MXPA00001194A - Thermosetting epoxy powder coatings having improved degassing properties - Google Patents

Abstract

Provided are low temperature curing thermosetting epoxy powder coating compositions comprising at least one non-crystalline epoxy resin, at least one crystalline epoxy resin, a curing agent, and a cure catalyst. Coatings made from such powder compositions exhibit a substantial reduction in bubble entrapment, enough to prevent a visible haze from forming in the coating, when cured at low temperatures demanded by brass substrates, while the powders from which the coatings are made still exhibit good storage stability and melt-processability.

Description

COATINGS OF EPOXY THERMO NON-CURTAIN POWDER THAT HAVE IMPROVED DEGASIFICATION PROPERTIES Field of the Invention This invention relates to a thermosetting epoxy powder coating composition and, in particular, to an epoxy powder coating composition adapted to prevent entrapment of bubbles within the coating film formed during curing. thermal at low temperatures.

BACKGROUND OF THE INVENTION Brass is a widely used building material for many articles of commerce. The brass formed parts usually require a transparent coating to improve their luster and appearance and achieve protection against wear or the environment. Liquid coatings carried by solvent have been tested for these purposes, but fail to deliver the operation needed. The solventborne coatings also contain alarmingly high levels of hazardous volatile organic compounds that have to evaporate during curing, making it necessary to contain and collect the vapor from the volatile ingredients which is rather expensive. Thermo-hardenable, melt-bonded powder coating compositions have also been used to coat brass parts. Powder coatings offer a number of advantages over liquid coatings. For example, the resistance to corrosion and scraping is much higher than that of liquid coatings. In addition, the powder coatings are virtually free of the harmful fumes organic solvents normally present in the liquid coatings and, consequently, give off a few, if they make it volatile during the curing, which eliminates the problems of solvent emission and the dangers to la-: health of workers employed in coating operations. Because brass parts, eg, brass-plated zinc die-cast parts, are susceptible to gas removal during heating, thermosettable powder coatings that are capable of cure at low temperatures, e.g. , less than about 177SC, are generally preferred to minimize the removal of gases from the substrate during curing and permanent healing of the finished coating. Low curing temperatures are also desired, since brass tends to discolour or stain at higher temperatures. Among commercially available low temperature thermosetting cured powder coatings, G_4A acrylic powder coatings have been the most widely used by the brass finishing industry. GMA acrylics offer coatings with exceptional softness and clarity, but they also suffer from a number of disadvantages, including low adhesion and rather high cost that is becoming increasingly difficult, to sustain by the brass finishing industry. Efforts have been made to replace GMA acrylics with traditional thermosetting epoxy powder coatings, such as those based on conventional non-crystalline epoxy resins, e.g., bisphenol A type epoxy resins, conventional curing agents for epoxy resins, .gr., dicyanodiamide, together with conventional catalysts, e.g., 2-methyl imidazole. While epbixes offer improved adhesion and reduced cost, curing these coatings to the desired low-temperature curing conditions usually leads to entrapment of bubbles within the finished coating, which is particularly troublesome from the point of view of appearance and film quality. Bubble entrapment is believed to occur during the application of the powder. In particular, as the powder coating is applied to the substrate, it is believed that air is trapped inside the powder particles as they are deposited on the substrate. When the dust starts to melt, flow and eventually cure, air must escape from the coating before the finished coating hardens or will become trapped as small bubbles dispersed through the coating. With traditional thermosetting epoxy powder coatings, the latter effect tends to occur at the desired conditions of low temperature cure, which is believed to be due, at least in part, to the rather high melt viscosities experienced. at those temperatures. In a transparent coating, these bubbles are especially problematic in that they tend to 'create an unwanted hazy appearance which interferes with the image clarity of the finished coating, i.e., the sharpness of image reflected by a surface-of the coating. Brass coatings, without. However, they must have a high image clarity and visually consistent to see through the finish coating as if a part of polished brass was visible.

What is needed is a thermosetting epoxy powder coating composition of less than about 177SC as well as preventing the entrapment of bubbles within the finished coating during heat curing at said low temperatures.

