WO1992014770A1 - Powder coating compositions - Google Patents

Abstract

Provided are thermosetting powder coating compositions comprised of blends of aromatic polyesters and all aliphatic polyesters derived from trans-1,4-cyclohexane-dicarboxylic acid and 1,4-butanediol, which, along with a blocked polyisocyanate, upon application to a substrate and curing provide a surface coating possessing good gloss, impact strength, flexibility and weatherability.

Description

POWDER COATING COMPOSITIONS

Field of the Invention

This invention belongs to the field of powder coatings. More particularly, this invention relates to a novel blend of polyester-based powder coating compositions.

Background of the Invention

Plastic materials used in the manufacture of powde coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.

Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, hav better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems i obtaining coatings which have, in addition to the above stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resultin in a relatively rough finish referred to as an "orange peelⁿ surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained frc thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coalings to be applied from organic solvent systems which are inherently 2 -

undesirable because of the environmental and safety problems occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 6 percent or less of the solvent-based coating compositio being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does no contact the article or substrate being coated obviously cannot be reclaimed.

In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compoεi- tions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.

Powder coatings based on aromatic polyesters are well-known but generally suffer from poor weather- ability.

Brief Description of the Drawing

Figure 1 is a graph of QUV weathering of the powder coatings of Comparative Example 1, and Examples 2 and 3, all of which are described in the Experimental Section below. "QUV" weathering is performed by exposing the coating to high intensity ultraviolet radiation, thereby simulating performance of the coating in the presence of sunlight, albeit on an accelerated basis. The plot indicated by bold dots is the coating of Comparative Example 1; the plot indicated by open-circled points is the coating of Example 2; and the plot indicated by "triangle-points" is the coating of Example 3. Percent gloss retention is plotted versus time.

Summary of the Invention

The present invention provides a novel blend of polymers having free hydroxyl groups, which, when combined with a cross-linking agent and cured, provides coatings which have superior gloss retention when exposed to ultraviolet radiation.

Detailed Description of the Invention

The present invention provides powder coating compositions having good to excellent gloss, impact strength (toughness), flexibility, and weatherability superior to that of coatings based on aromatic poly- esters. The time elapsed while subjecting powder coatings to QUV conditions which cause a 50% loss in gloss, is referred to herein as "GS_Q"_' AS is evident from Figure 1, the powder coating compositions of the present invention are far superior, with regard to gloss retention, than coatings based on aromatic polyesters. Thus, the present invention provides thermosetting powder coating compositions comprising:

(1) a novel blend of polymers having free hydroxy groups comprised of:

(a) about 10 to 80 weight percent of an aromatic polyester having a glass transition tempera¬ ture (Tg) of greater than 40°C, a hydroxyl - 4 -

number of about 20 to 200 and an inherent viscosity of about 0.1 to 0.5; and

(b) about 20 to 90 weight percent of poly(tetra- methylene trans-l,4-cyclohexanedicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5; and

(2) a cross-linking effective amount of a cross-linking agent.

The effectiveness of this novel blend becomes apparent in the comparison of accelerated QUV weathering of powder coatings formulated with (1) a blend of polymers containing hydroxy end groups and comprised of 50 weight percent of an aromatic polyester and 50 weight percent of poly(tetramethylene trans-1,4-cyclohexane- dicarboxylate) as described above, and (2) an aromatic polymer containing hydroxy groups. Each formulation contained a blocked polyisocyanate as cross-linking agent and 1 percent of a conventional ultraviolet light stabilizer in combination with 1 percent of a hindered amine light stabilizer (HALS). During QUV exposure, formulations of (1) and (2) retained 50% of 60^CC gloss after 810 and 280 hours, respectively. This data illustrates the superior weathering of coatings formulated with the novel blend of an aromatic polyester and pol (tetramethylene trans-1,4-cyclohexane- dicarboxylate) over coatings formulated with only an aromatic polyester.

