What Material Is a Rock Drill Accumulator Diaphragm Made Of? NBR, HNBR, and Polyurethane Explained

Every hydraulic rock drill accumulator relies on a diaphragm to separate its nitrogen gas charge from the hydraulic oil circuit, and that diaphragm is molded from a specific elastomer or thermoplastic compound. The material choice is not incidental: it determines how the diaphragm tolerates heat, oil, abrasive contamination, and the extremely high flexing frequency the part experiences in normal operation. This article looks at the materials most commonly used for rock drill accumulator diaphragms, how they differ, and what that means for the part’s expected behavior in service.

What Is a Rock Drill Accumulator Diaphragm Made Of?

In current production, a rock drill accumulator diaphragm is almost always a molded synthetic elastomer, most often nitrile rubber (NBR) or hydrogenated nitrile rubber (HNBR), or a cast or molded polyurethane (PU) component. Some parts catalogs still label this component a “leather cup,” a term carried over from earlier diaphragm and cup-seal designs; despite the name, the parts sold and installed today are not made of leather. Other names encountered across manufacturers and regions include “diaphragm,” “membrane,” and “gas separator,” all referring to the same functional part: a flexible barrier positioned between the nitrogen chamber and the oil chamber inside the accumulator body.

Why Does the Diaphragm Material Matter for a Rock Drill?

Inside a rock drill accumulator, the diaphragm sits in one of the more demanding operating environments found in a hydraulic circuit. It is in constant contact with hydraulic oil on one side and pressurized nitrogen on the other, it experiences the oil’s operating temperature (which rises well above ambient once the drill has been running), and it flexes through a full compression-and-release cycle every time the impact piston reverses, which happens at a high frequency for as long as the drill is in use.

A material that is a poor match for these conditions will show it in predictable ways: swelling or softening if it is not resistant enough to the hydraulic oil, hardening and cracking if it cannot tolerate the oil’s operating temperature, or fatigue cracks appearing well before the drill’s other components need attention if the compound does not hold up to repeated flexing. This is why diaphragm material selection is treated as a distinct engineering decision rather than an afterthought to the part’s shape.

The heat-related part of that picture has a well-documented chemical cause. Rubber researchers describe thermal-oxidative aging as a self-accelerating, free-radical chain reaction: heat first knocks a hydrogen atom loose from the rubber's molecular backbone, generating a reactive radical; that radical reacts with dissolved oxygen to form a peroxy radical, which then pulls a hydrogen atom from a neighboring chain and creates a hydroperoxide plus a fresh radical, feeding the cycle onward. Depending on the specific polymer, this chain reaction ends up favoring either additional cross-linking (which stiffens and can crack the rubber) or chain scission (which softens it and reduces strength). The practical takeaway is the same either way: heat and oxygen exposure over time change a diaphragm compound's mechanical behavior, independent of any single overload event, which is part of why a diaphragm's real-world life depends on sustained operating temperature and not only on peak pressure.

Photo1:Cross‑section structural diagram of diaphragm‑type accumulator

What Is NBR and Why Has It Been the Standard Diaphragm Material?

NBR, nitrile butadiene rubber, is a copolymer of acrylonitrile and butadiene and has been the general-purpose elastomer of choice for mineral-oil hydraulic seals for decades, including rock drill accumulator diaphragms. Its acrylonitrile content gives it strong resistance to mineral-based hydraulic oils without significant swelling, and it also provides a comparatively low permeability to nitrogen gas, which helps an accumulator hold its pre-charge over a longer interval between checks.

NBR compounds are typically rated for continuous use across a range of roughly –30°C to somewhere in the 100–120°C region, though the exact figures depend on the specific compound formulation used by the manufacturer. Because it combines solid oil resistance with moderate cost, NBR remains the default specification for many rock drill and hydraulic breaker diaphragms.

NBR's mechanical robustness under real working pressure is not just a manufacturer's claim; it is also the kind of behavior finite-element sealing studies are built to check. A recent study of NBR O-ring seals under high-pressure conditions, for example, modeled contact stress against media pressure and found it rose in step with pressure while remaining above it throughout the tested range — the basic condition a rubber seal has to meet to keep sealing rather than leaking. A diaphragm is a different shape and duty from an O-ring, but the underlying material behavior draws on the same rubber science, and it is one more reason NBR has stayed the incumbent choice for demanding hydraulic components generally.

Photo 2: Physical sample of molded NBR rubber accumulator diaphragm

What Is HNBR and How Is It Different from Standard NBR?

HNBR is produced by hydrogenating NBR, a process that removes most of the reactive carbon-carbon double bonds left in the polymer backbone. That detail connects directly to the aging chemistry described above: those double bonds are the main reactive sites where the heat-and-oxygen chain reaction gets started, so removing most of them removes most of the vulnerability. The practical effect is a compound that keeps NBR’s core strength, oil resistance, while gaining meaningfully better resistance to heat and long-term aging, along with higher tensile and tear strength.

