method of forming textured epoxy powder coatings on heat-sensitive substrates

Abstract: Thermosetting powder coating compositions adapted to form a grainy textured finish on heat sensitive substrates, especially wood products, such as particle board, conductive particle board, and medium density fiber board, without damaging the substrates comprise an epoxy resin, particularly bisphenol A type epoxy resins having low viscosities, either a catalytic curing agent comprising an imidazole, a substituted imidazole, or adduct of an imidazole or substituted imidazole and an epoxy resin, or a curing agent comprising an adduct of a polyamine and an epoxy resin, or a combination of the catalytic curing agent and curing agent, a texturing agent, a flow control agent, optional extender, and optional pigment. The thermosetting powder coatings have a cure time/temperature ranging from about 30 seconds at about 350°F peak substrate temperature down to about 20 minutes at about 225°F peak substrate temperature which provides for either a rapid cure or low bake temperature to prevent excessive heat exposure to the heat sensitive substrate, resulting in substantially no physical or chemical degradation of the heat sensitive substrate during baking. The powder coatings are applied to heat sensitive substrates, especially wood products, through electrostatic spraying onto the substrate followed by heating to cure the textured finish uniformly on the surfaces and edges of the substrate. Electrostatic spraying is performed with a triboelectric or corona spray gun and heat curing is performed with a high intensity infrared oven, a convection oven, or combination. Coated heat sensitive substrates having continuous grainy textured finishes that are excellent for hiding substrate surface defects and later appearing scratches are obtained.

Description: This invention relates to a process for coating of heat sensitive materials, such as wood substrates. These coatings are applied to the surfaces of wood substrates in particulate form and then fused and cured at relatively lower temperatures and/or relatively faster cure rates to form continuous dry film coatings without physical or chemical degradation of the substrate. More particularly, this invention relates to epoxy based powder coating compositions for wood substrates which are first applied by electrostatic spraying and next heat cured thereon at relatively lower temperatures and/or relatively faster rates to form durable grainy textured finishes without damaging the wood. Powder coating compositions are extremely desirable In painting articles In that they are virtually free of fugitive organic solvents conventionally utilized in liquid paint systems. Thus, powder coatings give off little, If any volatile material to the environment when heat cured. This eliminates numerous problems of air pollution and dangers to the health of workers employed in painting operations. Solventless fusion coating processes have been developed to apply such powder coatings on substrates in which the dry, finely divided, free flowing, heat fusible powders are deposited on the substrate and then fused and cured with external heating into a continuous protective or decorative film. Examples of such coating processes include electrostatic spray, fluidized bed, and electrostatic fluidized bed techniques, with electrostatic spray principally being used in the industry today. Electrostatic spray coating, however, has been problematic for coating of heat sensitive materials, such as wood, paper, cardboard and plastic substrates, since the curing takes place at rather high temperatures typically greater than 177°C (350° F) at normal oven dwell times, made necessary by the high curing temperatures of the paint powders and maintaining these high temperatures for a period of time to achieve the desired cure. Heat sensitive materials, however, cannot withstand such high temperatures for a continuous time necessary for powder coatings. For instance, the resinous binders and moisture that maintain the integrity of wood products, such as particle board or fiber board, tend to degrade and volatilize at temperatures required for curing powder coatings at acceptable rates. This causes the wood board to deteriorate in physical and/or chemical properties which is unacceptable. U.S. Pat. 5,387,442 (Kroeger et al.) discloses a method of electrostatic spray coating heat sensitive materials, such as plastic materials, wood, paper, cardboard, leathers and textiles with thermosetting powder paint compositions, which method is based on short repetitive cycles of high intensity infrared heating that are interspersed with cooling cycles. In this way, it is claimed that the heat necessary for causing melting, film formation and subsequent curing of the top powder coat does not have time to attack the heat sensitive material of the substrate in a damaging orworsening way, since the heat is rapidly dissipated during the cooling intervals. However, this method fails to teach how to avoid using thermosetting powder coatings having higher temperature cure and/or slower cure rate requirements altogether when coating heat sensitive materials. In Kroeger et al., it is shown that the substrate still sees temperatures on the average of about 199°C (390°F) for about 20 minutes to cure the powder coatings. Thermosetting powder coatings based on epoxy resins are known in the art. Examples of epoxy powder coatings for non-heat sensitive metal substrates where the substrates were heated to about 232°C (450°F) prior to electrostatic spraying the powders can be found in U.S. Pats. 4,568,606 (Hart et al.), 4,855,358 (Hart), and 4,857,362 (Hart). Epoxy powder coatings that produce grainy textured or wrinkled finishes for usually heat resistant metal substrates to better hide.substrate surface imperfections and scratches are known in the art as well. An