62 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 30 2O 10 -10 15 3O 45 Time, Seconds Figure 2. Typical compression stress-strain curves obtained with foams after repeated penetrations of the plunger. Depth of penetration: 0.0254 cm. speed of penetration: 0.254 cm/min. are substantially linear and remain constant even with repeated cycles of penetration. The constancy of these initial slopes, which are proportional to the shear modulus of the foam, indicates that no significant alteration of the foam structure occurs under the low-strain test conditions, at least insofar as its elasticity is concerned. The shear modulus is given by (8): a f Os - --- 3, bLo L(t) b = (2) In q - (q2 _ 1)/(q2 + 1) q-- R2/R, (3) where •r = the axial component of stress 3' = the axial component of strain Lo = the maximum immersion depth of the plunger f = the axial force acting on the plunger L(t) = the axial displacement of the plunger at time t R 2 = the radius of the container R• = the radius of the plunger. The elastic shear moduli calculated from our experimental results are summarized in

SHAVING FOAM VISCOELASTIC PROPERTIES 63 Table I Static Elastic Shear Modulus of Foam Low Strain Level With Repeated Penetration Elastic Shear Modulus, Grams/cm •, at Cycle* Foam 1 2 3 4 5 Average Deviation Flavor variant 1 4.11 4.29 4.31 4.33 4.31 4.27 0.09 Flavor variant 2 5.57 5.69 5.64 5.66 5.59 5.63 0.05 *Elastic shear modulus was calculated using equation (1). Table I. The results indicate that repeated penetration of the plunger does not affect the elastic shear modulus of the foam, suggesting that the foam structure was unaffected by the tests. MEASUREMENT OF THE VISCOELASTICITY OF FOAMS IN A CONSTANT RATE PLUNGER PENETRATION MODE In the second mode of operation, the plunger was driven into the foam at various constant cross-head speeds and the force measured as a function of time. A typical Slope 2 Max. Compression Force E 80 o m ¸ 60 4O 2O Slope 1 Yield Point i I I I I ß L 1 2 3 4 5 6 7 Compression Distance, CM Figure 3. A typical foam compression curve with larger penetration depth. Speed of penetration: 50.8 cm/min. cup diameter: 9 cm. plunger diameter: 2.72 cm.