312 JOURNAL OF COSMETIC SCIENCE cross-sectional area is calculated assuming an elliptical cross section. The micrometer was calibrated using standard calibration wires of known cross-sectional area (12). Test specimens for the laser-scan micrometer were prepared using a metal-tube sample- mounting system. This system uses a pair of hollow brass tubes with plastic sheaths on the inside and a stamping press (stock no. RS622-032) to crimp the fiber ends in the holders. The tubes are 1.4-cm long with an inner diameter of 0.1 cm and an outer diameter of 0.2 cm. The base of the press is a stainless steel template with a gauge length of 3.0 cm. The hair was threaded through two tubes, and the assembly was placed on the template. When crimped by mechanical pressure, a hair fiber is held securely by the crimped tubes. After measuring the cross-sectional area, the samples were directly transferred to the tensile tester. Tensile tests were conducted in deionized water using a Diastron © miniature tensile tester. The miniature tensile tester autosampler, MTT 600 series, from Diastron Ltd., UK, has a sample holder with a capacity of one hundred (100) specimens. Each specimen can be wetted i, sit•. The degree of deformation and the rate of extension were preset. Data from each specimen were collected and transferred automatically to a personal computer using the MTTWIN software included with the instrument. The software was also used to calculate the tensile properties of the fibers (13). Forty fibers per sample were tested, a number that earlier work has shown to be statistically acceptable (7). The sample length was 30 mm. The strain rate was 40%/min, that is, a crosshead speed of 12 mm/min was used. Samples were soaked for at least ten minutes to ensure complete wetting and saturation prior to tensile testing. SWELLING TEST Swelling was measured by determining the change in hair diameter in a 0.1 N solution of sodium hydroxide at room temperature (9). The "diameter" both dry and after swelling was measured using a Bausch & Lomb microscope fitted with a Digital Filar © eyepiece connected to a Microcode © meter from Boeckler Instruments. Hair fibers weathered for 300 hours at different humidity levels were tested. FTIR/ATR ANALYSIS The IR spectra of hair fibers exposed for 300 hours at different humidity levels were compared. IR spectra were obtained using a UMA 500 FTIR microscope from Bio-Rad. Samples were pressed between two optically matched diamond surfaces. The ATR (attenuated total reflectance) technique, with a depth of penetration of about 5 pm in hair, was employed. The ATR technique involves bringing the fiber in contact with a germanium crystal designed for total internal reflection of the incident radiation. The reflected beam is altered by absorption by the sample surface in contact with the reflecting medium, and this alteration or attenuation is analyzed. Transmission spectroscopy was not used be- cause the variation in hair sample diameter resulted in spectra unsuitable for comparison (•4).

PHOTODEGRADATION OF HUMAN HAIR 313 RESULTS AND DISCUSSION TENSILE TEST RESULTS A typical load-elongation curve obtained from the Diastron © tensile tester for an un- treated Piedmont hair fiber, immersed in deionized water, is shown in Figure 3. The wet tensile properties of untreated Piedmont hair fibers are presented in Table 1. Table II shows the percent loss in properties of the hair fibers after exposure to simulated solar radiation at various levels of relative humidity. The data for work-to-20%-strain and stress-at-20%-strain shown in block A of Table II are graphed in Figures 4 and 5. It is important to emphasize that significant differences in the mechanical properties will be observed only in the wet condition, where the contribution of hydrogen bonds to the mechanical properties is eliminated. Ef?•ct of length of exposure. Damage occurs with exposure to simulated solar radiation and increases with duration of exposure, at all humidity levels, in agreement with the results obtained by Ratnapandian (3), Leroy et al. (8) and Dubief (9). As shown in block B of Table II, exposure for 100 hours results in about a 20% decrease in many properties, whereas a decrease approaching 50% is characteristic of fibers exposed for 300 hours. On the other hand, the turnover point, which is the intersection of the yield and post-yield part of the load-elongation curve or the extension at which the [3-keratin begins to play a significant load-bearing role, and extension-to-break data show an increase of about 10% (block C, Table II). This may be due to the increased mobility of molecular chains that results from the loss of disulfide crosslinks. The gains in these two properties do not compound with length of exposure. Exposure to UV radiation of many polymers causes embrittlement and the formation of new crosslinks, which may be the limiting factor, as suggested by Wolfram (6). Efd•ct of change in humidity. The tensile properties of hair fibers exposed to simulated sunlight for any given time period depends on the RH of the atmosphere during exposure. Contrary to the proposed hypothesis that photolyric damage will increase 78.4 5S.S •t k6?"v 39.2 19.õ O.O O.O 16.50 32.S3 49.33 65.S3 % ext Figure 3. Typical load-elongation curve obtained for a single hair fiber on a Diastron © miniature tensile tester.