388 JOURNAL OF COSMETIC SCIENCE RELATIONSHIP OF SUPERCONTRACTION TO THE DEGREE OF HAIR STRAIGHTENING Table IV shows the relationship of hair supercontraction to the degree of hair straight- ening at different concentration ratios of DTDG to TGA. The results show that per- manent hair straightening is achieved by supercontraction above 5 % but that the surface smoothness is lost by a higher contraction of about 15%. In a practical application, a range of about 5 % to 8% supercontraction of hair has been recommended from the point of view of the mechanical property relationships (3). Figure 2a shows the relationship between the percentage of supercontraction, L c and the concentration ratio of DTDG to TGA. An approximately straight line is obtained for both reducing systems at pH 9.20 and 9.30, although the plots show scatter. The Lc decreases gradually with the increase of the concentration ratio. For TGA only, the values of L• are markedly dependent on the concentration of TGA and pH. Figure 2b shows the relationship of L• to the time of reduction using the bicomponent system in the concentration ratio of 1:5 and at pH 9.30. The curve initially increases gradually until about 25 rain reduction time, and then steeply increases. These results show that the extent of supercontraction is clearly dependent on the extent of reduction, which may be dependent not only on the chemical equilibria as expressed by. equations 3 and 4, but also on the rate of diffusion of the component molecules into the reducing sites of the structure controlling the supercontraction. It is of interest that the physical phenomenon of supercontraction is greatly affected by the reducing systems used, in spite of their small differences (Figure 1). DYEING TESTS FOR DAMAGE OF CURED HAIR Figure 3 shows the scanning electron micrographs in the longitudinal direction of the untreated and cured hairs. Less damage to the cuticles is observed on the fiber obtained Table IV Relationship of Hair Supercontraction to Degree of Hair Straightening Concentration ratio pH 9.20 pH 9.30 Calculated Lc • Degree of hair Lc • DTDG:TGA value (%) straightening 2 (%) Degree of hair straightening 2 0:5 0 18.0 Surface smoothness was lost 21.1 0:4 0 7.8 Complete/permanent 14.73 0:3 0 0.5 None 8.53 1:3 0.33 -- -- 7.9 1:4 0.25 -- -- 8.8 1:5 0.2 9.9 Complete/permanent 8.43 2:5 0.4 6.6 Complete/permanent 5.5 3:5 0.6 4.6 Partial/temporary 5.4 1:6 0.167 8.5 Complete/permanent 8.7 2:7 0.286 7.5 Complete/permanent -- 3:8 0.375 7.0 Complete/permanent 8.9 2:9 0.22 -- 9.9 4:9 0.44 4.0 Partial/temporary 8.4 Surface smoothness was lost Surface smoothness was lost Complete/permanent Complete/permanent Complete/permanent Complete/permanent Complete/permanent Complete/permanent Complete/permanent -- Complete/permanent Complete/permanent Complete/permanent Straight hair samples were used. Kinky hair samples were used. Samples were used for dyeing.

METHOD FOR PERMANENT HAIR STRAIGHTENING 389 [TGA](%) 4 2:5 ,3 , ,5 (a) *** 15[ (b) 20 "•ø d • 10• •. .. • s•, , , , . / / 2o 2s p / ' 0 0.1 0.2 0.3 0.4 0.5 0.6 [DTDGI/[TGA] Figure 2. (a) Relationships between the degree of supercontraction, Lc, and the concentration ratios of DTDG to TGA for bicomponent system (lower axis) at pH 9.20 (•) and pH 9.30 ([]), and the concen- tration of TGA for the TGA-only system (upper axis) at pH 9.20 (0) and pH 9.30 (O). (b) Relationship between Lc and the time of reduction, t in min: DTDG:TGA = 1:5 and pH 9.30. from the bicomponent system (b), while the smoothness of the surface is consider- ably lost for the fiber from the TGA-only system (c). For the case of highly contracted fiber (d), a crevice-like striation running along the fiber axis direction can be seen together with a significant disordering in the alignment of the cuticles. Figure 4 shows the optical micrographs of the transverse cross section for the untreated hairs and the cured hairs dyed with acid black. Each photograph designated as (a), (b), (c) and (d) corresponds to that of the sample in Figure 3. One of the present authors reported that the rate of diffusion of dyes into keratin fiber is increased by virtue of physical phenomenon such as supercontraction induced by the fine structure change