FAQ on outgassing: sources, implications, and solutions: part 1

This materials- and physics-based phenomenon is a non-issue for many designers but a major one for others.

If you have ever been in a new car with its distinctive smell of plastic, leather, carpet, and more, you’ve had a personal encounter with outgassing. In addition to smell, it is also seen, quite literally, as the annoying, hard-to-remove haze that collects on the inside of the windshield (and the lungs of occupants). Somewhat paradoxically, some people find the new car smell so attractive that you can buy it in a can (under $10) to “refresh” that sensation (Figure 1).

This FAQ will examine the phenomenon of outgassing (also known as “offgassing”), which goes far beyond its impact on new cars and their owners. It will examine its multiple facets, implications, assessment, and minimization. Understanding outgassing has become extremely important to more designers with the huge growth in low — and medium-Earth orbit (LEO and MEO) satellites such as CubeSats.

Q: Exactly what is outgassing?
A: Outgassing is a passive process in which a material — primarily but not exclusively a polymer, adhesive, rubber, or potting compound/epoxy — releases gas when exposed to heat and/or a vacuum. It can even be a factor for metals in some circumstances, although to a much smaller extent.

The gas or vapor that is released was previously dissolved, trapped, frozen, or absorbed in the solid. Outgassing includes multiple physics phenomena of sublimation and evaporation, which are phase transitions of a substance into a gas, as well as desorption (release of trapped molecules).

Q: Why and where is outgassing a concern?
A: It’s simple; it is a type of contamination. This gas will eventually condense on other materials and surfaces and could prevent them from fully implementing their primary function or even make them fail outright. A material’s outgassing performance is vitally important in areas with sensitive electronics, lenses (optics), mirrors, and windows, as it can result in partial degradation, permanent damage, or complete failure.

In the vacuum of deep space, outgassing has contributed to the degraded performance of image sensor assemblies (lenses and sensor) in space probes, prompting NASA to develop exacting procedures for evaluating materials prior to their use in space.

Q: Is outgassing only a concern for spacecraft due to the vacuum and, thus, the absence of pressure in space?
A: No, any high vacuum (HV) or ultrahigh vacuum (UHV) system needs to be aware of outgassing considerations (HV here does not mean “high voltage”). Outgassing affects HV and UHV laboratory chambers and experiments as well as advanced production processes; HV and UHV chambers are used in sputtering and vacuum deposition, for example, and its effects can impact many pieces of equipment and materials being processed in these chambers.

Q: What are some of the specific potential sources of the outgassed molecules? What molecule types are most likely to outgas?
A: All materials and surfaces are active sources of outgassing. Some materials are more likely to have outgassing than others. The worst are plastics, elastomers, and glues; porous ceramics and porous metals; most lubricants, heat transfer greases, and fingerprints; hair, skin cells, dust, food, and other body “residues.”

The most predominant gases and vapors that are outgassing from surfaces are water vapor, oil and grease vapor, solvents, and organic materials. Further, at the extremes of UHV systems, even the stainless steel used to manufacture much of the chamber itself outgases hydrogen and carbon monoxide to some degree.

Outgassing mechanism

Q: Does outgassing represent a single physical mechanism?
A: No, that would be too easy. Figure 2 shows four main mechanisms that contribute to outgassing.

1. Vaporization of the actual surface material itself (in metals this is negligible at most operating temperatures, but there are exceptions).
2. Desorption — this is the reverse process of adsorption; the release of molecules bound at the surfaces of the chamber and internal fixtures.
3. Diffusion — this is the movement of molecules from the inner structure of the material to the surface.
4. Permeation — this is the movement of molecules from the external atmosphere through the bulk to the vacuum surface.

The extent to which each contributes to the overall outgassing magnitude and type depends on the composition of the gas and the surface material (and its process history).

The next part of this article delves further into outgassing.

Related WTWH content

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Thermal dielectric filler eliminates contamination in high-voltage applications
Epoxy Preforms Have Low Outgassing Properties
Ah, that new car smell: NASA technology protects spacecraft from outgassed molecular contaminants

External references

Smithsonian History of Aviation Series, “Eye in the Sky: The Story of the Corona Spy Satellites”
CERN, “Physics of Outgassing” (1999)
Leybold GMBH, “How to reduce outgassing in vacuum systems“
Leybold GMBH, “Introduction to outgassing”
Accu-Glass Products, Inc., “Outgassing: What is it?”
Samtec, “What is outgassing and how is it tested?”
Samtec, “Outgassing FAQs Guide”
Wikipedia, “Outgassing”
NASA, “Outgassing”
Microwave Journal, “What is outgassing and when does it matter?”
Chemical Guys, “New Car Air Scent”