Improvement of Low-Temperature Flexibility of Hydrogenated Nitrile—Butadiene Rubber

Abstract

Abstract 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≥5) facilitate crystallization upon stretching, which leads to a poor low-temperature flexibility. Consequently, it is deduced that the TM≥5 content must be as low as possible to improve the low-temperature flexibility. Two methods were tried to decrease the TM≥5 content: (1) copolymerization with third monomers; and (2) addition of mercaptans. It has been confirmed that branched HNBRs made by each of the two methods have lower Tgs and improved low-temperature flexibility.