example of a wrinkled finish epoxy powder coating can be found In U.S. Pat, 4,434,819 (Schreffler et al.). In U.S. Pat. 5,212,263 (Schreffler), a powder coating adapted to form a textured finish is disclosed which contains an epoxy resin, a methylene disalicylic acid curing agent for the epoxy, and an adduct of imidazole and bisphenol A type epoxy resin as the cure catalyst. Generally speaking, the latter two patents show that it takes their respective epoxy powder coating formulations to melt and cure at peak substrate temperatures in excess of about 191°C (375°F) for about 10 minutes. EP-A-0503865 discloses powder coating compositions comprising an epoxy resin, a copolymer of (meth) acrylic acid and at least one C 1 -C 18 alkyl (meth)acrylate, together with a curing agent for the epoxy resin such as dicyandiamide. The composition may also include a minor amount, relative to the curing agent, of a cure accelerator such as methylimidazole. The compositions can be applied to plastics materials and give a smooth matt finish. A typical cure temperature is 180°C. Compositions of this type are disclosed In EP-A-0503864, wherein 8 parts of 2-methylimidazole are used to accelerate curing with 290 parts of a phenol novolak hardener. These compositions also contain a texturing agent. The exemplified powders are applied to steel substrates and cured at 180°C. GB-A-1475034 discloses the use of imidazole or imidazoline compounds to cure epoxy resins at temperatures of about 290°C to coat metal pipes. The compositions also contain a flow control agent and a thixotropic agent. Low temperature cure and/or rapid cure rate powder coatings for providing smooth finishes when electrostatically sprayed on wood products, such as TV stands made of medium density fiber board have been proposed, for example in US-A-4637954. Such smooth formulations contained an epoxy resin and an adduct of imidazole and an epoxy resin as a curing agent for the epoxy, but did not contain texturing agents. Despite having desirable cure properties for heat sensitive substrates, these powder coatings were found to be unsatisfactory for electrostatic spray coating on wood articles because of inadequate hiding of surface imperfections and Inadequate edge coverage provided. In addition, pinholes were visually evident In the final coatings as a result of volatiles escaping through partially cured films. This led to a discontinuous film which exposed the substrate and left it unprotected. What is needed is a process for powder coating, particularly electrostatic spray coating at lower temperatures and/or faster cure rates, on heat sensitive materials, particularly wood products, to produce continuous grainy textured finishes to adequately hide surface imperfections, without damaging or worsening the physical and/or chemical properties of the substrate coated therewith. The present invention consists in a method of coating a heat-sensitive substrate to provide a grainy texture film finish without damaging said substrate, which comprises the steps of: 1) applying to the substrate a thermosetting powder coating composition comprising (a) an epoxy resin; (b) at least one curing component selected from (i) 1 to 8 parts per hundred parts of resin (phr) of a catalytic curing agent comprising an imidazole, an adduct of an imidazole with an epoxy resin or a mixture thereof; and (ii) 5 to 40 phr of a low temperature curing agent comprising an adduct of a polyamine and an epoxy resin; (c) 1 to 20 parts per 100 parts of resin of a texturing agent selected from organophilic clays and (d) a flow control agent, 2) heating the powder-coated substrate to melt and flow and 3) curing said powder coating into a grainy textured dry film on the substrate at a peak substrate temperature cf not more than 177°C (350°F), the said imidazole, imidazole adduct and/or amine adduct serving the duel function cf curing agent and cure accelerator. By the method of the present invention the powder coating compositions are preferably electrostatically spray coated. The epoxy based powder coating compositions allow the powder to flow, film and subsequently cure at significantly lower temperatures and/or significantly faster rates in order to allow electrostatic spray coating on heat sensitive wood substrates without damaging the wood. The resulting powder coatings produce grainy textured finishes with increased hiding power and better coverage of edges on electrostatically spray coated wood substrates, giving a continuous coating that is aesthetically acceptable without any visual evidence of outgassing, bubbling, or pin hole formation. The epoxy resin is preferably selected from bisphenol A type epoxy resins having epoxy equivalent weights of between 600 750 and mixtures of such epoxy resins. The catalytic curing agent Is preferably selected from a substituted imidazole or an adduct of imidazole or substituted imidazole and a bisphenol A type epoxy resin. The texturing agent is preferably selected from an organophilic clay, rubber particles, and thermoplastic polymer particles. The flow control agent is preferably selected from an acrylic resin and benzoin. The composition can also Include extenders preferably selected from calcium carbonate and pigments. The powder coating composition can be cured on heat sensitive wood substrates at conditions of time and temperature that do not deteriorate the integrity of wood. The composition preferably has a cure time/temperature range of from about 30 seconds at 177°C (3500F) peak substrate temperature down to about 20 minutes at 107°C (225°F) peak substrate temperature. The low temperature and/or curing agent b(ii) which comprises an adduct of polyamine and epoxy resin can be combined with an optional secondary curing agent which comprises dicyandiamide and/or an optional catalytic curing agent which comprises an imidazole or an adduct of imidazole and