NBR (Nitrile Butadiene Rubber) is one of the oldest and most basic man-made elastomers used for sealing. It's a member of the R-Group of synthetic rubbers - which means it has an unsaturated hydrogen carbon chain. It can be formulated with differing levels of Acrylonitrile (ACN) which will impact the ultimate chemical resistance and temperature range capability of the specific compound. All NBR's generally have good levels of mechanical properties such as elongation at break, abrasion and compression set resistance.
HNBR (Hydrogenated Nitrile Butadiene Rubber) is produced by selective hydrogenation of the butadiene groups, and results in a material which has improved mechanical properties and a higher temperature capability. As with NBR, the ACN content within the specific compound will determine temperature and fluid resistance. The popular terms of “high”, “medium” and “low" nitrile are often used to indicate the ACN content although this can be too superficial to ensure robust performance in the application.
Why use NBR/HNBR?
NBR compounds are relatively easy to produce and are the most commonly used materials in the sealing industry, as well as being widely used for other products such as belts, rollers and hoses. An NBR seal is often the most cost effective choice for applications with moderately demanding requirements for temperature and fluid resistance.
HNBR compounds are more expensive but can offer significant advantages in some applications. In terms of fluid resistance (notably hydrogen sulphide and amines in oil and gas applications) and mechanical strength and overall life, for a modest increase in initial cost. Both materials can be processed by high (injection moulding) and low (compression moulding) volumes cost effectively.
NBR is ideal for applications sealing mineral and vegetable based oils and greases, and cold-to-warm water. It's also resistant to diesel, HFA/B/C hydraulic fluids and dilute acid, alkali and salt solutions. However, use with aromatic fuels or hydrocarbons, along with polar solvents (such as ketones and acetone) should be avoided, and ozone and weathering resistance is poor.
HNBR materials have slightly improved ozone resistance and are more durable in high temperature water and steam. General NBR grades are capable from -30°C (-22°F) to +100°C (212°F) with HNBR increasing to +150°C (300°F).
Through adjustment of the ACN level, capabilities as low as -60°C (-76°F) or up to +160°C (320°F) can be achieved (but not in the same compound).
This is an important consideration when specifying or choosing a material; not all NBR grades are equally capable and full application conditions should be considered even when replacing an existing seal.
Other R-Group materials include Chloroprene (CR), Butyl (IIR) and Styrene Butyl Rubber (SBR). These are similiar rubbers that have specific properties ideal for some applications; for example, CR has good low temperature properties whilst IIR has very low gas permeability and SBR has good abrasion resistance.