Dual curing of an epoxy resin with dicarboxylic acids
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Dual curing of an epoxy resin with dicarboxylic acids J. M. Morancho1 · X. Ramis1 · X. Fernández‑Francos1 · O. Konuray1 · J. M. Salla1 · À. Serra2 Received: 26 July 2019 / Accepted: 6 March 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
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. Keywords Dual curing · Epoxy networks · Diacids · Isothermal cure · Kinetics
Introduction Epoxy resins are used in a lot of applications such as adhesives, coatings, castings, electrical and electronic materials, encapsulation of semiconductor devices, matrix material for components, structural components [1–12], flame retardants [13, 14] and in cryogenic engineering [15–17] thanks to their mechanical and chemical properties, superior adhesion to various substrates, thermal stability, and electrical characteristics. Epoxy resins can be cured with several hardeners like amines and anhydrides [4, 18, 19]. Another hardeners used are acids [20–27] and amino acids [28, 29]. Mustata et al. [20] studied the curing and
* J. M. Morancho [email protected] 1
Thermodynamics Laboratory, Heat Engines Department, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain
Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain
2
the degradation process of diglycidyl ether of bisphenol A (DGEBA) crosslinked with stoichiometric amounts of terephthalic acid (ATFT) and 4-4′-biphenyldicarboxylic acid using triethylbenzylammonium chloride as catalyst. One of the diacids used is the same than in this work (ATF
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