The influence of drying routes on the properties of anisotropic all-cellulose composite foams from post-consumer cotton clothing

Biopolymer-based functional materials are essential for reducing the carbon footprint and providing high-quality lightweight materials suitable for packaging and thermal insulation. Here, cellulose nanocrystals (CNCs) were efficiently upcycled from post-consumer cotton clothing by TEMPO-mediated oxidation and HCl hydrolysis with a yield of 62% and combined with wood cellulose nanofibrils (CNFs) to produce anisotropic foams by unidirectional freeze-casting followed by freeze drying (FD) or supercritical-drying (SCD). Unidirectional freeze-casting resulted in foams with aligned macropores irrespective of the drying method, but the particle packing in the foam wall was significantly affected by how the ice was removed. The FD foams showed tightly packed and aligned CNC and CNF particles while the SCD foams displayed a more network-like structure in the foam walls. The SCD compared to FD foams had more pores smaller than 300 nm and higher specific surface area but they were more susceptible to moisture-induced shrinkage, especially at relative humidities (RH) > 50%. The FD and SCD foams displayed low radial thermal conductivity, and the FD foams displayed a higher mechanical strength and stiffness in compression in the direction of the aligned particles. Better understanding how drying influences the structural, thermal, mechanical and moisture-related properties of foams based on repurposed cotton is important for the development of sustainable nanostructured materials for various applications.