CA2103709A1 - 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

`` ` 210 3 7 9 RollUS 5 2 / O 1 17 POWDER COATING COMPOSITIONS

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

Back~round of the Invention Plastic materials used in the manufacture of po~der ;~
coatings are classified broadly as either thermosetting or thermoplas.ic. In the application of thermoplastic po-~der coatings, heat is applied to the coating on the substrate to melt the particles of the po~der coating and thereby permit the particles to flow together and fo_m a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tousher, more sesistant to solvents and detergents, ha~e better adhesion to metal substrates and do not soften ~hen exposed to ele~ated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coat~ngs which have, in addition to the above-stated desirable characteristics, good smoothness an~
flexibility. ~oatings prepared from thermosetting powder compositions, upon the application of heat, mz.
cure or set prior to forming a smooth coating, result_..g in a relati~ely rough finish referred to as an "oranae - peel" 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 coatings to be appliec from organic solvent systems ~hich are inherently ~ r~r~T~ ~U~T

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undesirable because of the envi~onment~l and sa~ety proble~s occasioned by the e~aporation of ~he solvent system. Solv~nt-based co~ing oompositions also su~fer from the disadvantage of rel~tively poor percent utilization, i.e., in some modes ot applicatiGn, oniy 60 perc~nt a~ less of the solvent-based coating composition being applied contacts ~he articls or subst~ate being coated. Thus, a substantial por~ion of ~olvent-based coatings can be wasted slnce that portion which does not lo contact the arLicle or substrate being coated o~viously cannot be reclaimed. . :
In ad~ition to exhibiting good gloss; impact strength and resistance to solvents and;chemicals, coating~ derived from thermo8etting coating compo~itio~s 1~ must p~ssess good to axcellent flexibility. For example, good flexibility is essential ~or powder ~oating compositions use~ to coat sheet (coil) steel wh~ch is destined to be for~ed or shaped into articles used in the ~anu~actu~e o~ various household appli~nces and automobiles wherein the sheet metil is ~lexed or bent at var~ouq angles.
Powder co~tings based on aromatic poly4s~ers a~e well-known but generally suffer fro~ poor weather-ability, U. S. Paten~ No. 4, 352,924 . d~s~-~ribes icertain crystalline polyaste~s use~ul in powcle~ coating co~positions. W0 8~5320 describes powder coati~g composi'cio~ co~pri61~d of an a~o2~phous polyester, a se~icrystalli~e pclyester, a~d a polyisocyanate crosslinking agent..

Brie~ ~s~ri~tion o~ th~ Draw~a .
~igure 1 is a gr~p~ o~ QUV weathering o~ t~e powder coating~ of Comparative Example 1, and Examples 2 and 3, e S l~ ~Jr - . . ; ~, .. . . , . ~ . .. .

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all of w~.ich are describcd in the ~xpe~imental Secticn below. "QW" weathering is per~ormed by expo~ing the ~-coating to high i n~ensity ultraviolet radiation, thereby simulating perfornl~nce o~ the coating in ~e presence Or 5 sunlight, albeit on an accel~rated basis. The plat indicated by bold dots is the coating of ComparativQ ; ~ -' - . : ,' . , ' ., :.

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: ' ' . , , PCTIUS ^,2/Ql 1 2 1 0 3 7 ~ ~ ROIUs 1 5 APR 199 Example 1; the plot indicated by open-circled points is the coating of Example 2; and the plot isdic ted by ~tri~ngle-points n is the coating of Example 3. Percent gloss retention is plotted versus time.
Summarv of _he Inventlon ~ he present invention provides a novel blend of polymers having free hydroxyl groups, which, when combined ~ith a cross-lin~ing agent And cured, provides coatings which have superior gloss retention when exposed to ultraviolet radiation.

