Electron beam modification and crosslinking: Influence of nitrile and carboxyl contents and level of unsaturation on structure and properties of nitrile rubber

The structural changes of nitrile rubber with varying nitrile contents, hydrogenated nitrile rubber and carboxylated nitrile rubber in the presence and absence of a polyfunctional monomer, namely trimethylolpropane triacrylate, at different doses of electron beam irradiation, were investigated with the help of FTIR spectroscopy (in the attenuated total reflectance mode), dynamic mechanical thermal analysis and sol–gel analysis. Solid-state NMR with gated high power decoupling technique was used to understand the mechanism of crosslinking of the irradiated samples. The allylic radicals generated in the butadiene chains react to form intermolecular crosslinkages. There was a significant decrease in the concentration of olefinic groups for the nitrile rubber on irradiation. This was also affirmed by the increase in the carbon resonances due to C–C linkages from the NMR technique, indicating more crosslinkages. The spectroscopic crosslink densities were determined and the results were compared with the swelling measurements. The variation in the crosslink clustering for rubbers with different acrylonitrile contents was explained using the NMR technique. The increase in crosslinking was also revealed by the increase in the percent gel content and dynamic storage moduli with radiation doses. The lifetime of spurs formed and the critical dose, an important criterion for overlapping of spurs, were determined for both the grafted and the ungrafted nitrile rubbers of different grades and compared using a mathematical model. The ratio of scissioning to crosslinking for nitrile rubber was determined using Charlesby–Pinner equation. The mechanical properties had also been studied for both the modified and the unmodified systems.