This week I would like to discuss water-based systems where the challenge is to apply the product effectively on a surface. This can be true for a high number of products ranging from coatings, adhesives, composites (for instance plastics), materials for 3D printing, deicing products and so on. The downstream processing and application of the product is by medium or high shear equipment, meaning either a roller, brush, a spray or the like. This calls for a shear thinning rheology system to stabilize the formulation and give the correct viscosity at each step.
Your toolbox of rheology modifiers is growing
There are currently a high number of alternatives to provide you with a solution, which can be grouped accordingly:
The first alternatives, soluble polymers, are the most common technology today. The reason for this is that there has not really been a widely used non-soluble alternative on the market which would give the correct viscosity levels at various shear rates, and at the same time, keeping the advantages of the soluble polymers. Many consider soluble polymers first, since this the technology is well known after many years of experience.
There are mainly three ways of a soluble polymer to thicken: hydrodynamic volume exclusion, by associative thickening and particle-particle interactions. Hydrodynamic volume exclusion simply means that the polymer chains occupy the space in the formulation which the other ingredients can’t take, reducing the flowability of the formulation. ASE polymers swell and uncoil in alkali conditions since functional groups on the polymer chains are charged which thickens the formulation. HASE polymers thicken by associative thickening which means that the hydrophobic parts of the polymer tend to interact with each other in hydrophilic environment since that lowers the overall free energy in the system. The third mechanisms, particle-particle interactions refer to dispersing effect. The polymer chains can adsorb on the solid particle creating repulsion between them and thus stabilizing the formulation.
The insoluble additives thicken the formulation by forming a network which expands throughout the formulation and stabilizes the other ingredients within that network. Clays and fumed silica interact with surrounding solvent (water) when wetted and form the network. Cellulose fibrils have, in addition, physical linkages in the network since the long fibrils can entangle with each other, offering extra strength for the network.
So many alternatives! Which one to choose when robustness is needed?
As mentioned above, the thickening mechanisms for the different rheology additives differ drastically. For the polymers, the mechanism is often a mixture of volume exclusion, associative thickening and particle-particle interaction. The downside of these thickening mechanism is that they are not so robust since they are dependent on the physico-chemical environment. To achieve the increased viscosity by volume exclusion, the pH needs to be alkaline. Similarly, a high amount of surfactants disturbs the associative thickening mechanisms, leading to lower viscosity. In addition, salts can disturb the adsorption of the polymers on the solid particles and thus interfere with the particle-particle interaction mechanism.
Insoluble additives instead form a physical network throughout the formulation and stabilize in that way. This is generally a robust way to stabilize a formulation. However, the strength of this network varies from additive to additive and cellulose fibrils have some advantages over the clays and fumed silica. Clays and fumed silica form the network based on the solvent – particle interactions (for example, the silanol groups on fumed silica). This means that the interactions are vulnerable for the changes in the chemical environment. Instead, cellulose fibrils form partly the network through their physical form, since the slender, long fibrils which can entangle with each other. This makes the network more stable against changes in pH, salinity or temperature. Insoluble rheology additives have some other advantages over the soluble polymers. For example, viscosity recovery after shear is quicker for the solid network. In addition, polymer chains might break down at very high shear conditions, whereas clays, fumed silica and cellulose fibrils withstand high shear conditions extremely well.
There are new alternatives out there now, which can create opportunities for you when you choose your rheology additive. The associative thickening methods became popular in the 1990’s and early 2000’s. Today, insoluble additives like clay and cellulose fibrils are making its entrance into the market, providing you with yet a new option. So, it’s up to you, next time you are re-constructing your rheology system or constructing something brand new.