WO1993004102A1

WO1993004102A1 - Thermosetting powder coating compositions

Info

Publication number: WO1993004102A1
Application number: PCT/US1992/007083
Authority: WO
Inventors: Robert Boyd Barbee; Brian Steven Phillips
Original assignee: Eastman Kodak Company
Priority date: 1991-08-27
Filing date: 1992-08-26
Publication date: 1993-03-04
Also published as: JPH07501353A; CA2115763A1; KR940702520A; EP0601079A1

Abstract

Provided are novel thermosetting powder coating compositions comprised of an aliphatic polyester, an aromatic polyester, and a self-blocked polyisocyanate. Also provided are shaped or formed articles coated with these compositions and cured.

Description

THERMOSETTING POWDER COATING COMPOSITIONS

Field of the Invention

This invention relates to certain novel thermo— setting powder coating compositions. More particularly, this invention provides a composition comprising a blend of an aromatic polyester, and aliphatic polyester, and a self—blocked polyisocyanate.

Background of the Invention

Thermosetting powder coating compositions are used extensively to produce durable protective coatings on various materials. Thermosetting coatings, when compared to coatings derived from thermoplastic composi¬ tions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates, and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above—stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder composi— tions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough or non-uniform finish. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The rough or non—uniform surface problem has caused thermo¬ setting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems sometimes 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 60 percent or less of the solvent—based coating composition 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 not contact the article or substrate being coated obviously cannot be reclaimed.

To produce smooth, glossy,uniform coatings, the polymeric materials constituting powder coating composi¬ tions must melt within a particular temperature range to permit timely and ample flow of the polymeric material prior to the occurrence of any significant degree of curing, i.e., cross—linking. Powder coating composi— tions which possess the requisite melting range provide smooth and glossy coatings upon being heated to cure the compositions. In addition to being smooth and glossy, coatings derived from thermosetting coating compositions should exhibit or possess good impact strength, hard— ness, flexibility, and resistance to solvents and chemicals. 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.

It is essential that powder coating compositions remain in a free—flowing, finely divided state for a reasonable period after they are manufactured and packaged. Thus, amorphous polyesters utilized in powder coating formulations desirably possess a glass transi¬ tion temperature (Tg) higher than the storage tempera¬ tures to which the formulations will be exposed. Semi- crystalline polyesters and blends thereof with amorphous polyesters also may be utilized in powder coating formulations. For this application, semi—crystalline polyesters desirably possess a significant degree of crystallinity to prevent caking or sintering of the powder for a reasonable period of time prior to its application to a substrate. Semi-crystalline polyesters used in powder coating formulations also must have melting temperature low enough to permit the compounding of the powder coating formulation without causing the cross—linking agent to react prematurely with the polyesters. The lower melting temperature of the semi- crystalline polyester also is important to achieving good flow of the coating prior to curing and thus aids the production of smooth and glossy coatings.

Finally, the production of tough coatings which are resistant to solvents and chemicals requires adequate cross-linking of the powder coating compositions at curing temperatures and times commonly employed in the industry. In the curing of powder coating compositions, a coated article typically is heated at a temperature in the range of about 325 to 400°F (163-204°C) for up to about 20 minutes causing the coating particles to melt and flow followed by reaction of the cross—linking (curing) agent with the polyester. The degree of curing may be determined by the methyl ethyl ketone rub test described hereinbelow. Normally, a thermosetting coating is considered to be completely or adequately cross—linked if the coating is capable of sustaining 200 double rubs. It is apparent that the use of lower temperatures and/or shorter curing times to produce adequately cross-linked coatings is very advantageous since higher production rates and/or lower energy costs can be achieved thereby.

