Method for powder coating

Abstract

A method of powder coating a substrate, the method comprising the steps of heating the substrate to a conditioning temperature, applying a powder to the substrate at a temperature lower than the conditioning temperature, and curing the powder in a decreasing temperature gradient.

Description

Method for Powder CoatingTECHNICAL FIELDThe present invention relates to methods and compositions for powder coating substrates. The invention has particular application in the coating of heat sensitive substrates and porous substrates but is not limited thereto.BACKGROUND ART

[0001] Powder coating is a well-established method of coating articles to provide high standards of finishes. Powder coatings are dry, finely divided, free flowing, solid materials at room temperature and at atmospheric pressure. They are typically an extruded thermoplastic or thermoset polymers which are ground or milled into a powder. The powder is then typically electrostatically applied to a substrate and cured with heat or UV radiation to form a coating layer.

[0002] Conventional powder coating compositions have for example, a softening temperature (Tg or glass transition temperature) of about 75 °C to 110 °C, and an incipient fusion start temperature (Tm). Such a composition may for instance cure at a substrate temperature of about 200 °C for at least 10 minutes; below these recommended temperatures the coatings have poor appearance, as well as poor physical and chemical properties. Generally low bake compositions cure at lower temperatures, for example in 3 to 5 minutes at 135 - 150 °C.

[0003] Conventional powder coatings are not suitable for heat sensitive substrates which are liable to for example, bend and crack at conventional curing temperatures, which may cause structural failure, poor uniformity of the coating layer and other defects from thermal stress. Furthermore, fibre rise from wood based substrates, such as MDF, solid wood, and veneered parts in some circumstances necessitates a pretreatment and / or post treatment such as priming, sanding and recoating.

[0004] In general, it is difficult to achieve smooth finishes with low temperature curing powder coating formulations. In view of the generally high reactivity of such coating compositions, the powder coating composition generally has a short time after melting before curing sets in. There is a desire for powder coating formulations and methods to achieve smooth finishes similar to liquid based systems with minimal orange peel.

[0005] A smooth finish can be defined as having a surface substantially free from irregularities, roughness or projections.

[0006] Many heat sensitive substrates consist of randomly oriented wood fibres resulting in open pre structures. When these substrates are heated, the air therein heats and expands increasing internal pressures. The wood fibres can also contain moisture which will seek to evaporate under these conditions. On curing of these materials, the increased pressure from the air and water vapour can create pinholes in the ultimate powder coating providing an unacceptable finish. Some plastics and phenolic moulding compounds are also susceptible to outgassing defects.

[0007] The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.

[0008] The term “ULB” (Ultra Low Bake I Low Temperature) and “Ultra” curing powder coating compositions is understood as to be based on powder coating types comprising a thermal initiated or radical initiated thermal curing principle for polymerisation reaction.

[0009] The term “UV Curable” includes coating compositions which are curable using electromagnetic radiation with a wavelength in the range of 100 to 445 nm, preferable provided by radiation sources with assigned frequencies between 1.5 PHz and 789 THz.

[0010] Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0011] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referenced to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0012] The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein. The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.SUMMARY OF INVENTION

[0013] In accordance with the present invention, there is provided a method of powder coating a substrate, the method comprising the steps of: heating the substrate to a conditioning temperature; applying a powder to the heated substrate at a temperature lower than the conditioning temperature; and curing the powder after powder application, wherein the conditioning temperature is within the temperature limitations of the heat sensitive substrate.

[0014] In the context of the present specification, the term temperature limitations of the substrate shall be understood to refer to the temperature at which the substrate undergoes degradation. It will be appreciated that different substrates will have different temperature limitations. It will be appreciated that heat sensitive substrates and porous substrates may have lower conditioning temperatures than non-heat sensitive substrates such as metals.

[0015] In accordance with the present invention, there is provided a method of powder coating a substrate, the method comprising the steps of: heating the substrate to a conditioning temperature; applying a powder to the substrate while reducing the temperature of the substrate; and curing the powder after powder application.

