Sustainable regulation of polylactic acid crystallization and toughness by bio-based epoxidized oils

Polylactic acid (PLA), one of the most widely used bio-based polymers, suffers from brittleness and low ductility, which limit its industrial applications. The introduction of plasticizers is therefore expected to overcome these drawbacks. This study investigates the effects of both chemically synthesized epoxy reagents and bio-based epoxidized oils, as additives (acting as plasticizers) for PLA, on its thermal behavior, crystallization kinetics, morphology, viscoelastic properties, optical transparency, and mechanical performance. PLA composites containing 15 wt% of four different plasticizers were prepared and systematically evaluated by differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR), polarized optical microscopy (POM), dynamic mechanical analysis (DMA), UV–visible transmittance measurements, and tensile testing. DSC results showed that all plasticizers decreased the glass transition temperature and cold crystallization temperature, indicating a clear plasticizing effect. Among them, the chemically synthesized diglycidyl ether (E-PT) markedly accelerated crystallization, shortening the half-crystallization time to one third of neat PLA. However, this excessive acceleration is likely to induce embrittlement and reduce long-term stability. In contrast, bio-based epoxidized oils (epoxidized linseed oil and epoxidized soybean oil) did not significantly alter crystallization kinetics but suppressed excessive crystal growth while increasing strain at break, and toughness. DMA measurements exhibited single tanδ relaxation peaks for all systems, suggesting relatively good compatibility without severe macroscopic phase separation. Tensile tests confirmed that PLA composites containing bio-based oils exhibited lower modulus but improved ductility and energy absorption capability compared with neat PLA, while maintaining practical optical transparency. These findings suggest that while synthetic epoxy reagents strongly promote crystallization, bio-based epoxidized oils act as effective and sustainable plasticizers, providing a more balanced modification that enhances mechanical stability without significantly compromising transparency or thermal behavior.