PERMANENT WAVING AND PERM CHEMISTRY 123 Figure 19. Reduced time plots for theoretical mechanisms derived from diffusion based assumptions. Figure 20. Reduced time plots for theoretical mechanisms derived from geometric based assumptions. the method holds up to validation studies by producing expected results for systematic changes in conditions. However, with this said, there are several method-related variables that can infl uence the absolute value of the rate.

JOURNAL OF COSMETIC SCIENCE 124 As mentioned earlier, it has been suggested that there is enhanced reactivity associated with strained disulfi de bonds. Table II shows results from a systematic set of experiments that were intended to investigate this idea. Intermittent stress relaxation experiments were performed as a function of the applied strain increment using single-source hair in combination with a 0.42 M, pH 9 ATG solution. Clearly, faster overall transformation results are obtained when using higher strain increments. Further to this point, an additional set of experiments were performed wherein both the rate and period of this deformation were altered. The use of a slower sample deformation rate (i.e., 0.25 inches/min vs. 0.5 inches/min) was observed to yield signifi cantly faster transformation rates. Similarly, repeated application of the strain every 15 s, rather than every 30 s, also led to faster rates. Both of these experimental conditions result in the hair being in a strained state for longer durations and may then be considered in line with the presumed proposition. Con- versely, it could be argued that in spending more time in a strained state, there is greater op- portunity for viscous relaxation, or indeed a yielding of the structure (if the stress begins to exceed the yield point). Further to this same point, static stress relaxation experiments were found to yield faster rates than the corresponding intermittent method. In short, although these fi ndings are in line with the premise, there may be other explana- tions for these outcomes. However, they do highlight the signifi cant contribution of these variables to the magnitude of the rates that result. As such, it is again emphasized that the SFTK approach appears to provide a convenient means of comparing relative transfor- mation rates that arise as a function of solution chemistry variables and/or hair type. However, these same rates should not be expected under real-life usage conditions. From the previous results, it is hypothesized that poor perm performance in resistant hair is a consequence of slow transformation rates that do not induce suffi cient bond breakage during treatment time. Accordingly, the SFTK approach would seem ideally suited for studying factors that may positively infl uence this state. For example, it would be antici- pated that elevated temperatures could induce faster perming rates. Indeed, in real-life salon conditions, it is common practice for the client to sit under an upright hair drier that provides this extra stimulus. It is noted that Wickett performed some preliminary experiments to illustrate this expected infl uence of temperature. One may also conceive of experiments to investigate the effect of swelling agents in formulations or perhaps various prewraps or other pretreatments that could alter diffusion. OXIDATION STEP After concentrating so much time on the breaking of disulfi de bonds, it now becomes neces- sary to consider their reformation. Following from the earlier chemistry discussion, if these bonds are cleaved by reduction, their restoration relies on oxidation. Whereas much has Table II SFTK Results as a Function of the Strain Increment Strain increment (%) Halftime (min) 0.5 20.4 1 17.0 1.5 11.6 2 4.7