EFFECTS OF WATER ON HEAT-STYLING DAMAGE 25 Table III Effects of Heat Treatment on the Tensile Properties of Hair: Comparison of Hair Treated with a “Wet” Heat-Protection Spray and Hair Treated with a “Dry” Heat-Protection Spray Measurement “Dry” heat-protection spray “Wet” heat-protection spray Mean differences in changes, “wet” spray versus “dry” spray Statistical signifi cance of the differences (Student’s t-test) Mean control Mean treated Mean change Mean control Mean treated Mean change Cross-sectional area (μm2) 4121 4087 −66 3669 3718 −18 48 0.7766 Young’s modulus (Nm−2) × 109 1.97 1.98 0.01 2.17 1.96 −0.26 −0.25 0.0007 Stress at 15% strain (gmf μm−2) × 10−3 8.65 7.54 −1.11 8.14 6.92 −1.33 −0.22 0.2437 Work at 15% strain (J) × 10−4 4.72 4.26 −0.49 4.19 3.57 −0.68 −0.19 0.3202 Post-yield gradient (gmf mm−1) 10.6 9.50 −1.13 9.96 7.47 −2.61 −1.48 0.0005 Break extension (% strain) 67.3 65.2 −2.14 64.1 63.9 −1.47 0.67 0.7354 Break stress (gmf μm−2) × 10−2 2.04 1.80 −0.23 2.04 1.56 −0.50 −0.27 0.0001 Total work (J) × 10−3 3.53 3.04 −0.52 3.07 2.31 −0.79 −0.27 0.1581 increase from fi ve minutes to 30 minutes. Clearly, after only a few seconds, water will have been removed from the hair, and the only way that oxidation of the tryptophan could occur would be through oxygen in the air. Light microscopy studies revealed that the presence of water, delivered from a “wet” heat- protection spray, did cause signifi cant extra structural damage during heat styling. Such damage had not been detected with the fl uorescence spectroscopy measurements. Previ- ous SEM studies have shown that the structural damage caused by heat treatment is dif- ferent on dry versus wet hair (4). On dry hair, repeated heat treatments with curling tongs caused mainly axial cuticle cracking and fusion of scale edges. On wet hair, the same treatments caused bulges or bumps in the cuticle scale faces and ripples or “half-domes” at the scale edges. The authors proposed that these distortions were caused by the hygro- thermal “fatiguing” of the wet cuticle. In good agreement with these previous studies, we also saw damage to the fi ber medulla in wet hair, which appeared to be related to the rapid boiling and evaporation of water. Tensile testing was used to get more precise and quantitative measures of structural dam- age. These experiments confi rmed that structural damage to hair caused by heat styling was greater in the presence of a “wet” versus a “dry” heat-protection spray. Decreases in
JOURNAL OF COSMETIC SCIENCE 26 tensile measures, such as Young’s modulus, were clearly greater in samples treated in the presence of the “wet” spray. The lack of any change in Young’s modulus on dry hair was in good agreement with similar studies in the literature (4). It now seems clear that the type of damage caused by heat treatments is different in wet versus dry hair. In dry hair, thermal treatments cause chemical damage and some struc- tural damage. However, in wet hair, thermal treatments cause the same chemical damage but considerably more structural damage, which causes signifi cant changes in the physi- cal properties of the hair. It is likely that the rapid evaporation of water from the hair is the main causal factor. The results of this study raise a number of important issues and opportunities in the area of heat styling: First, while our data confi rm that blends of insulating polymers can help protect hair from chemical damage, they also suggest that heat-protection sprays are best formu- lated with volatile carrier solvents, such as ethanol, rather than with water. As we have seen, dry hair will be less structurally damaged, and less weakened by the straightening irons. The idea of making “dry” heat-protection sprays has recently been patented (11). Second, this study suggests that straightening irons are best applied to blow-dried hair to minimize heat-styling damage. It is argued by some stylists that applying irons or tongs to wet hair gives a better straightening effect. Consumers also save time by not fi rst blow- drying their hair before straightening. These habits will, our data suggest, cause greater damage to the hair. Finally, it is now possible to buy straightening irons that are claimed to allow con- sumers to straighten wet, towel-dried hair (e.g. Remington Wet2Straight® straight- eners Vidal Sassoon Professional, Wet to Dry® hair straighteners). These appliances typically have steam vents to allow the rapid evaporation of water from the hair. More work needs to be done, our data suggest, to confi rm the safety of such appliances in terms of hair damage. Further work is clearly needed to investigate the effects of water on heat-styling damage. Electron microscopy studies of cross sections of heat-styled hair would iden- tify, with more clarity, the structural damage in hair ironed wet or dry. Differential scanning calorimetry could also be used to better characterize keratin changes (7). One would expect keratin denaturation to occur at lower temperatures in more damaged hair. Further work is also required to understand what the reactions are in heat-catalyzed tryp- tophan oxidation, particularly to understand the particular functions of water, oxygen, and free radicals in the reactions. Such studies might suggest new ways of chemically blocking protein damage. CONCLUSION This study has confi rmed that it is best to use straightening irons on dry hair to reduce structural damage. Furthermore, it is best to use a “dry” heat-protection spray to help keep damage to a minimum.
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