Organic Thickeners in Water- and Solvent-Based Paints

These are materials used to increase the viscosity of a liquid. Thickener additives provide paint with an appropriate consistency and help create sufficient paint film thickness on the surface to which the paint will be applied. At the same time, they prevent pigment settling during storage. Basically, thickening agents increase viscosity at moderate shear rates and thereby increase the paint's resistance to flow during pouring and mixing. For formulators, the suitable operation of rheology modifiers is generally challenging. These products are necessary additives that prevent excessive sagging and spattering during application, and they must provide a significant viscosity increase to the product that will affect the packaging viscosity. Because rheology control is particularly important in products sold directly to consumers. Paint packages used in homes can remain on store shelves for months or even years. For this reason, the formulation must prevent pigment settling in the can and ensure the paint can be applied properly. Emulsion paints are generally paints that are at appropriate application viscosity and therefore require the addition of thickeners that can prevent settling of pigments and other heavy ingredients. Ideally, the basic properties that rheology modifiers should have are: • Their effect on paint packaging viscosity should be minimal, • They should provide sag resistance control at high temperatures; • They should not adversely affect the final properties of the paint film. These additives must be carefully designed to provide high viscosity at low shear rates (for example, in the can) and low viscosity at higher shear rates (during application). To regulate the rheological properties of a paint, generally very small amounts of additive between 0.1 - 5% by weight are required. Rheological additives can be classified for use in solvent and water-based systems, but as a result of technological advances in water-based additives, these classifications are no longer particularly important. Because most thickeners can be applied to both water and solvent systems.

General Classification of Thickeners

I- Inorganic Thickeners Inorganic thickeners and organically modified inorganic thickeners are used in both water and solvent-based paints. • Bentonites • Silicas • Hectorites • Organoclays

II- Organic Thickeners

1. Organic Thickeners for Solvent-Based Systems • Indian oil derivatives • Polyureas • Metal soaps • Cellulose derivatives • Polyamides

2. Organic Thickeners for Water-Based Systems

• Non-Associative thickeners (interactive with aqueous phase) 1. Cellulose Derivatives • Hydroxyethyl Cellulose - HEC • Methyl Cellulose - MC • Carboxymethyl Cellulose - CMC

2. Acrylic Thickeners

3. Gum and Starch Derivatives (rarely used in the paint industry.) • Associative thickeners (interactive with other paint ingredients) 1. Hydrophobically modified polyether polyurethanes HEUR 2. Hydrophobically modified polyethers 3. Hydrophobically modified acrylic thickeners HASE 4. Hydrophobically modified cellulose ethers HMHEC

Comparison of thickeners used in solvent-based systems

Basic Properties of Organic Thickeners

In the literature, the different classifications of organic thickeners by manufacturer companies cause confusion. For example, organic thickeners and rheology modifiers are generally named as follows, and sometimes can be classified in this way.

NSAT and HASE are divided into two main categories:

• Low shear effect types that increase Stormer viscosity consistency, and • High shear effect types that increase ICI viscosity. High shear effect types show weaker effects in latexes. Therefore, a high-shear effect NSAT may not be the best choice for increasing ICI viscosity in high PVC / low latex content matte paint. However, if used at very high rates, results can be obtained. For stronger interaction, the use of low shear effect thickener additive will give better results. In low PVC / high latex content paints, a low shear effect thickener additive can interact very strongly as a primary thickener. In this type of paint, a high shear effect thickener would be a good option as a primary thickener. The basic thickener additives used to control the rheological properties of paints can be categorized as follows. • Associative Thickeners • Non-Associative Thickeners Interacting with Aqueous Phase

Associative Thickeners

1. Celluloses 2. Acrylics • Inorganic Thickeners 1. Clays • Swelling types • Nonswelling types 2. Fumed Silicas 3. Calcium Sulfonate • Organic Thixotropic Thickeners (includes celluloses and acrylics as well as Indian oil derivatives / polyamides)

Associative Thickeners

Associative thickeners can be characterized as hydrophobe - hydrophil - hydrophobe. In other words, a long hydrophilic center with two hydrophobic ends. One hydrophobic end will be attracted to the resin surface and the other end will play a role in the thickening mechanism. Here the other end can bind with another resin particle or with the hydrophobic end of another thickener molecule. Associative thickeners are water-soluble polymer-based polymers. These can be acrylic polymers, cellulose ethers or the highest quality nonionic products such as poly(ethylene glycol). They are coated with hydrophobic groups that are not water-soluble, such as fatty alcohols. In an aqueous environment or emulsion, these polymers form a network that increases viscosity. The water-soluble backbone polymer dissolves in water. Because the thickener interacts with the polymer surface, selection should be made with specific polymers in mind when choosing the appropriate thickener. Factors to be considered are: • Polymer particle size, • Polymer solids volume and • Polymer surface hydrophobicity. As polymer particle size changes, the total surface area can change significantly. This is very important in terms of associative thickeners. Since associative thickeners are surface-active, they work by binding with the surface of emulsion binder particles. In addition to the surfactant system, the particle size and size distribution of the emulsion can have significant effects on the efficiency and performance of thickeners. Associative thickeners provide excellent rub resistance and flow. Their anti-spattering properties in paint are a major advantage. However, they are sensitive to surfactants. Paints containing this type of thickener generally show lower viscosity at lower shear rates and higher viscosity at higher shear rates compared to paints using cellulosic thickeners.

