Extensional properties of human hair and permanent waving

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 maller 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