j. Cosmet. Sci., 51, 379-399 (November/December 2000) A curing method for permanent hair straightening using thioglycolic and dithiodiglycolic acids S. OGAWA, K. FUJII, K. KANEYAMA, K. ARAI, and K. JOKO, Research and Development Center, Milbon Co. Ltd., 2-I 7-2 Akagawa, Osaka 535-0005, Japan (S.O., K.F., K.K., K.A.), and Kyoto Women's University, Kitahiyoshi, Kyoto 605-850I, Japan (K.J. ). Accepted for publication September 2I, 2000. Presented in part at the 4th Scientific Conj•rence of the Asian Societies of Cosmetic Scientists, Bali, I999, and in Proceedings of SCAS as "A practical method for permanent hair straightening and the permanency related to supercontraction induced by microstructure change,"pp. 348-36 I. Synopsis A successful process of permanent hair straightening was achieved by applying a hot iron press technique to hair treated with an alkaline solution containing thioglycolic acid (TGA) and dithiodiglycolic acid (DTDG). Supercontraction of the hair was determined on the basis of fiber length. The effectiveness of straightening was evaluated as a function of supercontraction, which was dependent on the concentration ratio of DTDG to TGA, pH, time, and heat-treatment temperature. A range of around 5% to 8% supercontraction was found to be necessary to achieve successful permanent hair straightening, while at relatively higher contraction levels the smoothness of the fiber surface tended to be lost. About 89% of the initial content of cystine was retained in straightened hairs, and about 11% was converted to cysteic acid. X-ray diffraction and high-pressure differential scanning calorimetry were used to estimate the amount of o•-crystallites. The relationship between the melting enthalpy and the degree of supercontraction was discussed in relation to the stability in the straight configuration. Important conclusions include the observations that supercontraction of less than 10% is caused by randomization of the o•-helix and that permanency related to hair straightening is a result of the irreversible transformation of the o•-crystal into amorphous phase. INTRODUCTION Several hair-straightening processes have been applied to practical uses. A thioglycolate system has mostly been used as the reverse process of permanent waving, which includes an oxidation process after the reduction step under a straight configuration forced on kinky and curly hairs. In recent years, a hot iron press technique has become common and has been applied in hair straightening. Although all these processes are able to straighten hair to some extent, the hair will revert to its original or near original curly 379

380 JOURNAL OF COSMETIC SCIENCE state on washing or even on exposure to high humidity. As far as these practical methods are concerned, the set of hair in straight configuration is only temporary. Straighteners based on sodium hydroxide have been used for the straightening of hair of blacks as they are more effective than TGA-based products (1). More recently, a combined process consisting of chemical treatments and hot iron press had been used for permanent hair straightening (2). There are some commercial processes in this category. These practical methods include a heating process after the reduction of hair where keratin disulfide bridges are cleaved. So far nothing has been reported in the scientific literature on the reaction mechanism occurring in these treatments. In the practical application, there are many problems such as the differences in the cystine content of native hairs and in hair diameter from person to person, and also the differ- ences in the history of hairs as to coloring and perming. Optimum results in straight- ening for such a variety of hairs are a challenge. Our approach has been to study the effects of a reducing system composed of TGA and DTDG on the chemical and physical behavior of hair (2,3). Wong et al. (4) discussed the practice and general theory of permanent hair straight- ening. From their observations on fiber swelling and supercontraction in a variety of solutions, they proposed a model to explain permanent set in terms of molecular events such as changes in the secondary structure. The aim of this work was to analyze the reactions occurring during hair straightening, to investigate the possible factors responsible for unwanted results from treatments on hair damage, and to demonstrate the relationship between the degree of supercontraction and the amount of the ot-crystallites estimated by x-ray diffraction and high-pressure differential scanning calorimetry (5-8). In this paper, the chemical and physical char- acteristics of the cystine residues have been briefly discussed in relation to their location in the microstructure of hair keratin. EXPERIMENTAL MATERIALS The aqueous ammonium solutions of thioglycolic acid (TGA) and dithiodiglycolic acid (DTDG) used were commercial products, and they contained 50% TGA and 40% DTDG by weight, respectively. The monoethanolamine (MEA) used was an aqueous solution of 80% MEA, a commercial product. All other chemicals used in this study were of reagent grade. Two types of hair samples were collected from Japanese women who had not been exposed to chemical treatments such as perming and dyeing. Straight hair samples were used to study supercontraction induced by chemical and thermal treatments of hair. Kinky hair samples were used to estimate the efficiency of hair straightening. The fiber samples (0.5 g) were purified by immersion in a 5% (w/v) solution at pH 4.6 (50 ml) of poly(ethylene glycol lauryl ether) containing 20 mM EDTA for 1 h at 33øC. The hair was washed thoroughly with distilled water and dried. PREPARATION OF CURED HAIRS Cured fibers for both straight and kinky hairs were prepared by the following three step processes: (a) reduction, (b) heat treatment, and (c) oxidation.