INTERFIBER ADHESION 3 5 3 ELECTROBALANCE GLASS ROD Figure 2. Apparatus to measure interfiber adhesion. In a typical measurement the two fibers are brought into contact with each other at an angle of 90 ø, adjusted with a template. The stage is raised further to press the fibers together with the required force, which appears on the chart recorder as a negative value. After a controlled period of contact, the stage is lowered at the same rate of travel until the two fibers are separated. At this point the force of separation is recorded. Both human hair and nylon-6 fibers have been used in these studies. The latter were used principally in the early stages of the development of the method because of the better uniformity of their surface in comparison to hair fibers. Treatment solutions were applied by touching the fibers with a microsyringe, any excess being removed with a sorptive tissue paper. Treatments with Polymer JR-400 were carried out with a 1% aqueous solution of the polymer at 20øC or at 60øC for 0.9 ks. In some cases the spreading of the polymer solution was improved by a pretreatment with Tr-X-100 solution (1 drop in 200 ml water) followed by air drying prior to treatment with JR-400 solution. Synthetic sebum treatments were carried out by dip-coating the fiber at 20øC for 1.5 ks with solutions of appropriate concentrations (0.5%, 1.0%, 1.5%) in carbon tetrachlo- ride, followed by air drying and conditioning overnight at 65% RH and 21øC prior to the adhesion measurement. This technique measures the interfiber adhesion at a single contact point with a well- controlled contact force, which is applicable to fiber contacts at defined angles. RESULTS AND DISCUSSION Adhesion between nylon-6fibers. The nylon-6 fibers used in the following experiments have a smooth surface structure and were specially prepared without a spin finish. Typical recorder traces are shown in Figure 3. For each pair of fibers, 20 measurements were made in four different positions of the lower fiber between the glass rods. In a typical cycle the two fibers are brought into contact with each other (A) by moving the stage

354 JOURNAL OF COSMETIC SCIENCE FORCE {p.N) O - -IO -20 UNTR. •' FIBER POSITION Figure 3. Adhesion force curves for untreated nylon-6 fibers at 65% RH (normal force -2 mg). up. Further movement of the stage produces a contact pressure of-2 mg (B), which is seen as a negative force. At that point, the stage is reversed, the force begins to return to zero, and a positive force is developed until the fibers separate at (C) and the force drops back to zero. As mentioned earlier, the results of these measurements are to be used in interpreting radial compressibility of fiber bundles under the influence of relatively low contact forces, and it was therefore decided to measure interfiber adhesion forces at low normal forces. In the preliminary stages, adhesion forces at normal forces of 2, 4, and 8 mg were measured, but no significant dependence of adhesion forces on contact force was ob- served. Hence all subsequent measurements were made at a normal force of-2 mg. Fibers treated with JR-400 at 20øC show a large increase in adhesion force over that between untreated fibers (Table I). Large differences in adhesion force are observed at different positions along the lower fiber, as shown on the recorder trace in Figure 4. These differences suggest considerable nonuniformity in the distribution of the polymer on the fiber surface. This may be due to poor wetting by the JR-400 solution of the fiber surface, whose surface tension is considerably higher than the critical surface tension of nylon-6. Adhesion between human hair j•bers. Untreated human hair fibers show no adhesion at contact forces of-2 mg and tensions of 4 and 10 g on the lower fiber (Table II). This may be associated with the relatively much higher roughness of the cuticular surface. However, when the fibers were treated with JR-400 at 20øC, high interfiber adhesion was observed with no significant difference at the two tension levels. Recorder traces of these measurements, shown in Figure 5, indicate the lack of adhesion in untreated fibers and the high degree of nonuniformity in different positions for the treated fibers, where some regions show no adhesion forces at all. Again, this variation Table I Adhesion Forces Between Pairs of Untreated and JR-400-Treated Nylon-6 Fibers (contact force -2 mg) Treatment Force (•N) Untreated 4.3 + 0.9 JR-400 9.8 + 3.2 Each entry is an average of 20 measurements at the 95% confidence level.