Rheological behavior and multi-physical field coupling in gas-assisted extrusion of <i>Camellia oleifera</i> bran-filled polybutylene adipate terephthalate biocomposites

Camellia oleifera bran (CF), as a functional biomass filler rich in cellulose, has potential in enhancing the mechanical properties and functionalization of polybutylene adipate terephthalate (PBAT). However, the introduction of high-content CF may significantly alter the rheological properties of the PBAT matrix. This not only critically affects the melt rheology and product quality but also exacerbates challenges in conventional extrusion, such as pronounced extrudate swell, surface fracture, and processing limitations due to the elevated melt viscoelasticity. The underlying mechanism of this phenomenon requires in-depth research. To address these challenges, this study introduces gas-assisted extrusion (GAE), a novel method that reduces wall friction and stress concentration via the interface effects of a gas cushion layer. The influence of CF content (10%–40%) on the rheological response of PBAT melt during GAE was systematically investigated by combining experiments with multi-physical field numerical simulations. The results indicate that appropriate CF filling (10%–20%) is conducive to the formation of a stable gas cushion layer and improves surface quality (achieving an Ra as low as 0.0298 μm), while excessive CF content (30%–40%) enhances the melt elasticity, leading to an increased tendency of flow instability; gas pressure and the distribution of the gas cushion layer play a crucial role in controlling the shear field and the morphology of the product. This work elucidates the underlying multi-field coupling mechanism governing the GAE of CF/PBAT composites. The established framework provides fundamental insights for tailoring the processing of high-performance, biomass-reinforced biodegradable materials.