Field of the Invention

The present invention relates to methods of coating surfaces.

Background of the Invention Polymeric powdered coating compositions, such as certain paint compositions, are well-known [see, e.g., US Patent No. 5,470,912] and are used almost exclusively in industrial plants because their application requires large equipment, including conveyors, temperature controlled baking ovens and application booths. For example, acrylic clearcoat resins are usually applied on parts to be coated as a loose powder, after which the article is placed in a bake oven. These ovens bake the articles at temperatures between 100 °C and 200 °C for 20 to 30 minutes to melt the resin, eliminate bubbles and harden the resin by curing. Some powdered coatings can be applied using flame guns, for example thermoplastic powders. [See, e.g., US Patent No. 3,953,403; 4,999,225; 5,418,309 and 4,388,353]. There remains a need in the art for improved methods of applying powdered paints and other coatings and films to surfaces, particularly with respect to crosslinkable systems as provided for herein.

It has been found in accordance with the present invention that thermocurable compositions can be applied directly with a hot gaseous stream which enables coating of heat-sensitive materials and

provides a faster (e.g., one-step) coating process for metals and other materials.

In one aspect, the present invention provides a method of applying a meltable, powdered, thermocurable polymer composition to a surface in a substantially single application step. The method involves providing a polymer composition to a hot gaseous stream in a spraying apparatus, in which the temperature and velocity of the gas stream are sufficient to melt the composition and maintain flowability thereof while the composition is in the gas stream. The heated polymer composition is melt-sprayed onto the surface substantially simultaneously with entering the gas stream and, upon contact with the surface, is at least partially cured thereon. Advantageously, substantial curing occurs in the absence of a subsequent curing, e.g., oven baking, step.

In another aspect, the method of the invention permits application of a meltable, powdered, radiation-curable polymer composition to a surface. The method involves providing such a polymer composition to a hot gaseous stream in a spraying apparatus, in which the termperature and velocity of the gaseous stream are sufficient to melt the composition and maintain flowability thereof while the composition is in the gaseous stream. The heated polymer composition is sprayed onto the surface substantially simultaneously with entering the gaseous stream and, upon contact with the surface, forms a substantially uniform liquefied film thereon. By the term

"liquid", it is meant herein that the resin is at least partially melted. The coated surface is subsequently exposed to ultraviolet radiation or an electron beam, which acts to substantially cure the polymer

composition simultaneously with, or immediately subsequent to, said spraying. The radiation is applied while the film is at least in a partially liquefied state.

Brief Description of the Drawing

Fig. 1 is an illustration of a spray gun suitable for use in the method of the invention.

Detailed Description of the Invention The present invention includes a process for coating plastic material onto, for example, a flat sheet substrate such as rolled goods by way of introducing the powdered material into a hot gaseous stream, entraining the powdered material in the gas stream, the gas stream and material being heated to liquefy the powdered material, followed by directing the entrained liquefied, powdered material onto the substrate by directing the gas stream toward the substrate, coating the substrate with the material, and setting the material on the substrate as a solid phase adhered to the substrate. Such coating onto rolled goods is sometimes referred to as coil coating. The powdered material in general is a cross-linkable polymeric system which typically includes a polyester polymer, an epoxy polymer, a polyurethane polymer, an acrylic polymer or mixtures thereof. Most preferably, the step of setting the material includes crosslinking the polymeric system substantially contemporaneously with the step of coating the substrate.

The method of the present invention provides for the application of powdered polymers, specifically meltable, powdered polymer compositions, including low molecular weight polymers, via a

spray gun with substantially complete curing, without a separate curing/baking step. This substantially one-step method provides important advantages for the coating or painting of a variety of substrates.

A. The Polymer Composition

Any meltable or curable polymer composition which is a cross-linkable system in the form of a powder is useful in the method of the invention. The composition of the polymer is not a limitation on the present invention. However, the method of the invention is particularly well suited for use with a thermosettable polymer typically used in powder form. Suitable thermosettable polymer compositions include thermosetting paint polymers including, for example, polyester polymers, epoxy polymers, polyurethane polymers, acrylic polymers, and mixtures thereof.

