When ordering a replacement accumulator diaphragm for your Atlas Copco or Epiroc COP rock drill (part number 3115182200), one of the first decisions is material selection. The diaphragm is available in two primary compounds: NBR (nitrile butadiene rubber) and high-grade polyurethane. Both materials meet the functional requirements of the application, but their physical properties differ in ways that make each one better suited to specific operating conditions.
This article provides a detailed technical comparison to help mining engineers, maintenance planners, and purchasing managers make an informed material choice based on the actual conditions in their operation.
NBR (Nitrile Butadiene Rubber)
NBR is the traditional elastomer of choice for hydraulic accumulator diaphragms and is the standard specification for the original Atlas Copco 3115182200 component. Nitrile rubber is a copolymer of butadiene and acrylonitrile, where the acrylonitrile content determines the balance between oil resistance and low-temperature flexibility.
Key Properties of NBR
| Chemical Name | Nitrile Butadiene Rubber (Acrylonitrile-Butadiene Copolymer) |
| Hardness Range | 60–80 Shore A (typical for accumulator diaphragms: 70 Shore A) |
| Temperature Range | -30°C to +120°C (-22°F to +248°F) |
| Tensile Strength | 15–25 MPa |
| Elongation at Break | 300–500% |
| Tear Resistance | Good |
| Abrasion Resistance | Moderate |
| Oil Resistance | Excellent (mineral oils, synthetic esters) |
| Gas Permeability | Low |
| Compression Set | Good — maintains seal integrity over time |
Advantages of NBR for Rock Drill Accumulators
- Wide temperature range. NBR performs well from -30°C to +120°C, making it suitable for both cold-climate surface operations and hot underground mines where ambient temperatures can exceed 40°C.
- Excellent oil compatibility. NBR is inherently resistant to the mineral-based hydraulic oils (HLP 46, HLP 68) used in all COP rock drill systems. It does not swell, soften, or degrade in contact with these fluids.
- Low gas permeability. The acrylonitrile content in NBR provides an effective barrier against nitrogen permeation, helping maintain accumulator pre-charge pressure over longer intervals.
- Proven track record. NBR has been the standard material for hydraulic accumulator diaphragms in mining equipment for decades. Its behavior under cyclic loading is well understood and predictable.
- Cost-effective. NBR compounds are more widely available and less expensive to process than polyurethane, resulting in a lower unit cost for the finished diaphragm.
Limitations of NBR
- Moderate abrasion resistance — in contaminated hydraulic systems, particulate matter can erode the diaphragm surface faster than polyurethane
- Lower tear strength compared to polyurethane — if the diaphragm develops a nick or cut during installation or from debris, it will propagate faster
- Susceptible to ozone attack if exposed to air during extended storage
- Not suitable for phosphate ester or polyol ester hydraulic fluids (rarely used in mining rock drills)
Polyurethane (PU)
Polyurethane is an engineering polymer that combines some properties of rubber with some properties of plastic. For accumulator diaphragms, cast polyurethane (typically polyester-based) is used to produce a membrane with exceptional mechanical strength and abrasion resistance.
Key Properties of Polyurethane
| Chemical Type | Cast Polyester-Based Polyurethane Elastomer |
| Hardness Range | 80–95 Shore A (typical for accumulator diaphragms: 85–90 Shore A) |
| Temperature Range | -20°C to +80°C (-4°F to +176°F) |
| Tensile Strength | 30–60 MPa |
| Elongation at Break | 400–700% |
| Tear Resistance | Excellent — 3-5x greater than NBR |
| Abrasion Resistance | Excellent — up to 5x greater than NBR |
| Oil Resistance | Good (mineral oils); limited with some synthetic fluids |
| Gas Permeability | Moderate — slightly higher than NBR |
| Compression Set | Fair — may develop permanent deformation under sustained load |
Advantages of Polyurethane for Rock Drill Accumulators
- Superior abrasion resistance. In hydraulic systems where oil cleanliness is difficult to maintain (common in underground mining), polyurethane resists erosion from suspended particles significantly better than NBR.
- Exceptional tear strength. Polyurethane diaphragms are much more resistant to propagation of cuts and nicks, which can occur during installation or from sharp debris in the hydraulic circuit.
- Higher tensile strength. With tensile values 2-3x higher than NBR, polyurethane provides a greater safety margin against burst failure at maximum operating pressures.
- Extended service life in harsh conditions. Operations running COP rock drills at maximum percussion settings for continuous shifts may see longer diaphragm life with polyurethane due to its superior resistance to mechanical fatigue.
- Better extrusion resistance. Under high pressure, polyurethane is less likely to deform into gaps or clearances in the accumulator housing, maintaining seal integrity.
Limitations of Polyurethane
- Narrower temperature range. Polyurethane performance degrades above 80°C and below -20°C. In hot underground environments or operations where hydraulic oil temperatures run high, NBR may be the safer choice.
- Higher gas permeability. Polyurethane allows slightly more nitrogen permeation through the membrane, which may result in more frequent pre-charge topping required.
- Hydrolysis sensitivity. Polyester-based polyurethanes can degrade in the presence of water. If condensation or water contamination is present in the hydraulic system, NBR is more resistant.
- Higher cost. Polyurethane diaphragms cost more than NBR equivalents due to material and processing costs.
- Compression set. Over extended periods under static load, polyurethane may develop permanent deformation more readily than NBR.
Head-to-Head Comparison
| Property | NBR | Polyurethane | Winner |
|---|---|---|---|
| Temperature Range | -30°C to +120°C | -20°C to +80°C | NBR |
| Oil Resistance | Excellent | Good | NBR |
| Abrasion Resistance | Moderate | Excellent (5x) | PU |
| Tear Strength | Good | Excellent (3-5x) | PU |
| Tensile Strength | 15-25 MPa | 30-60 MPa | PU |
| Gas Permeability | Low | Moderate | NBR |
| Compression Set | Good | Fair | NBR |
| Water/Humidity Resistance | Good | Fair (hydrolysis risk) | NBR |
| Cost | Lower | Higher | NBR |
| Service Life (dirty oil) | Standard | Extended | PU |
Recommendation by Operating Condition
Based on our experience manufacturing accumulator diaphragms for COP rock drills across global mining operations, here are our material recommendations for specific scenarios:
Choose NBR When:
- Underground temperatures exceed 35°C regularly
- Hydraulic oil temperatures run above 70°C
- Cold-climate surface operations (below -20°C)
- Standard oil maintenance practices are in place
- Budget constraints require lowest unit cost
- Water contamination is possible in the hydraulic system
Choose Polyurethane When:
- Hydraulic oil cleanliness is difficult to maintain
- Rock drill operates at max percussion continuously
- Environmental temperature stays within -20°C to +30°C
- NBR diaphragms are wearing faster than expected
- Extended service intervals are a priority
- Total cost of ownership matters more than unit price
Our Manufacturing Approach
At Babacan Rubber, we manufacture the 3115182200 accumulator diaphragm in both NBR and polyurethane using compound formulations specifically developed for the high-frequency, high-pressure cycling environment of COP rock drill accumulators. Our compounds are not generic catalog materials — they are tailored formulations optimized for this exact application.
When you request a quote, specify your rock drill model, typical operating conditions (temperature range, drilling hours per shift, rock type), and oil maintenance practices. Our engineering team will recommend the optimal material choice and provide both options with pricing for your evaluation.
Both material options are manufactured in our ISO 9001:2015 certified facility, pressure-tested at 1.5x rated pressure, and ship with full material traceability documentation. For more information about the part itself, visit our 3115182200 product page or the cross-reference table.