Temperature-Dependent Mechanical Behaviour of Polylactic Acid Plasticised with a Bio-Based Chitin–Lecithin System

Polylactic acid (PLA) is a bio-based thermoplastic whose inherent brittleness limits mechanically demanding applications. This study investigates how a bio-based chitin–lecithin plasticiser system affects the mechanical behaviour of injection-moulded PLA as a function of additive composition, testing temperature, and annealing-induced crystallisation. Tensile and notched Charpy impact tests were performed under ambient conditions, complemented by dynamical mechanical characterisation, tensile testing at 40 and 50 °C, and DSC analysis after annealing at 130 °C for 2 h. Chitin alone increased Young’s modulus from 0.95 ± 0.08 to 1.26 ± 0.02 GPa but reduced ductility and impact strength. In contrast, ternary formulations showed increased deformability at low-to-intermediate additive contents, with fracture strain increasing from 6.87 ± 0.61% for neat PLA to 27.38 ± 3.73%. Dynamical mechanical characterisation showed only a small shift in the glass-transition temperature, while tensile deformability increased strongly as the testing temperature approached the glass-transition range. Annealing lead to crystallinities of approximately 43–45% in the ternary formulations and reduced fracture strain by 86–93%. These results indicate that the chitin–lecithin system promotes deformation of the amorphous PLA phase without a pronounced reduction in bulk glass-transition temperature, while crystallisation largely suppresses this effect.