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Low Outgassing Adhesive
Introduction to Low Outgassing Adhesive
Low outgassing adhesive is a critical component for high-altitude Unmanned Aerial Vehicles (UAVs), unmanned systems operating in a vacuum, or platforms equipped with highly sensitive payloads. These adhesives are designed to minimize the release of Volatile Organic Compounds (VOCs) and condensable species that can compromise sensitive electronics and optics.
The primary driver for specifying low outgassing materials is preventing molecular contamination. In aerospace and defense engineering, two metrics and one standard rule this specification:
- Total Mass Loss (TML): The percentage of the material’s original mass that is lost as volatile components when heated under vacuum.
- Collected Volatile Condensable Material (CVCM): The percentage of the lost mass (TML) that subsequently condenses onto a nearby, cooler surface. This is the critical factor for optical and sensor contamination.
For a material to be considered “low outgassing” or “NASA approved” for most high-reliability applications, it must typically pass the ASTM E595 test standard, requiring: TML ≤ 1.0% and CVCM ≤ 0.10%.
Developers should request the ASTM E595 certificate from the adhesive manufacturer. For stringent optical systems, some platforms require a CVCM as low as 0.05% or better.
Low outgassing adhesive from Dymax, the 9801.
Applications of Low Outgassing Adhesives in Unmanned Systems
The material choice is dictated by the component’s sensitivity, thermal profile, and structural requirements.
Optical Assemblies and Imaging Payloads
For Electro-Optical/Infrared (EO/IR) payloads, hyperspectral cameras, and laser rangefinders, contamination is an immediate performance killer. Optical adhesives must maintain radiometric accuracy and focal stability by preventing molecular film buildup on lenses or detector arrays. They must also manage the thermal expansion mismatch between metals, glass, and composite housings.
Low outgassing UV cure adhesives are favored for rapid bonding of lenses, fiber optics, and prisms due to their fast cure times and ability to be formulated as low viscosity adhesives that wick into tight gaps without excessive heat. Low outgassing silicone adhesives are used for flexible damping and strain relief around detector arrays to accommodate Coefficient of Thermal Expansion (CTE) mismatch.
Electronics Assembly and Avionics Bays
Adhesives are essential for securing components against the intense vibration and thermal cycling experienced by high-performance UAVs and UGVs, and they must provide predictable dimensional stability along with stable electrical and thermal properties.
Low outgassing epoxy adhesives (one- and two-part), such as PCB adhesive, are the workhorse for component staking, underfill, and heat sink attachment, offering superior mechanical strength and Glass Transition Temperature (Tg) stability. Low outgassing electrically conductive adhesives (typically silver-filled epoxies) are crucial for establishing EMI shielding continuity and grounding sensitive sensors, often serving as a reliable alternative to traditional soldering in delicate assemblies.
Structural Bonding and Airframes
While contamination within the structural laminate is less common, the bond lines near avionics bays or internal cavities must adhere to outgassing standards. They must provide high mechanical strength, resistance to fatigue, and minimal volatile release into adjacent payload bays.
Two-part low outgassing epoxy adhesives offer the high bond strength and durability needed for composite airframes, radomes, and control surfaces. Their minimal volatile release protects the sensitive electronics integrated into or near these structures.
Types of Low Outgassing Adhesive & Selection Considerations
| Adhesive Type | Key Benefit | Typical Application in UxS | Critical Consideration |
| Epoxy (Low Outgassing) | Highest mechanical strength, chemical resistance, high Tg. | Structural joints, component staking, heat sink bonding. | Requires heat cure (thermal stress); ensure full cure to meet TML/CVCM. |
| UV Cure (Low Outgassing) | Very fast cure speed, low shrinkage, precise dispensing. | Optical bonding, wire tacking, temporary fixturing. | Limited cure depth; must use dual-cure or thermal post-cure for shadowed areas. |
| Silicone (Low Outgassing) | Flexibility, wide thermal range, shock/vibration damping. | CTE mismatch relief, sensor potting/encapsulation. | Must be formulated to minimize siloxane migration (a common optical contaminant). |
| Electrically Conductive | Electrical grounding and continuity (EMI/RFI shielding). | RF component mounting, grounding plane connections. | Maintain stable conductivity under thermal stress and humidity. |
| Adhesive Tape/Film | Clean application, uniform thickness, reworkability. | Wiring harness securement, thermal interface layers. | Select polyimide or specialty acrylic/silicone variants for low outgassing performance. |
Engineering Considerations for Silicones & Ionic Purity
When considering low outgassing silicone adhesive, exercise caution. Standard, general-purpose silicones are notorious for releasing siloxane molecules, which readily condense onto and damage optics and sensitive electronic contacts. Always specify:
- Non-migratory siloxane content: Formulations specifically engineered to prevent the release of volatile siloxanes.
- Controlled Ionic Purity: For electronics assembly, adhesives must have low levels of mobile ions (Chlorine, Sodium, Potassium). High ion content can lead to corrosion and electrical leakage, particularly in humid environments.
By rigorously applying the ASTM E595 standard and selecting chemistries optimized for minimal volatile release, engineers ensure that unmanned systems maintain peak performance, even in the most demanding high-vacuum and high-temperature environments.