Illustrative of such resins are conventional crosslinkable or curable polymeric powder coating resin. Such coating resins which may be used and are presently known to the art and include, but are not limited to: epoxy coating resins, polyester coating resins, acrylic coating resins and the like. Conventional polymeric powder coatings may also include polyisocyanate crosslinked polyester coating resins, triglycidyl isocyanurate crosslinked polyester coating resins, polyester epoxy "hybrid"-type coating resins, triglycidyl isocyanurate crosslinked acrylic-type coating resins, polyisocyanate crosslinked acrylic-type coating resins, hydroxy alkylamine crosslinked polyester coating resins, hydroxy alkylamide crosslinked acrylic coating resins, amine crosslinked epoxy coating materials, anhydride crosslinked epoxy coating resins, tetramethoxymethyl glucoluril crosslinked

acrylics or polyesters, and uretidione crosslinked polyester-type coating resins. The resins may be either aromatic or aliphatic, as well as either blocked or unblocked materials, and for example, may include polyisocyanate-type materials blocked with caprolactam or methyl ethyl ketoxime.

The composition of this invention may also include various other constituents which are used in polymeric powder coating compositions. Further constituents which may be useful in the improved polymer coating compositions according to the present invention include catalyzing agents, flow control agents, coloring agents such as pigments or dyes, fillers, processing aids such as silica, catalysts, matting agents, and other conventional processing aids and additives known to those of ordinary skill in the art.

Conventional organic and inorganic pigments may be included in the compositions useful in the present invention. Suitable pigments include, without limitation, carbon black, ultra marine blue, dyes based on phthalocyanine, titanium dioxide, cadmium sulfide, cadmium sulfide selenide, nigrosine and the like.

Conventional catalyzing agents known to the art and sometimes referred to as "accelerators", may also be included in the polymer compositions. These agents can improve the cross-linking and/or curing of the polymeric powder coating resin material. Generally, these catalyzing agents lower the required temperature in applications wherein the coating composition is cross linked or cured by exposure to heat and/or reduce the time interval at an elevated temperature at which the coated article needs be exposed. Such conventional catalysts include, but are not limited to, stannous octoate, dibutyl tin dilaureate, dibutyl tin diacetate and the like.

Conventional materials which are used to control the surface appearance of the polymeric coating include matting agents which limit the surface gloss. Such matting agent materials include waxes, silicas, polytetrafluoroethylene, as well as other conventional materials not particularly denoted here. Processing aids which improve the process ability, and in the formation of the polymeric coating may be included in the compositions. For example, silica or benzoin are known to the art as processing aids.

The amounts of the above-identified conventional additives to the polymer compositions can be amounts which are conventional for such compositions. The selection and amounts of such agents are well within the skill of this art and do not limit this invention.

Suitable thermocurable polymer compositions for use in the method of the invention may be readily selected by one of skill in the art. When the powdered polymer is applied in a molten condition according to this invention, it adheres to any surface and it is not necessary that the composition be electrostatic or tribochargeable.

A particularly suitable polymer composition is defined herein as a thermocurable composition containing at least one base resin and either a second cross-linking base resin or a cross-linking agent. For example, the polymer composition may have a base resin selected from among carboxy-terminated polyester resins, hydroxyl-functional polyester resins, and glycidyl-functional acrylic resins. In a currently preferred embodiment, the cross-linking agent is a caprolactam- blocked isophorone diisocyanate crosslinker. In another embodiment, the composition may contain a second base resin such as a bisphenol A/epichlorohydrin epoxy resin.

Suitable UV-curable powder coating materials, including epoxy acrylates, urethane acrylates, polyester acrylates, unsaturated polyesters, diluted with styrene or with low-viscosity acrylates are likewise commercially available. Other suitable polymers for use in the method of the invention may be readily selected by one of skill in the art. See, also,

C. H. Hare (ed), "Protective Coatings", Technology Publ. Co., Pittsburgh, PA (1 994) for conventional information on polymer compositions. When the powdered polymer is applied in a molten condition according to this invention, it adheres to any surface and it is not necessary that the composition be electrostatic or tribochargeable as noted above in connection with thermocurable resins.

