Chain Transfer Agents in Cationic Epoxide Polymerizations: Kinetic and Physical Effects

摘要

UV/EB curing is a rapid and energy efficient method used to produce protective coatings, adhesives and sealants, templates for electronic devices, and even biomaterials such as tissue scaffolds. A variety of chemical polymerization mechanisms are initiated via radiation curing including free radical (both chain and step polymerization), cationic, and anionic. Free-radical polymerization is the most commonly used mechanism in UV/EB curing due to the low cost materials and high rates of polymerization. Cationic polymerization is used at lower volumes; anionic polymerization has been demonstrated to work in research laboratories, but to the author's knowledge has not yet been implemented in industry. Though cationic systems are more costly than the free-radical systems (in terms of monomers and photoinitiators), the use of cationic resins can be economically viable do to the tolerance of atmospheric oxygen. Free-radical systems are inhibited by molecular oxygen in both the active center generation step (via photoinitiation) and the subsequent polymerization.Conversely, cationic systems are unaffected by molecular oxygen in both the photoinitiation and polymerization steps. The oxygen-tolerating capacity of cationic systems allows for good cure of thin films with hard, fully cured surfaces without the use of expensive purging equipment and inert gases (N2, CO2, and Ar). Other characteristics that set cationic systems apart include: monomer side reactions with other nucleophilic compounds (water and alcohols) and long-lived active centers (virtually no chemical termination occurs in many systems).