vacuum permeation in elastomers; we solve o-ring problems

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

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

Vacuum Permeation