Introduction
Dispersion and stabilization of solid particles (pigments and fillers) in a liquid is a challenging process, taking place during the production of paints, colorants and inks. The whole process consists of three steps, starting from a dry powdery material that consists of agglomerates of the solid particles.
The three steps of the dispersion process.
Wetting of the particles takes place when the powdery solid is brought in contact with liquid. The solid particles are separated from each other by applying mechanical force. The separated particles must be stabilized to prevent flocculation.
In the following, the three steps are discussed separately even though they mostly occur simultaneously.
1. Wetting
During wetting, the air that is present at the surface of the particles and within the agglomerates is replaced by the liquid that is used in the dispersion process.
Wetting: solid-air interface transforms into solid-liquid interface.
A pre-condition for wetting is that the surface tension of the liquid is low enough compared to the surface energy of the solid particles1. Wetting agents are additives used to facilitate wetting in case the surface energy of the particles is too low2. The functionality of a wetting agent is to lower the surface tension of a liquid. The use of wetting agents can induce problems like foam, insufficient intercoat adhesion, increased water sensitivity and reduced film hardness.
In many cases, wetting agent is not needed in the dispersion process. Moreover, using wetting agent often retards wetting instead of improving it. Wetting, as well as the release of air from the system, proceeds faster when the viscosity of the liquid is lower. The importance of wetting is often underestimated, thus giving problems in the steps that follow in the dispersion and stabilization process.
2. Separation
Agglomerates — solid particles that are glued together — are split in the separation step. Because of the strong attractive forces between the solid particles, separating them requires a high amount of mechanical energy that is provided by suitable equipment.
Two principles are used to split agglomerates into smaller pieces.
A disk disperser, often called dissolver, works by means of shear forces. A disk, with teeth on the edge, rotates with high speed in a liquid mill base of high viscosity, thus shearing the particles apart.
In a bead mill, often called pearl mill, the impact principle (combined with crunching) is used to split agglomerates. Beads (pearls) collide with each other and agglomerates that are between the beads are hammered and crunched into smaller pieces.
Mill chamber of a bead mill and beads. (courtesy of WAB)
An axis with rotor blades rotates in the mill chamber that is partly filled with beads. The input of a bead mill is a predispersion that is made by using, for example, a disk disperser. The output of a bead mill is a dispersion of solid particles, with the desired particle size distribution, in liquid.
3. Stabilization
Particles that have been separated from each other must be stabilized to prevent flocculation, the spontaneous gluing together of solid particles in a liquid.
Separation and flocculation of solid particles in a liquid.
Solid particles are stabilized against flocculation by means of a polymeric additive called dispersant3,4,5. Dispersant molecules must adsorb strongly at the surface of the particles. This adsorption phenomenon is often referred to as anchoring. The groups that have a strong affinity for the surface of the particles are called anchoring groups.
The functionality of a dispersant is to provide colloidal stability. A dispersion is said to be stable from colloidal point of view when flocculation of separated particles is prevented because the particles repel each other.
Two principles of stabilization against flocculation are used.
First, all particles can have the same electrostatic charge, thus making them repel each other. This so-called electrostatic stabilization is critical and it depends upon key system properties like pH and the amount of electrolyte that is present. In waterbased paints and inks mostly anionic stabilization is used, implying that all particles have a negative charge6.
Secondly, the particles can be covered with a layer of polymeric tails that dissolve in the liquid that surrounds the particles, thus giving steric stabilization7. It turns out that steric stabilization is the most secure principle to use, in both solventbased and waterbased systems.
References
- Surface Tension & Surface Energy, Jochum Beetsma, 27 September 2019.
- Surface Active Agents (Surfactants), Marc Hirsch, 25 September 2015.
- Understanding Dispersants, Marc Hirsch, 19 February 2016.
- Dispersant Technology, George Deckner, 25 July 2014.
- Settle Down: Factors that Influence Pigment Settling and Stability, Ron Lewarchik, 12 May 2017.
- Dispersants for Electrostatic Stabilization, Jochum Beetsma, 7 October 2016.
- Dispersants for Steric Stabilization, Jochum Beetsma, 28 October 2016.
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NEED DISPERSING AND WETTING AGENT
very nice article
What is the benefit of using silanes in the pigment prepation.
Please if you can send your recommendations and the process of use .
Regards
Dear Mr/Mrs Sadek,
Silanes can be used during the production of pigments & fillers and/or during the dispersion process. The objective is to chemically bind one end of the silane molecules to the surface of the solid particles. A precondition is that part of the chemical groups of the silane molecules (mostly alkoxy-silane groups) are able to chemically react with chemical groups on the surface of the particles. In most cases this implies that the surface of the particles must contain hydroxyl (OH) groups.
Other reactive groups of the same silane molecules are able to react with the binder matrix of the system. In this way the solid particles become strongly attached to the matrix of the complete system, thus improving system properties.
Which silane & process to use depends upon the type of particles and the production and/or application method.
Jochum Beetsma.
Hi B.Dey,
You can search for dispersing and wetting agents in Prospector and contact the manufacturers with any questions.
Hope this helps!
Angie
UL Prospector team
Dear sir
Based on my comments. Is there anything you advise me to do to avoid the quantity of my paint shrinking after some days having used wetting agents? Or does it mean that the defoamer is not enough to tackle the l”atent” foam induced by the wetting agents?
Thank you.
Dear Emma,
Probably, the shrinkage is caused by the slow release of foam from your paint. A few remedies can be summed-up to minimize this effect: prevent additives that stabilize foam (like wetting agents), use a suitable defoamer, adjust the rheology of the paint (to assure that foam can escape more easily) and assure that no air is stirred-in the paint during production.
It depends on your composition and process what the most effective remedy is for you.
Greetings, Jochum.