[0016] In accordance with the present invention, there is provided a method of powder coating a substrate, the method comprising the steps of: heating the substrate to a conditioning temperature; applying a powder to the substrate at a temperature lower than the conditioning temperature; and curing the powder in a decreasing temperature gradient.

[0017] Advantageously, the present invention can obviate the need for an oven to cure the powder.

[0018] Advantageously, the step of curing the powder can be performed in the absence of an oven.

[0019] Advantageously, the present invention can obviate the need for a primer or sealer and the powder coating layer is preferably applied directly on the substrate delivering a one-step finish. Additionally, only one coating may be required to provide an acceptable finish without commonly known defects found in heat sensitive materials when processed with an increasing temperature gradient to achieve cure. In this way, a very efficient coating method is provided which may advantageously involve fewer steps than known powder coating processes for heat sensitive substrates. Further still, costly pretreatment steps may be avoided.

[0020] Advantageously, the conditioning phase of the process method can be done for both horizontal and vertical applications depending on specific requirements for the final coating on specific substrate types.

[0021] Advantageously, depending on the substrate properties, the present invention may eliminate the need for an electric charge to transfer the powder to the substrate surface.

[0022] The present invention can provide distinct advantages over methods of the prior art including:• As the temperature of the substrate is decreasing during powder application while at the same time the film continues to flow out and powder melting occurs from the bottom up;• Elimination and / or reduction in defects when processing certain substrates;• No ovens are required after powder application prior to UV or LED curing;• Less complexity in the process method;• Lower total applied cost compared to traditional methods; and• Reduction in process energy.

[0023] In the contest of the present specification, the term heating the substrate to a conditioning temperature shall be understood to mean conditioning the substrate.

[0024] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 100 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 110 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 120 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 130 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 140 °C. In an alternate form of the invention, the step of conditioning the substrate comprises conditioning the substrate to a temperature of at least 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises conditioning the substrate to a temperature of at least 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of at least 180 °C.

[0025] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between100 °C and 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 140 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 130 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 120 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 110 °C.

[0026] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 140 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 130 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 110 °C and 120 °C.

[0027] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 140 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 120 °C and 130 °C.

[0028] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 130 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 130 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 130 °C and 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 130 °C and 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 130 °C and 140 °C.

[0029] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 140 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 140 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 140 °C and 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 140 °C and 150 °C.

[0030] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 150 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 150 °C and 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 150 °C and 160 °C.

[0031] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 160 °C and 180 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 160 °C and 170 °C.

[0032] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature between 170 °C and 180 °C.

[0033] In one form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 100 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 110 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 120 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 140 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 150 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 160 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 170 °C. In an alternate form of the invention, the step of conditioning the substrate comprises heating the substrate to a temperature of about 180 °C.

[0034] Preferably, the step of conditioning the substrate is conducted in an enclosed area.

[0035] The step of conditioning the substrate may utilise any heating methods known in the art including hot convection air generated by gas or electricity, gas infrared, gas catalytic, infra red and combinations thereof.

[0036] It will be appreciated that the powder should melt on application to the heated substrate.

[0037] Powder deposition on the substrate may be performed by any method known in the art including electrostatic application for vertical or horizontal applications and scattering methods for horizontal applications.

[0038] It will be appreciated that the temperature at which the powder is applied should be greater than the melting point of the powder. It will be further appreciated that the conditioned substrate should have enough latent heat to complete the film forming process.

[0039] It will be appreciated that the temperature of the conditioning stage will be affected by a number of factors including heat sensitive parameters of the substrate, theheat required to melt the powder on application to the substrate, powder formulation and the thermal conductivity of the substrate.

[0040] In one form of the invention, the temperature of the substrate is decreased from the conditioning temperature to the powder application temperature by removing a source of heat. It will be appreciated that removing a source of heat will create a negative temperature gradient.

[0041] Without being limited by theory, it is believed that applying a powder to the substrate at a temperature lower than the conditioning temperature can obviate outgassing.

