Viscosity–temperature relationship of lipid‐based excipients amenable for spray congealing: Derivation of a rheological parameter with good correlation to particle size

The objective of this paper was to investigate the viscosity‐temperature relationship of lipid‐based materials (fatty alcohol, fatty acid, fatty ester and glycerides) amenable for spray congealing. The flow properties and viscosity–temperature relationship of cetyl alcohol, stearic acid, myristyl myristate, glyceryl monostearate, glyceryl dibehenate, hydrogenated soybean oil, hydrogenated vegetable oil and hydrogenated cottonseed oil were investigated using continuous ramping and temperature ramping tests. The viscosity–temperature data obtained was fitted into two existing viscosity–temperature models as well as a series of mathematical functions. The peak melting points of the materials were established by differential scanning calorimetry. Morphology and size of the spray‐congealed microparticles were analysed by scanning electron microscopy and laser diffractometry. Correlation analyses were conducted to explore the relationship between viscosity and microparticle median size. When molten, all eight lipid‐based materials used were Newtonian liquids. Viscosity of these materials decreased in a biphasic manner with increasing temperature which fitted a simple biexponential equation. The viscosity–temperature curves showed distinct transition points, from which mathematical manipulations generated a new rheological parameter termed Tp. Compared to the conventionally used viscosity values at spraying temperature, the Tp values of materials showed a stronger correlation with the median size of spray‐congealed microparticles produced. Practical applications: From this work, a new temperature‐independent constant (Tp) unique to each lipid‐based material was derived and found to have good correlation with the resultant spray‐congealed microparticle median size. This work aims to introduce Tp as a viable parameter of the rheology of lipid‐based materials for optimisation of formulation in spray congealation. Besides spray congealing, the application of Tp may be extended to other melt processes, such as melt granulation, melt extrusion and hot melt coating. (A) A typical biexponential temperature ramping curve showing the location of the characteristic transition point (Tp). (B) Graph of median size (μm) of spray‐congealed microparticles against Tp values.