The photocurable or radiation curable compositions useful in this invention may also include various other constituents, which may be selected by one of skill in the art as noted above. Generally, for example, if hydroxy ethyl acrylate monomer is reacted with an aliphatic poly- or diisocyanate, a relatively low viscosity urethane acrylic prepolymer is formed. Typical monomers used in this way are hexane diol diacrylate and trimethylol propane triacrylate. These monomers may be dissolved in one or more di- or polyfunctional acrylic monomers. This gives a system that, when dosed with a suitable photo initiator, may be converted at room temperature to hard, flexible, chemical-resistant films. Photo initiators are materials such as the aromatic ketone benzophenone. The molecule dissociates to provide a source of free radicals on exposure to high intensity ultraviolet radiation (usually 325-365 nanometers). Polymerization takes place via acrylic unsaturation and involves groups on both the prepolymer and the multi¬ functional monomers used as solvents.

Mono-functional monomers may also be added. They are lower in viscosity than are the multi-functional monomers. In controlled quantities, mono-functional monomers are useful for viscosity adjustment and control of cross link density. The acrylics are somewhat more reactive to ultraviolet light than are the methacrylics, which are, in turn, more reactive than the allyl and vinyl type monomers. Prepolymers are not confined to the urethane acrylics; epoxy acrylics are also available. These prepolymers are formed by the reaction of the oxirane linkages of low molecular weight bisphenol A resins with acrylic acid. Polyester acrylic prepolymers (prepared via the reaction of acrylic acid and hydroxy functional polyether and polyester resins) have also been employed in this way. In all of these systems, careful inhibition of the acrylic (at both the prepolymer stage and in the final coating) is vital to prevent unwanted polymerization. In general, compositions useful in connection with the present invention exhibit melting temperatures, that is become liquid at temperatures of 100°C or less. The gas temperature of the hot gaseous stream is thus typically 200°C or thereabouts as described below.

B. The Method of the Invention

According to the present invention, a conventional meltable, thermocurable polymer powder is introduced into a spraying device, such as a flame gun, plasma gun, hot air gun or hot gas gun. The spraying device is not critical so long as it is capable of melt-spraying the liquefied or powdered polymer to a surface via use of a hot gaseous stream. In many cases, a hot air gun may be preferred. Thus, the gaseous stream is desirably at a temperature at least twice that of the melting temperature of the polymer composition in degrees

Celsius (°C) . Preferably, the temperature of the stream is between about 100°C to about 600°C.

The velocity of the hot gas stream is such that it propels the polymer composition onto the desired surface or substrate. Desirably, the conditions of temperature and velocity in the hot gas stream are sufficient to melt the polymer composition and maintain its flowability while the polymer is in the stream. The heated polymer composition is sprayed onto any desired surface or substrate substantially simultaneously with entering the gas stream. Desirably, the surface is exposed to the spraying source for between about 0.1 to about 120 seconds, and more preferably between about 10 - 100 seconds. Preferably, the distance between the spraying apparatus and the surface is maintained at from about 1 to about 25 cm, and preferably from about 1 to about 10 cm. However, the exposure time and distance between the spraying source and the coated surface may be readily adjusted by one of skill in the art.

Preferably, the gas stream heats the polymer composition such that upon contact with the surface, the polymer composition is at least partially cross-iinked or cured so that it exhibits a glass transition temperature or melting temperature in degrees Celsius which is at least two times the melting temperature of the base resin forming the major component of the polymer composition. This causes the melted material to fuse and form a continuous polymeric coating on the surface. The curing (or partial curing) occurs in the absence of further heating and as a result of the heating effect of the hot gas stream on the polymer composition. Advantageously,

coatings applied according to the method of the invention do not require oven curing.

