MXPA99003892A - Insatured polyester powder coatings, with better surface cure - Google Patents

Abstract

Powdered polyester powder coatings, which can cure at low temperature, suitable for heat sensitive substrates, with improved surface curing, through the incorporation of active hydrogen groups in the unsaturated polyester molecule. Unsaturated polyesters containing active hydrogens are derived from polyfunctional carboxylic acids, ethylenically unsaturated, and polyols containing active hydrogens. Complete healing on the surface is obtained without the need for polyethylene waxes or oxygen-reactive curative resins that are commonly used to prevent atmospheric oxygen from inhibiting free-radical-induced healing on the surface of the coating.

Description

POLYESTER POWDER COATINGS 1NSATURED. WITH IMPROVED SURFACE CURE Field of the Invention This invention relates to powder coating, which cure at low temperature, based on unsaturated polyester resins, suitable for heat sensitive substrates. More particularly, this invention relates to such powder coatings in which active hydrogen groups are introduced into the unsaturated polyester resin, to prevent atmospheric oxygen from inhibiting curing on the surface of the coating film formed of the same BACKGROUND OF THE INVENTION Thermosetting powder coatings have gained considerable popularity in recent years over liquid coatings for a number of reasons. Powder coatings are virtually free of harmful fugitive organic solvents, normally present in liquid coatings and, as a result, provide, if any, few volatiles to the environment when they cure. This eliminates the problems of solvent emission and damage to the health of workers employed in coating operations. Powder coatings also improve work hygiene, since they are in dry solid form, without a disheveled liquid associated with them, which will adhere to workers' clothes and coating equipment. Likewise, they are easily collected in the case of spilling, without requiring cleaning and special containment supplies. Another advantage is that they are 100% recyclable. Sprayed powders are normally recycled during the coating operation and recombined with the original powder charge. This produces very high coating efficiencies and minimal waste generation. However, despite the many advantages, powder coatings have traditionally not been suitable for heat sensitive substrates, such as wood and plastic articles, due to the rather high temperatures required to melt and cure powders. . Recent efforts have focused on the development of powder coatings that allow polymerization or curing at lower temperatures, in order to reduce the amount of potential damage and thermal strain loads imposed on sensitive substrates. The powder coatings, which can be cured at low temperature, based on epoxy resin, have recently been proposed, as, for example, taught in the patent of E. U. A., No. 5,714,206. However, curing agents used to achieve low temperature cures are based on aliphatic or aromatic amines, which tend to become yellowish under heat. Likewise, epoxy coatings generally do not provide the durability and environmental resistance that powdered coatings, based on unsaturated polyester, normally provide. Unsaturated polyester powder coatings are both environmentally resistant and extremely reactive systems, which undergo rapid polymerization at low temperatures, making them particularly attractive for coating heat-sensitive substrates. However, a disadvantage with its use is that the polymerization, induced by free radical, or curing reaction, is easily inhibited along the surface of the coating on contact with air or, more precisely, oxygen. Atmospheric oxygen, which makes contact only with the surface of the coating film, while leaving the interior unaffected, is added to the terminal free radical, generated in the growth addition polymer and topped off, thus stopping further polymerization and leaving the surface of the uncoated coating film. Therefore, the surface remains soft and sticky and has inferior properties of the film, such as poor solvent resistance, dye resistance and surface hardness. Several approaches have been undertaken to minimize surface inhibition of air. For exampleAttempts have been made to incorporate polyethylene waxes into the unsaturated polyester powder formulations to provide an oxygen barrier layer on the surface of the film, which is finally polished and separated after curing. This approach works well with liquid coatings. However, in powder coatings, fast curing at low temperatures does not allow sufficient time for the wax to diffuse and rise to the surface of the film. Larger wax loads may be used, but this tends to cause the powders to block or sinter during storage and / or produce a rough unfavorable finish when cured. Another approach has been to incorporate reactive oxygen species into the coatings, as, for example, taught in International Publication (PCT) WO 93/19132. It is described therein that the unsaturated polyester powder coatings, which can be cured at low temperature, with a resin system composed of a mixture of polyester resin resins and allyl ether curatives, which are cured in the presence of initiators of free radical peroxide and cobalt salt catalysts. Air inhibition is prevented by the use of oxygen-reactive allyl ether-curing agents, which consume oxygen before it can interfere with the curing reaction. However, a disadvantage of such powders is that the curative agents used to achieve a good surface cure are semi-solids mostly liquid or waxy (low melting point) at room temperature. Liquid and semi-solid materials have only limited use in powder coatings, typically, when they are used beyond a small percentage, they tend to cause the powders to block or sinter in storage, giving the powders poor shelf stability and making them difficult to dose and spray during coating operations. The conversion of such materials into solids is costly and time consuming. Another disadvantage is that the production of these powders, which can cure at low temperature, is extremely difficult. since they have the tendency to react previously and harden in the extruder, during the traditional process of mixing the melt. It would be convenient and, therefore, a primary object of the invention to provide a coating of unsaturated polyester powder, which can cure at low temperature, and extrude the melt, suitable for heat-sensitive substrates, which exhibit excellent surface healing without the need for waxes or healing agents reactive with oxygen.

