Changes with time in mechanical properties of wool fibers

When a wool fiber is left in an unchanging environment, it will eventually relax to a molecular state in which its mechanical properties are constant. This is the aging process. Some aspects of the process for a fiber which has been immersed in water and then left in air at 65% RH and 20°C are examined. With time, the fiber becomes stiffer as seen by force-extension, torque-twist, and stress relaxation experiments. The process, while very slow after 3 years aging at 65% RH and 20°C is still not complete. The mechanical state reached after 3 years at 65% RH and 20°C can be reached in 24 hr under the conditions 75% RH and 60°C, i.e., by increasing the temperature and vapor pressure, but leaving the moisture content unchanged. Various experiments show clearly that both the matrix and microfibrils are involved in the process, although the relaxation of the polypeptide chain of the matrix is seen as the predominant change taking place. The increase in stiffness of the fully aged fiber results from the formation of a stable network of secondary bond interactions between the polypeptide chains.