MXPA96004793A - Thermosetting powder coating compositions - Google Patents

Abstract

Provided are novel thermosetting powder coating compositions which are useful for coating metal articles and protecting such articles from acidic corrosion. The compositions are comprised of an epoxy containing acrylic copolymer crosslinked with both an aliphatic dibasic acid and a blocked polyisocyanate.

Description

THERMOSTABLE COMPOSITIONS OF POWDER COATING This invention pertains to the field of thermosetting powder coating compositions. In particular, it refers to a powder coating with excellent resistance to chemical attack. The plastic materials used in the manufacture of powder coatings are classified broadly or as thermosets or thermoplastics. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and in this way the particles are allowed to flow together and form a smooth layer. Thermoset coatings, when compared to coatings derived from thermoplastic compositions, are generally tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the hardening of thermosetting coatings has created problems in obtaining coatings which have, in addition to the desirable characteristics indicated above, good smoothness and flexibility. Coatings prepared from heat-stable powder compositions, after the application of heat, can be cured or dried before forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating or finishing surface lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The problem of the "orange peel" surface has caused thermoset coatings to be applied from organic solvent systems which are inherently undesirable due to environmental and safety problems caused by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the drawback of relatively low percentage utilization, ie, in some modes of application only 60 percent or less of the solvent-based coating composition that is applied comes in contact with the article or substrate that should be covered. Therefore, a substantial portion of the solvent-based coatings can be discarded, since the portion that does not come into contact with the article or substrate that is coated can obviously not be regenerated. The present invention as described below pertains to suitable powder coatings for use in high performance automotive coatings. It is now foreseeable that the coatings that are subjected to exterior exposure provide the client with a finish that has excellent weather resistance and that also prevent or minimize damage by chemical attack and water stain due to weather conditions. It has now been found that a powder coating based on glycidyl methacrylate and having excellent weather resistance will show markedly improved resistance to chemical attack by the addition of polyisocyanate. Japanese Patent 60-120764 (Dainippon In and Chemicals, Inc.) discloses glycidyl methacrylate powder coatings based on a mixture of glycidyl methacrylate resins, polyester resins, and polyisocyanates. U.S. Patent 4,522,981 discloses acrylate copolymer having both blocked polyisocyanate and glycidyl groups. A glycidyl containing copolymer having free hydroxyl groups is reacted with a partially blocked polyisocyanate. U.S. Patent No. 4,539,218 discloses modified acrylate copolymer with glycidyl esters and isocyanate-blocked hydroxyalkyl acrylates. The patent of the United States N? No. 4,818,791 discloses a mixture of a polyester resin containing carboxyl and hydroxyl groups, a glycidyl methacrylate resin also containing hydroxyl groups, and polyisocyanates. U.S. Patent No. 4,824,909 describes a powder coating containing a mixture of a polyester resin, an acrylic resin containing hydroxyl and glycidyl groups, and polyisocyanates. The present invention provides thermosetting powder coating compositions comprised of an acrylic glycidyl resin, an aliphatic carboxylic acid, and a blocked polyisocyanate. The compositions, after application and hardening, provide coatings that have significantly improved chemical resistance to similar coatings that do not utilize a blocked polyisocyanate. This invention relates to a powder coating based on glycidyl methacrylate resins and hardened with an aliphatic dibasid acid and a polyisocyanate such as HULS 1530. This coating has excellent resistance to chemical attack. Therefore, the present invention provides a powder coating composition, comprising: A. about 55 to 71 weight percent of a glycidyl functionalized acrylic copolymer of 1. about 10 to 40 weight percent, based on the weight of the copolymer, of a monoethylenically unsaturated mohomer having the less an epoxy group, and 2. about 60 to about 90 weight percent, based on the weight of the copolymer, of at least one monoethylenically unsaturated monomer is free of epoxy groups, where the copolymer has an average molecular weight number of about 1,000 to 8,000, a weight average molecular weight of about 2,000 to 16,000 and a vitreous transition temperature of 40 ° C to 90 ° C, and B. about 9 to 14 weight percent of an aliphatic carboxylic acid having 2 to 3 carboxylic acid groups per molecule, and C. about 17 to 34 weight percent based on the total weight of A, B and C, or a blocked polyisocyanate.

