Self-Toughened Epoxy Resin via Hybridization of Structural Isomeric Curing Agents

Abstract

Fracture toughness is a key property of epoxy resins with a high glass transition temperature (Tg), used in carbon fiber/epoxy composites for aerospace applications. Conventional toughening methods rely on adding toughening agents, often compromising the processibility and thermal stability. This study introduces a simple self-toughening approach that enhances the fracture toughness without sacrificing other properties by controlling the cured epoxy network structure. Tetraglycidyl 4,4 '-diaminodiphenylmethane (TGDDM) epoxy resin was cured using mixtures of structural isomeric curing agents, 3,3 '- and 4,4 '-diaminodiphenyl sulfone (3,3 '- and 4,4 '-DDS), at ratios of 7:3, 5:5, and 3:7. The optimal 7:3 ratio produced a resin with 30% higher fracture toughness compared to TGDDM/3,3 '-DDS and 100% higher than the TGDDM/4,4 '-DDS system. The Tg of the self-toughened resin ranged from 241 to 266 degrees C, which was intermediate between the Tg values of the TGDDM/3,3 '-DDS and TGDDM/4,4 '-DDS systems. This improvement is attributed to the higher crosslink density and reduced free volume of the epoxy network. These findings demonstrate that simply mixing isomeric curing agents enables self-toughening, providing a practical and efficient strategy to enhance the performance of high-Tg epoxy resins in advanced composite applications.