310 Virgin Hair JOURNAL OF COSMETIC SCIENCE 1 min Bleach Smin Bleach Figure 7. Illustration of bleaching damage mechanism. 10min Bleach correlation between cuticle damage and the tensile properties of hair fiber, and their work has focused on chemical damage and the resultant morphological changes shown by SEM. Several other studies have proven that UV light decreases the wet tensile properties of hair (22). Figure 8 shows the results of our study that investigated the effect of UV exposure time on tensile strength. Over 40 hair fibers were used for each testing group to generate statistically sound data. The results are analyzed statistically in Figure 9. Figure 8 indicates that the tensile strength already shows a noticeable decrease after 200 hr of UV exposure, and it remains unchanged after 400 hours. Finally, there is another significant decrease after 1200 hr of exposure. The results illustrate that UV irradiation already attacks the hair cortex in a relatively short period of time, resulting in decreased tensile strength. However, much longer UV exposure times are needed to further dam­ age the cortex after the initial damage occurs. Although the result agrees with other published studies-which proved that UV light decreases the wet tensile properties of hair {22)-more analysis is needed for further understanding of these physical changes from a mechanistic perspective. Color measurement and gas sorption were employed to provide insight into the process of UV damage. Color measurement. Both hair proteins and pigment absorb UV light in the range of 254 to 350 nm. This unique nature helps hair protect itself from UV damage. However, in the process of protection from UV light, pigments are degraded and hair color is eventually "bleached." There are research papers published about the mechanisms of photochemical degradation of emuelanin (23). Since our main focus here is to understand at which exposure stage hair becomes significantly degraded, we measured the hair color 2.5 2 1.5 C) 1 C - � ns 0.5 - o T 1.11a T 1.143 T 1.049 I I o Hr 200 Hr 400 Hr Exposure Hour (Hr) Figure 8. Tensile strength of UV-damaged hair. I T o.sa1 1200 Hr

0.0025 .c 0.002 C) 00 0.0015 'cii � 0.001 1- 0.0005 POROSI1Y MEASUREMENT IN HAIR ■ 0 o_._------.------,-------,-------'---------' 1200 Hr 200 Hr 400 Hr Sample Name Virgin All Pairs Tukey-Kramer 0.05 Figure 9. Statistical analysis of tensile strength of damaged hair. 311 at different UV exposure times. Figure 10 and 11 and Table III indicate that hair color becomes significantly lighter after 200 hr, and shows relatively small change after 400 hr. There is then a much larger change after 1200 hr. The results from color measure­ ment correlate well with tensile strength analysis, which leads to the explanation that photodamage happens not only in the hair fiber periphery but also in the interior, even at 200 hr, and then damage seems to progress with a gradient to a lower level deeper inside, until reaching the stage where most of the peptide backbone has been oxidized. This explanation can be further supported by gas sorption analysis. Gas sorption. The UV irradiation damage of hair gives a much different SA and TPV (Table IV) than oxidative bleached hair. Figures 12 and 13 show the surface area (m2/g) and pore volume (cc/g) of hair versus UV irradiation times of 0, 200, 400, and 1200 hr. 10.0 8.0 � 6.0 4.0 0 2.0 0.0 200Hr 400 Hr 1200 Hr Exposure Hour (Hr) Figure 10. Total color change in UV-damaged hair.