HAIR DAMAGE DURING MULTIPLE OXIDATIVE DYEING AND SHAMPOOING 9 Interestingly, the dye-leaching rate does not always increase with progressively increasing amounts of fi ber damage resulting from consecutive dyeing. Figure 6 presents dyeing– leaching profi les of dyed European dark brown hair after each dyeing–leaching cycle. After the fi rst dyeing–leaching cycle, we observe an increase in the rate of dye loss. How- ever, the third, fourth, and fi fth consecutive dyeing–leaching cycles are accompanied by a decrease in the rate of dye loss. This trend is less noticeable in dyed white hair (data not shown). These results suggest that deposition of dyes during the initial dyeing steps occurs in pores and voids formed due to the degradation of melanin and other morphological components of hair. After the second dyeing process, dye molecules from subsequent dye- ing steps may granulate with pre-existing dyes already in the fi ber and form larger par- ticles. The likelihood that large dye particles dissolve slower could explain why the rate of dye loss decreases on consecutive dyeing–leaching cycles. CONCLUDING REMARKS The data generated in this study provide a fundamental understanding of the mecha- nisms involved in hair dye deposition and dye fastness. Key developments were also made Figure 5. DSC measurements of multiple dyed and leached hair fi bers for both (A) pigmented and (B) non- pigmented hair.

JOURNAL OF COSMETIC SCIENCE 10 in understanding hair fi ber damage resulting from the hair dyeing process as well as pro- gressive damage resulting from multiple dyeing cycles. A proprietary sample preparation technique of hair fi ber cross sections, used in combination with atomic force microscopy, allowed us to identify key areas of damage within the fi ber including the cell membrane complex (in both the cuticle and cortex) as well as other regions of the fi ber where we found small pores and other features consistent with physical damage. More than likely, these destructed regions provide exit points for existing hair dyes to leach from the fi ber. Utilizing DSC, we monitored structural changes of keratin protein as a result of hair dye- ing. In addition to examining the ultrafi ne structure of dyed hair, we also generated a signifi cant amount of practical data in regard to the kinetics of the leaching process dem- onstrating differences of leaching in different types of hair and providing real-time data for this commonly employed consumer procedure. This study confi rms the profound effects of rinsing/shampooing after dyeing in governing color fastness. Obviously, mitigating the effects of water and shampoo would help alleviate hair dye loss. As more porous, damaged hair is more susceptible to color loss, new technologies that minimize damage during dyeing could facilitate hair dye systems in which the dyes are less prone to undergo leach- ing during subsequent washing and rinsing steps. Formulators designing color protec- tion products could use the leaching method to quickly evaluate effects of new ingredients, shampoos, or hair treatments on dye leaching. The current work also demonstrates that multiple dyeing processes do not deteriorate hair color loss in spite of progressively dam- aging the hair fi ber structure. It is speculated that larger granular dye molecules are formed inside the hair structure during repeated multiple dyeing cycles. Therefore, any hair dye technologies that facilitate the granulation of dye molecules inside the hair fi ber should benefi t hair color retention. Figure 6. Dyeing–leaching profi les from dyed dark brown after each cycle of the dyeing–leaching process. The dye concentration in solution was determined by measuring the absorbance at 490 nm. The amount of the dye leaching from hair was calculated from the ratio of the absorbance to the weight of hair tress.