Plasticized PLA Composites Enhanced with Cellulose Nanoparticles for Improved Mechanical and Thermal Performance

Polylactic acid (PLA) is a bio-based polyester widely used for its good mechanical properties, biocompatibility, and inherent biodegradability, which enables end-of-life options such as recycling and industrial composting. Despite these advantages, its brittleness, slow crystallization rate, and low crystallinity limit broader application. This study aims to enhance the performance of PLA through the development of biodegradable composite material by incorporating bio-based plasticizers and cellulose nanofibers (CNFs). Triacetin (TA) and triethyl citrate (TEC) were used as plasticizers to improve toughness and processability, while CNFs were added at loadings of 1, 2, and 3 wt% to promote crystallization and thermal stability. The thermal and mechanical properties of the PLA composites were evaluated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensile testing. DSC results showed that the addition of TA and TEC slightly reduced the melting temperature by approximately 4 °C and significantly lowered the glass transition temperature (Tg) by about 20 °C, indicating enhanced chain mobility. The combined incorporation of plasticizers and CNFs resulted in smaller or less perfect crystalline structures, reducing the crystallinity index from 65% for neat PLA to 40–50% for the composites. TGA revealed improved thermal stability, with an increase of approximately 10 °C in the onset degradation temperature compared to neat PLA. Mechanically, plasticization reduced tensile strength from 63.75 MPa to around 20 MPa while markedly increasing elongation at break from ~8% to ~400%. Optimal CNF loadings were identified at 1 wt% for PLA/TEC and 3 wt% for PLA/TA systems.