Tris(alkoxycarbonylamino)triazine crosslinked waterborne...

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...

Reexamination Certificate

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C525S328800, C525S437000, C525S445000, C525S453000, C525S454000

Reexamination Certificate

active

06300422

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a thermosetting waterborne coating system. More particularly this invention is directed to optionally catalyzed coating systems utilizing hydroxy functional and/or carboxy functional resin binders and a (C
1
-C
6
alkoxy)carbonylamino triazine crosslinker.
BACKGROUND AND SUMMARY OF THE INVENTION
Amino formaldehyde resins are traditional crosslinkers which have been widely used for waterborne or solventborne coatings for nearly half a century. Coatings crosslinked with amino resins provide good cost-performance efficiency. However, these coating systems involve release of formaldehyde and acid-etching problems. With increasing concerns for the environment and public health, other crosslinkers for waterborne coatings have been developed by the coating industry to alleviate these problems. Typical crosslinkers of this type are aziridines, carbodiimides, oxazolines, epoxies, and silanes. However, waterborne coating systems using these compounds as the crosslinkers still present certain problems, including toxicity, short potlife, slow cure speed, or low cost-to-performance efficiency.
Tris(alkoxycarbonylamino) triazines (hereinafter, “TACT”) have been reported to be used as crosslinkers for solventborne coatings (WO 98/27166), water reducible coatings, or carboxylic acid functional aqueous coating binders applied as a base coat followed by a clear top coat through a wet-on-wet and then a bake process to develop a clear crosslinked coating (WO 98/44060).
The present invention is directed to a coating composition comprising an aqueous dispersion or solution of a hydroxyl-functional polymer component and a tris(alkoxycarbonylamino) triazine crosslinker. The polymer component(s) can optionally include carboxyl-functionality as well as hydroxyl-functionality. Further, the coating composition may optionally include catalysts, including Lewis or Brönsted acids, latent acid catalysts and tertiary amines, pigments, flow/leveling agents, dispersants, thickness and other rheology modifiers and/or coalescing agents.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a formaldehyde-free thermosetting aqueous coating composition comprising a tris(alkyoxycarbonylamino) triazine crosslinking agent and a waterborne hydroxy functional polymer, latex or waterborne solution/dispersion of a hydroxy functional polymer, with optional carboxyl functionality or a mixture of hydroxyl-functional polymers with or without carboxyl functionality. If the coating composition is in the form of a latex, the latex can be non-core/shell, core/shell, inter-penetrating network, or microdomain type.
Any of a wide variety of hydroxyl-functional polymers, such as acrylic, styrene acrylic, vinyl acrylic, polyester, polyether, polyurethane and alkyd resins can be used to formulate the present compositions. The species of hydroxy functional polymer is not critical provided that it is water dispersible (as a latex or in solution) and has a hydroxyl number of about 20 to about 140, more typically about 40 to about 80. By using one or more tris(alkyoxycarbonylamino) triazines as the crosslinker, the coating system can be provided as a thermoset, one-package system, which does not release formaldehyde or other toxic by-products during the thermoset process. The cure rate and other coating/coating composition properties of the coating system can be controlled by varying the concentration of crosslinker, by the nature of the hydroxy functional polymer, by the addition of carboxy functional polymer components, and/or by the addition of catalyst(s). Typically cure temperatures are about 250° F. (with catalyst) to about 425° F. At such temperatures the applied coating compositions cure in about 1 to about 5 minutes.
The use of TACT crosslinkers has been described for use in solventborne, water reducible coatings or carboxylic acid functional aqueous coating binders applied as a base coat followed by a clear top coat through a wet-on-wet/bake process. The present invention is based on Applicant's discovery that TACT compounds can be used effectively in waterborne coating compositions comprising the TACT crosslinker and hydroxyl-functional polymers alone (with or without carboxyl-functionality) or in combination with carboxy-functional film-forming polymers.
The use of the TACT crosslinkers in a water-based system in accordance with this invention allows for the formation of coating compositions that do not release formaldehyde on and do not include high levels of hazardous air polluting solvents (HAPS) or other volatile organic compounds (VOCs). Further, the molecular weight of the hydroxyl-functional polymer used in the coating system can be much higher than those polymers used in the water-miscible or water-reducible solvent systems previously developed. The use of higher molecular weight polymers in the coating composition provides more durable coatings. The weight average molecular weight of the polymer components for use in accordance with this invention can be about 3×10
3
to about 2×10
6
Daltons.
In formulating the present coating compositions TACT component can be in the form of an emulsion, or it can be blended directly with the aqueous hydroxyl-functional polymer. In one embodiment of the present invention the coating composition is prepared by forming a formaldehyde-free thermosetting aqueous coating composition, using the steps of forming an aqueous emulsion of a (C
1
-C
6
alkoxy)carbonylamino triazine crosslinker containing about 10% to about 60% by weight of the crosslinker, with dispersing agents and optional rheology modifiers, blending said emulsion with an aqueous dispersion or solution of a film-forming resin composition comprising a hydroxyl-functional polymer, so that the resulting coating composition has a total solids content of about 15 to about 70 percent by weight.
Generally, TACT is typically emulsified by dissolving about 30 weight percent to about 55 weight percent of the crosslinker in a minimum amount of an organic solvent and agitating the crosslinker solution with water and a surfactant. A variety of solvents are useful for emulsification of TACT step, including but not limited to alcohols, ethers and esters or mixtures of such solvents. Butanol, dipropylene monomethyl ether, and/or butyl acetate are typically employed. The surfactant component of the TACT emulsion are preferably anionic and/or nonionic surfactants, with nonionic surfactants being most preferred.
The solids content in the present coating composition is typically about 15 to about 60 percent by weight and the present coating compositions are formulated to have a weight ratio of hydroxyl-functional polymer to the (C
1
-C
6
alkoxy)carbonylamino triazine crosslinker of about 10:1 to about 100:1, respectively. In another embodiment of the invention the weight ratio of polymer to TACT is about 20:1 to about 50:1, respectively. In still another embodiment of the invention, the weight ratio of polymer to TACT is about 4:1 to 100:1, more typically, about 6:1 to 50:1.
The nature of the hydroxyl-functional polymer is not critical to the present invention, although the species selected for any given formulation can certainly effect coating cure rates and physical properties. Thus, for example, the hydroxyl-functional polymer can be a hydroxy functional acrylic, styrene acrylic, vinyl acrylic, polyester, polyether, polyurethane and alkyd resin polymers. The hydroxyl-functional polymer may also have some carboxyl-functionality itself or the hydroxy functional polymer component(s) can be blended with acid functional polymers. The polymers of the present invention may be in the form of either an aqueous dispersion or aqueous solution when mixed with the TACT crosslinkers.
The TACT crosslinking reaction during cure of the present compositions can be facilitated or accelerated in the presence of a catalyst. The catalyst can be in the form of Lewis acids, Brönsted acids such as organic acids, organo-tin compounds, tertiary amines, or ammonium or amine salts of sulfo

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