Both the aromatic polyester and the all-aliphatic polyester may be produced using well known polycondensa- tion procedures. - 5

Poly(tetramethylene trans-1,4-cyclohexane- dicarboxylate) may be prepared from 1,4-butanediol and the acid or dieεter of trans-l,4-cyclohexane- dicarboxylic acid. When the diester is used, excess glycol is preferably used during ester interchange and is removed under reduced pressure until the desired viscosity is obtained.

The preferred all-aliphatic poly(tetramethylene trans-l,4-cyclohexanedicarboxylate) polyester of this invention has a Tm in the range of about 110 to 150^CC, a hydroxyl number in the range of about 25 to 65, an acid number of not more than 10 and an inherent viscosity of about 0.10 to 0.40. This crystalline polyester component may also contain a branching agent, such as trimethylolpropane, to increase the cross-link density of the final coating in a concentration of up to 10 mol percent based on the total moles of polyol; i.e., up to 10 mole percent of trimethylolpropane residues and from 100 to 90 mole percent of 1,4-butanediol residues. As a further preferred aspect of this invention, up to about 10 mole percent of the 1,4-butanediol residues may be replaced with diol residues containing from 2 to about 10 carbon atoms. Examples of such glycol residues include residues of ethylene glycol, propylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-l,3-diol, 2,2-dimethyl-1,3-propanediol, 2-ethyl- -butyl-1,3- propanediol, 2-ethyl-2-iεobutyl-l,3-propanediol, 1,3- butanediol, 1,5-pentanediol, 1,6-hexanediol, thio- diethanol, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, 2,2,4,4-tetramethyl-l,3-cyclobutanediol, 1,4-xylylene- diol and the like. In this regard 2,2-dimethyl-l,3- propanediol is especially preferred. When trans-1,4- cyclohexanedicarboxylic acid is referred to herein, it is intended to mean at least 70% tranε-isomer. - 6 -

As noted above, the aromatic polyester (Component 1(a)) will necessarily have a Tg of greater than 40°C. The acceptably upper limit is generally dictated by th practicalities of the curing system; in other words, t upper limit could be as high as 150-180^βC, however, as the Tg of the aromatic polyester increaseε, the performance limitations of the oven used for curing becomes more critical.

The preferred aromatic polyester component of the composition provided this invention has a Tg greater than 55°C, a hydroxyl number in the range of about 25 t 80, an acid number of not more than 15 and an inherent viscosity of about 0.15 to 0.4. The aromatic polyester component preferably is comprised of (1) diacid residue of which at least 50 mole percent are terephthalic acid residues, (2) diol residues of which at least 50 mole percent are derived from 2,2-dimethyl-l,3-propanediol and (3) up to 10 mole percent, based on the total moles of (2) and (3), of trimethylolpropane residues. These preferred aromatic polyesters are commercially avail¬ able, e.g., under the names Rucote® 107 brand resin sol by Ruco Polymer Corp. and Cargill Reεin 3000 sold by Cargill, Inc.

The relative amounts of the aromatic polyester and the all-aliphatic polyester can be varied substantially depending on a number of factors such aε the particular polyeεterε employed, the cross-linking agent and the amount thereof being used, the degree of pigment loading, the properties required of the coatings to be prepared from the compositions, etc. As provided above the compositions of this invention comprise a blend cf about 10 to 80 weight percent of the aromatic polyester and 20 to 90 weight percent of the all-aliphatic poly¬ ester. The blend of polymers containing free hydroxy groups provided by this invention preferably is comprised of about 20 to 75 weight percent of the aromatic polyester and 25 to 80 weight percent of the all-aliphatic polyester.

Suitable curing or crosε-linking agentε for uεe with hydroxyl-functional polyesters are well known in the art. Preferred cross-linking agents include blocked isocyanates.

The blocked polyisocyanate compounds of the compositionε of thiε invention are known compoundε and can be obtained from commercial sources or may be prepared according to published procedures. Upon being heated to cure coatings of the compositions, the compounds are unblocked and the isocyanate groups react with hydroxy groups preεent on the amorphouε polyeεter and the all aliphatic polyester to cross-link the polymer chains and thus cure the compositions to form tough coatings. Examples of the blocked polyisocyanate cross-linking component include those which are based on isophorone diiεocyanate blocked with ε-caprolactam, commercially available under the tradenames Hϋls 1530 and Cargill 2400.

