PTFE vs PFA vs FEP vs PVDF: which fluoropolymer to pick

At a glance: nine fluorinated materials (PVF oxygen cell merges white and clear film; ETFE melting sources disagree; PVF dielectric constant is a pigmented film at 1 kHz)

Which fluoropolymer has the best chemical resistance?

The sources call PFA, FEP and PTFE the most resistant, but our data cannot rank any grade by test. PFA is described as resisting virtually all chemicals except molten alkali metals, gaseous fluorine and chlorine trifluoride at high temperature. FEP is described as inert to most chemicals except at high temperature, and attacked by high-pressure elemental fluorine and molten alkali metals. A safety data sheet says PTFE does not dissolve in water or organic solvents, but its own corrosion entry is rated unknown because no quantitative table was opened. ETFE has no chemical entry in our data.

ECTFE is listed as resisting most acids, alkalis and organic solvents, except hot amines and ketones. Its profile warns that small molecules such as water vapour, chlorine and hydrogen chloride can permeate a lining and reach the substrate; no profile holds permeation numbers.

PVDF carries a clear solvent warning. Arkema is quoted as saying Kynar homopolymers resist most acids, weak bases, halogens, oxidants and salts at ambient temperature, but dissolve in active solvents (among them tetrahydrofuran, dimethyl formamide and N-methyl-2-pyrrolidone). Acetone and cyclohexanone do not dissolve or substantially swell it at room temperature. The profile holds no alkali-attack test; strong bases can damage PVDF†, so ask about your base.

The rest are narrower. PVF white film showed no visible effect after 1 year in acids, bases or solvents at 25 °C. PCTFE has one chemical result: tensile strength at 77 K of 127.78 MPa before liquid-oxygen immersion and 125.10 MPa after 60 days, a mechanical result, not an ignition test. FKM resists fuels, oils, aromatics and many solvents but hardens and sets early in hot water or steam.

Which fluoropolymer handles the highest temperature?

PTFE and PFA are listed highest, at 260 °C, followed by FEP at 204 °C. PTFE melts at 327 to 335 °C, PFA at 302 to 310 °C and FEP at 245 to 275 °C. Those are the only three plastics in this set with a service limit above 200 °C. The partly fluorinated plastics are listed lower: PCTFE at 175 °C, ETFE at 150 to 160 °C, ECTFE at 149 to 150 °C and PVF white film at 107 °C. Their melting points sit higher: ECTFE 238 °C, PCTFE 210 to 212 °C, ETFE 210 to 265 °C (two Daikin documents for one grade disagree).

PVDF has no service temperature in our data. The Kynar 1000 Series melts at 169 to 171 °C, and its deflection temperature under a light load is 125 to 140 °C. Arkema rates cross-linked Kynar wire and tubing for continuous use to 180 °C, a special form.

In the cold, PTFE is listed down to -200 °C by a supplier, yet in tension tests its strain at break fell from 302.96 percent at room temperature to 2.51 percent at 20 K. ECTFE is listed down to -76 °C, PVF white film to -72 °C, and the FKM static seal limit is about -26 °C.

FEP, PTFE, PFA, ECTFE and PVF give trigger temperatures for hydrogen fluoride or other toxic fumes: above 250 °C for FEP, above 300 °C for PTFE (lung injury above 400 °C), and about 350 °C for PFA, ECTFE and PVF (in air). The PVDF, ETFE and PCTFE profiles give none. Melt processing runs above these triggers (the PFA profile has molten resin at 390 °C), so it needs ventilation or extraction, and tobacco must be kept away from the dust.

Which fluoropolymer is the strongest?

The highest numbers are PVF white film minimums of 55 to 62 MPa and one ETFE Tefzel film at 60.4 MPa; ECTFE reads 48.3 to 55.2 MPa; PVDF yields at 45 to 55 MPa and breaks at 35 to 50 MPa on a distributor page; PCTFE reads 34 to 39 MPa on its resin datasheet. The perfluorinated grades are the weakest: PTFE reads 18.15 MPa in one test and 23 to 27 MPa on supplier stock, PFA 21 MPa on film, FEP 20 MPa.

