Adhesive bonding is an assembly method favored by engineers for creating robust joints. However, industrial adhesives come with certain weaknesses—notably outgassing—which can create complications in some applications.
When materials are placed in a vacuum or low-pressure environment, adhesives may emit gases that can taint nearby surroundings or disrupt sensitive machinery.
This issue is especially problematic in aerospace applications, along with products that depend on optical components, optoelectronics, or photonics. Semiconductors and medical imaging tools—endoscopes, for example—are also vulnerable to these difficulties.
Substances released through outgassing can corrode and damage fragile electronic circuits. They can also haze over lenses and lead to performance degradation.
Satellites and other spacecraft are highly susceptible to outgassing-related issues. Photo courtesy Northrop Grumman Corp.
Out-of-This-World Problems
Outgassing causes major headaches for spacecraft engineers, since satellites and similar devices endure extreme temperature swings caused by direct solar radiation. The challenge is amplified because a craft’s colder surfaces tend to house vital components such as cameras, lenses, mirrors, and sensors.
“When a material heats up and releases condensable volatiles in a vacuum, those volatiles will migrate straight to the cold surfaces,” cautions Debbie Thomas, a senior materials engineer at Ball Aerospace, which operates NASA’s Outgassing Laboratory under contract at NASA’s Goddard Space Flight Center.
The lab tests materials by first exposing them to 50 percent humidity for 24 hours, then subjecting them to 24 hours inside a vacuum chamber at 125°C. Any condensable volatile substances escape through an opening in the chamber onto a collector plate maintained at 25°C. Afterward, the material’s weight is measured to determine total mass lost from vacuum heating, and the plates are weighed to quantify the collected volatile condensable material.
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“Outgassing refers to the gradual release of volatile compounds from materials, especially when exposed to heat, vacuum, or pressure shifts,” explains Lindsay Hampton Varma, global senior market segment manager for defense and space adhesive technologies at Henkel AG. “When left unchecked, outgassing can contaminate sensitive parts, lower product reliability, and interfere with coatings, optics, electronics, and precision assemblies.”
“In the aerospace sector, outgassing is particularly critical because contaminants have no way to disperse in a vacuum,” Varma adds. “Even minute emissions can settle on optical surfaces, sensors, solar arrays, and avionics, undermining mission performance and long-term dependability. For spacecraft and high-reliability aerospace systems, using low-outgassing materials is vital for contamination control and mission assurance.”
Beyond satellites and other spacecraft, low-outgassing adhesives are widely used in avionics and electronic assemblies, guidance systems, RF systems, sensors, solar panels, and thermal control systems.
Because they remain cold, mirrors and lenses in space telescopes are among the first surfaces where volatile compounds outgassed from a spacecraft’s materials will condense, leading to fogging. Photo courtesy NASA Goddard Space Flight Center
Growing Demand
Varma notes that demand is surging in aerospace applications such as autonomous drones, commercial space ventures, low-earth-orbit satellite constellations, CubeSats, electric vertical take-off and landing (eVTOL) aircraft, and space-based optical communications. “As systems grow smaller, more integrated, and more sensitive to contamination, low-outgassing performance is becoming increasingly essential,” she observes.
“Low-outgassing adhesives are needed across a wide range of applications and industries,” says Oliver Matyssek, product manager for hard disk drive and wafer-level micro-optics at DELO Industrial Adhesives. “Optical modules—including cameras, lidar sensors, and diode lasers—require low-outgassing adhesives to substantially reduce condensable outgassing on critical optical components.”
“Hard disk drives call for even tighter outgassing specifications to prevent any contamination within the extremely narrow gaps between the read-and-write heads and the spinning disks,” Matyssek adds.
“Developing low-outgassing adhesives hinges on three core formulation strategies,” Matyssek explains. “First, manufacturers choose raw materials with minimal volatile content and high-molecular-weight polymers that inherently resist vapor release. To accomplish this, catalyst systems and crosslinking density must be carefully managed to lock potential volatile compounds within the cured network.”
“Unreacted monomers are kept to a minimum by incorporating molecules with multiple, highly reactive functional groups that are more likely to become part of the network during curing,” Matyssek continues. “However, these requirements often clash with other adhesive properties such as low viscosity or high flexibility, posing significant formulation challenges that demand careful balancing and optimization.”
Low-outgassing adhesives are purposefully engineered to limit the release of volatile compounds both during curing and over the product’s entire service life.
“The key lies in cutting down the amount of low-molecular-weight components that can evaporate or migrate when exposed to heat or vacuum,” says Stephan Pröller, business development manager at Hoenle Adhesives GmbH. “Several formulation factors play a role, including high-purity raw materials with low residual monomer levels; optimized photoinitiator and catalyst systems; carefully controlled additive packages; and high crosslink density after curing.”
“Curing behavior matters just as much,” Pröller points out. “Even a well-designed adhesive can show elevated outgassing if curing is incomplete and reactive species remain trapped in the material.”
