Functional Melamine-Formaldehyde Cross-linked Cellulose Nanofiber Based Aerogels with Excellent Flame Retardancy for Thermal-Acoustic Insulation Applications

Biodegradable aerogels possessing flexibility and high strength are appealing for applications in construction, acoustic and thermal insulation. However, their susceptibility to flammability presents a significant challenge. Enhancing the flame retardancy of these aerogels has been a prominent focus of research, with the widespread use of inorganic fillers and layered materials for this purpose. In the current study, our objective is to fabricate cellulose nanofiber aerogels characterized by low density, exceptional flame retardancy, high mechanical properties, and thermal insulation. This is achieved through the cross-linking of melamine and formaldehyde under aqueous conditions using an eco-friendly freeze-drying process, followed by post-curing. The resulting aerogels demonstrate flexibility, effective sound absorption within the mid-frequency range, and outstanding flame retardancy (Limiting Oxygen Index ∼33%) with a non-flammable behaviour. The thermal conductivity of the fabricated melamine formaldehyde-modified cellulose nanofiber (MF-CNF) aerogels was 0.064 ± 0.014 W/m.K. MF-CNF aerogels exhibited a Time to Ignition (TTI) of 489 s, whereas pristine CNF aerogels only have 3 s. This improvement was attributed to the concurrent reductions in both the Peak Heat Release Rate (PHRR) and Fire Growth Rate (FIGRA) of MF-CNF aerogels. The straightforward melamine formaldehyde modification of CNF aerogels enhances their mechanical strength as well as fire resistance. These sustainable multifunctional aerogels hold great potential for a variety of real-life applications in the realm of buildings and its structures for ensuring fire safety and sound insulation.