In a sealed optical package or a vacuum system, an adhesive that releases trace volatiles will fog a lens or contaminate a detector months after assembly. When the substrate is plastic, which brings its own volatile load and adhesion challenges, a low-outgassing epoxy formulated for the job is the difference between a device that holds spec and one that drifts.

This article explains what outgassing is, how it is measured, and how to bond plastics with a low-outgassing epoxy.

What outgassing is and where it bites

Outgassing is the release of trapped gas, moisture, or unreacted monomer from a cured material, accelerated by vacuum, heat, or time. Every polymer does it to some degree. The problem starts when those volatiles condense on a nearby critical surface:

• Optical fogging. Condensate on a lens, window, or sensor face forms a haze that scatters and absorbs light.
• Electronic contamination. Deposits on contacts and traces cause leakage, corrosion, or intermittent faults.
• Vacuum system load. In evacuated instruments, outgassing raises base pressure and slows pump-down.
• Cosmetic and odor defects. Visible residue or smell on consumer products.

How it is measured

The standard reference is ASTM E595, which conditions a sample under vacuum at 125 C and measures Total Mass Loss (TML) and Collected Volatile Condensable Material (CVCM). The common acceptance thresholds are TML below 1.0 percent and CVCM below 0.1 percent. An epoxy meeting those limits is considered low outgassing. Specify the standard by name and confirm test data for the exact grade and cure schedule you plan to use.

Why plastics make it harder

Bonding plastics adds two complications on top of the outgassing requirement:

Low surface energy. Polyolefins such as polyethylene and polypropylene sit near 30 mN/m and resist wetting. Even easier plastics like ABS and polycarbonate need clean, sometimes activated surfaces.

Migrating additives. Plasticizers, mold-release agents, and slip additives move to the plastic surface over time. They interfere with adhesion and can themselves contribute to the volatile load of the finished assembly.

A large coefficient-of-thermal-expansion gap between plastic, adhesive, and any mating material also stresses the bond line under temperature change, the mechanism covered in how CTE mismatch causes adhesive bond failure.

Selecting and applying a low-outgassing epoxy for plastics

Grade selection. Choose an epoxy with published ASTM E595 data and adhesion suitable for your plastic. Toughened or semi-flexible formulations absorb CTE stress; rigid grades give higher modulus where the assembly is dimensionally stable. Where cure speed matters and the joint is accessible, a UV-curable adhesive with low-outgassing data may fit.

Surface preparation. Clean thoroughly with an appropriate solvent to remove release agents and oils. For low-energy plastics, add plasma or corona treatment to raise surface energy and create bonding sites. Handle prepared parts with clean gloves.

Cure completely. Incomplete cure is the largest single source of outgassing. Follow the recommended schedule exactly, using calibrated ovens for thermal cure or a radiometer-verified UV dose for UV grades. A post-cure bake can drive residual volatiles out before the assembly is sealed.

Validate against end use. Beyond the standard test, simulate the real environment, its temperature, vacuum level, and duration, and check the critical surfaces for condensate.

Store correctly. Keep epoxies within their specified storage conditions so the material does not degrade and raise its volatile content before use.

Incure supplies low-outgassing epoxies with ASTM E595 data and supports manufacturers on plastic surface prep, cure validation, and post-cure protocols. Email Us with your substrate and the sensitive surfaces you need to protect.

The role of post-cure

Even a fully reacted epoxy holds a small population of unreacted chain ends and absorbed moisture. A post-cure, holding the bonded assembly at an elevated temperature, often 60 to 100 C, for several hours after the primary cure, drives that residual volatile fraction out before the part is sealed into its final package. For assemblies destined for vacuum or sealed optical service, a post-cure and a vacuum bake step frequently make the difference between passing and failing an outgassing acceptance test. The trade-off is process time and the need for components that tolerate the bake, so it is specified where the contamination risk justifies it.

Which plastics bond cleanly

Low-outgassing epoxies bond well to polycarbonate, acrylic, ABS, and glass-filled engineering plastics such as PEEK and polyphthalamide after a solvent clean and light abrasion. Polycarbonate and acrylic need a solvent the plastic tolerates, since aggressive solvents craze them and the microcracks then trap and later release volatiles. Polyolefins, PTFE, and acetal need plasma or flame activation first; without it the bond is weak and the interface itself can become a leak path for outgassed species from the plastic bulk. Flexible PVC is a poor candidate because its plasticizer migrates continuously and adds to the assembly’s volatile load no matter how well the epoxy is cured.

Clean cure, clean assembly

A low-outgassing epoxy protects sensitive optics and electronics only when it is fully cured on a properly prepared plastic surface. The grade selection, the surface treatment, and the cure schedule all have to hold together, and the finished assembly has to be checked against its actual operating conditions.

Contact Our Team to discuss low-outgassing bonding for your plastic assemblies.

Visit www.incurelab.com for more information.