Coating Matters | Dispersions in Liquid Coating
- Published: June 06, 2017, By Mark Miller
Unlike non-functional particles that typically are filtered out, functional particles need to be included in the coated product, so special care must be taken to make sure the process is successful.
If you'd like to hear from Mark Miller's own lips rather than read his column, titled "Coating Matters | Dispersions in Liquid Coating," click on his podcast below:
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Liquids can be described as mayonnaise or beer. The Newtonian fluids are beer—they are self-leveling and if you turn a glass of beer upside down, the beer spills out. The non-Newtonian fluids are mayonnaise—without shear they will not move. Turn the mayonnaise jar upside down and it goes nowhere. But what happens when you have a wheat beer or a garlic aioli? The dispersed particles can cause coating defects you weren’t expecting.
Depending on the industry, particles in fluid coating can be common or unusual, but it is good to consider them if you coat fluids for a living, because the particles can add functionality and premium products. In the battery industry, cathodes and anodes are highly filled slurries, where particles are commonplace. This is also true with microspheres in adhesive coating and nanowires in ITO replacement film. The reality is particles are getting smaller and more important for functionality of active fluid coatings.
While non-functional particles in coating can create defects and are typically filtered out, functional particles need to be included in the coated product and special care needs to be taken to make sure the equipment is designed properly and the process is set up for success. There are a couple key items to consider when dealing with dispersions.
Residence time
Fluids that have particles dispersed throughout the liquid matrix have a residence time before they fall out of suspension. Have the fluid sit for too long and you will be coating a liquid without the functional component while it sits in the bottom of a pan or a slot die manifold that is too large for the application conditions.
Agglomeration
Particles can stick to each other if the initial mixing of the fluid with particles is done improperly, but it can also occur because of the coating process. Depending on the size and type of particles and the fluid and the loading of the particles, initial agglomeration can be avoided with proper mixing. Proper mixing may mean aggressive mills or simple agitation. Once the particles have been dispersed, they need to stay dispersed, so either movement, agitation, or chemical separators should be considered. For shear sensitive particles, excessive agitation or the shear of too narrow a slot gap can cause a agglomeration.
In each of these instances, the simulation of the flow of the dispersion through the system can provide insight into the excessive residence time, the shear zones or other areas of concern for agglomeration or particles falling out of suspension. So whether you are dealing with beer or mayonnaise, if the fluid is loaded with dispersed particles, dig deeper to understand the material reaction to the process and the design behind the equipment to ensure precise coating capability.
Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.
