Coating Matters | Everything You Wanted to Know about Coating But Were Afraid to Ask

Fluid coating is a world of science and art. Some things we know well, and some things have yet to be proven, and many things have yet to be investigated. So where do we start?

Fluid coating is the process of depositing one or more liquids that are balanced onto a substrate until the liquid is cured into a solid. This fluid-coated substrate provides some function that increases the value of the product above what the substrate would have been alone.

The key area of interest in fluid coating is the displacement of air at the liquid/substrate interface. Whether the liquid is 1 micron or 100 microns wet, the principles are the same.

Below 1 micron wet is typically the area of technology referred to as “thin film coating” and is fundamentally different. In thin film coating, the solid material that is placed on the substrate is built up in Angstrom levels until the thickness required is developed. In fluid coating, the solution, suspension, or dispersion is cured down to the required thickness. Therein lies the difference, one technology builds up, while our area of interest (fluid coating) coats thicker and cures down.

The world of fluid coating is vast and covers products from optical film to duct tape. Even so, the technology is based on the same concepts—move a fluid from a tank, through a coating station, deposit the fluid on the substrate, and cure in place. With this simple understanding, there are still complex questions to ask when developing a new-to-the-world product:

  • What coating method is best?
  • What process specifications will work for the product?
  • What design for coating equipment is appropriate?

What changes if the product is not new, but existing? The criteria that bubble to the surface focus on increased coating speed, thinner coating, more simultaneous layers, reducing defects, and increasing yield. With this multidisciplinary industry, coating process engineers need to collaborate with product chemists and equipment engineers to optimize the system available.

Whether the coating being developed is for a new fluid or an existing product, the keys still focus on fluid flow stability and displacing the air present at the substrate. To do both well, the process engineer needs to be concerned with surface energy and surface tension compatibility between the substrate and the fluid. Whether these work out naturally, or are forced with chemical or physical alteration, is only half the solution.

Once the fluid and substrate adhere, contaminants, flow disturbances, inhomogeneities in the fluid, or uncontrolled air flow may still cause coating defects. Defect formation may be connected to raw material variation or equipment instability. Everything has to be investigated and reviewed to understand and clarify a root cause.

In order to handle this massive data set for fluid coating understanding, simulations and modeling can provide predictive information. This set of interpolated results provide the basis for a designed experiment to empirically conquer coating defects in the field. Combining this scientific approach with in-field knowledge will provide the quickest and most robust response to the coating engineer’s dilemmas.

The best preparation for the coating process engineer to tackle the coating defects ahead is to ask the right questions:

  • What is the dynamic rheology of the fluid being coated?
  • What variability is inherent in the fluid and substrates involved?
  • How is the web controlled in the coating head position (tension or speed control)?
  • What is the simulated results of coating the specific fluid on the specific substrate at the suggested process conditions?
  • How big the coating window?
  • What quality and functional results are required of the final coated product?

This limited question set is a great starting point to lead the coating process engineer forward. Of course these lead to more in-depth questions. But with a strong basis, the coating defect solution is not far away.

The more you question, the more you know; the more you know, the more coating stops being an art and becomes a science!

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.

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