HAIR STRAIGHTENING USING AN AUTOMATED FLAT IRON 127 (ΔT ~ 1°C) for hair samples treated at lower temperatures (144°C–154°C). But signifi - cant Td shift (ΔT ~ 6°C) was observed in hair samples ironed at 175°C. Multiple heat/cool cycles in high-volume DSC pan. A simulation of multiple heating and cool- ing cycles was conducted in DSC pans. The hair sample was fi rst equilibrated at 58% RH at room temperature. The pan was then sealed and subjected to a number of heat/cool cycles at low temperature before the fi nal temperature scan. Cycle temperature was varied from 120°C to 170°C for either three or six cycles. The experiment was performed with ethnic Afro hair to identify more clearly the changes in the endotherm peak. In Figure 7, the denaturation peak temperature is plotted as function of cycle tempera- ture for three and six cycles. A nonlinear behavior was observed, indicating an onset of transition at around 140°C. In Figure 8, a similar plot is shown with the DSC data ob- tained with the ironed tress samples. Surprisingly, although heating and cooling rate were very different in both experiments, a similar nonlinear behavior was observed with Td, when cycle temperature reached the 150°C temperature range. WET ELASTICITY Average wet elasticity of hair samples B, C, D, and E (N ~ 50 fi bers per sample) is shown in Table III. A signifi cant decrease of wet elasticity was observed as iron cycle temperature increased. On the basis of t-test values, the silicone treatment did not affect signifi cantly the heat-induced elasticity change. A plot of wet elasticity versus Td showed a quite good correlation (R2= 0.76) (Figure 9). BIREFRINGENCE The hair fi bers displayed a range of birefringence values. For a 90 microns fi ber, blue- green colors were predominant in the samples treated at low temperature. Yellow-brown colors were predominant in the hair samples treated at higher temperatures. Birefringence Figure 7. Multiple heat/cool cycles in DSC pans for an untreated ethnic afro hair sample: Denaturation tempera- ture (Td) as a function of temperature for 3 and 6 cycles. {Note: Test data. Actual results may vary.}

JOURNAL OF COSMETIC SCIENCE 128 value histograms (N = 50) for hair samples treated with silicone (B, C, D, and E) are shown in Figure 10 and average values are shown in Table III. The curly samples (E) showed a broader distribution with a high birefringence average (μ ~ 0.008), whereas the straight samples had a skewed distribution with signifi cantly lower birefringence values (μ = 0.005–0.006). The broad spectrum of birefringence values within individual fi bers suggested that fi bers within the same hair tress might respond differently to heat, depending on their initial degree of keratin organization. A statistical t-test showed that the silicone-treated sam- ples were not signifi cantly different from the control samples. But there were signifi cant differences between the samples (B, C, D, and E) treated at different temperatures. Figure 8. Multiple heat/cool cycles in hair iron: Denaturation temperature (Td) as a function of cycle tem- perature (each symbol is an average value of 3 pans). Empty symbols are for the control tresses, bold symbols are for the treated tresses. Diamonds correspond tot he tresses treated for 2 iron cycles. Squares correspond to the tresses treated for 5 iron cycles. {Note: Test data. Actual results may vary.} Table III Summary Table Hair temperature Number of cycles Tensile test DSC Polarized Microscopy Wet Young modulus (/109 Pa) Td (oC) ΔH (J/g) Birefringence (×103) 122°C 10 Control 1.80 ± 0.29 174.4 ± 0.28 2.81 ± 0.51 7.7± 2.4 Silicone 1.73 ± 0.35 174.0 ± 0.15 4.26 ± 0.51 7.9 ± 2.2 144°C 10 Control 1.42 ± 0.25 172.9 ± 0.30 3.58 ± 0.16 6.4 ± 1.8 Silicone 1.40 ± 0.2 172.0 ± 0.31 3.65 ± 0.15 7.1 ± 1.9 154°C 5 Control 1.31 ± 0.23 173.1 ± 0.60 2.84 ± 0.50 5.9 ± 2.0 Silicone 1.21 ± 0.33 172.8 ± 0.83 2.59 ± 0.34 5.4 ± 2.1 175°C 2 Control 1.11 ± 0.31 169.9 ± 0.38 4.81 ± 0.11 5.8 ± 1.7 Silicone 0.95 ± 0.37 168.0 ± 0.71 2.02 ± 0.11 5.4 ± 1.8