JOURNAL OF COSMETIC SCIENCE 144 Figure 5. Effi cacy: Final recovery vs. temperature (0 - perfect hold 1 - no remaining hold). Figure 6. Torsional changes from heat damage (bleached hair, 60 sec. treatment). damage, this is suggestive of damage to the hydrophobic 18-MEA coating on the fi ber surface after exposure to high temperatures. DISCUSSION AND CONCLUSION The thermodynamics of water set have been well explained in terms of the glass transition and the process of aging (4,5,8–10). Water set is performed by shaping wet hair into the desired shape and holding it for some time after the water evaporates. We understand why this works by examining how the glass transition temperature depends on water in

2010 TRI/PRINCETON CONFERENCE 145 Figure 7. Elastic modulus after heat treatment. (a) Virgin. (b) Bleached. Figure 8. Break stress following heat treatment. (a) Virgin. (b) Bleached. the hair. The Tg of keratin fi bers has been well characterized using differential scanning calorimetry (11). When wet, the glass transition temperature is below 0°C, and so the fi ber acts like a viscoelastic fl uid. As the fi ber dries, the transition temperature increases, passing through room temperature and slowly rising to 50° or even 100°C, depending on the relative humidity. As an aid to understanding this effect, we have replotted the results of reference (11) which expresses Tg vs. the amount of water in the hair as Tg vs. relative humidity, assuming the fi ber has acquired an equilibrium amount of water at 24°C (Figure 10). To associate regain with relative humidity we use dynamic vapor sorption results taken on virgin hair in our laboratory, these agree quantitatively with classical values found in the literature (12). The longer the hair is held after it quenches during drying, the slower its relaxation and the less it recovers. The dependence of curl retention on hold time is an example of the