HAIR STRAIGHTENING USING AN AUTOMATED FLAT IRON 121 A 1-inch fl at iron was mounted on a fi xed holder. Two iron plates were brought into con- tact by a rigid holder connected to a force sensor (Imada, Northbrook, IL), measuring the contact force on the iron handle at a distance of 2 cm from the heating plates. The hair tress was pulled at a constant speed of 15 mm/s by the vertical motorized stage of a Tex- ture Analyzer (Stable Micro Systems Inc, Godalming, UK). The Texture Analyzer load cell measured the stretching force exerted on the hair tress during ironing. The study was conducted by keeping the contact force constant to achieve a tangential force (stretching force) of approximately 100 g for a 2 g tress. The hair temperature was measured using an infrared thermometer from Raytek (Santa Cruz, CA). The temperature of the iron was varied to achieve hair temperatures in a range of 122°C to 175°C. The hair temperature was measured at a distance of 1 cm above the fl at iron heating plates. Multiple iron cycles protocol. Curly hair tresses were washed and blow-dried. The hair was then dipped in 0.2% silicone dispersion for 1 min. When the hair was removed from the dispersion, excess liquid was squeezed out and the hair tress was blow-dried. Water served as the control treatment. Before ironing, the treated hair tresses were conditioned for 30 min in a 90% relative humidity (RH) chamber. The tresses were then ironed for one cycle of fi ve passes and stored overnight under ambient conditions. The next day, the tresses were washed and blow-dried before being conditioned at 90% RH for 30 min in prepara- tion for the next ironing cycle. This cycle was repeated 2 to 10 times, with the number of cycles being dependent on the temperature of treatment. Assessment of straightening permanency. Following each ironing cycle, the tresses were stored overnight in the laboratory at room temperature. The straightening permanency was assessed the next day when the hair tresses were washed once with 10% Sodium Laureth Sulfate (SLES) and blow-dried. They were then placed in a humidity chamber for 30 min at 90% RH with forced air fl ow. Tresses were photographed immediately after being taken out of the high- humidity chamber. Tresses were compared to the nonironed curly tress. DSC measurement. Hair sample DSC tests were performed with Q1000 TA Differential Calorimeter (New Castle, DE), using high-volume pans. These hermetic pans maintained constant water content throughout the temperature scan (80°C–250°C). Hair samples were cut into 1–2 mm pieces. Then 10 mg sample was placed in a pan and equilibrated for 2 days in a constant humidity chamber at room temperature. RHs ranging from 20% to 97% were obtained with saturated salt solutions (11). A completely dry hair sample (~0% RH) was Figure 1. Automated hair iron apparatus.

JOURNAL OF COSMETIC SCIENCE 122 obtained by leaving the pan with the hair sample in a dessicator containing Drierite for several days. Before sealing the DSC pan, a 200 cp polydimethylsiloxane oil droplet of 10 μl was added to the pan to act as a thermal medium according to method described by Cao (7,8). Unless specifi ed otherwise, the sample was equilibrated at 58% RH at room temperature. For each hair tress, DSC data was an average of three heat fl ow curve analyses (three pans). DSC analysis was also performed with ethnic Afro hair, which served as a model experiment the endothermic peaks of the undamaged peak were sharper and easier to quantify. A typical heat fl ow curve of undamaged ethnic Afro hair equilibrated at 58% RH is shown in Figure 2. The peak temperature and the area of the peak refer to the keratin denaturation temperature (Td) and the denaturation endothermic enthalpy (ΔH), respectively. Wet elasticity measurement. Single-fi ber tensile tests were performed using a 100-fi ber auto- mated tensile tester (Dia-Stron, Andover, UK). A total of 50 fi bers were analyzed per treatment. Tensile tests were performed with the fi bers immersed in distilled water in cassette holder. Maximum strain was 10%. The diameter of each fi ber was measured us- ing a laser-scanning instrument from Dia-Stron. The fi bers were immersed in deion- ized water for at least 30 min before measurement. Slope of the linear Hookean region was measured to calculate Young’s modulus of elasticity. Birefringence measurement. Because of the anisotropy of its keratin arrangement, hair fi bers display clear distinct interference colors in cross-polarized light, which can be analyzed to measure birefringence. Birefringence is a measure of structural organization of the keratin fi ber. The single fi bers used for tensile test were reused since they were strained in the reversible linear region. They were mounted on microscope slides with Permount medium (Fisher, Pittsburgh, PA) and a cover glass. They were examined on an Eclipse microscope Figure 2. Typical DSC curve for an untreated ethnic afro hair sample equilibrated at 58% RH, prior to seal- ing of the DSC pan.