Tailoring the polyphenol content of softwood bark residues for cellulose nanofibril barrier films

Softwood bark is an abundant but underutilized sidestream from forest industries with potential as sustainable raw material for value-added products. In this study, cellulose nanofibrils (CNF) were produced from Norway spruce and Scots pine bark residues after alkaline extraction and oxygen bleaching to adjust their polyphenol content. Four CNF grades were prepared by grinding and mechanical fibrillation of unbleached and mildly bleached spruce residues, and strongly bleached spruce and pine residues. All CNF hydrogels showed low residual fibre content, over 98% fines, and required less than 20 kWh/kg for fibrillation. Lower polyphenol content generally improved fibrillation efficiency, gel viscosity, and strength.CNF dispersions were cast into self-standing films that blocked more than 85% of UV light (below 350 nm). Bleaching markedly increased tensile strength and stiffness of spruce-based films, reaching up to 85 MPa and 4.3 GPa, while pine-based films showed lower mechanical performance despite similar polyphenol content. Oxygen transmission was below 10 cc∙(m²∙day)^-1 and water vapour transmission was 20–30 g∙(m²∙day)^-1 in all films, irrespective of their bleaching extent or bark origin. The results demonstrate that softwood bark can serve as raw material for CNF production using industry-relevant processes, enabling the fabrication of mechanically robust films for barrier applications.