MXPA98008879A - Coating powder, curable at low temperature, of a compone - Google Patents

Abstract

The present invention relates to: A one component thermosetting powder coating, which is cured at a temperature of about 107 to 149 ° C, consisting essentially of an extruded mixture comprising: an epoxy resin having a melt viscosity of about 200 to 2000 centipoise at 150 ° C and a Tg of about 35 ° C to about 55 ° C, and an epoxy resin adduct of an aliphatic polyamine which is solid at about 27 ° C and latent at about 71 ° C to about 104

Description

COATING POWDER, CURABLE AT LOW TEMPERATURE, OF A COMPONENT BACKGROUND OF THE INVENTION This invention relates to a powder coating system, in which a mixture of an epoxy resin and a low temperature curing agent is extruded as a component and sprayed to form a coating powder, curable at low temperature. The powder cures at a temperature of about 107 to 149 sc and produces a coating that has an exceptionally smooth surface with low or high gloss. This invention also relates to an electrostatic coating of large metal articles and wood and curing at low temperature to achieve a pleasant uniformity. It also relates to a method for the composition of a coating powder by extruding a mixture comprising an epoxy resin, having a low melt viscosity and a curing agent at a low temperature, such as a temperature of about 71 to 104 ° C. . Traditionally, the coating powders have been obtained by extruding a mixture of resins and curing agents, to obtain a homogeneous mixture and then grinding the extrudate and sieving the crushed product to obtain the particle sizes and particle size distribution desired . The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 932C. The achievement of a powder coating composition that cures on heat-sensitive substrates below 1492C in less than 5 minutes has been the goal in the industry. The curing of powder coatings on materials, such as wood, plastic and the like, has been limited by the fact that the extrusion of a mixture of a resin and a curing agent at a low temperature, i.e., one that is active at 1212C or less, will cause the coating powder to form a gel in the extruder, because the extrusion typically generates sufficient heat to raise the temperature to 932C or more. This problem has been avoided by extruding the resin and a small amount of catalyst or a low temperature curing agent, grinding the extrudate and then mixing the resulting powder with an additional amount of the curing agent, in powder form, according to the description of the patent application, commonly assigned, also pending, Serial No. 810/745, filed on March 4, 1997. The high temperatures that are generated by the extrusion of a curing agent with a resin, are caused, in part, by the fact that the temperature must be sufficient to facilitate the mixing of resins which initially have high viscosities of the melt. Even higher temperatures are produced by the friction arising from the mixture of the fused resins, still highly viscous, with curing agents that are solid at room temperature. Douglas S. Richart, states in his article published in the April 1996 issue of POWDER COATINGS, that the coating of wood with a curing powder at low temperature is immediately impossible, because the coating must be cured at a temperature below 93sc and the resin should have a flow temperature of about 10 to 20 degrees lower than that. Resins with lower viscosities of the melt, however, generally have lower glass transition temperatures. The presence of a resin having a very low glass transition temperature (Tg) in the powder extrudate increases the tendency to sinter and thus decreases the blocking resistance of the powder during storage at a normal maximum temperature of about 27sc.

SUMMARY OF THE INVENTION Therefore, it is an object of this invention to provide a method for extruding a coating powder, curable at low temperature, of a component.

It is a related object of this invention to provide a coating powder, curable at low temperature, of a component, which has satisfactory resistance to blockage, during storage at normal temperatures. It is a related object of this invention to provide a coating powder, curable at low temperature, of a component, for heat sensitive substrates and large metal articles. It is another object of this invention to provide a low temperature process to produce a smooth, high gloss coating on wood. These and other objects of the invention, which will become apparent from the following description, are achieved by a method for preparing a thermosetting powder coating, which comprises extruding a mixture of an epoxy resin having a melt viscosity of about 200 to 2000 centipoise, at 1502C, and a curing agent, which is solid at 27 C and latent at an extrusion temperature of about 71 to 104 sc, cool the extrudate and crush it.

