Surfactants and dispersants are used to disperse pigment, mineral and latex particles to increase suspension stability, minimize the viscosity contribution of particle aggregation or agglomeration, and provide higher solids content.

Adjusting paint rheology for optimum in-can stability and application properties normally requires the use of various types of thickeners, thixotropes, and so-called rheology modifiers. In terms of application features, Different types of thickeners are used to adjust the viscosity for ease of application for brush, roller, and spray applications, and to prevent problems in sagging, spattering, and leveling during application[1].

Commercial waterborne decorative paints are generally characterized by low and high shear viscosity. Low shear viscosity (also known as Stormer viscosity, measured in Krebs Units or KU) describes “in-can” or packing viscosity and other low shear processes such
as sag, levelling, pigment settling and syneresis. High shear viscosity (also known as ICI viscosity) is usually associated with coating application processes such as brushing, rolling, and spraying.

While it is a function of low and high shear viscosity when considered from
a rheological approach in terms of paint performance characteristics, it is a moderate shear viscosity that drives production quality control and user perception adof paint quality. Medium shear viscosity represents the apparent viscosity when the paint is mixed and transferred [2,3].

Paint performance depends on many properties, including but not limited to optical properties (light scattering, “hiding power”, etc.), freeze-thaw stability and “right” rheology. To optimize paint performance during storage, brushing, spraying and spreading, the constant shear rheological flow curve has the general shape shown in Figure 1, which differs significantly from the conventional rheological profile exhibited by pure colloidal suspensions (Newtonian solutions), especially in the low shear region[4].

Thickeners are carefully designed to favour high viscosity at low shear rates (e.g. in-pack) and low viscosity at higher shear rates (during application). The addition of very small amounts of thickener is sufficient to influence the rheological properties of a coating (usually between 0.1-5 wt%). Depending on the intended use, there are many types of thickeners, which are classified as coalescent, non-coalescent, organic, inorganic, etc. In this study, coalescent thickeners were examined in detail.

Associative Thickeners

Associative thickeners are polymers based on water-soluble polymers. These can be acrylate polymers, cellulose ethers, or, for quality nonionic products, poly(ethylene glycol). These are coated with water-insoluble hydrophobic groups, for example, fatty alcohols. In aqueous media or emulsion, these polymers form a network that increases viscosity. Hydrophobic ends are adsorbed on hydrophobic emulsion polymer particles or form micelle
structures with hydrophobes from other polymers. Since each coalescent thickening polymer contains at least two hydrophobic ends, the result is a three-dimensional network
in the emulsion to increase the viscosity. High and medium shear viscosities are mainly affected. Therefore, it improves splash prevention and brush drag more than many other thickeners. The water sensitivity is due to the increased use of surfactants required to stabilize the thickener emulsion. Increased water sensitivity leads to a reduction in scrub resistance. Heat stability is also not very predictable and can lead to problems if the formula is not properly tested. Associative thickeners are usually supplied as a latex dispersion or a viscous solution [5].

The most commonly used types of associative thickeners and their properties are as follows;

1. HASE, is often used, ASE stands for alkaline swellable emulsion and HASE stands for Hydrophobically modified Alkaline Swellable Emulsion. They are of low molecular weight but provide some hydrodynamic viscosity on neutralization. Alkali swellable acrylic emulsions
(ASE) are acid-based polymers and their thickening behavior depends on the pH of the solution. In the acid phase, the polymer chains are insoluble in water. They become soluble when placed in a basic medium. The increasing ionic strength (with added alkali) stretches the polymer chains due to electrostatic repulsion of charges and they swell with the amount of adsorbed water. Due to the expanding volumes of the swelling chains and the electrostatic repulsion of the charges, a network formation occurs. The network formation also depends on the molecular weight and charge density of the polymer. If the former is high and the latter low, additional entanglements occur in the chains. The molecular structure for ASE and HASE is shown in figure 2. [5,6].

2. Among the very best thickeners used for rheological purposes are those in the class known as HEURs – or Hydrophobically modified Ethylene oxide Urethane Rheology modifiers. Sometimes these are merely referred to as PU thickeners. These compounds provide excellent high film-build, leveling, reduced spatter and a non-flocculate thickening mechanism. These types are nonionic and exhibit poor sag resistance in most instances. Urethanetype associative thickeners network with themselves and the binder and sometimes even with the pigment. New technology significantly reduces the viscosity loss upon tinting. These HEUR, non-ionic-type thickeners deliver excellent flow and leveling, are free of both solvent and tin for compliance and offer viscosity stability. The viscosity stability is achieved through a new viscositybuilding mechanism. This technology is designed to have a lower molecular weight than the high-shear thickener and for tinted paints offers improved resistance to viscosity loss, improved sag resistance, and preservation of excellent flow [5,7]. The molecular structure for HEUR is shown in Figure 3.

