The Role of Polycarbodiimide in PET Monofilaments: An Investigation on Hydrolytic Stability, Crystallization Behavior, and Mechanical Properties

Polyethylene terephthalate (PET) monofilaments offer excellent performance, yet their susceptibility to hydrolytic degradation in high‐humid environments severely limits their long‐term application. Polycarbodiimide (PCDI) was incorporated as a hydrolytic stabilizer at varying contents (0–2.5 wt%) via a melt‐blending spinning process. The effect of PCDI contents on the hydrolytic stability, crystallization behavior, and mechanical properties of the monofilaments was systematically investigated. The results demonstrate that PCDI content at or below 1.5 wt% promotes lamellar lateral growth and molecular orientation compared to those of net PET monofilament. These microstructural modifications significantly enhance the monofilaments' hydrolytic stability and thermal resistance while maintaining excellent mechanical properties. Specifically, the PET/PCDI‐1.5 monofilament, with 1.5 wt% PCDI, exhibits a breaking stress of 441.39 MPa and a breaking elongation of 31.77%. After 14 days of hygrothermal aging, its retention rates of breaking stress and breaking elongation reach 80.37% and 57.35%, respectively, significantly higher than those of pure PET monofilament. However, exceeding 1.5 wt% PCDI leads to phase separation, disrupts lamellar order, and induces thermal degradation, thereby compromising the breaking stress. This study identifies the optimal PCDI content and clarifies the underlying microstructural regulation mechanism for developing durable PET monofilaments.