O+P Fluidtechnik 1/2025

DICHTUNGEN0304Change of

DICHTUNGEN0304Change of IRHD hardness of different compounds afterimmersion in H2 for 168 h at +23 °C and 70 MPaVolume change depending on specimens (left, 1 hour) andtime of measuring (right) after immersion in H2 for 168 h at+23 °C and 70 MPaThe upper temperature limit of plastics and elastomers is largelydetermined by the base material. At higher temperatures, mechanicalproperties decrease, and degradation is accelerated.Materials for sealing applications should be selected below theirupper temperature limit. Therefore in this study the change oftemperature limits is not considered, but rather the influence ofextreme application temperatures on the mechanical propertiesof sealing materials.3 MATERIALS AND METHODSIn this study we investigate four ethylene propylene diene monomerrubbers (EPDM), one fluoroelastomer (FKM) and one thermoplasticpolyurethane (TPU) as elastomer sealing materials.For the plastics side one ultra high molecular weight polyethylene(UHMW PE) and six high-performance plastics with a PTFEmatrixas listed in the Table below are tested. The compounds areselected according to common industrial practice and hydrogenmarket experience.3.1 COMPATIBILITY TESTINGA very established method for testing media compatibility is ISO1817. This standard specifies a procedure for determining the resistanceof vulcanized and thermoplastic rubbers to the effects ofliquids or vapors in accordance with specified conditions. Themethod involves immersing test specimens of elastomers in a testliquid or exposing them to a test vapor for a specified durationand temperature, and then measuring the changes in their mass,volume, hardness, tensile strength and elongation at break.The test method was enhanced to mirror hydrogen specificchallenges to the materials. The samples were immersed for 168hours at nominal working pressure (NWP, typically 70 MPa) andthen pressure was released to ambient conditions within lessthan one second [6]. This approach is based on the toughest requirementsfor RGD-testing, namely ISO 17268 and SAE J2600.Test samples of each material were loaded into a pressure vesselto be filled with gaseous hydrogen. Once pressurized, the vesselwas placed into a temperature chamber. A typical pressurevessel is shown in Figure 1. After 168 hrs, the final pressure wasrecorded before depressurizing to verify that the gas did not leakduring the test.The change in mass and volume was measured after one hourfor the first interval and a second time after 24 hours. In contrast,tensile testing was only carried out after 24 hours as the change ofmechanical properties was to be detected without the influenceof remaining swelling due to hydrogen.FORSCHUNG UND ENTWICKLUNGMaterialFamilyEPDMFKMCompoundNameHardnessTemperatureRangeE7T30 70 Shore A -45 °C to +150 °CE8T31 80 Shore A -45 °C to +150 °CE8T24 80 Shore A -50 °C to +150 °CEBT25 86 Shore A -50 °C to +150 °CV9T82 90 Shore A -45 °C to +200 °CTPU ZLT 93 Shore A -60 °C to +110 °CTurcon®PTFET01 22 Ball Ind.H. -200 °C to +260 °CT05 24 Ball Ind.H. -200 °C to +260 °CMF2 22 Ball Ind.H. -200 °C to +260 °CMF6 32 Ball Ind.H. -200 °C to +260 °CMF8 55 Shore D -200 °C to +260 °CUHMW PE Z80 29 Ball Ind.H. -200 °C to +80 °CMaterials used in this study28 O+P Fluidtechnik 2025/01 www.oup-fluidtechnik.de

3.2 COMPRESSION SET TESTINGFor elastomeric sealing materials, the compression set is one ofthe most important values. It is a method to evaluate the ability ofthe material to recover to its original shape after being compressedfor a period of time. Usually, the value is measured afterageing in air at a temperature dependent on the actual applicationtemperature. The surrounding media can improve or impairthe compression set of a material depending on the interactionbetween material and media. Tests were carried out in a hydrogenatmosphere to determine the influence of this gas on thecompression set of elastomers. All tests were performed accordingto ISO 815-1A in which the material is compressed betweentwo metal plates. Compression rate depends in this case on thehardness of the material as described in the standard.Sample materials were 13 x 6 mm buttons clamped betweentwo stainless steel plates. The fixture and compressed sampleswere loaded into a pressure chamber to be filled with hydrogen. Avacuum was pulled on the chamber to remove ambient air priorto pressurizing with gaseous hydrogen to 0.01 MPa. The filled gaschambers were then placed into a temperature-controlled chamberat the specified conditions for 72 hrs. Post-test material propertieswere measured upon test completion.4 RESULTSGiven the distinct nature of elastomers and plastics which resultsalso in different test results to be investigated, the results presentedin this chapter are separated by material type. Results of mechanicalproperties are obtained from S2 dog bone samples. Resultsfor volume and weight change are measured on O-Ringsamples, typically one hour after pressure release.4.1 ELASTOMERS4.1.1 ROOM TEMPERATURE TESTSWhen immersed at room temperature, the samples show smallchanges of volume and weight (see Figure 2). Weight change isclose to zero and therefore negligible. Volume increases for allelastomers, whereas the TPU material, ZLT and the FKM material,V9T82, show very small swell of much less than one percent.The EPDM materials with 80 Shore A show higher swell of around1.5 percent. Compared to compatibility tests in other fluids, thisis however still very low.Looking at the changes of tensile strength and elongation atbreak, it can be stated that hydrogen does not cause embrittlementof the elastomers. The change of IRHD hardness shown inFigure 3 further confirms this. Four out of the six materials displayedshow changes of less than five percent, which is negligiblein the elastomer world. The TPU material, ZLT, had an 11 percentchange in tensile strength and elongation values when comparedto pre-test results. The absorbed hydrogen seems to cause a slightsoftening of the material.Mechanical properties are normally measured on dog bonespecimens, whereas in real applications the seal is molded into ashape e.g. an O-Ring. Comparing the volume change of these twospecimen types produces interesting results, see Figure 4. All materialsshow a much greater volume change when tested as an S2sample; E8T31 swells three times more, EBT25 eight times moreand ZLT even 10 times more. The reason for this phenomenon liesin the manufacturing method of the samples. When produced asan O-Ring, the material experiences a greater amount of shearstress in the molding tool and the polymer network is more densethan in a test sheet. Therefore, the hydrogen molecules are able tomore easily enter a specimen cut from a test sheet. Thus, if a materialpasses a certification test based on results from a dog bonesample, the final seal will likely swell less in a real application.This phenomenon should be considered in design and certificationprocesses.0506DICHTUNGENVolume change after immersion in H2 for 168 h at differenttemperatures and 70 MPaChange of tensile strength and elongation at break afterimmersion in H2 for 168 h at different temperatures and70 MPawww.oup-fluidtechnik.de O+P Fluidtechnik 2025/01 29