With the development of the automotive and petroleum industries, rubber components are no longer required merely to be oil-resistant; they must also exhibit excellent resistance to heat, high temperature, high pressure, and oxidation. Conventional nitrile rubber (NBR) can no longer meet these demands. Although some applications have already been replaced by fluororubber, the latter is prohibitively expensive. Consequently, researchers have turned their attention to improving the performance of NBR, and hydrogenated nitrile rubber (HNBR) was developed precisely to satisfy this emerging need. HNBR boasts outstanding high-temperature resistance (130–180°C), cold resistance (−55 to −38°C), and superior mechanical properties, making it better suited than other polymers to the stringent requirements of the automotive industry. HNBR reinforced with ZnO and methacrylic acid (MAA) can be used to produce base compounds for V-belts, synchronous V-belts, multi-purpose V-ring seals, vibration isolators, as well as sealing rings, gaskets, and heat-resistant tubing. In oil drilling, rubber products are required to withstand extreme conditions, including high temperature, high pressure, and exposure to acidic, amine-containing, H₂S-, CO₂-, and CH₄-bearing vapors. Products made from HNBR, however, demonstrate exceptional resistance to acids, oils, and solvents. HNBR reinforced with ZnO and MAA is suitable for manufacturing drill protection casings and pistons for mud pumps. Moreover, HNBR can be processed into paper-like washers via a pulping method, which serve as sealing gaskets in both the petroleum and automotive industries. Compared with silicone rubber, fluororubber, and polytetrafluoroethylene, HNBR exhibits superior heat and radiation resistance, making it ideal for various rubber seals in power plants, as well as for hydraulic hoses, hydraulic seals, cable sheaths in power stations, printing and textile rollers, weapon components, aerospace seals and coatings, fuel bladders, and more. HNBR latex can be used as a surface coating (for painting), as an adhesive for textiles, paper, leather, metals, ceramics, and nonwoven fibers, and for producing foamed rubber and impregnated latex products. In addition, HNBR reinforced with ZnO/MAA, peroxides, or highly abrasion-resistant furnace black demonstrates overall performance that surpasses that of conventional HNBR.

II. Preparation of HNBR

There are three main methods for preparing HNBR: ethylene–acrylonitrile copolymerization, solution hydrogenation of NBR, and emulsion hydrogenation of NBR. When HNBR is prepared via ethylene–acrylonitrile copolymerization, the significant differences in the reactivity of the monomers during the copolymerization reaction (rAN = 0.04, rE = 0.8) make process control challenging, and the resulting polymer exhibits suboptimal performance; consequently, this method remains at the laboratory-scale research stage.

The emulsion hydrogenation method for NBR involves thermally decomposing p-toluenesulfonylhydrazine to produce diimide, which serves as an effective hydrogenation reducing agent. In 1984, WideMan first reported a process for preparing emulsion HNBR using diimide as the reducing agent, demonstrating that NBR latex can be directly converted into HNBR in the presence of oxidants such as hydrazine hydrate, oxygen, or hydrogen peroxide, as well as metal ion initiators like copper and iron. Subsequently, Parker et al. at the U.S. Goodyear Tire & Rubber Company employed the emulsion hydrogenation method to produce HNBR latex, with the following formulation (by mass): NBR latex, 100 g; CuSO4·5H2O, 0.008 g; sodium dodecyl sulfate, 0.15 g; antifoam agent; hydrazine hydrate, 15.6 g; and hydrogen peroxide, 16.66 g. The procedure entails charging NBR latex, CuSO4·5H2O, and a surfactant into a reactor, heating the mixture to 45–50°C, adding hydrazine hydrate, then gradually introducing hydrogen peroxide over the subsequent 7 hours while simultaneously adding the antifoam agent, followed by one hour of constant-temperature stirring to obtain HNBR. However, due to the high cost of the hydrogenation precursor (p-toluenesulfonylhydrazine) and the relatively slow hydrogenation rate, this method remains at the laboratory-scale trial stage.

