CRACKING OF HUMAN HAIR CUTICLES 147 formation is mainly limited to the outer part of the cuticles. This observation suggests that the stresses involved in crack formation are more intense at those cuticle portions near the outer hair surface. Since blow-drying and wetting involves swelling and deswelling of the hair fiber, the following experiment was carried out in order to test whether the phenomenon of swelling per se plays a role in crack formation. Several solvents with limited swelling capacity were used in the thermal cycling experiments instead of water. The solvents were ethanol, iso-propanol, and methanol these solvents have already been reported in the literature as poor keratin swelling solvents (14-16). The results showed that thermal cycling experiments with these solvents do not lead to crack production at all, indicating that cuticle swelling is a necessary phenomenon for cracks to occur. Increasing both the water-swelling and blow-drying times to periods longer than ten seconds did not have any effect on the number of produced cracks. Also, it was observed that non-swollen hair fibers, which were thermally cycled with water immersion time periods as short as five seconds, underwent cuticle cracking. In such short time periods of water immersion, only the cuticular system and a small portion of the cortex can be expected to swell. These observations indicate that cuticle cracking is not due to a thermal shock arising from rapid changes in cuticle temperature. It seems rather that cuticle cracking occurs because those outer cuticles sections lack elasticity to comply with the dimensional changes either of the swelling cuticle layers underneath or of the swelling cortex. In the case of blow-drying, the lack of elasticity in the outer cuticle sections will originate from rapid cuticle dehydration at high temperatures. Thus, it would appear that when hair is wet or dried at room temperature, all cuticle portions and the cortex contract in a synchronous manner. However, if during a water evaporation process only the outer cuticle sections contract more rapidly than those cuticle layers underneath or than the cortex itself, cracking will occur. Cracking during blow-drying takes place, thus, as a consequence of circumferential extension stresses set up on dry portions of cuticle by the swollen pressure of both the cuticle layers underneath and the cortex itself. It should be mentioned here that circumferential or "hoop" stresses are known to occur in cyclindrical pipes subjected to internal positive high pressures (17). Cracks at lower hair-swelling pressures may also occur if the cuticles lose their natural elasticity due to weathering. This might explain why cuticle vertical cracks are found at lower hair- surface concentrations in people who do not blow-dry their hair. The repetitive action of cuticle "rigidization" and water swelling set up on the outer cuticle sections by the absorption and desorption of water during thermal cycling did not lead immediately to crack formation. For instance, it was found that before the cracks became fully developed, they appeared first as sharp white lines on the cuticle surfaces (see Figure 5a) then, upon further thermal cycling, the white lines turned into full cracks. The appearance of these white lines indicates that before the cuticles crack, the mechanical energy accumulated by the circumferential tension stresses or "hoop stresses," is first dissipated by the formation of localized shear yield regions. This form of mechanical energy dissipation is a very common phenomenon that takes place in polymeric materials before they fracture (18). Increasing the water temperature during thermal cycling to about 50øC resulted in more diffuse and wider shear yield regions that did not turn into cracks even after the

148 JOURNAL OF COSMETIC SCIENCE .... x2.4k x2.4k 5a 5b Figure 5. Cuticles with shear yield regions formed before cracking takes place (5a) and with shear yield regions produced during thermal cycling with water at 50øC (5b). application of a high number of thermal cycles (see Figure 5b). Below this temperature, shear yield regions and cracks were always produced. Thus, increasing the water-swelling temperature during thermal cycling softens the cuticle proteins, preventing the shear yield regions from becoming full cracks. It should be mentioned here that those cuticles cracked thermally were seen to be easily broken during hair combing. For instance, Figure 6 shows a hair fiber from a tress that has been subjected to thermal cycles followed by combing. This micrograph shows that the removal of cuticles by abrasion occurs mainly at the cracked sites. EFFECTS OF BLOW-DRYING AND WATER TEMPERATURE The temperature at which air from the blow-dryer reaches the hair surface seemed to be crucial in the incubation and propagation of thermal cracks. In the trial experiments it was observed that the average number of cracks produced for a particular number of cycles was maximum when the hair surface temperature was maintained for about ten seconds between 75 ø and 95øC. In Figure 7 is shown the average number of cracks as a function of air temperature at the wet hair surface. In this figure it can be seen that temperatures lower than 50øC do not increase the average number of cracks already present in unexposed hair, while temperatures higher than 95øC lead rather to hair surface and bulk distortion. It is quite plausible, thus, that temperatures lower than 65øC do not produce the critical rate of water evaporation needed for the top part of the cuticles to contract and become rigid, while temperatures higher than 85øC might soften the cuticle proteins, releasing, thereby, the mechanical stresses by viscous flow. The temperature rate used during thermal cycling was found also to be an important parameter. For instance, if the hair surface temperature was increased at a very slow rate,