What does a photoinitiator do?

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The “Magic of Light”: How Photoinitiators Are Revolutionizing Curing Technology Today we’ll explore several commonly used photoinitiators in everyday life and their functions, hoping this information proves helpful.

Principles and Importance of Photoinitiators

Photoinitiators serve as the core component in photopolymerization systems. Within such systems—including UV adhesives, UV coatings, and UV inks—they absorb light energy at specific wavelengths, transitioning from the ground state to an excited state. Subsequently, through intersystem crossing, they reach the excited triplet state. Via unimolecular or bimolecular chemical reactions, they generate reactive fragments such as free radicals and cations, thereby initiating monomer polymerization and crosslinking curing. Their initiation efficiency directly determines the photopolymerization rate and influences product performance, making them crucial in radiation curing—an energy-saving, environmentally friendly “green technology.”

Free Radical Photoinitiators

Cracking Photoinitiators

Cracking photoinitiators (such as 1173, 184, etc.) absorb ultraviolet light quanta, causing the covalent bond between the carbonyl group and the adjacent carbon atom within the molecule to break. This generates primary radicals that initiate polymerization, cross-linking, and grafting reactions, rapidly curing the liquid into a film. Taking 184 (1-hydroxycyclohexylphenyl ketone) as an example, its photolysis mechanism is as follows: After absorbing light energy, it transitions from the ground state to an excited state, subsequently undergoing a Norrish I reaction. The covalent bond between the carbonyl group and the adjacent carbon atom in the molecular structure breaks, producing two types of radicals. Such photoinitiators are widely used in scenarios requiring rapid curing, such as photopolymer resins for 3D printing, enabling resin solidification within short timeframes to ensure printing precision and efficiency. In wood coatings, they rapidly form hard, wear-resistant coatings, enhancing production efficiency.

Hydrogen-Donating Photoinitiators

Hydrogen-donating photoinitiators (e.g., BP, benzophenone) undergo photoexcitation and abstract hydrogen atoms from hydrogen donors such as reactive monomers or low-molecular-weight prepolymers, forming reactive radicals that initiate polymerization. The reaction mechanism is as follows: The photoinitiator molecule is photoexcited to an excited state, abstracts a hydrogen atom from the hydrogen donor RH, and generates the photoinitiator radical XH・ and the monomer radical R・ (Figure 2). In practical applications, BP is commonly used in UV ink systems. When combined with amine accelerators, it enhances the initiation effect, improves ink curing speed and adhesion, and is widely used in the printing industry to meet high-speed printing demands.

Cationic Photoinitiators

Cationic photoinitiators include diazonium salts, diaryliodonium salts, and triarylsulfonium salts. Upon photoactivation, these molecules transition to an excited state and undergo a series of decompositions to generate hyperprotonic acids, initiating cationic polymerization of epoxy compounds, vinyl ethers, and similar materials. Taking the commonly used diaryliodonium salt I-250 as an example, photolysis can simultaneously induce ortho- and para-cleavage, generating both hyperprotonic acids and reactive free radicals. This enables simultaneous initiation of cationic and radical polymerization. Cationic photoinitiators find extensive application in electronic packaging for curing epoxy resins. Their curing process remains unaffected by oxygen inhibition, exhibiting strong “post-curing” capability to ensure long-term stability of electronic components in harsh environments. In optical fiber coating, they form uniform, highly adhesive layers that shield fibers from external environmental corrosion.

Special-Type Photoinitiators

Water-Based Photoinitiators

Water-based photoinitiators are synthesized by introducing ammonium salt or sulfonate functional groups into conventional photoinitiators. They primarily belong to the aryl ketone class, such as benzophenone derivatives and thioanthraquinone derivatives. They address the poor solubility of traditional photoinitiators in aqueous systems, making them suitable for eco-friendly products like water-based UV coatings and water-based inks. In architectural coatings, water-based photoinitiators enable water-based coatings to form high-performance films after light curing, meeting environmental requirements while delivering excellent weather resistance and decorative properties.

Macromolecular Photoinitiators

Macromolecular photoinitiators are obtained by incorporating conventional photoinitiators into macromolecular chains. They exhibit excellent compatibility with resins, remain non-migratory and non-volatile after curing, and reduce odor. When applied in automotive interior coatings, macromolecular photoinitiators prevent residual volatilization and odor associated with small-molecule photoinitiators. Simultaneously, they ensure the coating possesses superior mechanical properties and chemical resistance, thereby enhancing the quality of automotive interiors.

That’s all for today! I hope this article helps you grasp the fundamentals of photoinitiators more easily!

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