An adhesive that performs without issue at room temperature can become a contamination source the moment it is exposed to elevated temperature. Outgassing — the release of volatile compounds from a cured adhesive — is an often-overlooked failure mode that affects not only the adhesive bond itself but also sensitive components nearby. In electronics, optics, aerospace, and precision instruments, outgassing from adhesive systems can render an entire assembly nonfunctional.

What Outgassing Is

Outgassing refers to the release of gases or volatile organic compounds (VOCs) from a material under thermal or vacuum conditions. In cured adhesive systems, these volatiles originate from several sources:

• Residual unreacted monomers and solvents left over from incomplete cure
• Low-molecular-weight plasticizers that migrate out of the polymer matrix under heat
• Degradation byproducts produced when the polymer backbone or additives degrade at elevated temperatures
• Absorbed moisture that is driven off when the assembly is heated
• Processing aids such as release agents, mold lubricants, or reactive diluents that were not fully incorporated into the network

At room temperature, these species have low vapor pressure and remain trapped in the adhesive. As temperature rises, vapor pressure increases, diffusion rates accelerate, and volatile compounds migrate to the surface and enter the surrounding environment.

Why Outgassing Is Problematic

Contamination of Sensitive Surfaces

In optical assemblies, outgassing deposits thin films on lenses, mirrors, or sensors. These deposits scatter light, alter refractive properties, and can reduce optical throughput significantly. In high-power laser applications, even trace contamination can cause localized heating and catastrophic damage to optical surfaces.

In electronics, condensed outgassing products can coat connector contacts, printed circuit board traces, or sensor surfaces. Depending on the chemistry, these films may be insulating (causing contact resistance failure) or slightly conductive (causing leakage current or short circuit risks).

Bond Line Void Formation

When volatiles form within the adhesive bulk during cure or during service at elevated temperature, they can create voids in the bond line. These voids reduce the effective bonded area, concentrate stress at void boundaries, and provide pathways for moisture ingress. In adhesives that must form hermetic seals, internal void formation directly defeats the sealing function.

Mass Loss and Bond Degradation

Significant outgassing depletes the adhesive of plasticizers or low-molecular-weight network components that contribute to flexibility and toughness. As these components are lost, the adhesive becomes stiffer, more brittle, and more prone to cracking during thermal cycling.

Pressure Buildup in Sealed Assemblies

In hermetically sealed housings or encapsulated electronic modules, outgassing releases gas into a fixed volume. If the amount of outgassed material is significant, the resulting pressure increase can mechanically stress seals, lids, and enclosures. In extreme cases, it causes delamination or container rupture.

Email Us to discuss outgassing requirements and low-outgassing adhesive options for your application.

Measuring Outgassing

The standard test method widely referenced in aerospace and electronics is ASTM E595, developed by NASA. It measures:

• TML (Total Mass Loss): the percentage of initial mass lost by a sample after 24 hours in a vacuum at 125°C
• CVCM (Collected Volatile Condensable Materials): the percentage of mass that condenses on a collector plate held at 25°C

For space applications, accepted limits are typically TML ≤ 1.0% and CVCM ≤ 0.10%. Commercial electronics and precision instrument applications use these same figures as a common benchmark, though application-specific limits may differ.

Other relevant test methods include:

• ASTM E1559: a dynamic outgassing test that measures the rate of outgassing over time, useful for understanding how outgassing evolves during cure and service
• ISO 22007-4: thermal analysis methods that can characterize mass loss as a function of temperature and atmosphere
• Gas chromatography-mass spectrometry (GC-MS): identifies the specific chemical species outgassing from a material, essential for contamination analysis

Adhesive Chemistries and Their Outgassing Profiles

Not all adhesive chemistries outgas equally. Several factors influence outgassing potential:

Cure Chemistry and Completeness

Adhesives that cure by addition reactions without byproduct release (such as most epoxy and silicone systems) have lower inherent outgassing potential than condensation-cure systems (such as some silicones and polybenzoxazines), which release water or alcohol as cure byproducts. Incomplete cure dramatically increases outgassing because unreacted components remain in the film and release when heated.

Silicone Adhesives

Silicone adhesives are often specified in sensitive applications because of their thermal stability. However, some silicone formulations outgas siloxane oligomers (cyclic siloxanes such as D4 and D5) that condense on surfaces and interfere with adhesion, optical coatings, and electrical contacts. Low-outgassing silicone formulations specifically control siloxane oligomer content.

Epoxy Adhesives

Epoxy adhesives, when fully cured, have low outgassing from the base network. Problems arise from uncured reactive diluents, amine blush (surface reactions producing carbamate species), and volatile additives. Post-cure at elevated temperature drives off residual volatiles and is an effective strategy for reducing epoxy outgassing.

Acrylic and Cyanoacrylate Adhesives

These systems can have significant residual monomer content if not fully cured. Residual acrylate monomers are among the more volatile and odorous outgassing species from adhesive systems. High-temperature service of incompletely cured acrylic adhesives often reveals their outgassing potential.

Strategies for Minimizing Outgassing

Pre-Bake Before Assembly

Baking the cured adhesive before final assembly drives off volatile residuals in a controlled environment, so they do not outgas later onto sensitive surfaces. This is standard practice in aerospace assembly.

Use Post-Cure Steps

Following the full post-cure procedure — including any elevated temperature hold specified by the manufacturer — maximizes cure conversion and minimizes residual reactive species. Skipping post-cure leaves a larger volatile inventory available to outgas in service.

Select Low-Outgassing Formulations

Use adhesives specifically characterized and qualified to low-outgassing specifications. Ask for ASTM E595 or equivalent test data from the manufacturer. Do not assume that general-purpose adhesives meet outgassing requirements for sensitive applications.

Control the Thermal Environment

Moderate operating temperatures reduce outgassing rates even for materials that were not selected specifically for low outgassing. If the assembly can be designed to limit adhesive temperature, the outgassing flux will be correspondingly lower.

Incure’s Low-Outgassing Adhesive Range

Incure offers adhesive formulations characterized for outgassing performance at elevated temperatures. Products intended for electronics, optics, and other contamination-sensitive applications are tested to ASTM E595 or equivalent methods, with data available to support qualification processes.

Contact Our Team to review outgassing test data and identify the appropriate low-outgassing Incure adhesive for your assembly.

Conclusion

Outgassing from high-temperature adhesive systems is a real contamination and degradation risk in electronics, optics, aerospace, and precision manufacturing. It originates from residual volatiles, plasticizer loss, degradation byproducts, and moisture. Selecting adhesives with validated low-outgassing performance, completing full cure procedures, and pre-baking assemblies before final sealing are the practical measures that keep adhesive outgassing from becoming a field failure mechanism.

Visit www.incurelab.com for more information.