2010 TRI/PRINCETON CONFERENCE 143 water plays a larger role in bleached than virgin hair. Looking simply at extensional prop- erties, we see stiffening with heat for bleached hair (Figure 7) as well as a steady decrease in break stress with heat (Figure 8). All these results show that the oxidative damage makes the fi ber less resilient to heat treatments. One can also monitor chemical effects, in particular we have tracked changes in surface chemistry through contact angle measurement (7). We plot the cosine of the contact an- gle, which is negative for hydrophobic surfaces and positive for hydrophilic ones. We see the loss of resiliency for oxidatively damaged hair in Figure 9. But with or without prior Figure 3. Effi cacy: Initial set vs. temperature (1 - perfect set 0 - no set). Figure 4. Effi cacy: Duration of initial hold vs. temperature.
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
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