Fully bio-based epoxy resins derived from magnolol and varying furan amines: Cure kinetics, superior mechanical and thermal properties

Abstract

With rapidly shrinking petroleum resources and the deteriorating environment, the use of renewable green bio-based resources to replace petroleum-based raw materials and the development of new bio-based epoxy resins are hot topics of research today. In this study, a new bio-functional epoxy monomer based on magnolol (EMDBP) and two different curing agents based on furfuryl amine with various side groups (PDFA and BDFA) were synthesized and utilized to produce two sets of fully bio-based epoxy thermosets. The non-isothermal and isothermal curing kinetics of EMDBP/PDFA and EMDBP/BDFA were systematically investigated using different scanning calorimetry (DSC) according to the Friedman isoconversional differential method and model-fitting approach, respectively. The results indicate that the autocatalytic curing mechanism dominated the curing reaction of the fully bio-based curing system at a low conversion stage whereas diffusion controlled the curing reaction at a high conversion stage. With an attached benzene ring, BDFA has a more reactive amine group than PDFA. Thus, the curing reaction rate of the BDFA curing system was much faster than that of the PDFA curing system, which also increased significantly with temperature. Additionally, the glass transition temperature of the EMDBP systems (Tg, 151.3 °C and 168.4 °C) was increased by 71.3% to 95.7% compared to the diglycidyl ether of bisphenol A (E51) systems (Tg, 77.3 °C and 98.3 °C), which was also higher than those of most previously reported fully bio-based epoxy resins. Analysis by nanoindentation and tensile testing showed that the Young's modulus, hardness, tensile strength, and modulus of EMDBP systems were also greatly improved. Furthermore, thermogravimetric investigations demonstrated that EMDBP systems possessed excellent thermal stability at high temperatures, with a char yield of about 47.5 wt% at 800 °C. Consequently, these novel fully bio-based epoxy thermosets with superior heat resistance, mechanical properties, and thermal stability have the potential to replace commercial epoxy resins and be utilized under high temperature environments.