Detailed De _ri~tion of the Invention The present invention provides powder cozting 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 po~dercoatings to Q~V conditions which cause a 50~ loss in gloss, is referred to herein as ~G50~. As is evident from Figure 1, the powder coating compositions of the present invention are far superior, ~ith regard to gloss retention, than coatings based on aromatic polyesters.
Thus, the present invention provides thermosetting po~der 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 havins a glass transition tempera-ture (Tg) of greater than 40C, a hydroxyl :

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, : - : ~-~, 210 3 7 9 PR~T¦US 9 2 /AORl 11 '76 ~ ' ' number of ~bout 20 to 200 and n~ i~herent viscosity of about 0.1 to 0.5; and (b) about 20 to 90 weight percent of poly(tetra-methylene trans-1,4-cyclohexanedicarboxylate having a hydroxyl number of about 20 to 200, and an inherent viscosity of ~bout O.l to 0.5;
and (2) a cross-linking effective ~mount of 2 cross-linking ~gent.

~ he effectiveness of this novel blend becomes apparent in the comparison of accelerated 0 W weathering l; of powder coatings ~o_mulated with (i~) a blend of polymers containing hydroxy end groups ~nd comprised of 50 weight percent of an aromatic polyester and 50 weight percent o~ 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 l percent of a hindered amine light stabilizer (HALS). During Q W exposure, formulations of (1) and (2) retained 50~ of 60C 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 poly(tetramethylene trans-1,4-cyclohexane-dicarboxylate) over coatings formulated with only an aromatic polyester.
Both the ~romatic polyester and the all-aliphatic polyester may be produced using well kno~n polycondensa-tion procedures.

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P~T/~S 9 2 J ~1'1 f 2103709 ROIUS 15 APR l9~

Poly(tetr~ethylene trans-1,4-cyclohexane-dicarboxylate) ~ay be prep~red from 1,4-butaDediol and the ncid or diester of trans-1,4-cyclohex~ne-dicarboxylic acid. ~hen the diester is used, excess glycol is prefe~ably used during ester interchange and is removed unde~ reduced pressure until the desired viscosity is obtained.
~ he preferred ~ aliphatic poly(tetramethylene trar.s-1,4-cyclohexanedicarboxylate) polyester of this invention has a Tm in the range of about 110 to 150C, a hvàroxyl number in the range of about 25 to 65, an acid number of not more than 10 and an inherent ~iscosity of about C.10 to 0.40. This crystalline polyester component may ~lso contain a branching agent, such as trimethylolpropane, to increase the cross-link denslty of the final coating in a concentration of up to 10 mole 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 in~ention, 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-1,3-diol, 2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-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-1,3-cyclobutanediol, 1,4-xylylene-diol and the like. In this regard 2,2-dimethyl-1,3-propanediol is especially preferred. When trans-1,4-cyclohexanedicarboxylic acid is referred to herein, it is intended to mean at least 70~ trans-isomer.

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PCTllJS S2/01176 2103709 RQIUS 15 APR l99~

As noted above, the aromatic polyester ~Component l(a)) will nec~ssarily have a Tg of gre~ter than 40C.
~he acceptAbly upper limit is generally dictated by the practicalities of the curing system; in other words, the upper limit could be as high s 150-180C, however, as the Tg of the romatic polyester increases, the performance limitations of the oven used for curing becomes more critical.
~he preferred aromatic polyester component of the composition provided this invention has a Tg greater than 55C, a hydroxyl number in the range of about 25 to 80, an acid number of not more than 15 and ~n inherent viscosity of about 0.15 to 0.4. The aromatic polyester component preferably is comprised of (1) diacid residues of which at least 50 mole percent ~re terephthalic acid residues, (2~ diol residues of which at lèast 50 mole percent are derived from 2,2-dimethyl-1,3-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 sold by Ruco Polymer Corp. and Cargill Resin 3000 sold by Cargill, Inc.
The relative amounts of the aromatic polyester anc the all-aliphatic polyester can be varied substantiall-depending on a number of factors such as the particular polyesters 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 of about 10 to 80 weight percent of the Aromatic polyester and 20 to 90 weight percent of the all-~liphatic poly-ester. The blend of polymers containing free hydroxv groups provided by this invention preferably is .. . .. .

