Quick answer: PTFE O-rings resist virtually all chemicals (acids, bases, solvents, oxidizers) and seal from −270°C to +260°C — no elastomer spans this range. The critical limitation: PTFE has no elastic recovery (it cold-flows under compression). Solid PTFE O-rings work only for static seals at essentially constant temperature; they fail in dynamic and thermally-cycled applications. For dynamic service or frequent thermal cycling, specify spring-energized PTFE seals (metal spring maintains contact force as the PTFE jacket cold-flows). Use filled PTFE (glass 15–25%, carbon 15–25%) for better creep resistance; virgin PTFE for maximum chemical purity in pharmaceutical and semiconductor service.
PTFE (polytetrafluoroethylene, Teflon) is the only common sealing material that combines near-universal chemical resistance with service temperature from −270°C (approaching absolute zero) to +260°C continuous. No elastomer covers this temperature range; no elastomer is chemically inert to concentrated acids, strong bases, ketones, amines, and fluorinated solvents simultaneously. The constraint that limits PTFE is structural: it is a thermoplastic, not an elastomer. PTFE has no crosslinked molecular network and therefore no elastic recovery — it deforms permanently under compressive load (cold flow), cannot spring back when load is removed, and cannot re-establish sealing contact after thermal cycling. This single limitation determines whether solid PTFE or spring-energized PTFE is the correct design choice for a given application.
Material Chemistry: Why PTFE Behaves as It Does
PTFE is a linear polymer of tetrafluoroethylene (TFE) monomer — the repeating unit is (CF₂-CF₂)ₙ. Every carbon atom is bonded to two fluorine atoms; no hydrogen atoms are present in the backbone. This structure produces:
Chemical inertness: The C-F bond energy is 485 kJ/mol — the strongest single bond in organic chemistry. PTFE's surface is effectively inert to all nucleophilic, electrophilic, and radical chemical species. The fluorine atoms form a tight helical sheath around the carbon backbone, sterically blocking access to the C-C bonds. The only reagents that penetrate this sheath are: molten alkali metals (sodium, potassium), elemental fluorine (F₂) at elevated temperature, and fuming fluorosulfuric acid — all of which are extreme conditions encountered in specific chemical synthesis but not in industrial process equipment.
Thermal stability: The C-F and C-C bonds in PTFE are stable to +260°C continuous and +280°C short-term. Above +327°C, PTFE begins to depolymerize. At +400°C, it decomposes rapidly to release perfluoroalkene fragments (PFIB and others) that are acutely toxic — PTFE seals must never be used where they could contact an open flame or contact with surfaces above +280°C.
No elastic recovery (cold flow): PTFE has no crosslinked molecular network. The polymer chains are held together only by van der Waals forces — much weaker than the covalent crosslinks that give elastomers their elastic recovery. Under sustained compressive load, PTFE chains slide past each other irreversibly (cold flow or creep), permanently deforming toward the groove walls. The compressed PTFE fills the groove geometry but cannot return to its original shape when load is removed. This is not a defect — it is an intrinsic property of all uncrosslinked thermoplastics.
Temperature Range: The Broadest of Any Commercial Seal Material
| Temperature Range | PTFE Behavior | Application | Limitation |
|---|---|---|---|
| −270°C (approaching 0 K) | Functional — retains seal geometry | Liquid helium systems, cryogenic research | No elastomer survives; spring energization required |
| −196°C | Excellent | Liquid nitrogen service | Spring energization required |
| −183°C | Excellent | Liquid oxygen (LOX) service | Combustion risk with contamination — strict cleanliness required |
| −162°C | Excellent | LNG (liquefied natural gas) service | Spring energization recommended |
| −60°C to 0°C | Excellent | Cold service where elastomers stiffen | Solid PTFE acceptable for static |
| 0°C to +120°C | Excellent | General service | Solid PTFE static; spring-energized for dynamic |
| +120°C to +200°C | Excellent | High-temperature chemical service | Beyond FKM range for steam; PTFE holds |
| +200°C to +260°C | Excellent — full performance | High-temp reactors, aerospace exhaust | Approaching limit; FKM begins to degrade |
| Above +260°C | Begins to degrade slowly | Not recommended for sustained service | Depolymerization above +270°C |
Comparison to other materials at temperature extremes:
| Material | Low Temp Limit | High Temp Limit (continuous) |
|---|---|---|
| NBR | −40°C (standard); −55°C (low-temp) | +120°C |
| EPDM | −50°C | +150°C |
| FKM | −20°C (standard); −40°C (GLT) | +200°C |
| HNBR | −30°C | +150°C |
| VMQ | −60°C | +230°C (dry heat only) |
| FFKM | −15°C (standard); −40°C (low-temp) | +325°C |
| PTFE | −270°C | +260°C |
PTFE's −270°C lower limit is effectively unlimited by temperature — no known chemical or physical process degrades PTFE at cryogenic temperatures. At these temperatures, PTFE becomes harder and more rigid (modulus increases), which is why spring energization is required to maintain contact force.