[0042] Preferably, the step of applying a powder to the heated substrate is performed immediately after the step of heating the substrate to the conditioning temperature. This provides the additional benefit of decreasing the potential conditioning temperature.

[0043] In one form of the invention, the powder application temperature is up to 5 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 10 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 20 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 30 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 40 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 50 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 60 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 70 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 80 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 90 °C less than the conditioning temperature. In an alternate form of the invention, the powder application temperature is up to 100 °C less than the conditioning temperature.

[0044] It will be appreciated that the powder will begin melting on application to the substrate. In a negative temperature gradient, the surface of the substrate may becooler than the internal substrate temperature and the powder will melt from the bottom up.

[0045] The substrate may be heated to the conditioning temperature over a period of between 1 and 30 minutes depending on production requirements and substrate characteristics.

[0046] In one form of the invention, the source of heat is discontinued from the substrate when it reaches the desired conditioning temperature. Alternatively, the temperature of the substrate may be maintained at the conditioning temperature for a period of time including 1 minute 2 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes or 10 minutes.

[0047] In one form of the invention, the powder application temperature is about 140 °C. In an alternate form of the invention, the powder application temperature is about 130 °C. In an alternate form of the invention, the powder application temperature is about 120 °C. In an alternate form of the invention, the powder application temperature is about 110 °C. In an alternate form of the invention, the powder application temperature is about 100 °C. In an alternate form of the invention, the powder application temperature is about 90 °C. In an alternate form of the invention, the powder application temperature is about 80 °C.

[0048] The powder may have a melting point that allows for it to cure at lower temperatures. For example, the film forming resin and / or matting agent may have a melting point of less than 170 °C, less than 160 °C, less than 150 °C, less than 140 °C, less than 130 °C, less than 125 °C, less than 120 °C, less than 115 °C, less than 110 °C, less than 100 °C, less than 95 °C, less than 90 °C, less than 85 °C, less than 80 °C, less than 75 °C, less than 70 °C, less than 65 °C, less than 60 °C, or any range including any two of these values as endpoints.

[0049] The step of curing the powder may be carried out for any suitable time to allow the coating to fully or at least partially cure. The curing time may vary depending on the substrate, the powder composition, the powder thickness, formulation, formulation properties, ambient conditions, curing methods, or any combination of these factors.

[0050] The step of UV curing the powder may be carried out at any suitable temperature to allow the coating to fully or at least partially cure. In one form of theinvention, the curing temperature is less than 140 °C. In an alternate form of the invention, the curing temperature is less than 130 °C. In an alternate form of the invention, the curing temperature is less than 120 °C. In an alternate form of the invention, the curing temperature is less than 110 °C. In an alternate form of the invention, the curing temperature is less than 100 °C. In an alternate form of the invention, the curing temperature is less than 90 °C. In an alternate form of the invention, the curing temperature is less than 80 °C. In an alternate form of the invention, the curing temperature is less than 70 °C. In an alternate form of the invention, the curing temperature is less than 60 °C. In an alternate form of the invention, the curing temperature is less than 50 °C. In an alternate form of the invention, the curing temperature is less than 40 °C. In an alternate form of the invention, the curing temperature is less than 30 °C. In an alternate form of the invention, the curing temperature is less than 20 °C. In an alternate form of the invention, the curing temperature is less than 10 °C.

[0051] In one form of the invention, the curing temperature is about 140 °C. In an alternate form of the invention, the curing temperature is about 130 °C. In an alternate form of the invention, the curing temperature is about 120 °C. In an alternate form of the invention, the curing temperature is about 110 °C. In an alternate form of the invention, the curing temperature is about 100 °C. In an alternate form of the invention, the curing temperature is about 90 °C. In an alternate form of the invention, the curing temperature is about 80 °C. In an alternate form of the invention, the curing temperature is about 70 °C. In an alternate form of the invention, the curing temperature is about 60 °C. In an alternate form of the invention, the curing temperature is about 50 °C. In an alternate form of the invention, the curing temperature is about 40 °C. In an alternate form of the invention, the curing temperature is about 30 °C. In an alternate form of the invention, the curing temperature is about 20 °C. In an alternate form of the invention, the curing temperature is about 10 °C.

