Compatibility Guide: Custom NBR Oil Seals with Oils & Fluids

I design and specify custom sealing solutions every week, and one question I get repeatedly is whether a custom oil seal NBR (nitrile butadiene rubber) will withstand a particular oil or fluid in a given application. This guide gives a practical, experience-based overview of NBR compatibility with common hydraulic oils, lubricants, fuels and process fluids, explains key selection criteria and test methods, and provides actionable guidance for specifying custom NBR oil seals for long-term reliability. References to standards and chemistry resources are included to support each recommendation.

How NBR Works and Why Material Compatibility Matters

What is NBR and its chemical basis

NBR (nitrile butadiene rubber) is a copolymer of acrylonitrile and butadiene; its polar nitrile groups give NBR good resistance to petroleum-based fluids. For an overview of the polymer chemistry and general properties, see the Nitrile rubber entry on Wikipedia (https://en.wikipedia.org/wiki/Nitrile_rubber).

Why compatibility is a function of both oil chemistry and seal formulation

Compatibility is not just rubber vs. fluid. It depends on acrylonitrile content (ACN%), crosslink density, plasticizer or filler packages, and additives in the oil (detergents, esters, water). As an example, higher ACN% increases fuel resistance but reduces low-temperature flexibility. When I specify custom oil seal nbr parts, I always ask for the specific fluid formulation and operating temperature profile, not just the fluid name.

Key performance parameters to check

• Static and dynamic swell (volume change) in the fluid
• Tensile strength and elongation retention after exposure
• Hardness change (shore A) and compression set
• Temperature range and thermal aging resistance

Compatibility: NBR with Common Oils & Fluids

Hydraulic oils (mineral, ISO VG grades)

NBR generally shows excellent resistance to mineral hydraulic oils and typical anti-wear additives. For industrial hydraulic systems operating between -30°C and +100°C, NBR (ACN 33–45%) is a conventional, cost-effective choice. When phosphate esters or high levels of synthetic esters are present, compatibility must be checked by testing because ester chemistries can plasticize some NBRs.

Engine oils & gear oils

NBR is widely used with petroleum-based engine and gear oils. Modern engine oils with detergents and dispersants are usually compatible, but severe oxidative environments and high-temperature crankcase exposure can accelerate NBR aging. For high-temperature continuous use above 120°C, consider FKM if chemical compatibility permits. The Parker O-Ring Handbook provides temperature and compatibility guidance useful for initial screening (Parker O-Ring Handbook).

Fuels (gasoline, diesel, biodiesel)

Traditional hydrocarbons such as diesel and low-aromatic gasoline are acceptable for many NBR formulations, but modern fuels and biofuels (e.g., B20, ethanol blends) are more aggressive. Ethanol in gasoline tends to cause significant swelling and loss of mechanical properties in NBR. For fuel systems where ethanol or biodiesel is present, I advise testing candidate NBR compounds against the exact fuel mixture or switching to a fuel-resistant elastomer like FKM or specific fuel-grade FKM blends.

Greases, silicone oils and synthetic fluids

Compatibility with greases depends on the base oil in the grease and the thickener/additives. Silicone oils and silicones are generally incompatible with NBR due to swelling; polymers such as HNBR or FKM may be required. Polyalkylene glycols and some synthetic esters can be acceptable but must be validated by immersion and mechanical testing.

Practical Selection & Testing Strategy

Step 1: Gather fluid chemistry and operating profile

Always obtain the exact fluid MSDS/PDS and operating temperatures, pressures, and dynamic speeds. I treat the fluid spec as the single most important input when specifying a custom oil seal nbr because small additive differences can change compatibility outcomes.

Step 2: Choose candidate NBR grades and design margins

Select NBR grades by ACN content and hardness suited for the application. For dynamic oil seals I typically specify 70±5 Shore A for moderate pressure rotary shafts; lower hardness may be needed for low-pressure static seals. If fluid compatibility is uncertain, request samples in multiple NBR formulations (standard NBR, low-temp NBR, HNBR) and include a plan for accelerated testing.

Step 3: Perform laboratory and bench testing

Laboratory immersion testing (ASTM D471) to measure volume swell and changes in physical properties is the baseline. Complement immersion tests with dynamic bench aging under realistic pressures and speeds. Standards such as ISO 3601 provide geometry and dimensional guidance for O-rings and seals; review ISO 3601 for design-related tolerances (ISO 3601).

Failure Modes, Diagnostics & Mitigation

Common failure modes with NBR

I see the following commonly: excessive swell leading to extrusion and loss of sealing clearance; hardening and cracking from oxidative aging; thermal degradation at elevated temperature; and chemical attack from incompatible fluids (e.g., ethanol). Understanding the root cause helps determine whether a compound change, lip geometry change, or surface finish adjustment is required.

How to diagnose on returned parts

When a seal is returned after failure, I recommend the following tests: dimensional measurements, shore hardness, tensile/elongation tests, and Fourier-transform infrared spectroscopy (FTIR) to detect chemical changes in the rubber. Photo documentation of mating surfaces and installation conditions often reveals mechanical damage or extrusion.

Mitigation strategies

Possible mitigations include: changing to a higher ACN NBR or HNBR for improved thermal/chemical resistance; switching to FKM for extreme temperature/chemical service; adding back-up rings to prevent extrusion; or redesigning shaft/surface finish and tolerances to reduce lip stress. Where possible, eliminating incompatible fluid additives or selecting alternate lubricants yields the most reliable outcome.