SUMMARY OF THE INVENTION According to the present invention, a powder coating composition is provided, which can cure at low temperature and extrude in a melt, suitable for heat sensitive substrates, which comprises a particulate mixture, which forms a film. , of: A) an inert polyester resin, preferably an unsaturated polyester based on maleate or fumarate unsaturation, in which the resin contains one or more hydrogens per molecule; B) a free radical healing initiator; and C) a redox catalyst. The unsaturated polyester resin A) is preferably derived from a polyfunctional, ethylenically unsaturated carboxylic acid (or its anhydride), such as maleic anhydride or fumaric acid, and a polyol containing one or more active hydrogen atoms (methylene groups) or methino active) in the molecule. The unsaturated polyester resin A, thus formed, exhibits a decreased amount of air inhibition, which allows the surface to cure satisfactorily without the need for waxes or curative agents reactive with oxygen.

The present invention also provides a method for improving the surface curing of unsaturated polyester powder coatings, which can be cured at low temperature, by incorporating therein polyester resins in the aforementioned character, a method for coating heat sensitive substrates. with the powder coatings of the aforementioned character, without damaging the substrate, and heat sensitive articles, such as wood or plastic articles, which have on them powder coatings that have cured, of the aforementioned character.

Detailed Description of Preferred Modes Through this specification, all parts and percentages specified therein are by weight, unless otherwise indicated. Here, the "resin" is considered to be resin A), more, if any, resin D). Levels of other components are given as parts per hundred parts of the resin (per). The unsaturated polyester resins A), useful in the present invention, contain at least one site of ethylenic unsaturation and at least one active hydrogen site per molecule. The term "active hydrogen", used herein, means a hydrogen atom that is easily separated by free radicals and participates in the healing reaction. The unsaturated polyester resins A) are conveniently prepared by the condemnation of one or more ethylenically unsaturated polyfunctional carboxylic acids (or their anhydrides), having carboxyl functionalities of 2 or more, with one or more polyols containing hydrogens active, which have hydroxyl functionalities of 2 or more. Although the active hydrogen in the unsaturated polyester resin A) is typically supplied by the polyol, it can, instead, come from acids containing active hydrogens, used in conjunction with the unsaturated acid. In addition, while the ethylenic unsaturation is typically supplied by the acid, it is possible to supply it, instead, through the polyol. The ethylenic unsaturation can be provided in the polymer backbone or at the end of the chain. If supplied at the end of the chain, ethylenically unsaturated monocarboxylic acids (or their esters) are also used in the condensation reaction. Also, the unsaturated polyesters can be carboxyl or hydroxyl terminated, depending on the ratio of the monomer mixture. While these saturated reactivities generally do not participate in the healing reaction that proceeds primarily through the unsaturated groups, they are often used to achieve the desired chemical and mechanical properties in the final polymer. Examples of suitable polyfunctional, ethylenically unsaturated carboxylic acids (or anhydrides) include maleic anhydride, fumaric acid, itaconic anhydride, tetrahydrophthalic anhydride, nadic anhydride, dimeric methacrylic acid, etc. Maleic anhydride, fumaric acid, or mixtures thereof are generally preferred, due to economic considerations. It should be understood that when listing acids or anhydrides, any of these forms are considered for use here. Examples of monofunctional acids suitable for chain end unsaturation include acrylic acid, methacrylic acid, etc. Often, saturated and aromatic polyfunctional carboxylic acids (or their anhydrides) are used in conjunction with unsaturated acids to reduce the density of ethylenic unsaturation and provide the desired chemical and mechanical properties. Examples of suitable saturated and aromatic polyfunctional acids (or their anhydrides) include adipic acid, succinic acid, sebacic acid, phthalic anhydride, isophthalic acid, terephthalic acid, dimethylterephthalate, dimethylisophate, tetrahydrophthalic acid, hexahydrophthalic acid, cyclohexanedicarboxylic acid, Dodecane-dicarboxylic acid, trimellitic acid, pyromellitic anhydride, etc. Polyols with active hydrogens include polyols containing at least one active methylene group or an active methino group per molecule. If active hydrogens are supplied by the active methylene groups, the polyols may contain an active hydrogen atom attached to an allyl carbon or benzyl carbon. If the active hydrogens are supplied by methino-active groups, the polyols may contain an active hydrogen atom attached to a cyclohexyl or tertiary allyl-carbon. The allylic, benzylic, cyclohexyl or tertiary alkyl hydrogen atoms are easily separated during the free radical induced cure to form the allylic, benzylic, cyclohexyl and tertiary alkyl radicals, all of which promote healing in the surface of the coating film in an atmosphere open to the air. Examples of suitable polyols having an allylic hydrogen include trimethylolpropane monoallyl ether, trimethylolpropane diallyl ether, vinyl cyclohexanediol, etc. Examples of suitable polyols having a benzyl hydrogen include benzenedimethanol, etc. Examples of suitable polyols having a cyclohexyl hydrogen include cyclohexane-dimethanol, cyclohexanediol, etc.

