The evolution of structure and properties in high-tenacity poly(ethylene terephthalate) industrial fibers during thermal-oxidative aging process

Abstract

The changes in performance of the high-tenacity poly(ethylene terephthalate) (HT PET) industrial fibers during the thermal-oxidative aging process have been investigated over a broad temperature range from 120 °C to 200 °C. Additionally, the mechanism of the thermal-oxidative aging process has been studied by utilizing structural information obtained from the fibers at varying length scales. It was observed that a significant increase in breakage elongation when the fibers were subjected to temperatures above 160 °C for more than 8 h during the aging process. Macro defects induced by thermal-oxidative aging began to appear after 96 h of exposure at temperatures exceeding 160 °C. Microscopically, the crystal structure of fibers remained stable and did not show significant changes unless the aging temperature exceeds 160 °C. When subjected to lower thermal aging temperatures or shorter aging times, the molecular chains in the amorphous region relax and become disoriented. This leads to a significant decrease in amorphous orientation, lamellar long period, and an increase in elongation at break. As the temperature and aging duration time continue to increase, thermo-degradation and recrystallization become more prominent. Consequently, the overall macromolecular orientation initially decreases due to degradation but later increases due to the formation of crystalline structures within the fibers. These findings indicate that molecular degradation in ester group is the primary factor contributing to the breakage elongation changes, while the formation of newly crystalline structures within the fibers contributes to the prevention of breakage strength to some extent.