Fully degradable poly(L-lactic) acid (PLLA) and lignin blends were prepared using the melt blending method. The impact strength of PLLA was dramatically improved by 52.4% and 36.6% with the addition of 5 wt% and 10 wt% of lignin, respectively. Meanwhile, the Young's modulus was maintained. Polarized optical microscopy (POM) results indicated that lignin served as a nucleating agent for the heterogeneous crystallization of PLLA in blends, which was responsible for the improvement in the impact strength. The introduced lignin also promoted the cold-crystallization of PLLA, which was demonstrated by differential scanning calorimetry (DSC). The blends of PLLA with lignin are considered to be a promising material because of the improved toughness, the full degradability, and the lower price compared with pure PLLA.
To improve the mechanical properties of lignin-filled poly(L-lactic) composites, three silane coupling agents, 3-aminopropyltriethoxysilane (KH550), γ-glycidoxypropyltrimethoxysilane (KH560), and g-methyacryloxypropyltrimethoxysilane (KH570), were treated systematically with different solvents to modify the interfacial connections. The treatment of lignin with 2 wt.% aqueous KH550 solution was proved to be the most successful. Chemical bonding between the filler and the matrix was formed, according to the FTIR spectra. Furthermore, scanning electron microscope images showed that such treated lignin particles dispersed well in the composites. The tensile strength and Young's modulus of the composite improved significantly from 55.1 and 1589 MPa to 67.0 and 1641 MPa, respectively, with 5 wt.% treated lignin addition. Although its elongation at break decreased from 20.3 to 12.4% after 5 wt.% of the treated lignin was added, it was still better than that of poly(L-lactic acid) without any additive (10.3%).
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