Examples of suitable polyols having a tertiary alkyl hydrogen atom include ethyl propanediol, butylethyl propanediol, etc. As mentioned before, it is also possible to supply the active hydrogen through the carboxylic acid. Examples of suitable polyfunctional carboxylic acids with the active hydrogens (active methylene groups) include the sarboxylic acids comprising a malonyl hydrogen, such as malonic acid, etc., or an allylic hydrogen, such as nadic anhydride, tetrahydrophthalic anhydride, dimer acid, etc. Often, polyols without active hydrogens are used in the condensation reaction in conjunction with the active hydrogen containing polyols to provide the desired chemical and mechanical properties. Examples of suitable polyols without active hydrogens include ethylene glycol, dimethylene glycol, triethylene glycol, propylene glycol, neopentyl glycol, butanediol, dodecanediol, hydrogenated bisphenol A, bisphenol A / propylene oxide adducts, glycerol, trimethylolpropane, trimethylolet, etc. According to this invention, it is preferred that between about 10 and 100 mol% and even more preferred, than between 50 and 100 mol%, of the hydroxyl functionality, with respect to the total hydroxyl functionality of the monomers used for forming the resin A) of unsaturated polyester, whether provided by the active hydrogen containing polyol monomers. The unsaturated polyester resin A) can be formulated to have a crystalline or amorphous microstructure. Crystalline resins or mixtures of crystalline and amorphous resins are suitable for forming powder coatings with a lower viscosity of the melt and better flow behavior. It is well known in the art that certain alcohol and acid monomers impart crystallinity to the introduced polyesters. For example, symmetrically substituted linear monomers or cyclic monomers or their mixtures are generally used to form crystalline polyesters. Examples of suitable diols, which are known to promote crystallinity, include ethylene glycol, butanediol, hexanediol and cyclohexanedimethanol. Examples of suitable dicarboxylic acids known to do the same include terephthalic acid, adipic acid, dodecane dicarboxylic acid, and cyclohexanedicarboxylic acid. Mixtures of amorphous resins with active hydrogens and crystalline resins without active hydrogens, or vice versa, can be used to further improve the flow characteristics of powder coatings.

