Cationic cure mechanisms are an alternative approach to UV curing. This involves the photogeneration of ions, which initiate ionic polymerization. Although different monomers are normally required for this type of chemistry, the main difference lies within the photoinitiator selection. Because only a limited variety of monomers or oligomers are used in cationic systems (epoxy or vinyl-ether resins), formulating versatility is less than for free radical type systems.
This cationic process is not subject to oxygen inhibition as is the free radical mechanism, and it does not require a nitrogen blanket. Unlike free radical chemistry, cationic reactions continue after the UV light has been removed. These reactions also progressively penetrate into recessed areas inaccessible by line of sight to the radiation source. In this way cationic cured epoxy adhesive can be used effectively on substrates that are not transparent to UV radiation.
RESULTING PROPERTIES
The above table shows a starting formulation for a cationic cure epoxy laminating adhesive. Some polyurethane polyols can contribute tack in addition to retarding cure. Epoxidized polybutadiene has typically provided good adhesion to plastics although it does not retard cure. Formulations for cationic UV epoxy systems containing these additives are included in this formulation.
