Everyday life is full of formulations containing solid particles, pigments, beads or fillers. Depending on the application, the formulations may have a varying amount of solids. Common challenges with high solid content formulations are the settling of heavy particles or the floating of lighter ones. Therefore, it is important to ensure the stability of the solids suspended in a formulation. Especially those with high particle loading such as a coating formulation with matting agents, UV filters and other solids.
The stability and the performance of such formulations depend on how well the solids are suspended and how homogeneously they are dispersed and distributed. Both the short-term and the long-term behavior affects the quality of the formulation.
Cellulose fibrils, a biobased biodegradable and sustainable material, can counteract settling of particles in many applications and formulations. But before we get there, let’s discuss what causes settling of particles.
Heavy particles settle at the bottom of a formulation due to gravity. Less dense particles float on the surface of a formulation. Hence, it is necessary to have a formulation with enough gel strength to suspend the particles and hold them in one position counteracting the force of gravity as well as preventing floating. Even if the particles are suspended successfully, they tend to aggregate over time. This leads to an increase in the size and mass of particles. In such a situation, the mass of the aggregates might get so high that the gel strength of the formulation is not enough to maintain the heavier particles suspended. This can in turn lead to a settling of the particles after some storage time. Therefore, the key factor to counteract settling of particles is to ensure homogeneous dispersion of particles throughout the formulation during storage, transportation, handling and application.
A good dispersion is possible by using a high yield stress material that provides strong gel strength when added to a formulation, thus avoiding settling of the solid particles. This way, the particles will be dispersed, stabilized and homogeneously distributed in the formulation. An important part to remember is that the yield stress material must be used at the right concentration to provide the required gel strength. Some conventional stabilizers, other than cellulose fibrils, include cellulosic polymers, carbomers, bentonite and xanthan gum. Typical drawbacks with these materials are that their use is limited by their pH range, or they are oil-based chemicals. So the questions is then: how can cellulose fibrils contribute in this setting?
--> Read also: Dispersion of microfibrillated cellulose - a critical success factor
Cellulose fibrils, like MFC, forms a physical 3D network with high surface area when incorporated into a given system. This leads to high yield stress, good gel strength and high viscosity at rest at quite low concentrations. Read more about the high yield stress in this article.
MFC can maintain a stable suspension of solids provided it has been correctly and completely dispersed in the formulation. The suspended solids will be homogeneously dispersed throughout the formulation and throughout the physical 3D network.
Figure 1: 2g of Apricot Kernel Powder (Greentech product) with different particle sizes suspended in 2wt% MFC (Exilva, Borregaard). MFC maintain the homogeneous dispersion of the different particles, avoiding their settling, floating and aggregation/agglomeration.
In addition to providing the necessary gel strength and being of natural origin, cellulose fibrils is very robust regarding pH, temperature, salt and does not dissolve. It implies that MFC will do the job under the different conditions the formulation will have to undergo during production and use.
In earlier blog posts, we have discussed the potential of cellulose fibrils in different applications. By showing you some examples from these, I give you some ideas of where settling of solids and inhomogeneity is critical to the performance of the end product:
As a conclusion, cellulose fibrils and its high yield stress can provide stability of particles in many complex formulations containing high solid contents. Its high gel strength at low concentrations can allow improvement of the stability, performance and shelf-life of end-products, thus providing a new and sustainable tool in your tool box.
Editor's note: This post was originally published in November 2016. We've revamped and updated it for accuracy and comprehensiveness.