The most readily-available, and thus the preferred, blocked polyisocyanate crosε-linking agents or compoundε are those commonly referred to as ε-caprolactam-blocked iεophorone diisocyanate, e.g., those described in U.S. Patent Nos. 3,822,240 4,150,211 and 4,212,962, incorporated herein by reference. However, the products marketed aε ε-caprolactam-blocked iεophorone diiεo¬ cyanate may consiεt primarily of the blocked, difunctional, monomeric iεophorone diisocyanate, i.e., a mixture of the ciε and trans isomerε of 3-isocyanato- methyl-3,5,5-trimethylcyclohexyliεocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional tri er thereof or a mixture of the monomeric, dimeric and/or trimeric formε. For example, - 8 -

the blocked polyisocyanate compound used as the cross- linking agent may be a mixture consisting primarily of the ε-caprolactam-blocked , difunctional, monomeric isophorone diisocyanate and the ε-caprolactam-blocked, trifunctional trimer of isophorone diisocyanate. The description herein of the cross-linking agents as "polyisocyanates" refers to compounds which contain at least two isocyanato groups which are blocked with, i.e., reacted with, another compound, e.g., ε-caprolactam. The reaction of the isocyanato groups with the blocking compound is reversible at elevated temperatures, e.g., about 150°C, and above, at which temperature the isocyanato groups are available to react with the hydroxyl groups present on the free hydroxy groups of the polyeεter to form urethane linkages. The amount of the blocked diisocyanate crosε- linking compound preεent in the compoεitionε of this invention can be varied depending on several factors such as those mentioned hereinabove relative to the amount of components (l)(a) and (1) (b) which are utilized. Typically, the amount of cross-linking compound which will effectively cross-link the hydroxy- containing polymers to produce coatings having a good combination of properties is in the range of about 5 to 30 weight percent, preferably 15 to 25 weight percent, based on the total weight of components (l)(a) and (l)(b) and the crosε-linking compound.

The powder coating compoεitionε of this invention may be prepared from the compositionε deεcribed herein by dry-mixing and then melt-blending components (l)(a) and (1) (b) and the blocked polyiεocyanate compound, along with other additives commonly used in powder coatings, and then grinding the solidified blend to a particle size, e.g., an average particle size in the range of about 10 to 300 microns, suitable for producing - 9 -

powder coatingε. For example, the ingredients of the powder coating compoεition may be dry blended and then melt blended in a Brabender extruder at 90° to 130°C, granulated and finally ground. The melt blending εhoul be carried out at a temperature sufficiently low to prevent the unblocking of the polyisocyanate cross- linking compound and thus avoiding premature crosε- linking. To minimize the expoεure of the blocked polyisocyanate to elevated temperatures, componentε (l)(a) and (l)(b) may be blended prior to the incorporation therein of the blocked polyiεocyanate compound.

Typical of the additiveε which may be preεent in the powder coating compoεitionε include benzoin, uεed t reduce entrapped air or volatileε, flow aidε or flow control agentε which aid the formation of a smooth, glosεy εurface, catalysts to promote the cross-linking reaction between the isocyanate groups of the crosε- linking agent and the hydroxyl groupε on the polymers, stabilizers, pigments and dyes. Although it is possibl to cure or crosε-link the compoεition without the use o a catalyst, it is usually desirable to employ a catalys to aid the croεε-linking reaction, e.g., in an amount o about 0.05 to 2.0 weight percent cross-linking catalyεt baεed on the total weight of components (l)(a) and

(l)(b) and the cross-linking agent. Suitable catalysts for promoting the croεε-linking include organo-tin compoundε εuch aε dibutyltin dilaurate, dibutyltin di aleate, dibutyltin oxide, εtannous octanoate and εimilar compoundε.

