Your white UV coating skins over glossy on top and stays soft a millimeter down. Or the opposite problem: a clearcoat that cures fine, but you are paying premium-photoinitiator money for a job a cheaper initiator would have done. Both are usually the same mistake — the wrong acylphosphine oxide for the film you are curing. TPO and BAPO are close cousins, but the difference between a mono- and a bis-acylphosphine oxide decides how deep the cure reaches, how a pigmented or white system behaves, and what you spend per kilo.
The short version: TPO and BAPO are both Type I acylphosphine oxide photoinitiators that absorb into the long-UV and near-visible and photobleach as they react, which is what lets them cure thick and pigmented films. The structural difference drives everything else. TPO is a *mono*-acylphosphine oxide (one acyl chromophore); it is cheaper, more soluble, and usually enough for clear and thin films and good general LED surface-plus-through cure. BAPO is a *bis*-acylphosphine oxide (two acyl groups on the phosphorus); it carries a markedly higher molar extinction and absorbs further toward visible, so it cures deeper, thicker, and through pigmented or white systems better than TPO. BAPO is the active sold as Irgacure 819 / Omnirad 819. It also costs more and is less soluble. Most production formulas do not choose one outright: they pair an acylphosphine oxide for depth with a surface-cure hydroxyketone, and high-end white systems often run TPO and BAPO together.
The structural difference, and why it is the whole story
Both molecules are Type I (cleavage) initiators: absorb a photon, split homolytically, and the fragments add across acrylate double bonds to build the network. (If “Type I” needs grounding, see what a photoinitiator is and Type I vs Type II.) What separates them is how many acyl chromophores hang off the phosphorus.
TPO is diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, CAS 75980-60-8. It has one trimethylbenzoyl group on the phosphine oxide, so on cleavage it generates a benzoyl and a phosphinoyl radical — two initiating species per molecule.
BAPO is phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, CAS 162881-26-7, and it is the active sold as Irgacure 819 / Omnirad 819. It carries two trimethylbenzoyl groups. That second acyl chromophore roughly doubles the molecule’s absorptivity and lets it keep generating phosphinoyl-type radicals through successive cleavages, so a single BAPO molecule does more initiating work and absorbs more strongly than a single TPO molecule. The bis structure is the reason BAPO out-cures TPO at depth and in pigment.
Absorbance and cure depth
A photoinitiator only works at the wavelengths it absorbs, and how deep it cures depends on how that absorbance is distributed across the film. TPO absorbs in the long-UV with a peak near 380 nm and a useful tail out toward roughly 420 nm. BAPO absorbs across a broader band, with meaningful absorption from about 365 nm and a tail reaching further into the violet/near-visible, near 420 to 440 nm, and it does so with a higher molar extinction coefficient than TPO.
Higher extinction sounds like it should hurt depth — a strongly absorbing initiator soaks up photons at the surface before they reach the bottom. The reason BAPO still cures deeper is photobleaching, covered next. The practical reading: for a thin clearcoat, TPO’s absorbance is plenty and the extra cost of BAPO buys you little. For a thick section, a deep 3D-printed part, or anything loaded with pigment, BAPO’s combination of high initial absorbance plus aggressive bleaching drives a more complete through-cure. Confirm depth of cure on your own film build; absorbance curves predict the trend, not your exact part.
Why both photobleach — and why that matters for white
Acylphosphine oxides have a property most other photoinitiators lack: they photobleach. They start strongly colored-absorbing in the violet, and as they cleave and react, their absorption in that band collapses. The top layer goes optically clear once it has cured, so incoming light is no longer blocked at the surface and keeps reaching deeper material. The cure front effectively walks down through the film. (RadTech proceedings on phosphine oxide photoinitiators detail this bleaching-and-deep-cure behavior.)
That behavior is exactly what a white or pigmented coating needs. In a system loaded with titanium dioxide, the pigment competes with the photoinitiator for every photon and scatters the rest, so light struggles to reach the bottom of the film. A photobleaching initiator clears its own path: the cured surface stops absorbing and lets photons through to the still-wet depth. BAPO is the standard pick for high-opacity white and pigmented furniture coatings and screen inks because it pairs the strongest bleaching with the highest extinction. Both TPO and BAPO are also low-yellowing relative to older chemistries, which is part of why they win in white where any added color shows immediately.
