Degradation Behavior of NBR Seals Exposed to Halogenated Olefins under Coupled Thermal and Mechanical Loads: A Multiscale Experimental and Computational Study

Halogenated olefins are promising low-GWP substitutes for traditional halocarbons in refrigeration, fire protection, and energy storage, but their compatibility with elastomeric seals poses a potential safety risk in process systems. This study systematically investigates the degradation behavior of nitrile butadiene rubber (NBR) exposed to five representative halogenated olefins-namely, 2-bromo-3,3,3-trifluoropropene (2-BTP), 2-chloro-3,3,3-trifluoropropene (R1233xf), 2,3,3,3-tetrafluoropropene (R1234yf), cis-1,3,3,3-tetrafluoropropene (R1234ze-(Z)), and trans-1,3,3,3-tetrafluoropropene (R1234ze-(E))-under simulated service conditions (0-70 °C, 0-2000 N compressive load, 15-45 days). Overall, the performance evolution of NBR exhibits a strong dependence on environmental conditions, and is closely correlated with the molecular architecture of halogenated olefins. 2-BTP caused the most severe damage (compression set: 31.18%; hardness loss: 14.38%; compressive strength loss: 41.35%), while fluorinated olefins showed weaker effects. Microscopic and spectroscopic analyses confirmed surface roughening, microcracking, halogen penetration, and formation of C-X (X = F, Cl, Br) bonds. Molecular dynamics simulations revealed that thermal energy weakens intermolecular interactions, whereas compressive stress induces chain orientation and free-volume redistribution, synergistically enhancing diffusion. The diffusion coefficients follow R1234ze-(E) > R1234yf > R1234ze-(Z) > R1233xf >2-BTP. Density functional theory calculations confirm that halogen radicals attack NBR's CC and -CN groups via low-barrier pathways. These findings elucidate the degradation mechanisms of NBR seals and provide practical engineering guidance for material selection and failure prevention, contributing to safer process design with halogenated olefins.