01-03-2011, 09:49 AM
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HYDROGENATED NITRILE BUTADIENE RUBBER
PROPERTIES AND APPLICATIONS
HNBR is widely known for its physical strength and retention of properties after long-term exposure to heat, oil, and chemicals. Depending on filler selection and loading, HNBR compounds typically have tensile strengths of 20 – 31 MPa when measured at 23°C. Compounding techniques allow for HNBR to be used over a broad temperature range, -40° to 165°C, with minimal degradation over long periods of time. For low-temperature performance, low ACN grades should be used; high-temperature performance can be obtained by using highly saturated HNBR grades with white fillers. As a group, HNBR elastomers have excellent resistance to common automotive fluids (e.g., engine oil, coolant, fuel, etc.) and many industrial chemicals. Like NBR, fluid and chemical resistance improves as the ACN content is increased.
The unique properties attributed to HNBR have resulted in wide adoption of HNBR in automotive, industrial, and assorted, performance-demanding applications. On a volume basis, the automotive market is the largest consumer, using HNBR for a host of dynamic and static seals, hoses, and belts. HNBR has also been widely employed in industrial sealing for oil field exploration and processing, as well as rolls for steel and paper mills.
CHEMISTRY AND MANUFACTURING PROCESS
The basic structure of an HNBR elastomer is provided in Figure 1. As outlined below, the process begins with the production of an emulsion-polymerized NBR. This polymer is then dissolved in an appropriate solvent. After the dissolution process is complete, the addition of hydrogen gas, in conjunction with a precious metal catalyst at a designated temperature and
pressure, brings about a selective hydrogenation to produce a “highly saturated nitrile” (HSN) polymer. Even today, HNBR is still sometimes referred to as “HSN”.
BASIC RECIPES
There are a wide variety of acrylonitrile (ACN) content polymers available in the HNBR market today. They range from approximately 17 to 50% ACN. The ACN content not only controls fluid resistance but also impacts the low-temperature performance. If the ACN content of the polymer is increased, the volume swell of the associated compound will decrease while the low-temperature flexibility will become poorer. Alternatively, if one decreases the ACN level of the polymer, the associated compound will have higher volume swell and improved low-temperature flexibility.
Likewise, as the hydrogenation level is increased, the heat and ozone resistance improves but the dynamic hysteresis increases. If you decrease the hydrogenation level, the heat and ozone resistance is not as good but the dynamic hysteresis improves significantly. The other
characteristic imparted by the hydrogenation level is the type or selection of the appropriate cure system.
Lastly, the wide range of Mooney viscosities available permits the compounder to choose a product which best suits their specific method of manufacturing (e.g., compression, transfer, or injection molding vs. extrusion). Today, HNBRs range in Mooney viscosity from 50 to approximately 150 when measured at ML(1+4)@100°C.