Match it to your LED: 395-405 nm
Most UV-LED lines today peak at 385, 395, or 405 nm, and the acylphosphine oxides are the photoinitiators that actually absorb there. This is where they pull away from hydroxyketones, whose absorption falls off above the mercury i-line (365 nm) and which go nearly blind by 400 nm. TPO covers 385 and 395 nm LED well and reaches into 405 nm; BAPO’s broader tail makes it the more reliable absorber across the full 395 to 420 nm violet band, which is part of why it holds up in deep and pigmented cure on long-wavelength LED. Under a broadband medium-pressure mercury lamp, both run well. Pull the emission spectrum from your lamp datasheet and overlay the photoinitiator’s absorption curve from its Technical Data Sheet (TDS) before you commit; a 365 nm-biased package under a 405 nm LED under-cures no matter how long you expose it.
TPO vs BAPO: head to head
| Property | TPO (mono-acylphosphine oxide) | BAPO (bis-acylphosphine oxide) |
|---|---|---|
| Structure | One trimethylbenzoyl group on the phosphine oxide (mono) | Two trimethylbenzoyl groups on the phosphine oxide (bis) |
| Chemical name | Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide | Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide |
| CAS | 75980-60-8 | 162881-26-7 |
| Sold as | TPO (generic) | The active of Irgacure 819 / Omnirad 819 |
| Absorption | Long-UV, peak ~380 nm, tail to ~420 nm | Broader, ~365 nm into the violet, tail to ~440 nm |
| Molar extinction | High | Higher (roughly double; two chromophores) |
| Through / deep cure | Good | Deeper — the thick-section choice |
| Pigmented / white cure | Workable | Preferred for white (TiO2) and pigmented |
| Photobleaching | Yes | Yes — stronger initial absorbance, bleaches as it reacts |
| Solubility | Better in common monomers/oligomers | Lower; solid, can be solubility-limited |
| Relative cost | Lower | Higher |
| Typical use | Clear and thin films, general LED surface + through cure | Thick, pigmented, white, deep-section parts, 3D resins |
Pairing with a surface-cure photoinitiator, and beating oxygen inhibition
Acylphosphine oxides are strong at depth, but the very surface of a free-radical film fights a different problem: oxygen inhibition, where atmospheric oxygen scavenges radicals at the air interface and leaves a thin, tacky skin even when the bulk cured hard. The standard answer is a blended package: a fast surface-cure hydroxyketone such as PI-1173 (comparable to Darocur 1173) or PI-184 (comparable to Irgacure 184) at the top for the air interface, plus an acylphosphine oxide underneath for depth. A typical clear UV coating runs a hydroxyketone with TPO; a pigmented or white build runs a hydroxyketone with BAPO. High-end white systems sometimes carry both TPO and BAPO to balance early surface speed against deep through-cure.
The surface package usually wants a tertiary-amine synergist as well, which scavenges oxygen and pushes the tack-free point to a lower dose. The full diagnosis-and-fix ladder for a sticky cured surface is in our guide to why UV resin and coatings stay tacky after curing. Beyond that, a deeper background on the acylphosphine oxide family sits in what TPO is and where it fits.
Cost, and the honest case for TPO
BAPO is the more capable initiator at depth and in pigment, but capability you do not use is just cost. BAPO sits at the higher end of the acylphosphine oxide price range and is less soluble, so it can demand more formulation work to keep it in solution. TPO is cheaper, dissolves more easily, and for a clear, thin, low-build coating it cures the surface and through the film perfectly well on a mercury or LED line. Reaching for Irgacure 819-grade BAPO on a 15-micron clearcoat is paying for through-cure you do not need.
Two honest caveats on color. First, while both photobleach and are low-yellowing compared with older photoinitiators, neither is truly color-free in a water-clear system: residual photoinitiator, byproducts, and especially any amine synergist you add for surface cure can contribute initial color and after-yellowing on UV exposure. Always confirm yellowing on your own substrate and at your real loading before committing a clearcoat line. Second, overloading any acylphosphine oxide to force a deep cure can leave unreacted initiator that bleeds color or odor later, so dial loading to what the dose and film actually require rather than the maximum.
