REVIEW OF CHLORINE-HAIR INTERACTION 365 tion (9, 13,18,22). The increased capacity is most likely due to the formation of sulfonic acid groups which result from disulfide bond scission and carboxyl groups which result from the cleavage of peptide bonds. The acid binding capacity of chlorinated wool has been found to be lower than that of unreacted wool. This reduction has been attributed to the loss of amino groups in the formation of chloramines and to the possibility of bonds forming between existing amino groups and sulfonic acid groups (13). Chlorination has been shown to enhance the affinity of wool for all types of commercial dyes, although the dye retention generally has not been improved (16,47-57). The increased affinity has been attributed to the damage imparted by chlorine to the fiber surface layers, enabling greater dye penetration into the interior of the fibers (16,55). SURFACE PROPERTIES Shrinkage of woolen goods when wetted (and agitated) has been shown to arise from the directional character of the fiber's cuticular structure. In particular, shrinkage has been linked to the difference in the values of the coefficients of friction measured with the fibers sliding "against" the scales and the fibers sliding "with" the scales (pba-pbw). This difference is known as the differential friction effect, or DFE. Frictional studies on keratin fibers subjected to chlorine treatment show that this treatment leads to a reduc- tion in the value of DFE and, consequently, to a decrease in the tendency of the fabric to shrink upon wetting. This reduction in DFE results from the "with" scale coefficient of friction increasing more than the "against" scale coefficient of friction with treatment (58-63). Makinson (64) proposed that chlorination degrades the keratin material within the cuticle, which softens the scales and gives rise to an increase in the values of the coefficient of friction and a decrease in the value of DFE. Changes in friction have been found to occur more rapidly with chlorination in the acid region than in the neutral or alkaline regions (63,65,66). Postchlorination microscopical studies of keratin fibers show a change in the surface structure (2,9,49,59,62,63,67-72). Acid chlorination appears to react in a more sur- face-specific manner than the neutral or the alkaline chlorination surface detail after acid chlorination often is barely distinguishable (2,9, 55,63). Fiber surfaces appear to be less affected as the pH of the chlorination treatment is increased, and the scale structure appears to remain intact longer (2,9,63). Degradation of the fiber surface probably proceeds by a reaction first noted by All- w/Srden (72). The reaction can be observed through microscopy during treatment of the fibers with chlorine water and involves the formation of bubbles or sacs on the surface of the scales. The Allw/Srden reaction has been shown to occur under a variety of condi- tions and over a wide range of pH (16,55,56,73-75). The formation of a jelly layer between the unattacked cortex and the attacked outer surface of wool fiber has been observed and with extended chlorination, a collapsed scale structure has been found (58,64,76). This reaction requires that the epicuticle be intact and sufficient area of each cuticle cell be exposed on the surface of the fiber to allow the sacs to form. In human hair, where only one-sixth of each cuticle cell is exposed on the surface, as compared to about three-fourths exposed in the case of wool, such sacs are less likely to form. In- stead, the whole surface of the hair fiber appears to be raised. (73,77). A mechanism for the Allw6rden reaction has been proposed (73,78) in which as the

366 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS reagent diffuses into the cuticle, the protein lying under the epicuticle dissolves, even- tually causing the epicuticle to lose its adherence to the substructure. As the solubility of the protein increases with treatment, an osmotic pressure is generated which results in the formation of sacs. If the degraded protein is of high enough molecular weight, it will not diffuse through the epicuticle. As the reaction proceeds further, however, the molecular weight of the protein decreases so that protein segments are able to diffuse through the epicuticle, causing the AllwiSrden sacs to fiatten. The osmotic active sub- stances of the cuticle have been found to contain more than half of their sulfur in the form of cystine, whereas only 10 percent of the sulfur is in this form in the degradation products which have passed into the solution. This suggests that the presence of cystine is important in keeping protein chains large enough to inhibit their passage through the epicuticle. WEIGHT LOSS The reaction of halogens with keratin fibers has been found to cause considerable weight loss (2,6,20,22,48-50). As a function ofpH, the weight loss has been found to reach a minimum between pH 4 and 7, where sometimes even a slight increase in weight has been seen (2,22). Maximum weight loss has been observed in the acid region, below pH 4 (2,6,22,50). These differences in weight loss have been attributed to differences in the extent of disulfide bond oxidation and peptide bond cleavage. In the acid region, extensive peptide bond cleavage and disulfide bond oxidation have been noted, which could be expected to lead to considerable weight loss. Between pH 4 and 7, where peptide bond cleavage is low but oxidation of cystine is high, a small loss of material due to the former could be expected to be compensated for by the latter, i.e., by an uptake of oxygen by the disulfide group. In the alkaline region, peptide bond cleavage is extensive, but there is very little cystine oxidation. This combination can be expected to result in a net weight loss which, however, will be less than expected in the acid region. MECHANICAL PROPERTIES Changes in tensile properties have been used to determine the extent of structural damage produced by halogenation in keratin fibers (45,49,50,55,58,59,79- 84). Gen- erally, strength and elongation decrease with increase in halogen treatment (30,49,50,59,84). Nondestructive methods for studying the effect of halogen treat- ments on tensile properties also have been used which examine the change in the amount of work (2,12, 18,55,80,82) or force (58,81) required to extend a wet fiber by a given length. These measurements have been carried out over a range of extensions (15-30 percent). The work or force required to stretch the fibers is greatly reduced after acid and neutral chlorination treatments. There is less change in these values with chlorination in the alkaline region. The changes in work seen by Houff et al. (82) are related to the amount of cystine converted to cysteic acid. CONCLUSIONS The distinguishing characteristic of the reaction of halogens with keratin fibers is that the reaction depends strongly on pH. The rate of diffusion of halogens in keratin fibers