GAS PERMEABILITY: Teflon® encapsulated O-ring
All plastics have some permeability to gases. In the case of the FEP/PFA encapsulation of the ENCAP-O-SEALTM Teflon® encapsulated O-ring, however, gases and vapors will permeate at a considerably lower rate than for most other plastics. The primary permeation mechanism is intermolecular migration. “Because permeation is basically a molecular movement, we might expect temperature rises to increase permeability. The data bears this out, as shown in the Gas Permeability of “Teflon®” FEP Resins chart. For example, increasing the temperature only 63oF (20oC) will almost double the permeation rate through plastic parts. The Permeability of FEP Resins chart lists permeability data for various gases and vapors in terms of grams per 100 square inches of surface area per 24 hours and corrected for 1 mil thickness. The rate of permeation is linearly dependent on the thickness of the plastic component as shown in the Water Vapor Transmission Rate chart for the moisture transmission through FEP film of various thicknesses. The numbers listed in the Permeability of FEP Resins must therefore be multiplied by the thickness of the plastic part to determine the over-all level of permeation.” (Journal of Teflon®, Dec. 1964, Vol. 22) Consequently, while highly corrosive gases do not attack the FEP/PFA encapsulation of the ENCAP-O-SEALTM Teflon® encapsulated O-ring, they may eventually permeate through and damage the elastomer core and hence affect the mechanical properties. In addition to chemical resistance, these permeability effects must be considered in sealing systems. See How to Choose Application Specific Encapsulated O-rings.
1) Note that the permeation data listed in table 1 is based on 1 mil thickness of FEP/PFA encapsulation.
* The transmission rate significantly decreases when the thickness of encapsulation is increased. * See H2O vapor transmission rate chart.
2) Test method: ASTM D-790-59T (at 1atm).
3) Test method: ASTM E-96-53T (vapor pressure).