Mechanical Properties of Hair 232 Table 1. Percentage change in tensile parameters on successive permanent wave treatments calculated from data in Tate et al.34 Treatment Parameter Perm 1 Perm 2 Perm3 Young’s Modulus 86 54 27 Yield Stress 82 52 37 Work to 20% 68 46 61 Stress to Break 70 52 30 Work to Break 54 35 27 Hu32 investigated the effects of five-minute heat treatments by measuring effects on both breaking stress and the work to 20% extension (W20). Results with virgin Asian hair are shown in Table 2. The data indicate that while breaking stress was significantly reduced by the five-minute heat treatment at 130°C and 160°C, W20 actually increased, indicating a slight stiffening of the fiber. This stiffening in the yield region may be due to slight supercontraction of the fiber as a result of moderate heat treatments.37 Mitchell and Feughelman found little difference in yield stress but large changes in post-yield stress when wool fibers, which had been held for 1 hour extended in water in the yield region at temperatures up to 100°C, were relaxed and extended at 20°C.38 This may be due to loss of disulfide bonds on heating that are not extended before post-yield region is reached. Hu32 also investigated the effect of heat treatment on virgin and relaxer treated African American hair, the latter of which was timed for 18 minutes and performed with a commercial hair relaxer (Table 3).