Introduction to Photoinitiator 184
Photoinitiator 184 (CAS 947-19-3), chemically 1-hydroxycyclohexyl phenyl ketone and widely known under the trade names Irgacure 184 and Omnirad 184, is one of the most widely used ultraviolet (UV) photoinitiators in the coatings and printing industries. It belongs to the alpha-hydroxyketone (AHK) family of “Type I” photoinitiators: on exposure to UV light it undergoes a clean, intramolecular bond cleavage that generates the radicals which start free-radical polymerisation.
Its combination of high reactivity, low odour, good solubility and low yellowing has kept photoinitiator 184 a reference product in UV-curable coatings, inks, adhesives and photoresists for decades. This article covers its identity, properties, photochemistry, production, specification, applications and sourcing, and explains the one characteristic that governs where it can — and cannot — be used: its UV absorption profile.
PolyBlueChem can supply photoinitiator 184 (CAS 947-19-3); see the product page for the current specification.
Chemical Identity and Structure
Photoinitiator 184 is a small, symmetric alpha-hydroxy ketone: a benzoyl group joined to a cyclohexane ring that carries a hydroxyl group at the point of attachment. That arrangement — a carbonyl adjacent to a hydroxyl-bearing carbon — is exactly what enables the alpha-cleavage that defines Type I photoinitiators.
| Property | Value |
|---|---|
| Common names | Photoinitiator 184; Irgacure 184; Omnirad 184 |
| Chemical name | 1-Hydroxycyclohexyl phenyl ketone |
| CAS number | 947-19-3 |
| Molecular formula | C13H16O2 |
| Molecular weight | 204.26 g/mol |
| Class | Type I alpha-hydroxyketone (AHK) photoinitiator |
| SMILES | OC1(CCCCC1)C(=O)c1ccccc1 |
Physical and Chemical Properties
| Property | Typical value / behaviour |
|---|---|
| Appearance | White crystalline powder |
| Melting range | 45–50 °C |
| Purity | ≥ 98% (by GC) |
| Solubility | Readily soluble in common monomers, oligomers and organic solvents |
| Odour | Low |
| Yellowing | Low tendency |
| UV absorption | Strong in the UV-C/UV-B region, weaker above 300 nm |
| Loss on drying | ≤ 0.5% |
Photoinitiator 184 is a low-melting solid, which is convenient because it can be handled and dissolved easily in typical UV formulations. Its good solubility and low odour and colour make it a favourite for clear and light-coloured systems.
How Type I Photoinitiators Work
In a UV-curable system, the coating or ink contains monomers and oligomers that will form the polymer network, and a photoinitiator that starts the reaction when it absorbs UV light. Photoinitiators are classified by how they generate radicals:
- Type I (unimolecular). The excited initiator breaks apart on its own (alpha-cleavage), producing two radicals directly. Alpha-hydroxyketones such as photoinitiator 184 are Type I.
- Type II (bimolecular). The excited initiator abstracts a hydrogen from a co-initiator (for example benzophenone with an amine), producing radicals by a two-molecule interaction.
For photoinitiator 184 the alpha-cleavage splits the molecule into a benzoyl radical and a 1-hydroxycyclohexyl radical. Both are reactive and can initiate polymerisation, which helps make the initiator efficient. Because the cleavage is intramolecular, it does not depend on the initiator finding a co-initiator, so Type I AHK photoinitiators are simple and robust to use. They are also valued for producing relatively little odour and yellowing after cure, because their photolysis fragments tend to be bound into the polymer network rather than left as volatile small molecules.
UV Absorption and Cure Depth
The one property that most limits where photoinitiator 184 is used is its absorption spectrum. It absorbs strongly in the UV-C and UV-B regions but only weakly above about 300 nm. The practical consequences are important.
| Formulation type | Suitability of photoinitiator 184 | Reason |
|---|---|---|
| Clear, thin coatings and inks | Excellent | UV reaches the initiator throughout the film; high surface reactivity |
| Clear, moderately thick films | Good with adequate UV output | Limited penetration of the longer wavelengths it absorbs |
| Pigmented or highly filled films | Limited on its own | Pigments absorb/scatter UV; longer-wavelength initiators are needed |
| Thick or deeply cured sections | Limited on its own | Red-shifted initiators (such as TPO) cure through the depth better |
| White and light-coloured systems | Good | Low yellowing contribution |
This is why photoinitiator 184 is often combined with a longer-wavelength initiator in demanding systems: 184 delivers fast surface cure, while a red-shifted initiator (TPO) or a bifunctional AHK (photoinitiator 127) provides through-cure. The related reading below compares these options in detail.
