PTFE Membranes – Beauty is in the Eye of the Beholder
By: Tom Frankel
Post Date: October 21st 2015
What is the point of putting a PTFE coating over an EPDM membrane? Is it to look pretty?During our extensive research into the use of PTFE with diffuser media 10 years ago, we looked at many different options to make diffusers operate better, longer and more efficiently, including blending PTFE powder into the uncured EPDM gum rubber. There didn’t seem to be a point to sandwich PTFE inside of the membrane. We wanted it on the outside of the membrane. At the time, we thought the benefit of PTFE was going to be the non-stick property, like a frying pan. We learned that while SSI’s PTFE coated diffuser membranes do appear to slow the rate of fouling, there was a larger benefit that we had not fully appreciated during the development process.
We noticed something funny while testing our PTFE membranes in a lab tank. There was a period over one summer when people were taking vacations and we left a tank full of water next to an open window, and when we came back we emptied the tank and there was one diffuser with an EPDM membrane and one diffuser with a PTFE membrane. Both were covered with a thin algae film. We set them on the counter to dry, and when we came back the a few days later, we saw something interesting. The thin film on the EPDM membrane had dried and cracked in a uniform diamond pattern, but the thin film on the surface of the PTFE coated membrane was still intact. There was no shrinkage or crazing of the film on the PTFE membrane like the EPDM membrane.
SSI’s PTFE coated EPDM membranes don’t swell measurably with the equipment we have in a shallow water bath, but plain EPDM membranes do. The EPDM membrane had absorbed just enough water in the bath to shrink, causing the surface film to crack. The PTFE membrane absorbed so little or no water, so that there was no shrinkage of the substrate product under the film. This set us on a path of research which included independent accelerated aging tests, creep (rubber stress/strain) tests, and field tests.
We installed PTFE membranes at a perfume factory in North Carolina that have run 5 years, where the original EPDM membranes lasted only months. We replaced EPDM membranes at a textile factory in India that had operated with EPDM membranes less than a year, and has also run about 5 years with the first set of PTFE membranes. We threw PTFE membranes into competitive pilot tests at paper mills in Canada and the northern US, we submitted samples to laboratories in the US and Europe for testing, and we participated in aggressive foul air experiments in California which had destroyed every membrane material including fluorinated silicone and VitonTM , but PTFE worked. The list goes on and on and includes practically every industry including pharmaceuticals, oil refineries, automobile manufacturing, and of course, municipal wastewater.
Water is an enemy of an EPDM membrane, with depth and temperature as secondary factors affecting lifetime. It is just a theory, but it seems that EPDM membranes that have a long run time in municipal applications may do so, precisely because they are run all the time and continuously dried out. EPDM membranes that are allowed to soak over a period of time are likely to swell and suffer dynamic changes including changes in thickness and hardness. Some of you may have observed this in swing zones, where it may be difficult to turn diffusers on after a long dormancy. Or in SBR’s where the run time of diffusers is typically 50% or less of a conventional plant, so why don’t EPDM membranes in an SBR last twice as long? It’s been our experience that they don’t even last as long as diffusers operated continuously.
In the next post, we’ll show some photos of SSI PTFE membranes that have had a long run time. We will also share some physical properties and performance metrics for those diffusers, to show you how these products are holding up in a wide variety of applications over time.
Thanks for your interest and we hope that SSI can do something for your wastewater treatment plant soon.