Synthesis and kinetics study of cycloaliphatic epoxide coatings

Cycloaliphatic epoxides are the most widely used epoxide resins in the field of cationic UV-curable coatings due to their fast cure response, low potential for skin irritation, low shrinkage, and low dielectric constant. The ultraviolet-curing (UV-curing) kinetics of cycloaliphatic epoxide/polyol coatings, especially the “dark-cure” process, was investigated using real-time FT-IR spectroscopy with an optic fiber UV-curing system. The kinetic data showed that the rate of curing was dependent on hydroxyl equivalent weight, R-value, type of epoxide, and photoinitiator. The synergistic effect of water and polyols on the UV-curing kinetics was explored. Both water and polyol can be proton donors, chain transfer agents, and chain termination agents in the cationic UV-curing process. The UV-curing rate of a formulation with polyol reached a maximum at lower relative humidity than that of a formulation without polyol. Mechanisms for the UV-curing reaction at low, intermediate, and high relative humidity were proposed.; In addition, siloxane functionalized polyols were synthesized by reacting tetraethyl orthosilicate (TEOS) or 3-isocyanatopropyltriethoxysilane (IPTES) with ϵ-caprolactone polyols and used as co-crosslinkers/modifiers for the epoxide/polyol coatings. In comparison with the epoxide/unmodified polyol coatings, the incorporation of TEOS-functionalized polyol increased the curing rate. However, the addition of IPTES-functionalized polyols into the formulation inhibited the curing speed. The effect of relative humidity on the UV-curing reaction of cycloaliphatic epoxide coatings was lowered by the incorporation of the TEOS-functionalized polyols.; The synthesis of a novel norbornyl epoxidized linseed oil, which can be used as a resin for cationic UV-curable coating, was also investigated. Epoxidation of Dilulin was performed using three different approaches: (1) peracetic acid, (2) dioxirane (OXONE), and (3) hydrogen peroxide. Out of the three approaches, the hydrogen peroxide epoxidation was preferred on the basis of yield and ease of purification. The norbornylized linseed oil was synthesized via Diels-Alder reaction with cyclopentadiene. The extent of norbornylization was controlled and ranged from 20–30%. The norbornylized linseed oil was epoxidized using hydrogen peroxide with a quaternary ammonium tetrakis-(diperoxotungsto) phosphate(3-) as a phase transfer catalyst. The UV-curing rate of epoxynorbornylized linseed oil was lower than that of cycloaliphatic epoxide, but higher than epoxidized linseed oil. The curing rate was substantially increased when divinyl ether was incorporated into the formulation. Of the three divinyl ethers (Diethyleneglycol divinyl ether (DEGDE), cyclohexane dimethanol divinyl ether (CHDMDE), and triethyleneglykoldivinylether (TEGDE)) used, coating with TEGDE showed the highest curing rate, and coating with CHDMDE showed the lowest curing rate.