Structure–Property–Degradation Relationships in PHBV Biocomposites Filled with Cellulose, Walnut Shell, and Wood Fibres

PHBV composites incorporating 10 and 20% by weight of cellulose, walnut shell particles, or wood filler were produced under consistent extrusion and injection-moulding conditions for comparative analysis. The study assessed morphology, wettability, water absorption, mechanical properties at 23 and 80 °C, thermal decomposition, and alterations in mechanical properties following exposure to a detergent solution at 60 °C. Among the mean values reported, PHBV 20W exhibited the highest modulus at 23 °C (7403 MPa), while PHBV 10C demonstrated the highest impact strength (8.9 kJ/m2). At a filler content of 20% by weight, the tensile strength was observed to be lower than that of the matrix. After 35 days, water absorption varied from 1.3% for PHBV to 14.7% for PHBV 20W. Thermogravimetric analysis (TGA) results revealed that the direction of the shift in the temperature at 5% mass loss was filler-dependent, with values ranging from 229.2 to 272.2 °C, compared to 267.2 °C for PHBV. Exposure to detergent resulted in reductions in strength and elongation, with PHBV 20R and 20W experiencing a marked decrease in modulus.