PVDF's flexural modulus of 1.66 to 2.31 GPa is higher than FEP at 0.52 GPa and PFA at 0.54 to 0.59 GPa, so it is stiffer than those two, but its elongation at break, 20 to 100 %, is far below the 300 percent of PFA film. ETFE is listed at 250 to 600 %, ECTFE at 200 to 250 % and PVF white film at 90 to 110 % as minimums.

In the cold, at 77 K PTFE reached 86.22 MPa and PCTFE film reached 127.78 MPa, while PFA reached 130 MPa at -198 °C. These ground tests used different specimens: they show that these plastics gain strength when cold and that PTFE and PCTFE lose most of their stretch, not which is best. Creep, cold flow, wear and fatigue are not in our data.

Which fluoropolymer is the best electrical insulator?

On dielectric constant alone, PTFE, PFA and FEP lead, at about 2: PTFE reads 2.1, PFA 2 to 2.03 and FEP 2.03, near 1 MHz or without a frequency on the sheet. ETFE reads 2.6 at 1 MHz and ECTFE 2.5. Loss factor and resistivity matter too, and our data has them for only a few grades: FEP has a dissipation factor of 0.0006 at 1 MHz, PTFE a volume resistivity of 10^17 to 10^18 ohm cm, and ECTFE 10^15 ohm cm at 50 percent relative humidity.

Breakdown strength depends on thickness, so it cannot be ranked. PTFE reads 30 to 45 kV/mm on a 0.5 mm specimen. PFA reads 80 to 260 kV/mm, from 80 kV/mm on 0.25 mm to 260 kV/mm on 0.025 mm film. FEP reads 80 kV/mm with no thickness stated, and ECTFE 19.7 kV/mm on a 1/8 inch sample. PVDF and PCTFE have no dielectric data. The 11 for PVF white film is a pigmented film at 1 kHz, so do not use it to rank PVF.

Which fluoropolymer survives atomic oxygen and radiation in space?

Atomic oxygen: on the MISSE 2 flight PTFE eroded at 1.4e-25 cm3/atom, which its profile calls about twenty times slower than Kapton H. PFA eroded 1.2 times and FEP 1.4 times as fast as PTFE, PCTFE 5.9 times, ETFE 6.8 times, PVDF 9.1 times and ECTFE 12.6 times. ECTFE measured 1.79e-24 against 3.00e-24 for Kapton H.

Radiation, in one list (kGy; property, atmosphere and source differ, so the grades cannot be ranked). PTFE: a 1968 NASA guide says it is useless after one or two megarads (about 10 to 20 kGy) in air, and loses 88 percent of its elongation at 8 megarads (80 kGy) in vacuum; NASA Glenn calls it significantly more susceptible than FEP. PFA: embrittlement above 10 kGy, a supplier gamma tolerance of 5 to 10 kGy, and marked loss of elongation at 10 Mrad (100 kGy) in an AGC test; the PTFE and PFA figures are the same order of magnitude but defined differently. FEP: no dose figure; Hubble's retrieved film lost most of its stretch in long exposure to thermal cycling and radiation combined. ETFE: elongation fell slowly to 200 kGy and dropped sharply above about 400 kGy in air (one electron-beam abstract, no per-dose values). PVDF: a vendor statement of essentially unchanged tensile strength after 100 Mrad (1,000 kGy) of gamma at 50 °C in vacuum, with slightly lower impact strength and elongation. PCTFE: said to degrade like PTFE, no dose figure. ECTFE: no data. FKM: a 66.7 percent compression set after 1 x 10^7 rad of gamma, where the source says a compound above 40 percent set needs a lower dose.

Where is each fluoropolymer used?

