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Formulating optimized aqueous dispersions using: surface active additive triangulation.


The formulation of aqueous pigment dispersions is enabled by the use of surface active agents to wet, disperse, and stabilize solid particles or pigments in water as well as to provide letdown compatibilization, defoaming, and application performance. Typically, formulations contain at least two, and often three or more, surface active components that are combined to provide the optimal properties. These different components can provide synergistic benefits, but, just as often, they can function antagonistically, requiring extensive trial and error testing by a formulator to identify a final formulation. Understanding the functionality of the different surface active components is a critical first step in streamlining formulation development, minimizing development work, and effectively trouble-shooting performance hiccups.


A complicating factor in this understanding is the proprietary composition of many additives and pigment surface chemistries. The absence of this important information severely inhibits a formulator's ability to apply both theory and experience from one formulation to the next. "Black box" testing and trial and error become a leading method of product development and require a significant amount of work to be done to develop every formulation. In spite of this limitation, there do exist ways to streamline this work. Although exact additive chemistries may be trade secrets, general chemistry classes and attributes are often shared by suppliers, and this information enables some general guidelines to be proposed. The form and function of the different surface active additives can be described generally and their probable interactions predicted, thus providing a pathway for elimination of low-value experimentation and faster formulation development.


The role of surface active agents in an aqueous dispersion is multifaceted. These additives are the enabling technology in the stabilization of the solid pigment; they are not only essential in making the dispersion, but they also help optimize the milling process, compatibilize the dispersion in letdown, and optimize handling properties. The primary functions and contributions of surface active chemistries in a dispersion generally can be broken down as follows:

* Wetting of the air-solid, air-water interface to enable proper deaeration of dry pigment, eliminating the detrimental effects of entrapped air on both milling efficiency and foam,

* Dynamic stabilization of newly created liquid-solid interfaces to optimize milling efficiency and color development,

* Stabilization of the dispersed particles for optimal dispersion shelf life and handling as well as the necessary performance in applications,

* Compatibilization of the dispersion for letdown into other systems and minimizing of shock and related issues, and

* Controlling foam during processing and application.

Stabilization of the particles is the most critical function in dispersion preparation; however, the other properties are also required for optimal performance. This contribution is particularly evident when considering that a dispersion is typically not just a formulation; it is a process as well. Wetting of the dry solid may be of little importance in the use of the dispersion in its final application, but it is a vital necessity in the initial dry pigment incorporation and is essential for milling efficiency. The dispersion process is classically described as three unique steps that are required to obtain a stable dispersion (1):

* Wetting of the dry solid,

* Milling of the solid to optimal particle size, and

* Stabilization of the solid. For many dispersion end uses, though, it is appropriate to recognize a fourth step:

* Letdown or formulation into application.

Application performance is a critical step that is recognized in dispersion performance evaluations, but it is often not considered as a fundamental step in dispersion preparation. When considering the surface active chemistries used in the optimization of the process and dispersion stabilization, it is necessary to consider also how they will impact the final application. Therefore, these four steps become intertwined during the development of a dispersion formulation.

The complete process requires a specific mix of contributions from the surface active chemistries used in formulation of the dispersion. The ideal situation would be one in which a single additive serves all functions. However, as will be described later, the attributes required in one step are often at odds with the attributes needed for another, making multicomponent formulation a critical necessity to meet all performance targets. When matching up the dispersion steps with the functional contributions of the surface active additives (Table 1). it can be seen that, while many functions/attributes overlap, the criticality of the contributions vary with each step.
Table 1--Relationship between Dispersion Process Steps and
Surface Active Additive Functions

 A) surface B) Dynamic C) Robust,
 tension mobile static
 reduction stabilization stabilization

1) Wetting of Critical Helpful
the dry solid

2) Milling to Helpful Critical Helpful
reduce particle

3) Helpful Critical
of trie

4) Letdown and Critical

 D) E) Foam
 Compatibiization control

1) Wetting of
the dry solid

2) Milling to Critical
reduce particle

3) Helpful
of trie

4) Letdown and Critical

Additives used to enhance performance in one step will often have little to no impact on other steps. This is well understood with defoam-ing additives, which are critical in the high-energy milling process, but are often detrimental in later steps, particularly because they can promote defects in the final application. (2) Somewhat less evident is the use of surface active additives for surface tension reduction and dynamic stabilization during these steps. These additives can have significant downstream impacts, both synergistic and antagonistic, on the stabilizing chemistries and in letdown compatibility. Developing a functional understanding of these interactions is essential to formulation optimization.