DETAILED DESCRIPTION OF THE INVENTION In order to describe the proportions of the components in the compositions of this invention, the term "resin" includes the resin per se and the curing agent, but not the catalyst. Also, the term "one component coating powder" means that the powder is formed completely by grinding and sieving only one extrudate of a mixture of resin, curing agent, catalyst and additives. Epoxy resins, which are suitable for the purposes of this invention, have an equivalent weight of about 100 to 700. Mixtures of such epoxy resins can be used. A suitable mixture may comprise an epoxy resin, having an equivalent weight between about 100 and 400, and one having an equivalent weight between 400 and 700, in a weight ratio of 1:99 to 99: 1. The resins are exemplified by, but are not limited to, those produced by the reaction of epichlorohydrin and a bisphenol, for example, bisphenol A and bisphenol P. The low viscosities of the melt of these resins facilitates the extrusion thereof in mixing with a curing agent, additives and pigments at about 71 to 1042C. The preferred viscosity of the melt is about 300 to 1000 centipoise. The viscosity of the melt of resins having a low Tg, ie from about 35 to 552C, is suitable for the purposes of this invention. Epoxy resins known as EPN (epoxy-phenol-novolac) and ECN (epoxy-cresol-novolac) and those obtained by the reaction of isopropylidenediphenol (bisphenol A) with epichlorohydrin, are suitable for the purposes of this invention. Bisphenol A epoxides are sold under the trademarks of ARALDITE GT-7071, GT-7072, EPON 1001 and EPON 2042. ARALDITE GT-6259 is the trademark for an ECN resin. The use of a crystalline epoxy resin can improve the flow characteristics of the molten coating powder. A particularly convenient flow is achieved when a crystalline epoxy resin constitutes about 5 to 20% by weight of the total amount of the epoxy resin used in the powder formulation. The performance of a coating powder of this invention deteriorates as the level of the crystalline epoxy resin increases, because the relatively low equivalent weights of these resins and the preferred amount of such resin is about 10% or less for that reason. A crystalline epoxy resin, having a melting point between about 80 and 150ac is preferred. A crystalline epoxy resin having an equivalent weight of about 185 and sold by Shell under the trademark RSS 1407, is suitable for the purposes of this invention. When resins having a Tg of about 35 to 40sc are used in this invention, the sintering of the powder is prevented, allowing the temperature in the extruder to rise to activate the low temperature curing agent for a sufficient time to raise the viscosity of extruding beyond the sintering point and then cooling the extrudate rapidly to about 10-20 c, before crumbling and grinding and storing the powder at such a temperature to avoid a further accumulation of viscosity by continued curing. Another way to avoid sintering the powder when using resins with low Tg, is pre-mixing the resin with a crystalline or non-crystalline curing agent, which has an average particle size of about 5 microns and does not become liquid in the extruder. The powdered agent does not react with the resin, as easily as the same curing agent does in the form of flakes. A specific example of a curing agent that can thus be used in the powder form, is sold under the trademark of ANCAMINE 2441. The low temperature curing agent, according to this invention, is one that will be active at a temperature of 107 to 149se and you can select from the many that are commercially available, but an epoxy adduct of an aliphatic polyamine (including cycloaliphatic polyamines) having a primary, secondary or tertiary amino group, or a combination of such amino groups, is a suitable curing agent for the purposes of this invention. Examples of such curing agents include hardeners sold under the trademarks of PF LMB 5218 (Ciba Geigy), ANCAMINE 2337 XS, ANCAMINE 2014 AS and ANCAMINE 2441 (Air Products &; Chemicals). An epoxy adduct of an aromatic polyamine, such as methylene dianiline, is also a suitable curing agent for the purposes of this invention. It is preferred that the functionality of the adducts reaction mixture is 2 or less and it is particularly preferred to use a difunctional epoxy compound. The amount of the low temperature curing agent is about 2 to 40 parts per hundred parts of the resin (per) and the preferred amount is about 5 to 