Curing Behavior and Properties of Active Ester Cured Cycloaliphatic Epoxy Resins

Cycloaliphatic epoxy resins are known for their synthesis process that eliminates hydrolyzable chlorine, thus avoiding the hydrolysis of hydrolyzable chlorine to hydroxyl groups in conventional epoxies, which is very favorable for the preparation of low dielectric epoxy resins. The curing process using active ester generates no hydroxyl groups, also indicating a promising approach for producing low dielectric epoxy resins. However, the resistance of cycloaliphatic epoxy to nucleophilic reactions restricts it from reacting usually only with anhydrides. This study investigates the reactivity of the active ester with cycloaliphatic epoxy, demonstrating its capability to undergo complete curing reactions at temperatures near 150 °C. Factors influencing this process, including the volatilization of the catalyst DMAP and its reactivity, are examined. Two types of cycloaliphatic epoxies (3,4-epoxycyclohexylmethyl-3′,4'-epoxycyclohexane carboxylate (EEC) and bis (3,4-epoxycyclohexylmethyl) adipate (BEA)) are combined with an active ester hardener, triacetyl resveratrol (TAR), to prepare epoxy resins. The activation energies for the curing reactions are determined, and the optimal concentration of catalyst 4-dimethylaminopyridine (DMAP) is identified for each resin. The use of catalysts at specific concentrations results in resins with superior mechanical properties (91.7 and 71.4 MPa for EEC/TAR and BEA/TAR), excellent thermal stability (Td5% of 354 and 347 °C), and good dielectric properties (dielectric constant of 3.68 and 3.95 at 10 MHz), outperforming anhydride-cured cycloaliphatic epoxy resins. These findings expand the selection of hardeners for cycloaliphatic epoxy resins and highlight the potential of the prepared resins in electronic packaging materials, offering enhanced properties suitable for various applications.