CUTICLE DECEMENTATION AND BUCKLING 219 4kx Sky 277 Figure 6. Surface of a hair fiber half of which was treated by capillarity with a 3% aqueous solution of hydrolyzed wheat protein polysiloxane copolymer. strengthen and give more elasticity to the endocuticular regions, while the second one will substantially reduce the circumferential shear and compression stresses. The occur- rence of these two phenomena will, certainly, account for the observed lack of individual cuticle damage when the fibers are subjected to low and high extensions (7-40%) under swollen conditions (see Figures 5a and 5b). The results also suggest that after the endocuticular regions have been strengthened and expanded by swelling, the cementing junction between the cortex and cuticular envelope becomes the weakest link, which breaks by the shear stresses set up at high extensions. Once the cuticular envelope and cortex are separated along the fiber, extension stresses act alone on the swollen cuticular envelope, producing the long, deep transversal cracks observed in Figures 5a and 5b. EFFECTS OF FIBER PRETREATMENT Hair fibers that were pretreated with 3% aqueous solutions of glycerin or propylene glycol without rinsing, followed by a drying period of six hours, did not present severe patterns of cuticle lifting and buckling when strain-cycled. However, if the hair fibers were rinsed before cycling, cuticle decementation took place. These experiments indicate that the presence of small amounts of low-vapor-pressure swelling solvents in the hair fibers helps to swell and plasticize the cuticle, preventing lifting and buckling. Treat- ment of hair fibers with a 3% w/w aqueous solution of a cationically modified glucon- amide compound (11) was also effective in preventing cuticle lifting and buckling, even

220 JOURNAL OF COSMETIC SCIENCE when the fiber was rinsed after treatment. The preventive effect of some cosmetic actives like these probably arises as a consequence of the presence of moisture or swelling solvents in the hair, which were already seen to inhibit cuticle lifting and buckling. It was also observed that lifted and buckled cuticles returned to their normal appearance after immersion in water a similar observation was also made by Reutsch et aL (10). Decementation of the cuticles could, however, be easily brought back with a lower number of strain cycles at very low strain levels. This was not the case when the lifted cuticle cells were treated with a hydrolyzed wheat protein polysiloxane copolymer (12). This cosmetic active, which crosslinks upon drying, was seen to form a flexible thin film capable of recementing the lifted cuticles, increasing the decementing strain threshold. The thin film was also seen to be very smooth, i.e., in Figure 6 it is shown that the half portion of a hair fiber treated with this protein/copolymer is smoother than the other half, which was untreated. It is interesting to note that polymeric materials have been used in the past to stabilize the intercellular cement of wool (17). An even stronger cuticle-cementing polymer was cystine polysiloxane (13) this protein- silicone copolymer when used at 2% or 3% w/w levels was capable of rendering lifted cuticles very resistant to redecementation. For instance, it was observed that lifted and buckled cuticles returned to their natural position upon water immersion however, when the hair fibers were tightly knotted, the cuticles lifted again. This was not the case when the cuticles were treated with a 3% aqueous solution of cystine polysiloxane prior to knotting (see Figures 7a and 7b). Other polymers tested at 3 % w/w in an aqueous solution were polyacrylate polymer and polyethylenimine (PEI). These polymers were seen to penetrate between the spaces left by the decemented cuticles and to cause swelling of the cuticle even when the hair was dry. Both polymers seemed to reduce cuticle lifting in virgin fibers but failed to reglue the already decemented ones. Also, after blow-drying of the hair, both polymers seemed 7b Figure 7. Surfaces of two hair fibers treated as follows: 7a treated with 200 extension cycles, 20% extension, at 10% RH, then water immersed and knotted 7b treated with the same process as the fiber in 7a but with a 3% aqueous solution of cystine polysiloxane before knotting.