The basic properties of associative thickeners are:

• They provide flow and surface smoothness, • They cause very little paint spattering in roller application, • They provide better sag resistance, • They generally exhibit better gloss than HEC, • They create higher film structure, • They are lower in cost than HEC, • They are generally easy to supply as liquids, • They are resistant to enzymes, • They provide single-coat coverage.

The basic problems of associative thickeners are:

• They are pH-sensitive, • They are sensitive to surfactant HLB, • They are sensitive to surfactant type depending on the total paint formula (anionic, nonionic, nonylphenol, octylphenol are commonly used surfactant types), • They are sensitive to latex particle size, • They are sensitive to film-forming (coalescent) type, • They are sensitive to glycol type and • They have greater water sensitivity. Associative thickeners are generally found in the market as a latex dispersion or as a viscous solution. Their types are given below.

1. HASE:

Hydrophobically modified acrylic thickeners, generally called HASE, are frequently used. HASE stands for hydrophobically modified "Alkali Swellable Emulsion".

2. HEUR:

Among the best thickeners used for rheological purposes are HEUR or, in expanded form, hydrophobically modified ethylene oxide urethane rheology modifier, known in this class. Sometimes they are simply called PU thickeners. These compounds provide excellent high film formation, smooth surface finish, reduced spattering and a non-agglomeration thickening mechanism. These types are nonionic and in most cases show weak sag resistance. Urethane-type associative thickeners form networks with themselves and binders and sometimes even with pigments.

3. HEURASE:

Due to certain deficiencies, HEURASE-type thickeners were developed. These alkali-soluble terpolymers are produced by emulsion polymerization of a carboxyl-functional monomer, a water-insoluble monomer, and a hydrophobic-terminated urethane-functional ethoxylate monomer. Different products and properties can be obtained by varying the ratio of the three monomers and the type of hydrophobe, degree of ethoxylation and the like. These materials are supplied to the market as aqueous emulsions of water-insoluble polymers. When a base such as ammonium hydroxide is added, the polymer swells, combines strongly with water to dissolve and consequently provides a thickening effect. HEURASE-type thickeners (water-soluble polymers) are relatively long-chained among polymers. Additionally, there are many carboxyl anions distributed along the polymer backbone that repel each other. Hydrophobic interaction provides the thickening mechanism in this type of polymer. The HEURASE thickener family can also be mixed with other thickeners, so the paint's rheology has quite unique properties.

4. HEAT:

Aminoplast Associative Thickeners: HEAT - also called hydrophobically modified ethoxylated aminoplast thickeners - is a new class of associative thickeners. The aminoplast linkage is made by using an aminoplast instead of a diisocyanate. In most cases, aminoplast linkage is more hydrophilic and more water-soluble than diurethane groups. The ability to add hydrophobe at very high levels is a special characteristic of aminoplast chemistry and allows the production of associative thickeners that show resistance to viscosity loss when glycols or surfactants are added to paint systems, such as during paint coloring with concentrated color pastes.

5. HMHEC:

Hydrophobically modified celluloses, abbreviated as HMHEC, are cellulosic thickeners. Along the backbone of the structure with hydrophobic modification in some branches, several long-chain alkyl groups are inserted. These molecules create viscosity increase through a combination of various hydrophobic groups. These paints have higher viscosity at high shear rates and therefore achieve better film formation and higher opacity.

Non-Associative Thickeners Interacting with Aqueous Phase

 

