Why Low-Outgassing Matters in Optical and Vacuum Seals
Low-outgassing elastomers are essential in optical, sensor, and vacuum assemblies where even microscopic contaminants can cloud a lens or shift a measurement.
In vacuum or cleanroom environments, volatile compounds released from typical rubber materials can condense on optical surfaces or interfere with coatings — causing loss of clarity, drift, or false readings.
Selecting low-outgassing elastomers for optical and vacuum seals ensures long-term stability, optical cleanliness, and system reliability. These compounds are engineered to minimize total mass loss (TML) and collected volatile condensable materials (CVCM) as defined by ASTM E595 and NASA standards.
How Outgassing Occurs in Elastomers
Outgassing is the release of trapped volatiles (unreacted monomers, plasticizers, or residual solvents) when elastomers are exposed to heat or vacuum.
At atmospheric pressure, these vapors dissipate harmlessly. In a vacuum or sealed optical system, however, they condense on cold surfaces, leading to:
- Optical fogging or film buildup
- Sensor drift or loss of calibration
- Adhesion failure in coated lenses or mirrors
- Particle contamination and long-term instability
Reducing outgassing is a combination of material choice, formulation purity, and post-molding processing.
Comparing Elastomer Options for Low Outgassing
| Material | Outgassing Performance | Advantages | Considerations |
|---|---|---|---|
| FKM (Fluoroelastomer) | Excellent | Low vapor pressure, chemical and radiation stability | Slightly stiff; higher molding temperature |
| Silicone (High-Purity LSR) | Good | Flexible, clean molding, transparent | Moderate permeability; may require vacuum bake |
| EPDM | Fair | Economical, clean molding | Not ideal for deep vacuum or optical use |
| Butyl (IIR) | Excellent | Gas-tight; low vapor transmission | Limited temperature and chemical range |
Pro Tip: FKM remains the gold standard for low-outgassing seals in optics, photonics, and vacuum instrumentation, while high-purity silicone serves well in flexible, single-use optical systems.
Design and Process Tips for Low-Outgassing Seals
1. Specify Purity and Post-Cure Requirements
Post-curing removes residual volatiles from vulcanization. Baking at 200°C for 4–8 hours under vacuum or nitrogen purge can dramatically lower outgassing levels.
2. Use Cleanroom Molding
Class 1000 or better environments reduce particulate contamination. Combine with no-parting-line tooling to prevent flash that can trap volatiles.
3. Avoid Additives and Plasticizers
Choose compounds without processing aids or colorants that can sublimate under heat. Pure fluorinated or perfluorinated formulations are most stable.
4. Validate with Standardized Testing
Use ASTM E595 or ECSS-Q-ST-70-02C methods to verify Total Mass Loss (TML) < 1.0 % and Collected Volatile Condensable Material (CVCM) < 0.1 %.
5. Consider Vacuum Pre-Bake
A pre-bake can remove surface residues, enhancing optical and vacuum cleanliness before assembly.
Applications That Depend on Low-Outgassing Seals
- Camera and imaging modules — prevent lens fogging and optical distortion
- Vacuum chambers and analyzers — maintain clean, stable internal pressure
- Aerospace optics — ensure clarity and coating integrity under radiation
- Medical diagnostic sensors — eliminate false readings due to vapor contamination
Each of these relies on stable, contamination-free seals that perform at the micro scale.
Why Low-Outgassing Elastomer Selection Drives Reliability
Low-outgassing elastomers directly determine optical clarity, sensor accuracy, and long-term system stability.
Even the best mechanical seal fails if chemical volatility introduces contaminants or outgassed films.
By combining high-purity FKM compounds with cleanroom molding and vacuum post-processing, engineers can achieve consistent, contamination-free performance at the micron level.
Need a low-outgassing elastomer for your optical or vacuum seal?
We specialize in cleanroom molding and material formulation for low-outgassing seals — engineered for optical performance, vacuum integrity, and repeatable precision.
Contact us to learn more.
