Expanding lignin thermal property space by fractionation and covalent modification

Abstract

To fully exploit kraft lignin's potential in material applications, we need to achieve tight control over those key physicochemical lignin parameters that ultimately determine, and serve as proxy for, the properties of lignin-derived materials. Here, we show that fractionation combined with systematic (incremental) modification provides a powerful strategy to expand and controllably tailor lignin property space. In particular, the glass transition temperature (T_g) of a typical kraft lignin could be tuned over a remarkable and unprecedented 213 °C. Remarkably, for all fractions the T_g proved to be highly linearly correlated with the degree of derivatisation by allylation, offering such tight control over the T_g of the lignin and ultimately the ability to ‘dial-in’ this key property. Importantly, such control over this proxy parameter indeed translated well to lignin-based thiol–ene thermosetting films, whose T_gs thus covered a range from 2–124 °C. This proof of concept suggests this approach to be a powerful and generalisable one, allowing a biorefinery or downstream operation to consciously and reliably tailor lignins to predictable specifications which fit their desired application.