VISCOELASTIC PARAMETER MEASUREMENTS 12 3 RESOLUTION OF COMPLIANCE CURVE-2nd RESOLUTION PETROLATUM •10 I I I I I I I I I I00 150 200 250 300 750 400 450 500 Time (t) sec Figure d. II RESOLUTION OF COMPLIANCE CURVE-3rd RESOLUTION PETROLATUM 0 0 60 80 I00 I 0 140 160 Time (t) sec Figure 7.

124 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Comparison of Viscoelastic Parameter Values for Petrolatum* Parameter Literature** Literature*** Experimental •i Values (seconds) 1360-2059 1230.0 914.7 38-487 245.0 222.2 2-57 58.8 76.9 qqi Values (poise X 10 -8) qq0 19.00-278.00 0.132 1.74 Xl• 13.50-55.70 0.059 0.35 Xl 2 0.98-33.40 0.024 0.33 qf]3 0.06--6. 13 0.024 O. 11 J, Values (cm 2 dynes- • X 10 6) J0 0.757-0.355 42.2 13.8 J• 0.315-1.050 93.5 25.9 J2 o. 100-0.452 41.5 6.79 J3 0.024-0.388 24.7 6.79 * Melting point 38-39øC. ** Reference 15. *** Reference 3. To check the model and the resolved values of the various parameters, the compliance values at different times were calculated back with the help of Equation 10. These values are superimposable on the experimental curve (Figure 4), thus confirming the model and the resolved values. A similar study was performed on PEG 1500, and the compliance curve obtained is shown in Figure 8. The resolution of this curve required only two steps, suggesting the presence of only two Voigt units in contrast to three such units with petrolatum. The model is shown as an inset in Figure 8, while the various parameters obtained are shown in Table II. The regenerated compliance curve, shown as filled circles in Figure 8, is again superimposable. COMPARISONS WITH THE MODIFIED CREEP APPARATUS As mentioned earlier, besides the rheogoniometer (2) which is used mainly in testing engineering materials, the modified creep apparatus test (3) is so far the only other method capable of demonstrating the viscoelastic nature of semisolid materials. It is basically a rotovisco-type viscometer with a concentric bob-cup geometry and differs from other bob-cup type viscometers, e.g., the Stormer viscometer, in its unique ability to use very low levels of stress because of the air-bearings in its design. The following points provide a comparison of our method with those of the modified creep apparatus test. 1. Since the cup and bob of the creep test are an integral part of the instrument and the material needs to equilibrate in the cup after filling, the number of observations which may be taken with this device in a given period is limited.