Adhesion On Wood And Intercoat Adhesion for wood coating
Since the early 1970s when radiation-curing technology started to develop, mainly in the field of printing ink, the technology has known considerable success. One of the most successful areas has been wood coating, specifically the furniture and parquet flooring industries
As with conventional coatings, it is necessary in radiation-curing systems to apply several layers of coating to get a satisfactory finish. Consequently, in addition to considering adhesion on wood, intercoat adhesion and substrate preparation must be considered. Sanding of the wood is very important to achieve an optimum adhesion. A pretreated surface will aid adhesion and will allow a nice coating with a minimum film thickness to be obtained. Sanding is normally done with aluminum-oxide sanding belts.
Primer systems on wood can be based on 100%-UV products or on UV-curable waterborne products. Some flexible epoxy acrylates based on bisphenol A give very good results. Formulations based on these products penetrate less into the porous surface of the wood, which is sometimes a problem because of migration of uncured parts. When a very high adhesion level is required, the amount of epoxy acrylate can be increased up to 60%.However, in this case the formulation or the rollers need to be heated because of the high viscosity of these products.
Several waterborne systems also give excellent results in primer formulations. Acrylic emulsions and polyurethane acrylates in dispersion (UV-PUD) combine very low viscosity with good adhesion. Both the emulsions and the dispersions need to be physically dried before curing. With the UV-PUD primer formulation excellent adhesion is obtained on oak, beech, ash, iroko, afromosia, rovema, wenge, cabreuva, ovangkol, doussie afrika, cabreuva vermeila and roveme. Only on teak was the adhesion insufficient.
Sometimes polyisocyanates are used to improve the adhesion on difficult types of wood. By blending 5% of an aliphatic polyisocyanate with a UV primer it is possible to crosslink UV material that is not irradiated by UV light or is inhibited by chemicals in the wood. The viscosity of these blends can be stabilized by adding small amounts of acid functional acrylates.