Cationic latex coatings

Cationic latex coatings

Patent: US 5312863 A


A novel coating composition, exhibiting improved adhesion to anionic substrates and a process for its preparation and use, is disclosed. The coating contains an aqueous dispersion of a cationic polymeric binder. The polymeric binder is preferably prepared by the polymerization of at least one monoethylenically unsaturated monomer, having amine functionality, in the presence of at least one nonionic or amphoteric surfactant, followed by the subsequent neutralization of the polymer using selected acids. Coatings containing the cationic latex polymeric binder and selected cationic pigment dispersants are provided. In addition, by selecting certain process conditions and reactive pigments, completely cationic aqueous coating compositions, which maintain the advantages of a water-based system while exhibiting excellent stain blocking, corrosion resistance, water sensitivity resistance and adhesion to wood and alkyd surfaces, and which are competitive in their performance with conventional solvent based alkyd primers and paints, are disclosed.

State of the prior art

Conventional aqueous latex coatings are anionic. They contain anionic latex polymer binders which are typically prepared by aqueous emulsion polymerization using non-ionic surfactants, anionic surfactants or combinations thereof. These anionic latex polymer binders are combined with pigments and extenders and dispersed with anionic pigment dispersants to form the aqueous latex coating. The anionic polymeric binders typically contain anionic functional groups such as sulfate and carboxylate groups. It is known in the art that further functionalization of these anionic latex polymers with amines, acetoacetate, or amides such as for example ethylene urea derivatives can assist in the adhesion of coatings containing these polymeric binders. A review article Developments in Ureido Functional Monomer for Promoting Wet Adhesion in Latex Paints; R. W. Kreis and A. M. Sherman, Water-Borne and Higher-Solids Coating Symposium Feb. 3-5, 1988, New Orleans, La., discusses the use of these functionalities to obtain adhesion under wet conditions. The following U.S. Patents disclose the use of such functional groups to assist in the adhesion of aqueous latex coatings to substrates: U.S. Pat. Nos. 3,356,627; U.S. Pat. Nos. 3,404,114, 3,637,565; U.S. Pat. No. 4,248,754; U.S. Pat. No. 4,254,003 and U.S. Pat. No. 4,375,440.

U.S. Pat. No. 3,356,627 is directed to aqueous blends of two water insoluble polymers, one of which being formed from 1 to 15 percent of a polar monomer selected from alcoholic hydroxyl, amino, carboxylic and amide and ureido groups, for use as coating and impregnating compositions having improved adhesion to wood. The '627 patent teaches formulating pigmented compositions using pigment dispersants and stabilizing auxiliary surfactants of the non-ionic, cationic or anionic type. The rheological characteristics of the paint may be accomplished by reacting the carboxylic substituents on the polymer with ammonium hydroxide to form the ammonium carboxylate of the ester copolymer. The aqueous dispersion paint is ordinarily adjusted to an alkaline state of pH of 7.5 to pH 10 using ammonium hydroxide.

U.S. Pat. No. 3,404,114 is also directed to the preparation of latex polymers which purport to yield adherent films. The polymers are formed from about 1-25 percent of an unsaturated carboxylic acid, 50-98 percent of monovinylidene monomer and 1-25 percent of an alkylamino alkylester of an unsaturated ester. The polymer is formed by emulsion polymerizing the unsaturated carboxylic acid and a portion of the monovinylidene monomer followed by the addition of a liquid nitrogenous neutralizing reagent, such as for example ammonia or ammonium hydroxide to raise the pH of the system to a pH of at least 7 and preferably pH 7.5 to 8.5. After the pH adjustment, a second charge of monomers consisting of the remainder of the monovinylidene component and the alkylamino alkylester of an ethylenically unsaturated carboxylic acid. In order to form stable polymer dispersions the '114 patent discloses the use of nonionic or anionic surfactants at a concentration of from about 0.1 to 10 weight percent of the monomer mixture.