20 per cent. Increasing levels of the curing agent reduce the gel time and, therefore, increase the orange peel (rough) type effect. A catalyst can be used at a level of approximately 0.1 to 5 parts per hundred parts of resin, preferably about 0.2 to 2 per, to accelerate the curing reaction with the low temperature curing agent. Preferred catalysts for this invention are the imidazoles and their epoxy adducts, these imidazoles have the general formula: R1 wherein R1, R2, R3 and R4 are, independently, hydrogen, alkyl, aryl, or any substituent that is not reactive with the epoxy resin. For the purposes of this invention, the term "imidazole" is used herein to mean both substituted and unsubstituted imidazoles. Imidazoles, by themselves, tend to be insoluble in epoxy resins. Thus, epoxy adducts are obtained to make them more compatible with the epoxy system of this invention. Suitable adducts of these imidazoles with an epoxy resin of bisphenol A are commercially available from Shell Chemical Company, under their trademark EPON, for example, EPON P-101, and also from Ciba-Geigy Corporation, under their designation HT 261. Examples of suitable imidazoles include imidazole, 2-methyl-imidazole and 2-phenyl-imidazole. For enhanced color stability, 2-phenyl-imidazole, which is available from SWK Chemical Co., is preferred. A particularly hard surface is obtained when a combination of the curative agent ANCAMINE 2441 and 2-phenyl-imidazole is used to cure an epoxy resin having a mean Tg and melt viscosity. Although the applicants are not bound by any theory, an imidazole adduct is believed to be an epoxy resin through the epoxy ring opening, which results in the epoxy oxygen bonded to the C = N bond of the imidazole ring. The adduct imidazole moves from the epoxy group to another, as it facilitates the epoxy ring openings and the curing reactions. A tertiary amine, such as triethylamine diamine, available under the trademark ACTIRON SI 27071 from Synthron Chemicals, is another type of cure catalyst that can be used in this invention. Various gloss levels of the cured coating can be achieved through the selection of epoxy resins, curing agents, curing catalysts and the relative amounts of each. A low gloss can be achieved, for example, by the use of a combination of dicyandiamide (a slow-acting agent) and a fast-acting agent, to adjust competitive reactions. A thermally cured and melted powder coating, obtained from an epoxy resin, having an equivalent weight of about 400, can have a brightness level of 60s as low as about 10, while a brightness level of about 90 it can be achieved when the equivalent weight is around 650 and the curing agent and healing catalyst are changed. The coating powder may also contain a flow control agent in the range of about 0.5 to 2.0 per. Examples of flow control agents include MODAFLOW, poly (alkyl acrylate) products and SURFYNOL, acetylenic diols; they can be used simply or in combination. Antioxidants can also be used at a concentration of about 0.5 to 2.0 per cent to prevent discoloration of the coatings even at relatively low cure temperatures, suitable for the purposes of this invention. Examples of the antioxidants that are useful in this invention include sodium hypophosphite, tris- (2,4-di-t-butyl-phenyl) phosphite (sold under the trademark IRGAFOS 168) and bis / [monoethyl (3 , Calcium 5-di-t-butyl-4-hydroxybenzyl) phosphonate] (sold under the trademark IRGANOX 1425). Mixtures of antioxidants can be used. The powder of this invention can be used in coating glass, ceramics and graphite-filled composites, as well as metal substrates, such as steel and aluminum. Much heat is wasted by heating thick articles or large metal, in order to raise the surface temperature to the high levels required by the coating powders. The particular utility of the powder of this invention, however, in the coating of heat sensitive substrates, such as plastics, paper, cardboard and wood, makes them appear as a commercially viable alternative to liquid coatings that have been used almost universally in the past. For the purposes of this invention, wood is defined as any lignocellulosic material, coming from trees or other plants and whether it is in the natural form, ground or made in plywood, particle boards or fibreboards of various densities. It is exemplified by sawed wood, panels, molding boards, lining boards, oriented lacing boards, hardboard and medium density fibreboard (MDF). The particle board can be