Chemical Resistance: Near-Universal Inertness
Chemical families PTFE resists (complete list for practical purposes)
- Strong mineral acids: Concentrated H₂SO₄, concentrated HCl, concentrated HNO₃ (fuming and non-fuming), HF (hydrofluoric acid), H₃PO₄, HBr, HI
- Strong bases: Concentrated NaOH (caustic soda), KOH, Ca(OH)₂, concentrated NH₃ (ammonia), organic amines at any concentration
- Organic solvents: Acetone, MEK, methanol, ethanol, isopropanol, ethyl acetate, butyl acetate, toluene, xylene, benzene, chloroform, methylene chloride, carbon tetrachloride, THF, DMF, DMSO
- Oxidizing agents: H₂O₂ (any concentration), concentrated HNO₃, Cl₂, ClO₂, ozone, peracetic acid (PAA), sodium hypochlorite (any concentration)
- Hot steam: Any temperature within PTFE's service range
- Pharmaceutical process fluids: Virtually all APIs, buffer solutions, fermentation media, organic intermediates
- Semiconductor process chemicals: HF/BOE, SC-1 (H₂O₂/NH₃), SC-2 (H₂O₂/HCl), SPM (H₂SO₄/H₂O₂), TMAH developer, organic solvents
Specific incompatibilities (the exceptions)
| Chemical | PTFE Behavior | Severity | Notes |
|---|---|---|---|
| Molten sodium (Na) | Attacked — dehalogenation | Severe | Elemental alkali metals only |
| Molten potassium (K) | Attacked | Severe | Same mechanism as sodium |
| Elemental fluorine (F₂), > 200°C | Slow attack | Moderate | At elevated temperature and high purity F₂ |
| Chlorine trifluoride (ClF₃) | Attacks PTFE | Severe | Specialty semiconductor chemical |
| Fuming fluorosulfuric acid | Marginal resistance | Moderate | Oleum-type chemistry |
For standard industrial chemical processing, pharmaceutical manufacturing, semiconductor wet bench chemistry, and food processing, PTFE incompatibilities are not encountered. The exceptions above are specific to fluorination chemistry and alkali metal reduction processes.
PTFE vs FKM and FFKM chemical resistance comparison
| Chemical | FKM (Type A) | FFKM | PTFE (solid) |
|---|---|---|---|
| Strong acids (HCl, H₂SO₄ < 50%) | Good | Excellent | Excellent |
| Concentrated bases (NaOH > 10%) | Poor (dehydrofluorination) | Excellent | Excellent |
| Amines (MEA, DEA, aniline) | Poor (dehydrofluorination) | Excellent | Excellent |
| Ketones (MEK, acetone) | Poor (30–80% swell) | Excellent | Excellent |
| Esters (ethyl acetate) | Poor (20–50% swell) | Excellent | Excellent |
| Hot steam (> +121°C) | Poor (hydrolysis at C-H sites) | Excellent | Excellent |
| Concentrated HF | Fair (marginal) | Excellent | Excellent |
| Aromatic solvents (toluene) | Good | Excellent | Excellent |
| Petroleum oils and fuels | Excellent | Excellent | Excellent |
The chemical resistance advantage of PTFE over FKM is primarily in the chemical families where FKM undergoes dehydrofluorination (strong bases, amines) or polarity-driven swell (ketones, esters). Against petroleum, aromatics, and mild acids, FKM and PTFE perform similarly — and FKM has the significant advantage of elastic recovery.
