Hydrogenated Nitrile Butadiene Rubber (XNBR)
Description:
XNBR is a nitrile rubber (NBR) with the addition of a carboxyl group to saturate the butadiene segment of the carbon polymer backbone. The added carboxyl group greatly improves the resistance of NBR to abrasive and tear wear without affecting its oil and solvent resistance or its thermal stability. For this reason, XNBR based parts are usually applied in dynamic assembly such as seals and rod wipers.
Common Uses:
XNBR is found in:
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V- belts
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High abrasion resistance shoes
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Printing rolls
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Rice dehusking rolls
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Oil extraction industry artifacts
XNBR Properties
Standard Compound: -20°C (-4°F) to 100°C (212°F)
Special Compound (High temp): 125°C (257°F)
Hardness: 50 to 90 Shore A
ASTM D1418 Designation: XNBR
ASTM D2000 Designation: BG, BK, CH
Standard Colors: Black
Optional Colors:
Production
XNBR is produced by a crosslinking of carboxylated nitrile rubber with a standard preparation by a sulfur recipe, or in some cases metal oxides/peroxides.
Advantages
XNBR performs well in aliphatic hydrocarbon, vegetable/mineral oil, greases, diesel, water, dilute acids, alkali, and salt solutions.
Limitations
XNBR rubber compounds are susceptive to aromatic hydrocarbons, chlorinated hydrocarbons, ketones, acetic acids, ethylene esters, strong acids, and brake fluids with a glycol base.
Frequently Asked Questions About XNBR Rubber
1. What is XNBR?
Carboxylated Nitrile Butadiene Rubber (XNBR) is a nitrile rubber (NBR) modified by adding carboxyl groups (-COOH) to the polymer backbone. This carboxylation creates additional crosslinking sites and raises tensile strength, modulus and, above all, abrasion resistance.
Oil, solvent and thermal behaviour remain broadly in the NBR range, but they are not unchanged and depend on the specific compound. XNBR is designated according to ASTM D1418.
Important: XNBR should not be confused with HNBR. These are two different modifications of NBR. Carboxylation improves mechanical properties, while hydrogenation improves heat and ozone resistance.
2. How does XNBR differ from standard NBR?
XNBR offers markedly higher resistance to abrasion and tearing than standard NBR, along with higher tensile strength and modulus. These improvements come from metal-carboxylate ionic crosslinks formed during curing, alongside the conventional covalent network.
The size of the wear improvement is not a fixed number. Multiples such as 2–3 times are only meaningful when the test method and the reference NBR compound are both stated. Different tests and comparison compounds produce different results.
Oil, solvent and thermal behaviour remain broadly in the NBR range, but they are not identical. Water and polar-medium sensitivity, low-temperature flexibility and compression set must be verified for the specific compound.
3. What are the typical applications of XNBR?
XNBR is commonly used in applications where resistance to wear and tearing is critical, including:
- Hydraulic rod seals and rod wipers
- V-belts and timing belts
- Industrial conveyor belts used in abrasive environments
- Printing and ink rolls
- Rice dehusking and agricultural processing rolls
- Medical examination and surgical gloves
- High-wear footwear soles
- Mining equipment seals
4. In which industries is XNBR commonly found?
XNBR is commonly used in the following industries:
- Power transmission, including belts and hoses
- Printing, including rolls and blankets
- Agriculture, including rice mill rolls and equipment seals
- Oil and gas extraction
- Medical glove manufacturing
- Mining and heavy industry
- Automotive manufacturing
5. What products are typically manufactured using XNBR?
Common XNBR products include V-belts, high-abrasion footwear, O-rings for hydraulic service, hydraulic seals, rod wipers, printing rolls, rice dehusking rolls, medical gloves and components for oil extraction equipment.
6. What is the temperature range of XNBR?
The following figures provide general guidance for the XNBR polymer family and should not be treated as a specification:
- Standard XNBR: approximately -25°C to +100°C
- Specially formulated grades: short-term or intermittent exposure toward +125°C
The higher figure applies only to specific compounds under limited exposure. It is not a general working temperature for XNBR.
XNBR’s low-temperature flexibility is somewhat reduced compared with standard NBR because of its carboxylic crosslinking. For low-temperature service, standard NBR may be more suitable. Always confirm the range using the specific compound datasheet and real operating conditions.
7. What hardness levels are available for XNBR?
XNBR compounds are typically available in hardness levels ranging from 50 to 90 Shore A. High-hardness grades of 80–90 Shore A are commonly used in dynamic sealing applications where wear resistance is critical.
8. How is XNBR manufactured?
XNBR is produced through the emulsion copolymerization of acrylonitrile, butadiene and a small amount of acrylic or methacrylic acid. The carboxyl groups (-COOH) become part of the polymer backbone.
Common curing systems include:
- Sulfur-based systems
- Metal oxides such as ZnO, which form metal-carboxylate ionic crosslinks
- Peroxide systems for specialty grades
The combination of covalent crosslinks and metal-carboxylate ionic crosslinks produces XNBR’s enhanced mechanical properties.
9. What substances is XNBR resistant to?
XNBR generally performs well when exposed to:
- Aliphatic hydrocarbons
- Vegetable and mineral oils
- Petroleum greases
- Diesel fuel
- Cold and warm water
- Dilute acids, alkalis and salt solutions
- Animal fats and oils
10. What are the limitations of XNBR?
XNBR is generally unsuitable for environments involving:
- Aromatic hydrocarbons such as benzene and toluene
- Chlorinated hydrocarbons
- Ketones, esters and ethers
- Acetic acid
- Strong acids and oxidizers
- Glycol-based DOT 3 and DOT 4 brake fluids
- Ozone and prolonged outdoor exposure
Its limited ozone resistance is similar to that of standard NBR.
11. What is the difference between NBR, XNBR and HNBR?
All three materials are nitrile rubbers. XNBR and HNBR are different modifications of NBR and are designed to improve different properties.
| Property | NBR | XNBR | HNBR |
|---|---|---|---|
| Abrasion resistance | Baseline | Substantially better than NBR. The exact improvement depends on the test method and reference compound. | Similar to NBR, with improved performance at high temperatures. |
| Heat resistance | Up to approximately +100°C | Approximately +100°C, with limited excursions toward +125°C in specially formulated compounds. | Up to approximately +150°C in selected grades. |
| Relative cost | Baseline | Higher than NBR | Generally higher than both NBR and XNBR. Actual costs depend on grade, volume and market conditions. |
| Best use | General oil-resistant sealing | High-wear dynamic seals, rolls and belts | High-temperature and chemically demanding service |
Choose XNBR when mechanical durability and abrasion resistance are the main priorities. Choose HNBR when thermal and chemical durability are more important.
12. Is XNBR suitable for medical examination gloves?
Yes. Carboxylated NBR latex is widely used for disposable medical examination gloves. It avoids the natural rubber latex proteins associated with latex allergies.
Carboxylation also enables effective crosslinking under the dipping and curing conditions used to manufacture thin-film gloves. Any claim regarding XNBR’s share of the global glove market should be supported by a current market source.