PTFE vs PCTFE for Seats, Seals and Oxygen Service

Updated October 13, 2026

PTFE and PCTFE side by side

The two fluoropolymers look alike on a bench and behave very differently under a sealing load. PTFE is softer, takes more heat and has the better oxygen ignition record. PCTFE is harder, creeps far less, absorbs essentially no water and blocks gas better. The table pulls typical values from named data sheets; none are specification minimums, and sources test to different methods, so compare rows rather than decimals.

The deformation row is the one that decides most seat and seal designs. At almost the same stress and time, the PTFE sheet deformed five to six times as much as the PCTFE stock, and half of that PTFE deformation did not recover.

Cold flow and creep: why seats move

PTFE cold flows at room temperature under modest stress; PCTFE does not to any comparable degree. That single difference explains why PTFE seals usually need a design that keeps load on them and why PCTFE is chosen for seats that must hold a precise sealing line.

• Bolted joints. A plain PTFE gasket relaxes after torque, so bolt load drops over hours and days. Retorque procedures, filled PTFE, or a confined groove that limits extrusion are the usual answers.
• Valve seats. A soft seat that flows under the closing load develops a wider, shallower land and a changing leak path. PCTFE's higher hardness and low deformation keep the land geometry, which is why it is widely used for poppet and regulator seat inserts in high pressure gas service.
• Temperature. Creep accelerates with temperature for both, and PCTFE runs out of temperature first: Angst+Pfister lists 150 C long term for its PCTFE stock against 260 C for the PTFE above. Hot service with low creep needs a filled PTFE or another polymer, not PCTFE.

Filled PTFE, with glass, carbon or bronze, reduces creep markedly but changes chemical and oxygen behavior. A NASA MSFC study of a brass filled PTFE shows how much the filler matters in oxygen: it reported no reactions in 80 additional gaseous oxygen impacts at 98 J and 10,000 psia, but four reactions in the additional 80 liquid oxygen impacts, and the authors stressed that the metal chosen for the filler drives the result.

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Cryogenic sealing

Both stay usable at cryogenic temperature, which is why both appear on LOX, LN2 and liquid hydrogen hardware. The design problem at cryogenic temperature is shrinkage, not brittleness: a polymer seal contracts much more than the steel or aluminum body around it, and the seal load falls as the joint cools.

• PCTFE's low creep and higher modulus mean a pressed-in seat holds interference better through thermal cycles. Angst+Pfister describes its PCTFE as ideal for liquid gases, particularly liquid air and liquid oxygen, and lists a linear expansion of 4.5 to 7.0 x 10^-5 per C from 23 to 200 C.
• PTFE is more forgiving of surface finish and minor damage because it conforms, but needs a spring energizer, Belleville stack or confined geometry to keep sealing load after cooldown.
• Leak checks at ambient temperature do not prove a cryogenic joint. Specify a cold shock or cryogenic proof and leak test where the program requires one.

Oxygen compatibility: published ignition data

For oxygen hardware, PTFE has the stronger published ignition numbers, while PCTFE is chosen where its shape stability matters more and the configuration has been assessed. Neither is "oxygen compatible" in the abstract; compatibility depends on pressure, temperature, geometry, cleanliness and ignition mechanisms in the specific system. The test framework, including NASA-STD-6001 and the full test list, is on our engineering plastics page; the data below is what the two materials actually show in published work.

• Autogenous ignition (ASTM G72). A NASA Marshall paper by Herald, Frisby and Davis, citing data from Chou and Giedorowicz with a 1,500 psia starting pressure, lists PTFE above 425 C, the test's upper limit, and Kel-F 81 PCTFE at 404.3 C. A NASA White Sands study by Smith (Journal of ASTM International, 2009) measured PTFE AITs of 446 C in 21 percent oxygen and 439 C in 100 percent oxygen, a change of under 10 C.
• Heat of combustion. The same Marshall paper lists 1,517 cal/g for PTFE and 2,556 cal/g for Kel-F 81, so PCTFE releases roughly two thirds more energy if it does ignite.
• Mechanical impact. ASTM G86 covers mechanical impact in ambient liquid oxygen and pressurized liquid and gaseous oxygen. In the Chou and Giedorowicz comparison as reported by Marshall, PTFE was the only one of the fluorinated polymers listed, which included Kel-F 81, to pass the ambient pressure liquid oxygen mechanical impact test.
• Pneumatic impact (ASTM G74). The White Sands study found PTFE did not ignite at 9,500 psi in 21 percent oxygen but ignited at 1,500 psi in 100 percent oxygen, passing at 1,000 psi. Adiabatic compression is a real threat even to PTFE in pure oxygen.

Practical reading: PCTFE's lower AIT and higher heat of combustion are the price paid for its dimensional stability. Programs accept that trade for seats in regulators and valves when the configuration, pressure and test data support it. Always cite the specific test data, lot and cleaning level in the oxygen compatibility assessment, and see precision and oxygen cleaning for cleaning and packaging after machining.

Permeability and moisture

PCTFE is the better barrier. Daikin describes NEOFLON PCTFE as the fluororesin with the highest gas barrier property, and Fluorotherm lists PCTFE permeability of 1.5 x 10^-10 for oxygen, 0.18 x 10^-10 for nitrogen and 56.4 x 10^-10 for hydrogen, all in cc cm per cm2 s atm, with 0.00 percent water absorption by ASTM D570. Hydrogen is the outlier: it permeates PCTFE more than 30 times faster than oxygen, which matters for small seals and thin films in hydrogen and helium service.

