Any question?
Help needed?

28 July 2020

Maurizio Lugli

Gel rheology: improving productivity and ROI

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 wide characterization of the rheological properties of the original gel and the five variants was the goal of this project.
The final project result can be resumed with the following rheogram. The data in figure 1 are showing the flow behavior of the 3 samples.

Fig.1 The viscosity curve of the reference gel ( two different lots) and the final new formulated gel.

Some detail on the flow property

What does that mean in terms of physical look 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: lower the viscosity is, better the user feeling will be.
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 stands up (bad feeling 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 mean 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 reach the Yield Stress limit, the gel mechanical structure broken, the gel start to flow and the viscosity quickly drop.

Yield Stress measurement

Fig.2 The Yield Stress measurement of the reference gel (two different lots) and the final new formulated gel.

The Yield Stress measurement is really sensitive to small variation of the sample.It is also affected by the recent stresses the sample was subjected to (time dependency). The fig 2 is showing 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 experience in manipulating the gel, let him to modulate properly 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 samples 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 flow as soon as it is touched by fingers or handled tools. It start to flow for an applied deformation that cannot be control by hands.This explain why an experienced formulator can achieve very similar products when they are flowing, while cannot keep under control the products behavior at rest. A deeper evaluation of the six gels has been done at rest condition, measuring their viscoelastic properties. The linear visco-elastic 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 mechanical behavior of gels, soft gels, cream and other compounds. Good skilling in formulating and simple measurements tools could not be enough to achieve the goal. Surely the knowledge of basic rheology concepts is a must.


Download the Technical Note