epoxy resin, as well as the texturing agent and flow control agent. This curing agent is preferably selected from an adduct of a primary or secondary polyamine and a bisphenol A type epoxy resin. The other listed ingredients are the same as those previously mentioned. This alternative composition preferably also has a cure time/temperature range of from about 30 seconds 177°C (350°F) peak substrate temperature down to about 20 minutes at 107°C (225°F) peak substrate temperature. The wood substrates can include, without limitation, hardwood, particle board, electrically conductive particle board and medium density fiber board. The thermosetting powder coating compositions used In the method of the present invention contain as one component, an epoxy resin. The epoxy resins useful in the thermosetting powder coatings of this invention are solid resins or blends of solid and small amounts of liquid resins up to about 10 wt. %, which resins are the reaction products of a diol and a halohydrin. Suitable epoxy resins useful in the practice of this invention are exemplified by, but not limited to, the reaction products of bisphenol A and epichlorohydrin. Generally, the bisphenol A type epoxies used herein are of the type 1 to type 9 form, with the low viscosity type 3 or less epoxy resins being most preferred. The useful bisphenol A type epoxy resins have an epoxy equivalent weight ranging between about 400 and 2,250, preferably an epoxy equivalent weight of between about 550 and 1,100, with an epoxy equivalent weight of between about 600 and 750 being most preferred. Preferred epoxy resins include those sold under the tradename Araldite GT 7013 (type 3) and Araldite GT 7072 (type 2) by Ciba-Geigy Corporation, which are both 4,4'-Isopropylidene-diphenol-epichlorohydrin type epoxy resins. The thermosetting powder coating compositions used in the method of the present invention contain as another component, a catalytic curing agent. In the present invention, the catalytic curing agent serves the dual function of curing agent and cure accelerator. No other curing agents need to be present in the powder coating compositions. The advantage of this catalytic curing agent component is that it alone allows the powdered coatings to cure at much lower cure temperatures or at ultra rapid cure rates than ever thought possible with textured powder coatings, thereby allowing such compositions to be coated on heat sensitive materials, especially wood substrates, without deteriorating the physical and/or chemical properties of the substrate. The catalytic curing agents which may be employed in the thermosetting powder coating compositions used in the method of this invention are an imidazole, a substituted imidazole, or adducts of an imidazole or substituted imidazole and an epoxy resin or quaternary ammonium salts thereof, and mixtures of any of the aforesaid materials. Most preferably a substituted imidazole of 2-methylimidazole is used. Another suitable substituted imidazole is 2-phenylimidazole. The imidazole or its derivatives, i . e ., substituted imidazoles, that are adducted to epoxy resins, are preferably adducted to bisphenol A type epoxy resins. The imidazoles or imidazoles which are adducted to the epoxy resin and their derivatives have the general formula: where R 1 , R 2 , R 3 , and R 4 are H or any substitution which is non-reactive with the epoxy resin, with the R's if not H typically being alkyl, e.g., methyl, aryl, e . g ., phenyl, or alkaryl groups, and with R 2 , R 3 and R 4 preferably being H groups and R 1 preferably being a methyl group, i . e ., 2-methylimidazole. A preferred imidazole and epoxy resin adduct is sold under the tradename Epon Curing Agent P-1 01 by Shell Chemical Company. Another preferred imidazole adduct of a bisphenol A type epoxy resin is sold under the tradename HT 3261 by Ciba-Geigy Corporation. The imidazole or substituted imidazole residue, e.g., a 2-methylimidazole residue, typically comprises between about 5 and 50 wt. % of the imidazole/epoxy resin adduct composition, preferably between about 30 and 40 wt. %, with about 33 wt. % being most preferred. It is believed that the epoxy component of the imidazole adduct readily promotes incorporation of the otherwise insoluble imidazoles in the epoxy resin component system. It is also believed that the formation of the adduct reduces the melting point of the imidazole, thereby lowering the cure reaction temperature between the epoxy groups and the adduct. It is further believed that the adduct when used alone without other curing agents allows the curing of the epoxy resin component to occur at significantly lower temperatures or at significantly higher rates. This allows for these powder coatings to be used on heat sensitive wood substrates without exposure of the substrate to excessive heat which tends to deteriorate the integrity of the wood. In general, curing is believed to occur as the imidazole or imidazole adducted epoxy or quaternary ammonium salt thereof performs a ring opening operation on the epoxy ring of the epoxy resin component. This results in bonding between the epoxy group of the epoxy resin component and the C=N bond of the imidazole ring and formation of a quaternary ammonium saft with a highly reactive negatively charged oxygen available for further ring opening operations. The imidazole or adducted imidazole acts as a catalyst, moving from epoxy group to epoxy group as it grows and facilitates further epoxy group ring opening and epoxide cure reactions. The catalytic cure agent is used at between 1 and 8 parts per hundred resin (phr), most preferably between 1 and 4 phr. In another embodiment, the powder coating composition used in the method of the present invention contains a low temperature curing agent, either as the sole curing agent instead of the aforesaid catalytic curing agent, or in combination with the aforesaid