Nanoparticles extracted from plants or textile waste are promising candidates for the design of sustainable materials. In this thesis, I explored how nanoparticles extracted from trees and from Kevlar and cotton textile wastes can be processed to form lightweight composite foams. The heat transfer and other functional properties such as electromagnetic shielding have been related to the structure, composition, and processing of the composite foams. 

Specifically, upcycled aramid nanofibers (upANF_A) with a small diameter were derived from Kevlar yarn. The upANF_A could be combined with wood-based cellulose nanofibrils (CNF) to produce moisture-resilient anisotropic foams with a very low thermal conductivity perpendicular to the aligned nanofibrils. The very low radial thermal conductivity was related to the strong interfacial phonon scattering between the very thin upANF_A and CNF in the hybrid foam walls. 

Aqueous dispersions of multiwalled carbon nanotubes (MWCNT) and cellulose nanocrystals (CNC) were used to form anisotropic foams with an anisotropic heat transport and orientation-dependent electromagnetic interference shielding efficiency (EMI-SE). The low-density (31 kg m^–3) CNC-MWCNT hybrid foams with 22 wt% MWCNT were mechanically robust along the axial direction (Young’s Modulus of 1200 kPa). The foams displayed an absorption-dominated EMI-SE of up to 41–48 dB and transferred heat favorably along the axial direction compared to the radial, meaning that this material could be useful in devices that require directional heat management and electromagnetic shielding.

A novel wet-foaming with subsequent freeze-casting process was developed to produce air- and ice-templated foams based on methylcellulose, CNF, and tannic acid. The air- and ice-templated foams displayed a high specific compression stiffness compared with other CNF-based materials while maintaining good insulation properties. 

Hybrid foams based on CNC extruded from cotton textile waste and wood-based CNF were prepared by freeze-casting in combination with two different solvent removal routes: supercritical drying and freeze drying. The nanoparticles in the foam walls of the freeze-dried foams were more densely packed, and the foams were mechanically stiffer and more resistant to moisture, whereas the supercritically dried foams displayed a significantly larger surface area. This highlights how the processing techniques govern the structure of a material, which in turn affects its properties.