21Q37~9 ~û~ls 15 AP~ ~197g~

comprised of about 20 to 75 weight percent of the ~romatic polyester ~nd ~5 to 80 weight percent of the all-aliphatic polyester.
Suitable curing or cross-linking a~ents for use with hydroxyl-functional polyesters re well kno~ in the ~rt. Preferred cross-linking agents include blocked isocyanates.
The blocked polyisocyanate compounds of the compositions of this invention ~re kno~ compounds and can be obtained from commercial sources or may be prepared according to published procedures. ~pon being heated to cure coatings of the compositions, the compounds are unblocked and the isocyanate groups react with hydroxy groups present on the amorphous polyester l; and the all aliphatic polyester to cross-link the polymer chains and thus cure the compositions to form toush coatings. Examples of the blocked polyisocyanate cross-linking component include those which are based on isophorone diisocyanate blocked with E-caprolactam, commercially a~ailable under the tradenames HUls 1530 and Cargill 2400.
~ he most readily-~ilable, and thus the pre~erred, blocked polyisocyanate cross-linking agents or compounds are those commonly referred to as E-caprolactam-blocked isophorone 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 p-oducts marketed as -caprolactam-blocked isophorone dii,o-cyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyan~te, i.e., a mixture of the cis And trans isomers of 3-isocyanato-methyl-3,5,5-trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, ~ ~ S ~

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PCTIUS 92/ Ol 1~6 2103709 ~OIUS 1~ APR 1992 the blocked polyisocyanate compound used as the cross-linking agent may be a mixture consisting prLmarily of the - caprolactam-blocked, difunctional, monomeri~
isophorone diisocyanate and the E-caprol ctam-blocked, trifunctional t~imer 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., E-caprolactam. ~he reaction of the isocya~ato groups with the blocking compound is reversible at elevated temperatures, e.g., about 150C, and above, at which temperature the isocyaDato groups are available to react with the hydroxyl groups present on the free hydroxy groups of the polyester to form urethane linkages.
The amount of the blocked diisocyanate cross-linking compound present in the compositions of this invention can be varied depending on several factors such as those mentioned hereinabove relative to the amount of components (l)~a) and (l)(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 cross-linking compound.
The powder coating compositions of this invention may be prepared from the compositions described herein by dry-mixing and then melt-blending components (l)(a) and (l)(b) ~nd the blocked polyisocyanate compound, alo~g with other additives commonly used in pouder 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 producir.g ~ ,~,`~ S~

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~ ~CTIUS 92/ Ol 17 - 2103709 ROIUS 15 At'~

powder coatings~ For example, the ingredients of the powder coating composition may be dry blended ~d then melt blended in a ~rabender extruder at 90 to 130C, granulated and finally ground. The melt blending should be cArried out at a temperature sufficiently low to prevent the unbloc~ing of the polyisocyaDate cross-linking compound and thus voiding premature cross-linking. To minimize the exposure of the blocked polyisocyanate to elevated temperatures, components (l)(a) and (l)(b) may be blended prior to the iDcorporation therein of the blocked polyisocyanate compound.
Typi~al of the additi~es which may be present in the powder coating compositions include benzoin, used to reduce entrapped air or volatiles, f~ow aids or flow control agents which aid the formation of a smooth, glossy surface, catalysts to promote the cross-linking reactio~ between the isocyanate groups of the c-oss-linking agent and the hydroxyl groups on the polyme-s, stabilizers, pigments and dyes. Although it is possible to cure or cross-link the composition without the use of a catalyst, it is usually desirable to employ a c2talyst to aid the cross-linking reaction, e.g., in an amount of about 0.05 to 2.0 weight percent cross-linking catalyst based on the total weight of components (l~(~) and (l)(b) and the cross-linking agent. Suitable catalysts for promoting the cross-linking include organo-tin compounds such as dibutyltin dilaurate, dibutyltin dimaleate, dibutyltin oxide, stannous octanoate and similar compounds.
~ he powder coating compositions preferably cont~in a flow aid, al50 referred to as flow control or le~ieling agents, to enh~nce the surface appearance of cured coatings of the powder coating compositions. Such flo~
aids typically comprise acrylic polymers and a-e a~ail-S~S~ S~

