j. Soc. Cosmet. Chem., 36, 39-52 (January/February 1985) Effect of chemical and hurnectant treatments on the mechanical and fractographic behavior of Negroid hair Y. K. KAMATH, S. B. HORNBY, and H.-D. WEIGMANN, Textile Research Institute, P.O. Box 625, Princeton, NJ 08542. Received October 31, 1984. Presented at the Annual Meeting of the Society of Cosmetic Chemists, New York, December 6-7, 1984. Synopsis The fracture behavior of Negroid hair, both untreated and treated with relaxers and humectants, was studied to clarify the causes of fiber breakage at low levels of extension. Microscopic observation of the fiber reveals frequent twists, with random reversals in direction along the length, and high ellipticity. Measurements of the effect of tensile load on axial angle of untwisting of specimens with a single twist indicate that failure at low extensions is due to the initiation of cracks at flaws near the twists, which relieves torsional stresses in these regions. Extension at failure is higher in wet fibers, probably because plasticization relaxes these stresses. Treatment of fibers with solutions of humectants such as polyacrylic acid or glycerin has been found to reduce premature failure significantly. However, pretreatment with relaxers and thioglycolic acid eliminates the beneficial effects of the humectant treatment. Scanning electron microscopy of fracture ends reveals a predominance of step fractures and fibrillated ends, indicating a large number of flaws. Fatiguing hair fibers seems to accentuate fiber damage, leading to a predominance of fibrillated fracture ends. INTRODUCTION Negroid hair presents severe problems in manageability because of its kinky structure. The highly entangled hair mass is difficult to comb (1) and requires much higher forces in grooming procedures than straight hair, so that it is subjected to higher degrees of mechanical damage. The widely used practice of "hot combing" to detangle the hair mass, especially in combination with the use of relaxers, seems to introduce extensive damage to the hair fiber. This is reflected in the tensile mechanical properties, a significantly higher fraction of fibers breaking at low elongations (premature failures) than hair fibers of other ethnic origin. In some cases the extent of damage is so severe that even manual squeezing and rubbing results in the breakage of fibers in half wavelength sections. This observation suggests that combing and stretching probably damages the fibers in the regions of twist. Microscopic examination of the fiber shows frequent twists along its length with no preferred directionality (2). Subjecting such a fiber to tensile deformation involves development of torsional stresses in the regions of twist, the magnitude of which will depend on the angle of twist and the applied tensile force. In earlier work a method was developed to study torsional effects in the tensile loading of such specimens in- 39

40 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS volving a single region of twist (2). The extent of untwisting of the fiber under continuous loading was measured in terms of the angle of rotation. In many instances fiber failure seems to have been preceded by the generation of a crack in the region of twist, although this could not be established with certainty. Combing a hair mass involves the application of low levels of tensile force similar to those in a tensile fatigue experiment with a fiber. Because of entanglements, these forces are generally higher in the case of Negroid hair than in Caucasian hair. To simulate the effect of combing, an apparatus was designed to subject single fibers to tensile fatigue at various load levels (2). It should be noted that the tensile fatigue procedure gives rise to torsional fatigue in the regions of twist, which can enhance damage in these regions of the fiber. This may be responsible for the breaking of fibers into half wavelength sections on manual squeezing. The mechanical and fractographic behavior of untreated Negroid hair has been reported earlier (2). In this communication these studies have been extended to include the effects of chemical treatments and humectants. EXPERIMENTAL MATERIALS These studies were carried out on hair obtained from a 31-year-old male. The hair sample was cleaned with sodium dodecyl sulfate (12.5%), exhaustively washed with distilled water, and conditioned at appropriate relative humidity at 2 iøC. EVALUATION METHODS Tensile mechanical properties were measured with 50-mm long specimens extended at 50%/min (strain rate 0.0083 s-•). Wet measurements were carried out on a sample soaked in water for 2 hours. The apparatus and experimental procedures for the study of torsional effects in tensile loading and tensile fatigue have been described earlier (1). Fatigue experiments extended to 11 kc, with applied loads ranging from 10 to 40 g. HAIR TREATMENTS Treatment with Creme Relaxer. Single fibers were taped to a plastic board and an alkaline permanent creme relaxer (Revlon Realistic © , super strength formulation) was applied for 20 min with stroking motions of the fingers to straighten the hair. During this procedure about 8% of the fibers broke. The remaining fibers were rinsed with distilled water to remove the relaxer and were subsequently treated with a neutralizer shampoo (Revlon Realistic © Herbal Deep Clean Shampoo). The fibers were then thoroughly rinsed with distilled water, blotted dry, and conditioned at 65% RH and 21øC. This sample will be referred to as "superrelaxed hair." Treatment with Thioglycolic Acid. Hair samples in the form of small tresses were immersed in a 5.5% solution of thioglycolic acid (pH = 9.3, adjusted with ammonium hy- droxide) at 35øC for 20 min. During this treatment an attempt was made to straighten the fibers by stroking gently with gloved hands. Subsequently, the tresses were rinsed