Industrial Production
Photoinitiator 184 is an alpha-hydroxy ketone, and the industrial logic is to build the phenyl cyclohexyl ketone framework and then introduce the alpha-hydroxyl group. A representative approach is Friedel–Crafts acylation of benzene with a cyclohexanecarbonyl derivative to give cyclohexyl phenyl ketone, followed by alpha-hydroxylation (for example via the alpha-bromo ketone) to give 1-hydroxycyclohexyl phenyl ketone. Producers use optimised, proprietary routes, but the chemistry centres on the same two ideas: forming the aryl ketone, then oxidising the carbon adjacent to the carbonyl.
| Stage | Purpose | Notes |
|---|---|---|
| 1. Acylation | Form the phenyl cyclohexyl ketone core | Friedel–Crafts acylation of benzene with a cyclohexanecarbonyl source and a Lewis acid catalyst |
| 2. Alpha-hydroxylation | Introduce the alpha-hydroxy group | Typically via alpha-halogenation followed by hydrolysis |
| 3. Isolation and purification | Obtain a clean, low-colour solid | Crystallisation/distillation; colour control is important |
| 4. Drying and finishing | Meet the specification | Flaking/prilling a low-melting solid; assay by GC |
| 5. Quality control | Confirm purity and colour | GC assay, melting range, loss on drying |
Specifications and Quality Control
Photoinitiator 184 is controlled by GC purity, melting range and moisture, with colour and absence of residual starting materials as additional concerns for demanding applications.
| Parameter | Typical specification |
|---|---|
| Appearance | White crystalline powder |
| Purity (GC) | ≥ 98.00% |
| Melting range | 45.0–50.0 °C |
| Loss on drying | ≤ 0.50% |
| Colour | Low colour, consistent with a white powder |
For inks and coatings intended for sensitive uses (for example food packaging or low-migration applications), the level of residual starting material, the migration behaviour of the photoinitiator and its photolysis products, and the purity profile all matter, so buyers should specify the grade accordingly.
Grades and Forms
- Standard grade: the white crystalline powder/flakes used in general UV coatings and inks.
- Low-migration or high-purity grades: for sensitive packaging and medical applications.
- Liquid blends and one-pack photoinitiator blends: 184 is frequently supplied in blends with other initiators (for example TPO or 127) to deliver combined surface and through-cure.
- Dispersed forms: for water-based and other systems where a powder is difficult to incorporate.
Applications: UV-Curable Coatings and Inks
| Application | Role of photoinitiator 184 |
|---|---|
| Clear coatings (wood, plastic, metal) | Fast surface cure, low yellowing, low odour |
| Printing inks (offset, flexo, inkjet) | Rapid cure for high-speed printing |
| Overprint varnishes | Fast, colour-stable surface cure |
| Adhesives (laminating, pressure-sensitive) | Cure on demand with UV; good clarity |
| Photoresists and electronics | Image-wise cure for fine patterns |
| 3D printing (SLA/DLP) | Initiation of photopolymer resins |
Photoinitiator 184 is particularly strong for surface cure: it reacts quickly at the top of the film, giving a tack-free, well-cured surface. That is why it is often the “surface” component of a blended initiator system, paired with a longer-wavelength initiator that cures through the depth.
Photoinitiator 184 vs 127 vs TPO
These three photoinitiators solve different parts of the cure problem, and they are frequently combined. The table summarises the comparison.
| Property | PI 184 | PI 127 | TPO |
|---|---|---|---|
| Type | Type I, monofunctional AHK | Type I, difunctional AHK | Type I, acylphosphine oxide |
| Absorption | Mainly UV-C/B; weak above 300 nm | Similar to 184 | Extends further into the UV-A (to about 380–400 nm) |
| Strength | Fast surface cure, low odour, low yellowing | Low migration, high reactivity, low odour | Through-cure; works in pigmented and thick films |
| Typical role | Surface cure in clear systems | Low-migration systems | Depth cure, white/pigmented systems |
The practical pattern is to use 184 for surface cure and to add TPO (or another red-shifted initiator) where depth or pigmented cure is needed, or to use 127 where low migration is the priority. A fuller comparison is given in the related reading below.
Formulation and Handling Guidance
- Dosage. Photoinitiators are used at low levels (typically a few percent of the formulation), tuned to the light source, the film thickness and the pigment load; confirm the starting point with the supplier.
- Dissolution. 184 dissolves readily in common monomers and oligomers; ensure full dissolution to avoid haze and uneven cure.
- Blending. Combine with a red-shifted initiator for through-cure, and check for any interaction with pigments or light stabilisers.
- Light source match. Match the initiator to the lamp (mercury, LED) spectrum; 184 works well with lamps emitting in the UV-B/C, while LED systems at longer wavelengths may need more of a red-shifted co-initiator.
- Storage. The low-melting solid should be stored cool, dry and away from light; keep containers sealed to avoid caking and colour change.
- Oxygen inhibition. As with all free-radical UV systems, oxygen can inhibit cure at the surface; formulation and process measures address this.
