The Stress Relaxation of Fibrous Materials

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Abstract
An apparatus has been built to study the relaxation behavior of wool, silk, and nylon as a function of reagent temperature, concentration, and hydrogen ion concentration. In distilled water all three materials show the same behavior over most of the time range studied—that is, a gradual relaxation of perhaps one-third of the initial stress, attributable in all likelihood to the destruction of hydrogen bonds. When the water is replaced by sodium bisulfite solution at pH 6.5, the rate of stress relaxation of wool keratin increases markedly. This change is at tributed to the scission of the disulfide cross-links in this material by the sulfite ion, in view of the finding that silk and nylon, which are free of cystine, are not affected by sodium bisulfite. Further experiments on wool have been made to show the effects of solutions of hydrochloric acid, urea, thioglycolic acid, and cysteine. The addition of 0.1M HCl to an extended fiber re laxing in water produces a sharp drop in the stress; but the stress returns to its previous value when the acid is replaced with water. When 0.1M hydrochloric acid is replaced by sodium bisulfite solution at pH 1.2, the rate of stress relaxation is much slower than that observed with sodium bisulfite at pH 6.5. The addition of 10M urea to a fiber relaxing in water produces a relatively slow decay of stress. When the 10M urea solution is replaced by a solution of 0.1M sodium bisulfite in 10M urea, the rate of stress relaxation is more rapid than that produced by sodium bisulfite alone. Cysteine and thioglycolic acid solutions at pH 6.5 produce an approxi mately exponential decay of stress when added to wool fibers previously relaxed in water.

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