HAIR DAMAGE 273 -+ 0.01 (waved hair) and 0.16 -+ 0.0! (untreated hair) to 0.09 -+ 0.01 and 0.06 _+ 0.01, respectively. Destabilization of the fiber structure in HC1 solution can be inter- preted as due to breaking of salt linkages. The reduction of rigidity ratios is accompa- nied by an increase in the logarithmic decrement in water from 0.40 to 0.56. Reduction with 0.1M TGA followed by alkylation with iodoacetic acid reduced the softening temperature from 242øC to 225øC and increased the expansion at 256øC from 54% to 86% in TMA experiments. However, no shift in low-temperature transition was detected. Lowering of the high-temperature transition corresponds to melting of the o•-helices and has been interpreted as being consistent with the idea that disulfide crosslinks are converted to thiol groups by reducing agents, resulting in a structure that is less stable. Samples not reacted with iodoacetic acid, to block reoxidation, did not differ from the control, untreated hair. Similar dependencies were noted in DTA exper- iments with reduced wool fibers (44). DYEING There is a genuine scarcity of published information on the effect of dyeing on the physical properties of hair. Among three types of dyeing compositions, permanent (oxi- dative), semipermanent, and surface, only the oxidative systems are able to produce irreversible, deleterious changes in the fiber structure. Such alterations are expected to be similar to those produced by bleaching, although on a smaller scale. This hypothesis seems to be substantiated by torsional measurements of hair (6). Measurement of the torsional modulus at 65% RH shows no significant difference between control (1.02 + 0.09 ß l0 n pascals) and blond (1.07 -+ 0.05)' 10 • pascals) or black-dyed (1.08 -+ 0.13)' 10 • pascals) hair. Rigidity ratios are slightly reduced to 0.23 + 0.01, as compared to those obtained for untreated hair (0.26 + 0.01), and probably reflect a decrease in crosslink density caused by the action of hydrogen peroxide. Bleaching produces more extensive degradation of the fiber integrity by reducing the rigidity ratio to 0.14 + 0.01. Although no numerical data are available, oxidative dyeing should also result in an increased swelling capacity. Probably the most important aspect of the damaging effect of dye compositions on the properties of hair is their ability to form dye deposits on the fiber surface which can considerably increase the frictional coefficient and, consequently, combing forces (20). The measurements of dry and wet combing work revealed a few hundred percent in- crease in combing forces for hair treated with oxidative dye compositions (20). This problem, however, can be circumvented by the post-dyeing application of suitable con- ditioners. ALKALINE RELAXING OR STRAIGHTENING Hair straightening formulations for African-type hair which employ strong alkali such as sodium hydroxide (or combination of carbonates and guanidine) as active ingredient are referred to as hair relaxers. The interaction of such compositions with hair involves primarily alkaline fission of disulfide bonds which can occur either by hydrolysis or nucleophilic substitution of sulphur by hydroxide ion, or-elimination consisting of the initial abstraction of the ot proton followed by heterolytic cleavage of the -S-S-

274 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS bond, or lB-elimination involving abstraction of the [3 proton followed by elimination of the unstable disulfide anion and formation of dehydroalanine (41). Other hair-structure damaging reactions might be related to peptide-bond hydrolysis. Alkaline scission of the disulfide bond brings about a reduction in half-cystine content (as high as 72% (39)), with a concurrent increase in the free thiol content to 70.3 •mol/mg. Destabilization of the relaxed hair structure is evident from torsional mechanical anal- ysis (6). The torsional modulus measured in hair at 65% RH was reduced from (1.02 + 0.09) ß 10 • pascals to (0.91 + 0.11) ß 10 • pascals by alkaline relaxing. A decrease in wet fiber strength is more pronounced and is reflected in a decrease in the rigidity ratio in water from 0.26 + 0.01 to 0.06 --- 0.02, with a concomitant increase in the logarithmic decrement from 0.40 + 0.05 to 0.65 - 0.04. SEM analysis of relaxed hair, with cuticle cells removed by formic acid treatment and sonication, revealed extensive damage to the cortex manifested as fraying of large-fiber bundles (39). Reduced crosslink density of the cuticle and cortex also leads to increased swelling, which makes the fibers more susceptible to surface damage during normal handling procedures. PHOTODEGRADATION Prolonged exposure of keratin to sunlight or UV irradiation leads to destructive changes in the keratin structure (45-47). Mechanistic studies suggest interaction of light with disulfide bonds (47-51), deamination or decarboxylation of aminoacids (52), disorien- tation of hydrogen bonds, and chemical alterations of the aromatic nuclei in tyrosine and phenylalanine (53). The recent application of ESCA to investigate the process of UV oxidation of hair revealed an extensive conversion of S(II) to S(IV), which is consis- tent with the hypothesis that UV light is primarily responsible for the oxidation of sulfur in weathering of human hair (22). Beyak et al. (47) used the stress value at 15% elongation (yield point at 15% strain), determined on the basis of stress-strain curves, to assess the extent of damage after a few months exposure to solar radiation. They demonstrated that the decrease in the yield point at 15 % strain for virgin hair may be as high as 45.1% after a total radiation dose of 3.52 ß 10 4 langleys (approximately a 2- month exposure to sunlight in summer). Bleach-modified hair underwent similar damage, with the yield point declining by 36.1% after a total radiation dose of 3.31 ß 10 4 langleys. The change in the yield point at 15% strain was found to be linear with the logarithm of the dose in the range of 1-4 ß 10 4 langleys. The deterioration in the tensile properties of hair were of the same magnitude in artificial-UV irradiation experiments. EXPOSURE TO SOLVENTS, DETERGENTS, AND OTHER CHEMICALS--SUPERCONTRACTION Solvents can alter the physical properties of hair by extracting materials which play a structural role, or by interacting with the macromolecular components to induce con- formational changes to a more stable energetic form. Although commercially available hair-care products are based on water, lower alcohols (ethanol or isopropanol), and their mixtures, interaction of hair with other solvents was also investigated. Humphries et (21) studied the effect of diethyl ether, hexane, methanol, ethanol, and chloroform on