Effects of high-lignin-loading on thermal, mechanical, and morphological properties of bioplastic composites

The present study investigates the effects of high-lignin-loading on properties of lignin/polyethylene-co-vinyl acetate (EVA) rubber composites. Results from mechanical (quasi-static and cyclic) and rheological investigations revealed a brittle-ductile transition around a lignin volume fraction of φ_L = 0.59 coinciding with a twofold increase in steady-shear viscosity. Towards higher lignin contents, a 36% increase in dynamic stiffness C_dyn from φ_L = 0.59 (C_dyn ≈ 350 N mm⁻¹) to φ_L = 0.71 (C_dyn ≈ 550 N mm⁻¹) was observed by load increase tests (LIT). In addition, analyses of the ultra-micro-hardness revealed less indentation creep towards high-lignin-loading. At φ_L = 0.59, a pronounced relaxation endotherm superimposed on the glass transition (T_g) was observed, which was ascribed to molecular confinement to occur at highly loaded composites. At this point, the molecular weight (M_w) of lignin increased considerably. These results were explained by the different role of lignin at high volume fraction, i.e. a change from lignin as mere stiffness-inducing filler to a strength-imparting and fatigue-resistant matrix component which was supported by morphological analysis.