Anionic surfactant effect on the structural and thermal insulation properties of crosslinked-cellulose nanofiber foam and its superhydrophobic treatment

Sustainable and environmentally friendly cellulose nanofiber (CNF) has unique advantages and properties for preparing porous materials for various applications. This study reports a CNF foam developed via an environmentally friendly, expeditious and cost-effective process using sodium dodecyl sulfate (SDS) and citric acid (CA) as an anionic surfactant and a bio-based green crosslinker. Incorporating 0.5 wt% SDS into CA-crosslinked CNF foam significantly enhances its porous structure, indicating the highest porosity (>99.24 %) and a very low density (18.59 mg/cm³). The performance improvement is attributed to the formation of robust ester bonds through chemical crosslinking, which stabilizes the foam's microstructure and enhances its mechanical strength. Additionally, the SDS facilitates better foaming and reduces surface tension, promoting uniform distribution of pores. The foam shows better mechanical properties and antioxidant behavior than the neat CNF foam. Moreover, adding SDS into the CA-crosslinked NC foam exhibits lower thermal conductivity (0.029 W/mK) and diffusivity than commercial insulation foams, including polyurethane foam. Finally, a superhydrophobic CA-crosslinked CNF foam is made by a water-based hydrophobic treatment (water contact angle = 150°), indicating its waterproof behavior.