PATHWAYS OF HAIR BREAKAGE 237 RESULTS AND DISCUSSION HAIR BREAKAGE DURING COMBING OF TRESSES Eight-gram hair tresses made from 14-inch dark brown European hair were both square­ cut about two inches from the bottom (so the ends were of the same length) and angle-cut about two inches from the bottom to the tip (the ends were of varying lengths) and combed vigorously for 100 strokes, starting each comb stroke at 17 .8 cm from the bottom of each tress. The broken hair fragments were all collected on a large piece of white plastic and separated by length into five groups and counted. See the data of Table I and the Experimental section for details. The data of this experiment (Table I) show significant differences among hair lengths and between the two types of cut. Clearly more hairs are broken by crosscutting the hair where the ends are essentially of the same length. This technique of crosscutting hair provides more hairs at the end of the comb stroke and therefore a higher end peak force and thus more breakage. But, clearly another important factor is the larger number of shorter broken hair fragments than longer fragments. Only 3% to 5% of the broken fragments are longer than 17 .8 cm (the comb stroke), while about 70% of the broken hairs are less than 1.27 cm. For both types of cut, only two broken hair fragments are longer than the comb stroke length. Therefore, most of the breakage occurs in short fragments at or near where the comb interfaces with the hair rather than above where the comb interfaces with the hair as one would expect from tensile loading. Thus the comb is not functioning as a device to hold and stretch the hairs as in tensile fracture, but a more complex interaction is occurring at snags at or near where the comb interfaces with the hair, consistent with the conclusion of Brown and Swift (3). There­ fore, to understand hair breakage during combing, the key to these interactions is to understand the snags where the highest combing forces and hair breakage are encoun­ tered. ENTANGLEMENTS IN SNAGS AND PATHWAYS FOR HAIR BREAKAGE Nearly 30 years ago, Brown and Swift (3) photographed hair snags in an SEM and related the entanglements to the fractured ends of human hairs. These scientists illustrated crossover entanglements and looped hairs, but also described how these entanglements lead to cuticle disruption caused by the abrasive actions of hair on hair rubbing during Table I Combing Hair Tresses and Breakage by Hair Length Number of hair fragments Broken-hair lengths Cross-cut Angle-cut 17 .8 cm or longer 2 2 12.7 to 17.8 cm 33 16 6.4 to 12.7 cm 63 23 1.27 to 6.4 cm 192 64 d.27 cm 754 238

238 JOURNAL OF COSMETIC SCIENCE combing, which seriously damages the hairs, leading to longitudinal splitting of the fibers. In the current study, snags were created by combing one- and two-gram tresses of very curly steam-set hair and slightly wavy Caucasian hair, which were photographed using a digital camera with a close-up lens. These photographs show that crossover hairs in advance of the moving comb and behind the moving comb create snags in which: • hairs can wrap and loop around other hairs and around comb teeth (single and multiple teeth) to create very tight entanglements with hairs in stressed positions (see Figures 3, 4, and 5), • hair fibers can cross over taut hairs that are wrapped around comb teeth, creating a situation with a high probability for breakage (see Figures 3, 4, and 5). Considering the above conclusions from photographs and the prior literature (1-4), the following mechanisms or pathways for hair breakage were hypothesized: 1. Impacting or compressing and extending one hair fiber against another taut hair (Figures 3-5). This condition may involve almost any angle of wrap from a simple perpendicular crossover to one hair fiber completely looped around another (3). 2. Impacting or compressing, extending, and abrading a hair fiber against a taut hair fiber (Figures 3-5). This is essentially pathway 1 with abrasion. 3. Extending and compressing or impacting hairs with flaws or cracks and/or chemically weakened hair. If the fibers have been sufficiently weakened, extension alone may cause breakage. COMPRESSION FORCES DURING COMBING All the above pathways for hair breakage suggest that compression forces in addition to extension are involved in breakage and that in two of these pathways (2 and 3 ), abrasion is also important. In other words, these pathways are consistent with Brown and Swift's Figure 3. Slight wavy Caucasian hair snag beneath advancing comb.