Rheological+Testing+Of+Creams,+Lotions+and+Ointments

__**Rheological Testing Of Creams, Lotions and Ointments **__  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: //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 in the at-rest resistance to deformation - in simple terms: the stiffness - of the products. 
 * Texture and application property assessment
 * Stability tests
 * Scale-up and batch-to-batch comparisons
 * Benchmarking and competitor analyses
 * <span style="font-family: 'Arial','sans-serif'; font-size: 10pt;">Process design
 * <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 7.5pt;">Complex Modulus of Cream, Lotion and Ointment <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">  ||

<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 7.5pt; line-height: 115%;">*Complex Modulus of Cream, Lotion and Ointment //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">Yield Stress //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;"> 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 //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">do //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;"> flow, but at a very low rate //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">creeping flow //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">. <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;"> <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">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). <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;"> //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">Phase angle // //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">(δ or "delta") //<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">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.

<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 7.5pt; line-height: 115%;">*Phase angle increases as the yield progresses. <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 10pt;">It seems reasonable to expect that a "stiffer, stronger" sample is therefore a more structured sample. However, the results for the oinment 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.