PTFE is sintered rather than melt processed, and ASTM D3294 includes a porosity requirement for molded PTFE sheet and shapes, which tells you voids are a known quality variable. For thin diaphragms or films holding helium or hydrogen, that is a reason to ask for permeation data rather than assume PTFE behaves like PCTFE. Neither source set gives a directly comparable PTFE permeability figure at the same conditions, so if permeation drives the design, ask the material producer for data at your gas, temperature and thickness.

Seat and seal uses in valves

These are common practice, not rules. For purchased valves, see check and relief valves and gaskets and packings.

Specifications

PTFE is controlled by resin specifications and a product specification for molded stock; PCTFE by one classification system. Confirm the current revision with ASTM before putting one on a drawing.

One caution about D1430: ASTM notes PCTFE properties depend on the crystallinity produced by processing and annealing, and the property values in the standard apply only to specimens molded under its procedure. They are not design values for finished parts. Two PCTFE parts of the same resin can differ in clarity, hardness and toughness depending on how the stock was molded and cooled.

Machinability

PCTFE machines more like a hard plastic and holds tolerance; PTFE machines easily but is hard to hold. Notes that matter:

• PTFE moves. It is soft, springs back from the tool, and cold flows in a tight fixture. Light clamping, sharp tools, and measuring after the part has relaxed are standard. Avoid tight tolerances on thin PTFE walls.
• PCTFE chips. It is harder and can chip at sharp edges and thread roots. Sharp, polished tools and light finishing passes give the best edges on seat lands.
• Annealing. Boedeker's post-machining air annealing guidelines are 3 hours to 525 F for PTFE and 3 hours to 225 F for PCTFE, each holding 60 minutes per 1/4 inch of thickness and cooling at 50 F per hour, with parts fixtured and critical dimensions finish machined after annealing.
• Contamination. For oxygen parts, keep hydrocarbon coolants and marking inks off the part or plan for cleaning to the specified level, and bag the parts as cleaned.

Small seats and inserts are typical Swiss turning work.

Choose PTFE when, choose PCTFE when

Choose PTFE when:

• Service temperature runs above about 150 C.
• Oxygen ignition margin drives the design and the geometry can confine the seal against cold flow.
• You need the lowest friction or the broadest chemical resistance.
• The seal is spring energized or otherwise kept under load.

Choose PCTFE when:

• A seat or seal must hold a precise land under pressure without creeping.
• Gas permeation or moisture pickup must be minimal.
• The part sees cryogenic cycles and must keep its interference fit.
• Your oxygen compatibility assessment accepts PCTFE's ignition data for the pressure and configuration.

Cost and availability drivers

PTFE is a commodity fluoropolymer with many resin producers and wide stock availability in sheet, rod, tube and skived film. PCTFE resin comes from a short list of producers, so stock is less common, especially in large diameters or thick sheet. Drawings that still call out a discontinued trade name such as Kel-F 81 force a substitution question on every order. Virgin versus reprocessed resin, filled grades, oxygen cleaning, and lot traceability to the resin producer all add cost and time.

Callout examples

• MATERIAL: PCTFE PER ASTM D1430, UNFILLED, VIRGIN RESIN. NEOFLON M-300 SERIES OR APPROVED EQUAL. CERT TO STATE RESIN PRODUCER, GRADE AND STOCK LOT. CLEAN FOR OXYGEN SERVICE PER [PROGRAM SPEC] AND DOUBLE BAG.
• MATERIAL: PTFE, UNPIGMENTED, MOLDED SHAPE PER ASTM D3294 FROM ASTM D4894 RESIN (CONFIRM CURRENT REVISIONS WITH ASTM). NO REPROCESSED RESIN.
• SEAT MATERIAL SUBSTITUTION NOT PERMITTED WITHOUT WRITTEN APPROVAL. OXYGEN COMPATIBILITY ASSESSMENT REFERENCE: [DOCUMENT NUMBER].

State the inspection temperature for tight seat dimensions, any annealing step, and the surface finish on sealing lands.

Get PTFE or PCTFE seats and seals quoted

Send the seat or seal drawing with the material callout, oxygen cleaning level and packaging requirement through a CNC quote request. We flow the resin, lot traceability and cleaning requirements down to the supplier, check the cert against the drawing, and ship with the cert package. For cleaning as a separate service, see precision and oxygen cleaning. For the high temperature engineering plastics, see PEEK vs ULTEM and the materials hub.

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• Precision Cleaning and Oxygen Service Cleaning
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Sources

• Daikin, NEOFLON PCTFE M-300H technical data sheet (20 Feb 2025)
• Daikin, NEOFLON PCTFE M-300P technical data sheet (March 2018)
• Angst+Pfister, APSOplast PCTFE technical data sheet (19 Jun 2024)
• Freudenberg FST, PTFE W-FLON data sheet (rev 6, 19 Jan 2024)
• Plastics International, PTFE data sheet
• Fluorotherm, PCTFE properties
• Herald, Frisby and Davis, NASA MSFC, Oxygen Compatibility of Brass-Filled PTFE Compared to Commonly Used Fluorinated Polymers
• Smith, NASA WSTF, Effect of Oxygen Concentration on AIT and Pneumatic Impact Ignitability of Nonmetallic Materials (2009)
• NASA White Sands Test Facility, ASTM G72 test description
• ASTM D1430-22, classification system for PCTFE plastics
• ASTM D4894-19(2024), granular PTFE molding and ram extrusion materials
• ASTM D4895-18, PTFE resin produced from dispersion
• ASTM D3294-22, PTFE molded sheet and molded basic shapes
• Boedeker Plastics, plastic annealing guidelines