Pigments are solid, insoluble powders that are used to provide color to various materials including paints, coatings, and inks. They are available commercially in a wide range of colors and chemistries. While the options for pigments can seem endless, all of them have something in common. They must be properly dispersed in order to provide optimal performance and appearance in inks, paints, and coatings.
What is Pigment Dispersion?
Pigment dispersion is the process of suspending insoluble pigments in a liquid medium, typically water or organic solvent, so that they can be used in inks paints and coatings. Traditionally, pigment dispersion is comprised of three main steps; wetting, deagglomeration, and stabilization. These three steps are outlined in further detail below.
When solid pigments are first added to the liquid medium they are agglomerated and air is entrapped in the surface of the pigment agglomeration. In order for proper wetting to occur the air entrapped in the pigment needs to be replaced with the liquid medium. The use of a wetting agent speeds up the process and ensures complete wetting of the pigment.
Once complete wetting of the pigment particles is achieved, the large agglomerations of pigment must be broken up to reduce the particle size. This is commonly achieved through mechanical shear via the grinding mill. Reduction of the surface tension in the liquid medium allows for faster deagglomeration and a more homogeneous particle size in the dispersion.
The deagglomerated pigment particles now must be stabilized to prevent reagglomeration. In this step, dispersants are added that will adsorb onto the pigment particles and keep them separated. Stabilization of pigments in suspension can be classified as either electrostatic or steric.
Electrostatic dispersants carry an ionic charge. They interact with the surface of the pigment particles to produce like charged surfaces, typically cationic. These like charges on the surface of the pigment particles repel each other and keep the pigment in stable suspension. Changes in pH and addition of ionic material can destabilize particles suspended with electrostatic dispersants.
Steric dispersants consist of molecules with an anchoring head group that is attracted to the pigment particles and tails that are attracted to the liquid phase of the dispersion. The head groups adsorb onto the surface of the pigments and the tails orient themselves outward from the surface. The protruding tails keep the pigment particles separated and suspended. Steric stabilization is incredibly stable at high salt concentrations and over a wide pH range.
Why are Pigment Wetting Agents & Dispersants Important?
In many cases, the pigment is the most expensive ingredient in an ink, paint, or coating formulation. Optimization of the wetting and dispersion of pigments ensures optimal appearance and physical properties in the formulation while using the least amount of pigment possible. Selecting the right wetting agent and dispersant for your formulation is critical for producing high quality inks, paints, and coatings while keeping production costs down.
Surfactants as Wetting Agents and Dispersants
Surfactants, or surface active agents, are molecules that reduce the surface tension at the interface between two substances. In the case of wetting and dispersing pigments, a reduction of surface tension between the solid pigment particles and the liquid carrier allows for faster and more efficient wetting and deagglomeration. Additionally, surfactants can adsorb onto the surface of pigment particles and provide steric stabilization of the dispersion. Commercially, there are many types of non-ionic surfactants and anionic surfactants that are used today as wetting agents and dispersants in inks, paints, and coatings. These surfactants are compared below based on performance characteristics.
Reduction of Surface Tension
Selection of the appropriate surfactant for pigment wetting and dispersion should be based on the liquid carrier (aqueous or organic solvent), the pigment, and the end use application. While there is no "one size fits all" solution, fluorosurfactants provide the highest level of surface reduction at the lowest addition rate, are anti-foaming, and can provide a cost savings over more traditional nonionic surfactants and anionic surfactants.
Capstone™ - Fluorosurfactant Wetting Agents
Capstone™ fluorinated surfactants are powerful pigment wetting agents that are ideally suited for water-based paints, coatings, and inks. Capstone™ can be added at a much lower rate than traditional surfactants (typically 0.01-0.05%) and often eliminate the need for other additives like defoamers. The elimination of other wetting agents and additives typically results in a cost savings.
Commercially Available Capstone™ Grades
The grades of Capstone™ listed below are designed to provide optimal pigment wetting for inks, paints, and coatings while eliminating the need for other wetting agents and defoamers.