Controlling the Thermodynamic Stability of Melt-Compounded PLA as Opportunity to Achieve 3D Printing Automotive Items with Medium Lifetime

In view of the future estimation of the life-time of 3D printed automotive components, this paper evaluates the thermodynamic stability of controlled-nucleated poly (lactic acid) (PLA), focusing on formulations that maintain good mechanical behavior after 4 years of storage under controlled conditions. PLA with 0.5% D-lactide and low molecular weight, which has optimal melt flow at 3D printing, was nucleated using either a sulfonic acid derivative (heterogeneous nucleation) or a PLA grade with 4% D-lactide (stereocomplex or racemic nucleation). Since the earliest signs of thermodynamic instability manifest as changes in chemical structure, which alters thermal behavior, this study focuses on FTIR, DSC analysis and some functional properties such as impact resistance and heat deflection temperature (HDT). The initial properties were compared with those measured 4 years later. Due to heterogeneous nucleation, the bi-modal melting of neat PLA turned into a mono-modal peak, which remained stable over 4 years. Initially, the mono-modal melting of racemic nucleated PLA transitioned into a bi-modal pattern over time, proving its long-term thermodynamic instability. Because 3D printing requires mono-modal melting, it was concluded that racemic crystallization is unsuitable for the used PLA modification with respect to future 3D printing of medium-life automotive components. Crystallinity shapes long-term mechanical performance; therefore, the process must be conducted under selected conditions.