SUMMARY OF THE INVENTION Therefore, a primary object of this invention is to provide a thermosetting epoxy powder coating composition that does not suffer from the above disadvantages Another object of this invention is to provide a thermosetting epoxy powder coating composition adapted to to cure at low temperatures, as well as to prevent entrapment of bubbles and the resultant visible nebulosity within the finished coating during curing at said low temperatures. Yet another object of this invention is to provide an oxidisable powder coating composition having A relatively low viscosity when melted to cure so that it can flow easily, coalesce and degas at low temperatures before reaching a hardened state A further object of this invention is to provide a thermosetting epoxy powder coating composition of the type an It is mentioned above that it is processable in motionless fusion in an extruder under conventional conditions and stable to storage at ambient temperatures. A related object of this invention is to provide a coated substrate, preferably a transparent coated brass substrate, having a thermosetting powder coating composition of the aforementioned type, coated and cured thereon while the substrate is at a generally less than the degassing temperature and / or degradation of the substrate, with the resulting finished coating, • notoriously, having little or no bubble entrapment or visible nebulosity resulting therein. The above objects of the invention are achieved through an improvement in a low temperature thermosetting epoxy powder coating composition, especially in a transparent composition that is essentially free of opaque pigments and fillers. The powder coating composition according to this invention comprises "a particulate film-forming mixture of a non-crystalline epoxy resin, a curing agent for the epoxy resin, and a curing catalyst. A crystalline epoxy resin is included in the composition to improve the low temperature degassing properties of the composition and thereby reduce or eliminate bubble entrapment within the coating film formed therefrom during curing, particularly at more low cure temperatures demanded by certain substrates, especially those made of brass, however without subtracting from the ability of the composition to be melt processed and stored in a conventional manner.The thermosetting powder coating composition of this invention, therefore, it is useful to provide essentially free coatings of nebulosity on substrates that demand lower curing temperatures, such as brass parts. Our discovery in the present that the addition of small amounts of crystalline resin prevents the entrapment of bubbles at low curing temperatures in the previous formulation was truly unexpected. There is no indication of this unexpected benefit in the literature. The Patent of E.U.A. 5,414,058 (Ono) makes no mention of improvements in powder degassing. Furthermore, no prior art related to mixtures of non-crystalline and crystalline epoxy resins for use in thermosetting powder coatings that achieve the novel film properties described herein is known. By achieving the above objects of this inventionA method is also provided for obtaining a coating essentially free of haze, preferably a transparent coating, having few or no optical bubble defects on a substrate surface susceptible to degassing and / or degradation during heating at elevated temperatures. , preferably on a surface made of brass, comprising applying a low temperature powder coating composition of the aforementioned type on a substrate surface, and curing the powder coating on the substrate at temperatures sufficient to cure the composition of powder coating and lower than the degassing temperature and / or degradation of the substrate. Also provided by this invention is a substrate susceptible to degassing and / or degradation during heating, preferably a brass substrate, coated, preferably with a transparent coating, with a layer essentially free of haze of a powder coating composition. Cured thermosetting epoxy which in its uncured state comprises the composition of the aforementioned type, The various objects, features and advantages of this invention will become more apparent from the following description and appended claims.