2103709 ROIIJS 15 APR ~

able from se~eral suppliers, e.q., Modaflow from Monsanto Company ~nd Acronal from BASF. Other flow control agents ~hich may be used iDclude Modarez ~FP
available from Synthron, EX 486 a~ailable from Troy Chemical, BYK 360P available from BYK Mallinkrodt and Perenol F-30-P available from HenXel. A specific flo~
aid is an acrylic polymer having a molecular weight of about 17,000 and contnining 60 mole pexcent 2-ethylhexyl meth~crylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present may preferably be in the range of about 0.5 to 4.0 weight percent, based on the total weight of components (l)~a) and (l)(b) and the cross-linking agent.
The powder coating compositions~may be deposited on various metallic and non-metallic substrates by kno~-n techniques for powder deposition such as by means of a po~der gun, by electrostàtic deposition or by deposition from a fluidized bed. In fluidized bed sintering, a preheated article is immersed into a suspension of the powder coating in air. The particle size of the po~der coating composition normally is in the range of 60 to 300 microns. The powder is maintained in suspension bi p ssing air through a porous bottom of the fluidized bed chamber. The articles to be coated are preheated to about 250 to 400F (about 121 to 205C) and then brought into contact ~ith the fluidized bed of the powder coating composition. The contact time depends on the thickness of the coating that is to be produced and typically is from l to 12 seconds. The temperature of the substrate ~eing coated causes the powder to flo~ and thus f~se together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated ~rticle also affects cross-linking of the coating composition and results in the formation of a tough coating ha~ing a good combination of properties.
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210 3 7 0 9 ~OIus 12~ AP~

Coatings having a thickness between 200 ~nd 500 microns may be produced by this method.
. The compositions also may be applied using an electrostatic process wherein a powder coating composi-tion having a particle size of less than 100 microns,preferably about 15 to 50 microns, i5 bloun by means of compressed air i~to an applicator in which it is charged with a voltage of 30 to 100 kV by high-voltage direct current. The charged particles then are sprayed onto the grounded article to be coated to which the p~rticles adhere due to the electrical charge thereof. The coated article is heated to melt and cure the powde~ particles.
Coatings of 40 to 120 microns thickness may be obtained.
Another method of applying the powder c~ating lS compositions is the electrostatic fluidized bed process ~hich is a combination of the two methods described above. For example, annular or partially annular electrodes 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 400F, or cold, is exposed briefly to the fluidized pouder. The coated article then can be heated to effect cross-linking if the article was not preheated to a temperature sufficiently high to cure the coating upon contact of the coating particles with the article.
The powder coating compositions of this invention may be used to coat articles of various shapes and sizes constructed of heat-resistance materials such as glass, ceramic and various metal materials. The compositions are especially useful for producing coatings on articles constructed of metals and metal alloys, p~rticularl-steel articles.