The Cold Flow Problem: When Solid PTFE Works and When It Fails
Cold flow mechanism in detail
PTFE's cold flow (creep) under compressive load is not uniform — it follows the groove geometry. When an PTFE O-ring or ring is compressed in a groove with rigid metal walls on three sides (groove bottom and two side walls) and the mating surface on the fourth:
- The compressive load causes the PTFE to creep into the groove geometry
- The PTFE fills the groove profile — contacting the groove bottom and both side walls
- In this fully-grooved state, the PTFE has achieved a static equilibrium — three rigid walls prevent further creep in those directions; the fourth side (the mating surface contact) is the only remaining deformation path
- Under sustained, unchanging conditions, the cold-flowed PTFE can maintain adequate sealing contact
Where cold flow causes failure:
- Thermal cycling: At elevated temperature, PTFE expands and cold-flows to fill the groove more completely. When cooled, it cannot contract back to original dimensions (no elastic recovery). Effective compression decreases with each thermal cycle. After 10–20 thermal cycles, compression may be insufficient for reliable sealing.
- Dynamic service (any motion): In a reciprocating or rotary seal, each motion cycle displaces the cold-flowed PTFE at the contact zone — the displaced material cannot return. PTFE in a dynamic groove leaks within hours to days.
- Pressure cycling: Pressure-induced changes in contact geometry require elastic recovery to maintain the seal. PTFE cannot compensate for pressure-induced groove expansion or dimensional changes.
Where solid PTFE static sealing works:
- Flanges maintained at essentially constant temperature (no startup/shutdown cycling)
- Face seals with rigid metal housings on all groove sides
- Applications where re-torquing or reshimming after initial installation is acceptable
- Chemical service where elastomer alternatives are all incompatible
Quantitative cold flow data
PTFE creep under 3.5 MPa compressive stress at various temperatures (measured by change in compressed thickness):
| Temperature | Creep at 100h | Creep at 1,000h | Effect on Compression |
|---|---|---|---|
| +25°C | 3–5% thickness loss | 8–12% | Moderate — may be acceptable for static |
| +100°C | 8–15% thickness loss | 20–30% | Significant — compression set equivalent |
| +150°C | 15–25% thickness loss | 35–50% | Severe — re-torquing required |
| +200°C | 25–40% thickness loss | 50–70% | PTFE is flowing continuously; not suitable |
Glass-filled PTFE (25% glass fiber) reduces these creep rates by approximately 50% across all temperatures while slightly reducing chemical resistance. For static PTFE seals at elevated temperature, glass-filled grades are standard.
Spring-Energized PTFE Seals: The Dynamic Solution
Spring-energized PTFE seals combine a precision-machined PTFE jacket (U-cup or lip profile) with a metal spring that provides continuous radial contact force. The spring compensates for PTFE cold flow — as the PTFE jacket creeps under contact load, the spring expands to maintain contact force. The sealing force is spring-determined, not PTFE-elasticity-determined.
Spring types and force characteristics
| Spring Type | Force Profile | Deflection Range | Best Application |
|---|---|---|---|
| Cantilever (single-turn) | Moderate constant force | Limited — best for controlled compression | Precision instrumentation valves; face seals |
| Canted-coil | Near-constant force over wide deflection range | Wide — accommodates wear | General dynamic; reciprocating pumps; rotary |
| Helical (open-coil) | High force; increases with deflection | Moderate | High-pressure static and slow dynamic |
| V-spring (multiple leaf) | High force; predictable preload | Limited | Valve seats; high-load static |
Canted-coil spring advantage: The canted-coil spring provides near-constant contact force over a wide range of PTFE jacket wear and deflection. As the PTFE jacket wears or cold-flows, the canted coils straighten slightly, maintaining force across the full deflection range. This makes canted-coil spring-energized seals self-compensating for both initial PTFE creep and long-term wear.
Spring material selection by temperature
| Spring Material | Temperature Range | Chemical Resistance | Cryogenic |
|---|---|---|---|
| 302/304 Stainless Steel | −200°C to +315°C | General service | Good — austenitic SS retains ductility at cryogenic |
| 316 Stainless Steel | −200°C to +315°C | Better Cl⁻ resistance | Good |
| Inconel 718 | −270°C to +650°C | Excellent — nickel-chromium alloy | Excellent — standard for cryogenic and high-temp |
| Elgiloy (Co-Cr-Ni) | −270°C to +550°C | Excellent | Excellent — used in LNG and LOX service |
| Hastelloy C-276 | −200°C to +550°C | Best chemical resistance | Good |
For LNG (−162°C), LOX (−183°C), LN₂ (−196°C), and liquid hydrogen (−253°C) service, Inconel 718 or Elgiloy springs are standard — austenitic stainless steel retains ductility but Inconel provides better fatigue life at cryogenic temperatures with long-stroke cycling.
