In vacuum systems and precision optics, a bonded joint can fail in a way that leaves the adhesive perfectly intact: volatile material bleeds off the curing epoxy and condenses on a cold mirror or sensor. Incure Epo-Weld™ HTE-5351 is a high-temperature epoxy formulated to keep that from happening.

What HTE-5351 is

Epo-Weld™ HTE-5351 is a two-component epoxy for structural bonding and potting in demanding environments. It holds properties across roughly -65°C to 205°C (-85°F to 400°F), resists a wide range of chemicals, and delivers strong mechanical performance. Its defining feature is compliance with NASA low-outgassing requirements, which makes it usable in vacuum chambers, space hardware, and optical assemblies where outgassed contamination would degrade performance.

The formulation also wets and bonds technical substrates well, including metals and optical materials such as glass and sapphire windows, when they are properly prepared.

Key properties and what they mean

• Low outgassing. In vacuum, a standard adhesive can release volatiles that condense on nearby cold surfaces, fogging optics and coating sensors. A low-outgassing grade is qualified against total mass loss and collected volatile condensable material limits.
• Wide temperature range. Properties hold from cold-soak conditions to a 205°C ceiling, covering the thermal swings seen in orbit and in vacuum process equipment.
• Chemical resistance. The cured matrix resists solvents, fuels, and dilute acids and bases.
• Strong mechanical bond. HTE-5351 provides the shear and tensile strength needed for structural mounts, not just sealing.

Where HTE-5351 fits

• Aerospace and defense: bonding structural brackets, optical benches, and sensor mounts on spacecraft and high-altitude platforms.
• Vacuum process equipment: bonding and potting components inside deposition, etch, and analytical chambers.
• Optical instrumentation: mounting windows, prisms, and filters where alignment must hold through temperature drift and outgassing must be controlled.
• Semiconductor manufacturing equipment: securing components in tools that operate under vacuum.
• Scientific instruments: bonding elements in spectrometers and detectors sensitive to surface contamination.

Bonding optical and dissimilar materials

Glass and sapphire are smooth, low-porosity surfaces, so mechanical keying is minimal and surface cleanliness carries the bond. Solvent-clean thoroughly and, where the joint will see moisture or thermal cycling, use a primer. When an optic is bonded into a metal cell, the expansion mismatch loads the joint at every temperature change and can distort the optic. Keep bond areas small and symmetric, control the bond line, and load the adhesive in shear. This is the same mechanism explained in how CTE mismatch causes adhesive bond failure.

If you are bonding an optic where wavefront distortion is a concern, Email Us with the mount geometry and temperature range.

Surface preparation, mixing, and cure

Degrease metals, abrade to fresh material, and wipe again before bonding. Clean optical surfaces with a lint-free wipe and a clean solvent. Meter the two parts at the specified ratio and mix until completely uniform, then let the mix de-air briefly before applying. HTE-5351 develops its full properties and its low-outgassing behavior through an elevated-temperature cure; follow the recommended ramp and hold, and add a vacuum bake-out afterward if the application requires it.

Failure modes and prevention

• Contamination on cold surfaces: incomplete cure or no post-bake. Verify the oven profile and add a bake-out.
• Optic distortion after cycling: asymmetric or oversized bond footprint. Use small, symmetric bond pads and control the gap.
• Weak adhesion to glass or sapphire: surface contamination. Improve cleaning and add a primer.
• Voids in a potted section: trapped air. Pour in thin passes and degas the mixed adhesive.

Understanding the outgassing specification

Low-outgassing grades are qualified against two numbers measured under vacuum at elevated temperature: total mass loss, which is how much material the cured adhesive gives up overall, and collected volatile condensable material, which is the fraction of that loss that will redeposit on a cold surface. The second number is the one that fogs an optic. A grade such as HTE-5351 is formulated and tested to stay under the widely used screening limits, but the result you get depends on completing the cure and, for the most sensitive applications, adding a vacuum bake-out afterward to drive off the last of the volatiles before the hardware sees a clean optical surface. An under-cured joint can pass a strength test and still fail outgassing.

Bonding thin optics without distortion

A window or filter bonded into a metal cell will bend if the bond shrinks unevenly during cure or if the mount pinches it as temperature changes. Use three small, symmetric bond pads rather than a continuous ring, keep each pad thin and equal in volume, and let the adhesive cure at a stable temperature. Where wavefront quality is critical, measure the optic before and after bonding to confirm the mount is not introducing figure error.

How HTE-5351 compares

Choose HTE-5351 when low outgassing is a requirement. For flexibility at high temperature, HTE-5354 is the better choice; for dedicated shock and vibration potting, HTE-5355. If you are also considering thin emissive coatings for hot metal surfaces, see the high emissive ceramic coatings. For a general comparison of adhesive options against other joining methods, see which adhesive is stronger for heavy-duty repairs.

Next steps

Match the grade to your outgassing spec, temperature range, and substrates. Incure’s technical team can review the application and recommend a grade and cure schedule. Contact Our Team to get started.

Visit www.incurelab.com for more information.