Detailed Description of Preferred Modes Through this specification, all parts and percentages specified herein are by weight unless otherwise stated. Also, in the present, the non-crystalline epoxy resin component plus the crystalline epoxy resin component are considered to be the "resin system" and equal to 1 0 parts. The levels of other components are calculated as parts in relation to 100 parts of the resin system ("phr"). Furthermore, as used herein, the term "non-crystalline epoxy" (otherwise referred to as "amorphous epoxy") broadly defines epoxy resins that do not show, or trace, of crystallization or melting point as determined by the calorimetry of differential scan (DSC). While the term "crystalline epoxy" used herein denotes crystalline as well as semi-crystalline materials and broadly defines epoxy resins with a crystallization or melting point discernible by DSC. In accordance with the present invention, thermosetting epoxy powder coatings having improved degassing properties at low temperature are provided and provide essentially bubble-free and cloud-free coatings when cured at such low temperatures, while powder those which are made the coatings still have good storage stability and melt processing capacity. The epoxy powder coating compositions of this invention typically include a non-crystalline epoxy resin, a curing agent, and a catalyst, with the improvement being that a small amount of crystalline epoxy resin sufficient to prevent bubble entrapment inside. of the cured coating film formed therefrom is included in the resin system of the composition in place of some non-crystalline epoxy resin. By "bubble-free", what is implied is that trapped bubbles are reduced to a degree such that a visible nebulosity can not be discerned by human sight without assistance in the coating. It is possible that a closer inspection may reveal minute bubbles within the reverse; However, are not they enough to interfere with? the aesthetic appearance and clarity desired of the coating as required by conventional standards. Non-crystalline epoxy resins useful herein include, without limitation, bisphenol A-type epoxy resins which are diglycidyl ethers of bisphenol A, usually produced by the reaction of epichlorohydrin and bisphenol A. Bisphenol-type epoxy resins used present are preferably solid resins having a TG by DSC of greater than about 40aC, such that powders made from said resins are storage stable, preferably greater than about 55eC, an epoxy functionality of about 2. or greater, preferably about 2 to 4, and epoxy equivalent weight of about 600 to 1100, preferably about 600 to 750. In accordance with the present invention, the non-crystallized epoxy resin is mixed with a sufficient amount of epoxy resin crystalline resin to reduce or eliminate bubble entrapment during low temperature curing.The crystalline epoxy resins useful in the present preferably are solid resins having a Tm by DSC greater than the Tg of the non-crystalline resin, preferably greater than about 90aC, so that the powders made from said resins can be melt processed in a conventional extruder without a) causing substantial processing delays while crystalline resins are expected to be recrystallized, and b) substantial reduction in storage stability of the powder as a result of destruction of crystal structures, "an epoxy functionality of about 2 or greater, preferably around 2 to 3, and an epoxide equivalent weight of about 50 to 500, preferably about 100 to 300. These resins can be produced by conventional techniques, such as by reacting epichlorohydrin with a di-monomer polyhydride known to have limited rotation with epichlorohydrin. Examples of suitable crystalline epoxy resins include, without limitation, prepolymers selected from the group consisting of diglycidyl ether of tetramethylbisphenol, diglycidyl ether of bisphenol S, 2,5-di-t-butylbenzene ether, 4- diglycidyl, diglycidyl ether of hydroquinone, diglycidyl ether of 2,5-di-t-butylhydroquinone, diglycidyl ether of terephthalic acid, diglycidyl isophthalate, triglycidyl isocyanurate and epoxypropoxydimethylbenzyl acrylamide, which are also described in the US Pat. . 5,414,059, the teachings of which are hereby incorporated by reference herein in their entirety. The diglycidyl ether of tetramethylbisphenol is a crystalline epoxy resin particularly preferred in this invention. The amount of crystalline substance incorporated in the powder coating composition is critical to the success of this invention. Only a minimum threshold amount of the above crystalline resin is required to produce the desired results. While this lower limit is critical, it may vary depending on the particular crystalline substance employed, in general, as little as about 5% by weight of crystalline resin, based on the total weight of crystalline and non-crystalline epoxy resin, will be sufficient to reduce or eliminate bubble entrapment within the coating film when cured at the desired low temperatures. Generally, it has been found that less than about 5% by weight of crystalline resin, effective bubble reduction can not be achieved in most cases. Quantities greater than this minimum level of crystalline resin, of course, can be employed, although to some extent the increase in degassing properties at low temperature is substantially reduced by a substantial decrease in the melt processing capacity and shelf stability of the powder composition so that the powder composition becomes impractical for commercial use. It has generally been found that when the crystalline resin exceeds an upper limit of about 15% by weight, the storage stability of the powder coating, in particular, tends to become extremely low as a result of severe blockage at ambient temperatures, making the powder impractical for commercial use. Therefore, in a preferred embodiment of this invention, the amount of crystalline epoxy resins is generally about 5-15% by weight, based on the total weight of the crystalline and non-crystalline epoxy resins. A curing agent it is typically