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~` 2103709 ~:XP~RIMENTAL S ECq ION
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The com~onents of the compositions according to this i~ven~ion ~ay be mixed by dry blending in a Henschel ~ixer, followrd by compounding in a 2SK-30 Extruder (Werne~ & Pfleiderer) at 110-130C, grinding, and screening to obtain powder ~ith average particle ~izc of ab~ut 3g microns.
The powdered compositions were el~ctostatically deposited on the substrate by use of a powder gun.
After deposition, the powder was heated to a temperature sur~icient L~ cause its partic}es to flow and ~use tosether to form a smooth, uniform s~rface. Coatings -~ere prepared on 7.62 cm by 22.86 cm of 20-gauge, 1~ polis~ed, cold roll ste~l, the surface of which has been zinc phospha~ed (Bo~.deri~e 37, The Par~er Company).
The artificia} weatherability of the coatings was detarmined by exposure of the coated panels in a Cycl~c Ultraviolet Weathering ~ester ~Q W) with ~13 nm fluorescent tube~. The te5t condition was 8 hours Or light at 70~C ~nd 4 hours of condensation at 45OC.
The flexibility of the coating~ was determined in .acco~dance wi~h ASTM 4145-83 a~ ambient emperature by bendinq or f~lding a coated p2nel back against itsel~, using a hydraulic jack pre~surized at 700 kg pe~ square cen~imeter, until the apex o~ ~he bend is as ~lat as can be ~easonaDly achieved. Thi~ initial bend is ~e~erred t~ as OT meaning that there i~ nothing ~z~ro thick-ne~es) between the bent por~ions o~ the panel. The bend is examined using a lOX magni~ying gla~s and, i~
~ractures of the coating are observed, the panel is bent a ~econd tim~ ~lT) to ~or~ a thr~e-lay~r ~andwich. Th-second bend is inspecte~ for coating ~acture and this p~ocedur~ i~ repeated, forming 4- J 5-, 6-, etc. layer sandwiches, until a bend exhibit~ no fractur~ of ~he , .
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coa~ing. The re~ult o~ each ~end tes~ i3 th~ minimum thickness (min~mu~ T-bend) of the b~nd which does not give any frac~ur~s o~ the coating. Although the bend test used is excessively :~;evere ~or most purposes ~or 5 which coated articl2s are used, lt provides a means to compare the ~lexibilities of different powder coating compositions.
Impact strength was deter~ined by using a G~dner Lab~ratory, Inc., ~mpact Tester. A weight is dropped ~ithin a slide ~ube from a specified height to hit a punch having a 1.59 centi~eter dla~ete~ he~ispherical nose wh~ch is driven into the front ~coated face) or :~
back of the panel. The highest impact which does not cracX the coating is recorded in cm-kg, front and 15 rev~rsa .
~ wenty degree and 60 degree gloss was measured using a gloss ~eter (Gardner Laboratory, Inc.) according to ASTM D-523.
A11 inherent Viscositih~ ~e~e determin~ ~t ~5C in a ~60~4~ ky weight) mixture of phenol~tetrachloroethane at a conc~ntration of o . 5 g.~lOO mL. Acid and hydroxyl numbers a~e deter~ined by titration and are reported herein ~s ~g of KOH consumed for each gram of polymQr.
The mslting te~pcratures (T~) are determined by dif~erential scanning calorimetry (DSC) on the s~ond heating cycle at ~ scanning rate o~ 2~ C per minute arter t~e sample h~s been heated to ~elt and guenche~ to below th~ glass transition temperature of the polymer.
Tg value arè reported as the midpo mt of the tran~ition 0 aIId Tm at peaks ~ transitions .
The p~ncil hardness of a coating is that o~ the hard~s~ that will not cut into the coating accor~ing to AST~ 3363-74 (reapproved 1980). The results are expressed according to th~ ~ollowing ~ca1e: ~so~test) 5 ~gST~

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~`^` 21037~9 -- 13a 6~3, 5B, 4~, 3~, 2B, B, HB, F, H, 2H, 3H, 4~, 5~, 6H
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~ 210 3 7 0 9 RO¦US 1 ~ R~R 1792 The conical mandrel is performed by beDding the panel over 15 seconds using a Gardner L~boratory, Inc., conical mandrel of specified size accordiDg to ASTM-522-85. A pass or fail is recorded.