Naturally, in the above composition, it should be appreciated that the total weight percentage of A, B and C will be equal to 100%. Glycidyl copolymer resins, that is, component A, are well known in the art and are commercially available, for example, as PD 7610, PD 6300 and PD 1700 manufactured by Anderson Chemical Development. Typical compositions are described in United States patents 4,042,645 and 4,346,144, incorporated herein by reference and are generally composed of from about 10 to 40 weight percent of a monoethylenically unsaturated monomer having at least one epoxy group and 60 to 90 weight percent of one or more monoethylenically unsaturated monomers that do not have epoxy groups. Preferred monomers containing epoxy groups are glycidyl acrylate and glycidyl methacrylate. Preferred monomers which do not have epoxy groups are styrene and esters of acrylic or methacrylic acid such as methyl methacrylate, n-butyl methacrylate. Monomers having hydroxyl functionality, such as hydroxyethyl methacrylate, can also be used. The glycidyl copolymer resin preferably has a number average molecular weight of from 1,000 to 8,000, more preferably from 2,000 to 5,000, and a weight average in molecular weight of preferably 2,000 to 16,000, more preferably from 4,000 to 12,000, as determined by gas chromatography. gel permeation in tetrahydrofuran. The glycidyl copolymer resin can be prepared by conventional solution, emulsion or bead polymerization techniques using conventional polymerization catalysts. In general, aliphatic crosslinking agents with carboxy functionality are C3-C30 alkyl, alkenyl, or alkynyl compounds with two or more carboxylic acid functional groups. Preferred carboxy-functional cross-linking compounds can be described by the formula where n is an integer from 1 to 10. Examples of such crosslinking agents with carboxy functionality include alkyl polycarboxy compounds, such as dodecanedioic acid, azelaic acid, adipic acid, 1,6-hexanedioic acid, succinic acid, pimelic acid, acid sebacic, maleic acid, citric acid, itaconic acid, aconitic acid and the like. Additional examples of suitable aliphatic dicarboxylic acids include 1,2-, 1,3-, and 1,4-cyclohexanedicarboxylic acid. Aliphatic dicarboxylic acids can be used as crosslinking agents for the glycidyl copolymer resin. The most readily available and therefore preferred blocked polyisocyanate crosslinking agents or compounds are those commonly referred to as isophorone diisocyanate blocked with e-caprolactam, for example those described in U.S. Patent Nos. 3,822,240, 4,150. .211 and 4.212.-862, incorporated herein by reference. However, products marketed as isophorone diisocyanate blocked with e-caprolactam may consist mainly of the monomeric isophorone diisocyanates, difunctional, blocked, that is, a mixture of the cis and trans isomers of 3-isocyanato-methyl-3,5,5-trimethylcyclohexyl, the difunctional dimer, blocked thereof, the trifunctional trimer, blocked therefrom or a mixture of the monomeric, dimeric and / or trimeric forms. For example, the blocked polyisocyanate compound used as the crosslinking agent may be a mixture consisting mainly of the monomeric, difunctional isophorone diisocyanate blocked with e-caprolactam and the trifunctional trimer blocked with isophorone diisocyanate e-caprolactam. The description presented here of the crosslinking agents as "polyisocyanates" refers to compounds containing at least two isocyanate groups, which are blocked, i.e., reacted with another compound, for example e-caprolactam. The reaction of the isocyanate groups with the blocking compound is reversible at elevated temperatures, for example normally at about 150 ° C, and more. Alternatively, the blocked polyisocyanate can be an effective crosslinking amount of an adduct of 1,3-diacetydin-2,4-dione dimer of isophorone diisocyanate and a diol having the structure where R1 is a divalent radical of 1-methylene-1,3,3-trimethyl-5-cyclohexyl, ie, a radical having the structure R2 is an aliphatic, cycloaliphatic, araliphatic or aromatic divalent residue of a diol; and X is a 1,3-diacetidin-2,4-diondiyl radical, that is, a radical having the structure O M -N N- c / 0 where the ratio of NCO groups with respect to OH in the adduct formation is approximately 1: 0.5 to 1: 0.9, the molar ratio of diacetyldinedione to diol is from 2: 1 to 6: 5, the content of free isocyanate groups in the adduct is not more 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 adducts of 1,3-diacetydin-2,4-dione dimer of isophorone diisocyanurate and a diol are prepared according to the procedures described in U.S. Patent 4,413,079, incorporated herein by reference, by reacting the diacetidine dimer of isophorone diisocyanate, preferably free of isocyanurate trimers of isophorone diisocyanate, with diols in a reagent ratio giving an isocyanate: hydroxyl ratio of from about 1: 0.5 to 1: 0.9, preferably 1 : 0.6 to 1.08. The adduct preferably has a molecular weight of 1450 to 2800 and a melting point of about 85 to 120 ° C. The preferred diol reagent is 1,4-butanediol. An adduct of this type is commercially available under the name Hüls BF1540. The amount of the blocked polyisocyanate crosslinking compound present in the compositions of this invention can be varied depending on several factors such as those mentioned above with respect to the amount of components A, B and C that are used. Typically, the amount of crosslinking compound that will effectively crosslink the polymers to produce coatings having a good combination of properties is in the range of about 17 to 34 weight percent, preferably 24 to 31 weight percent, based on weight total of components A, B and C. After heating to harden the coatings of the composition, the blocked polyisocyanates are deblocked and it is believed that the isocyanate groups react with the hydroxy groups that are formed during the opening of the epoxy ring by the crosslinking aliphatic acid. The additional crosslinking provides improved resistance to acid attack.