Spring-energized PTFE seal applications
- Cryogenic valve stems: LNG transfer valves, LOX valve stems, liquid nitrogen distribution — no elastomeric O-ring can function; spring-energized PTFE is the standard solution
- Reciprocating chemical pumps: Ketone, amine, or strong acid service where FKM would degrade and PTFE cold flow would cause failure in a standard O-ring groove
- Analytical instrument valves: HPLC injectors, GC valves — ultra-low friction (PTFE coefficient of friction 0.04–0.10), zero extractables, solvent-resistant
- Semiconductor process equipment dynamic seals: Clean room compatible; low outgassing; chemical-resistant to HF, acids, and oxidizers
- High-vacuum rotary feedthroughs: PTFE's low outgassing (vs. elastomers) allows base pressures < 10⁻⁸ mbar; spring force maintains contact at vacuum-induced contact stress reduction
- Food and pharmaceutical pumps: FDA-compliant PTFE grade with no extractables; handles PAA sanitization, concentrated caustic, and solvent contact
Groove design for spring-energized PTFE: Spring-energized seals require a custom groove profile — they cannot be installed in a standard O-ring groove. The U-cup or lip geometry requires a groove that accommodates the PTFE jacket in its uncompressed state and allows the spring to expand against the sealing surface. Groove design must follow the seal manufacturer's specification exactly — incorrect groove geometry prevents spring expansion or causes jacket buckling.
PTFE Grades and Fillers
Virgin PTFE provides maximum chemical resistance but has lower wear resistance and higher creep than filled grades. Fillers modify properties for specific applications:
| Filler | Filler Content | Creep Reduction | Wear Improvement | Chemical Resistance | Application |
|---|---|---|---|---|---|
| Virgin (unfilled) | 0% | Baseline | Baseline | Maximum | Maximum chemical resistance; pharmaceutical |
| Glass fiber | 15–25% | 50% reduction | 2–3× | Slight reduction | General industrial; hydraulic backup rings |
| Carbon/graphite | 15–25% | 40% reduction | 3–5× | Slight reduction | Dry-running; rotary seals |
| Bronze | 25–40% | 60% reduction | 5–10× | Reduced (not for acids) | Heavy-load hydraulics; non-corrosive media |
| Carbon fiber | 10–15% | 45% reduction | 4–6× | Minimal reduction | High-performance dynamic; semiconductor |
| PEEK particles | 10–20% | 40% reduction | 3–4× | Minimal reduction | High-temp; broad chemical resistance |
| Graphite (natural) | 10–15% | 35% reduction | 3× | Minimal reduction | Food and pharma (FDA compliant) |
Bronze-filled PTFE limitation: Bronze particles dissolve in oxidizing acids, chlorinated solvents, and some aggressive process media. Bronze-filled PTFE is used for hydraulic backup rings in petroleum fluid service but should not be used in contact with aggressive process chemistry.
Semiconductor-grade PTFE: For semiconductor use (wafer processing, liquid chemical delivery), ultra-high-purity virgin PTFE is required — no fillers. Metallic fillers introduce trace metal contamination; glass fibers introduce silica contamination — both unacceptable at the ppb level. Semiconductor PTFE O-rings and seals are manufactured in cleanroom conditions and packaged per SEMI F57.
PTFE O-Ring Compression, Hardness, and Low-Friction Behavior
PTFE is much harder and stiffer than elastomeric O-rings. A solid PTFE O-ring does not have a Shore A hardness in the same sense as rubber — typical PTFE has a Shore D hardness of 50–65, roughly equivalent to 90–95 Shore A but with no elastic recovery. This means PTFE seals require higher compression (squeeze) to achieve initial sealing contact.
| Parameter | Virgin PTFE | Glass-Filled PTFE | Elastomeric O-Ring (Reference) |
|---|---|---|---|
| Hardness | Shore D 50–55 | Shore D 55–65 | Shore A 70–90 |
| Compression rate (static) | 20–30% | 18–25% | 15–22% |
| Coefficient of friction | 0.04–0.10 | 0.08–0.15 | 0.30–0.80 (rubber on metal) |
| Elastic recovery | None | None | 20–50% after compression |
The low coefficient of friction is an advantage in dynamic spring-energized seals and valve applications, but it is a liability in solid O-ring grooves because PTFE cannot generate the same static friction force that helps an elastomer resist blowout. For this reason, solid PTFE O-rings are always installed in fully constrained grooves where metal walls prevent extrusion.
