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