catalytic curing agent. The low temperature curing agent may be selected from commercially available curing agents that are adducts of polyamines and epoxy resins, preferably adducts of an aliphatic primary or secondary polyamine and a bisphenol A type epoxy resin. An example of suitable low temperature curing agent that is an epoxy adduct of an aliphatic polyamine Is sold under the tradename PF LMB 5218 Hardener by Ciba-Geigy Corporation. Another preferred low temperature curing agent comprises an adduct of a secondary aliphatic polyamine and a bisphenol A epoxy resin which is sold under the tradename Ancamine 2014AS by Air Products & Chemicals. When present, the low temperature curing agent is used in an amount ranging between 5 and 40 phr, and, preferably between 20 and 30 phr. If used in combination with an aforesaid catalytic curing agent, for Instance, a bisphenol A epoxy adduct of 2-methylimidazole, the catalytic curing agent is preferably employed in an amount ranging up to 6 phr, and, preferably between 2 and 4 phr. Another curing agent that can be used in combination with the low temperature curing agent to enhance the curing properties is dicyandiamide. A suitable dicyandiamide curing agent is sold under the tradename Dyhard 100S by SKW Chemicals. If present, the dicyandiamide curing agent is used in the powder coating composition in an amount ranging up to 8 phr, preferably between 2 and 8 phr, and, more preferably between 4 and 6 phr. The powder coating composition used in the method of the present invention does not include methylene disalicyclic acid as a curing agent. In addition to the above components, the thermosetting powder compositions used in the method of the present invention include as another component, a texturing agent for achieving the desired grainy textured effect of the finish. The texturing agents which may be employed are organophilic clays, such as an organophilic clay sold under the tradename Bentone 27 and Bentone 38 by NL Chemicals, which are trialkylarylammoniumhectorite and tetraalkylammoniumsmectite, respectively. The amount of texturing agent used determines the coarseness or fineness of the texture. The texturing agent is used in a range between 1 and 20 phr, and, most preferably between 2 and 10 phr. It is believed that the texturing agent contributes to the high viscosity and low melt flow of the powder coating composition leading to the textured finish and, thus, provides for better edge coverage and hiding of surface imperfections of wood substrates. The thermosetting powder coatings used in the method of the present invention include as another component, a flow control agent. The flow control agents which may be employed in the thermosetting powder coating compositions are exemplified by, without limitation, acrylic resins. These acrylic resins are generally liquids which have been converted to powder form by absorption onto silica-type materials. A preferred flow control agent is sold under the tradename Resiflow P-67 acrylic resin by Estron Chemical, Inc., which is a 2-propenoic acid, ethyl ester polymer. Another preferred flow control agent is sold under the tradename Benzoin by DSM, Inc., which is a 2-hydroxy-1,2-diphenylethanone crystalline solid that is believed to keep the molten coating open for a suitable time to allow outgassing to occur prior to the formation of the hard set film. The flow control agent is generally used in a range between 1 and 5 phr, preferably between 1.5 and 2.5 phr. Fumed silica and aluminum oxide may also be employed as a powder dry flow additive. An example of fumed silica is sold under the tradename Cab-O-Sil by Cabot Corporation. An example of aluminum oxide is sold under the tradename Aluminum Oxide C by Degussa Corporation. In addition, the thermosetting powder coating compositions may contain as another component, pigments. Any of the usual pigments may be employed in the thermosetting powder coating of the Invention to obtain the desired color and opacity. Examples of useful pigments for the black textured powder coatings Include, without limitation, carbon black and black iron oxide. A preferred carbon black pigment is sold under the tradename Raven 22 and Raven 1255 by Columbian Chemical Company. An example of a useful pigment for white textured powder coatings Include, without limitation, titanium dioxide. The pigment Is used In a range up to about 100 phr, more preferably between about 1 and 4 phr for a black textured finish and between about 15 and 80 phr for a white textured finish. The thermosetting powder coating compositions may contain as another component, extenders or fillers. Since a textured finish is desired, the extender loading can be rather high to lower the melt flow of the powder coating and allow the molten coating to cure while retaining some of the finish of the powder particles as applied. The level of extenders can also be used to control the coarseness or fineness of the finish. The extenders which may be employed in the thermosetting powder coating compositions of the present invention are exemplified by, without limitation, calcium carbonate, barium sulfate, wollastonite, and mica. The extender is generally used in a range up to 120 phr, more preferably between about 10 and 80 phr. In addition to the above components, the thermosetting powder coating compositions used in the method of this invention may also contain the usual additives common to powder coatings. These additives include, without limitation, gloss control waxes, such as polyethylene, slip additives, such as Teflon® (polytetrafluoroethylene), siloxanes, and the like. The thermosetting powder coating compositions of this invention are prepared by conventional techniques employed in the powder coatings art. Typically, the components of the powder coating are thoroughly blended together and then melt blended In a extruder. Melt