EXA~PLE 1 This example illustrates the typical procedure for preparing the all-aliphatic polyesters of this in~en-tion. A 3000 mL, 3-necked, round bottom fl~sk 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 moles) and 10 mL of titanium te~raiso-propoxide/2-propanol solution (100 ppm Ti). The flask and contents were heated under nitrogen atmosphere to a temperature of 170C at which point methanol began to distill rapidly from the flasX. After the reaction mixture was heated with stirring ~t this temperature for about 1 hour, the temperature was increased to 200C for 2 hours, raised to 215C for 4 hours, and then to 235C.
After 3 hours at this temperature, ~ ~acuum of 10 mm o' mercury was applied over a period of 12 minutes.
Stirring was continued under 10 mm of merrury at 235C
for about 3 hours to produce a low melt ~isrosity, colorless polymer. The resulting polymer has an inherent ~iscosity of 0.30, a melting point of 130C, and a hydroxyl number of 30.

EXA~P~E 2 A powder coating composition was prepared from the following materials:

~l~rD~ T~lTr r~r~_ `~: 2103709 81~3 ~ Polyester of Example 1;
243.7 g Rucote 107, ~ polyester ~ased prima~ily on te_ephthaliC acid and 2,2-dimethyl-1,3-5propanediol;
75.0 g Ca~rolactam-blocked isophorone poly-lsocyan.ate (~uls 153~);
4.0 g Dibutyltin dilaur~te; .
4.0 g ~enzoin;

6.0 g Modaflow ITI;
.0 g Tinuvin 144; and 4.0 g TinuYin 234.

The above ~aterials ~ere ~lt-blended in an AP~
t~in screw extruder at llO~C, ground in a Bantam ~111 t~
which a s~'ea~ o~ liquid nitrogen is fed and classi~ied 2S through a 170 mesh screen on a KEK ~ent~ifugal sifter.
The finely-divid~d, powd6r coating composition obtai~d had an avera~e ~article size o~ about 50 microns.
The powder coating compo~itlon prepared in Example 2 was applied elect~ostatically to one side of the 7.62 c~ by 22~ a6 c~ par.els described above. ~he cGating ~as the~ cured (cross-linked) by heating the coated panels at 177C in an oven for 25 minutes. The curcd coatinqs are about 50 microns thick.
ThQ coatings o~ the panels had both front and back impact strengths of >1~4 c~-kg, 20 ~nd 60 gloss value~
8~ a~d ~, respcctively, and a pencil hardness ~ ~B.
The coatad pan~ls passed a 0.318 cm c~nical m~ndrel test and had a T-bend fl~xibility value o~ 1. After 780 - hours of QUV exposure, the coati~g r~tained 50~ o~ ~he 60~ gloss.~ .

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XAMPL~ 3 Using t~e procedure described in Example 2, a powder coating compo6ition was prepared from ~he following marerials:

163.7 g Polyester of ~xample l;
163 ~ 7 g Rucote 107, a p<:lyegter describçd in 1 û Examp l e 2;
72. ? g cap~olacta~blocked i~ophorone d~ isocyanate lS 6. 0 g Dibutylt~n dilaurate;
4. ~ g Benzoin;
6. 0 g Moaaflow III;

Using the procedure o~ Example 2, panels we~e coated wi~h ~his powder coating comDosition and the coatings were c~red and evaluated. The coatings had both front ~nd bac~ L~paCt ~trengths of ~184 cm-kg inch-pounds and 20~ and 60 glosY values of 72 and 91, re~pectively, and a pencil hardness of B. The coated panels passed a 0.3~7 c~ conical ~andrel and had a T-bend ~lex}bility ~alue or 1. After 810 hours of Q W
expGsure, t~e coati~g retained 50t of the 600 gloss.

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A po~der coating composition was prepared ~rom the ~ollowing materials:

