Mechanical Properties and Soiling Resistance of Paper with Polyurethane Coating Reinforced with Cellulose Nanomaterials

Recently, enhancing the performance of polyurethane (PU) coatings with cellulose nanomaterials (CNM) has been actively researched. Cellulose nanomaterials exhibit considerable potential to increase the mechanical strength of PU coatings due to their high aspect ratios and elastic moduli. In this study, PU reinforced with CNM was coated onto paper to enhance the paper's mechanical strength and soiling resistance. To investigate the reinforcing effect, two different CNM, cellulose nanocrystals (CNC) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TOCN), were selected, and suspensions with different ratios of PU and CNM were prepared. After coating the paper with each of them, the mechanical properties of the paper, including tensile strength, folding endurance, and soiling resistance, were evaluated. The mechanical strength and anti-soiling performance of the PU-CNM coated papers were greatly enhanced. Especially, PU-TOCN had superior properties as a durable paper coating despite a low TOCN concentration, less than 2%, because the TOCN crosslinked with PU via polyaziridine. Furthermore, the PU-CNM coating protected the paper from being contaminated, which was confirmed by scanning electron microscopy and energy dispersive X-ray mapping. Consequently, durable paper exhibiting soiling resistance was fabricated by coating the paper with PU-TOCN suspensions.