Patent Publication Number: US-2006009591-A1

Title: Powder coating compositions containing anhydride end-caped crystalline polyesters

Description:
This invention relates to thermosetting coating compositions containing a carboxylic acid anhydride end-capped crystalline polyester in powder coating applications, and more particularly in low temperature cure powder coating applications.  
      Powder coating applications are considered green chemistry process because their compositions greatly reduce or completely eliminate the organic solvents used in liquid paints. When they are thermally cured, no or very little volatile organic compounds (VOC) are released to the surrounding environment.  
      A typical thermoset powder coating formulation consists of a polymeric binder, curative, pigment, flow aid, degassing agent and curing catalyst. Among all binders polyesters are widely used because they provide excellent weathering resistance and mechanical properties as well as good appearance. Polyester resins have either hydroxyl or carboxyl groups at their chain ends. Hydroxyl functional polyesters are typically formulated with a blocked isocyanate curative to form a polyester-urethane powder coating. Carboxyl functional polyesters may be formulated with a crosslinker such as triglycidyl isocyanurate, TGIC, polyepoxy resins and other compounds or polymers which contain functional groups reactable with carboxylic acid groups to form thermosetting networks. Powder coating compositions using carboxylated polyester and TGIC curative give coatings with good exterior durability, and the powder coatings using polyepoxy resins as curatives are mainly for interior applications. The polyester resins in powder coating compositions usually possess a glass transition temperature of above 50° C., which allows the powder coating composition to have storage stability without sintering.  
      Powder coating formulations are typically mixed, extruded, pulverized, classified and electrostatically applied onto substrates. The coated parts are then baked at elevated temperatures.  
      Currently, the great majority of polyesters used in heat-curable powder compositions are amorphous polyesters. When the polyester is amorphous, it is difficult to prepare perfect heat-curable powdered compositions because they have to meet often contradictory criteria. Thus, these powders may not re-agglomerate during handling, transportation and storage, which implies that the amorphous polyester must possess a sufficiently high glass transition temperature (T g ). On the other hand, in order for the powder particles to be able to coalesce and to form a perfectly homogeneous and uniform coating, it is necessary for the T g  of the polyester to be sufficiently low to ensure a low viscosity in the molten state which itself ensures good wetting of the pigments and other solid materials accompanying the polyester in the formulation of the said heat-curable powder compositions.  
      Moreover, the powder must be capable of melting at the stoving temperature in order to form an even film before the crosslinking reaction begins which results in the final curing. In order to obtain good spreading of the molten film over the surface of the substrates, it is therefore necessary for the viscosity of the polyester in the molten state to be sufficiently low. This is because a very high viscosity in the molten state prevents good spreading of the molten film and is reflected by a loss in the evenness and gloss of the coating. Finally, the rate of the crosslinking reaction is controlled by varying the temperature, the amount and/or the nature of the curative and that of the curing catalyst, which is optionally used.  
      For all these reasons, it Is not generally recommended to produce coatings from compositions based on such amorphous polyesters by stoving at temperatures below 160° C. for approximately 10 to 20 minutes.  
      There is increasing market interest in applying powder coatings on heat-sensitive substrates such as wood, plastics and medium-density fibreboard (MDF). In order to fulfil these needs, cure temperatures must be lowered to below 150° C. However, at low temperatures, powders tend to cure incompletely and do not flow effectively. Incomplete cure and poor flow can cause many property defects such as adhesion failure, poor chemical resistance, poor mechanical properties, orange peel, etc.  
      Accordingly, it is an object of the present invention to provide thermosetting powder coating compositions which, upon application and curing at low temperatures, provide finishes which overcome all these negative aspects.  
      It now has been surprisingly found that a binder composition comprising a carboxylic acid group containing amorphous polyester, a particular carboxylic acid anhydride end-capped semi-crystalline polyester and a glycidyl group containing polyphenoxy resin curative, when applied and cured at temperatures of from 80 to 150° C. for 5-30 minutes, affords coatings having good adhesion to metallic and non-metallic surfaces, excellent flow, outstanding flexibility and chemical resistance. The composition can optionally contain one or more curing agents having functional groups reactable with the polyesters&#39; carboxylic acid groups,  
      Powder coating compositions based on semicrystalline polyesters have already formed the subject of a certain number of publications in the form of papers and patents, in particular U.S. Pat. Nos. 4,352,924, 3,387,214, 4,937,288 and 4,973,646.  
      EP 521992B1 describes a powder coating composition containing an amorphous and a semi-crystalline polyester resin. The semi-crystalline resin has an acid number of from 10 to 70 mg KOH/g. The purpose of adding the crystalline polyester is to improve the flow of the powder coating.  
