Fabrication of Flame Retarded Cellulose Aerogel with Hydrophobicity via MF/MTMS Double Cross-Linking

In order to overcome the disadvantage of highly flammable and poor mechanical performance of cellulose aerogel, herein, cellulose nanofibrils (CNF) were cross-linked by melamine formaldehyde (MF), and further cross-linked and surface decorated by methyltrimethoxysilane (MTMS) to obtain flame retarded compound aerogels. FT-IR spectra confirmed that cross-linking reaction occurred between MF/MTMS and CNF matrix. The morphological analysis demonstrated that the pore size shrinked, whereas the pore amount of compound aerogels increased after cross-linking, thereby resulting in the improvement of mechanical performance. Via double cross-linking, the compound aerogels also exhibited excellent hydrophobicity (contact angle up to 132.3°) and flame retardant properties. The limited oxygen index (LOI) value of double cross-linked specimen (Si-CNF/MF) increased from 19.5% to 37.1%, and the vertical combustion test (UL-94) reached V-0 grade. Microcalorimetry measurement showed that the peak heat release rate (pHRR) and total heat release (THR) of Si-CNF/MF decreased by 50.6% and 64.3% in comparison with pure CNF aerogel, respectively. The analysis results of char residue showed that both condensed phase and gas phase flame retardant mechanisms occurred during combustion, and synergistic effect existed between MF and MTMS. Moreover, the very low thermal conductivity of compound aerogels permitted their application as heat preservation materials.