Coatings World - April 2017

An Efficient Approach to Dispersing Pigments
April 2017 www.coatingsworld.com Coatings World | 41

anti-corrosion characteristics. Inorganic pigments normally have a high refractive index, meaning they have a greater ability to scatter light. 3 Therefore, these pigments are very good at “hiding” the surface under the coating layer. Oxide compounds such as titanium dioxide and iron oxides are typical examples of inorganic pigments. Carbon blacks are technically classified as inorganic pigments, but they require a different anchor group to adsorb onto the pigment surface. 4

Organic pigments are intensely colored, therefore they are incorporated solely for their coloristic properties. Organic pigments are classified as azopig-ments, polycyclic pigments, and anthraquinone pigments. 4 They tend to have a smaller particle size than inorganic pigments, thus making them more transparent. They have a tendency to dissolve when moisture is present, which causes them to migrate and chalk to the surface. Because of their smaller particle size, organic pigments are normally more difficult to disperse compared to inorganic pigments. Because most inorganic pigments have polar surfaces, they are much easier to wet out.

Dispersing Technology

In order to understand wetting and dispersing agents, a fundamental knowledge of the dispersing process is required. Dispersing agents stabilize deflocculated pigment particles. For stabilization of these particles to occur, the dispersant must be able to overcome van der Waals attractions that are constantly moving pigment particles back together.2 The pigment dispersion process can be broken down into three steps: wetting, de-agglomeration, and stabilization.

• Step one: Wetting
The first step of the dispersion process
consists of wetting the pigments by a liq-
uid. The liquid spreads over the pigment
surface and fills the voids and pores of
the pigment, displacing any remaining
air pockets.2 For a pigment to be wet-
ted by a liquid, the surface tension of the
liquid must be lower than the surface
energy of the pigment.1 This interaction
between pigment and liquid is described
below by the Young equation. A liquid
with a low surface tension typically wets
pigments better than one with a higher
surface tension.

• Step two: De-agglomeration After the pigments are wetted, they are broken down to achieve small particle sizes with a large surface area. This yields a higher color strength, which is more cost-efficient for paint manufacturers.

In order to grind down to smaller particle sizes, more energy is required. To break up agglomerates and increase the surface area (ΔA), an increased energy input (ΔW) is required (Equation 2).1 This energy is proportional to the surface tension (Y) of the dispersion. The smaller the surface tension, the greater the surface area will be for a certain amount of energy. 3

When dispersing, agglomerates are broken down into primary particles and small aggregates. When breaking down agglomerates, only physical bonds are being interrupted. Figure 3 shows the typical range of energy content of one mole for different types of chemical and physical bonds.

These energies range from 40-50 kJ/ mol, meaning 40,000 to 50,000 joules are required to break these physical bonds. 4

If aggregates are milled, then approximately 600 to 1000 kJ/mol are required to break these chemical bonds. 4 This is about ten times the energy it takes to grind agglomerates.

• Step three: Stabilization Consequently, when there are large surface areas and small pigment particles, the energy is very high and thermodynamically unstable. Solid particles will always gravitate towards each other in Brownian motion to minimize their surface area and return to a lower energy state that is more stable2 (Figure 4).

If these particles are not well stabilized, then they will flocculate back together. To achieve good pigment stabilization, the dispersing agent must be able to adsorb onto the surface of the pigment. Therefore, the additive must have anchor groups with high affinity for the pigment surface.