The Mechanical Properties of Permanently Set and Cystine Reduced Wool Fibers at Various Relative Humidities and the Structure of Wool

Abstract

The mechanical properties of disulfide reduced wool fibers and “permanently” set wool fibers have been measured over a range of humidities and times. For the permanently set fibers, it was found that at extensions in the yield region over the humidity range 20% to 100%, a fixed amount of stress, which is independent of relative humidity and time, is removed from the value of the stress corresponding to the same extension and humidity for a normal wool fiber. This fixed stress change is associated with ability of the crystalline β-phase in an extended fiber to reverse to the α-phase on retraction. For disulfide reduced fibers a progressive reduction in stiffness was observed with reduction of disulfide content. At 15% extension if Ft is the stress of an unreduced fiber at any time t and F't is the stress on a reduced fiber, then from previous results [9] Ft = ( a – br) ( t), where a and b are constants independent of t and the relative humidity r. The results presented show that F't may be expressed by F't = ( a' – br) ( t) where a' is a constant independent of t and r. Further ( a – a') is proportional to the difference of the disulfide content of the normal and reduced fibers. On the basis of these results a model that is an extension of the series zone model is proposed. It is suggested that the disulfide density of the matrix varies in a regular manner alongside the microfibrils in the fiber direction. Further, that the disulfide content and temperature, possibly by entanglement or disulfide interchange, control the “viscosity” of the differing sections of the matrix.