MECHANICAL PROPERTIES OF HAIR 73 It was observed, in general, that the F(1) values of 96 -+ 3 I•m diameter hairs reach equilibrium in about 1.5 minutes after immersion in de-ionized water (see section A-B of curve 1 in Figure 2). De-immersion of the fiber from water leads first to a stage of rapid recovery of F(1) (see section B-C of curve 2 in Figure 2, where it can be seen at the inflection point that the fiber recovers 85% of its initial F(1) in about five minutes). After this rapid recovery period, the fiber goes through a second long stage where the remaining 15% of F(1) is recovered rather slowly in about six hours, as portrayed in Figure 3. The observed transition between the first rapid and second long-term recovery stages described above is certainly associated with the turning point that exists between Fickean and viscoelastic water diffusion as already proposed by Watts (29,30). In Figure 3 are shown also the effects of relative humidity on the rates of recovery that appear to be faster for higher relative humidities. Presented in Figure 4 are the IRs at different relative humidities measured two hours after de-immersion from de-ionized water. By comparing Figures 3 and 4 it can be seen that for the entire range of humidities, F(1) reaches its original equilibrium value long before the indices of relaxation do. This time difference for F(1) and IR to reach equilibrium appears to be of the same nature as that observed by Sram et al. (25) and Marsh et al. (39). These authors found that fiber weight changes due to moisture absorption reach equilibrium faster than do their length and electrical conductivities. c 0 2 4 6 9 t0 t4 J6 ill TIHE [Hsl Figure 2. Percentage of F(1) variation as a function of time measured in hair fibers immediately after immersion, (i), and de-immersion, (d), under the following conditions: in de-ionized water, (i) -(1), (d) - (2) in 5% lysine, (i) - (3 /'x), (d) - (4) in 5% silk amino acids, (i) o (3 *), (d) - (5) in hydrolyzed wheat proteins and wheat oligosaccharides, (i) - (3 O), (d) - (6).

74 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aO OF F(fl RECOVERY _ tO ø 2 4 6 9 t0 t2 t4 t6 t8 20 TINE (HOURS] Figure 3. Percentage of F(1) variation as a function of time after two hours of de-immersion from de-ionized water at 60% RH, (1) 40% RH, (2) and 10% RH, (3) and from solutions of 5% hydrolyzed wheat proteins and wheat oligosaccharides, (4) and 5% silk amino acids, (5), both at 60% RH. The phenomena described above implies that the soft feel sensation and relative ease in manageability of wet hair after one or two hours of drying is due to its 85% recovery of F(1) combined with its long-term great deformability. Hysteresis cycle measurements of F(1), IR, and length dimensions at different equilibrium relative humidities (see Figures 5, 6, and 7) showed, on the other hand, that there was no difference in the equilibrium mechanical properties of the fiber before and after immersion in de-ionized water as expected. The immersion of hair fibers in aqueous solutions of LiCI, NaCI, CaCl 2, and MgCI2 resulted in time variations of length and F(1)s whose direction of variation was very much dependent on the salts concentrations and the moisture levels at which the hair fibers were conditioned. For instance, hair fibers conditioned from 0% to 5% RH and immersed in saturated solutions of LiCI presented length increments and decrements in their F(1)s (see Figure 8). Alternatively, if the same fibers were conditioned above 10% RH and immersed in saturated LiCI solutions, they showed instead contractions in length and increments in their F(1)s, which then leveled off with the immersion time (see Figure 8). For hair fibers conditioned at a particular moisture level, the observed increments in their F(1)s and length dimensions upon immersion decreased in the following order: LiCI, CaCI 2, NaCI, and MgCI 2, i.e., the lesser the moisture content in hair, the stronger, in the order described above, that the salt had to be in order to produce contractions in length and increments in F(1). These effects are well illustrated in Figure