PTFE is used as cable and wire insulation, including expanded PTFE in the trailing cable of the ISS Mobile Transporter, which has a 10-year mission life; a NASA Glenn prediction of surface cracking over that life rests on ground tests, not flown data. PFA is used for high-temperature wire and cable insulation, including plenum cable, and for tubing, valve linings and fittings in corrosive and semiconductor service. FEP is used for wire and cable coating films, tubes, and pipe and valve linings and sleeves.

ETFE goes into wire and cable for aerospace, automotive and appliances, and building film sold as Texlon and Sumiflon-E. ECTFE is used for chemical-industry pipes, tanks and linings, cable jacketing, semiconductor wet benches, and gaskets for liquid oxygen and other propellants. PVDF is a lithium-ion electrode binder (Kureha) and a cross-linked wire and heat-shrink tubing material.

PCTFE is a cryogenic seal material: it was the preferred inner layer of a liquid-oxygen hose among four fluoropolymers in one study, and its valve seats for oxygen regulators were tested. PVF white film is a protective film on photovoltaic module backs, aircraft cabin walls and ceilings, and insulating bags. FKM is a seal material for fuel, oil and chemical equipment, including jet-fuel O-rings.

Terms used

Fluoropolymer

a plastic whose carbon chain carries fluorine atoms. The carbon-fluorine bond is strong, which is why these plastics resist many chemicals.

Melt-processible

can be melted and shaped by extrusion or moulding like an ordinary thermoplastic. Our profiles describe PFA and FEP this way; PTFE is usually shaped from powder and machined from stock†.

Dielectric constant

how much a material stores electric energy compared with vacuum. A low value, near 2, helps signals travel with less loss in cables.

Dielectric strength

the electric field a thin sample withstands before it breaks down. It falls as the sample gets thicker, so compare only values taken at the same thickness.

Atomic oxygen erosion yield

the volume of plastic one oxygen atom removes in low Earth orbit, in cm3 per atom. Lower means the plastic wears away more slowly.

Total ionizing dose

the radiation energy a material absorbs. Our profiles print it in kilogray (kGy), megarad (Mrad) or rad. One megarad is 10 kilogray†.

What to check before you order

1. Ask for a chemical-resistance table at your chemical, concentration and temperature, and which basis the temperature limit uses (continuous, 20,000 hours, typical).
2. For PFA, say whether you need a resin grade or a film, and ask for the values of that grade and the test standard behind each strength and dielectric value.
3. For insulation, ask for dielectric constant, loss factor and volume resistivity at your frequency, temperature and thickness.
4. Give the radiation dose with its unit and atmosphere, and ask for property data after that dose.
5. For PVDF, list every solvent and base the part will touch. For linings, ask for permeation data; for PTFE seals, creep and cold-flow data.
6. For liquid-oxygen or cryogenic seals, ask for a liquid-oxygen ignition and impact compatibility report at your temperature and pressure, and for seat stability data.
7. Plan ventilation or extraction for any melt processing, because it runs above the fume trigger temperatures listed in the profiles.

What do we not know yet?

• The profiles hold no quantitative chemical-resistance table. PTFE's entry is rated unknown, ETFE has no chemical entry, PCTFE has only a liquid-oxygen strength result, and no profile covers liquid-oxygen ignition or impact compatibility.
• Electrical data is uneven: PVDF and PCTFE have no dielectric constant or strength, and loss factor and resistivity exist for only a few grades.
• Strength values are supplier typical values or single tests on different forms. Creep, cold flow, wear, fatigue and permeation are not in the data, and Wikipedia calls PCTFE the lowest water-vapour transmission plastic with no number.
• Radiation figures come from different sources, units and atmospheres, and no common test ranks all nine.
• PVDF has no service temperature, outgassing value or hydrogen-fluoride trigger temperature, and only the Kynar 1000 Series is recorded.
• Relative cost order, PFAS rules and PTFE cold flow under seal load matter in real selection†; none is in the profiles. FKM has no atomic oxygen or outgassing value.

Questions people ask