UV DAMAGE ON GRAY HAIR 109

110 JOURNAL OF COSMETIC SCIENCE 12 10 6 i .............. 10.9 •: i r•DL EIDb BIDE •DYI ..... -3.69 9.34 # 3, OMC 1 •33 ---- f i o.61 o• -0.42 # 2, CATC 4,42 # 4, SLS Gray Hair Sample Figure 2. Change in color index values of gray hair tresses after UV irradiation. color. This demonstrates that CATC is a very good UV absorber and protects hair from color fading. CHANGE IN CROSS-SECTIONAL AREA WITH EXTENSION As a solid fiber, it is known that the cross-sectional area of a hair fiber decreases with an increase in its length during the extension process. Since human hair consists mainly of ot-keratin, a biological polymer, it is expected that the hair fibers exhibit viscoelastic properties. Therefore, the contraction in the cross-sectional area of a hair fiber during its extension process would not exactly be inversely proportional to the amount of its extension. In order to find and establish the exact correlation between the length extension and the contraction of the cross-sectional area during the elongation process, we measured the center cross-sectional areas at different extensions for 100 single brown and gray hair fibers. The plot of the length extension rs. average contraction of the central cross- sectional area is presented in Figure 4. It was found that the correlation between the length extension and the average contraction of the center cross-sectional area is a statistically linear function. The determined ratio of extension to contraction is 1 to 0.758, which indicates that as a hair fiber is extended by 10%, its cross-sectional area only contracts about 7.58%. This coefficient was very useful, and used in calculations of the revised cross-sectional area at the strain-to-break.