Safety, Health and Environment
- Skin and eye irritation. Photoinitiators can be skin and eye irritants, and some are sensitisers; avoid contact and use appropriate gloves, eye protection and ventilation.
- Dust. Avoid inhaling dust when handling the powder; follow the SDS.
- Uncured material. Handle uncured formulations with care, since the photoinitiator is reactive before curing.
- Environment. Prevent release to drains and watercourses; dispose of waste in line with local regulations.
Regulatory Status
Photoinitiator 184 is an established industrial chemical. Its use in food-contact and other regulated applications is subject to specific rules, and buyers should confirm the grade and documentation for the intended use:
- SDS and per-batch COA.
- Food-contact or migration statements where relevant.
- REACH registration and TSCA status.
Market and Supply Overview
- Demand driver. Driven by UV-curable coatings, inks and adhesives, which continue to displace solvent-based systems on environmental and productivity grounds.
- Production base. Made by a number of specialty photoinitiator producers, mainly in Asia and Europe.
- Price drivers. Raw-material and manufacturing costs for the aryl ketone route, energy, and the grade and purity required.
- Quality differentiators. GC purity, colour, absence of residual starting materials and low migration behaviour.
- Packaging. Typically 20–25 kg fibre drums or boxes with inner liners.
Procurement and Sourcing Guide
- Purity and colour. Confirm the GC purity and the colour, since these drive cure performance and appearance in clear systems.
- Grade. Match the grade to the application, especially for food-contact or low-migration uses.
- Form. Choose powder, blend or pre-dispersed form to suit your process.
- Documentation. Require the SDS, per-batch COA and any food-contact/migration statements.
- Consistency. Ask for batch history if colour or purity is critical.
PolyBlueChem can supply photoinitiator 184 (CAS 947-19-3) as a bulk raw material. To request current specifications, a COA or a quotation, contact the PolyBlueChem team.
How UV Curing Works: From Light to Network
Understanding where photoinitiator 184 fits means understanding the UV-curing process as a whole. A UV-curable formulation is a mixture of reactive monomers and oligomers (the “vehicle”), a photoinitiator and small amounts of additives such as stabilisers, pigments and adhesion promoters. Curing happens in three steps:
| Step | What happens | Relevance to PI 184 |
|---|---|---|
| 1. Absorption | The photoinitiator absorbs a UV photon and is promoted to an excited state | PI 184 absorbs strongly in the UV-B/C region |
| 2. Radical formation | Excited PI 184 undergoes alpha-cleavage into two radicals | Unimolecular (Type I) cleavage, efficient and robust |
| 3. Polymerisation | The radicals attack the acrylate double bonds; the network grows and cures | Fast surface cure; fragments bound into the network |
The process is extremely fast — often a fraction of a second — which is why UV curing competes with thermal drying on production speed. The critical variables are the light source (its wavelength and intensity), the film thickness, the pigment load and the photoinitiator, and all four must be matched. If the light does not reach the initiator, or the initiator does not absorb the light available, cure suffers.
Photoinitiator 184 in LED-Curing Systems
The shift from traditional mercury lamps to UV-LED curing is one of the biggest trends in the coatings and printing industry. UV-LED lamps are more efficient, longer-lived and free of mercury, but they emit in a narrow band at longer wavelengths (commonly around 385–395 nm) rather than across the UV spectrum. This has direct consequences for photoinitiator selection.
- The mismatch. Photoinitiator 184 absorbs strongly in the UV-C/UV-B but weakly at the wavelengths emitted by UV-LED lamps, so used alone it cures LED systems relatively poorly.
- The remedy. LED-curable formulations pair 184 with a red-shifted initiator such as TPO (or another acylphosphine oxide) that absorbs efficiently at LED wavelengths, giving both surface and through-cure.
- Formulating for LED. The combination is tuned to the specific LED wavelength, the film thickness and the line speed; there is no single universal recipe.
For a formulator, the practical message is that photoinitiator 184 remains valuable in LED systems, but as part of a blend rather than on its own. This is one of the clearest examples of why choosing a photoinitiator is a question of matching absorption to the light source, not of picking the “strongest” product.
Low-Migration Photoinitiators and Food Packaging
In packaging and other sensitive applications, a cured coating must not release initiator residues or photolysis products into the packaged goods. This “migration” concern has driven the development of low-migration photoinitiators and of guidelines for their use.
- Why migration happens. Small, unreacted initiator molecules or their fragments can diffuse out of the cured film over time; the higher the residual level and the smaller the molecule, the greater the risk.
- How it is controlled. Using higher-molecular-weight or difunctional initiators, ensuring complete cure (so little initiator remains), and selecting grades and systems validated for the application reduce migration.
- Where photoinitiator 184 stands. It is a relatively small, monofunctional molecule and is not the lowest-migration option; where low migration is critical, a higher-molecular-weight or difunctional AHK such as photoinitiator 127 is often preferred, or a low-migration grade and a validated formulation is used.
