New insights into the molecular structure and dynamics of a recyclable and ionically crosslinked carboxylated nitrile rubber (XNBR)

Abstract

Ionic crosslinking offers a route to rubber reprocessability due to ion pairs’ dynamism, with recent studies focusing on tensile properties recovery. However, this research aims to provide, for the first time, a comprehensive overview of the recyclability of carboxylated nitrile rubber (XNBR), spotlighting changes in molecular dynamics through multiple recycling cycles beyond tensile tests. A uniquely recyclable XNBR, incorporating ZnO as a multifunctional additive, was designed alongside a simple, scalable, two-step recycling process. Evidence of the delicate balance between crosslink density and molecular entanglements that affects the dynamics of the recycled material was found. Recycling also restricts the molecular dynamics near ionic domains; attributed to a higher crosslink density (from 3.69×10-5molcm−3 in the pristine sample to 6.00×10-5molcm−3 after the third cycle), caused by a decreased ionic clusters size (aggregation number drops from 12.2 to 6.9). Remarkably, negligible differences (<10%) in compressive fatigue behavior and an enhanced chemical resistance in different solvents (up to 350% increase in motor oil) were also observed, ensuring suitable performance in conditions closer to service. Overall, this study demonstrates the feasibility of XNBR recycling and provides a broad understanding of this material at the molecular level.