THERMAL TREATMENTS WITH A CURLING IRON 21 Fi g ure 5. Continued. was already observed during ren minutes of curling of wet hair under normal load (no added tension). As expected, cuticular damage increases with increasing load during the curling of wet hair fibers for a longer time. As the tension increases from 10 g to 30 g during thermal exposure, there is increased compression and complete disintegration of the surface cuticle cell at the fiber/metal interface, fine-line cracking as well as severe radial cuticular cracking, and various levels of scale edge fusion. These "typical" damage phenomena are shown in Figure 6d-i. Extremely severe disintegration of the surface cuticle cell into an unrecognizable, fused enriry was observed at the f/m interface. This fusion is probably caused by the high temperature flow of the cell proteins plasticized by water at the f/m interface under rhe influence of the applied load ( 10-30 g). These rests have shown that adhering to the recommended guidelines for curling (dry hair, normal tension, short-term curling) results in minimal damage to the cuticula. However, disregarding these guidelines increases the potential for severe surface damage, which culminates in rhe disintegration of the curicula, especially when rhe hair is in rhe wet stare. EFFECT OF HEAT CURLING ON TENSILE MECHANICAL PROPERTIES or DRY HAIR Specimens from the same tress of dark brown European hair were used to examine rhe

22 JOURNAL OF COSMETIC SCIENCE Figure 6. Typical topography of wet hair fibers curled for ten minutes under 0 g tension (a-c) and 10, 20, and 30 g constant tension (d-i). effects of "repeated, cyclical short-term curling/cooling under normal tension" on the mechanical properties of dry hair. The untreated hair fibers served as control. For the curled sample, a small bundle of hair fibers was curled 200 rimes for ten seconds each time, using the Phillips HP 4480 curling iron, which subjects the hair to a temperature of 110°C. The fibers were extended to break on a Diastron machine (at 65% RH, 21 °C). Table I shows the results. In examining the data in Table I, we find a trend toward slight increases in breaking strength, work to 20% with no statistical significance. A slight reduction in extension to break suggests rigidification of the fiber structure. This can be due to the introduction of crosslinks by dehydration reactions at a higher temperature. This is especially true of a large increase in the post-yield modulus. The post-yield modulus, which reflects the interference of crosslinks in the matrix with the a ➔ � transition of helical structures, shows a significant increase after the hair has been exposed co repeated, cyclical short-term curling/cooling. Therefore, one might hypoth­ esize that this may be due to thermally activated crosslinking in the matrix during curling, and as a result of this crosslinking, the transformation of the polypeptide chains from the a-helical configuration to the �-pleated sheet arrangement may be inhibited. EFFECT OF HEAT CURLING ON TENSILE FATIGUE BEHAVIOR An extensive study investigated the effects of thermal treatment under normal tension