When a seal has to survive 900 degrees F of dry heat, a stream of hydraulic fluid, a swing from cryogenic to ambient temperature, or a strong acid bath, the material choice stops being a preference and becomes the whole engineering problem. A gasket that works fine on a water flange will char, swell, embrittle, or extrude out of the joint under any of those conditions. The way forward is rarely a single "better" rubber. It is matching the specific failure mode of the environment to a material that resists it, then designing the gasket geometry so the material is loaded the way it wants to be loaded.
This article walks through the demanding conditions Cannon Gasket sees most often, the non-metallic materials that hold up in each, and the temperature, durometer, and chemical limits that decide whether a design works or fails in service.
High heat and steam: flexible graphite and compressed fiber
Sustained high temperature destroys most elastomers. EPDM tops out near 300 degrees F, Viton (FKM) near 400 to 450 degrees F, and even silicone is generally limited to about 450 degrees F for continuous service. Above that range, the practical choices are mineral and carbon-based sheet materials, not rubber.
Flexible graphite is the workhorse for high heat and steam. Pure expanded graphite handles continuous service to roughly 850 to 900 degrees F in oxidizing atmospheres and far higher in steam or inert conditions, with essentially no creep relaxation over time. It conforms to imperfect flange faces, recovers a usable amount after compression, and does not harden the way a fiber sheet can. For thin sections or to add handling strength, graphite is laminated to a stainless foil or tang insert. Graphite gaskets with a metal core are a standard answer for steam headers, exhaust flanges, valve bonnets, and heat-exchanger service where bolt-load retention matters more than chemical breadth.
Compressed non-asbestos fiber sheet, the category often referenced by the Klingersil trade name, fills the gap between rubber and graphite. These aramid or glass fiber sheets bound with an NBR or SBR binder typically carry continuous ratings in the 400 to 600 degrees F range depending on grade, with good resistance to steam, oils, and mild chemicals. They are inexpensive, cut cleanly, and seal well at moderate flange loads, which makes them a common default for general industrial steam and process piping. The trade-off versus graphite is creep: fiber sheets relax under sustained heat, so bolted joints may need re-torque after the first heat cycle.
- Saturated and superheated steam: flexible graphite, or metal-reinforced graphite for high-pressure flanges.
- General hot process piping to roughly 500 degrees F: compressed fiber sheet, re-torqued after first heat-up.
- Exhaust and combustion flanges: graphite with a stainless tang or eyelet for blow-out resistance.
Aggressive chemicals and cryogenic service: PTFE and Kapton
PTFE (polytetrafluoroethylene) has the broadest chemical resistance of any common sealing material. It is inert to nearly all acids, bases, and solvents across roughly the full pH range, with the well-known exceptions of molten alkali metals and elemental fluorine. Its useful temperature window runs from about minus 350 degrees F up to roughly 500 degrees F, which is why it shows up in both chemical and cryogenic applications. The catch with virgin PTFE is cold flow: under constant load it creeps, so a joint can lose bolt tension over time. Two design responses solve this. Filled grades, using glass, carbon, or barium sulfate, cut creep substantially while keeping most of the chemical resistance. Expanded PTFE (ePTFE), a microporous structure, conforms to rough or glass-lined flanges at low seating stress and is the usual pick for fragile, large-diameter, or fiberglass equipment. PTFE-based PTFE washers apply the same chemistry to fastener isolation and load spreading in corrosive assemblies.
Where the demand is a polymer that holds dimensional stability and dielectric strength across an extreme temperature range, Kapton (polyimide film) earns its place. Polyimide stays usable from cryogenic temperatures up to about 400 degrees F continuous, with short excursions higher, and it does not melt. It is thin, tough, and an excellent electrical insulator, which makes Kapton washers and die-cut polyimide parts a common choice in vacuum systems, aerospace electronics, and high-temperature electrical isolation where a thicker elastomer would not fit or would outgas.