3. HEURASE (Hydrophobically modified Ethoxylate Urethane Alkali Swellable Emulsions) type thickeners are produced by emulsion polymerization of alkali-soluble addition terpolymers, a carboxyl functional monomer, a water-insoluble monomer, and a hydrophobic terminated urethane functional ethoxylate monomer. Different products and characteristics can be achieved by varying the ratio of the three monomers and the type of hydrophobic, degree of ethoxylations and so forth.

These materials are supplied as aqueous emulsions of water-insoluble polymer. When a base, such as ammonium hydroxide is added, the polymer swells, becomes soluble and strongly associates with water – hence its thickening action. The HEURASE types of thickeners (water-soluble polymers) have relatively long chains within the polymer.

There are also many carboxyl anions scattered along the polymer backbone, which repel one another. The hydrophobe interaction is what is responsible for the thickening mechanism in this type of polymer. The HEURASE family of thickeners can also be blended so that the rheology of the coating can be unique [5,7].

4. Aminoplast Associative Thickeners – There is a new class of associative thickeners called HEAT – Hydrophobically modified Ethoxylated Aminoplast Thickener. The aminoplast linkage is done by use of an aminoplast instead of a diisocyanate. The aminoplast linkage in most cases is more hydrophilic and more water-soluble than the diurethane groups. The ability to add very high levels of hydrophobe is a special property of aminoplast chemistry, and it
allows the production of associative thickeners that resist viscosity loss when glycols or surfactants are added to coating systems, as happens during tinting of paints with concentrated colorants [5,8].

5. Hydrophobically Modified Polyether (HMPE) – There are new high shear modifiers on the market based on HMPE that are VOC (volatile organic compounds) and APEO(alkylphenol) free that are easy to incorporate and handle. They have a high degree of efficiency in building highshear viscosity but also provide medium shear viscosity contribution. Consequently, lower incorporation levels are needed to achieve the same viscosity target as compared to some currently existing HMPEs or HEURs [5].

6. HMHEC – The hydrophobically modified cellulosic (HMHEC) are cellulosic thickeners, which have a hydrophobe modification on some branches – several long chain alkyl groups have been introduced along the backbone of the structure. These molecules build viscosity by association of the various hydrophobic groups. These paints have higher viscosity at high shear rates, and therefore get better film build and hiding [5,6,8]. The molecular structure for HMHEC is shown in Figure 4.

Thickener performance, especially for synthetic associative thickeners, depends on interactions with latexes and surface-active paint ingredients such as surfactants and dispersants. However, each of the four types of organic thickeners in the following table may be characterized by several attributes that are fairly independent of paint type and composition[5].

Results

In this study, thickener types and their properties used for decorative water-based paints in the paint industry were investigated, and types of coalescence thickeners and effects are given comparatively.

It was seen that thickener types can be selected according to in-can (packaging) viscosity, application performance, and stability during shelf life in accordance with the desired rheological profile in terms of paint quality characteristics. When these properties are analyzed in detail, the selection of the thickener type to be used in the formulation is very important since quality problems such as flowing, spattering, leveling failures, difficulty in application, settling and separation should be prevented.

It is important for the developers to acquire the polymer structure of the thickener to be used, its rheological effects at low-medium-high shear viscosity regions, and its interaction with any raw material in advance as well as the durability effects in terms of the robustness of the developed formulation.

References:
[1] McGonigle F., Cuillo P. A., Industrial Minerals and Their Uses, 1996, 138.
[2] Reuvers AJ. Control of rheology of water-borne paints using associative thickeners. Prog Org Coatings 1999.
[3] Overbeek A, Bückmann F, Martin E, Steenwinkel P, Annable T. New generation decorative paint technology. Prog Org Coatings 2003.
[4] Larson G. L., Dyk V. K. A., Chatterjee T., Ginzburg V. V., Associative thickeners for waterborne paints: Structure, characterization, rheology, and modeling, 2022.
[5] Koleske V. J., Springate R., Brezinski D., Additives Handbook, 52-57, 2011.
[6] Kastner U., The impact of rheological modifiers on water-borne coatings, 2001.
[7] Calbo, L.J., Ed., Handbook of Coatings Additives; Marcel Dekker: New York, 1992.

Sinem Kulak Boyraz
R&D Executive
Marshall Boya ve Vernik Sanayi A.Ş.