Photoinitiators, also known as photosensitizers or photocuring initiators, are compounds that absorb light energy under ultraviolet or visible light irradiation and generate active free radicals, cations, or acidic active centers. They are key components in UV adhesives, UV coatings, UV inks, photocurable resins, 3D printing resins, and some hydrogel crosslinking systems, directly affecting curing speed, curing depth, surface-dry state, yellowing degree, and final material properties.

Keywords: photoinitiator; UV curing; free-radical photoinitiator; cationic photoinitiator; TPO; 184; 1173; 907; BAPO; LED curing; photopolymerization

1 Basic Role of Photoinitiators

1.1 Conversion of Light Energy into Chemical Reaction

(1) Light absorption

Photoinitiator molecules contain specific chromophoric structures that can absorb ultraviolet or visible light energy. Common absorption ranges include the UV region of 250–420 nm and parts of the visible-light region of 400–800 nm. Matching the light source wavelength with the absorption band of the photoinitiator is the basic requirement for curing efficiency.

(2) Excited-state formation

After absorbing light energy, the photoinitiator transitions from the ground state to an excited singlet state, and may then form an excited triplet state through intersystem crossing. Excited-state molecules have higher energy, making chemical bonds more prone to cleavage, hydrogen abstraction, electron transfer, or photoacid generation.

(3) Generation of active centers

In the excited state, photoinitiators generate free radicals, cations, or strongly acidic active species. These active centers further initiate polymerization, crosslinking, or ring-opening reactions of monomers, oligomers, or resins, rapidly converting liquid or semi-liquid systems into solid materials.

1.2 Functions in Photocuring Systems

(1) Determining curing speed

The absorption capacity, quantum efficiency, and active-center generation efficiency of a photoinitiator directly affect the polymerization rate. The higher the initiation efficiency, the easier it is for the system to achieve surface drying and bulk curing within a short exposure time.

(2) Influencing curing depth

In thick coatings, dark inks, highly filled resins, and 3D printing materials, light entering the system can be absorbed or scattered by photoinitiators, pigments, fillers, and resins. Excessively strong photoinitiator absorption may cause rapid surface curing but insufficient deep curing; weak absorption may lead to low overall reaction efficiency.

(3) Affecting material appearance

Some photoinitiators are slightly yellow themselves or generate colored by-products after cleavage. In transparent coatings, optical adhesives, dental resins, and light-colored materials, yellowing, odor, migration, and residues should be carefully controlled.

(4) Affecting application safety

Food packaging, medical materials, biohydrogels, and dental materials are more sensitive to photoinitiator residues and migration. For such systems, curing speed should not be the only priority; biocompatibility, low toxicity, and low extractable residues should also be considered.

2 Main Types of Photoinitiators

2.1 Free-Radical Photoinitiators

Free-radical photoinitiators are mainly used in acrylates, methacrylates, unsaturated polyesters, and some vinyl systems. Their active centers are free radicals, and the polymerization speed is fast. They are the most widely used type in UV coatings, UV inks, UV adhesives, and 3D printing resins.

(1) Type I photoinitiators

Type I photoinitiators are also known as cleavage-type photoinitiators. After absorbing light energy, the molecule undergoes homolytic bond cleavage and directly generates two or more free radicals. This type has a direct reaction pathway and high initiation efficiency, and is commonly used in systems requiring rapid curing. Typical products include 1173, 184, TPO, TPO-L, BAPO, and 907.

(2) Type II photoinitiators

Type II photoinitiators are also known as hydrogen-abstraction photoinitiators. After absorbing light, they usually do not cleave directly; instead, they abstract hydrogen atoms from amines, alcohols, or other hydrogen donors to form radicals capable of initiating polymerization. Typical products include benzophenone, ITX, and DETX. These systems are commonly used in inks, colored coatings, and surface curing, but generally require amine co-initiators.

2.2 Cationic Photoinitiators

(1) Reaction mechanism

Cationic photoinitiators generate strongly acidic active species after light absorption, such as superstrong protonic acids or Lewis acids. These acidic centers can initiate cationic ring-opening polymerization of epoxy resins, oxetanes, and vinyl ethers.

(2) Typical types

Common cationic photoinitiators include diaryliodonium salts and triarylsulfonium salts. Their systems are often used in epoxy UV curing, electronic encapsulation, protective coatings, and low-shrinkage curing materials.

(3) Application characteristics

Cationic curing systems have weak oxygen inhibition, low curing shrinkage, and certain dark-reaction characteristics, meaning polymerization can continue after light exposure stops. However, these systems are easily affected by moisture, alkaline components, amines, and alkaline impurities on filler surfaces.

2.3 Visible-Light Initiation Systems

(1) Blue-light initiation systems

Camphorquinone is commonly used in dental resins and blue-light curing systems, usually together with tertiary amine co-initiators. It is suitable for visible-light irradiation, but yellowing and amine residues need to be controlled.

