Strong and transparent film of naturally aligned softwood holocellulose fibers

Mildly delignified softwood holocellulose fibers featuring native tracheid fiber cell wall structure and high hemicellulose content are prominent building blocks for wood derived fiber-based materials. However, preserving the natural alignment of long softwood fiber is challenging since top-down structure-retaining delignified softwood is unstable as extensive removal of lignin from intercellular space induces cracking and disintegration of wood structure. Here we report the use of chemical crosslinking pretreatment to improve the intercellular bonding between softwood fibers, therefore preserving the integrity of the naturally aligned softwood fibers after delignification. The crosslinked softwood veneer was delignified with peracetic acid and further densified into transparent and high-density film by thermal compression. The obtained transparent film of naturally aligned softwood holocellulose fibers showed high optical transmittance of 71 %, high haze of 85 %, strong optical anisotropy, as well as high tensile strength of 449 ± 58 MPa and high Young's modulus of 49.9 ± 5.6 GPa. This study provides a facile approach to preserve the natural alignment of softwood fibers for the fabrication of high performance holocellulose fibers-based materials.