390 JOURNAL OF COSMETIC SCIENCE x•oo x•oo (a) (b) (c) (d) Figure 3. Electron micrographs of longitudinal views of the untreated and the supercontracted hairs. Degree of supercontraction, Lc, for cured hair obtained by ming the reducing system in different concen- tration ratios, DTDG:TGA: (a) untreated (b) 8.4%, 1:5 (c) 8.5%, 0:3 (d) 14.7%, 0:4. (13). Hair that has been damaged by perming and bleaching is also known to be more easily dyed than undamaged hair (1,14). It can be expected, therefore, that it is possible to estimate qualitatively the extent of damage to hairs by the difference in the distri- bution of dye among the treated fibers in the radial direction of the transverse fiber cross section. As compared with the cured samples with approximately the same levels of supercontraction, the peripheral region of the sample obtained from the TGA-only system (c) is more densely dyed than the DTDG-existence system (b), although the dye concentration of both treated samples is clearly higher than that of the untreated sample (a). A higher level of supercontraction leads to a dense dyeing (d) and even to an irregular surface as seen in Figure 4d. It is noted that dye front on the cross section of the fiber treated with the DTDG-containing system exists in the cortex cells near the cuticular region and that the treatment does not affect the overall cortex, while treat- ment with either the TGA-only system (c or d) facilitates dye penetration in the cortex cells to shift the dye front to the inner cortex region, and the cortex and surface morphology appear significantly changed. These facts suggest that deformation of the

METHOD FOR PERMANENT HAIR STRAIGHTENING 391 (a) (c) (d) Figure 4. Optical micrographs of cross-sectional views of the untreated and the supercontracted hairs dyed with acid black. The samples are the same as described in Figure 3. cell membrane complex in both cuticlar and cortical cells occurs. It should be empha- sized that the present test method of dyeing is a useful technique to characterize hair damage induced by chemical and physical treatments, especially for the peripheral regions of fiber. It is further important to note that as compared with the TGA-only system, the reducing system including DTDG gives much less damage to the fiber. RELATIONSHIP OF SUPERCONTRACTION TO THE AMOUNT OF tx-CRYSTALLITES Figure 5 shows the PDSC curves of the cured hair s,amples. The PDSC response of the untreated hair is designated as curve a, whereby a sharp peak can be observed at about 183øC, which is ascribed to the melting or denaturation of the c•-crystallites (6,7). For the sample obtained from the TGA-only system, there is no significant endotherm, as