One of the questions we are often asked is “In the case of filters, what's the difference Between an ePTFE Membrane vs. a PTFE Finish?”
What is the Difference Between PTFE and ePTFE?
Commonly known as Teflon®, PTFE, which is short for polytetrafluoroethylene, is a synthetic fluoropolymer of tetrafluoroethylene (that’s the TFE part). In addition to being hydrophobic, which means it repels water, PTFE is resistant to high temperatures; it’s unaffected by most chemicals and compounds, and it offers a surface to which almost nothing will stick.
Due to these characteristics, PTFE is of interest in various dust collection scenarios. This leads us to the two PTFE-related questions we are asked most often here at Sly: “What's the Difference Between an ePTFE Membrane vs. a PTFE Finish?” and “Which is best—an ePTFE Membrane or a PTFE Finish?” The first question is easy to answer, but the answer to the second question is, “It depends.”
Types of Dust Collection
For dry dust collectors, which use baghouse filters, there are two common options--shaker systems (these are old systems that are becoming rarer every day), in which the collection bag is shaken to remove caked-on particles, and pulse jet (also known as compressed air cleaning), in which a high-pressure blast of air is used to remove dust from the bag.
Most baghouses use long, tubular-shaped bags made of woven or felted fabric as a filter medium. For applications with relatively low dust loading and gas temperatures of 250 °F (121 °C) or less, pleated, nonwoven cartridges are also sometimes used as filtering media instead of bags.
Types of Filter Bag Media
Concerning the materials used to create the filter media, many options are available. These materials tolerate different temperatures, provide different levels of collection efficiency, support different abilities to withstand abrasive materials, and offer different chemical compatibilities.
Media options (which may be provided in woven and/or felted form) include cotton, polyester, high-efficiency micro denier felts, polypropylene, nylon, acrylic, aramid, fiberglass, P84 (polyimide), PPS (polyphenylene sulfide), and even Teflon® (fluorocarbon fiber). (You can discover more details about filter bag media options on our website.)
Types of Filter Bag Finishes
Once you’ve selected a media for your filter bags, your next choice will be whether or not to apply a finish. Using an appropriate finish (or combination of finishes in some instances) can significantly improve your bag life, cake release, and protection from harsh application conditions.
Types of finishes include singed, glazed, fire retardant, acid-resistant, spark-resistant, antistatic, and oleophobic, to name but a few.
PTFE may be applied as a finish in two different ways—as a thin membrane or as a coating/bath. (You can discover more details about filter bag finishes on our website.)
Types of PTFE Finishes
Let’s start by considering a baghouse filter in the form of a felted polyester bag. When the bag is in use, some of the dust particles will work their way into the media. This is called depth loading filtration. When the bag is shaken, or a compressed air pulse is actuated to remove caked-on particles, some of the particles will fall into the hopper and be removed from the system, but others will remain embedded in the fabric. Over time, more and more particles will become entrained deep into the pores of the media and begin to blind the filter media, which will degrade the filter's performance in future cycles.
An ePTFE membrane may be applied to regular and pleated bags formed from woven and felted media. Such a membrane is microscopically thin (think “plastic food wrap” to provide a visualization) and is applied in the factory to the outside surface of the bag. In this case, the membrane will greatly increase the efficiency of the bag (where “efficiency” in this context refers to the number and size of the dust particles being filtered). If an unfinished polyester bag obtains 99% efficiency for particles of two microns and larger, for example, the addition of an ePTFE membrane may result in 99.99% efficiency for particles down to 1 micron and smaller, depending on the dust and operating conditions. Furthermore, the slick, non-stick properties of the ePTFE membrane mean that shaking the bag or applying a pulse jet will cause most caked-on dust to be removed and eliminate or minimize depth filtration and blinding for the life of the membrane (these membranes will deteriorate over time; also, to maximize their longevity, they should not be used in conjunction with abrasive dust particles).
Although an ePTFE membrane is a type of finish, some people regard the term “PTFE finish” to refer to bathing or spraying a liquid coating of PTFE onto the filter media. In this case, the media’s fibers are individually encapsulated in PTFE. A PTFE finish of this type will not increase the efficiency of the filtration, and the bag may still become depth loaded, but if a pulse jet is employed, the bag will clean easier due to the slick coating the PTFE provides on the fibers.
Which is Best: An ePTFE Membrane or PTFE Finish?
A bag augmented with an ePTFE membrane may see an increase in efficiency as much as 10X or more, will be easy to clean, and won’t suffer from depth loading. Also, an ePTFE membrane is advantageous in the case of sticky, oily dust. By comparison, a non-membrane bag treated with a PTFE finish won’t experience an increase in efficiency and will still become depth loaded, but it will be easier to clean than if the finish were omitted.
In the past, in some cases, the choice between an ePTFE membrane and a PTFE finish was driven by cost because membranes were expensive, but the price of membrane bags has fallen in recent years.
All this may prompt the question: “If you can’t beat an ePTFE membrane for efficiency and for preventing depth loading, and if the price of a membrane bag has fallen so it costs only slightly more than a bag with a PTFE finish, then why would you not opt for the ePTFE membrane?” The answer is that you can’t use a membrane in an environment where the dust is abrasive because—if you do—you won’t have a membrane for long. In the case of abrasive dust, a PTFE finish is the way to go.
Having said this, selecting the most appropriate combination of filter media and filter finish (or finishes) is a multi-dimensional problem, and the optimal answer is dependent on many factors. The best solution is to ask the experts at Sly to evaluate your unique dust collection situation and offer their expert advice.