Introduction
Dispersing solid particles in a liquid is an important process in the production of paints and inks. Several types of additives can be used in the dispersion process in which solid particles, like pigments and fillers, are separated from each other and stabilized in a liquid.
Two categories of such additives, wetting agents and dispersants, are often mentioned in one breath. However, the two materials differ strongly with respect to the role they play in the process and with respect to the chemical composition and structure of the molecules the additives are composed of.
Functionality
It is important to have a clear view on what each component of a paint or ink should do. The job a raw material, like an additive, must do in a system is called its functionality. The functionality describes in a few words what the system developer would like the additive to do. A check question that can be asked if you are not sure about the functionality of a component: What will happen if I remove the additive from the formulation?
Wetting agents
Wetting is the first step in the dispersion process. The solid raw materials that are used in paints and inks – pigments and fillers – are often supplied as powders. The solid particles are glued together in clusters, called agglomerates. During wetting, the air that surrounds the particles in an agglomerate is substituted by liquid.
Wetting will take place when the surface tension of the liquid is low compared to the surface energy of the solid particles1. Wetting will not occur when the surface tension is too high. The surface tension can be lowered by adding a wetting agent. This additive does its job because the molecules adsorb and orient at the liquid-air interface.
Wetting agents show this behavior because the molecules consist of two parts. The hydrophobic part points towards the air and the hydrophilic part sticks into the liquid. An interface between a liquid and air is often referred to as a ‘surface’. Wetting agents are therefore often called surfactants, ‘agents’ that are active at a surface2.
There is a wide variety of wetting agents that is used in paints and inks.
Dispersants
Solid particles attract each other because of Van der Waals forces. Energy is therefore needed to separate the particles from each other in the second step of the dispersion process. Particles in a liquid will move to one and other and glue together because of the attractive forces. This spontaneous and undesired gluing together of solid particles in a liquid is called flocculation. Solid particles that have been separated from each other are stabilized against flocculation by using a dispersant3,4. These additives do their job because the molecules are designed as such that they adsorb at the solid-liquid interface and assure repulsion between the particles. The repulsive forces, resulting from the dispersant, must be stronger than the attractive Van der Waals forces.
Repulsion can result from two mechanisms that may be used separately or in combination. Particles repel each other when they all carry a charge of the same sign. This so-called electrostatic stabilization is obtained when the dispersant molecules carry a charge. Steric stabilization is obtained when all particles are covered with tails of sufficient length that dissolve in the liquid that surrounds the particles. The soluble tails are part of the dispersant molecules.
Many companies develop and supply dispersants for both solventbased and waterbased systems.
Overview
Wetting agents and dispersants are additives that are used in dispersion processes. Both are interface additives but there are fundamental differences between the two categories of materials.
System developers want to predict how additives will behave in their systems. The behavior of wetting agents and dispersants is governed by their chemical composition and structure. Useful information can be obtained by studying the documentation that describes an additive, like TDS (technical data sheet) and SDS (safety data sheet).
The surface composition of the solid particles that are used, as well as the composition of the continuous liquid phase, must also be known in order to select the right additives and to predict the behavior of each additive in a system.
References
- Surface Tension & Surface Energy, Jochum Beetsma, 27 September 2019.
- Surface Active Agents (Surfactants), Marc Hirsch, 25 September 2015.
- Dispersant Technology, George Deckner, 25 July 2014.
- Understanding Dispersants, Marc Hirsch, 19 February 2016.
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL Solutions or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL Solutions does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL Solutions does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
Hi.
I am making commercial acrylic paint .Can you please guide me with , what additives to use and in what propotion if i want to make 1 litre of paint.
Thanks
Dear Vinay,
On the UL Prospector website, you can find a wide variety of starting formulations for paints: on the home page ‘Paints & Coatings’ of UL Prospector, click ‘Formulations’. Then you can choose what type of starting formulation you would like to see.
Success with your search!
i am making Acrylic and Polyester white for can coating, how to make more opasity ink without incresing the pigment white
Dear Adhi, optimum opacity (and color strength) of white pigment is obtained when the particles are separated and stabilized well. Please check your dispersion process (preferably a bead mill is used) and the type and amount of dispersant. Also, use a high-quality TiO2 pigment.
Greetings, Jochum.
Thanks for arranging this, Rita.
Greetings, Jochum.
Can you combine a weting agent and a dispersant on the same formulation or do you need to use just a dispersant?
Hi, Rafael:
A wetting agent can be combined with a dispersant in one formulation, and this is done in practice. A dispersant is always needed (to prevent flocculation). A wetting agent is only needed when the surface tension of the liquid mill base is too high to give wetting of the solid particles.
Thanks for reading,
Jochum.