Voids in a cured adhesive bondline are sites where adhesive is absent — replaced by air, vapor, or gas. Each void in the bondline represents an absence of load transfer capability at that location and a stress concentration site at its boundary. Small, infrequent voids may have negligible effect on joint performance; a bondline with high void content or large voids fails well below its designed strength. Understanding how voids form during curing is the first step to preventing them.
Why Void Formation Matters
Voids in adhesive bondlines affect performance through two mechanisms. First, they reduce effective bond area. If the total void area is 10% of the bond area, the remaining 90% of intact adhesive carries the full applied load — average stress on the intact adhesive is 11% higher than the nominal design stress. For large void fractions, this effective area reduction alone can bring the joint below strength requirements.
Second, voids act as stress concentration sites. Circular voids in a stressed solid amplify local stress by a factor of approximately 3 (stress concentration factor Kt ≈ 3 for a circular hole in a uniaxial stress field). Under fatigue or impact loading, these high-stress zones initiate cracks that propagate through the surrounding adhesive, causing failure at loads well below what an equivalent void-free joint would require.
In environmental durability, voids provide internal reservoirs for moisture condensation and chemical accumulation. Voids connected to the joint edge allow moisture and corrosive species to penetrate deep into the bondline through the void network.
Sources of Void Formation During Cure
Entrapped Air During Application
The most common source of voids in production is air trapped during adhesive application and joint assembly. When an adhesive bead is dispensed and the joint is closed, air between adhesive islands must escape to the joint edges before the adhesive seals. If the adhesive advance front traps air pockets before they can escape — due to fast closing speed, irregular bead pattern, or high adhesive viscosity — those trapped air pockets become permanent voids in the cured joint.
Bead pattern design significantly affects air entrapment. A single central bead dispenses from one side of the joint and must push air ahead of it toward the edges. Multiple parallel beads can trap air between them when the beads merge. An X or asterisk pattern dispenses from the center outward, allowing air to escape radially. The pattern that minimizes air entrapment depends on joint geometry and assembly orientation.
Closing speed affects air expulsion. Slow, gradual joint closure allows air more time to escape before the adhesive seals. Rapid assembly of large joints is more likely to trap air.
Vacuum bonding eliminates air entrapment by evacuating the joint cavity before adhesive flow. For precision optical and electronic applications where any void is unacceptable, vacuum-assisted assembly is the standard approach.
Moisture and Volatile Outgassing
Absorbed moisture in the adhesive, substrates, or fillers becomes steam at elevated cure temperatures. If the cure temperature exceeds 100°C and moisture is present, steam bubbles form within the adhesive. High-boiling-point solvents retained in the adhesive from manufacturing also volatilize during cure, creating solvent vapor bubbles.
If these vapbles form and grow before the adhesive has gelled, they may rise to the surface and escape — leaving pinholes or surface irregularities but no internal voids. If they form after gelation, when the adhesive is no longer mobile, they create internal voids frozen in place by the rigid network.
Pre-drying adhesive and substrates before application eliminates absorbed moisture that would create steam voids. For moisture-sensitive substrates (wood, certain composites, hygroscopic plastics), oven drying before bonding is an important step that is often overlooked.
Cure Shrinkage Void Formation
During crosslink network formation, the adhesive contracts slightly as monomers pack more closely together than they were in the pre-cured liquid state. This cure shrinkage is typically 1–5% by volume for epoxy systems. In constrained bondlines, this shrinkage cannot freely contract the full adhesive volume; the substrate prevents contraction in the bondline plane. The adhesive must accommodate the shrinkage strain in the through-thickness direction, and if adhesion to one substrate is weaker than the cure shrinkage stress, a partial disbond or gap void forms at that interface.
This is particularly problematic in potting applications where thick adhesive layers cure in contact with low-adhesion substrates or release-coated surfaces. Voids form at the adhesive-mold surface interface as the adhesive pulls away during shrinkage.
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Carbon Dioxide Evolution from Chemical Reactions
In polyurethane adhesives, water reacts with isocyanate groups to produce CO₂ gas as a byproduct. If this reaction occurs in the bondline before gelation, CO₂ bubbles may rise to the surface. If moisture levels are high or if the adhesive is being cured in a humid environment, CO₂ generation can be substantial. In two-component urethane systems, off-ratio mixing (too much isocyanate relative to polyol) increases the isocyanate available to react with moisture and increases CO₂ generation.
Some epoxy cure chemistry also generates small amounts of volatile byproducts. Aminosilane hardeners at elevated temperatures can produce volatile amine fragments; anhydride systems can generate volatiles if cure temperature is too high.
Filler Porosity and Pre-mixed Voids
Adhesives formulated with hollow microballoon fillers (for weight reduction), certain porous mineral fillers, or gas-filled spacer particles incorporate internal porosity intentionally. The processing challenge is preventing fracture of these fillers during mixing and dispensing, which releases their gas content and creates voids in the adhesive matrix.
Two-part adhesives mixed with high-shear equipment can entrain air into the adhesive stream during mixing. Static mixers, if not properly sized for the adhesive viscosity, create turbulent mixing zones that entrain air. Vacuum mixing or low-shear mixing protocols reduce air entrainment during mixing.
Detection and Measurement of Voids
Ultrasonic C-scan is the standard non-destructive method for void detection in adhesive bondlines. Ultrasonic pulses transmitted through the bondline are reflected at air-adhesive interfaces; voids appear as bright spots in the C-scan image. Resolution depends on the ultrasonic frequency and the acoustic properties of the adhesive and substrates.
X-ray radiography detects voids in adhesives that have sufficient radiographic contrast with the adhesive and substrates.
Cross-section destructive examination cuts the joint and examines the cross-section under magnification, revealing internal void size, distribution, and location within the bondline.
Incure’s Void Control Solutions
Incure provides guidance on application patterns, assembly procedures, and cure profiles that minimize void formation for specific adhesive products. Products are formulated to minimize outgassing during cure and are available in viscosity grades suited to different void-control requirements.
Contact Our Team to discuss void formation prevention for your adhesive bonding process and identify Incure products and processing recommendations that minimize void content for your application.
Conclusion
Void formation during adhesive curing results from trapped air during assembly, moisture and volatile outgassing during cure, cure shrinkage void formation at weak interfaces, CO₂ evolution from polyurethane chemistry, and air entrainment during mixing. Voids reduce effective bond area and concentrate stress, degrading both short-term strength and long-term fatigue resistance. Preventing voids requires appropriate bead pattern design for air expulsion, pre-drying of moisture-sensitive materials, low-shrinkage adhesive selection, process controls for mixing and application, and vacuum-assisted assembly for the most demanding applications.
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