The Role of Surfactants in Aqueous Pigment Dispersion
Surfactants are critical components of aqueous systems because they provide a wide range of performance attributes. In aqueous pigment dispersions such as paints or colorants, surfactants must facilitate the milling of pigment and provide adequate stabilization of the dispersed pigment, while at the same time ensure letdown compatibility and optimal application performance. Rarely is just one surfactant used in a pigment dispersion; more often, two or three surfactants are added in order to achieve specific performance and process attributes. A formulator must balance these requirements, managing the complex interactions between the system components to avoid the frustrating cycle of adding one surfactant to resolve one deficiency only to cause a new problem to tackle. This article will review the basic types of surfactants commonly found in an aqueous pigment dispersion and provide guidance on when and how each is best employed to minimize formulation development and troubleshooting.
Understanding Surface Active Agents
Aqueous dispersion is the stabilization of insoluble solids in an aqueous medium using surface active agents. Classically this process is described in three steps:1
- Wetting of the dry solid;
- Milling to optimal particle size;
- Stabilization of particles.
In aqueous dispersion, dispersants are the enabling surface active chemistry, but additional surfactants are widely used and known to significantly impact the dispersion process as well as the formulation performance. The key is to understand how surfactants are used most effectively.
The word surfactant is a general term that encompasses all surface active agents. The language used within industry is not always clear and can lead to communication issues in discussions between formulators and suppliers. Surfactants are often described based on their benefit within a system as opposed to the core functional role they may offer; therefore, the same chemistry may be described in a variety of ways to different audiences. A simple alcohol ethoxylate may be called an emulsifier, co-dispersant, lubricity aid, foamer, wetting agent or a surfactant, depending on the nature of the conversation. Understanding the specific role that a surfactant is required to perform within a complex system is often open to interpretation. Even when working with a surfactant of known chemistry, a formulator who is trying to understand the impact of that surfactant within a fully formulated system must rely on a substantial amount of guesswork and trial and error. Recognizing that a formulator is faced with both known and unknown additive chemistries, it is critical to categorize these in very general terms based on their core functionality. In aqueous pigment dispersion, there are often three key categories of surface active chemistries: dispersants, stabilizing surfactants (often termed co-dispersants or grind aids) and nonstabilizing surfactants (often termed wetting agents). Each of these will be described, and additional aspects of the role of stabilizing surfactants will be reviewed.
Dispersants are the enabling surface active agent in a pigment dispersion. Designed to function at the pigment/water interface, they provide two key attributes: affinity for the pigment surface and a stabilizing force to keep particles separated. There is a very wide range of dispersants available for aqueous dispersion, however many of the chemistries are trade secrets and leave formulators with little option beyond extensive trial and error testing. Although time consuming, this work is very well spent because selecting an optimal dispersant often minimizes the need for further formulation. Where this is not viable or where inventory minimization or cost concerns drive dispersant selection, formulation with other surfactants becomes a very valuable tool to improve both performance and process.
Dispersants typically fall into two general groups: commodity polymers and high-performance dispersants. Commodity polymers are typically based on lower-cost monomers and polymerization processes, have higher molecular weights and often provide electrostatic stabilization. The identification of an optimal commodity dispersant will often provide a formulator with adequate dispersion stability but may result in suboptimal pigment performance and challenges in the dispersion process. High-performance dispersants are typically based on specialty monomers, more complex production processes, lower molecular weights, and commonly steric or electrosteric stabilization. An optimal high-performance dispersant will often provide a formulator with exceptional dispersion and letdown stability with adequate color development and milling efficiencies. A comparison of the relative performance of these two classifications of dispersants can be seen in Figure 1.
For many formulators, finding an optimal high-performance dispersant will often minimize the amount of further formulation work needed. Dispersant cost can be an important factor in determining the time investment necessary to optimize a system based on a commodity dispersant. When formulation is necessary, the use of additional surfactants is often the next step. In Figure 1, the key attributes needed to meet each performance criteria are shown in italics. For example, a commodity dispersant polymer that shows weak performance in milling efficiency or color development is often best formulated with a secondary additive that can provide additional interfacial tension reduction or provide dynamic stabilization – a lower-molecular-weight, stabilizing chemistry that can facilitate faster particle size reduction. Alternatively, if color development and milling are adequate, sometimes a small amount of a wetting surfactant is all that is necessary to improve pigment cut-in and reduce any interfacial tension gradients that may aggravate letdown shock. Formulation is a valuable tool to improve performance, but it must be stressed that dispersant selection is the first and foremost priority. Identification of an optimal dispersant often minimizes both formulation work and additive usage.
When formulation is necessary, both stabilizing and nonstabilizing surfactants are often used to optimize the properties in Figure 1. Three different types are typically found in aqueous dispersions.
1. High-Hydrophile Lipophile Balance (HLB) Stabilizing Surfactants
Surfactants based on highly ethoxylated structures, ionic character or a combination of the two are often employed to improve stabilization characteristics of an aqueous dispersion. Dispersion stability, color and viscosity stability with aging, as well as letdown compatibility are often the attributes best improved with stabilizing surfactants. Care must be taken as these materials are typically foamy and often difficult to utilize in the dispersion process. They are also emulsifying and can negatively impact other formulation components such as associative thickeners. Impact on defoamer usage, formulation rheology and coating water resistance are common problems experienced with overdosing of these materials.
