Let’s start with a definition: Rheology is the study of flow and deformation of materials under applied forces which is routinely measured using various methods.
More simply put, rheology is part of our daily life, observed in things like yogurt that has fruit suspended or shower gel coming out of a bottle.
In waterborne paints, rheology is determined only after all the ingredients have been mixed thoroughly into the final product. Your specifications should already be set for your target viscosity.
At this point, you need to select what type of thickener you need to use. You will start to see which type of thickener and the different shear level required. These methods of incorporation are extremely important as the higher the shear, the more potential foaming issues may have to be addressed. Figure 1 below illustrates four specific attributes that the viscosity of your product addresses.
In the above example, you see the direct relationship that paint rheology has on specific attributes. The thickener selection and chemistry type determine the shear rate needed to incorporate into the paint.
After you chose your ingredient for viscosity build, levelling and sagging directly affect the final paint’s performance.
With the newest paint and coating formulations reducing volatile organic compounds (VOC), waterborne paint formulations have become extremely complex. Older versions of many coatings contained a mere five to seven ingredients and were loaded with glycol ethers. That is no longer the case, with zero-VOC paints flooding the market. Chemists must ensure that EVERY ingredient is VOC-free. So today, the chemical manufacturers have modernized additives to meet these requirements.
So, as you look for solutions to your rheology targets, search UL Prospector for a list of different thickeners and review the traits that each have. Your choice of additives, such as wetting aids, dispersing aids, and the products that address sagging and viscosity consistency, will affect that overall formulation.
Now, let’s discuss what additives are available and how they affect these flows. Most people refer to these as associative thickeners.
Rheology modifiers
Rheology modifiers that thicken by volume exclusion include cellulosic ethers and alkali soluble (or swellable) emulsions (ASE). Cellulosic ethers are nonionic, water soluble polymers derived from natural fibers. They thicken by absorbing water and creating chain entanglement and flocculation.
There are basically three types that are commonly used in waterborne paints to control overall viscosity and flow. They are:
- ASE = Alkali swellable emulsions
- HASE = Hydrophobically modified ASE
- HEUR = Hydrophobically modified ethoxylated urethane resins
Thickener selection affects many things with your paint, such as adequate covering power, good film build, brush-ability and viscosity. Then flow, sag and leveling are also affected. Even paint splatter resistance can be controlled with the correct thickener!
As an example, in medium and high PVC paints, the HEUR thickeners also improve water resistance and even wet scrub resistance in the final paint.
Associative and non-associative thickeners
When you need viscosity stability during colorant addition, there are some cases where the use of a combination of both associative and non-associative thickeners help tailor the needs of the rheological profile for the best performance.
Evonik explains that “associative thickening involves non-specific interactions of hydrophobic end groups of a thickener molecule both with themselves and with components of the coating. The thickener produces a reversible, dynamic network of thickener molecules and other components of the coating. The thickening effect is caused by interactions of the hydrophobic end groups of the thickener with other components of the formulation.”3
Evonik further describes non-associative thickening as “thickening by an entanglement of water-soluble, high molecular weight polymer chains. The effectiveness of a thickener is mainly determined by the molecular weight of the polymer. Formulations thickened non-associatively have pseudo-plastic rheology with highly elastic properties. This produces good stabilization against settling out and low sagging even with high build coatings.”3
Surfactants are normally used in waterborne paints for different effects to the coating. These can affect the efficiency of your thickeners as well, so if you are experiencing problems, look at this possibility.
Adding universal colorants to your paint can also disrupt and contribute to a lower viscosity due to the normal glycol ethers that are used in these products.
Formulators’ experience shows that many suppliers now offer different modifiers and suggestions for use of their products.
A ULProspector.com search for Thickeners/Rheology Modifiers will yield more than 950 results. This can be overwhelming to many formulators. Everything is resin/emulsion/polymer driven. You might ask your supplier which “thickener” works best with your choice of resin/emulsion/polymer.
In a prior article I wrote on formulating,2 one of the items I addressed was how the final product should perform. If you need anti-sag for heavily-applied paint, you might want to choose a modifier like TAFIGEL® PUR 60 from Munzing Chemie. TAFIGEL PUR 60 is a liquid white, turbid rheology modifier that is a non-ionic polyurethane in butyl triglycol/water (VOC-free). It is an associate thickener that builds up and stabilizes viscosity. It is commonly used in interior and exterior paints, latex paints, anti-corrosive paints, emulsion plasters, adhesives and joint fillers.
If you are developing a low- or Zero-VOC product, make sure you chose a thickener that is VOC-free as well.
Then there are the “old standbys,” like ACRYSOL™ RM-725 Rheology Modifier, which is a solvent-free (manufactured without added solvent), HEUR rheology modifier. This low shear (KU) building associative thickener offers an excellent balance of sag resistance and leveling, particularly in PVA, VAE and acrylic binder-based paints.
Regardless of your choice, make sure you check your product’s viscosity AFTER you let it set a day. Sometimes your product may have a viscosity swing with age. If you experience this, re-mix your product and check again. If it does not return to your specified viscosity, you may need to either add an additional product, or change your modifier completely.
References:
- Journal of Surface Coatings Australia: The role of thickeners in optimizing coatings formulations [PDF]
- Formulating Waterborne Coatings: A Checklist
- Evonik Technical Background: TEGO® ViscoPlus Rheological Additives [PDF]
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Since when was butyl triglycol not a VOC?
(Volatile organic compounds (VOC) means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.)
Unless you’re thinking it falls under the CPR regulations but; “Coatings and industrial applications are not regulated under the Consumer Product Rule and the LVP-VOC exclusion does not apply to these products.”
TAFIGEL® PUR 60 is a liquid white, turbid rheology modifier that is a non-ionic polyurethane in butyl triglycol/water (APE and VOC free). It is an associate thickener that builds up and stabilizes viscosity. It is commonly used in interior and exterior paints, latex paints, anti-corrosive paints, emulsion plasters, adhesives, and joint fillers.
this is as stated by Munzing, the manufacturer of the product.
“Your choice of additives, such as wetting aids, dispersing aids, and the products that address sagging and viscosity consistency, will affect that overall formulation”
I would like to know how exactly wetting aids can affect the viscosity consistency of overall formulation?
Thank you in advance.
everything you add to a formulation makes a difference.
if you have “everything” you desire in your formulation and then start to adjust the viscosity, depending on the system, you can have an incompatible reaction with in your formulation.
If you have already formulated the coating, adjusted the viscosity, and feel you need a wetting additive for better overall penetration etc, make certain that additive is compatible with your system or you will experience a viscosity swing in your system.