Air Drying Thermosetting Aqueous Epoxy-Acrylic Copolymer Coating Systems

The present invention relates to thermosetting aqueous epoxy-acrylic copolymer coating systems which are adapted to cure on air drying. Such coating systems are two package systems which cure upon admixture with one another and are particularly adapted for the painting of porches and floors, including cementitious substrates.

Aqueous coating compositions of the general type under consideration are in commerce, being produced under the disclosure of U.S. Pat. No. 3,719,629 issued Mar. 6, 1973. These known compositions are not only expensive, but they exhibit poor chalking resistance, and poor tint retention. They are also marginal from the standpoints of (1) applicability to porches; (2) film hardness; and (3) cracking resistance.

In the aqueous coating compositions of said U.S. Pat. No. 3,719,629, the epoxy resin component is combined in an aqueous medium with an aminoethylated vinyl polymer having pendant aminoethyl groups, these groups having the formula: ##STR1## and the cure is between the oxirane (epoxy) group in the epoxy resin and the amino hydrogen atoms in the aminoethyl group pictured. In the present invention, the amino hydrogen atoms which are essential in the prior art are not present. Moreover, the production of the pendant aminoethyl groups in the prior art required the use of expensive and hazardous ethylene imine, and this is avoided in this invention.

From the broader standpoint, and in addition to the aqueous epoxy system noted previously, other resin systems have been employed to provide air drying porch and floor paints. Thus, the systems of this invention are intended to replace two existing aqueous systems, one based on an alkyd resin and another based on an aqueous copolymer latex, and also to replace an organic solvent-based epoxy system. The invention exhibits significant superiority to all three as will be documented hereinafter.

A two package system is employed in the present invention. In one component is placed an acid-neutralized tertiary amine copolymer in solution in a water-miscible alcoholic solvent. The other component contains the epoxy resin, desirably together with a catalyst for the epoxy-tertiary amine reaction, in solution in organic solvent which is at least partially water-miscible, and which is preferably alcoholic. Before applying the coating, the two separately stable solution components are mixed together to form an aqueous mixture of limited pot life, and this aqueous mixture is coated upon the surface to be painted. The water which is present in the aqueous mixture is normally added to the first component prior to admixture of the two components, but all or a portion of the water can be added after the mixture is formed. The first component also desirably includes a surfactant for maintaining the epoxy resin in relatively stable emulsion in the aqueous mixture which is provided.

Typical coatings produced by the present invention have a pot life of at least about 8 hours, usually about 12-14 hours, and can be applied by roller, brush, or air spray. The applied aqueous coatings dry to the touch in air at room temperature in about 6-7 hours and become water and solvent resistant in about 8-12 days. Recoat properties are comparable to that of conventional porch and floor paints. Importantly, polymer solubility is obtained using acids like lactic or acetic acid which are less toxic than anionic systems solubilized with amines, and generally less than 10% of nonobjectionable alcoholic solvents are needed which is advantageous from several well known standpoints, such as cost, hazard, pollution, and the like.

Referring more particularly to the tertiary amine copolymers, these are organic solvent-soluble copolymers comprising three copolymerized monoethylenically unsaturated components, namely: (1) nonreactive monomer; (2) hydroxy-functional monomer; and (3) tertiary amine monomer. While small amounts of other monomers may be present, such as monoethylenic carboxylic acids, monoethylenic amides or N-methylol derivatives thereof, or polyethylenic hydroxy-functional materials such as polyesters, these are not necessary.

The nonreactive monoethylenically unsaturated (monoethylenic) monomer should constitute at least about 40% of the copolymer in order to provide desirably physical characteristics. Styrene is the preferred nonreactive monoethylenic monomer, but vinyl toluene is also useful. Methyl methacrylate, acrylonitrile, and vinyl acetate will further illustrate the nonreactive monomers which are useful. A portion of a flexibilizing nonreactive monomer is also desirably present, such as ethyl acrylate or methacrylate or butyl acrylate or methacrylate. These flexibilizing monomers can be more generally defined as C.sub.2 -C.sub.18 alkanol esters of monoethylenic carboxylic acids.

In preferred practice, the nonreactive monomer component will constitute from about 50% to about 80% of the weight of the tertiary amine copolymer, typically about 65%.

The monoethylenic hydroxy-functional monomer is used in an amount of from 5-35% of the weight of the tertiary amine copolymer, preferably from 10-25%, and it is important for various reasons. First, the hydroxy functionality assists in achieving water solubility with the aid of a solubilizing acid. Second, the presence of the hydroxy group is important to the ultimate cure, and this factor may be only partially satisfied by the presence of the alcoholic solvent since the bulk of the thermosetting cure takes place in the solid film after the water and volatile solvent have been removed. Appropriate monomers are illustrated by hydroxy ethyl acrylate or methacrylate, and simple variations thereof, such as hydroxy propyl acrylate. Polyhydric monomers such as trimethylol propane monoacrylate are also useful, and so are unsaturated alcohols like allyl alcohol and oleyl alcohol, especially when the rest of the monomers are not acrylic, as when large amounts of styrene and/or vinyl acetate are used.

The term "hydroxy" refers to the alcoholic group, and does not embrace related functionalities in which a terminal OH is present, such as the phenolic OH group, the SiOH group, or the N-methylol group.

A limited proportion of polyethylenic polyester having hydroxyl functionality may be present as disclosed in U.S. Pat. No. 3,506,601 dated Apr. 14, 1970, so long as it does not interfere with water solubility. This material, if used, will constitute a portion of the hydroxy monomer component.

