Curing Properties of Oxetane Derivatives: 3-Ethyl-3-hydroxymethyl-oxetane Formulated with Epoxides

UV-cationic-curing, based on the photo-generation of acid by and consecutive cationic polymerization, was first proposed in 1970's (1). Since then, wide variety of cationically polymerizable monomers such as epoxides and vinyl ether derivatives have been investigated (2). Among these, formulations based on cycloaliphatic epoxy resins (such as '-epoxy cyclohexane-carboxylate: EGG) are known to give cured coatings with high thermal capability, excellent adhesion and good chemical resistance. The curing rate of EGG is rather high, but still much lower than that for radical ones. Thus, it has been desired to develop novel cationic formulations keeping the good performance capabilities, while at the same time possessing higher reactivity competitive with radical system. One approach was addition of alcoholic compounds. Crivello first studied the positive effect of alcohol on cure rate in UV-cationic polymerization of EGG (3). In an industrial application, formulation of polyether- or polycaprolactone-polyols is widely used as effective method to improve the curing rate and modify the cured coating property (4).
Another approach proposed by our laboratory is an introduction of a four membered cyclic ether;
Oxetane, possessing rather high ring strain energy (107 kJ/mol) and basicity (pKa = 2.02) (5). In the UV cationic polymerization, oxetane derivatives were found to exhibit quite different polymerization characteristics from those for epoxides (6). In spite of rather slow initiation, propagation of oxetane polymerization proceeded smoothly until high conversion of oxetane ring and gave higher molecular weight polymer. In the formulation with epoxy monomer, the slow initiation of oxetane monomer was improved drastically and exhibited fairly fast polymerization. Previously, we demonstrated that 3-ethyl-3-hydroxymethyloxetane (OXA), possessing oxetane ring and methylol group in the molecule, effectively increase the curing rate of EGG (7). The hydroxy unit in OXA should work as chain transfer agent in the formulation and resulted in a cross-linked network. Because of hydrophilic nature of OXA, relatively high amount of water will permeate into the formulation under humid conditions. The effect of water should be very important issue for the formulations, but not studied experimentally so far. In this study, using the samples kept under high humid condition, the consequence of contaminated water on the curing properties of OXA and EGG formulations was evaluated. The roll of hydroxy groups in water and/or OXA in the polymerization step was also investigated in conjunction with the curing mechanism of ECC.