      U.S. Pat. No. 4,937,288 describes a powder coating system having an improved flow property that consists of a carboxyl functional acrylic polymer, a crystalline polyester and a beta-hydroxyalkylamide curing agent. The crystalline polyester has an acid number of from about 150 to 750 mg KOH/g. The system is limited to beta-hydroxyalkylamide curative and is not related to low-temperature cure powder coatings.  
      U.S. Pat. No. 6,407,181 describes a powder coating composition based on a glycidyl (meth)acrylate copolymer and a low-viscosity carboxylic acid functional polyester, which provides a smooth, weatherable, reduced gloss coating for use on heat sensitive substrates. According to that patent, the low-viscosity carboxylic acid functional polyesters are generally linear but may be capped with trimellitic anhydride to provide a functionality of 4. The said polyesters have acid numbers between 20 and 60 mg KOH/g, and glass transition temperatures between 40 and 800C. Depending upon the gloss reduction desired, the low viscosity polyesters can be replaced by or blended with a crystalline polyester. Neither the crystalline polyester nor the preparation of it was described in the patent. The glycidyl (meth)acrylate containing copolymer is used as the hardener.  
      According to the current invention there are provided a thermosetting coating composition comprising a binder wherein 100 parts by weight of this binder comprises:  
      (A) 1 to 50 parts by weight of a carboxylic acid group containing amorphous polyester having an acid number of from 15 to 100 mg KOH/g.  
      (B) 1 to 50 parts by weight of a carboxylic acid group containing semi-crystalline polyester, said polyester comprising the reaction product of an anhydride of a polybasic organic carboxylic acid and a hydroxyl group containing semi-crystalline polyester.  
      (C) 1 to 90 parts by weight of a glycidyl group containing polyphenoxy resin having an epoxy equivalent weight of 150 to 1500 g/equiv.  
      (D) 0 to 85 parts by weight of a glycidyl group containing (meth) acrylate copolymer having an epoxy equivalent weight of 1.0 to 5.0 milli-equivalents of epoxy/gram of polymer.  
      (E) 0 to 20 parts by weight of a curing agent different from (C) and (D) and having functional groups reactable with the polyester (A) and (B) carboxylic acid groups.  
      Said thermosetting compositions are particularly suitable for low temperature cure powder applications on metal and heat sensitive substrates, which provide excellent coating appearance and mechanical properties.  
      The current invention solves the problems of low flow, poor appearance and inferior mechanical properties encountered in conventional low temperature cure powder coatings.  
      The carboxyl functional amorphous polyester (A) used in the composition according to the invention is preferably composed of, referring to the polyacid constituents, from 50 to 100 molar percent of terephthalic or isophthalic acid or their mixtures and from 50 to 0 molar percent of another aliphatic, cycloaliphatic or aromatic polyacid, and, referring to the polyol constituents, from 40 to 100 molar percent of neopentyl glycol and from 60 to 0 molar percent of another aliphatic and/or cycloaliphatic polyol. Branching of the amorphous polyester can be obtained by incorporation of a polyacid or polyol.  
      The carboxyl functional semi-crystalline polyester (B) used in the composition according to the invention is preferably obtained from the ring-opening reaction of the anhydride group of a trimellitic and/or pyromellitic anhydride with a hydroxyl group containing semi-crystalline polyester (b). This hydroxyl-group containing semi-crystalline polyester (b)which is preferably composed of, referring to the polyacid constituents, from 70 to 100 molar percent of terephthalic acid, 1,4-cyclohexanedicarboxylic acid or a linear chain dicarboxylic acid containing 4 to 16 carbon atoms and from 30 to 0 molar percent of another aliphatic, cycloaliphatic or aromatic polyacid, and, referring to the polyol constituents, from 70 to 100 molar percent of a cycloaliphatic or a linear chain aliphatic polyol containing 2 to 16 carbon atoms and from 30 to 0 molar percent of another aliphatic or cycloaliphatic polyol. This hydroxyl-group containing polyester (b) preferably has an hydroxylnumber of 15 to 70 mg KOH/g.  
      The glycidyl group containing polyphenoxy resin (C) used in the composition according to the invention is preferably from the Bisphenol A or from the phenol or cresol novolac type.  
      The glycidyl group containing acrylic copolymer (D) optionally used in the composition according to the invention is preferably prepared from 10 to 90 molar percent of a glycidyl group containing monomer and from 90 to 10 molar percent of one or more other monomers copolymerizable with the glycidyl group containing monomer.  
      The curing agent having functional groups reactive with the carboxylic acid groups of the polyesters (E) optionally used in the composition according to the invention is preferably a polyepoxy or β-hydroxyalkylamide compound.  
      The carboxyl functional amorphous polyesters (A) of the present invention have an acid number from 15 to 100 mg KOH/g and preferably from 30 to 70 mg KOH/g.  
      Preferably, the carboxyl functional amorphous polyesters have: 
          a number averaged molecular weight ranging from 1100 to 15000 and more preferably from 1600 to 8500, measured by gel permeation chromatography (GPC)     a glass transition temperature Tg) from 40 to 80° C., measured by Differential Scanning Calorimetry according to ASTM D3418 with a heating gradient of 20° C. per minute     an ICI cone and plate viscosity according to ASTM D4287-88, measured at 200° C. ranging from 5 to 15000 mPa.s.        

