Sunlight Exposure on Vat‐Dyed Polysulfonamide Fibers: Analysis of Mechanisms and Effects

Abstract In this paper, the sunlight resistance of vat‐dyed polysulfonamide (PSA) fibers was systematically studied, with particular focus on the mechanisms of UV‐induced degradation and the protective role of vat dyes. Vat‐dyed PSA with Vat Red 31 (PSA/VR), Vat Blue 6 (PSA/VB), and Vat Violet 13 (PSA/VV), were evaluated for UV absorption, free radical formation, color stability, and mechanical properties. UV absorption analysis revealed that PSA/VR exhibited the highest integrated UV absorption (1657.3), followed by PSA/VB (1355.2) and PSA/VV (1126.2), with lightfastness ratings ranging from 4 (PSA/VR) to 5 (PSA/VV). Electron spin resonance (ESR) analysis confirmed that free radical generation followed the order: undyed PSA < PSA/VR < PSA/VV < PSA/VB, indicating a direct correlation with UV absorption. Colorimetric analysis showed that undyed PSA fibers darkened over time (ΔK/S = −1.43), whereas vat‐dyed PSA fibers exhibited a photodarkening effect, with PSA/VV demonstrating the highest color retention (ΔK/S = −0.81). X‐ray photoelectron spectroscopy (XPS) analysis revealed significant amide bond degradation in PSA/VR (CO–NH content reduction from 14.85% to 3.63%), whereas PSA/VV exhibited better stability (retaining 6.38%). The results indicate that vat dyes enhance the photostability of PSA fibers, with PSA/VV offering superior sunlight resistance, making it a promising candidate for applications requiring extended UV exposure.