Improvement of low-temperature flexibility of hydrogenated nitrile-butadiene rubber

Improvement of low-temperature flexibility of Hydrogenated Nitrile Butadiene Rubber (HNBR) is a requirement for industrial applications. In this study, the relationship between low-temperature flexibility and polymer structure-such as higher order structure and monomer unit distribution along a polymer chain-was examined for HNBRs having various nitrile contents, using X-ray diffraction, nuclear magnetic resonance, computer simulation, and other methods. It was found that the length of tetramethylene sequence in a polymer chain is the key factor for improving the low-temperature flexibility Tetramethylene sequences having five or more than five continuously linked tetramethylene units (TM greater than or equal to 5) facilitate crystallization upon stretching, which leads to a poor low-temperature flexibility. Consequently, it is deduced that the TM greater than or equal to 5 content must be as low as possible to improve the low-temperature flexibility. Two methods were tried to decrease the TM greater than or equal to 5 content: (1) copolymerisation with third monomers; and (2) addition of mercaptans. It has been confirmed that branched HNBRs made by each of the two methods have lower T(g)s and improved low-temperature flexibility.