The most useful polyester resins here are the solid materials at room temperature, so they can be easily formulated into non-blocking powders. Preferably, the polyester resins also remain solid during normal storage and do not exhibit virtually cold flow at temperatures up to about 322C. Preferred resins further have a glass transaction temperature (Tg) and / or a melting point (Tm) below the flow temperature required for the preservation of heat sensitive substrates, usually between about 71 to 1492C. These unsaturated polyesters typically have a weight-average molecular weight (Mw) ranging from about 400 to 10,000, and preferably from about 1,000 to 4,500. The degree of unsaturation, preferably unsaturation of maleate or fumarate, is typically between about 2 and 20% by weight of resin A) of unsaturated polyester and preferably about 4 and 10% by weight. Also, when the unsaturated polyester is functional hydroxyl or acid functional, it depends on the molar ratio of -OH / -COOH of the monomer mixture. Usually, the functional hydroxyl resins have a hydroxyl number of about 5 to 100 mg KOH / gram resin. Acid functional resins typically have an acid number of about 1 to 80 mg KOH / gram resin. The free radical initiators B) are used to generate the free radicals in the active hydrogens and to initiate the curing (by means of homopolymerization) of the unsaturated polyesters A). Since surface healing is achieved thermally, the free radical initiators useful herein, are selected from traditional thermal initiators, such as peroxides and azo compounds. Examples of peroxide initiators include diacyl peroxides, such as benzoyl peroxide, peroxy esters, peroxy ketals, such as 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, esters of peroxy, dialkyl peroxides, ketone peroxides, etc. Examples of suitable azo initiators include the peroxy compounds of azobis (alkyl nitrile), etc. Standard photoinitiators can also be used in conjunction with thermal initiators for photo-activated healing (ie, by ultraviolet light). In general, the amount of free radical initiator B) used in the powder coating composition of the present invention ranges from about O.l to 10 per and preferably between about 1 and 5 per. Preferably, the free radical initiators B), used here, are solids. Of course, if they are liquid, as with any of the other materials used in the UV curable powder. they can be converted into solids by absorption over an inert filler before use, as is well known in the art. Still, liquids should be avoided when possible. The C) redox catalysts are also used in powder coatings, to induce the generation of free radicals from the initiators through a reduction-oxidation reaction. As redox catalysts, the transition metal compounds based on fatty acids or oils can be used. Examples of suitable metals include cobalt, manganese, lead, copper and vanadium. Cobalt-containing compounds, especially cobalt salts of monocarboxylic (ie, fatty) acids, for example, cobalt octoate, cobalt neodecanoate, cobalt naphthenate and cobalt octadecanoate, are more preferred. During curing, on the surface of the coating, even the free radicals formed at the active hydrogen sites, tend to react with atmospheric oxygen to form hydroperoxides (ie, inactivated peroxide initiators), which stop the healing reaction. Still, the hydroperoxides thus formed, due to their location, are now easily decomposed in the presence of the cobalt salts to restart the healing of the free radical, thus allowing the curing to proceed to its termination on the surface. The redox catalysts C) are generally used in the powder coating of this invention, in amounts of less than about 1.0 per cent, and preferably in the range between 0.1 and 0.5 per cent. In accordance with this invention, it may be convenient to include curable resins D) copolymerizable in the powder coating resin system, rather than some of the unsaturated polyester resin A). Since the unsaturated polyester resins A) are self-healing (by means of homopolymerization) due to the presence of active hydrogen sites in the polyester molecule, they do not require curative agents to achieve the desired core. Still, it has been found that if the curative agents D) reactive with oxygen are used in conjunction with the resins A) containing active hydrogen, the surface cure can still be further improved. The curative agents D) reactive with oxygen, useful herein, include ethylenically unsaturated resins and preferably have two unsaturation sites per molecule. Examples of such curative resins include oligomers or polymers having vinyl ether, vinyl ester, allyl ether, allyl ester, acrylate or methacrylate groups. Examples of suitable allyl esters include the reaction product of allyl alcohol and phthalic anhydride, such as diallyl phthalates, iso-diallyl phthalates and p-diallyl phthalates, etc. Examples of suitable allyl ethers include the reaction product of allyl propoxylate and hydrogenated methylene diisocyanate, etc. Examples of suitable vinyl ethers include divinyl ether urethanes, such as those formed by the reaction of the hydroxybutyl vinyl ether with the diisocyanates. Examples of suitable methacrylates or acrylates include methacrylated or acrylated urethanes, such as those formed by the reaction of the methacrylate or hydroxypropyl or hydroxypropyl acrylate with the diisocyanates, etc. The curative agents D), such as the unsaturated polyesters A), can be formulated to have a crystalline or amorphous microstructure. This will depend on the selection of monomers used in the formation reaction, as is well known in the art, and the properties of the desired flow and final coating.

