Over the past decade, low-temperature-cure powder coating chemistries - including ultraviolet (UV), infrared (IR), near infrared (NIR) and thermal cure types - have received a great deal of attention from powder coating researchers around the world due to their immense potential for application on heat-sensitive substrates such as medium density fiberboard (MDF). The prevailing practice is to finish MDF substrates with laminates or liquid coatings. However, both of these options are expensive, time-consuming and labor-intensive. Liquid coatings must be applied in multiple layers with long holding times between coats for drying and sanding, which affects productivity. They also pose problems with odor and emissions. Lamination limits design flexibility due to difficulties in adhesion on sharp edges, corners and curved surfaces.
In contrast, powder coatings are one-coat, environmentally friendly products that allow a seamless finish in range of color, gloss and texture on all types of surfaces, including flat, curvilinear and sharp edges. They also offer improved productivity and considerable savings on process costs compared to both liquid coatings and laminates.1-7The extensive design capabilities possible with powder coatings have made this coating type an excellent choice for MDF finishing, which is reflected in the growing interest of MDF coaters on this subject. However, inconsistencies in the density, moisture level, heat distortion temperature and porosity of MDF substrates have led to various problems related to the appearance and quality of the powder coated finish.
Fortunately, continued advances in low-temperature-cure powder coatings, along with a better understanding of the various factors that affect coating quality, are making it possible to overcome these problems and achieve trouble-free application and a uniform finish on MDF.
Powder Coating Challenges
Powder coatings were originally developed for use on metals, which are tolerant of high cure temperatures and provide good electrical conductivity on the surface for efficient grounding and optimum coating adhesion. MDF and other wood surfaces have low conductivity and cannot tolerate high cure temperatures, and therefore cannot be coated using conventional electrostatic techniques. While board conductivity can be enhanced by applying a coat of conductive liquid powder, this option introduces an extra step to the process, thereby affecting productivity. Clearly, a different solution is needed.Coating MDF is a delicate process that requires significant planning and attention to detail to achieve consistent results. The four main parameters that can affect the final coating quality are the powder quality, the nature of the MDF, the application line and the product specification. Obtaining the right synergy among all of these influential variables is key to achieving the desired finish.
Low-Temperature-Cure Powders
For application on heat-sensitive substrates like MDF, the powder must cure below 302°F (150°C) or even 212°F (100°C). Several approaches have been developed to achieve this goal, ranging from low-temperature-cure conventional chemistries to radiation-curable evolving chemistries.A vast number of published articles and patents have confirmed the ability of UV-curable technologies to produce glossy, smooth coatings on MDF within three to five minutes of process time.8The main advantages of UV-cured powders include higher productivity and space savings due to shorter curing times at lower temperatures, excellent surface finish and film properties, a reduced risk of damage to the MDF due to a reduced peak temperature and, most importantly, a reduced sensitivity to outgassing. However, UV technologies also have a number of drawbacks, including an inability to produce thick or matte coatings, shade specificity (particularly yellow shades), instability in tropical climates, higher material costs and higher capital equipment requirements. UV systems also present processing difficulties during conventional extrusion due their lower glass transition temperature. These limitations have deterred the large-scale commercialization of this technology.
In comparison, standard thermal-cure powders do not have any limitations in shades, effect finishes, coating thickness, product stability in warm climates, object shapes, etc., and they can be adapted to conventional extrusion and application equipment. Their only drawback is their higher curing temperature. For this reason, the ideal approach to ensure broad-based acceptability would be to develop a suitable low-temperature-cure technology in the existing thermal-cure chemistries.
In a review of the research work conducted with low-temperature-cure powders, systems using glycidyl methacrylic (GMA) acrylics - a mixture of amorphous and crystalline epoxy and polyester resin systems, along with suitable curing catalysts - have been most successful. The GMA acrylics provide excellent weatherability, film appearance, film clarity and color retention, as well as the absence of byproducts during cure and the lowest curing temperature. Commercial application of these low-temperature-cure technologies has been realized at several automotive plants, including General Motors, BMW and Chrysler, and many more are on the horizon in near future to conform to more stringent emission regulations.
Correll, et. al., has described several techniques used to produce low-temperature-cure thermoset powders suitable for application on wood substrates.9-12Muthiah, et. al., reported a single-stage processing technique in which a mixture of self-crosslinking epoxy resin and a low-temperature curing agent resin is co-extruded at 212 to 230°F (100 to 110°C) and subsequently cooled, pulverized and classified to produce a low temperature cure (LTC) powder that cures between 212 to 302°F (100 to 150°C).13The powder produces a smooth, glossy finish on heat-sensitive substrates such as MDF.
