Abstract:

Objective Bio-based polyamide 56 (PA56) fibers experience photo-oxidative degradation during prolonged ultraviolet exposure. This degradation leads to a decline in mechanical properties and service life, thereby restricting their outdoor use. Enhancing the Ultraviolet (UV) aging resistance of these fibers can prolong their service life in outdoor environments. For this purpose, TDI-TiO₂ composite particles were produced by modifying TiO₂ nanoparticles with toluene diisocyanate (TDI), and TDI-TiO₂/PA56 composite fibers were fabricated. The structure, thermal properties, and UV aging behaviors of the fibers were systematically investigated.

Method TDI was adopted to modify TiO₂ nanoparticles so as to produce TDI-TiO₂ composite particles which enabled the preparation of TDI-TiO₂/PA56 composite fibers by melt-blending and spinning processes. Experiments determined the optimal modification conditions for TDI and TiO₂. The morphology, chemical structure, crystallization behavior and thermal stability of the composite fibers were characterized analyzed. UV accelerated aging tests exposed fibers for 160 h. Subsequent measurements evaluated changes in mechanical properties and specific viscosity, comprehensively assessing the UV aging resistance of the composite fibers.

Results This study modified nano-TiO₂ using TDI. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses confirmed that TDI grafted onto the TiO₂ surface. scanning electron microscopy (SEM) images showed that TDI modification significantly improved the dispersion of TiO₂ in the PA56 matrix, and a TDI to TiO₂ ratio of 1∶10 yielded the best dispersion. During melt-blending, the unreacted isocyanate groups in TDI reacted with the terminal amino groups of PA56 chains to form urea linkages, creating covalent bonds between TiO₂ nanoparticles and the polymer matrix which greatly enhanced TiO₂ dispersion and suppress its photoactivity. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests showed that the addition of TDI-TiO₂ lowered the crystallinity of PA56 from 24.8% to 20.9% without altering its crystal structure. (TG) analysis indicated a slight decrease in the thermal stability of the PA56/TDI-TiO₂ composite fibers. Meanwhile, the char residue increased to 1.43 wt%, demonstrating better carbonization ability at high temperature. After 160 h of UV aging, the PA56/TDI-TiO₂ composite fiber exhibited significantly superior aging resistance compared to PA56 fiber or the PA56/TiO₂ blend. Specifically, the modified fiber maintained a tensile strength retention of 45.1%, with an absolute value of 1.84 cN/dtex. This value was significantly higher than that of pure PA56 (1.26 cN/dtex) and the unmodified PA56/TiO₂ blend (1.72 cN/dtex). This represented an improvement in mechanical property retention of 20.8% over the pure PA56 system. Characteristic viscosity measurements revealed that PA56/TDI-TiO₂ fibers underwent the smallest molecular weight loss, decreasing only from 0.521 dL/g to 0.409 dL/g. By contrast, PA56 fibers lost much more molecular weight, dropping sharply from 0.541 dL/g to 0.168 dL/g. FT-IR analysis detected new absorption peaks for oxygen-containing groups (e.g., conjugated imides and carboxylates) in the aged PA56 and PA56/TiO₂ fibers, which indicated photo-oxidative chain scission. In contrast, the FT-IR spectra of PA56/TDI-TiO₂ fibers showed no major new peaks. This confirmed that the modified material effectively resisted photochemical degradation. SEM observations further supported these results, showing that PA56/TDI-TiO₂ fibers developed the fewest surface cracks and micro-pores after aging. In conclusion, the addition of TDI-TiO₂ moderately reduced the crystallinity of PA56 and lowered its initial pyrolysis temperature, but these changes did not impair its melt processing properties.

Conclusion Surface modification of nano-TiO₂ with TDI produces TDI-TiO₂ composite particles, and PA56/TDI-TiO₂ composite fibers are fabricated by melt-blending and spinning these particles with PA56 which resists UV light. TDI modification improves the dispersion of TiO₂ and enhances its compatibility with the PA56 matrix. Although adding TDI-TiO₂ slightly reduces the crystallinity of PA56 and lowers its initial thermal decomposition temperature, it does not harm the melt processing. After 160 h of UV aging, the composite fiber maintains a tensile strength of 1.84 cN/dtex. It retains its mechanical properties 20.8% better than pure PA56 or the unmodified PA56/TiO₂ blend. Its performance is significantly superior. Adding TDI-TiO₂ effectively slows the photo-oxidative degradation of PA56 chains. This result confirms the material's suitability for outdoor applications.

Key words: bio-based polyamide 56, bio-based fiber, ultraviolet-resistance, titania, toluenediscocyanate modification, melt blending, composite fiber