The effectiveness and efficiency of an ethylene/acrylate copolymer in toughening semicrystalline and amorphous PLA through melt blending is studied. The mechanical properties, phase morphologies, miscibilities, and toughening mechanisms of the blends are assessed. The ethylene/acrylate impact modifier effectively improved the impact strength of the blends, regardless of the PLA type. 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. The relatively low BDT temperature obtained for the PLA blends renders the ethylene/acrylate copolymer impact modifier a desirable additive to toughen PLA for use in cold temperatures.magnified image

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Factorial design analyses and numerical optimization are performed to establish material compositions (wood flour content, particle size, and impact modifier content) and mechanical property relationships for PLA/wood flour composites. Numerical optimization produces two scenarios based on materials compositions to manufacture composites with similar mechanical properties as unfilled PLA. High wood flour and impact modifier contents are required for composites made with fine wood flour particles, whereas the formulation requires low wood flour content and excludes impact modifiers for composites with coarse wood flour particles. These optimization solutions are validated experimentally.magnified image

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A fracture mechanics analysis based on the J-integral method was adopted to determine the resistance of composites with various concentrations of wood-flour and ethylene acrylate copolymer (EAC) to crack initiation (J in ) and complete fracture (J f ). The J in and J f energies of unmodified poly(lactic acid) (PLA)/wood-flour composites showed the deleterious effect of incorporating wood fibers into the plastic matrix by significantly decreasing the fracture toughness of PLA as the wood-flour content increased. The reduced fracture toughness of the matrix induced by adding brittle wood-flour into PLA was well recovered by impact modification of the composites with EAC. Microscopic morphological studies revealed that the major mechanisms of toughening were 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.