The Role of Residual Lignin in Microfibrillated Cellulose in Properties of Polylactic Acid Biocomposites

Microfibrillated cellulose (MFC) derived from wood sources is a biodegradable and eco-friendly reinforcing material for polymer composites. However, the high polarity of MFC is a challenge in homogeneous distribution into the hydrophobic PLA matrix, which limits its reinforcing efficiency. In this study, lignin-containing MFC (LMFC) with different residual lignin contents was prepared to investigate its dispersion behavior and reinforcing effect in polylactic acid (PLA). The aspect ratio and neutral sugar composition of LMFC remained similar regardless of lignin content, whereas the dispersion degree in PLA, quantified using a log-normal distribution model, increased from 24.2% to 35.1% with increasing lignin content. Mechanical testing showed that LMFC incorporation enhanced tensile strength and elastic modulus while reducing elongation at break. Higher residual lignin content in LMFC positively affected the tensile strength of the LMFC–PLA composites. Dynamic mechanical analysis revealed an increase in storage modulus and a decrease in loss factor with higher lignin content and LMFC loading (1–10 wt%), indicating enhanced interfacial interactions. Differential scanning calorimetry showed reductions in glass transition temperature (5–8 °C) and cold crystallization temperature (8–16 °C) compared to neat PLA. These findings indicate that residual lignin in LMFC enhances dispersion and interfacial interactions in PLA, leading to improved mechanical and thermal performance and highlighting its potential as an effective reinforcing component in sustainable biocomposites.