CATIONIC CONDITIONING COMPOUNDS 77 _ • •o 040 I =3o I ._o -2o I I o treated II ,. II : II treated Conditioner Treatments (Effect of conditioners on extension to failure) Figure l l. oexrension eo breal{ in hair fibers treuted with cationic polymeric und monomeric quaternctry compounds. encountered in normal grooming practices (-10% extension). As a matter of f act, such radial cracking would occur only under extreme conditions, which are irrelevant to normal grooming practices. Therefore, this effect should not be construed as a drawback of treatments with MCCs. In practical situations, the hair fiber would pull out of the hair follicle before such severe radial cracking can occur, and therefore, may not be of any practical significance. In our earlier work (1), we had discussed that during longitudinal extension of untreated/ unaltered hair at intermediate RH and extremely slow extension, which allows the dissipation of stresses, cohesive/intracuticular failure can occur within the weakly cross- linked endocuticle. It is proposed that this type of slow extension causes shear stresses between the layers of different composition and extensibility within the cuticle cell. This can lead to failure within the easily extensible (weakly cross-linked) endocuticle, its delamination, and lifting of the inextensible (highly cross-linked) upper layers (A-layer and exocuticle) of the edge of the cuticle cell. It should be noted that this cohesive failure within the cuticula is greatly dependent on the relative humidity and the extension rate, and does not occur during normal tensile loading in the Instron or at high extension rates in the Diastron, not allowing slow stress dissipation and partial recovery. Swift (5) suggests that the frequent appearance of granular material underneath, of lifted, chipped- away, or broken-off cuticles, is associated with endocuticular debris. This would indicate that the endocuticular layer is indeed a region of weakness within the cuticle cell.

78 JOURNAL OF COSMETIC SCIENCE Figure 12. (a,b) This alternate path of stress release is most often severe radial cracking of the cuticula. However, we feel that there is a strain rate and moisture dependence in order for this endocuticular failure to occur. SEM observations reveal that scale lifting of the extended, cationic-reinforced cuticula occurs with an intact scale edge. The "clean" nature of scale lifting suggests intercu- ticular, adhesive failure at the interface between cuticle cells (Figure 13a,b). In this case, the intercellular cement has become the stress release medium, but always at high levels of extension when greater stress is exerted on the cuticula. This may be a beneficial effect of the cationics. Some hair fibers display either none or only random scale lifting at high levels of extension, again suggesting diffusion of the conditioners into the non-kerati- nous components of the CMC and the endocuticle, thereby increasing overall cohesion within the cuticula and preventing cohesive failure within the endocuticle as well. EFFECTS OF CATIONIC POLYMERS ON CREEP These studies have shown that single and multiple applications of cationic polymers result in their adsorption onto the hair fiber surface, due to attraction of their positive