CATIONIC CONDITIONING COMPOUNDS 75 i -- fi2 Z • 1 where•=X•,•2 X2 and•=X• +X• The number of fibers showing scale lifting is denoted by X• and X2, and n• and n 2 are the number of fibers in each group (in this case 10). From the normal probability distribution, the critical value ofz for the 95% confidence level was found to be -1.645. Values of z lower than - 1.645 indicate significant treatment effects. The z values for the three MCCs investigated in this work are shown in Table I. The numbers indicate that treatment effects are significant for common and extreme scale lifting in all cases except PQ-10 (1 x treated). For random scale lifting, treatment effects were significant for HPG (10x treated) and CETAB (10x treated) only. POLYMERIC VERSUS MONOMERIC MCCs The cuticular reinforcement effect of polymeric (PQ-10) and monomeric (CETAB) cationics is shown in Figure 10, where percent decrease in the number of fibers showing different types of scale lifting is plotted. Because polymeric cationics adsorb mostly onto the surface, the number of applications makes a difference in the amount of cationic residue left on the fiber and in the consequent reinforcement effect. Since monomeric cationics diffuse rather quickly and in greater amounts, single and multiple applications do not show a significant difference in the scale-lifting behavior. The low-molecular- weight CETAB appears to resist scale lifting more effectively than PQ-10 after a single application. However, the polymeric PQ-10 seems to reinforce the surface cuticle more effectively after multiple applications than the low-molecular-weight CETAB. The cu- mulative effect becomes important. The behavior of HPG derivative is similar to that of CETAB. This suggests that it is a fast-absorbing polymer and that the fiber surface is close to saturation in a single treatment. Table I z Statistics Types of scale lifting Hair fibers treatment Random Common Extreme PQ-10 (1 x treated) 0.0000 0.0000 - 1.4907 PQ-10 (10x treated) -1.4907 -2.9277* -4.4721' HPG (1 x treated) - 1.4907 -2.2361' -2.9277* HPG (10x treated) - 1.8787' -2.5820* -2.9277* CETAB (1 x treated) - 1.4907 -2.5820* -3.2817' CETAB (10 x treated) - 1.8787* -2.5820* -3.6515* Zo.o5 (95% confidence level) = -1.645. * Statistically significant.

76 JOURNAL OF COSMETIC SCIENCE 100 8O 60 40 2O o + 10X Polyquaternium-10 treated • Untreated + lx Polyquaternium-10 treated • lx CETAB treated • 10x CETAB treated Random Common Extreme Type of Scale Lifting Figure 10. Cuticular reinforcement effect of cationic polymeric (PQ-10) and monomeric (CETAB) qua- ternary compounds. EFFECT OF MCCs ON BREAK EXTENSION The increased extension-to-break observed for fibers treated once and ten times with conditioners is shown in Figure 11. All cationic treatments lead to an increase in extension-to-break compared to the untreated fiber. In the case of CETAB, one and ten treatments do not seem to make much difference. With polymeric cationics, a single treatment leads to a significant increase in extension-to-break compared to that of the untreated fiber. Fibers treated ten times with polymeric cationics show a lower exten- sion-to-break compared to those treated only once. This suggests that the single treat- ment with polymeric cationics softens the fiber better than multiple treatments. In the latter case, accumulation of the cationic polymer seems to erabrittle the cuticular sheath, possibly by crosslinking through hydrogen and hydrophobic bonds. Alternate mechanism of stress release for the MCC-reinforced cuticula during fiber extension. As demonstrated in this study, improved cuticular cohesion induced by MCCs appears to inhibit scale lifting as a mechanism of stress release of the cuticula during extension. Scale lifting is either completely eliminated or delayed until greater stresses are exerted at much higher extension levels. However, whenever scale lifting cannot occur as a mechanism of stress release of the cuticula, alternate paths are found to dissipate the energy. SEM observations have shown that an alternate path is often the severe radial cracking of the cuticula (Figure 12a,b). However, it has to be pointed out that this cuticular cracking occurs only at the highest levels of extension and does not occur at extension levels