incorporated in the powder coating of this invention to crosslink the epoxy resins at the epoxy sites and provide the desired thermoenduring properties to the coating, even though it is possible to cure the coating without a curing agent. The curing agents useful herein are preferably solid materials having at least two functional groups reactive with the epoxy groups. Examples of suitable curing agents include, without limitation, dicyanodiamide, bisphenol A, bisphenol S, epoxy adduct of bisphenol A of an aliphatic polyamine having a primary or secondary amino group, with dicyanodiamide being preferred. Generally, the curing agent is used in an amount of 0.7-1.7 equivalents, preferably 1.1-1.4 equivalents of the functional group per one equivalent of the epoxy group present in the powder coating composition. Typically, this results in a scale of about 3 to 7 phr of curing agent in the powder coating composition, preferably about 4.5 to 5.5 phr. While it is possible to cure or crosslink the powder coating without the use of catalysts, it is usually desirable to employ a curing catalyst in the powder coating composition of this invention to allow the curing reaction to progress to commercially acceptable regimes. The curing catalysts useful herein are preferably solid materials known to promote an epoxy ring opening function and the formation of ether bonds between the epoxy resins. Particularly preferred catalysts include, without limitation, 2-methyl imidazole, 2-phenyl imidazole, as well as bisphenol A epoxy adducts of the aforementioned imidazoles if lower / faster cured temperatures are desired. In general, the amount of - íe - Catalysts employed in the powder coating range from about 0.01 to 0.3 phr, preferably, about 0.05 to 0.1 phr. The powder coating composition of this invention can be transparent, i.e., non-pigmented or unfilled, or contain conventional pigments and fillers to impart the desired color and opacity to the coating film, even when the benefits of this invention are achieved. in the most effective way in transparent formulations. By "transparent", it is implied that the powder coating composition is essentially free of opaque pigments and fillers, so that it will produce cured coating films which are essentially transparent.In addition to the above components, the composition of The thermosettable powder coating of this invention can contain the usual additives such as, without limit, conventional dry flow additives, flow control agents, degassing agents, antioxidants, UV absorbers, light stabilizers, etc. However, it is particularly important that the coating powders possess the ability to cure at low temperatures without entrapment of bubbles within the cured coating film formed from the former powders. The substrates susceptible to degassing n and / or degradation during heating, such as brass parts, usually require a curing temperature of less than about 177aC, preferably between about 163aC and 177'SC, within commercially reasonable times, v.gr ., 30 minutes or less, preferably 15 minutes or less, while still producing coating films essentially free of bubbles and free of haze. Preferably, the powder coating composition of this invention will experience little or no bubble entrapment through the complete cure program. The powder coatings of this invention are prepared in the usual manner. First, an intimate mixture is formed by dry blending together all the ingredients of the formulation in a mixer. The dry mix is then melt-blended in a mixing extruder with heating above the melting point of the resin and other ingredients, when necessary, so that the extrudate is a complete and homogeneous mixture. Extrusion is preferably carried out at temperatures either lower or closer to the Tm of the crystalline epoxy resin for efficient melt processing and desired storage stability. The gaseous or super critical fluid, e.g., C02, can be charged to the extruder for better control of extrusion temperatures. The extruded composition is then rapidly cooled and solidified and then broken into chips. The chips are then milled in a mill - with cooling and, as necessary, the particles are sifted and sorted according to size. The average particle size desired for electrostatic application is generally between about 20 and 60 micrometers. Once dry, the free-flowing powders of this invention, which now contain at least one non-crystalline epoxy resin and at least one crystalline epoxy resin, are produced, ready for application to a substrate to be coated. The powder coatings of this invention can then be applied to the substrate by any conventional powder coating technique, even when electrostatic application, e.g., electrostatic spraying, is generally preferred. In electrostatic spraying, electrostatic spraying stands are usually used, which house banks of corona discharge or triboelectric spraying guns and a recovery system for recycling excessively sprayed powders to the powder feed. The substrate is heated, at least on the surface, at the time of application and / or subsequently up to a temperature equal to or higher than the temperature necessary to cure the powder coating and lower than the degassing temperature and / or degradation of the substrate, so that the Coating particles are sufficiently cast, flow and form a continuous coating film, and then cure to a thermoset state without degrading the substrate. The heating can be done in infrared ovens, convection ovens or a combination of both, even though infrared ovens are preferred. The time and temperature of the final curing will vary somewhat depending on the coating powders used and the conditions of use. However, irrespective of the curing time and temperatures employed, provided that the powdered ingredients have been sufficiently fused from curing, the coating films generated on the substrates will have a visually consistent appearance and will be free of trapped bubbles which interfere with the appearance aesthetics and the image distinction required by conventional standards.

