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Coatings Ingredients
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Coatings Ingredients
Pigment dispersion : Wetting & Dispersing Agents Selection

The Ultimate Guide to Selecting Wetting & Dispersing Agents

Bright, vivid & durable colors along with high durability are one of the most important parts of the coatings’ performance. Formulators use pigments or fillers to bring desired color & performance in the paints. But keeping these solid particles stable in a liquid is a complex mechanism which includes appropriate: wetting, dispersing and stabilizing of solid particles in the system. Enhance your coatings quality while lowering down the production cost by:
  • Understanding key characteristics of dispersion, and
  • Learning the criteria for selecting the most suitable wetting or dispersing agent for your coating formulation.

Overview

Importance of Pigment Dispersion

Importance of Pigment Dispersion

The dispersion process is crucial throughout coatings manufacture, storage, and application. Proper dispersion influences many parameters, such as:

  • Appearance (color strength, transparency, gloss, etc.), and
  • Paint physical properties (rheology, stability flocculation, etc.)

Solving your problems at manufacturing stage can dramatically reduce the final paint quality. Understanding this process is a great help to prevent many coatings defects.

The clear understanding of stages of dispersion process - wetting, separation and stabilizing – helps you win the half battle already. You can go in-depth to understand the basics of dispersion here.

Dispersion and Stabilization of Pigments and Fillers - Essential Concepts


Influence of Grinding Liquid Medium


In addition to the characteristics of 3 stages of dispersion, we must take into consideration the influence of the grinding liquid medium as well.

  • In solventborne systems, the wetting and dispersing agent must be soluble in the grinding liquid medium. Solubility and solvent polarity are important parameters to check.
  • In waterborne systems, the liquid phase is quite polar (due to the water) and together with the solubility, the pH is also an important parameter to check.

Now, once you understand the underlying science behind the root causes w.r.t dispersion instability, deciding if you should adjust your ingredients or pigment dispersion process is easy. In this guide, we explain to you how dispersants work and provide you with tips and guidelines for selecting the best one for your system.


What is dispersant and how does it work?

What is dispersant and how does it work?

Dispersant or dispersing agent is defined as a surface-active chemical that has a solvating action on the solid particles (pigments or fillers) to be dispersed and thus promotes formation of dispersion by dispersing or suspending them. Dispersants help maintain a state of dispersion by preventing settling or aggregation.

Dispersant molecules must adsorb strongly at the surface of the particles. This adsorption process is often referred to as anchoring, and the groups, sticking to the surface, are called anchoring groups*.

Anchoring Process
Anchoring Process of Dispersant Molecule

Apart from being strongly adsorbed at the surface of the solid particles, the dispersant has to give colloidal stability. Dispersion is said to be stable from colloidal point of view when flocculation of separated particles is prevented because the particles repel each other.

*Functional groups, such as amine, amides, sulfonate, phosphate, for firm anchoring onto pigment surfaces are present in the polymer structure3. Amine-functional anchoring groups are efficient in apolar systems and interact with the pigment surface through hydrogen bonding, dipole or London-van der Waals interactions. Acid or amid functionalities are typically preferred as anchoring moiety for various aqueous applications.

Our Coatings Selector features a full range of dispersing agents available today for all types of coating and ink formulations (aqueous, solvent-based, high-solid or 100% solids systems and for pigment concentrates). You can check tech profile for each product, ask for samples or discuss your case with producer’s tech staff.


Different Types of Dispersing and Wetting Agents

Different Types of Dispersing and Wetting Agents

There are many dispersing and wetting agents chemistries today in the coatings market, nevertheless we can classify them as below.
  1. Conventional Wetting and Dispersing Agents (High Mw, Low Mw)
  2. Polymeric Wetting and Dispersing Agents
  3. Ionic and Non-ionic Dispersing Agents

Conventional Wetting and Dispersing Agents


Mainly low molecular weight, they are based on polyesters, polyamides, polyglycols and fatty acid chemistry (FAME). They have general characteristics as listed below:

  • Surfactant effect, reduction of solid / liquid interface surface tension
  • Anchoring groups adsorbed at the pigment surface
  • Good compatibility with the media
  • MW = 500 ~ 2,000 g/mol

Other key features of this type of dispersants include:

  • Excellent wetting power 
  • Grinding / dispersing time reduction
  • Anti-sedimentation
  • Effective against flooding and floating
  • Action mode: Mainly electrostatic, few steric hindrance
  • Recommended for inorganic materials and waterborne systems suitable for organic pigments

Surfactant type - They have an excellent compatibility and excellent water dispersibility. They offer many alternatives to replace the APEO products (alkylphenol ethoxylated). Reducing the surface tension improves the wetting process.


