Design, processing and properties of lightweight foams from cellulose nanofibers
Abstract: Foams are applied in many areas including thermal insulation of buildings, flotation devices, packaging, filters for water purification, CO2 sorbents and for biomedical devices. Today, the market is dominated by foams produced from synthetic, non-renewable polymers, which raises serious concerns for the sustainable and ecological development of our society. This thesis will demonstrate how lightweight foams based on nanocellulose can be processed and how the properties in both the wet and dry state can be optimized.Lightweight and highly porous foams were successfully prepared using a commercially available surface-active polyoxamer, Pluronic P123TM, cellulose nanofibers (CNFs), and soluble CaCO3 nanoparticles. The stability of wet and dry composite foams was significantly improved by delayed aggregation of the CNF matrix by gluconic acid-triggered dissolution of the CaCO3 nanoparticles, which generated a strong and dense CNF network in the foam walls. Drying the Ca2+-reinforced foam at 60 °C resulted in moderate shrinkage but the overall microstructure and pore/foam bubble size distribution were preserved after drying. The elastic modulus of Ca2+-reinforced composite foams with a density of 9 – 15 kg/m3 was significantly higher than fossil-based polyurethane foams.Lightweight hybrid foams have been prepared from aqueous dispersions of a surface-active aminosilane (AS) and CNF for a pH range of 10.4 – 10.8. Evaporative drying at a mild temperature (60 °C) resulted in dry foams with low densities (25 – 50 kg/m3) and high porosities (96 – 99%). The evaporation of water catalyzed the condensation of the AS to form low-molecular linear polymers, which contributed to the increase in the stiffness and strength of the CNF-containing foam lamella.Strong wet foams suitable for 3D printing were produced using methylcellulose (MC), CNFs and montmorillonite clay (MMT) as a filler and tannic acid and glyoxal as cross-linkers. The air-water interface of the foams was stabilized by the co-adsorption of MC, CNF and MMT. Complexation of the polysaccharides with tannic acid improved the foam stability and the viscoelastic properties of the wet foam for direct ink writing of robust cellular architectures. Glyoxal improved the water resistance and stiffened the lightweight material that had been dried at ambient pressure and elevated temperatures with minimum shrinkage. The highly porous foams displayed a specific Young’s modulus and yield strength that outperformed other bio-based foams and commercially available expanded polystyrene.Unidirectional freezing, freeze-casting, of nanocellulose dispersions produced cellular foams with high alignment of the rod-like nanoparticles in the freezing direction. Quantification of the alignment with X-ray diffraction showed high orientation of CNF and short and stiff cellulose nanocrystals (CNC).
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