What Are UV Photoinitiators? Science, Function, and EU Regulation in Nail Gel Systems
UV photoinitiators are chemical molecules that absorb ultraviolet or visible light and generate reactive radicals that initiate polymerisation, transforming liquid nail gel into a solid cross-linked polymer network. Without UV photoinitiators, modern gel nail coatings would not cure, would remain mechanically weak, and would fail in professional use.
This article provides a complete scientific explanation of UV photoinitiators in nail gel systems, including their photochemical mechanism, functional necessity, formulation levels, toxicological considerations, European Union regulatory restrictions such as the prohibition of TPO, and the rationale for combining multiple photoinitiators in one formulation.
1. Fundamental Photochemistry of UV Photoinitiators
UV photoinitiators operate by converting light energy into chemical reactivity. When exposed to radiation in the approximate range of 320–420 nm—typical for UV and LED nail lamps—the photoinitiator absorbs a photon and transitions into an excited electronic state. From this excited state, the molecule undergoes bond cleavage or hydrogen abstraction to produce free radicals.
These radicals react with carbon–carbon double bonds present in acrylate monomers and oligomers, initiating a free-radical chain polymerisation. As polymer chains grow and cross-link, a dense three-dimensional network forms. This network is responsible for the hardness, adhesion, solvent resistance, and long-term durability of cured nail gel coatings.
Two principal mechanistic categories exist:
- Type I photoinitiators (α-cleavage): directly fragment into radicals after light absorption. Examples include acyl phosphine oxides such as TPO, TPO-L, and BAPO.
- Type II photoinitiators: form radicals through interaction with a co-initiator, commonly an amine. Camphorquinone-based systems are typical representatives.
Type I systems dominate modern nail gels because they generate radicals efficiently and support rapid curing even in pigmented or thick layers.
2. Why UV Photoinitiators Are Essential in Nail Gels
Acrylate oligomers themselves absorb very little light in the emission range of nail curing lamps. Without a dedicated photoinitiator, polymerisation would be negligible. UV photoinitiators therefore serve two indispensable roles:
- Efficient absorption of lamp radiation
- Conversion of absorbed energy into initiating radicals
If photoinitiator concentration is too low, curing remains incomplete. This leads to soft coatings, reduced wear resistance, poor adhesion, and elevated exposure to residual monomers that may contribute to skin sensitisation.
3. Radical Polymerisation Kinetics and Oxygen Inhibition
The polymerisation reaction initiated by UV photoinitiators proceeds through initiation, propagation, and termination steps. Reaction rate depends on radical concentration, monomer reactivity, temperature, and oxygen presence.
Atmospheric oxygen can quench free radicals at the coating surface, producing the well-known oxygen inhibition layer. Modern photoinitiator systems mitigate this effect by generating radicals at higher rates or through synergistic initiator combinations, allowing efficient curing even in thin surface regions.
4. Typical Photoinitiator Concentrations in Nail Gel Formulations
Industrial formulation practice and toxicological evaluations converge on similar concentration ranges for total photoinitiator loading:
- Transparent or lightly pigmented gels: 1–3 %
- Highly pigmented colour gels: 2–5 %
- Builder or sculpting gels: up to approximately 5 %
These percentages represent the sum of all photoinitiators in the formulation rather than a single compound.
5. Why Multiple UV Photoinitiators Are Used Together
Modern LED nail lamps emit narrow spectral peaks, commonly around 365 nm and 395–405 nm. No single photoinitiator absorbs efficiently across this full range. Combining photoinitiators enables:
- Broader spectral absorption
- Improved curing depth in opaque systems
- Faster polymerisation kinetics
- Reduced oxygen inhibition at the surface
This multi-initiator strategy is now standard in professional nail gel chemistry and is critical for reliable curing across different lamp technologies.
6. EU Regulation and the Prohibition of TPO
Trimethylbenzoyl Diphenylphosphine Oxide (TPO, CAS 75980-60-8) was historically one of the most efficient UV photoinitiators for LED curing. However, hazard classification identified TPO as a CMR Category 1B reproductive toxicant.
Under Regulation (EC) 1223/2009, substances classified as CMR 1 are automatically prohibited in cosmetic products unless a specific derogation is granted. No derogation was approved, leading to a full EU cosmetic prohibition of TPO effective 1 September 2025.
Consequently, compliant nail gel formulations now rely on alternative photoinitiators such as TPO-L and BAPO, which provide comparable curing efficiency without current cosmetic prohibition.
7. Toxicological and Sensitisation Considerations
Photoinitiators can act as skin sensitisers if uncured residues contact the skin. Safety therefore depends strongly on complete polymerisation and controlled professional application. Key risk-management principles include:
- Ensuring full curing under appropriate lamp conditions
- Minimising residual monomer and initiator migration
- Selecting lower-sensitisation photoinitiator systems
- Maintaining correct professional handling procedures
These toxicological considerations drive ongoing reformulation toward newer phosphinate-based photoinitiators.
8. Technical Table of Major UV Photoinitiators
| INCI Name | Chemical Name | Abbrev. | CAS | λmax (nm) | Typical Use % | EU Status |
|---|---|---|---|---|---|---|
| Hydroxycyclohexyl Phenyl Ketone | (1-Hydroxycyclohexyl)phenyl-methanone | HCPK | 947-19-3 | ≈333 | 0.5–2 % | Allowed |
| 2-Hydroxy-2-methylpropiophenone | α-Hydroxy ketone | HMPP | 7473-98-5 | ≈333 | 0.5–2 % | Allowed |
| Benzil Dimethyl Ketal | Dimethoxy acetophenone derivative | BDK | 24650-42-8 | ≈350 | 0.5–2 % | Allowed |
| Camphorquinone | Bornane-2,3-dione | CQ | 10373-78-1 | ≈468 | 0.2–1 % | Allowed |
| Trimethylbenzoyl Diphenylphosphine Oxide | Acyl phosphine oxide | TPO | 75980-60-8 | ≈380 | 1–5 % | Prohibited EU (2025) |
| Ethyl Trimethylbenzoyl Phenylphosphinate | Acyl phosphinate | TPO-L | 84434-11-7 | ≈371 | 1–4 % | Allowed |
| Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide | Bis-acyl phosphine oxide | BAPO | 162881-26-7 | ≈395 | 0.5–3 % | Allowed |
| Irgacure 907 | Morpholino ketone photoinitiator | 907 | 71868-10-5 | ≈369 | 0.5–3 % | Allowed |
| Isopropylthioxanthone | Thioxanthone derivative | ITX | 5495-84-1 | ≈385 | 0.1–1 % | Limited relevance |
| Benzophenone | Diphenyl ketone | BP | 119-61-9 | 254 / 365 | 0.1–1 % | Limited cosmetic use |
9. Future Trends in UV Photoinitiator Chemistry
Future development in nail gel technology is expected to prioritise lower sensitisation potential, improved LED curing efficiency, reduced migration, and full compliance with evolving EU and UK cosmetic regulations.
Conclusion
UV photoinitiators are the essential photochemical engine of modern nail gel systems, enabling rapid curing, durable coatings, and professional performance. Scientific understanding of their mechanism, concentration ranges, toxicology, and regulatory status—especially the EU prohibition of TPO—is critical for safe and compliant nail gel formulation.
