Photoinitiators: Types, Selection, and UV/LED Cure Uses

A UV coating that cures glassy-hard on top but stays tacky underneath. An LED lamp that the old mercury-bulb photoinitiator simply will not respond to. A white ink that will not through-cure because the pigment is eating the light. Every one of these is a photoinitiator problem — and solving it is mostly a matter of picking the right initiator chemistry for the light source, the film, and the color.

The short version: A photoinitiator is a molecule that absorbs UV or visible light and converts it into the reactive species (usually free radicals) that start polymerization, curing a coating, ink, or adhesive in a fraction of a second. They divide into two mechanisms: Norrish Type I initiators *cleave* under light to form radicals directly (they work alone and excel at deep/through cure), and Norrish Type II initiators *abstract a hydrogen* from a co-initiator — almost always an amine synergist — to form radicals (they cure the surface and are cheaper, but need a partner). Selection comes down to the light source (mercury vs LED wavelength), clear vs pigmented, surface vs deep cure, yellowing, and migration/odor limits. The major families are acylphosphine oxides, α-hydroxyketones, α-aminoketones, benzophenones, thioxanthones, and specialty oxime esters and titanocenes.

What is a photoinitiator?

In a UV/LED-curable formulation, the resin and monomers will not polymerize on their own at room temperature; they need radicals to kick off the chain reaction. The photoinitiator is the light-sensitive component that supplies them. It absorbs photons at a specific wavelength, reaches an excited state, and then either splits apart (Type I) or pulls a hydrogen from a donor (Type II) to generate the radicals that crosslink the film in milliseconds. Matching the initiator’s absorption to the lamp’s output — and getting enough light to the bottom of the film — is the whole game. The underlying photochemistry is described at primary sources such as PubChem.

Type I vs Type II photoinitiators

Most real formulations blend a Type I for through-cure with a Type II/amine package for surface cure, tuned to the lamp and the film.

The photoinitiator families

Acylphosphine oxides — deep cure, low yellowing, LED-friendly

These absorb well into the near-UV/violet, which makes them the backbone of LED curing, and they photobleach (lose color as they react) for low final yellowing. TPO and the liquid TPO-L are the workhorses; BAPO (819) is the bis-acyl version for deep and pigmented through-cure.

α-Hydroxyketones — clean surface cure

Low-odor, low-yellowing Type I initiators that give excellent surface cure in clear coatings, overprint varnishes and clear inks. Photoinitiator 1173 (liquid) and Photoinitiator 184 (solid) are the standards, often blended with an acylphosphine oxide for balanced surface-plus-depth cure.

α-Aminoketones — power for pigmented & deep films

Strong Type I initiators that drive cure in white and pigmented systems and thicker films. Photoinitiator 907, Photoinitiator 369, the biphenyl morpholino-propanone (307) and the methylbenzyl morpholino-butanone cover this space.

Benzophenones — economical Type II surface cure

The classic, low-cost Type II initiators; used with an amine synergist for surface cure of coatings and inks. Benzophenone (BP), 4-methylbenzophenone (MBZ), 4-chlorobenzophenone (CBZ), 4-benzoylbiphenyl (PBZ), 4-benzoyl-4′-methyldiphenyl sulfide (BMS) and the high-efficiency 4,4′-bis(diethylamino)benzophenone.

Thioxanthones — for pigmented & deeper UV

Type II initiators that absorb at longer UV wavelengths, so they cure through pigment better; paired with an amine synergist. ITX (2-isopropylthioxanthone) and DETX (2,4-diethylthioxanthone).

Benzil ketals, benzoylformates, glyoxylates & anthraquinones

A broad Type I/II toolbox for specific cure, odor and yellowing profiles: benzil dimethyl ketal (BDK/651), methyl benzoylformate (MBF), methyl 2-benzoylbenzoate (OMBB), 2,2-diethoxyacetophenone (DEAP), the phenylglyoxylate ester blend and 2-ethylanthraquinone (2-EAQ).

Oxime esters & titanocenes — electronics, color filters, visible light

High-sensitivity specialty initiators for photoresists, color filters and visible-light cure: carbazole oxime-ester, octanedione benzoyloxime (OXE-01) and the visible-light titanocene initiator (784).

