Preparation of lignin-containing cellulose nanofibers from walnut shell using deep eutectic solvent for nanotube conductive film

A deep eutectic solvent (DES) was employed to purify cellulose by depolymerizing agricultural waste walnut shells (WS) at 90 degrees C from 1.5 to 3.0 h. The pretreatment achieved an 88.57% delignification rate and yielded 33.62% cellulose solids (CS), with 77.63% cellulose and 13.11% lignin contained in CS. The DES pretreatment exposed functional groups on the surface of cellulose and resulted in a 24.7% increase in crystallinity. Suspension with a homogeneous distribution of lignin-containing cellulose nanofibers (LCNF) was made by ultrasonic dispersing (650 W, 30 min), and the average length of 1406 nm, the zeta-potential of -17.65 mV. Higher lignin content led to cross-linking effect which produced a heightened degree of entanglement of LCNF while increasing the UV-shielding property and thermal stability. Additionally, LCNF stabilized the dispersion of multi-walled carbon nanotubes (MWCNT) in water, and served as a carrier for MWCNT conductive networks, which showed a tensile strength of 63.86 MPa and a conductivity of 15.63 S/m. The composite film exhibited potential applications in real-time monitoring of pressure sensing.