Characterization of Kraft Lignin Fractions Obtained by Sequential Ultrafiltration and Their Potential Application as a Biobased Component in Blends with Polyethylene

Fractionation of Kraft lignin from black liquor is necessary to reduce its heterogeneity, which exerts negative effects upon lignin-commercial polymer composite materials. In this work, Kraft lignin was dissolved in acetic acid to get low viscosity lignin solution and fractionated into specific molecular weight fractions by sequential ultrafiltration with different molecular weight cut-offs. The fractionated lignins were characterized, and their suitability in polyethylene–lignin composites was evaluated. After ultrafiltration with 5k and 3k Da cutoff, fractionation resulted in three fractions of differing molecular weights and lower polydispersity than the original Kraft lignin (F1 (7010 g/mol), F2 (3540 g/mol), and F3 (1890 g/mol)). The results from spectroscopic characterization of lignin (31P, 13C, and 2D-HSQC NMR) and thermal stability analysis (TGA and DSC) indicated that the contents of various linkages and functional groups and the thermal properties of each fraction varied as a function of its molecular weight. Compared to F1 and F2, F3 (lowest molecular weight and high quantity of phenolic hydroxyl groups) most positively contributed to the mechanical properties of a polyethylene–lignin composite.