High molecular mass dispersants (Mw approx. 5000-30,000 g/mol) – They are most widely used in industrial paints. As compared to low molecular mass dispersants, they typically provide:

  • Superior performances,
  • Workability,
  • Best coloristic properties,
  • Gloss,
  • Film transparency,
  • Film integrity,
  • Low risk of being extracted from the dried film, and
  • Minimal side effects.

High Versus Low Molecular Mass -based Dispersants


High molecular mass-based additives tend to be more system specific as compared to low molecular mass dispersants and a careful selection and evaluation procedure is required.

Oligomers of medium high molecular mass (Mw around 1000-2000 g/mol) typically show widest range of compatibility and superior (fast) pigment wetting properties in comparison to high molecular mass products, whereas still outperforming low molecular mass products in dispersion stability and film consistency properties.

With few exceptions, monomolecular surfactant-based additives are less commonly used as dispersing agent. This is because, this group typically is inferior in contributing to dispersion stability and has high risk on effecting film properties, such as:

If you struggling at achieving the desired characteristics - hardness, resistance to water and solvents or adhesion even you have the right chemistry, then it may come from an overuse of dispersants. Take this quick tutorial to save on troubleshooting and understand if you are using optimum level of dispersants (and hence, lower formulation costs).

Free Tutorial - Dispersant: How to Quickly Find the One You Need?


Polymeric Dispersants and Wetting Agents


Main dispersing agents that are polymeric include polyacrylates, polyester, polyether or polyurethane-based systems. The classification of polymeric types wetting agents and dispersants is based on their:

  • Anchoring mechanism 
  • Chemical structure (polyacrylic, polyurethane, copolymer…), and
  • Molecular weight

This type is also influenced by the polymer design (linear, branched, star designed) and the polymerization process (controlled polymerization process types offer high performances products but are also more expensive). Their key characteristics include:

  • Polymeric type: many anchoring groups
  • Large choice of chemistry
  • Large choice of polymer design and molecular weight
  • Mw = 5,000 ~ 50,000 g/mol

Further, polymeric wetting and dispersing agents offer several benefits as listed below.

  • Excellent wetting power
  • Grinding / dispersing time reduction
  • Very effective for the long term stabilization
  • Action mode: steric hindrance
  • Polyvalent family (waterborne, solventborne, organic or inorganic material)

Polyacrylic Acid Based / Polyacrylates - Polyacrylic acid-based dispersants are usually lower in molecular weight (and also in cost) in comparison with the other structures. They are particularly recommended in waterborne coatings to increase the pigment load of inorganic material. Very nice cost effective product. Ammonium and sodium salt are typical products for latex paints - higher in molecular weights, they can offer a better compatibility.

Polyurethanes - They are excellent for the millbase viscosity reduction. As a consequence, PU dispersants enhance the pigment load and reduce the dispersing time. The flexibility of this structure (backbone, branched chains, anchoring groups) allows the design of various structures for many solventborne and solvent-free systems.

View the Full Range of Polymeric Dispersing and Wetting Agents Available today »

Controlled Polymerization Technology (CPT)/ Living Chain Growth


This polymerization process allows the manufacturer to make very fine adjustment on the polymer chain, which is not the case with the classical step-growth process (condensation polymerization is a random process).

Wetting and dispersing agent polymerized with this process are very similar batch to batch, which is not the case of classical condensation where the molecular weight can vary significantly from one batch to the other. Very effective but more expensive products.

The table below compares the properties of conventional and polymeric wetting / dispersing agents.

