Rheological and Sensory Evaluation of Work Softening and Recovery of Pharmaceutical White Soft Paraffins

Abstract

Continuous shear and creep viscometry were used to investigate the effect of work softening and recovery on the rheological properties of four grades of white soft paraffin BP. In continuous shear, using a cone and plate viscometer, the apparent viscosities of two samples increased with recovery time after working, which indicated partial structural recovery. Apparent viscosities of unworked samples were generally lower than those of the worked samples due to elastic recovery and/or sample fracture of the unworked samples during shear in the viscometer. Viscoelastic results were analyzed to obtain initial elastic compliances and residual viscosities. These data indicated that the loss of consistency during working and recovery after working were mainly viscous phenomena. Continuous spectra of retardation times derived for worked and unworked soft paraffins showed that working caused little irreversible structural breakdown. Structural recovery, indicated by concentrations of retardation mechanisms in the continuous spectra, occurred for at least 240 hr. after work softening. A sensory evaluation of white soft paraffin in different states of working was correlated with discrete viscoelastic parameters and continuous shear yield stresses of the material; no correlation was found with continuous shear apparent viscosity data.