Influence of Carbon Black on Crack Damage of Sealing Rubber Material Exposed to High-Pressure Hydrogen Gas

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Two carbon black-filled ethylene-propylene-diene-methylene linkage (EPDM) rubbers which have a different particle size were exposed to hydrogen gas; then, the critical pressures at the initiation of internal cracks (pF) were evaluated. It was obtained that pF = 4 - 5 MPa for two CB-filled rubbers by optical microscope observation. The pF values were estimated in terms of fracture mechanics under the assumption that bubbles were formed in the rubber structure after decompression, and these grew with the elapsed time; consequently, cracks initiated from the stress concentration of the bubbles. When an internal pressure of the bubble (Π) was assumed to be equal to the applied hydrogen pressure (p), the pF values were successfully estimated in terms of the critical internal pressure (ΠF), since ΠF showed good coincidence with pF for unfilled and silica-filled rubbers. In contrast, pF of the CB-filled rubbers was overestimated by this method, because ΠF was lager than pF. Since the CB-filled rubbers contain carbon black as filler, much hydrogen is absorbed in the rubbers compared with the unfilled and silica-filled rubbers. Because the increase of Π originated from the absorbed hydrogen by CB was underestimated, it was inferred that pF showed smaller than ΠF. Therefore, it was supposed that Π > p for the CB-filled rubbers, i.e., the CB-filled rubbers experienced more pressurization due to additional hydrogen.