Property Conventional Polymeric
System Waterborne ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Solventborne ⭐ ⭐ ⭐ ⭐ ⭐
Pigment Organic ⭐ ⭐ ⭐ ⭐ ⭐
Mineral ⭐ ⭐ ⭐ ⭐ ⭐
Electrostatic Stabilization High Low
Steric Hindrance Stabilization Low High
Pigment load Low - Medium High
Final pigment paste quality Low - Medium High – Very high
Versatility Medium High
Price Low - Medium High – Very high

Pigment dispersion & stabilization root causes


Ionic and Non-ionic Dispersants


For use in waterborne coatings, anionic charged and non-ionic dispersing agents can be considered. Excellent wetting and dispersion performance in mill-bases for dispersion paints can be achieved using a combination of sodium- or ammonium-polycarboxylate and polymeric non-ionic surfactant additive. A main non-ionic additive is alkyl phenol ethoxylate (APE) and more precisely nonyl phenol ethoxylate, NP 10 (ethylene glycol chain of 10 units). Due to toxicity concerns, NP 10 is being replaced now with an APE free non-ionic, possessing same HLB-value.

HLB stands for hydrophilic lipophilic balance and is used as indicative value for comparing nonionic surfactants from different hydrophobic nature. The HLB value can be calculated from the percentage hydrophilic components in a molecule, divided by 5.

Effect of the dispersing agent on color development PBk in a stoving enamel
Effect of the Dispersing Agent on Color Development PBk in a Stoving Enamel.
High molecular Mass Dispersant (L), Right Low Molecular Mass Dispersant (R)

Related to the high degree of ionic dissociation in water, applying combinations of anionic and cationic dispersants in aqueous systems should be avoided; reaction between the anionic and cationic products may result in insolubility and changed surface activity.

Cationic, amine functional dispersants are successfully used in solvent-borne systems, for instance to support the wetting and dispersing process. Due to the low degree of dissociation the effect of the electronic charge is less evident in a-polar systems.

Find the Suitable Surfactant Chemistry Offering Powerful Wetting Characteristics »


Select the Right Dispersing Agent for Your Formulation

Select the Right Dispersing Agent for Your Formulation

Selecting the best wetting and dispersing agent for a system may look complicated first, but many clues can orientate our choice. Then, through a serial of simple lab tests, it will be possible to select the best one.

The selected wetting & dispersing agent must be effective at the 3 steps of the dispersion process - Wetting, Separation, Stabilizing, or at least not have any unwanted negative effects.

Dispersion Time Evolution

For the selection process of dispersing agent for a particular coating formulation, following characteristics are to be considered for a selection approach:

  • Solvent composition: polarity of the liquid phase
  • Pigment characteristics: ease of wetting
  • Binder system: compatibility


#1. Role of Solvents in Pigment Wetting


Solvent characteristics largely contribute to ease pigment wetting as well as required stabilization mechanism.

  • Polar solvents, such as water, show high surface tension and therefore poor wetting properties for low surface tension substrates, as typical for organic pigments. Thus, additional wetting support is required, through the use of specific wetting agents or pigment dispersing agents, showing strong wetting activity.

  • On the other hand, most inorganic pigments show much higher surface tension and are easily wetted by water; therefore having no need for additional wetting agent. 

Although apolar solvents, such as mineral spirits and xylene, show low surface tension characteristics, the interfacial tension with pigment surfaces (air/ water surface) is high and requires further wetting adjustment. The preferred solution is selecting dispersing agents, contributing to pigment wetting and dispersion stability, by steric stabilization.


#2. Pigment - Role of Chemistry and Surface Treatment


Typically, untreated inorganic pigments show high surface tension and are easily wetted by water; therefore, having no need for additional wetting agent. The selected pigment dispersing agent best provides electrostatic stabilization for pigment dispersions in polar media. However, for use in apolar systems, dispersing agents with wetting and stabilizing properties are required.

Organic pigments typically demonstrate hydrophobic character, low surface tension and require dispersants providing optimal wetting and stability properties. This is true for aqueous as well as apolar solvent based systems.

Organic pigments have higher oil absorption than inorganic pigments. This will have a direct consequence on the additive demand, and of course on the formulation cost. To select the best dosage, test should be done using the recommended dosage, then ¼ more and ¼ less, and compare the results.

Indicative Dosage, % Solid Wetting & Dispersing Agent on Solid Pigment
Titanium Dioxide 1.5 – 3.0 %
Iron Oxide 2.5 – 4.0 %
Phtalocyanine 15 – 25 %
Organic Red 15 – 30 %
Organic Violet 15 – 35%
Carbon Black, regular 15 – 20 %
Carbon Black, high channel 15 – 50 %

#3. Good Binder Compatibility Means Good Dispersion


The non-adsorbing moieties of the dispersing agent should demonstrate excellent compatibility with the binder system. This is essential for optimal stability in the liquid phase as well as best film performance. Any incompatibility may result in pigment flocculation in the wet paint, even during film formation.

