J. Cosmet. Sci., 55, 13-27 CTanuary/February 2004) Effects of thermal treatments with a curling iron on hair fiber S. B. RUETSCH and Y. K. KAMATH, TRI/Princeton, Princeton, NJ 08542. Accepted for publication October 2, 2003. Synopsis The effect of curling hair with a curling iron has been investigated. Possibilities of thermal damage with repeated curling according to, and in violation of, the manufacturer's specifications have been studied. The propensity of hair surface to damage depends on the moisture content of the hair, and these experiments have been conducted in both wet and dry conditions, with and without application of tension, and with short or prolonged times. Scanning electron microscopic (SEM) examination revealed that fibers treated under the dry condition (50% RH) show radial and axial cracking along with scale edge fusion. Similar thermal treatment on wet hair resulted in severe damage of the type described above, as well as bubbling and buckling of the cuticle due to the formation and escaping of steam from the fiber. Fibers subjected to repeated curling in the dry condition show slight increases in tensile mechanical properties, characteristic of a crosslinked fiber. Fibers treated with conditioners show an improvement in characteristic life, especially in the case of low-molecular-weight conditioners, such as CETAB, which can penetrate into the hair fiber (shown by TOP-SIMS analysis). INTRODUCTION Temporarily curling hair by application of heat is one of the grooming practices used for hair styling. This work examines the effects of such thermal treatments with a curling iron on the cuticular structure of the hair fiber. Manufacturers of curling irons claim that no damage is inflicted upon the hair fiber if the curling iron is used according to the specified treatment conditions, such as dry hair, short curling times, and normal tension (no applied load). The process of curling, by its very nature, has the potential for hair damage if the specifications are ignored. Therefore, we carried out a multifaceted study involving the recommended conditions, as well as "severe" conditions that ignored the specifications of the manufacturer of the curling iron. The protocols we investigated were the effects of: 1. Repeated, cyclical short-term curling/cooling of dry hair under moderate tension as specified by the manufacturer of the curling iron 2. Prolonged curling of dry hair under increased tension 3. Repeated, cyclical short-term curling/cooling of wet hair 13

14 JOURNAL OF COSMETIC SCIENCE 4. Prolonged curling of wet hair under increased tension with no regard to the specifications. Obviously, repeated curling was adopted to understand the cumulative effect of this grooming procedure on hair damage. The other purpose of this study was to understand the combined effect of hygrothermal and mechanical stress on the structure of the hair fiber. EXPERIMENT AL MATERIALS Hair. Root sections of untreated (not chemically treated) dark brown European hair (14" long) was obtained from DeMeo Brothers, New York. Curling iron. We used Philips Styling Comb Model HP-4480 (Courtesy of Philips) under various conditions. The temperature gradient was measured along the length of the curling iron with a thermocouple. Avoiding the top and the bottom, the middle region of the curling iron used in this study ranged in temperature from 110° to l 20°C. METHODS Individual dry or wet hair fibers were used in this study. Each fiber had direct contact with the hot metal surface, and the temperature of the curling iron was not affected by the single fiber, as in the case of a hair tress. It is assumed that the single hair fiber will be much more severely affected by the hot metal surface of the curling iron than a hair tress, which tends to absorb and dissipate the heat at the fiber/metal interface, thereby reducing the temperature of the hot metal surface. Four separate experiments were carried out, each to establish the nature of damage to the hair fiber at the exact location where the hair fiber surface and hot metal are in contact with each other. In each of the experiments, individual fibers were mounted in parallel to assure direct contact of specific lengths and areas of fibers with the hot metal. Four curling protocols adopted in this work are detailed below. (I) Repeated cyclic curling/cooling of dry hair fibers. Individual fibers were mounted in parallel and subjected up to 50, 100, 150, and 200 times to ten-second intervals of thermal treatment with a curling iron interrupted each time by cooling (20 fibers for each category). This experiment simulates curling of hair fibers approximately every two, three, and seven days during a given year. The fibers were held at the root ends and the middle/tip sections were curled, simulating the way the hair would be curled during normal styling practices. At least ten fibers were examined in the SEM for each curling frequency. After the thermal treatment, the side of the curled hair fiber, which had been in direct contact with the heated metal, was examined. (2) High-load and long-time curling of dry hair fibers. In the first experiment, individual fibers were taped together at the root and tip sections and then strung over the heated curling iron for ten minutes under various levels of tension. While the control fibers were allowed to hang freely without an applied load, other fibers were individually suspended under an increasing load of 10 g, 20 g, and 30 g (20 fibers for each category). At least ten hair fibers were examined for each category. The increasing loads were