoxymethylmelamine, pigments and extenders, primer surfacers can be formulated which show good corrosion resistance over bare steel, an excellent balance of hardness, flexibility, anti-chipping properties, good topcoat hold-out and appearance.

These primer surfacers can be baked for 30 min at from 110.degree. C. to 200.degree. C., still retaining their basic properties. The primers can be applied by conventional or electrostatic spraying. To further improvve application and flow properties, rheology control agents and flow agents can be used. Silica or clay pastes are quite successful for rheology control to prevent pinholing and sagging, and water soluble or dispersable linear or branched polyethylene glycols or polypropyleneglycols are efficient for flow properties. The primer surfacers show excellent initial and wet adhesion over bare steel and cathodic electrode-posited primers. Adhesion and hold-out of conventional topcoats (polyester, alkyd, acrylic) is very good over broad temperature ranges. Hold-out is the resistance to interpenetration at the interface between the layers of primer surfacer and topcoat applied in organic solvents.

The following are examples describing manufacture of epoxy ester graft acrylic dispersions. Parts, proportions and percentages are by weight except where indicated otherwise.

Following are examples describing manufacture of primer surfacer paints or finishes with a content of organic volatiles of no more than 5%.

Following are examples describing manufacture of finishes paints, application and properties:

The above mixture is ground to a fineness smaller than 15 .mu.m. Next there is added deionized water to obtain a package viscosity of 100 to 150 sec DIN cup 4 at 20.degree. C.

The procedure of Example 5 was repeated with the exception that the resin dispersion of Example 2 was replaced by resin of Example 1.

The procedure of Example 5 was repeated with the exception that the melamine resin Cymel 303 was replaced by Cymel 350, another melamine resin.

The procedure of Example 5 was repeated with the exception that the dimethylethanolamine was replaced by DMAMP 80 (2-dimethylamino-2-methyl-1-propanol) from IMC Co.

The procedure of Example 5 was repeated with the exception that reactive diluent Pluriol P 900 (polypropylene oxide, molecular weight 900) was replaced by Luphen 1320 from BASF (branched polyethyleneglycol).

The procedure of Example 5 was repeated with the exception that Aerosil 200 was replaced by Bentone EW bentonite clay from NL Chemicals.

The procedure of Example 5 was repeated with the exception that the P:B ratio was changed from 74.3/100 to 50/100.

The products of Examples 5-11 were reduced with deionized water to a spray viscosity of 20 seconds DIN cup 4 at 20.degree. C. The coating composition obtained was sprayed over a cataphoretic electrocoated panel at a thickness of 35 .mu.m dry film thickness and cured at a temperature of 170.degree. C. for 25 minutes.

The obtained primer film was coated with different topcoats including alkyd/melamine/acrylic/melamine color coat--clear coat resulting in excellent adhesion gloss, flow and DOI. Test results on the coating properties are given in Table I in terms of:

Buckholz hardness: DIN 53153

Erichson flexibility: DIN 53156

Salt spray: DIN 50021. The rust creepage from the scribeline is measured in mm.

All Examples were submitted to the humidity cabinet according to DIN 50017 for 240 hours over bare steel and 480 lhours over electrocoated panels. In neither of the tests did any blistering occur. Good chip resistance was also observed.