Microstructural and mechanical properties of biocomposites made of native starch granules and wood fibers

Novel biocomposites were fabricated using preforms of unmodified starch powder and wood pulp fibers. Stacks of preforms were consolidated using thermo-compression (TCM) and ultrasonic compression moldings (UCM). The characterization of the microstructure of the biocomposites showed that TCM enabled a better preservation of the crystallinity of starch granules during their welding than UCM. However, UCM allowed a significant gain in processing time. For the best set of forming and material parameters, the composites exhibited an elastoplastic response with strain hardening. Their Young's modulus, flexural strength and strain at ultimate stress reached up to approx. 6 GPa, 70 MPa, and 8%, respectively. The best properties were associated to the partial preservation of the native crystallinity of starch and lowered porosity. Bleached and fibrillated fibers with a large aspect ratio also contributed to the enhancement of composite properties. These effects were explained by a better starch fiber interface and the presence of a network of connected fibers within the composites.