Powder coating systems based on hydroxyl polyesters and caprolacta —blocked polyisocyanate cross—linking agents have been used extensively in the coatings industry. The most widely used caprolactam—blocked polyisocyanates are those commonly referred to as e—caprolactam—blocked isophorone diisocyanate, e.g., those described in U.S. Patents 3,822,240, 4,150,211, and 4,212,962. However, the products marketed as e—caprolactam—blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i.e., a mixture of the cis and trans iso ers of 3—isocyanatomethyl—3,5,5— tri ethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, di eric and/or trimeric forms. For example, the blocked polyisocyanate compound used as the cross—linking agent may be a mixture consisting primarily of the e—caprolactam—blocked, difunctional, monomeric isophorone diisocyanate and the e—caprolactam—blocked, trifunctional trimer of isophorone diisocyanate. 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 polyester to form urethane linkages, thereby cross—linking or curing the coating composition. During the curing process using an e—caprolactam— blocked polyisocyanate as described above, e—caprolactam is liberated from the powder coating compositions. To eliminate the presence of e—caprolactam from the work¬ place, adducts of the 1,3-diazetidine—2,4—dione dimer of isophorone diisocyanate and diols have been developed for use as cross—linking agents in powder coating compositions. Such adducts and powder coating composi¬ tions containing the adducts are described in the literature such as, for example, U. S. Patent 4,413,079, German OLS 3,328,133, and the Journal of Chromatography, 472 (1989) 175—195. While these oligomeric cross- linking agents avoid the liberation of e—caprolactam, they possess the disadvantage of not being as reactive as the e—caprolactam—blocked polyisocyanates when used in combination with commercially—available, amorphous polyesters. Thus, powder coatings based on amorphous polyesters commonly used in the powder coating industry and adducts of the 1,3—diazetidine—2,4—dione dimer of isophorone diisocyanate must be heated at higher temperatures, e.g., 400°F (204°C) as compared to 350°F (177°C) for caprolactam—blocked isophorone diisocyanate, and/or for longer periods of time to provide adequately cured coatings. However, the use of such higher temperatures does not produce the degree of cross- linking necessary to impart to the cured coating a satisfactory combination of properties, especially resistance to chemicals and solvents.

Further, the use of these self—blocked isocyanates also generally provides a coating having poorer physical properties, such as impact resistance, than do the corresponding compositions cross—linked with caprolactam—blocked isophorone diisocyanate.

We have discovered that an aliphatic polyester derived from 1,4—cyclohexanedicarboxylic acid and 1,4— butanediol can be blended with an aromatic polyester and formulated into a powder coating with a self—blocked isocyanate and which has better cure and physical properties than coatings not containing the aliphatic polyester.

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 at 60° is plotted versus time in hours. This plot illustrates the unexpectedly superior weathering properties of the compositions of the present invention relative to compositions containing no aliphatic polyester component.

Summary of the Invention

The present invention provides a novel thermo¬ setting powder coating composition comprised of a blend of an aromatic polyester and an aliphatic polyester, poly(tetramethylene trans—1,4—cyclohexane—dicarboxylate) and a self—blocked crosslinking agent. The cross- linking agent is an adduct of 1,3—diazetinine—2,4—dione and a diol. The powder coating compositions of the present invention and the coatings derived therefrom were found to possess superior cure and physical properties, especially weather resistance.

Detailed Description of the Invention

The present invention provides a thermosetting powder coating composition which comprises an intimate blend of

(1) A novel blend of polymers containing free hydroxy groups comprised of: (a) about 10 to about 80 weight percent of an aromatic polyester having a glass transition temperature (T_g) of greater than about 40"C., a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; and

(b) about 90 to about 20 weight percent of poly(tetramethylene trans—1,4—cycle— hexanedicarboxylate) 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 an adduct of the 1,3—diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol having the structure

OCN-R¹-[χ-R¹-NH-C-0-R²-0-C- •lNH-R^J'4-X- ■R -NCO n wherein

R¹ is a divalent l—methylene—l,3,3—trimethyl—

5-cyclohexyl radical, i.e., a radical having the structure

R² is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and X is a 1,3—diazetidine—2,4—dionediyl radical, i.e., a radical having the structure

wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70 to 130°C.

The powder coatings of the present invention provide coatings with improved impact strength, improved flexibility, improved weatherability, and a higher degree of cure at a lower cure temperature than a system without the aliphatic polyester.

In the above composition, both the aromatic poly— ester and the aliphatic polyester may be produced using well known polycondensation procedures.

Poly(tetramethylene trans—1,4—cyclohexanedicar— boxylate) may be prepared from 1,4—butanediol and the acid or diester of trans—1,4—cyclohexanedicarboxylic acid. When the diester is used, it is preferred that some excess glycol is utilized during the ester interchange reaction and is removed under reduced pressure until the desired viscosity is obtained.

The preferred aliphatic poly(tetramethylene trans- 1,4-cyclohexanedicarboxylate) polyester has a T_m in the range of about 110° to 160°C, 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.

