Triketone-Modified Lignin and the Corresponding Biomass-Based Diketoenamine Resins: Synthesis and Properties

This work reports the preparation of environmentally benign polymeric materials by employing biomass-based raw materials in the synthesis of recyclable thermosetting resins. With water-soluble lignin (possessing -COOH groups) as a feedstock, lignin-triketone (OL-TK) is prepared and cross-linked with tris-(2-aminoethyl)-amine (TREN) to result in the corresponding lignin-based diketoenamine resins. The resin shows a glass transition temperature (<i>T</i> _<i>g</i> ) of about 165 °C, which is higher than the values reported for other diketoenamine resins and other lignin-based vitrimers due to the relatively rigid structure of lignin and diketoenamine groups. Nevertheless, the lignin-based diketoenamine resin shows high brittleness, leading to failure in the mechanical tests. Further studies employ OL-TK as a reactive modifier for a furanic dicarboxylic acid (FDCA)-based diketoenamine resin. The lignin-containing diketoenamine resins exhibit some features of vitrimers, including satisfactory stress relaxation at 180 to 220 °C, thermal recycling ability, and chemical degradation/recycling characteristics. The addition of 3 wt % OL-TK brings positive effects on enhancing the mechanical properties and suppressing the creep at high temperatures of the pristine resin and does not alter its thermal properties and thermal recycling and reprocessing features. A high OL-TK fraction of 7 wt %, although further increasing the mechanical property and stress relaxation of the vitrimers, would result in lignin aggregation in the thermally recycled sample. High OL-TK content is negative for the recycling performance of the vitrimers.