Where standard NBR’s continuous-use ceiling tends to sit in the 100–120°C range, HNBR compounds are commonly rated to roughly 150°C, while retaining cold-temperature flexibility not far behind NBR’s. This combination makes HNBR a common upgrade choice for rock drills that run hot, operate on long duty cycles, or work in climates with a wide temperature swing between a cold start and a fully warmed-up hydraulic system. The trade-off is cost: HNBR compounds are more expensive to produce than standard NBR.

“HNBR” also is not one fixed recipe. Research comparing several commercial HNBR grades has linked two formulation variables to real performance differences: a higher Mooney viscosity (a measure related to molecular weight) generally raises the vulcanizate’s crosslink density and mechanical strength, while a higher acrylonitrile content improves oil resistance but also raises the compound’s glass transition temperature — meaning it stiffens sooner as the diaphragm cools. The same body of research found that grades hydrogenated to a very high degree, on the order of 99%, showed the best resistance to heat aging and the best retention of compression set and tensile strength after prolonged exposure at 120°C. The practical implication is that two diaphragms both correctly labeled “HNBR” are not guaranteed to perform identically; the degree of hydrogenation and the specific formulation matter as much as the base polymer name.

What Is Polyurethane (PU) and When Is It Used Instead of Rubber?

Polyurethane, often abbreviated PU or referred to by the elastomer designation AU, is a different class of material from NBR and HNBR; rather than a conventional vulcanized rubber, it is typically a cast or molded thermoplastic or elastomeric polymer built around urethane linkages. Its defining strength is abrasion and tear resistance that is generally well ahead of standard rubber compounds, along with high tensile strength.

That combination makes PU diaphragms a common choice where the hydraulic oil is difficult to keep perfectly clean — a realistic condition on many drilling and mining sites — since PU resists erosion from suspended particles better than NBR does. The trade-off runs the other way on temperature: PU compounds are typically limited to a lower continuous-use ceiling, often cited around 80°C, than NBR or HNBR, and can stiffen more noticeably in cold conditions. PU is therefore usually selected for its wear resistance in moderate-temperature, contamination-prone conditions rather than as a heat-resistance upgrade.

Photo 3: Appearance comparison between polyurethane diaphragm and rubber diaphragm sample

Where Do FKM and Other Specialty Compounds Fit In?

FKM, a fluoroelastomer sometimes known by the trademarked name Viton, extends the usable temperature range well beyond NBR and HNBR, with continuous-use ratings commonly cited as high as 200°C, along with broader chemical resistance that covers some fluids and additives that attack nitrile-based rubbers. It is not the default choice for most rock drill diaphragms, however, because its low-temperature flexibility is noticeably weaker than NBR’s or HNBR’s, and it costs considerably more.

In practice, FKM tends to appear in rock drill or hydraulic breaker applications only where a specific condition calls for it: unusually high sustained oil temperatures, a hydraulic fluid outside the standard mineral-oil range, or a customer specification that requires it. Materials such as EPDM or standard silicone, while common elsewhere in sealing, are generally unsuitable for this application because they do not resist mineral hydraulic oil well, and are rarely if ever specified for accumulator diaphragms in rock drills.

A continuous-temperature rating is also not the whole story at the high end. Research on FKM seals in other high-temperature sealing equipment has shown that elevated temperature causes stress relaxation over time — the material's sealing force gradually eases even when the temperature itself stays within the compound's rated range, and prolonged exposure compounds the effect. The same caution applies to chemical exposure: research on HNBR sealing materials has found measurably faster degradation in high-temperature acidic conditions than in plain hydraulic oil at a similar temperature, with the degree of hydrogenation and the specific filler system both affecting how much resistance the compound retains. Neither finding changes the basic material choice for an ordinary mineral-oil rock drill circuit, but both are reminders that a temperature number alone does not fully describe how a compound will hold up, and that unusual fluids or sustained heat are worth flagging to a supplier rather than assuming any oil-resistant elastomer will do.

How Do These Materials Compare Side by Side?

The table below summarizes the general characteristics discussed above. Actual performance always depends on the specific compound formulation used by a given manufacturer, so it should be read as a guide to typical positioning rather than a specification for any individual product.

Photo 4: Physical comparison of NBR, HNBR and PU replacement diaphragms

How Does Material Choice Affect Fatigue Life Under High-Frequency Flexing?

A rock drill accumulator diaphragm does not usually fail because it is torn or punctured on day one; it fails because it flexes through a full compression cycle at a very high frequency, cycle after cycle, for as long as the drill is running. This makes flex-fatigue resistance — essentially how well a material tolerates repeated bending and stress reversal without developing cracks — at least as important as the material’s static strength figures.

Elastomer fatigue research generally evaluates this behavior in one of two ways. A crack-nucleation approach tracks how many load cycles an initially flaw-free sample can withstand before a crack first appears, typically correlated against a strain-based or energy-based load parameter. A crack-growth approach instead starts from a sample with a small existing flaw and measures how quickly that flaw extends per cycle. The classic framework behind the second approach, first applied to rubber by Rivlin and Thomas and still the standard reference point in the field, treats crack growth as governed by the tearing energy — the strain energy released per unit of new crack surface — rather than by stress or strain amplitude alone. One useful consequence of that framework is the idea of a fatigue threshold: below a certain tearing energy, an existing crack in a given compound essentially stops growing under cyclic load, which is part of why small, well-controlled flaws in a molded diaphragm do not necessarily doom it to early failure.

