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
1999-06-28
2002-02-26
Buttner, David J. (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...
C525S444000, C428S413000
Reexamination Certificate
active
06350821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to low gloss, ultraviolet-durable, matte powder coating compositions and a method for preparing such matte powder coating compositions, and more particularly to matte powder coating compositions including two carboxyl-functional polyester resins having a difference in acid values of from about 200 to about 350 mg KOH/g.
BACKGROUND AND SUMMARY OF THE INVENTION
Powder coatings have become increasingly important because they give off very little volatile material to the environment during application and during cure. Typically, any such emissions are limited to minor amounts of by-products from the curing reaction, such as blocking agents or volatile condensation products.
There is also an increasing need for matte powder coatings exhibiting ultra-low gloss and ultraviolet-durable characteristics. Matte finishes in powder coatings typically may be achieved by crosslinking two epoxy resins with a curing agent, such as a mono- or di-salt of a polycarboxylic acid and cyclic amidine. The two resins employ incompatible crosslinking mechanisms, where the first resin has a fast cure time, and the second resin cures at a slower rate and has a different shrinking or expanding coefficient than the first resin. The resulting micro-roughness of the coating surface scatters light, providing a low-gloss surface. These coatings are typically not, however, ultraviolet-durable or stable such that the components of the coating will degrade when exposed to ultraviolet light.
Other known matte powder coatings employ triglycidyl isocyanurate (TGIC) as a crosslinking agent in a polyester resin system. The most common approach of lowering the gloss of this type of coating is to extrude two different resin systems that differ only in their level of reactivity. The two resin systems are processed independent of one another, and are blended together after grinding of the powder coating is completed. The combination of a high reactivity powder with a low reactivity powder causes an incompatibility in the system, lowering the gloss. This type of powder coating, however, is limited to gloss ranges only as low as 30% gloss at a 60° angle. It would, therefore, be desirable to produce matte powder coating systems that yield lower gloss ranges and can be processed by a single extrusion step.
The present invention provides a matte powder coating composition including a first carboxyl-functional polyester resin, a second carboxyl-functional polyester resin, and an epoxide-functional crosslinking agent, where the difference in acid values between the first and second carboxyl-functional polyester resins is from about 200 to about 350 mg KOH/g.
The present invention also provides a matte powder coating composition comprising a first carboxyl-functional polyester resin having an acid value of from about 30 mg KOH/g to about 80 mg KOH/g. The coating also includes a second carboxyl-functional polyester resin having an acid value of from about 300 mg KOH/g to about 330 mg KOH/g and an epoxide-functional crosslinking agent. The first and second carboxyl-functional polyester resins are extruded together in a single step process.
The present invention additionally provides a method of preparing a matte powder coating composition comprising the steps of dry blending together two carboxyl-functional polyester resins and an epoxide-functional crosslinking agent, producing a substantially homogeneous mixture. The mixture is next melt blended in an extruder in a single step process, producing an extrudate. The extrudate is then pulverized. The difference in acid values between the two carboxyl-functional polyester resins is from about 200 mg KOH/g to about 350 mg KOH/g.
The present invention further provides a coated substrate coated by the method including applying a matte powder coating composition to the substrate. The matte powder coating comprises a first carboxyl-functional polyester resin, a second carboxyl-functional polyester resin, and an epoxide-functional crosslinking agent, and the difference in acid values between the first and second carboxyl-functional polyester resins is from about 200 to about 350 mg KOH/g. The method also includes curing the matte powder coating composition.
DETAILED DESCRIPTION
The polyester resins of the present invention have an average of two or more carboxyl groups per molecule. Such polyester resins are obtained by the condensation reaction between a poly-functional acid component and a polyol component. The equivalents of acid are in excess so that an acid-functional polyester is formed. According to the present invention, at least two carboxy-functional polyester resins are employed in the powder coating composition. In a preferred embodiment, the difference in acid values between the polyester resins is between about 200 to about 350, and most preferably between about 265 to about 345. The first polyester resin preferably has an acid value of about 30 mg KOH/g to about 80 mg KOH/g. In a preferred embodiment, the first polyester has an acid value of about 35 mg KOH/g. In addition, the powder coating composition includes a second polyester resin that has a substantially higher acid number, preferably an acid value of about 280 mg KOH/g to about 380 mg KOH/g, and more preferably about 300 mg KOH/g to 330 mg KOH/g. In a most preferred embodiment, the second polyester resin has an acid value of about 320 mg KOH/g.
The poly-functional acid component used to prepare each of the polyester resins comprises compounds having two or more carboxyl groups or their anhydrides. Such compounds may be alkylene, aralkylene, or aromatic compounds. Dicarboxylic acids and anhydrides of dicarboxylic acids are preferred. Acids or anhydrides with higher functionality may be used when some branching of the polyester is desired. When tri-functional compounds or compounds of higher functionality are used, it is also possible to include mono-functional carboxylic acids or anhydrides of monocarboxylic acids, such as versatic acid, fatty acids, or neodecanoic acid, so long as the poly-functional acid component has an average functionality of at least about two.
Illustrative examples of compounds having two or more carboxyl groups or anhydrides of such compounds include, without limitation, phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, tetrachlorophthalic anhydride, hexahydrophthalic anhydride, pyromellitic anhydride, succinic acid, azelaic acid, adipic acid, 1,4-cyclohexanedicarboxylic acid, citric acid, oxalic acid, maleic acid, malonic acid, maleic anhydride, glutaric acid, pimelic acid, trimellitic anhydride, and combinations of these.
The polyol component used to make the polyester resins also has an average functionality of at least about two. The polyol component may contain mono-, di-, and tri-functional alcohols, as well as alcohols of higher functionality. Diols are preferred as the polyol component. Alcohols with higher functionality may be used when some branching of the polyester is desired, and mixtures of diols and triols are also 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, 1,4-butanediol, neopentyl glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, hydrogenated bisphenol A, hydroxyalkylated bisphenols, and combinations of these.
The methods of making polyester resins are well-known. Polyesters are typically formed by heating together the polyol and poly-functional acid components, with or without catalysis, while removing the by-product of water in order to drive the reaction to the desired extent of completion. A small amount of a solvent, such as toluene, may be added in order to help remove the water azeotropically. If added, such solvent is preferably removed from the polyester product before powder coating formulat
BASF Corporation
Buttner David J.
Selby Jennifer Loope
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