The powder coating compositions "are particularly suitable for application to metal substrates, particularly brass, susceptible to degassing and / or degradation during heating, since the above powders are adapted to cure at relatively low temperatures, they are also suitable for application to other types of heat-sensitive substrates, such as wood substrates, e.g., hardwood, hardboard, laminated bamboo, wood composites, particle board, electrically conductive particulate board, density fiber board high, edia or low, masonite board, and other substrates that contain a significant amount of way, as well as plastics, eg, ABS, PPO, SMC, BMC, polyolefins, polycarbonates, acrylics, nylons and other copolymers that They will usually rip or degas when coated and heated with traditional heat-curable powders, along with paper, cardboard, and c thermally resistant metallic components and components that have a metallic or non-metallic heat sensitive appearance and possibly have a variable mass. The coatings of this invention are also suitable for typical heat-resistant substrates, such as high temperature metals, steels, and other alloys, glass, ceramics, carbon and graphite. This invention will now be described in more detail through specific examples.

EXAMPLE 1 Transparent Epoxy Powder Coatings Derived from 0, 1, 5, 10 and 20 Parts of Crystalline Resin The following ingredients were melt-mixed together in a manner provided in the Table below, to provide powder coatings derived respectively of 0, 1, 5, 10 and 20 parts of crystalline epoxy and 100, 99, 95, 90 and 80 parts of non-crystalline epoxy, to demonstrate the improvement in bubble entrapment provided by this invention PHR INGREDIENTS A B C D MIX IN DRY IN AN AMAZER UNTIL HOMOGENEOUS Araldite GT 70131 (Non-crystalline Epoxy 100 99 95 90 80 Epon RSS 14072 (Crystalline Epoxy 0 1 4 10 20 Dihard 100S3 (Curing Agent) 5 5 5 5 5 2-Methyl Imidazole (Catalyst) 0.1 0.1 0.1 0.1 0.1 INGREDIENTS PHR BDE Oil Baysilone (Leveling Agent) 0.4 0.4 0.4 - 0.4 0.4 Benzoin (Degassing Agent) 0.8 0.8 0.8 0.8 0.8 Tinuvin 9004 (UV Absorber) 2 2 2 2 2 Tinuvin 6225 (UV absorber) 1 1 1 1 1 AFTER MIXING IN EXTRUDER IN DOUBLE SCREW TO 82SC THE EXTRUDE IS COOLED AND THEN BREAKS IN CHIPS AND CHARGES ARE CHARGED AND 0.5% OF ALUMINUM OXIDE C6 A A BRINKMANN MILL IS THEN MILLED TO DUST AND BROUGHT TO MESH -200 Fitted Notes from Table 1 Araldite GT 7013 is a bisphenol A type epoxy resin having a Tg of 551C and an epoxy equivalent weight of 650-725, and an epoxy functionality of 1.9 to 2, sold by Ciba Specialty Chemicals. 2 Epon RSS 1407 is a crystalline epoxy resin of diglycidyl ether of tetramethylbisphenol having a Tm of 108SC, an epoxy equivalent weight of 166, and an epoxy functionality of 2, sold by Shell Chemical. 3 Dihard 100S is a dicyanodiamide curing agent. sold by SKW Chemical. 4 Tinuvin 900 is a benzotriazole uv absorber sold by Ciba Specialty Chemicals. s Tinuvin 622 is a benzotriazole UV absorber, sold by Ciba Specíalty Chemicals. 6 Aluminum Oxide C is a dry flow additive of smoked alumina sold by Sullivan Associates.

