Coating Matters | The Fundamentals of Coating

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Let’s go back…way back…back to the basis of what fluid coating is and why the converting industry cares about the process of placing a fluid onto a substrate for products around the world. When someone in the coating industry is trying to create, coat, and solidify fluids, there are some fundamentals that they simply need to know.

These basics are rooted in the understanding of rheology (see Technical Report: A Rheology Lesson), interactions at the interface between fluid and substrate, and the turbulent world of fluids (see Coating Matters | It’s a Turbulent World). Like those of you reading this, I have spent many years learning, testing, and developing an understanding of this awesome world of fluid dynamics, and it never ceases to amaze me. Consulting with multiple companies across various product applications (from duct tape to optical film and beyond), my understanding of fluid coating phenomena always leads me back to the basics.

So what are we trying to accomplish when we coat a fluid onto a substrate? Whether the coating is thick (>100 microns) or thin (<12 microns), the key is uniformity. Coating heads can transport a fluid in a continuous roll-to-roll application narrow (5 cm) or wide (150 cm+) to a substrate with little variation across the width of the substrate. The less the variation, the better the final product performance will be.

Take a battery for instance: the anode and cathode slurry need to be coated both in the crossweb and downweb direction with precision to avoid hot spots. If a battery is compiled with high and low areas in the coating, the battery will burn up during use (see Coating Matters | Battery Coating Technology).

The fluid being coated onto the substrate is replacing air (see Coating Matters: How to Avoid Bubble Defects). The substrates of the final products can cover everything from paper in masking tape to nonwoven substrates in dryer sheets to polyester films in optical materials to foils in battery applications. Because of the diversity, understanding the interaction of the substrate to the fluid is critical. In a few applications, a fluid is coated on top of another fluid that has not yet cured, so understanding of fluid-fluid interfacial chemistry also can be important.

In yet another application (casting), fluid is coated onto a solid roll or belt only to be removed at the end of the process as a cured sheet of cast fluid that does not stay attached to the roll or belt. Substrates can be coated continuously or discretely (see Coating Matters | Discrete Coating Techniques and Equipment), but both processes require an understanding of the interaction between the fluid coating and solid substrate.

So if you know your substrate, then you also better understand your fluid. The viscosity versus shear rate (rheology) of the fluid along with the viscoelastic nature of the fluid (see Coating Matters | Understanding Rheology) control the fluid interaction with the substrate. The range of fluid characteristics cover water (Newtonian fluid) to polymers (highly viscoelastic fluid). Turning this fluid into a useful product requires some form of curing. Curing is the process step where energy is introduced to the fluid to cause it to solidify (see The Role of Energy in Coating). The interaction between coating and curing is the key for the formulation chemist. A more liquid coating chemistry may allow for better coating capability, but a more solidified chemistry allows for less energy in the curing process. We never said coating was easy!

To control the distribution of the fluid precisely, a coating head needs to meter the fluid accurately and precisely. While an open pan is reasonable for many applications, more capability is provided by a precision manufactured manifold present in slot die coating (see Coating Matters | How to Select a Fluid Coating Method). Even with a precise coating method, a window of process-ability needs to be determined for a given product (see Coating Matters: Open Up the Coating Process Window). This coating process window is the basis of your product robustness, and in turn the profitability of the product. All these fundamentals lead to financial success—I told you this is important.

Ultimately, when the product and process of fluid coating is understood, a control plan for the system can be established and improvements can be researched to develop optimum product performance and financial return (see Process Troubleshooting). Follow the fundamentals and the road will lead you 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; mark@coatingtechservice.com; www.coatingtechservice.com.


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