Thymol-Derived Trifunctional Epoxy Novolac from Controllable Synthesis to Performance-Advantaged Thermosets

Abstract

In order to develop biobased epoxy resins with superior performance, this study highlights the synthesis, characterization, and properties of a biobased thymol-derived epoxy novolac and related thermosets. Thymol is cocondensated with 2,6-bis(hydroxymethyl)-p-cresol under catalysis of an acidic ion-exchange resin to yield a novolac (TPF) in an easy and controllable way. TPF is found to mainly consist of condensation products with three phenolic nuclei including two from thymol and one from 2,6-bis(hydroxymethyl)-p-cresol connected to two methylene bridges. Then, TPF is efficiently converted into a titled epoxy novolac (TEP) through O-glycidation. TEP and a curative (4,4′-diaminodiphenyl sulfone (44DDS) or methyl nadic anhydride (MNA)) are formed together to afford thermosets expressing highly desirable properties such as high T_g and low water absorption. A correlation is established between thermosetting network structures and their exhibited T_g, thermal stability, dynamic thermomechanical, dielectric and adhesion properties, solvent resistance, thermal diffusivity, and water absorption. Furthermore, TPF is used as a curative for TEP, to result in a TEP/TPF thermoset with significantly increased content of thymol blocks. This thermoset exhibits even superior properties, such as very low water uptake (<1%), decreased density, and good adhesion (>10 MPa), as well as moderate heat resistance (T_g = 136 °C). To conclude, TPF and TEP could be readily prepared in a more controlled manner, and their based epoxy thermosets exhibit multiple properties of interest with more tunability in network structures and ultimate performance, providing a robust candidate for performance-advantaged, value-added applications to advance sustainable epoxy chemistry and technology.