However, in another embodiment, the surface on which the powdered polymer composition is melt-sprayed with the gun may be preheated and the composition cured during or after said application.

In another embodiment the surface spray coated by the method of the invention may be subsequently subjected to infrared (IR) rays to heat the coating. This step may substitute for the use of a dryer, or be used in conjunction therewith. Suitable IR-beamers are commercially available. Currently, medium wave IR rays are preferred, which heat the substrate more quickly. Desirably, the substrate is exposed to the IR radiation source for between about 0 to about 1 20 seconds after being coated with the composition, and more preferably between about 10 - 100 seconds. Preferably, the distance between the IR source and the surface is maintained at about 1 to about 24 cm. However, the exposure time and distance between the IR source and the coated surface may be readily adjusted by one of skill in the art. However, this radiation step may be performed while the film coating is still hot. This separate drying step may be the only drying step applied to the coated surface, or it may be used to achieve further hardening of the polymer coating partially cured following coating by the method of the invention.

C. The Substrate Material Substrates specifically contemplated within the present invention include substrates such as polymer film or sheet substrates. Other substrates are further contemplated as hereinafter described.

D. The Spraying Apparatus

Suitable spray guns for use in this invention may be readily adapted from among the flame, plasma combustion (inert or spiked), high velocity oxygen fuel (hot air) and detonation guns known in the art. For example, suitable spray guns are commercially available from

Plastic Flamecoat ^® Systems (Big Spring, TX), Alamo Supply Company, Inc. (Houston, TX), UTP Welding Materials, Inc. (Houston, TX) . These commercial flame spray guns may be readily modified for use in the method of the invention by lowering the flame temperature to meet the requirements of the present invention, i.e., to provide a gaseous stream at a suitable temperature range, that allows the polymer to be applied in powdered or liquefied form.

For example, the gaseous stream may be at a temperature about twice that of the melting temperature of the polymer in degrees Celsius (°C), or between about 100°C to about 600°C so that the polymer can be applied in liquified form.

Fig. 1 provides a schematic of a suitable spray system. A suitable spray gun for use in the method of the invention permits the powdered polymer to be projected in powder (or liquefied) form from an adjustable close distance and to be deposited on a surface in a molten or powdered condition. For example, a spray gun of the invention is preferably a hot air generator or convector capable of delivering hot air with variable temperature and output. Preferred velocity of the gaseous stream can be between about 30 to about 3000 mph, and preferably between 60 to 600 mph. Feed rates of the polymer composition into and through the spray gun can be typically about 1 to about 60 lbs/hour, depending on nozzle size and gas velocity. Such a gun may further have a separate or combined spray

and dryer nozzle so that the spray and drying features may be used contemporaneously or alternately as desired. The distance from the output nozzle of the gun from the substrate may be between about 1 to about 40 cm. In a preferred embodiment, these features are combined, permitting simultaneous spraying of the heated polymer composition onto the surface so that, upon contact with the surface, the polymer composition is at least partially cured. This curing takes place to a glass transition temperature or melting temperature (°C) which is at least two times that of the base resin which forms the composition.

In contrast to the coating methods and oven curing techniques of the prior art, the equipment used in the method of the invention has a reduced size and thus greater flexibility, e.g., it may be a single unit or a set of units, movable or fixed. Thus, the method of the invention allows the coating of static or large surfaces, or small surfaces even for retouching purposes, by applying the paint or other coating materials melted at a temperature lower than that required for cross-linking.

Example 1 - Thermal Spray Coating of Acrylic Clearcoat Composition

A. Preparation of Clearcoat

An acrylic clearcoat formulation of the following composition with a melting point of about 74°C was prepared using the components described in Table I.

Table

Synthacryl7 VSC 146 resin 743 parts by weight Additol7 VXL 1381 hardener 224 parts by weight Additol7 LH 627 agent 30 parts by weight Benzoin 3 parts by weight

Total 1000

The VSC 146 is a glycidyl-functional acrylic resin containing methacrylic acid, acrylic acid and ethyl acrylate as major components with alkyl oligoethylene glycol crotonate as a minor component. The VXL 1 381 is an anhydride hardener which is dodecaryl, dicarboxylic acid anhydride. The Additol7 LH 627 product is a flow control agent.

