Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-10-20
2001-06-26
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S407100
Reexamination Certificate
active
06251485
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a stable aqueous dispersion that forms a water-resistant fast-setting coating when applied to a suitable substrate.
Coating are generally used to provide a protective barrier for applications such as floors, automobiles, exteriors and interiors of honses, as well as painted roads. Protective coatings for floors, for example, have been known since the mid 1950s. Many of the early coating materials were applied using petroleum- or naphthene-based solvents, and as such were undesirable due to the toxicity and flammability of these solvents.
Water-based synthetic emulsion compositions such as styrene resin emulsions, styrene-acrylate copolymer resin emulsions, and acrylate emulsions, developed in the early 1960s, gradually replaced organic solvent-based compositions. Although the water-based compositions are preferable to organic solvent-based compositions for safety and environmental reasons, coatings prepared from water-based compositions require long drying times. During drying, the coatings are susceptible to a number of events that may produce a defective coating, such as drips, sags and runs. Moreover, until the coating is set, it is vulnerable to contamination by dust or water contact, which may also lead to a marred coating. These effects can be minimized by the use of volatile cosolvents which accelerate the drying process, but the cosolvents reintroduce environmental problems that the water-based systems were designed to address.
In addition to the environmental concerns, one of the challenges that remains is to develop latexes that coalesce at room temperature, without leaving a tacky surface.
In view of the deficiencies in the art, it would be desirable to provide a coating that rapidly becomes resistant to marring, defects, and contamination during drying, using a stable aqueous dispersion that does not require environmentally unacceptable cosolvents or crosslinking curatives.
SUMMARY OF THE INVENTION
The present invention addresses a need in the art by providing a way to prepare coatings that set quickly upon contact with a substrate. Accordingly, in one aspect, the present invention is a coated material comprising a substrate having a surface and a coating thereupon, wherein the coating is prepared by any of the steps of:
a) contacting the surface of the substrate with a stable aqueous dispersion that contains a polymer having pendant strong cationic groups and pendant weak acid groups; or
b) contacting the surface of the substrate with a stable aqueous dispersion containing a first polymer having pendant strong cationic groups, and a stable aqueous dispersion of a second polymer having pendant weak acid groups, the contact of the polymers with the surface being made in any order or concurrently;
with the proviso that when the coating is prepared by the method of (a), the surface of the substrate is, or is treated to be, sufficiently basic so that the stable aqueous dispersion sets in less time than the time required for a latex that only has pendant strong cation groups or pendant weak acid groups to set.
In a second aspect, the present invention is a method of preparing a fast-setting coating on a substrate having a surface comprising either of the steps of:
a) contacting the surface of a substrate with a stable aqueous dispersion that contains a polymer having pendant strong cationic groups, and pendant weak acid groups; or
b) contacting the surface of the substrate with a stable aqueous dispersion that contains a first polymer having strong cationic groups, and a stable aqueous dispersion that contains a second polymer having weak acid groups, the contact of the polymers with the surface being made in any order or concurrently; with the proviso that when the fast-setting coating is prepared by the method of (a), the surface of the substrate is, or is treated to be, sufficiently basic so that the stable aqueous dispersion sets in less time than the time required for a latex that only contains pendant strong cation groups or pendant weak acid groups to set.
In a third aspect, the present invention is a stable aqueous dispersion comprising a polymer having strong cationic groups, weak acid groups, and structural units formed from the polymerization of a non-interfering monomer, wherein the ratio of structural units formed from the polymerization of the non-interfering polymerizable monomer to strong cationic groups and the weak acid groups is from about 70:30 to about 99:1, with the proviso that the strong cationic groups are associated with non-alkaline counterions.
DETAILED DESCRIPTION OF THE INVENTION
The fast-setting coated material of the present invention can be prepared by contacting a substrate with a stable aqueous dispersion that contains a polymer having structural units formed from the polymerization of: a) a polymerizable strong cationic monomer, and b) a polymerizable weak acid monomer. In this aspect of the present invention, the substrate is, or is treated to be, sufficiently basic that the stable aqueous dispersion sets in a time that is less than the time required for a latex that only contains pendant strong cation groups or pendant weak acid groups to set.
The term “sufficiently basic” refers to sufficiency of amount of base as well as base strength. A coating “sets” or is “dry-to-the-touch” when it has formed a skin with sufficient mechanical integrity such that no portion of the skin is removed when it is touched lightly with a finger, and no portion is washed off the substrate when rinsed under a light stream of water.
As used herein, the term “polymerizable strong cationic monomer” refers to a monomer that contains ethylenic unsaturation and a cationic group having a charge that is independent of pH. Similarly, the term “polymenizable weak acid monomer” refers to a monomer that contains ethylenic unsaturation and an acid group having a pK
a
in the range of about 2 to about 10. The term “structural units formed from the polymerization of . . .” is illustrated by the following example:
In addition to structural units formed from the polymerization of a polymerizable strong cationic monomer and a polymerizable weak acid monomer, the polymer also preferably includes structural units formed from the polymerization of a polymerizable non-interfering monomer. The term “polymerizable non-interfering monomer” is used herein to refer to a monomer that does not adversely affect the fast-setting nature of a coating prepared from the stable aqueous dispersion of the polymer.
Polymerizable weak acid monomers that are suitable for the preparation of stable aqueous dispersion used to prepare the water-resistant fast-setting coating include ethylenically unsaturated compounds having carboxylic acid, phenolic, thiophenolic, or phosphinyl functionality. Preferred polymerizable weak acid monomers include acrylic acid, methacrvlic acid, itaconic acid, &bgr;-carboxyethyl acrylate (usually as a mixture of acrylic acid oligomers), vinylbenzoic acid, and 2-propenoic acid: 2-methyl-, (hydroxyphosphinyl) methyl ester. Acrylic acid and methacrylic acid are more preferred weak acid monomers.
The polymerizable strong cationic monomer is associated with a non-alkaline counterion, which may be, for example, halide such as chloride, bromide, or iodide, as well as nitrate or sulfate. As used herein, the term “non-alkaline counterion” refers to a counterion that does not cause sufficient ionization of the weak acid to render the stable aqueous dispersion unstable. Thus, a bicarbonate counterion would not be appropriate if a carboxylic acid were the weak acid, since this counterion would, in sufficient quantity, raise the pH of the stable aqueous dispersion to an unstable level. For example, for a stable aqueous dispersion containing 2.3 mole percent each of a quaternary ammonium salt and a carboxylic acid, the presence of a sufficient amount of a bicarbonate counterion to raise the pH of the latex to above 5.5 would cause the dispersion to become unstable.
Suitable polymerizable strono cationic monomers include s
Harris John K.
Rose Gene D.
Schmidt Donald L.
Cameron Erma
The Dow Chemical Company
Willis Reid S.
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