270 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Rubbing of dry fibers results in cuticle loss near the fiber tips, although it does not produce the same degree of scale damage. SEM examination of tangled hair has demon- strated that when hairs become twisted around each other, grooves can form in the cuticle surface, and in some cases cuticle can be removed. In dynamic combing experi- ments, abrasion and deformation of the fibers has been shown to cause stripping of cuticle and fracturing of the fibers. It appears that the types of fracture that result from this treatment yield longitudinal splitting. The average rate at which the scale margins recede down the fiber by combing is about 0.5 •m/2000 comb strokes. Structural deformation of hair fibers under various conditions of mechanical stress were studied by SEM (26). Hair samples were clamped between the crosshead and load cell of an Instron tensile tester and extended at various rates until fracture occurred. Five main types of fracture, related to the degree of internal damage as a result of weathering, were observed. These were: (1) clean transverse fracture in the case of less weathered hair samples (2) clean transverse fracture with some disturbance of culticle (3) part of the fracture is transverse but the remainder tails off with segments of cortex pulled off (also, the damage to cuticle is more severe in the form of lifting as well as longitudinal and circumferential splitting) and (4, 5) fracture ragged with cortex separating into fibrillar elements (such a fracture pattern was observed in the case of severely weathered hair (26)). Negroid hair is especially prone to damage during grooming procedures because of a high frequency of kinks along the fiber axis and its ribbonlike cross-sectional shape (38). Such a unique fiber configuration leads to extensive entanglements which increase the forces applied during combing. The fracture behavior of negroid hair was studied by tensile-fatigue measurements and SEM (38). These studies revealed that frequent twists with random reversals in direction and pronounced flattening in the region of twist lead to stress concentrations during tensile deformation. Fiber failure at low extensions is due to the initiation of cracks at numerous weak points near the twists. SEM showed the predominance of step fractures and a large proportion of fibrillated ends which reflect poor cohesion between cortical cells. In contrast, undamaged caucasian hair usually produced smooth transverse fractures. BLEACHING Although the purpose of bleaching is to eliminate or lighten the natural hair color by the reaction of an oxidizing agent with the melanin pigment, side reactions signifi- cantly affect the properties of the keratin fibers. It is well established that hydrogen peroxide in conventional bleaching and oxidative hair dye formulations (alkaline media at pH 10 and above) leads to oxidative cleavage of disulfide bonds and the formation of cysteic acid as the end product (29). According to aminoacid analysis, extensive bleaching can reduce the content of half-cystine residues from 13.9% to 5%, which corresponds to approximately a 64% reduction in the disulphide bond linkages of the protein molecules (10,39). Bleaching also reduces the concentration of free sulfhydryl groups (for example, from 2.46 •mol/mg to 0.3 •mol/mg (39)). Qualitatively similar results were reported by Robbins et M. (40) and Wolfram (41). The oxidative scission of disulfide bonds results in decreasing the crosslink density of the protein and provides additional anionic sites in the form of cysteic acid residues. Both reduction in covalent crosslink density and an increase in hydrophilicity contribute to the increase in swelling

HAIR DAMAGE 271 (liquid retention) (10), alkali solubility (9), and sorption of metal ions (9) as well as polymers (34). For bleached hair, liquid retention was found to be about 42-43% in the pH range 3-5, with increases of up to 52% in the pH region of 5-7.5.(10). For untreated hair, an increase in swelling above 31% is observed at pH values above 9. The phenomenon of increased swelling and its pH dependence for bleached hair has been explained by the charge-rearrangement mechanism postulated for oxidized wool by Thomson and O'Donnel (42). According to this mechanism, strongly acidic cysteic acid residues displace carboxylic groups from their salt links with ionized basic groups. The carboxyl groups remain essentially unionized at pH 4 and below, due to the increase in negative charge density following the formation of cysteic acid residues. Above pH 5, the displaced carboxylic groups begin to titrate, bringing about an increase in swelling. In addition, it has also been proposed that the oxidative destruction of the melanin granules might result in the formation of discrete voids within the fiber structure which contribute to the swelling characteristics (t0). Moisture absorption (regain) parallels the trends recorded in swelling measurements and increases with the extent of bleaching over a wide range of humidities, 17-94%. At 61% RH regain increases from 13.5% for intact hair to 14.8% for bleached fibers (t0). The wet strength of keratin fibers is determined to a great extent by the concentration of disulfide bonds which form crosslinks. On the other hand, the strength of dry fibers is not appreciably influenced by the density of covalent crosslinks and depends largely on Van der Waals forces and ionic interactions between the peptide chains. Conse- quently, for bleached hair, in which the disulfide bonds have undergone oxidative scis- sion by hydrogen peroxide or peracetic acid, a steady decrease in wet strength (yield stress, stress at breakpoint, and 20% index) with increased time of bleaching is ob- served (t0). For example, the yield and stress at break decrease from t. lg/den to .99 g/den and from 1.9 g/den to 1.4 g/den, respectively, for hair bleached with H202 at pH 10 for 60 min. The reduction in fiber strength is accompanied by an increase in the extension to break by a few percent, from 51.5-53.5% to 57.5-59.5%. The modulus, ultimate strength, and breaking extension of dry fibers are virtually unaffected by the same treatments. Since the hydration of bleached hair is strongly pH-dependent, it is also reflected in the mechanical performance of wet fibers. Bleached hair exhibits a region of maximum mechanical stability between pH 3 and 5, where the displaced carboxylic groups are undissociated and those ionized are bound in salt links. Above pH 5 the ionization of free carboxyl groups occurs, leading to increased fiber hydration and reduction in fiber strength (10). Another parameter deduced from stress-strain relations, the hysteresis ratio at constant temperature, was found to be independent of the number of bleaching treatments, but the absolute value of W2o decreases by as much as 35% with increasing bleaching (11). Bleaching raises the temperature of the second-order phase transition from 46øC to 56øC. An increase in the transition temperature was also reported for H202-oxidized hair in TMA measurements (21). Similar trends are evident from dynamic mechanical measurements (6). The torsional modulus of bleached hair, at 65% RH, was (1.05 --- 0.005) ß 10 TM pascals and does not differ from that determined for intact hair ((1.02 ___ 0.09) ß 10 TM pascals). For wet hair, the decrease in rigidity ratio (the ratio of wet to dry torsional modulae) from 0.26 ---