Specialty Nitrile Elastomers - Carboxylated addition of carboxylic acid groups to the nitrile polymer's backbone significantly alters processing and cured properties. The primary reason for including carboxylation is to provide a network of ionic bonds that supplement conventional sulfur or carbon vulcanization bonds. The additional ionic network is a series of metallic-carboxyl reactions. The result is significantly increased strength, measured by improved tensile, tear, modulus and abrasion resistance.
Carboxylated nitriles are used in high-abrasion applications, such as rolls, conveyor belting, hose covers, oil well drilling parts, high modulus wear seals and premium footwear soles. Offsetting these benefits, there are losses in compression set, resilience and some low-temperature properties. Another effect of the carboxylation is usually a 12 to 16 point hardness increase in most recipes.
Working with carboxylated nitrile elastomers (XNBR) requires special compounding considerations. Customary peroxide or sulfur cure systems common to conventional nitrile elastomers react identically with the butadiene network in the carboxylated nitrile elastomers. However, there are now the acid-base reactions of the carboxylation with metallic oxides, metallic salts, amines and a wide variety of other acid-reactive materials. These form a second crosslinking network. Water is the primary catalyst for the carboxyl-metallic reactions and must be treated as an “ultra” accelerator. Many of the compounding ingredients contain water, including the polymer itself, and this must always be kept in mind when working with any carboxylated nitriles. Volatilizing the water through heat generated during mixing is the best way for removing it. Desiccants such as calcium oxide do not work, because of high reactivity with the carboxylation causing premature ionic crosslinking. The figures below represent the basic dual nature of the carboxyl-zinc ionic bonds and the sulfur cross-linking. 
 |