EXPERIMENTAL AND NUMERICAL ANALYSES OF THERMO-OXIDATIVE AGING IN NBR AND EPDM RUBBER WITH DIFFERENT LENGTHS

Abstract

Acrylonitrile-butadiene rubber (NBR) and ethylene-propylene-diene monomer (EPDM) are widely used as sealing elements in various industries. These elements are subject to thermo-oxidative aging when exposed to high temperatures and oxygen in their applications. Merging the rubber dimensional aspects and interaction of oxygen with the polymer matrix results in complex aging effects. Knowledge about this process is crucial for performing an assertive lifetime prediction and better component selection for specific requests. This work aims to understand the mass transport during the oxidation of NBR and EPDM rubbers by employing numerical analyses for mass transport considering heat diffusion for different sample lengths. Experimental tests considered rubber lengths from 10 to 120 mm, up to 90 aging days, and aging temperatures ranging from 80 °C to 140 °C. Experimental tests determined oxygen solubility to implement the gas concentration as a boundary condition for numerical analyses, where EPDM shows oxygen solubility 74.9% greater than NBR rubber. Furthermore, negative energy solubilization indicates a decrease in oxygen solubility with temperature increasing. Simulation results showed a reduction in oxygen concentration at the inner sample, which leads to diffusion and reaction processes reaching a steady state in this region with higher concentration at the surface. NBR shows higher reaction rates, where the diffusion-limited oxidation (DLO) effect starts at 120 °C, while EPDM samples did not present this behavior. Shorter specimens showed higher reaction rates compared to the larger ones. Fourier Transform Infrared - Attenuated Total Reflectance (FITR-ATR) spectroscopy, indentation modulus, and thermogravimetric analysis (TGA) confirmed the simulation results.