Outgassing in Aerospace Design
Some materials will release gas or vapor when placed in a vacuum. This is called outgassing. The gas or vapor can be absorbed, dissolved, or frozen, and creates a challenge to maintaining a clean area in high vacuum environments.
Designing in regards to outgassing is especially crucial in the aerospace industry. Outgassed products can condense onto lenses and other visual sensors, and can also have adverse effects on solar cells, thermal radiators, and electronics.
Plastics and other organic materials will often outgas since they may contain components that easily release into a vacuum, especially when in a high temperature environment. The solution to this problem is the use of durable and heat resistant materials which will not release vapors into the vacuum.
Finding Quality Materials
While many agencies such as NASA or the European Space Agency have their own tests for outgassing, the common industry standard is the ASTM Test Method E595-93. The specific name of this test is the “Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment.”
The test involves heating the material sample to 125°C while placed in a vacuum. The mass of the material is documented both before and after the test and presented as total mass loss percentage, or TML%. The lower the loss percentages are, the more resistant to outgassing a material will be.
Polyimide In Aerospace
Polyimide is one of the most frequently used materials in the aerospace industry because of its light weight, thermal stability, chemical resistance, and versatile mechanical properties. Polyimide shims—including brand names such as Kapton®, can be used at temperatures reaching 400°C and have very low outgassing rates. Kapton® has a TML of 0.96% which compares favorably to similar aerospace polymer materials such as Vespel® (1.09%), Celazole® (2.50%), and unfilled Torlon® (1.85%).
AccuTrex Laminated Polyimide Shims – a combination of polyimide foils and a special laminating adhesive - meet aerospace specifications in regards to ESD protection, EMI shielding, and resistance to oil and gas fluids as well as harsh external environments. Accutrex laminated shims have been tested according to ASTM E595, and have shown a TML% of 0.073+/-0.025%.