For products used in mechanical and electrical applications, understanding a product’s outgassing performance is frequently just as crucial as understanding other technical details.
The process of outgassing is the process in which a non-metallic substance, such as a polymer, adhesive, rubber, or potting compound/epoxy, will release a gas when exposed to heat and/or a vacuum. Other materials may become inoperable as a result of this gas’s eventual condensation on them.
In the vacuum of deep space, outgassing (also known as “offgassing”) has contributed to the degraded performance of charge-coupled-device (CCD) sensors in space probes, which led NASA to create exacting protocols for examining materials before they are used in space. Continue reading to find out more information about outgassing.
Table of Contents
What is Outgassing?
A non-metallic material will release a gas when heated or placed in a vacuum, such as a polymer, adhesive, rubber, or potting compound/epoxy. This process is known as outgassing. Other materials may become inoperable when this gas eventually condenses on them.
In areas with delicate electronics, lenses (optics), mirrors, and windows, a material’s outgassing performance is crucial for preventing damage to those components. Anything used in the International Space Station, anything implanted in a person to prevent infection, anything used in hospitals, or components used with image sensors are all excellent examples of this.
While independent testing for outgassing is available, Samtec currently uses NASA data to determine whether a series is thought to be a low outgassing product and can be used in situations where outgassing of a product may be a problem.
How Is It Tested?
NASA uses the ASTM E595-77/84/90 test to evaluate a material’s performance in environments with high heat or extreme vacuum. The steps to this test include:
- Exposure of the parts to 25°C at a 50% Relative Humidity for 24 hours
- Products are then weighed
- Exposure in a chamber at 125°C for 24 hours at a vacuum no less than 5×10-5 torr
- Chamber contains a cooling collector plat at 25°C to collect gas condensate
- Products are then weighed again with the collector plate
The materials are weighed before and after the experiment, and if the total mass lost (TML) and the collected volatile condensible materials (CVCM) are below 1.0% and 0.10%, respectively, the materials are considered to have passed.
Four Main Mechanisms Which Contribute to Outgassing
- Vapourisation of the actual surface material itself (in metals this is negligible at typical operating temperatures)
- Desorption — this is the reverse process of adsorption; the release of molecules bound at the surfaces of the chamber and internal fixtures
- Diffusion — this is the movement of molecules from the inner structure of the material to the surface
- Permeation — this is the movement of molecules from the external atmosphere through the bulk to the vacuum surface
According to the gas’s composition as well as the surface material’s (and its history’s) composition, each of these influences outgassing to varying degrees. These contributions are added together to form outgassing rates.
Final Words
Especially below Medium Vacuum, outgassing frequently contributes the most to a system’s gas load, which lowers the achievable ultimate pressure. Vapourization, desorption, diffusion, and permeation are a few of the processes that cause it. According to the vacuum level, water vapor makes up the majority of outgassing in HV, whereas hydrogen is most frequently produced when working with metals at UHV.
Although the measurement technique and sample preparation are the main causes of variations in outgassing rates, it would be beneficial to develop a standard for rate measurement techniques.
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