Polytetrafluoroethylene (PTFE) resin is a highly effective material for seal consumers due to its extremely low friction, high heat tolerance and chemical inertness.
With the right additives, PTFE resin can perform even better in terms of strength, thermal performance, chemical resistance and abrasion.
However, there are a few design considerations when using PTFE resin, particularly when combined with glass fiber and bronze.
This blog will examine PTFE fillers to enhance PTFE performance.
Below, we’ll explore how Teflon is processed for sealing purposes, and why we sometimes see variation in surface quality and/or cracks in finished Teflon seals.
There are different grades of Fluoropolymers that can be used to manufacture seals. There are melt processable fluoropolymers, which are rarely used in the seal manufacturing process due to cost, and granular PTFE.
Melt processable fluoropolymers allow for injection molding, and exhibit many of the same characteristics as granular PTFE. But the first grade doesn’t allow for the flexibility of molding and machining, which is why most of our seals are made from granular PTFE.
Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Website and was written by Nathan Wells, application engineer, Engineered Polymer Systems Division.
Heavy duty equipment moves industry forward in all climates, from the sunny Caribbean to icy Greenland. Effective, reliable sealing is what allows hydraulic systems in heavy duty equipment to do work, no matter the temperature. Reliable sealing solutions allow cylinders on dump trucks and excavators to move icy, frozen tundra, and allow actuators on subsea valves to operate 5,000 - 20,000 feet below the surface of the ocean. We depend on these seals for our safety and productivity, so a little chilly weather is no reason to call it quits.
Most objects shrink as they get cold, with few exceptions, such as water. This applies to all matter in the universe. Materials shrink at different rates, and this is a measurable property called the Coefficient of Thermal Expansion (CoTE). Thermoset elastomers and thermoplastics shrink roughly 5 times more than metals for a given temperature change. This means at cold temperatures, seals shrink more than their housings, and thus have less “squeeze” to make a tight seal.
To make matters worse, elastomers also harden as the temperature drops. At some temperatures, known for each material as its Glass Transition Temperature (abbreviated ‘Tg’), seals become rock hard and brittle … like glass. We don’t make seals out of glass for a reason; they wouldn’t work. In order to keep seals springy and resilient, we need to specify materials with a Tg below the coldest temperature a system will see.
In very high pressure, low temperature applications, there is one additional concern. Applying pressure to seals effectively raises the Tg of the material by about +1°C per 750 PSI. This is called Pressure-Induced Glass Transition and is the reason high pressure seals fail slightly above their measured Tg.
The term “plastics” is generic way of describing a synthetic material made from a wide range of organic polymers. Organic polymers describes a man-made substance that is formulated using polymer chains to create what we commonly refer to as…(you guessed it), plastics.
Before plastic, leather had been used to create Backup ring devices behind O-rings. Leather allows fluids to be retained, providing lubrication for the O-ring when the system was running dry.
The problem with leather was that it could become dry and shrink away from the sealing service, exposing the elastomer to same pressure it was intended to protect against.
With the advent of polymers, a piece of plastic could be cut or formed into the exact shape to allow for zero extrusion gap, and for continued protection for the O-ring.
Some polymers were very brittle. Since they needed to be deformed to allow for installation into solid glands, the cut of the plastic could nibble at the O-ring, causing premature failure of the element it was supposed to be protecting.
When PTFE moved out of the lab and into industrial use, it quickly found itself adjacent to the O-ring. PTFE offers extrusion resistance and, at the same time, doesn’t erode or nibble at the O-ring due to the “softness” of the polymer.(Hardness between 55 and 65 Shore D)
Given the composition of PTFE, or Teflon, it could be utilized as a sealing element to protect Backup rings and conform to the shaft. The bonus was it was generally easy on shafts (depending on the filler added to the PTFE).
There are some negative aspects to Teflon that needed to be overcome by early engineers. First, it has a fairly high rate of Thermal expansion which, by its own nature, could often times lose contact with the sealing surface. This meant some kind of loading was necessary to ensure contact.
PTFE is as tough as other polymers, so the fact that it could seal on a shaft made it vulnerable during installation for tears or nicks on sealing surface.
Second, if it were stretched during installation, the material had to be sized back to its original shape due to its poor elastic properties.
Better known as Teflon in the industry, Polytetrafluoroethylene is widely used in practically every industry on and off the planet (and even beneath its surface!)
This material’s primary claim to fame is its resistance to most chemicals. It inherently has an extremely low coefficient of friction, it’s easily machined from rods, tubes, or compression-molded shapes.
It’s one of the few polymers that are approved for medical implants due to its inertness to bodily fluids — the immune system principally ignores its presence in the body.
Moving away from the body, you’ll find PTFE or Teflon products in medical
The search for the ideal Polytetrafluoroethylene (PTFE) gasket has been elusive. Competing applications and workplace variables have led to the creation of myriad solutions, yet none that has proven fully adaptable and appropriate for universal adoption.
Garlock Sealing Technologies considered this to be a critical yet entirely solvable shortcoming. And it is against this backdrop that in 2016, they set out to compile a comprehensive list of attributes for the ideal PTFE gasket — a wish list, as it were — in order to build a better gasket.
Working with a third-party survey development company, Garlock developed an exhaustive questionnaire that probed every aspect and functionality of PTFE gaskets, testing and adjusting the questions until they had a workable, finalized version.