Compounding choices affect this fatigue behavior in ways that are not always intuitive. Crosslink density is a good example: increasing it generally improves fatigue resistance up to a point, but pushing it too far reduces the rubber's ability to absorb strain and can shorten fatigue life rather than extend it, so a well-designed diaphragm compound is a balance rather than simply “as much crosslinking as possible.” Research on rubber compounding for accumulator diaphragms specifically has also shown that filler dispersion and the broader additive package can shift gas permeability and flex-crack resistance considerably even within the same base polymer — in one published study, refining the filler dispersion in an NBR diaphragm compound measurably reduced nitrogen permeability and increased the number of flex cycles the material withstood before cracking, evaluated using standardized flex-cracking (De Mattia-type) and gas-permeation test methods. Environmental exposure compounds all of this: elevated temperature, humidity, and ambient ozone are all documented to accelerate crack initiation and growth in elastomers by reducing elongation capacity and attacking the material's surface, which is one more reason sustained oil temperature matters for diaphragm life independent of its effect on aging chemistry.

The practical implication is that two diaphragms of the “same material” on paper are not guaranteed to have the same fatigue life; the specific compound formulation, and how it was tested, can matter as much as the base polymer name. Material selection sets the ceiling on fatigue performance; the compound's formulation, the molding quality, and whether the accumulator's nitrogen pre-charge is set correctly — since incorrect pre-charge changes how far the diaphragm has to travel on every cycle — determine whether the part actually reaches it.

How Does the Operating Environment Influence Which Material Performs Best?

No single material is the best choice in every condition, which is why rock drill diaphragms are still commonly available in more than one compound for the same model. A few general patterns are worth keeping in mind:

• In cold climates, or on equipment that sits outdoors overnight before a cold start, a material with good low-temperature flexibility, such as standard NBR or HNBR, is usually a safer choice than a PU compound that stiffens more at low temperatures.
• In hot climates, or on machines run at a high duty cycle where hydraulic oil temperature climbs well above ambient, HNBR’s higher continuous-use temperature rating gives it more margin than standard NBR.
• On sites where keeping hydraulic oil consistently clean is difficult, such as many underground or high-dust surface operations, PU’s abrasion resistance can outweigh its more limited temperature range.
• Where the hydraulic fluid itself is unusual, for example certain fire-resistant or synthetic fluids, chemical compatibility with that specific fluid should be confirmed before assuming a standard NBR or HNBR compound will perform as expected.

Because these factors can pull in different directions, the right material is a judgment based on the specific site’s conditions rather than a fixed rule that applies to every rock drill everywhere.

How Can You Identify the Material of an Existing Diaphragm?

When a diaphragm needs to be replaced, it helps to confirm its material rather than assume it based on appearance, since color and texture are not standardized across manufacturers and are not a reliable way to identify a compound. The most reliable source is the original equipment manufacturer’s parts catalog or service documentation for that rock drill model, which typically lists the specified material alongside the part number.

Where that documentation is not available, a supplier familiar with the specific rock drill brand and model can often identify the material from the OEM part number, and in some cases from a physical sample of the failed diaphragm. Asking for written confirmation of the material grade, rather than relying on a verbal description, is a reasonable precaution before committing to a replacement, particularly if the operating conditions on your site fall outside typical ranges.

Photo 5: Technician verifies diaphragm material specification against parts documentation

Selected technical references consulted for this article:

1. Gao Y., Xiao P., Luo Z., et al. “Research Progress on Thermal-Oxidative Aging of Rubber Materials.” China Rubber Industry, 2026, 73(7): 548–556.
2. Xue Y. “Study on High-Temperature Performance of Hydrogenated Nitrile Rubber and Its Application.” Master’s thesis, Yangzhou University, 2023.
3. Jin G. “Study on Fatigue Behavior and Microscopic Mechanism of Polyurethane Elastomers.” PhD dissertation, Beijing University of Chemical Technology, 2025.
4. Liang J., Yan L., Tang S., et al. “Effect of StronWi Powder TNK on Performance of Rubber Diaphragm Compound in Energy Accumulator.” China Rubber Industry, 2024, 71(8): 599–603.
5. Cao J., Ma L., Shi Q., et al. “Study on Sealing Performance of NBR O-Rings under Room-Temperature High-Pressure Conditions.” China Rubber Industry, 2026, 73(7): 483–490.
6. Huang X., Sui Z., Hao Y., Li Q., Yan C. “High-Temperature Mechanical Behavior of FKM Fluororubber and Its Effect on the Sealing Performance of Downhole Packers.” Experimental Technology and Management, 2026, 43(4): 130–136.
7. Zhang X., Jin X., Jia D., et al. “Corrosion Behavior and Performance Optimization of Hydrogenated Nitrile Butadiene Rubber Sealing Materials under High-Temperature Acidic Environments.” Lubrication Engineering, published online ahead of print, 2026.