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

Abstract

AbstractLignin 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 non‐catalyzed conditions using a low boiling point solvent and elevated pressure. The properties of the retrieved fractions are investigated by 31P‐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 (Mn), weight‐average molar mass (Mw), and dispersity (Đ) significantly reduced. The chemically modified lignin is characterized by 31P 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°C.