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al6.60 ~ ~u~ote lQ7, a poly~ste~ described in Example 2;
la3.40 g Caprolacta~-blocked isophorone ~iis~cyanate (Huls 1530);
o.ao g Dibutyl~in dilaurate;
o.oo g 3enzoin;
. 10 15.00 g Mod~flow III;
400.00 g ~itanium dioxide;
10.00 g Tinuvin lC4 ' 10.00 g T~nuvi~ 2 Using the procedure of ~xample 2, panels w~re coat~d with this powder coating composition and the coatings wa~e cured and eva1uated. The ooatings had both front and back impa~t strengths o~ ~184 cm-kg and 200 and 60 gloss values of 85 and 9S, respecti~ely, and j~. a pencil hardness of H. The c~ated panels passe~ ~
O.317 c~ conical ~,andrel and had a T-bend flexibility ~; val~e of 6. Af~er 280 ~ours of Q W exposure, the ;~j. coatin~ retained 50% of the 50 gloss.
Thus, the coatin~s of Examples 2 and ~ p~ssess a G~o of 780 hours and 810 hours, respective1y. The coating of Compa~ative Exampl~ 1 pos~csses a G~o Of 280 hours.
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Claims (19)

We Claim:

1. 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 determined at 25°C in a 60/40 (by weight) mixture of phenol/tetrachloroethane at a concen-tration of 0.5 g/mL of about 0.1 to 0.5;
and (b) about 20 to 90 weight percent of poly-(tetramethylene trans-1,4-cyclohexane-dicarboxylate) having a trans-isomer content of at least 70% trans-, having a hydroxyl number of about 20 to 200, and an inherent viscosity determined at 25°C
in a 60/40 (by weight) mixture of phenol/tetrachloroethane at a concen-tration of 0.5 g/mL of about 0.1 to 0.5 dL/g; and (2) 5 to 30 weight percent based on the weight of components (1)(a) and (1)(b) and the cross-linking agent of a cross-linking agent.

2. The composition of Claim 1, wherein the cross-linking agent is a blocked polyisocyanate.

3. The composition of Claim 2, wherein the blocked polyisocyanate is .epsilon.-caprolactam-blocked isophorone diisocyanate.

4. The composition of Claim 1, further comprising one or more cross-linking catalysts.

5. A thermosetting coating accomposition 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 determined at 25°C in a 60/40 (by weight) mixture of phenol/etrachloroethane at a concen-tration of 0.5 g/mL of about 0.1 to 0.5;
wherein said aromatic polyester is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues, (2) diol residues of which at least 50 mole percent are 2,2-dimethyl-1,3-propanediol residues, and (3) up to 10 mole percent, based on the total moles or (2) and (3), of trimethylolpropane residues; and (b) about 20 to 90 weight percent of poly-(tetramethylene trans-1,4-cyclohexane-dicarboxylate) having a trans-isomer content of at least 70% trans-, having a hydroxyl number of about 20 to 200, and an inherent viscosity determined at 25°C
in a 60/40 (by weight) mixture of phenol/tetrachloroethane and at a concentration of 0.5 g/mL of about 0.1 to 0.5 dL/g; wherein up to about 10 mole percent of the polyol residues are comprised of trimethylolpropane residues;
and (2) 5 to 30 weight percent based on the weight of components (1)(a) and (1)(b) and the cross-linking agent of a cross-linking agent.

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

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

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

9. 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 determined at 25°C in a 60/40 (by weight) mixture of phenol/tetrachloroethane at a concen-tration of 0.5 g/mL of about 0.1 to 0.5;
and (b) about 20 to 90 weight percent of poly-(tetramethylene trans-1,4-cyclohexane-dicarboxylate) having a trans-isomer content of at least 70% trans-, having a hydroxyl number of about 25 to 65, and an inherent viscosity determined at 25°C in a 60/40 (by weight) mixture of phenol-tetrachloroethane at a concentration of 0.5 g/mL of about 0.1 to 0.4 dL/g; and (2) 5 to 30 weight percent based on the weight of compenents (1)(a) and (1)(b) and the cross-linking agent of a cross-linking agent.

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

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

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

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

14. A coating which results from the application of a thermosetting powder coating composition of Claim 5 to an article followed by heat-curing.

15. A coating which results from the application of a thermosetting powder coating composition of Claim 9 to an article followed by heat-curing.

16. An article coated with the cured powder coating composition of Claim 1.

17. An article coated with the cured powder coating composition of Claim 5.

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

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