114 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS LOAD CELL I COMB 2 i COMB INSTRON CROSSHEAD Figure 2. Comb holder for double-comb combing force measurements. Twenty measurements of combing force were obtained on each tress under each condi- tion. Averages and standard deviations were calculated using the logarithms of the combing forces because the distribution of these forces was slightly skewed towards higher values. In all cases, 95% confidence limits are given. RESULTS AND DISCUSSION COMBING FORCE CURVES A typical combing force curve obtained using method I with a single comb is shown in Figure 3. The force level remains low during the traversal of the comb through the tress (midlength force, ML) until the free end of the tress is reached, at which point the force increases sharply (end-peak force, EP) and drops precipitously as the comb clears the tress. The fundamental processes that give rise to the midlength force are: 1) comb-hair friction, 2) compression of fibers into the spaces between the teeth of the comb, and 3) separation of the tress as the teeth of the comb move through it. The latter two pro- cesses contain elements of fiber bending and interfiber friction and adhesion. In the case of wet hair tresses, even though comb-hair and hair-hair friction may be reduced by lubrication, swelling of the fibers and the forces necessary to separate fibers against the surface tension forces of the liquid holding the fibers together can predominate, leading to an overall increase in the midlength force. The end-peak force results from entanglements formed by the free fiber ends. It is not clear to what extent against-scale friction and interlocking of scale edges contribute to the end-peak force. Interfiber friction forces can be significant in this region because of high normal forces between the fibers in the entangled regions. In experiments in which the tress was combed prior to measurement, end-peak forces are significantly lower than in those cases where the tresses were not precombed. Thus precombing is obviously

COMBING FORCE MEASUREMENT ! 15 COMBING FORCE (mN) I00 I I I i 0 50 I00 150 200 DISTANCE (rnrn) Figure 3, Combing force for an untreated hair tress (20 cm) at 65% RH as a function of distance traversed by the comb (single comb). useful in enhancing the contribution of adhesive forces in combing as compared to the forces of fiber disentanglement. COMB-HAIR FRICTION An attempt was made to determine the magnitude of comb-hair friction and compres- sion using the arrangement shown in Figure 4a. Basically, this arrangement is similar to that in Figure 1, with an auxiliary comb mounted on the frame of the Instron below the measuring comb which is connected to the load cell. The hair tress is carefully put into these combs so that the same fiber bundles pass through the spaces between the aligned teeth of the two combs as shown in Figure 4b. The lower comb compresses the fibers between its teeth, and these prealigned fiber bundles then pass through the spaces between the teeth of the measuring comb. This fiber bundle arrangement eliminates the tress separation contribution to the midlength force, and the upper comb thus measures mainly the forces of comb-hair friction and compression. A typical force curve obtained from this measurement is shown in Figure 5. The average midlength force read from these charts is significantly lower than that observed in Figure 3 (the tress was the same), and the end-peak force is almost eliminated. To see if tress length had any effect on these forces, measurements were made on tresses of three different lengths. Midlength forces obtained with and without the second comb are shown in Table I. The data in Table I indicate that comb-hair friction constitutes about half of the mid- length force obtained in a single comb measurement. As would be expected, the mid- length force is independent of the tress length.