PTFE O-Ring Cord Stock and Custom Lathe-Cut Rings
Because PTFE cannot be injection-molded or compression-molded like rubber, most solid PTFE O-rings are lathe-cut from extruded PTFE cord stock. Key points:
- Cord stock is supplied in continuous lengths (common diameters 1.5 mm to 25 mm) and vulcanized/welded or glued into rings, or cut to length and spliced
- Lathe-cut rings are machined from tubular PTFE stock to precise inside diameter and cross-section — better dimensional control than spliced cord
- Custom sizes are practical because no mold tooling is required; lead time is typically 7–15 days
- Joint quality: Welded PTFE joints are weaker than the base material; for critical static seals, specify seamless lathe-cut rings or verify weld tensile strength
PTFE cord stock is also used to make large-diameter gaskets and flange seals on-site, where the installer cuts and joins the cord to match the flange geometry.
Teflon-Coated O-Rings vs Solid PTFE O-Rings
"Teflon-coated O-rings" are not the same as solid PTFE O-rings. There are two common constructions:
- FEP- or PFA-encapsulated elastomeric O-rings — a Viton or silicone core is wrapped in a thin fluoropolymer jacket. The elastomer core provides elastic recovery; the jacket provides chemical resistance. These are used when some elasticity is needed but the fluid attacks standard FKM or VMQ. See our FEP encapsulated O-rings guide for details.
- PTFE-coated metal or elastomer seals — a thin PTFE layer applied by spray, dip, or plasma coating to improve release, reduce friction, or add chemical barrier. These are not standard O-rings and are usually custom engineered.
| Feature | Solid PTFE O-Ring | FEP-Encapsulated FKM/VMQ |
|---|---|---|
| Elastic recovery | None | Good (from elastomer core) |
| Chemical resistance | Near-universal | Excellent (limited by jacket pinholes) |
| Dynamic service | Not suitable | Limited dynamic capability |
| Cost | 1–2× elastomer O-ring | 5–15× elastomer O-ring |
| Temperature limit | −270°C to +260°C | −60°C to +205°C (FKM core) |
For most chemical-static applications, solid PTFE is simpler and more reliable. Choose encapsulated elastomer only when the seal must recover from thermal cycling or conform to less-than-ideal flange surfaces.
Outgassing and Vacuum Performance
PTFE's low outgassing makes it a preferred material for vacuum and high-vacuum sealing — it outperforms all standard elastomers by a significant margin.
ASTM E595 Outgassing Comparison
| Material | TML (Total Mass Loss, %) | CVCM (Collected Volatile Condensable, %) | Suitable Vacuum Level |
|---|---|---|---|
| Virgin PTFE | 0.01–0.05% | 0.001–0.005% | UHV (< 10⁻⁹ mbar) |
| Glass-filled PTFE (25%) | 0.02–0.08% | 0.003–0.010% | HV (< 10⁻⁶ mbar) |
| FKM (standard) | 0.10–0.40% | 0.02–0.10% | Medium vacuum (< 10⁻³ mbar) |
| EPDM (standard) | 0.40–1.20% | 0.05–0.30% | Rough vacuum only |
| VMQ (silicone) | 0.50–2.00% (D3–D5 oligomers) | 0.10–0.50% | Not suitable for HV |
| FFKM (semiconductor grade) | 0.02–0.10% | 0.003–0.020% | HV/UHV |
| NBR (standard) | 0.80–2.50% | 0.10–0.50% | Rough vacuum only |
ASTM E595 criterion: TML < 1.0% and CVCM < 0.1% for general aerospace/vacuum use; TML < 0.1% and CVCM < 0.01% for sensitive optical and semiconductor applications. Virgin PTFE meets the stringent semiconductor standard without compound-specific qualification — a significant advantage over FKM and FFKM, which require lot-specific outgassing testing.
VMQ (silicone) in vacuum: Silicone O-rings release cyclic siloxane oligomers (D3, D4, D5) with measurable vapor pressure at room temperature. These deposit as SiO₂ films on metal surfaces in the vacuum system after O₂ plasma exposure, permanently contaminating optical surfaces and process chambers. Silicone is excluded from any vacuum application below 10⁻³ mbar.