The improvement marketed in acid attack by the addition of polyisocyanate to a powder coating of glycidyl methacrylate is shown in Table 1 below. Coatings containing less than 17% polyisocyanate have a worse resistance to acid attack than coatings having from 17 to 34% polyisocyanate. Thus, a further aspect of the present invention is to provide a method for protecting a metal article comprising coating the article with the compositions of the present invention, followed by heat setting. Also conventional ultraviolet light stabilizers, such as Tinuvin 234, and hindered amine photo stabilizers, such as Tinuvin 144. In addition, conventional dyes and pigments such as R960 titanium dioxide pigment marketed by Du Pont may be used as well. as catalysts. The components of the compositions according to this invention can be mixed by dry blending in a Henschel kneader, followed by combination in a ZSK-30 extruder (Werner &Pfleiderer) or APV twin-screw extruder at 100-130 ° C, crushing, and screening to obtain powder with average particle size of approximately 35 microns. The powder can be electrostatically deposited on the substrate by the use of a powder gun. After deposition, the powder is heated to a temperature sufficient to cause its particles to flow and melt together to form a smooth uniform surface. Coatings were prepared on 3-inch by 9-inch panels of cold-rolled steel polished, of caliber 20, whose surface has been phosphated with zinc (Bonderite 37, The Parker Company). The melt mixture should be run at a sufficiently low temperature to prevent release of the polyisocyanate crosslinking compound and thus prevent premature crosslinking. Typical additives that may be present in the powder coating compositions include benzoin, flow aids or flow control agents that contribute to the formation of a smooth, glossy surface, stabilizers, pigments and dyes. The powder coating compositions preferably contain a flow aid, also referred to as flow control or leveling agents, to improve the surface appearance of hardened coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are available from various suppliers, for example MODAFLOW from Monsanto Company and ACRONAL from BASF. Other flow control agents that can 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. An example of a specific flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing 60 mole percent of 2-ethylhexyl methacrylate residues and about 40 mole percent of 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 the resin component, and the crosslinking agent. The powder coating compositions can be deposited on various metallic and non-metallic substrates (eg, thermoplastic or thermosetting compound) by known powder deposition techniques, 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 in a suspension of the air powder coating. The particle size of the powder coating composition is usually in the range of 60 to 300 microns. The powder is kept in suspension by passing air through a porous lower part of the fluidized bed chamber. The articles to be coated are preheated to approximately 250 ° to 400 ° F (approximately 121 to 205 ° C) and then contacted with the fluidized bed of the powder coating composition. The contact time depends on the thickness of the coating that must be produced and is typically from 1 to 12 seconds. The temperature of the substrate to be coated causes the powder to flow and thus melts together to form a seamless, continuous, uniform, smooth