Quick answer
When an engineer selects a UV-curable adhesive or coating, the decision is driven by the final cured properties the application demands — bond strength, chemical and thermal resistance, flexibility, hardness, low cure shrinkage — not by cure speed. The UV source (lamp or LED plus delivered dose) is the enabler and constraint: it must spectrally match the formulation's photoinitiator and deliver enough energy to cure it through. Cure time is usually the least binding factor, because the source is sized to deliver the required dose — typically in the range of 250–1000 mJ/cm² for adhesives and coatings.
This article frames the choice the way engineers actually make it: properties first, UV source as the gate. Substrate-family deep-dives and a photoinitiator-selection guide come as follow-ups (marked (coming) below).
1. The selection hierarchy: properties first, source as the gate
- Functional requirement drives the material. What must the cured layer withstand or do (e.g. bond glass to metal that survives heat and solvents; a scratch-resistant clear coat; a flexible bond to plastic)? That requirement points to a substrate chemistry and formulation.
- The chosen material dictates a UV-source constraint — its photoinitiator defines the usable wavelength band, and its chemistry defines the dose needed for full cure.
- Throughput/time is tuned, not decisive — you reach the required dose by choosing source power and line speed (dose = irradiance × time). It rarely overrides a property requirement.
Practical consequence for tooling: an engineer benefits most from selecting substrates by end-property first, then checking UV-curability — not from starting at the cure curve.
2. End-property axes (the selection drivers)
- Hardness — reported as Shore A/D; rigid systems reach e.g. Shore A > 75 or Shore D > 60.
- Adhesion and the peel-vs-shear trade-off — densely cross-linked (rigid) systems give high shear strength plus chemical/thermal resistance but low peel strength; more flexible systems give high peel and impact strength and better adhesion to plastics, but less heat and chemical resistance.
- Chemical / thermal / moisture resistance — rises with cross-link density; urethane-acrylate oligomers are used to add chemical, water and heat resistance together with adhesion.
- Cure shrinkage and internal stress — in free-radical cure, monomers move from van-der-Waals to covalent-bond spacing, causing volume shrinkage → internal stress → reduced adhesion. Shrinkage is itself an end-property concern, not just a process detail.
| Profile | Shear / chemical / thermal | Peel / impact / plastic-adhesion |
|---|---|---|
| Rigid (high cross-link) | High | Low |
| Flexible | Lower | High |
(Sources: AZoM properties guide; ScienceDirect urethane-acrylate study; PMC nano-SiO₂ coating study.)
3. Chemistry families and their property profiles
Two dominant UV-cure chemistries behave very differently — the choice is again property-driven:
| Property | Free-radical (acrylate) | Cationic (epoxy) |
|---|---|---|
| Cure speed | Fast, versatile | Slower |
| Oxygen | Inhibited (tacky surface skin) | Insensitive |
| Cure shrinkage | High → internal stress | Low (ring-opening offsets shrinkage) |
| Adhesion | Lower, esp. on metals | Excellent (≈100 % reported on aluminium) |
| Dark / post cure | Stops when light is removed | Continues (up to ~24 h); often needs post-thermal for full performance |
| Typical use | Broad coatings/inks/adhesives | Low-stress, metal bonds, high-performance |
(Sources: EpoxySet; Longchang; Chase/Resin Designs — vendor-neutral consensus across multiple suppliers.)
4. The UV-source constraint (the gate)
- Photoinitiator ↔ wavelength match. Efficiency hinges on aligning the source's peak emission with the photoinitiator's peak absorption; a mismatch of even 10–20 nm significantly reduces photon absorption.
- Mercury vs LED bands. 365 nm is the mercury i-line; adhesives formulated for mercury lamps carry photoinitiators absorbing 340–380 nm, so 365 nm LEDs are compatible without reformulation. 395 nm is the mainstream LED line; 385 nm balances surface cure and penetration; 365 nm penetrates deeper into clear/lightly-pigmented layers (less risk of an undercured tacky underlayer).
- Photoinitiator types. Type-I phosphine oxides (TPO, BAPO) are standard for 385/395 nm; α-hydroxyketones suit 365 nm.
- Cure depth and oxygen inhibition. Free-radical surface cure is oxygen-inhibited → a tacky, undercured skin. Thin films (
10 µm) are 3–4× less surface-reactive than thick films (30 µm), because bulk polymerization raises viscosity and limits O₂ diffusion. Mitigate with inert gas (N₂/CO₂) over the surface. - Dose vs irradiance. Irradiance (W/cm²) is power per area; dose / energy density (J/cm²) is the time-integral of irradiance; a radiometer verifies in production. Irradiance figures must always state the wavelength range they apply to (a common omission).
(Sources: Incure / UVET wavelength + photoinitiator guides; RadTech Guide to UV Measurement; RadTech Arceneaux oxygen-inhibition paper; UV+EB wood-coatings article.)
5. Practical implications
- Match spectrum before optimizing dose — a 10–20 nm lamp↔photoinitiator mismatch wastes photons regardless of total dose.
- Always specify dose with its wavelength band (e.g. "X mJ/cm² in UVA 365 nm"), not a bare mJ/cm² number.
- Plan oxygen-inhibition mitigation for thin films / surface-cure-critical coatings.
- Property-driven source choice: for metal bonds or low-stress optical assemblies, cationic epoxy's low shrinkage and dark cure is often the right property answer even though it cures slower.
Cross-references
- UV Curing - overview and how it works - the general topic this article supports.
- (coming) Substrate-family deep-dives — acrylate vs epoxy vs silicone property profiles
- (coming) Photoinitiator selection guide (Type-I/II, wavelength bands)
- (coming) Oxygen-inhibition engineering for thin-film coatings
- (coming) UV dose & irradiance measurement how-to (radiometer, wavelength-band reporting)
Sources
- AZoM — Properties of UV Curable Adhesives, Sealants, Coatings
- ScienceDirect — UV-curing conditions & urethane acrylate coatings (peer-reviewed)
- PMC/NCBI — UV-curable coating with nano-SiO₂ (peer-reviewed)
- RadTech — Guide to UV Measurement (Energy Density) (RadTech is the radiation-curing industry standards body)
- RadTech — Mitigation of Oxygen Inhibition (Arceneaux)
- UV+EB Technology — UV-curable wood coatings
- Incure / UVET — UV LED wavelength & photoinitiator-matching guides
- EpoxySet / Longchang / Chase — cationic vs free-radical UV cure