Abstract: A treatment for reducing friction of seat belts which is characterized by containing therein a polyether polyester having an average molecular weight of 2000-15000 which is obtained from a polyether diol and an aliphatic carboxylic acid, and preferably further containing a silicone compound. This treatment can be applied to either seat belt yarns in the yarn-making step or to seat belt webbing. The resulting seat belt is superior in slip properties and retains them even after wear. It is also superior in retractiablity and wear resistance (durability).

Abstract: To provide a polyamide fiber such as polyhexamethylene adipamide fiber having high tenacity and excellent tenacity retention after vulcanization and GY fatigue life. This fiber is a high tenacity polyamide fiber, preferably a polyhexamethylene adipamide fiber comprising at least 95% by mole of hexamethylene adipamide units and having a sulfuric acid relative viscosity of at least 3.0, said polyamide fiber satisfying the requirements: (a) differential birefringence .delta..DELTA.n.gtoreq.-5.times.10.sup.-3 to 0.times.10.sup.-3, (b) long period (Dm) in the direction of fiber axis: Dm.gtoreq.105 A, and long period (De) in the radial direction of fiber: De=90 to 130 A, (c) main dispersion peak temperature (T.alpha.) in mechanical loss tangent (tan.delta.) curve as obtained by viscoelastic measurement: T.alpha..gtoreq.125.degree. C., and preferably further satisfying the requirements: (d) birefringence .DELTA.n.gtoreq.60.times.10.sup.-3, (e) crystal orientation function fc.gtoreq.0.