Selecting UV-curing adhesives, coatings and potting compounds

UV-curing adhesives, coatings and potting compounds combine short process times with a cure that can be controlled deliberately. Which material class fits, however, does not depend on the available UV wavelength alone. Substrate transmission, layer thickness, pigmentation, filler content, oxygen contact and shadowed areas decide whether a free-radical, cationic or dual-curing system is suitable. The selection aid below sorts the material classes by application, process limits and the post-cure they require.

The curing reaction itself is covered on the fundamentals page on how UV curing and photopolymerization work; how to lay out a bonding or potting process in the joint is shown under designing UV bonding and potting processes.

In UV curing, radiation activates a photoinitiator and starts the cross-linking of the material. A detailed account of reaction pathways, irradiance and dose is given on the specialist page on reaction pathways, irradiance and dose in UV curing.

For material selection, what matters here is above all the chemistry, the optical properties and the accessibility of the entire reaction zone.

Two levels of distinction matter when choosing a material: the chemistry of the binder system – free-radical versus cationic – and the decision between single-cure and dual-cure systems for geometrically demanding parts.

On the radiation source, a note is enough here: mercury vapour lamps emit broadband, UV LEDs narrowband at discrete peaks (365, 385, 395 and 405 nm). All that matters for material selection is whether the absorption band of the photoinitiator in use is actually hit. The sources themselves are compared on the technology page on UV LEDs for UVA, UVB and UVC.

The matrix assigns each typical requirement the material system to examine first and the limit at which that system fails.

Whether the chosen source actually delivers the radiation needed at the part is only shown by a measurement: which sensor measures which quantity reliably is covered under selecting suitable UV sensors; why two instruments can show different readings at the same source is explained under spectral mismatch.

A handful of assumptions come up again and again in material selection and can lead to incomplete cure or to customer complaints. The general misconceptions about lamp power, dose and reciprocity are covered on the fundamentals page on the limits of dose and the reciprocity law:

• „Oxygen inhibition affects the whole volume“: dissolved oxygen in the bulk is consumed quickly under irradiation; the real problem is the continuously re-diffusing oxygen layer at the surface, which leaves tacky, uncured surfaces especially at low irradiance.
• „An acrylate formulated for 365 nm cures tack-free with any 365 nm LED“: for a tack-free surface, several common acrylate adhesive systems need additional radiation in the 220–260 nm range to overcome oxygen inhibition – a requirement that LED systems emitting only at 365 nm cannot meet, and one that has to be taken into account when selecting the formulation.
• „A line of sight between lamp and bond line is optional“: in areas shadowed by UV-opaque parts (metals, ceramics, filled plastics) a pure UV material stays liquid, even when directly illuminated areas next to it are fully cured.

Source ageing adds to this: irradiance and spectrum shift over operating time, so a released process window has to be re-measured regularly – see ageing of UV lamps and UV LEDs.

Filled or pigmented formulations behave fundamentally differently from clear systems. Opaque pigments such as carbon black or titanium dioxide absorb part of the UV radiation the photoinitiator needs before it reaches deeper layers; inorganic fillers such as glass beads or silica additionally scatter the light and reduce the intensity with depth exponentially.

The result is a cure gradient in which material near the surface cures completely while the core stays liquid. The Lambert-Beer law describes this quantitatively through the penetration depth D_p, at which the intensity has fallen to 1/e of its value, and the critical energy E_c needed as a minimum to initiate polymerization.

For pigmented systems it is worth moving to longer wavelengths (385 or 405 nm instead of 365 nm), because many pigments scatter and absorb these less strongly. Simply raising the dose helps only up to a point: it can over-cross-link and yellow the near-surface region without improving the cure in the core. Layer thickness also affects the shrinkage stress that builds up during cure: typical values are 1–2 % linear and 2–5 % volumetric shrinkage, which in thicker layers acts three-dimensionally and can impose mechanical stress on adjacent components when the cure is faster than the stresses can relax.

For geometrically demanding parts with undercuts or UV-opaque materials (metals, ceramics, filled plastics), dual-cure systems have become established. They combine a UV primary cure with a secondary mechanism: UV plus moisture cure for shadowed areas without additional heat input, UV plus thermal post-cure for highly loaded bonds, and UV plus anaerobic cure for inactive substrates.

Cationic epoxy systems offer an alternative rooted in the chemistry itself: after the exposure ends, their onium salt photoinitiators continue an acid-catalysed dark reaction and can therefore reach shadowed areas without a second curing mechanism – at the cost of lower initial strength and higher moisture sensitivity than acrylates. For applications with no line of sight at all between light source and bond line, research is also looking experimentally at non-optical excitation such as X-rays, which does not require a direct line of sight; that approach is at the research stage and not in broad industrial use.