This formulation was mixed, extruded at a temperature of between 1 1 5°C to 120°C from a twin screw extruder and the extrudate was ground to powder. The melting point of this composition is about 74°C.

B. Application Examples

The powder polymer composition prepared as described above was injected into a spray gun and entrained in hot air at a temperature of approximately 240°C. The composition was thermally sprayed from a spray gun nozzle about 3-4 inches from the steel, wood, polycarbonate plastic sheet and ABS plastic sheet substrates. The powder was also thermally sprayed on "plaster board" used in construction of walls of buildings for about 60 seconds. The distance

of the end of the gun from the substrate surface was about 3.75 inches.

The melt-sprayed coating according to the present invention had a 53 micron thickness on steel and when tested, had less than 80 inch lb mechanical resistance in direct and reverse impact testing.

The chemical resistance was less than 100 MEK double rubs [The Powder Coating Institute, #8 Recommended Procedure for Solvent Cure Test] and the pencil hardness was 2B [ASTM Designation D3363-74 (Reapproved 1 989) Standard Test Method for Film Hardness by Pencil Test].

As a control, a similarly coated steel panel was subsequently baked at 140°C for 30 minutes. Mechanical resistance improved to greater than 1 60 inch lb direct and greater than 80 inch lb reverse, chemical resistance was better than 100 MEK double rubs and pencil hardness was > 2H.

Using the method of the invention, a 6" x 3" "Q" coupon was sprayed with the acrylic clearcoat powder prepared as described in A above using hot air as described above and subsequently heated with 490°C hot air for 7 seconds. The coating of 48 microns thickness had greater than 1 60 inch-lb mechanical resistance in direct and reverse impact testing and pencil hardness of H; chemical resistance was better than 100 MEK double rubs. This acrylic clearcoat was thus cured without baking in an oven. The cured coating was smooth and glossy.

Example 2 - Thermal Spray Coating of Hybrid Po yester-Epoxy Paint

The following experiment made use of a polymer composition having a melting point of about 60°C containing the components listed in Table II.

Table

Alftalat AN 783 polyester 39%

Epon 2002 resin 18%

Additol XL 496 resin 3% Titanium Dioxide 39.5%

Benzoin 0.5%

The Alftalat ^® AN 783 product is a carboxy-terminated (saturated carboxylated) polyester resin, the major constituents being terephthalic acid and neopentyl glycol and minor constituents being adipic acid, ethylene glycol and trimethylol propane. Epon ^® 2002 epoxy resin is a bisphenol A/epichlorohydrin, epoxy resin. Additol ^® XL 496 product is a hydroxylate polyester resin for flow control. The above composition is likewise suitable for application as described in connection with example 1 .

Example 3 - Polyester-Polyurethane Resin

A polyester-polyurethane resin having a melting point of about 63°C was formulated of the following components in Table III:

Table III

Alftalat ^® AN 745 resin 49.2%

Isocyanate Vestigon ^® B1 530 agent 7.8% Additol* XL 496 resin 3%

Titanium Dioxide 39.5%

Benzoin 0.5%

The AN 745 product is a hydroxyl functional polyester resin with major constituents being terephthalic acid and neopentyl glycol and with minor constituents being adipic acid, isophthalic acid and trimethylol propane. The Vestagon ^® B1530 product is a capprolactan- blocked isophorone diisocyanate crosslinking agent. The Additol product is as described above in Example 2.

The formulations were melt compusided in a twin-screw extruder and the extrudate was powdered. The powder was thermally sprayed, according to this invention, 8" x 4" polycarbonate and ABS plastic plates from a nozzle to substrate distance of about 2.5 inches. The air temperature in the spray gun was 240°C and the temperature of the plastic material rose to 40°C. The coatings were partially cross-linked (cured) upon contact with the coupons and plates. The metal plates were subsequently heated with 460°C air for one minute to fully cross-link the resins. The temperature of the back of the plates was 140°C to 1 50°C.