Typically,

Dispersant Performance According to Your End-use Application


End-use Application Conventional Polymeric
Polyacrylic acid Polyurethane Polyacrylates CPT
Architectural (Interior) ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Architectural (Exterior) ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Automotive ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Can / Coil ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
General Industry ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Printing ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Wood / Flooring ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Resin Containing Concentrates ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Resin Free Concentrates ⭐ ⭐ ⭐ ⭐ ⭐ ⭐
Universal Pigment Concentrate ⭐ ⭐ ⭐ ⭐ ⭐ ⭐ ⭐

It can be concluded that: selecting the right wetting and dispersing agent is a compromised based on many parameters.

  • First, the system itself (waterborne or solvent borne)
  • Then the pigment (organic, mineral, fine, rough, transparent…)
  • And finally, the end-use application

In some formulation, changing the wetting & dispersing agent is a really positive choice, enhancing the paint quality. Products from the CPT offer excellent results, but is the cost reasonable in the considered formulation? The right choice will be based on these test results of course, but also on the specification to reach, not only in terms of paint quality, but also economically.

For fast troubleshooting, go one step further theory & boost your pigment dispersion skills with day-to-day practical methods. Take this exclusive course with coatings expert Jochum Beetsma to fine-tune your ingredients selection as well as avoid unfocussed trials.

Practical Tips for Pigment Dispersion & Stabilization


Role of Dispersants in Preventing Sedimentation

Role of Dispersants in Preventing Sedimentation

In general, solid particles in a liquid are pulled down because of gravitational force. The cause for this phenomenon is that most pigments and fillers have a density that is higher than the density of the liquid that surrounds the particles. The process of particles sinking in a liquid, called sedimentation, can give problems during storage. In the extreme case, particles can cluster together on the bottom of the can. This phenomenon, resulting in the formation of hard sediment, is called settling.

Sedimentation of a Solid Particle in a Liquid
Sedimentation of a Solid Particle in a Liquid

Several properties govern the speed of sedimentation:

  • An important factor, governing how fast a particle will go down, is the density of the particle or, to be more precise, the difference in the density of the particle and the density of the surrounding liquid (ρp - ρl).
  • The second factor is size: big particles sink faster than small particles.
  • The third factor is the viscosity of the surrounding liquid. Liquid has to make place when a particle goes down: liquid must flow around the particle to fill-up the space where the particle comes from. This flow of liquid is easier when the viscosity of the liquid is lower. A low viscosity of the liquid will, therefore, result in faster sedimentation.

Related Read: Get clarity around the core fundamentals for efficient management of the rheological profile of your product

Sedimentation is mainly a problem that occurs during storage of the system. During storage, the system stands still and the main force acting on the system is the gravitational force. Sinking of particles can be prevented by arranging a physical network in the system.

Reversible Physical Network Preventing Sedimentation
Reversible Physical Network Preventing Sedimentation

Because of the physical network, the particles behave as if they are frozen-in during storage when the applied force is low. During storage, the liquid paint or ink can be considered as a solid system when the physical network is strong enough. As soon as enough force is applied, for example by mixing, brushing, pumping or spraying, the physical network breaks down, and the system behaves as a liquid.

A physical network can be built-up in a liquid system by using rheology additives, often referred to as associative thickeners. The materials are called ‘associative’ because they attach themselves onto fellow molecules or they adsorb partly at the surface of particles that are present in the system.

The physical network, obtained by using rheology additives, has to be strong enough to resist the gravitational force during storage. On the other hand, the network must be weak enough to be broken down as soon as enough force is applied.


How dispersants work with different pigments?

How dispersants work with different pigments?

Pigments are usually the most expensive raw material in paint systems and can only show their full color strength if optimally dispersed. High performance dispersants can provide the required color quality with the minimum amount of pigment and, thus, help to minimize raw material costs.

Easily Wetted Pigments Vs Difficult to Disperse Pigments


The necessity to contribute to wetting depends on the pigment liquid phase characteristics. Easy wetted pigments, like Titanium Dioxide (TiO2) in water, do not require additional wetting support, so emphasize on the contribution of the dispersant is very much on stabilization effect.