      The acid constituent of the amorphous polyester, according to the present invention, is preferably from 50 to 100 molar percent composed of terephthalic or isophthalic acid or their mixtures and from 0 to 50 molar percent of another polyacid constituent selected from one or more aliphatic, cycloaliphatic or aromatic polyacids, such as: fumaric acid, maleic acid, phthalic anhydride, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, succinic acid, adipic acid, glutaric acid, pimelic acid, suberic acid, azealic acid, sebacic acid, 1,12-dodecanedioic acid, trimellitic acid or pyromellitic acid, etc., or the corresponding anhydrides.  
      The glycol constituent of the amorphous polyester, according to the present invention, is preferably from 40 to 100 molar percent composed of neopentyl glycol and from 0 to 60 molar percent of another glycol constituent selected from one or more aliphatic or cycloaliphatic glycols such as: ethylene glycol, propylene glycol, 1,4-butanepolyol, 1,6-hexanepolyol, 1,4-cyclohexanepolyol, 1,4-cyclohexanedimethanol, 2-methyl-1,3-propanepolyol, 2-butyl-2-ethyl-1,3-propanepolyol, hydrogenated Bisphenol A, hydroxypivalate of neopentyl glycol, trimethylolpropane, ditrimethylolpropane, pentaerythritol, etc.  
      The carboxyl functional semi-crystalline polyesters (B) used in the present invention preferably have a carboxyl number from 30 to 120 mg KOH/g and more preferably from 50 to 100 mg KOH/g and most preferable from 70 to 100 mg KOH/g.  
      Preferably, the carboxyl functional semi-crystalline polyesters are further characterized by: 
          a number averaged molecular weight ranging from 1100 to 17000 and more preferably from 1400 to 11200     a fusion zone from 50 to 150° C., measured by Differential Scanning Calorimetry (DSC) according to ASTM D3418 with a heating gradient of 20° C. per minute     a glass transition temperature (Tg) of below 40° C., measured by Differential Scanning Calorimetry (DSC) according to ASTM D3418 with a heating gradient of 20° C. per minute     a degree of crystallinity, measured by Differential Scanning Calorimetry (DSC) according to ASTM D3415 of at least 5 J/g and preferably at least 10 J/g     an ICI (cone/plate) viscosity according to ASTM D4287-88, measured at 100° C. of at least 10 mPa.s.        

