PERMANENT WAVING: POST-YIELD SLOPE 689 POST YIELD SLOPE VARIATION WITH TIME TYPICAL ALKALINE PERMANENT WAVE. ROOM TEMPERATURE . }• •/• ,o o i i i i i •, BROMATE pH 6.5 O ch THIOGLYCOLATE pH 9.2 uJ 0.2 o 10 2o 3o 4o • , • , I , I , I 0 10 20 30 40 Time (Minutes) Figure 3. Variation of post-yield slope with time during typical alkaline wave. Humidity variations have little effect on the post-yield slope. Hamburger and Morgan (7) have shown that there was little difference between the post-yield slope of a hair immersed in water and one conditioned at 65% R.H. This had also been shown for wool at 22øC over the relative humidity range of 0-100% (13). The post-yield slopes of all stress-strain curves for these conditions are roughly parallel. In actual experi- mental practice, no variations were observed for the relative humidity range of 40- 60% commonly encountered in the salon. While the Hookean and yield regions in- volve extensive contributions from hydrogen bonds, the stiffness of the post-yield region depends upon disulfide crosslinks which are unaffected by water at room temperature (7). The post-yield slope is strongly dependent upon temperature, and this has been shown for wool fibers in water over the range of 0-100øC (13). With increasing temperature, the post-yield slope decreases. It has been shown by Rebenfeld et al. (2) that untreated hairs undergo a transition at about 85øC where the disulfide bonds reach such a degree of instability that the disulfide-rich matrix shows a rapidly increasing flow component with increasing temperature. Partial reduction lowers this value to 66øC. While the slight variation in room temperature (20-25øC) does not affect the post-yield slope analysis, the contribution of temperature to results obtained with heat activated acid waves cannot be ignored. Air oxidation of the reduced hair sample might conceivably be a third factor affecting the post-yield slope analysis. Since oxidation restores the disulfide bond, the post-yield slope would increase with increasing oxidation. In practice air oxidation is not an efficient process for the restoration of disulfide (7, 11, 12), and indeed samples were exposed to air overnight without an appreciable increase in post-yield slope. Thus in practice these external variables do not make a significant contribution to the post-yield slope under waving conditions. The possible exception is the temperature contribution to matrix flow in heat activated acid waving.

690 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS THE EFFECT OF HAIR FIBER DIAMETER It is well known to those who practice permanent waving that fiber diameter will affect the time of processing. Coarse hair is often described as "resistant" and may require higher strength waving solutions, or pre-softening with waving solution to decrease processing time. The post-yield slope itself represents the stiffness modules of the post-yield region. While we have obtained the slope values by simple graphical interpolation and expressed the values in arbitrary units, we recognize that the slope is commonly expressed in dynes/cm 2--the stress divided by the cross-sectional area of the fiber. The variation of slope values with diameter can be minimized by selecting hair from a single individual, though of course there is a Gaussian distribution of fiber diameters even on an individual head. In general we averaged the slopes from 5-10 single hair samples for each set of experimental conditions, though in certain instances corrections were made for diameter to compare results between different hair sources. For a single head, the standard deviation for slope values was approximately - 10%. The change in arbitrary post-yield slope with fiber diameter and its effect upon the time of processing can be seen in Figure 4. A single mandrel was wrapped with l0 hairs, saturated with waving solution and placed in a plastic bag at 46øC for each time point. The waving solution was a commercial acid wave of pH 6.5. The initial arbitrary slope increases with increasing fiber diameter where the values of diameter represent the wide axis of the hair's elliptical cross-section. The time variation shown in this graph would suggest that the hairs of 64 and 95/am would process in 10-15 min, while the 112/am hair would require over 20 min if we use the same reasoning that we applied to the alkaline wave analysis in Figure 3. We tested this result by evaluating the processing time for an acid wave on 49 heads in the salon and asking the operators to classify the hair according to the designations of VARIATION OF POST YIELD SLOPE WITH TIME COMMERCIAL ACID WAVE pH 6.5 10 HairslMa • 64• diame Irel • 0.8 • 0.6 UJ •.4 o m diameter 0.2 ! 95,• diameter 10 20 30 40 TIME (MINUTES) Figure 4. Variation of post-yield slope with time for hairs of varying diameter.