Effect of species type and sanding sequence on the adhesion strength and the interphase region of Polyurethane coating and veneer

The purpose of this research was the investigation of sanding sequence and coating thickness effect on adhesion strength of coating in two species: beech and elm. For this purpose, beech and elm veneers with 0.6 mm thickness were laminated on MDF with 16 mm thickness by UF and hot press. The surface of the laminated panels was sanded by two different sequences; the soft (180-280 grits) and rough (150-180 grits) one. Before the coating application, the surface was sanded with 400 grits. Then they were coated with polyurethane in two forms of one and two-layered coating. After that, the adhesion strength test was done on specimens according to ASTM D4541. To have a more precise investigation of the interphase region of coating-veneer, photomicrographs were captured by fluorescent microscopy to analyze and calculate the effective penetration parameter of the coating into the wood texture. The results showed that two species respond to sanding sequences differently and a thicker coating layer increased the adhesion strength. Beech as a diffuse-porous wood had the finer texture and higher density compared with elm. Therefore, the adhesion strength of the beech veneer was declined by 13.8 % and reaches 2.59 MPa due to the use of a rough sanding sequence. However, these sequences caused the opposite result in elm veneer and led to a 25.7 % adhesion improvement (3.23 MPa). The study on microphotographs and the effective penetration parameter stated that an even thickness of the coating layer and effective penetration reduction has been possible when the rough sequence and soft one are applied on elm and beech, respectively.