Agglomerates
- Published: April 24, 2014, By Mark Miller
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Agglomeration is a dispersion issue that can cause headaches for coating operators. Fundamentally, an agglomerate is a collection of particles that stick together and create a larger particle that can get stuck in narrow gaps or cause coating defects. Why is this such an issue?
Ideally, people who want to utilize dispersions for functional reasons in a coating have to determine the loading of the particle that provides the most use for the final product. The particles are dispersed in a solvent and mixed to a stable regime. The key to a stable dispersion is to make sure that the initial mixing is adequate. This mixing needs to keep the solid particles separated and suspended throughout the entire fluid preparation, pumping, coating and drying process.
It is natural for the dispersion to fall out of suspension because the solid particles are attracted to one another. This attraction leads to agglomeration, or a collection of microscopic particles that form a macro-scale issue for coating. Initial good mixing may need to be buffered with physical or chemical additives to keep the particles separated. Surfactants that act as dispersing agents are helpful because they surround the particles and impart a charge that repels the particles. However, with higher loading levels, even this chemical adjustment can be overcome. Alternatively, physical molecules can be added to buffer the suspension. In lower concentrations the polymers may actually encourage agglomeration because of free ends attaching to open areas of the particle. In higher concentrations, the polymers form a more solid barrier for protecting the particles from attachment.
So what happens if agglomeration occurs? It depends on where the agglomeration occurs. At the tank, agglomeration can cause a filter to plug quickly. One way to check this is to setup a recirculation system and monitor the fluid as it passes between the tank and pump. If agglomeration is recognized, then surfactants may need to be added.
If agglomeration is occurring at the coating head, coating defects may be presented on the web. Agglomeration may show up as continuous coating streaks or solid particles that release over time. Continuous coating streaks cause a more difficult coating issue because a whole section of the coating may need to be scrapped. While in point defects, the part may be removed to create a full web product (minus this area). In slot die coating, this may occur because of increased shear because of a narrow gap at the lip exit. Altering the shim thickness may help alleviate this issue. If the shim size is set, then more mixing may need to be added prior to the coating head.
Understanding whether agglomeration is a concern for your product will reduce process development efforts down the road. Before finalizing the dispersion chemistry, test the coating and work in a reciprocal fashion between the process and product engineer. These two people need to stick together so the particles do not!
If you are interested in discussing this concept further, contact Mark D. Miller, Founder and CEO of Coating Tech Service, LLC (www.coatingtechservce.com) at This email address is being protected from spambots. You need JavaScript enabled to view it. or (612) 605-6019.
