Synthetic fiber processing agents and synthetic fibers

Abstract

The invention relates to a synthetic fiber treatment agent that improves both heat resistance and storage stability; and a synthetic fiber to which the synthetic fiber treatment agent is coated. The synthetic fiber treatment agent comprises: a smoothing agent (A); a non-ionic surfactant (B); an ionic surfactant (C) containing a phosphoester compound (Cl); and an alcohol compound (D). The phosphoester (Cl) compound comprises at least one compound selected from the phosphoesters Pl, P2, P3, and P4, which are respectively represented by specific chemical formulas. When the total ratio of p-nucleus NMR integrals for phosphoesters P1 to P4 is 100%, the ratio of p-nucleus NMR integrals for phosphoester P1 is 7% or less.