Nanocellulose-reinforced polyurethane as flexible coating for cork floor

Cork materials have the property of elastic deformation under pressure due to their unique closed, porous, cell structure with thin cell wall; and they accordingly need a surface coating that matches their deformation during there being used as floor materials. However, traditional wood coatings are hard, brittle, and mostly suitable for stiff wood surfaces, which are difficult to be directly adapted to the requirements of flexible cork. Although waterborne polyurethane (WPU) has the advantage of property tailorability, its fully applicable to cork materials are still a challenge. Herein, this study utilized a simple physical blending of waterborne polyurethane and nanocellulose with high aspect ratio, to develop a high-strength coating that matches the elastic deformation requirements of cork materials. When both two kinds of nanocellulose, poplar wood nanocellulose (WCNF) with an aspect ratio of 800 and bacterial nanocellulose (BCNF) with an aspect ratio of 2000, was individually added into the WPU emulsion at 0.5 wt%, the tensile strength and elastic modulus of the nanocellulose-modified polyurethane coating increased by 48.58 % and 118.88 %, 54.50 % and 120.48 %, respectively, compared to those of the pure WPU coating (14.09 MPa and 312.74 MPa). It is notable that both the nanocellulose-modified coatings showed no signs of damage on the cork surface even they underwent continuous bending over 2000 cycles, which indicates their excellent flexural fatigue resistance. Interestingly, such nanocellulose-modified coatings maintain comparable transparency, gloss, adhesion, solvent resistance and even present slightly improved hardness and wear resistance than their pure WPU coatings. Overall, the two nanocelluloses are comparative to the modification of the coatings, both of which could match the elastic deformation requirements of cork materials. Therefore, the nanocellulose physically blended polyurethane technology provides a simple and effective solution for the design and property tailoring of flexible coatings for cork floor.