High mechanical strength, flame retardant, and waterproof silanized cellulose nanofiber composite foam for thermal insulation

With a growing focus on sustainable building thermal regulation for buildings, cellulose foams have emerged as promising materials due to their low thermal conductivity and biodegradable properties. However, their flammability and hygroscopic nature limit practical applications. This is attributed to the abundant hydroxy groups of cellulose. In this study, a sustainable, simple, and cost-effective method was proposed for the synthesis of multifunctional thermal insulation materials based on cellulose nanofiber composite foam with hydrophobic, flame retardant, and thermally insulating performance. As a result, the cellulose nanofiber composite foam showed a high mechanical modulus (6.3 ± 0.3 MPa), high compression strength (0.78 ± 0.10 MPa), and specific modulus (246.2 ± 34.4 MPa·cm³·g⁻¹). The homogeneous three-dimensional (3D) porous network structure of cellulose nanofiber composite foam resulted in outstanding thermal insulation capabilities (LOI values of 60.7 ± 3.2, UL-94 V-0 rating) and low thermal conductivity (36.3 ± 0.8 mW·m⁻¹ K⁻¹). Furthermore, the incorporation of phytic acid (PA) imparted high flame retardancy, while cellulose nanofiber composite foam modified with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (POTS) maintained outstanding hydrophobicity (static water contact angle of 145.5° ± 0.2°) even under harsh environmental conditions. In this way, it is believed that cellulose nanofiber composite foam with light weight, high mechanical strength, thermal insulation, high flame retardancy, and hydrophobicity has great potential in thermal insulation materials.