Thermally Insulating and Moisture‐Resilient Foams Based on Upcycled Aramid Nanofibers and Nanocellulose

Abstract Low‐density foams and aerogels based on upcycled and bio‐based nanofibers and additives are promising alternatives to fossil‐based thermal insulation materials. Super‐insulating foams are prepared from upcycled acid‐treated aramid nanofibers (upANF_A) obtained from Kevlar yarn and tempo‐oxidized cellulose nanofibers (CNF) from wood. The ice‐templated hybrid upANF_A/CNF‐based foams with an upANF_A content of up to 40 wt% display high thermal stability and a very low thermal conductivity of 18–23 mW m⁻¹ K⁻¹ perpendicular to the aligned nanofibrils over a wide relative humidity (RH) range of 20% to 80%. The thermal conductivity of the hybrid upANF_A/CNF foams is found to decrease with increasing upANF_A content (5–20 wt%). The super‐insulating properties of the CNF‐upANF_A hybrid foams are related to the low density of the foams and the strong interfacial phonon scattering between the very thin and partially branched upANF_A and CNF in the hybrid foam walls. Defibrillated nanofibers from textiles are not limited to Kevlar, and this study can hopefully inspire efforts to upcycle textile waste into high‐performance products.