Preparation of flame-retardant lignin-containing wood nanofibers using a high-consistency mechano-chemical pretreatment

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.