Dual curing of an epoxy resin with dicarboxylic acids

Abstract

In this work, we have studied the preparation and characterization of a new family of thermosets based on off-stoichiometric diacid-epoxy formulations in the presence of 1-methylimidazole as initiator. Tri-glycidyl para-amino phenol has been used as epoxy resin and isophthalic (AIFT), and terephthalic (ATFT) acids have been used as diacids. The curing has been analyzed isothermally at different temperatures by calorimetry, using an isoconversional method and the Šestak–Berggren equation to determine the activation energy, the frequency factor, and the reaction orders. The thermal–mechanical properties of the partially cured and fully cured materials were also determined by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The analysis of the isothermal curing by infrared spectroscopy by Fourier transform allowed monitoring the reacting groups during the process. Two peaks appeared during the isothermal curing in the DSC. The first one is associated with the reaction of the carboxylic groups of the diacids with the epoxy groups, and the second one is related to the homopolymerization of the excess of epoxy groups. ATFT reacts less with the epoxy groups than AIFT, but accelerates more the homopolymerization and the isothermal curing ends earlier than in the systems with AIFT. Incomplete solubilization of the diacid monomers led to incomplete carboxylic reaction and excess of epoxy homopolymerization. In addition, two phases could be observed.