Buying TPO and BAPO
RawSource supplies both acylphosphine oxide photoinitiators for coatings, ink, adhesive, and 3D-resin formulators, in drums and bulk, with Certificate of Analysis (CoA) documentation: TPO for clear, thin, and general surface-plus-through cure, and BAPO (comparable to Irgacure 819 / Omnirad 819) for deep, thick, pigmented, and white systems, alongside the surface-cure hydroxyketones PI-1173 and PI-184 for blended packages. Tell us your lamp type and peak wavelength (mercury, or 365 / 385 / 395 / 405 nm LED), your film build and whether it is clear or pigmented, and your cure-depth and yellowing targets, and request a sample to qualify cure on your own line.
Frequently asked questions
What is the difference between TPO and BAPO?
Both are Type I acylphosphine oxide photoinitiators, but TPO is a *mono*-acylphosphine oxide (one trimethylbenzoyl group, CAS 75980-60-8) and BAPO is a *bis*-acylphosphine oxide (two trimethylbenzoyl groups, CAS 162881-26-7). The second chromophore gives BAPO roughly double the molar extinction and broader absorption into the violet, so it cures deeper and through pigmented or white films better than TPO. TPO is cheaper, more soluble, and usually sufficient for clear and thin films.
Is BAPO the same as Irgacure 819?
BAPO (phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, CAS 162881-26-7) is the active compound sold under the trade names Irgacure 819 and Omnirad 819. Those trade names are referenced here only to identify the comparable generic active; the names are the property of their owner (IGM Resins) and no affiliation or endorsement is implied.
Which photoinitiator is best for white or pigmented coatings?
BAPO is the usual choice for white and pigmented systems. It combines the highest molar extinction in the acylphosphine oxide family with strong photobleaching, so as the surface cures it stops absorbing and lets light reach the still-wet depth past the titanium dioxide or pigment that would otherwise block it. It is also low-yellowing, which matters most in white. Pair it with a surface-cure hydroxyketone for the air interface.
Why do both TPO and BAPO photobleach, and why does it help?
Acylphosphine oxides absorb strongly in the violet, then lose that absorption as they cleave and react. The cured surface turns optically clear, so incoming light is no longer blocked at the top and keeps driving the cure deeper. That self-clearing behavior is what lets a thick or pigmented film cure all the way through instead of only skinning over, and it is why these initiators dominate deep and white systems.
Can I use TPO instead of BAPO to save money?
For clear, thin, low-build coatings, often yes: TPO cures the surface and through the film well at lower cost and with easier solubility, so BAPO’s deep-cure advantage goes unused. Switch to BAPO when the film is thick, deeply pigmented or white, or a deep-section 3D part, where TPO cannot drive a complete through-cure. Many formulas blend the two to balance cost against depth. Validate cure on your own film build before deciding.
Which works better on a 395 or 405 nm LED?
Both absorb in the 395 to 405 nm band, which is why acylphosphine oxides are the standard initiators for UV-LED lines, where hydroxyketones go nearly blind. TPO covers 385 and 395 nm well and reaches into 405 nm; BAPO’s broader tail toward 440 nm makes it the more reliable absorber across the full violet band and the better pick when you also need deep or pigmented cure at those wavelengths. Overlay your lamp’s emission spectrum with the photoinitiator’s absorption curve to confirm.
Editorial note. This article is general technical guidance for coatings, ink, adhesive, and 3D-resin formulation professionals. Cure speed, through-cure, surface tack, pigmented and white cure, and yellowing depend on your specific resin and oligomer, photoinitiator package and loading, lamp type and wavelength, film thickness and pigmentation, line speed, and curing atmosphere, and must be validated on your own system; the Certificate of Analysis (CoA) governs the grade you buy. Photoinitiators are industrial chemicals; review the current Safety Data Sheet (SDS) and use appropriate PPE before handling. Trade names (Irgacure, Omnirad) are referenced only to identify the comparable generic active and are the property of their respective owner (IGM Resins); no affiliation or endorsement is implied. Products are sold for industrial and professional use only. Nothing here is a medical, health, or safety claim. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
