Bubbles forming in curing epoxy are a visible sign that something has gone wrong in the mixing or application process. For cosmetic epoxy applications, bubbles ruin the appearance. For structural bonds and potting applications, bubbles create voids that reduce effective bond area, concentrate stress, and compromise moisture and electrical isolation. The frustrating aspect of bubble formation is that it is frequently attributed to “the epoxy bubbling” — as if the material is inherently defective — when the actual cause is almost always a process issue that can be identified and corrected. Understanding the mechanism behind each type of bubble formation leads directly to the corrective action.
Cause 1: Air Entrapped During Mixing
The most common source of bubbles in cured epoxy is air introduced during the mixing of the two components. When resin and hardener are combined and stirred, the stirring action folds air into the mixture. Vigorous mechanical mixing — a high-speed drill-mounted mixer, aggressive manual stirring, or any technique that creates a vortex or froth — entraps far more air than gentle hand-mixing.
The entrapped air is distributed as fine bubbles throughout the mixed material. If the epoxy has low viscosity or adequate pot life, these bubbles rise and escape from the surface before gel time. If the viscosity is high, pot life is short, or the poured layer is deep, the bubbles are immobilized before they can escape.
Mixing gently, with a flat paddle stirrer and a scraping motion rather than a whipping motion, avoids creating a vortex or froth in the first place. Letting the mixed epoxy rest in the mixing container for two to five minutes before pouring allows bubbles to rise and pop at the surface before the material is transferred to the mold or housing.
Cause 2: Moisture Reacting with the Hardener
Some epoxy hardeners — particularly amine hardeners — react with atmospheric moisture. This reaction can produce CO₂ as a byproduct, which forms bubbles within the curing epoxy. The reaction is more pronounced at higher humidity and is characteristic of specific hardener types (benzyl dimethyl amine and other tertiary amines are known for this behavior).
Moisture on the substrate surface can also react with the adhesive at the bond line, creating a thin layer of bubbles at the interface — a failure mode that looks like poor adhesion but originates from moisture reaction.
Using substrates that have been dried before bonding — particularly porous substrates like wood, ceramics, or foam that absorb ambient moisture — addresses this at the source. Storing hardener components in sealed containers minimizes moisture exposure, and for moisture-reactive hardener systems in high-humidity environments, switching to a formulation with lower moisture sensitivity or working in a humidity-controlled space closes the gap entirely.
If you need formulation guidance for high-humidity environments where moisture-reactive bubbling is a problem, Email Us — Incure can recommend moisture-tolerant epoxy systems for your application.
Cause 3: Outgassing from the Substrate
Porous or gas-absorbing substrates — foam, balsa wood, porous ceramics, green (uncured) concrete — release gas from their pore structure as epoxy wets the surface. The gas cannot escape downward through the substrate, so it migrates upward through the epoxy, forming bubbles that concentrate at the surface or within the adhesive layer.
Sealing porous substrates with a thin preliminary coat of low-viscosity epoxy before the structural application closes off this path: the sealing coat penetrates the surface pores and gels before the structural layer goes on, blocking the gas escape route. The seal coat needs to fully cure before the structural coat is applied. For potted electronic assemblies specifically, outgassing and trapped-air bubbles behave somewhat differently than in open coatings — see Incure’s guide to achieving a void-free epoxy potting fill in complex geometries for dispensing and vacuum techniques suited to dense assemblies.
Cause 4: Trapped Solvent or Volatile in the Epoxy
If the epoxy formulation contains solvents or reactive diluents that volatilize during cure, and the evaporation pathway is blocked by the enclosing geometry, solvent vapor forms bubbles within the curing mass. This mechanism is more common with solvent-containing primer formulations or solvent-modified epoxy systems than with fully formulated two-part structural adhesives.
Confirming that solvent-containing primers or coatings have fully dried before applying epoxy over them, allowing the specified flash time after solvent application, and avoiding confined geometries where vapor cannot escape all address this directly.
Cause 5: Exothermic Cure Creating Thermal Runaway
In deep pours of fast-cure epoxy, the exothermic heat of reaction can cause the interior temperature of the curing mass to rise dramatically. If the temperature rise is sufficient to volatilize trace solvents, to cause depolymerization at the surface, or to exceed the boiling point of any liquid constituent, vapor-phase bubbles form within the hot curing mass.
Limiting pour depth for fast-cure formulations, using slow-cure hardeners for large-volume applications, and spreading the mixed epoxy in thin layers — letting each layer gel and cool before adding the next — keeps the exotherm in check. A thermocouple in the curing mass for the first production run confirms whether temperature runaway is occurring — the same exothermic mechanism that Incure’s guide on how exotherm in potting compounds damages electronics covers in more detail for deep-pour electronic encapsulation.
Cause 6: Surface Tension at the Application Surface
When epoxy is applied as a coating and small bubbles rise to the surface and pop, they leave a small crater rather than a smooth surface. This is a surface tension effect: the bubble pops before the epoxy surface has gelled, and the resulting crater does not self-level before cure is complete.
For coating applications, passing a propane torch or heat gun briefly over the surface immediately after pouring reduces surface viscosity and lets the surface self-level before gel. The heat needs to stay brief — excessive heating causes yellowing or cures the surface non-uniformly.
Post-Application Bubble Removal
For potting applications where bubbles are visible in the freshly dispensed compound before gelation, brief vacuum exposure — placing the filled assembly in a vacuum chamber at low pressure for thirty to sixty seconds — draws bubbles to the surface where they pop. Returning to atmospheric pressure ensures the compound fills any voids left by escaped bubbles. This is the most reliable method for void elimination when mixing and dispensing technique alone is insufficient, and it applies directly to the high-temperature compounds discussed in Incure’s guide to potting electronics in high-temperature compound without voids.
Contact Our Team to discuss bubble elimination, formulation selection for low-bubble applications, and vacuum dispensing processes for your epoxy bonding or encapsulation application.
Visit www.incurelab.com for more information.