Design Guidelines for PTFE Seals
Solid PTFE O-ring groove design
Solid PTFE O-rings require different groove dimensions than elastomeric O-rings:
| Parameter | Solid PTFE O-Ring | Elastomeric O-Ring |
|---|---|---|
| Compression rate (static) | 20–30% (higher to compensate for cold flow) | 15–22% |
| Groove fill rate | 70–80% (leave room for cold flow expansion) | 65–85% |
| Mating surface Ra | ≤ 0.4 μm (cannot conform to rough surfaces) | ≤ 0.8 μm (conforms elastically) |
| Groove corner radius | ≥ 0.25 mm (PTFE is stiffer and more sensitive to sharp edges) | ≥ 0.10 mm |
| Thermal cycling tolerance | Poor — require re-torquing | Good — elastic recovery compensates |
Surface finish requirement: Because solid PTFE cannot elastically conform to surface irregularities, the mating surface must be smoother than for equivalent elastomeric seals. Ra ≤ 0.40 μm (16 μin) is the practical minimum for solid PTFE static sealing. Below Ra 0.10 μm, the surface may be too smooth for consistent PTFE contact — at very low roughness, the PTFE surface rests on the raised tips of the machined surface rather than making broad contact.
Spring-energized PTFE seal groove
Spring-energized seal groove dimensions are seal-specific — they depend on the PTFE jacket geometry, spring force curve, and operating conditions. Always obtain groove design drawings from the seal manufacturer. Common general parameters:
- Groove width: Set by jacket width + operational clearance (typically ±0.05 mm per side)
- Groove depth: Set by uncompressed jacket height + spring deflection range
- Surface finish on dynamic contact surfaces: Ra 0.10–0.25 μm (same as elastomeric dynamic seals)
- Lead-in chamfer: 15°–20°, length ≥ 2× jacket width (PTFE jackets are sensitive to installation damage)
Cost Comparison
| Seal Type | Relative Unit Cost | Notes |
|---|---|---|
| Solid PTFE lathe-cut O-ring | 1.0–2.0× round elastomeric O-ring | Lathe-cut from cord — no mold tooling; custom sizes practical |
| Glass-filled PTFE backup ring | 0.5–1.5× | Standard component; low cost per piece |
| Spring-energized PTFE (standard grades) | 5–15× elastomeric O-ring | Custom groove design required; machined PTFE jacket |
| Spring-energized PTFE (cryogenic Inconel spring) | 15–40× elastomeric O-ring | Inconel spring and clean-room qualification add cost |
| FFKM O-ring (for comparison) | 10–50× NBR O-ring | FFKM has elastic recovery; spring-energized PTFE does not |
Spring-energized PTFE seals cost significantly more than standard O-rings, but in the applications where they are correctly specified (cryogenic, dynamic with aggressive chemistry), they replace a component that cannot be executed any other way. The cost comparison should be against the next-best alternative (which typically fails), not against a standard O-ring.
FAQ
Q1: What is the temperature range of PTFE O-rings?
PTFE operates continuously from −200°C to +260°C in static service. With spring energization, sealing is functional at −270°C (liquid helium temperature). The +260°C continuous limit applies to sealing service — PTFE begins to depolymerize above +270°C and releases toxic perfluoroalkene degradation products if exposed to open flames or surfaces above +350°C. PTFE is the only common seal material functional at both cryogenic (LN₂, LOX, LH₂) and high-temperature (> +200°C) service.
Q2: Are PTFE O-rings chemically resistant to everything?
Virtually everything in industrial practice. PTFE resists all concentrated mineral acids (HF, H₂SO₄, HCl, HNO₃), all strong bases (concentrated NaOH, KOH), all common organic solvents (acetone, MEK, alcohols, esters, chlorinated solvents, aromatics), oxidizing agents (H₂O₂, peracetic acid, hypochlorite), and hot steam. The only chemicals that attack PTFE under any conditions are: molten alkali metals (sodium, potassium), elemental fluorine at high temperature, and chlorine trifluoride — all of which are exotic conditions outside standard industrial processing.
Q3: Can solid PTFE O-rings be used for dynamic seals?
