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J. Soc. Cosmet. Chem., 38, 125-140 (March/April 1987) Extensional properties of human hair and permanent waving F.-J. WORTMAN and I. SOUREN, Deutsches Wollforschungsinstitut, Veltmanplatz 8, D-51 Aachen, West Germany. Received November 21, 1986. Synopsis The mechanical properties of hair fibers in extension which, from an "engineering" point of view, are expected to control the set imparted to a hair fiber during waving were investigated. An experimental method that combined static and dynamic testing was used to assess to what extent the extensional moduli can be used to follow the course of a treatment and to predict its results. The calculations for fiber recovery were based on linear viscoelastic theory as expressed by Denby's equation and compared to the bending set measured for fiber loops. Assessment of the results on the basis of the two-phase model for hair indicates that reduction mainly affects the properties of the continuous, o•-helical filaments (phase C) in the hair fiber, while the properties of the matrix phase (phase M) are largely unchanged unless severe conditions (i.e., 1 M thioglycolic acid, pH 9) are applied. No length increase was acquired by the fibers during extension testing, but some cases of length contraction were observed on release. The recovery observed for the fiber loops was in all cases smaller than the recovery expected from the change of the extensional moduli. This systematic deviation with theory is attributed to a non-uniform degree of reduction and modulus change over the fiber cross-section. INTRODUCTION A wealth of general information exists on how satisfactory permanent set in keratins, e.g., set in wool fabrics or permanent waves in human hair, can be efficiently gener- ated. The chemistry of setting processes and the changes in the chemical and physical properties of wool and hair with treatment have been studied in great detail and were reviewed for wool by Maclaren et al. (1) and for human hair by Gershon et al. (2) and by Robbins (3). In contrast, there seems to be a less detailed knowledge of the fundamental "engi- neering" mechanics of the formation of chemical set of extensional or bending deforma- tions, of their stabilization, and of the physical parameters controlling the set of a hair curl. This, despite the obvious fact that a wave can only be formed by the application of chemicals or heat and the deformation desired. Concentrating on one special type of set, namely the permanent waving of human hair, which is essentially the chemical set of a bending deformation, a study was undertaken to investigate those mechanical properties of hair fibers that are expected to control the set imparted to a hair fiber during a waving process. 125