Catalytic hydrogenation improves the chemical and thermal resistance of NBR
Hydrogenated NBR elastomer is produced from acrylonitrile-butadiene rubber, a copolymer of acrylonitrile (ACN) and 1,3-butadiene. As the double bonds of the butadiene monomer units remain in the NBR main chain, this material is susceptible to chemical influences, thermal degradation and oxidation. This disadvantage can be eliminated by means of catalytic hydrogenation to form HNBR.
Hydrogenation of NBR
As well as the reaction conditions, the choice of catalyst is a critical factor for the hydrogenation reaction. For the quantitative hydrogenation of NBR, the catalyst must have a sufficiently high activity and a high selectivity, so that only the olefinic double bonds, not the nitrile groups, are hydrogenated. Heterogeneous palladium or homogeneous rhodium(I) catalysts are best suited to NBR hydrogenation.
Heterogeneously catalysed hydrogenation
On an industrial scale, the heterogeneous hydrogenation of acrylonitrile-butadiene rubber is carried out using palladium as the active metal. Complete hydrogenation can be achieved with 0.01 to 1.0 weight percent palladium. If Group 1 to 6 metals, e.g. calcium, titanium, zirconium or vanadium, are used as promotors, complete hydrogenation can also be achieved with a smaller amount of palladium.
Transformation to HNBR usually takes place at a temperature of 50°C and under 50 bar hydrogen pressure, in polymer solutions containing 10 weight percent rubber. The catalyst is then separated off by filtration or centrifuging Once it has been separated off, it can in principle be reused, although its activity will be reduced.
Homogeneously catalysed hydrogenation
Only Wilkinson's catalyst and RhH(PPh3)4 are used on an industrial scale for this type of NBR hydrogenation. Both catalysts bring about a quantitative hydrogenation of the C=C double bonds, without any reduction in nitrile groups.
The reaction is generally performed at temperatures from 100 to 150°C and under a pressure of 30 to 85 bar. In the lower pressure range, complete hydrogenation takes more than ten hours. The reaction time can be reduced to a few hours by increasing the pressure. Once they have been recovered, homogeneous catalysts are inactive and cannot be reused without being chemically modified.
Properties of HNBR
Unlike NBR, hydrogenated NBR elastomer has high heat and ageing resistance, which means that it is frequently used both in sealing technology and also in dynamically challenging applications such as synchronous belts. HNBR is also resistant to mineral oils, animal and vegetable oils and fats, flame-resistant hydraulic fluids (HFA, HFB and HFC) and some refrigerants. In long-term operation (up to 1000 hours), fully hydrogenated HNBR grades can withstand temperatures of up to 150°C. Partially hydrogenated HNBR can be crosslinked both with sulfur and accelerators and with peroxides, whereas fully hydrogenated HNBR can only be crosslinked with peroxides.
Hydrogenated NBR elastomer – advantages at a glance:
- Good media resistance/chemical resistance
- High mechanical and dynamic strength
- Suitable for use with hot water and steam
- Ozone and weathering resistance
- Good low-temperature flexibility
- Low compression set
- High ageing resistance
- Thermal stability from -40 to +150°C
The properties of HNBR, especially its oil resistance, are determined by the acrylonitrile content in the vulcanisates. For good low-temperature performance, HNBR grades with a low ACN content and specific termonomers are recommended, whereas high-temperature resistance, a common requirement in sealing technology, can be achieved by using highly saturated HNBR elastomers.
Hydrogenated NBR elastomer – possible applications
In the automotive industry, HNBR is a well-established standard material for toothed belts, not least because of its ageing resistance and dynamic load-bearing capacity. It is also recommended as a starting material for many static and dynamic seals and hoses that come into contact with engine and transmission oils and coolants. Hydrogenated NBR elastomer is also commonly used in oil and gas exploration, since it has very good resistance to hydrogen sulfide (H2S) and amine-based stabilisers. In addition, HNBR is the best solution for dairies and other applications involving contact with very fatty media in the food sector.
In sealing technology, hydrogenated NBR elastomer is primarily used where elevated temperatures and/or high mechanical strength are required. HNBR offers a broad spectrum of applications, in both the process industry and the food industry. For example, hydrogenated NBR elastomer is an ideal sealing material for valve seats or in espresso machines.