INTERFIBER ADHESION 3 61 RECORDING ELECTROBALANCE ,---SINGLE HAIR O mmx 15 mm I Tnchworm STAGE Hook -----m.. Fine Kevlor © •,• Filoment Figure 11. Apparatus for interfiber adhesion measurement by the pull-out method. nated this problem. The force of withdrawal was recorded while the "Inchworm" stage (Burleigh Instruments, Inc.) was moving downwards. In each measurement, about 5 mm of the fiber were pulled out of the bundle. The nature of the curve depends on the treatment of the assembly and will be discussed in detail below. The recorder trace was integrated electronically to obtain an average force acting on the fiber. RESULTS AND DISCUSSION Adhesio, force carves. The force of withdrawal of a fiber from a fiber bundle is a function of the number of points of contact and the normal force acting at these points. Since the number of points of contact is dependent on the length of the fiber in contact with the assembly, the force would be expected to decrease as the length of the fiber withdrawn increases. However, in these studies the length of fiber withdrawn (5 mm) is too short to have a perceptible effect on the withdrawal force. Occasionally such a decrease is indeed observed, but not often enough to take it into consideration. The dependence of the withdrawal force on the normal force acting at the points of contact should result in an increase in the withdrawal force as the number of fibers in the cylinder, i.e., the packing density, increases, provided the number of contact points remains constant. Although an increase in force is clearly seen in the force curves shown in Figure 12, it cannot be attributed to an increase in normal force. This aspect is further discussed below. It should be noted that these forces of interfiber friction cannot be used to obtain frictional coefficients, since the normal force and the number of contact points are not known. The nature of the force curve is altered considerably by changing the fiber geometry, by "perming" for example, or by surface treatments. Untreated clean fibers show low forces of withdrawal with a faint stick-slip pattern (Figure 12), which is accentuated by increasing fiber packing density. Setting the hair in a wavy pattern (wavelength 2-4 cm) by reduction (perming) results in a pronounced stick-slip pattern and an overall increase

362 JOURNAL OF COSMETIC SCIENCE FORCE(mg) 70- 60- 50- 40- 30- 2C UNTREATED HAIR O. 12B g/cm 3 . • t• - 0.03 g/crn $ O • ,,.,5 mm L LENGTH OF FIBER MOVED Figure 12. Withdrawal force curves for untreated human hair fibers at various packing densities. in the force of withdrawal (Figure 13a). This is due to an increase in the "springiness" of the assembly and a consequent increase in the normal forces at the points of contact, when such an assembly is packed into the cylinder. The stick-slip phenomenon is particularly pronounced in swatches that had been permed and treated with sebum, as seen in Figure 13b, reflecting the combined effects of the increased normal forces at the points of contact due to waving and the increased interfiber adhesion due to capillary forces caused by sebum deposition at these points of contact between fibers. Efj•ct of fiber packing density. As pointed out above, the force of withdrawal of a single fiber from an assembly depends on the lateral force acting on the assembly. This lateral force is proportional to the normal force acting at the points of contact between the fibers. An indirect way of increasing this lateral force would be to increase the packing density in the cylinder. In order to study the effect of packing density on the withdrawal force, a series of hair swatches (about ! 50-mm long) containing various amounts of hair were prepared. The cylinders were packed with hair from these swatches taken near their tip ends. The swatches were cut, and the remaining parts of each swatch were saved to repeat these measurements after treatment with synthetic sebum (1% solution in CC14). Fifteen to twenty measurements were made on each swatch to obtain a reliable average force value. The fibers withdrawn were randomly chosen throughout the area of the cylinder. The average withdrawal forces are shown in Table VI as a function of packing density before and after treatment with sebum. The data in Table VI are also shown graphically in Figure 14. It can be seen that the withdrawal forces increase up to a packing density of-0.3 g/cm 3 and then remain essentially constant over the range studied. Treatment with sebum causes a considerable increase in withdrawal force, which follows a packing density dependence similar to that of the untreated material.