Q & A: Comprehensive Guide to Low Outgassing Adhesives

Join Venkat Nandivada, Manager of Technical Support, and Rohit Ramnath, Senior Product Engineer, as they answer questions about low outgassing epoxy adhesives.

Q: What is outgassing, and why is it a concern for adhesives?

A: Outgassing is the release of chemical species over time from a fully cured adhesive. These released materials may include volatile organic compounds (VOCs), unreacted monomers or oligomers, residual solvents, or other constituents from the original formulation. In sensitive applications, particularly those involving optics, vacuum systems, and space environments, outgassing can deposit residues on critical surfaces, cause fogging of lenses and sensors, alter pressure levels within sealed systems, and potentially affect electrical performance. Even if the adhesive bond remains structurally sound, contamination from outgassing can render the entire assembly unusable.

Q: What substances are typically released during outgassing?

A: The specific substances released depend on the adhesive chemistry. For epoxy adhesives, common outgassed species include water vapor, carbon dioxide, and molecules composed of carbon, hydrogen, and oxygen. If the curing agent is amine-based, nitrogen-containing compounds may also be released. For UV-curable adhesives, unreacted monomers, oligomers, and photo initiators can contribute to outgassing. In most standard testing, the total quantity of released material is measured rather than identifying specific chemical species.

Q: How is outgassing measured, and what standards apply?

A: The industry standard for measuring outgassing is ASTM E595, originally developed by NASA for qualifying materials used in space applications. In this test, a fully cured sample is exposed to 125°C for 24 hours at a vacuum level of approximately 10⁻⁵ to 10⁻⁶ torr. The test measures three parameters: Total Mass Loss (TML), which indicates the percentage of mass lost from the sample; Collected Volatile Condensable Materials (CVCM), which measures condensable residue that could deposit on surfaces; and Water Vapor Recovered (WVR), which accounts for moisture content. To meet NASA low outgassing requirements, a material must have less than 1% TML and less than 0.1% CVCM. If TML exceeds 1%, the material can still qualify if TML minus WVR is less than 1%.

Q: What is the typical range of outgassing values for adhesives?

A: Adhesives not formulated for low outgassing can exhibit TML values as high as 5% or more, with CVCM potentially reaching 0.5% to 1%. In contrast, specialty low-outgassing formulations can achieve TML values less than 1%  and CVCM values below 0.1%.  Importantly, low outgassing performance is achievable across multiple chemistries, including epoxies, UV-curable systems, and even certain high-purity silicones.

Q: Does the ASTM E595 test guarantee performance in all applications?

A: The ASTM E595 test provides an excellent baseline for qualifying materials, but it may not fully represent all application conditions. The test is conducted at 125°C, but some applications operate at significantly higher temperatures (200°C or above) where outgassing rates may increase. Similarly, the test vacuum level of 10⁻⁵ to 10⁻⁶ torr is suitable for many space applications, but ultra-high vacuum environments operating  as low as 10⁻¹² torr are far more sensitive. At such extreme vacuum levels, even trace amounts of outgassing that would be negligible under standard test conditions can cause significant problems. For demanding applications, engineers should use ASTM E595 results as a guideline and conduct additional testing under conditions that match their specific operating environment.

Q: Are there tests available to identify exactly what substances are being outgassed?

A: Yes. While ASTM E595 measures how much material is outgassed, it does not identify what is being released. For applications requiring this information, such as ultra-high vacuum systems where specific contaminants must be avoided, RGA (Residual Gas Analysis)  can identify the specific chemical species being released. This type of testing is particularly relevant for applications operating at vacuum levels as low as 10⁻¹² torr, where the identity of outgassed materials matters as much as the quantity. Master Bond does not perform RGA testing in-house, but customers can arrange third-party testing when needed. For example, Master Bond EP30-2 was analyzed for the Laser Interferometer Gravitational-Wave Observatory (LIGO) project: https://www.masterbond.com/case-study/ep30-2-qualified-ligo-optical-assembly, wherein RGA data was critical.

Q: What applications require low outgassing adhesives?

A: Low outgassing adhesives are essential in space and satellite systems where vacuum exposure and sensitive instrumentation require contamination-free materials. Optical assemblies (including cameras, sensors, telescopes, and laser systems) require low outgassing to prevent fogging and maintain clarity. High-vacuum scientific equipment, semiconductor manufacturing, and aerospace electronics also demand these materials. Any application where residue formation could interfere with measurements, optical transmission, electrical performance, or system pressure must consider outgassing characteristics.

Q: How does cure schedule affect outgassing performance?

