What is a photoinitiator, and what are its main processes and applications?
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Photoinitiators, also known as photosensitizers and photocuring agents, are substances that can generate free radicals under light exposure and thereby initiate polymerization reactions. Based on the type of radiation absorbed, photoinitiators can be classified into ultraviolet (250–420 nm) initiators and visible light (400–700 nm) initiators. Depending on their molecular structure, they exhibit different properties; therefore, paint and ink manufacturers need to select appropriate photoinitiators based on their specific requirements. Among them, 184 is efficient and resistant to yellowing; it focuses on surface curing. It can be used together with light initiators such as TPO that exhibit low yellowing tendency in white-based systems. It is primarily applied in furniture and wood varnishes, floor coatings, electronic product coatings, adhesives, automotive interiors, home decorations, paper glazes, plastic products, and more ； The 1173 synthesis process is relatively simple and its cost is low; it provides good curing effects when used in combination with materials such as TPO. It is primarily applied in furniture and wood coatings, floor coatings, coatings for electronic products, home decorations, paper varnishes, and plastic products. Photoinitiators are mainly divided into two categories: radical polymerization photoinitiators and cationic polymerization photoinitiators, among which radical-type photoinitiators are the most widely used. Free-radical photoinitiators are further subdivided into cleavage-type photoinitiators and hydrogen-abstraction-type photoinitiators based on their mechanism of generating free radicals. Therefore, photoinitiators, formally known as UV-curing photoinitiators, can be classified into three categories based on their mechanism:
① Pyrolytic initiators: Pyrolytic radical photoinitiators refer to those in which the photoinitiator molecule absorbs light energy and transitions to an excited singlet state; through intersystem crossing, it reaches an excited triplet state. In either the excited singlet or triplet state, the molecular structure becomes unstable, and the weak bonds within it undergo homolytic cleavage to generate primary active radicals, which initiate the polymerization and cross-linking of oligomers and reactive diluents. Pyrolytic radical photoinitiators are mostly arylalkyl ketone compounds, including benzoin and its derivatives (benzoin ethers), benzoyl and its derivatives (such as 651), acetophenone derivatives (such as DEAP), α-hydroxyketone derivatives (such as 1173, 184, 2959), α-aminoalkylacetophenones (907, 369), and acylphosphine oxides (TPO, TPO-L, 819). ② Hydrogen-abstraction type photoinitiators (photosensitive initiators): These initiators generate free radicals through hydrogen-abstraction reactions, such as BP. Its hydrogen abstraction mechanism is as follows: the excited photoinitiator molecules abstract hydrogen atoms from hydrogen donors such as reactive monomers and low-molecular-weight prepolymers, turning them into active free radicals, which in turn initiate the polymerization reaction. . The main ones are benzophenones and their derivatives, thioxanthones (ITX, DETX), and anthraquinones (2-EA). ③ Cationic photoinitiators: This is another very important category of photoinitiators, including diazonium salts, diaryl iodides, triaryl sulfides, alkyl sulfides, ferroaromatic salts, sulfonyl oxoketones, and triaryl siloxanes, among others. Its basic functional characteristic is that photoactivation brings the molecules to an excited state, after which a series of decomposition reactions occur, ultimately yielding super-strong protonic acids (also known as Brønsted acids or Lewis acids) that serve as active species for cationic polymerization, thereby initiating the polymerization of epoxides, vinyl ethers, lactones, acetals, cyclic ethers, and similar compounds. Cationic photoinitiators can be divided into salts, metal-organic compounds, and organosilanes, among which iodide salts, sulfide salts, and iron aryl compounds are the most representative.
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