Binderless hierarchical natural fibre composites with localised cellulose nanocrystals and tailored wet processing for improved mechanical and thermal properties

Binderless natural fibre composites are attractive for circular manufacturing since the removal of a synthetic matrix improves recyclability and end-of-life processing. However, their applications are often constrained by weak interfacial bonding and limited mechanical performance. This study presents a scalable approach to strengthen binderless luffa fibre composites by combining localised surface reinforcement with cellulose nanocrystals (CNCs) and tailored wet processing conditions. CNCs were introduced by immersing luffa layers in a CNC suspension, enabling diffusion into the porous network and subsequent accumulation at fibre-fibre contact regions during hot pressing, resulting in localised interfacial reinforcement. The process exploits the self-bonding of lignocellulosic fibres under controlled moisture and elevated temperature to mobilise lignin and promote hydrogen bonding. Compared to neat luffa panels, a 280 % increase in peel strength and a 49 % improvement in interlaminar shear strength (from 2.47 to 3.68 MPa) were obtained, alongside substantial improvements in flexural strength and modulus. CNCs further improved interfacial interactions, with FTIR evidences reconfiguration of O–H hydrogen bonding interactions under wet CNC processing, while DSC and TGA confirm reduced chain mobility (higher Tg) and delayed thermal decomposition. The synergistic effects of CNC integration and optimised processing parameters provide a scalable route to high-performance environmentally friendly natural fibre composites without synthetic binders.