1. Cellulosic Thickeners (Celluloses)

Cellulose is a naturally occurring polysaccharide that can form many water-soluble ethers. Celluloses are nonionic water-soluble polymers. In the paint industry, the most commonly used thickeners are cellulosic thickeners. They have various types as follows. • Methyl cellulose (MC) • Hydroxyethyl cellulose (HEC) • Ethyl hydroxyethyl cellulose (EHEC) • Hydroxypropyl cellulose (HPC) • Hydroxypropyl methyl cellulose (HPMC) • Hydrophobically modified hydroxyethyl cellulose (HMHEC) Hydroxyethyl cellulose (HEC) is a water-soluble polysaccharide commonly used to thicken many matte and semi-matte construction paints. It is available in a series of different viscosity grades and paint thickened with HEC exhibits excellent colorant compatibility and storage stability. The paint properties dependent on the molecular weight of HEC are: • Thickening efficiency, • Flow and surface smoothness, • Spattering resistance, • Film-forming property and • Sag resistance. HEC and other non-associative water-soluble polymers thicken the aqueous phase of the paint and the resulting viscosity is related to the hydrodynamic volume of the water-soluble polymer (WSP) and the degree of chain entanglement of these polymers in solution. Cellulosic thickeners can be used alone or combined with other thickeners to achieve specific rheological properties. Cellulose ethers are produced at various molecular weights and in a range of viscosity grades. With 2% aqueous solution they can be produced from very low molecular weight with approximate viscosity from 10 cps to approximately 100,000 cps at the highest molecular weight grade. Low molecular weight grades are generally used as protective colloids in emulsion polymerization of paint latexes. The most common grades used as thickeners are between 4,800 and 50,000 cps. Various degrees of hydroxyl substitution also exist and therefore it is important that paint manufacturers be aware of various options and related effects when selecting thickeners of this type. This situation occurs through the effect of hydrogen bonding where the mixture actually initiates an increase and rise in viscosity. The effect here is quite strong. Most cellulose ether suppliers introduce surface-treated versions of their powder thickeners to the market to allow them to disperse in water before beginning to dissolve or hydrate. Without such a treatment, the outer layer of the particles will immediately begin to hydrate and gel when added to water, causing large agglomerates to form. Dissolution of these surface aqueous agglomerates will be difficult.

Surface treatment temporarily renders the surface insoluble and allows complete dispersion of the powder thickener before dissolution begins. The time between initial wetting of the powder thickener and the beginning of dissolution is called "hydration time". Hydration time depends essentially on two factors:

• Water temperature. The higher the temperature, the faster the hydration rate. • pH - Higher pH means accelerated hydration time. Conventional celluloses are high molecular weight polymers that provide thickening mainly through chain entanglement. At high shear forces, polymer chains are oriented, causing lower viscosity and shear-thinning behavior. When shear forces extract chains that are highly viscoelastic, the viscosity immediately increases rapidly upon recovery, which does not provide the desired level of smooth surface finish. Application properties such as brushing, film formation and roller spattering are directly related to thickener selection. Additionally, flow properties such as surface smoothness and sag resistance are greatly affected by the thickener. Hydrophobically modified celluloses (HMHEC) are cellulosic thickeners with hydrophobic modification in some branches. Here several long-chain alkyl groups are inserted along the backbone of the structure. These molecules create viscosity through a combination of various hydrophobic groups. These paints have higher viscosity at high shear rates and therefore achieve better film formation and opacity. HMHEC-type cellulosic thickeners provide good resistance to pigment settling, excellent spreadability, sag resistance, flow and surface smoothness combined with gloss. They allow a wide variety of application techniques and have no adverse effect on the film formation of small particle size latexes. Good film formation, good water and corrosion resistance and good gloss are produced. This special type of associative thickener has gained advantage with vinyl acetate copolymer systems. Performance is quite close to associative thickeners, but without formulation complexity.

2. Acrylic Thickeners (Acrylics)

Acrylic emulsion polymers are used as thickeners in some systems. Water-soluble acrylates are used for this purpose. These are polymers in acid or salt form. These types of soluble thickeners should be used with consideration that they will remain as part of the finished film and are always water-sensitive. Polyacrylic acids are used quite successfully as thickeners. The fact that they are thickening emulsions by adjusting the pH of the prepared paint tank is an important advantage of these. However, because they are pH-sensitive, the viscosity of the paint in the tank will change depending on any change in pH. When considering these characteristics, in the use of these types as thickeners, it should not be forgotten that in addition to system pH, they are also sensitive to many other additives entering the formula. When aramid fiber is added to paints, the degree of reinforcement will depend on the length of the fibers used and the amount of loading. Viscosity formation and sag resistance are not greatly affected by fiber length; however, reinforcement increases with fiber length and provides reinforcement without affecting thixotropy. Other factors such as moisture level and degree of fibrillation also affect fiber performance in paints. For water-based paints, wet versions of the fiber provide improved dispersion and application properties. When reinforcement is desired with minimal effect on viscosity, short fibers are recommended.   M.Namık Kayaalp Chemical Engineer Ecelak Boya ve Kimya San. Tic. Ltd. Şti.    

References 1. PCI, Paint and Coatings Indutry, Additives Handbook, 2013 2. Newton, D.S. (Ed.), Paint Technology Manual Part 4, Chapman and Hall, London 3. Paint Formulation, J.Boxal, Geoge Godwin Ltd.-Londo London 1980 4. Additives for Coatings, Johan H. Bieleman, WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000