The types of surface active chemistries used in formulation development can vary widely, but there are common chemistries and attributes that are useful in building general classifications. These will not fit every surface active additive found in use; however, the vast majority of surface active additives will fall into these categories.


Dispersants are the stabilizing agents in a pigment dispersion and are commonly based on polymeric, and often complex, structures. As such, they are typically comprised of proprietary chemistries that present significant challenges for formulators if complementary chemistries are being sought. Dispersants can be grouped into categories based upon general functionality that can help guide appropriate formulation.

Ionic Polymers: The majority of dispersants in aqueous dispersions are based on inexpensive anionic polymers. Acrylic acids, naphthalene sulphonates, and other commodity polymers are commonly used. Their chemistries and suitability for specific pigments, solids, or applications may differ, but their attributes are generally similar. The polymers are typically of higher molecular weights and are slow to orient at interfaces, offering little to no wetting or dynamic stabilization. However, anionic polymers offer strong stabilization characteristics at equilibrium. The stabilization is ionic and the resultant dispersions are typically prone to having higher viscosities and lower dispersion solids limits; they are also susceptible to letdown incompatibilities and shock. Grind resins and binders used in pigment dispersion are a specific class of stabilizing polymers that provide additional letdown benefits in film formation. They serve the role of dispersants in a pigment dispersion and they will be referred to as dispersants in this discussion.

High-Performance Dispersants: The chemistry types encompassed by this group vary significantly, but, in general, high-performance dispersants differ from commodity dispersants in several ways. The high-performance dispersants are lower in molecular weight and offer greater mobility. While still poor for wetting, they do provide better efficiency during milling. High-performance dispersants also typically employ ethylene oxide chains to provide nonionic steric stabilization characteristics. Either in comb structures or in block groups, this nonionic functionality helps to reduce dispersion viscosities, allowing increased solids loadings, and to reduce sensitivity to letdown incompatibility and shock.

Nonionic, Surfactant-like Dispersants: This class of dispersants is comprised of lower-molecular-weight A-B- or A-B-A-type structures that contain a large ethylene oxide chain block that provides nonionic steric stabilization. While offering low-dispersion viscosities, dynamic stabilization during milling, and excellent letdown shock resistance, this class of dispersants does not provide strong static stabilization and is often recommended for use in resin- or binder-containing systems or in combination with another anionic polymeric dispersant. This group of dispersants overlaps with traditional stabilizing and emulsifying surfactants and shares many of the characteristics offered by high-HLB ethoxylated surfactants.

These three classes of dispersants can be viewed as a continuum of characteristics, as shown in Figure 1, where their relative performance attributes are mapped.


In general, dispersants are designed for stabilization. The attributes that optimize that performance characteristic include a higher degree of ionic character, higher molecular weight, and strong intermolecular interactions. These attributes are often not beneficial in optimal surface tension reduction or dynamic behaviors in wetting and stabilization, reducing their benefits in the other stages of the dispersion process. The higher-molecular-weight anionic polymers, like grind resins and commodity copolymers, tend to be poorest for dispersion process benefits. Although suitable for stabilization, they are most often formulated with surfactants to improve the other attributes. Higher-performance dispersants are typically designed to improve upon commodity polymers, incorporating better stabilization, and also enhancing benefits in the other steps of the process. Still, they are often improved through formulation with surfactants, but to a lesser degree than are the commodity dispersants. Lower-molecular-weight dispersants, typically modified ethoxylates, block polyglycol copolymers, and similar chemistries tend to have the most surfactant-like performance. Wetting and milling process attributes can be improved with formulation, but this is often of minimal necessity. These types of dispersants, however, are typically weak in stabilization and are often recommended as co-dispersants to be used jointly with anionic polymers in resin-containing or binder-containing dispersions. (3)


The surfactants commonly used in formulation also lend themselves to similar characterization because they fall into distinct groupings based on functionality.