- Documentation. For food-contact use, the relevant migration data, statements and regulatory approvals must be confirmed for the specific grade and use.
This is why “which photoinitiator?” cannot be answered by reactivity alone: for packaging and medical uses, migration and regulatory compliance often outweigh raw cure speed.
Handling, Storage and Shelf Life
- Low-melting solid. Photoinitiator 184 melts at about 45–50 °C, so store it cool to keep it solid and free-flowing and to avoid caking during hot weather.
- Light and heat. Keep containers sealed and away from light and heat, which can affect colour and cause gradual degradation.
- Moisture. Store dry; avoid moisture uptake that could affect the product.
- Dust and hygiene. Avoid generating and inhaling dust; use appropriate ventilation and personal protective equipment, following the SDS.
- Shelf life. The product is stable for a defined shelf life when stored correctly; confirm the shelf life and retest policy with the supplier.
Frequently Asked Questions
What is photoinitiator 184 and what is its CAS number?
Photoinitiator 184 is 1-hydroxycyclohexyl phenyl ketone, a Type I alpha-hydroxyketone UV photoinitiator; its CAS number is 947-19-3. See the product page for specifications.
Is photoinitiator 184 the same as Irgacure 184?
Yes. Irgacure 184 and Omnirad 184 are trade names for 1-hydroxycyclohexyl phenyl ketone, the same substance as photoinitiator 184.
What is the molecular formula of photoinitiator 184?
C13H16O2, with a molecular weight of 204.26 g/mol.
What does “Type I” photoinitiator mean?
It means the initiator generates radicals by unimolecular cleavage (alpha-cleavage) upon absorbing UV light, rather than by abstracting hydrogen from a co-initiator as Type II initiators do.
What is the main limitation of photoinitiator 184?
Its absorption is strongest in the UV-C/UV-B region and weak above about 300 nm, so it works best in clear, relatively thin films; pigmented or thick films usually need a red-shifted initiator such as TPO as well.
Is photoinitiator 184 low-migration?
Being a relatively small molecule, it is not the lowest-migration option; where low migration is essential, a higher-molecular-weight initiator such as photoinitiator 127 may be preferred. Grade and application must be matched.
What is photoinitiator 184 used for?
Mainly in UV-curable clear coatings, printing inks, overprint varnishes, adhesives, photoresists and 3D-printing resins, where fast surface cure with low odour and low yellowing is wanted.
Can photoinitiator 184 be used on its own?
Yes for clear, thin films; for pigmented, filled or thick films it is usually blended with a longer-wavelength initiator to achieve through-cure.
Does photoinitiator 184 work with UV-LED lamps?
Poorly on its own, because it absorbs weakly at the longer wavelengths LED lamps emit. In LED systems it is used together with a red-shifted initiator such as TPO.
What is the melting point of photoinitiator 184?
About 45–50 °C, which is why it is stored cool to keep it solid and free-flowing.
Is photoinitiator 184 odour-free after curing?
It is chosen for its low odour and low yellowing, because its photolysis fragments tend to be bound into the cured network rather than left as volatile molecules.
How is photoinitiator 184 supplied?
Usually as a white crystalline powder or flakes, and also in liquid or one-pack blends with other photoinitiators such as TPO.
What light source is best for photoinitiator 184?
Lamps that emit strongly in the UV-B/C region, such as traditional mercury lamps, suit 184 well; for UV-LED lamps that emit at longer wavelengths, it is used with a red-shifted co-initiator.
What is the CAS number of Irgacure 184?
947-19-3 — the same as photoinitiator 184, since Irgacure 184 is a trade name for 1-hydroxycyclohexyl phenyl ketone.
Is photoinitiator 184 a Type I or Type II initiator?
Type I. It generates radicals by unimolecular alpha-cleavage on absorbing UV light.
Can photoinitiator 184 be used in water-based UV formulations?
It is used in UV systems, including dispersed forms for water-based and other formats where a powder is difficult to incorporate; the exact approach depends on the formulation, so confirm it with the supplier.
Why is colour important for photoinitiator 184?
Because it is used in clear and light-coloured coatings, where a batch with high colour or impurities would tint the film; low colour is a key quality parameter.
References and Disclaimer
References. Substance identification data for 1-hydroxycyclohexyl phenyl ketone (CAS 947-19-3); standard photochemistry of alpha-hydroxyketone Type I photoinitiators and of radical UV curing; manufacturer technical data for the powder grade. Specifications quoted are representative and should be confirmed against the current certificate of analysis.
Disclaimer. This article is provided for general technical and commercial information only and does not constitute a specification or a warranty of fitness for any particular use. Users are responsible for confirming regulatory status, safe handling and suitability of the material for their own application.