coating. The temperature of the preheated article also effects crosslinking of the coating composition and results in the formation of a tough coating having a good combination of properties. By this method, coatings having a thickness between 200 and 500 microns can be produced. The compositions can also be applied using an electrostatic process, where a powder coating composition having a particle size of less than 100 microns, preferably from about 15 to 50 microns, is blown by means of compressed air in an applicator in which They are charged with a voltage of 30 to 100 kV by high voltage direct current. The charged particles are then sprayed onto the crushed article to be coated to which the particles adhere due to the electrical charge thereof. The coated article is heated to melt and harden the powder particles. Coatings of 40 to 120 microns thick can 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 in the air fed to a fluidized bed to produce an electrostatic charge, such as 50 to 100 kv. The article to be coated, either hot, for example 250 ° to 400 ° F, or cold, is briefly exposed to the fluidized powder. The coated article can then be heated to effect crosslinking, if the article was not preheated to a temperature high enough to harden the coating after contact of the coating particles with the article. The powder coating compositions of this invention can be used to coat articles of various shapes and sizes constructed of heat-resistant materials, such as glass, ceramic and various metallic materials. The compositions are especially useful for producing coatings on articles made of metals and metal alloys, particularly steel articles. As an alternative process to the melt / extrusion / grinding / mixing process, it is also possible to mix and crush the components, preferably using a hammer-mill type sprayer, for example a Bantam micro-sprayer hammer mill equipped with a 0.010"slot screen, which operates at a rotor speed of about 8000-14,000 rpms.The composition can then be crushed in a jet mill to obtain a useful powder composition.The stage of grinding in jet mill is preferably carried out Using a fluid energy type mill, a Microjet Fluid Energy Mill Model 4, a 4"mill operating at nominal values of 60-100 psi and 40-70 scfm (standard cubic foot) is preferred. per minute), which has a feed rate of 2 to 16 lbs / h. Alternatively, you can also use a TROST air impact sprayer, which operates at 10-18 scfm (standard cubic feet per minute) at 80-100 psi, which has a feed rate of 0.2 to 2 lbs / h. Other air jet mills capable of performing the process include those manufactured by Micron Powder Systems and Sturtevant. Additional examples of formulation methods, additives, and powder coating application methods can be found in User's Guide to Powder Coatinq, 2nd edition, Emery Miller, editor, Society of Manufacturing Engineers, Dearborn, (1987). Brightness at 60 degrees is measured using a gloss meter (Gardner Laboratory, Inc., Model GC-9095) in accordance with ASTM D-523. The test procedure to evaluate the sensitivity of coatings to chemical attack consists of applying 6 drops of 36% H2SO4 on the coating and heating it at 60 ° C for 30 minutes and observing any spot developed. The spots are classified according to the following objective classification scale: 2 = severe, 4 = pronounced, 6 = moderate, 8 = light, and 10 = no change. The coatings of this invention are further illustrated by the following examples.

