120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS COMBING FORCE (raN) 3000-- 2500 1500 I000 EP-i ML- t,2 ML-2 EP-2 50 I00 150 DISTANCE (mrn) Figure 7. Combing force curves for untreated hair at 30% RH. 200 ment and thus lower end-peak forces. A spacing of 100 mm was chosen for best repro- ducibility of EP-2, which is a much more reliable property than the unpredictable EP-1 and can thus be used advantageously in characterizing hair tresses and hair care formu- lations. Combing force curves for a hair tress at 65% RH and for the same tress in the wet condition are shown in Figure 8. At 65% RH, small midlength forces ML-1,2 and ML-2 are observed, with relatively high end-peak forces EP-1 and EP-2. As discussed above, wetting the tress causes a large increase in the midlength forces and essentially complete disappearance of the end-peak forces. Wet combing of Negroid hair shows the opposite behavior: wet combing forces are lower than the forces for dry combing (6). Extreme curliness of the fibers prevents them from being held together by surface tension forces of the liquid which therefore do not make any contribution to the combing forces (midlength). Furthermore, reductions in torsional and bending moduli lead to a lowering of the wet combing force. It can be assumed that torsional and bending contributions to the midlength combing forces of Caucasian hair are minimal and that surface tension forces therefore make the major contribution to the wet combing force (midlength) of Caucasian hair. In an effort to establish the contribution of surface tension forces to midlength and end-peak forces, combing measurements were made on a tress at 65% RH, after wet-

COMBING FORCE MEASUREMENT 121 COMBING FORCE (rnN) .. ?'"' ......... .^•%., •.- H20 GAF-755 i• 0 50 I00 150 200 DISTANCE (ram) COMBING FORCE (mN) - 2000 - 1500 !000 5OO Figure 8. Combing force curves for a hair tress wet with water and a 1% solution of GAF-755 (left ordinate). Curve for GAF-755 is arbitrarily displaced along the abscissa. Curve for the same tress at 65% RH is also shown (right ordinate). ting with water, and after wetting with a 1% solution of Triton X-400 (TR-X-400), a quaternary ammonium surfactant used extensively in hair conditioners (Figure 9). A considerable decrease in the combing forces is observed on lowering the surface tension of the liquid (water: 72 raN/m 1% TR-X-400:27 mN/m). The combing force curve for the tress wetted with a 1% Gafquat-755 (cationic polymer) solution shown in Figure 8, on the other hand, does not show a decrease. The surface tension of the Gafquat-755 solution (67 mN/m) is similar to that of water. These experiments appear to reflect the significant role that surface tension forces play in the wet combing of hair tresses. However, it has been pointed out that the substantivity of Triton-X-400 to hair reduces the coefficient of friction of wet hair (7), so that the decrease in combing forces after wetting with Triton X-400 may not be due entirely to the decreased surface tension. As a matter of fact, preliminary experiments with nonsubstantive surfactant solutions (1% sodium dodecyl sulfate) did not produce similar reductions in combing forces. This aspect will be explored in more detail in a future investigation. COMBING FORCE MEASUREMENTS DURING HAIR DRYING Blow drying wet hair with hot combs is becoming a widely used grooming technique. The process involves combing the hair while continuously removing moisture. These conditions were simulated by combing a wet tress at various intervals of time in the environmental chamber at 60øC. Combing force curves for time periods up to 0.3 ks are shown in Figure 10. End-peak forces begin to appear at about 0.3 ks, reaching a peak at 0.4 ks, suggesting the removal of most of the water held by surface tension forces and the release of free ends from adhesive interactions to form entanglements. Note that the combing force curve at 0 ks shows a small end-peak force which is not observed in