Optimizing Curing Profiles of One-Component Epoxy Resins with Small Molecule Thermal Latent Curing Agents Containing Reversible Urea Bonds.

Abstract

This study focused on the synthesis of small-molecule thermal latent curing agents (TLCs) containing reversible urea bonds and their utilization in one-component epoxy resins (OCERs). The TLCs were synthesized by reacting aromatic monoisocyanates with linear aliphatic amines with primary and secondary amino groups and characterized by NMR, FTIR, TGA, and DSC. The TLCs were then incorporated into diglycidyl ether bisphenol A (DGEBA) resin at 60 °C to formulate OCERs and assess their effectiveness. Optical microscopy of the OCERs showed uniform dispersion of the TLCs. Cure kinetics, evaluated by dynamic and isothermal DSC and rheological analysis, revealed that the stability of urea bonds of the TLCs was affected by phenyl group substitution and the amino group type. Urea bonds formed from dichlorophenyl isocyanate (DCPI) with secondary amino groups showed the lowest thermal stability. The TLC-5 synthesized from DCPI and triethylenetetramine (TETA), was most effective, initiating the curing of DGEBA at 120 °C. OCER-5, prepared with TLC-5 and DGEBA and cured at 140 °C, was analyzed by DMA, showing a T_g of 135 °C. This study highlighted the importance of molecular structure in optimizing TLC stability and efficacy, guiding the design of improved TLCs for epoxy applications in composites and adhesives.