Burnish resistant powder coating composition

Stock material or miscellaneous articles – Composite – Of polyamidoester

Reexamination Certificate

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C428S425800, C428S482000, C427S195000, C427S385500, C427S386000, C427S388200

Reexamination Certificate

active

06472071

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
This invention relates to a powder coating composition, and more particularly a powder coating composition that exhibits excellent resistance to degradation and is burnish resistant.
Powder coating compositions are now being used in a wide variety of painting techniques. They have become increasingly desirable, particularly in the automotive and track industries, for various reasons including those related to ecology, health and safety. For example, powder coating compositions can reduce the amount of volatile solvents used as compared to liquid paints. After curing, typically using heat, only a very small amount, if any, of volatile solvent is given off to the environment.
Various powder coating compositions have been suggested. For example, a powder coating composition with excellent exterior durability can be prepared by reacting an acid group-containing acrylic polymer and a curing agent, triglycidyl isocyanurate (TGIC). Such coating compositions, however, often have poor stability and physical properties.
U.S. Pat. No. 4,499,239 to Murakami et al. proposes a composition comprising 60 to 97 percent by weight of a linear polyester resin having an acid number of 15 to 200 mg KOH/g and 3 to 40 percent by weight of a glycidyl group-containing acrylic polymer, and optionally is modified with a vinyl monomer such as methyl methacrylate. Powder coating compositions comprising a copolymer of glycidyl methacrylate, an ethylenically unsaturated compound, and a crosslinking agent formed in an anhydride of a dicarboxylic acid are proposed in U.S. Pat. Nos. 3,758,632, 3,781,379, 2,888,943 and 4,091,049 to Labana et al.
These compositions, however, tend to provide high gloss finishes. Powder coating compositions having a low gloss (matte) finish are becoming more desirable for use in automotive interiors, and for wheel rims, bumpers and the like. A common way to reduce gloss is to incorporate a filler such as talc or silica into the composition. It is believed that the gloss is lowered by the formation of minute surface irregularities on the coating. Another alternative is proposed in U.S. Pat. Nos. 5,436,311 and 5,525,370 to Hoebeke et al. which proposes a low gloss composition using a high percentage of methyl methacrylate in the composition. Other low gloss compositions are proposed in U.S. Pat. Nos. 4,242,253 to Yallourakis, and U.S. Pat. No. 5,491,202 to Umehara et al.
These low gloss compositions, however, are often not resistant to burnish. Namely, an article having low gloss will change to a high gloss after marring, scratching or rubbing. There is also a need to improve weatherability and durability.
Thus, there remains a need for a low gloss powder coating composition that has excellent resistance to degradation, namely is durable and weather resistant, and is burnish resistant.
SUMMARY OF THE INVENTION
To this end, the present invention provides a burnish resistant and durable powder coating composition comprising a polyester resin having hydroxyl and carboxyl groups and a glycidyl-containing acrylic copolymer wherein the polyester resin has a glass transition temperature (hereinafter “T
g
”) less than about 55° C. and the glycidyl-containing acrylic copolymer has a number average molecular weight greater than about 8000.
In another embodiment, the present invention provides a powder coating composition comprising about 20 to 90 percent by weight of a polyester resin having hydroxyl and carboxyl groups, preferably having a T
g
of less than about 55° C., about 5 to 60 percent by weight of a glycidyl-containing acrylic copolymer and about 2 to 20 percent by weight of a blocked polyisocyanate derivative.
In still another embodiment, the present invention provides a method of preparing a burnish resistant coating on an article of manufacture. The method comprises applying to the article a powder coating composition comprising a polyester resin having hydroxyl and carboxyl groups and a glycidyl-containing acrylic copolymer and curing the coated article of manufacture preferably at a temperature of about 120° C. to 220° C.
DETAILED DESCRIPTION OF THE INVENTION
As summarized above, the powder coating comprises a polyester resin including carboxyl and hydroxyl functionality and a glycidyl-containing acrylic copolymer. The polyester resin includes carboxyl and hydroxyl functionality, i.e., it is a “bi-functional” polyester. The amount of polyester, by weight is preferably from about 20 to 90 percent, and preferably is about 25 to 75 percent by weight of the powder coating composition.
With respect to the carboxyl portion of the polyester resin, the starting acid and alcohol components thereof may be known conventional dibasic and polybasic acids, and dihydric and polyhydric alcohols, respectively. Typical examples of the acid component include terephthalic acid, isophthalic acid, phthalic acid, methylterephthalic acid, trimellitic acid, pyromellitic acid, adipic acid, sebacic acid, succinic acid, maleic acid, fumaric acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, and the anhydrides thereof. Typical examples of the alcohol component include ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, isopentyl glycol, bishydroxyethyl terephthalate, hydrogenated bisphenol A, an ethylene oxide adduct of hydrogenated bisphenol A, a propylene oxide adduct of hydrogenated bisphenol A, trimethylolethane, trimethylolpropane, glycerol, pentaerythritol and 2,2,4-trimethylpentane-1,3-diol. Monoepoxy compounds can also be used as one glycol component. The polyester resin preferably has an acid value of about 5 to 100 mg KOH/g, and more preferably about 10 to 30 mg KOH/g.
With respect to the hydroxyl portion, the above alcohol components may be used, namely ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, isopentyl glycol, bishydroxyethyl terephthalate, hydrogenated bisphenol A, an ethylene oxide adduct of hydrogenated bisphenol A, a propylene oxide adduct of hydrogenated bisphenol A, trimethylolethane, trimethylolpropane, glycerol, pentaerythritol and 2,2,4-trimethylpentane-1,3-diol. The polyester resin has a hydroxyl value of about 5 to 100 mg KOH/g, preferably about 10 to 30 mg KOH/g. Preferably, the polyester resin is a softer resin, namely it has a T
g
of less than about 55° C.
The blocked isocyanate derivative reacts With the hydroxyl group of the polyester. The amount of blocked polyisocyanate derivative is preferably from about 1 to 30 percent, and more preferably about 2 to 20 percent by weight of the total powder coating composition. Typical blocked isocyanates include products obtained by blocking the free isocyanate groups present in aliphatic, aromatic and alicyclic isocyanates such as hexamethylene diisocyanate, xylylene diisocyanate and isophorone diisocyanate or adducts of these. isocyanates with compounds having active hydrogen, with conventional blocking agents such as methanol, isopropanol, butanol, ethyl lactate and e-carprolactam. Examples of the compounds having active hydrogen are various low-molecular weight compounds such as ethylene glycol, propylene glycol, butylene glycol, trimethylopropane, glycerol, ethylenediamine and hexamethylenediamine, and various high-molecular-weight compounds such as polyols, polyethers, polyesters and polyamides.
Another class of blocked isocyanates derivatives are uretdione hardeners. Suitable uretdione hardeners are self-blocked uretdione groups containing polyisocyanates. These uretdione group-containing organic polyisocyanates can be prepared in accordance with well-known methods by dimerization of the corresponding organic polyisocyanate. See Saunders, J. H. and Frisch, K. C.,
Polyurethane: Chemistry and Technology, Part I
, pages 91-94 (1962) and U.S. Pat. No. 4,522,975 the disclosure of which is incorporated by reference herein in its entirety. Any suitable organic polyisocyanate, or mixture of polyisocyanates, which is susceptible to d

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