THERMAL TREATMENTS WITH A CURLING IRON 25 crosslinks in the form of salt linkages is quite well known (2). Even though we have not examined the heat-curled/conditioner-treated hair in the SEM, relevant work of others (3) suggests that the cuticle-cracking tendency is significantly reduced by conditioning agents. Recently published work (4) on the effect of conditioning compounds on the scale-lifting behavior of the cuticle supports this concept further. MECHANISMS OF HYGROTHERMAL DAMAGE The stresses acting on a hair fiber draped on a heated mandrel like the curling iron are shown in Figure 8, where the hair fiber is shown schematically at high magnification. Under dry conditions, the outside of the fiber is under tension and the inside is under compression. Dehydration due to heat makes the f/m contact area rigid and brittle so that an attempt at straightening it can generate radial cracks in the fiber. The cuticle cells are curved. The force of compression tries to flatten them, and brittleness leads to the formation of axial cracks. This is shown in Figure 8, displaying a cross section through the center line (black dashed line). Wetting of the fiber plasticizes the keratin, and the fiber deforms much more easily and to a greater extent than in the dry condition. Endocuticular material is softened to an Tension Compression Neutral Plane Fi gur e 8. Stresses acting on a hair fiber draped on a heated mandrel like the curling iron.

26 JOURNAL OF COSMETIC SCIENCE extent where it can be squeezed out at the cuticle edge, masking the scale structure along the squeeze line. Entire cuticle cells can be displaced and cracked into pieces (Figure 6). It is very likely that this process damages the cuticle sheath parallel to the major axis of the fiber because of the preferred orientation of the fiber with the major axis parallel to the curling iron. CONCLUSIONS DRY HAIR Thermal treatments by use of curling irons according to the manufacturer's specifica­ tions (dry hair, short times, and normal tension) result in minimal damage to the "dry" hair fiber. Prolonged (ten-minute) contact times combined with increased tension (10- 30 g) lead to compression, disintegration, radial cracking, and scale edge fusion of the surface cuticle cell at the f/m interface. WET HAIR Repeated short-term curling and wetting results in less damage to the cuticula, com­ parable to that observed in the "dry" fiber. Long-term effects lead to distortion of the cuticle cell due to trapped moisture expanding in the form of steam in the fiber, forming bulges or bumps in the scale faces and ripples at the scale edges. This type of damage occurs all around the fiber and is not restricted to the f/m contact zone. With wet hair, prolonged (ten-minute) contact times under increased tension lead to damage similar to that of the cuticula of dry hair, although under these wet conditions the damage is considerably more severe than in the case of the "dry" hair fiber. The high temperature flow of water-plasticized cell proteins creates mutilated and distorted cuticle cells when recommendations by the manufacturer are ignored and abusive practices are carried out. MECHANICAL PROPER TIES Thermal treatments by use of a curling iron according to the manufacturer's specifica­ tions (dry hair, small skein, short-term curling, and normal tension) showed, quite surprisingly, that repeated, cyclical short-term curling/cooling leads to changes in spe­ cific mechanical properties of the hair. The data showed that after repeated short-term curling/cooling, the post-yield modulus of the hair fibers had increased, possibly due to thermally induced crosslinking of components of the cortical domains. FATIGUE RESISTANCE Exposure to heat increased characteristic life (0), indicated by improved fatigue resis­ tance. This behavior is likely to be the result of crosslinking in the interior of the hair fiber as well as in the cuticula due to thermally induced dehydration. The presence of the conditioning compounds enhances this heat-induced crosslinking in the form of salt