Utilization of lignin fractions in UV resistant lignin-PLA biocomposites via lignin-lactide grafting

Lignin was fractionated with several organic solvents and fractions were utilized for UV resistant lignin-PLA composites. First, soda lignin (SL) was sequentially fractionated into six fractions: ethyl acetate (F1), 2-butanone (F2), methanol (F3), acetone (F4), dioxane/water (F5), and an insoluble fraction (INS). Molecular weight of the fractions increased from F1 to F5 and phenolic hydroxyl contents decreased with increasing molecular weight of fractions. Five lignin fractions (SL, F1, F3, F5, and INS) were grafted with l-lactide to produce lignin-grafted poly-l-lactide (PLLA) copolymers. Conversion ratio of l-lactide to PLLA chains increased from 88.3% for F5-PLLA copolymer to 91.2% for F1-PLLA copolymer as the content of hydroxyl groups in the fraction increased, while the molecular weight of the copolymers showed the reverse tendency. Each copolymer was mixed with PLA 2002D, and mechanical and optical properties of the composites were investigated. Composites of F1, F3, and F5 showed a tensile strength around 65 MPa, which is similar to that of neat PLA. The elastic modulus increased from 2197.7 for F1 to 2503.4 MPa for F5. According to the investigation of UV-VIS transmittance of the composite films, composites of F3 and F5 showed better UV blocking ability than the other composites, and this UV blocking ability increased with increasing concentration of lignin copolymer.