Deep Eutectic Solvent-Extracted Lignin as an Efficient Additive for Entirely Biobased Polylactic Acid Composites

This work reports the fabrication and characterization of entirely biobased composites made with polylactic acid (PLA) and deep eutectic solvent (DES)-extracted lignin. White fir sawdust and corn stover were used as feedstocks to extract sawdust lignin (SDL) and corn stover lignin (CSL). Commercial alkali lignin (CAL) was also used as a baseline. PLA/lignin composites were fabricated using a twin-screw extrusion process followed by compression molding. Characterizations of the composites were conducted using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and tensile testing. Overall, the results revealed that PLA/DES lignin biocomposites significantly outperform their PLA/CAL counterparts. SEM results indicated the absence of microcracks in PLA/DES lignin morphologies and better lignin dispersion and smaller lignin agglomerates compared with PLA/CAL composites. The initial thermal degradation temperature of PLA slightly dropped by 8–27 °C with the addition of 5–15 wt % DES lignin, compared with a significant reduction of 89–124 °C when incorporating CAL. DSC analysis showed a maximum drop of about 5 and 15 °C in the melt temperature of PLA when DES lignin and CAL lignin, respectively, were added. Moreover, SDL increased the PLA’s crystallinity several folds. The tensile strength and elongation at break of PLA/DES lignin composites were significantly greater than those of PLA/CAL composites, up to a maximum of 1 order of magnitude. The better performance of PLA/DES lignin biocomposites was associated with the high purity, ultrafine particle size, low heterogeneity, and low molecular weight of DES lignin. The results suggest DES lignin as a potential feedstock for entirely biobased high-performance materials.