blending is generally carried out in the temperature range of 60 to 82°C (140 to 180°F) with careful control of the extruder temperature to minimize any curing and gelation from taking place In the extruder. The aforesaid extruder temperatures are lower than the typical cure temperatures of the powder coating composition that can begin initial curing at temperatures starting at 93°C (200°F). The extruded composition usually in sheet form, after cooling, is ground in a mill, such as a Brinkman mill or a Bantam hammer mill, to achieve the desired particle size. Particulate size also determines the coarseness or fineness of the texture of the hard set film coating that is produced. Generally, the particle size ranges from 250 µm or 60 mesh (for coarse) to 74 µm or 200 mesh (for fine), depending upon the desired texture. The thermosetting powder coating compositions of the present invention have very low cure temperature properties or very rapid cure rates. These properties provide a powder coating composition that can be readily applied, especially by electrostatically spraying, to heat sensitive materials, particularly wood products, while limiting the substrate heat exposure so as to not cause damage to the substrate. The cure time/temperature range of the thermosetting powder coatings of this invention range from about 30 seconds at 177°C (350° F) peak substrate temperature down to about 20 minutes at 107°C (225°F) peak substrate temperature. Cure times anywhere from about a few seconds to about 30 minutes can be considered normal or acceptable oven dwell times for electrostatic spray lines. Furthermore, at these cure conditions, no significant outgassing is noticed from the wood substrate that could degrade the substrate integrity as well as form large noticeable craters and popping defects in the continuous hard cured textured finish. This film continuity is especially surprising as pinholes are much larger than the variations in the surface profile of the textured coating. It is also surprising that a smooth fine texture can be formed on unprimed surfaces, such as wooden particle board, without any evidence of outgassing. In addition, the high viscosity and low melt flow of these compositions that are provided for texturing allows the cured powder coating to uniformly cover and hide not only the face of the wood substrate, but also unexpectedly the edges of wood substrates, which are highly porous and, thus, most difficult to uniformly coat in an electrostatic spray coating process. The preferred method used to apply the low temperature cure thermosetting powder coating compositions on the heat sensitive substrates is by electrostatic spraying. The preferred heat sensitive substrates to be coated are wood substrates. The method of the present invention accordingly will be discussed hereinafter with reference to electrostatic spraying methods on wood substrates. However, it should be understood that other fusion coating methods can be used and other heat sensitive substrates, such as plastic, paper, and cardboard, and composites and components with heat sensitive aspects, can be coated with the powder coatings of the present invention. Likewise, heat resistant substrates may be coated with the powder coatings of the present invention, such as metal, steel and other alloys, including building panels, rebars, pipelines, cold coil springs, and steel strapping, glass, ceramic, including ceramic tiles, carbon and graphite. Electrostatic spraying of powder coatings is based on the principle of electrostatic charging. In electrostatic spraying, the powder particles receive charges by one of the two following methods. In the corona method, the powder coating particles are passed in a carrier gas stream through a corona discharge in a corona spray gun, and charge is transferred from the ionized discharged air molecules to the powder particles, whereby the powder particles become electrostatically charged. In the triboelectric method, use is made of the principle of frictional electricity. The powder particles rub against a friction surface of polytetrafluoroethylene, eg, Teflon®, In the tribo gun and are given an electrostatic charge which is opposite In polarity to the charge of the friction surface. Reference can be made to US-A-4,747,546 (Talacko) for an example of an electrostatic spray gun which can perform both methods of powder particle charging. After charging, the powder particles are ejected as a cloud through the spray gun nozzle by virtue of their charge and output carrier gas pressure to the vicinity of a grounded target substrate. The charged spray particles are attracted to the grounded substrate by virtue of the difference in their respective charges. This causes the particles to deposit as a uniform coating on the desired substrate, covering the entire substrate including faces and edges. The charged powder adheres to the substrate for a period generally sufficient to permit conveying the coated article to an oven. A subsequent bake, or curing, process in the oven transforms the powder into a uniform, continuous coating having the desired characteristics bound to the substrate. In the method of the present invention, a heat sensitive substrate to be coated, preferably a wood article, is supported and moved on a grounded conveyor line extending through a spray station of an electrostatic spray coating apparatus. Either flat line or vertical line electrostatic spray equipment may be used, although a flat line is preferred. In a flat line apparatus, the conveyor belt can be impregnated with conductive metal strips or entirely covered with a metal sheet. Since the wood substrate is relatively non-conductive, typically only having about 3-10 wt. % free water for conductivity, the conductive metal placed behind the article more readily attracts the charged particles and rapidly discharges them to ground. This