Though the double bonds within nitrile’s butadiene segments are needed for cross-linking, they are also the main attack sites for heat, chemicals, and oxidation. As part of an ongoing effort to engineer more resistant compounds, a new class of nitrile was developed in the 1980s. Initially known as highly saturated nitrile (HSN), this class is now more commonly called hydrogenated nitrile butadiene rubber (HNBR), or just hydrogenated nitrile.
As you might guess, hydrogenated nitrile results from the hydrogenation of standard nitrile. Hydrogenation is the process of adding hydrogen atoms to the butadiene segments. Adding hydrogen greatly reduces the number of carbon-to-carbon double bonds that would otherwise be weak links in the polymer chain. Why are double bonds weak? It stems from valence, or the ability of an atom to form one or more energy bonds with neighboring atoms. A carbon atom can form four distinct covalent bonds. Because carbon has this valence of four, it is most “satisfied” when it has actually formed four single bonds (a state known as saturation) rather than two single bonds and a double bond. A satisfied, saturated atom is more stable, so a compound composed largely of saturated carbons is less reactive and more resistant to chemical attack.