The amount of the curative agent D) in relation to the resin A) of unsaturated polyester will also depend on the selection of the materials used. Usually, such materials are employed in equivalent stoichiometric amounts, to allow interlacing during curing to proceed to substantial termination, although the excess of either of them may be used when convenient. In the present invention, the curative agents D), if used, typically comprise up to 50% by weight of the resin and preferably up to 20% by weight. Common additives may also be employed in the powder coatings of this invention. For example, the powder coatings formed in accordance with this invention can be clear (ie, non-pigmented) or can contain up to 200 per, although generally about 120 per or less, conventional fillers and / or pigments. Examples of suitable fillers include calcium carbonate, barium sulfate, wollastonite, mica, china clay, infusoria earth, benzoic acid, low molecular weight nylon, etc. Examples of suitable pigments include inorganic pigments, such as titanium dioxide, and organic pigments, such as carbon black, etc.

The other common additives, such as gloss control agents, flow control agents, dry flow additives, anti-cratering agents, texture forming agents, light stabilizers, etc., are typically present in a total amount of up to about 15 per. Examples of suitable gloss control agents include polyethylene waxes, oxidized polyethylenes, polyamides, tefions, polyamides, etc. Examples of suitable flow control agents include acrylic resins, silicone resins, etc. Examples of suitable dry flow additives include fumed silica, alumina oxide, etc. Examples of suitable anti-cratering agents include benzoin, benzoin derivatives, phenoxy plasticizers and low molecular weight phthalates, etc. Examples of texturizing agents include organophilic clays, interlaced rubber particles, multiple healing agents, etc. Examples of suitable light stabilizers include clogged amines, clogged phenols, etc. The coating powders are produced in the usual manner. The components are mixed together dry and then mixed in the melt in a single screw or twin screw extruder, with heating above the melting point of the resin system. The extruded composition is rapidly cooled and chunked into pieces, milled in a mill, with cooling, and, when necessary, the particles are screened and sorted according to their size. The average particle size desired for the electrostatic application is typically between about 20 to 60 microns. Extrusion is preferably carried out between about 82 and 1212C, to minimize any cure and gel formation in the extruder taking place. Gaseous or supercritical fluid, such as gaseous or supercritical CO 2, can be charged to the extruder to reduce extrusion temperatures. This is particularly convenient with powders containing crystalline materials. Once the dry, free-flowing powders are produced, they are ready for application on a substrate to be coated. The powder coatings are applied in the usual manner, for example electrostatically, to the substrate. Electrostatic spray booths housing banks of corona discharge or triboelectric spray guns and recirculators are usually employed to recycle the sprayed powder back into the powder charge. The applied powders are then exposed to sufficient heat for the powders to melt, flow in a continuous film, and cure. The substrate can be heated at the time of application and / or subsequently to effect flow and cure. The heating is usually carried out in convection or infrared ovens or in a combination thereof. The powders of this invention are formulated to melt, flow and obtain a complete cure, even on the surface, at extraordinarily low temperatures and / or fast speeds, while still being melts that can be extruded, making them especially suitable for heat-sensitive substrates, without the risk of thermal damage to the substrate , such as a brittle state, loss of integrity, deformation and other physical and / or chemical degradations, during healing. The curing temperature of the composition is usually from about 149 ° C or below, and typically even from 121 ° C or below, temperatures consistent with the application of the coating powder compositions to wood or plastic products. Of course, healing depends on time as well as temperature; however, a complete cure at the above temperatures can be achieved with a reasonable time, for example in about 30 minutes or less. Preferred powder coatings of the invention can perform a complete cure between about 121 and 149 seconds, in about 5 minutes or less, which is safe for most heat-sensitive applications. A "complete cure" is a degree of cure achieved in which additional time at elevated temperature will not improve the properties of the coating, once cooled to ambient temperatures. Examples of suitable heat-sensitive substrates, useful herein, include wood, such as hardwood, hardboard, laminated bamboo, wood composites, such as particle board, electrically conductive particle board, low density fibreboard, medium or high, Masonite board, laminated bamboo and other substrates that contain a significant amount of wood. Any of the wood based substrates can be filled or primed with materials, such as UV fluids, powder coatings or coatings that carry water or solvents, to improve softness and reduce film formations. Other heat-sensitive substrates include plastics, such as ABS, PPO, SMC, polyolefins, polycarbonates, acrylics, nylons and other copolymers that will usually coil or be purified from gases when coated and heated with traditional powders that can heat cure together with paper, cardboard, composites and metal components with a heat-sensitive appearance, etc. Powder coatings are also applicable to customary heat resistant substrates, such as metal, steel and other alloys, glass, ceramics, carbon, graphite, etc. In summary, the present inventors have found that the incorporation of a compound containing one or more active hydrogens into the unsaturated polyester resin molecule itself, significantly improves the surface curing properties of the cured coating, without the need for waxes or curative agents reagents with oxygen. While not wishing to be bound by the theory, it is believed that the inclusion of the compound containing active hydrogen in the polyester molecule installed allows the generation of free radicals that have greater stability and are less susceptible to permanent deactivation on contact with oxygen atmospheric. When free radicals react with oxygen and are capped, they easily start again in the presence of redox catalysts. The curing of the coating film can, therefore, now proceed to the termination along the surface. The invention will now be described in more detail by means of the specific examples.

