Spray-Dried TEMPO-Oxidized Cellulose Nanofibrils and Their Reinforcing Performance in Polyvinyl Alcohol

TEMPO-oxidized cellulose nanofibril (TEMPO-CNF) suspensions were subjected to high-pressure homogenization (HPH) at different pass numbers and subsequently spray-dried using a two-fluid nozzle. Transmission electron microscopy and atomic force microscopy demonstrated a progressive decrease in TEMPO-CNF fibril length and the aspect ratio with increasing HPH pass number. In addition, the viscosity and degree of polymerization of TEMPO-CNF suspensions decreased with increasing HPH pass number. The average particle size of the spray-dried TEMPO-CNFs decreased with increasing HPH pass number, as shorter fibrils led to the formation of smaller particles. Spray-dried TEMPO-CNF powders obtained from different HPH pass numbers were incorporated into a polyvinyl alcohol (PVA) matrix at a loading level of 5 wt % via solvent casting. Compared to neat PVA, the incorporation of TEMPO-CNFs processed with 1 HPH pass reduced the tensile strength by 31%. TEMPO-CNFs subjected to 5 and 10 HPH passes increased the tensile strength to 61 and 69 MPa, respectively. A similar trend was observed in the tensile modulus of elasticity, which decreased from 4.6 GPa for neat PVA to 2.7 GPa in the 1-pass sample but increased to 5.5 and 5.2 GPa in the 5- and 10-pass samples, respectively, without compromising the transparency of the PVA films.