The solution hydrogenation of NBR involves hydrogenating an NBR solution in the presence of precious-metal catalysts—palladium, calcium, and rhodium—using hydrogen gas. The acrylonitrile content in the NBR molecular chain determines its oil resistance; during hydrogenation, only the double bonds in the diene units are selectively reduced to saturated bonds, while the nitrile groups (–C≡N) on the side chains of the acrylonitrile units remain unhydrogenated. Currently, two main types of catalysts are employed for this hydrogenation process: heterogeneous supported catalysts and homogeneous coordination catalysts. The earliest heterogeneous supported catalyst was the Pd/C catalyst with carbon as the support, which exhibits high selectivity and a maximum hydrogenation conversion of 95.6%. However, during the hydrogenation reaction, the diene rubber, which has a strong affinity for carbon black, tends to adsorb onto the carbon-black surface, leading to agglomeration of the carbon black under stirring and resulting in residual carbon-black particles in the HNBR, which can adversely affect its vulcanization characteristics. Japan’s Zeon Corporation has adopted a Pd/SiO2 catalyst with SiO2 as the support, which has now been commercialized. With both types of supported catalysts, residual catalyst materials or processing aids used in the polymerization reaction may adhere to the support surface or become entrapped in the micropores, causing a sharp decline in catalytic activity and thereby limiting the catalyst’s reusability.

Currently, three types of homogeneous coordination catalysts are commonly used: palladium-, rhodium-, and iridium-based catalysts. Palladium-based catalysts are stable in water and air, easy to store and transport, and can be repeatedly recovered and reused; however, their activity and selectivity are relatively poor. Iridium-based catalysts exhibit exceptionally high activity and selectivity in the hydrogenation of NBR, making them one of the focal points in the development of HNBR. Rhodium-based catalysts offer the highest levels of both activity and selectivity; for instance, the RhH(PPh3)3 complex can catalyze the hydrogenation of NBR in either solution or emulsion, with benzene as the solvent, reaction temperatures ranging from 80 to 160°C, hydrogen pressures from 0.05 to 3 MPa, and reaction times of 2 to 10 hours. To enhance catalyst stability, PPh3 is added, typically at loadings of 0.05% to 20% for the catalyst and 1% to 25% for PPh3 (based on the NBR feed), with a mass ratio of the two components ranging from 0.6:1 to 20:1. This catalytic system demonstrates high activity and selectivity, achieving a minimum hydrogenation conversion of 95%. However, rhodium is a scarce and expensive resource, necessitating its recovery and reuse in large-scale production; some studies have reported that triaminosilane can adsorb up to 81% of the residual rhodium remaining in HNBR.

III. Conclusion

While retaining NBR’s outstanding oil resistance, HNBR also exhibits excellent heat resistance (withstanding temperatures up to 150°C) and ozone resistance. As a result, its application scope has expanded beyond the traditional domains of NBR, thereby exerting some competitive pressure on specialty rubbers such as chlorinated polyethylene and chlorosulfonated polyethylene. From an economic standpoint, HNBR’s selling price remains relatively high for now, but it is still considerably lower than that of fluororubber. Moreover, owing to its low stiffness, favorable processing characteristics, and low density, HNBR allows for higher filler loadings; in addition, HNBR products have only about half the volume per unit mass compared with fluororubber. Consequently, HNBR is poised to serve as a substitute for fluororubber and other specialty elastomers. Although HNBR has been produced abroad for many years and its applications continue to broaden, domestic production of HNBR is still virtually nonexistent, underscoring the urgent need for us to seize the opportunity to develop this material. Preliminary information indicates that more than 2,000 secondary-production wells in China’s Daqing and Shengli oilfields require submersible pumps capable of operating at well depths of 2,000 meters and temperatures around 140°C. If NBR impellers are used, their service life is only a few months—or even shorter—whereas HNBR impellers can last for over a year. Furthermore, Nanjing Rubber Products Factory No. 1 alone requires more than 30 metric tons of HNBR annually. In the production of HNBR, catalytic hydrogenation of NBR is the key technology; however, solution-phase hydrogenation of NBR entails stringent process requirements. Lanzhou Petrochemical Company is currently conducting research in this area, though low rhodium recovery rates result in relatively high costs. In recent years, research on hydrogenation and hydroformylation using water-soluble biphasic catalysts has focused on the separation, recovery, and recycling of precious-metal catalysts. Once the application of such catalysts can be extended to the hydrogenation of polymers, they can be employed for the hydrogenation of NBR. To date, we have synthesized two water-soluble phosphine ligands and are actively pursuing related experimental trials.