EXAMPLE 1 Preparation of the Unsaturated Polyester Resin, Containing Active Cryohexyl Hydrogens 1 mole (144 g) of 1,4-cyclohexane-dimethanol, was charged in a 0.5 liter resin kettle, equipped with a partial condenser, total condenser , agitator, nitrogen inlet and temperature controller. While introducing a stream of nitrogen at a rate of 25-30 ml / min and stirring, the temperature rose to 1252C. Next, 0.5 mol (74 g) of phthalic anhydride, 0.6 mol (69.6 g) of fumaric acid, and 50 ppm of 4-methoxy-phenol (catalyst) were added to the copper. Still under agitation and nitrogen sparge, the temperature rose slowly to 180 ° C, while the esterification water was collected. When 85 to 90% of the theoretical distillate had been collected, a vacuum was applied to remove the rest of the water. The resin was then discharged into a container, cooled and ground into flakes. The recovered amorphous resin had the following characteristics: Properties Example 1 Glass Transition Temperature (Tg) 33. OSC Fusion Point (Tm) 43. 62C Acid Number (mg KOH / g resin) 45 Viscosity ICI @ 175 ° C 3250 cps Molecular Weight (Mw) 2, 500 Example 2 Preparation of Polyester Powder Coating nsaturated with Active Ciconhexyl Hydrogens The following were mixed together ingredients in the manner and amounts given, to form powder coatings (A, B).

Parts in Weight Ingredients MIX IN DRY IN THE MIXER UP TO HOMOGENEITY Unsaturated Polyester (from Example 1) 100 Comparative Unsaturated Polyester1 100 Lupersol 23XL (peroxide initiator) 2 5.0 5.0 Cobalt Neodecanoate (Redox Catalyst) 0.5 0.5 Resiflow P-67 (Acrylic Flow Agent) * 1 1.4 1.4 Uraflor B (Agent against crater formation) 4 0.8 1.0 TiPure R-960 TÍO2 (Pigment) 5 20 20 MIXING MASS FLOW IN EXTRUDER OF DOUBLE SCREW TO 71-82 ° C COOL EXTRUDED AND BREAK IN PIECES LOADING A BRINKMANN MILL AND MOLTING A DUST CLASSIFYING IN A MESH OF -140 Footnotes: ** The comparative resin is an acid functional unsaturated polyester, similar to the unsaturated polyester prepared in Example 1, but not containing no active hydrogen groups in the polyester backbone. Instead, it is based on fumaric acid, phthalic anhydride and neopentyl glycol, instead of 1,4-cyclohexane-dimethanol. 2 Lupersol 231XL is a free radical initiator of peroxy ketal ketal, based on 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, sold by Elf Atochem. 3 Resiflow P-67 is an acrylic flow control agent, sold by Estron Chemical. 4 Uraflow B is a benzoin agent against crater formation, sold by Estron Chemical. 5 TiPure R-960 is a white titanium dioxide pigment, sold by DuPont.

