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
2000-10-11
2003-07-29
Sellers, Robert E. L. (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...
C525S172000, C525S173000, C525S176000, C525S508000
Reexamination Certificate
active
06599993
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to powder coating compositions that provide a low gloss, textured coating.
BACKGROUND OF THE INVENTION
Powder coating compositions have become increasingly important because they give off very little or no volatile material to the environment when cured. Typically, any such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation products.
Powder coatings have found use as both decorative coatings and protective coatings. In particular, powder coatings have been used to provide textured or matte surface coatings for various articles. Matte finishes have been obtained by adding a matting agent that may be a reactive component or may be inert. In the first instance, a matte finish can be obtained by including reactant or reactants that introduce incompatibility, such as a bicyclic amidine or reactants having a different rate of reaction. Alternatively, an inert powder, such as chalk or feldspar, can be added. These methods, however, have produced inconsistent results. Separation of the coating has been a problem. Problems have also be encountered in recycling the powder overspray. In addition, each method requires additional expense from incorporation of another material.
Textured finishes are similarly obtained by adding a texturing agent, for example pulverized poly(vinylidene fluoride) (PVDF) or an inorganic material such as coarse aluminum oxide. Schreffler, U.S. Pat. No. 5,212,263, incorporated herein by reference, discloses obtaining a grainy textured finish by combining a bisphenol A-type epoxy resin, methylene disalicylic acid, and an imidazole bisphenol A epoxy resin. Patel, U.S. Pat. No. 5,554,681, incorporated herein by reference, discloses a textured coating obtained from a powder coating composition including a solid silicone resin and an acrylic flow modifier that is a combination of an acrylic resin and a silica. The silicone resin and the acrylic flow modifier can be used in a powder coating based on a carboxyl-functional polyester resin and a solid bisphenol A epoxy resin.
Each of the methods of the prior art for obtaining matte or textured coatings requires addition of components that are relatively expensive compared to the resins used in the powder coating. Thus, two expensive additives must be included to obtain a textured, matte finish.
In addition to considerations of appearance, powder coating compositions must produce coatings that have integrity and durability. In some applications, such as for appliance coatings, the coating should have good stain resistance. Branched polyester resins have been marketed to the powder coatings industry as additive resins that may be included in powder coatings in amounts of one to three percent by weight, based on the weight of the total vehicle [“total vehicle” meaning all of the resinous or polymeric reactive components], to provide improved stain resistance. No effect on appearance was observed for these amounts.
SUMMARY OF THE INVENTION
The present invention provides a powder coating composition that includes at least about 9% by weight, based on total vehicle weight, of a branched, low molecular weight, carboxylic acid-functional polyester and at least one vehicle component reactive with carboxylic acid groups, preferably an epoxide-functional material. A process for obtaining a matte, textured coating includes steps of applying the powder coating composition of the invention to a substrate and curing the applied coating composition. As used herein, “matte” or “low gloss” refers to a coating having a gloss of 10% or less, preferably 5% or less, as measured at a 60° angle according to ASTM D523. As used herein, “textured” means a granular appearance.
DETAILED DESCRIPTION OF THE INVENTION
The powder coating composition that produces the low gloss, textured coating includes at least an epoxide-functional material and at least about 9% by weight, based on the total vehicle weight, of a branched, low molecular weight polyester.
The polyester resin of the invention has an average of at least about 2.2 carboxylic acid groups and may have an average of up to about 6 carboxylic acid groups. In a preferred embodiment, the polyester resin has a number average molecular weight of from about 425 to about 1050.
Such polyester resins may be obtained by the condensation reaction between a polyol component and a poly-functional carboxylic acid component, in which at least one polyol compound of the polyol component has three or more hydroxyl groups and/or at least one carboxylic acid compound of the poly-functional carboxylic acid component has three or more carboxylic acid groups. The poly-functional carboxylic acid component may also have carboxylic acid anhydrides of carboxylic acid groups. The equivalents of the carboxylic acid and/or anhydride of carboxylic acid are in excess so that an acid-functional polyester is formed.
The polyol component used to make the branched, low molecular weight polyester has an average functionality of at least two, and preferably more than two. The average functionalities of the polyol component and the carboxylic acid component are selected to provide a polyester resin that has at least about 2.2 carboxylic acid groups, on average, per molecule and up to about 6 carboxylic acid groups, on average, per molecule. The polyol component may contain mono-functional, di-functional, tri-functional, and higher functional alcohols. Mixtures of diols and triols are preferred as the polyol component. Examples of useful polyols include, without limitation, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butylene glycol, glycerine, trimethylolpropane, trimethylolethane, pentaerythritol, neopentyl glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,4-cyclohexane dimethanol, hydrogenated bisphenol A, and hydroxyalkylated bisphenols. The carboxylic acid component may likewise include mono-functional, di-functional, tri-functional, and higher functional carboxylic acid compounds or anhydrides thereof. Suitable examples include, without limitation, phthalic acid, isophthalic acid, terephthalic acid, hexahydrophthalic acid, pyromellitic acid, succinic acid, azeleic acid, adipic acid, 1,4-cyclohexanedicarboxylic acid, citric acid, trimellitic acid, and anhydrides thereof.
Methods for making polyester resins are well-known. Typically, the polyester is formed by heating together the polyol and carboxylic acid components, with or without catalysis, while removing the by-product of water to drive the reaction toward completion. A small amount of a solvent, such as toluene or xylene, may be added to aid in removing the water through distillation. If added, such solvent is preferably removed form the polyester product before the powder coating formulation is begun. The amounts of polyol materials and carboxylic acid materials are selected and apportioned to produce a polyester having a number average molecular weight of from about 425 to about 1050. The polyester equivalent weight is preferably from about 150 to about 300, preferably from about 150 to about 250, and particularly preferably from about 175 to about 215. The average functionality of the polyester resin (i.e., the average number of carboxylic acid groups per macromolecule of the acid-functional polyester) is at least about 2.2, preferably at least about 2.5, preferably at least about 3, and may be up to about 6, preferably up to about 4. In one preferred embodiment the average functionality of the polyester resin is from about 3 to about 4. The polyester resin should be a solid at room temperature. Preferably, the polyester resin should have a glass transition temperature of at least about 50 to 55° C. to prevent sintering of the powder coating composition. Suitable branched, low molecular weight polyester resins are commercially available. One preferred product is XP 5600, available from Ruco Polymer Corporation, Hicksville, N.Y., 11802.
The powder coating composition further includes at lea
Norris William C.
Rademacher Josef
BASF Corporation
Budde Anna M.
Sellers Robert E. L.
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