362 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VI Mechanical Properties of Bleached and Permed Hair Work to Breaking Yield stress extend 20% stress Work to Bleach No. of Modulus (GN/m 2 (MJ/m 2 (GN/m • break treatment perms (GN/m 2) X 10 -2) X 10-x) X 10-x) (Mj/m •) Unbleached 0 2.8 + 0.23* 6.7 -+ 0.76 4.4 -+ 0.71 3.3 -+ 0.31 2.6 -+ 0.31 1 2.4 -+ 0.43 5.5 +- 0.89 3.0 -+ 0.50 2.3 -+ 0.43 1.4 -+ 0.35 2 1.5 -+ 0.32 3.5 -+ 0.95 2.0 - 0.51 1.7 -+ 0.41 0.91 -+ 0.21 3 0.76 -+ 0.17 2.5 -+ 0.66 2.7 -+ 0.65 1.0 -+ 0.22 0.69 + 0.12 4 hour, 0 2.5 -+ 0.23 4.9 + 0.69 2.7 -+ 0.27 2.8 -+ 0.32 2.2 -+ 0.32 6% H202 1 1.5 --+ 0.23 3.2 --+ 0.54 1.7 --+ 0.44 1.5 -- 0.29 0.88 -- 0.32 2 1.1 --+ 0.22 2.4 --+ 0.59 1.5 --+ 0.44 1.4 --+ 0.24 0.76 + 0.14 3 0.52 --+ 0.1 1.4 --+ 0.38 1.8 --+ 0.47 1.0 --+ 0.22 0.72 --+ 0.20 * 95% conficence limits. Fatigue behavior. Hair is routinely exposed to mechanical stresses during combing and brushing, causing repeated low level extensions, mostly within the elastic limit of the fiber. Constant load fatiguing, described earlier, is used as an approximate simulation of the mechanical tensile damage caused by combing. Fatigue data are expressed as number of cycles to failure for a specimen at a given load. Such data can be presented in terms of cumulative failure probability, as given by equation 4. F(x) = A(x) n (4) Where F(x) is the cumulative probability of failure, x is the number of cycles-to-failure, and A is a constant. The exponent n and fatigue half life (hf), which is the number of cycles required for half the specimens to fail, can be used to characterize the damaging effects of grooming treatments on hair. Typical logarithmic plots for unbleached hair after various numbers of perms are shown in Figure ! 1. The slope of these plots representing the exponent n and the fatigue half life are shown in Table VII. The usefulness of n and hf in evaluating the damaging effect of a treatment or a combination of treatments is clear from Table VII. Reductions in slope and in fatigue half life indicate that the first perm has a greater damaging effect on bleached hair than on unbleached hair. A second perm brings about a precipitous drop in the fatigue half life, suggesting extensive damage. These parameters seem to become less sensitive with additional treatments as the fibers become progressively weaker. It should be noted that perming involved substantial bending effects as a result of winding on perm rods. This methodology shows promise for the evaluation of fiber damage by a combination of treatments. An attempt will be made to extend these studies to evaluate preventive treatments involving various hair care products. SECONDARY DAMAGE FROM GROOMING OF BLEACHED HAIR Changes in the nature and properties of the substrate are of primary concern in the manufacture or consumption of hair cosmetics. While appearance and manageability improvement are the initial goals of any modification of hair surface or structure, it must be kept in mind that such changes can lead to irreversible damage, and efforts must be made to achieve an optimum balance between desired effect and resulting damage.

EVALUATION OF HAIR DAMAGE 363 No Perm • Perm 2 Perms 3 Perms Unbleached Hair No Perm • Perm 2 Perms 3 Perms Bleached Hair Figure 10. Force-extension curves for hair with repeated perm treatments.