Waste-derived Mycelium materials for non-structural and semi-structural applications
Creators: name: Jones, Mitchell
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401602
subject: Composite and hybrid materials

description: Mycelium composites are materials that are produced by allowing natural fungal growth to bind lignocellulosic substrates into a single mass resembling any given mould geometry, typically possessing foam-like mechanical properties. Fungi are specifically used due to their growth characteristics, which constitute an expanding web-like structure of filaments comprising chitin-ß-glucan cell walls and a heterotrophic growth process which digests and bonds substrates under ambient conditions. This cheap and environmentally sustainable bio fabrication method, which can be used to upcycle agricultural by-products and wastes into green materials, is experiencing increasing research interest and commercialisation globally. This thesis systematically explores biological optimisation of the manufacturing process, expansion to non-structural functional applications, specifically as thermally safer and cost competitive alternatives to highly flammable petroleum- and natural gas-derived insulation and panelling materials, such as synthetic foams like extruded polystyrene and engineered woods containing flammable resorcinol- and polyvinyl acetate-based resins, and improvement of mycelium material mechanical properties. Initially, biological optimisation of the manufacturing process was investigated, with the suitability of several fungal species and agricultural by-products assessed for use as matrix and filler phases in mycelium composites and for chitin-ß-glucan polymer generation. While no inherent fungal characteristics, such as hyphal types, pathogenicity or association- or taxonomic-based classifications proved reliable predictors of growth performance, several high performing species were identified for use in the remaining research. Selected species included white-rot fungi for use as a composite binder and soil- and water-associated fungi with high cell wall concentrations of structural polymers, such as chitin and chitosan. Solid agricultural by-products were then assessed for their suitability as composite fillers. Rice hulls, sugarcane bagasse and wheat straw proved to be very poor nutrients, demonstrating a need for nutrient supplementation using wheat grains to achieve sufficient bonding within the biological composites. However, the sugarcane by-product blackstrap molasses was an exceptional nutrient for chitin-ß-glucan polymer generation, outperforming even common laboratory nutrients, such as malt extract. Highly nutritious nutrients were associated with larger hyphal diameters and significant anastomosis, which generated pseudo-laminar sheets of mycelium. A two-part investigation was then completed to produce fire resistant mycelium composite materials. The previously undocumented thermal degradation and fire reaction properties of the mycelium matrix phase were first investigated. Mycelial biomass exhibited a three-stage degradation process typical of biological materials, but superior fire reaction properties to other competing thermoplastic polymers, such as polymethyl methacrylate and polylactic acid. The fibrous structure of mycelium was retained following pyrolysis, albeit with a reduction in its diameter and cell wall thickness, and mycelium exhibited certain flame-retardant properties, such as a high char residue and water vapour release. Mycelium composite materials were then produced utilising various combinations of agricultural and industrial by-products with a high silica content, such as rice hulls and glass fines, as the composite substrate filler phase. The composites produced exhibited outstanding fire safety properties, with lower average and peak heat release rates and longer estimated time to flash over than the extruded polystyrene foam and particleboard synthetic construction material references. They also released significantly less smoke and CO2 and had very low raw material substrate costs. In addition to being low in cost, the substrate materials were responsible for the improvements in fire performance with rice hulls yielding significant char and silica ash and composites containing glass fines exhibiting the best fire performance because of their significantly higher silica concentrations and low combustible material content. Finally, investigations were completed into the improvement of the mechanical performance of mycelium through mild alkaline extraction and hot pressing to form nanopapers primarily comprising polysaccharides including fungal structural polymers, such as chitin and chitosan. The nanopapers produced exhibited much higher tensile strength than most existing mycelium materials, with comparable properties to paper and some plastics, but were weakened by inorganic Ca and organic lipid impurities within the nanopapers. Mycelium-derived nanopapers exhibited hydrophobic surface properties with high water advancing contact angles resulting from the presence of lipid residues within the nanopapers. These could be removed, and the surface properties subsequently tuned through HCl or H2O2 treatments. These investigations have demonstrated that mycelium-derived materials have a range of useful functional properties and could be used as low-cost and environmentally sustainable alternatives to synthetic polymers in a range of non-structural and semi-structural applications.