PTFE and industrial rubber are the two workhorse families of soft-material sealing, and a maintenance engineer at a chemical skid builder picks between them on three numbers: continuous service temperature, chemical compatibility with the process fluid, and whether the joint is static (flange, vial cap) or dynamic (reciprocating rod, rotary shaft) [S1].
Filled PTFE — PTFE compounded with glass, carbon, graphite or bronze — runs continuously to roughly 260 °C and tolerates virtually all process chemicals except molten alkali metals and elemental fluorine; EPDM caps near 150 °C, NBR near 110 °C, FKM (Viton-class) near 200-230 °C, and silicone near 200-230 °C [S3]. Where the temperature and chemistry permit, PTFE seals are usually specified; where elastic recovery at low bolt load matters, industrial rubber is the default.
Static Joint Anatomy: PTFE-Faced Rubber Laminates as a 2025-2026 Workhorse
A common screw-cap septa build used in 2025-2026 lab and GC consumables is a 0.05 mm PTFE skin bonded to a 1.53 mm red-rubber core, in sizes AK73816-00015 / -00018 / -00020 / -00024, with the PTFE face giving chemical resistance and the red-rubber core giving resealability under low cap torque [S2]. The same pattern is sold as 8 mm rubber/PTFE seal packs of 100, listed on chemical catalogues under CB11000512 [S1].
For the flange engineer, the same logic translates: a pure PTFE envelope gasket (e.g. Garlock Style 3500) handles aggressive media but creeps under sustained bolt load, so it is paired with a rubber or flexible-graphite core; an NBR or FKM bonded door seal in a process analyser is chosen instead when the joint is a closed groove and the temperature stays inside the elastomer window. Choice of laminate vs monolithic gasket is therefore a function of bolt-load retention versus chemical attack on the wetted face.
Dynamic Sealing: Filled-PTFE Step Rings vs O-Rubbers on Reciprocating Rods
For hydraulic cylinder piston rods, a step-ring (Glyd-ring) pack stacks a filled-PTFE slip ring against an O-ring energiser; the O-ring provides radial pre-load and compensates for PTFE wear, while the filled-PTFE ring carries the rod at low friction (filled PTFE dynamic friction typically falls in the 0.05-0.15 range, against 0.2-0.6 for a lubricated NBR O-ring) [S3].
Two stacked step rings on the same rod can deliver effectively zero leakage in slow reciprocating duty, because the second ring scavenges the film that the first ring leaves on the surface; the penalty is harder assembly tolerance and a need for harder rod finishes (Ra ≤ 0.2 μm) than a pure rubber stack demands. Where reciprocation is slow, pressures exceed 25-40 MPa, or the fluid is a low-viscosity solvent that would extract a rubber's plasticiser, PTFE step rings are the right call; for general-purpose pneumatics at 0.5-1.0 MPa, a wiper plus NBR O-ring set is cheaper and more forgiving.
Decision Matrix: Where Each Material Wins and Where It Loses
Use the matrix below as a first cut. Pick the column whose row-matches the application's worst-case condition, not the average. [S1]
Pure PTFE / filled PTFE: continuous service to ~260 °C, near-universal chemical compatibility, very low friction, but elastic recovery below 20 % so it needs continuous mechanical loading; cold flow under sustained bolt load is the main failure mode, so PTFE envelope gaskets are used as a face on a stronger core. Industrial rubber (NBR / EPDM / FKM / silicone): elastic recovery above 80 % at room temperature, low cost, easy moulding into O-rings and gaskets, but capped at 110-230 °C depending on family and attacked by hydrocarbons (NBR weakness) or strong acids (EPDM weakness). PTFE-faced rubber laminate: 0.05 mm PTFE skin (S2) over a 1.5 mm red-rubber core, used for screw-cap septa and analyser access doors, giving chemical resistance on the wetted face and elastic recovery in the bulk. Other elastomer choices (PU, HNBR, FFKM) sit in the cost/temperature gap between FKM and PTFE; FFKM perfluoroelastomer closes most of PTFE's chemical window while keeping rubber-class elasticity, at a price roughly 10-30× NBR.
Process Compatibility: Solvents, Acids, Steam and Cryogenics
PTFE is attacked only by molten alkali metals, elemental fluorine, and a few fluorinated compounds at high temperature; in practice this means any hydrocarbon, acid, oxidiser, or solvent stream is PTFE-safe. NBR handles mineral oil and petroleum fuels well but swells in ketones and aromatics; EPDM handles steam, hot water, brake fluid and phosphate esters well but fails in petroleum; FKM handles fuels, lubricants and many chlorinated solvents up to about 200-230 °C. For mixed-service cleaning-in-place lines that cycle between hot caustic and hot acid, PTFE gaskets or PTFE-faced rubber are the path of least surprise. [S2]
Cryogenic service flips the logic. PTFE is rated down to roughly -200 °C, but its thermal expansion is high and it becomes brittle, so LNG and LN2 seals more commonly use PTFE-jacketed gaskets with a non-metallic core rather than monolithic PTFE; rubber elastomers with low-T grades (silicone down to ~-60 °C, FKM low-T grades to ~-26 °C) cover the moderate cold range, and below that the seal turns to metal.
Standards and Verification: FDA, USP, NACE and Fire-Safe
For pharmaceutical, food and drinking-water contact, virgin PTFE grades routinely carry FDA 21 CFR, USP Class VI and EU 1935/2004 compliance, with material certificates available from major resin suppliers; for O& H2S service, elastomer selection is constrained by NACE MR0175 / ISO 15156 limits on hardness, swell and extractables, and FKM is the common upper-tier choice where PTFE is also acceptable. [S3]
Fire-safe flange gaskets typically combine a graphite or non-asbestos fibre core with an outer PTFE or rubber ring — the outer ring keeps the joint sealed in normal service, the core provides the fire-resistant barrier; the same composite principle underlies many 2025-2026 fire-tested soft gaskets. Buyers should match the joint certificate (e.g. API 6D fire test for valves, or EN 13555 for flange gaskets) to the application class rather than assume any soft gasket is fire-safe.
Cost, Lead Time and 2026 Sourcing Reality
On per-piece cost, NBR O-rings sit at the bottom of the curve, FKM roughly 3-5× NBR, silicone 2-3×, and PTFE step-rings 5-15× depending on filler; the cost gap narrows on small machined components and widens on large gaskets. Lead time is broadly similar across the families for standard sizes (stock O-rings in metric and AS568, stock PTFE billets, stocked septa SKUs like AK73816-00015 to -00024 [S2]), but custom moulded elastomers extend to 6-10 weeks while machined PTFE parts run on CNC with 2-3 week turn.
Use PTFE selection criteria: 5 gates buyers lock before 2026 RFQ release when the candidate is a filled-PTFE part, and consider engineering plastic vs polyurethane elastomer: spec frame for resin class selection when the choice sits between a thermoplastic, a thermoset rubber, and a PU elastomer. The two pieces together cover most soft-material spec calls a 2026 process engineer faces.
Trackable signals for the next 90 days: revised FKM low-T grade datasheets from the three major fluoroelastomer suppliers, and any update to EN 13555 gasket test factors for PTFE-faced laminates as it intersects with the new EU industrial-emissions package. Either of these will shift the matrix above by one row.
For component-level specifications, see industrial adhesive.