The aliphatic polyester component may also contain a polyol branching agent such as trimethylolpropane, to increase the crosslink density of the coating. As a preferred embodiment of the present invention, up to about 10 mole percent of the 1,4—butanediol is replaced with a glycol having 2 to 12 carbon atoms. Examples of such glycols include 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—tetramethy1—1,3—cyclobutanediol, 1,4—xylylene— diol and the like. When trans—1,4—eyelohexane- dicarboxylic acid is referred to herein, it is intended to denote a mixture comprised of at least 70% trans isomer. The preferred aromatic polyester component of the composition provided by this invention has a T_g greater than 55°C, a hydroxyl number in the range of about 25 to 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 residues of which at least 50 mole percent are terephthalic acid residues, (2) glycol residues of which at least 50 mole percent are derived from 2,2—dimethy1— 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 comercially available under the tradenames Rucote 107 and Cargill Resin 3000.

A further preferred aliphatic poly(tetramethylene trans—1,4—cyclohexanedicarboxylate) has a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5; wherein up to 10 mole percent of the polyol residues are comprised of trimethylolpropane residues.

The relative amounts of the aromatic polyester and the aliphatic polyester can be varied substantially 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 desired from the cured coating, etc. As described above, the compositions of the present invention comprise a blend of about 10 to 80 weight percent of the aromatic polyester and about 20 to 90 weight percent of the aliphatic polyester. The blend of polymers containing free hydroxy groups provided by this invention is preferably comprised of about 20 to 75 weight percent of the aromatic polyester and 25 to 80 weight percent of the aliphatic polyester. It should be thus appreciated that components (1) (a) and (1) (b) will always total 100 percent.

The adducts of the 1,3—diazetidine-2,4—dione dimer of isophorone diisocyanate and a diol are prepared according to the procedures described in U.S. Patent 4,413,079, incorporated herein by reference, by reacting the diazetidine dimer of isophorone diisocyanate, preferably free of isocyanurate trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives as isocyanto:hydroxyl ratio of about 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8. The adduct preferably has a molecular weight of 1450 to 2800 and a melting point of about 85 to 120°C. The preferred diol residue (R²) is a C₂—C_Z diol, most preferably the residue of 1,4— butanediol. Such an adduct is commercially available under the name H ls BF1540. The amount of the cross—linking adduct present in the compositions of this invention can be varied depending on several factors such as those mentioned hereinabove relative to the amounts of aromatic polyester and aliphatic polyester utilized. Typically, the amount of cross—linking adduct 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 10 to 25 weight percent, based on the total weight of the aromatic polyester, the aliphatic polyester and the cross—linking compound.

The cross—linking component of the compositions may contain a minor amount, e.g., up to about 30 weight percent based on the total weight of the cross—linking component, of another blocked polyisocyanate such as those which are based on isophorone diisocyanate blocked with e—caprolactam, commercially available as Hϋls B1530, Ruco NI—2 and Cargill 2400, and phenol— blocked hexamethylene diisocyanate. The presence of minor amounts of such blocked polyisocyanates has been found to provide good cross—linking at temperatures as low as 325°F (163°C) with the liberation of very minor amounts of the blocking agent, e.g., e—caprolactam. The powder coating compositions of our invention may be prepared from the compositions described herein by dry—mixing and then melt—blending the aromatic polyester, the aliphatic polyester and the cross—linking adduct, 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 powder coatings. For example, the ingredients of the powder coating composition may be dry blended and then melt blended in a ZSK twin—screw extruder at 90 to 130°C, granulated and finally ground. The melt blending should be carried out at a temperature sufficiently low to prevent the conversion of the cross—linking adduct to a reactive form and thus avoid premature cross—linking. To minimize the exposure of the cross-linking adduct to elevated temperatures, the amorphous and semi- crystalline polyesters may be blended prior to the incorporation therein of the cross—linking agent.

Typical of the additives which may be present in the powder coating compositions include benzoin, used to reduce entrapped air or volatileε, flow aids or flow control agents which aid the formation of a smooth, glossy surface, catalysts to promote the cross—linking reaction between the isocyanate groups of the cross— linking agent, and the hydroxyl groups on the polymers, 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 catalyst 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 the amorphous and semi—crystalline polyesters 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.

Conventional ultraviolet light stabilizers such as TINUVIN 234, and hindered a ine light stabilizers such as TINUVIN 144, may also be utilized. The powder coating compositions preferably contain a flow aid, also referred to as flow control or leveling agents, to enhance the surface appearance of cured coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are available 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—ethylhexyl methacrylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present may be in the range of about 0.5 to 4.0 weight percent, based on the total weight of the amorphous and semi- crystalline polyesters and the cross—linking agent.

