HAIR STRAIGHTENING USING AN AUTOMATED FLAT IRON 123 (Nikon, Melville, NY) using cross-polarized illumination and the method described by McCrone et al. (12). The fi ber was oriented 45° to the vibration direction of the crossed polars to obtain the brightest interference color. The main interference colors were observed close to the center axis, which were then compared to a Michel–Levy chart to obtain the value of retardation (optical path difference). The retardation and fi ber thickness were related to the birefringence according to the following equation: λ = 1000 × t × (Δn) where λ is the retardation in nm, t is the fi ber diameter in micron, and Δn is the birefrin- gence. About 40 to 50 fi bers were examined for each treatment. Statistical analysis was performed using Minitab software. RESULTS EFFECT OF MOISTURE ON THERMAL α-KERATIN DENATURATION The denaturation temperature and the enthalpy as a function of initial humidity during sample equilibration are shown in Table I for undamaged ethnic Afro hair. The denatur- ation temperature decreased as moisture increased, which is consistent with the data of Cao and Leroy for Caucasian hair (8). In contrast, the value of denaturation enthalpy did not vary signifi cantly. When the matrix keratin was plasticized by water, the denatur- ation transition of the crystalline α-keratin microfi laments occurred at lower tempera- tures. In the next sections, the initial moisture of the hair tresses was controlled by placing them in a 90% RH chamber before ironing. STRETCHING FORCE Curly hair was treated as described in the section Multiple Iron Cycles Protocol. Stretching force was recorded as a function of time at each pass. Figures 3A and B represent the stretching force through one ironing cycle for samples treated with water and silicone, respectively. In the water-treated sample, the force was higher in the fi rst pass (F = 130 ± 6 g) and decreased as the hair fl attened. In the sample treated with silicone, the stretching Table I Denaturation Temperature, Td and Enthalpy, ΔH as Function of Initial RH of the Hair Sample (Equilibration) for Ethnic Afro Hair Sample Equilibrated at Different RHs Initial RH (%) Td (°C) ΔH (J/g) 0 213.2 ± 2.2 5.0 ± 1.6 21 186.0 ± 0.1 5.3 ± 0.3 33 181.7 ± 0.3 6.7 ± 0.6 43 180.0 ± 0.3 5.3 ± 0.4 58 177.1 ± 0.4 5.0 ± 0.5 75 167.6 ± 0.4 5.1 ± 0.3 86 166.6 ± 1.8 5.5 ± 0.4 97 163.5 ± 0.4 4.6 ± 0.8

JOURNAL OF COSMETIC SCIENCE 124 force was lower in the fi rst pass (F = 90 ± 2 g) and stayed low. Figure 4 shows the images taken with a stereomicroscope at 0.67× magnifi cation just after the fi rst and fi fth pass of the control tress. In the water-treated tress, the fi bers were misaligned and showed a disorga- nized packing (Figures 4A and B). In contrast, with the silicone-treated tress (Figures 4C and D), regular fi ber alignment was observed. Figure 3. A) Stretching force during ironing as a function of time and pass number for the control tress. Symbols , , , , correspond to pass #1, 2, 3, 4, 5 respectively. {Note: Test data. Actual results may vary.} B) Stretching force during ironing as a function of time and pass number for the silicone treated tress. Sym- bols , , , , correspond to pass #1, 2, 3, 4, 5 respectively. {Note: Test data. Actual results may vary.} Figure 4. Fiber alignment pictures after ironing pass #1 and after ironing pass #5 for the control tresses (A, B) and for the silicone treated tresses (C, D) respectively. {Note: Test data. Actual results may vary.}