Chapter Cationic chemistry
Cationic chemistry Cationic curing is a relatively small but growing area of interest in the UV curing industry, mainly due to the inherently more expensive materials that are used compared with free radical systems, but it brings a new set of parameters and a different chemistry to widen the scope and application of UV curing. Cationic curing involves the use of UV energy to generate a protonic acid from the cationic photoinitiator. Unlike free radicals, which promote a chain polymerization of acrylate-type monomers, the protonic acid will initiate a ring opening polymerization of epoxy resins, forming polyethers. These resins bring properties to UV-cured coatings that differ from those of acrylates, and cationic curing has found several niche markets such as metal decorating, adhesives, and inks where the advantages of excellent adhesion, insensitivity to oxygen, and good chemical resistance of the polymer are required. Cationic curing, compared with free-radical curing, provides coatings that
Are insensitive to oxygen during cure• Give very low shrinkage• Give excellent adhesion and chemical resistance• Are subject to thermal post-cure• Produce a more uniform and stable polymer• Are of an acidic nature•
The process is easily inhibited by trace amounts of basic materials such as amines, urethanes, fillers, and some basic pigments such as magenta 57:1.