Prefinished oak flooring surface checking and physical properties

Aluminum oxide (Al2O3) in varnish formulation is one of the major improvements that have appeared in finishing systems over the last decade, increasing wear and scratch resistance. An important problem encountered with prefinished flooring strips is surface checking. The flooring industry suspects that high Al2O3 content in varnish reduces the flexibility of the film, resulting in cracks in the film when the wood shrinks and swells with seasonal variation in humidity. The varnish industry is inclined to think that too broad moisture content (MC) variation in wood results in shrinkage or swelling bringing the film to its elastic limit. The present study was conducted to shed light on this issue. Oak being a dominant species in the wood flooring market, white oak was selected for this study. This species is also prone to splitting due to its porous structure. The main objective of this study was to determine if AI,03 content in varnish and/or high variation of MC in white oak flooring strips are responsible for the surface checking observed in the flooring industry. A secondary objective was to determine the effect of Al2O3 content in film and wood MC on wear resistance, impact resistance, and hardness of the film. Flooring samples were submitted to aging cycles, i.e., simulated alternating summer and winter conditions. The increase of wood MC resulted in lower presence of varnish in the checks because the swelled fibers kept the varnish at the surface during the varnishing process. Anatomical analysis of the checks has shown that more than 80 percent of the checks were initiated at a ray location, relating checks to wood anatomy instead of varnish Al2O3 content. The second initiator of checks was the transition in annual rings (12%). Wear resistance and hardness of the film were found to be significantly related to both Al2O3 content in the varnish and to wood MC.