Lignin, a renewable aromatic polymer, has significant potential as a building block for thermoset materials. This study examines two different methods for modifying lignin to enhance its reactivity: carboxymethylation and oxidative carboxylation. Both approaches introduce carboxylic acid groups, which serve as chemical handles for crosslinking during curing. These groups are particularly useful because they allow thermosets to form ester linkages, making the material more recyclable at the end of its life.
Carboxymethylation selectively modifies lignin’s hydroxyl groups under alkaline conditions, adding carboxyl functionalities while keeping the core lignin structure intact. This method provides controlled modification but is influenced by lignin’s solubility and structural variability. Oxidative carboxylation, on the other hand, uses acetic acid and hydrogen peroxide to introduce carboxyl groups through ring opening and side-chain oxidation, leading to more extensive structural changes.
Each approach has trade-offs. Carboxymethylation maintains lignin’s structural integrity and offers precise functionalization, but it requires additional reagents that become part of the final material. In contrast, oxidative carboxylation does not introduce extra molecules—only the oxidant is used—making it a more sustainable option. Additionally, lignin modified through oxidative carboxylation is nearly 100% bio-based.
By comparing these two strategies, this study provides valuable insights into how lignin can be tailored for thermoset applications. Understanding the impact of these modifications on lignin’s reactivity and structure will help in designing more sustainable and high-performance lignin-based thermosets.