Dynamic Wetting Agents: These surfactants are characterized by a low HLB and a structure that offers minimal intermolecular and intramolecular interactions. As such, they provide negligible stabilization, have minimal interactions with the dispersant or other surfactants. and are nonmi-cellar. They provide efficient reduction of surface tension, excellent wetting behavior, and little other impact on the dispersion. Acetylenic diols are the classic chemistry used for this performance, but other proprietary chemistries can be used to good effect. Wetting of the dry pigment is critical to eliminate trapped air and prepare the dispersion for milling (Figure 2). (4)


Ethoxylates: These surfactants, a classic formulation tool in aqueous pigment dispersion. are based on ethoxylation of an alcohol or other hydrophobic moiety. They provide basic steric stabilization benefits in a dispersion. but, more importantly, they also offer the dynamic stabilization attributes that enhance milling efficiency. These additives often engage in moderate intermolecular and intramolecular interactions, providing strong interactions with other system components. This can result in synergistic benefits, but selection is critical to avoid competitive behaviors with the dispersant chemistries. These additives also vary significantly with the level of ethoxylation, or HLB. At high HLBs, 16+, they begin to function similarly to nonionic dispersants by providing greater benefits in stabilization characteristics. At lower ethoxylation levels (HLBs of 12-15), the benefits in milling efficiency and letdown compatibilization are dominant. At even lower HLB values, ethoxylates begin to function more similarly to wetting agents. Ethoxylated alkylphenols (APEs) are the classic chemistry used; however, with the growing efforts to eliminate APEs, newer chemistries are available that offer equivalent or better performance attributes.

The surfactant continuum is best related to eth-oxylation level or FILB and is mapped in Figure 3 in a similar manner to that of dispersants (Figure 1), by looking at the range of attributes they offer.


The formulation of these various additive types can be challenging, particularly as many interact strongly with other additives. Finding the right combination can involve significant work, but mapping out their properties in this way allows us to create some tools that may make this easier. Both the dispersant and surfactant continuums share similarities and, in particular, they overlap significantly on the ends of the continuums where nonionic dispersants and high-HLB ethoxylated surfactants serve very similar functions. In many cases. the form and function of a high-HLB ethoxylated surfactant and a nonionic dispersant are indistinguishable. If we join these ends and view these continuums in relationship to one another, a visual tool can be created to serve as a guide to formulation. In Figure 4, the visual tool takes the shape of a triangle, where each of the points represents the core attributes necessary for the steps of dispersion discussed earlier.


Higher-molecular-weight ionic polymers provide basic stabilization for flocculation control and stability. Lower-molecular-weight nonionic dispersants, and/or high-HLB ethoxylated surfactants, provide dynamic stabilization characteristics optimal for milling benefits as well as compatibilization in letdown. Dynamic wetting agents provide surface tension reduction and efficient wetting of pigment surfaces. These three attributes can be combined to provide optimal performance for both dispersion and process. It may not always be easily apparent that one can visualize dispersions in this manner: however, typical formulations do reflect these exact conibinations of additives/attributes. Paint grinds seen in many published starting point formulations are often comprised of an acrylic acid polymer dispersant (equilibrium stabilizing), alcohol ethoxylate (dynamic stabilizing), and an acetylenic diol (dynamic wetting). Resinated colorants also commonly follow a similar path, utilizing an anionic acrylic or styreneacrylic resin, a nonionic co-dispersant, and a wetting surfactant.

The points of the triangle represent the three fundamental attributes necessary for an optimal dispersion, but often additives are composed of balanced properties. High-performance dispersants commonly offer a mix of equilibrium- and dynamic-stabilizing attributes falling on the right-hand line somewhere between the points of the triangle. Further formulation of a high-performance dispersant is less critical than a high-molecular-weight commodity dispersant, but, nonetheless, the triangle offers guidance here as well. To reach the center of the triangle for optimal performance, a high-performance dispersant is often best paired with only a dynamic wetting agent.

