Residual lignin-based poly(butylene succinate) composites for sustainable packaging

Owing to its complex and variable structure, lignin is still most commonly used through combustion. However, its aromatic nature makes it an attractive renewable resource to produce chemicals via lignin depolymerisation. Although significant progress has been made in lignin depolymerization, the resulting by-product, lignin solid residue, remains largely unexplored due to its intricate and highly recalcitrant structure. Rich in condensed structures, this solid residue is difficult to further fragment. Instead, it can be directly valorised as a filler in the fabrication of lignin-based composites. In this study, commercial technical lignin (both crude and pure grades, referred to as CL and PL) was hydrothermally depolymerized to obtain solid and liquid fractions rich in condensed structures and phenolic compounds. The solid fraction, named residual lignin (RL), was melt-mixed with poly(butylene succinate) (PBS), a biodegradable polymer, to create high lignin content composites (30% w/w) for packaging applications. The results were compared with CL/PL-PBS composites. Spectroscopic studies confirmed weak interactions between lignin and PBS polymers, which was further explored through microscopic analysis. Water contact angle measurements indicated a decrease in wettability for the RL-PBS sample, which also maintained its antioxidant properties compared to the CL/PL-PBS composites. Regardless of the type of lignin used (virgin or residual), there was no significant difference in the mechanical properties (Young's modulus, tensile strength and elongation at break). Overall, this research demonstrates the use of the lignin depolymerisation by-product RL as green filler for fabricating lignin-based biocomposites suitable for packaging applications.