Example 4 - Thermal Spraying of UV-Cure Clearcoat Composition A. Preparation of UV Clearcoat

A UV-curable formulation of the following composition of Table I was prepared.

Table IV

Alftalat ^® VAN 1743 Resin 60-85%

Additol ^® VXL 1385 Resin 10-30% Radical Initiator 3%

Flow Control Additive 1 %

Thixotropic Agent 2%

The Alftalat ^® resin is an unsaturated polyester the major components of which are terephthalic acid, neopentyl glycol, and maleic anhydride. The Additol7 product is a polyurethane acrylate containing hydroxypropyl acrylate, isophorone diisocyanate, and 2-ethyl 2- hydroxymethyl 1 ,3 propanediol. B. Spraying of Coating

The formulation having a melting point of about 82-93°C, was thermally sprayed on wood by injection into hot air of temperature 170°C, 200°C, 220°C, 245°C, 295°C and 370°C. The powder was also thermally sprayed with 245°C hot air on metal (steel), polycarbonate and ABS plastic. In each case, the distance between the nozzle of the spray gun and the substrate surface was about two inches. The coatings sprayed on wood at hot air temperatures of 245°C, 295°C and 370°C were UV cured by passing under a UV beamer (IST-Strahlentechnik

Metz)-200-U CK with 80 W/cm, emission maximum 360 nm, with two passes. The UV cure was performed with and without preheating for 30 seconds using a Medium wave IR-beamer Heraeus MMS 2000/32 kw/m ² at 25 cm distance for 30 seconds. Subsequent testing of the coated wood panels of 8" x 3 1 /2" revealed high degree of cross linking with little or no change on exposure to butyl acetate for 3 minutes and xylene for 30 minutes, for both IR-heated and non-heated samples.

In another application, the present invention is directed to a method of providing a cured coating to an architectural substrate such as a bridge, building, house, free standing structure, walkways, objects or the like. The process steps are to entrain a powder composition into a hot gaseous stream, with a gas temperature sufficient to melt said powder composition to form a liquefied material followed by applying the stream to the substrate to form a liquefied, that is, at least partially melted, coating thereon. The liquefied coating is then cured, either by way of the application of radiation, in the case of radiation-curable powder compositions, or the coating is self-curing in the case of the thermocurable powder compositions. The curable resins used in accordance with the present invention are cured by way of cross-linking as described further herein. In general, the term curable as used herein refers to a crosslinkable material and the terms "melted", "liquid", or "liquefied" mean at least partially melted to a liquid state unless the text explicitly indicates otherwise.

The present invention has numerous advantages not before realized in connection with powder coatings. For one aspect, it is noted below, that it is possible to deposit thinner, uniform films due to the "splatting" effect. As used herein a substantially uniform film of a given thickness is one that is between 50% and 200% of that

thickness throughout. It is possible to apply substantially uniform thin films with the present invention, say less than 50 microns in thickness with films of 10-40 microns being preferred; but perhaps more pertinent to the substrates contemplated herein, it is also possible to build thicker coatings, either substantially uniform or otherwise of 50-

100 microns to 100-200 microns or perhaps 200-1000 microns in thickness if so desired.

Due to the high spray stream velocities employed, say 30- 3000 miles per hour with 60 to 600 mph being preferred and the temperatures involved, typically the hot gas stream having an average temperature at least twice the melting point of the powder composition, e.g. 1 50°C to 300°C, the liquefied powder splatters on the application surface to form a substantially uniform coating as noted above, and this leads to further advantages. For one, there is more flexibility in selecting powder resin compositions. Higher molecular weight, typically tougher and more durable resins may be selected if so desired since their higher viscosity is not as much as a limitation in forming a uniform film, as it is when one merely relies on the formation of the film by liquefaction of the powder coating in an oven. Higher molecular weight coatings, besides having enhanced toughness are typically more durable and have more corrosion resistance.