Another technology that has gained popularity is a thermosetting composition consisting of a GMA acrylic, curing agents and specialized catalysts.14,15These low-temperature chemistries have been successful in lowering the temperature of powder coatings to around 140°C (284°F) without affecting storage stability. However, this technology still limits the use of powder coatings to select qualities of MDF, and it cannot be applied directly to natural wood.
Recent work has been directed toward reducing the curing temperature to around 248°F (120°C) or lower by using epoxy resins or GMA acrylic resins.16The coatings that result from these novel compositions can be cured at 176 to 257°F (80 to 125°C) and have performance properties equal to higher-temperature powder coating compositions prepared with the same epoxies and conventional curing agents.
Yet another development is a composition based on polyester acrylic chemistry.17The powder cures at a temperature of about 230°F (110°C). While several technological options are possible in principle, the formulation must have the right chemistry and catalyst combination to achieve the desired finish.
MDF Characteristics
While the powder selection is important, finishing professionals also should have a full background of the characteristics of the MDF before attempting to powder coat an MDF substrate. MDF is made from hardwood or softwood fibers that are bonded with an adhesive glue (such as urea formaldehyde or phenol formaldehyde resins) under heat and compressed under pressure. Sometimes fibers are steam treated to remove the lignin glue and are subsequently heat pressed with synthetic resins. The nature of the MDF substrate has a strong bearing on the overall finish and appearance achievable with powder coating. Even if the initial finish is attractive and acceptable after painting, problems such as warping or cracking can appear over time. Depending on the method of manufacture, the porosity level, density, moisture level and dimensional strength vary from one grade of substrate to another, and these characteristics all influence the final finish.Density. The density of MDF is a measure of its compactness and is somewhat linked to porosity. Depending on the manufacturing process, MDF density can vary from 600 to 900 kg/m3. Generally, low-density MDF has more voids in the resin fiber matrix; these voids open up at high temperatures, leading to the release of trapped air or volatiles and therefore bubbles and blisters in the finish.1High-density MDF is usually bonded with phenol-formaldehyde or isocyanate and is denser with less porosity. The minimum density required to achieve a smooth finish is 750 kg/m³. At lower densities, the surface absorption of the MDF is higher and a fine-textured powder coating is required.
Moisture Content.The typical moisture level of an MDF substrate can vary from 3 to 12%. However, the ideal moisture level should be around 4 to 8% to achieve consistent powder coating application, outgassing control and defect-free coatings. Moisture levels below 4% affect powder deposition by electrostatic means, while moisture levels above 8% lead to blistering and possible warping. If the moisture level of MDF exceeds 10%, a coat of moisture-sealing conductive primer should be applied to control outgassing. It is important not only to select MDF with proper moisture content, but also to store MDF in a controlled environment so that the original moisture content is maintained.
Fiber Size.The fibers in MDF should be short and thin to ensure a smooth, glossy finish. Long fibers are only partially bound and tend to rise to the surface, resulting in film defects such as roughness and low gloss. It is difficult to get a smooth surface even after sanding if the fiber size is too long.
Temperature Resistance. MDF suitable for powder coating must resist the peak temperature required for melt, flow and curing of the applied powder without losing its structural integrity. Additionally, it should release the minimum amount of free formaldehyde at cure temperatures.
Batch-to-Batch Consistency. To achieve consistent coating application, finish and gloss, the MDF boards must have a uniform surface from lot to lot or part to part. Vendor control and agreement on the acceptable substrate quality is extremely important.
Application Process
Satisfactory powder coating on MDF is also greatly influenced by the application process and the design of application line. The processing steps, dwell time at each stage, and application conditions and parameters must be optimized with respect to the powder quality and MDF substrate. It is almost impossible to achieve a good quality powder coating on MDF with an old metal coating line. MDF parts should be transported from one station to other by an overhead conveyor or a flat belt ground conveyor. The painting process involves four discrete stages - namely sanding, preheating, powder application and full curing.Sanding. The MDF substrate is cut into pieces of the required size, and these pieces are sanded on both the top and bottom, as well as the edges and corners, to make all surfaces feel smooth. Sanding dust and foreign particles are removed by air blast or vacuum to prevent surface defects.
