- May 15, 2013, Mark Miller
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When a fluid hits a substrate, pushing through the barrier of air, the fluid tends to flow predictably. This phenomenon is a function of the interfacial tensions between a fluid, the displaced air, and the substrate being contacted. Whether the fluid wets the surface of the substrate and spreads, or de-wets and retracts, is determined by the interactive forces involved. The energies that are inherent in the substrate and fluid can be manipulated, but not without consequences.
Fluids tend to like themselves more than they like attaching to a substrate. One way to improve this situation is to have active ends to the substrate or fluid. This means that there is excess energy on the substrate or unfinished molecules in the fluid.
To create an active substrate, energy can be added to the web through corona, plasma, flame, vacuum, or chemical treatment. Too much excess energy, however, and the fluid may not know when to stop and not bead-up properly on the web. This excess energy also can damage the substrate and lead to optical or physical consequences.
This balance of energy and chemistry leads us to the discussion of contact angles. While contact angles can be measured between a specific substrate and fluid under a microscope, it does not tell us what happens in the process of a high-speed coating operation. This is further complicated by the roughness of the substrate or impurities in the fluid. Many times an estimation can be done, but process experimentation provides the best feedback.
Visualization of the coating bead on the backside of a fluid coating operation can provide more insight into the world of adhesion than watching the resulting web coating. Many times, however, this view into the underside of the coating station is not physically possible. If possible, start with the fluid exit and work your way to the coated sample to see if defects, impurities, or wetting issues can be controlled at the fluid/substrate interface.
Fluids and substrates interact in one of two ways:
- The contact angle is less than 90 deg and the fluid wets the substrate (hydrophilic) or
- The contact angle is greater than 90 deg and the fluid does not wet the substrate (hydrophobic).
This starting point should lead the process or product engineer to consider what options to use to control wettability. Drastic wetting issues may require material changes, while minor issues may require an increase in the energy of the substrate. Fluid coatings greater than 1 micron wet follow the behavior of thin film coatings and can be controlled by this surface energy and material design. Coatings less than 1 micron wet are possible, but require non-intuitive process applications to overcome surface tension disturbances.
Faster and thinner coatings are the wave of the future—learn how to ride the wave to success!
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.