Particles in Fluid Coating
- Published: August 12, 2014, By Mark Miller
With the modern coating era upon us, the need to coat functional materials has grown exponentially. Whether you are looking to coat flexible electronics, solar panels, optical films, or energy storage devices, the need for fluids that provide transmission of energy is important. So how can fluids provide this superior performance? Particles.
Dispersions and suspensions of functional particles on the micron and nanometer size can provide these optical and energy capabilities, as long as they coat successfully. That is the point to emphasis – if the fluids are coated successfully. With the size of the particles dropping to dimensions that require microscopes to visualize, the predictive behavior of the flow and the combinatorial nature of the particles moves from macro understanding to molecular understanding. Because particle filled fluids are complex fluids, more in-depth understanding of fluid flow is required to overcome potential obstacles associated with complex fluid flow. This new world of complex fluid flow, combined with small-scale particle behavior, leads fluid coating operators to reach into fluid flow understanding in the finite element analysis range of understanding.
Typical particle filled fluid issues include particle settling, agglomeration, and viscosity shifts. Each issue is recognized as a defect in coating and requires a unique adjustment to overcome.
Particle Settling. The residence time of the particle in suspension needs to be understood to develop equipment that can place the fluid (with the particles) onto the substrate prior to the particle falling out of suspension. As with agglomeration and viscosity shifts, understanding the flow pattern of the fluid within coating equipment provides an understanding of flow vortices and dead zones where particles settle, accumulate, and drop out of suspension. Identification and reduction of the number and size of these zones provide the best defense for particle settling.
Agglomeration. Because of the small relative size of the particles to the bulk flow, collections of particles into larger “clumps” can lead to uneven distribution of the solids onto the substrate, misalignment of the particles in the coating, and streak defects. The uneven distribution is recognized as a coat weight variation crossweb. The misalignment of the particles is presented as a morphology difference within the structure of the final coated product. This misalignment can provide visual and physical variation within the product that reduces functionality. Misalignment can be observed through microscopy. A good example of the importance of morphology is in particle alignment (typically seen in polarizing films). If the flow pattern and particle alignment is random, then the polarizer will not provide the filter required for functionality. The final issue, streak defects force the operator to look at tighter filtration, larger gaps, and static mixing elements to reduce continuous coating defects on the substrate.
Viscosity Shifts. If particles are not evenly distributed within a fluid, the viscosity can vary and coating flows can be uneven. For some products this is an annoyance, for others it is essential. Take battery coating for instance: if the viscosity varies because the particles fall out of suspension over time, then the final coated density will vary for the battery. With a varied coat density comes a varied energy density. This can develop into hot spots in the battery and potentially run-away heat that can damage the battery. To overcome viscosity shifts, it is important to work with reasonably sized batches for production and continuously mix the fluid while not introducing air into the system.
Coating complex fluids requires more rheological information, modeling, and fluid flow understanding. Whether you are interested in coating nanowires, quantum dots, or thin film energy products, deeper fluid coating understanding is required. Those companies that look deep into the final coated morphology and work their way back to rheological understanding will succeed in this brave new world.
