Natural Latxa Sheep Wool as an Environmentally Friendly Substitute for Specific Organic Fibers in Railway Friction Materials: A Preliminary Approach

Abstract

The present work characterizes a natural sheep wool and compares its properties to those of a range of three different, well-known organic fibers that are typically used in the friction material production: aramid, cellulose, and polyacrylonitrile (PAN) fibers. The characterization work consisted of the study of the fibers alone, the nonpressed friction materials, and the final railway brake blocks. By these means, it was proven that the wool obtained from latxa sheep (Ovis aries) is a suitable replacement for cellulose because the wool presented slightly better thermal resistance in thermogravimetric analysis, and the particle retention ability of both fibers was similar. The latxa sheep wool was also found to delay the curing reaction to a similar degree as the PAN fiber. Finally, the friction material including sheep wool presented physical and mechanical properties similar to those including cellulose and PAN fibers, and no major differences were found in friction with changing speeds or temperatures. In addition, the inclusion of sheep wool led to the lowest speed and force fade and to a good recovery behavior after the speed and temperature phases, only second to the friction material using PAN fiber. An average spread in the instantaneous friction coefficient and a slightly higher wear are reported for the material containing sheep wool. Finally, no significant differences were found on the friction surfaces or on the wear mechanism when their microstructure was analyzed using scanning electron microscopy.