The powder coating compoεitions preferably contain a flow aid, also referred to as flow control or levelin agents, to enhance the surface appearance of cured coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are avail- 10

able from several suppliers, e.g., Modaflow from Monsanto Company and Acronal from BASF. Other flow control agents which may be used include Modarez MFP available from Synthron, EX 486 available from Troy Chemical, BYK 360P available from BYK Mallinkrodt and Perenol F-30-P available from Henkel. A specific flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing 60 mole percent 2-ethylhexy methacrylate residues and about 40 mole percent ethyl acrylate residueε. The amount of flow aid preεent may preferably be in the range of about 0.5 to 4.0 weight percent, baεed on the total weight of componentε (l)(a) and (l)(b) and the croεε-linking agent.

The powder coating compoεitionε may be depoεited o variouε metallic and non-metallic εubεtrateε by known techniqueε for powder deposition such as by means of a powder gun, by electrostatic deposition or by depoεitio from a fluidized bed. In fluidized bed sintering, a preheated article iε immerεed into a suεpension of the powder coating in air. The particle size of the powder coating composition normally is in the range of 60 to 300 microns. The powder iε maintained in suεpenεion by paεεing air through a porous bottom of the fluidized be chamber. The articles to be coated are preheated to about 250° to 400°F (about 121° to 205^βC) and then brought into contact with the fluidized bed of the powder coating composition. The contact time depends o the thickness of the coating that iε to be produced and typically is from 1 to 12 seconds. The temperature of the substrate being coated causeε the powder to flow an thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated article alεo affects cross-linking of the coating compoεition and results in the formation of a tough coating having a good combination of properties. 11

Coatingε having a thicknesε between 200 and 500 icronε may be produced by this method.

The compositionε alεo may be applied using an electrostatic procesε wherein a powder coating compoεi- tion having a particle size of less than 100 microns, preferably about 15 to 50 microns, is blown by means of compresεed air into an applicator in which it iε charge with a voltage of 30 to 100 kV by high-voltage direct current. The charged particleε then are sprayed onto the grounded article to be coated to which the particle adhere due to the electrical charge thereof. The coate article is heated to melt and cure the powder particles.

Coatings of 40 to 120 microns thickness may be obtained.

Another method of applying the powder coating compositions is the electrostatic fluidized bed proceεε which iε a combination of the two methodε deεcribed above. For example, annular or partially annular electrodeε are mounted over a fluidized bed so as to produce an electrostatic charge such as 50 to 100 kV. The article to be coated, either heated, e.g., 250° to 400°F, or cold, iε expoεed briefly to the fluidized powder. The coated article then can be heated to effec croεε-linking if the article waε not preheated to a temperature sufficiently high to cure the coating upon contact of the coating particles with the article.

The powder coating compositionε of thiε invention may be uεed to coat articleε of variouε shapes and size constructed of heat-reεiεtance aterialε such as glass, ceramic and various metal materials. The compoεitionε are especially useful for producing coatings on article conεtructed of metals and metal alloyε, particularly εteel articleε. EXPERIMENTAL SECTION

The components of the compositionε according to thiε invention may be mixed by dry blending in a Henschel mixer, followed by compounding in a ZSK-30 Extruder (Werner & Pfleiderer) at 110-130^βC, grinding, and screening to obtain powder with average particle size of about 35 micronε.

The powdered compoεitionε were electoεtatically depoεited on the substrate by use of a powder gun.

After deposition, the powder was heated to a temperature sufficient to cause its particles to flow and fuse together to form a smooth, uniform surface. Coatings were prepared on 3 inch by 9 inch panels of 20-gauge, poliεhed, cold roll steel, the εurface of which haε been zinc phosphated (Bonderite 37, The Parker Company).

The artificial weatherability of the coatings waε determined by expoεure of the coated panelε in a Cyclic Ultraviolet Weathering Tester (QUV) with 313 nm fluoreεcent tubeε. The test condition waε 8 hourε of light at 70°C and 4 hours of condensation at 45°C.

