Photoinitiator 184 vs 127 vs TPO: Choosing a UV Photoinitiator
Photoinitiator 184, 127 and TPO are three of the most widely used initiators in UV-curable coatings, inks and adhesives, yet they are chosen for different reasons. This guide compares them on chemistry, absorption, cure behaviour, migration and cost, so that a formulator can select — or blend — deliberately.
For the full background on photoinitiator 184, see our main article on photoinitiator 184 (CAS 947-19-3).
At a Glance
| Property | Photoinitiator 184 | Photoinitiator 127 | TPO |
|---|---|---|---|
| CAS number | 947-19-3 | 474510-57-1 | 75980-60-8 |
| Chemical type | Type I monofunctional alpha-hydroxyketone | Type I difunctional alpha-hydroxyketone | Type I acylphosphine oxide |
| Absorption | Strong UV-C/B; weak above 300 nm | Similar to 184 | Extends into UV-A (approx. 380–400 nm) |
| Surface cure | Excellent | Excellent | Good |
| Through / deep cure | Limited | Limited (but difunctional) | Excellent |
| Migration | Moderate | Low (higher molecular weight, difunctional) | Higher |
| Odour / yellowing | Low | Low | Can yellow more, especially with amines |
| Typical role | Surface cure in clear systems | Low-migration and high-reactivity systems | Depth cure; pigmented and thick films; LED systems |
What Each One Is Good At
- Photoinitiator 184: fast surface cure, low odour and low yellowing in clear, relatively thin coatings and inks. It is a reference choice for clear overprint varnishes and general clear coatings.
- Photoinitiator 127: a difunctional AHK with low migration and high reactivity, favoured where the cured film must not release residues — for example packaging and other sensitive applications.
- TPO: an acylphosphine oxide that absorbs further into the UV-A, giving through-cure in pigmented, filled or thick films and excellent performance with UV-LED lamps.
The Absorption Question
Photoinitiator selection is fundamentally about matching the initiator’s absorption to the available light.
| Situation | Best starting point | Why |
|---|---|---|
| Clear, thin film, mercury lamp | 184 (or 127) | Strong absorption in the UV-B/C the lamp provides |
| Pigmented or thick film | TPO (often with 184) | Longer-wavelength absorption penetrates the film |
| UV-LED curing (approx. 385–395 nm) | TPO + 184 | TPO absorbs the LED output; 184 adds surface cure |
| Low-migration requirement | 127 | Higher molecular weight and difunctional |
| White or light-coloured coating | 184 (colour-stable) | Low yellowing contribution |
Blending: Why 184 and TPO Are Often Used Together
A single photoinitiator rarely does everything well. Blending is the standard solution, and the most common blend is 184 plus TPO:
- 184 provides the surface cure, reacting quickly at the top of the film to give a tack-free finish.
- TPO provides the through-cure, absorbing longer wavelengths that penetrate the film, which is essential in pigmented, filled or thicker coatings and in LED systems.
- The balance is tuned to the film thickness, pigment load and light source; there is no universal ratio.
Where low migration is the priority, 127 is used instead of, or alongside, 184, because its higher molecular weight and difunctionality reduce the amount of mobile residue.
Cost, Supply and Practical Notes
- Cost. The three initiators have different costs; TPO and 127 are generally more expensive than 184, so the blend is designed to meet performance at acceptable cost rather than to use the most expensive product everywhere.
- Purity and colour. For all three, GC purity and colour matter, since they affect cure and appearance.
- Regulatory. Food-contact and other regulated uses set limits and require specific grades and migration data, which may rule some products out for some applications.
- Re-validation. Any change in the initiator system — even between similar products — should be re-validated against the actual cure and service tests.
Frequently Asked Questions
Is photoinitiator 184 the same as TPO?
No. 184 is a monofunctional alpha-hydroxyketone that absorbs mainly in the UV-B/C; TPO is an acylphosphine oxide that absorbs further into the UV-A and gives better through-cure.
Which is better for pigmented coatings?
TPO, often blended with 184. TPO’s longer-wavelength absorption penetrates pigments and thick films, while 184 improves surface cure.
Which has the lowest migration?
Photoinitiator 127, because it is difunctional and of higher molecular weight, which reduces the amount of mobile residue in the cured film.
Why are 184 and TPO often blended?
Because 184 cures the surface quickly while TPO cures through the depth; together they give both a tack-free surface and full through-cure.
Do they all work with UV-LED lamps?
TPO works well with the longer wavelengths of UV-LED; 184 does not absorb them strongly, so in LED systems it is used with TPO or another red-shifted initiator.