Vacuum environments impose a material requirement that most terrestrial applications do not: low outgassing. In atmospheric applications, the small quantities of volatile compounds that migrate out of adhesive polymer networks are diluted into the surrounding air and have no consequential effect. In vacuum — particularly in spacecraft, vacuum processing chambers, and optical systems operating under hard vacuum — these same volatiles condense on nearby cold surfaces, contaminate optical coatings and sensor surfaces, degrade the performance of neighboring materials, and in cryogenic systems can cause functional failures of critical hardware. Ultra-high temperature epoxy in vacuum applications must be selected and processed not just for thermal and mechanical performance, but for a low-outgassing profile that does not compromise the system it is part of.
What Outgassing Is and Why It Matters
Outgassing is the release of absorbed and dissolved gases, residual solvents, low-molecular-weight polymer fragments, plasticizers, and other volatile compounds from a solid material in a vacuum environment. Every organic polymer outgasses when exposed to vacuum; the question is how much and what species are released.
The primary concern in sensitive applications is total mass loss (TML) — the fraction of the adhesive’s mass that is released in vacuum at a defined temperature — and collected volatile condensable materials (CVCM) — the fraction of outgassed material that condenses on a collector surface at a defined lower temperature. CVCM is particularly relevant for optical and sensor applications because it represents the fraction of outgassed material that will deposit on cold surfaces and contaminate them.
The ASTM E595 test method (used in NASA and aerospace standards) measures TML and CVCM at 125°C in 10⁻³ torr vacuum for 24 hours, with a collector plate held at 25°C. Standard acceptance criteria for space materials in most programs are TML ≤ 1.0 percent and CVCM ≤ 0.10 percent. Ultra-high temperature epoxy formulations for space and vacuum applications must be selected from those with documented ASTM E595 data meeting these criteria.
Why Ultra-High Temperature Epoxy Outgasses Differently Than Standard Epoxy
Standard two-part epoxy adhesives — bisphenol A resins with aliphatic amine hardeners — contain several sources of volatile material: residual solvent if carrier solvent is used in the formulation; low-molecular-weight epoxy oligomers that did not participate in the cure reaction; excess amine hardener if mixed with excess; and moisture absorbed after cure from the environment.
Ultra-high temperature epoxy systems based on bismaleimide or cyanate ester chemistry have different outgassing profiles. They are typically solvent-free solid or semi-solid materials (particularly film adhesives), which eliminates solvent outgassing. Their higher functionality and crosslink density produce fewer residual low-molecular-weight oligomers after cure, because the multifunctional monomers react more completely than lower-functionality standard epoxy components.
However, BMI and cyanate ester systems can outgas volatile compounds related to their specific chemistry — small molecule byproducts of the cure reaction in some cyanate ester formulations, or residual monomer in under-cured BMI systems. The complete outgassing profile of an ultra-high temperature epoxy formulation must be measured by ASTM E595 testing on the cured specimen, under the cure conditions planned for the application, rather than assumed from the chemical class.
Post-cure at elevated temperature before installation in vacuum significantly reduces outgassing. The post-cure drives off residual volatiles under atmospheric conditions where they are harmless, reducing the TML and CVCM measured subsequently in vacuum. The post-cure temperature and duration should be chosen to drive outgassing completeness rather than just to develop mechanical properties; in some cases, a higher or longer post-cure than required for mechanical performance is specified to reach the outgassing acceptance criterion.
Outgassing in Thermal Vacuum Environments
The outgassing behavior of a material changes with temperature. At higher temperatures — above the ASTM E595 test temperature of 125°C — volatile species in the polymer have higher vapor pressures and higher diffusion coefficients, producing faster and more complete outgassing. In space applications where the adhesive will reach temperatures above 125°C during orbital operation (sunlit surfaces on a spacecraft in low Earth orbit can reach 80°C to 150°C depending on absorptivity and power dissipation), the outgassing rate at the actual service temperature exceeds the ASTM E595 characterization temperature.
For critical optical or detector systems, ASTM E595 testing at the actual service temperature provides more relevant data than the standard 125°C test. If the ultra-high temperature epoxy will be at 200°C in service, testing at 200°C for 24 hours provides a more conservative and application-relevant characterization of the contamination risk.
Thermal cycling in orbit — from the cold shade temperature to the warm sunlit temperature on each orbital period — drives cyclic outgassing as the adhesive warms and volatiles diffuse out faster, then slows as it cools. Over hundreds to thousands of orbital cycles, this cyclic outgassing continues until the volatile reservoir in the adhesive is depleted. Pre-baking the adhesive-bonded assembly in vacuum at the maximum expected service temperature before system integration depletes the volatile reservoir under controlled conditions before the assembly is integrated with sensitive components.
For ASTM E595 data for specific ultra-high temperature epoxy formulations, or for guidance on pre-bake schedule development for low-outgassing qualification, Email Us and Incure can provide the relevant data and protocol support.
Applications in Optical Systems
Precision optical instruments — telescope mirror mounts, laser cavity components, interferometer assemblies, and optical fiber end terminations — use structural adhesive to bond mirrors, lenses, and optical elements to their mounting structures. In vacuum-operated instruments, outgassing contamination on optical surfaces reduces transmission, scatters light, changes the optical path length through the contaminating film, and in some cases permanently damages the optical coating.
Ultra-high temperature epoxy for optical bonding applications must combine low outgassing with adequate mechanical performance — vibration resistance, thermal stability through the instrument’s operating temperature range — and in many cases, compatibility with the specific adhesive cure, inspection, and qualification requirements of the optical system design.
The dimensional stability of the cured adhesive is also important for optical bonding: any post-cure shrinkage or creep under load can shift the optical element position and alter the alignment of the instrument. Low-creep formulations post-cured to high conversion are preferred for precision optical mount applications.
Vacuum Chamber Applications
Industrial vacuum processing equipment — physical vapor deposition systems, electron beam evaporators, sputtering systems, and high-vacuum test chambers — uses adhesives for internal component assembly: sensor mounting, viewport frame sealing, and fixture assembly within the chamber. At the reduced pressures of these systems — from 10⁻³ torr for process vacuum to 10⁻⁹ torr for research applications — outgassing from adhesives affects pump-down time, system base pressure, and contamination of processed substrates.
Ultra-high temperature epoxy with low ASTM E595 TML and CVCM values, selected from products that have been characterized in vacuum applications, minimizes the outgassing contribution from bonded components to the chamber’s baseline pressure. Post-baking of bonded assemblies before installation in the chamber depletes volatile reservoirs and reduces pump-down contribution.
Contact Our Team to discuss outgassing data, pre-bake protocol development, and ultra-high temperature epoxy selection for vacuum, space, and precision optical applications.
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