The powder coating compositions may be deposited on various metallic and non-metallic substrates by known techniques for powder deposition such as by means of a powder gun, by electrostatic 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 powder coating composition normally is in the range of 60 to

300 microns. The powder is maintained in suspension by passing air through a porous bottom of the fluidized bed chamber. The articles to be coated are preheated to about 250 to 400°F (about 121 to 205°C) and then brought into contact with 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 1 to 12 seconds. The temperature of the substrate being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated article also affects cross—linking of the coating composition and results in the formation of a tough coating having a good combination of properties. Coatings having a thickness between 200 and 500 microns may be produced by this method.

The compositions also may be applied using an electrostatic process wherein a powder coating composition having a particle size of less than

100 microns, preferably about 15 to 50 microns, is blown by means of compressed air into 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 particles adhere due to the electrical charge thereof. The coated article is heated to melt and cure the powder particles. Coating of 40 to 120 microns thickness may be obtained. Another method of applying the powder coating compositions is the electrostatic fluidized bed process which 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 400°F, or cold, is exposed briefly to the fluidized powder. 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-resistant 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, particularly steel articles.

The compositions and coatings of our invention are further illustrated by the following examples. The inherent viscosities (I.v.; dl/g) referred to herein were measured at 25°C using 0.5 g polymer per 100 mL of a solvent consisting of 60 parts by weight phenol and 40 parts by weight tetrachloroethane. Melt viscosities (poise) were determined using an ICI melt viscometer according to ASTM D4287—83. Acid and hydroxyl numbers were determined by titration and are reported herein as mg of KOH consumed for each gram of polymer. The glass transition temperatures (Tg) and the melting tempera— tures (Tm) were determined by differential scanning calorimetry (DSC) on the second heating cycle at a scanning rate of 20°C per minute after the sample was heated to melt and quenched to below the Tg of the polymer. Tg values are reported as the midpoint of the transition and Tm at peaks of transitions. The weight average molecular weight (Mw) and number average molecular weight (Mn) were determined by gel permeation chromatography in tetrahydrofuran (THF) using a poly¬ styrene standard and a UV detector. Coatings were prepared on 3 inch by 9 inch panels of 24—gauge, polished, cold roll steel, the surface of which has been zinc phosphated (BONDERITE 37, The Parker Company) . Impact strengths were determined using an impact tester (Gardner Laboratory, Inc.) according to ASTM D2794-84. A weight with a 5/8-inch diameter, hemispherical nose was dropped within a slide tube from a specified height to drive into the front (coated face) or back of the panel. The highest impact which did not crack the coating was recorded in inch—pounds, front and reverse.

The solvent resistance and the degree of cure (cross-linking) of the coatings were determined by a methyl ethyl ketone (MEK) rub procedure in which coated panels were rubbed with a two-pound ball peen hammer wrapped with cheese cloth approximately 0.5 inch thick. The cloth was wetted with MEK every 50 double strokes. The rubbing was continued until bare metal is observed or until 200 double rubs are completed. The result of each MEK rub procedure is reported as the number of double rubs required for the observation of bare metal or 200, whichever is less.

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 pressurized to 20,000 pounds per square inch (psi) , until the apex of the bend is as flat as can be reasonably achieved. This bend is referred to as 0T meaning that there is nothing (zero thicknesses) between the bent portions of the panel. The bend is examined using a 10X magnifying glass and a pass is recorded if no fractures of the coating are observed. If fractures of the coating are observed, the panel is bent a second time (IT) to form a three—layer sandwich. The second bend is inspected for coating fracture and this procedure is repeated, forming 4—, 5—, 6—, etc. layer sandwiches, until a bend exhibits no fracture of the coating. The result of each bend test is 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 compositions.

The 20 degree and 60 degree gloss are measured using a gloss meter (Gardner Laboratory, Inc. , Model GC— 9095) according to ASTM D-523.

The acid numbers and hydroxyl number are determined by titration and reported as mg of KOH consumed for each gram of resin. The pencil hardness of a coating is that of the hardest pencil that will not cut into the coating according to ASTM 3363—74 (reapproved 1980) . The results are expressed according to the following scale: (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest) .

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

The artificial weatherability of the coatings was determined by exposure of the coated panels in a Cyclic Ultraviolet Weathering Tester (QUV) with 313 nm fluorescent tubes. The test condition was 8 hours of light at 70°C and 4 hours of condensation at 45° C.