Results Each powder formulation mentioned above was electrostatically sprayed with a corona discharge gun on separate panels of polished brass and a sufficient amount to obtain 0.07-0.08 mm dry film on the panel after curing. After powder application, for bubble test purposes, the panels were first cured at 121SC for 5 minutes, allowed to cool to room temperature, and then further cured at 163SC for 3 minutes to generate a clear coating film on the substrate. These panels were then evaluated to determine bubbles and film clarity. The operating results for the individual coating powders and the coating films formed therefrom are given in the Table below. PROPERTY A B C D E Clarity 1 Bubbles Bubbles Without Bur- Without Bur- Without Bubbles Bujas bu-jas Blocking2 4 A 4 6 - 10 Limit Notes of Table 1 The clarity data for each sample were collected using a Nikon optical microscope at 200 times of amplification through which the bubbles were observed. No bubbles means that virtually no bubbles were observed. 2 The blocking digit for each sample was collected by placing the powder in a test tube and subjecting the powder to a weight of 100 g for 24 hours at 60 aC. • -The powders were classified on a scale of 1 to 10, with 1 being free-flowing powder after 24 hours, and 10 being completely sintered material. The blocking resistance of approximately or less is Commically acceptable.

The results demonstrate the scale on which the approach of the invention is effective to eliminate bubble entrapment for powder coatings. At 1% crystalline resin, the coating still contains bubbles after curing. At 5% and more, the coatings are bubble-free. At 20% crystalline resin, the coating exhibits extreme blocking that would not make it practical for commercial use.

EXAMPLE 2 Transparent Epoxy Powder Coatings Derived from 0 and 5 Parts of Crystalline Resin The following ingredients were mixed together by melting in the same manner as in Example 1, to provide powder coatings respectively derived from 0 and 5 parts. Crystalline epoxy and 100 and 95 parts of non-crystalline epoxy to further demonstrate the utility of this invention.

PHR INGREDIENTS G Araldite GT 7013 (Non-Crystalline Epoxy) 100 95 Epon RSS 1407 (Crystalline Epoxy) 0 5 Home ine OTB1 (Curing Agent) 5 5 2-Phenyl Imidazole (Catalyst) 0. 05 0. 05 Baysilone Oil (Leveling Agent) 0.4 0.4 Benzoin (Degassing Agent) 0.8 0.8 INGREDIENTS PHR F G Tinuvin 90Q4 (UV Absorber) 2 2 Tinuvin 6225 (UV Absorber) 1 1 Aluminum O C (Post-mixed) 0. fifty . 5 Fitted Notes of Table 1 Casamine OTB is an agent for curing l- (o-tolyl) biguanide sold by Swan Chemical, Results The individual coatings were tested luegp in the same manner as in Example 1. The results are given in the Table a with inuación.

PROPERTY G Clarity Bubbles Without Bubbles Block 4 4 The above results confirm that the incorporation of at least 5% crystalline epoxy resin towards the powder coating compositions eliminates bubble entrapment and prevents a haze from developing visibly on the surface of the coating when cured at safe temperatures for parts of the coating. Brass From the foregoing, it will be seen that this invention is a well adapted to achieve all the purposes and objects set forth above, together with other advantages that are obvious and inherent. Since many variations of the invention can be made without departing from the scope thereof, the invention is not intended to be limited to the embodiments and examples described, which are considered to be purely exemplary. Accordingly, reference should be made to the appended claims to determine the true spirit and scope of the invention, in which exclusive rights are claimed,

Claims (2)

