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j. Soc. Cosmet. Chem., 48, 123-126 (March/April 1997) Mechanism of split-end formation in human head hair j. ALAN SWIFT, Department of Textiles and Fashion, De Montfort University, Leicester LE1 9BH, England. Accepted for publication May 1, 1997. Synopsis The satisfactory prevention or repair of "split ends" in long-hair styles continues to present a significant challenge to the toiletries industry. The mechanics by which they form has not been clarified previously. Combing is a common requirement, without which few split ends are formed (1,2). Splitting always occurs in the plane containing the hair's major elliptic diameter. A theoretical analysis supports the notion that during the combing of tangled hairs, high shear stresses are developed about the major elliptic diameters of the fibers. Shear fracture about this diameter and the further propagation of the fracture front towards the hair tip adequately explain the characteristic primary morphology of this cosmetically undesirable terminal bifurcation of the hair. Amongst the various factors that influence split-end formation, two are highlighted in that they offer prospects for simple modification to the cosmetic advantage of reducing the incidence of split ends. One is about reducing frictional interactions between hairs by lubrication and the other concerns improvements in the longitudinal shear plasticity within the hair's bulk. Of frequent concern among those with long-hair styles are "split ends." The satisfactory elimination of this cosmetically undesirable manifestation of hair damage remains a major challenge for the toiletries industry. Split ends form more readily in hair that has been treated with harsh chemical agents (as in bleaching or permanent waving or oxidative dyeing) or that has been exposed to excessive sunlight. They do not form spontaneously but require the input of mechanical energy most commonly thought to be associated with brushing and combing (1). Many of the factors that exacerbate the formation of split ends are known (1,2), but yet the sequence of mechanical events underlying their generation has not been described previously. In his extensive investigations of human head hairs with the scanning electron microscope (SEM) the author has noted that terminal splits involve a primary bifurcation extending away from the hair's extremity in a longitudinal plane invariably through the fiber's diameter. Moreover, given that most hairs are elliptical in section, this initial split without exception occurs through the major elliptic diameter. It is this form that provides the clue to the mechanical events that were involved in the hair's longitudinal fracture. During regular combing of hair, "snagging" often occurs as the comb approaches the ends of the hair, an effect that increases in severity with the natural length of the hair (3). Such high terminal comb forces are encountered during the first passage of the comb through the hair, but this force diminishes with successive passages of the comb and as 123