Nitrile (NBR) provides excellent resistance to petroleum oils and gasoline as well as mineral and vegetable oil. It also offers strong resistance to heat aging — often a key advantage over natural rubber. Nitrile rubber performs well in carburetor and fuel pump diaphragms, aircraft hoses, seals and gaskets as well as oil-lined tubing. Due to its versatility, nitrile is used in applications involving not only oil and fuel resistance, but those applications requiring resistance to heat, abrasion, water, and gas permeability.
Additional features/information about NBR (nitrile):
- Good resistance to acids and bases except those having strong oxidizing effects
- Poor resistance to the swelling action of oxygenated solvents such as acetone and ketones
- Higher acrylonitrile contents increase solvent resistance but decrease low-temperature flexibility
- Compounding to improve low-temperature flexibility decreases oil and solvent resistance
- Reinforcing materials are required to obtain high strength because it does not crystallize on stretching
- With compounding, it is possible to get a fairly good balance between low creep, good resilience, low permanent set and good abrasion resistance
- Tear resistance and electrical insulation properties are inferior to natural rubber
Nitrile Rubber Properties
Nitrile rubber is a copolymer of butadiene and acrylonitrile (ACN). The butadiene segment of nitrile allows for the compounds elasticity and low temperature flexibility. It also contains the 'unsaturated' double bond that becomes the site for cross-linking during vulcanization. This unsaturated double bond will also be the main attack site chemicals, heat, and oxidation.
The second segment of nitrile rubber, acrylonitrile, accounts for the compounds fuel and oil resistance as well as its hardness, abrasion resistance, and tensile strength. By increasing the acrylonitrile content of a compound, better heat resistance and gas impermeability can be achieved. The ACN content of Nitrile rubber material can range from 18% to 45% with a general purpose nitrile compound usually containing around 34%. By selecting an elastomer with the appropriate acrylonitrile content in balance with its other properties, we are able to use nitrile rubber in a wide variety of applications depending on individual needs.
|Common Names||Buna-N, Nitrile, NBR|
|ASTM D-2000 Classification||BF, BG, BK|
|Military (MIL-STD 417)||SB|
|Chemical Definition||Butadiene Acrylonitrile|
|Durometer Range (Shore A)||20-95|
|Tensile Range (P.S.I)||200-3000|
|Elongation (Max %)||600|
|Resilience — Rebound||Good|
|Solvent Resistance||Good to Excellent|
|Oil Resistance||Good to Excellent|
|Low Temperature Usage (F°)||-30° to -40°|
|High Temperature Usage (F°)||Up to 250°|
|Aging Weather — Sunlight||Poor|
|Adhesion to Metals||Good to Excellent|
Comments — Nitrile (Buna-N) is a general purpose oil resistant polymer, with good solvent, oil, water and hydraulic fluid resistance. Additionally, Nitrile rubber exhibits a good compression set, abrasion resistance, and tensile strength. Nitrile should not be used in applications involving highly polar solvents such as acetone, MEK, ozone, chlorinated hydrocarbons and nitro hydrocarbons.
Nitrile Rubber Applications
Nitrile rubbers material properties make it an excellent solution for sealing applications. The popularity of nitrile material results from its excellent resistance to petroleum products and its ability to be compounded for service of temperatures up to 250°F. With these temperature resistances, the right nitrile rubber compounds can withstand all but the most severe automotive applications. Other applications that benefit from nitrile rubbers properties which can be custom compounded and molded include:
- Oil resistant applications
- Low temperature applications
- Automotive, marine and aircraft fuel systems
- Nitrile roll covers
- Hydraulic hoses
- Conveyer belting
- Nitrile tubing
Nitrile Rubber Chemistry and Compounding
The production of nitrile material takes place through an emulsion polymerization system. This emulsion process begins when the monomers (butadiene and acrylonitrile) are introduced along with water, emulsifier/soap, radical generating activator and other ingredients into polymerization vessels. The result of this emulsion process is a polymer latex yield that is coagulated using various materials such as calcium chloride and aluminum sulfate to form the crumb rubber that is then dried and compressed into bales.
Like most other unsaturated thermoset elastomers, Nitrile compound formulations require added ingredients and further processing to be rendered useful in different applications. The additional additives typically include plasticizers, reinforcement fillers, protectants, and vulcanization packages.
The differing amount of additive materials and levels of acrylonitrile will determine the differences in the various compounds that can be formulated. Timco rubber has the capabilities to compound nitrile rubber specifically to your individual needs. We are able to vary polymerization temperatures to create "hot" and "cold" polymers. Also, by varying the Acrylonitrile and butadiene ratios we can specifically formulate your nitrile compound for the necessary fuel resistance and low temperature requirements. Nitrile elastomer compounds can also be hydrogenated in order to reduce the chemical reactivity of the polymers backbone, thereby significantly improving the materials heat resistance.
By adjusting the acrylonitrile (ACN) content in a nitrile compound, a number of different results can be achieved. Because of its polarity, the ACN level determines several key properties of the nitrile material such as; oil and solvent resistance, abrasion resistance, and also low-temperature flexibility/glass transition temperature. A higher concentration of ACN will provide improved solvent, abrasion and oil resistance along with a higher glass transition temperature. The table below summarizes properties common to many conventional NBR polymers. The direction of the arrows signifies an improvement in the values.
|Nitrile Rubber with Lower Acrylonitrile Content||Nitrile Rubber with Higher Acrylonitrile Content|
|Cure Rate w/Sulfur Cure system||→|
|Compatibility w/Polar Polymers||→|
|←||Cure Rate w/Peroxide Cure System|
|←||Low Temperature Flexibility|
General Types of Nitrile Rubber
Hot Nitrile Rubber
Hot NBRs are polymerized at temperatures ranging from 30° to 40°C in a process that yields highly branched polymers. This branching supports good tack and strong bonding in adhesive applications. By processing this kind of polymer and creating a physically entangled structure, the end product also exhibits a significant improvement in hot tear strength when compared to a cold-polymerized counterpart. Hot NBR has a natural resistance to flow, making it an excellent candidate for compression molding and sponge applications. Other applications where Hot Nitrile Rubber proves useful include thin walled or complex extrusions where shape retention is important.
Cold Nitrile Rubber
Depending on the balance of linear-to-branched configuration desired, cold nitrile is polymerized at temperatures ranging from 5° to 15°C. These lower polymerization temperatures will yield more linear polymer chains. The reactions are conducted in processes known as continuous, semi-continuous, or batch polymerization.
Cold NBR compounds span a wide variety of compositions. They can contain Acrylonitrile in ranges from 15% to 51%. The Mooney values of these Nitrile materials range from a very tough 110 to pourable liquids, with 20-25 as the lowest practical limit to maintain a solid material. These Nitrile compounds are made with an array of chemical compositions, coagulants, molecular weight modifiers, stabilizers and emulsifier systems. Third monomers can also be added to the polymer background to provide advanced performance; with each variation of the compound providing a specific function to the overall Nitrile material.
Crosslinked Hot Nitrile Rubber
Crosslinked Hot NBRs are branched polymers that become further cross-linked through the addition of a di-functional monomer. These nitrile rubber compounds are typically used in molded parts to provide sufficient molding forces, or back pressure, in order to eliminate any trapped air. An additional use is to provide increased shape retention or dimensional stability for extrusion. As a result, crosslinked hot nitrile proves to be more efficient in the extrusion and vulcanization of intricate shaped parts. These NBRs will also add impact resistance, dimensional stability and flexibility for PVC modification.