How to counteract formulation settling issues

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.

What can cause settling of particles – and how to avoid it?

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 

Why cellulose fibrils prevents settling so effectively

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.

Applications and product types where MFC have enhanced performance

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:

• In cosmetics, solid particles may include inorganic particles for optical effects on the skin; UV filters; pigments in color cosmetics; or encapsulated particles with active molecules or fragrances. MFC will counteract settling of such particles. Also, MFC can provide additional benefits as mentioned in one of my earlier blog posts. One specific example is the soft focus effect.

• In home care, there is a range of products called abrasive cleaning products, and they rely on the mechanical cleaning action of what is called abrasive particles such as small mineral or metal particles; fine steel wool; copper; or nylon particles. They remove accumulations of dirt and soil found in small areas. MFC will maintain these particles homogeneously dispersed even at extreme pH conditions.

• In paints and coatings, Pigment Volume Concentration (PVC) is a number that represents the volume of pigment compared to the volume of all solids. This is expressed as a ratio of pigment to binder. It is possible to have a high particle loading making the use of MFC in such products very relevant. Also, MFC can provide better surface wetting and enhanced performance like improved mud-crack resistance. Again, some of the solids that needs to be stabilized against settling are pigments, UV filters, matting agents and more. In a range of products, there can also be other materials to be suspended such as fire retardants, anti-fouling polymers and waxes

• In concrete, MFC can prevent bleeding and segregation of cement (a complex mixture of chemicals, fines, and heavy particles). 
  
  

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.