Fiber Size and Loading Dependent Defect Evolution Governs the Strength and Moisture Uptake of Melt Processed Bamboo Fiber/Polyhydroxyalkanoate Composites

ABSTRACT The coupled effects of fiber mesh grade and loading on defect formation and property evolution remain insufficiently understood in melt‐processed bamboo fiber (BF)/polyhydroxyalkanoate (PHA) composites. Here, a 3 × 3 full‐factorial design was employed using 40, 80, and 120 mesh BF at loadings of 10, 20, and 30 wt%, with neat PHA as the control. BF incorporation increased the tensile modulus from 0.10 to 0.33 GPa and the flexural modulus from 0.35 to 1.39 GPa, whereas tensile strength decreased from 9.69 MPa for neat PHA to 5.91–9.40 MPa in the composites, indicating defect sensitive failure. Microscopy and density analysis showed that higher BF loading promoted agglomeration, microvoid formation, and interfacial micro‐gaps, while finer BF improved dispersion but increased wetting demand. Fourier transform infrared spectroscopy indicated no detectable covalent bonding, and dynamic mechanical analysis confirmed stiffness enhancement accompanied by greater viscoelastic heterogeneity. The 80 mesh/20 wt% BF composite provided the most balanced processing window by combining sufficient interfacial contact with limited defect connectivity. These results establish a size–loading–defect–property framework for optimizing stiffness, strength retention, and moisture resistance in biobased BF/PHA composites.