Experimental Section

Example 1

This example illustrates the typical procedure for preparing the aliphatic polyesters of the invention. A 3000mL, 3—necked, round—bottom flask equipped with a stirrer, a short distillation column, and an inlet for nitrogen, is charged with dimethyl cyclohexane— dicarboxylate (1259.7 g, 6.29 mol), 1,4—butanediol (997.5 g, 11.08 mol), trimethylolpropane (73.9 g, 0.55 mol) and 10 mL of titanium tetraisopropoxide/2—propanol solution(100 ppm Ti) . The flask and contents are heated under nitrogen atmosphere to a temperature of 170°C at which point methanol begins to distill rapidly from the flask. After the reaction mixture is heated with stirring at this temperature for about 1 hour, the temperature is increased to 200°C. for 2 hours, raised to 215°C. for 4 hours, and then to 235°C. After 3 hours at this temperature, a vacuum of 10 mm of mercury is applied over a period of 18 minutes. Stirring is continued under 10 mm of mercury at 235°C. for about 3 hours to produce a low melt viscosity, colorless polymer. The polymer has an inherent viscosity of 0.30, a melting point of 130°C, and a hydroxyl number of 30.

Example 2

A powder coating composition is prepared from the following materials:

141.0 g Polyester of Example 1;

423.0 g RUCOTE 107, a polyester based primarily on terephthalic acid and 2,2—dimethyl—

1,3— ropanediol; 136.0 g Self—blocked isophorone polyisocyanate

(Huls BF 1540) 280.0 g Ti0₂ 7.0 g Dibutyltin dilaurate;

7.0 g Benzoin; 10.5 g MODAFLOW III (flow aid Monsanto); 7.0 g TINUVIN 144; and 7.0 g TINUVIN 234.

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

The powder coating composition prepared in Example 2 was applied electrostatically to one side of the 3 inch by 9 inch panels described above. The coating was cured (i.e., cross-linked) by heating the coated panels at 177°C. in an oven for 20 minutes. The cured coatings are generally about 50 microns thick.

The coatings on the panel have both front and back impact strengths of >160 inch—pounds, 20° and 60° gloss values of 84 and 94, respectively, and a pencil hardness of F. The coated panels pass a 0.125 inch conical mandrel test and have a T—bend flexibility value of 1. After 520 hours of QUV exposure, the coating retains 50% of the 60° gloss.

Example 3

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

Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings were cured and evaluated. The coatings have both front and back impact strengths of >160 inch—pounds and 20° and 60° gloss values of 82 and 94, respectively, and a pencil hardness of F. The coated panels pass a 0.125 inch conical mandrel and have a T—bend flexibility value of 0. After 500 hours of QUV exposure, the coating retains 50% of the 60° gloss.

Example 4

This example illustrates the typical procedure for preparing the aliphatic polyesters of this invention using 1,4—cyclohexanedicarboxylic acid. A 3000 mL, 3— necked, round—bottom flask equipped with a stirrer, a short distillation column, and an inlet for nitrogen was charged with 1,4—cyclohexanedicarboxylic acid (1100.3 g, 6.39 mol), 1,4-butanediol (629.77 g, 6.68 mol), 1.5 g of butanestannoic acid (FASCAT 4100) catalyst. The flask and contents are heated under nitrogen atmosphere at

200° C during 60 minutes and maintained at 200° C for 2 hours. The temperature was then increased to 215° C for 2 hours and then to 235° C for 8 hours to produce a low melt viscosity, colorless polymer. The polymer has an inherent viscosity of 0.25, a melting point of 155° C, and a hydroxyl number of 36.

Example 5

Using the procedure described in Example 2, a powder coating was prepared from the following materials:

204.2 g Polyester of Example 4; 612.5 g RUCOTE 107, a polyester described in

Example 2; 183.4 g Self—blocked isophorone diisocyanate (Huls BF 1540) ; 10.0 g Dibutyltin dilaurate; 10.0 g Benzoin; 15.0 g MODAFLOW III; 400.0 g Ti0₂; 10.0 g TINUVIN 144; and 10.0 g TINUVIN 234.

Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings were cured and evaluated. The coatings have both front and back impact strengths of >160 inch—pounds and 20° and 60° gloss values of 84 and 96, respectively, and a pencil hardness of F. The coated panels pass a 0.125 inch conical mandrel and have a T—bend flexibility value of 0. After 600 hours of QUV exposure, the coating retains 50% of the 20° gloss.

