j. Cosmet. Sci., 50, 69-77 (March/April 1999) JUL 6 1999 LIBRARY OF SCIENCE & MEDICINE Plastic yielding and fracture of human hair cuticles by cyclical torsion stresses MANUEL GAMEZ-GARCIA, Croda North American Technical Center, 180 Northfield Ave., Edison, NJ 08832. Accepted for publication March 31, ] 999. Synopsis Plastic yielding in the form of crazing and shear bands was found to occur in human hair cuticles subjected to cyclical torsion stresses. This type of damage appeared in the form of helicoidal longitudinal strips around the main axis of the hair fiber following sections of maximum shear stress during twisting. The hair regions with shear bands and crazing were approximately 30 microns wide and 2 to 3 millimeters long and gave the appearance that the hair had partially lost its cuticles. SEM analysis revealed, however, that the cuticular material was still there, and that rather the damaged cuticular regions had lost their structure and bound- aries because they were filled with microvoids, microcracks, and sometimes very narrow long vertical cracks. This type of plastic deformation was found to be a mechanism for dissipating mechanical energy in cuticles in response to the torsional shear stresses that are expected to attain a maximum value at the hair surface. Shear band and craze formation was found to be very sensitive to the moisture content in hair, and at high relative humidities it did not occur at all. Analysis of hair from a panel of 100 individuals showed that shear band and craze formation is also frequently found in hair collected from panelists who employed only conventional grooming practices. The effbct of shear bands and crazes on split end formation is also discussed. INTRODUCTION It is well known that daily grooming practices such as toweling, combing, derangling, blow drying, etc., can produce thermal and mechanical stresses in hair (1,2). They can lead to different types of damage in the cuticles as well as in the cortex. Among the most known types of cuticular and cortical damage caused by grooming practices are cuticle abrasion and "split ends" formation (3,4). Although it is known that combing and derangling of dry hair contribute to the appearance of "split ends," the mechanism of damage is not clearly understood. Recently, it has been proposed that fracture at the hair tips by longitudinal shear stresses during derangling may explain "split end" formation The author's present address is Amerchol Corporation, 136 Talmadge Road, P.O. Box 4061, Edison, NJ 08818-4061. 69

70 JOURNAL OF COSMETIC SCIENCE (5). Cracks and hair fracture are, certainly, two necessary steps during "split end" formation however, the occurrence of longitudinal cracks such as those observed in "split ends" rarely takes place in hair by the action of longitudinal stresses (6,7). In this paper it is shown that cyclical torsion stresses with strain levels beyond the elastic limit (8) produce long longitudinal cracks identical to those observed in hair fibers with split ends. The appearance of these cracks in the analyzed hair fibers was found to result from localized plastic deformation in the cuticles preceded by the formation of shear bands and crazing. Shear band and crazing are the main phenomena accompanying localized shear stresses in polymeric materials (9,10). Shear bands involve strain-localized softening of the material without changes in its density. Craze formation involves, on the other hand, a localized loss of cohesion with significant decreases in density examples of crazing are microvoid and microfracture formation in the material. In the following paragraphs it will be shown that all these types of plastic deformation also occur in human hair cuticles--in particular, at the hair tips and on cuticular regions undergoing split end formation. EXPERIMENTAL METHODOLOGY The hair fibers selected for these experiments were from a subject whose hair was washed only with a 10% aqueous solution of SLS for a period of one year the fibers had a diameter of approximately 82 + 11 !•m. Only portions of hair close to the root and with no damage at all in its cuticles were chosen. The selected hair fibers were then subjected to torsion cycling. Cyclical torsion was applied to the hair fibers by using a 2-V DC reversible motor whose speed and number of turns in each direction were regulated with an electronic controller. The shaft of the motor was attached to one end of the hair fiber while the other hair end was fixed to a bracket. Each torsion cycle consisted in applying to a single hair fiber between 4 and 40 turns/inch, first clockwise and then counter- clockwise. A total of 20 of these cycles was applied to each of at least five fibers. It should be pointed out here that, when the number of cycles was lower than ten, only traces of surface damage were observed. When the number of cycles was increased to 20, the cuticle damage patterns became fully developed in all cases. For this reason, unless otherwise stated, all torsion experiments were carried out using 20 cycles. Three dif- ferent relative humidities (30%, 65%, and 100%) were chosen to carry out the experi- ments. The torsion-fatigued fibers were analyzed by SEM, and their damage patterns were compared with those found in hair from a panel of 100 individuals. The panel consisted mostly of women with Caucasian brown hair that had never been treated chemically. At least ten fibers per each individual were analyzed by SEM. RESULTS EFFECTS OF HAIR TORSION AT LOW RELATIVE HUMIDITIES Low levels of fiber twisting, i.e., four to eight turns per inch applied to hair at 10% RH resulted in torsional deformations that were predominantly viscoelastic and reversible. For instance, a single cycle of six turns/inch applied to hair caused a delay in the