Outgassing is the release of trapped gases or vapors from a solid or liquid material into a surrounding vacuum or low-pressure environment. It occurs when a material is placed in a vacuum, causing the lower pressure to force volatile substances—like water vapor, manufacturing solvents, and chemical additives—to escape and evaporate into space.
The Mechanism: How Outgassing Happens
- Desorption: Gas molecules physically bound to the surface of the material detach and enter the vacuum.
- Diffusion: Trapped gas molecules deep inside the material move to the surface before releasing.
- Decomposition: The material chemically breaks down under heat or radiation, creating new volatile compounds.
- Vaporization: Liquid or solid components within the material boil off due to the low pressure.
Why Outgassing is a Major Threat in Space
In the vacuum of space, these released gases do not simply float away safely. They create a localized “atmosphere” around the spacecraft, leading to serious technical failures:
- Optical Contamination: Escaping chemical vapors condense onto cold surfaces like camera lenses, mirrors, and star trackers, clouding the optics and ruining scientific data.
- Power Degradation: Contaminants deposit onto solar arrays, creating a film that blocks sunlight and reduces power generation.
- Electrical Arcing: The localized gas cloud can cause high-voltage electronics to short-circuit, spark, or suffer destructive electrical breakdowns.
- Thermal Control Issues: The condensed films alter the thermal properties of specialized radiative coatings, causing the spacecraft to overheat or overcool.
Material Causing Outgassing
Polymeric (organic) materials are the primary cause of outgassing in vacuum and space environments. While nearly all materials can release trapped molecules under low pressure, non-metallic polymers pose the highest risk because they contain volatile organic compounds (VOCs), trapped solvents, moisture, and unreacted additives from manufacturing.
High-Risk Materials That Cause Outgassing
- Rubbers and Elastomers: Standard Room-Temperature Vulcanized (RTV) silicones and nitrile rubbers outgas heavily. They release low-molecular-weight silicone oils and plasticizers.
- Adhesives and Glues: Epoxies, polyurethanes, and acrylic binders trap residual solvents and curing agents. They release these volatiles when subjected to the vacuum of space.
- Polymer Matrix Composites: Carbon fiber, fiberglass, and aramid (Kevlar) fabrics bound with epoxy resins absorb structural moisture and atmospheric gases. They release water vapor and hydrocarbons.
- Lubricants and Greases: Industrial wet lubricants, thermal greases, and sealing oils evaporate or break down easily under high temperatures and low pressure.
- Plastics and Insulation: Polyvinyl chloride (PVC), standard nylon, and certain polycarbonates outgas plasticizers and unreacted monomers. Cable jackets and shrink tubing are common culprits.
- Paints and Coatings: Conformal coatings, varnishes, marking inks, and decorative lacquers contain volatile chemical solvents that escape continuously in a vacuum.
- Porous Metals and Ceramics: Though not polymers, materials with micro-porosity can trap moisture, hydrogen, or carbon monoxide internally, which slowly leaks out over time.
What Do These Materials Actually Release?
When placed in a vacuum, these materials typically release water vapor, residual solvents, unreacted monomers, plasticizers, and hydrocarbons. These vapors can condense onto cold optical lenses, solar arrays, or high-voltage electronics, causing system degradation or failure.
Space Industry Screening Standards
To be qualified for space flight, materials must be tested using the ASTM E595 standard protocol. A material is only considered safe by agencies like NASA if it passes two strict limits:
- Total Mass Loss (TML): Must be less than 1.0%.
- Collected Volatile Condensable Material (CVCM): Must be less than 0.1%.
Conformal Coatings are widely used in Space Sector – Reason listed here.