HNBR (Hydrogenated Nitrile) Rubber Technical Guide

1. What Is HNBR?

HNBR (Hydrogenated Acrylonitrile-Butadiene Rubber, ASTM D1418 designation HNBR) is produced by the selective catalytic hydrogenation of the carbon-carbon double bonds in the butadiene portion of NBR. This process, developed by Bayer (now Arlanxeo) in the 1980s under the trade name Therban and by Zeon Corporation as Zetpol, converts the unsaturated butadiene segments into a saturated polyethylene-like backbone while preserving the oil-resistant acrylonitrile (ACN) groups.

The result is an elastomer that retains NBR's excellent oil and fuel resistance while gaining:

• • Significantly improved heat resistance (150°C continuous vs 100–120°C for NBR)
• • Excellent tensile strength (20–30 MPa, among the best of oil-resistant elastomers)
• • Superior dynamic fatigue resistance (critical for timing belts)
• • Outstanding H₂S, CO₂, and sour gas resistance (critical for oilfield)

HNBR is often called the "gap filler" between NBR and FKM — it offers roughly half the performance gain from NBR to FKM at about 1/3 of FKM's cost.

2. HNBR Grades by ACN Content and Hydrogenation Level

Hydrogenation level: Commercially, >95% saturation is considered "fully hydrogenated" and is required for 150°C service. Grades with 90–95% hydrogenation are sometimes offered at lower cost for less demanding applications.

3. Mechanical, Thermal, and Physical Properties

Key strength — Tensile: HNBR delivers 20–30 MPa tensile strength, which is higher than both standard NBR (10–25 MPa) and FKM (8–17 MPa). This is due to the crystallinity of the hydrogenated polyethylene segments, which act as strain-induced reinforcing domains.

Key strength — Abrasion: With DIN abrasion values of 70–120 mm³, HNBR matches or exceeds NBR and significantly outperforms FKM (120–200 mm³). This is critical for dynamic applications like timing belts and rotating shaft seals.

4. Chemical and Environmental Resistance

Critical differentiator — H₂S/CO₂ resistance: HNBR is one of the very few elastomers that combines oil resistance with outstanding resistance to hydrogen sulfide (H₂S) and carbon dioxide (CO₂) at elevated temperatures and pressures. This is because the saturated backbone has no double bonds for H₂S to attack — the same double bonds that make NBR vulnerable to H₂S crosslinking and embrittlement. This gives HNBR a dominant position in oilfield downhole applications. Testing per NACE TM0297 and ISO 23936-2 (sour service elastomers) consistently shows HNBR outperforming both NBR and FKM in H₂S environments.

5. Key Applications

5.1 Automotive Timing Belts

HNBR is the dominant material for modern automotive timing (synchronous) belts. The combination of 150°C heat resistance, excellent dynamic fatigue life, high tensile strength (belt tensile cords are fully supported), and good oil and coolant resistance makes it ideal for the under-hood environment. Modern engines run hotter (130–150°C oil sump temperatures in turbocharged engines), and HNBR timing belts survive the full service interval (typically 100,000–160,000 km) under these conditions.

5.2 Oilfield Downhole Equipment

In oil and gas drilling and production, HNBR is extensively used for:

• • Stator elastomers in mud motors (PDM): Carboxylated XHNBR provides the best balance of abrasion resistance, H₂S/CO₂ resistance, and dynamic fatigue for progressive cavity drilling motor stators. Typical hardness 55–65 Shore D.
• • Blowout preventer (BOP) components: Packers, ram seals, annular elements (HNBR works where NBR ages too quickly and FKM is compromised by amines).
• • Packer elements and seal stacks: For high-pressure/high-temperature (HPHT) wells exceeding 150°C and 10,000 psi.
• • Wireline and slickline sealing elements
• • Downhole tool seals (frac plugs, bridge plugs)

5.3 Automotive Air Conditioning Systems

HNBR is the standard material for automotive AC system seals and O-rings, particularly with R-134a and R-1234yf refrigerants and PAG lubricants. HNBR's excellent compatibility with PAG oils and minimal explosive decompression resistance (when rapidly depressurized from refrigerant pressures) makes it preferred over NBR.

5.4 Industrial Seals and Gaskets

HNBR is used in demanding industrial sealing applications: mechanical seal secondary seals and bellows, hydraulic cylinder rod seals for high-temperature systems (>100°C), heat exchanger gaskets in oil and chemical processing, and compressor valve plates and seals.

6. HNBR vs NBR vs FKM: Cost-Performance Matrix

Selection logic:

• • NBR sufficient → Most cost-effective; use it if environment allows
• • NBR marginal on heat/H₂S/ozone → HNBR bridges the gap at moderate premium
• • Environment beyond HNBR's 150°C or extreme chemical exposure → FKM (at 3–5× HNBR cost)

7. Processing and Formulation

7.1 Cure Systems

HNBR is cured primarily with peroxide (dicumyl peroxide at 2–8 phr, or bis(t-butylperoxy-isopropyl)benzene for lower odor). Coagents (TAC, TAIC, TMPTMA, ZDMA) are used to improve crosslink density and compression set. Sulfur cures are not used for HNBR since the saturation of the backbone leaves few sites for sulfur crosslinking.

7.2 Carbon Black and Filler Selection

N770/N774 carbon blacks are preferred for compression set resistance. N330/N550 balance reinforcement and heat aging. N990 (MT) black is used for high compound hardness with minimal Mooney impact. Silica reinforcement is also applicable when lower heat buildup is required.

7.3 Antioxidants

Despite HNBR's inherent oxidation resistance, additional antioxidant protection (ZMTI, TMQ, 4,4'-bis(alpha,alpha-dimethylbenzyl)diphenylamine) improves long-term heat aging at 150°C. The combination of ZMTI + diphenylamine antioxidant with an MBI synergist is a typical protective package.

8. Standards and Specifications

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Nanjing Yuhang Rubber Co., Ltd. manufactures high-performance HNBR O-rings, gaskets, molded seals, and custom components for automotive, oilfield, and demanding industrial applications. Our HNBR compounds are formulated for high-temperature (150°C), sour gas (H₂S/CO₂) resistance, and dynamic sealing applications. ISO 23936-2 and NORSOK M-710 compliant grades available with full test certification.

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