Technical Bulletin 171 PTFE Coated EPDM Diffuser Membranes: Benefits and Limitations
Membrane diffuser technology has become the standard for high efficiency transfer of oxygen into wastewater. This difficult service has utilized many materials for membrane construction with premium low oil EPDM having emerged as one of the most reliable materials throughout the industry. Even premium EPDM can occasionally experience difficulty or failure so search for the perfect membrane continues. Several alternates to EPDM have performed well in specific sites but to date premium EPDM is the clear standard for efficiency, economies, and life.
One of the specialty membranes promoted over the past few years is “PTFE (best known brand name Teflon®) Membranes”. This product has great market appeal as everyone knows PTFE is resistant to even severe operating conditions in household use (eggs on frying pan etc.). Why not make a PTFE membrane and solve all membrane diffuser problems? This program had been pursued and “PTFE Membranes” are currently being marketed for diffusers as a solution to all membrane problems. What does an analysis of this product actually suggest?
- PTFE membranes marketed in the industry are perhaps mislabeled. Membranes marked under this label are actually EPDM membranes.
- The face of the EPDM diffuser disc is coated with a molecular thin layer of PTFE compound! Body of the diffuser membrane is 100% EPDM.
- This thin layer of coating is an interesting composition of approximately 97% polyurethane with about 3% PTFE dispersed into the layer on the outside of the EPDM membrane.
- PTFE compound coated EPDM membranes are available from multiple sources.
- In a few specialty service applications, PTFE coated membranes have offered benefits vs premium EPDM.
- Even for specialty PTFE coated applications it has been demonstrated the base EPDM product must be a premium EPDM as PTFE coating on poor quality EPDM is not effective.
- Cost to employ PTFE coated rubber is a premium of about 100%. Typical 9” disc membrane cost of $3 – $5 while PTFE coated would be $6 – $10 each.
- Thin coating has limitations.
- Coated on outside of membrane only.
- Once perforated, the smooth outside coating is impaired at the perforations.
- Most fouling occurs inside the perforations and no PTFE present to help in slits as only surface coating.
- Thin film subject to abrasion if any grit, attached growth carriers, etc.. in the wastewater.
- PTFE/polyurethane thin coating has different physical characteristics from base polymer EPDM:
- Rubber stretches and returns to form while PTFE/PU has little elasticity
- Thermal Expansion rate different
- Bonding dissimilar materials
- Should I consider this thin film coated EPDM for a project? Absolutely this specialty material should be considered when proper information or test results available.
- When EPDM demonstrated failures.
- When an opportunity to test multiple specialty membranes in the wastewater determined PTFE coated or other types membrane offer best value. (field test units
- Cost not a major consideration and the PTFE coating considered an insurance policy to reduce risk of fouling!
- Cases where in-situ acid cleaning may not be possible.
- Note – PTFE coated EPDM membranes should not be applied in any petrochemical applications as petrochemical attacks the base EPDM structure of the membrane.
- Are there options to thin coated EPDM rubber membranes of 97% PU, 3% PTFE?
- Matrix™ – polyurethane PTFE membrane with 100% of product PTFE/PU compound. Same composition throughout entire membrane thickness. Same PTFE/PU surface inside slits. Membrane protection inside surface.
- Polyurethane (no PTFE) is a standard membrane particularly when oily or petrochemical wastewaters
- Silicone or other specialty polymers may be considered
- Why not make 100% PTFE membrane?
- Technically possible and has been tested
- Very hard durometer and poor bubble uniformity
- Cost 9” disc at unit cost of up to $100 each
- Large bubble size because of surface energy of PTFE (contact angle). Fine bubbles gather at opening and release as large bubbles.
- Units do not seal well in holders
EDI recommends each project be reviewed for membrane technology! Our 40 year history with membrane technology suggests premium EPDM is a superior choice for most municipal wastewaters and many industrial projects. Evaluation of membrane types can be helpful when test rigs can be installed with real field data accumulated.
EDI considers the PTFE thin coated membrane a viable specialty membrane product and available for applications when the material has demonstrated benefits. EDI does not promote this product as superior to EPDM or polyurethane standard products without data or customer preference to employ PTFE coated EPDM membranes as insurance policy.