Photoinitiator is a substance that can absorb radiant energy and produce active intermediates with initiator polymerization ability through chemical changes. It is also the main component required by any UV curing system. Photoinitiators can be divided into hydrogen abstraction type and cracking type; Hydrogen scavenging type is a bimolecular photoinitiator, which needs to cooperate with a compound containing active hydrogen (generally called co initiator) to form free radicals through hydrogen scavenging reaction; Cleavage photoinitiator is a single molecule photoinitiator, which is decomposed into free radicals after laser.
(1) Hydrogen abstraction type: take xylenone (BP) as an example. When benzophenone is used alone, olefin monomers cannot be photopolymerized, and their needs to become photoinitiators are different. The reaction mechanism is different alkyl and aryl. When extracting hydrogen atoms from alcohols and ethers, oxygen can easily quench the excited benzophenone. When extracting hydrogen atoms from amines, ketones form excited complexes with amines immediately after they form excited states, which avoids energy transfer to oxygen molecules. Therefore, amine systems are not easy to be quenched by oxygen. Compared with alcohol ether systems, it also reduces the possibility of energy transfer to monomers. Therefore, amine system is generally used in practical application. In addition to benzophenone, this kind of photoinitiator also has anthraquinone and thioxanthone, such as 2-isopropyl thioxanthone, which is often used in UV ink.
(2) Pyrolysis type: Taking benzoin ethers as an example, benzoin ether was once the most widely used photoinitiator, which is characterized by the direct decomposition of excited grams into total free radicals. The generated free radicals can initiate monomer polymerization. Benzoin ether has a short excited state life and is not easy to be quenched by oxygen or styrene, so it can be used for the polymerization of styrene. However, benzoin ether has different degrees of thermal decomposition even in the absence of light, and the storage stability is not good. Generally, stabilizer and polymerization inhibitor should be added. At present, benzoin ether is commonly used.
Principles for selecting photoinitiators:
a. High light absorption efficiency in UV range;
b. Good relative stability;
c. Low cost.
