This study examined the mechanisms of toughening the brittle bio-based poly(lactic acid) (PLA) with a biodegradable rubbery impact modifier to develop biodegradable and cost effective PLA/wood-flour composites with improved impact strength, toughness, high ductility, and flexibility. Semicrystalline and amorphous PLA grades were impact modified by melt blending with an ethylene-acrylate copolymer (EAC) impact modifier. EAC content was varied to study the effectiveness and efficiency of the impact modifier in toughening the semicrystalline and amorphous grades of the PLA. Impact strength was used to assess the effectiveness and efficiency of the EAC in toughening the blends, whereas the toughening mechanisms were determined with the phase morphologies and the miscibilities of the blends. Subsequent tensile property analyses were performed on the most efficiently toughened PLA grade. Composites were made from PLA, wood flour of various particle sizes, and EAC. Using two-level factorial design the interaction between wood flour content, wood flour particle size, and EAC content and its effect on the mechanical properties of the PLA/wood-flour composites was statistically studied. Numerical optimization was also performed to statistically model and optimize material compositions to attain mechanical properties for the PLA/wood-flour composites equivalent to at least those of unfilled PLA. The J-integral method of fracture mechanics was applied to assess the crack initiation (Jin) and complete fracture (Jf) energies of the composites to account for imperfections in the composites and generate data useful for engineering designs. Morphologies of the fractured surfaces of the composites were analyzed to elucidate the failure and toughening mechanisms of the composites.

The EAC impact modifier effectively improved the impact strength of the PLA/EAC blends, regardless of the PLA type. However, the EAC was more efficient in the semicrystalline grades of PLA compared to the amorphous grade. The semicrystalline blends showed decreased tensile strength and modulus with increased impact modifier content. In contrast, the ductility, elongation at break, and energy to break increased significantly. Mechanisms of toughening of PLA with EAC included impact modifier debonding, fibrillization, crack bridging and matrix shear yielding resulting in a ductile behavior. Increasing the EAC content in PLA/wood-flour composites enhanced the impact strength and elongation at break, but reduced the tensile modulus and strength of the composites. Composites with fine wood particles showed greater improvement in elongation at break than those with coarse particles;​ an opposite trend was observed for impact strength, tensile modulus and tensile strength. Numerical optimization produced two scenarios based on materials compositions to produce composites with similar mechanical properties as unfilled PLA. These optimization solutions were successfully validated experimentally. The crack initiation (Jin) and complete fracture (Jf) energies of unmodified PLA/wood-flour composites showed the deleterious effect of wood fiber incorporation into the plastic matrix by significantly decreasing the fracture toughness of PLA as the wood flour content increased. By contrast, impact modification of wood plastic composites with EAC significantly increased both the resistance to crack initiation (Jin) and complete fracture (Jf). Microscopic morphological studies revealed that the major mechanisms of toughening was through the EAC existing as separate domains in the bulk matrix of the composites which tended to act as stress concentrators that initiated local yielding of the matrix around crack tips and enhanced the toughness of the composites.