The flexibility of the coatings was determined in accordance with ASTM 4145-83 at ambient temperature by bending or folding a coated panel back against itself, using a hydraulic jack preεsurized at 20,000 poundε per square inch (psi), until the apex of the bend iε aε flat as can be reasonably achieved. Thiε initial bend iε referred to aε OT meaning that there iε nothing (zero thickneεεes) between the bent portions of the panel. The bend is examined uεing a 10X magnifying glaεε and, if fractureε of the coating are obεerved, the panel is bent a second time (IT) to form a three-layer sandwich. The second bend iε inεpected for coating fracture and thiε procedure iε repeated, forming 4-, 5-, 6-, etc. layer εandwicheε, until a bend exhibits no fracture of 13

the coating. The reεult of each bend test iε the minimum thickness (minimum T-bend) of the bend which does not give any fractures of the coating. Although the bend test used is excessively severe for most purposes for which coated articles are used, it provides a means to compare the flexibilities of different powder coating compoεitionε.

Impact strength was determined by using a Gardner Laboratory, Inc., Impact Tester. A weight is dropped within a slide tube from a specified height to hit a punch having a 5/8 inch diameter hemispherical nose which is driven into the front (coated face) or back of the panel. The highest impact which does not crack the coating is recorded in inch-pounds, front and reverse. Twenty degree and 60 degree glosε waε meaεured uεing a gloεε meter (Gardner Laboratory, Inc.) according to ASTM D-523.

All inherent viscosities were determined at 25°C in a (60/40 by weight) mixture of phenol/tetrachloroethane at a concentration of 0.5 g/100 mL. Acid and hydroxyl numbers are determined by titration and are reported herein aε mg of KOH conεumed for each gram of polymer. The melting temperatureε (Tm) are determined by differential scanning calorimetry (DSC) on the εecond heating cycle at a scanning rate of 20^βC per minute after the sample has been heated to melt and quenched t below the glass transition temperature of the polymer. Tg values are reported as the midpoint of the transitio and Tm at peaks of tranεitionε. The pencil hardness of a coating is that of the hardest that will not cut into the coating according to ASTM 3363-74 (reapproved 1980). The results are expresεed according to the following εcale: (εofteεt) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6K (hardest). 14 -

The conical mandrel is performed by bending the panel over 15 secondε uεing a Gardner Laboratory, Inc. conical mandrel of specified size according to ASTM-522-85. A pass or fail is recorded.

EXAMPLE 1

This example illuεtrateε the typical procedure for preparing the all-aliphatic polyesters of this inven- tion. A 3000 mL, 3-necked, round bottom flaεk equipped with a stirrer, a short distillation column, and an inlet for nitrogen, was charged with dimethyl cyclo- hexanedicarboxylate (1259.7 g, 6.29 mol), 1,4-butanediol (997.5 g, 11.08 mol), trimethylolpropane (73.9 g, 0.55 moleε) and 10 mL of titanium tetraiεo- propoxide/2-propanol solution (100 ppm Ti) . The flask and contents were heated under nitrogen atmosphere to a temperature of 170^βC at which point methanol began to distill rapidly from the flask. After the reaction mixture was heated with εtirring at this temperature for about 1 hour, the temperature waε increased to 200°C for 2 hours, raised to 215°C for 4 hours, and then to 235°C. After 3 hours at thiε temperature, a vacuum of 10 mm of mercury waε applied over a period of 12 minuteε. Stirring waε continued under 10 mm of mercury at 235°C for about 3 hours to produce a low melt viscoεity, colorless polymer. The resulting polymer has an inherent viεcoεity of 0.30, a melting point of 130°C and a hydroxyl number of 30.

EX.AMPLE 2

A powder coating composition was prepared from the following materials: 81.3 g Polyester of Example 1;

243.7 g Ruσote 107, a polyester based primarily on terephthalic acid and 2,2-dimethyl-l,3- propanediol;

75.0 g Caprolacta -blocked isophorone poly¬ isocyanate (Hulε 1530); 4.0 g Dibutyltin dilaurate;

4.0 g Benzoin;

6.0 g Modaflow III;

4.0 g Tinuvin 144; and

4.0 g Tinuvin 234.

