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
2000-05-16
2003-07-29
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S437000, C525S440030, C525S934000, C427S195000, C427S385500, C427S386000
Reexamination Certificate
active
06599992
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 external durability and excellent mechanical properties while having low or substantially zero VOC levels.
Powder coating compositions are now being used in a wide variety of painting techniques. They have become increasingly desirable, particularly in the automotive and truck industries, for various reasons including those related to ecology, health and safety. For example, powder coating compositions can potentially 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. Moreover, there is a concern about the potential environment impact of the use of TGIC. It is also common to use blocked polyisocyanate crosslinking agents. For example, caprolactam-blocked isophoronediisocyanates and toluene 2,4-diisocyanate are described in U.S. Pat. No. 3,822,240 to Schmitt et al., U.S. Pat. No. 4,150,211 to Muller et al. and U.S. Pat. No. 4,212,962 to Schmitt et al.
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 monobasic acid 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.
There, however, remains a need for a powder coating composition that has excellent resistance to degradation, and eliminates the need for potentially hazardous modifiers, curing agents (e.g., TGIC) or crosslinking agents (e.g., blocked polyisocyanates).
SUMMARY OF THE INVENTION
To this end, the present invention provides a powder coating composition. The powder coating composition comprises a polyester resin having both hydroxyl and carboxyl groups, a curing agent for the carboxyl groups of the polyester resin, and an uretdione hardener which reacts with the hydroxyl groups. The curing agent for the carboxyl groups is preferably a compound having active hydrogens, and for example is a beta-hydroxy(1)alkylamide. Alternatively, an aromatic compound containing glycidyl groups can be used, and for example, is a di or triglycidyl ester of terephthalic or trimellitic acid or mixtures of both.
DETAILED DESCRIPTION OF THE INVENTION
As detailed above, the powder coating composition of the present invention comprises a polyester resin having both hydroxyl and carboxyl groups, a curing agent for the carboxyl groups of the polyester resin, and an uretdione hardener which reacts with the hydroxyl groups.
The polyester resin includes carboxyl and hydroxyl functionality, i.e., it is a “bi-functional” polyester. With respect to the carboxyl portion, 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 amount of polyester, by weight is preferably from about 70 to 97 percent, and more preferably is 80 to 95 percent by weight of the powder coating composition. The polyester resin preferably has a hydroxyl value of about 5 to 100 mg KOH/g, more preferably 7 to 40 mg KOH/g, and an acid value preferably of about 5 to 100 mg KOH/g, and more preferably 7 to 40 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 curing agent for the carboxyl groups is preferably a compound having active hydrogens. Exemplary compounds having active hydrogens are the beta-hydroxyalkylamides. They provide a crosslinked polymer network which is hard, durable, corrosion resistant and solvent resistant. It is believed the beta-hydroxyalkylamides cure the coating (i.e., is the curing agent for the carboxyl group) through an esterification reaction with the carboxy-containing compounds forming multiple ester crosslinks. The hydroxyl functionality of the hydroxyalkylamide should be on an average basis of at least 2, preferably greater than 2, and more preferably from greater than 2, up to and including 4, in order to obtain an optimum curing. The beta-hydroxyalkylamide curing agents are described in U.S. Pat. No. 4,937,288 to Pettit et al. the disclosure of which is incorporated herein by reference in its entirety. The amount of beta-hydroxyalkylamide curing agent by weight is preferably about 0.5 to 10 percent, and more preferably is 1 to 7 percent by weight of the total powder coating composition.
The beta-hydroxyalkylamides can be prepared, for example, by reacting a lower alkyl ester or mixture of esters of carboxylic acids with a beta-hydroxyalkylamine at a temperature ranging from ambient temperature up to about 200° C. depending on the choice of reactants and the presence or absence of catalyst. Suitable catalysts are basic catalysts including e.g., sodium methoxide, potassium methoxide, sodium butoxide, potassium butoxide, sodium hydroxide, and potassium hydroxide present preferably in amounts of about 0.1 to about 1 percent by weight based on weight of alkyl ester. A particularly suitable commercially available &bgr;-hydroxyalkylamide is Primid XL-552 or Primid QM-1260 available from EMS-Primid, Switzerland.
Alternatively the curing agent for the carboxyl groups can be an aromatic compound containing glycidyl groups. Exemplary compounds include di or triglycidyl esters of terephthalic or trimellitic acid or mixtures of both. A specific example is Araldite PT 910 available from Ciba Specialty Chemicals Inc., Switzerland.
The uretdione hardener reacts with the hydroxyl groups. 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 to O'Connor the disclosure of which is incorporated by reference herein in its entirety. Any suitable organic polyisocyanate, or mixture of polyisocyanates, which is susceptibl
Brunmair Christoph Alfred
Kaiser Rudolf Leopold
Reichhold Inc.
Short Patricia A.
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