Towards sustainable and on-demand adhesives: Catalyst-free preparation of lignin-based covalent adaptable networks with superior bonding and recyclability

Polymer materials today face significant challenges, notably the utilization of green and low-carbon feedstock, the development of eco-friendly preparation processes, and effective material recycling. A promising solution to these challenges lies in the direct synthesis of covalent adaptable networks (CANs) from biomass sources, like lignin. However, the development of unmodified lignin CANs via catalyst-free methods has been a challenging task. Here, we report the successful preparation of lignin-based CANs (TERs) via phthalate monoester transesterification. In this system, lignin (PB1000) serves as hard segment and crosslinker, while the soft segment consists of a biomass diol (PripolTM 2033). By blending the ratio of hard to soft segments, we were able to tune the mechanical properties of the TERs (with lignin content ranging from 10 to 50 wt% and crosslink density increasing from 3600 mol/m3 to 47900 mol/m3). Monoester bonds within the TERs are highly dynamic at elevated temperatures (with an activation energy of 169.2 kJ mol-1), facilitating material recycling without the need for catalysts. Furthermore, TERs can be chemically recycled via alkaline solutions at 80 °C. Notably, we demonstrate a potential application for this work in the form of a TER-based adhesive. In addition to its excellent adhesion properties, the TER adhesive exhibits thermal repair ability, removability, and degradability properties. This work provides a green and sustainable approach towards tackling the challenges associated with recycling of thermoset plastics.