• Wavelength shift from 365 nm towards 385, 395 and 405 nm: UV LEDs reach a higher wall-plug efficiency at longer wavelengths, but this calls for reformulated photoinitiator systems matched to the wavelength, because classic initiators developed for mercury lamps can lose efficiency at 405 nm.
• More dual-cure formulations: new product lines combine UV with moisture cure in order to reach shadowed areas without additional heat input; the technical consequence is an additional, time-separated post-cure step in production planning.
• Photoinitiators without a critical classification: in response to the REACH classification of TPO, alternative aliphatic multi-functional initiators with lower cytotoxicity are being developed, which means existing formulations have to be re-assessed for cure behaviour and absorption whenever a raw material changes.
• Miniaturization in electronics and optics: smaller part geometries with tighter cavities increase the need for dual-cure systems and precise dose control, because flood curing reaches its geometric limits.

How the irradiance and UV dose this requires are captured at the part is covered by the product overview of UV measuring instruments and radiometers.

Selection starts with the question of which wavelength actually reaches the adhesive in the bond line – determined by substrate transmission, shadowed areas and the spectral match between lamp and photoinitiator – and derives from that whether a single-cure acrylate is sufficient or whether a dual-cure or cationic system is needed for unexposed zones. Only once application, technical problem, measurand and process quantity and the suitable technology have been clarified does a concrete product selection become meaningful.

What spectrum the existing source really delivers only shows in a measurement at the part; measured spectra of common lamp types are available for comparison in the spectral database for UV lamps.

The following selection is a referenced compilation of commercially available products based on publicly accessible manufacturer data sheets. It does not claim to be complete; the figures for wavelength and chemical base are taken from the respective technical data sheets and should always be verified against the current data sheet before a process is designed. Each product name links to the manufacturer document evaluated – technical data sheet, product data sheet or product page. Opsytec does not sell adhesives, coatings or potting compounds; this overview serves technical orientation only.

Selection criterion: The chemical base and the absorption range of the photoinitiator have to match the available source; bond gap, substrate transmission and the required initial strength then decide between a free-radical acrylate, a cationic epoxy and a dual-cure system. Limit: In shadowed zones and behind UV-opaque joining parts a pure UV acrylate stays liquid – a secondary mechanism is needed there. Typical application: Fixturing and joining in electronics, optics, medical technology and battery assembly, where second-range cycle times and locally confined heat input are required.

Selection criterion: What governs the choice is layer thickness, the required surface hardness and whether the surface has to cure tack-free – that decides between accepting oxygen inhibition, inerting the atmosphere or using a formulation designed for it. Limit: Pigmented and highly opaque coatings cure incompletely in depth; simply raising the dose causes over-cross-linking and yellowing near the surface. Typical application: Conformal coatings on printed circuit boards, furniture and component coatings on web lines, and print finishing.

Selection criterion: Potting height, filler content and whether radiation can reach the entire potting volume are decisive; for optoelectronic parts, refractive index and resistance to yellowing are added. Limit: As potting height and filler content grow, a cure gradient with a liquid core develops – above a few millimetres, dual-cure or cationic systems with a dark reaction are preferable. Typical application: Encapsulation of sensors, connectors and optoelectronic assemblies, moisture-stable potting.

How are wavelength, irradiance and dose related?
The wavelength decides whether the photoinitiator is excited at all, the irradiance sets the rate at which radicals or acid are formed, and the dose is the time integral of that irradiance. Higher irradiance does not make up for a wavelength mismatch, and a dose figure alone does not let the process be reconstructed. This is derived – including the limits of the reciprocity law – on the fundamentals page on the process variables of UV curing.

What distinguishes free-radical from cationic UV curing?
Free-radical acrylate systems cure within seconds but are oxygen-sensitive at the surface and show a linear shrinkage of 1–2 %. Cationic epoxy systems are insensitive to oxygen, continue to post-cure in the dark after exposure ends and shrink less, but they react sensitively to humidity and build initial strength more slowly.

When is a dual-cure system required?
As soon as part of the reaction zone has no direct line of sight to the radiation source – undercuts, cavities, bond areas behind metal, ceramic or filled plastics. The secondary mechanism (moisture, heat or anaerobic) cures those areas afterwards, but requires a separate post-cure step in the process plan. Cationic epoxies reach shadowed areas partly without a second mechanism through their dark reaction.

