J. Cosmet. Sci., 64, 119–131 (March/April 2013) 119 Progressive hair straightening using an automated fl at iron: Function of silicones ANNE DUSSAUD, BHAVNA RANA, and HUI TUNG LAM, Momentive Performance Materials, Tarrytown, NY 10591. Accepted for publication July 23, 2012. Synopsis An automated hair iron was built with which the hair temperature, contact force of the iron against the hair tress, and gliding speed were controlled. The changes in keratin were characterized by several techniques including differential scanning calorimetry, birefringence measurements, and wet tensile tests. Undamaged curly hair was ironed for several iron cycles at temperatures ranging from 120°C to 175°C and washed between each iron cycle. Irreversible straightening of curly hair was observed and depended on the temperature and the number of cycles. The birefringence data suggested that the straightening was related to a gradual decrease of the microfi lament organization. Silicone treatment did not signifi cantly affect the course of microfi lament denaturation, but it improved the quality of straightening. It enhanced the fi ber alignment under the gliding action of the iron. Progressive thermal straightening may be a promising method to achieve permanent smoothing of curly hair without chemical treatment. Ironing at the onset temperature (~154°C), before substantial disulfi de bond scission occurred, seemed to be a good compromise between process speed, straightening performance, and hair integrity (i.e., reduced loss of cross-linking). INTRODUCTION Thermal treatments for hair styling are becoming increasingly popular with consumers both at home and in hair salons. High-temperature irons with maximum temperatures of 250°C seem to provide better straightening permanency. However, since these ironing treatments are often combined with chemical treatments, it remains unclear how the chemical and heat contributions to straightening decouple. Controlled studies to address the process of thermal straightening are still needed. The mechanisms involved in setting treatments of keratin fi bers have been discussed exten- sively in the literature, especially for wool. With chemical treatments (alkaline or reducing agents), it is usually accepted that straightening involves breaking chemical bonds, followed by reforming new bonds while the keratin fi bers are held straight (1,2). The permanent setting of wool can occur in the absence of chemical treatments when wool is held extended in boiling water (3). Feughelman showed by x-ray and birefringence measurement that it was Address all correspondence to Anne Dussaud at Anne.dussaud@momentive.com

JOURNAL OF COSMETIC SCIENCE 120 related to an irreversible physical modifi cation of the microfi lament keratin conformation induced by the coupled action of heat, strain, and water (4). More recently, differential scanning calorimetry (DSC) analyses of human hair have shown that thermal denaturation of α-keratin occurred between 140°C and 200°C depending on the water content of the samples (5–8). The denaturation is characterized by an endotherm corresponding to a transition from a crystalline to a more amorphous state. Although hair can reach those high temperatures during ironing, there are signifi cant differences between the heat transfer processes in an iron versus a hermetic DSC pan. The hair iron involves multiple heating and cooling cycles, and the heating rate in the iron is usually much faster than in a DSC experiment. In addition, in an iron, the moisture content around the heated portion of hair decreases with time and hair fi bers undergo deformation and stress. Here the objective was to develop a controlled iron experiment to study the effect of mul- tiple iron cycles on the straightening permanency of curly hair. Since holding the fi bers straight is key to straightening, another aim of this study was to elucidate the contribu- tion of silicones to the process. Silicones are often used in hair thermal treatment because of their thermal stability (9). Because of their low surface tension, silicones form fi lms reducing iron gliding force they condition the hair and produce increased fi ber–fi ber interaction with an anti-frizz effect (10). An automated iron was built with which the hair temperature, contact force of the iron against the hair tress, and gliding speed were controlled. The stretching force was re- corded by a load cell. Fiber alignment could be monitored by a camera mounted on a stereomicroscope. The changes in keratin were characterized by several techniques in- cluding DSC, birefringence measurements, and wet tensile tests. The initial moisture of the hair tress was controlled. All tests were performed without the use of chemical relax- ers or cross-linking agents. EXPERIMENTAL MATERIALS We purchased 2 g undamaged naturally curly hair tresses and ethnic Afro hair swatches from International Hair Importers & Products (Glendale, NY). The silicone emulsion used in the study was made by Momentive (Columbus, OH). The silicone was an alkyl-modifi ed amodimethicone from Momentive, trade named Silsoft* AX® conditioning agent with International Nomenclature of Cosmetic Ingredients (INCI) name Bis-cetearyl amodimethicone. The silicone was emulsifi ed with nonionic emulsifi ers to form a stable emulsion with a particle size of around 350 nm. METHODS Automated iron. In a manual iron experiment, the heat transfer process controlling the hair alteration is quite variable since it depends on manual factors, including gliding speed of the heating elements, contact pressure, and the amount of hair between heating plates. An automated iron was built to control those variables (Figure 1).