Disclosing the UV aging mechanisms of polyamide 66 industrial fiber on the base of the multi-scale structural evolutions

This study investigated the changes in the properties of polyamide 66 (PA66) industrial fibers during UV aging and explored the underlying UV aging mechanisms through structural data analysis across varying length scales. The findings reveal a three-stage progression in the deterioration of mechanical properties due to UV aging. Initially, in the pre-aging stage (t ≤ 1 day), both mechanical properties and microscopic structural parameters remain stable, showing no significant changes. Following this, UV-induced degradation and concurrent recrystallization in the amorphous region result in a decline in mechanical parameters as the aging process continues, albeit with varying magnitudes across the stages. A notable decrease in tenacity and elongation at break is observed in the mid-aging stage (1 day < t ≤ 49 days). While the formation of low-melting-point microcrystals enhances crystallinity and crystalline size, the predominant effect of molecular chain scission due to UV exposure leads to a significant increase in carbonyl group content and a marked decrease in intrinsic viscosity. Furthermore, the formation of microcrystals hinders oxygen diffusion, resulting in the gradual stabilization of intrinsic viscosity and carbonyl content in the late-aging stage (t > 49 days), which significantly slows the deterioration of mechanical properties. These results indicate that UV-induced molecular degradation is the primary driver of changes in break elongation, while the development of new crystalline structures within the fibers contributes to the preservation of mechanical properties.