RHEOLOGICAL STUDIES OF NEW CREAM BASES 229 stirring manually at room temperature or with the aid of a high-speed agitator. Ointment Base 601 is suitable for the preparation of o/w creams. We prepared three cream bases with 75, 80, and 85% of water, and added 0.1% of a mixture of methyl and propyl esters of phydroxybenzoate as preservative. For the rheological measurements of the different cream bases we used the RVT-model with the T-shaped spindles B and E. As specimen calculations, the experimental values of the measurement of the cream base with 75% of water and spindle B are detailed in Table VI. From these a rheogram (Fig. $) is constructed as detailed in page 227. By extrapolating the straight line through the points in the auxiliary diagram, a value for S O is obtained (---- 24). From Smax, % is calculated by multiplying by a value (98.1) obtained from Table II, section 2 and, from S o, ,f is found by multiplying by a value (97.8) obtained from section 1, Table II. To calculate the viscosity (U), we first obtained the rotational speed corresponding to d¾/dt = 98 B.R.U. (= 43 rpm) from Table IV. On the up-curve, S is read off at 43 rpm (---- 78). U is then obtained using equation (XV): U = constant- (S- So) where the constant can be found in Table V. The calculations are recorded in Table VI. Table VI Results with 75% water in Ointment Base 601 Apparatus: RVT Spindles: B Temperature: 20.5øC rpm 0.5 1 2.5 5 10 20 5O 100 x 1 rain 100 x 2 min 100 x 3 min scale reading up-curve down-curve So 24 Smax 45 30 38 + 43 60 49 67 56 80 69 88 85 'rs •- 4414 B.S.U. 'rt ---- 2347 B.S.U. U9s = 53.8 P Static yield value ('rs) = 98.1 x 45 = 4414 'r ---- shearing stress. Yield value ('rf) ---- 97.8 x 24 ---- 2347. Plastic viscosity (Ugs) ---- 53.8 P.

230 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • o