The evaluation of different rheological agents and gelling components, testing new available materials, can lead to an economic saving. Obviously, if the original properties are kept in the final product. This tech note reflect exactly a case like this.
AlfatestLab has been asked to evaluate five formulation variants of a standard product, a pharmaceutical gel available on the market since a long time. A thorough characterisation of the rheological properties of the original gel and the five variants was the goal of this project.
The final project outcome can be summarised with the following rheogram. The data in figure 1 shows the flow behaviour of the 3 samples.
Fig.1 The viscosity curve of the reference gel ( two different batches) and the final new formulated gel.
Some detail on the flow property
What does this mean in terms of the physical appearance of the gel? Well, the high shear end of the curve (right side) can be related to when the gel is spread/sheared on the skin or between hands: the lower the viscosity, the better the feel will be for the user.
The low shear end of the curve, low shear rate, is related to when the gel must move slowly, for example when it has just been squeezed from the containing tube and, if the viscosity is not high enough, the gel can pour away from the skin instead of standing up (not a good feel for the user).
The curve tell us that there is another well known and hidden rheological property that the gel has, the Yield Stress. If you virtually extend the left end side of the curve toward 0 s-1 (pay attention to the log/log scale of the diagram), the viscosity will tend to infinite. That simply means that the gel, at rest, will behave like a solid.
Anyway, to measure the Yield Stress, we need another type of test. We will not constrain the gel to flow, applying an increasing shear rate as we have done in the flow curve test: in this test, a shear stress gradient will be applied. When the applied shear stress reaches the Yield Stress limit, the mechanical structure of the gel breaks, the gel starts to flow and the viscosity quickly drops
Yield Stress measurement
Fig.2 The Yield Stress measurement of the reference gel (two different batches) and the final new formulated gel.
The Yield Stress measurement is very sensitive to small variations of the sample. It is also affected by the recent stresses the sample was subjected to (time dependency). Fig.2 shows how much difference can be found between 2 different production batches of the reference sample (P-ref and ref) and the new formulation P-Var.
The 6 six formulated gels
Getting back to the first stage of the project, let’s have a look at the results given by the five different evaluated formulations. In figure 3, the viscosity profile of the six products (var(s) and ref) do not show relevant difference between the samples. That is a good point; the final goal should be achieved quickly. The formulator’s experience in manipulating the gel help him properly modulate the components to get comparable products.
Fig.3: The viscosity profile of the six products
In reality, looking at figure 4, we can see that the Yield Stress results for the six products show some significant differences between different sample formulations.
Fig 4: Yield Stress results for the six products
Conclusion
In fact, the Yield Stress is a material characteristic which cannot be easily detected by our senses. A soft gel material usually flows as soon as it is touched by fingers or handled with tools. It starts to flow due to an applied deformation that cannot be controlled by hand .This explains why an experienced formulator can achieve very similar products when they are flowing, but cannot keep the products’ behaviour at rest under control. A deeper evaluation of the six gels has been made at rest conditions, measuring their viscoelastic properties. The linear viscoelastic region as well as the mechanical spectra, the rebuilding time and the response function of the temperature can complete the profile of the gel. Several rheological parameters can be used to understand, simulate and improve the mechanical behaviour of gels, soft gels, cream and other compounds. Good skills in formulating and simple measurement tools may not be enough to achieve the goal. Surely the knowledge of basic rheology concepts is a must.
