Coating processes – Nonuniform coating – Metal – glass – or ceramic base
Reexamination Certificate
1995-06-06
2001-10-16
Henderson, Christopher (Department: 1713)
Coating processes
Nonuniform coating
Metal, glass, or ceramic base
C052S309100, C524S832000
Reexamination Certificate
active
06303186
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for improving the dirt-resistance and gloss retention of a coalescent-free coating composition.
1. Background of the Invention
When coatings incorporate an emulsion-polymerized polymer as binder, there is a need for a soft binder polymer, i.e., a polymer with a glass transition temperature (Tg), and, more particularly, a minimum film formation temperature (MFFT), lower than the temperature of application of the coating, so that facile film formation via fusion of the emulsion-polymerized polymer particles can be achieved; at the same time there is a need for the surface hardness, dirt-resistance, outdoor durability, etc. characteristic of a harder polymer, i.e., a polymer with a Tg higher than the temperature of use of the coating. One solution to this problem is using a high Tg coatings binder formulated with a coalescent or volatile plasticizer so as to temporarily achieve the facile film formation characteristics of a low Tg binder along with the coatings film properties of a higher Tg polymer after the coalescent has evaporated. However, concerns regarding the odor and toxicity of some organic solvent coalescents and current and projected legislation limiting the amount of volatile organic compounds allowable in coatings have made low or no coalescent coatings desirable.
2. Description of the Prior Art
U.S. Pat. No. 4,421,889 discloses dispersion paints with high wet adhesion which are obtained by incorporating into the binder from 0.5 to 10%, by weight, acetoacetic acid ester monomers and by incorporating into the paint from 3% to 20%, by weight relative to the total weight of the paint, organic solvent film consolidation agents.
The problem faced by the inventor was to provide a coating composition substantially free from coalescent, the dry coating having improved dirt resistance and improved gloss retention. Improved dirt resistance and improved gloss retention are defined relative to the dirt resistance and gloss retention of a coating composition containing a binder with a Tg sufficiently low to permit facile film formation without the use of coalescent.
SUMMARY OF THE INVENTION
This invention is directed to a method for providing improved dirt pick-up resistance and improved gloss retention for a coalescent-free aqueous coating composition containing an aqueous emulsion-polymerized polymeric binder having a glass transition temperature from about −35 C. to about +25 C., wherein the binder contains from about 2% to about 20%, by weight based on the weight of said polymeric binder, of at least one copolymerized ethylenically-unsaturated active methylene monomer. Preferred as an active methylene monomer is an ethylenically-unsaturated monomer bearing acetoacetate functionality.
Examples of monomers useful for introduction of acetoacetate functionality are acetoacetoxyethyl acrylate, acetoacetoxyethyl methacrylate (AAEM), acetoacetoxypropyl methacrylate (AAPM), allyl acetoacetate, acetoacetoxybutyl methacrylate, 2,3-di(acetoacetoxy) propyl methacrylate and the like. In general, any polymerizable hydroxy functional monomer can be converted to the corresponding acetoacetate by reaction with diketene or other suitable acetoacetylating agent (See e.g.
Comparison of Methods for the Preparation of Acetoacetylated Coating Resins
, Witzeman, J. S.; Dell Nottingham, W.; Del Rector, F. J. Coatings Technology; Vol. 62, 1990, 101 (and references contained therein)).
The polymeric binder also contains from about 80% to about 95% by weight, based on the weight of the polymeric binder, of at least one copolymerized ethylenically-unsaturated monomer. For example, acrylic ester monomers including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate; acrylamide or substituted acrylamides; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrollidone; acrylonitrile or methacrylonitrile; may be used. Low levels of copolymerized ethylenically-unsaturated acid monomers such as, for example, 0.1%-7%, by weight based on the weight of the polymeric binder, acrylic acid, methacrylic acid, crotonic acid, phosphoethyl methacrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid, sodium vinyl sulfonate, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride may be used.
The polymeric binder used in this invention is a substantially thermoplastic, or substantially uncrosslinked, polymer when it is applied to the substrate, although low levels of deliberate or adventitious crosslinking may be present. When low levels of precrosslinking or gel content are desired low levels of multi-ethylenically unsaturated monomers such as, for example, 0.1%-5%, by weight based on the weight of the polymeric binder, allyl methacrylate, diallyl phthalate, 1,4-butylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, and the like, may be used. It is important, however, that the quality of the film formation is not materially impaired.
The glass transition temperature of the polymeric binder is from about −35 C. to about +25 C., as measured by differential scanning calorimetry (DSC). The emulsion polymer samples are dried, preheated to 120 C., rapidly cooled to −100 C., and then heated to 150 C. at a rate of 20 C./minute while data is being collected. The Tg is measured at the midpoint using the half-height method.
The emulsion polymerization techniques used to prepare such dispersions are well known in the art. Conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as alkali or ammonium alkyl sulfates, alkyl sulfonic acids, fatty acids, and oxyethylated alkyl phenols. The amount of surfactant used is usually 0.1% to 6% by weight, based on the weight of total monomer. Either thermal or redox initiation processes may be used. Conventional free radical initiators may be used such as, for example, hydrogen peroxide, t-butyl hydroperoxide, ammonium and/or alkali persulfates, typically at a level of 0.05% to 3.0% by weight, based on the weight of total monomer. Redox systems using the same initiators coupled with a suitable reductant such as, for example, isoascorbic acid and sodium bisulfite may be used at similar levels.
The particle size of the emulsion-polymerized polymeric binder is from about 50 nanometers to about 500 nanometers in diameter. The particles may be composed of two or more phases such as, for example, core/shell particles, core/shell particles with shell phases incompletely encapsulating the core, corelshell particles with a multiplicity of cores, interpenetrating network particles, and the like.
In addition to the polymeric binder, the coalescent-free aqueous coating composition may contain conventional coating components such as, for example, emulsifiers, pigments, fillers, anti-migration aids, curing agents, wetting agents, biocides, anti-foaming agents, colorants, waxes, and anti-oxidants.
The coalescent-free aqueous coating composition is free from volatile organic compounds. “Coalescent-free” as used herein means that the coating composition does not contain “volatile organic compounds” defined herein as organic compounds having a boiling point at atmospheric pressure of less than about 250 C., as may materially contribute to pollution, toxicity, and/or volatile organic content of the coating compositions. “Coalescent-free” composition is not to be taken as excluding compositions which contain very low levels of deliberately or adventitiously added volatile organic compounds such as, for example, volatile organic compounds incorporated in certain commercial initiator or surfactant compositions, volatile organic compounds formed during the polymerization reaction, and volatile organic compound impurities; in any event the level of volatile organic co
Bors Daniel A.
Hermes Ann R.
Rokowski Joseph M.
Speece David G.
Bakule Ronald D.
Henderson Christopher
Rohm and Haas Company
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