Still another advantage of the method of the present invention is that the melt-application method minimizes the need for the surface preparation since the high velocity application method promotes adhesion. Thus, a rusted metal surface could be coated without sand blasting, or a road surface could be minimally prepared. In fact, the present invention may be particularly useful where lead paint

abatement (containment) is a problem since it is possible to simply coat the surface to contain the leaded paint. So, also the method is relatively weather-insensitive and may be used under moist conditions. The present invention may also be used in connection with virtually any roadway surface to be provided with highway markings, whether it is asphalt, concrete, or even a wooden, brick or steel surface as may be found on a bridge. Advantages of the present invention include the fact that the method is relatively weather- insensitive, particularly to moisture or temperature which are often problems with conventional paint application means; as well as the fact the present invention requires virtually no curing time.

In still another aspect, the present invention is directed to a method of providing a thin film cured powder coating to a substrate. The process steps are to entrain a powder composition into a hot gaseous stream, with a gas temperature sufficient to melt powder composition to form a liquid material followed by applying the stream to the substrate to form a liquefied, that is at least partially melted, coating thereon. The liquefied coating is then cured, either by way of the application of radiation, in the case of radiation-curable powder compositions, or the coating is self-curing in the case of the thermocurable powder compositions. The curable resins used in accordance with the present invention are cured by way of cross- linking as described further herein. As used herein, the term curable refers to a crosslinkable polymeric system and the terms "melted", "liquid" or "liquefied" mean at least partially melted to the liquid state unless the text explicitly indicates otherwise.

Typically, powder coating formulations have in their unmelted state particles of approximately 20 microns or more in average diameter. It is thus extremely difficult to produce uniform coatings having thicknesses of 50 microns or less by conventional powder coating means. In accordance with the present invention the high velocities, melting of the powder formulations prior to impact with the substrate and so on introduce substantial flattening of the particles so it is possible to produce substantially uniform films of 50 microns or less; such as 40 microns or less, 25 micron or even 10 micron substantially uniform films. The application phenomenon, sometimes referred to as splatting, in connection with thermoplastic powder coatings, conserves material and also minimizes the need for surface preparation, promotes adhesion, and allows for the application of relatively high molecular weight, relatively tough, powder compositions.

The present invention is likewise suitable as a method of providing a cured coating to a temperature sensitive substrate. The process steps are to entrain a powder composition into a hot gaseous stream, with a gas temperature sufficient to melt powder composition to form a liquid material followed by applying the stream to the substrate to form a liquefied, that is at least partially melted, coating thereon. The liquefied coating is then cured, either by way of the application of radiation, in the case of radiation-curable powder compositions, or the coating is self-curing in the case of the thermocurable powder compositions. The curable resins used in accordance with the present invention are cured by way of cross- linking in any event as described further herein. In general, the term curable as used herein refers to a crosslinkable material and the terms

"melted", "liquid", or "liquefied" mean at least partially melted to a liquid state unless the text explicitly indicates otherwise.

The temperature-sensitive substrates specifically contemplated within the present invention include wooden substrates such as furniture and the like or polymer substrates. Wooden substrates may include hardwoods incorporated as part of a piece of furniture such as maple substrates, oak substrates and the like as well as veneered substrates which are generally wood/adhesive composite structures including plywood as well as decorative veneered articles. There is of course a wide variety of such substrates, all of which are temperature sensitive in the sense that they do not readily withstand temperatures of 200°C or more.