Amine synergists (co-initiators)

Every Type II system needs a hydrogen donor; aminobenzoate amine synergists are the standard. Ethyl 4-(dimethylamino)benzoate (EDB), 2-ethylhexyl 4-(dimethylamino)benzoate (EHA) and isoamyl 4-(dimethylamino)benzoate (IAEB) accelerate surface cure and help overcome oxygen inhibition.

How to select a photoinitiator

Work through five questions. Light source: mercury (broad UV) or LED (typically 365/385/395/405 nm)? LED needs initiators that absorb at longer wavelengths — acylphosphine oxides (TPO/BAPO) and thioxanthone/amine packages. Clear or pigmented: pigment competes for light, so white and colored systems need strong, longer-wavelength initiators (BAPO, 907, thioxanthones). Surface vs deep cure: combine a surface-curing Type I/Type II (184/1173, benzophenone+amine) with a deep-curing acylphosphine oxide. Yellowing: acylphosphine oxides and hydroxyketones photobleach for low yellowing; some aminoketones and Type II/amine systems yellow more. Migration, odor & regulatory: for food-contact and low-odor work, choose low-migration or polymeric grades and minimize residuals. Validate the final package on your line — initiator choice is confirmed by cure trials, not theory.

Where photoinitiators are used

Buying photoinitiators in bulk

RawSource sources the full photoinitiator range direct from producers — acylphosphine oxides, hydroxy- and amino-ketones, benzophenones, thioxanthones, benzil ketals, oxime esters and amine synergists — about 30 grades in all. Tell us the lamp (mercury or LED wavelength), the system (clear or pigmented), the cure target (surface/through), and any yellowing, odor or migration limits, and we will quote the right initiator or blend with CoA, TDS and SDS. Many photoinitiators are supplied as fine powders or liquids and ship under their specific hazard classification. Related building blocks for these systems are mapped in the amines guide (amine synergists and curing amines).

Frequently asked questions

What is a photoinitiator?

A photoinitiator is a molecule that absorbs UV or visible light and generates the radicals (or, in cationic systems, acids) that start polymerization, curing coatings, inks and adhesives almost instantly under a UV or LED lamp.

What is the difference between Type I and Type II photoinitiators?

Type I (cleavage) initiators split into radicals directly on absorbing light and work alone, giving efficient deep/through cure. Type II (hydrogen-abstraction) initiators must pull a hydrogen from a co-initiator — usually an amine synergist — to form radicals; they are economical surface-cure initiators with broad UV absorption.

What photoinitiator works with LED curing?

LED lamps emit at longer wavelengths (often 365–405 nm), so LED systems use initiators that absorb there: acylphosphine oxides such as TPO and BAPO, often combined with thioxanthone/amine synergist packages, especially for pigmented films.

What are TPO, 184, and 1173 used for?

TPO (an acylphosphine oxide) gives deep/through cure with low yellowing and is LED-friendly; Photoinitiator 184 and 1173 are α-hydroxyketones that give clean, low-odor surface cure in clear coatings and inks. They are frequently blended for balanced surface-plus-depth cure.

Why do Type II photoinitiators need an amine?

Type II initiators cure by abstracting a hydrogen atom; the amine synergist (such as EDB, EHA or IAEB) is that hydrogen donor, and it also helps overcome oxygen inhibition at the film surface.

How do I choose a photoinitiator for a pigmented (white) system?

Pigment absorbs and scatters UV, so pigmented systems need strong, longer-wavelength initiators that cure through the film — typically BAPO and α-aminoketones (such as 907) with thioxanthone/amine packages. Confirm by through-cure trials at your film thickness.

Editorial note. This article is general technical guidance for industrial and professional buyers and formulators. Mechanisms, family characteristics and selection guidance are typical, generalized references to validate on your own line; the Certificate of Analysis and Technical Data Sheet govern the grade you buy. Nothing here is a safety or efficacy claim. Photoinitiators have specific handling, light-sensitivity and hazard requirements — always consult the current Safety Data Sheet (SDS) before handling, and confirm regulatory and migration status for your application and jurisdiction. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.

RawSource Editorial

Commercial & Sourcing Desk