Toagosei Annual R&D Report “TREND” – Issue No. 2

Oxetane compounds are cationic curable resin components designed for photo-cationic curing and cationic ring-opening polymerization. Unlike radical UV curing, cationic UV curing is not inhibited by oxygen in the air, enabling oxygen-inhibition-free curing in solvent-free UV-curable resin systems. This makes cationic curing particularly useful where no oxygen inhibition at the surface is required. The highly basic, strained four-membered cyclic ether ring gives oxetane compounds polymerization behavior that differs from conventional epoxy resin, making them valuable ring-opening polymerization resin components for a UV-curable epoxy system.

For formulators developing cationic epoxy UV curing materials, oxetane compounds can be used as a cationic curable additive, epoxy resin additive, or reactive diluent for epoxy formulations. In the formulations described in this technical review, low-viscosity monofunctional oxetane compounds substantially reduced formulation viscosity without lowering curability at the tested addition level. These materials therefore have potential as low-viscosity UV resin components and low-viscosity epoxy additives.

Bifunctional oxetane compounds maintained high surface curability and improved acetone resistance immediately after UV curing. In comparison, the alicyclic epoxy resin used alone showed a significant post-cure contribution and limited acetone resistance immediately after irradiation. The addition of bifunctional oxetane compounds promoted rapid crosslinking and the formation of a strong polymer network, improving performance immediately after curing.

Oxetane alcohol contains both an oxetane ring and a hydroxyl group and can function as a low-viscosity diluting component, a polymerization accelerator, and a crosslinking component in cationic curing formulations. In a model photo-cationic polymerization system, adding a small amount of epoxy compound accelerated the disappearance rate of the oxetane ring by more than sevenfold. This result demonstrates the complementary reactivity of oxetane and epoxy components in cationic curing and shows how their combination can support rapid network formation.

Oxetane chemistry is also attractive for the design of a low-shrinkage UV resin. A bifunctional xylylene dioxetane compound was reported to exhibit low curing shrinkage of 3.9% during photo-cationic polymerization. By combining low viscosity, cationic ring-opening, crosslinking, oxygen tolerance, high surface curability, and relatively low shrinkage, oxetane compounds provide a versatile platform for the development of advanced photo-curing materials.

The technical review describes the molecular design and synthesis of oxetane compounds, their polymerization behavior and reaction mechanisms, their interaction with epoxy compounds, and their application to photo-cationic curing systems.