How to compare rheology additives: the example of nanocellulose/Cellulose fibrils and HEUR
Ole Martin Kristiansen | October 16, 2018
As a researcher, to have an overview of the alternatives available in your area of profession is of importance. In the landscape of rheology, new alternatives are emerging. In my short review today, I will grasp on the subject of similarities and potential synergies between two of the candidates you should note down: nanocellulose and hydrophobically modified ethoxylated polyurethanes (HEUR). Here are my hints and tips on how to understand these two technologies better.
The HEUR and its typical characteristics
Polyurethanes are a popular field of technology, and the hydrophobically modified urethanes are present as a rheology modifier. Well known under the name HEUR these products are based on isocyanates and are widely used in applications where the focus is good flow characteristics. HEURs are synthetically derived products where the chemistry enables fine tuning of the properties. Traditional HEURs are typically associative systems, restricting molecular movement/motion, giving increased viscosity. It is in many ways creating a "physical network" interacting with its surroundings. Different variants of HEUR can display both Newtonian and non-Newtonian characteristics, depending on how the polymer chains are constructed.
The HEURs are very good at controlling flow which is making it a popular additive in coating applications. It displays good splatter resistance and can build good KU (krebs unit) viscosity and display good ICI viscosity, depending on its composition. A potential area which needs some trial and error when you use the HEUR, is to study the ability of your product system absorb the hydrophobes of the HEUR. This is very important in relation to achieving the molecular motion restriction. So, for instance the wrong latex surface chemistry may give the HEUR challenges to work.
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 nanocellulose/cellulose fibrils and its typical characteristics
The two most striking differences between HEUR and nanocellulose or cellulose fibrils are their raw material and solubility. The fibrils is derived from a natural source, while HEUR is synthetically derived. Moreover, fibrils consists of non-soluble fibres whereas HEUR is a water-soluble polymer.
HEUR and nanocellulose/cellulose fibrils are also different when it comes to flow behavior and stabilizing effect in coatings; sag resistance vs. leveling. Nanocellulose/cellulose fibrils typically displays different properties within the field of stability than HEUR due to its insoluble fibres and gives a very efficient anti-sag resistance.
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 range of HEUR, creating potential use in a wider range of systems. Users of the nanocellulose/cellulose fibrils products 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 nanocellulose/cellulose fibrils and HEUR
Nanocellulose/cellulose fibrils and HEUR are quite good companions in, for instance, improving the performance of a decorative or an industrial coating. First of all, the fibrils also improves low shear viscosity and high shear viscosity in spray coatings compared to HEUR. As discussed, the nanocellulose/cellulose fibrils 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. You may also find synergies from the physical presence of fibers from the nanocellulose/cellulose fibrils and molecular motion restriction of the HEUR. In applications with a high amount of pigments and specific wishes on flow-leveling end-point results, these two additives may very well create a good foundation for success.
All in all, the nanocellulose/cellulose fibrils 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.