CATIONIC CONDITIONING COMPOUNDS 81 88- 66- 88 44 22 0 T,rne Under Constant L•ad (h i i i ! ie tl ii • I I plC ø Figure 14. Continued. as "in-depth" studies were carried out, yielding similar and reproducible results. A typical example of the effects of cationic treatments on improving creep resistance is givrn hrrr Creep studies were carried out under the same environmental conditions used for the scale-lifting studies. Untreated hair fibers and hair fibers exposed to one and ten appli- cations of cationics were individually mounted and freely suspended under a constant tension of 40 g. Their creep (extension) was measured as a function of time up to 96 hours. Figure 14a shows creep measured in fine, medium, and coarse untreated/unaltered hair fibers (controls). The coarse hair fibers were not affected by the constant tension of 40 g within the time span of 96 hours, and measurable creep was not observed. However, hair fibers of medium and fine diameters did show increasing creep with time. Some of the fine hair fibers failed shortly after application of tension. Figure 14b shows creep measured in hair fibers exposed to one application of the polymeric PQ-10. As observed for the control, the coarse hair fibers showed no creep with a constant tension of 40 g within 96 hours, while hair fibers of medium and fine diameters showed increasing creep under tension as a function of exposure time. The levels of creep were generally lower than those observed for the controls. One of the fine hair fibers failed shortly after application of tension. Creep measured in unaltered hair fibers, which had been exposed to ten applications of PQ-10, is shown in Figure 14c. Surprisingly, the medium-diameter hair fibers displayed

82 JOURNAL OF COSMETIC SCIENCE Aco.½x=3632 pm 2 Aco.½x =1256 pm 2 Acuticl½=1395 pm 2 Acuticl½=707 2 Acuticl e/Acortex = .38 Acuticl e/Acortex = .56 Figure 15. Relative areas of cortex and cuticula for coarse and fine hair fibers. either no creep or only negligible creep under a constant weight of 40 g in 96 hours. The creep observed in hair fibers of fine diameter levels off and stops completely after 24-48 hours. This is in sharp contrast to the creep behavior of hair fibers without the cationic treatment. The low levels of creep in fine fibers, especially in the early stages of the experiment, strongly suggest that multiple cationic applications improve creep resistance The cationic-induced, improved cuticular cohesion apparently stops movement of the cuticle cells relative to each other when the applied load generates low levels of shear deformation in the cuticular layers. The weight of 40 g apparently is adequate to generate the creep phenomenon in fine fibers. Obviously, increased weight has to be used for fibers of coarse diameter to obtain comparable results. Stress concentrations seem to be important. Sample calculations shown in Figure 15 indicate that the stress concentrations in the cortex and cuticle of fine fibers can be much higher than in coarse fibers. The difference in the cuticle/cortex ratio of coarse and fine hair fibers is reflected in their creep behavior. These studies indicate that in fine hair, at very low strain rates, a reinforced cuticular sheath exerts a significant restraining effect on the extension of the cortex, contrary to generally accepted concepts that the cuticle does not participate in the tensile deformation of the fiber. In a typical tensile test, at higher rates of extension (-50%/min), the effect of the cuticle is not seen. However, at the low extension rates used in creep experiments, the effect of the cuticle manifests itself in the tensile deformation of the fiber. CONCLUSIONS SCALE LIFTING Our studies suggest that besides surface binding, cationic surfactants and polymers may also diffuse into the hair fibers. While the high-molecular-weight cationics may be restricted to penetration for limited distances into the cuticula by way of the nonkera- tinous regions •such as•the CMC"and endocudcie), longer a-nct muit•ie treat•n•ntS