Defense sealing for fuel, hydraulic, and lubricant service runs through NBR (nitrile butadiene rubber) at 28-50% ACN, with continuous service between -40°C and +120°C and tensile strength of 1,500-3,000 psi [S4].
The trade-off is fixed: higher acrylonitrile (ACN) lifts oil and fuel resistance but stiffens the compound in cold conditions, and lower ACN preserves flexibility at the cost of hydrocarbon defense [S5]. That single knob is the first call an engineer makes on a defense spec.
ACN content and the oil-versus-flexibility trade-off
Commercial NBR sits in a 28-34% ACN window for general industrial sheet, with 33% ACN the common UK stocking grade for gaskets, seals and liners in petroleum contact [S3]. Above 40% ACN, nitrile tolerates aggressive aromatic fuels and additives, which is the range defense fuel systems typically demand [S5]. Below 30% ACN, low-temperature flexibility improves but swell in gasoline and diesel rises measurably, so the part is limited to lightly contaminated lubricant service [S5].
The practical defense rule: spec medium-to-high ACN (33-50%) for fuel-line, bladder, and fuel-handling seals; reserve low ACN (28-30%) only for hydraulic actuators operating below -20°C where aromatic exposure is limited. For rail and transit variants of this same ACN-versus-temperature decision, see the related NBR selection map for rail service bands.
Hardness, tensile and the published property envelope
Defense procurement sheets typically call out Shore A 60-80 as the working range; 50A is softer and conforms to irregular flange faces under low bolt load, 70A resists extrusion in higher-pressure hydraulic cylinders, and 80-90A is reserved for high-pressure dynamic sealing [S3][S4]. Published tensile strength for NBR runs 1,500-3,000 psi depending on grade and filler system, with HNBR pushing that ceiling to roughly 4,000 psi [S4][S6].
For construction equipment exposed to similar oil and abrasion duty but on a different cost cycle, the parallel NBR selection map for construction service bands walks through the same Shore A-versus-pressure decision. The comparison that matters in defense qualification: a 70A NBR at 2,500 psi tensile is roughly the cost baseline, and HNBR at the same hardness is typically priced 2-3x higher with a -40°C to +150°C service window and improved ozone resistance [S6].
Defense failure modes: what actually kills NBR seals
Three failure patterns dominate defense NBR returns: ozone cracking on stored diaphragms, compression set loss on long-idle hydraulic actuators, and explosive decompression in high-pressure gas-charged accumulators. NBR is rated "Poor" for ozone and weather resistance, so any NBR part stored outdoors or fitted to exterior hardware needs a protective cover or an HNBR/EPDM substitution [S4]. Compression set for NBR compounds is "Good" but not excellent; seals held under constant deflection for 12+ months will take a permanent set above 25% and leak on re-pressurisation [S4].
Fluid compatibility is the most spec-sensitive area. NBR handles petroleum oils, mineral oils, vegetable oils, hydraulic fluids, petrol and diesel as baseline [S3]. It is rated only "Fair" against acids and ketones, which eliminates it from any circuit carrying brake fluid (DOT 3/4/5.1 is glycol-ether) or ester-based synthetics, and forces the spec toward EPDM or FKM in those branches [S4]. For the general nitrile rubber properties and grade catalogue, the published property envelope is the first cross-check.
HNBR as the upgrade path, not the default
Hydrogenated nitrile (HNBR) is the direct upgrade when NBR's envelope is breached. Published HNBR tensile reaches 4,000 psi with superior tear resistance and fatigue life, and the saturated backbone adds ozone and heat resistance that standard NBR cannot match [S6]. HNBR continuous service is typically rated -40°C to +150°C, lifting the upper ceiling by roughly 30°C over NBR.
The trade-off engineers should write into the spec: HNBR costs 2-3x NBR at the same geometry, so the default defense compound stays NBR at 33-50% ACN for fuel and hydraulic circuits, and HNBR is reserved for under-hood, high-temperature, or ozone-exposed locations. Where the elastomer choice is broader than the nitrile family, the industrial rubber selection reference sets the wider material map, and silicone rubber properties cover the high-temperature and weathering branch that nitrile cannot.
Specification checklist for a defense NBR drawing
A defensible NBR callout on a military drawing should pin six values: base polymer (NBR or HNBR), ACN content (e.g. 33% or 45%), Shore A hardness (60, 70, or 80), tensile strength minimum (typically 2,000-2,500 psi), continuous service temperature (matching the -30°C to +120°C NBR window or -40°C to +150°C HNBR window), and fluid compatibility list naming each media the seal will see [S3][S4][S6].
Add the test methods: ASTM D2000 for material classification, volume swell after 70 hours at the service fluid and temperature, and compression set after 22 hours at the upper service temperature. Without these, the callout buys a generic NBR part that may pass the receiving inspection and fail in the field. For the comparison baseline against other elastomer families used in defense, EPDM rubber properties cover the weather and water-ozone branch, and plastic versus rubber selection sets the broader polymer decision.
Track three signals over the next two quarters: HNBR price compression as hydrogenation capacity comes online, any revision to defense fuel specifications (JP-8 additives, F-34 variants) that would push ACN content higher, and the qualification status of bio-based or melanin-filled NBR composites that recent RSC work has shown can match petrochemical-NBR tensile at lower carbon footprint [S7].