Mild H2O2 Oxidation of Fungal Mycelium for Enhanced Interfacial Compatibility in Waterborne Polyurethane Composite Films

Petroleum-derived synthetic fillers and solvent-borne polymer systems remain major contributors to industrial carbon footprints and environmental pollution. Fungal mycelium grown on agricultural by-products offers a renewable, biodegradable, and low-cost alternative to conventional fillers; however, its hydrophobic and chemically heterogeneous hyphal surface limits interfacial compatibility with polar polymer matrices. Here, we report a mild and scalable surface functionalization strategy for Aspergillus oryzae mycelium as a bio-based filler in waterborne polyurethane dispersion (PUD) composite films. The strategy employs dilute aqueous hydrogen peroxide (H2O2, 1–3 wt %, 40 °C, 1 h), a green oxidant that generates only water and oxygen upon decomposition, enabling environmentally benign surface modification without toxic by-products. FT-IR and X-ray photoelectron spectroscopy (XPS) confirmed that H2O2 treatment selectively introduced carbonyl/carboxyl-related functional groups onto the hyphal surface while preserving the chitin–glucan structural framework. Treatment at 2 wt % H2O2 (HP2) provided the optimal balance between surface functionalization and structural integrity. Composite films incorporating 1 wt % HP2-treated mycelium (HP2-1) exhibited tensile strength and elongation at break 64.1% and 49.3% higher, respectively, than neat PUD. At 5 wt % loading, HP2-5 reduced water uptake from approximately 120% for neat PUD to approximately 45%. Laser diffraction analysis of the composite dispersions showed a decrease in D50 from ∼41 μm (Raw-5) to ∼9 μm (HP2-5), indicating substantially improved filler dispersion. By combining a bio-based filler, a low-VOC waterborne polymer matrix, and a reagent-efficient oxidation process with benign by-products, this work presents a practical potential for sustainable coating and flexible film applications.