Different Types of Thickeners Used in Water-Based Paints

Different Types of Thickeners Used in Water-Based Paints Surfactants and dispersants are used to disperse pigment, mineral and latex particles to increase suspension stability, prevent particle agglomeration, minimize viscosity contribution and provide higher solids content. To achieve the desired rheological profile, box stability and application properties of paint, adjustment of paint rheology normally requires the use of various types of thickeners, thixotropes and, in other words, rheology modifiers. From an application properties perspective; different types of thickeners are used to enable viscosity adjustment for brush, roller and spray applications, and to prevent problems that may occur in flow, splashing and spreading during application [1]. Commercial water-based 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-package" or packaging viscosity and other low shear processes such as flow, spreading, pigment settling and syneresis. High shear viscosity (also known as ICI viscosity) is generally associated with coating application processes such as brushing, rolling and spraying. When water-based paint performance properties are evaluated from a rheological perspective, although they are a function of low and high shear viscosity, it is the medium shear viscosity that directs production quality control and the user's quality perception of the paint. Medium shear viscosity represents the apparent viscosity when paint is mixed and transferred [2,3]. Paint performance depends on many properties including, but not limited to, optical properties (light scattering, opacity, etc.), freeze-thaw stability and appropriate rheology. To optimize paint performance during storage, brushing, spraying and spreading, constant shear rheological flow curves, particularly in the low shear region, have the general shape shown in Figure 1, which differs significantly from the traditional rheological profile (Newtonian solutions) exhibited by pure colloidal suspensions [4]. [caption id="attachment_157695" align="aligncenter"] Figure 1: Ideal relationship between viscosity, shear rate and paint properties[4][/caption]Thickeners are designed to support high viscosity at low shear rates (for example; in-package) and low viscosity at higher shear rates (during application). Addition of thickeners in very small quantities is sufficient to affect the rheological properties of a coating (typically this amount is between 0.1-5% by weight). Many types of thickeners are available depending on the intended use, and these are classified into categories such as associative, non-associative, organic and inorganic. In this study, associative thickeners have been examined in detail.

Associative Thickeners

Associative thickeners are polymers based on water-soluble polymers. These can be acrylate polymers, cellulose ethers or poly(ethylene glycol) (PEG) for quality nonionic products. These are coated with hydrophobic groups that are insoluble in water, such as fatty alcohols. In aqueous medium or emulsion, these polymers form a network that increases viscosity. Hydrophobic ends adsorb onto hydrophobic emulsion polymer particles or form micellar structures with hydrophobes from other polymers. Since each associative thickener polymer contains at least two hydrophobic ends, a three-dimensional network forms in the emulsion, which increases viscosity. Primarily high and medium shear viscosities are affected. Therefore, it improves splashing prevention and brush drag more than many other thickeners. Water sensitivity arises from increased use of surfactants needed to stabilize the thickener emulsion. Increased water sensitivity leads to reduction in scrub resistance. Heat stability is also not very predictable and can cause problems if the formula is not tested appropriately. Associative thickeners are generally supplied as a latex dispersion or viscous solution [5].

