VISCOELASTIC BENDING RECOVERY OF HAIR 37 of the same material in which the bending stiffness of the fi laments, B∞, is reduced to relative values of 70%, 50%, and 30%, respectively, while that of the matrix ΔB is kept unchanged. Figure 4 shows that damage to the elastic fraction of fi ber bending stiffness is expected to signifi cantly promote and stabilize the set for the aging fi ber, while the ef- fect in the non-aging fi ber will eventually be lost. In practice, however, the benefi cial ef- fects of fi lament damage are expected to be offset through collateral processing damage to the matrix and other morphological components, as well as the expected higher suscepti- bility of damaged hair to humidity and temperature changes, which will remove the benefi ts of physical aging. Figure 3. Recovery values of aging hair at log t = 11 (t 3000 years), considered as representing “equilib- rium,” fi nal recovery, Rf, for all practical purpose, vs water content. A straight line is fi tted through the data, for which the equation is given on the graph in the usual x/y-notation. The broken line marks the extrapola- tion range. Figure 4. Bending recovery curves of non-aging (broken lines) and aging hair (solid lines) at 65% RH and tA=10 min, in which the elastic bending stiffness of the fi laments, B∞, has been reduced from the initial, relative value of 1 to 0.7, 0.5, and 0.3, respectively. The curves for the aging material are marked accord- ingly.

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