Using this final questionnaire, Garlock conducted extensive interviews at 15 major chemical processor companies, speaking with 20 engineers responsible for process operations, projects, maintenance and reliability. The goal was simple: to discover the ideal characteristics and their relative importance that engineers sought in a PTFE gasket.
After several months of data collection, Garlock analyzed the feedback and noted the most popular responses:
From those answers, Garlock drew the following conclusions, representing the most desirable and essential PTFE gasket characteristics:
Garlock used this feedback in developing a next generation PTFE gasket — GYLON EPIX. Featuring a hexagonal surface profile, GYLON EPIX offers superior compressibility and sealing for use in chemical processing environments. Its enhanced surface profile performs as well or better than existing 1/16″ or 1/8″ gaskets, allowing end-users and distributors to consolidate inventory, lower the risk of using incorrect gasket thicknesses and reduce stocking costs.
GYLON EPIX checks off the most desirable gasket attributes:
GYLON EPIX with its raised, hexagonal profile allows it to perform the job of both traditional 1/16” and 1/8” gaskets. It accomplishes this by combining the bolt retention of the former with the forgiveness for bad flange conditions of the latter, a truly innovative feature for PTFE sheet gasketing.
Polymer wear rings were developed to offer an alternative to dissimilar metal wear rings.
One of the advantages to using a polymer material such as nylon or filled-Teflon instead of a metallic bearing . Whereas when you use bronze or metallic bushings, these materials are prone to point loading on the edges of the bearing.
This property of polymer bearings combined with solid lubricants can yield a product that is much less likely to damage moving components.
Polytetrafluoroethylene (PTFE) has an interesting history and has been used in a number of ways over the years. There is, of course, the best-known PTFE brand Teflon® – the miracle cookware coating. But PTFE is also used in aerospace and computer wiring, and it’s even coated the fiberglass dome of the Hubert H. Humphrey Metrodome in Minneapolis. But the Metrodome isn’t the only sports-related application for PTFE. Because it’s so slippery and resistant to extreme temperature changes and chemical reactions, PTFE is an excellent component for many parts such as: bearings, gears, and of course, seals – like those used in so many motorsport vehicles. So, what role does PTFE play in the exciting world of motorsports?
TRITEC Seal’s PTFE rotary lip seals can be found in the race cars of nearly
When it comes to designing and developing seals, the aerospace and industrial industries need a basis to allow production anywhere in the world.
One of the first PTFE (Teflon) standards, AMS3678, describes Teflon and the addition of fillers. This was used in conjunction with Mil-R-8791, which is one of the Mil specs describing a backup ring device.
The origin of all these specs dates back to the creation of the O-ring.
In 1939, Niels A. Christensen was granted a U.S. Patent for “new and useful improvements in packings and the like for power cylinders.” These referred to improved packing rings made of “solid rubber or rubber composition very dense and yet possessive of great liveliness and compressibility.” These products were suitable for use as packings for fluid medium pistons (liquid or air). The improved packing ring is the modern O-ring.
There was a progression of standards for the O-rings created by individual countries, such as AS568, BS 1806, DIN 3771, JIS B2401, NF T47-501, and SMS 1586. Eventually, AS568 became more accepted in the industry.
The backup ring was originally created to help improve the O-ring’s ability to resist extrusion. Teflon was widely used as one of the materials for backup ring devices. Standards were created to unify the production of this Teflon device.
The progression of standard changes has led to AMS3678/1 for Virgin PTFE through AMS3678/16. These standards describe a group of Virgin- and filled-PTFE materials accepted by the industry for manufacturing seals and back-up ring devices.
Mil-R-8791 was canceled in February 1982. This spec was superseded with AS8791, which eventually evolved into AMS3678.
AMS3678 is a tool used by customers and Teflon suppliers to create uniformity in the manufacturing and processing of seal and bearing materials. The standard is inclusive of most of the compounds upon which the industry was built.
When customers approach with an old “mil spec”, they are pushed to the new AMS spec which is currently active. Eclipse manufactures to the spec so their customers will have the confidence that they manufacture to a known standard.
When crossing custom materials from well-known sources, customers are driven to an accepted spec that is equivalent to the original source of the material. This helps customers sell their products with internationally-known materials rather than custom, home-grown compounds that are often intended to single source those materials.
There are several qualifications of the spec that suppliers must observe. This includes dimensional stability tests. This test ensures the material has been properly annealed, and that the seal or backup ring will fit and function as it was originally intended.
Eclipse is uniquely qualified to supply parts to the latest AMS3678 specification. They understand the scope of the specification which allows us to ship parts with fully traceable certification.
AMS3678 helps validate a material to a customer to ensure they get the same material processed the same way with each order. Beyond this, there are other ways to determine what makes a part process-capable.
During the June 17-23 event in Paris, Freudenberg showcased a new high temperature, fireproof material; an Omegat OMS-CS cap seal; and new ethylene propylene diene monomer (EPDM) and a fluoroelastomer (FKM) developmental material.
“Our aerospace customers strive continuously to be faster, safer and more efficient, which in turn requires us to innovate to help them reach those goals – a challenge we enthusiastically embrace,” said Vinay Nilkanth, vice president, Global Mobility Sector, Freudenberg Sealing Technologies. “The launch of several new products aimed at improved performance underscores Freudenberg’s commitment