Adhesion of coating films on laser engraved wood surface

The adhesion of two coating systems – hard wax oil (oil-based) and PAM lak (water-based) – were evaluated on laser-engraved wood surfaces of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), and pedunculate oak (Quercus robur L.). Laser engraving was performed at two laser powers (8% = 11 W; 16% = 22 W) and three raster densities (10, 20, and 30 lines·mm⁻¹) for each power level. Adhesion was assessed using the pull-off test. The oil-based coating generally showed lower adhesion to the wood surface compared to the water-based coating. In contrast, several combinations of engraving parameters on spruce (8 × 20) and oak wood (8 × 10, 16 × 10) increased oil-based adhesion but tended to reduce water-based adhesion. On the other hand, the adhesion of the water-based coating was significantly reduced on beech wood (16 × 30) and oak wood (8 × 30, 16 × 20 and 16 × 30). In some cases, adhesion of the water-based coating exceeded the cohesive strength of the modified wood surface layers, leading to cohesive failure within the wood.