Ravi designs seals for chemical metering pumps at a manufacturer outside Pune. Last spring he pulled two datasheets side by side and nearly killed a project.
The PTFE gasket read Shore D 57. The POM housing read Shore D 80. His conclusion was immediate and wrong: PTFE was too soft to seal against 12 bar.
The pump has now run for eighteen months on that PTFE gasket. Nothing about the hardness number changed.
What Ravi had missed is that PTFE hardness isn’t the property a seal is asked to deliver. Indentation resistance tells you how far a sharp point sinks into a surface. A seal cares about compression, extrusion, and chemical survival, and hardness speaks to none of them.
That misunderstanding is the most common one in fluoropolymer selection. PTFE is the only common engineering plastic where the hardness value is not the selection lever at all.
This guide gives you PTFE hardness on every scale that reports it, shows how each filler grade moves the number, places PTFE beside POM and nylon on one shared measurement, and explains the property that actually limits a PTFE part. For the underlying measurement systems, start with our complete guide to plastics hardness.
What PTFE Hardness Actually Measures
Hardness is resistance to indentation under a defined load. That is the whole property. It doesn’t measure stiffness, tensile strength, wear resistance, or load capacity, and it never has.
PTFE gets reported on four different scales, which is where most of the confusion starts:
- Shore D (ASTM D2240, ISO 868, DIN 53505) is the standard durometer scale for PTFE and the one most datasheets quote.
- Ball indentation hardness (ISO 2039-1 and DIN 53 456, test condition H 358/30) is the European method preferred by the seal industry, reported in MPa or N/mm².
- Rockwell R (ASTM D785) is rarely used for PTFE at all, for reasons covered below.
- Vickers (5.9 to 6.5 HV) appears mostly in microhardness research.
PTFE ball indentation hardness and PTFE Shore D hardness answer the same question with different indenters. The first rule of reading either is that a value without its scale, its standard, and its filler grade is not a specification. It is a number. Two labs can measure the same material and disagree simply because one used ASTM D2240 and the other used ISO 868, or because one tested a virgin rod and the other a glass-filled sheet.
Comparing a PTFE datasheet against another plastic and unsure if the numbers are even on the same scale? Send us both datasheets and our technical team will tell you which values are comparable and which are not. Request a grade review and get a response within 24 hours.
PTFE Hardness by Grade: Virgin, Modified, and Filled
This is the table the rest of the internet doesn’t publish. Reference databases report PTFE Shore D hardness from 11 to 70 across dozens of grades, with an average near 57 . That spread is exactly why an average tells you nothing. The table below comes from compound datasheets and test guidelines, and every value is labelled with its scale and filler content.
| Grade | Shore D | Ball indentation (N/mm²) | Friction vs virgin | Chemical inertness | Food-contact suitability |
|---|---|---|---|---|---|
| Virgin (unfilled) PTFE | 50–65, typical 55–60 | 23–37, average ~32 | Baseline (µ 0.04–0.10) | Universal | Yes (21 CFR 177.1550, USP Class VI) |
| Modified (2nd generation) PTFE | 56–60 | ~30 | Similar | Universal | Yes |
| 15–25% glass fibre | 60–67 | ~33 | Higher | Reduced (alkalis attack glass) | Grade-dependent |
| Carbon / carbon fibre | 61–66 | ~34 | Higher | Reduced | Grade-dependent |
| Carbon-graphite | ~64 | ~35 | Similar to carbon | Reduced, conductive | Usually not |
| 40–60% bronze | 64–69 | ~41 | Higher | Reduced (acids) | No |
| 20% polyimide | ~65 | ~38 | Higher | Reduced | Grade-dependent |
| 40% polyimide | ~70 | ~42 | Higher | Reduced | Grade-dependent |
A few things in that table deserve emphasis.
Virgin PTFE hardness settles at Shore D 50 to 65 per ASTM D2240, with 55 to 60 as the realistic band . The German fluoropolymer industry association pro-K sets a minimum of Shore D 54 and 22 N/mm² ball indentation for virgin material, which is why so many datasheets cluster at the bottom of that range.
The filler ladder is monotonic. Glass filled PTFE hardness lands around Shore D 60 to 67, carbon filled PTFE hardness around 61 to 66, and bronze filled PTFE hardness around 64 to 69. Hard polymer fillers like polyimide push further, reaching Shore D 70 at 40% loading. Fillers typically raise PTFE hardness by 10 to 30%.
One direction check matters. Carbon fibre, glass fibre, and bronze reliably increase hardness. Molybdenum disulphide and graphite, however, are reported both ways: some sources show them lowering hardness while still improving wear performance, and pro-K’s own processing guidance treats them as neutral to positive. The honest answer is that the effect depends on the specific compound and loading, so verify on the datasheet for the grade you are buying.
PTFE Rockwell hardness deserves a warning. Credible values sit at roughly R40 to R58, which puts PTFE at or below the practical floor of the Rockwell R scale. That’s why most references decline to report PTFE on Rockwell at all.