1 - A thermosetting epoxy powder coating composition in the form of particles, comprising a film-forming mixture of: a) at least one non-crystalline epoxy resin; b) at least one crystalline epoxy resin in an amount sufficient to reduce or eliminate it. the entrapment of bubbles within the cured coating film, determined from the powder coating composition; and, c) a curing agent for facilitating the curing of the composition to a thermoset state, wherein the incorporation of the crystalline epoxy resin reduces or eliminates entrapment of bubbles within the cured coating film formed from the powder coating composition, when the composition is cured to a thermoset state at temperatures below about 177SC. 2. The composition according to claim 1, wherein the crystalline epoxy resin is incorporated in an amount sufficient to reduce or eliminate entrapment of bubbles within the cured coating at temperatures below about 177SC, while in an insufficient amount. to cause the resistance to blockage of the composition to be reduced to levels that are not commercially practical. 3. The composition according to claim 1, wherein at least 5% by weight of a) plus b) is crystalline epoxy resin. 4. The composition according to claim 3, wherein the total amount of crystalline epoxy resin does not exceed about 15% by weight of a) plus b). 5. The composition according to claim 1, wherein the composition is a transparent composition that is essentially free of opaque pigments and fillers. 6 - The composition according to claim 1, wherein the at least one non-crystalline epoxy resin has a Tg greater than about 40SC. 7. The composition according to claim 1, wherein the at least one crystalline epoxy resin has a Tm greater than about 90eC. 8. The composition according to claim 1, further comprising a catalytic amount of a curing catalyst. 9, - The compliance composition of claim 1, wherein the crystalline epoxy resin is selected from the group consisting of diglycidyl ether of tetramethylbisphenol, diglycidyl ether of bisphenol S, 2,5-diol ether, 5-butylbenzene-1-diglycidyl, diglycidyl ether of hydroquinone, diglycidyl ether of 2,5-di-t-butylhydroquinone, diglycidyl ether of terephthalic acid, diglycidyl isophthalate, epoxypropoxydimethylbenzylacrylamide and triglycidyl isocyanurate. 10. The composition according to claim 1, wherein the crystalline epoxy resin is diglycidyl ether of tetramethylbisphenol. 11. The composition according to claim 1, wherein the non-crystalline epoxy resin is an epoxy resin of bisphenol A type, 12. The composition according to claim 1, wherein the curing agent is dicyanodiamide. 13. The composition according to claim 8, wherein the curing catalyst is

2-methyl imidazole. 14. A thermosetting epoxy powder coating composition in the form of particles, comprising a film-forming mixture of: a) at least one non-crystalline epoxy resin having a Tg greater than about 40aC, Jb) at least an epoxy-crystalline resin having a Tm greater than about 90 ° C in an amount sufficient to reduce or eliminate entrapment of bubbles within the cured coating film, formed from the powder coating composition; and, c) a curing agent to facilitate the curing of the composition to a thermoset state; wherein the incorporation of the 'crystalline eppxi resin reduces or eliminates the entrapment' of bubbles within the cured coating film, formed from the powder coating composition when the composition is cured to a thermoset state at temperatures below about 177SC. 15. The composition according to claim 14, wherein the non-crystalline epoxy resin is an epoxy resin bisphenol A type, "the crystalline epoxy resin is diglycidyl ether of tetramethylbisphenol, the curing agent is dicyanodiamide, the The composition further comprises an imidazole curing catalyst, and the composition is essentially free of opaque pigments and fillers so as to form a clearcoating film after curing.Lé.-A substrate coated with the powder coating composition accordingly. with claim 1. 17. A substrate coated with the powder coating composition according to claim 14, 18. A brass substrate coated transparent with the powder coating according to claim 5, and wherein The resulting coated brass substrate contains bubbles trapped in an insufficient quantity to produce a nebulosity. visible in the cured coating film, formed from the powder coating. 19. A method for obtaining a coating essentially free of bubbles and free of visible haze on a substrate susceptible to degassing and / or degradation during heating, which comprises applying the powder coating composition according to claim 1 on " a surface of the substrate, and curing the powder coating on the substrate at temperatures below the degassing temperature and / or degradation of the substrate until the coating reaches a thermoset state, wherein the resulting coating film on the substrate is without a visible nebulosity 20. The method according to claim 19, wherein the substrate is brass, the powder coating composition is transparent, and the powder coating composition is cured at temperatures below about 177aC.