Interfacial-engineered "top-down" construction of bulk hydrophobic biomass boards with integrated super hydrophobicity and robust mechanical properties

Driven by carbon neutrality and petroleum depletion, renewable bio-based composites with environmental durability are urgently needed. However, the inherent hydrophilicity of agricultural waste and the poor durability/formaldehyde emission of traditional surface modifications limit engineering applications. This study proposes a “top-down” interfacial engineering strategy to resolve biomass–hydrophobic particle compatibility, achieving inherent hydrophobicity without adhesives. Using absolute ethanol as a low-polarity carrier, silane-modified hydrophobic silica (SiO₂) was uniformly dispersed in wheat straw fibers, with cationic starch as an interfacial bridging agent. The effects of SiO₂ particle size (30 nm–30 μm) and addition level (10%–50%) on morphology, mechanics, and wetting were investigated. With 30 nm SiO₂ at 25% (N30–25), the board achieves superhydrophobicity (water contact angle 150°, sliding angle 15°) and high bending strength (22.08 MPa), balancing hydrophobicity and reinforcement. Mechanistically, cationic starch forms a 3D cross-linked network via electrostatic attraction and hydrogen bonding. The nano-SiO₂ and micro-fiber bundles create a micro/nano hierarchical rough structure that traps air to form a stable Cassie-Baxter air cushion. Environmental durability tests confirm the bulk hydrophobic network remains stable under mechanical abrasion, water erosion, and UV aging. This work provides a theoretical basis for high-value agricultural waste utilization and a pathway for durable, eco-friendly green composites.