The aforementioned powders (A, B) were electrostatically sprayed using a triboelectric gun on medium density fibreboard (MDF) separated by 1.27 cm, which was preheated under quartz infrared (IR) lamps, of medium intensity, to a surface temperature of 121 to 132 C, before application of the powder. The re-surfaced surfaces were then exposed to quartz IR lamps for about 30 to 60 seconds (ie, until the surface temperature of the coating reached 149-1602C) in an open atmosphere for melting and the flow of the powders into films melted, smooth and continuous, and thermally activated free radical induced cure to harden the films. The performance properties of the individual coating powders (A, B) and the coating films formed therefrom are given below.

Properties B Gel time at 204 ° C (sec.) Hot plate melt flow at 190 ° C (mm) 45 126 Resistance to MEK (50 double rubs) Do not detach excessive detachment Adhesion Excellent Poor Softness (roughness formation) Mild Moderate The above results show that the comparative powder formulation (B), which does not contain active hydrogens in the polyester resin itself, exhibited poorer surface healing properties (ie, lower resistance to rubbing by methylethyletone (MEK). )) than the powder formulation (A), which contains the active hydrogens obtained according to the present invention.

Example 3 Preparation of an Unsaturated Polyester Powder Coating with Active Ciciohexyl Hydrogens The following ingredients, in the given amounts, were mixed together by the same method used in Example 2, to form powder coatings (C, D).

Parts in Weight Ingredients Installed polyester (from Example 1) 90 Aropol 7501 Comparative Unsaturated Polyester1 90 Diallyl isophthalate (Allyl ester curative) 10 10 Lupersol 231 XL (peroxide initiator) 6.0 5 Cobalt Neodecanoate 0.5 0.5 Resiflor P-67 (Acrylic flow agent) 2.3 2.3 TiPure R-960, T02 (Pigment) 20 20 Footnotes of the Table * 'The comparative resin Aropol 7501 ,. sold by Ashland Chemical, is an unsaturated polyester resin, which is believed to contain no active hydrogen groups in the polyester molecule The aforementioned powders (C, D) were electrostatically sprayed and cured on separate MDF boards, by the same method used in Example 2. The performance properties of coating powders (C, D) and coating films formed of them are given below.

Properties D Gel time at 204 ° C (sec.) 14 Flow of the hot plate melt at 190 ° C (mm) 15 17 Resistance to rubbing by MEK (50 double rubs) No detachment Detached moderate Adhesion Excellent Good Softness (roughness formation) Moderate Dense The above results demonstrate that the comparative powder formulation (D) exhibited poor surface healing properties (ie, poor rub resistance with MEK) than the active hydrogen-containing powder formulation (C) obtained in accordance with present invention.

Example 4 Preparation of an Unsaturated Polyester Resin Containing Active Allyl Hydrogens 2 moles (104 g) of neopentyl glycol were reacted with 1 mole (83 g) of isophthalic acid, 0.6 mole (45.6 g) of tetrahydrophthalic anhydride, 0.6 mol (34.8 g) of fumaric acid, in the presence (50 ppm) of 4-methoxy-phenol, under the same procedure used in Example 1.

The recovered amorphous resin had the following characteristics: Properties Example 4 Glass transition temperature (Tg) 43 sc Melting point (Tm) 472c Acid number (mg KOH / g resin) 42 Viscosity ICI @ 175 ° C 500 cps Molecular weight (Mw) 2, 500 Example 5 Preparation of an Unsaturated Polyester Powder Coating with Active Hydrogens The following ingredients were mixed together in the amounts given, by the method used in Example 2, to form a powder coating (E). Ingredients Parts in Weight E Unsaturated Polyester (from Example 4) 100 Lupersol 231 XL (Peroxide initiator) 5.0 Cobalt Neodecanoate (Redox Catalyst) 0.5 Resif low P-67 (Acrylic Flow Agent 1.4 Uraflow B (Anti crater agent) 1.0 TiPure R-960, TiO2 (Pigment) The powder (E), mentioned above, was sprayed and cured on an MDF board by the method used in Example 2. The performance properties are given below.