Comparative Example 1

A powder coating composition was prepared from the following materials:

968.0 g RUCOTE 107, a polyester described in

Example 2; 232.0 g Self-blocked isophorone diisocyanate (Huls BF 1540) ; 12.0 g Dibutyltin dilaurate;

12.0 g Benzoin; 18.0 g MODAFLOW III 480.0 g Ti0₂; 12.0 g TINUVIN 144; and 12.0 g TINUVIN 234.

Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings were cured at 177°C for 20 minutes and evaluated. Coated panels have front impact of 60 inch- pounds and back impact of 10-inch pounds. The 20° and 60° gloss values were 86 and 96, respectively, and the pencil hardness is F. The coated panels fail a 0.125 inch conical mandrel test and have a T—bend flexibility value of 6. After 220 hours of QUV weathering exposure, the coating retains 50% of the 60° gloss.

Claims

Claims We claim:

1. A thermosetting powder coating composition which comprises an intimate blend of

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

(a) about 10 to about 80 weight percent of an aromatic polyester having a glass transi¬ tion temperature (T_g) of greater than about 40°C. , a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; and

(b) about 20 to about 90 weight percent of poly(tetramethylene trans—1 ,4—cycle— hexanedicarboxylate) 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 an adduct of the 1,3—diazetidine—2,4—dione dimer of isophorone diisocyanate and a diol having the structure

wherein

R¹ is a radical having the structure ^CH3\ CH - !

^C ^2^"

R² is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and

X is a 1,3-diazetidine—2,4-dionediyl radical having the structure

2. The thermosetting powder coating composition of claim 1, wherein the aromatic polyester component has a T_g greater than 55°C, a hydroxyl number in the range of about 25 to 80, an acid number of not more than 15, and an inherent viscosity of about 0.15 to 0.4.

The thermosetting powder coating composition of claim 2, wherein the aromatic polyester component is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues and (2) glycol residues of which at least 50 mole percent are derived from 2,2—dimethyl—1,

3— propanediol.

4. The thermosetting powder coating composition of claim 1, wherein the aliphatic polyester component has a T_m in the range of about 110° to 160°C, 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.

5. The thermosetting powder coating composition of claim 4, wherein in the aliphatic polyester component, up to about 10 mole percent of the 1,4— butanediol residues are replaced with a glycol residues having 2 to 12 carbon atoms.

6. The thermosetting powder coating composition of claim 1, wherein in component (2) , R² is the residue of a C₂-C₈ diol.

7. The thermosetting powder coating composition of Claim 6, wherein in component (2) , R² is the residue of 1,4—butanediol.

8. A shaped or formed article coated with the cured composition of claim 1.

9. A thermosetting powder coating composition which comprises an intimate blend of

(1) A novel blend of polymers containing free hydroxy groups comprised of: (a) about 10 to about 80 weight percent of an aromatic polyester having a glass transi¬ tion temperature (T_g) of greater than about 40°C, a hydroxyl number of about 20 to 200 and an inherent viscosity of about 0.1 to about 0.5; wherein the aromatic polyester component is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues, (2) glycol residues of which at least 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 trimethylol— propane residues, and

(b) about 20 to about 90 weight percent of poly(tetramethylene trans—l,4—cycle— hexanedicarboxylate) having a hydroxyl number of about 20 to 200, and an inherent viscosity of about 0.1 to 0.5; wherein up to 10 mole percent of the polyol residues are comprised of tri¬ methylolpropane residues; and

wherein R¹ is a radical having the structure

X is a 1,3—diazetidine—2,4—dionediyl radical having the structure

wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to

1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70 to 130°C.

10. The thermosetting powder coating composition of claim 9, wherein the aromatic polyester component has a T_ε greater than 55°C, a hydroxyl number in the range of about 25 to 80, an acid number of not more than 15, and an inherent viscosity of about 0.15 to 0.4.

11. The thermosetting powder coating composition of claim 9, wherein the aliphatic polyester component has a T_m in the range of about 110° to 160°C, 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.

12. The thermosetting powder coating composition of claim 11, wherein in the aliphatic polyester component, up to about 10 mole percent of the 1,4— butanediol residues are replaced with a glycol residues having 2 to 12 carbon atoms.

13. The thermosetting powder coating composition of claim 9, wherein in component (2), R² is the residue of a C₂—C_g diol.

14. The thermosetting powder coating composition of Claim 6, wherein in component (2), R² is the residue of 1,4—butanediol.

15. A shaped or formed article coated with the cured composition of claim 9.