Curing powder coatings with UV energy instead of heat energy opened up new possibilities for powder coatings in a wide range of applications. Most UV powders cure between 100 and 120°C and do so quite rapidly making UV powders more attractive, especially for heat sensitive substrates (HSS) like MDF, wood, paper and plastics. The application of a two step coating process with IR- and UV-radiation allows for complete separation of the melt and flow stage and the curing stage leading to a smooth film at relatively low temperature. However, for non-conductive substrates such as wood, wood-based products and plastics, the quality of the powder application depends largely on substrate pretreatments to render them more conductive. This study showed that in order to increase the conductivity of wood-based products such as MDF, it is essential to have a minimum of 4% moisture content (MC) within the substrate. Also, when MC is high (10% or higher) the conductivity of the substrate is sufficient to allow powder application without preheating. On the other hand, when the MC is very low, around 2 to 3%, heat pretreatments did not result in good powder application indicating that preheating is not the only determining parameter for powder adhesion on the substrate. For the most commonly distributed MDF products with a MC between 5 and 8%, excellent powder application can be obtained following heat pretreatments.
Optimisation of powder curing was out of the scope of the present study but excellent results were achieved in most cases in terms of impact resistance, pencil scratch resistance and solvent resistance.
Thermal powder was successfully applied and cured on different sets of MDF samples. Results compared very well with those obtained with the UV powder in terms of impact resistance, pencil scratch resistance and MEK solvent rub resistance. However, the integrity of the product needs to be evaluated in order to ensure that there is no serious deterioration of the finished product retains all of its original mechanical and physical properties.