BENDING PROPERTIES OF HAIR FIBERS 365 350 300 250 200 15o 100 5O • ß N = 19.69 (10 ? / R 4) + 71.47 ß R 2 = 0.8496 0 i ! i ! i i 0 2 4 6 8 10 12 10 ? / R 4 (IJrn '4) Figure 8. Mean number of strokes N of the pendulum versus 1//•4 (/•: mean radius of hair fiber) for different natural hair fibers. ambient air (11). This test makes it possible to evaluate the influence of water content in air on the bending behavior of natural hair fibers. Due to its chemical and physical structure, hair can easily absorb water. The water molecules have a plasticizing effect on the keratin structure of hair. Because of their small size, water molecules can diffuse inside hair fibers, separate the keratin molecules, and reduce intermolecular interactions or change their nature (12). After reaching an equilibrium between hair and ambient humidity, the mechanical properties of hair are thus altered. Since the water-sensitive amorphous area of keratin is particularly involved during bending, the number of pendulum strokes can be measured in relation to the water content of ambient air. The analysis was based upon four series of four samples, each made up of 39 similar hair fibers. The hair fibers were previously washed with a shampoo. The measurements were taken in a temperature- and humidity-controlled chamber that was set to 25øC and to an appropriate relative humidity. Before being tested, the hair fiber samples were stored for 12 hours in the conditioning chamber. In order to measure the samples at 100% RH, they were immersed in distilled water for ten minutes before measurement in the controlled environmental chamber set to 90% RH and 25øC. The same samples were measured at different levels of relative humidity. The test was performed with virgin hair, bleached hair, and both waved and bleached hair (conditions are given in the Appendix). The numbers of pendulum strokes were then plotted against the level of relative humidity, as shown in Figure 9. The data confirmed that the moisture conditions played a key role in the bending properties of hair. Between 20% RH and 90% RH, the hair fibers became less rigid as the relative humidity increased irrespective of the type of hair. Since virgin hair is less

366 JOURNAL OF COSMETIC SCIENCE 400 z 350 300 250 200 150 ---- virgin hair - - ß - - bleached hair ß both waved and bleached hair 0 20 40 60 80 100 % of relative humidity (at 25øC) Figure 9. Strokes number N versus relative humidity (at 25øC) for three different types of hair (virgin, bleached, and both waved and bleached). rigid than bleached hair at 20% RH and up to 90% RH, both types of hair appear equivalent at 90% RH, with the stroke number values reaching a maximum. At 20% RH both waved and bleached hair and bleached hair are similar, but the former becomes less and less rigid as the relative humidity increases without reaching a maximum at 9O%. Those results indicate that from 20% RH to 90% RH, the active sites for plasticizing effect are progressively used. Above 90% RH, in the case of virgin or bleached hair, all the sites are occupied, whereas, in the case of both bleached and permed hair, the modifications of the structure are more numerous and more sites are available for water penetration. INFLUENCE OF COSMETIC TREATMENTS In the field of hair research, this technique is a useful test in evaluating the stiffening or weakening of hair fibers after a particular cosmetic treatment. It may also be useful in better understanding the possible structural modifications--or their prevention-- induced by bleaching, permanent waving, etc. Two types of treated hair were studied: bleached hair and both waved and bleached hair. The treatment conditions are given in the Appendix. The results (Figure 9) show alterations in the bending properties according to the cosmetic treatment performed. Both permanent waving and bleaching treatments alter the disulphide bond structures and consequently modify the mechanical properties of the fibers. At low relative hu-