Dispersions Revisited

Published: December 14, 2013, By Mark Miller

If you'd like to hear from Mark Miller's own lips rather than read his Coating Matters blog post titled, "Dispersions Revisited," click on his podcast below:

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To understand the forces involved in developing a stable coating on a substrate, key features of the coating need to be viewed from the internal perspective of the fluid. Within the world of coating, it is common to have a solid suspended in a liquid – referred to as a colloidal dispersion. The distribution of fine particles within a liquid needs to be a stable system. This stability is fleeting! Many times the solid particles like themselves more than they like the fluid and agglomerate. Sometimes the particles settle and the final dried product has an irregular loading within the microstructure of the coating. To combat these interests of the solid particles, we need to understand what forces are for us and what forces are against us.

Solubility: Solubility parameters (found in polymer handbooks and chemical engineering textbooks) for solvents and polymers provide the key to understanding whether the solid particle will become a dispersion within a specific solvent. The closer these two parameters are to each other, the more they like each other and the better the solid particle is solvated.

Molecular Weight: The size of the particle on a molecular level is important to whether it disperses in a given solvent or not. The dispersed solid particles are typically viewed as a coil, or spring, with constant movement, entanglement, and rotation. The “size” of the particle is a combination of the molecules size in addition to the space the molecule takes up by moving and entangling with other molecules. Branching within a given polymer particle can also have a strong effect on the ability to disperse the solid particle within the fluid.

Particle-Particle Interactions: There are many forces that act on a solid particle within a fluid medium. Imagine the solid particle as one magnetic ball floating in a pool of solvent with other magnetic balls nearby. Individually, the ball has gravity pulling it down and viscous resistance holding it up. The ball will spin, and move from temperature changes and vibrations that the pool of solvent experiences. You also have to remember that the balls are attracted to each other (in the fluid coating world these are referred to as Van der Waals force). These attractive forces need to be overcome with some physical or chemical improvement in the system. The particle forces have three critical parameters – physical distance between the particles, innate energy of the particle, and potential energy at the surface of the particle.

Dispersion Stability: The solid particles need to stay suspended through fluid preparation, pumping, coating, and drying. To develop stability throughout the process may require pH modifiers, increasing solvent use, or adding chemical modifiers to reduce chemical forces. Even if the dispersion is stable in the tank, verifying that the fluid is properly wetting the substrate is important. Morphological and microscopic review of the coated material is suggested to understand the resultant coated product.

Having a molecular view of the fluid coating will help you produce substrate coated product wide, fast, and in one pass that holds up to internal and external forces throughout the coating process. Keep this molecular view in mind and many of the macro problems you may encounter will be eliminated.