normal or treated to increase its electrical conductivity. Wood having a moisture content of 3 to 10% by weight is suitable for this purpose of the invention. A porous particle board, precoated with a liquid, conductive, and cured coating composition, can also serve as a substrate for the coating powder of this invention. For example, a soft coarse powder of 50.8 to 76.2 microns was achieved in a previous thermally cured coating or by UV radiation, with a thickness of 12.7 to 25.4 microns. Pigments, optical brighteners, fillers such as calcium carbonate and bentonite clays, texturizing agents such as particulate rubber and other conventional additives may also be present. A particularly convenient textured finish can be obtained by the addition of about 14 to 20 per rubber to the coating composition, along with calcium carbonate, in a rubber to carbonate ratio of from about 0.7: 1 to 1.5: 1 by weight. The titanium dioxide, in an amount of approximately 5 to 50 per or more, is an example of a pigment that can be used. An optical brightener, exemplified by 2,2 '- (2,5-thiophendiyl) -bis [5-t-butyl-benzoxazole [, sold under the trademark UVITEX OB, may be present at approximately 0.1 to 0.5 per. The coating powder of this invention can be applied by any conventional powder coating method. The panels with flat and grooved surface can be covered by triboelectric guns on a flat line conveyor that has electrically conductive bands around the circumference of the conveyor belt. A suitable flat line powder coating apparatus comprises a conveyor extending through a powder coating booth, in which a wooden article supported and moved by a conveyor belt is triboelectrically coated by a plurality of pistols, located adjacent to each other, and in one or more rows. The article that carries the powder is then transported through a curing oven that has several heating zones, some of which are heated by IR lamps, others by heat convection and still others by a combination of the two. The speeds of the coating and curing lines may be the same or different, depending on the length of the curing furnace. The line speed through the powder application box can be about 1.52 to 45.72 meters per minute, but it is preferable at 30.5 meters per minute. The line speed through the curing furnace, on the one hand, can be about 1.52 to 6.10 meters per minute, depending on the furnace temperature and the particular coating powder used. The curing temperature may vary from about 1072 up to, but not including, the decomposition temperature of the powder. It is preferred to maintain the cure temperature within the approximate range of 107 to 1492C and still more preferred the cure temperature of 107 to 1212C is maintained. It is preferred that the coating speeds and the cure line be adjusted to the length of the oven so that they are balanced. The pre-heating of the panel, before the coating stage, it is preferred in some cases, for example, to help the powder reach its flow temperature in the first zone of the furnace and also to minimize gasification during curing. The oven can have several heating zones of the IR and convection types and also a combination of the two. The film thickness of the cured coating is at least about 25.4 microns and can be as large as 203 microns or even greater, if there is a practical need for this. Film thicknesses of about 101.6 to 152.4 microns are achieved regularly by the method of this invention. The gel time of the coating powder of this invention was measured in accordance with Specification D-3451 of ASTM (14) wherein a small amount of powder was dropped onto a hot plate at 149sc and collided with a tongue depressor until that continuous filaments were formed, which break easily, when the depressor rose from the sample. The time elapsed for this to occur was measured in seconds and it is gel time. The blocking resistance of a coating powder was tested by placing about 2.54 cm of powder in a tube, placing a 100 gram charge on the top of the powder and heating at 43 cs for 24 hours. When removing the contents of the tube, the degree of sintering was measured on a scale of 1 to 10, 1 being completely free flow and 10 being without flow. Powders that have a value less than 5 are acceptable. The hot plate melt flow (HPMF) of the powder coating composition of this invention was measured by placing a powder pellet, having a diameter of 12.7 mm and 6 mm thick, on a 190 hot plate. ± 2sc with an inclination angle of 352.

