727 PHYSICOCHEMICAL PROPERTIES OF TEXTURED HAIR breakage (54–57). Tensile strength measurements of hair help us to gain insight into the mechanical strength of hair as well as the physicochemical properties of its structural components. A number of different parameters can be determined from the experimental measurements, which include Young modulus, break stress, and percentage strain at break. For demonstration, the break stresses at 65% RH and 100% RH are provided in Table II. The data obtained in this study were compared to previously published studies and found to have agreement at both climate conditions (6). Nevertheless, care should be taken when comparing tensile strength data since strain rate can affect the measured values (58). Not surprisingly, higher humidity results in lower stress break values due to water obstructing the hydrogen bonds that normally form in the matrix component of the cortex of dry hair. It should also be pointed out that African hair was found to break at lower strains. Another aspect of tensile strength experiments is examining the hair morphology at the fracture point in the fiber. Researchers at TRI Princeton conducted a fairly comprehensive study on the fracture behavior of African hair (from one subject) in the 1980s (7). Overall, they concluded that a series of different fracture patterns could be observed in hair when it undergoes breakage in a tensile test: smooth, step, angle, fibrillate, and split end. African hair was shown to experience all types of fracture, while the predominant fracture pattern in Caucasian hair was reported as a smooth fracture. Our results indicate that African Table II Break Stress (MPa) Data for African and Caucasian Hair From This and Other Studies at Two Different Climate Conditions Studya 65% RH 100% RH African Caucasian African Caucasian A 191 188 156 165 B 180 178 160 155 C 148 184 94 162 D 112 180 — — This study 155 ± 28 194 ± 13 122 ± 23 180 ± 16 a The African hair reported is tightly curled African hair. Source: Data from Studies A, B, C, and D, reported for four different locations, were published in a review article by Wolfram (6). Figure 16. FESEM micrographs of a step fracture in Caucasian hair at (A) low and (B) high magnifications.

728 JOURNAL OF COSMETIC SCIENCE hair has unique fracture behavior, possibly due to its higher lipid levels or other biological components not yet identified. A typical fracture pattern observed in Caucasian hair is found in Figure 16A. Such a pattern can be classified as a step fracture. In this case, there are two steps within the step fracture. In some cases, we observed that this type of fracture can be a relatively uniform cut along the axis of the fiber (containing only one step). Figure 16B contains a closer view of the step fracture in Figure 16A. The overall characteristics of the extended, broken fibrils were observed in all the Caucasian hair samples we examined. Figure 17A presents an example of an angle or slanted step fracture pattern in tightly curled African hair. It is immediately apparent—as compared to Caucasian hair—that there are shorter broken fibrils in tightly curled African hair. A close-up view of one of the fracture zones (Figure 17B) reveals shorter extended fibrils that appear to be heavily coated with a nonfibrillar substance, perhaps lipids. CONCLUSION In this work, we investigated the physicochemical properties of textured hair, specifically African hair with CI values of 0.45 ± 0.08 and 0.63 ± 0.13. General morphological and fine structural details of textured hair were compared with Caucasian hair, and the unique geometrical aspects of the fibers were qualitatively described. The lipid distribution levels were determined by examining cross-sections of hair fibers where it was found that African hair had greater quantities of lipids than Caucasian hair. These data were corroborated by DVS data that demonstrated that the water uptake of African hair is less than that found in Caucasian hair. The morphology and fine structure of hair were further examined by preparing cross-sections of hair. African hair did not contain a well-developed medulla, such as that found in Asian or Caucasian hair. More than likely, this is due to the slower growth rate of African hair, which allows the cortex to become more fully developed. Tensile strength studies yielded break stress data in agreement with previous studies in the literature. We also examined the fracture patterns of fibers subjected to tensile testing with FESEM. African hair experiences various types of fracture patterns when subjected to mechanical tensile strain. African hair is unique from Caucasian hair in terms of the dimensions of the fibril remnants that are present after fiber fracture. Figure 17. FESEM micrographs of (A) an angle or slanted step fracture in African hair and (B) a close-up view of a fracture region.