Abstract

Deep eutectic solvents (DESs) can be used as green media for lignocellulosic nanofiber production. However, typically, a high liquid-to-lignocellulose ratio is required (i.e. the consistencies of biomass are low, usually limited from 1 to 5 wt%), which greatly reduces the sustainability of DES use. In the present work, wood sawdust was first anionized using a reactive DES (formed from sulfamic acid and urea) at a high mass consistency (with gradually increased sawdust consistencies up to 60 wt%) in the pre-milling and post-heating processes. Anionic sawdust nanofibers (ASNFs) were then produced after mild nanofibrillation (two passes) using a microfluidizer. The processes of producing ASNFs were fast (1 h of pretreatment) and the ASNFs was obtained with high sulfate group contents (up to 3.1 mmol g⁻¹). Besides, the ASNFs possessed a well-individualized structure with a homogeneous size distribution (3–5 nm in width). More importantly, films fabricated from ASNFs achieved strong flame resistance and good mechanical strength. Due to the presence of lignin, ASNF films were able to block over 90% of UV light (at a wavelength of 200–400 nm). The investigated functionalities of ASNFs led the lignocellulosic biomass residuals to be converted to high value-added bioproducts. Therefore, the reactive DES treatment combined with milling and heating process offers a practical and sustainable route for multi-functional biomaterial production, which can be potentially utilized, even at an industrial scale.