j. Cosmet. Sci., 49, 141-153 (May/June 1998) The cracking of human hair cuticles by cyclical thermal stresses MANUEL GAMEZ-GARCIA, Croda North American Technical Center, 180 Northfield Avenue, Edison, NJ 08832. Accepted for publication May 15, 1998. Synopsis Cycles of wetting and blow-drying were applied to hair fibers and resulted in the formation of multiple cracks on the hair cuticles. The peculiarity of these cracks was that they always appeared aligned parallel to the longitudinal axis of the hair fiber. The cracks appeared to be initiated at the end of the cuticles close to the cortex and propagated invariably towards the outer cuticle edges. The maximum growth length of each crack was seen to be limited to the size of one cuticle. Crack formation did not only occur at the outer edges of the cuticles but also took place in the second and third overlaid hidden cuticle sections. The results show that these cracks form when the external portions of the cuticles undergo drastic reduction in their hydration water. Under these conditions the outer cuticle portions become rigid and brittle and crack by the action of circumferential tension stresses arising from the swelling pressure of both the cuticle layers underneath and the cortex itself. Hair cuticle analysis from a panel of 100 individuals showed that these cracks are present in the hair of people who commonly blow-dry their hair and appear to a much lesser extent in the hair of subjects who do not practice this type of grooming process. The combing of hair fibers presenting this type of cracking was seen to result in the breakage of large portions of cuticle. The effect of some cosmetic actives on the formation of these cracks is also discussed. INTRODUCTION Hair cuticles represent the hair structure components most exposed to various grooming and environmental stresses during the life of a hair fiber. The outer cuticles at the hair surface are frequently subjected to harsh stresses such as abrasion, UV light, bleaching, and blow-drying (1-4). It is well known that once the cuticles are one or two centimeters away from the hair root, patterns of breakage and abrasion start to appear at the cuticle edges (5). By the time the hair is about 14 or 20 cm long, if not enough care is taken, the cuticles might be totally absent at the tips, giving rise to the earlier formation of split ends (6). It is mostly because of these reasons that the effects of combing abrasion on hair cuticles have been thoroughly studied in the past. There are, however, other grooming practices, such as hair blow-drying, whose effects on the cuticle degradation process are still poorly understood. As it is well known, hair blow-drying is nowadays a common practice by many people. When keratin fibers are heated, not only water adsorbed to hair by capilarity is rapidly evaporated but there is also a rapid loss in the free and bound water of the hair (7,8). The rapid loss of hydration water may cause morphological changes within the cortex due to 141

142 JOURNAL OF COSMETIC SCIENCE destabilization of the keratin crystalline regions (9). The consequences of this type of process in hair cuticles after cyclical thermal stresses have not yet been analyzed. This paper represents part of a study whose aim is to reproduce patterns of cuticle damage found in a panel of 100 individuals (10). This article reports the production of small longitudinal cracks by cyclical thermal stresses. The cracks were mainly found in the hair cuticles of subjects who frequently blow-dry their hair. EXPERIMENTAL METHODOLOGY The panel of 100 individuals participating in this study was mainly composed of women with Caucasian brown hair never treated chemically. A total of ten fibers per each individual was analyzed. The different patterns of cuticle damage found in these indi- viduals such as cuticle decementation, abrasion, craters, and cracks were classified and quantified. Cycles of mechanical tension, torsion, bending, and thermal stresses were then applied to single hair fibers in order to reproduce such patterns (10). Only the cyclical thermal stresses were seen to reproduce the types of cracks mentioned above, and therefore, only the experimental conditions related to this type of stress will be described here. The other types of damage and experimental conditions are described elsewhere (10-12). Each thermal cycle consisted of immersing single hair fibers for a period of ten seconds into de-ionized water followed by ten seconds of blow-drying. The temperatures used during blow-drying and measured at the level of the wet hair surface varied between 30 ø and 120øC. The number of applied wetting/blow-drying cycles varied between five and 100 as required. A total of ten fibers per each set of thermal cycles was analyzed for short longitudinal cuticle cracks. The number of these cracks per millimeter of hair was then counted. From this data, means and standard deviations were calculated. Prior to the thermal cycles the fibers were thoroughly washed with SLS and rinsed with tap water. The hair used in the single-fiber experiments was from a subject whose hair was washed only with a 10% SLS aqueous solution for a period of one year. Sections of hair fibers three inches long and cut close to the root were used in the experiments. The hair fibers were subsequently cut into two snippets 1.5 inches long each one snippet was used as a control while the other one was subjected to thermal cycling. All selected hair fibers presented an average diameter of 82 + ll l•m. Caucasian virgin brown hair from International Hair Importers in the form of tresses was also used to study effects of combing abrasion on wet/blow-dry cycled hair. Aqueous solutions of glycerin, propylene glycol, polyquaternium ll, cetrimonium chloride, steralkonium chloride, and hydro- lyzed wheat protein polysiloxane copolymer (13) at a 2% w/w were also used as wetting solutions during thermal cycling. These solutions were prepared in order to test the effect of some cosmetic actives on cuticle cracking. After thermal cycling, the fibers were prepared for SEM analysis. RESULTS AND DISCUSSION SHORT LONGITUDINAL CUTICLE CRACKS FOUND IN A HAIR ANALYSIS FROM A PANEL OF 100 INDIVIDUALS SEM analysis of hair fibers pertaining to subjects in the panel showed that most of them presented different numbers of short longitudinal cracks in their cuticles. The popula-