UV-curable adhesives
Light delivery, cure depth, oxygen inhibition and shadow cure, with bounded LOCTITE AA 3494 and Permabond 130UV examples.
Tags: consumables, adhesives, assembly
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Henkel LOCTITE AA 3494 fixtures on glass slides in ≤10 s with a Zeta 7500 at 6 mW/cm², and <5 s with a D bulb at 50 mW/cm², both measured at 365 nm. At 100 mW/cm² measured at 365 nm, the separate surface tack-free test gives 45–60 s with mercury arc versus 150–180 s with a D bulb. Verify source spectrum, bondline light and actual cure. Henkel, LOCTITE AA 3494 TDS, p. 1
Light initiates a product-specific reaction
Light initiates polymerization in the illuminated adhesive. Match wavelength and intensity to the product. Some grades respond to visible light or include a secondary cure. Permabond, UV and visible-light adhesives
Light curing can accelerate assembly, but a short exposure time is not an independent material property. Lamp spectrum, light delivered to the joint, exposure duration, bond geometry and adhesive formulation form one process. Changing the lamp or a transparent component can change the cure even when the product name and timer setting remain unchanged. Permabond, Curing UV adhesive.
Wavelength, irradiance and dose
| Quantity | Meaning at the process boundary |
|---|---|
| Wavelength or spectrum, nm | Which light reaches and can excite the formulation's photoinitiator system |
| Irradiance, mW/cm² or W/cm² | Incident radiant power per area at the measurement position |
| Exposure time, s | Duration of illumination |
| Radiant exposure, J/cm² | Time integral of irradiance, often called dose in curing practice |
For constant irradiance, H = E × t. An assumed 100 mW/cm² for 10 s gives 1 J/cm². Check the wavelength band, dose at the joint and formulation response before comparing sources. Opsytec, Selecting UV-curing adhesives, coatings and potting compounds, wavelength, irradiance and dose discussion
Equal dose does not guarantee equal results. Dymax describes minimum irradiance requirements for overcoming surface inhibition: a low irradiance applied longer can deliver the same integrated energy while leaving a tacky surface. A dose target therefore does not erase irradiance or spectral requirements. Dymax, How to overcome oxygen inhibition in light-curable adhesives.
The substrate is part of the optical path
The relevant light is what reaches the adhesive after passing through the assembly. A visibly clear plastic can block a useful UV band; a compatible visible-light-curing formulation may offer another route. Dymax's technology guide distinguishes UV, visible and combined UV/visible curing and describes UV-blocked substrates. Visible appearance alone does not establish transmission at the cure wavelength. Dymax, Guide to selecting and using light-curing materials, UV/visible-light discussion.
The light nearest the source can cure material before deeper regions receive enough radiation. Bondline thickness, absorbance, pigment, tint and part geometry consequently matter. A solid skin is not proof that a deep potting volume is cured. An opaque insert or overlapping metal feature can create a shadow regardless of exposure on the surrounding surface. Dymax, Determining when full adhesive cure is accomplished; Permabond, cure-process variables.
Surface cure and bulk cure are different
In susceptible free-radical formulations, oxygen can interfere with polymerization at the air interface and leave partially reacted material. Dymax describes this as oxygen inhibition. Its severity depends on the formulation, irradiance, spectrum and exposure. Some bonding formulations intended for enclosed joints may not achieve a tack-free exposed surface under the same process. Dymax, Oxygen inhibition.
Shorter-wavelength output can favor surface cure while longer-wavelength output can favor penetration in a compatible formulation. That is a process tradeoff, not a universal instruction to select the shortest wavelength. A broad-spectrum lamp and a narrow-band LED are not interchangeable merely because both have output near 365 nm. Tack-free time also differs from time to complete cure. Permabond, Curing UV adhesive; Dymax, surface-cure limitations.
Named example: LOCTITE AA 3494
The current Henkel PDF carries a January 2015 technical header. It identifies modified acrylic, one-component UV/visible-light adhesive and defines fixture as reaching 0.1 N/mm² shear strength. Its glass-microscope-slide fixture results and exposed-surface tack-free results are separate tests:
| Published test | Reported time |
|---|---|
| Fixture, Zeta 7500 black light, 6 mW/cm² measured at 365 nm | ≤10 s |
| Fixture, electrodeless D bulb, 50 mW/cm² measured at 365 nm | <5 s |
| Tack-free surface, mercury arc, 100 mW/cm² measured at 365 nm | 45–60 s |
| Tack-free surface, electrodeless D bulb at that same measured irradiance | 150–180 s |
A single-band irradiance reading leaves the rest of the lamp spectrum unspecified. Qualify the actual lamp and joint. Henkel, LOCTITE AA 3494 TDS, p. 1
Henkel's adhesive-application directions specify at least 40 mW/cm² at the bondline and exposure of five to six times the fixture time at that irradiance. They also call for cooling temperature-sensitive substrates and allowing bonds to cool before service loads. These are named-product instructions, not limits for every UV adhesive. AA 3494 TDS, p. 3.
Secondary cure must be an explicit product feature
Increasing external exposure cannot deliver photons through an opaque barrier. A shadowed joint needs a supported secondary mechanism or a geometry that provides optical access. “Dual cure” must identify the mechanism and its own conditions; it does not mean that all hidden adhesive cures as quickly as illuminated material. Opsytec, single- versus dual-cure selection.
Permabond 130UV, in its provisional TDS Revision 4, August 6, 2025, is a light-curable ethyl cyanoacrylate with moisture cure for small shadow areas. It lists tack-free times of ≤1 s at 150 mW/cm² and ≤5 s at 25 mW/cm², both using a 405 nm spot LED. Its tabulated shear strengths, however, are based on 24-hour moisture cure only. The light exposure and mechanical-test preparation must not be combined into an invented instantaneous strength claim. Permabond 130UV TDS, p. 1.
Establishing a repeatable process
Measure light with an appropriate radiometer and preserve the measurement position, spectral band, distance and exposure conditions. Confirm cure in representative parts, including the least illuminated region. Dymax describes comparing measured properties, such as hardness development, with the product's expected behavior and building margin into the exposure process. Surface appearance alone cannot replace that assessment. Dymax, Determining when full adhesive cure is accomplished.
Prepare clean, compatible substrates and hold the intended gap during cure. Include fixtures, dispensing variation and substrate transmission in the process definition. The adhesive and lamp must be selected together; a published lap-shear value is not an allowable load for an arbitrary assembly. Permabond, UV adhesive selection and process overview.
Protect uncured adhesive from unintended room light and follow its own storage instructions. Permabond 130UV, for example, specifies 2–7°C storage and protection from room lighting. Light exposure during handling and the secondary moisture mechanism both belong in its process planning. Handling precautions for the adhesive and operation of the curing source come from their respective safety documents. No universal dose, cure depth, service temperature or lifetime is established here. Permabond 130UV TDS, p. 2.