The above materialε were melt-blended in an APV twin screw extruder at 110°C, ground in a Bantam mill to which a stream of liquid nitrogen is fed and classified through a 170 mesh screen on a KEK centrifugal sifter. The finely-divided, powder coating composition obtained had an average particle size of about 50 microns.

The powder coating composition prepared in Example 2 waε applied electrostatically to one side of the 3 inch by 9 inch panels described above. The coating was then cured (cross-linked) by heating the coated panels at 177°C in an oven for 25 minutes. The cured coatings are about 50 microns thick.

The coatings on the panels had both front and back impact strengths of >160 inch-pounds, 20° and 60° gloss valueε of 82 and 98, reεpectively, and a pencil hardness of HB. The coated panelε paεεed a 0.125 inch conical mandrel teεt and had a T-bend flexibility value of 1. After 780 hourε of QUV expoεure, the coating retained 50% of the 60° gloεε. EXAMPLE 3

Using the procedure described in Example 2, a powder coating composition waε prepared from the following materialε:

163.7 g Polyester of Example 1;

163.7 g Rucote 107, a polyester described in Example 2;

72.7 g Caprolactam-blocked isophorone diisocyanate (Hulε 1530) ; 6.0 g Dibutyltin dilaurate;

4.0 g Benzoin;

6.0 g Modaflow III;

Using the procedure of Example 2, panels were coated with thiε powder coating compoεition and the coatings were cured and evaluated. The coatings had both front and back impact strengths of >160 inch-pounds and 20° and 60° glosε valueε of 72 and 91, respectively, and a pencil hardness of B. The coated panels passed a 0.125 inch conical mandrel and had a T-bend flexibility value of 1. After 810 hours of QUV exposure, the coating retained 50% of the 60° gloεε.

COMPARATIVE EXAMPLE 1

A powder coating compoεition waε prepared from the following materials: 816.60 g Rucote 107, a polyeεter described in Example 2;

183.40 g Caprolactam-blocked isophorone diisocyanate (Huls 1530);

10.00 g Dibutyltin dilaurate;

10.00 g Benzoin;

15.00 g Modaflow III;

400.00 g Titanium dioxide; 10.00 g Tinuvin 144

10.00 g Tinuvin 234

Uεing the procedure of Example 2, panelε were coated with thiε powder coating compoεition and the coatingε were cured and evaluated. The coatingε had both front and back impact εtrengthε of >160 inch-poundε and 20° and 60° gloεε valueε of 85 and 95, reεpectively, and a pencil hardneεε of H. The coated panelε paεεed a 0.125 inch conical mandrel and had a T-bend flexibility value of 6. After 280 hourε of QUV expoεure, the coating retained 50% of the 60° gloss. Thus, the coatingε of Examples 2 and 3 possesε a G₅₀ of 780 hours and 810 hourε, reεpectively. The coating of Comparative Example 1 poεεeεεeε a G^ of 280 hourε.

Claims

CLAIMSWe Claim:

1. A thermoεetting coating compoεition in the form of a powder compriεing

(1) a blend of polymers containing free hydroxy groups comprised of:

(a) about 10 to 80 weight percent of an aromatic polyester having a glasε transition temperature of greater than 40°C, a hydroxyl number of about 20 to 200 and an inherent viscoεity of about

0.1 to 0.5; and

(b) about 20 to 90 weight percent of poly- (tetramethylene tranε-1,4-c clohexane- dicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5; and

(2) a cross-linking effective amount of a croεε- linking agent.

