POLYTETRAFLUOROETHYLENE (PTFE) SEALS
PTFE minimizes friction while maximizing chemical and temperature resistance.
Chain reaction: Whereas the ethylene
structure features four hydrogen atoms
bonded to two carbon atoms (Figure 1),
tetrafluoroethylene features four fluorine
atoms bonded to carbons (Figure 2). The
double bond between the carbons can
become a single bond, allowing the
formation of a polytetrafluoroethylene
chain (Figure 3).
Even outside the sealing industry, PTFE (better known as Teflon®) is a fairly familiar material. Its use as a “non-stick” surface for cookware is legendary. As recognizable as it may be, however, it may not be as obvious just what makes PTFE so useful. Here’s a look at its form and function, as well as an overview of typical fillers used with PTFE, a peek at the main fabrication techniques used to produce PTFE parts, and a glimpse of how varied those parts can be.
Teflon® is but one trade name (others include Algoflon® from Ausimont USA and Polyflon® from Daikin) for polytetrafluoroethylene (PTFE). PTFE is a completely fluorinated polymer manufactured when the monomer tetrafluoroethylene (TFE) undergoes free radical vinyl polymerization. As a monomer, TFE is made up of a pair of double-bonded carbon atoms, both of which have two fluorine atoms covalently bonded to them. Thus the name: “tetra” means there are four atoms bonded to the carbons, “fluoro” means those bonded atoms are fluorine, and “ethylene” means the carbons are joined by a double bond as in the classic ethylene structure. (Ethylene has hydrogen atoms attached to the carbons, as in Figure 1, but TFE has fluorine in place of the hydrogen, as in Figure 2.) When TFE polymerizes into PTFE, the carbon-to-carbon double bond becomes a single bond and a long chain of carbon atoms is formed, as in Figure 3. This chain is the polymer’s backbone, and it gives PTFE its “poly” quality.
With a ratio of four fluorine atoms to every two carbon atoms, the backbone is essentially shielded from contact. It’s almost impossible for any other chemical structure to gain access to the carbon
atoms. This gives PTFE extraordinary chemical resistance. It’s tough for a solvent or other agent to degrade the backbone if the carbon is “out of reach.” Even if an agent could gain access, the carbon-to-fluorine bonds have high bond disassociation energy, making them almost unbreakable.
What makes PTFE so slippery? By its very nature, the fluorine in PTFE repels everything. As part of a molecule, fluorine is decidedly “anti-social.” It wants to get as far away from other molecules as possible. Anything getting close is automatically repelled, and repelled molecules can’t stick to the PTFE surface.
The inability of other materials to stick to PTFE makes it perfect for applications requiring a low coefficient of friction. The only thing slicker than PTFE is ice! Because they are essentially self-lubricating, PTFE parts are ideal for applications in which external lubricants (such as oils and greases) can’t be used.
As the most chemically resistant thermoplastic polymer available, PTFE is inert to almost all chemicals and solvents, allowing PTFE parts to function well in acids, alcohols, alkalis, esters, ketones, and hydrocarbons. There are only a few substances harmful to PTFE, notably fluorine, chlorine trifluoride, and molten alkali metal solutions at high pressures.
PTFE can also withstand a wide range of temperatures (-300° to 500° F, -184° to 260° C). Because it’s non-flammable and doesn’t dissipate heat, PTFE is often used as a thermal insulator (as in welding equipment). At the other extreme, PTFE is widely used in very cold environments (such as space). Other important properties include resistance to both weathering and water absorption. PTFE can also act as an electrical insulator.
Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no memory and is subject to creep (also known as cold flow). Creep is the increasing deformation of a material under a constant compressive load. This can be both good and bad. A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal is compromised. Compounding fillers are used to control unwanted creep, as well as to improve wear, friction, and other properties.
PTFE fillers don’t act like elastomer fillers, which become chemically bonded to the elastomer. With polytetrafluoroethylene, the high shear modulus fillers are encapsulated and bound by the low shear modulus PTFE. Here’s an overview of PTFE fillers:
- Glass is the most common filler for PTFE. Widely used in hydraulic piston rings, glass gives good wear resistance, low creep, and good compressive strength. Glass also has excellent chemical compatibility. The major disadvantage is that glass-filled PTFE compounds are abrasive to mating surfaces, especially in rotary applications.
- Molybdenum disulfide (MoS2) improves wear resistance and further lowers the coefficient of friction. “Moly” is typically combined with other fillers (such as glass and bronze).
