Preparation and characterization of aminated co-solvent enhanced lignocellulosic fractionation lignin as a renewable building block for the synthesis of non-isocyanate polyurethanes

Lignin is a major bioresource from 2nd generation biorefinery process streams and from pulp and paper production that is still severely underutilized nowadays. Its natural polymeric and aromatic structures make it suitable as a potential building block for biopolymer production. Due to the growing environmental and health concerns over conventional polyurethane (PU) manufacturing processes that utilize toxic isocyanates, there is a great interest to develop a non-isocyanate PU (NIPU) route. Herein, we demonstrate a novel strategy that uses highly pure lignin isolated from co-solvent enhanced lignocellulosic fractionation (CELF) pretreatment of poplar wood to produce biobased NIPUs. In this strategy, hardwood poplar is firstly fractionated via a CELF pretreatment to produce a clean lignin stream that is rich in phenolics. The CELF lignin was then aminated by a Mannich reaction, and the aminated CELF lignin was finally reacted with bicyclic carbonates to yield an advanced NIPU. The structure, mechanical, and thermal properties of these novel NIPUs were elucidated by FTIR, NMR, tensile test, dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). The mechanical properties of these lignin based NIPUs could be tuned from highly rigid to elastic polymers by simply modifying the lignin constituents of the polymer. The thermal stabilities of NIPUs were improved because of the addition of aminated lignin, and NIPU containing 55 and 23 wt% lignin showed the highest tensile strength (~1.2 MPa) and elongation at break (~140%), respectively. The results obtained in this study demonstrate that the reaction of cyclic carbonate with aminated lignin represents an interesting pathway for the synthesizing of lignin based NIPU with a relative high lignin content.