Relation of Lubricant Structure to Frictional Properties : Polyoxyalkylene Monoether Lubricants on Filament Yarns

Abstract

The fiber-to-metal frictional behavior of polyester, polyamide, and polypropylene multifilament yarns lubricated with random copoly (oxyethylene-oxypropylene) monoethers having number average molecular weights ranging from 360 to 4200 has been studied over the sliding speed range of 0.01 to 400 m/min and temperatures from 21 to 225°C. It has been found that striking similarities exist between the frequency dependence of dynamic mechanical properties of the bulk lubricant polymers and the speed dependence of dynamic fiber-to-metal friction exhibited by the copolyether lubricants. Plots of yarn tension vs. sliding speed pass through a maximum, and the speed at which friction reaches its maximum value, Umax, is inversely related to the lubricant molecular weight ( Mn). By plotting Umax against the reciprocal average molecular weight, a straight line of positive slope is obtained in agreement with a transformed expression of the Fox-Loshaek equation relating reciprocal molecular weight and the lubricant glass transition temperature. These indications of a fundamental relationship between the maximum in the friction versus sliding-speed curve and the frequency of the maximum for the dynamic mechanical loss modulus of the lubricant polymer are further substantiated by the temperature dependence of Umax, which follows an Arrhenius type equation. At low sliding speeds, below 10 m/min, the fiber-to-metal friction of the copolyoxyalkylene monoether lubricants is largely governed by the hydrodynamic effect of the lubricant film. Thus, the relation between the friction force F, speed U, and the steady state viscosity of the lubricant η can be described by the expression F = k(η U)1/ n, where the constant k is dependent on the fiber polymer, and the constant n is independent of the fiber type.