Silicone vs PTFE O-Rings: Choosing the Right Seal Material

Quick answer

Choose silicone o-rings when you need a flexible, FDA-compliant seal for food, beverage, or medical contact across roughly -80°F to +450°F. Choose PTFE o-rings when the seal must survive aggressive chemicals or extreme temperatures from -350°F to +500°F, and accept that the material is much harder and less forgiving to install.

Temperature range is the first filter

Start by writing down the lowest and highest temperature the seal will see in service, not just the normal operating temperature. Silicone handles -80°F to +450°F (-62°C to +232°C), which covers most food processing, beverage, and general industrial equipment. PTFE stretches from -350°F to +500°F (-212°C to +260°C), so it is the material to reach for in cryogenic lines, high-temperature process equipment, or anywhere a silicone seal would harden or degrade. If your application swings between both extremes, PTFE is usually the safer starting point, but confirm the mating surfaces can handle a harder seal.

Chemical exposure decides the rest

Silicone is not a good match for many solvents, fuels, and strong acids, while PTFE resists a very broad range of aggressive chemicals. If the seal will contact oils, fuels, or concentrated chemicals, PTFE is the more reliable choice. If it will contact water, mild cleaners, or food products, silicone is typically fine and easier to work with. When in doubt, test a sample in the actual media before committing to a full production run.

Durometer changes how the seal installs

Durometer measures hardness on the Shore A scale, and it affects both sealing force and installation difficulty. A 70 durometer silicone o-ring is firm enough to hold its shape but still flexible enough to stretch over a groove or gland. A 22 durometer PTFE o-ring is much softer on the scale but behaves very differently: PTFE is stiff and does not stretch, so it is usually installed as a solid ring rather than stretched into place. Plan your assembly sequence around that difference, and do not assume a PTFE ring can be flexed on like a rubber one.

FDA compliance and color coding

If the seal touches food, beverage, or pharmaceutical product, look for FDA-compliant material. Silicone o-rings are commonly supplied in red for this reason, which makes them easy to identify on a line and helps with visual inspection. PTFE is also used in food and chemical applications, but verify the specific part meets your compliance requirement rather than assuming it. Color is a helpful cue, not a substitute for the material documentation.

Sizing: AS568 dash numbers and cross-section

AS568 dash numbers, often called dash number o-rings, encode the cross-section and inner diameter so you can match a replacement without measuring every time. A dash 358 ring, for example, is a 0.21" cross-section with a 6.725" inner diameter, while a dash 221 is a 0.139" cross-section with a 1.421" inner diameter. Always confirm the dash size against the groove dimensions, and remember that switching materials does not change the dash size but can change how the ring seats.

Decision rules for silicone vs PTFE o-rings

• Food, beverage, or medical contact: Pick an FDA-compliant silicone o-ring such as a red 70 durometer ring when the seal touches product and temperatures stay within -80°F to +450°F. Confirm FDA compliance on the part before ordering.
• Aggressive chemicals or extreme cold and heat: Choose PTFE when the media would attack silicone or when service temperatures run from -350°F up to +500°F. Verify the mating surfaces can handle a harder, non-stretching ring.
• Tight installation space: Silicone stretches over grooves and glands; PTFE does not, so plan the assembly order and clearances before you buy. If you cannot install a solid ring, silicone is the practical choice.
• Replacement sizing: Match the AS568 dash number and cross-section to the existing groove rather than measuring the old ring, which may be deformed. Dash number plus cross-section is the reliable match.

Product facts

• 0.21" CS X 6.725" ID Silicone O-Rings | AS568-358 | Dash 358 | FDA Compliant | 70 Durometer | Red (11 / Pkg.): Silicone o-ring, AS568-358, 0.21" cross-section by 6.725" inner diameter, 70 durometer, red, FDA compliant, rated -80°F to +450°F. A good fit for food and beverage equipment or general sealing where flexibility and FDA compliance matter more than extreme chemical resistance.
• 0.139" CS X 1.421" ID PTFE O-Rings | AS568-221 | Dash 221 | 22 Durometer (300 / Pkg.): PTFE o-ring, AS568-221, 0.139" cross-section by 1.421" inner diameter, 22 durometer, rated -350°F to +500°F. The choice when the seal must resist aggressive chemicals or extreme temperatures, with the tradeoff of a stiffer ring that will not stretch into place.

Which material fits your application

Best for

• Food, beverage, and pharmaceutical equipment needing FDA-compliant seals
• General industrial sealing where flexibility and easy installation matter
• Applications with moderate temperatures from -80°F to +450°F

Not for

• Service with aggressive solvents, fuels, or strong acids, where PTFE is the better choice
• Cryogenic or high-temperature processes beyond silicone's range
• Assemblies where a hard, non-stretching ring cannot be installed