      The acid constituent of the semi-crystalline polyester, according to the present invention, is preferably from 70 to 100 molar percent composed of terephthalic acid, 1,4-cyclohexanedicarboxylic acid or a linear chain dicarboxylic acid containing from 4 to 16 carbon atoms such as succinic acid, adipic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, 1,10-decanedioic acid, 1,11-undecanedioic acid, 1,12-dodecanedioic acid, 1,13-triadecanedioic acid, 1,14-tetradecanedioic acid, 1,15-pentadecanedioic acid, 1,16-hexadecanedioic acid, etc. used in a mixture or alone, and from 30 to 0 molar percent of another aliphatic, cycloaliphatic or aromatic polyacid such as fumaric acid, maleic anhydride, phthalic anhydride, isophthalic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, ect.  
      The glycol constituent of the semi-crystalline polyester, according to the present invention, is preferably from 70 to 100 molar percent composed of a cycloaliphatic or linear-chain aliphatic polyol containing 2 to 16 carbon atoms such as 1,4-cyclohexanepolyol, 1,4-cyclohexanedimethanol, hydrogenated Bisphenol A, 2,2,4,4-tetramethyl-1,3-cyclobutanol or 4,8-bis[hydroxymethyl]tricyclo[5.2.1.O]decane, ethylene glycol, 1,3-propanepolyol, 1,4-butanepolyol, 1,5-pentanepolyol, 1,6-hexanepolyol, 1,7-heptanepolyol, 1,8-octanepolyol, 1,9-nonanepolyol, 1,10-decanepolyol, 14-tetradecanepolyol, 1,16-hexadecanepolyol, etc., used in a mixture or alone, and from 30 to 0 molar percent of another aliphatic glycol such as propylene glycol, neopentyl glycol, 2-methyl-1,3-propanepolyol, 2-butyl-2-ethyl-1,3-propanepolyol, hydroxypivalate of neopentyl glycol, etc.  
      The glycidyl group containing polyphenoxy resin (C) used in the composition of the present invention is preferably selected from the Bisphenol A based epoxy resins, phenol or cresol epoxy novolacs.  
      Bisphenol A based epoxy resins are typically prepared from the reaction of Bisphenol A and epichlorohydrin, wherein the excess of epichlorohydrin determines the number average molecular weight of the epoxy resin. See W. G. Potter: Epoxide Resins, Springer-Verlag, New York (1970) and Y. Tanaka, A. Okada, I. Tomizuka in C. A. May, Y. Tanaka (eds.): Epoxy Resins Chemistry and Technology, Chapter 2, pp.9-134, Marcel Dekker, New York 1973.  
      The phenol and cresol epoxy novolacs are generally prepared by the acid-catalyzed condensation of formaldehyde with either phenol or cresol.  
      Epoxidation of the novolacs with epichlorohydrin furnishes the epoxy novolacs. Commercially available epoxy resins such as Epikote 1055 from Shell, Araldite GT7004 or Araldite ECN9699 from Ciba, D.E.R.664 from Dow and EPON 2002 from Shell are typical examples of glycidyl group containing polyphenoxies.  
      The glycidyl group containing acrylic copolymers (D) optionally used in the composition of the present invention have an epoxy equivalent weight of 1.0 to 5.0 milli-equivalents of epoxy/gram of polymer.  
      Preferably, the glycidyl group containing acrylic copolymers are further characterized by: 
          a number averaged molecular weight ranging from 1000 to 15000     a glass transition temperature (T g ) from 40 to 85° C. measured by Differential Scanning Calorimetry (DSC), according to ASTM D3418 with a heating gradient of 20° C. per minute     an ICI cone and plate viscosity at 200° C. of at least 100 mPa.s        

      The glycidyl group containing monomer used to make the acrylic copolymer of the present invention is preferably used in molar percentages ranging from 10 to 90 and is preferably selected from the group of glycidyl acrylate, glycidyl methacrylate, methyl glycidyl methacrylate, methyl glycidyl acrylate, 3,4-epoxycyclohexylmethyl(meth)acrylate and acrylic glycidyl ether. They can be used alone or in mixtures of two or more.  
      Other monomers copolymerizable with the epoxy group containing monomer are preferably used In molar percentages ranging from 10 to 90 and selected from: 
          40 to 100 mole percentage of acrylic or methacrylic ester monomers such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, n-decyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, n-hexyl methacrylate, isoamyl methacrylate, allyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, 2-ethylbutyl methacrylate, cinnamyl methacrylate, crotyl methacrylate, cyclohexyl methacrylate, cyclopentyl methacrylate, methallyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, 2-phenylethyl methacrylate and phenyl methacrylate.     0 to 60 mole percent of other ethylenically unsaturated copolymerizable monomers such as styrene, alkyl-substituted styrenes and chloro-substituted styrenes, acrylonitrile, vinyl chloride and vinylidene fluoride and vinyl acetate.        

