Depolymerised lignin oil: A promising building block towards thermoplasticity in polyurethanes

The use of lignin as building block in the preparation of lignin-based thermoplastics has been a great challenge for the scientific community. Indeed, both the heterogeneous branched structure and the high hydroxyl functionality of lignin favours crosslinking reactions leading to the formation of thermosetting chemical structures. Some attempts have been made to circumvent these limitations, although the approaches required labour-intensive and time-consuming modifications of lignin and used only a relatively low amount in the materials. To tackle this problem, the present work employs lignin hydrogenolysis oil (LHO) which contains a low hydroxyl functionality oligomeric fraction suitable for the synthesis of thermoplastic polyurethanes (TPUs) via a two-step polymerization process. A series of TPUs were synthesized to investigate the impact of lignin content as well as soft segment molar mass on the chemical and thermo-mechanical properties of the materials. The study demonstrates that the use of high lignin content (38–46 wt. %) and high molar mass of the polyol (polypropylene glycol, PPG, Mn=4 kg mol−1) enhanced the mechanical properties of the obtained TPUs compared to counterpart polymers synthesized with lower molar mass of PPG. Additionally, LHO-based TPUs were applied as hot-melt adhesives for wood and their bonding strengths were evaluated by single lap shear tests. The lap shear adhesiveness was found to increase with increasing LHO concentration and decreasing molar mass of PPG. The developed methodology could lead to the preparation of robust thermoplastic lignin-based polyurethanes with a wide range of desired properties.