- Carbon (powder or fiber) imparts excellent compression (low deformation under load) and wear resistance, good thermal conductivity (heat dissipation), and low permeability. Carbon-filled PTFE compounds are not as abrasive as glass-filled compounds, but they are still more abrasive than polymer-filled compounds. Carbon-filled compounds have excellent wear and friction properties when combined with graphite. Carbon fiber lends better creep resistance than carbon powder, but fiber is more expensive.
- Graphite is a crystal modification of high purity carbon. Its flaky structure imparts excellent lubricity and decreased wear. Graphite is often combined with other fillers (especially carbon and glass).
- Bronze (a copper-tin alloy) lends excellent wear resistance and thermal conductivity. Bronze-filled materials have higher friction than other filled PTFE compounds, but that can be improved by adding moly or graphite. Bearing and piston ring applications often use compounds containing 55% bronze – 5% moly. Bronze-filled compounds have poorer chemical resistance than other PTFE compounds.
- Stainless steel supplies high wear resistance and load bearing capability, along with better chemical resistance than bronze-filled PTFE. Stainless steel is especially good in steam service.
- Wollastonite (calcium silicate) is a mineral filler giving properties similar to glass (minus the abrasiveness). The FDA has approved it for food service.
- PPS (polyphenylene sulfide, trade name Ryton®) was the first polymeric material used to improve PTFE’s wear and abrasion properties. PPS-filled compounds also exhibit excellent deformation and extrusion resistance, making them good for use in back-up rings.
- Ekonol® is a thermally stable aromatic polyester. When blended with PTFE, it produces a composite material with excellent high temperature and wear resistance. Ekonol® will not wear mating metal surfaces, making it good for rotary applications. Ekonol®-filled materials are also good for food service.
- Polyimide is another polymeric filler offering superior wear and abrasion resistance. Polyimide-filled PTFE compounds have about the lowest friction properties of all filled PTFE materials, so they provide great performance in non-lubricated (dry) applications. They will not abrade mating surfaces (even soft materials such as brass, stainless steel, aluminum, and plastic). Polyimide is one of the most expensive PTFE fillers, however.
Other fillers include calcium fluoride (CaF2), which is specifically used in hydrofluoric acid (HF) service, and alumina (Al2O3), which can improve the mechanical properties of compounds destined for high voltage applications. Alumina-filled compounds are very abrasive.
Raw PTFE polymer is available as a powder, and this powder must be processed to produce a useful product. There are three main techniques used to process PTFE: compression molding (hydraulic or isostatic), net molding, and extrusion.
Hydraulic compression molding involves pouring PTFE granules between two metal tubes. A third tube with a slightly smaller outside diameter (O.D.) than the largest tube’s inside diameter (I.D.) and a slightly larger I.D. than the smallest tube’s O.D. is inserted between the tubes. The assembly is placed in a hydraulic press, where pressure is applied to the middle tube, thus compressing the PTFE. The compaction is typically 4 to 1. In order to achieve uniform compaction and billet density, the maximum recommended length for a hydraulic compression molded billet is 7". The billet must be sintered (heated to a high temperature, then slowly cooled) to develop PTFE’s optimum physical properties.
Isostatic compression molding is accomplished by compressing PTFE powder between a metal tube and a rubber bladder. The rubber bladder is inflated with very high hydraulic pressure, causing the PTFE powder to be compressed radially. Compression takes place between the I.D. and the O.D. of the billet rather than from the ends (as with hydraulic compression molding). Again, the billet must be sintered to achieve maximum properties.
Net molding involves the formation of PTFE parts in a hydraulic press. The parts must be relatively simple and have uniform wall thickness. After the parts have been formed and removed from the mold, they are said to still be in a “green” state. That is, the parts are still very fragile and can be broken with light hand pressure. They must be sintered to achieve maximum properties. Net molded parts have about one-half to two-thirds of the physical properties achieved by machining a part from a compression-molded billet. Net molded parts usually exhibit an “orange peel” surface appearance.
Extrusion is the third main fabrication method. During the extrusion process, PTFE powder is heated and forced under high pressure through a series of dies to produce either a rod or a tube. Because the PTFE is heated to its gel state, the extruded product does not need to be sintered (as required with compression molded and net molded products). One of the disadvantages of extruding is that the hot PTFE cools quickly. The result is a more amorphous structure with less tensile strength and poor elongation properties.
Because of its resistance and versatility, PTFE can be used to produce a wide variety of parts.
- O-rings may be machined from virgin (unfilled) PTFE. Because PTFE has poor memory, PTFE O-rings undergo high compression set and are usually rendered ineffective after a few thermal cycles.
- Back-up rings may be machined from a variety of grades of unfilled and filled PTFE. Virgin and glass-filled PTFE back-up rings are primarily used in media.