Nitrile vs Viton Rubber: Which Seal Material Should You Specify?
Nitrile (NBR) and Viton (FKM) are the two most widely specified elastomers for oil and chemical sealing applications — but they are not interchangeable, and choosing the wrong one is an expensive mistake. Nitrile is the standard choice for petroleum oil, hydraulic fluid, and fuel resistance up to 120°C, at a relatively low cost. Viton is the engineering upgrade for aggressive chemicals, aromatic fuels, and temperatures up to 200°C, at roughly 10–15 times the price.
The decision between them is not simply about which is “better.” It is about matching the material’s chemical resistance window and temperature ceiling to your actual service conditions. This guide gives you the technical comparison and a clear decision framework — so you specify the right seal first time.
What Is Nitrile Rubber (NBR)?
Nitrile rubber — formally acrylonitrile-butadiene rubber (NBR) — is a synthetic elastomer produced by copolymerising acrylonitrile (ACN) and butadiene. The acrylonitrile content, typically between 18% and 50%, governs the balance of oil resistance and low-temperature flexibility: higher ACN content gives better oil resistance but reduces cold-temperature performance.
NBR is the global standard for oil and fuel sealing. It offers excellent resistance to petroleum-based oils, mineral hydraulic fluids, greases, and aliphatic hydrocarbons. Paired with its good abrasion resistance and mechanical properties, NBR is specified for the vast majority of hydraulic system seals, O-rings, gaskets, and hose liners across engineering, automotive, and industrial applications.
Key properties of nitrile rubber:
| Property | Value |
|---|---|
| Temperature range | -40°C to +120°C (standard grades) |
| Oil and mineral hydraulic fluid resistance | Excellent |
| Abrasion resistance | Good |
| Tensile strength | 10–20 MPa (compound dependent) |
| Shore A hardness range | 40–90 |
| Typical industrial grade | 60–70 Shore A |
| Ozone and UV resistance | Poor |
| Ketone resistance (MEK, acetone) | Poor |
| Aromatic hydrocarbon resistance | Limited |
What Is Viton Rubber (FKM)?
Viton is the registered trade name of Chemours (formerly DuPont) for their range of fluoroelastomers. The generic designation is FKM under ASTM D1418 and ISO 1629. Other manufacturers produce FKM under different trade names (Fluorel, Tecnoflon, DAI-EL), but Viton is the most widely recognised.
FKM compounds contain fluorine atoms bonded to the polymer backbone — typically 66% fluorine by weight in standard grades. This fluorine content is what gives Viton its exceptional resistance to heat, aggressive chemicals, fuels, and oxidising environments. The trade-off is cost: FKM is significantly more expensive than NBR, and some grades require elevated curing temperatures that add manufacturing complexity.
Key properties of Viton (FKM) rubber:
| Property | Value |
|---|---|
| Temperature range | -20°C to +200°C (standard grades; specialist grades to +230°C) |
| Oil, fuel and chemical resistance | Excellent — including aromatic and oxygenated fuels |
| Ozone and UV resistance | Excellent |
| Steam resistance | Poor above 150°C — a common misapplication |
| Ketone resistance (MEK, acetone) | Poor |
| Tensile strength | 10–15 MPa |
| Shore A hardness range | 60–90 |
| Typical industrial grade | 70–75 Shore A |
Nitrile vs Viton: Side-by-Side Comparison
| Property | Nitrile (NBR) | Viton (FKM) |
|---|---|---|
| Max continuous temperature | +120°C | +200°C |
| Min temperature | -40°C | -20°C |
| Petroleum oil resistance | Excellent | Excellent |
| Aromatic fuel resistance | Limited | Excellent |
| High-ethanol fuel (E85+) resistance | Poor | Good |
| Mineral hydraulic fluid resistance | Excellent | Excellent |
| Phosphate ester hydraulic fluid (Skydrol) | Poor | Good |
| Ozone and UV resistance | Poor | Excellent |
| Steam resistance | Moderate (to 120°C) | Poor above 150°C |
| Ketone resistance (acetone, MEK) | Poor | Poor |
| Amine resistance | Moderate | Poor |
| Low-temperature flexibility | Excellent | Moderate |
| Relative cost | Low | Very High (10–15× NBR) |
| Typical applications | Hydraulic seals, O-rings, fuel hose liners, general sealing | Chemical processing seals, aerospace fuel systems, high-temp applications |
When to Use Nitrile — and When Not To
Nitrile rubber is the correct choice for the majority of oil and hydraulic sealing applications where service temperatures remain below 120°C and the media is petroleum-based. It performs reliably in:
- Mineral hydraulic systems (HM, HV, HLP grades)
- Engine oil and gearbox oil sealing
- Fuel systems using standard diesel and petrol (low ethanol content)
- Pneumatic systems with oil-lubricated air
- General industrial O-ring and gasket applications
Where nitrile fails — and this matters for specification:
Nitrile has zero resistance to ozone and UV. Seals, extrusions, and gaskets left exposed to outdoor environments will crack within months — sometimes weeks in high-ozone urban environments. If the seal is static and indoors, this is rarely an issue. If it is exposed, weather-resistant compounds must be used.