[0052] In one form of the invention, the powder may be cured with actinic radiation. Actinic radiation is electromagnetic radiation that is capable of initiating a chemical reaction and includes the use of LEDs, UV light, IR light, visible light, X-ray and gamma radiation. Preferably the powder is cured with UV radiation. UV radiation may be provided with a UV lamp or an LED lamp. Suitable powders include those made of:epoxy, epoxy / polyester hybrids, polyester, acrylic, polyurethane, amorphous, semicrystalline and crystalline. Alternatively, or in addition thereto the powder may be cured with heat.

[0053] Where the powder is cured with heat, the temperature is preferably below the conditioning temperature.

[0054] Where the powder is cured by UV radiation, the powder may comprise UV photo initiators as is known in the art.

[0055] On melting of the powder, the powder should remain in its molten state until the curing stage, where it hardens. In this fashion, should any outgassing occur prior to the curing stage, the molten powder may flow to recover any crater or pinhole that the outgassing may have formed prior to curing.

[0056] Preferably the powder has a low viscosity above its melting point.

[0057] In one form of the invention, there is provided the additional step of drying the substrate.

[0058] The step of drying the substrate will be conducted prior to the step of heating the substrate to the conditioning temperature.

[0059] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 90 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 80 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 70 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 60 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 50 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 40 °C.

[0060] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 40 °C and 90 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to atemperature between 40 °C and 80 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 40 °C and 70 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 40 °C and 60 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 40 °C and 50 °C.

[0061] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 50 °C and 90 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 50 °C and 80 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 50 °C and 70 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 50 °C and 60 °C.

[0062] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 60 °C and 90 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 60 °C and 80 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 60 °C and 70 °C.

[0063] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 70 °C and 90 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 70 °C and 80 °C.

[0064] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature between 80 °C and 90 °C.

[0065] In one form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 30 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 40 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 50 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to atemperature of about 60 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 70 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 80 °C. In an alternate form of the invention, the step of drying the substrate comprises heating the substrate to a temperature of about 90 °C.

[0066] The step of drying the substrate may be conducted for between 1 and 10 minutes. In one form of the invention, the step of drying the substrate is conducted for about 1 minute. In an alternate form of the invention, the step of drying the substrate is conducted for about 2 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 3 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 4 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 5 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 6 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 7 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 8 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 9 minutes. In an alternate form of the invention, the step of drying the substrate is conducted for about 10 minutes.

[0067] In one form of the invention, there is provided the additional step of: applying a base coat powder to the heated substrate, prior to the step of applying a powder to the heated substrate at a temperature lower than the conditioning temperature.

[0068] In one form of the invention, there is provided the additional step of: applying a base coat powder to the heated substrate, prior to the step of applying a powder to the substrate while reducing the temperature of the substrate.

[0069] It will be appreciated that the base coat powder should melt and flow before the step of: applying a powder to the substrate while reducing the temperature of the substrate.

[0070] Where the method comprises the application of two coats, it becomes possible to increasingly tailor the powder coating to provide increased functionality. Some powders have greater adherence to substrates than other. Some powders have greater UV resistance than others. Properties of powder coatings may be manipulated. The base coat may comprise an ultra-low bake powder, or an actinic radiation powder and the top coat may comprise an ultra-low bake powder, or an actinic radiation powder.

[0071] Specific combinations can include UV base and UV top, thermal base and UV top. The top coat may also comprise combinations of UV and LED.

[0072] Heat sensitive substrates may be prepared from plastics, wood, wood composites, particle board, low density fibreboard, medium density fibreboard, high density fibreboard, particle board, wood laminate, chipboard, substrates with cellulosic fibres, textiles, plastic material, fibre cement, paper faced gypsum, paper foil, bakelite and phenolic moulding compounds. The substrate may for instance comprise pressed wood fibres and / or particles and typically a binder resin.

