HAIR STRAIGHTENING USING AN AUTOMATED FLAT IRON 129 IRONING AT THE ONSET TEMPERATURE (T = 154°C) An experiment conducted at the onset temperature (154°C) for three iron cycles is shown in Figure 11 and Table IV. DSC data showed no signifi cant change of Td but a 40% de- crease in enthalpy in both samples. However, straightening was much better with sili- cone because of a better fi ber alignment during setting. DISCUSSION AND CONCLUSION The progressive, irreversible straightening of curly hair was a function of temperature, occurring faster at higher temperatures than at lower temperatures. The birefringence data suggested that the straightening was related to a gradual decrease of the microfi la- ment organization (i.e., decrease of birefringence). This was induced by heating the fi bers above a threshold temperature while holding the fi bers straight. The silicone did not Figure 9. Wet elasticity versus the denaturation temperature (Td) for the curly hair samples ironed multiple times shown in Table III. {Note: Test data. Actual results may vary.} Figure 10. Histograms of birefringence values of fi bers obtained from silicone treated tresses ironed multi- ple times. Y-20 = 10 iron cycles at 122°C, Y-30 = 10 iron cycles at 144°C, Y-35 = 5 iron cycles at 154°C, and Y-40 = 2 iron cycles at 175°C. {Note: Test data. Actual results may vary.}

JOURNAL OF COSMETIC SCIENCE 130 signifi cantly affect the course of microfi lament denaturation, but it improved the quality of straightening. By forming a fi lm on the hair, the silicone treatment enhanced the slip between fi bers, allowing a regular packing and alignment under the gliding action of the iron. This kept the fi bers in a more aligned conformation. As shown by DSC analysis, the keratin denaturation temperature depended on moisture. In particular, denaturation temperature increased signifi cantly at reduced moisture levels (10%). During one ironing cycle, hair moisture level decreased to a point where the denaturation temperature was higher than the hair temperature. At each ironing cycle, moisture was restored to further modify the keratin organization toward a straight con- formation. The hair ironing process presented some analogy with the permanent set of wool using boiling water or steam (4). In the case of wool, where water and strain were necessary to cause the keratin microfi lament transition, the extension strain needed to be maintained for a suffi cient time to produce irreversible setting. In the case of hair ironing, a similar process occurred by iteration, restoring water at each cycle. The shift of denaturation temperature, Td, has been interpreted in different ways in the literature. It could be due to a plasticizing effect or loss of cross-linking density. For ex- ample, it was shown that bleaching, by causing a loss of cross-linking density in the matrix (higher swelling, lower wet elasticity), leads to a decrease of the keratin denatur- ation temperature (5,13). Here, since the decrease of the denaturation temperature, Td, was correlated with a decrease of the wet elasticity, a loss of cross-linking density might have occurred in the matrix. The nonlinear decrease of Td as ironing temperature in- creased, shown by both the iron and the multiple DSC heat and cool cycle experiments, suggested that the rate of disulfide bond scissions sharply increased as heat cycle Figure 11. Tress photograph after 3 iron cycles at 154°C, 1 wash, and storage for 30 min at 90% RH. {Note: Test data. Actual results may vary.} Table IV DSC Analysis for Curly Hair Samples Ironed for Three Cycles at 154°C Td (oC) ΔH (J/g) Initial 172 ± 1 5.7 ± 0.6 3× control 171.7 ± 0.2 3.3 ± 0.2 3× silicone 172.2 ± 0.5 3.7 ± 0.4