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
1998-09-18
2001-06-19
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...
C525S934000, C528S045000, C528S080000, C528S081000, C528S296000
Reexamination Certificate
active
06248843
ABSTRACT:
The present invention relates to powdered coating compositions which can be cured at low temperatures with or without the use of a catalyst. More particularly, the present invention relates to branched hydroxyl terminated oligoesters which when cross-linked provide improved performance properties at low curing temperatures even without catalyst.
BACKGROUND OF THE INVENTION
Thermosetting powder coating compositions are well known in the art and are widely used as coatings for electric appliances, bicycles, garden furniture, accessories for the automotive industry, general metal parts and the like. Thermosetting powders consist of a mixture of a primary resin and one or more cross-linkers, often called hardeners or curing agents. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. In so far as possible, each particle contains all of the ingredients in the formulation. The powder is applied to the substrate, usually but not limited to a metal, and fused to a continuous film by baking.
A problem with powder coating compositions is that they frequently have low glass transition temperatures (T
g
) which will lead to agglomeration or sintering of the powdered coating particles when they are stored at elevated temperatures for a prolonged duration of time. The agglomerated powder coating can cause problems during application and may require re-milling, which itself can cause further problems by, for example, adversely affecting the particle size distribution of the re-milled powder coating. Often, powder coating compositions which are formulated to avoid problems with agglomeration and sintering do not provide optimal coatings properties.
It is an object of the invention to provide a powder coating composition which has a relatively high glass transition temperature, will resist agglomeration during storage and which composition will cure without a catalyst at a relatively low temperature.
It is an object of the invention to provide a powdered coating composition which will maximize film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss, yet also provide a coating composition with a relatively high glass transition temperature and which will cure without a catalyst at relatively low temperatures.
It is another object of the invention to provide a powdered coating composition which can be cured at temperatures as low as about 150° C. without the use of an effective amount of catalyst.
It is another object of the invention to provide a powder coating composition with a desirable melt viscosity.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.
SUMMARY OF THE INVENTION
The present invention provides a powder coating composition that will not readily agglomerate during storage and can be cured at temperatures as low as about 140° C. with the use of a urethane catalyst and at 145° C. to about 155° C. without the use of a catalyst.
The powder coating composition of the invention comprises a unique combination of a branched oligoester polyol and cross-linking agent which when cured results in a coating with desirable hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss. The branched oligoester polyol has a unique combination of branched structure, number average molecular weight, hydroxyl number, and acid number which provides a relatively high glass transition temperature, and hence, agglomeration resistance. When the latter branched oligoester polyol is cured with a triazole blocked isocyanate, the combination of branched oligomer and isocyanate provides a coating binder with good performance characteristics even without catalysts. The invention provides a reactivity and high rate of cure at lower temperatures without sacrificing storage stability to agglomeration or sintering.
The branched oligoester polyol has a Tg of at least about 40° C. to about 80° C., a number average molecular weight of from about 1000 to about 7500 daltons, a hydroxyl functionality of about 1.5 to about 5.0, a hydroxyl number of from about 15 to about 250 and an acid number of about 1 to about 25, and in an very important aspect, an acid number of about 5 to about 7. In another important aspect, the branched oligoester will have a viscosity of from about 20 to about 90 poise at about 200° C.
The powder coating composition of the invention comprises the branched oligoester polyol and triazole blocked isocyanate powder coating crosslinking agent each in relative amounts which are effective for providing crosslinked coating compositions with a pencil hardness of at least about H, a direct impact resistance of at least about 80 in lb and a reverse impact resistance of at least about 80 in lb at a binder thickness of about 0.8 to about 2.5 mils when curing is conducted on ground steel panels of a thickness of 0.032 inches at temperatures as low as from about 140° C., preferably about 145° C. to about 155° C. The powder coating composition of the invention which comprises the branched oligoester polyol and triazole blocked isocyanate has a Tg of from about 40° C. to about 70° C. In an important aspect the powder coating composition comprises from about 18 to about 97 weight percent of the branched hydroxyl terminated oligoester, based on the weight of branched oligoester polyol and crosslinking agent.
The branched oligoester polyol may be synthesized by forming a generally linear hydroxyl terminated oligoester diol by reacting a diol and a diacid and then reacting the resulting hydroxyl terminated oligoester diol with less than a stoichiometric amount (relative to the hydroxyls on the oligoester) of a polyacid having a carboxyl functionality of at least about 3. This less than stoichiometric amount provides some carboxyl groups to the oligomer, but its more important purpose is to generally provide complex branching of the oligoester polyol so that oligomer chains extend in some cases, from all of the carboxyl functionality of the polyacid and some of the polyacids are interconnected by oligomer chains. In an important aspect, the carboxyl functionality from the polyacid reacted with the oligoester is not more than about 15% of the equivalents of the stoichiometric amount of carboxyl equivalent needed to react with all of the hydroxyl groups of the oligoester. In an important aspect, the ratio of hydroxyl terminated oligoester diol to triacid is from about 9.0:1 to about 30:1, preferably about 10:1 to about 20:1.
In an important aspect of the invention, the hydroxyl terminated diol is the reaction product of an aliphatic diol (open chain or cycloaliphatic) and an aromatic diacid, diacid halide, or diacid anhydride, such as terephthalic acid, which provides a hydroxyl terminated oligoester diol having aromatic groups. Alternatively in this aspect, the acid may be a straight chain or cycloaliphatic diacid, diacid anhydride or diacid halide, and the diol may be hydroquinone to provide the oligoester with aromatic monomers along its main chain.
In another important aspect, the diol used for the oligoester diol is a straight chain aliphatic or cycloaliphatic diol and the diacid is a cycloaliphatic diacid, diacid anhydride, or diacid halide, which monomers provide an oligoester diol having cycloaliphatic groups.
In yet another aspect, if the diacid, diacid anhydride or halide and diol used to make the oligoester diol are both straight chain, an aromatic monomer having hydroxyl and carboxyl functionality may be used to improve properties of the ultimate coating composition. These aromatic monomers having hydroxy and carboxy functionality include ortho, meta, and parahydroxybenzoic acid. While not intending to be bound by any theory, it appears that the ring and straight chain combination, or aroma
Bronk John Michael
Panandiker Kamlesh Pai
Pont James Duncan
Fitch Even Tabin & Flannery
McWhorter Technologies Inc.
Short Patricia A.
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