avoids an appreciable rise in the surface potential of the wood, thereby eliminating back ionization and allowing the particles to provide a uniform and thick powder coating on the wood. Electrostatic spray guns, either corona or tribo guns, are arranged over the wood substrate traveling through the spray station for coating. In the method of the present invention, tribo guns are preferred. The application of the powder coating by triboelectric guns is especially preferred where the wood substrate to be coated has grooves and ridges, since the charge picked up by friction is much smaller than that picked up by the corona discharge, thereby reducing the Faraday effect. A plurality of tribo guns are situated adjacent one another and in one or more tiers. The powder is forced into the guns under about 276 kPa (40 psi) pressure and air at about 138 kPa (20 psi) pressure is passed into the powder conduits just before the powder passes into the spray nozzles. As the substrate travels into the spray station, the aforesaid low temperature cure thermosetting powder coating compositions of the present invention are electrostatically spray coated onto the wood. In a flat line, the one exposed face of the wood and the edges are coated with powder in a single pass. A second pass after curing is required to cover the other face. The line speed through the spray station in a flat line can be from 1.52 to 61 metres (5 to 200 feet) per minute, preferably 6.1 to 30.5 metres (20 to 100 feet) per minute, most preferably from 9.14 to 15.24 metres (30 to 50 feet) per minute. A flat line triboelectrostatic spray apparatus sold by Nordson Corporation can be used. Reference can be made to U.S. Pats. 4,498,913; 4,590,884; 4,723,505; 4,871,380; 4,910,047; and, 5,018,909, all assigned to Nordson Corporation, for examples of flat line electrostatic spray equipment. The substrate may also be pre-heated above ambient temperature or unheated prior to electrostatic deposition. With a cold sprayed substrate, very high intensity heating is required to melt the coating and then cure it upon the wood. The continuous exposure to high intensity heat tends to deteriorate the substrate. If the substrate is pre-heated and sprayed, however, then the melting and curing process occurs much faster, thereby enabling faster cure in the baking cycle. This also eliminates continuous exposure of the wood to high intensity heat, since in the electrostatic spraying step, the substrate is not exposed to heat. Preferably, the board is pre-heated to temperatures in the range between 66 and 149°C (150 and 300°F), more preferably between 82 and 121°C (180 and 250°F). After the substrate is electrostatically spray coated, it is conveyed while bearing the powder to an oven for melting, film formation, and curing to form a heat set hard textured finish on the wood article. The powder coatings of this invention are preferably cured on the wood article along the cure time/temperature range of from about 30 seconds at 177°C (350°F) peak substrate temperature (flat line) down to about 20 minutes at 107°C (225°F) peak substrate temperature (vertical line). The curing ovens used preferably have several heating zones, some of which are heated by infrared lamps, others by heat convention, and still others by a combination of these two. An oven sold under the tradename Triad Speedoven by Thermal Innovations Corporation can be used for this purpose. In a flat line where conveyor speed is important and rapid cures are desired, the preferable oven residence time is less than 2 minutes to achieve the desired cure. In a vertical line, where oven residence time can vary, the powder coatings can be cured in accordance with the entire cure time/temperature range. In the method of the present invention, the dry thickness of the cure continuous grainy textured film ranges from 25.4 to 203 µm (1 to 8 mils) thick, preferably 50.8 to 152 µm (2 to 6 mils) thick. The heat sensitive wood substrates electrostatically spray coated in this invention include, without limitation, hardwood, particle board, medium density fiber board (MDF), electrically conductive particle board (ECP) with conductive carbon black particles incorporated throughout the board, masonite board, or any other wood products. ECP is sold by Boise Cascade Corporation. Wood substrates having a moisture content of between 3 and 10 wt. % are suitable for this invention. The wood substrates can also be pretreated with conductive coatings as are well known In the art to enhance electrostatic sprayability. The coated wood substrates are generally used to make articles having flat or profiled surfaces, such as computer furniture, business furniture, ready to assemble furniture, kitchen cabinets, and other wood products. The invention will be further clarified by a consideration of specific examples which are intended to be purely exemplary of the invention. All parts and percentages specified herein are by weight unless otherwise stated. Example 1 Black Coarse Textured Epoxy Powder Coating Composition A black coarse grainy textured thermosetting powder coating composition was prepared in accordance with the present invention by initially blending the following ingredients listed in Table 1. Table 1 Ingredient Phr Material Use Araldite GT 7072 Epoxy Resin 100 Bisphenol A/ Epichlorohydrin Type 2 Epoxy Epoxy Resin Epon Curing Agent P-101 2.0 2-Methyl imidazole Adduct (33 wt. % Imidazole Residue) Curing Agent Resiflow P-67 1.4 Acrylic Resin How Additive Benzoin 0.8 2-Hydroxy-1,2-DiphenylEthanone Flow Additive Bentone 38 10 Organophilic Clay Texturing Agent Raven 22 2.5 Carbon Black Pigment Calcium Carbonate 80 Calcium Carbonate Extender The ingredients were then melt blended in an extruder at temperatures below 82°C (180°F). The extruded material was mixed with from 0.1 to 0.3 wt. % based on the weight of the extruded material, particularly