EFFECT OF CURLING IRONS 239 treatment of hair with hair dyes, bleaches, and permanent lotions, as well as photo- degradation or even combing. The latest modification of the technique involved the application of a treatment to a localized segment of a hair tress, while leaving the rest of the fibers intact as an internal reference (32). Similarly, in the analysis of the thermal degradation of hair, curling irons were applied to a relatively small selected area on each hair tress as illustrated by Figure 9. Consequently, the combing traces revealed the appearance of a peak in the thermally treated area, as a result of fiber damage. The sensitivity of the combing evaluations can further be enhanced by subtracting the untreated control traces from the curves obtained after the treatment. The effect of thermal treatments at 164øC for light-brown fibers is illustrated in Figures 10a and 10b. The fibers were exposed to curling irons continuously for 15 min, followed by two shampooings, then evaluated by combing measurements. A total treatment time of 60 min represents, thus, four heating cycles. A gradual increase in combing forces, probably related to thermal decomposition of the surface lipid layer, is observed. Similar trends were evident in the analysis of combing traces obtained for other types of hair, such as unpigmented white, Piedmont, and Asian. Figure 10b provides the combing- work difference values, which were obtained by integrating the area of the combing curve where thermal treatment was administered and subtracting this value from the similarly obtained value prior to thermal exposure. Additionally, Figure 10b contains a plot for both, a i-in- and a 1.5-in-diameter curling iron. The data for the 1.5-in- diameter iron (Figure 10b) were determined by integrating the thermally exposed areas of the curves in Figure 10a. Recognize, also, that the 1.5-in-diameter curling iron results in much higher combing-work difference values than the corresponding i-in-diameter iron values. This difference is due to much larger integration boundaries provided by the larger iron, which provides the same extent of damage as the smaller iron but covers a much larger surface area of the hair tress. The 1.5-in.-diameter curling iron was used for all the data reported in this communication. Figure 11 presents the temperature de- pendence of combing work, for both intermittent and continuous modes of thermal treatment, for a relatively short period of time (3 min). These data indicate that, in general, the use of a higher heating temperature results in higher combing-work values, although the effect is not very pronounced. The data also demonstrate that the inter- mittent mode inflicts a greater amount of damage to hair, at least in the short time span of this experiment (3 min). One factor, which could have contributed to the larger extent of damage, is the increased handling of samples treated in the intermittent mode, which involves rinsing, towel-drying, and low-temperature hair dryer drying after each heating cycle. The total number of handling cycles for intermittently treated fibers is thus twelve. In contrast, the samples subjected to a continuous thermal treatment were only rinsed and dried once after the continuous 3-min treatment. Finally, it should be pointed out that shampooing is an important step in the evaluation of thermally degraded hair. It was frequently observed, at short exposure times, after the thermal treatment but before shampooing, that hair could actually show a small decrease in combing forces. This could presumably be related to the presence of hair lipids, possibly brought to the fiber surface at elevated temperatures by diffusion from the fiber interior. These lipids are solubilized by shampoo detergents, removed by rinsing with water, and most likely unmask the thermally damaged hair surface, characterized by increased friction.

240 JOURNAL OF COSMETIC SCIENCE 180 (a) lOO 120 20 o -2o 60 min 0 25 S0 75 100 125 150 175 Distance (mm) (b) 800 700 ß 600, 500. 400, 300. 200 ß 100, 0 10 I 1/2" Diameter Iron 1" Diameter Iron 20 30 40 50 60 70 Time (min) Figure 10. (a) Combing curve differences for light-brown fibers monitored after 15, 30, 45, and 60 min of curling iron application (164øC). (b) Combing work difference for light-brown fibers as a function of thermal exposure time reported for two curling irons with different barrel diameters. CONCLUSIONS The thermal treatment of hair with curling irons, at temperatures ranging from 130øC to 170øC, was found to result in significant damage to a variety of fiber types, including white (completely unpigmented), Piedmont, Caucasian light-brown, and Asian hair. The extent of damage was quantified in terms of (a) hair chromophore decomposition determined by fluorescence spectroscopy and Hunter colorimetry, and (b) hair surface modification determined by combing measurements. Both Trp and its oxidation prod- ucts (kynurenines) were affected. As judged by a reduction of its emission intensity, Trp was found to undergo decomposition in all types of hair, with the first-order kinetic rates