Changing the Shape of Hair 184 remarkable effect relaxing can have on hair breaking in fatigue testing see Table 3 in Chapter 7. Depilation This chapter is focused on changing the shape of hair and perhaps the ultimate way to change the shape of hair is to remove it! Depilatories function by weakening the hair to the point that it can easily be removed by rubbing with a washcloth. Most depilatories use salts of thioglycolic acid to weaken the hair. From the standpoint of reactive chemistry the only significant differences between an ATG permanent wave as described above and a depilatory are pH and counter ion.82 Depilatories are usually pH 12–12.5 rather than pH 9.2–9.5 so ammonia can’t be used and calcium is the most common counter ion. Yablonski and Williams83 used a traveling microscope to study both diametrical and longitudinal swelling of hairs treated with depilatories. With untreated hair, longitudinal swelling is very limited due to the water impermeable nature of the keratin microfibrils.1,59,84 As the hair is broken down by the depilatory, length swelling occurs. Figure 17 shows the swelling data of Yablonski and Williams as rate of diametrical and length swelling (A) and extent of length swelling (B) in the presence of a depilatory. Note that longitudinal swelling does not begin until cross- sectional swelling is nearly complete. The point of maximum rate of longitudinal swelling was reported to be a reproducible value that is correlated with depilatory efficacy. Other methods have been used to measure depilatory efficacy. And older method was to observe a hair fiber in a light microscope under crossed polarizers. As the depilatory works its way into the hair, the birefringence from the oriented α-helical proteins of the microfibrils disappears and the hair “goes dark.” This method is interesting but not easy to quantify. Elliot85 developed an instrument that repeatedly applied a load to 10 hairs for 10-second intervals and measured the depilation time as the time it took the hairs to break. Data were reported on a wide variety of depilatory conditions.
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