o-rings and seals

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

Outgassing and Weight Loss of Elastomers

outgassing of elastomersOutgassing in elastomers is the release of volatile materials when the elastomers are heated. Outgassing affects vacuum performance. Outgassed components may include water vapor from the reaction of acid scavengers (i.e., MgO) and acid during the curing process of elastomers. Also, low-molecular-weight species of antioxidants and UV stabilizers, unreacted polymer residues and degraded products may appear as outgassed components. Many traditional seal materials contain small amounts of low-molecular-weight plasticizers, or process aids, that can volatilize under vacuum conditions. Water vapor and carbon dioxide are also absorbed into seal elastomers exposed to air.

Weight loss measurements are often used as an indicator of outgassing in elastomers. Specific values for vacuum weight loss are dependent on the elastomer and sample conditioning.

Heating an o-ring seal for several hours at 100C has been shown to reduce outgassing significantly. This "bake-out" procedure is less important for seals made from modern fluorocarbon and perfluorocarbon elastomers as the manufacturing process includes a several-hour post-curing step at temperatures above 200C. Vacuum baking can provide even less outgassing.

Outgassing Rates of Elastomers and Various Materials

Material RATE*
(10-9 mbar/sec-cm2)
RATE**
(10-8 Torr-liter/sec)
Stainless Steel 13.5  
Steel, Chrome Plated 7.1  
Stainless Steel,
Electropolished
4.3  
Aluminum 6.3  
Nitrile 3,500 300
KEL-F 40 4
Silicone 18,000 2,000
Fluoroelastomer 1,140 2,000
Fluoroelastomer, baked 4 0.2
Perfluoroelastomer   0.3
PTFE 300 400
Polyimide 900 80
Pyrex glass 7.4  

* Leybold Inficon, Inc.-"Vacuum" 1997
** Peacock, R.N., J. Vac. Sci. Technol., Jan./Feb. 1980

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