376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS than those of samples chlorinated in either pH 7 or pH 10 solutions. At ten cycles of treatment and above, the differences between frictional values of the pH 7 and the pH 10 samples were also statistically significant. When examining the effect of cycles of treatment, the greatest increase in the friction values of the samples chlorinated in pH 2 solutions occurred by five cycles of treatment. After five cycles the values fluctuated and, in the case of blond hair, dropped to a significantly lower value at 30 cycles. The friction values of the samples chlorinated in pH 7 solutions showed a continual increase with cycles of chlorination, while those of samples chlorinated in pH 10 solution showed no significant change with cycling. The strong pH dependence of frictional properties in chlorine treatments observed here is similar to that found in wool (11, 12). The alteration of friction is generally attributed to the oxidation of disulfide bonds in the cuticular layers, resulting in the softening of the surface and the destruction of scales. An important frictional prope.rty of viscoelastic fibers is that under suitable conditions of testing, they produce a stick-slip profile. Several typical profiles obtained in this work are given in Figure 1. These profiles clearly indicate the marked alteration in the prop- erties of the fiber surface with increased acidity and cycles of chlorination. Since all friction tests were carried out at a constant speed of traverse (0.5 in/min), a change in the tension profiles could only arise from a change in the state of the surface of the fibers. A comparison of profiles clearly indicates a transition from a hard and elastic surface of the control and less severely chlorinated specimens to a softened and plastic surface of the more severely chlorinated specimens. The amount of time spent in the upward traverse of the profile (stick) as a fraction of the whole was assessed to quantify the changes in the stick-slip behavior. Data acquisition by computer is more sensitive than the recorder system of a tensile tester. Therefore, there can be measurable "sticking" in what appears to be a straight force-time plot. The average values of this parameter (percent stick) are given in Table I. Both hair types gave similar results. There were no significant differences in percent stick noted be- tween pH levels of the control fibers. In pH 2 solutions, percent stick increased rapidly with cycles of chlorination, indicating a softening of the fiber surface. In pH 7 solu- tions, it increased gradually, and in pH 10, it showed a significant increase only at 30 cycles. The transition from hard elastic to soft plastic surface is verified by examination of the surface morphology of friction-tested samples in the scanning electron microscope. The specific areas on the surfaces examined were those where the fibers had actually rubbed during friction tests. The control samples (Figure 2a) and the samples chlorinated in alkaline solutions (Figures 2b and 2c) show a definite scale structure, and the damage due to the friction test is minimal. In the samples chlorinated in neutral (pH 7) solutions, the deformation due to friction is clearly visible. The transition from a hard to soft surface is gradual. After 15 cycles of treatment (Figure 3a), the fibers exhibit an altered surface with a greatly reduced scale definition. By 30 cycles (Figure 3b), the fibers have lost all scale definition. The frictional data indicates a rapid alteration of the surfaces of hair fibers chlorinated in pH 2 solutions. This result was substantiated by observation of fiber surfaces under SEM. After five cycles of treatment (Figure 4a), the surfaces have lost much of their scale definition, and the damage due to rubbing reaches the underlying cuticle layers.

CHLORINATION OF HAIR AND pH 377 pH I0 -- - -" CONTROL CYCLES 30 CYCLES pH 7 CQNTROL --------- ---- ,- - -- - -- , 5 CYCLES 30 CYCLES pH2 • ' - CONTROL 5 CYCLES 30 CYCLES Figure 1. Examples of Instron © force patterns illustrating the effect of pH and cycles of chlorination. Figure 4b shows little further change in the fiber surfaces with additional cycles of chlorination, a result supported by the frictional data presented earlier. These changes in surface structure are similar to those seen in the chlorination of wool (13-17).