Advantages of powder coating include thicker coatings without running or sagging, faster production cycles, and much lower emissions of volatile organic compounds. In most cases, key elements of powder paint systems are natural gas-fired convection ovens, or gas-fired catalytic infrared systems. The use of powder coatings for product finishing is expanding into new markets. These finishes are expanding from metallic products only to other surfaces, notably medium-density fiberboard (MDF).
Growing in market penetration
Powder paint systems were introduced more than 30 years ago. Since then, a growing number of product manufacturers have taken the step away from spraying liquid paint. Liquid has inherent challenges, including the need for very expert application in order to get a uniform coating without sagging or thin spots. Another is the ever-tightening requirements to reduce and capture volatile organic compound emissions from solvent carriers to comply with strict VOC air emission and occupational health rules.
Early adopters of the powder coating technique included manufacturers of appliances, HVAC equipment, lawn and garden equipment, metal furnishings, agricultural machinery and tools. Today, an estimated 15% of industrial coating is done with powder, and that number is growing rapidly. Expansion into new applications is feeding that growth.
The method involves the application of coating powders through either a hand-held or robotic nozzle. The coating itself is a very fine and uniform powder made up of polymer resins, pigments, flow control agents and other additives. Powder is electrically charged as it exits the spray gun. The target object is electrically grounded, causing the charged powder to adhere evenly to the surface. Painted products are then placed in a batch oven, or carried through a conveyor oven that melts the powder and fuses it to the product.
Powder coatings are extremely variable in their requirements for heating the coated surface at a prescribed temperature for a specific period of time. Often controlled cooling at the end of the oven cycle completes the curing of the surface. Various resin types require different thermal treatments. Many newer powder products are designed for lower cure temperatures
Less material waste
Importantly, powder coating virtually eliminates the huge evaporative losses experienced with liquid spray coating. With liquid spray, up to 80% of the volume of paint is lost to evaporation in the curing process, and to overspray in the booth. With powder coating, more than 80% of the powder adheres to the target, and even the overspray can often be collected and reused, assuming there is not a mixture of pigments used in the spray booth.
Infrared gaining in popularity
Curing systems using gas-fired catalytic infrared panels are gaining in popularity for parts or all of the process. Theseanels can be placed along the conveyor line, either in an open configuration or in an enclosed tunnel. Infrared curing is faster than convection ovens and can be closely adjusted to the size and shapes of the target objects being coated. In some cases, infrared is used to gel the paint on the objects before they enter a conventional convection oven. This shortens the bake time, allowing more production throughput.
Although powder coating was initially used only on metallic objects, techniques have been perfected for powder coating other materials, including plastic, wood, and especially medium density fiberboard (MDF). The MDF opportunity is important because this material is widely used in furniture, cabinets, recreational items, paneling, containers and shelving. The key to successfully powder coating MDF is to preheat the material, which increases its electrical conductivity, allowing it to be effectively grounded to cause the powder to adhere. Here again, the use of catalytic infrared panels makes this process effective.
Infrared designed for powder
One of the major providers of catalytic infrared heating systems is Heraeus-Vulcan. This company offers natural gas-fired systems specifically designed for the powder coating industry. According to company spokesman Mike Chapman, infrared systems offer design flexibility, allowing a thermal treatment area to be adapted for various coatings and product types. Chapman also notes the short cure times for infrared versus convection oven systems. Infrared systems can also be created to allow for product pretreatment before a convection oven, and controlled cooling at the other end of the process.
Chapman stresses that powder coatings for MDF products is an important trend. He explains, “The first part of the process is to heat the MDF to around 50° C (122° F) to draw moisture to the surface. This makes it conductive. So it is very important to use MDF that has a moisture content between 5% and 8%. The other issue with powder coating MDF is the need for quality. It needs to be dense with a strong internal bond so it can withstand the rapid heating and cooling that it will undergo in the powder coating process. Poor quality MDF will split and crack on the edges where there are cut surfaces when it is powder coated.”
Single coat process possible
He points out that until recently MDF has required two coats, a primer and a top coat. “This system has been adequate for most applications, except for products used in high humidity environments.” He states that powder coating resin manufacturer DSM has worked with Heraeus-Vulcan to develop a single-coat system which is non-porous and completely seals the MDF product.
Chapman offers Ekoltech in Slovakia as an example of an industrial user that has changed from liquid to powder coating with catalytic infrared systems. The company is a major supplier to global household products retailer IKEA. He says, “They are in the process of moving from liquid to powder for a number of their products. They have two separate gas catalytic IR oven systems which have very high output capacity. Their measured cost of poor quality (COPR) has dropped dramatically.” In addition to MDF, Chapman indicates that other non-metallic products being powder coated now also include certain fiber-reinforced plastic (FRP) products, and plywood and engineered wood products such as oriented-strand board (OSB). For the wood products, pre-heating is required.
Method of the Future
Users of these systems are motivated by a desire to reduce emissions and waste, improve finished product quality, and shorten production cycles. All of these results are being achieved with powdered coatings applied with natural gas-fired catalytic infrared systems. If you are currently using a liquid paint coating system, powder represents a major potential saving in production time, improvement in product quality, and a reduction in emissions. It’s worth considering.
The Original Posted by Gas Technology