j. Soc. Cosmet. Chem., 42, 129-131 (March/April 1991) A comment on "Bending relaxation properties of human hair and permanent waving performance" M. FEUGHELMAN, School of Fibre Science & Technology, University of New South Wales, Kensington, NSW 2033, Australia. Received January 1991. Synopsis Wortmann and Kure (1) have developed a model to explain the greater ease of setting of a human hair fibre in a bent configuration as against a low extension. Their proposal for a distribution ofYoung's moduli from the surface to the core of the fibre, based on diffusion-controlled chemical breakdown during the reduction procedure, does not appear to be applicable to smaller diameter alpha-keratin fibres such as wools and cashmere goat hairs. Their model takes no account of the two-phase composite nature (4) of the cortex of alpha-keratin fibres. A simple model for setting of a fibre in a bent configuration based on the two-phase concept (2) leads to a universal explanation for the greater ease of set in bending as against set at low extensions. In a recent paper Wortmann and Kure (1) were able to explain the set obtained in the permanent waving of human hair in terms of the bending stiffness variation in single hair fibres during the reduction and reoxidation steps. They analysed the difference between change of bending stiffness and change of extensional stress during the chemical reduction step applied to the fibre. This analysis indicated that during reduction the chemical breakdown results in a distribution of Young's moduli from the surface to the core of the fibre. The analysis suggested that the action of the reducing agent is affected by its diffusion rate into the fibre, and that the bending stiffness, being mainly depen- dent on the fibre moduli towards the surface of the fibre, is more rapidly relaxed than the extensional stress under similar reducing conditions. The whole analysis is completely self-consistent and leads to a straightforward expla- nation of the relation between bending and extensional performance of human hair due to the action of reducing agents. The observed set in bending, which is more easily attained than extensional set at low strain levels, can be directly explained on the basis of Wortmann and Kure's model. When other mammalian alpha-keratin fibres, such as coarse- and fine-diameter wool and cashmere goat hairs, are examined (2), the same relationship holds for the setting of these fibres in bending set as against set at low strain extension. At low extensions, typically around 1% strain for wool fibres held in distilled water at 100øC, the stress in the wool fibres even after one hour is still 62% of its initial value in water at 100øC and 35% of the initial value in water at 20øC (3). The results indicated that a low extensional 129

130 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS set would be obtained in water at 100øC even after one hour because of the presence of a high proportion of the stress returning the fibre to its native length. However, set in bending for the same fibres is substantial within five minutes. Similar results have been reported for fine cashmere goat hairs with diameters as low as 16 microns (2). In all the experiments quoted for these latter fibres, the diameters are considerably lower than for human hair fibres and the setting medium is distilled water at 100øC. Under these circumstances, because of the size of water molecules and the diameters of the fibres, it is difficult to propose the creation during setting of a distribution of Young's moduli from the surface to the fibre core produced by the limited diffusion of the reducing medium (water at 100øC). The more rapid relaxation of bending stiffness as against extensional stress does not appear to be favoured in these circumstances. The model produced by Wortmann and Kure (1), while it is quite consistant with the observed behaviour of human hair fibres in a setting medium such as ammonium thioglycollate, does not appear to be applicable to smaller diameter alpha-keratin fibres in water at 100øC. In Wortmann and Kure's (1) model, although the hair fibres are recognised as being mechanically anisotropic with a higher Young's modulus in the fibre axis direction, it is assumed that a fibre prior to setting is uniform in mechanical properties across its radial section. No consideration is given to the fact that mechanically the cortex of alpha-keratin fibres acts as a two-phase composite (4) consisting of water-impenetrable rods (C) oriented parallel to the fibre axis and embedded in a water-penetrable medium (M). With the fibre in water, the water-penetrable phase M is mechanically considerably weaker than the rods, C, which contain the organised alpha-helices of the keratin fibres. Figure 1. (a): The two phases, C and M, equally distorted by bending of the fibre. This is the typical situation in the bending of an untreated alpha-keratin fibre in water at room temperature. (b): With the weakening of phase M by chemical and/or heat treatment, the tension and compression strains in phase C are removed by distortion of phase M.