If you find a listing claiming R118, or an M-scale value, or a “Rockwell hardness 55 Shore D” entry, you’ve found a data error, not a material property. Our guide to Rockwell hardness in plastics explains how those errors arise.
Why PTFE Is Soft, and Why That Is Not a Defect
PTFE’s chains are smooth and sheathed in fluorine atoms, which shield the carbon backbone and produce exceptionally weak intermolecular forces. There is no cross-linking to stiffen the network. The result is a low modulus of elasticity, about 0.8 GPa against nylon’s 1.7 GPa and POM’s 2.8 GPa.
That softness arrives bundled with the properties that actually get PTFE specified. The same structure that makes it easy to indent makes it inert to nearly every industrial chemical and solvent, gives it one of the lowest coefficients of friction of any solid, and holds service temperature from −260 °C to +260 °C continuous with a melting point of 327 °C.
Answering the question directly: yes, PTFE is soft, and it is chosen despite that, never because of it. A supplier that tells you PTFE is the wrong material for impact or structural load is being accurate, not modest. Hardness was never going to predict tensile strength, elongation at break (300 to 400% for virgin PTFE), dielectric strength, or chemical resistance, and those are the properties PTFE buyers actually depend on.
For the broader argument about what hardness does and does not predict, our plastic hardness material selection guide works through the failure modes where a hardness specification actively makes a part worse.
Is PTFE the Softest Engineering Plastic?
On ball indentation, yes, by a wide margin.
| Material | Ball indentation (MPa) | Modulus (GPa) | Coefficient of friction |
|---|---|---|---|
| PTFE | ~30 | 0.8 | 0.04–0.10 |
| PA6 | ~160 | n/a | 0.25–0.50 |
| PA66 | ~160–170 | 1.7 | 0.25–0.50 |
| POM | ~140–170 | 2.8 | 0.20–0.35 |
Treat those figures as an order-of-magnitude contrast rather than exact values. Test method and grade shift the absolute numbers between sources, but the direction never moves: PTFE sits roughly five to six times softer than nylon or acetal on the same measurement scale. Our engineering plastic hardness comparison carries the full side-by-side.
But note the scale trap. PTFE is quoted on Shore D, POM and PC usually on Rockwell M, and PA66 and ABS on Rockwell R. Those lanes don’t convert into each other, and comparing a Shore D value to a Rockwell M value isn’t a comparison at all.
For the POM side of that equation, our POM hardness breakdown works through the scales that material is reported on.
And the property that runs the other way is friction, where PTFE outperforms every material above it by a factor of three or more.
Inside the fluoropolymer family, PTFE isn’t even the outlier. FEP hardness reads about Shore D 56, PTFE about 55, and PFA hardness about 60, then ETFE 63 to 75 and PCTFE 75 to 80. PFA is the melt-processable alternative when a slightly harder, more creep-resistant fluoropolymer is acceptable. If you’re still working out how the D scale reads at all, our Shore hardness scale guide walks through it.
Cold Flow: The Property That Actually Limits a PTFE Part
Here is the insight that no datasheet row communicates. PTFE’s total deformation under load has three parts: elastic deformation that recovers instantly, viscoelastic deformation that recovers over time, and permanent plastic deformation, which is cold flow.
PTFE recovers only about half of its total deformation after the load is removed. The other half stays. Cold flow happens at room temperature, well below yield, and it is the real design limit for PTFE seals, gaskets, and structural supports.
Deformation and hardness are coupled. Research on PTFE microhardness shows that mechanical deformation reduces the measured microhardness by up to 30%, with a measurable drop at strains as low as 2%. Compressed specimens tested for wear showed roughly 20% larger scar volumes at about 6% strain, while the coefficient of friction stayed essentially unchanged.
Read that carefully. A PTFE part in service reads softer than a fresh specimen, and a deformed part wears faster. Hardness still doesn’t predict the wear, because friction didn’t move.
The fix is also not what most people assume. pro-K’s tribology guidance is explicit that cold flow and wear are separate problems with separate solutions: modified PTFE improves cold flow, while wear should be addressed through fillers. And because PTFE is not elastic, a seal needs an energiser, an O-ring or a spring, to maintain contact. That is why a reliable sealing design looks at compressive deformation, counterface roughness (Ra 0.1 to 0.2 µm for low-viscosity gaps), and energiser design rather than at the durometer reading.
Owen, a reliability engineer at a chemical plant in Rotterdam, chased a leak on a PTFE-lined valve for two months. The gasket had extruded and lost compression set.
His first instinct was to specify a harder, more heavily filled grade. The right answer was the opposite: a modified PTFE with better cold-flow resistance under continued load, supported by a proper anti-extrusion ring. Naming those two mechanisms separately is what solved it.
What Changes a PTFE Hardness Reading
If a PTFE lot reads differently from the one your part was validated on, the cause is almost always one of these.
- Filler type and loading is the largest single lever, and the reason a “PTFE hardness” chart without a filler column misleads.