No — solid PTFE O-rings are not suitable for any dynamic application (reciprocating, rotary, or oscillating). Cold flow causes the compressed PTFE to deform irreversibly into the groove geometry; each dynamic stroke displaces the cold-flowed PTFE from the contact zone faster than new material can flow in. Leakage begins within hours to days. For dynamic applications requiring PTFE chemical resistance, spring-energized PTFE seals with metal springs (canted-coil, cantilever, or helical) are the engineering-correct design.
Q4: What is the difference between virgin PTFE and filled PTFE?
Virgin PTFE is unfilled homopolymer — it provides the maximum chemical resistance (all fillers reduce chemical resistance to some degree) but has the highest cold flow rate and lowest wear resistance. Filled PTFE incorporates glass fiber, carbon, bronze, graphite, or other materials to improve dimensional stability under load (reduce creep) and wear resistance. For pharmaceutical and semiconductor applications where chemical purity is paramount, virgin PTFE is required. For mechanical backup rings, bearing pads, and industrial dynamic seals, glass-filled or carbon-filled PTFE is preferred.
Q5: Is PTFE the same as Teflon?
Teflon is DuPont's (now Chemours's) registered trade name for PTFE — the same relationship as Viton (DuPont's trade name for FKM) or Kalrez (DuPont's trade name for FFKM). PTFE is the generic chemical name per IUPAC nomenclature. Other manufacturers produce PTFE under different trade names. Specifying "PTFE" rather than "Teflon" allows multi-source procurement without restriction to a single supplier's brand.
Q6: What are PTFE backup rings and how are they used with elastomeric O-rings?
PTFE backup rings (anti-extrusion rings) are flat rings installed on the low-pressure side of an elastomeric O-ring in high-pressure grooves. The PTFE backup ring bridges the clearance gap between the bore and rod/piston, preventing the softer elastomeric O-ring from extruding into the gap under pressure. The elastomeric O-ring (typically NBR or FKM) provides the sealing contact force; the PTFE backup ring provides the extrusion resistance. This combination is standard in hydraulic cylinders above 150 bar. PTFE backup rings are not O-rings — they are flat rings of rectangular or step-cut cross-section, made from glass-filled or bronze-filled PTFE.
Q7: When should I choose PTFE over FFKM for a static chemical seal?
PTFE for static seals in aggressive chemistry when: (1) temperature exceeds FFKM's practical limit (> +300°C); (2) chemistry includes chemicals that attack even FFKM (molten alkali metals); (3) cryogenic service (< −15°C, below FFKM's practical lower limit); or (4) cost is the primary constraint and elastic recovery is not required. FFKM for static seals when: (1) thermal cycling is significant (FFKM elastic recovery re-establishes sealing after each cycle; PTFE cannot recover); (2) surface finish on the flange face is imperfect (FFKM conforms elastically; PTFE requires Ra ≤ 0.40 μm to achieve broad contact); (3) the application has occasional dynamic loading or vibration that would cause PTFE cold flow under variable-direction loading. Cost comparison: PTFE static O-rings cost 1–2× round elastomeric O-rings; FFKM costs 10–50× — the PTFE cost advantage is significant for non-cycling static applications where cold flow is not a concern.
Q8: How do I specify a PTFE O-ring to ensure chemical purity for pharmaceutical or semiconductor use?
Specify the PTFE grade explicitly on the purchase order: (1) "Virgin PTFE" — no fillers, made from homopolymer PTFE resin only; (2) request a Certificate of Conformance (CoC) confirming compliance with FDA 21 CFR §177.1550 (PTFE for food contact) or USP Class VI (pharmaceutical biocompatibility) as applicable; (3) for semiconductor applications, request SEMI F57 compliance or equivalent ICP-MS trace metal analysis (typically specifying metals < 0.1 ppb by ICP-MS); (4) for vacuum applications, request ASTM E595 outgassing data (TML and CVCM values from the specific production lot). Do not accept material certified only by generic trade name — "Teflon grade" without specification number does not guarantee food-grade or semiconductor-grade purity. Request lot-traceable CoC documents with physical property test data from the production batch, not from a master qualification file.
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Need PTFE O-rings or spring-energized PTFE seals? Contact our engineering team with your operating temperature, fluid chemistry, and whether the application is static or dynamic — we supply virgin and glass-filled PTFE lathe-cut O-rings for static service and spring-energized PTFE seals with 316 SS, Inconel 718, and Elgiloy springs for cryogenic and dynamic applications. MOQ 1 piece; 7–15 day lead time; FDA-compliant and semiconductor-grade PTFE available with material certification.