[0073] Examples of plastic substrates include polyester, polycarbonate, polyolefin, polyamide, polyethylene, polypropylene, cellulosic, polystyrene, poly(ethylene naphthalate), nylon, polylactic acid, poly(ethyleneterephthalate) (PET), polycarbonate, engineering polymers such as poly(etheretherketone) (PEEK), polycarbonate acrylobutadiene styrene (PC / ABS), ethylene-propylene-diene monomer (EPDM), thermoplastic olefin (TPO), polyurethane (PU), polypropylene oxide (PPO), polyethylene oxide (PEO), polyethyleneterephthalate and polyamide,

[0074] Substrates that are particularly suitable for coating with the method of the present invention include specific wood coating markets such as residential housing, commercial buildings, retail and institutional where the composition of the invention may be used include furniture e.g. in assembled form or flat pack, such as tables, storage systems, chairs, cabinets, sanitary products and all other suitable joinery based products.

[0075] The powder coating compositions of this embodiment are typically suitable for outdoor and / or indoor applications.

[0076] The substrate may have a variety of shapes in both 2D and 3D geometry with flat and curved surfaces including routed and moulded parts.

[0077] Advantageously, the method of the invention may obviate the need for a sanding step after the powder is cured.

[0078] Advantageously, the method of the invention may remove the need for a primer or sealer prior to application of the powder as is known in the art.

[0079] In one form of the invention, the powder is crystalline as is known in the art. Alternatively, the powder may be amorphous.

[0080] Preferably, the powder has a low melting point. In one form of the invention, the melting point is about 50 °C. In an alternate form of the invention, the melting point is about 55 °C. In an alternate form of the invention, the melting point is about 60 °C. In an alternate form of the invention, the melting point is about 65 °C. In an alternate form of the invention, the melting point is about 70 °C. In an alternate form of the invention, the melting point is about 75 °C. In an alternate form of the invention, the melting point is about 80 °C. In an alternate form of the invention, the melting point is about 85 °C. In an alternate form of the invention, the melting point is about 90 °C. In an alternate form of the invention, the melting point is about 95 °C. In an alternate form of the invention, the melting point is about 100 °C. In an alternate form of the invention, the melting point is about 105 °C. In an alternate form of the invention, the melting point is about 110 °C. In an alternate form of the invention, the melting point is about 115 °C. In an alternate form of the invention, the melting point is about 120 °C. In an alternate form of the invention, the melting point is about 125 °C. In an alternate form of the invention, the melting point is about 130 °C.

[0081] Preferably the powder has a low viscosity. Preferably the viscosity is between 0.1 and 30 Pascal. Advantageously, a low viscosity may enable the molten powder to overlay any defects such as pinholes prior to completion of the curing cycle.

[0082] The composition of the present invention may comprise further components as is known in the art including but not limited to flow agents, PTFE, peroxides, amorphouspolyesters, acrylated polyesters, acrylic oligomers, aromatic urethane oligomers, pigments, fillers, acrylated epoxies, unsaturated vinyl resins, unsaturated allyl resins, unsaturated acrylate resins, acrylated aliphatic oligomers, and additives for outgassing and adhesion.BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:Figure 1 is a temperature profile curve for a conventional powder coating method;Figure 2 is a temperature profile curve for a powder coating method in accordance with an embodiment of the invention; andFigure 3 is a process flow chart.DESCRIPTION OF EMBODIMENTS

[0084] Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification individually or collectively and any and all combinations or any two or more of the steps or features.

[0085] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0086] In Figure 1 there is proved a standard ULB I Low Temperature powder temperature profile for the curing stage after application of powder. The powder isapplied at a temperature in the range of 15 - 25 °C (in some cases an increased substrate application temperature may be used to assist with electrostatic deposition of powder) and the film forming and melting follows as the temperature increases to 130 °C after powder application. The substrate is maintained at 130 °C as the coating cures.