Properties Gel time at 204 ° C (sec) 8 Melt flow in hot plate at 190 ° C (mm) 105 Resistance to MEK (50 double rubs) Mild detachment Adhesion Good Smoothness Wrinkle formation From the foregoing, it will be seen that this invention is well suited to achieve all of the aforementioned purposes and objects, together with the other advantages that are evident and inherent. Since many possible variations of the invention can be made without departing from its scope, the invention does not intend to be limited to the modalities and examples described, which are considered to be purely exemplary. Therefore reference should be made to the appended claims to assess the true spirit and scope of the invention, in which exclusive rights are claimed.

Claims (20)

1. CLAIMS 1. A powder coating composition, which is in particulate form and comprises a film-forming mixture of: a) an unsaturated polyester resin, containing an active hydrogen; b) a free radical initiator; and c) a catalyst.
2. 2. The composition of claim 1, wherein the unsaturated polyester resin is derived from at least one polyfunctional, ethylenically unsaturated carboxylic acid, or its anhydride, and at least one polyol containing an active hydrogen.
3. 3. The composition of claim 2, wherein said at least one polyol containing active hydrogen is selected from polyols containing allylic, benzylic, cyclohexyl or tertiary alkyl hydrogens.
4. 4. The composition of claim 2, wherein said at least one polyfunctional, ethylenically unsaturated carboxylic acid, or its anhydride, is selected from a fumarate, a maleate, or mixtures thereof.
5. 5. The composition of claim 2, wherein between 10 and 100 mole% of the hydroxyl functionality, relative to the total hydroxyl functionality of the polyol monomers, used to form the unsaturated polyester resin A), is provided by This at least one polyol containing active hydrogen.
6. The composition of claim 1, wherein the catalyst is a redox catalyst, comprised of a metal compound, based on a fatty acid or oil.
7. 7. The composition of claim 6, wherein the redox catalyst is a cobalt salt of a fatty acid.
8. 8. The composition of claim 1, further comprising: d) a curative resin, ethylenically restored, copolymerizable.
9. 9. The composition of claim 1, wherein this composition is free of curative resins, ethylenically restored, copolymerizable.
10. 10. The composition of claim 1, wherein the free radical initiator is a thermal initiator, selected from a peroxide or azo compound.
11. 11. A powder coating composition, which is in particulate form and comprises a film-forming mixture of: a) an unsaturated polyester, having at least one group of maleate or fumarate per molecule and at least one group of active hydrogen per molecule; b) a thermal peroxide free radical initiator; and c) a redox catalyst, selected from a metal compound, based on a fatty acid or oil.
12. 12. The composition of claim 11, which further comprises: d) a curative resin, ethylenically restored, copolymerizable.
13. 13. The composition of claim 11, wherein the active hydrogen is selected from the allylic, benzyl, cyclohexyl, tertiary alkyl or malonyl hydrogen.
14. 14. The composition of claim 11, wherein the active hydrogen is derived from a polyol containing an allylic, benzyl, cyclohexyl or tertiary alkyl polyol.
15. 15. The composition of claim 14, wherein the polyol containing the active hydrogen is selected from trimethylolpropane-monoallyl ether, trimethylolpropane diallyl ether, vinyl cyclohexanediol, benzene dimethanol, cyclohexane dimethanol, cyclohexane diol. , methyl propanediol or propanediol of butylethyl.
16. 16. The composition of claim 11, wherein the active hydrogen is derived from a polyfunctional carboxylic acid, containing an allyl or malonyl hydrogen.
17. 17. The composition of claim 16, wherein the active hydrogen containing the polycarboxylic acid is selected from malonic acid, nadic anhydride, tetrahydrophthalic anhydride or dimeric acid.
18. 18. A method for improving the surface cure of a powder coating composition, based on unsaturated polyester resins, this method comprises: a) incorporating into the composition an unsaturated polyester resin, having at least one group of maleate or fumarate per molecule and at least one group of active hydrogen per molecule, this active hydrogen is supplied by an allylic, benzyl, cyclohexyl, tertiary alkyl or malonyl hydrogen; and b) curing the composition with heat, in the presence of free radical peroxide initiators and cobalt salts, also contained in this composition.
19. 19. A heat-sensitive substrate, having the powder coating of claim 1, coated and cured thereon.
20. 20. The re-coated substrate of claim 19, wherein the heat sensitive substrate is an article containing wood, plastic or paper.