Why do UV adhesives often stay liquid in shadowed areas?
Free-radical curing systems need a direct line of sight to the light source, because activation of the photoinitiator relies on photons actually arriving. Areas behind UV-opaque parts, metals or in undercuts do not receive sufficient irradiation and do not cure unless a second curing mechanism such as moisture or heat post-cure is provided.

What role does oxygen play in UV curing?
Oxygen quenches radicals into unreactive peroxy radicals and therefore acts mainly as a surface effect, since dissolved oxygen in the bulk is consumed quickly under irradiation, while oxygen diffusing in continuously at the interface with air delays a tack-free cure. Cationic systems are unaffected by this, but are sensitive to moisture instead.

How do pigmentation and layer thickness affect the cure?
Pigments such as carbon black or titanium dioxide absorb and scatter part of the radiation before it reaches deeper layers; as the layer grows thicker this creates a cure gradient with a cured surface and a liquid core. Moving to longer wavelengths (385 or 405 nm instead of 365 nm) helps, whereas simply raising the dose does not – it causes over-cross-linking and yellowing near the surface without materially improving the cure in the core.

What irradiance does a UV adhesive require?
There is no blanket answer – what governs this is the adhesive's data sheet and the absorption range of its photoinitiator. Typical figures lie between a few tens and a few hundred mW/cm² in the matching wavelength range, together with a minimum dose.

Which standards are relevant for UV potting compounds?
For reactive resin compounds and potting systems the IEC 60455 family applies, in particular IEC 60455-2:2023 for test methods such as degree of cure and mechanical properties. In medical technology, ISO 10993 for biological evaluation and sector-specific approval standards from automotive and aerospace apply in addition, depending on where the potted part is used.

• ISO 10993-1:2018/2025 – biological evaluation of medical devices, part 1; the revised edition EN ISO 10993-1:2026 has been available since 15 January 2026.
• IEC 60455-2:2023 – test methods for reactive resin compounds (potting compounds), 4th edition 2023.
• ASTM D1002 / D3163-01(2023) – lap shear strength of metal and of rigid plastic adhesive joints.
• UV-Curable Adhesives Market – Mordor Intelligence, data as of 2026; market volume and segment shares.
• UV-Curable Coatings Market Report – Grand View Research, 2025; global market figures for UV coatings.
• RoHS exemption 4(f)-IV – Clarifications on 2027 Expiration, UV+EB Technology, 2026; regulatory status of mercury lamps.

An overview of the regulations relevant to UV applications is given under guidelines, norms and standards in UV.

The following scientific publications are the primary basis for the statements made in this report on reaction kinetics, the reciprocity law and measurement methodology. For full manufacturer data sheets and standards documents, please refer to the sources cited above.

• Wydra JW, Cramer NB, Stansbury JW, Bowman CN: The reciprocity law concerning light dose-relationships applied to BisGMA/TEGDMA photopolymers. Dental Materials 30(6):605–612, 2014.
• Palagummi SV, Hong T, Wang Z, Moon CK, Chiang MYM: Resin Viscosity Determines the Condition for a Valid Exposure Reciprocity Law in Dental Composites. Dental Materials, 2019/2020.
• Sangermano M, Roppolo I, Chiappone A: New Horizons in Cationic Photopolymerization. Polymers 10(2):136, 2018.
• Eibel A, Fast DE, Gescheidt G: Choosing the ideal photoinitiator for free radical photopolymerizations. Polymer Chemistry 9:5107–5115, 2018.
• Lang M, Hirner S, Wiesbrock F, Fuchs P: A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization. Polymers 14(10):2074, 2022.
• Bennett J: Measuring UV Curing Parameters of Commercial Photopolymers used in Additive Manufacturing. Additive Manufacturing 18:203–212, 2017.
• Stadler E, Eibel A, Fast D, Freißmuth H, Holly C, Wiech M, Moszner N, Gescheidt G: A versatile method for the determination of photochemical quantum yields via online UV-Vis spectroscopy. Photochemical & Photobiological Sciences 17:660–669, 2018.
• Noè C, Hakkarainen M, Sangermano M: Cationic UV-Curing of Epoxidized Biobased Resins. Polymers 13(1):89, 2021.
• Bauer F et al.: Free-Radical Photopolymerization. Polymers, 2022.

Editorial status: 2 September 2026.

Whether the released process window is still met at the source actually in use only becomes clear from a measurement at the part. We measure the spectrum, irradiance and dose of your curing process and advise on suitable measurement technology – from the flat UVpad spectroradiometer that travels through the machine, through the RMD Pro radiometer and the XT series UV curing sensors for process monitoring, to UV irradiation chambers for reproducible curing trials in the laboratory. Send us your question.