CUTTING OF BEARD HAIR 585 50- 45- 40- '•' 35- • 30- c-- 0•.,, .• )5- o 20- 15- 10- x x o o x x o-o Fiber 1 x-x Fibee 2 1 2 3 4 5 6 7 8 9 10 Force to Cut (g) Figure 4. Cutting rate vs. cutting force (cantilever) the data of Mitchell and Feughelman (5). The fact that the cutting force is less dependent on relative humidity than the shear or tensile modulus sug- gests that these moduli-even at a rate of 0.5 in./min-are not the predomi- nant factors in beard hair cutting. Instead, the f-t-c might be more closely related to stress propagation or to the creation of new surface area, than to the viscoelastic properties of the fiber. Hollander and Casselman (1) made creep measurements on scalp hair, which they then extrapolated to beard hair, to determine the rate of softening

586 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS * - * Shear Modulus x - x Cutting Force o - o Tensile Modulus 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 10 20 30 40 50 60 70 80 90 100 %R.H. Figure 5. Normalized toodull or cutting force vs. l•er cent R.H. of the hair with in, creased temperature. Their extrapolated value was 21/2 to 3 min for complete hydration at 120øF. The hydration studies reported below were performed as follows: (1) a fiber was cut !0 times at ambient humidity to determine an equilibrium dry