A: Cure schedule has a significant impact on outgassing. A fully cured adhesive will outgas less than a partially cured one because unreacted components are primary contributors to outgassing. For two-part room-temperature curing epoxies, achieving 90-95% cure may require five to seven days at room temperature. Adding heat, even moderate temperatures of 150-200°F (65-95°C), can accelerate cure and improve cross-linking, resulting in potentially lower TML and CVCM values. For UV-curable adhesives, ensuring complete cure with proper lamp intensity, wavelength (typically around 365 nm), and exposure time is critical. A post-cure heat treatment at 80-100°C for one to two hours can further improve outgassing performance even for UV systems that appear fully cured.

Q: Can heat curing make the difference between passing and failing outgassing specifications?

A: Yes, absolutely. In some cases, heat curing can push a product from failing to passing the NASA low outgassing requirements. The degree of improvement depends on the specific chemistry, the temperature used, and the duration of heat exposure. For applications where heat curing is not possible due to temperature-sensitive components, this can present a significant challenge. However, many formulations can achieve acceptable outgassing performance with relatively modest heat exposure. 80°C (175°F) is often sufficient to meaningfully improve results.

Q: Do fillers affect outgassing performance?

A: There is no definitive rule that fillers increase or decrease outgassing. In theory, adding inorganic fillers like aluminum oxide reduces the organic content of the adhesive, which might be expected to reduce outgassing. However, in practice, Master Bond offers both filled and unfilled versions of the same base chemistry that achieve very similar outgassing numbers. The formulation as a whole matters more than filler content alone. 

Q: Are there trade-offs when formulating for low outgassing?

A: Interestingly, low outgassing performance does not require sacrificing other properties. Master Bond offers low-outgassing products with glass transition temperatures exceeding 400°F (204°C).  For applications where structural strength is critical, formulations with lap shear strengths greater than 4,000 psi on aluminum are also available. The primary trade-off is often in processing rather than final properties: achieving optimal outgassing performance typically requires heat curing, even for room-temperature curing chemistries. For applications where heat exposure is not feasible, this can be a limitation. However, there are formulations designed to achieve good outgassing performance with room-temperature cure alone.

Q: Are there UV-curable adhesives that meet NASA low outgassing requirements?

A: Yes, Master Bond offers numerous UV-curable formulations that meet ASTM E595 requirements. The key requirement is ensuring complete cure, which means providing unobstructed access for UV light (typically at 365 nm wavelength) to reach all adhesive surfaces. Shadows or blocked areas will result in incomplete cure and compromised outgassing performance. For assemblies where partial shadowing is unavoidable, dual-cure systems are available that initiate with UV exposure and complete curing with heat as low as 180°F (80°C). These dual-cure products can also meet NASA low outgassing standards when processed correctly.

Q:  What products does Master Bond offer for ultra-high vacuum applications?

A: For demanding applications including ultra-high vacuum systems, Master Bond EP30-2 and EP21TCHT-1 have been successfully used by customers in environments operating at 10⁻¹² torr and have had their outgassing rates independently measured and validated. While NASA low outgassing testing provides a useful guideline, customers working in ultra-high vacuum applications often perform their own testing under application-specific conditions. Master Bond's technical support team can help identify appropriate products based on specific vacuum levels, operating temperatures, and other application requirements.

Q: What environmental factors beyond temperature can affect outgassing in service?

A: Several factors can influence outgassing performance in actual service conditions. Higher operating temperatures will generally increase outgassing rates—if an application operates at 220°C rather than the 125°C test temperature, higher mass loss may occur. Vacuum level matters significantly: outgassing that causes no issues at 10⁻³ torr may be problematic at 10⁻¹² torr. Chemical exposure can alter outgassing characteristics as well. 

Q: How can I tell if outgassing is causing problems in my assembly?

A: One indicator is visible contamination or residue forming on optical surfaces, sensors, or other critical components—often appearing as fogging or haze that develops over time during service. In vacuum systems, unexpected pressure changes or difficulty maintaining target vacuum levels may indicate outgassing. Measurement drift in sensitive instruments can also suggest contamination from outgassed materials. If these symptoms appear and an adhesive is present in the assembly, switching to a NASA low outgassing qualified product is recommended to minimize the risk of continued contamination.

Master Bond provides an extensive portfolio of epoxies, silicones, and UV-curable systems that meet or exceed the ASTM E595 requirements. For a comprehensive directory of our qualified products, please visit our NASA Low Outgassing Certification page.

Contact Master Bond if you require further information or have specific adhesive application questions. Our technical team is available to discuss your engineering requirements.