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Home > O-ring, Seal Design > Vacuums > Permeation

Vacuum Permeation

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)

The permeation process

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

 

Material He O2 H2O N2 CO2
Nitrile 8 2.5 760 0.1 25
EPDM 25-30 16-18 - 6-7 85
VMQ 250 75-450 8,000 200 2,000
FVMQ 140 80 - 40 400
FKM 9-22 1-2 40 0.05-0.7 5
FKM* 30 3 - 2 -
FFKM (PFE) 60-80 6-8 90-100 8-12 -
KEL-F - 0.1 - 0.1 0.5
PTFE - 0.04 - 0.14 0.12
Polyimide 1.9 0.1 - 0.03 0.2

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

Next Topic Outgassing and Weight Loss of Elastomers

 

 

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