Ultra-light-weight, anti-flammable and water-proof cellulosic aerogels for thermal insulation applications

Cellulosic aerogels (CNF) are considered naturally available thermal insulating materials as substitutes for conventional polymeric aerogels owing to their extensive sources, low density, low thermal conductivity, sustainability and biodegradability. However, cellulosic aerogels suffer from high flammability and hygroscopicity. In this work, a novel P/N-containing flame retardant (TPMPAT) was synthesized to modify cellulosic aerogels to improve their anti-flammability. TPMPAT/CNF aerogels were further modified by polydimethylsiloxane (PDMS) to enhance the water-proof characteristics. Although the addition of TPMPAT and/or PDMS slightly increased the density and thermal conductivity of the composite aerogels, those values were still comparable to the commercial polymeric aerogels. Compared with pure CNF aerogel, the cellulose aerogel modified by TPMPAT and/or PDMS had higher T_-10%, T_-50% and T_max, which indicated that the modified cellulose aerogels have better thermal stability. TPMPAT modification made CNF aerogels highly hydrophilic, while TPMPAT/CNF aerogel modified by PDMS became a highly hydrophobic material with a water contact angle (WCA) of 142°. Pure CNF aerogel burned rapidly after ignition, showing a low limiting oxygen index (LOI) of 23.0% and no UL-94 grade. In contrast, both TPMPAT/CNF-30% and PDMS-TPMPAT/CNF-30% showed self-extinction behaviors with a UL-94 V-0 grade, implying high fire resistance. Combined with high anti-flammability and hydrophobicity, the ultra-light-weight cellulosic aerogels show great potential for thermal insulation applications.