584 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Torsion Modulus of Waved Hair a at Various Moisture Conditions Torsion Modulus, 10 •ø dynes/cm • Hair In Lot No. 41% RH 58% RH 65% RH 81% RH 93% RH Water 405 1.16 1.05 0.96 0.68 0.37 0.14 404 1.27 1.13 1.09 0.80 0.44 0.17 403 1.34 1.23 0.99 0.81 0.39 0.12 402 1.24 1.11 0.91 0.76 0.40 0.13 Mean 1.25 1.13 0.99 0.76 0.40 0.14 All measurements were made at 21 :k IøC. The results for fibers tested at 65% RH and immersed in water were obtained by the direct twist method, all other results by the pendulum method. • Permanent waving process corresponded to standard type of commercial treatment except that process time was extended to 90 minutes. Table IV Creep Properties of Waved Hair •* under Torsion at Various Moisture Conditions Creep in 5 Minutes at a Constant Couple of 3.1 dyne-cm, Turns per cm Length b Hair Lot No. 65% RH In Water 405 0.01 0.10 404 0.01 0.10 403 0.01 0.12 402 0.01 0.09 Mean 0.01 0.10 All measurements were made at 21 :k 1 øC. The results for fibers tested at 65% RH and immersed in water (creep) were obtained by the direct twist method. • Permanent waving process corresponded to standard type of commercial treatment except that process time was extended to 90 minutes. b A couple of 3.1 dyne-cm produces a fiber twist in a typical hair of about 0.5 turn per cm at 65% RH and about 3-4 turns per cm in a wet fiber. that moisture acts to decrease them. Most interesting, however, is the finding that waved hair on the average tends to exhibit greater stiffness at low humidities. The effect of permanent waving in increasing the torsional stiffness is seen in most, but not all, samples hair lot 402 is an example of a case in which the permanent waving treatment produced no significant alteration in the "dry" torsional properties. The damping capacities of the hair given by the logarithmic decre- ment are, within experimental error, similar in the waved and unwaved fibers and are not reported here. The creep properties of the waved

TORSIONAL PROPERTIES OF HAiR 585 hair obtained by the direct twist method are shown in Table IV. The application of the chemical waving treatment does not seem to alter the flow properties appreciably at low humidities, the creep being low in any event. In the wet state, however, the permanent-waved hair is even more plastic and less stiff than before treatment. These results may explain the value of permanent waving in the hair setting process. The fibers assume the set configuration more easily in the wet state, and the ease of deformation is further enhanced with wet waved hair. The hair flows more easily under the confiõurational strain, especially when waved. Drying to normal low humidities restores the fiber stiffness and elasticity to higher levels with hair containing a permanent wave, the torsional stiffness offers at least equal, and often greater, resistance to deformation than unwaved hair. THE BEHAVIOR OF HAIR TRESSES AS COILS The previous section started with consideration of the behavior of an ideal elastic spring and showed that the hair fiber is not an ideally elastic substance. For this reason, at least, application of the simple theory cannot be expected to hold in a quantitative way. It may, none- theless, be useful to examine equation 1 and the information on the creep behavior of hair in a qualitative sense and to consider the extension of hair tresses in coil form under gravitational loading in a variety of circumstances. From equation 1, one would expect a hair coil to become more easily deformed under load as the torsional stiffness (G) decreased. One important way in which the torsion modulus of human hair can be altered is by change in the ambient humidity. As the data presented indicate, a hair fiber at 40% RH is about three times as stiff in torsion as at 90•0 RH thus increasing the humidity decreases the shear modulus markedly. Additionally, the hair fiber shows substantially more creep at higher moisture contents. It would be expected that hair coils exposed in more humid atmospheres, therefore, would extend to a greater degree under stress or under gravitational forces than those similarly exposed under drier conditions. This conclusion is no great surprise to any woman who knows from extensive experience that her hair set is poorer and tends to be lost in damp weather. It might also be expected that imposition of the set and ready coil formation in the setting operation would be favored by lower torsion modulus, i.e., at higher moisture content, since the fibers can be de- formed more easily. Creep of the fibers in the moist state also favors