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Pigment dispersion : Wetting & Dispersing Agents Selection

Bright, vivid and durable colors are one of the most important parts of the coatings’ aesthetic definition. Formulators are using pigments in order to bring color in the paint. 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.

Make your task easier by learning the selection criteria for choosing best product for your coating formulation which could not only enhance your product’s quality but also lower down the production cost!

How Does Pigment Dispersion Takes Place?


The dispersion process is crucial. Indeed, it will influence many parameters such as the appearance (color strength, transparency, gloss...) but also the paint physical properties (rheology, stability flocculation…). Keeping problems at this stage can dramatically reduce the final paint quality. Understanding this process is a great help to prevent many coatings defects.


The 3 Stages of Pigment Dispersion


Mastered wetting, dispersing and stabilizing are the keys for high quality pigment dispersed coating.

Let's understand them thoroughly:

Wetting:


Wetting


 The formulator introduces the solid pigments into the liquid phase. In this phase, air and moisture entrapped at the pigment surface are displaced to the grinding medium liquid phase. The pigment / air interfaces become pigment / liquid interface. To proceed, the liquid needs to wet the pigment surface.

 » Understand Substrate Wetting Theory Here! 

Wetting & dispersing additives have an influence on this stage, by modifying the surface tension at the interface they can help and accelerate the pigment wetting.

Dispersing:


Dispersing


The grinding equipment (dissolvers, grinding mills, sand mills…) introduce mechanical energy in the system. This shear forces energy breaks and separates the pigment agglomerates in smaller particles. This new created surface is wetted by the grinding medium liquid. The dispersing energy (related to the dispersing time) is an extremely important parameter to reach an optimal separation of all pigment agglomerates to the ideal primary pigment particles.

Wetting & dispersing agents lower the forces between the pigment particles and help in crushing the agglomerates in smaller particles


Stabilizing:


Stabilizing


Naturally the newly obtained small particles have a tendency to re-agglomerate. It is the “flocculation”. Uncontrolled aggregates and flocculates have poor influence on the paint quality and can reduce the paint stability, reduce the color strength and gloss, or change the paint’s rheology. Wetting & dispersing agents will prevent this unwanted effect and provide a long-term stabilization.


There are mainly two mechanisms of stabilization:

Electrostatic Stabilization

Steric Stabilization

Electrostatic Stabilization

  • Pigment particles have the same surface charge
  • Charges are arranged in a double layer causing the repulsion
  • Van-der-Waals forces cause the attraction
  • Mainly for inorganic pigments and dispersions in water
  • Stability can be affected by high salt concentrations

Steric Stabilization

  • Liquid phase soluble polymer chains adsorbed to the pigment particles through the anchoring groups
  • Strong stabilizing mechanism
  • OK for waterborne and solventborne systems

It is clear that pigment dispersion process results in three important stages : Wetting, Dispersing, Stabilizing. A correct Wetting & Dispersing agent should fulfill the requirement for all these three stages.

Solve Pigment Dispersion Issues Faster


Continue Reading to find out more on:



Dispersing / Wetting Agent Chemistries


In addition to the characteristics of the 3 stages, we must take in consideration the influence of the grinding liquid medium :

  • 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.

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

 Features  Benefits
  • 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
  • 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


Types of Dispersing / Weting Agents


Surfactant type


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

 » Explore All Available Surfactant-like Dispersing and Wetting Agents Here! 

Modified Fatty Acid type


Acting like emulsifier, they are giving excellent results for universal pigment concentrates.

Phosphoric Acid Ester


Polyether and polyester structure, excellent for inorganic material dispersion.

Polymeric Types


The classification of polymeric types wetting and dispersing agents is based on their anchoring mechanism, their chemical structure (polyacrylic, polyurethane, copolymer…) and their 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).

As polymer designed, polymeric wetting and dispersing agent can offer very high quality:
  • Polymeric type : many anchoring groups
  • Large choice of chemistry
  • Large choice of polymer design and molecular weight
  • Mw = 5 000 ~ 50 000 g/mol
  • 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)

 » Check Out All Polymeric Type Wetting Agents Available Today!

Polyacrylic Acid Based


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.

Polyurethanes


Excellent for the millbase viscosity reduction. As a consequence, 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.

Polyacrylates


They have similar properties to the polyurethane ones. Higher in molecular weights, they can offer a better compatibility where the polyurethane structure is not ok. Suitable structure for waterborne and solventborne systems.

CPT : Controlled Polymerization Technology / 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.


Select Right Wetting & Dispersing Agent


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, or at least not have any unwanted negative effects.


Dispersion Time Evolution


General Comparison between Conventional and Polymeric Type


There are many wetting & Dispersing agents in the coatings market, nevertheless we can classify them as Conventional and polymeric Wetting and dispersing agents. 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


Wetting and Dispersing Demand


Usually, 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.

In order 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 %


Additive 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


Hence, 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.


Testing the 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:

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


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


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


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


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.

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