Kinetic evaluation of the crosslinking of a low-temperature cured biobased epoxy-diamine structure

Abstract

Bis(oxiran-2-ylmethyl) adipate, synthesized by adipic acid, was investigated for its capability to polymerize at ambient temperature when blended with 1-(2-aminoethyl) piperazine. The object of the current work is associated to the kinetic analysis of the aforementioned epoxy-amine system, qualifying it as a potential candidate in the adhesion field of manufacturing industries. Polymerization kinetics were followed by Differential Scanning Calorimetry measurements, carried out both under dynamic and constant temperature conditions. The first gave rise to the total heat released through polymerization and the glass transition temperatures of the initial monomers (Tgo) and the crosslinked network (Tgoo), found to be 356 J/g, −69.7°C and 1.8 °C, respectively. The later revealed the effective crosslinking of the system at 25, 40 and 60 °C under reasonable periods of time. Cyclical dynamic-dynamic and isothermal-dynamic DSC experiments were further performed, giving results that follow the Di Benedetto equation, with Tgo and Tgoo values very close to the experimentally calculated. According to Vyazovkin isoconversional method applied on dynamic DSC measurements, the activation energy found to be independent of the conversion degree throughout the curing process and equal to 61.4 kJ/mol, indicating a single-step reaction. Various autocatalytic reaction models were selected to reproduce constant heating rate experiments, all displaying very accurate results, while fitting parameters from the aforementioned simulations, along with kinetic parameters derived from Vyazovkin method, were utilized for the estimation of isothermal curing predictions. Isothermal predictions were validated through comparison to both solely isothermal DSC measurements and data gathered in cyclical isothermal-dynamic experiments. Consequently, the nth order with an autocatalytic constant (Kcat) reaction model Cn was selected for the construction of a Time-Temperature-Transformation plot, revealing optimum curing conditions for the investigated temperature range. The superiority of the present epoxy thermoset system lies in the monomers' organically originated nature, along with its low-temperature curing cycle, enhancing environmental consciousness and saving energy for generations to come.