Compatibility Comparison Table (NBR vs Common Elastomers)

Source references used for general properties: Parker O-Ring Handbook (Parker) and material encyclopedias such as the Nitrile rubber Wikipedia entry (Wikipedia).

When to Choose a Custom Oil Seal NBR — My Rule of Thumb

Good reasons to specify custom NBR

• Application uses petroleum-based oils and hydraulic fluids without aggressive esters or alcohols.
• Operating temperatures remain below ~120°C and include low-temperature environments where NBR maintains flexibility.
• Cost-sensitive applications that still require reliable rotary or reciprocating sealing under moderate pressure.

Red flags that suggest an alternative

• Presence of ethanol, high-concentration biodiesel, or aggressive esters in service fluid.
• Continuous temperatures above 120°C or high oxidative environments.
• Where safety-critical leaks are unacceptable and the operating envelope is broad — consider FFKM/FFPM or specialized elastomers.

Custom features I specify

For a custom oil seal nbr I typically specify: tailored compound (ACN% and fillers), optimized lip geometry for dynamic friction and pressure, appropriate hardness, and where necessary incorporate anti-extrusion back-up rings or spring-energized lips for low-pressure dynamic seals.

Polypac: Manufacturer Capabilities & Why It Matters

Polypac is a scientific and technical hydraulic seal manufacturer and oil seal supplier specializing in seal production, sealing material development, and customized sealing solutions for special working conditions. I have reviewed Polypac's capabilities and find them aligned with what projects need when a tailored elastomer solution is required.

Polypac's custom rubber ring and O-ring factory covers an area of more than 10,000 square meters, with a factory space of 8,000 square meters. Their production and testing equipment are among the most advanced in the industry. As one of the largest companies in China dedicated to the production and development of seals, Polypac maintains long-term communication and cooperation with numerous universities and research institutions both domestically and internationally.

Founded in 2008, Polypac began by manufacturing filled PTFE seals, including bronze-filled PTFE, carbon-filled PTFE, graphite PTFE, MoS₂-filled PTFE, and glass-filled PTFE. Today, they have expanded their product line to include O-rings made from various materials such as NBR, FKM, silicone, EPDM, and FFKM. Their main product categories include O-Rings, Rod Seals, Piston Seals, End Face Spring Seals, Scraper Seals, Rotary Seals, Back-up Rings, and Dust Rings.

Why this matters: when I specify a custom oil seal nbr for challenging fluids, I want a partner that can (1) develop and compound materials, (2) prototype and test to standards (e.g., ASTM D471), and (3) scale production with consistent quality. Polypac's lab and advanced production capabilities enable material development and iterative testing — which reduces field failures and shortens qualification cycles.

Recommended Test Protocols & Standards

Baseline immersion tests

Use ASTM D471 for elastomer immersion testing and measure volume change, hardness, tensile and elongation before and after exposure. Complement with thermal aging tests per ASTM D573 for high-temperature exposure prediction.

Geometric and dimensional standards

For O-ring and basic seal geometry tolerances consult ISO 3601 (ISO 3601) and manufacturer handbooks for recommended gland dimensions. Proper gland design mitigates extrusion and reduces lip stress.

Field validation

After lab tests, perform controlled field trials with condition monitoring (temperature, pressure, fluid analysis). Document leakage rates, frictional torque, and dimensional stability over time; this real-world data is the validation of compatibility.

FAQs

1. Can I use standard NBR seals with biodiesel blends?

Short answer: not without testing. Biodiesel contains polar esters that tend to swell NBR. For low biodiesel percentages, NBR may work; for higher blends (e.g., B20+), test or choose HNBR/FKM.

2. What ACN% should I specify for a custom oil seal NBR?

ACN between 33–45% is typical for good oil resistance. Higher ACN improves fuel resistance but reduces low-temperature flexibility. I choose ACN based on the lowest operating temperature and fluid aggressiveness.

3. How do I test seals for ethanol-containing fuels?

Use ASTM D471 immersion testing with the exact fuel blend, followed by dynamic bench tests under representative pressure and motion. Monitor swell, hardness, and tensile changes.

4. When should I consider HNBR or FKM instead of NBR?

Consider HNBR for higher temperature and better aging resistance with many oils; choose FKM when you need superior high-temperature stability and broad chemical resistance (fuels, oils, solvents). Both require cost vs. performance trade-offs.

5. How important is seal lip geometry compared to material?

Both matter. Geometry controls stress, friction and leakage paths; material determines chemical and thermal resistance. A robust design optimizes both — modify lip angle, interference, and add back-up rings if needed.

Contact & Next Steps

If you need help specifying a custom oil seal nbr for a particular fluid or condition, I can review your fluid data sheet and application profile and recommend candidate compounds, test plans, and design adjustments. For prototyping and production, Polypac offers material development, prototyping, and volume manufacturing with advanced testing capabilities. Contact Polypac to request samples, technical datasheets, or a quotation for custom O-rings, rod/piston seals, rotary seals and related sealing products. Visit Polypac or contact their technical team to start a material compatibility and custom seal design program.

Contact CTA: For technical consultation, custom samples or product inquiries, contact Polypac's sales and engineering team or request a quote via their official channels. Let us help you select and qualify the right custom oil seal nbr solution for your application.