Rheological properties of white soft paraffin

Abstract

White soft paraffin has been used in pharmacy for nearly a century (1). The physical nature varies, depending upon the source of the crude petroleum, the type and degree of refining, and on any subsequent blending processes. The major constituents are n-paraffins, iso-paraffins and naphthenes (2-5), which form a colloidal gel type structure containing oil, amorphous waxes, and discrete crystals (6).Many procedures are employed to investigate the rheological properties of semisolids. Continuous shear rheometry is used widely to investigate pharmaceutical materials (7-15) although many workers have reported disadvantages for this type of procedure (16)(17)(18)(19)(20)(21)(22).Oscillatory viscometers, which operate over a wide range of shear and strain rate, have many theoretical advantages for testing pharmaceutical materials, but little work has been reported (23,24).Creep viscometry is used especially for investigation of viscoelastic materials which yield at high stresses and rates of shear, for example emulsion and semisolid systems (23,(25)(26)(27)(28)(29)(30). The theory of linear viscoelasticity (31, 32) and the method of analysing creep curves (27, 33) are documented well.There is some controversy in the reported rheologieal properties of soft paraffin, and similar paraffin-greases. The material has been reported to be elastic (34), non linear viscoelastic (35) and linear viscoelastic (36, 37) in small strain experiments. Continuous shear rheograms have been published (38) whilst ejection of the paraffin from a cone-plate gap has been reported (34). The hysteresis loop of soft paraffin rheograms may be due to thixotropy (39) or irreversible shear breakdown (40).The purpose of this work is to establish whether a sample of white soft paraffin is a) viscoelastic or non viscoelastic; b) if viscoelastic, is it linear or nonlinear? and c) *) Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14-15, 1970. whether hysteresis loops, with or without pour points, obtained in continuous shear experiments indicate thixotropy, irreversible shear breakdown, or are artifacts due to expulsion of the material from the viscometer. The influence of temperature on the rheology of the material will be considered.The sample of white soft paraffin (Loveridge, Southampton) is of B. P. standard.Melting point ranges of the crystals in the sample were determined using a Kofler hot stage mounted on a Cooke polarising microscope, and with a capillary melting point apparatus (Gallenkamp). To determine whether the shear breakdown within soft paraffin is due to thixotropy or to irreversible shear breakdown, the FerrantiShirley viscometer, with large cone, was partially loaded so that the sample did not reach the edge of the cone, thus avoiding a hysteresis loop forming due to expulsion of material from the cone-plate gap. A flow curve was obtained, using Regime (1) at 25.5 ~ The sample was left on the viscometer at this temperature and a further curve obtained after 72 h.All...

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