Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols

A series
of fully renewable triphenols (TPs) with various number
of methoxy group substituents (n = 0–6) were synthesized using
lignin-derived phenols (guaiacol and 2,6-dimethoxyphenol) and aldehydes
(4-hydroxybenzaldehyde, vanillin, and syringaldehyde). The structural
evolution from TPs to epoxy thermosets was followed by nuclear magnetic
resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy.
Thermomechanical properties of the resulting epoxy thermosets were
investigated by dynamic mechanical analysis (DMA), tensile analysis
(TA), and thermogravimetric analysis (TGA). Increasing the content
of methoxy groups decreased the glass transition temperature (132–118
°C) and glassy modulus (2.7–2.2 GPa). Thermal stability
of high-methoxy-content thermosets was reduced due to electron-donating
effects and higher oxygen content. Conversions and isolated yields
of TPs significantly decreased as the number of methoxy substituents
increased, which markedly determined the feasibility of TPs as precursors
for polymers. This work widens the synthesis route of fully lignin-derived
polyphenols, yielding polymers with thermomechanical properties comparable
to bisphenol A (BPA) based materials. Evaluation of methoxy substitution
provides insight for the selection of lignin-derived monomers.