(2) Aqueous photoinitiation systems

Irgacure 2959, LAP, and Eosin Y can be used in hydrogels, cell encapsulation, and bioprinting systems. These systems have higher requirements for water solubility, cell compatibility, and mild light exposure.

(3) Photosensitizer synergistic systems

Some visible-light systems do not rely on a single initiator. Instead, they consist of a photosensitizer, amine electron donor, thiol, or other additives. Their efficiency depends on the matching among light absorption, electron transfer, and radical generation.

Table 1 Main Types and Characteristics of Photoinitiators

3 Performance Differences Among Common Photoinitiators

3.1 1173 and 184

(1) 1173

1173, or 2-hydroxy-2-methylpropiophenone, is a liquid Type I free-radical photoinitiator. It has good compatibility and is suitable for low-viscosity coatings, varnishes, and adhesive systems. Because its absorption band is relatively short-wavelength, it usually needs to be combined with long-wavelength photoinitiators in thick-film or 405 nm LED systems.

(2) 184

184, or 1-hydroxycyclohexyl phenyl ketone, is a commonly used low-yellowing Type I photoinitiator. It is suitable for transparent coatings, varnishes, UV adhesives, and some acrylate systems. It cures quickly and has a light color, but its ability is limited in deep curing or highly light-shielding systems.

3.2 TPO, TPO-L, and BAPO

(1) TPO

TPO is an acylphosphine oxide photoinitiator with long-wavelength absorption capability. It is suitable for 365 nm, 385 nm, 395 nm, and some 405 nm LED curing systems. It is widely used in thick films, colored coatings, and 3D printing resins, but yellowing and residues should be considered in transparent systems.

(2) TPO-L

TPO-L is a liquid acylphosphine oxide photoinitiator with good compatibility and formulation operability. Compared with solid TPO, TPO-L is more suitable for low-viscosity coatings, photocurable adhesives, and transparent-system blends.

(3) BAPO

BAPO is a bisacylphosphine oxide photoinitiator with strong long-wavelength absorption and high initiation efficiency. It is suitable for thick films, dark systems, and high-opacity systems. Its disadvantages are its inherent color and relatively high yellowing risk, so its dosage should be carefully controlled in light-colored or optical systems.

3.3 907, Benzophenone, and Thioxanthones

(1) 907

907 is an α-aminoketone Type I photoinitiator with high initiation efficiency. It is commonly used in colored systems, inks, and coatings. It has good adaptability to pigment systems but may cause yellowing, odor, or migration problems.

(2) Benzophenone

Benzophenone is a typical Type II photoinitiator and must be used with amine hydrogen donors. It is low-cost and widely used, suitable for some coatings and ink systems, but oxygen inhibition and migration problems are relatively obvious.

(3) ITX and DETX

ITX and DETX are thioxanthone photosensitizers with good long-wavelength absorption capability. They are suitable for colored inks and LED curing systems. They are valuable in pigment-shielded systems, but color, odor, and migration risks need to be controlled.

4 Key Factors in Photoinitiator Selection

4.1 Light Source Wavelength

(1) Mercury-lamp systems

Traditional mercury lamps have multi-band output and broad compatibility. 184, 1173, benzophenone, ITX, and some acylphosphine oxides can all be used in mercury-lamp systems.

(2) LED systems

LED light sources have narrow output bands, commonly including 365 nm, 385 nm, 395 nm, and 405 nm. In LED systems, photoinitiators must have effective absorption at the corresponding wavelength. TPO, TPO-L, BAPO, and some thioxanthones are more suitable for long-wavelength LED systems.

(3) Visible-light systems

Dental materials, hydrogels, and bioprinting often use blue or green light. These systems usually use camphorquinone, LAP, Eosin Y, and similar initiators, often together with amine or thiol additives.

4.2 Light Transmittance of the Curing System

(1) Transparent systems

Transparent varnishes, optical adhesives, transparent resins, and hydrogels should prioritize low-yellowing, low-residue, and low-odor photoinitiators. Excessively strong absorption may cause fast surface curing but insufficient deep reaction.

(2) Colored systems

Pigments, dyes, and carbon black absorb or block light, reducing curing depth. Colored systems usually require initiators with strong long-wavelength absorption, such as TPO, BAPO, ITX, and DETX.

(3) Highly filled systems

Inorganic fillers cause light scattering and shielding. In highly filled resins, dental composites, and 3D printing composites, the light source, filler particle size, refractive-index matching, and photoinitiator absorption curve should be considered together.

4.3 Oxygen Inhibition

(1) Tacky surface

Free-radical polymerization is susceptible to oxygen inhibition. Oxygen can capture radicals and form low-activity peroxy radicals, resulting in incomplete surface curing, tackiness, or reduced rub resistance.

