Effect of Desulfonation of Lignosulfonate on the Properties of Poly(Lactic Acid)/Lignin Composites

To utilize the lignin generated by the paper industry and reduce the cost of poly(lactic acid) (PLA), PLA/lignin composites were prepared from PLA and different ratios of lignosulfonate (LS) or desulfonated lignosulfonate (DLS) particles using a casting method. The physicochemical properties of the lignins were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), gel permeation-high performance liquid chromatography (GPC), and elementary analysis. The results indicate that the sulfur content of the original LS was successfully reduced to half by desulfonation to produce DLS, which was found to have a higher thermal stability and a lower average molecular weight than LS. Additionally, the thermal stability, crystallization, compatibility, mechanical, hydrophobicity, and optical properties of the PLA/lignin composites were also meticulously evaluated. Comparison of the PLA/DLS and PLA/LS composites revealed that the incorporation of DLS into PLA improved compatibility, thermal stability (T5% and Tmax), and hydrophobicity, while the mechanical properties remained almost unchanged. In addition, both PLA/DLS and PLA/LS exhibited UV light absorption capacity. Finally, the low-rate addition of both LS (10%) and DLS (5%) accelerated the crystallization of PLA, but crystallization was delayed with higher lignin content.