The swelling behaviour of hair studied through the structural change of keratin protein during the permanent waving treatment

Filament of human hair is formed from α-keratin protein and its physical property is predominantly dominated by the structure of microfibril (also known as intermediate filaments (IF)). It is known that human hair is swollen by permanent waving (pw) treatment which consists of the reducing process and following oxidizing process, but a detail in the swelling behaviour remains still unclarified. The present work was devoted to the analysis of the swelling behaviour of hair through the structural change of IF during pw treatment, where 1.0 mol/L ammonium thioglycolate solution (pH 9.25) was employed as reducing reagent. The structure of IF was represented in terms of its microstructure, which is given by α-helix content in keratin chain, and its macrostructure, which is given by alignment of IF along human hair axis, and the structures were studied by SAXS and CP/MAS ¹³C NMR and others. It is shown that the microstructure and macrostructure of IF simultaneously start to change at an initial stage of the pw treatment without any induction period and both structures are sufficiently swollen at that stage. Furthermore, it is shown that the microstructure and macrostructure of IF is partly destructed by reducing treatment, but the destructed structures are considerably restored by following oxidizing treatment.