Is kynar (PVDF) a good hydrogen gas barrier?  

Posting by jim on April 17, 2008 at 15:41:42.

Hydrogen gas can damage magnets by the process called Hydrogen decrepitation. We plan to use a skeleton structure made of Kynar to hold the magnets and then do an overmolding to cover the whole structure with
Kynar, the same material as the skeleton structure.

Is Kynar (PVDF) a good hydrogen gas diffusion barrier? Is Kynar impermeable to hydrogen gas? What is the minimum thickness of Kynar required to protect the magnets from hydrogen gas?

The finished rotor with magnets embedded inside will be used in an environment with water, hydrogen gas and up to 5% of hydrogen peroxide at temperature of about 40 degrees Celsius. Are there other issues I should be concerning about?

Many thanks for sharing your thoughts with us,


          follow up posts
    On 04/17/2008 rodney posts: There are various high performance polymer materials for coating, lining, relining and field joint applications, that have a good permeation and chemical resistance against Hydrogen at high pressure. Below we list some permeability figures of some common high performance polymers. Note that for sake of comparing the material performance relative to - for example - Ethylene Tetrafluoroethylene, the permeability figures are insufficient. Visit the gas diffusion in polymers and metals section of the website to obtain more detailed information.

    Permeability figures at:

    System Temperature: 310 Kelvin
    System Pressure: 1 BAR
    Hydrogen Partial Pressure: 1 bar

    Ascending Hydrogen Permeation in SI Units [mstp3 / m3 x bar] x [m2/s]

    Liquid Crystalline Polymer (LCP): 1.0 E-14
    Fluoropolymer (PVDF): 4.0 E-13 [4 E-13 for KYNAR POLYMER]
    Polyethylene Terephthalate (PET): 5.8 E-13
    Parylene: 6.9 E-13
    Chloro Trifluoroethylene (CTFE): 8.7 E-13
    Ethylene Tetrafluoroethylene (ETFE): 9.0 E-13
    Polypyrole: 1.0 E-12
    Polyether Etherketone (PEEK): 1.0 E-12
    High Density Polyethylene (HDPE): 2 E-12
    Polysulfone (PSF): 8.6 E-12
    Polydimethylsiloxane Rubber (PMDS): 2.1 E-10

    Good luck,
    [responses: 1]

      On May 24, 2008 at 22:58:38 jim posts: Thanks! May be polyethylene is another good suggestion since the environment contains some degree of moisture (50-70% humidity)? Is polyethylene a good permeation barrier for water vapour, hydrogen sulphide, carbon dioxide, methane, propane? What about the difference of gas and vapour diffusion / solubility / permeability in low, medium polyethylene, high, ultra high linear polyethylene, Dyneema? Does the hydrogen permeation rate in polyethylene (PE) or polyvinylidene fluoride (PVDF) change if water diffusies simultaneously? I think I have to choose between high density polyethylene and polyvinylidene fluoride


      p.s. do PE or PVDF show stress cracking or chemical corrosion under the load of the chemical described?
      [responses: 0]

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