Boron/nitrogen flame retardant additives cross‐linked cellulose nanofibril/montmorillonite aerogels toward super‐low flammability and improved mechanical properties

Low‐flammability freeze‐dried cellulose nanofibril (CNF)/sodium montmorillonite (MMT) aerogels with improved mechanical properties were fabricated via a facile cross‐linking of boric acid (BA) and melamine‐formaldehyde (MF) resins. Scanning electron microscopy analysis showed that BA cross‐linking reduced the interspacing of layered CNF/MMT aerogels whereas introduction of MF formed polymeric fibrils that connected the layers. These changes on microstructures resulted in the improvement on compressive mechanical properties of the cross‐linked aerogels. Moreover, the boron (B)/nitrogen (N) containing flame retardant cross‐linkers greatly increased the limiting oxygen index values that could reach 85% and leveled the UL‐94 rating from no rating to V‐0. Cone calorimetric results suggested that BA and MF induced a synergistic effect on the flame retardant properties of the CNF/MMT aerogels. However, the thermal conductivity was little affected because pore structure and size was not substantially modified. This simple approach fabricated highly flame‐resistant and mechanically strong CNF‐based aerogels that could be used in various engineering fields.