      The curing agent (E) optionally used in the composition in accordance to the present invention, having functional groups reactive with the polyester&#39;s carboxylic acid groups, is preferably selected from: 
          polyepoxy compounds, which are solid at room temperature and contain at least two epoxy groups per molecule, such as, for example, triglycidyl isocyanurate (TGIC) diglycidyl terephthalate, triglycidyl trimellitate, or mixutres thereof, and Araldite FP910 or PT 912 both manufactured by Ciba.     β-hydroxyalkylamides which contains at least one, preferably two bis(β-hydroxyalkyl)amide groups such as those mentioned in U.S. Pat. Nos. 4,727,111, 4,788,255, 4,076,917, EP 322,834 and EP 473,380.        

      The carboxylic acid group containing amorphous polyester (A) and the hydroxyl group containing semi-crystalline polyester (b), according to the present invention, may be prepared using conventional esterification techniques well known in the art. The polyesters are preferably prepared according to a procedure consisting of one or more reaction steps.  
      For the preparation of these polyesters, a conventional reactor equipped with a stirrer, an inert gas (nitrogen) inlet, a thermocouple, a distillation column connected to a water-cooled condenser, a water separator and a vacuum connection tube are used.  
      The esterification conditions used to prepare the polyesters are conventional, namely a standard esterification catalyst, such as dibutyltin oxide, dibutyltin dilaurate, n-butyltin trioctoate, sulfuric acid or a sulphonic acid, can be used in an amount from 0.05 to 1.50% by weight of the reactants and optionally, color stabilizers, for example, phenolic antioxidants such as Irganox 1010 (Ciba) or phosphonite- and phosphite-type stabilizers such as tributylphosphite, can be added in an amount from 0 to 1% by weight of the reactants.  
      Polyesterification is generally carried out at a temperature which is gradually increased from 130° C. to about 190 to 250° C., first under normal pressure, then, when necessary, under reduced pressure at the end of each process step, while maintaining these operating conditions until a polyester with the desired hydroxyl and/or acid number is obtained. The degree of esterification is monitored by determining the amount of water formed in the course of the reaction and the properties of the obtained polyester, for example, hydroxyl number, acid number, and viscosity.  
      The carboxylic acid group containing semi-crystalline polyester (B) is generally obtained by the ring opening reaction of the anhydride group of a carboxylic acid anhydride with the hydroxyl group of the hydroxyl group containing semi-crystalline polyester (b) at a temperature of from 120 to 200° C.  
      To the carboxylic acid group containing amorphous and semi-crystalline polyesters, crosslinking catalysts can optionally be added. These catalysts are added in order to accelerate crosslinking reactions of the thermosetting powder composition during curing. Examples of such catalysts include amines (e.g. 2-phenylimidazoline), phosphines (e.g. triphenylphosphine), ammonium salts (e.g. tetrabutylammonium bromide or tetrapropylammonium chloride), phosphonium salts (e.g. ethyltriphenylphosphonium bromide (BETP) or tetrapropylphosphonium chloride). These catalysts are preferably used in an amount of from 0.1 to 5% with respect to the weight of the binder.  
      The carboxylic acid group containing semi-crystalline polyesters used in the composition according to the present invention is more preferably a carboxylic acid anhydride end-capped semi-crystalline polyester, prepared from the ring-opening reaction of the anhydride group of at least one, and preferably two mole(s), of a carboxylic acid anhydride, e.g. trimellitic anhydride, with one mole of hydroxyl functional prepolymers having hydroxyl numbers of from 15 to 70 mg KOH/g, which was prepared from the polycondensation of: 
          1,4-cyclohexanedicarboxylic acid and a linear chain aliphatic C2-C16 polyol, or     a linear chain aliphatic C4-C16 polyacid and a linear chain aliphatic C2-C16 polyol or a cycloaliphatic polyol        

      Particularly preferred carboxylic acid anhydride end-capped semi-crystalline polyesters are those prepared from the ring opening reaction of two moles of anhydride, especially trimellitic anhydride, with one mole of hydroxyl functional prepolymers prepared from the condensation of: 
          1,4-cyclohexandicarboxylic acid and ethylene glycol, 1,4-butanepolyol or 1,6-hexanepolyol, or     1,12-dodecanedioic acid and ethylene glycol, 1,4-butanepolyol, 1,6-hexanepolyol or 1,4-cyclohexanedimethanol, or     1,4-cyclohexanedimethanol and succinic acid, adipic acid or azelaic acid.        

