Coating Matters | Membrane Process Technology
- Published: March 11, 2015, By Mark Miller
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Here are a few theories to guide your decision making for achieving the thin coatings needed for filtration.
Like with everything else in the coating world, membranes perform better the thinner they are. Thin coatings or castings, in this case, require a level of understanding in fluid flow that may seem daunting. But don’t worry, the concepts can be boiled down to a level where a handful of theories can guide your process decision making!
In water filtration membranes, this optimum thin cast thickness allows for higher water flow (or flux), which reduces the amount of energy required to push water through the filtration process. However, as the filter is cast thin, more imperfections are recognized.
Just imagine a gel within a bathtub. Like a thick casting or coating, when the tub is full, the gel is buried at the bottom. As you drain the tub, you start to recognize the gel, and it eventually peeks up above the water line. So it is when you coat or cast thinner. Imperfections rise to the top.
With water filtration membranes, the thicknesses approach the molecular level, so variation in the molecular weight of the developed polymer can be recognized. These imperfections develop holes, which are definitely an issue when you are trying to control flux!
So what do you do when you need a thin casting, but the flaws stick out and cause problems? You can’t just let these holes persist, or the filter will fail. The goal needs to be thin casting with zero holes.
As you can imagine, casting a filter in a free state, thinner and thinner, becomes tricky and complicated. So what is done in practice is a carrier substrate is utilized. This base layer of woven or nonwoven fabric carries the filter through the process. The strength of the cast membrane is then supported by this secondary porous substrate.
What’s unique and fascinating about filtration casting is that the cure typically is not the same as curing of fluids in most roll-to-roll coating systems. In water filtration manufacturing, the cast fluid forms a thin film by reaction with another fluid in a tank. The resulting polymer matrix is the complex sheet that is a membrane.
But the fun doesn’t end there! Besides needing a thin filter for increased fluid flow and reduced flaws for pinhole free operation, filters need to stay clean longer. Fouling, or the clogging of the filter over time, can be reduced by adding functional coatings to the water filtration membrane. These functional coatings repel the items that would stick and cause the filter to clog through hydrophilicity, roughness reduction, and static charge, while allowing the water to pass through the filter. In both the casting and coating steps, polymeric formation in-process are presented and show how reactive chemistry can occur before, during, and after the coating process.
Ultimately, coating technology provides the science behind improved filtration results, including higher water flux, excellent resistance to damaging media, and even rejection of salt in desalinization treatment. Membrane process technology can also provide insights into other coating techniques and systems.
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.
