J. Cosmet. Sci., 66, 31–56 (January/February 2015) 31 Prediction of bending set, wave effi cacy, and hair damage using an extensional permanent waving treatment and the 20% index value YUZO UENO and HIDEO NAMIKI, Department of Biology, School of Education, Waseda University, Tokyo, Japan. Accepted for publication November 23, 2014. Synopsis To predict “wave effi cacy” as evaluated by hairdressers, an extensional permanent waving treatment was per- formed on human hair fi bers using various wave lotions manufactured in Japan. Glass columns devised for the purpose were equipped with a tensile tester in order to increase the measurement accuracy. Notably, the observed set agreed with the theoretical set. In addition, the data for the extensional set exhibited good cor- relation with the bending set and the wave effi cacy assessed in a beauty parlor, and hair damage was estimated by the characteristic change in the 20% index. The following facts were experimentally determined. First, the Young’s modulus of the hair fi bers after extensional permanent waving treatment continually decreased with an increase in the reduction of the fi bers and then abruptly decreased at 80% reduction. Second, the reduction of hair treated with the ammonium salt of thioglycolic acid followed pseudo fi rst-order kinetics only during the initial stage of the reaction, independent of the pH level. Third, the 20% index of the indi- vidual virgin hairs remained constant in water at 30°C and also correlated with the Young’s modulus of the hair after extensional permanent waving treatment. INTRODUCTION Many studies regarding the physical and chemical treatment of wool and human hair have been presented and reviewed by Arai (1) and Robbins (2). However, these studies provide limited information that hairdressers can apply to practical permanent waving treatments in actual beauty parlors. There is a signifi cant need in beauty parlors to be able to predict wave effi cacy using a given wave lotion and estimate hair damage. The purpose of the present study was to achieve this goal using a simple method. There- fore, quick extensional permanent tests were performed using commercially available wave lotions under conditions that simulated treatments given in beauty parlors, and the results were used to predict the bending set and wave effi cacy and determine the 20% index (3) for hair damage. Address all correspondence to Yuzo Ueno at yuzo@hm.catv.ne.jp.