Synergistic Effect of a Chitosan/Micro–Nano‐Fibrillated Cellulose in Surface Sizing of Kraft Paper

ABSTRACT The present study introduces a novel approach to surface sizing (SS) of kraft paper by directly comparing oxidized starch (OS), chitosan, and a blended solution of micro–nanofibrillated cellulose (MNFC) with chitosan. This is the first investigation to benchmark chitosan and chitosan–nanocellulose‐based sizing against OS, the most widely used surface sizing agent. Remarkably, the chitosan–MNFC blend achieved substantial improvements in paper properties even at very low application levels (0.35–0.50 g/m 2 ), demonstrating both cost‐effectiveness and sustainability. At a 0.50 g/m 2 dose, tensile strength, tensile energy absorption (TEA), and stretch in the machine direction increased by 46%, 50%, and 111%, respectively. The burst factor improved by up to 1.8 times, and air permeance resistance increased by 2100%–3680% compared to unsized paper. Scanning electron microscopy confirmed that the chitosan–MNFC blend formed a denser and smoother surface than the other sizing agents. These findings establish the chitosan–MNFC system as a promising, sustainable alternative to starch‐based surface sizing.