2. The compoεition of Claim 1, wherein the croεs- linking agent iε a blocked polyisocyanate.

3. The composition of Claim 2, wherein the blocked polyisocyanate iε ε-caprolactam-blocked iεophorcr.e diiεocyanate.

4. The compoεition of Claim 1, further comprising cr.e or more cross-linking catalysts. 5. A thermoεetting coating compoεition in the form of a powder compriεing

(1) a blend of polymerε containing free hydroxy groupε comprised of:

(a) about 10 to 80 weight percent of an aromatic polyester having a glass transition temperature of greater than 40°C, a hydroxyl number of about 20 to

200 and an inherent viscosity of about 0.1 to 0.5; wherein said aromatic polyester is comprised of (1) diacid residueε of which at least 50 mole percent are terephthalic acid residueε,

(2) diol residues of which at least 50 mole percent are 2,2-dimethyl-l,3- propanediol residues, and (3) up to 10 mole percent, based on the total moles of (2) and (3), of trimethylolpropane residues; and

(b) about 20 to 90 weight percent of poly- (tetramethylene trans-1,4-cyclohexane- dicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.

5; wherein u to about 10 mole percent of the polyol residueε are compriεed of trimethylol- propane reεidueε; and

(2) a croεε-linking effective amount of a croεε- linking agent.

6. The composition of Claim 5, wherein the crosε- linking agent iε a blocked polyisocyanate.

7. The composition of Claim 6, wherein the blocked polyisocyanate is ε-caprolactam-blocked isophorone diisocyanate.

8. The composition of Claim 5, further comprising one or more cross-linking catalystε.

9. A thermoεetting coating compoεition in the form of a powder comprising

(1) a blend of polymers containing free hydroxy groups comprised of:

(a) about 10 to 80 weight percent of an aromatic polyester having a glasε tranεition temperature of greater than 40^βC, a hydroxyl number of about 20 to

200 and an inherent viscosity of about 0.1 to 0.5; and

(b) about 20 to 90 weight percent of poly- (tetramethylene trans-l,4-cyclohexane- dicarboxylate) having a hydroxyl number of about 25 to 65, and an inherent viscosity of about 0.1 to 0.4; and

(2) a crosε-linking effective amount of a croεs- linking agent.

10. The composition of Claim 9, wherein the crosε- linking agent iε a blocked polyisocyanate.

11. The composition of Claim 10, wherein the blocked polyisocyanate is ε-caprolactam-blocked isophorone diisocyanate.

12. The composition of Claim 9, further comprising one or more cross-linking catalysts.

13. A thermosetting coating composition in the form of a powder comprising

(1) a blend of polymers containing free hydroxy groups comprised of:

(a) about 10 to 80 weight percent of an aromatic polyester having a glass transition temperature of greater than 40°C a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to 0.5; wherein said aromatic polyester is comprised of (1) diacid residueε of which at least 50 mole percent are terephthalic acid reεidueε, (2) diol residues of which at least 50 mole percent are 2,2-dimethyl-l,3- propanediol residueε, and (3) up to 10 mole percent, baεed on the total oleε of (2) and (3), of trimethylolpropane reεidueε; and

(b) about 20 to 90 weight percent of poly-

(tetramethylene tranε-1,4-cyclohexane- dicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viεcoεity of about 0.1 to 0.5; wherein up to about 10 mole percent of the polyol - 22 -

reεidueε are compriεed of trimethylol¬ propane reεidueε; and

(2) a crosε-linking effective amount of a croεε- linking agent.

14. The composition of Claim 13, wherein the croεε- linking agent is a blocked polyisocyanate.

15. The composition of Claim 14, wherein the blocked polyisocyanate is ε-caprolactam-blocked iεophorone diisocyanate.

16. The composition of Claim 13, further comprising one or more cross-linking catalystε.

17. A coating which results from the application of a thermosetting powder coating compoεition of Claim 1 to an article followed by heat-curing.

18. A coating which reεultε from the application of a thermoεetting powder coating compoεition of Claim 5 to an article followed by heat-curing.

19. A coating which reεultε from the application of a thermoεetting powder coating compoεition of Claim 9 to an article followed by heat-curing.

20. A coating which reεultε from the application of a thermoεetting powder coating compoεition of

Claim 13 to an article followed by heat-curing.

21. An article coated with the cured powder coating compoεition of Claim 1.

22. An article coated with the cured powder coating compoεition of Claim 5.

23. An article coated with the cured powder coating composition of Claim 9.

24. An article coated with the cured powder coating composition of Claim 13.