Eun-Kyoung Park  Geun-Seok Choi  Ki-Won Song

Vol. 57,  No. 3, pp. 125-132, Jun.  2020
10.12772/TSE.2020.57.125

wool fat skin-care cosmetic products steady shear rheology yield stress shearthinning viscosity linear viscoelasticity modified form of the Cox-Merz rule temperaturedependent viscoelasticity

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The principal objective of the present study has been to systematically investigate the rheological properties of wool fat as a main ingredient of skin-care cosmetic products. Using a strain-controlled rheometer, the steady shear flow properties of commercially available wool fat were measured over a wide range of shear rates, and the linear viscoelastic properties of the material in small amplitude oscillatory shear flow fields were measured over a broad range of angular frequencies. In addition, the temperature dependency of the linear viscoelastic behavior was investigated over a temperature range most relevant to usual human life. The main findings obtained from this study are summarized as follows : (1) Wool fat exhibits a finite magnitude of yield stress and a marked non-Newtonian shearthinning flow behavior. The appearance of a yield stress and a shear-thinning feature can be explained by the crystalline networks existing within the material. A yield stress is correlated with consistency whereas a shear-thinning viscosity is associated with spreadability. (2) The linear viscoelastic behavior of wool fat is dominated by an elastic nature rather than a viscous nature. This behavior can be interpreted by the orientation of hydrocarbon chains inside a monodomain. This linear viscoelastic feature could become a criterion to assess the storage stability of cosmetic products at rest or at very small deformations. (3) A modified form of the Cox-Merz rule provides a good ability to predict the relationship between steady shear flow properties (nonlinear behavior) and dynamic viscoelastic properties (linear behavior) for wool fat. (4) The storage modulus and loss modulus show a qualitatively similar tendency to sharply decrease with an increase in temperature. At a temperature range higher than 40°C, both of the two moduli exhibit a more pronounced temperature dependency while the difference between the two moduli becomes smaller. In the case of the storage modulus, an unexpected plateau region is observed over a temperature range between 25 and 30 °C.