1.1 Overview of Photoinitiators
Photoinitiators are the "engine" of UV-curing technology-they are a class of compounds that, upon exposure to ultraviolet (UV) or visible light, absorb light energy and undergo photochemical reactions to generate reactive species (free radicals or cations). These active species then initiate chain-growth polymerization of monomers or prepolymers containing unsaturated groups. It is this instantaneous "light-to-chemistry" conversion that transforms a liquid formulation into a solid crosslinked network within seconds or even milliseconds, enabling applications such as coating, adhesive bonding, and 3D shaping.
The performance of photoinitiators directly determines the curing speed, conversion rate, yellowing tendency, deep-curing capability, and final mechanical properties of the cured product. Based on the type of reactive species generated upon irradiation, photoinitiators are broadly divided into two major categories: cationic photoinitiators and free-radical photoinitiators. Our company maintains complete technical expertise and supply capabilities for both product families, meeting diverse industrial needs ranging from UV mercury lamps to LED light sources, from clear systems to highly pigmented formulations, and from thin coatings to thick-film curing.
1.2 Cationic Photoinitiators
1.2.1 Chemical Classification and Structural Characteristics
Cationic photoinitiators decompose upon light irradiation to generate protonic acids or Lewis acids, which subsequently initiate cationic polymerization. They are primarily suitable for the polymerization of electron-rich monomers such as epoxides, vinyl ethers, and oxetanes. Currently commercialized cationic photoinitiators mainly include the following types:
Diazonium salts (e.g., aryl diazonium tetrafluoroborates): The earliest commercialized cationic initiators, but they release nitrogen gas upon decomposition, which tends to cause bubble formation, and their thermal stability is relatively poor. They are now mostly used in specific systems.
Diaryliodonium salts: These exhibit high photoactivity, with absorption maxima primarily in the 250–300 nm range. Their absorption can be extended to longer wavelengths with appropriate sensitizers. They have better thermal stability than diazonium salts and are among the most widely used cationic photoinitiators.
Triarylsulfonium salts: They offer high photolytic acid-generation efficiency, excellent thermal and storage stability, and slightly longer absorption wavelengths than iodonium salts. They also exhibit low corrosion toward metal substrates, making them highly desirable in electronic packaging, optical fiber coatings, and related applications.
Alkylsulfonium salts: These have good solubility and are suitable for certain specialty monomer systems, but their thermal stability is moderate.
Ferrocenium salts: They feature unique absorption spectra with some response in the visible region, enabling visible-light curing, but their cost is relatively high.
Sulfonyloxy ketones and triarylsilyl ethers: These are non-ionic cationic initiators that generate acid upon photolysis to initiate polymerization. They exhibit "dark-reaction" characteristics, which are beneficial for deep curing of thick films.
1.2.2 Core Advantages and Application Areas
Cationic UV curing offers significant advantages over free-radical UV curing:
No oxygen inhibition: Cationic active species are not quenched by oxygen, so surface curing is not affected by atmospheric oxygen-particularly advantageous for thin coatings and systems where oxygen exclusion is difficult.
Low polymerization shrinkage: Ring-opening polymerization of epoxy monomers results in low volumetric shrinkage, which benefits adhesion and dimensional accuracy.
Post-irradiation curing ("dark curing"): The active centers in cationic polymerization have relatively long lifetimes, so polymerization can continue for some time even after the light source is removed, facilitating curing of thick films and shaded areas.
Excellent adhesion to various substrates including metals, glass, and plastics.
Our company offers cationic photoinitiator products covering diaryliodonium salts, triarylsulfonium salts, and certain specialty ferrocenium salts, compatible with UV mercury lamps, LED-UV (365 nm, 385 nm, 395 nm), and low-power UV sources. They are widely used in:
Electronic adhesives (underfill, dam-and-fill, COG encapsulation)
UV-curable epoxy coatings (metal and glass decoration)
3D printing photopolymer resins (cationic systems)
Optical fiber coatings
Offset printing plates
1.3 Free-Radical Photoinitiators
1.3.1 Chemical Classification and Typical Varieties
Free-radical photoinitiators currently represent the largest volume and most diverse category in the UV-curing market. Upon irradiation, they generate free radicals via cleavage (Type I) or hydrogen abstraction (Type II), initiating polymerization of acrylates, methacrylates, unsaturated polyesters, and other monomers. Major varieties include:
Benzoin and its derivatives (e.g., benzoin methyl ether, benzoin ethyl ether): Early classic initiators, but their thermal stability and yellowing issues have limited their use.
Benzil derivatives (e.g., diphenylethanedione): Used as Type II hydrogen-abstraction initiators, requiring tertiary amine co-initiators.
Dialkoxyacetophenones (e.g., α,α-dimethoxy-α-phenylacetophenone, DMPA): Type I cleavage initiators with high photolysis efficiency, suitable for clear varnishes.
α-Hydroxyalkylphenones (e.g., 1173, 184): Currently the most widely used Type I initiators. They produce benzoyl and alkyl radicals upon photolysis, offering fast curing, excellent surface cure, and relatively low yellowing.
α-Aminoalkylphenones (e.g., 907, 369): These have red-shifted absorption, making them suitable for pigmented systems. They also contain tertiary amine structures that reduce oxygen inhibition.
