Imidazole and chromium acetylacetonate as additives for the cure and fracture toughness of epoxy resins and their composites

The effects of additives such as 2-undecyl-imidazole (C₁₁Z) and chromium acetylacetonate (Cr(acac)₃) were examined on the curing behavior and fracture toughness of tetraglycidyldiaminodiphenyl methane/diaminodiphenyl sulphone (TGDDM/DDS) epoxy resins and their composites. The C₁₁Z additive alone reacted with TGDDM epoxy resins at about 127°C and increased the resin viscosity, resulting in an acceptable resin content for composite processing. Further addition of Cr(acac)₃ to TGDDM/DDS/C₁₁Z formulation increased the fracture toughness 5.7 times compared to the typical TGDDM/DDS/BF₃MEA epoxy formulation used for the preparation of laminates. The interlaminar fracture toughness of the laminates prepared by TGDDM/DDS/C₁₁Z/Cr(acac)₃ formulation was only twice as much as that prepared by typical TGDDM/DDS/BF₃MEA. This was due to the fiber bridging contribution to the interlaminar fracture toughness. Based on the experiment, this fiber bridging contribution was only dependent on the fiber content. Thus, the interlaminar fracture toughness is approximated by the sum of the fracture toughness of epoxy matrix and the estimated fiber bridging contribution.