Experimental section Example 1 - Preparation of powder coating composition containing 0% HULS 1530 polyisocyanate A powder coating composition was prepared from the following materials: 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 6.0 g Benzoin; and 6.0 g MODAFLOW III.

The above material was melt blended in an APV twin-screw extruder at 110 ° C, ground in a Bantan mill to which a stream of liquid nitrogen was fed, and sorted through a 170 mesh screen on a centrifugal screen KEK The finely divided powder coating composition obtained had an average particle size of about 50 microns. This powder coating composition was electrostatically applied to one side of a 3 inch by 9 inch panel described above. The coating hardened (lattice) by heating the coated panel at 177 ° C in an oven for 20 minutes. The coating on the panel had a pencil hardness of F, a gloss value at 60 ° of 87, and an acid etch rating of 7 to 60 ° C.

Example 2 - Powder coating preparation based on PD 7610 and containing 8% Huís 1530 A powder coating composition was prepared from the following materials: 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 50.0 g HULS 1530 crosslinking agent; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of F, a brightness value at 60 ° of 94, and an acid attack rating of 7 to 60 ° C.

Example 3 - Powder coating preparation based on PD 7610 and containing 14% Huís 1530 A powder coating composition was prepared from the following materials: 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 100.0 g HULS 1530; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of F, a brightness value at 60 ° of 94, and an acid attack rating of 8 to 60 ° C.

Example 4 - Preparation of powder coating based on PD 7610 and containing 20% Huís 1530 A powder coating composition was prepared from the following materials: 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 150.0 g HULS 1530 crosslinking agent; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of 2H, a gloss value at 60 ° of 100, and an acid attack rating of 10 to 60 ° C.

Example 5 - Preparation of powder coating based on PD 7610 and containing 25% Huís 1530 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 202.0 g HULS 1530 crosslinker; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of 2H, a gloss value at 60 ° of 102, and an acid attack rating of 10 to 60 ° C.

Example 6 - Powder coating preparation based on PD 7610 and containing 30% Huís 1530 A powder coating composition was prepared from the following materials: 497.1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 252.0 g HULS 1530 crosslinker; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of 3H, a brightness value at 60 ° of 99, and an acid attack rating of 10 to 60 ° C.

Example 7 - Powder coating preparation based on PD 7610 and containing 32% Huís 1530 A powder coating composition was prepared from the following materials: 497. 1 g PD 7610 glycidyl copolymer; 102.9 g Dodecanedioic acid; 275.4 g HULS 1530 crosslinker; 6.0 g Dibutyltin dilaurate; 6.0 g Benzoin; 6.0 g MODAFLOW III flow adjuvant.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of 3H, a gloss value at 60 ° of 98, and an acid attack rating of 10 to 60 ° C.

Example 8 - Powder coating preparation based on PD 7000 and containing 0% Huís 1530 A powder coating composition was prepared from the following materials: 826.9 g PD 1700 glycidyl copolymer; 173.3 g Dodecanedioic acid; 10.0 g Benzoin; 20.0 g EX 486 Flow Adjuvant (Troy Chemical Co.); 10.0 g TINUVIN 144, and 20.0 g TINUVIN 234.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of H, a gloss value at 60 ° of 105, and an acid attack rating of 8 to 60 ° C.

Example 9 - Preparation of powder coating based on PD 1700 and containing 17% Huís 1530 A powder coating composition was prepared from the following materials: 826.7 g PD 1700 glycidyl copolymer; 173.3 g Dodecanedioic acid; 210.6 g Huís 1530 crosslinker; 10.0 g Dibutyltin dilaurate 10.0 g Benzoin; 20.0 g Flow adjuvant EX 486; 10.0 g TINUVIN 144, and 20.0 g TINUVIN 234.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of 3H, a gloss value at 60 ° of 100, and an acid attack rating of 10 to 60 ° C.

Example 10 - Powder coating preparation based on PD 6300 and containing 0% Huís 1530 A powder coating composition was prepared from the following materials: 993.0 g PD 6300 glycidyl copolymer; 207.0 g Dodecanedioic acid; 10.0 g Benzoin; 20.0 g Flow adjuvant EX 486; 10.0 g TINUVIN 144, and 20.0 g TINUVIN 234.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of H, a brightness value at 60 ° of 94, and an acid etch rating of 8 to 60 ° C.

Example 11 - Powder coating preparation based on PD 6300 and containing 17% Huís 1530 A powder coating composition was prepared from the following materials: 828.4 g PD 6300 glycidyl copolymer; 171.6 g Dodecanedioic acid; 206.5 g Huís 1530 crosslinker; 10.0 g Dibutyltin dilaurate; 10.0 g Benzoin; 20.0 g Flow adjuvant EX 486; 10.0 g TINUVIN 144, and 20.0 g TINUVIN 234.

Using the procedure of Example 1, panels with this powder coating composition were coated and the coatings hardened and evaluated. The coatings have a pencil hardness of H, a gloss value at 60 ° of 103, and an acid attack rating of 10 to 60 ° C.

