Synergistic Effects of Lignin Type and Polyethylene Glycol on the Structure Property Relationships of PLA/Lignin Biocomposites

The growing electric vehicle (EV) industry necessitates sustainable material solutions to reduce reliance on petroleum-based plastics. This study investigates lignin-reinforced polylactic acid (PLA) biocomposites for sustainable EV components, focusing on rearview mirror covers. Alkaline lignin (AL) and sodium lignosulfonate (LS) were incorporated into PLA at 0–3 wt.% with polyethylene glycol (PEG400) as a compatibilizer. The objective was to develop a bio-based composite suitable for non-structural EV components, specifically a rearview mirror cover. The composites were characterized via scanning electron microscopy (SEM), universal testing machine (UTM), thermogravimetric analysis (TGA), and melt flow rate (MFR) testing. Results revealed that LS-PLA composites exhibited superior mechanical strength (38.6 MPa tensile strength at 1 wt.%) and thermal stability (onset degradation: 347.9 °C) compared to AL-PLA, attributed to enhanced interfacial compatibility. PEG increased MFR by 40–60% but reduced tensile strength by 15–20% due to plasticization. Optimal performance was achieved with 1 wt.% LS, demonstrating balanced processability (MFR: 12 g/10 min), thermal resilience, and economic viability (IRR: 28.5%). These biocomposites reduce petroleum dependency while meeting EV industry standards, supporting circular economy goals.