How Carbodiimide Modulates Oligomer Migration and Molar Mass in Recycled Poly(lactic acid): A Study Using UPLC-QTOF-MS^E

Abstract

The recycling of polymers for direct food contact faces initial challenges due to the loss of molar mass and the generation of Non-Intentionally Added Substances (NIAS), among which oligomers are prominent. In this study, carbodiimide (CDI) was employed as an antihydrolysis agent and chain extender to prevent oligomer formation in the recycling of poly(lactic acid) (PLA). Mechanical recycling was performed on wet and dried PLA with varying concentrations of CDI using corotating intermeshing twin-screw extrusion. CDI contributed to the increase in polymer molar mass and influenced the kinetics of its nonquiescent crystallization. Moisture played a key role in this process, as CDI can react either with water molecules, preventing hydrolytic degradation, or with available sites on the PLA molecule, resulting in increased molar mass and a decrease in the oligomer content. Oligomer migration tests were conducted using three food simulants: ethanol 10%, acetic acid 3%, and ethanol 95%. Ultraperformance liquid chromatography coupled with a quadrupole timeof-flight mass spectrometer was utilized for qualitative and quantitative PLA oligomer analyses. Electronic structure calculations based on the GFN-xTB Hamiltonian were employed for structural optimizations and energies to understand migration and interaction processes in the described systems. Sixteen different PLA oligomers were detected in the food simulant samples. Linear oligomers were identified in all simulants, while cyclic were detected mainly in the 95% ethanol simulant. The antihydrolysis and chain-extending effects of CDI in recycled PLA reduced the total oligomer concentration in both wet and dry samples, improving the physicochemical properties of PLA and ensuring food safety.