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

1. Alkali-swellable emulsion (ASE) and Hydrophobically modified Alkali-swellable emulsion (HASE) are frequently used. They are of low molecular weight but provide some hydrodynamic viscosity upon neutralization. ASE are acid-based polymers and their thickening behavior depends on the pH of the solution. In the acidic state, polymer chains are not soluble in water. When placed in a basic environment, they become soluble. Increased ionic strength (with added alkali) causes electrostatic repulsion of charges to extend the polymer chains and they swell with the amount of adsorbed water. A network forms due to the expanding volumes of the swollen chains and electrostatic repulsion of charges. Network formation also depends on the molecular weight of the polymer and its charge density. If the first is high and the second is low, additional entanglements occur in the chains. The molecular structure pattern for ASE and HASE is shown in Figure 2 [5,6]. 2. Among good thickeners used for rheological purposes, Hydrophobically modified Ethylene oxide Urethane (HEUR) rheology modifiers, belonging to the class known as, are noteworthy. Sometimes these are referred to simply as Polyurethane (PU) thickeners. These compounds provide excellent film formation, spreading, less splashing and a non-flocculant thickening mechanism. These types are nonionic and in most cases show weak sagging resistance. Urethane-type associative thickeners form networks with themselves, with binders and sometimes even with pigments. New and evolving technologies significantly reduce viscosity loss during colorization. In this regard, HEUR, nonionic type thickeners provide excellent flow and balance, do not require either solvent or tin for compatibility, and provide viscosity stability. Viscosity stability is achieved through a new viscosity generation mechanism. This technology is designed to have a lower molecular weight than high shear thickeners and provides improved resistance to viscosity loss for colored paints, improved sagging resistance and good preservation of flow [5,7]. The molecular structure for HEUR is shown in Figure 3. [caption id="attachment_157696" align="alignleft"] Figure 2: Molecular structure of ASE and HASE type thickeners in aqueous
solutions[6][/caption][caption id="attachment_157697" align="alignnone"] Figure 3: Molecular structure of HEUR type thickener in aqueous
solutions[6][/caption]  3. Hydrophobically modified Ethoxylated Urethane Alkali-swellable emulsion (HEURASE) type thickeners are produced by emulsion polymerization of alkali-soluble additional terpolymers, a carboxyl functional monomer, a water-insoluble monomer and a hydrophobic terminal urethane functional ethoxylate monomer. Different products and properties can be obtained by varying the ratio of three monomers and the hydrophobe type, degree of ethoxylation, etc. These materials are supplied as aqueous emulsions of water-insoluble polymers. When a base such as ammonium hydroxide is added, the polymer swells, becomes soluble and combines strongly with water - thus creating the thickening effect. HEURASE type thickeners have relatively long chains. Also along the polymer backbone there are many carboxyl anions that are distributed and repel each other. Hydrophobic interaction is what is responsible for the thickening mechanism in this type of polymer. The HEURASE thickener family can be blended so that the paint's rheology can be quite unique [5,7]. 4. There is a new class of associative thickeners called Hydrophobically modified Ethoxylated Aminoplast (HEAT) thickener. The aminoplast linkage is made by using an aminoplast instead of a diisocyanate. Aminoplast linkage is in most cases more hydrophilic than diurethane groups and more soluble in water. The ability to add hydrophobes at very high levels is a special characteristic of aminoplast chemistry and allows the production of associative thickeners that resist viscosity loss when glycols or surfactants are added to paint systems, as occurs during colorization of paints with concentrated colorants [5,8]. 5. Hydrophobically Modified Polyether (HMPE) - On the market there are new high shear modifiers based on HMPE that are free of volatile organic compounds (VOC) and alkylphenol (APEO), easy to incorporate and use. They have a high degree of efficiency in creating high shear viscosity but also provide medium shear viscosity contribution. As a result, some currently available HMPE's require lower association levels to reach the same viscosity target compared to HEUR's [5]. 6. Hydrophobically modified cellulosics (HMHEC) are cellulosic thickeners with hydrophobic modification in some of their branches. Several long-chain alkyl groups have been added along the backbone of the structure. These molecules create viscosity by the coming together of various hydrophobic groups. These paints have higher viscosity at higher shear rates and therefore better film formation and opacity are achieved [5,6,8]. The molecular structure for HMHEC is shown in Figure 4. [caption id="attachment_157698" align="aligncenter"] Figure 4: Molecular structure of HEC and HMHEC type thickeners in aqueous solutions[6][/caption]Thickener performance, particularly for synthetic associative thickeners, depends on interactions with surfactant paint components such as latexes and surfactants and dispersants. However, each of the four organic thickener types listed in Table 1 below can be characterized by various properties that are fairly independent of paint type and composition [5]. [caption id="attachment_157699" align="aligncenter"] Table 1: Impact of some organic thickeners on paint quality properties[5][/caption]

Conclusions

In this study, thickener types and properties used for decorative water-based paints in the paint industry were investigated, and the types and effects of associative thickeners were presented comparatively. It was observed that thickener types can be selected according to the desired rheological profile in terms of paint quality properties; in-package (packaging) viscosity, application performance and stability throughout shelf life. When these properties are examined in detail, it is necessary to prevent quality problems such as flow, splashing, spreading problems, difficulty of application, settling and separation, so the selection of the type of thickener to be used in the formulation is very important. Paint formulation developers should have good prior knowledge of many points such as; the polymer structure of the thickener they will use, its rheological effects on viscosity in low-medium-high shear regions, any interactions it may have with raw materials, its effect on external facade durability properties. This knowledge is important for the stability of the formulation to be developed.   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 Research and Development Specialist Marshall Boya ve Vernik Sanayi A.Ş.