U.S. Pat. No. 3,637,565 is directed to latex compositions having improved adhesion to polar substrates. The cationic lattices are prepared by emulsion polymerization at pH below pH 7 of a primary or secondary amino alcohol ester of an alpha-beta ethylenically unsaturated carboxylic acid or a primary or secondary amino alcohol half ester of an alpha, beta ethylenically unsaturated dicarboxylic acid with at least one other polymerizable ethylenically unsaturated substantially water insoluble monomer. Preferably a nonionic surfactant alone or optionally a nonpolymerizable low molecular weight cationic surfactant is used during the polymerization. The cationic latex may be converted into a stable anionic latex by adding a nonionic surfactant, if such surfactant is not added prior to the polymerization of monomers, to stabilize the latex during subsequent pH adjustment to a pH greater than 8, and usually from pH 9 to pH 10, preferably by the addition of ammonium hydroxide. The anionic latex so formed is compatible with conventional formulating ingredients used to produce coating and impregnating compositions and the resofting product so formed is disclosed to contribute wet and dry adhesion to dried film compositions or coatings. The latex is disclosed as being useful for preparing coating and impregnating compositions especially useful for adhesion to polar substrates such as paper coatings, fabric coatings and the like. When preparing an aqueous paint dispersion using the latex the '565 patent discloses the use of wetting and dispersing agents such as polyphosphates and pyrophosphates or anionic and nonionic surfactants.

U.S. Pat. No. 4,248,754 is directed to aqueous dispersions of polymer particles for adhesion to a substrate under moist conditions. The polymer comprises polar groups, selected from amine, ureido and thioureido groups; a proportion of monomer units derived from acrylamide and methacrylamide; and a proportion of poly(alkylene oxide).

U.S. Pat. No. 4,254,003 is also directed to an aqueous dispersion of anionic polymer particles useful in paint compositions exhibiting good moisture resistance. The polymers comprise polar groups selected from amine, ureido and thioureido groups and poly(alkylene oxide) chains.

U.S. Pat. No. 4,357,440 is directed to the use of 2 hydroxy 3-t-butylamino-1 -propyl methacrylate as a wet adhesion aid for latex paints to improve adhesion of the latex coating to the substrate.

It is also known that adhesion between anionic substrates and cationically dispersed materials can occur through columbic interactions and the formation of ionic bonds. B. Alince Performance of Cationic Latex as a Wet-end Additive TAPPI, Vol. 60 (12/1977) discloses that the deposition of amino and quaternized latexes on anionic pulp fibers occurs due to columbic interactions which are pH dependent.

U.S. Pat. No. 3,926,890 discloses the preparation and use of quaternary functional latexes to provide adsorption to substrates such as pulp, and paper. These polymers are prepared by the emulsion polymerization of a haloalkyl ester of acrylic or methacrylic acid with other monoethylencially unsaturated compounds and /or a conjugated diene in the presence of nonionic and/or cationic surfactants and then treating the copolymer with a basic nitrogen containing compound to form the quaternary ammonium salt.

Japan Patent Disclosure 56-13174 is directed to binders for anionically charged glass fibers. The binders are quaternary functional latexes.

U.S. Pat. No. 4,399,254 is directed to cationic lattices useful for thermoplastic and thermosetting applications. The invention relates to the use of cationic surfactants wherein the gegenion for the surfactant is derived from methane sulfonic acid. The patent discloses the use of these lattices for the manufacture of paper, textiles, adhesives and the like where adhesion to anionic substrates is required.

Japan Patent Disclosures 59-155413 and 60-32860 disclose cationic, amine functional dispersions made in water-solvent solutions as solution polymers. They are converted into dispersions by neutralization with acids such a formic, acetic or hydrochloric acid. they also contain grafted polyethylene oxides as stearic stabilizers. Pigmented coatings made by dispersing the pigments directly in the polymeric cationic dispersion are disclosed as providing good adhesion to anionic substrates such as for example alkyds, asphalt block, PVC, concrete, ceramic tile and glass.

Other references disclosing polymeric coatings for adhesion to various substrates include U.S. Pat. No. 4,7610,526; Japan Patent Disclosures 52-6748; 58-23969; 57-63366 and 62-187702.

U.S. Pat. No. 4,710,526 is directed to cement admixtures containing an aqueous emulsion of an alkaline-curable polymer which has excellent adhesion flexibility and waterproofing properties. The cement may be used as a paint. The polymer comprises from about 25 to 99.5 percent of a hydrolytically stable acrylate, from 0.5 to 15% of an alkaline-curable cationic quaternary ammonium salt monomer and other optional ingredients. The '526 patent discloses the use of all conventional types of surfactants with a preference for nonionic or cationic surfactants.

