The Plasticity of Wool

Abstract

The variations in plasticity among the fibers of a single staple of wool have been shown to be associated with variations in swelling in water and in 100% formic acid; as a rule, fibers with the lowest plasticity (primary follicles) show the least swelling, and fibers with the highest hest plasticity (secondary follicles) the greatest swelling. In the great majority of cases such differences of plasticity and swelling do not seem to be governed by difference in sulfur content, i.e., in the degree of cross-linking with cystine. They are, however, dependent on the relative proportions of accessible (amorphous) and inaccessible (crystalline) material in the fibers, the proportions of accessible material being deduced from the gain in dry weight of the fibers after exposure to D₂O. Fibers showing the highest plasticity and swelling contain the greatest proportion of accessible material, and vice versa. Further, fibers of high plasticity contain more tyrosine than fibers of low plasticity, suggesting that the bulky side chains of tyrosine are one cause of main chain disorder in wool, just as in the case of silk. The above observations have an important bearing on the assessment of wool quality. Using 20 Cape merino wools, which had been classified as good, ordinary, or common in quality by expert appraisers in South Africa, it has been shown that the good quality wools have the highest plasticity, even when comparison is restricted to groups of fibers of similar thickness. Since the behavior of wool at various stages of processing is de termined to some extent at least by the plasticity of the fibers, and since there is no necessary connection between plasticity and fineness of fiber, it must be concluded that the quality of wool cannot be defined in terms of dimensional characteristics alone. Not until breeding policy is based on knowledge of the substance of fibers as well as of their dimensions can there be any precise and effective link between the producer and user of wool.