You know that wonderful new car smell? Theres a name for that phenomena: outgassing.
The dictionary definition of outgassing is the release of gas from a solid material, and can be a major concern when it comes to plastics and elastomers. To make matters more interesting, the outgassed material not only contaminate the air we breathe, but can contaminate surfaces.
Outgassing as a Critical Characteristic
Outgassing is particularly critical in the semi-conductor and aerospace applications where purity is an absolute must, or in any field where highly sensitive instruments are used. It is especially a problem in high temperature and/or vacuum conditions where polymer seals are commonly used. For these types of applications, there are outgassing criteria involving the total mass loss and the collected volatile condensable materials. That leads us to our next topic: testing.
Testing for Outgassing
So, how do you determine the level of outgassing for a particular material?
There are a variety of methods for obtaining outgassing data, but the most widely accepted seems to be ASTM Test Method E595-93, “Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment.
In this test, a sample of the material in question is placed in a vacuum at elevated temperature for a
certain amount of time. The mass of the specimen is measured before and after the test to obtain the
percent total mass loss (TML). The percent collected volatile condensable materials (CVCM) is measured by placing a cooled plate near the specimen to collect condensing material.
Test Data
Both NASA and the ESA (European Space Agency) have online databases of test data on material
outgassing, and many plastics manufacturers make this data available for their customers. Keep in
mind, however, that the actual outgassing of a material can vary greatly according to the cure, mix, and differences in formulation.
Low Outgassing Seal Polymers
Seals are found in so many different aerospace, semiconductor, and pharmaceutical applications where high purity is a critical matter. There are low outgassing polymers that can be considered for these applications. First, lets define what exactly low outgassing means.
A low outgassing material is normally defined as one for which
the TML = 1.0% and the CVCM = 0.10%.
A low outgassing material is normally defined as one for which the TML = 1.0% and the CVCM = 0.10%. However, there will be different requirements for different applications but this provides an excellent starting point for narrowing down what kinds of polymers can be used.
Below is a list of some of the low outgassing polymers that are commonly used seal materials:
- Acetal
- PEEK
- Polyamide-imide
- Polyimide
- Antistatic PTFE (tradename Teflon)
- PCTFE
Conclusion
Outgassing can be a serious problem in certain applications that require sealing solutions, but there are various low outgassing polymer options available.
Useful links:
NASA Discussion on Outgassing, http://www.nasa.gov/topics/technology/features/outgas-tech.html
NASA outgassing database: https://outgassing.nasa.gov/
ESA outgassing database: http://esmat.esa.int/Services/outgassing_data/outgassing_data.html
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