The crucial component of the tertiary amine copolymer is the monoethylenic tertiary amine monomer. Typical tertiary amine monomers are dimethyl aminoethyl acrylate or methacrylate, dimethyl aminopropyl acrylate or methacrylate, diethyl aminomethyl acrylate or methacrylate, and diethyl aminoethyl or aminopropyl acrylate or methacrylate. The corresponding crotonates and tertiary amino esters of other monoethylenic carboxylic acids are also useful. Additionally, the corresponding amides are useful such as dimethyl aminoethyl methacrylamide.

The proportion of the tertiary amine monomer can vary in the same way as the hydroxy monomer, namely, from 5-35% by weight of the copolymer, preferably from 10-25% by weight.

Water miscible organic solvents which are useful in this invention are preferably alcoholic and are illustrated by ethanol, propanol, isopropanol, n-butanol, isobutanol, 2-ethoxy ethanol and 2-butoxy ethanol.

The specific nature of the solubilizing acid used to disperse the amine copolymer in water is of secondary significance. Inorganic acids, such as hydrochloric acid or sulfuric acid, are useful, though not preferred. It is presently preferred to employ phosphoric acid, glycolic acid (hydroxy acetic acid), or acetic acid, but other acids can be used, such as formic acid or carbonic acid. Propionic acid is also useful.

The solubilizing acid is used in an amount to provide a colliodal dispersion having a preferred pH in the range of pH 6.0-7.0, though a pH in the range of pH 3-8 broadly useful.

The epoxy resins which are used in this invention can be broadly referred to as resinous polyepoxides, and those which possess a 1, 2-epoxy equivalency in the range of about 1.4 to about 2.0 are preferred. Polyepoxides which are diglycidyl ethers of bisphenols are particularly preferred, especially those having an average molecular weight in the range of about 350 to 4000. A diglycidyl ether of bisphenol A having an average molecular weight of about 390 will be used to illustrate the invention.

The polyepoxide component is itself conventional, as discussed in U.S Pat. No. 3,719,629 referred to previously.

The polyepoxide is desirably used in an amount providing approximate stoichiometry between epoxy groups in the polyepoxide and tertiary nitrogen atoms in the amine copolymer, .+-. 50%, more preferably .+-. 20%.

The organic solvent component of the package containing the polyepoxide is intended to provide desirably liquidity, particularly since the polyepoxide is normally a viscous liquid or a solid at room temperature. Since it is desired that the polyepoxide by stably dispersed in the aqueous mixture which is ultimately produced, the organic solvent should be at least partially water miscible, but the partial miscibility which is intended need merely be that which is consistent with the ultimate stable dispersion which is contemplated, and it is more accurate to refer to the form of the dispersion as an emulsion since the presence of the polyepoxide component in the aqueous mixture which is formed is such as to induce considerably opacity to the liquid mixture.

A surfactant is normally included in the polyepoxide solution in order to stabilize the aqueous emulsions which are formed. Appropriate surfactants may be nonionic or anionic and the selection of surfactants appropriate for the emulsification of epoxy resins in water is well known and is not a feature of this invention. Surfactants are not needed, however, since the cationic copolymer itself functions to help to maintain the epoxy resin in relatively stable emulsion.

It is also permissible to include catalysts for the tertiary amine-epoxy curing reaction. This reaction is itself well known, and thus the selection of catalysts for the reaction is not of primary concern. Appropriate catalysts are Lewis acids, such as dibutyl tin dilaurate. Indeed, no catalyst is needed, and none is used in the preferred form of the invention.

The systems of this invention can be applied clear or pigmented, but pigmented systems containing titanium dioxide rutile at a pigment to binder ratio of 0.3:1.0 are preferred.

The invention is illustrated in the following examples.

1360 parts of 2-ethoxy ethanol are heated to 120.degree. C. in a reactor equipped with an agitator, a reflux condenser, a thermometer, and a nitrogen inlet tube for sparging the reaction mixture.

Monomers and catalysts are premixed in an addition tank. 710 parts of styrene, 360 parts of hydroxy ethyl acrylate, 630 parts of butyl acrylate, 350 parts of dimethyl aminoethyl methacrylate, 24 parts of azobisisobutyronitrile and 24 parts of tertiary butyl perbenzoate are thus premixed and slowly added to the reactor over a period of 3 hours. The temperature in the reactor is maintained at 120.degree. C. during the period of addition and for one hour thereafter.

To complete monomer conversion, 10 parts of tertiary butyl perbenzoate are added and the reaction continued for one hour at 120.degree. C. and then 10 more parts of tertiary butyl perbenzoate are added and the reaction continued for 2 additional hours at the same temperature.

The product is then cooled to about 70.degree. C. and 50 parts of 2-ethoxy ethanol are added and the final copolymer solution is filtered to provide a solution having a Gardner viscosity of X-Y, a Gardner-Holdt color of 8-9 and a nonvolatile solids content of 57.7%.

The above is ground at high speed and then slowly add, with slow agitation, 12 parts of acetic acid. After continuing the agitation for 3 minutes, add 558.11 parts of water containing 1 part of 37% aqueous formaldehyde and 1.5 parts of a biocide. The formaldehyde and the biocide prevent the growth of mold and are optional. 1.0 part of defoamer (Drew Chemical Company L-475) may also be used, but is not necessary.

Upon admixture of the solutions of Examples II and III, one obtains an aqueous emulsion which has a pot life of about 8 hours. Coating this emulsion by brushing on a concrete surface followed by air drying at room temperature for 8 hours provided a painted surface which was hard enough to be walked upon without being marred.

The advantages of the invention will be apparent from the tabulated data presented below in which X indicates a positive feature, 0 indicates a negative feature, and 1 indicates neither.