- February 26, 2013, Mark Miller
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When you look at the process of fluid coating onto substrates, there are many ancillary pieces of equipment that affect the final coating quality of the product, be it an adhesive tape, optical film, or automotive battery. If we dig back deep enough into the process—beyond where the fluid hits the film, foil, or paper; beyond the coating head; beyond the fluid delivery system—we find ourselves in the fluid mixing room. What happens in the back room needs to be understood, because it can have a profound effect on what gets coated onto the substrate.
Fluid mixing is a critical step in the fluid coating process. While the fluid to be mixed needs to move through an impeller and develop turbulence, too much entrained air can cause coating defects (see Coating Matters | How To Avoid Bubble Defects). A couple items need to be kept in mind when sizing and specifying a mixing vessel:
- Many fluids can be handled by mixers, but 500,000 cP is an upper limit for viscosity.
- Processing speeds of the coating line can dictate the amount of time available to develop uniform mixing.
- When developing suspensions, 75% solids (by weight) is an upper limit for concentration of solids.
The proper balance of fluid flow through the mixing element with shearing action developed through agitation needs to be maintained. The sizing of the mixing impeller along with power put into the fluid determines this balance of flow and shear. As an example, if a large mixing element is run at a slow speed, you will have more flow, but less shearing action. Inversely, if a small mixing element is run at fast speeds, you will have less flow, but more shearing action. This dynamic mixing system affects the fluid before the fluid has a chance to encounter the coating head.
Fluid mixing is not relegated to the backroom mix chambers and the closed doors of the solvent room, however. Mixing can play a major role right before application to the substrate. Fluids in the coating world can have laminar or turbulent flow regimes. If the fluid is turbulent in nature, additional mixing may not be required. However, if the fluid is viscoelastic in nature, and the Reynolds number is low, the flow will be laminar and mixing needs to be encouraged.
In the case of higher viscosity fluids, an in-line mixer can be added prior to the coating head to ensure that the fluid is dispersed evenly in the distribution of the fluid onto the substrate. This is especially true with heated fluids, as the temperature profile of the fluid within the feed hose can vary. In-line mixing (with enough residence time) can distribute not only the fluid but the temperature profile of the fluid. Remember that viscosity changes drastically with temperature, and if there is a profile of temperature within a fluid delivery system, there also will be a profile of viscosity.
Whether tank mixing or in-line mixing is used (or both), proper mixing of the fluid prior to coating will reduce coating defects, improve cross-web uniformity, and produce an improved fluid coated product.
Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. He is the author of PFFC's Coating Matters column. Contact him at 715-456-9545; firstname.lastname@example.org; www.coatingtechservice.com.