Acylphosphine oxides (e.g., TPO, 819): Featuring long-wavelength absorption (up to 405 nm), they are suitable for pigmented systems and LED curing. Their photobleaching effect benefits deep-layer curing.
Benzophenone and its derivatives (e.g., BP): Typical Type II hydrogen-abstraction initiators that require tertiary amine co-initiators. They provide excellent surface cure and are low in cost.
Thioxanthones (e.g., ITX, DETX): Absorption can reach beyond 400 nm, commonly used in pigmented ink systems in combination with amine co-initiators.
Anthraquinones: Rarely used alone; more often employed as photosensitizers or auxiliary initiators.
1.3.2 Selection Guidelines and Application Advantages
The core advantages of free-radical photoinitiators are fast curing speed, high initiation efficiency, excellent cost-performance, and the availability of options across different wavelength bands to match various light sources (especially LEDs). Based on the formulation system, our company can supply products suitable for:
Clear varnishes and transparent topcoats: Recommended α-hydroxyalkylphenones (184, 1173) or acylphosphine oxides (TPO) to balance surface and deep curing.
White, high-titanium-dioxide pigmented systems: Long-wavelength absorbing acylphosphine oxides (819 or TPO-L) are required to overcome the UV shielding effect of the pigment.
UV inks (flexo, screen, offset): Typically combinations of benzophenone, thioxanthones, and amine co-initiators to achieve a balance between rapid surface drying and through-cure.
UV inkjet inks: Low-viscosity, low-yellowing, highly reactive α-aminoalkylphenones (e.g., 907, 369) combined with TPO.
Our free-radical photoinitiator product line covers all major chemical types listed above. We also provide customized blending recommendations based on the customer's light source type (mercury lamp, electrodeless lamp, LED), pigment type, film thickness, and curing speed requirements.
1.4 Photosensitizers – Broadening the "Spectral Window" of Photoinitiators
1.4.1 Why Are Sensitizers Needed?
Although onium salt cationic photoinitiators (diaryliodonium salts, triarylsulfonium salts) have relatively strong absorption in the short-wavelength UV region (<300 nm), their absorption in the long-wavelength UV and visible regions (above 300 nm) is extremely weak. This leads to two practical challenges:
Conventional medium-pressure mercury lamps have strong emission in the 300–400 nm range, but onium salts cannot fully utilize this energy, resulting in wasted light energy and relatively low curing speeds.
LED-UV light sources (365 nm, 385 nm, 395 nm, 405 nm) emit only narrow-band long-wavelength UV light, to which onium salts absorb even more weakly, making them almost unusable directly.
1.4.2 Mechanism of Photosensitizers
Photosensitizers are a class of compounds that themselves possess strong absorption at longer wavelengths (e.g., thioxanthones, anthraquinones, acenaphthene derivatives, coumarin ketones, etc.). Their mechanism of action is as follows:
The sensitizer (S) absorbs light energy and transitions to an excited state (S*). The excited sensitizer then transfers energy to the onium salt initiator (On⁺) via electron transfer, energy transfer, or hydrogen abstraction, promoting the decomposition of the onium salt to generate active cations or radicals, thereby initiating polymerization.
This process effectively decouples the functions of light absorption and active-species generation, allowing onium salts that are otherwise "inactive" in the long-wavelength region to be efficiently utilized. For free-radical UV systems, in formulations with dark pigments (e.g., carbon black, phthalocyanine blue), sensitizers can also compensate for the "inner-filter effect" of pigments that absorb incident light, thereby improving curing efficiency.
1.4.3 Sensitizer Solutions Offered by Our Company
We provide a variety of photosensitizers compatible with both cationic and free-radical photoinitiators, including but not limited to:
Thioxanthones (ITX, DETX, CPTX)
Anthraquinones
Coumarin ketones
Specially designed electron donor–acceptor complex systems
Through rational combination of sensitizers, we can achieve:
For cationic systems: Extending the effective excitation wavelength of iodonium or sulfonium salts to 365–405 nm, adapting to LED light sources, and achieving curing depths of several millimeters (deep curing of thick films).
For pigmented free-radical systems: Significantly improved curing speed and surface drying in formulations containing pigments such as titanium dioxide and carbon black.
Overall enhancement of curing efficiency, reduction of photoinitiator dosage, cost savings, and minimized residual odor and yellowing.
1.5 Our Comprehensive Advantages
In the field of photoinitiators, our company not only offers a full range of cationic, free-radical, and photosensitizer products but also possesses the following core capabilities:
Systematic formulation support: From single initiators to combined initiating systems, from light-source matching to process optimization-we provide comprehensive technical consultation.
Quality consistency: Strict quality control procedures ensure high batch-to-batch consistency in activity, solubility, color, and storage stability.
Regulatory compliance and safety: Products comply with environmental regulations such as REACH and RoHS, and complete MSDS documentation is available.
Whether you require a fast-curing free-radical system, a low-shrinkage high-adhesion cationic system, or an LED-curing solution with deep-layer curing capability, we can provide the most suitable photoinitiator solution for your needs. Please feel free to contact our technical team for product samples and recommended formulations tailored to your specific application.