Experiments on Influence of Depressurization Rates and Test Temperatures on Polymers in High- and Low-Pressure Cycling Hydrogen Environments As Applicable to the Hydrogen Infrastructure
Authors: Menon Nalini C. , Nissen April , McArthur Keri , McNair James , Mills Bernice , Simmons Kevin 
Published: November 2024

Abstract
Polymers used in hydrogen transportation, production, storage, and dispensing operations of the hydrogen infrastructure are subject to demanding performance temperatures (−60°C to +140°C) and pressures (0.9 MPa to 87 MPa), under static and cycling conditions of hydrogen exposure. Cycling exposures which include pressurization and depressurization stages can particularly affect properties of these soft materials. Other factors such temperature of exposure in hydrogen environments can also play an influential role on polymer degradation behaviors. In this work, we evaluated the influence of varying rates of depressurization (1, 10, 20, 40 MPa/min and uncontrolled) with model elastomer compounds exposed to high-pressure hydrogen cycling (17 MPa to 87 MPa) at ambient temperature. The goal was to develop an understanding of factors that affect rapid gas decompression in elastomers, which is a phenomenon common in hydrogen fueling operations. Cycling was followed by ex-situ characterization for changes in properties. Dynamic Mechanical Thermal Analysis (DMTA), density, compression set, Attenuated Total Reflectance-FTIR (ATR-FTIR), nanoindentation, and X-ray computer tomography were characterization techniques used to compare polymers before and after cycling. Polymer degradation in the form of internal damage was found to increase with rate of depressurization. EPDM showed the most dependence on rates of depressurization, compared to FKM and HNBR formulations.
Additionally, filled, and unfilled model compounds of EPDM, FKM, HNBR, and NBR were tested in high-pressure (17 MPa to 87 MPa) and low-pressure (10 MPa to 31 MPa) cycling conditions at −40°C and +85°C. These experiments were performed at a fixed depressurization rate. The goal of these experiments was to better understand temperature effects under pressure cycling conditions for elastomeric polymer seals. Filled formulations of EPDM, HNBR, and NBR exhibited increased compression set and decreased storage modulus under cold cycling exposures to a greater extent than when cycled at ambient. For filled polymers cycled at low pressures at 85°C and −40°C, FKM showed the most resistance to blistering. HNBR and NBR showed heavy swelling and blistering under both these conditions. Micro CT imaging of one of the polymers (EPDM) subjected to high-pressure cycling at 85°C showed great damage in the form of cracks in the center of the sample. Filled formulations exhibited decreased compression set and storage modulus under hot cycling exposures to a greater extent than with cold and ambient cycling. The findings from these studies will help build a strong understanding of polymer behaviors in cycling hydrogen under rapid gas decompression and thermal conditions encountered in fueling operations and storage. Proper material selection for appropriate use-conditions within components is also enabled.