2. Mid-HLB Stabilizing Surfactants (Grind Aids or Grind Surfactants)
Surfactants based on mid-HLB ethoxylates are very common in dispersion applications, and materials like nonylphenol 9 EO and similar surfactants were, for many years, the standard chemistry in this grouping. Nonionic surfactants of 8-15 EO blocks and an HLB of 10-14 provide a degree of stabilization that can improve letdown compatibility, milling efficiency and small particle stabilization with far less of the foam and water sensitivity issues found with anionic or higher-HLB nonionic stabilizers. They are often employed in the dispersion process, as foam is typically manageable. Overdosing can still result in similar problems with water resistance, foam and rheology, but mid-HLB surfactants are often more forgiving than higher-HLB stabilizers. Surfactants in this category are often described as grind aids or grind surfactants.
3. Wetting Agents
Surfactants based on low-HLB, minimally or nonethoxy-lated structures are often employed as wetting agents. These types of surfactants are often nonmicellar with no stabilization attributes. They are used solely for interfacial tension reduction with benefits in dry pigment deaeration and reduction in letdown shock.
Formulation direction should stem from the deficiencies encountered with the dispersant of choice. For stabilization concerns, the use of co-dispersants and higher-HLB surfactants is warranted if it is no longer possible to conduct further evaluations to find a better dispersant. Usage should be minimized, and a ladder study is always the best approach to find the minimal use level to avoid new problems.
Often it is suitable to try a grind surfactant instead. The stabilization benefits of a lower-HLB surfactant may be adequate to resolve the dispersant deficiencies and typically provide additional process benefits that a co-dispersant may not. With any stabilizing surfactant it is critical to reserve its usage and evaluation until after the dispersant is selected. The inclusion of surfactant as routine during dispersant evaluation often will lead to false negatives. Figures 2 and 3 show general performance trends with a grind surfactant.
Figure 2 shows a generalized representative of the types of performance differences seen in the evaluation of different dispersants. Often the dispersants will show different levels of achievement of the pigment performance attributes such as color, gloss, hiding or particle size, relating to the suitability of the dispersant’s chemistry and characteristics for the pigment and formulation. Ideally the top dispersant is selected, but other factors may drive selection of a slightly lower-performing dispersant. The use of a grind surfactant can significantly alter the apparent performance of a dispersant, as seen in Figure 3. Grind surfactants will typically provide faster pigment particle size reduction during milling and dispersion,2 resulting from better wetting of the dry pigment, as well as improved dynamic stabilization attributes; however, final performance attributes can vary significantly. Both improved and reduced performance is possible. This creates significant value in proper formulation but presents a danger to a formulator. A stabilizing surfactant should very rarely be used when evaluating dispersants because false negatives could result in lost opportunity and unnecessary work. Only after identification of the preferred dispersant should further formulation be conducted.
Figure 4 shows the results of an experiment with the dispersant of an orange pigment (PO 5), a commodity acrylic dispersant with and without two different grind surfactants. While both grind surfactants show initial benefit, Grind Aid 2 caused a significant reduction in color development compared to the blank with no surfactant, while Grind Aid 1 showed significant benefit.
In formulations with additional surfactants, use level is also a very important consideration. Ladder studies to identify the optimal use level are very important, as overdosing of surfactant can cause secondary issues like foam and water sensitivity. More is not always better, and it is common to find a level where additional surfactant provides no further benefit. Figure 5 shows this in the evaluation of a phthalocyanine blue (PB 15:3) pigment with a commodity acrylic dispersant and two different grind surfactants across a range of 0 to 2.25 wt% use level. Both Grind Aids A1 and A2 appear to achieve optimal utility at a loading of 1.5 wt%, and additional loading provides no further benefit. The performance of Grind Aid A1 is significantly better, even compared to a 2.25 wt% loading of Grind Aid A2. Performance is driven by the match of surfactant to dispersant, and a poor match cannot be improved with additional additive.
Optimization of an aqueous dispersion is commonly achieved by using surfactants, but as the data in the figures suggests, it is not a straightforward endeavor. Dispersants are surfactants, and it is reasonable to expect that the addition of additional surfactants may involve interaction, both synergistic and antagonistic, with the primary dispersant. The former creates opportunity for the formulator, but the latter creates problems and unnecessary work. It is critical that any formulation be conducted with the awareness of the potential issues, and, ideally, the addition of other surfactants should be left until after the dispersant has been selected. It is often far easier to identify and optimize the use of additional surfactants than it is to identify a dispersant.
Surfactants are both the enabling chemistry as well as valuable formulating tools in aqueous pigment dispersion. The use of these materials should always follow the identification of the primary surfactant – the dispersant. The pairing of the dispersant to the solids to be dispersed and the chemistry and performance needs of the formulation will determine what deficiencies exist and what types of additional surfactant may be necessary. Commodity dispersants often require a higher degree of formulation than high-performance dispersants, but both classes are typically improved with additional surfactant. Regardless of the type, the dispersant should always be evaluated with the use of no or minimal additional stabilizing or grind surfactants, as interactions and performance impact can be significant. Identify the dispersant first, evaluate what type of additional surfactant may be necessary, and use a ladder study to determine the minimal usage necessary to achieve the desired performance.
The author would like to acknowledge the work of Mike Pauley and Timothy Smith of Air Products and Chemicals, Inc.
1 Parfitt, G.D. Dispersion of Powders in Liquids, Elsevier Science, New York, 1969.
2 Winkler, J. Dispersing Pigments and Fillers, Vincentz Network, Hanover, 2012.