      These trimellitic anhydride end-capped semi-crystalline polyesters have a sharp melting zone, a high degree of crystallinity, a high reactivity towards reactable groups of crosslinkers, and properties that are different from those found in the semi-crystalline polyesters known from anterior arts.  
      Used in the thermosetting powder coating compositions of the present invention, they provide high reactivity, excellent flow and good storage stability,  
      The glycidyl group containing acrylic copolymer (D) can be prepared by conventional polymerization techniques, either in mass, in emulsion, or in the solution of an organic solvent. The nature of the solvent used is very little of importance, provided that it is inert and that it readily dissolves the monomers and the synthesized copolymer. Suitable solvents include toluene, ethyl acetate, butyl acetate, xylene, etc. The monomers are generally copolymerized in the presence of a free radical polymerization initiator (benzoyl peroxide, dibutyl peroxide, azo-bis-isobutyronitrile, and the like) in an amount representing 0. 1 to 4.0% by weight of the monomers.  
      To achieve a good control of the molecular weight and its distribution, a chain transfer agent, preferably of the mercaptan type, such as n-dodecylmercaptan, t-dodecanethiol, iso-octylmercaptan, or of the carbon halide type, such as carbon tetrabromide, bromotrichloromethane, etc., can also added in the course of the reaction. The chain transfer agent is usually used in amounts of up to 10% by weight of the monomers used in the copolymerization.  
      A cylindrical, double walled reactor equipped with a stirrer, a condenser, an inert gas (nitrogen, for example) inlet and outlet, and metering pump feeding systems is generally used to prepare the glycidyl group containing acrylic copolymer. Polymerization is carried out under conventional conditions. Thus, when polymerization is carried out in solution, for example, an organic solvent is first introduced into the reactor and heated to the refluxing temperature under an inert gas atmosphere (nitrogen, carbon dioxide, and the like) and a homogeneous mixture of the required monomers, the free radical polymerization initiator and the chain transfer agent, when needed, is then added to the solvent gradually over several hours. The reaction mixture is then maintained at the indicated temperature for certain hours, while stirring. The solvent Is then removed from the copolymer obtained in vacuo.  
      The binder system of the thermosetting composition of the invention is generally composed in such a way that for each equivalent of carboxyl group present in the amorphous polyester (A) and semi-crystalline polyester (B), there is between 0.3 and 2.0 and preferably between 0.6 and 1.7 equivalents of epoxy groups from the polyphenoxy resin (C), optionally the acrylic copolymer (D) and the curing agent (E). The particular thermosetting polyester blend (A) and (B), can be obtained by dry blending the amorphous and the semi-crystalline polyester using a mechanical mixing procedure as available for the premixing of the powder paint constituents.  
      Alternatively, the amorphous and the semi-crystalline polyester can be blended in the melt using the conventional cylindrical double-walled reactor or by extrusion such as the Betol BTS40.  
      In addition to the essential components described above, compositions within the scope of the present invention can also include flow control agents such as Resiflow P-67 (Estron), Modaflow (Monsanto), Acronal 4F (BASF), etc., and degassing agents such as Benzoin (BASF) etc. To the formulation UV-light absorbers such as Tinuvin 900 (Ciba), hindered amine light stabilizers represented by Tinuvin 144 (Ciba), other stabilizing agents such as Tinuvin 312 and 1130 (Ciba), antioxidants such as Irganox 1010 (Ciba) and stabilizers of phosphonite or phosphite types, can also be added.  
      Both pigmented and clear lacquers can be prepared. A variety of dyes and pigments can be utilized in the composition of this invention. Examples of useful pigments and dyes are: metallic oxides such as titanium dioxide, iron oxide, zinc oxide and the like, metal hydroxides, metal powders, sulphides, sulphates, carbonates, silicates such as ammonium silicate, carbon black, talc, china clay, barytes, iron blues, lead blues, organic reds, organic maroons and the like.  
      The components of the composition according to the invention may be mixed by dry blending in a mixer or blender (e.g. drum mixer). The premix is then homogenized at temperatures ranging from 50 to 120° C. in a single screw extruder such as the BUSS-Ko-Kneeter or a twin screw extruder such as the PRISM or APV. The extrudate, when cooled down, is ground to a powder with a particle size ranging from 10 to 150 μm. The powdered composition may be deposited on the substrate by use of a powder gun such as an electrostatic CORONA gun or TRIBO gun. On the other hand, well known methods of powder deposition such as the fluidized bed technique can also be used. After deposition the powder is heated to a temperature between 80 and 150° C., causing the particles to flow and fuse together to form a smooth, uniform, continuous, non-cratered coating on the substrate surface.  
      The following examples are submitted for a better understanding of the invention without being restricted thereto. 
    
