Lignin's effect on cellulose fiber bonding and inter-fiber interface design for enhancing densified wood

We hypothesize that the existence of an "optimal bonding layer" at the cellulose fiber interface, where the bonding force between cellulose fibers is maximized at a critical lignin content, resulting in the highest tensile strength of densified wood. Specifically, the tensile strength of densified wood reached a peak value of 347.5 MPa at a lignin content of 1.99 %, suggesting an optimal bonding force between cellulose fibers at this concentration. To validate this inference, atomic force microscopy (AFM) force spectroscopy was employed to simulate the bonding force between cellulose fibers by measuring the adhesion forces between cellulose nanofibrils (CNF)-modified microspheres and lignin-containing CNF films. The results showed that the adhesion force was maximized when the lignosulfonate content was 2 %. These results support the concept of an "optimal bonding layer". Based on these findings, polyvinyl alcohol (PVA) was incorporated into the bonding interface between cellulose fibers, resulting in a substantial enhancement in tensile strength, reaching 770.6 MPa at the optimal PVA content. This study provides insights into the mechanism of cellulose fiber bonding enhancement in densified wood, offering a theoretical foundation for the development of high-performance cellulose fiber materials.