TORSIONAL BEHAVIOR OF HAIR 97 where, for example, bleached or oxidatively dyed hair is unscrupulously waved or the relaxer is applied not only to the new growth but is spread over hair that had already been alkali-straightened. It is not surprising, in view of what was said above about the treatment conditions, that the values of rigidity ratios are low. However, it is the practical significance of this observation that is important. Clearly, hair fibers of such low torsional moduli are ill-equipped to effectively support "antigravitational" config- urations such as curls. A wave imparted to such fibers is likely to lose most of its curvature within seconds of rinsing, offering at best an appearance of a "frizz." Alkaline relaxing converts a substantial quantity of combined cystine into lanthionine cross links. It has been generally assumed that it is the resistance of these new cross links to the action of mercaptans or sulfites used in wave preparations that prevents the relaxed hair from taking a wave. The explanation, however, for the resistance to waving seems to be more physical than chemical. In fact, alkali-straightened hair can be successfully waved if the relaxing treatment is carried out carefully to prevent excessive hair damage. It is worth pointing out that elimination of salt linkages by exposing hair to acidic conditions is not always associated with hair weakening. A case in point are the results obtained on treatment of hair with naphthalenesulfonic acid (NSA). The data shown in Table IV (along with 820 and HC1 as reference readings) indicate that NSA-treated fibers are more torsion-resistant to pH 1 environment than the intact, fibers to water and twice as rigid as intact fibers in 0.1 N HC1. Upon drying, the HCl-treated hair does not fully recover its original rigidity, thus suggesting that even under relatively dry conditions, the coulombic interactions continue to contribute to the rheological properties of hair. On the other hand, the NSA-treated fibers exhibit rigidity ratios of well over 1. The sorption of the NSA moleties into the hair structure introduces a substantial quantity of bulky groups, causing an increase in the internal viscosity of the structure, thus more than compensating for the loss of salt linkages. Additionally, NSA lowers the moisture-binding capacity of hair (10), which independently leads to higher fiber rigidity. The retention of adequate fiber pliability in the wet state in combination with increased rigidity at ambient conditions of humidity is clearly a characteristic desired in a hair-setting agent. Further stiffening of the fibers, which we have shown to occur as a result of heating, should also be beneficial. The combined effect of these is shown in Figure 6, which depicts the setting characteristics at 40 ø Table IV Effect of Naphthalenesulfonic Acid (NSA) on the Torsional Properties of Hair Testing Medium Rigidity Ratio Logarithmic Decrement (8) Air, 65% RH 1 0.20 (0.01) H20 0.26 (0.01) 0.40 (0.05) 0.1 N HC1 0.16 (0.01) 0.44 (0.11) Air, 65% RH 0.88 (0.02) 0.22 (0.02) 0.1 N NSA 0.32 (0.01) 0.41 (0.04) Air, 65% RH 1.11 (0.02) 0.21 (0.01) Values in parentheses are standard deviations.

98 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5O 40 3O 2O 10 Set Intact Hair NSA Treated Hair 40oC 70øC Setting Temperature Figure 6. Heat setting of NSA-treated hair. and 70øC of NSA-treated fibers using the coil-setting technique (11). The results are also in accordance with data obtained in an earlier (12) investigation on hair setting. CONCLUSIONS 1. By simple capillary insertion, the pendulum technique can be used for measurement of torsional properties of fibers in both air and liquid media. 2. The results of torsional measurements of hair of different diameter indicate that the hair cuticle is plasticized by water to a greater extent than the hair cortex. 3. Dry heating of hair leads to a small but measurable and long-lasting increase in torsional rigidity. 4. Changes in the torsional rigidity of fibers attendant upon their chemical modifi- cation provide valuable information regarding wet configurational stability of hair and the potential for their settability. REFERENCES (1) H. Bogaty, Torsional properties of hair in relation to permanent waving and setting, J. Soc. Cosmet. Chem., 18, 575 (1967). (2) M. Feughelman, A two-phase structure for keratin fibers, Text. Res. J., 29, 223 (1959). (3) J. B. Speakman, The reactivity of the sulphur linkage in animal fibers--Part I. The chemical mechanism of permanent set, J. Soc. Dyers and Colourists, 52, 335 (1936).