Influence of powder addition to macrogol ointment Japanese pharmacopeia on the rheological properties.

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To understand the influence of powder properties on the viscoelasticity of a powder-filled semisolid, 8 powders (zinc oxide, titanium dioxide, hydrated sillicone dioxide, synthetic aluminum silicate, magnesium stearate, talc, cornstarch and glycyrrhetic acid) were added to macrogol ointment (MGO), and the viscoelasticity of the powder-filled MGO was investigated using the oscillation method at the rheological ground state and the continuous shear method at a high shear rate.As determined by the oscillation method, storage modulus (G') and loss modulus (G'') were increased in all cases by the addition of powders, suggesting the formation of a bridge structure through the particles. Though the increasing rate of G' or G'' on the weight fraction was different from powder to powder, a good linearity was maintained between logG' or logG'' and the square root of the contacting surface area (√(SA)), irrespective of the powder species. A master curve between log G' or log G'' and mean surface distance could be drawn. These simple relationships were more obvious than in the case of vaseline, studied previously. For this difference, it was presumed that in MGO, individual powder properties such as rigidity or lubricating function were masked by the more thickly adhered layers around the particles, and these thicker layers were formed due to the high consistency of MGO.As determined by the continuous shear method, the structures formed in MGO were susceptible to being ruptured easily. Thus, the powder properties seemed to have a stronger influence on the rheological properties at a higher shear rate than those at the rheological ground state.

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