Kinetics and Temperature Dependence of the Chemical Stress Relaxation of Wool Fibers

Abstract

The stress relaxation process of keratin fibers may be divided into two stages, the first rapid stage involving hydrogen bond breakdown and the second slower stage being connected with disulfide bond scission. The temperature dependence of the second disulfide-controlled stage of stress relaxation of wool fibers from extensions of 20 and 40% was investigated at pH 7.0, both in the presence and absence of the sulfhydryl- blocking agent N-ethylmalimide (NEMI). By plotting the relaxation modulus E_r as a function of temperature, a transition at 70°C was observed for untreated wool fibers. An increase in the SH content by partial reduction of the wool cystine decreases this transition temperature, as was expected from previous evidence of the SH dependence of the transition temperature under stress obtained from stress-strain curves [20]. By normalizing the stress at any time with a stress value in the presence of the SH-group blocking agent, it was possible to eliminate the first stage of stress relaxation without losing any component of the second stage. The viscoelastic behavior of the second stage was found to approximate that of a single Maxwellian element. The controlling mechanism in 'this stage appears to be a disulfide interchange catalyzed by free. SH groups, with an activation energy of approximately 23 kcal mole, which is independent of the strain level.