Likewise, a wide variety of polymeric materials are temperature sensitive and advantageously provided with a durable coating in accordance with the present invention. For example, linear polyesters which may be employed as a thermoplastic substrate in the form of film or sheet in practicing the present invention include polyesters of alkyl glycols and aromatic acids such as: poly(alkylene terephthalates) having the repeating unit

where X = 2-10 and preferably 2-4 and n is an integer throughout this section; copolymers including (alkylene isophthalates) having the repeating unit

where X = 2-6 and preferably 2 or 4; poly(alkylene 4,4'bibenzoates) having the repeating unit

where X = 2-10 with X = 2-6 being preferred; poly(alkylene 2,6 naphthalene-dicarboxylates) having the repeating unit

where X = 2-10 and preferably 2-4; poly(alkylene sulfonyl-4,4'-dibenzoates having the repeating unit

where X = 2-10, preferably 2-6; poly(p-phenylene alkylene dicarboxylates) having the repeating unit

where X = 1 -8 and preferably 1 -4;

Poly(p-xylylene aklylene dicarboxylates) having the repeating unit

where X = 1 -10 and preferably 2; as well as Poly(p-phenylene dialkylene terephthalates) having the repeating unit

where X = 1 -5 and preferably 1 , 2 and 4.

As will be appreciated by those of skill in the art, the foregoing list is by no means exhaustive and it is sometimes desired to employ terepolymers and linear polyesters with even more monomers. Particularly desirable terepolymers might include poly(alkylene terephthalate-co-4,4'-bibenzoate), and poly(alkylene 4,4'-bibenzoate- co-2,6-naphthalene dicarboxylates) . These polymers are disclosed in United States Patent Nos. 3,008,934, 4,082,731 and 5,453,321 as well as European Application No. 0 202 631 .

The list of specific polymers within other classes of thermoplastic polymers, such as polyolefins, polyurethanes, polγacetals and so on would be similarly extensive yet not exhaustive.

Other classes of temperature-sensitive polymer substrates where the present invention is particularly useful includes rubbers and elastomers. Such elastomers may be thermosetting or thermoplastic and include natural rubber based materials. Thermoplastic elastomers which may be suitable as substrate components include block copolymers as noted in Table A below.

TABLE A. THERMOPLASTIC BLOCK COPOLYMERS

Soft or elastomeric Typical

Hard segment, A segment, B Structure polystyrene polybutadiene, A— B A polyisoprene

poly( α -methylstyrene) polybutadiene, A— B—A polyisoprene

polystyrene poly(ethylene-cø-butylene) A— B— A

polyethylene poly(ethylene-co-butylene) A— B— A

polystyrene polydimethylsiloxane A— B— A

poly( a -methylstyrene) polydimethylsiloxane A— B— A and (A—

polysulfone polydimethylsiloxane (A-B) _n

poly(silphenylene siloxane) polydimethylsiloxane (A-B) _n

polyurethane polyester or polyether (A B) _n

polyester polyether (A-B) _n

polycarbonate polydimethylsiloxane (A--B) _n

polycarbonate polyether (A-B) _n

The three commercially important elastomer block copolymers are poly(styrene-elastomer-styrene), thermoplastic polyurethanes, and thermoplastic polyesters.

Particularly preferred commercially available block copolymer thermoplastic elastomers appear in Table B below.

TABLE B. TRADE NAMES AND MANUFACTURERS OF THERMOPLASTIC ELASTOMERS

Trade Name Manufacturer Type Hard Soft segment segment

Kraton D Shell Chemical triblock (S-B-S or S-I-S) S B or l Co.

Solprene 400 Phillips Petroleum branched (S-B)„ (S-I)„ S B or l Co.

Stereon Firestone Co. triblock (S-B-S) S B

Tufprene Asahi triblock (S-B-S) s B

Europrene Enichem triblock (S-B-S) or (S-I-S) S B or l SOL T

Kraton G Shell Chemical triblock (S-EB-S) s EB Co

Elexar Shell Chemical triblock (S-EB-S)and( S-B-S) s EB or B Co.

Riteflex Hoechst Celanese polyester poiyether

S = Polystyrene; B= Polybutadiene

I = Polyisoprene, EB = Poly(ethylene-co-butylene)

Riteflex is a multiblock (A-B)n type elastomer wherein, A the hard segment is poly(butylene terephthalate) and B, the soft segment is poly(tetramethγlene ether); note that n denotes an integer.