PTFE

What PTFE Is

PTFE is the polymer of tetrafluoroethylene, a two-carbon molecule in which every available bond is occupied by fluorine. The repeating unit is a carbon chain wrapped in a continuous sheath of fluorine atoms. That sheath is the source of every defining property: fluorine forms one of the strongest single bonds in organic chemistry, and the dense, symmetric shield shields the carbon backbone from chemical attack, blocks intermolecular adhesion (so almost nothing sticks to it), and produces an extremely low surface energy. The result is a material that is chemically near-inert, self-lubricating, electrically insulating, and stable across a very wide temperature band.

The discovery was accidental. On 6 April 1938, Roy Plunkett, a young DuPont chemist searching for a non-toxic refrigerant, found that a cylinder of tetrafluoroethylene gas had spontaneously polymerized into a waxy white solid, with the iron of the cylinder wall acting as the catalyst. DuPont patented the material, branded it Teflon, and first deployed it during the Second World War to seal equipment in the Manhattan Project that handled corrosive uranium hexafluoride. Commercial fluoropolymer production followed, and Teflon became a household name through non-stick cookware coatings in the post-war decades, even though cookware is a small fraction of total PTFE consumption.

Mechanically and thermally, PTFE behaves unlike a normal plastic. It has a crystalline melt point near 327 degrees Celsius, but above that point it does not flow: its melt viscosity is so high (on the order of 10 to the 11 poise) that it behaves like a sinterable powder rather than a moldable melt. This is why PTFE is shaped by compression molding and sintering, ram extrusion, and paste extrusion rather than by injection molding. The same property makes it dimensionally stable but also gives it a relatively high creep, or cold flow, under sustained load, which is the main weakness virgin PTFE designers must engineer around.

Industrially, PTFE is one member of the broader fluoropolymer family. Where true PTFE cannot be melt-processed, chemically related resins fill the gap: PFA (perfluoroalkoxy) and FEP (fluorinated ethylene propylene) are melt-processible and injection-moldable while retaining most of PTFE's chemistry; PVDF (polyvinylidene fluoride) trades some chemical resistance for higher mechanical strength and weldability. PTFE remains the benchmark for chemical inertness, the widest service temperature, and the lowest friction, which is why it dominates aggressive chemical, sealing, and dielectric applications despite its higher processing cost.

The scale of use is large. PTFE is consumed as gaskets and sheet seals, lined pipe and vessels, bearings and bushings, wire and cable insulation, valve seats and stem packing, laboratory and semiconductor labware, and architectural membrane. Global fluoropolymer demand runs into hundreds of thousands of tonnes per year, with PTFE the single largest component. The essence of engineering selection is matching the specific grade (virgin or a particular filler) and the fabrication route to the dominant requirement, because no single PTFE grade is optimal for chemical, mechanical, and electrical duty at once.

Virgin and Filled Grades

PTFE is supplied as virgin (unfilled) resin or as compounded grades carrying 5 to 60 percent of a functional filler. Virgin PTFE offers the best chemical resistance, the lowest friction, the highest dielectric strength, and food-contact compatibility, but it is soft and creeps under load. Fillers are added precisely to overcome that weakness: they raise compressive strength, cut deformation under load, improve wear life, and add thermal conductivity, at the cost of some chemical and electrical performance. The table below compares the mainstream grades on the properties that drive a bearing, seal, or insulation decision.

Virgin PTFE is the reference grade. It is chosen wherever chemical purity, absolute corrosion resistance, dielectric performance, or FDA food and pharmaceutical compatibility is the priority, and wherever the part is not heavily loaded. Its weakness is creep: under sustained compression a virgin PTFE seal slowly deforms and can lose preload, so designs use it with metal back-up rings, springs, or limited unsupported spans. It is the default for chemical gaskets, lined components, beaker and labware, and primary wire insulation.

Glass-filled PTFE (commonly 15 or 25 percent) is the workhorse mechanical grade. Glass fiber roughly doubles compressive strength, cuts deformation under load by about two thirds, and markedly improves wear resistance against softer metals, while keeping most of PTFE's temperature range and fair chemical resistance. The key caution is that glass is attacked by hydrofluoric acid and hot concentrated sodium hydroxide, media that virgin PTFE itself resists, so glass-filled grades must not be used where those chemicals contact the part.

Carbon and carbon-graphite grades raise the PV limit, add a degree of electrical conductivity (useful for dissipating static), and provide self-lubrication for dry-running and reciprocating service. Bronze-filled PTFE (40 to 60 percent bronze) gives the highest thermal conductivity of the common grades, raising it from about 0.25 W/m·K for virgin PTFE toward roughly 0.5 to 1.5 W/m·K, which lets heat escape the sliding interface in heavy-load, high-speed bearings; its trade-off is that the metallic filler rules it out of corrosive and food contact. ePTFE is a structurally distinct grade: solid PTFE stretched into a microporous node-and-fibril network, soft and conformable, used as compressible gasket tape, breathable membrane, and filtration media rather than as load-bearing stock.