Nitrile also has limited resistance to aromatic hydrocarbons and performs poorly with modern high-ethanol fuel blends (above E20). The growing adoption of E85 and E10 fuels has caused a measurable increase in premature nitrile seal failures in older fuel systems originally designed around standard petrol. If your fuel system will see ethanol-blended fuels regularly, Viton is the safer specification.
When to Use Viton — and When Not To
Viton (FKM) is the correct choice when service conditions exceed what nitrile can handle — either in temperature, chemical aggression, or outdoor exposure. Justify the cost premium with one or more of these conditions:
- Continuous service temperatures above 150°C
- Exposure to aromatic fuels, halogenated solvents, or aggressive chemicals
- Phosphate ester hydraulic fluids (Skydrol) — critical in aerospace
- Outdoor applications requiring long-term ozone and UV resistance
- Long service life requirements where a seal failure causes significant downtime cost
Where Viton fails — the most common misapplication is steam. Engineers assume that a high-performance material will resist steam, but Viton performs poorly in saturated steam above 150°C and hot water service. EPDM is the correct choice for steam and hot water. Using Viton in a steam application will result in premature failure and seal degradation at significant material cost.
Viton also has poor resistance to ketones (acetone, MEK, methyl ethyl ketone) and amines — despite its otherwise broad chemical resistance. Always check the specific chemical resistance data for the media in question before specifying.
Which Should You Specify? A Decision Framework
Use this logic to determine the correct material for your application:
| Condition | Specify |
|---|---|
| Petroleum oil or mineral hydraulic fluid, temperature below 120°C | Nitrile (NBR) |
| Petroleum oil or mineral hydraulic fluid, temperature 120°C–200°C | Viton (FKM) |
| Standard diesel or petrol fuel (low ethanol) | Nitrile (NBR) |
| High-ethanol fuel blends (E20 and above) | Viton (FKM) |
| Aromatic solvents or halogenated chemicals | Viton (FKM) |
| Steam or hot water service | EPDM (neither NBR nor FKM) |
| Ketones (acetone, MEK) | EPDM (neither NBR nor FKM) |
| Outdoor/UV-exposed static seals | Viton (FKM) or EPDM |
| Phosphate ester hydraulic fluid (Skydrol) | Viton (FKM) |
| Budget is the primary constraint, conditions are mild | Nitrile (NBR) |
| Seal failure would cause significant downtime or safety risk | Viton (FKM) |
When conditions sit on the borderline — for example, a hydraulic system running at 110°C with occasional peaks to 130°C — specify Viton. The cost premium is marginal against the cost of an unplanned seal replacement in a live system.
The Cost Equation
Viton O-rings and seals typically cost 10 to 15 times the equivalent nitrile component. For a standard BS 1806 / ISO 3601 O-ring in a common size, the unit cost difference may be a few pence versus a few pounds — which sounds manageable. However, in a system requiring hundreds of seals, or where bespoke cut gaskets are involved, the cost difference is significant.
The correct approach is to calculate the total cost of ownership, not just the component cost:
- What is the cost of a seal failure in your system? (downtime, fluid loss, contamination, safety incident)
- What is the expected service life of each material under your conditions?
- How frequently does the system require planned maintenance?
In many applications, the answer is that nitrile is entirely adequate and the Viton premium is unnecessary. In others — particularly chemical processing, aerospace, and high-temperature energy applications — Viton’s extended service life easily justifies its cost.
