Application of nonlinear viscoelastic analysis by the oscillation method to some pharmaceutical ointments in the Japanese pharmacopeia.

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Abstract
Several kinds of ointments in the Japanese pharmacopeia were tested in an oscillation rheometer and the results were analyzed by the nonlinear method proposed by Onogi. The stress (.sigma.) curve obtained by appplication of sinusoidal strain (.gamma. = .gamma.0sin.omega.t) was approximated by a Fourier series, and the first order function (.sigma.1st) was regarded as the linear part. A nonlinearity parameter Dnl was defined as the difference between observed .sigma. and .sigma.1st. Dnl showed different .omega. dependency for each ointment, and seemed to be related to the pattern of the rheogram. Ordinal viscoelastic parameters such as storage modulus (G''), loss modulus (G'''') and loss tangent (tan .delta.) were calculated from the 1st function. The .omega. dependencies of these parameters were also different for each ointment. The influence of the weight fraction of solid (fs) or liquid component (1-fs) on these parameters was examined for simple ointment and macrogol ointment. A bending point was observed on the log G'' vs. fs plot, which might correspond to the occurrence of stiff bridge structures in the vehicles. Tan .delta. of the simple ointment was found to increase, in spite of the increase of solid component. This was presumed to be due to a greater increase of G'''' than of G''. This example shows that the increase of fs does not always result in a relative increase of rheological elasticity. Continuous shear measurement was performed with a Ferranti-Shirley viscometer and several parameters were compared with those obtained by the oscillation method. The correlation seemed to be closer when the shear rate was smaller.