    
     EXAMPLE A  
      A trimellitic anhydride end-capped crystalline polyester was prepared based on the following mixture of ingredients:  
                                                   Ingredients   Parts by weight (g)                                                    Dodecanedioic acid   4127.2           1,4-butanepolyol   1784.2           Trimellitic anhydride   721.3           Fascat 4102 1     13.2                           1 Fascat 4102 is butyltin tris(2-ethylhexanoate) available from Atofina.             
 
      The dodecanedioic acid, 1,4-butanepolyol and Fascat 4102 were charged to a reaction vessel and heated under a nitrogen atmosphere until all the ingredients melted. The temperature of the reaction mixture was gradually increased to 220° C. The mixture was stirred and held at 220° C. until an acid value of below 5 mg KOH/g was obtained. The reaction mixture was then cooled to 170-190° C. followed by the addition of the trimellitic anhydride. The temperature was maintained until the reaction mixture became clear and an acid value of 70-80 mg KOH/g was obtained. Vacuum was applied to remove water to push the polycondensation to completion. The reaction mixture was transferred from the vessel to a receiving container and allowed to cool to room temperature to give a solid product. The resultant polyester had an acid number of 78 mg KOH/g, a melt viscosity at 100° C. measured by Brookfield Cone and Plate Viscometer of 5.5 poise, a melting temperature measured by Differential Scanning Colorimeter (DSC) of 58.9° C.  
     EXAMPLE B  
      A trimellitic anhydride end-capped crystalline polyester was prepared based on the following mixture of ingredients:  
                                                   Ingredients   Parts by weight (g)                                                    Dodecanedioic acid   3692.0           1,6-hexanepolyol   2143.8           Trimelltic anhydride   729.3           Fascat 4102   12.6                      
 
      The dodecanedioic acid, 1,6-hexanepolyol and Fascat 4102 were charged to a reaction vessel and heated under a nitrogen atmosphere until all the ingredients melted. The temperature of the reaction mixture was gradually increased to 230° C. The mixture was stirred and held at 230° C. until an acid value of below 5 mg KOH/g was obtained. The reaction mixture was then cooled to 170-190° C. followed by the addition of the trimellitic anhydride. The temperature was maintained until the reaction mixture became clear and an acid value of 70-80 mg KOH/g was obtained. Vacuum was applied to remove water to push the polycondensation to completion. The reaction mixture was transferred from the vessel to a receiving container and allowed to cool to room temperature to give a solid product. The resultant polyester had an acid number of 76 mg KOH/g, a melt viscosity at 100° C. measured by Brookfield Cone and Plate Viscometer of 6.5 poise, a melting temperature measured by Differential Scanning Colorimeter (DSC) of 60.0° C.  
     EXAMPLE C  
      A carboxylic acid group containing amorphous polyester was prepared based on the following mixture of ingredients:  
                                                   Ingredients   Parts by weight (g)                                                    Terephthalic acid   5389.1           Adipic acid   598.8           Neopentyl glycol   4130.2           Trimelltic anhydride   1198.8           Fascat 4102   25.0                      
 
