THERMOMECHANICAL ANALYSIS OF HAIR 363 I I I I 50 I00 150 200 TEMPERATURE (øC) Figure 3. Penetration thermogram for human hair: 20øC/rain, 5-g load hair is exhibited as a rapid probe displacement. A typical penetration (softening) thermogram for untreated human hair is shown in Fig. 3. The transition temperatures and the degree of softenings are quite dis- tinct from other keratin and protein materials studied (2, 3). There was a softening at 59øC which accounted for 3.0% of the sam- ple thickness and a second at 242øC amounted to 24%. Even at 327øC, 40% of the hair diameter has not been melted and hence penetrated at this applied force. Dimensional Temperature-induced dimensional changes parallel and perpendicu- lar to the fiber axis were determined for hair over the temperature range from --50 to 325øC. Actual extension (longitudinal axis) and expansion (transverse axis) thermograms are shown in Figs. 4 and 5. Along the fiber axis, hair exhibits two contractions the first begins at 72øC and amounts to 0.6% of the original sample length while the second begins at 224øC and is 2.9%. At 242øC, a melting occurs and the sample rapidly elon- gates. Both of the longitudinal contractions were time-dependent since holding the temperature anywhere above the initiation temperature re- sulted in complete contraction. Energies of activation for the two con- tractions were calculated to be 12 kcal for the 60øC contraction and 90 kcal for the 224øC contraction. These data suggest that the contraction at 60øC involves only side-chain motion, whereas the 90 kcal at 224øC indicates macromolecular motion which is consistent with the disorder-

364 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •- 25,• -- o w I I I I 50 I00 150 200 TEMPERATURE (øC) Figure 4. Extension thermogram for human hair: 20øC/min, 2.3-g load I I 1 I00 150 200 TEMPERATURE (*C) ! 25O Figure 5. Expansion thermogram for human hair: 20øC/min, 2.3-g load ing of the o•-helix. The dimensional changes that accompany these con- tractions were not immediately reversible. Studies are underway to de- termine the long-term reversibility of these transitions. In the transverse direction, untreated hair exhibited a 54% irreversible expansion begin- ning at 256øC. A summary of the TMA inflection temperatures and dimensional changes for untreated hair are shown in Table I. The data were treated statistically to determine the reproducibility of the sample and measure- ment system. The standard deviation for each inflection temperature as well as the linear displacement of the probe at the end of the transition are shown. The low standard deviations reflect the precision of the sys- tem.