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Permeation Data

The solubility and diffusion of a gas through an elastomer affects the pump-down time as well as the ultimate vacuum achieved in a vacuum seal. The gas permeation coefficient of a polymeric material is the volumetric flow rate of a gas under steady-state conditions through a unit surface area of unit thickness at unit pressure difference.

Q = K A (P1 - P2) / d

Q is the permeation rate (cm3/sec)

K is the permeation coefficient [(cm3 cm)/(sec cm2 atm)]

A is the area (cm2)

P1 - P2 is the pressure gradient (atm)

d is the thickness (cm)

Permeability is a fundamental property of an elastomer. It is often desired to measure the rate of transmission of a vapor or liquid through the elastomer. This is called "transmission." In general: Swelling decreases permeation rate High pressures decrease permeation rate (reduction of free volume) Higher temperatures increase diffusion rate and permeation rate Inorganic fillers eliminate diffusive passages and lower permeability Larger molecules of gas lower diffusion rate

Permeation Data for Various Polymers - unit expressed in 10^-8 sccm-cm/(sec-cm²-atm)
*Highly fluorinated compound

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