Preheating. MDF is a non-conductive substrate and therefore must be preheated to facilitate grounding for electrostatic application of the powder. Preheating controls outgassing by allowing excess water and free formaldehyde to escape while retaining optimum moisture on the surface to ensure uniform surface conductivity. Preheating also brings the board to a temperature at which the deposited powder particles melt and flow out to maximize wetting, which ensures good adhesion to the MDF surface. The temperature must be controlled carefully; temperatures that are too low can affect both the powder deposition and adhesion, while temperatures that are too high can completely remove the moisture from the round and sharp edges, thereby preventing full coating coverage on these areas. Using the right preheating time and temperature is critical to achieving the best application and powder adhesion to the substrate. Preheating times and surface temperatures can vary from 5 to 15 minutes at 212 to 266°F (100 to 130°C), depending on the powder quality, the type of finish required and the type of substrate used. In general, smooth coatings require higher preheating temperatures than textured coatings.
Powder Application. To get uniform coatings, the line should be conveyorized. The distance between the preheat oven and spray booth also should be optimized to ensure the right surface temperature at the spray gun point. The board temperature at gun point usually is 18 to 36°F (10 to 20°C) lower than that of the preheating temperature. To ensure a uniform finish, automatic guns with manual touch-up points are recommended. The powder coating thickness usually varies from 4 to 6 mils.
Curing. Since the thermal conductivity of MDF is poor, it is difficult to achieve uniform temperature distribution on all parts of an MDF substrate unless the oven is carefully designed with respect to the profile of MDF, including alignment, fresh air intake ratio, recirculation air changes, etc. Non-homogeneous temperature distribution will affect the powder deposition and finish, and can cause defects such as warping and board cracking. Both preheating and curing are carried out either in a convection oven or an IR oven or a combination of both. A combined IR/convection oven allows more control over the rate of heat up and attainment of surface temperature during the melting stage. IR energy rapidly raises the temperature of both the powder and the board to a level sufficient to melt and cure the powder. Due to the low thermal conductivity of MDF, the temperature in the core of the board remains far below that of the surface, thereby helping to reduce the overall heat stress and related damage on the MDF. Shorter curing times and temperatures also reduce heat stress on MDF.
Curing times and effective board temperatures may vary from 5 to 10 minutes at 284 to 320°F (140 to 160°C) for low-temperature thermoset powder coatings. A well-compacted MDF surface can be heated to 302°F (150°C) with no substrate damage. Completeness of cure at application thickness is important to ensure effective moisture barrier properties. At low coating thicknesses, MDF can absorb moisture from outside and equilibrate at a 10 to 12% moisture level, which can cause post-cracking of the coating. It is therefore essential to coat both the sides and edges of MDF at a thickness that will provide thorough sealing.
Application Requirements
Powder coated MDF panels are used most often in the kitchen and bathroom cabinet, home furniture and office furniture manufacturing industries because of their design flexibility, wide array of colors and outstanding chemical and stain resistance. The overall quality and finish expectations are similar to those of high-pressure laminates. The performance standards stipulated by U.S. Kitchen Cabinet Manufacturers Association (KCMA) and European Furniture Manufacturing Association (FIRMA) are the benchmarks followed in these industries. These specifications cover numerous requirements, including resistance to chemicals, stains, scratches, abrasion and heat shrinkage, as well as high hardness and flexibility. Color, uniformity, gloss, completeness of cure by solvent rub test, accelerated weathering (QUV) and humidity resistance are also important. KCMA specifies conformance to stain resistance against tea, coffee, vinegar, mustard, tomato ketchup, edible oil, grape juice, alcohol, detergent, felt pen ink, and so on, while FIRMA insists on specific tests, such as boiling water resistance and heat aging. All of these standards must be met to ensure a high-quality finish.Future Challenges
Despite advances in low-temperature-cure powders and MDF manufacturing, a number of challenges remain. A lack of standardization in MDF composition and manufacturing practices has resulted in considerable variation in the quality of MDF available around the world in terms of density, grain size, moisture level and dimensional stability. This challenge is further compounded by variations in temperature and humidity among tropical, arid and rain-fed regions, all of which have an adverse impact on MDF paintability with powder coatings.The existing thermal cure powder coating systems offer a satisfactory solution for select qualities of MDF. However, a future challenge is to improve on the robustness of this technology to enable application on all types of MDF substrates, regardless of inherent variations in quality, while meeting the desired aesthetics and performance characteristics equivalent to those of high-pressure laminates and overlays. Yet another challenge is to develop a powder coating that can produce a woodgrain finish in one stroke. Further development also is required to produce a low-temperature-curing resin chemistry that is stable in tropical climates but capable of curing at temperatures as low as 248°F (120°C) or below, so that problems relating to bubbling and MDF core deformation can be overcome.
Much has been learned about achieving a satisfactory powder coating application on MDF substrates. However, additional work remains to raise the finish standard and visual appearance of powder coated MDF to match the look of commonly used laminates and vinyl overlays.
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