Lignin-based benzoxazines: A tunable key-precursor for the design of hydrophobic coatings, fire resistant materials and catalyst-free vitrimers

Abstract

The meaningful concept of circular economy has prompted the scientific community to re-think the design of materials. In this work, lignin-based benzoxazines (LBZs) thermosets with tuneable properties were synthesized in agreement with the principles of Green Chemistry, following a straightforward two-step synthetic route. The reactivity of a soda lignin was enhanced by its esterification with phloretic acid, a naturally occurring phenolic acid. Thereafter, the resulting structure enriched in phenolic rings with no ortho substituents, was used to prepare LBZs precursors with a bio-based primary amine via a Mannich-like ring-closure reaction. The structure and properties of each LBZ precursor were confirmed by multiple NMR techniques, Fourier transform infrared spectroscopy, elemental analysis, gel permeation chromatography, differential scanning calorimetry, rheology, and solubility assays. Lignin-based polybenzoxazine with high lignin mass fraction (between 46 and 66 wt.%) were obtained following curing via a vacuum compression molding manufacture process. Lignin-based thermosets exhibit a single-phase confirmed by X-ray computed tomography analysis, a Tg ranging from 136 to 197 °C, and storage moduli from 0.5 to 3.1 GPa. The features of each lignin-based precursor were tuned depending on the structure of the bio-based primary amine used to close the oxazine ring. Stearylamine confers good processability and yields hydrophobic coatings with a water contact angle reaching 91°. Furfurylamine-based LBZs generate high-Tg (197 °C) and fire-resistant materials with a peak of heat release rate of no>47 W.g−1. Finally, ethanolamine produces lignin-based thermosets capable of internally catalysed bond exchange via transesterification, providing vitrimeric properties such as fast stress relaxation (τ*200 °C = 233 s) and a complete circularity.