(2) Improvement methods

Surface drying can be improved by increasing light intensity, extending exposure, using high-activity initiators, adding amine co-initiators, increasing resin functionality, curing under inert atmosphere, or introducing thiol-ene systems.

(3) Balance requirements

Improving oxygen inhibition should not rely only on increasing initiator or amine co-initiator dosage. Excessive initiator may increase yellowing and migration, while excessive amines may cause odor and reduced weather resistance.

4.4 Yellowing, Odor, and Migration

(1) Yellowing

Yellowing can originate from the inherent color of the initiator, cleavage products, oxidation of amine co-initiators, or overexposure. Transparent materials and light-colored coatings should focus on controlling this issue.

(2) Odor

Some small-molecule initiators and their cleavage products are volatile or leave residual odor. Low-odor coatings, food packaging inks, and medical materials should prioritize low-migration or macromolecular initiation systems.

(3) Migration

Food-contact materials, dental materials, and biomaterials are more sensitive to migration. Migration risks can be reduced by lowering dosage, improving conversion, and selecting polymerizable or macromolecular photoinitiators.

5 Selection Strategies in Different Application Scenarios

5.1 UV Coatings and Varnishes

Transparent UV coatings usually require low yellowing, high transparency, and good surface drying. 184 and 1173 can be used as basic initiators, while TPO-L can improve LED compatibility and thick-film curing ability. For outdoor weather-resistant systems, high-yellowing initiators and excessive amine co-initiators should be avoided.

5.2 UV Inks and Colored Coatings

Ink systems contain many pigments, limiting light penetration. TPO, BAPO, ITX, and DETX are often used to enhance long-wavelength absorption and deep curing. Surface curing can be improved through Type II initiators and amine co-initiators, but food packaging inks require careful evaluation of migration and odor.

5.3 Photocurable Adhesives

Adhesives must balance interfacial curing, thick-layer penetration, and bonding stability. Transparent adhesives can use low-yellowing initiators, while thick adhesive layers can be combined with TPO-L or BAPO. In shadowed areas, light/thermal dual-curing or light/moisture dual-curing systems can be used.

5.4 3D Printing Resins

In 3D printing, photoinitiators affect curing depth, interlayer bonding, and printing accuracy. Excessive initiator can cause overcuring, loss of detail, and dimensional deviation; insufficient initiator can result in low interlayer strength, molding failure, or insufficient post-curing. 405 nm printing systems commonly use TPO, BAPO, LAP, and similar initiators.

5.5 Hydrogels and Bioprinting

Hydrogel and bioprinting systems place greater emphasis on water solubility, cell compatibility, and mild light exposure. LAP, Irgacure 2959, and Eosin Y/triethanolamine systems are widely used. These systems require simultaneous control of light dose, initiator concentration, and cell damage.

Table 2 Key Photoinitiator Selection Points in Different Application Scenarios

6 Common Problems and Adjustment Directions

6.1 Tacky Surface

(1) Causes

Mainly related to oxygen inhibition, insufficient surface radicals, insufficient light intensity, insufficient exposure time, or insufficient amine co-initiator.

(2) Adjustments

Light intensity can be increased or exposure time extended; surface-dry initiation systems can be strengthened; an appropriate amount of amine co-initiator can be added; or curing can be performed under an inert atmosphere.

6.2 Insufficient Deep Curing

(1) Causes

Usually caused by mismatched light-source wavelength, pigment shielding, excessive film thickness, filler scattering, or unreasonable initiator absorption distribution.

(2) Adjustments

Long-wavelength absorbers such as TPO, TPO-L, and BAPO can be selected; film thickness can be reduced; pigment and filler systems can be optimized; or multi-band light sources can be used.

6.3 Obvious Yellowing

(1) Causes

High-yellowing initiators, oxidation of amine additives, residual cleavage products, or excessive exposure may all cause yellowing.

(2) Adjustments

The dosage of easily yellowing components such as TPO, BAPO, and 907 can be reduced; low-yellowing initiator blends can be used; and excessive amine additives should be avoided.

6.4 Odor and Migration

(1) Causes

Residual small-molecule initiators, volatile cleavage products, insufficient resin conversion, or additive migration can all cause odor and migration.

(2) Adjustments

The curing conversion rate can be increased; low-migration or macromolecular photoinitiators can be selected; total dosage can be reduced; and residues can be reduced through post-curing.

Table 3 Common Problems in Photocuring and Adjustment Directions

7 Selection of Common Photoinitiators and Supporting Components

Table 4 Selection of Free-Radical Photoinitiators

Table 5 Selection of Co-Initiators and Cationic Photoinitiators

Photoinitiator selection should be based on light-source wavelength, resin type, film thickness, pigments and fillers, oxygen inhibition, yellowing, and migration risks. Transparent systems emphasize low yellowing and low residues; colored and thick-film systems emphasize long-wavelength absorption and deep curing; hydrogels and biomaterials focus more on water solubility, mild curing, and biocompatibility.