Table 1 Example 1 2 3 4 5 6 7 8 9 10 11 Amount of polyisocyanate% 0 8 14 20 25 30 32 0 17 0 17 Classification of chemical attack 7 7 8 10 10 10 10 8 10 8 10

Claims (2)

CLAIMS 1. A powder coating composition, comprising: A. about 55 to 71 weight percent of a glycidyl functionalized acrylic copolymer comprised of 1. about 10 to 40 weight percent, based on the weight of the polymer, of a monoethylenically unsaturated monomer having at least one epoxy group, and 2. about 60 to 90 weight percent, based on the weight of the copolymer, of at least one monoethylenically unsaturated monomer that is free of epoxy groups, wherein the copolymer has a numerical average molecular weight of about 1,000 to 8,000, a weight average in molecular weight of approximately 2,000 to 16,000 and a vitreous transition temperature of 40 ° C to 90 ° C, and B. about 9 to 14 weight percent of an aliphatic carboxylic acid having 2 to 3 carboxylic acid groups per molecule, C. about 17 to 34 weight percent based on the total weight of A, B and C, or a blocked polyisocyanate. The composition of claim 1, wherein the acrylic copolymer functionalized with glycidyl has a number average molecular weight of about 2,000 to 5,000 and a weight average molecular weight of about 4,000 to 12,000. 3. The composition of claims 1 or 2, wherein the aliphatic carboxylic acid is a compound of the formula where n is an integer from 1 to 10. The composition of claims 1, 2 or 3, wherein the aliphatic carboxylic acid is selected from the group consisting of dodecanedioic acid, azelaic acid, adipic acid, 1,6-hexanedioic acid , succinic acid, pimelic acid, sebacic acid, maleic acid, citric acid, itaconic acid, aconitic acid, or 1,2-, 1,3-, or 1,4-cyclohexanedicarboxylic acid. The composition of any one of claims 1 to 4, wherein the blocked polyisocyanate is isophorone diisocyanate blocked with e-caprolactam. The composition of any one of claims 1 to 5, wherein the aliphatic carboxylic acid is dodecandioic acid and the blocked polyisocyanate is isophorone diisocyanate blocked with e-caprolactam. A molded or formed article coated with the hardened composition of any one of claims 1 to 6. 8. A method for protecting a metal article against acid corrosion, comprising applying a powder coating composition comprising: A. about 55 to 71 weight percent of a glycidyl functionalized acrylic copolymer comprised of

1. about 10 to 40 weight percent, based on the weight of the polymer, of a monoethylenically unsaturated monomer having at least one epoxy group, and

2. about 60 to 90 weight percent, based on the weight of the copolymer, of at least one monoethylenically unsaturated monomer that is free of epoxy groups, wherein the copolymer has a numerical average in molecular weight of about 1,000 to 8,000, a weight average in molecular weight of approximately 2,000 to 16,000 and a vitreous transition temperature of 40 ° C to 90 ° C, and B. about 9 to 14 weight percent of an aliphatic carboxylic acid having 2 to 3 carboxylic acid groups per molecule, C. about 17 to 34 weight percent based on the total weight of A, B and C, or a blocked polyisocyanate; followed by thermo-hardening the coated article. The method of claim 8, wherein the acrylic copolymer functionalized with glycidyl has a number average molecular weight of about 2,000 to 5,000 and a weight average molecular weight of about 4,000 to 12,000. The method of claim 8 or 9, wherein the aliphatic carboxylic acid is a compound of the formula where n is an integer from 1 to 10. The method of claim 8, 9 or 10, wherein the aliphatic carboxylic acid is selected from the group consisting of dodecanedioic acid, azelaic acid, adipic acid, 1,6-hexanedioic acid , succinic acid, pimelic acid, sebacic acid, maleic acid, citric acid, itaconic acid, aconitic acid, or 1,2-, 1,3-, or 1,4-cyclohexanedicarboxylic acid. The method of claim 8 or 9, wherein the carboxylic acid is dodecanedioic acid and the blocked polyisocyanate is isophorone diisocyanate blocked with e-caprolactam.