JOURNAL OF COSMETIC SCIENCE 580 DISCUSSION TANGLE FORMATION INVOLVING HAIR FIBERS LOOPED OVER OTHER HAIRS AND SEVERE BENDING DEFORMATIONS The number and complexity of tangles increase with hair fi ber curvature (10,11), produc- ing higher combing forces as shown by Epps and Wolfram (11) in the midlength and end peak regions of combing force curves. Even with relatively straight-to-wavy hair, in dry combing, tangles form near the tip ends from end wrapping, leading to a higher end peak force with a relatively low midlength force as shown by Kamath and Weigmann (13). Brown and Swift (2) examined the combing of Caucasian hair tresses in the scanning electron microscope (SEM) and observed that severe bending deformations in hair tangles are involved in hair breakage. They observed further that “the tangle tightened to the extent that a few individual hairs began to break and this occurred predominantly at a loop.” Since this type of break occurred frequently, they examined hair fi ber loops near the root end and tip end of the fi bers and noted two types of loop breakage. One type of loop involved pulling both ends of a hair fi ber so that virtually no slippage occurred. Near the root end they observed smooth fractures, but as the site of the loop moved down toward the tip they observed more longitudinal splitting or step fractures until fi brillation oc- curred near the tip end. With another type of loop, they attached one end and pulled on the other end, which resulted in the hair moving slowly as it tightened over the wire. In this case they always observed longitudinal splitting due to high stresses building at the crack tip having a strong tendency to diverge in the axial direction. Robbins (3) observed similar effects by looping hair fi bers over other hairs and fracturing by impact loading. IMPACT BREAKAGE OR PULLING A COMB OR BRUSH THROUGH A TANGLE WITH BREAKAGE Impact loading involves tangle formation and the grooming force to brush through a tangle. It has been demonstrated that hair fi bers can be broken more easily by impact with virtually no increase in length (as in tensile loading) and that hair fi bers break more readily by impact loading than by tensile extension (3). Therefore, hairs can be broken by impact (3), which involves rapidly pulling a brush or a comb through a tangle where one or more hairs are looped over another hair or by pulling through a diffi cult snag of looped hair (2). Whether or not it is necessary for the fi ber to be previously weakened by wear or by fatiguing may be determined by the type of fracture because Kamath and Weigmann (13) have described different types of hair fractures, specifying some conditions, and Robbins et al. (14) have described in more detail conditions that produce different types of hair fractures. Furthermore, some hair fi bers broken in tress combing experiments have been shown to provide smooth breaks (3) indicative of minimum prior damage (13,14) from wear or fatiguing. KNOTS FORM IN HIGH CURVATURE HAIR AND ARE EASILY FRACTURED Knot formation is also related to fi ber curvature, being highest in African-type hair (4). Khumalo et al. (4) demonstrated that more broken hairs are formed on African hair by

HAIR BREAKAGE 581 consumers using their own combing devices than by Caucasians or Asians. Furthermore, these scientists found 13% of the fi bers from two African subjects had knots that they concluded lead to hair breakage. Robbins (3) later demonstrated that hair fi bers with knots break under impact more easily than hair fi bers without knots and that when break- age occurs it is at the knot. The conclusion is that breakage occurs at knots because of severe bending deformations analogous to severe bending at fi ber loops in tangles as sug- gested by Brown and Swift (2), causing the fi bers to break more easily by impact or by breaking through the tangle. Apart from knot formation, in very curly hair with high ellipticity torsional deformation can make a signifi cant contribution to hair breakage, because such fi bers have suffered signifi cant damage during normal daily grooming in the regions of twist. These regions are further stressed by the torsional deformations that occur when the hair passes through the teeth of the comb. These fi bers can fail by a combination of torsional stresses working in tandem with relatively low tensile loads. Such effects are unlikely to be of much importance in the case of fi bers that are straight or have only a slight curvature. TREATMENTS AND WEATHERING: CHEMICAL DAMAGE INCREASES BREAKAGE AND CONDITIONERS DECREASE BREAKAGE Chemical damage by perms (5), bleaches (6), permanent dyes (7), straighteners (8), and sunlight exposure (9) weaken hair and increase interfi ber friction and abrasive damage, leading to more tangle formation and to more hair breakage. Straighteners do decrease curvature and in that manner decrease tangles, but they often weaken hair to the extent that they make the hair brittle. On the other hand, surface treatments such as condition- ers, which make hair combing and brushing easier, have been shown to produce less breakage (10). RELATIVE HUMIDITY OR WATER CONTENT OF THE HAIR Relative humidity or the amount of water in the fi bers can also affect hair combing forces and hair breakage (10). Epps and Wolfram (11) demonstrated that the work of combing highly coiled African hair is lower when the hair is wet than when it is dry, while the reverse holds for wavy-to-straight Caucasians hair (10,11). But this is true for all highly coiled hair versus straight-to-wavy hair (15). For example, highly coiled hair, such as from a permanent wave or highly coiled African, Caucasian, or even Asian hair, provides signifi cantly higher combing (11,15) or brushing forces in the dry state than in the wet state and more dry-state than wet-state breakage. This effect occurs because water breaks some of the hydrogen bonds and salt linkages, resulting in relaxation of the curl, which reduces tangles. On the other hand, straight-to-wavy Caucasian or Asian hair produces higher midlength combing forces in the wet rather than the dry state (3), but a higher end peak force at moderate-to-low relative humidity, explaining why this type of hair provides more long- segment breaks when wet (9), but more short segment breaks when dry (10). Therefore, when we consider the total number of grooming strokes during the day, we must consider three states of relative humidity. The fi rst state considers the number of grooming strokes