Combination of Mocrowave Drying and UV Curing for Waterborne Lacquers on Wood Surface

Since April 2001, new EU regulations have become applicable regarding handling emissions from
solvents based on organic compounds. The VOC Guideline of the European Union also provides for regulations in the fields of wood coating and wood impregnation. Therefore, emission limits apply to facilities using more than 5 t of solvents annually, for both registered and fugitive emissions. As is generally known, the limit values to be complied with in any place where spray application must be used (e.g. lacquer-coating of profiled parts, lacquer-coating of finished parts) can only be observed — according to current knowledge — by applying waterborne coating systems. That is the reason why the application of low-solvent or solvent-free coating systems will eventually lead to drastic success. Meanwhile, materials on a waterborne basis with several resins are available whose surface properties are hardly inferior to the solvent-containing coating materials or are even superior to them. In spite of this, waterborne lacquers only become accepted in the furniture industry extremely slowly. Their main disadvantage is seen in the correlation between water as a thinner and the hygroscopic properties of all wood-based materials as the substrate, whereas swelling processes narrow the technological window and require pre-treatment adapted to the respective substrate, e.g. by sanding as well as by intensive intermediate sanding in applying multi-layer coating. Hence, the swelling effects of the wood surface remain to be the fundamental hindrance for the swift market introduction of waterborne lacquers. One AIF-funded research project* found out that the aqueous phase of a waterborne lacquer on the surfaces that have been pre-treated in various ways requires — depending on the species of wood, on chemical additions and on the kind of pre-treatment — different time periods in order to penetrate into the surface. Such period may last between a few seconds up to a quarter of an hour on veneer surfaces and longer on MDF surfaces. This allows to deduce that the swelling process of the surface may substantially be influenced by a forced lacquer-drying procedure. New, qualitatively demanding waterborne coating systems have been developed recently. Despite adapted recipes and due to the high evaporation rate of water, the drying process of waterborne coating systems still represents an energy-consuming and cost-intensive stage in processing, which needs to be optimised within the context of the overall process design.