When the pellet melts and runs under the plate, the length of the flow was measured in millimeters. The distance of the flow depends on the initial viscosity of the melt, the rate of the reaction, the temperature at which the test is conducted and the type and amount of catalyst. The Hoffman scratch resistance of the coated articles obtained according to this invention was measured with the By-Gardner scratch tester. The Taber abrasion resistance of the coated articles obtained in accordance with this invention was measured in accordance with ASTM D-4060 using CS-10 wheels and a load of 1000 grams. The invention is more specifically described in the following working examples, where the parts are by weight, unless indicated otherwise.

EXAMPLES 1-3 Three resins of increased viscosity and increased Tg, as shown in Table A, were compounded in molten form with polyamine adducts and the other components shown in Table 1 in a twin screw extruder. The extrudate was cooled between water cooled rolls, broken into chips and then ground into a powder. The powder passing through a 200 mesh screen was electrostatically coated on Q panels of cold rolled steel and cured at 1 92c for 5 minutes to obtain a film thickness of 50.8 to 76.2 microns. As shown in Table 2, the superior solvent resistance and blocking resistance of Examples 1-3 demonstrate that the most rapid cure was achieved using this technology. The extrudate temperatures and the properties of the uncured and cured powders are given in Table 2. TABLE A TABLE 1 LMB 5218 Epoxy / Polyamine; ** EPON P-101 Epoxy / lidazole; *** Sodium hypophosphite TABLE 2 Table 2 shows the significant reduction in the temperature that the extrudate reaches when a low viscosity epoxy resin is used. The shorter gel times for the powders containing the more viscous resins are caused by the partial curing of the resin in the extruder at higher temperatures.