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The two newest low-outgassing adhesives are Vitralit E-1672 and Vitralit UH 1640. The former works best for active alignment tasks, while the latter is tailored for lens bonding in products like cameras, microscopes, and telescopes.
Low-outgassing adhesives help stop optical lenses from fogging up. Illustration courtesy Hoenle Adhesives GmbH
Standardized Testing
Outgassing can be measured using established test methods. ASTM E595 is the main standard, focusing on total mass loss and volatile condensable substances under specific vacuum and heat conditions.
For detailed breakdowns of which specific chemicals are emitted in what quantities, gas chromatography paired with mass spectrometry is typically used.
“Outgassing tests for NASA and ESA are usually carried out by independent testing facilities or aerospace qualification labs,” notes Pröller. “Meeting standard outgassing criteria is often just the first step. Many optical manufacturers also run fogging assessments or optical clarity checks.”
“How an adhesive cures has a big impact on its outgassing behavior,” Matyssek states. “Curing it partially or at too high a temperature can leave unreacted parts or cause breakdown products, both of which add to gas emissions.”
“On the other hand, applying higher heat can sometimes help eliminate trapped gases formed during curing,” Matyssek continues. “UV-curable adhesives have a clear benefit because they set quickly at room temperature, which limits both heat damage and volatile formation.”
DELO has introduced Photobond FB4151 for aircraft interior use. This UV/light-curable acrylate sets in seconds and delivers strong bonding (about 20 MPa) to many plastics and metals while staying flexible. Its outgassing levels are low, and it contains no solvents.
“Adhesives that meet NASA E595 and MIL-STD-883 Method 5011 and have low ions and minimal gas emission are crucial for assembling and protecting electronics in space and aerospace systems,” explains Virginia Hogan, Dymax Corp.’s senior business development manager for aerospace and energy.
Products listed under MAPTIS, like Dymax 9773, give engineers trusted choices for securing components, filling gaps, and encapsulating parts—all offering reliable results in vacuum conditions and high-stakes environments,” Hogan states.
According to Hogan, materials free of ions and low in outgassing are built to withstand demanding low-Earth-orbit missions and next-gen spacecraft.
“Keeping ion levels low helps avoid corrosion or tiny conductive crystal growth on circuit board traces and solder points, which could otherwise cause electronics to fail in critical missions.” Hogan says.
“Even small traces of contaminant ions—like chlorides, sodium, or potassium—can pull in moisture or form pathways for electricity that lead to rusting or short circuits,” Hogan adds. “These residues become especially problematic in dry or vacuum-like settings, especially when systems face repeated temperature swings.”
Keeping ionic contamination at bay helps maintain electrical insulation strength and prevents failures tied to corrosion, making sure high-cost equipment like avionics, defense gear, and satellites last over time.
“UV-curable adhesives harden instantly when hit with ultraviolet or visible light, allowing exact application and immediate handling, which cuts down on mistakes and prevents parts from moving,” Hogan explains.
“On production lines dealing with sensitive parts like chip-scale packages, ball-grid arrays, or tiny-pitch components, adhesives need to stay put until fully cured,” Hogan notes.
“Thanks to its thickness, Dymax 9773 sticks right where it’s placed, even on sloped or vertical surfaces, resisting sagging for up to three days at 90°F before curing,” Hogan states.
“This well-controlled curing process cuts rework and speeds up production, making it perfect for reinforcing circuit boards or alternatives in aerospace electronics,” Hogan adds.
Components such as LiDAR sensors and other advanced driver-assistance systems require assembly using adhesives that produce minimal outgassing. Illustration courtesy DELO Industrial Adhesives
Common Mistakes
The best-known outgassing standard is ASTM E595, originally created by NASA for testing materials used in spacecraft and vacuum settings. It’s often called just the NASA outgassing test.
“ESA’s equivalent is ECSS-Q-ST-70-02C,” Pröller explains. “If more in-depth outgassing data is needed, ASTM E1559 is used, particularly for modeling spacecraft contamination.”
“Some engineers think passing a general outgassing test means the material will work everywhere,” remarks Pröller. “Passing ASTM E595 or ECSS is helpful, but these tests are just initial screens.”
“Real-world uses can involve different heat levels, long usage times, intense light power, humidity, or placement close to delicate optics,” Pröller adds. “Even materials that meet standard thresholds may need extra testing in their specific optical setup.”
Pröller also notes that engineers often only focus on the glue itself, ignoring how important curing quality and process cleanliness are. But the curing method greatly affects outgassing.
Engineers should keep these three steps in mind:
- Improve the curing process. “Getting full cure is vital,” emphasizes Pröller. “Making sure you get the right UV dose, proper light exposure angles, thermal post-cure, and validating each step helps reduce leftover volatiles.”
- Use just enough adhesive. “Sticking to the minimum required amount lowers the total volatile content in the system,” advises Pröller.
- Choose bake-out steps when suitable. “In top-tier optical or aerospace builds, sometimes parts with cured glue go through controlled bake-out cycles before putting everything together,” Pröller adds.
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