Rheology of Creams, Ointments and Lotions
A simple Oscillation Stress Sweep test reveals the significant differences in the behaviour of a lotion, a cream and an ointment.
The test involves applying an increasing oscillating stress to the sample whilst monitoring two rheological parameters: Complex Modulus and Phase Angle. From this we can obtain valuable information that can be used for:
- Texture and application property assessment
- Stability tests
- Scale-up and batch-to-batch comparisons
- Benchmarking and competitor analyses
- Process design
Complex Modulus (G*) is a measure of the resistance to deformation of the sample. We can clearly see that as the oscillatory stress is increased the samples maintain their resistance to deformation until they reach a critical range of stresses, whereupon they undergo a rapid transistion from a high to low modulus. The plateau values reveal the significant differences between the products in the resistance to deformation - in simple terms: the stiffness - of the products.
Complex Modulus of Cream, Lotion and Ointment
Yield Stress is a useful practical measure of the stress required to induce flow in a product. In actual fact, when exposed to stresses below the yield stress, viscoelastic materials such as creams, lotions and ointments do flow, but at a very low rate creeping flow. An assessment of the various methods of quantification of yield stresses is outside of the scope of this article, however, an inspection of the curves reveals a low relative yield stress for the lotion (a pre-requisite for pourability) and a significantly higher relative yield stress for the ointment (pre-requisite for wash-resistance).
Phase angle (δ or "delta") is a measure of the presence of elastic (or "gel") structure within a material - a low value indicating greater elastic behaviour and vice-versa. The drop in complex modulus in the yielding process is matched by an increase in phase angle. This tells us that there is a break-up of internal elastic structure in the sample as the imposed stress is increased.
Phase angle increases as the yield progresses
It seems reasonable to expect that a "stiffer, stronger" sample is therefore a more structured sample. However, the results for the ointment reveal that this is not necessarily the case. It has a higher complex modulus and a higher yield stress than the cream and lotion but its plateau phase angle is also very much higher - that is, it exhibits less elasticity or gel structure.
Would you like a profiling such as this performed on your products and maybe those of your competitors?