# UV Curing (DD-640)

> UV curing is photopolymerization. A UV-curable ink, coating or adhesive is a liquid mix of small reactive molecules (monomers and oligomers) plus a few percent of photoinitiator. When the photoinitiator absorbs UV light, it breaks apart into reactive species that link the molecules into a solid, crosslinked network, often within a fraction of a second to a few seconds. There are two main chemistries. Free-radical systems, mostly acrylates, form radicals and stop curing almost as soon as the light goes off, and oxygen in the air can slow them at the surface. Cationic systems, mostly epoxies, form a strong acid and can keep curing after the light is gone, which is called 'dark cure'. The light source matters. Traditional mercury-vapor lamps emit a broad spectrum that includes a lot of infrared heat. UV LEDs emit a narrow band, often near 365-395 nm, and produce much less heat, but the formula has to include a photoinitiator that absorbs at that wavelength. Because there's little or no solvent to dry off, UV curing is fast and usually uses less energy than heat drying. The trade-off is that the light has to reach the material, which is why UV-cured pipe liners need translucent glass-fiber tubes (see CIPP).

- Department: Science (also Tech)
- First published: 2025-08-20
- Last updated: 2025-08-20
- Permalink: https://detailenjoyer.com/case/uv-curing/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-640-F01**: Photoinitiators turn UV light into reactive species. _(Write-up · Exhibit A (The recipe))_
- **DD-640-F02**: Cationic UV coatings can keep curing in the dark. _(Write-up · Exhibit B (Two chemistries))_

## Write-up

Liquid to solid in seconds. UV light sets off photoinitiators that harden resins and inks.

1. **The recipe**: UV-curable liquids mix reactive monomers and oligomers with a small amount of photoinitiator.
2. **Two chemistries**: Free-radical (acrylate) systems stop curing when the light goes off. Cationic (epoxy) systems keep curing in the dark.
3. **Oxygen trouble**: Oxygen in the air can slow free-radical curing at the surface.
4. **Lamps versus LEDs**: Mercury lamps give off a lot of infrared heat. UV LEDs emit a narrow band with much less heat.
5. **Matching the light**: The photoinitiator must absorb at the lamp's wavelength, or the material won't cure.

## Related case files

- [CIPP (DD-676)](https://detailenjoyer.com/case/cipp/): UV light cures some pipe liners
- [UV Face (DD-654)](https://detailenjoyer.com/case/uv-face/): Another case about ultraviolet light

## Reel vs. record

- **Note**: 'Minimal heat generation': this depends on the lamp. Traditional mercury-vapor UV lamps give off a lot of infrared heat. UV LEDs run much cooler.
- **Note**: 'Within seconds': often true for thin layers, but thick or heavily pigmented layers cure more slowly, because UV has to get through the material.
- **Missing**: there are two main chemistries. Free-radical acrylates stop curing when the light goes off. Cationic epoxies form an acid and can keep curing afterward ('dark cure').
- **Missing limit**: the light has to reach the material, so shadowed areas and opaque parts can stay uncured. That's why UV-cured pipe liners use translucent glass fiber.
- **Note**: 'Energy efficiency': UV curing usually beats heat-drying, partly because there's little or no solvent to evaporate. But the photoinitiator has to be matched to the lamp's wavelength.

## Video: UV Curing: Liquid to Solid in Seconds

- Duration: 25.5 s · Published: 2025-08-20
- File: https://detailenjoyer.com/assets/videos/DNmFLlwRGYq.mp4

### Transcript

DID YOU KNOW? UV light curing is a process where ultraviolet radiation rapidly hardens or cures materials like adhesives, coatings, and inks by triggering a photochemical reaction. When exposed to UV light, specialized photoinitiators in the material absorb the energy and generate reactive species that start polymerization, transforming liquid resins into solid polymers within seconds. This technique offers fast curing times, energy efficiency, and minimal heat generation. NOW YOU KNOW!

## Questions & answers

**Q: What absorbs the UV light in a curable resin?**  
A: It turns light into reactive species. _(Source: Write-up · Exhibit A (The recipe) · RadTech: The UV & EB curing process; Opsytec: UV curing & photopolymerization fundamentals)_

**Q: Which chemistry can keep curing after the light is off?**  
A: Cationic systems 'dark cure'. _(Source: Write-up · Exhibit B (Two chemistries) · RadTech Europe: UV/EB curing brochure (2018); Opsytec: UV curing & photopolymerization fundamentals)_

**Q: What can slow free-radical curing at the surface?**  
A: Oxygen inhibition. _(Source: Write-up · Exhibit C (Oxygen trouble) · Opsytec: UV curing & photopolymerization fundamentals; RadTech Report (Jun 2013): The state of UV-LED curing, an investigation of chemistry and applications)_

**Q: Which UV source gives off the most heat?**  
A: Mercury lamps emit a lot of infrared. _(Source: Write-up · Exhibit D (Lamps versus LEDs) · RadTech Report (Jun 2013): The state of UV-LED curing, an investigation of chemistry and applications)_

**Q: What must a photoinitiator match?**  
A: Otherwise it won't absorb the light. _(Source: Write-up · Exhibit E (Matching the light) · RadTech Report (Jun 2013): The state of UV-LED curing, an investigation of chemistry and applications)_

## Sources

- [RadTech: The UV & EB curing process](https://radtech.org/the-uv-eb-curing-process/)
- [RadTech Europe: UV/EB curing brochure (2018)](https://www.radtech-europe.com/uploads/Bestanden/rte_uveb_brochure_2018_0.pdf)
- [Opsytec: UV curing & photopolymerization fundamentals](https://www.opsytec.com/uv-applications/uv-processes/uv-curing-photopolymerization)
- [RadTech Report (Jun 2013): The state of UV-LED curing, an investigation of chemistry and applications](https://radtech.org/magazinearchives/Publications/RadTechReport/jun-2013/The%20State%20of%20UV-LED%20Curing%20-%20An%20Investigation%20of%20Chemistry%20and%20Applications.pdf)

## Cite this case file

Easterling, Marcus. "UV Curing." The Detail Department (DetailEnjoyer), case file DD-640, last updated Aug 20, 2025. https://detailenjoyer.com/case/uv-curing/