Coatings World - August 2017

Four Steps to Effective Pigment Dispersions

Deagglomeration

In this phase of the dispersion process, pigment agglomerates are separated into smaller aggregates and primary particles.

The lower the surface tension of the vehicle in which the pigment is being incorporated, the lower the energy that will be needed to disperse the pigment. Deagglomeration is achieved through the use of mechanical energy developed by the use of high-speed dispersers and various types of grinding equipment. A Cowles blade fitted on the shaft of a high-speed mixer can be an efficient means of dispersing a pigment. High Shear blades are a highly recommended type of Cowles blade (Figure 4) which is well suited for pigment dispersion. Other types of blades include a High Vane blade or a Combination blade which also can be used depending on the viscosity of the dispersion and the need to move/blend the raw materials during processing.

In order to ensure a good laminar flow and increase the efficiency of dispersion it is recommended that the blade diameter is approximately 1/3 of the tank diameter, and also that the blade is approximately 0.5 to 1.0 diameters off the bottom of the tank. The recommended tip speed for a system with viscosity between 70-100 Krebs units is between 4,000 – 6,000 fpm. The following equation can be used to determine the tip speed: Shaft RPM x 0.262 x blade diameter (inches). Pigments with a harder texture can be more finely dispersed by using a media mill, which produces significantly more shear, such as horizontal, vertical and basket mills. In order to disperse the pigment to a nano level, the use of 0.3- 0.5 nm grinding media is recommended.

Stabilization

Due to the increased surface area of the solid particles during the deagglomeration/grinding stage, the pigments that are deagglomerated need to be stabilized in order to avoid issues such as flocculation, color shift, sedimentation and stability loss. The stabilization process takes place by incorporating dispersion additives which achieve stabilization through the following mechanisms.

Electrostatic Stabilization

Used in water-based systems and mostly with inorganic pigments, additive molecules adhere to the pigment surface, through ionic bonding, hydrogen bonds, and/or dipole interaction and cause the particles to repel each other through electrostatic forces. Pigments with high conductivity may not be stabilized through electrostatic stabilization. Zeta potential (the potential difference existing between the surface of a solid particle immersed in a conducting liquid, e.g., water, and the bulk of the liquid), serves as a reference as to how stable the formula will be. Pigment dispersions with a potential between + 30 mV and - 30 mV have a high probability of being unstable. The pH, once the pigment has been incorporated into the formula, can be a good indicator of stability; dispersions with pH values between 4 and 7. 5 are more prone to have dispersibility/stability issues, indicated by the Zeta potential value as it will most likely be between + 30 and - 30 mV. The addition of a pH modifier can be necessary in some cases.

Formulas where the dispersion must be acidic due to the final application having a pH lower than 4 is recommended, and for alkaline dispersions a pH higher than 7. 5 is good. The closer to zero the Zeta Potential is, the more prone re-agglomeration will be with WB dispersions. Electrostatic stabilization is accomplished by dispersion agents with cationic or anionic molecular groups, e.g., quaternary ammonia salts and alkylpolyamines (cationic) or polycarboxylic acids and sulfonated organic substances (anionic).

Steric Stabilization

Used in both water- and solvent-based systems, the additive anchoring groups will adhere to the pigment surface. The compatibility of the system is dependent on the functional segments of the polymer that compose the hydrophobic portion. Regarding steric stabilization, the additive will physically reduce the mobility of the pigment particles and therefore avoid/minimize flocculation or re-agglomeration. This type of stabilization is accomplished mainly with nonionic dispersing agents.