[0087] This conventional method may require additional stages for heat sensitive substrates to avoid defects, including: a. Conditioning of the substrate; b. Application of liquid primers; c. Two powder coats with sanding of defects in between coats; and d. Reliance of additional holes in the substrate to reduce internal substrate pressures and provide outlets for VOCs.

[0088] In Figure 2, there is provided a temperature profile curve for a method of powder coating in accordance with an embodiment of the present invention. The substrate is heated to 180 °C in a heater. The substrate is conveyed out of the heater at the 8 minute stage, initiating the decreasing temperature gradient. The substrate enters the powder coating application booth at about 120 °C and the powder is applied. Immediately after powder application, the powder on the substrate melts and flows to form a uniform surface based on the characteristics of the formulation i.e. gloss, satin, matt, smooth or textured (e.g. rippled or wrinkled). During this phase of decreasing temperature gradient, no additional heating sources from a traditional oven is required unlike the method of the prior art in Figure 1 . Once the powder has reached an optimal stage in terms of film forming and aesthetics (see the 15 minute mark) the substrate enters the UV Zone for a UV Flash Cure. In Figure 2, this is shown at the 80 °C mark. A small increase in temperature occurs due to the UV energy. If the source of UV radiation is from an LED, no increase in temperature should be observed. Once curing is ceased (see the 18 min mark), the temperature drops rapidly.

[0089] In Figure 3, an embodiment of the present invention 10 is described. A substrate is loaded 12 onto a conveyor line as a single component of multiple hung products for vertical application at a loading zone. The conveyor method could be oneof many options including but not limited to continuous or power and free function. The substrate is conveyed to a drying stage 14 which increases the temperature to about 60 °C. The substrate is conveyed to the conditioning stage 16 where it is heated to a setpoint temperature in the range from 100 °C to 180 °C. The substrate is conveyed to the first powder application stage 18 at a temperature of about 90 °C to 120 °C. The powder instantly melts when it is applied to the heated substrate. The substrate is conveyed to the UV curing stage 20. The substrate is allowed to cool to ambient using different methods and conveyed to the unloading stage 22.

[0090] For each application the exposure time and process temperatures will be determined by the substrate characteristics in combination with the powder formulation.

[0091] Additionally, productivity requirements for the process will also determine other variables and combinations of exposure time and pre-heat / conditioning temperature.

Claims

CLAIMS1 . A method of powder coating a substrate, the method comprising the steps of: heating the substrate to a conditioning temperature; applying a powder to the substrate at a temperature lower than the conditioning temperature; and curing the powder in a decreasing temperature gradient.

2. A method of powder coating a substrate in accordance with claim 1 , wherein the step of conditioning the substrate comprises heating the substrate to a temperature between 100 °C and 180 °C.

3. A method of powder coating a substrate in accordance with claim 1 or claim 2, wherein the temperature at which the powder is applied is greater than the melting point of the powder.

4. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein the step of applying a powder to the heated substrate is performed immediately after the step of heating the substrate to the conditioning temperature.

5. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein the step of curing the powder is performed immediately after the step of applying a powder to the substrate.

6. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein the surface of the substrate is cooler than the internal substrate temperature and the powder melts from the bottom up.

7. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein the powder is cured with actinic radiation or heat.

8. A method of powder coating a substrate in accordance with claim 7, wherein the step of curing the powder with actinic radiation is conducted without an oven.

9. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein there is provided the additional step of drying the substrate prior to the step of heating the substrate to the conditioning temperature.

10. A method of powder coating a substrate in accordance with claim 8, wherein the step of drying the substrate comprises heating the substrate to a temperature between 30 °C and 90 °C.

11. A method of powder coating a substrate in accordance with any one of the preceding claims, wherein there is provided the additional step of: applying a base coat powder to the heated substrate, prior to the step of: applying a powder to the heated substrate at a temperature lower than the conditioning temperature.

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