The main group of dispersants as used in white waterborne dispersion paints is sodium-polycarboxylate. Indeed, this dispersant provides excellent electrostatic stabilization, but provides only limited wetting activity.

However, another important pigment – Carbon Black – is difficult to disperse and stabilize, primarily due to their notoriously low surface charge and poor wetting characteristics. Carbon black pigments provide excellent color and hiding power and can ultimately improve coating performance.

But at the same time, carbon black is generally considered to be the most time consuming and difficult pigment to disperse. This is especially true for waterborne systems because water is very polar, has high surface tension, and there is little interaction between the binder and the pigment. These properties require the use of a highly efficient wetting and dispersing agent.

Organic pigments are high in tint strength and brightness, but they are very difficult to disperse and stabilize because of small particle size. The small particle size causes following issues inhibiting wetting and dispersant adsorption :
  • Increased flocculation
  • Non-uniform surface structure
  • Low surface energy

Dispersants as offered for organic pigments in water demonstrate strong wetting support, as well as stabilization activity. A wide range of products is offered, however, having in common of offering surfactant (reduction interfacial tension pigment and liquid phase) as well as strong stabilization properties.

Can science-based approach help you go faster in selection process? One possible approach to quickly predict the most compatible dispersant/pigment pairing is by using Hansen Solubility Parameters. Read this case study utilizing 2 carbon black grades (Raven 5000 Ultra II and Raven 5100 Ultra) and 2 dispersants (Tego® Dispers 761 W and CLiQSPERSE® 149), where compatibility predictions were made and validated via practical experiments.

CASE STUDY
Optimizing Coating Performance via Predictive Compatibility Parameters of Carbon Blacks and Dispersants

» READ CASE STUDY

Now that you have learned about all the selecting criteria, we now focus on the quick tips which are useful for you to test the efficiency of your selected chemistry.

How can you evaluate the efficiency of Wetting & Dispersing Agent?

How can you evaluate the efficiency of Wetting & Dispersing Agent?

The wetting & dispersing agent has a significant influence on the paint properties. It has a direct impact on the particle size, and then, its efficiency can be evaluated by checking the right parameters.

To complete its validation, the wetting & dispersing agent must follow a serial of laboratory tests as discussed below.

  1. Compatibility with the System - Mix the wetting & dispersing agent with the system, without pigments. It should be perfectly compatible with the other formulation component. If not, try adjusting the pH or the polarity.

    Dispersing agent compatibility

  2. Pigment Shock - After the dispersion, make a simple poor-out of a small amount of paint diluted (10-20% in solvent or water). Pigment shock results of a poor pigment stabilization.

    Pigment Shock

  3. Draw down - Make a simple draw down and check the quality of the application - Color strength, transparency, gloss, general aspect. Incompatible wetting and dispersing agent can lead to many defects like seeding.

    Draw down

  4. Rub out (for color mix, or pigment concentrates in a base paint) - In order to check the flooding, a simple rub out test can be done. After short drying time when the film is nearly dry, with the finger rub a part of the paint surface. The color should be the same as the unrubbed part.

    Rub Out Test

  5. Storage Stability - Paint samples are stored at cold temperature (-5°C to 5°C) and high temperature (40°C to 60°C) for one or two weeks and the previous tests are realized, then the results are compared with the original ones and the sample stayed on the shelves.

    A perfect product should not show any significant variation regarding the storage conditions.

So, what are you waiting for? Let’s start formulating!

Our Coatings Selector features a full range of dispersing agents available today for all types of coating and ink formulations (aqueous, solvent-based, high-solid or 100% solids systems and for pigment concentrates). You can check tech profile for each product, ask for samples or discuss your case with producer’s tech staff.



We would like to acknowledge our experts Jochum Beetsma, Johan Bieleman & Vincent Makala for providing technical information needed to develop this page.

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3 Comments on "Pigment Dispersion: Wetting & Dispersing Agents Selection"
Sukdeb S Aug 18, 2021
Hi! I request a list of recommended dispersing additives for high jetness black paints. It would help us conducting a few lab trials for checking suitability for our specialty CB
Thaer M Oct 11, 2018
interesting information for research and development chemists
Fozia A Oct 11, 2018
Very informative article

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