Rheological evaluation of petroleum jelly as a base material in ointment and cream formulations with respect to rubbing onto the human body

The objective of the present study is to systematically characterize a nonlinear viscoelastic behavior of petroleum jelly in large amplitude oscillatory shear flow fields correspondent to the rubbing condition onto the human body. With this aim, using a strain-controlled rheometer, the dynamic viscoelastic properties of commercially available petroleum jelly have been measured at 37oC (body temperature) over a wide range of strain amplitudes at several fixed angular frequencies. In this article, the strain amplitude dependence of the dynamic viscoelastic behavior was firstly reported in detail from the experimentally obtained data and then the results were explained from a structural view-point of petroleum jelly. Nextly, a comparison of elastic and viscous properties was made in small and large strain amplitude ranges and then these results were discussed in depth with a special emphasis on their importance in actual usage situations (i.e., rubbing onto the human body or skin). Main findings obtained from this study can be summarized as follows : (1) Both the storage modulus and loss modulus show a linear behavior only within an extremely small strain amplitude range ( ) and exhibit almost an equivalent strain limit of linear response ( ). (2) Both the storage modulus and loss modulus demonstrate a qualitatively similar strain-dependent nonlinear behavior (i.e., strain-thinning feature), even though the storage modulus shows a stronger dependence on strain amplitude than does the loss modulus. (3) As the strain amplitude is increased, the difference between the storage modulus and loss modulus is gradually decreased and subsequently a viscous property becomes superior to an elastic property at sufficiently large strain amplitude range. (4) A large amplitude oscillatory shear flow behavior can provide a plentiful information for a better understanding of the complicated rheological behavior of semi-solid ointment-like materials in their actual application onto the human body or skin.