Microfibrillated cellulose (MFC) displays a rheology profile which is similar to a variety of known rheology additives but can also give additional benefits. To further explain the properties of MFC in relation to the rheological landscape, I will grasp on the subject of similarities and synergies between MFC and hydrophobically modified ethoxylated polyurethanes (HEUR), another commonly used rheology modifier.
The HEUR and its typical characteristics
The HEUR products are polyurethanes (based on isocyanates) and are widely used in applications where the focus is on good flow characteristics. HEURs are synthetically derived products where the chemistry enables fine tuning of the properties. HEUR is typically an associative rheology additive, meaning that its hydrophobic end-groups can interact with its surroundings or itself to create a physical network. Different variants of HEUR typically display both Newtonian and non-Newtonian characteristics, depending on how the polymer chains are constructed.
The HEURs are very good at controlling flow, making it a popular additive in, for instance, coating applications. It displays good splatter resistance but at the same time, it tends to have more problems related to stability, both in use and on the shelf. Several attempts are being made to construct the polymer chains in such a way that HEUR can improve stability but this typically affects the flow behaviour negatively.
The pH window for using HEUR is typically between 4 and 7-8. At any lower or higher pH than this, the product may lose some of its performance characteristics.
The MFC and its typical characteristics
The two most obvious differences between HEUR and MFC are their raw material and solubility. MFC is derived from a natural source, while HEUR is synthetically derived. Moreover, MFC consists of non-soluble fibres and fibrils whereas HEUR is a water soluble polymer.
HEUR and MFC are also different when it comes to flow behaviour and stabilising effect in coatings; sag resistance vs. levelling. MFC typically displays better properties within the field of stability than HEUR and gives excellent sag resistance. It can at the same time affect the flow and levelling properties, but HEURs can often be used together with the MFC to create a better relationship between sag resistance and levelling.
Another difference between the two is the pH range where these products can be used. The MFC can be used from pH levels 1 to 13, while the HEUR have its functional area between pH 4-8. This enables MFC to function outside the spectrum of HEUR. Users of the product may experience valuable freedom both with regard to the processing/handling (pH may vary across different process steps) and in the final products.
Potential synergies of MFC and HEUR
MFC and HEUR are quite good companions in, for instance, improving the performance of a decorative or an industrial coating. First of all, MFC also improves low shear viscosity and high shear viscosity in spray coatings compared to HEUR. As discussed, MFC will at any time provide you with a very good improvement of stability (both in use and at rest), while HEUR will ensure good flow properties. In applications with a high amount of pigments and specific wishes on flow-levelling end-point results, these two additives may very well create a good foundation for success.
All in all, MFC and its multifunctionality is not only a direct alternative to HEUR but can in many cases be used simultaneously, taking advantage of the strength of both systems.
Coming from a natural source, MFC can both improve the functionality of your products and increase their sustainability.