Why Did My Nitrile Seal Fail?
This is one of the most common technical queries Delta’s team receives. Nitrile seal failures fall into a small number of recurring patterns:
Thermal Degradation
The seal has been operating at or above its rated temperature. Nitrile hardens, loses elasticity, and cracks under sustained heat above 120°C. The fix is either to reduce operating temperature or upgrade to Viton.
Chemical Attack
The media contains a component outside nitrile’s resistance range — most commonly aromatic solvents, oxygenated fuel additives, or cleaning agents introduced during maintenance. Always verify the full chemical composition of the media, not just the primary fluid.
Ozone Cracking
Characteristic surface cracking perpendicular to the direction of stress. Caused by ozone exposure in outdoor or poorly ventilated environments. Common in seals near electrical equipment (which generates ozone). Viton or EPDM resolves this.
Wrong Hardness for the Application
A seal that is too soft extrudes under pressure; one that is too hard fails to seal adequately under low bolt load. See the Shore hardness guide for application-specific hardness recommendations.
Extrusion Damage
The O-ring has been forced into the clearance gap at the dynamic or static sealing face, causing nibbling and tearing at the seal edge. This is a system design issue (excessive clearance gap) — but upgrading to a harder compound (80–90 Shore A) or adding back-up rings resolves it regardless of material.
Frequently Asked Questions
What is the difference between nitrile and Viton rubber?
Nitrile (NBR) and Viton (FKM) are both oil-resistant elastomers, but Viton offers significantly higher temperature resistance (up to 200°C vs 120°C for nitrile) and broader chemical resistance, particularly to aromatic fuels and aggressive solvents. Nitrile is considerably less expensive and is the standard choice for most petroleum oil and hydraulic sealing applications. Viton is specified when service conditions exceed what nitrile can reliably handle.
Is Viton better than nitrile for O-rings?
Not universally — it depends on the application. For standard hydraulic systems using mineral oil below 120°C, nitrile O-rings perform excellently and are the cost-effective choice. Viton O-rings are the correct specification for high-temperature service, aromatic or oxygenated fuel systems, aggressive chemical environments, and applications requiring long-term ozone resistance. Specifying Viton where nitrile is sufficient adds unnecessary cost.
When should I use Viton instead of nitrile?
Specify Viton over nitrile when: operating temperature exceeds 120°C continuously; the media contains aromatic hydrocarbons, halogenated solvents, or high-ethanol fuel blends; the application requires outdoor UV and ozone resistance; or the fluid is a phosphate ester hydraulic fluid such as Skydrol. The 10–15x cost premium is justified when seal failure carries significant consequences.
Why does nitrile rubber fail in fuel systems?
Modern fuel blends containing high proportions of ethanol (E20 and above) cause nitrile to swell and degrade — nitrile has limited resistance to oxygenated fuel additives. Older fuel systems designed around standard petrol may experience seal failure as fuel compositions change. Viton is the recommended upgrade for fuel systems regularly using high-ethanol blends.
Can Viton be used with steam?
No — steam is one of Viton’s key weaknesses. Despite its broad chemical resistance, Viton performs poorly in saturated steam above 150°C and hot water service. EPDM is the correct elastomer for steam, condensate, and hot water applications.
What is FKM rubber?
FKM is the generic ISO designation for fluoroelastomers — the material class that includes Viton (Chemours), Fluorel (3M/Dyneon), and Tecnoflon (Solvay). All FKM compounds share the same fundamental chemistry: a fluorinated polymer backbone providing heat and chemical resistance. Viton is simply the most recognised brand name in this class.
For most oil and hydraulic sealing applications below 120°C, nitrile remains the technically correct and cost-effective choice. Where temperature, chemical aggression, or long service life requirements push beyond that envelope, Viton delivers the performance margin that justifies its cost. The key is matching the material to the actual service conditions — not defaulting to Viton for reassurance, or to nitrile for cost savings without checking the application requirements.
Delta Rubber supplies both nitrile and Viton in sheet, O-ring, extrusion, and custom gasket form. For material selection advice on a specific application, contact the team directly — or browse the full rubber seals and O-rings range at deltarubber.co.uk.
