Quick answer
The photoinitiator (PI) is what converts UV photons into the radicals (or acid) that start the cure — and its choice drives surface-vs-through cure, color/yellowing, and food-contact migration, all of which are end-relevant. Three selection questions:
- Does the PI's absorption band match your lamp peak? (carry-over from Selecting UV-Curable Adhesives & Coatings — a 10–20 nm mismatch wastes photons.)
- Type I (cleavage, self-sufficient) or Type II (needs an amine co-initiator)?
- Food contact? Then favour low-migration, high-molecular-weight PIs.
Companion to the chemistry-family deep-dive (Acrylate, Epoxy & Silicone).
1. Type I vs Type II
| Type I (cleavage) | Type II (H-abstraction) | |
|---|---|---|
| Mechanism | Unimolecular α-cleavage → directly generates two radicals (one reactive) | Excited PI abstracts an H/electron from a co-initiator (amine synergist) |
| Examples | TPO, BAPO (acyl-phosphine oxides), α-hydroxyketones | Benzophenone, thioxanthone (ITX) |
| Self-sufficient? | Yes | No — needs amine synergist |
| Note | TPO-L absorbs near ~380 nm → cures thick / heavily-pigmented layers (incl. TiO₂) | Benzophenone abs. max ~290 nm; ITX abs. maxima 259 + 383 nm → long-wavelength sensitization with amine |
Amine synergists absorb in the 280–310 nm region and pair with aromatic ketones that absorb above 300 nm (thioxanthones, 4-phenyl-benzophenone). Benzophenone + amine gives high reactivity plus surface and depth cure.
(Sources: Stanford Adv. Materials (TPO/ITX/DETX); NB Inno (TPO-L); PMC phosphine-oxide study + Wiley TPO/BAPO kinetics — peer-reviewed.)
2. Wavelength matching (the gate)
PI absorption must align with the source's peak emission. Practical bands:
- Mercury i-line 365 nm — legacy formulations carry PIs absorbing 340–380 nm.
- LED 385 / 395 / 405 nm — favour longer-absorbing PIs (TPO/BAPO near 380 nm) for both LED compatibility and thick/pigmented through-cure.
- Short-absorbing PIs (benzophenone ~290 nm) suit broadband mercury but are weak under common LEDs unless paired/sensitized.
See foundation §4 for the full wavelength-match logic.
3. Surface vs through cure
- Surface cure is oxygen-limited (radical systems) → tacky skin without enough surface-active PI or inert-gas mitigation.
- Through/depth cure needs PIs that absorb where light penetrates — longer-wavelength PIs (TPO) reach deeper, especially through pigment.
- Combining a fast surface PI with a deeper-curing PI is a common formulation lever (out of scope here; see (coming) oxygen-inhibition article).
4. Color / yellowing
The PI must not yellow the cured film. For clear or outdoor coatings, PIs and synergists are selected for low photo-yellowing; residual aromatic ketones/amines are common yellowing culprits.
5. Food contact & low-migration
Unreacted residual PIs and monomers are typically low molecular weight and can migrate into food → a safety concern for packaging. Mitigations:
- Shift to higher-molecular-weight / polymeric / oligomeric PIs (>1000 Da) — lower volatility, lower migration, no photo-yellowing.
- Ensure full cure (residual = migratable).
- Regulatory anchor: under FDA rules, migration of each cleared monomer/PI in the cured layer is permitted up to ~1 ppm.
(Sources: MDPI/PMC migration studies — peer-reviewed; UV+EB food-packaging article; Cork Industries FDA-clearance overview.)
Cross-references
- UV Curing - overview and how it works - the general topic this article supports.
- Selecting UV-Curable Adhesives & Coatings: End-Properties First — parent overview
- UV-Curable Chemistry Families: Acrylate, Epoxy & Silicone — sibling deep-dive
- (coming) Oxygen-inhibition engineering for thin-film coatings
- (coming) UV dose & irradiance measurement how-to
Sources
- Stanford Advanced Materials — TPO / ITX / DETX selection
- NB Inno — TPO-L vs other photoinitiators
- PMC — Near-UV Type I phosphine-oxide photoinitiators (peer-reviewed)
- Wiley J. Appl. Polym. Sci. — TPO/BAPO initiation kinetics (peer-reviewed)
- MDPI Molecules / PMC — photoinitiator migration in food packaging (peer-reviewed)
- UV+EB Technology — food-packaging-compliant inks & set-off migration
- Cork Industries — UV/EB FDA clearance for direct food contact