Coating Matters | Optical Films Require Multiple Layers

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When coating optical films, should the fluid layers be coated and dried or built up via deposition?

Optical films cover a wide variety of coated applications in the world today. With the need for better reflection properties in solar panels, transmissive glass with touchscreen conduction capabilities, and increased battery life in all electronic gadgets, optical films have become the leading edge coated product in the converting industry today. But what makes these coatings unique?

Many of the optical coatings require an even coating of a sub-micron layer of metal particles. This suspension of metal in a fluid is prone to agglomeration and shear sensitivity. So you can understand the need to design processing equipment that places the fluidized metal suspension on the substrate without coating defects. The question arises: When these materials are coated thin enough, should the metal be coated and dried or built up via deposition? There is a lot that goes into this decision, but speed of the process is a major consideration.

Coating one layer typically is not enough for an optical film. The reflectivity of the film is one important factor, but it needs to be balanced with a coating for transmissivity, a coating for anti-static control for handling purposes, and a coating for anti-glare and scratch resistance. These multiple coatings must process well and work well together as a final formed product.

Because multiple layers need to be coated, one concept is to reduce the overall number of handling steps by reducing the coating passes. This can be accomplished with multi-llayer coating capability. Multi-layer coating, however, is not a trivial procedure. The fluids of interest must not mix when stacked and must be able to be cured sufficiently.

In multi-layer coating, there is simultaneous application of two or more layers of fluid. This requires that each fluid has a separate fluid delivery system for pre-metered control into a multi-manifold slot die. The liquids are layered outside the slot exit because one wants to reduce the effect of pressure on the miscibility of the fluids. Surface energy of the top layer needs to be lower than the surface energy of the bottom layer. This can be modified through surfactants.

Once the fluid is released onto the substrate, concern over the contact interface is paramount. Recirculation near the interlayer can cause the fluids to mix and not operate properly. Pre-metered flow control and viscosity balance between fluids is important. One way to deal with fluids that tend to mix is to introduce a barrier fluid layer to avoid unwanted diffusion.

Previously utilized when photographic film was produced, multi-layer coating allows the reduction in process steps and allows for increased yield. During the photographic film era, upwards of 20 layers of fluid were coated simultaneously! While not currently the standard, multi-layer coating of optical films is worth the investigation.

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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