- Filler dispersion and compound quality can shift the number between suppliers using the same nominal recipe.
- Crystallinity and the sintering or molding cycle, including whether the part was molded, machined, or skived.
- Temperature, since hardness falls as service temperature climbs toward the 260 °C limit.
- Deformation history, because a part already under load reads lower.
- Test conditions, meaning the standard used, the specimen thickness, and the durometer dwell time.
Our guide to measuring plastic hardness covers the procedure side of those variables in detail.
Keeping a PTFE Spec Meaningful at the Supply Stage
A hardness specification only survives procurement if it says what it means. “PTFE, Shore D 57” is incomplete. The version an inspector can verify reads like this: “PTFE, virgin, Shore D 55 to 60 per ASTM D2240, 6 mm specimen, 23 °C, lot certificate stating specific gravity and ash content.”
That sentence names four things an incomplete spec leaves out:
- The scale and standard
- The specimen condition
- The filler type and loading
- The lot documentation that corroborates it
The check on an incoming lot isn’t the hardness value itself. Filler content, filler type, and dispersion are what move a PTFE reading, so the diagnostic quantities are specific gravity, a direct proxy for filler loading, and ash content, cross-referenced against the lot number. A low reading on a filled grade nearly always means the filler changed, not the polymer.
Working from a drawing that specifies a PTFE hardness value you cannot source? Send us the number and the application. If your drawing calls for Shore D 65 on a virgin PTFE seal, that number is not achievable, and we will tell you so and propose the grade that delivers what the part actually needs. Tell us the number and the application and we’ll come back with branded prime fluoropolymer resin, full COA documentation, 25 kg minimum order, FOB Shanghai, within 24 hours.
PTFE Hardness FAQ
What is the hardness of PTFE?
Unfilled PTFE reads Shore D 50 to 65, typically 55 to 60. PTFE ball indentation hardness runs 23 to 37 MPa with an average around 32 MPa (ISO 2039-1), against an industry minimum of 22 N/mm². Rockwell R values sit at roughly R40 to R58.
What is the Shore D hardness of PTFE?
Shore D 50 to 65 for unfilled material. Filled grades run from about 60 up to 70 depending on the filler and its loading.
What is the Rockwell hardness of PTFE?
Roughly R40 to R58, which places PTFE at or below the practical floor of the Rockwell R scale. That is why many references decline to report PTFE on Rockwell at all, and why any listing claiming R118 or an M-scale value should be treated as a data error.
Does filling make PTFE harder?
Glass fibre, carbon, carbon fibre, bronze, and polyimide all raise hardness, with bronze and polyimide moving it furthest. MoS₂ and graphite are reported both ways and depend on the compound, so confirm on the specific grade datasheet.
Is PTFE hard or soft?
Soft. On ball indentation it is roughly five to six times softer than POM or nylon, and it is selected in spite of that, for chemical inertness and low friction.
Is Teflon hardness different from PTFE hardness?
No. Teflon™ is a Chemours brand name for PTFE, so the two describe the same polymer. A value published under either name still needs its scale, its standard, and its filler grade to be usable.
What is PTFE hardness vs POM hardness?
They aren’t comparable at face value. POM is normally quoted on Rockwell M and PTFE on Shore D, and those scales don’t convert. On a shared ball indentation measurement, PTFE reads about 30 MPa against 140 to 170 MPa for POM.
Is PTFE harder than HDPE?
Both are reported on Shore D, and the answer depends entirely on filler grade. Compare them only with the filler content and standard stated for each.
Is filled PTFE still chemically resistant?
Less universally than virgin material. Fillers are the chemical weak point: glass is attacked by strong alkalis and bronze-filled grades have reduced acid resistance. Virgin PTFE is the reference for universal chemical resistance and for FDA and USP Class VI compliance.
Does PTFE hardness change with temperature?
Yes. It falls as service temperature rises toward the continuous limit of 260 °C.
The Bottom Line
PTFE hardness is a small number with a large amount of context behind it. Five points carry the decision:
- Shore D 50 to 65 is the unfilled baseline, and 55 to 60 is the realistic band.
- The filler, not the polymer name, sets the hardness value, and a value without its filler grade is not a specification.
- PTFE is the softest common engineering plastic on ball indentation, and that is not the reason parts fail.
- Cold flow, not indentation resistance, is the property that limits a PTFE part, and it needs modified PTFE rather than a filler.
- Filled grades buy hardness with chemical inertness, purity, and counterface life.
State the scale, the standard, the filler, and the specimen condition, and a PTFE hardness number becomes useful. Leave any of them out and it becomes the kind of number that nearly cost Ravi his project.
Choosing between virgin and filled PTFE for a seal, gasket, or chemical-service part? Suzhou Yifuhui New Material supplies branded prime fluoropolymer grades including Teflon™ (a Chemours brand of PTFE), Dyneon™, Polyflon™, and Fluon®, with COA documentation that states filler type and content. Request a quote for a response within 24 hours.
