HAIR AND WOOL 778 acid composition from wool. A comparison of a human hair sample with those from several breeds of wool indeed shows the hair to be richer in cystinc and proline and poorer in alaninc, leucine, tyrosine, phenyl- alaninc, glutamic and aspartic acids, lysine and arginine. Real, but smaller, differences are seen among the various wool samples (17). 3. Acid Binding, g/Ioisture Regain and Swelling The acid-binding of hair, which is an index of the number of basic groups present, is slightly lower for hair than for wool (18) this is in accord with the lower basic amino acid content of hair, noted above. Wool and hair have practically identical moisture-adsorption iso- therms at relative humidities up to 95%. Here, the regain* curves Hookean region I I I I I i I i i I _ _11 yield region Post.yield region STRAIN Figure & A typical keratin fiber stress-strain curve diverge (Fig. 1) in saturated water vapor, wool had a regain of 33-34% (19) and hair of about 30% (20). The difference in saturation swelling is probably a function of the extent of restraint imposed by crosslinking. The effect is shown still more strikingly (Fig. 2) when the additional swelling caused by transfer from water to a strongly acidic medium is examined: Cotswold wool (21) and 64's Merino wool (22) exhibit a volume swelling of about 6% when transferred from water to HC1 at pH 1, whereas human hair swells about 3•% under the same conditions (23). 4. g/Iechanical Properties The fiber tensile properties vary within an animal species and even from one fiber to another within a lock. One may nevertheless state * "Regain" is the moisture content based on the dry weight of the material.

774 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS broadly that hair is somewhat stiffer, stronger, and more extensible than wool, both in air and wet. Typical data are given in Table I. The lower elastic modulus of wool indicated by such extensometric data is due partly to the perturbing effect of fiber crimp a greater rate of stress relaxation, made possible by the lower degree of crosslinking, may also play a part. The dynamic moduli of hair and Lincoln wool, as determined by sonic measurement, are closely similar (26). Another mechanical parameter exhibiting a difference between wool and hair is the post-yield turnover point. When wool fibers are ex- tended in water at different temperatures, the location of this point on the strain axis (Fig. 3) is constant up to a characteristic temperature and then shifts to higher strains this effect has been ascribed to a second- order transition in the keratin (27). As shown in Table I, the extension at which the post-yield slope begins is similar for wool and hair at room temperature. However, the transition temperature is 72 øC for B. A. fleece wool (1), 70øC for Merino wool, and 85øC for hair examined under the same conditions (28). It appears that a higher temperature Table III Fiber Mechanical Properties in pH 9.2 Buffer--Effect of Acid (34) Fiber Human Hair 64's Wool Acid- Acid- Property Intact Treated* Intact Treated* Stress at 20% extension, mg cm -2 0.49 0.23 0.44 Stress at break, mg cm-2 1.28 0.46 1.18 Energy to break, gem -2 X 10-4t 30 11 30 Extension to break, % 58 51 51 0.13 0.19 4 30 * 0.04 N H2SO4, 16 hours at the boil. I Per unit volume. Table IV Acid Hydrolysis: 0.5 g Fiber in 25 ml Solution, 105øC, 18 Hours Residue Weight, % Fiber 6 N HC1 6 N H2SO• Lincoln wool 0.06 0.06 Caucasian hair Brown 2.5 2.2 White 1.0 0.7 Negro hair 2.7 2.3