Microwave-assisted fractionation and functionalization of technical lignin toward thermoset resins

Lignin is the most abundant aromatic biopolymer, with a potential to serve as a building block of rigid and thermally stable bio-based materials. However, it is still underutilized because of the heterogeneous and not fully understood chemical structure. Here, technical softwood Kraft lignin is refined in to narrow-dispersity and relatively low molar mass fractions by microwave-assisted processing, followed by microwave-assisted allylation and further application in lignin-based thermosets. This microwave processing is carried out under noncatalyzed conditions using a low boiling point solvent and elevated pressure. The properties of the retrieved fractions are investigated by P-31-NMR, heteronuclear single quantum coherence spectroscopy-NMR, SEC, differential scanning calorimetry, and thermogravimetric analysis. The extraction yield of the selected lignin fraction is around 25%, with the number-average molar mass (M-n), weight-average molar mass (M-w), and dispersity (D) significantly reduced. The chemically modified lignin is characterized by P-31 NMR and FTIR, which provides evidence of the introduction of the allyl moieties. The analyses demonstrate that 90 +/- 3% of the hydroxyl groups in fractionated lignin are successfully allylated. Subsequently, the allylated lignin is cross-linked through thermally induced thiol-ene chemistry to produce lignin-based thermosets. The final thermosets exhibit a storage modulus of 4050 +/- 60 MPa and a Tg of 105 +/- 5 degrees C.