Grind surfactants, specifically designed for dispersion applications, typically offer a mix of surfactant attributes spanning both wetting and stabilization characteristics. Similar to high-performance dispersants, grind surfactants fall on the line between the points of the triangle. Formulations with ionic dispersants are often served well by formulation with a grind surfactant to achieve the combination of all three key attributes, particularly in terms of milling efficiency and optimization of the pigment performance properties, such as color development. Figure 5 describes the benefits typically contributed by a grind surfactant in a milling operation when combined with a static stabilization dispersant like a grind resin.



The first step in the formulation of a dispersion is always the selection of the stabilization chemistry--the dispersant--based on cost-performance decisions, the pigment chemistry to be dispersed, and the end-use application needs. Supplier recommendations are critical to facilitating this step, and many suppliers offer guides for their customers to help them find the correct dispersant. Experimentation with the recommended dispersants is necessary to find the top candidates for final formulation development During the experimentation, it is critical to remove all other surfactants, as they can mask performance deficits or adversely impact dispersant performance. Basic formulation testing for flocculation prevention, color stability, and aged stability are the leading criteria for dispersant performance.

Once the leading dispersant candidate(s) are identified, the next step is to approximate where the additive falls on the triangle. Is it a commodity anionic polymer, an electrosteric comb polymer, or a formulated product? Most suppliers will share enough general chemistry information to allow an estimate of where it falls on the triangle. Based on this approximation, a formulator can use the triangle to get a jump-start on what attributes can be added via formulation to optimize the dispersion's performance. In Figure 6, the triangle is redrawn with the general chemistry types that provide the three performance aspects necessary for an optimal dispersion. When evaluating the chemistries selected and used in each formulation, it is important to consider what contribution each makes in the dispersion process and match up complementary additive types to maximize performance and limit cost.


Formulating Strategies for Optimizing a Grind Resin or Commodity Anionic Polymer

By way of example. if the dispersant selected is an anionic polymer or a grind resin, the following choices can be considered to reach the optimal performance in the center of the triangle. A nonionic co-dispersant can enhance stability and letdown compatibility, but an additional low-HLB, dynamic wetting agent will also be necessary for adequate wetting (Figure 7a). Alternatively, a choice directly across the triangle, a grind surfactant (Figure 7b), will offer balanced performance and allow a single additive solution.

[figure 7a omitted]

[figure 7b omitted]

Formulating Strategies for Optimizing a High-Performance Dispersant

If the dispersant selected is a high-performance product or an electrosteric polymer, the choices become different. Using an additional anionic polymer or a nonionic co-dispersant will often have minimal benefit and overlapping performance. A grind aid will serve to enhance wetting and milling efficiency, but a dynamic wetting agent may be all that is typically needed (Figure 8).



Aqueous pigment dispersion development is a time-consuming endeavor. There is significant theory and literature available to the bench chemist that can guide development, but its value is hindered by the proprietary nature of many of the components and the complexity of the formulations. A general overview of the functions of basic surface active additives in pigment dispersion may help streamline the process by eliminating trial and error testing of additives with overlapping or competing performance attributes. The triangle format of the key surface active additive contributions offers a method to expedite formulation development using publicly available product characteristics. Although nothing can eliminate the need to test and evaluate formulations in the lab, this new approach should help reduce the workload necessary to reach the desired result. CT


The author wishes to thank Wim Stout and Rick Cuddeback, both of Air Products and Chemicals, Inc., for their contributions to this work.


(1.) Parfitt, G.D., Dispersion of Powders in Liquids, Elsevier Science, New York, 1969.

(2.) Snyder, J.M., Reader, CJ., and Hegedus, C.R., Ink World, 17 (5), 70 (2011).

(3.) Patton, T.C., Paint Flow and Pigment Dispersion, John Wiley & Sons, New York, 1979.

(4.) Rosen, M.J., Surfactants and Interfacial Phenomena, John Wiley & Sons, New York, 1989.


K. Michael Peck, Air Products and Chemicals, Inc., Allentown, PA;
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Author:Peck, Michael
Publication:JCT CoatingsTech
Date:Mar 1, 2014
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