Japan Patent Disclosure 52-6748 is directed to an aqueous dispersion of an olefin polymer, organic solvent, high molecular weight water soluble compound and a nitrogen containing ring compound, tertiary amine or caprolactan. The dispersion is prepared without the use of an emulsifying surfactant. This is disclosed as being beneficial for the water resistance and bonding of the product.

Japan Patent Disclosure 58-23969 is directed to a soft finishing agent for textiles composed of a cationic emulsion of a polymeric quaternary ammonium salt.

Japan Patent Disclosure 57-63366 is directed to a method of electrodepositing a paint composed of a copolymer formed from dimethyl aminoethyl methacrylate, acrylic acid alkylesters, alpha beta monoethylenically unsaturated N-alkoxy methylated or N- methylolated monomers of carboxylic acid anhydride and optionally with other alpha beta monoethylenic unsaturated monomers. Acid is used as a neutralizing agent and water as a diluent. The dimethylaminoethyl methacrylate is disclosed as being essential to obtain adhesion with an alkyd melamine resin or acryl melamine resin paint which is subsequently applied over the above copolymer. Diethyl amino(meth)acrylate, dibutyl aminoethyl (meth)acrylate or their reaction products with primary or secondary amine type copolymers are disclosed as having inferior bonding properties with the overcoated paint coating as compared with the dimethylamino ethylmethacrylate copolymer of the invention. Any organic acid including formic acid, acetic acid, propionic acid and lactic acid are disclosed as being useful to neutralize the amino group of the dimethylamino ethyl methacrylate in the copolymer.

Japan Patent Disclosure 62-187702 is directed to a cationic emulsion formed by adding a polymerizable quaternary ammonium salt to an emulsion polymerization system containing a vinyl monomer for adhesion to negatively charged objects.

Since most water soluble staining agents are anionic, they can be effectively complexed, in an ion exchange content, with cationic materials to render the staining agents insoluble, such as for example by trapping them in the primer coat when it dries, such that the stains are prevented from migrating into water based topcoats.

U.S. Pat. No. 3,847,857 discusses this concept in more detail. The patent discloses two types of polymer dispersions which can entrap and insolubilize stains. The less preferred polymer type (Type 11) can be employed as a binder. This material is prepared as a copolymeric dispersion of non-crosslinked to slightly crosslinked, thermoplastic, film forming spherical particles of from 0.1 to 1 micron in diameter formed from a mixture of from 5 to 70 weight percent of one or more monomers containing an amine or quaternary ammonium group in salt from, from 0 to 50 weight percent of one or more polyethylenically unsaturated crosslinking monomers, and 0 to 89 weight percent of one or more monethylenically unsaturated monomers of neutral or nonionic characteristics, the counter ion of the salt being a metal counter ion in water such as those derived from boron, chromium, molybdenum and tungsten.

These polymers may be made by anionic, cationic, or non-ionic type surfactant. The polymerization can be carded out under neutral, acidic or alkaline conditions. After emulsion polymerization, the pH of the dispersion may be adjusted to whatever condition of neutrality, acidity or alkalinity is desired, the pigment may be dispersed using water soluble and swellable colloidal bodying agents and an auxiliary surfactant to stabilize the dispersion. This auxiliary surfactant may be a non-ionic, anionic or cationic surfactant. The '857 patent does not suggest that any particular type of surfactant is important; nor does it suggest any importance to the selection of the neutralizing acid type, pH, pigment dispersant type on the adhesion of coating containing the polymers. The coating composition of the present invention is an improvement to the materials of the '857 patent.

It is an object of the present invention to provide a stable, aqueous, cationic coating composition which exhibits improved performance over conventional anionic latex coatings, and is competitive in performance with organic solvent based alkyd primers for application to anionic substrates.

It is a further object of the invention to prepare an all cationic aqueous coating composition containing a cationic latex polymeric binder and a cationic pigment dispersant which does not compete with the cationic latex polymer binder for anionic binding sites on the substrate.