      The terephthalic acid and adipic acid were charged to a reaction vessel containing molten neopentyl glycol and Fascat 4102 at a temperature of 150° C. and under a nitrogen atmosphere. The temperature of the reaction mixture was gradually increased to 230° C. The mixture was stirred and held at 230° C. until an acid value of below 5 mg KOH/g was obtained. Vacuum was applied to remove water to push the polycondensation to completion. The reaction mixture was then cooled to 170-190° C. followed by the addition of the trimellitic anhydride. The temperature was maintained until the reaction mixture became clear and an acid value of 70-82 mg KOH/g was obtained. The reaction mixture was transferred from the vessel to a receiving container and allowed to cool to room temperature to give a solid product. The resultant polyester had an acid number of 80 mg KOH/g, a melt viscosity at 175° C. measured by Brookfield Cone and Plate Viscometer of 90 poise, a glass transition temperature measured by Differential Scanning Colorimeter (DSC) of 63.3° C.  
     EXAMPLE D  
      A glycidyl group containing acrylic copolymer was prepared based on the following procedure:  
      800 parts of n-butyl acetate are charged in a 5-litre, double walled flask equipped with a stirrer, a water-cooled condenser, an inlet for nitrogen and a thermocouple attached to a thermo-regulator. The flask content is then heated and stirred continuously while nitrogen is purged through the solvent. At 125° C. a mixture of 91 parts of tert-butylperoxybenzoate in 200 parts of n-butyl acetate are fed in the flask during 215 minutes with a peristaltic pump. 5 Minutes after the feeding, the other pump is started to feed the mixture of 284 parts of glycidyl methacrylate, 312 parts of butyl methacrylate and 312 parts of methyl methacrylate within 180 minutes. The total synthesis time was 315 minutes.  
      After evaporation of the n-butyl acetate an acrylic copolymer with following characteristics was obtained:  
                                                      ICI viscosity @ 200° C.    400 poise           M n     2600                      
 
     EXAMPLE E  
      A pigmented low temperature cure powder coating composition was prepared based on the following mixture of ingredients:  
                                   Ingredients   Parts by weight (g)                                        Example C   136       Crystalline Polyester prepared in Example A   34       EPON 2002   170       TiO 2     150       Resiflow P-67   5       Benzoin   2       BETP   1.5                  
 
      The ingredients were pre-mixed, melt blended at 90° C. in a Prism TSE 16 PC Twin Screw Extruder. The extrudate was chilled and broken into flakes, which were particulated, classified and electrostatically sprayed onto metal or MDF panels and cured at 130° C. for 20 minutes. The properties of the resultant coating are reported in Table 1 below.  
     EXAMPLE F  
      A pigmented low temperature cure powder coating composition was prepared based on the following mixture of ingredients:  
                                   Ingredients   Parts by weight (g)                                        Example C   127.5       Crystalline Polyester prepared in Example B   42.5       EPON 2002   170       TiO-2   149.5       Resiflow P-67   5       Benzoin   3.5       BETP   2                  
 
      The ingredients were pre-mixed, melt blended at 85° C. in a Prism TSE 16 PC Twin Screw Extruder. The extrudate was chilled and broken into flakes, which were particulated, classified and electrostatically sprayed onto metal or MDF panels and cured at 130° C. for 20 minutes. The properties of the resultant coating are reported in Table 1 below.  
     EXAMPLE G  
      A pigmented low temperature cure powder coating composition was prepared based on the following mixture of ingredients:  
                                   Ingredients   Parts by weight (g)                                        Example C   127.5       Crystalline Polyester prepared in Example B   42.5       EPON 2002   150       Glycidyl acrylic copolymer of Example D   20       TiO 2     149.5       Resiflow P-67   5       Benzoin   3.5       BETP   2.0                  
 
      The ingredients were pre-mixed, melt blended at 90° C. in a Prism TSE 16 PC Twin Screw Extruder. The extrudate was chilled and broken into flakes, which were particulated, classified and electrostatically sprayed onto metal or MDF panels and cured at 130° C. for 25 minutes. The properties of the resultant coating are reported in Table 1 below.  
                                   TABLE 1                                      Reverse   MEK           Example   Gloss       Impact   Double   Visual                                         No.   60°   20°   DOI   (in. lb.)   Rubs   Appearance                                                 E   99.5   94.2   80   160   &gt;100   smooth       F   99.0   95.0   80   160   &gt;100   smooth       G   100.0   94.2   80   80   &gt;100   fairly                               smooth