about 0.2% of dry flow additive of Aluminum Oxide C aluminum oxide and then ground into powder. The powder particles were sieved using 149µm (100 mesh) screen and the coarser particles were discarded. The powder particles were then electrostatically sprayed with a corona discharge gun onto both metal and wood panels and cured in an oven at about 107°C (225°F) peak substrate temperature for about 20 minutes. Gel time and hot plate melt flow were tested on the powder coating. Direct impact pencil hardness, flexibility, MEK resistance and gloss were tested on the metal panels and MEK resistance on the wood panels. The coating thickness was 50.8 to 30.5µm (2 to 3 mils). The resulting properties are given in Table 2. Table 2 Property Result Gel Time at 204°C (400°F) (sec) 15 Hot Plate Melt Flow at 191°C (375°F) (mm) 13 Direct Impact (in-lbs) Nm (160) 18 Reverse Impact (in-lbs) Nm (160) 18 Mandrel Flexibility Pass 4-8mm (3/16") MEK (50 double rubs) Slight Rub Off MEK (on wood) Slight Rub Off 60° Gloss 0.8-1.5 Pencil Hardness (gouge) H Texture Coarse Example 2 Black Fine Textured Epoxy Powder Coating Composition A black fine grainy textured thermosetting powder coating composition was prepared in accordance with the present invention by initially blending the following ingredients listed in Table 3. Table 3 Ingredient Phr Material Use Araldite GT 7072 Epoxy Resin 100 Bisphenol A/ Epichlorohydrin Type 2 Epoxy Epoxy Resin Epon Curing Agent P-101 3.0 2-Methyl Imidazole Adduct (33 wt. % Imidazole Residue) Curing Agent Resiflow P-67 1.4 Acrylic Resin Flow Additive Benzoin 0.8 2-Hydroxy-1,2-DipnenylEthanone Flow Additive Bentone 38 10 Organophilic Clay Texturing Agent Polyethylene Grade 6A 2.0 Polyethylene Wax Raven 1255 3.0 Carbon Black Pigment Calcium Carbonate 30 Calcium Carbonate Extender The ingredients were then melt blended in an extruder at temperatures below 82°C (180°F). The extruded material was mixed with about 0.2 % dry flow additive of Aluminum Oxide C aluminum oxide and then ground Into powder. The powder particles were sieved using 74µm (200 mesh) screen and the coarser particles were discarded. The powder particles were then electrostatically sprayed with a corona discharge gun onto metal panels and cured In an oven at about 107°C (225°F) peak substrate temperature for about 20 minutes. Gel time and hot plate melt flow were tested on the powder coating. Direct impact pencil hardness, flexibility, MEK resistance and gloss were tested on the metal panels. The coating thickness was 51 to 76µm (2 to 3 mils) thick. The resulting properties are given in Table 4. Table 4 Property Result Gel Time at 204°C. 400°F (sec) 10 Hot Plate Melt Flow at 191°C 375°F (mm) 13 Direct impact (in-lbs) Nm (160) 18 Mandrel Flexibility Pass 3.175mm (1/8") MEK (50 double rubs) No Rub Off 60° Gloss 4.8 Pencil Hardness (gouge) 2H Texture Fine Example 3 Triboelectric Spraying Of Black Fine Textured Epoxy Powder Coating Composition On Wooden Particle Board The black fine textured thermosetting powder coating composition of Example 2 was trioboelectrically sprayed with tribo guns on a particle board wood substrate using a Nordson Corporation flat line electrostatic spray apparatus in accordance with the present invention under the following conditions listed in Table 5. Table 5 Electrostatic Coating Processing Conditions Type of Substrate Particle Board Substrate Temperature Room Temperature (≈24°C, 75°F) Line Type Flat Line Costing Line Speed 9.14 m/min (30ft/min) Type of Electrostatic Charging Triboelectric Air Flow Pressure 276 kPa (40Psi) Atomization Pressure 138 kPa (20Psi) Measured Powder Charge 2-3 µAmps Curing Line Speed 1.524m/min (5ft/min) Type of Substrate Particle Board Oven Size 3.66m (12 ft) long (using only first 1.83m 6 ft or 3 IR zones) Peak Subsuate Temperature 177°C (350°F) Maximum Time At Peak Temperature 30 sec Oven Exit Substrate Temperature 99°C (210°F) MEK (50 double rubs) Slight-Moderate Rub Off Example 4 Triboelectric Spraying Of Black Fine Textured Epoxy Powder Coating Composition On Wooden Electrically Conductive Particle Board The black fine textured thermosetting powder coating composition of Example 2 was trioboelectrically sprayed with tribo guns on electrically conductive particle board wood substrate using a Nordson Corporation flat line electrostatic spray apparatus in accordance with the present invention under the following conditions listed in Table 6. Table 6 Electrostatic Coating Processing Conditions Type of Substrate ECP Board Substrate Temperature Pre-Heated Pre-Heat Line Speed 1.524m/min (5ft/min) Pre-Heat Oven Exit Substrate Temperature 103°C (218°F) Substrate Temperature Prior to Coating 77°C (170° F) Line Type Flat Line Coating Line Speed 30.5m/min (100ft/min) Type of Electrostatic Charging Triboelectric Air Flow Pressure 276 kPa (40 Psi) Atomization Pressure 138 kPa (20 Psi) Measured Powder Charge 2-3 µAmps curing Line speed 305 cm/min (10ft/min) Oven Size 305 cm (12 ft) long (using only first 152 cm (6 ft) or 3 IR zones) Oven Exit Substrate Temperature 99°C (210°F) MEK (50 double rubs) Moderate Rub Off Example 5 Corona Discharge Spraying Of Black Fine Textured Epoxy Powder Coating Composition On Wooden Particle Board The black fine textured thermosetting powder coating composition of Example 2 was electrostatically sprayed with corona guns on a particle board wood substrate using a vertical line electrostatic spray apparatus in accordance with the present invention under the following conditions listed in Table 7. Table 7 Electrostatic Coating Processing Conditions Type of Substrate Particle Board Substrate Temperature Pre-Heated Pre-Heat Oven Exit Substrate Temperature (210°F) 99°C Substrate Temperature Prior to Coating (210°F) 99°C Line Type Vertical Line Type of Electrostatic Charging Corona Discharge Measured Powder Charge 60 KV Curing Temperature (225°F)107°C Curing Time 20 min Teak