Outgassing — the emission of volatile chemical species from a cured adhesive into the surrounding environment — is a concern that engineers sometimes overlook when specifying epoxy for enclosed medical device assemblies. The visible properties of the cured adhesive look right: it is hard, non-tacky, correctly colored, and dimensionally stable. But invisible to visual inspection, volatile compounds continue to desorb from the cured polymer network for hours, days, or weeks after cure, filling the enclosed space with trace quantities of organic vapors and water vapor. In open assemblies ventilated to ambient air, these vapors dissipate harmlessly. In enclosed assemblies — sealed housings, hermetically packaged components, confined spaces with optical elements or sensitive sensors — the accumulated volatiles cause corrosion, contamination, fogging, or biological effects that standard adhesive property specifications do not predict.
What Outgasses from Cured Epoxy
The volatile species that emit from cured epoxy after cure include: residual unreacted monomers and hardener components that did not fully react during cure, diluents and plasticizers incorporated in the formulation, reaction byproducts generated during the cure chemistry, antioxidants and stabilizers that are volatile at elevated temperatures, and water absorbed from the environment during or after cure.
Residual monomer outgassing is proportional to the degree of cure — the fraction of reactive groups that completed the crosslinking reaction. Fully cured adhesive (95 percent or greater conversion) has much lower residual monomer content than partially cured adhesive. Post-cure at elevated temperature is the most effective way to reduce residual monomer outgassing: the additional heat drives the cure reaction further toward completion, consuming the reactive species that would otherwise outgas in service.
Diluents — low-molecular-weight chemicals added to reduce adhesive viscosity for application — are often the largest contributors to outgassing if they do not fully react into the network. Reactive diluents (those with epoxide or amine groups that participate in the crosslinking reaction) become part of the cured network and do not outgas. Non-reactive diluents remain mobile within the network and continue to desorb throughout the adhesive’s service life.
Water vapor is a significant outgassing component for any epoxy stored or processed in a humid environment. Absorbed moisture in the adhesive at the time of cure produces bubbles in thick sections (if the cure is too fast), remains in the cured network, and releases over time. Low-humidity storage of the adhesive before use and a controlled bonding environment minimizes absorbed moisture entering the product.
Why Outgassing Matters in Enclosed Medical Device Assemblies
In sealed electronic packaging inside medical devices — ASIC packages, pressure sensor assemblies, optical module enclosures — volatile organic compounds from adhesive outgassing accumulate in the sealed gas volume. At sufficient concentrations, these vapors condense on cold surfaces, depositing organic films that contaminate contact surfaces, increase contact resistance, or block optical paths.
Optical fogging is a visible failure mode from adhesive outgassing in sealed optical assemblies. Volatile adhesive components condense on the cooler optical surfaces — lenses, windows, detector surfaces — and deposit a thin film that scatters and absorbs light, reducing signal intensity and changing calibration. In a sealed diagnostic instrument with optical elements bonded near the adhesive, fogging may not appear for weeks or months after assembly, and then progressively degrades instrument performance without a visible cause.
Contact corrosion in hermetically sealed electronic packages occurs when outgassed halide-containing compounds — residual catalysts, hardener reaction byproducts — deposit on metal surfaces. Even sub-ppm concentrations of halide vapors in a sealed package contribute to corrosion of fine gold or aluminum bond wire, contact pads, and metal-to-ceramic seals over years of service.
Biological impact of outgassed species is relevant for devices where the enclosed space eventually opens to a fluid pathway or the patient environment. EtO-sterilized devices accumulate residual EtO and its reaction products within enclosed spaces; similarly, other volatile adhesive components in a sealed space that is later opened to patient contact contribute to the chemical environment the patient is exposed to.
For low-outgassing medical epoxy formulations with ASTM E595 TML and CVCM data for enclosed device applications, Email Us — Incure can provide outgassing characterization data matched to your enclosed assembly requirements.
Quantifying Outgassing: ASTM E595 and Other Methods
ASTM E595 is the standard test for measuring total mass loss (TML) and collected volatile condensable material (CVCM) from materials heated under vacuum. The test heats a sample at 125°C for 24 hours in a vacuum chamber, collecting condensed volatile material on a cold plate above the sample. TML is the fraction of the original sample mass lost; CVCM is the fraction that condenses on the collector plate.
Standard requirements for low-outgassing materials in sensitive applications (originally developed for spacecraft materials) specify TML below 1.0 percent and CVCM below 0.1 percent. For medical device assemblies with optical or electronic sensitive components in enclosed spaces, the same standards are often applied as a material qualification criterion.
Thermal desorption GC-MS provides a more detailed analysis: it identifies the specific volatile compounds emitting from the adhesive at service temperatures, allowing targeted evaluation of whether the identified species are problematic for the specific enclosed environment (optical, electronic, or biological).
Water vapor regain (WVR) — the fraction of mass loss attributable to absorbed moisture rather than organic volatiles — is a complementary measurement that characterizes how much the adhesive outgasses water under test conditions. High WVR indicates high moisture content in the as-tested material; for enclosed assemblies, this moisture release contributes to humidity buildup and condensation.
Minimizing Outgassing in Production
Post-cure bake-out before final closure of sealed assemblies is the most effective production technique for minimizing outgassing from enclosed adhesive bonds. After the adhesive is cured and before the assembly is hermetically sealed or closed, a bake-out at 100°C to 120°C under low humidity or vacuum for several hours drives off the residual volatiles that would otherwise outgas in service.
The bake-out temperature must be compatible with all other materials in the partially assembled device — it must not exceed the temperature tolerance of any component already in the assembly. If the bake-out temperature must be lower than ideal because of temperature-sensitive components, a longer bake-out time at lower temperature achieves most of the volatile reduction, though less efficiently than a higher temperature bake.
Contact Our Team to discuss low-outgassing medical-grade epoxy selection, ASTM E595 data, bake-out protocols, and post-cure procedures for enclosed medical device assemblies with optical, electronic, or patient-contact sensitivity.
Visit www.incurelab.com for more information.