How PTFE Is Made

Because PTFE does not melt-flow, it cannot be injection molded; it is shaped by powder-metallurgy-style routes followed by sintering. The choice of route is set by the geometry: solid blocks and rod, thin-wall tube and tape, complex shapes, or porous membrane each have a preferred process. The table below summarizes the mainstream methods, the resin type they use, and the products they yield.

Compression molding is the foundational technique. Granular resin is poured into a die, cold-pressed at pressures from roughly 10 to 100 MPa to consolidate the powder into a coherent preform, then ejected and sintered in an oven. Sintering is the critical step: the preform is heated above the 327 degrees Celsius melt point, typically to 360 to 380 degrees Celsius, held to allow the particles to coalesce, then cooled in a controlled ramp. Sintering quality, not brand, governs the porosity, dielectric strength, and dimensional stability of the finished part, which is why specifiers should ask about the fabricator's process as well as the resin grade.

Ram extrusion feeds granular resin into a heated die where successive charges are compacted and sintered against the previously formed material, producing continuous rod and heavy-walled tube. ASTM D1710 specifies extruded PTFE rod, heavy-walled tubing, and basic shapes with wall thickness of 1.6 mm or greater, made from the granular resin of ASTM D4894. Paste extrusion handles fine-powder (dispersion) resin per ASTM D4895: the powder is blended with a volatile liquid lubricant, formed into a preform, and extruded into thin-wall tube, unsintered tape (the familiar plumber's thread-seal tape), and wire coatings; the lubricant is then driven off and the part sintered.

Skiving converts a sintered billet into film by mounting it on a lathe-like mandrel and peeling a continuous sheet with a blade, producing film from about 0.03 mm up to several millimeters thick for gaskets, liners, and tapes. Biaxial stretching produces ePTFE: a paste-extruded, calendered tape is rapidly stretched below the melt point so the matrix opens into a microporous network of nodes connected by fibrils. This process, commercialized by W. L. Gore in the 1970s, underlies expanded gasket tape, breathable membrane, and filtration media. Finished PTFE stock of any route is commonly machined (turned, milled, drilled) into final parts, since PTFE machines cleanly with sharp tools.

Chemical and Thermal Limits

PTFE's selling point is its chemical and thermal envelope, but that envelope has defined edges, and filled grades shrink it. Understanding both the strengths and the few genuine exceptions prevents the most expensive selection mistakes: a chemical attack that perforates a lining, or a thermal overload that decomposes the polymer and releases toxic gas.

Chemical resistance. Virgin PTFE resists essentially all industrial acids, bases, solvents, and oxidizers across its full temperature range. It is the standard lining for vessels and pipework carrying hydrochloric, sulfuric, hydrofluoric, nitric, and mixed acids, as well as aggressive organic solvents that swell or dissolve other polymers. The documented exceptions are narrow: molten alkali metals (sodium, potassium), elemental fluorine and certain fluorinating agents at elevated temperature, and chlorine trifluoride, all of which can attack the carbon-fluorine bond. PTFE also does not bond or glue easily and must be chemically etched (for example with a sodium-naphthalene treatment) before adhesives will hold.

Filled-grade caution. A filled grade is only as resistant as its weakest constituent. Glass-filled PTFE is attacked by hydrofluoric acid and by hot concentrated sodium hydroxide, even though virgin PTFE shrugs both off, because the exposed glass fiber dissolves. Bronze, carbon, and metal fillers likewise narrow chemical compatibility. The rule for corrosive or high-purity service is to specify virgin PTFE for any wetted surface, and reserve filled grades for mechanical duty where the media is benign.

Thermal envelope. PTFE has a maximum continuous service temperature of 260 degrees Celsius (500 degrees Fahrenheit) and tolerates short excursions to about 300 degrees Celsius. At the cold end it remains tough and usable to roughly -200 degrees Celsius, giving one of the widest service windows of any polymer. Above 400 degrees Celsius decomposition becomes appreciable and the evolved gases are toxic, so machining and any thermal work require local ventilation, and smoking near PTFE dust is prohibited. The table below summarizes the temperature behavior that matters for design.

One subtle but important point: a high rated temperature does not mean full strength at that temperature. PTFE softens progressively as it heats, with flexural modulus falling from roughly 650 N/mm2 at room temperature to about 200 N/mm2 at 100 degrees Celsius and near 45 N/mm2 at 260 degrees Celsius. A seal or bearing rated for 260 degrees Celsius must therefore be derated for load at high temperature, or stiffened with a filler, or backed by a metal support, rather than relied upon at full mechanical capacity at the top of its thermal range.

