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.

BENDING RELAXATION PROPERTIES 131 When a fibre is bent in water, not only is tension and compression developed towards the surface of the outside and inside of the bend, but shear stress is set up in both phases. Torsional data (5) suggest that the stiffness of the water-penetrable phase, M, is almost completely dependent on the presence of disulphide bonds. With the application of ammonium thioglycollate at pH 9.2, the structure of phase M would be extremely mobile due to the rate of thiol-disulphide interchange. This high rate results from the formation of a high density of charged thioIs due to the breakdown of disulphides to thioIs and the ionisation of thioIs at the high pH. A complete elimination of longitu- dinal tension and compression in the rods, C, by the distortion of the water-penetrable phase M can occur (see Figure 1). This possible mechanism of elimination of the mechanical energy stored in a bent fibre is not available to a fibre under pure extension, where the longitudinal strain of the phase C rods cannot be simply transferred to the weakened phase M. This mechanism for set in bending has been discussed in a more detail elsewhere (2) and represents a model of more universal application than that proposed by Wortmann and Kure (1), whose model, however, is certainly consistent with the behaviour of human hair in the permanent waving procedure. REFERENCES (1) F.J. Wortmann and N. Kure, Bending relaxation properties of human hair and permanent waving performance, J. Soc. Cosmet. Chem., 41, 123 (1990). (2) M. Feughelman, The mechanism of set in bending of alpha-keratin fibres, in Proceedings of the 8th International Wool Textile Research Conference, Vol. 1 (Christchurch, New Zealand, 1990), p. 517. (3) M. Feughelman, Permanent set in single wool fibres and the process of recovery of extension, in HI Congrgs International de la Recherche Textile Lainiere, Vol. 2 (Paris, 1965), p. 245. (4) M. Feughelman, A note on the water impenetrable component of alpha-keratin fibres, Textile Res. J., 59, 739 (1989). (5) M. Feughelman and T. Mitchell, The torsional properties of single wool fibres. Part III. Disulphide reduced and permanently set wool fibres, Textile Res. J., 34, 593 (1964).