EXAMPLES 4-6 The use of non-adduct imidazoles as a cure catalyst, the practice of the invention without a cure catalyst and the use of a high temperature curing agent in combination with a curing agent at low temperature and a catalyst, are illustrated by the compositions and their properties, shown in Tables 3 and 4.

TABLE 3 * Me = Methyl; ** Sodium hypophosphite TABLE 4 The properties of the coating powder, which contains the imidazole as the catalyst, is recommended for use where fast cures are required, as in the coating of rebar, for example; it has the best cure, as indicated by its resistance to methyl ethyl ketone (MEC), and its flow properties are extraordinarily good.

Example 7-9 The following examples illustrate the performance of curing agents containing secondary and tertiary amino groups. The coating powder was passed through a 200 mesh screen and sprayed from a trobo-loading gun onto a panels of medium density fibreboard 2.54 cm thick (MDF), which had been previously heated in an oven 176.52c for 10 minutes, at a surface temperature of 115.52c. The powder coating was cured by heating the panels to 176.52c for 5 minutes to reach a surface temperature of 1382C TABLE 5 * Sodium hypophosphite TABLE 6 Comparative Example 1 A two-component epoxy powder coating composition described in Example 9 of the pending, commonly assigned application filed March 4, 1997 as Serial No. 810,745, was prepared for comparison with the composition of this invention. The Components A b Y of the formulation shown in Table 7 were extruded separately and then ground and sorted in the usual manner. Sixty-five parts by weight of Component A and 35 parts of Component B were mixed to form a powder coating composition of Comparative Example 1. The powder of Comparative Example 1 was applied to a wood panel that had previously been heated to a The panel temperature was 104 to lyse and cured at a panel temperature of 132-138sc for about 5 minutes.

The properties of the cured coating are shown in Table 8. TABLE 7 TABLE 8 EXAMPLES 10-12 The following examples illustrate the combined effects of the curing agents having different epoxy resin regimes and resins having low and high equivalent weights in the gloss of the powder coatings of this invention. The coating powder was passed through a 200 mesh screen and sprayed from a tribo-magazine gun onto panels of medium density fibreboard (MDF) 2.54 cm thick, which had been preheated in an oven. at 176.52C for 10 minutes at a surface temperature of 1152C. The powder coating was cured by heating the panels to 176.52c for 5 minutes, to reach a surface temperature of 1382C. TABLE 9 Weight Eq. ~ 400; ** Weight Eq. ~ 650; *** Sodium hypophosphite.