Key Specification Parameters

A PTFE datasheet lists physical, mechanical, thermal, and electrical properties, but only a handful drive most selection decisions. The table below collects the typical values for virgin PTFE with their test methods; filled grades shift these numbers, generally trading higher strength and conductivity for lower chemical and electrical performance. Treat these as representative ranges for initial selection and confirm against the specific supplier datasheet before design.

Coefficient of friction is PTFE's signature property: at 0.05 to 0.09 it is the lowest of any solid material, and uniquely the static and dynamic values are nearly identical, so PTFE bearings do not "stick" at start-up. This is why it is the bushing and slide material of choice where lubricants cannot be used. Note that friction rises slightly with sliding speed, and that fillers raise it modestly in exchange for wear life.

Dielectric properties are exceptional and stable. The dielectric constant of about 2.1 is among the lowest of all solids and barely changes with frequency up to gigahertz ranges or with temperature up to 300 degrees Celsius, while dielectric strength runs from 20 kV/mm in thick molded stock to 60 to 80 kV/mm in thin skived tape. Combined with negligible water absorption (under 0.01 percent), this makes PTFE the insulation standard for coaxial cable, high-frequency circuit substrates, and aerospace wiring.

Flammability is effectively nil for an organic polymer: PTFE has a limiting oxygen index near 95 percent, meaning a surrounding atmosphere would need 95 percent oxygen to sustain a flame, and it carries a UL94 V-0 rating. It does not propagate fire. The caveat is thermal decomposition above 400 degrees Celsius, which releases toxic fluorinated gases; PTFE is fire-safe in service but hazardous if overheated.

Creep and deformation under load are the parameters that virgin PTFE designers watch most closely, because they are PTFE's true weakness. Virgin PTFE deforms measurably under sustained compression, so creep (cold flow) and deformation-under-load figures, not tensile strength, often govern a seal or bearing design. Filled grades exist primarily to address this: glass and bronze fillers can cut deformation under load by roughly two thirds, which is why mechanical PTFE is almost always filled.

Selection Decision Factors

To turn the preceding chapters into a specific order, follow the decision sequence below. Most PTFE selection errors come from choosing a grade for one property (say, low friction) while ignoring a constraint that actually governs (chemical attack on the filler, or creep under load). These seven steps can serve as a fixed RFQ template.

1. Dominant duty: Decide first whether the part is primarily a chemical barrier, a dielectric, or a load-bearing mechanical element. This single decision drives the virgin-versus-filled choice more than any other factor.
2. Media and chemical compatibility: List every chemical the part will contact, including cleaning agents. For corrosive, high-purity, or food media, specify virgin PTFE; never put glass, bronze, or carbon fillers in contact with hydrofluoric acid, hot caustic, or food.
3. Grade and filler: If mechanical, match the filler to the failure mode: glass (15 to 25%) for general wear and creep, bronze (40 to 60%) for high load and heat dissipation, carbon or carbon-graphite for dry running and PV limit, MoS2 for low-load dry bearings.
4. Thermal and load combination: Confirm both the service temperature (continuous limit 260 degrees Celsius) and the load at that temperature, then derate or add filler or metal backing because PTFE softens with heat. State the PV product for bearings.
5. Form and process: Choose the stock form and fabrication route: machined billet or skived sheet for dimensional stability, ram-extruded rod and tube per ASTM D1710, paste-extruded thin-wall tube and tape, or ePTFE for conformable gaskets and membrane.
6. Standards and compliance: Specify the resin standard (ASTM D4894 granular, ASTM D4895 dispersion) and end-product standard (ASTM D1710 or ISO 13000), plus any FDA 21 CFR 177.1550, USP Class VI, or 3-A sanitary requirement for food, pharma, and medical service.
7. Total cost of ownership: Weigh material and machining cost against service life. PTFE costs more than commodity plastics, but in corrosive sealing and lining duty its lifespan and avoided downtime usually justify it; the wrong grade, by contrast, fails early and contaminates product.

One last dimension is supplier serviceability and traceability: confirm the resin grade and class on the certificate, the sintering process used by the fabricator, batch traceability, and (for regulated work) the compliance certifications. At the resin level the established names are The Chemours Company (Teflon), Daikin (Polyflon), AGC (Fluon), 3M Dyneon, and Solvay (Algoflon), with W. L. Gore the reference for expanded PTFE membrane and sealant tape. For machined stock, the fabricator's sintering quality matters as much as the brand, since it sets porosity, dielectric strength, and dimensional stability over the life of the part.