EFFECTS OF WATER ON HEAT-STYLING DAMAGE 23 Figure 4 shows that the damage to the medulla and cortex is much less severe in hair that has been heat treated when dry, versus hair that has been heat treated when wet. This was a pattern also seen in the experiments using the heat-protection sprays. Medulla damage was consistently greater on hair that had been treated with the “wet” products versus the “dry” products. Lower structural damage in dry versus wet hair was in good agreement with previous SEM studies on thermal damage to hair (4). SINGLE-FIBER TENSILE TESTING Tensile testing was performed on the same fi bers used in the fl uorescence spectroscopy work and in light microscopy for the single-dose experiments with “wet” and “dry” products. Figure 5 illustrates the differences in the changes in Young’s modulus. Clearly, the reduc- tion in Young’s modulus caused by heat treatment is much greater in hair treated in the presence of the “wet” heat-protection spray. No signifi cant change in modulus (p0.05, Student’s t-test) was observed in hair treated in the presence of the “dry” heat-protection spray. Our data are in good agreement with previous work on the effects of repeated thermal treatments on dry hair (4). These previous investigations also showed no signifi cant changes in Young’s modulus after treatment. Unfortunately, changes in the mechanical properties of heat-treated wet hair were not reported. Figure 4. Typical light microscopy images of hair fi bers, illustrating the increased structural damage ob- served in hair treated with straightening irons for 12 × 5 seconds (×60 magnifi cation). A, C, and E are focused on the cuticle and surface. B,D, and F are focused on the cortex and medulla.
JOURNAL OF COSMETIC SCIENCE 24 Table III shows that both the post-yield gradient and the break stress were reduced by heat treatment in the presence of the “dry” spray (p0.05, Student’s t-test), but in good agreement with the Young’s modulus data, the reductions were signifi cantly greater in the presence of the “wet” spray. Table III also shows that no signifi cant differences were found in the changes in stress at 15% strain, work at 15% strain, and break extension in hair treated in the presence of “wet” versus “dry” sprays. In the case of work at 15% strain and break extension, this might be explained by the fact that these data are calculated without normalization with fi ber cross-sectional areas. It is likely that the greater variability in this data is part of the reason why no statistically signifi cant differences were seen. DISCUSSION In this study we have investigated how the presence of water affects the chemical and physical damage to hair caused by thermal treatments with straightening irons. Fluorescence spectroscopy studies were performed to investigate how heat treatment damaged the hair proteins, specifi cally the amino acid tryptophan. These experiments showed that, in good agreement with other published data (5), heat treatment at 185°C for a cumulative treatment time of 60 seconds could reduce tryptophan levels by 35%. In agreement with other fl uorescence spectroscopy studies (9), we did see good protective effects from our prototype heat-protection spray containing vinylpyrrolidone/vinyl ace- tate co-polymer, quaternium-70, and bis-PEG/PPG-20/20 dimethicone. Interestingly, the effects were only seen when quite a thick layer of product had been put on the hair after repeated applications. It is likely that clumping of the fi bers and the presence of thick insulating fi lms helped provide extra thermal protection. Interestingly, in this study we saw no difference in tryptophan damage in wet versus dry hair. Our data, suggest, therefore, that tryptophan oxidation is not sensitive to the pres- ence of water and occurs simply in the presence of oxygen from the air. This conclusion would fi t observations made by McMullen and Jachowicz (5). These workers were able to show that thermal degradation of tryptophan at 164°C increases as treatment times Figure 5. Changes in Young’s modulus after heat treatment. Comparison of the effects on hair treated with a “dry” heat-protection spray and a “wet” heat-protection spray.
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