Synthesis of Tannic Acid-ESO Microgel via In Situ Melt Reaction for Reinforcing Heat-Resistant PLA Fibers

Poly(lactic acid) (PLA) is a biobased polymer with extensive potential for application in the fiber industry. However, its slow crystallization rate and low crystallinity limit its heat resistance, rendering it insufficient for many practical applications. In this study, heat-resistant tannic acid-epoxidized soybean oil (TA-ESO)/PLA composite fibers reinforced with TA-ESO microgel are successfully fabricated via melt spinning. The TA-ESO microgel functions as a physical cross-linker, establishing a cross-linked structure within the PLA matrix. The incorporation of TA-ESO microgel substantially influences the isothermal crystallinity of the TA-ESO/PLA composite and effectively reduces spherulite size. Moreover, the TA-ESO microgel enhances the mechanical properties of as-spun TA-ESO/PLA fibers, increasing tensile strength from 0.80 cN/dtex of pure PLA to approximately 1.00 cN/dtex. The hot-drawn process significantly improves both the mechanical properties and thermal stability of TA-ESO/PLA fibers. The tensile strength of TA-ESO0.5/PLA-DR-3.0 reaches 3.04 cN/dtex, nearly three times that of as-spun fibers. Following hot-drawn at various draw ratios, the Tg of TA-ESO0.5/PLA fibers increased from 62.9 to 86.9 °C. The TA-ESO0.5/PLA-DR-3.0 fiber exhibits excellent heat resistance, with its boiling water shrinkage rate dramatically reduced from 61.32% of as-spun TA-ESO0.5/PLA fiber to 8.90% (83.44% of PLA to 13.96% of PLA-3.0), while exhibiting minimal curling. This study provides a method to significantly increase the use temperature of PLA fiber, facilitating their further development and potential applications.