Fuel, oil, and wide temperature swings: fluorosilicone and FKM
Some environments combine hydrocarbon exposure with a temperature range too wide for a single conventional rubber. Fuel systems are the classic case. Standard silicone has an outstanding temperature range, roughly minus 65 to 450 degrees F, but it swells badly in fuels and oils. Standard FKM resists fuel well but stiffens and loses sealing force in deep cold.
Fluorosilicone (FVMQ) is engineered for exactly this conflict. It keeps the low-temperature flexibility of silicone, staying serviceable down to about minus 75 degrees F, while adding resistance to jet fuel, gasoline, and petroleum oils. That combination is why fluorosilicone gaskets and o-rings are specified throughout aerospace and military fuel systems that see both high-altitude cold and engine-bay heat. Fluorosilicone is softer and less abrasion-resistant than FKM, so it is chosen for static seals and controlled environments rather than dynamic or high-pressure duty.
When the temperature floor is not as severe but chemical and fuel breadth is the priority, FKM remains the benchmark. Viton (FKM) gaskets hold continuous service to about 400 to 450 degrees F with broad resistance to fuels, oils, and many chemicals, and specialty FKM grades extend low-temperature performance when needed. For oil and gas service, material selection also runs through standards such as NACE MR0175 for sour (H2S) environments and the API and ASME flange and valve specifications that govern the joint itself.
Electronics enclosures: EMI conductive elastomers
A different kind of challenging environment is electromagnetic. Electronics enclosures often need a gasket that does two jobs at once: keep moisture and dust out, and maintain electrical continuity across the seam so the housing acts as a Faraday shield. A standard insulating rubber gasket breaks that continuity and lets emissions leak through the slot.
EMI conductive elastomers solve this by loading a silicone or fluorosilicone base with conductive filler, such as silver-plated aluminum, silver-plated copper, nickel-graphite, or pure carbon. The result is a gasket that seals environmentally to an IP rating and provides shielding effectiveness that can exceed 100 dB across a wide frequency band. Filler choice is a real engineering decision: it sets both the shielding level and the galvanic compatibility with the flange metal, since a poorly matched filler and housing pair can corrode at the interface in salt or humid environments. EMI and RFI shielding gaskets are common in avionics, telecom, medical instrumentation, and defense housings. Where the same enclosure also has biocompatibility or cleanliness requirements, an FDA-compliant conductive silicone can be specified using materials that meet FDA 21 CFR 177.2600 and USP Class VI, the same standards that govern FDA medical-grade silicone gaskets.
Geometry matters as much as material
The right polymer still fails if the gasket is shaped wrong for the joint. A few design levers come up repeatedly in hard service:
- Durometer: a soft 40 to 50 Shore A elastomer conforms to rough or warped flanges at low bolt load, while a harder 70 to 80 Shore A resists extrusion and blow-out under pressure. The flange flatness and available seating stress decide which way to go.
- Profile: o-rings to AS568 dash sizes seat in a defined groove and are predictable to design; flat die-cut gaskets suit flat-face flanges and large or irregular shapes; raised beads or molded cross-sections concentrate seating stress where a flat sheet cannot.
- Reinforcement: a metal core, tang, or eyelet adds blow-out resistance and handling strength to graphite and PTFE without giving up the facing material's chemical or thermal resistance.
- Thickness: thinner gaskets creep and relax less and hold bolt load better; thicker gaskets conform to worse surfaces. The thinnest section that still seals the actual flange condition is usually the right answer.
Matching the material to the condition
The pattern across all of these cases is the same. Identify the dominant stressor, heat, chemical attack, fuel exposure, cold, pressure, or electromagnetic leakage, then choose the material whose known limits clear that stressor with margin, and finally set the geometry so the material is loaded correctly. Flexible graphite and compressed fiber for heat and steam, PTFE and polyimide for chemicals and extreme temperature span, fluorosilicone and FKM for fuel and wide ranges, and conductive elastomers for shielding each earn their place because of specific, measurable properties, not because one is universally better than another.
If you are working a joint that does not fit a standard catalog material, send us the operating conditions, the media, the temperature and pressure range, the flange or groove dimensions, and any standards you have to meet. We will recommend a material and gasket design built for that service and quote it against your drawings.