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J. Soc. Cosmet. Chem., 41, 123-139 (March/April 1990) Bending relaxation properties of human hair and permanent waving performance F.-J. WORTMANN, Deutsches Wollj'brschungsinstitut, D-5 I Aachen, Federal Republic of Germany, and N. KURE, KAO Corporation Tokyo Laboratories, 2-I-3 Bunka Sumida-ku, Tokyo 131, Japan. Received May 4, 1988. Synopsis The bending properties of human hair were investigated under various conditions related to those applied during a permanent waving process. Permanent set values were calculated from fiber bending stiffnesses in the reduced and reoxidized states on the basis of linear viscoelastic theory as expressed by Denby's equation and shown to be in agreement with the bending set measured for fiber loops. The analysis of the difference between the change of the bending stiffness and that of the extensional stress during reduction suggests that a non-uniform Young's modulus distribution is induced in the fiber during the reducing step. The evalua- tion of two simple types of distributions (linear and quadratic) leads to results that are consistent with the experimental data, giving sensible estimates for the local moduli of the reduced fiber at the fiber core and at the surface. The results show that only for severe reducing conditions does the reductant reach the fiber core in a sufficient concentration to substantially reduce the local modulus. INTRODUCTION As shown by the reviews of Gershon et al. (1) and Robbins (2), the chemistry of the permanent setting process and the changes in the chemical and physical properties of human hair with a reducing treatment have been widely studied, and the basic chemical mechanism of permanent set as due to the cleavage and the subsequent recombination of sulfur cross links has generally been accepted. However, studies on the mechanisms connecting the chemical reaction between a reducing agent and the hair fiber material, occurring on a molecular level, with the macroscopic set of a hair in a curl of a perma- nent wave are still lacking comprehensiveness. One possible approach to this subject, from the macroscopic point of view, is to relate fiber set to physical quantities, like the extensional and bending modulus, character- izing the mechanical properties. This kind of study has been undertaken by DeJong (3) for wool and in a previous paper by one of the authors (4) for human hair, applying linear viscoelastic theory to the extensional properties of hair fibers treated under setting conditions. In both studies, good correlations were found between the actual set and the theoretical set, calculated from the extensional moduli of the treated fibers. 123