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HNBR rubber compounds

HNBR advantages

  • Excellent media resistance
  • Excellent heat and oxidation stability
  • High mechanical strength
  • Improved wear resistance
  • Good in (ultra-pure) water and steam
  • Temperature range from –30 °C to +160 °C

HNBR rubber, Properties & Applications

HNBR is a hydrogenated version of NBR (nitrile) rubber. NBR is a copolymer of ACN and 1,3-butadiene. The NBR backbone still contains double bonds remaining from the 1,3-butadiene monomer units. These double bonds are also prone to chemical attack, thermal degradation and oxidation. To improve these properties, NBR can be hydrogenated in a catalytic hydrogenation process to form HNBR.

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 °C to 150 °C, and 160 °C in oil, 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.

HNBR is used in a wide range of applications, including automotive (coolant, AdBlue, oil, fuel systems), industrial applications, oil and gas industries, food and pharma, and medical applications.

Chemistry & Manufactering

The basic structure of an HNBR elastomer is provided in Figure 1.

The production 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, starts the selective hydrogenation of the double bonds in the butadiene building blocks in the polymer chain. The degree of hydrogenation may vary from 80 to 99.5% to yield different products with different properties.

HNBR Producers & Products

The hydrogenation of various diene-containing elastomers has been well known for many years. Berthelot began the earliest experiments in 1869, with many other workers carrying out numerous experiments under different conditions and with different catalyst systems. Hydrogenated nitrile butadiene rubber or HNBR was first developed in the late ‘70s and early ‘80s. Though commercial production did not begin until 1984, there were numerous companies looking at the feasibility of producing this type of elastomer. Of those who chose to evaluate this polymer, three companies emerged: Bayer Corporation, Zeon Corporation, and Polysar. Zeon Corporation was the first to commercialize HNBR in March of 1984 with Bayer and Polysar very close behind. Zeon Corporation’s initial manufacturing site was in Takaoka, Japan. The other initial manufacturing site was Polysar’s in Orange, Texas. However, Polysar eventually sold that business to Bayer Corporation (now Lanxess) who now owns and operates that facility. Zeon Chemicals and Lanxess now have many different HNBR grades available, with varying ACN-content (typically ranging from 17 to 50 mol%), Mooney viscosity (ranging from 30 to 150 Mooney when measured at ML(1+4)@100°C), and residual unsaturation (typically 0.5 to 20%). Moreover, special types for high or low temperature applications have been developed. HNBR specialties include carboxylated HNBR, known as XNBR (typically used when very good wear resistance is needed), and types modified with acrylates.

Choice of polymer

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 the specific method of manufacturing the customer is using (e.g., compression, transfer, injection moulding, or extrusion).

Cure systems

HNBR elastomers are typically cured with either peroxide or sulphur/sulphur-donor cure systems. Comparisons of sulphur/sulphur-donor and peroxide cured HNBR compounds indicate that peroxide curing provides better compression set and heat resistance. Because HNBR has fewer highly reactive allyl position hydrogens versus other diene-based elastomers, such as NBR and SBR, it is necessary to add 50-100% more peroxide in order to produce good curing characteristics. Many types of peroxides are available for curing HNBR. However, it is important to select one that is the most suitable based on the process and cure temperature that will be utilized to produce the finished parts. Since peroxides have different molecular weights and decomposition temperatures, it is imperative to select the correct one or one can greatly affect the processability and cost-effectiveness of producing the finished goods in question. As in all peroxide cured material, vulcanization in the presence of oxygen causes reversion and thus leaves a sticky surface on the cured part. Therefore, one must take care when using pressureless cures and purge the autoclave prior to pressuring up for curing. When sulphur cure is used, the degree of hydrogenation must be less than 95% in order to have sufficient double bonds available for the cross-linking reaction. In general, cure speed also depends on the percentage of hydrogenation. When compared to SBR or NBR, the curing speed tends to be slower; therefore, to increase the curing speed a secondary accelerator should be employed in combination with the primary accelerator. Long curing times are required when thiazole based or sulfenamide-based, primary accelerators are used. To speed up the curing process, a small quantity of guanidine-based or thiuram-based secondary accelerator is used in combination with the primary accelerator. Even when using a thiuram-based primary accelerator, the addition of a thiazole based secondary accelerator will shorten the time required for curing. The use of dithiocarbamate as the primary accelerator is undesirable since the scorch time will be shortened.

Meer informatie over HNBR rubber compounds

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