PTFE Coated Membranes – What’s the Story?
By: Doreen Tresca
Post Date: April 16th 2020
ORIGIN OF PTFE COATED MEMBRANES
In 2010, SSI was granted US Patent #7674514, Multiple layered membrane with thin fluorine containing polymer layer. This protected SSI’s extensive R & D work into multi-layer PTFE membranes used in diffused aeration for wastewater treatment. We developed the multi-layer technique through a year of research and testing. We adopted internationally accepted methods of testing coating adhesion and resistance, involving special tapes.
During this initial phase or research, it was quickly understood that the barrier property of the product was the most important performance aspect. The ability to protect the EPDM substrate from ingress of fats, oils, greases and solvents, and to prevent the EPDM substrate from leaching plasticizers, losing weight and elasticity, and hardening prematurely was more important than protecting the membrane from calcium scaling.
PTFE coated membranes offer two main benefits – Barrier Property and Anti-Fouling.
Barrier Property – The PTFE surface layer becomes the ‘skin’ of the membrane, protecting the membrane from acid, solvent, fats, oils and greases. It also locks in waxes and oils inside of the membrane called plasticizers. Without this layer, liquids can soak into the rubber (typically 1% to 2% for water, varies for solvents). When they soak in, they weaken the vulcanized rubber and cause it to flow, creep (stretch) and deform. They also carry away plasticizers, stiffening the membrane.
The anti-fouling aspect is more self-explanatory. A sticky and rough membrane surface makes it easier for bacteria and solids from the wastewater process to form a sort of ‘concrete’ on the surface of the membrane. Over time, in some applications, this can affect diffuser performance, by creating a tortuous path for bubbles to rise (causing coalescence and larger bubbles, hence lower transfer efficiency) or by increasing headloss and power consumption. Fouling doesn’t occur at the same rate on all membranes, so head losses can be different from grid to grid, and from tank to tank. This pressure gradient is sometimes substantial enough to cause maldistribution of air between zones or tanks. The cleaner you can keep the membrane, the more stable and energy efficient your process will be.
SSI’s PTFE coating has a lower modulus and a lower elongation at break than the EPDM substrate it is attached to. This means that the coating stretches more easily than the substrate, and it cannot stretch as far. It is good that it stretches more easily than the substrate, because the substrate controls the extent of stretching, and it is limited, so that the coating rarely if ever approaches its maximum elongation. This is why you rarely if ever see wrinkles between the coating and the substrate even after years of service. Bonding issues would likely be much more severe with a single layer coating, but SSI has not seen them with our multi-layer approach.
It is a good idea to avoid sandblasting or high pressure hosing of PTFE membranes. In tanks with a lot of grit or sugar sand, especially near pillars or walls, where swirling and scrolling can take place, it is possible to blast some of the coating off.
SSI’s multi-layer PTFE coated membranes is a proven technology.
Since its development, PTFE quickly attracted attention from some of the most desperate clients, who had tried EPDM, Silicone, and PU without success. Many of these plants were dairies and paper mills, but hot countries of the Middle East were also early adopters.
Ft. Worth Texas, North Texas Municipal, Albuquerque New Mexico, Aurora Illinois, Tacoma Washington and many others worldwide tried the PTFE coated membranes. All of the original membranes sold to these clients are still in service without any replacements, and many of them are over 10 years old. Industrial clients have also seen tremendous benefits, particularly in dairy, slaughterhouse, landfill leachate, and pulp and paper applications, where they quickly became a “must have” and have proven to outlast uncoated EPDM membranes in those applications.
A key factor in this product’s extended lifetime is a dramatic reduction in plasticizer extraction from the membrane over time. The barrier properties of the PTFE coating have been proven through a study by Akron Rubber Laboratories, a leading independent expert in the field of rubber research. We and many of our clients have also observed in the field that PTFE coated membranes remain flexible with stable weight and dimensions over a long period of time in service.
SSI Aeration pioneered and introduced the multi-layer coated membrane that includes a layer for substrate barrier protection. Competitors paid close attention and have been busy ever since. Our competitors who want to sell PTFE coated membranes are required to coat in a single layer. Most of the commercially available single layer coatings are not suitable for the aggressive service duty of a diffuser membrane (submerged in wastewater, perforated in 6000+ places, and repeatedly stretched and relaxed).
Static single layer coatings applied after an alcohol rub are more suitable for things like shielding car or train window gaskets from UV exposure, not from a dynamic application like diffused aeration.
Single layer coatings tend to lack full coverage of the EPDM substrate, according to SSI lab observations under 10x magnification with a microscope. The barrier property is of significant and probably primary importance.
Everyone knows that the monetary cost of owning an aeration system is primarily in power consumption. It is orders of magnitude more costly than the cost of purchasing and installing the system, no matter how disruptive and annoying it is to have to change your aeration system.
A better membrane which impedes plasticizer loss and chemical/solvent/FOG attack and scaling mitigation properties is a good idea for all wastewater treatment plants and types, at the right price.
SSI’s was first and now has 15 years of experience with PTFE coating technology. Our disc diffuser membranes are coated by robot. We optimized the purchasing and application of our coating years ago, and it has never been more affordable to purchase SSI PTFE than it is today. As a rule, PTFE adds no more than 10% to the cost of an aeration system and usually much less than that.
MOVING AWAY FROM PFOA’s
While SSI PTFE coated membranes do not contain PFOA, we have developed a newer formulation based on Polyether ketone (PEEK) instead of PTFE. PEEK is a solid lubricant which has anti-fouling properties similar to PTFE. The product appears to be working as well or better than the PTFE product, and we are transitioning to the solid lubricant product as a standard by 2022.
SSI offers a 5 year warranty on all PTFE and PEEK coated EPDM diffuser products, subject to standard terms and conditions.