Glossy paints containing emulsion polymers of vinyl acetate

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

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C524S524000, C524S563000, C524S564000

Reexamination Certificate

active

06656998

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to glossy paint formulations containing blends of latexes of hard and soft polymers of vinyl acetate. In another aspect it relates to paints capable of forming coatings having improved resistance to blocking without diminished gloss. In still another aspect it relates to a method of making non-blocking semi-gloss paint.
The need today for architectural coating materials free from volatile organic content (VOC) for both safety and health reasons is well documented. The switch to water-borne or high solids paints has answered this need, but there still remains a problem with respect to paints where surface luster is important, such as in satin, semi-gloss and high gloss paints. Such paints need to have good blocking resistance to keep painted surfaces, such as on doors and windows, from sticking. This problem has been addressed by incorporating organic solvents, but this approach partially defeats a major advantage of using water-based paints. While the blocking problem can be alleviated by incorporating “hard” polymers, i.e. polymers having a relatively high glass transition temperature (T
g
), into the formulation, this step has generally resulted in the reduction of gloss values in the finished coating.
Daniels in “Vinyl Acetate Polymers”,
Encyclopedia of Polymer Science and Engineering
, Vol. 17, p. 393-425 (1989) states that it has long been the practice to modify latex film properties by blending one polymer latex with another, for example, making a “soft” polymer less blocking with a “hard” polymer. As examples, he describes compatible solvent blends of poly(vinyl acetate) with poly(ethyl methacrylate), cholesterol and derivatives thereof, poly(butyl acrylate), poly(ethylene oxide) with poly(methyl methacrylate), and terpolymers of acrylonitrile, methyl acrylate and Na methylpropenyl sulfonate. The issue of gloss is not addressed.
Snyder, U.S. Pat. No. 5,308,890 (1994) discloses an approach to the blocking problem in multi-stage latex polymers by incorporating into the multi-stage latex polymer a latex which is not film-forming at ambient temperature. The nonfilm-forming polymers named are copolymers of butyl acrylate or ethyl acrylate with methyl methacrylate and methacrylic acid and/or styrene (with T
g
(glass transition temperature) values above room temperature). Film gloss was not an issue. Never-the-less, it was observed by Patel et al. in “Characterization of Latex Blend Films by Atomic Force Microscopy”,
Polymer
, Vol. 37, No. 25, pp. 5577-82 (1996) that the incorporation of a latex of a hard polymer into the latex of a soft polymer increases surface roughness of the polymer film in proportion to the hard polymer in the blend. The polymers studied were copolymers of butyl acrylate and methyl methacrylate. A similar observation was made by Fream & Magnet in “Low VOC, High Performance Coating Formulation”,
Farg och Lack Scandinavia
, 1/1998, pp. 4-9. These authors discussed the effects of blending two carboxylated styrene acrylate latexes, one having a T
g
of 57° C. and the other a T
g
of 0° C., in coating formulations. Although a reduction in film surface tack was noted, there was also considerable reduction in gloss.
Friel, U.S. Pat. No. 5,731,377 (1998) discloses paints based on emulsion polymer blends of two different polymers having relatively high and low T
g
values, e.g. one polymer having a T
g
above 20° C. and the other a T
g
below 15° C. The purpose given for making such blends is to improve blocking resistance. Although an extensive list of possible monomers is given, the only polymers described as suitable for both the high and low T
g
polymers are copolymers of butyl acrylate and styrene and/or methacrylic acid and/or methyl methacrylate, and copolymers of ethylhexyl acrylate, styrene and acrylonitrile for high T
g
polymers. No data or evaluations are given on the effect of high T
g
polymers on paint gloss. It is clear from the foregoing references that the problem of eliminating VOC from paint formulations while improving blocking resistance without loss of gloss in the paint surface has heretofore not been solved.
BRIEF SUMMARY OF THE INVENTION
We have now discovered that a water-borne glossy paint can be made from a blend of latexes of vinyl acetate polymer and vinyl acetate copolymer having relatively high and low T
g
values, respectively, both polymers being film-forming at room temperature, with marked improvement in blocking resistance but without significant reduction of gloss. In many cases gloss is actually improved which is very surprising in view of the prior art.
The vinyl acetate polymer is preferably a homopolymer but can contain up to 15 weight percent of comonomer such as other vinyl esters and acrylates. This vinyl acetate polymer must have a dry glass transition temperature (T
g
) above room temperature (23° C.) but a wet T
g
below room temperature. This polymer must also be film forming at room temperature. These vinyl acetate polymers are unique in this respect and differ functionally as well as chemically from the high T
g
(“hard”) polymers used in the prior art to enhance blocking resistance in glossy paints containing soft polymers.
The copolymer of vinyl acetate can be a copolymer of vinyl acetate and ethylene or a copolymer of vinyl acetate and an alkyl acrylate, the alkyl group preferably containing 1 to 8 carbons. The copolymer is also film forming at room temperature, having a T
g
less than 20° C. The proportion of high T
g
polymer of vinyl acetate in the blend with the low T
g
copolymer of vinyl acetate is generally in the range of 5 to 75 weight percent, preferably 10 to 60 weight percent, based on the combined polymer solids. Latex blends in which from 20 to 50 weight percent of the polymer solids are from the high T
g
polymer of vinyl acetate are demonstrated in the data of the examples herein and are still more preferred.
This invention, which is embodied in both composition of matter and the process steps taken to form the composition, has application only to paints in which gloss or luster is important. These paint applications include satin and semi-gloss paints as well as high gloss paints. For satin paints, the ASTM gloss (60°) gloss value will be above 10. For semi-gloss paints, the gloss value will be above 30. The preferred systems will be semi-gloss paints, however, the compositions of this invention can also be employed for satin as well as high gloss applications. Glossy in the invention refers to paint surfaces having gloss values in the range of satin, semi-gloss and high gloss applications. Flat paints would have gloss values (ASTM 60° gloss) less than 10 and are outside the scope of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The large market for architectural coatings (paint) has been continually undergoing changes in product type and formulation due to increasing need and regulation for lower VOC (volatile organic content) products. Initially (and continuing), the decrease in VOC paints involved a switch from solvent based to water-borne or high solids paints. The limitation on high solids due to viscosity problems has resulted in even more effort being expended on the development of water-borne paints. Nevertheless, for water-borne paints where gloss is important (e.g. satin, semi-gloss, high gloss) VOC addition is still necessary because the desired combination of properties (especially blocking resistance) has not been achievable without VOC addition. In water-borne paint, it has been well-recognized that a combination of relatively low T
g
and high T
g
polymer emulsions are necessary to yield film formation and blocking resistance at low to no VOC. This combination, however, leads to decreased gloss and thus is only viable in flat (low gloss) paints. With flat paints, the higher filler loading improves blocking resistance and gloss is not an issue. The present invention applies to glo

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