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
JOURNAL OF COSMETIC SCIENCE 146 phenomenon known as aging in the study of glassy systems (13). For heat styling, we know that considerable thermal work must be done to rapidly remove water and that the amount of this work is proportional to the amount of water in the fi ber (14). In heat styl- ing, the hair is driven towards Tg in two ways: by driving off the plasticizing water, rais- ing Tg and by heating the hair towards the elevated Tg. Once the hair is removed from the iron, its temperature plunges, quenching the fi ber. In this description, it matters not at all how long the hair is held far above the melting temperature. Rather it is the time spent near and below Tg under mechanical force that determines effi cacy. The quenching process will proceed in almost the same way for any iron temperature far above 150°C. Having understood that temperatures above 150°C play little role in improving perfor- mance, we now turn to the role of heat and water in producing damage. The evidence from a single cycle of heating in this work shows little damage from heating wet fi bers. This should not be taken as an endorsement of the practice of heating wet fi bers as con- siderable literature has shown this to be quite damaging after multiple treatments (1,6). The use of multiple rounds of treatment prior to testing for mechanical damage will be an important follow up to this report. Chemically damaged hair, however, shows signifi cant effects from a single heat treat- ments. And these effects are dramatic at temperatures above 200°C. At these temperatures Figure 9. Contact angle following heat treatment. Figure 10. Tg vs. RH at 24°C.
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