Substrate Temperature (225°F) 107°C Maximum Time At Peak Temperature 20 min MEK (50 double rubs) Slight Rub Off Exemples 6 to 8 White Textured Epoxy Powder Coating Compositions Three white grainy textured thermosetting powder coating compositions were prepared in accordance with the alternative embodiment of the present invention by initially blending the following ingredients listed in Table 10. Table 10 Phr Ingredient Ex.6 Ex.7 Ex.8 Material Use Araldite GT 7072 70 70 70 Bisphenol A/ Epichlorohydrin Type 2 Epoxy Epoxy Resin Ancamine 2014AS 30 30 30 Epoxy and Polyamine Adduct Low Temp. Curing Agent Epon P-101 ---- 2.0 ---- Epoxy and 2-Methyl Imidazole Adduct (33 wt. % Imidazole Residue) Catalytic Curing Agent Dyhard 100S ---- ---- 5.0 Dicyandiamide Curing Agent Resiflow P-67 1.4 1.4 1.4 Acrylic Resin Flow Additive Benzoin 0.8 0.8 0.8 2-Hydroxy-1,2-DiphenylEthanone Flow Additive Bentone 38 2.0 2.0 2.0 Organophilic Clay Texturing Agent TiPure R902 30 30 30 Titanium Dioxide Pigment The aforesaid ingredients were then melt blended in an extruder at temperatures below 82°C (180°F). The extruded material was mixed with about 0.2 % dry flow additive of Aluminum Oxide C aluminum oxide and then ground into powder. The powder particles were sieved using 74µm (200 mesh) screen and the coarser particles were discarded. The powder particles were electrostatically sprayed with a corona discharge gun onto metal panels and then cured in an oven at about 107°C (225° F) peak substrate temperature for about 20 minutes. Gel time and hot plate melt flow were tested on the coated metal panels. The coating thickness was 51 to 76µm (2 to 3 mils) thick. The powder particles were also electrostatically sprayed with corona discharge onto preheated medium density fiber boards that were preheated to 121 to 132°C (250 to 270° F) peak substrate temperature for about 2 minutes and then cured in an infrared oven at about 149°C (300°F) peak substrate temperature for about 40 seconds. MEK resistance and texture were evaluated on the coated wood board. The resulting properties are given in Table 11. Table 11 Property Result Ex. 6 Ex. 7 Ex. 8 Gel Time at 204°C, (400°F) (sec) 11 8 9 Hot Plate Melt Flow at 191°C (375°F) (mm) 18 16 13 MEK (on wood) Moderate Rub Off Slight Rub Off No Rub Off

Claims: A method of coating a heat-sensitive substrate to provide a grainy texture film finish without damaging said substrate, which comprises the steps of: 1) applying to the substrate a thermosetting powder coating composition comprising (a) an epoxy resin; (b) at least one curing component selected from (i) 1 to 8 parts per hundred parts of resin (phr) of a catalytic curing agent comprising an imidazole, an adduct of an imidazole with an epoxy resin or a mixture thereof; and (ii) 5 to 40 phr of a low temperature curing agent comprising an adduct of a polyamine and an epoxy resin; (c) 1 to 20 parts per 100 parts of resin of a texturing agent selected from organophilic clays and (d) a flow control agent, 2) heating the powder-coated substrate to melt and flow and 3) curing said powder coating into a grainy textured dry film on the substrate at a peak substrate temperature of not more than 177°C (350°F), the said imidazole, imidazole adduct and/or amine adduct serving the duel function of curing agent and cure accelerator. A method according to claim 1 in which said epoxy resin (a) is selected from bisphenol A type epoxy resins having epoxy equivalent weights of from 550 to 1,100, or mixtures thereof. A method according to claim 1 or claim 2 in which said catalytic curing agent b(i) comprises 2-methylimidazole. A method according to claim 1 or claim 2 in which said catalytic curing agent b(i) is an imidazole adduct of an epoxy resin comprising an adduct of 2-methylimidazole and a bisphenol A type epoxy resin. A method according to any preceding claim wherein the powder coating composition comprises: from 1 to 5 phr of said flow control agent (d). A method according to any preceding claim wherein the powder coating composition comprises: from 5 to 40 phr of said low temperature curing agent b(ii); and from 1 to 6 phr of said catalytic curing agent b(i). A method according to any preceding claim wherein the powder coating composition further comprises: (e) a secondary curing agent of dicyandiamide. A method according to claim 7 wherein the powder coating composition further comprises between from 2 to 8 phr of dicyandiamide. A method according to any preceding claim in which said low temperature curing agent b(ii) comprises an adduct of a primary or secondary polyamine and a bisphenol A type epoxy resin. A method according to any preceding claim wherein said curing component (b) comprises both said catalytic curing agent (I) and said low temperature curing agent (ii). A method according to any preceding claim wherein the powder coating composition further comprises: (f) up to 100 phr of a pigment; and (g) up to 120 phr of an extender. A method according to any preceding claim wherein the cure time ranges from about 30 seconds at 177°C (350°F) peak substrate temperature down to about 20 minutes at 107°C (225°F) peak substrate temperature. A method according to any preceding claim in which said heat sensitive substrate is composed of wood. A method according to claim 13 in which said wood substrate is selected from hardwood, particle board, masonite board, electrically conductive particle board, or medium density fiber board. A method according to any preceding claim in which said powder coating is applied in step (a) electrostatically.

Inventors: CORRELL GLENN D, DALY ANDREW T, HALEY RICHARD P, HORINKA PAUL R, KOZLOWSKI JOSEPH J, MUTHIAH JENO, CORRELL, GLENN D., DALY, ANDREW T., HALEY, RICHARD P., HORINKA, PAUL R., KOZLOWSKI, JOSEPH J., MUTHIAH, JENO
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1997
Main classification: C09D5
IPC codes: C08G59, C09D163, C09D5