TABLE 10 Examples 13-17 Coating powders, formed as shown in Table 11, were prepared by extrusion through a die assembly in an extruder, having a 12-pin rotor operating at 300 rpm and whose front zone is at 1202C and the rear area is cold. The extrudate was cold-crushed and milled to pass through a 200-mesh screen. The resulting powder was electrostatically sprayed from a tribo-loading gun onto medium density fibreboard (MDF) panels with a thickness of 2.54 cm. they had previously heated in an oven to 176.52c for 10 minutes at a surface temperature of 115.52c. The powder coating was cured by heating the panels to 176.52c for 5 minutes to reach a surface temperature of 138sc. The thickness of the coating was from 101.6 to 178 microns.

TABLE 11 * Sodium hypophosphite TABLE 12

Claims (51)

1. CLAIMS 1. A method for preparing a thermosetting powder coating, which comprises extruding a mixture of an epoxy resin, having a melt viscosity of about 200 to 2000 centipoise and a low temperature curing agent, which is solid at about 27sc, at a temperature from less than about 7sc to about 104sc, cooling the extrudate and grinding it. The method of claim 1, wherein the low temperature curing agent is an epoxy resin adduct of a polyamine. 3. The method of claim 1, wherein an imidazole, having the general formula: R1 wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin, is added to the extrusion mixture as a catalyst. 4. The method of claim 1, wherein the curing agent is in powder form. 5. . The method of claim 1, wherein the extrusion temperature is about 71 to 1042c. The method of claim 1, wherein the epoxy adduct of an imidazole, having the formula: wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin, is added to the extrusion mixture as a catalyst. The method of claim 1, wherein about 5 to 20 percent of the epoxy resin is crystalline. The method of claim 3 wherein R1, R2, R3 and R4 are, independently, hydrogen, alkyl, aryl or alkaryl. 9. The method of claim 3, wherein R2 is methyl or phenyl and R1, R3 and R4 are hydrogen. 10. The method of claim 6 wherein R2 is methyl or phenyl and R1, R3 and R4 are hydrogen. 11. The method of claim 1, wherein the curing agent is an epoxy adduct of an aliphatic polyamine. The method of claim 1, wherein the epoxy resin has an equivalent weight of about 350 to 700. The method of claim 1, wherein the epoxy resin has an equivalent weight of about 400. 14. The method of claim 1, wherein the epoxy resin has an equivalent weight of about 650. 15. A thermosetting coating powder, which cures at a temperature of about 10 7 to 1492C and comprises an epoxy resin having a viscosity of the melt from about 200 to 2000 centipoise at 1502C and a low temperature curing agent, which is solid at about 272C. 16. The powder of claim 15, wherein the curing agent at low temperature is an epoxy resin adduct of an aliphatic polyamine. 17. The powder of claim 16, wherein the aliphatic polyamine contains a primary amino group. 18. The powder of claim 15, further characterized by an imidazole, having the general formula: wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin. 19. The powder of claim 15, further characterized by an epoxy adduct of an imidazole, having the general formula: wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin. The powder of claim 15, wherein about 5 to 20 weight percent of the epoxy resin is crystalline. 21. The powder of claim 16 wherein the aliphatic polyamine contains a secondary amino group. 22. The powder of claim 18 wherein R1, R2, R3 and R4 are, independently, hydrogen, alkyl, aryl or alkaryl. 23. The powder of claim 18, wherein R2 is methyl or phenyl and R1, R3 and R4 are hydrogen. 24. The powder of claim 19 wherein R2 is methyl or phenyl and R1, R3 and R4 are hydrogen. 25. The coating powder of claim 15, wherein the amount of the curing agent is from about 2 to 40 parts per hundred parts of the resin. 26. The coating powder of claim 18, in which the amount of the imidazole is about 0.2 to 5 parts per hundred parts of the resin. 27. The coating powder of claim 19, wherein the amount of the epoxy adduct of the imidazole is about 0.2 to 5 parts per hundred parts of the resin. 28. The coating powder of claim 15, wherein the epoxy resin has an equivalent weight of about 100 to 700. 29. The coating powder of claim 15, wherein the epoxy resin has an equivalent weight of about 400. 30. The coating powder of claim 15, wherein the epoxy resin has an equivalent weight of about 650. The powder of claim 15, wherein the epoxy resin is a mixture of one having an equivalent weight between about 100 and 400, and another that has an equivalent weight between approximately 400 and 700, in a weight ratio of 1:99 to 99: 1. 32. A method for coating wood, which comprises electrostatically spraying a heat-stable coating powder onto a surface of the wood and heating the powder to a cure temperature of 107 to 1492C, approximately, this powder comprises an epoxy resin having a viscosity of the melt of approximately 200 to 2000 centipoises, at 150sc, and a low temperature curing agent, which is solid at about 27sc. 33. The method of claim 32, wherein the low temperature curing agent is an adduct of an epoxy resin of an aliphatic polyamine. 34. The method of claim 32, further characterized by an imidazole, having the general formula: wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin. 35. The method of claim 32, further characterized by an epoxy adduct of an imidazole, having the general formula: R1 wherein R1, R2, R3 and R4 are, independently, hydrogen or any substituent that is not reactive with the epoxy resin. 36. The method of claim 32, wherein about 5 to 20 weight percent of the epoxy resin is crystalline. 37. The method of claim 32 wherein R1, R2, R3 and R4 are, independently, hydrogen, alkyl, aryl or alkaryl. 38. The method of claim 32, wherein R 2 is methyl or phenyl and R 1, R 3 and R 4 are hydrogen. 39. The method of claim 33 wherein R 2 is methyl or phenyl and R 1, R 3 and R 4 are hydrogen. 40. The method of claim 32, wherein the amount of the imidazole is about 0.2 to 5 parts per hundred parts of the resin. 41. The method of claim 33, wherein the amount of the epoxy adduct of the imidazole is about 0.2 to 5 parts per hundred parts of the resin. 42. The method of claim 32, wherein the epoxy resin is a mixture of one having an equivalent weight between about 100 and 400, and another having an equivalent weight between about 400 and 700, in a weight ratio of 1. : 99 up to 99: 1. 43. The method of claim 1, wherein the epoxy resin is a mixture of one having an equivalent weight between about 100 and 400, and another having an equivalent weight between about 400 and 700, in a weight ratio of 1. : 99 up to 99: 1. 44. An article, having a thermoset epoxy powder coating composition, with the powder coated and cured thereon, this composition, in powder form, comprises a particulate mixture of: an epoxy resin, having a viscosity of the melt of about 200 to 2000 centipoise at 150sc and a low temperature curing agent, which is about 27SC solid. 45. The article of claim 44, wherein the amount of the curing agent is about 2 to 40 parts per hundred parts of the resin. 46. The article of claim 44, wherein the low temperature curing agent is an epoxy resin adduct of an aliphatic polyamine. 47. The article of claim 44, wherein the epoxy resin has an equivalent weight of about 100 to 700. 48. The article of claim 44, wherein the epoxy resin has an equivalent weight of about 400. 49. The article of claim 44, wherein the epoxy resin has an equivalent weight of about 650. 50. The article of claim 44, wherein the epoxy resin is a mixture of one having an equivalent weight between about 100 and 400. and another that has an equivalent weight between about 400 and 700. in a weight ratio of 1:99 to 99: 1. 51. The article of claim 44, wherein the article is obtained from wood.