Effect of reprocessing on thermal, rheological, mechanical and surface features of polybutylene succinate-based bio-composites

Following the principles of the circular economy, which aims to recycle and reuse bio-derived and biodegradable materials such as polybutylene succinate (PBS), this study evaluated the effect of reprocessing cycles of both pure PBS and an industrial agri-food waste filler (30% BSG brewer's grains, size <100 microns). The characterization results showed that the BSG filler increased melt viscosity (especially at low rheological frequencies), reduced structural order, and decreased stability compared to pure PBS during processing cycles. Indeed, pure PBS was stable throughout all five reprocessing cycles, while the PBS/BSG bio-composite lost its mechanical performance after three cycles. Indeed, the r4/5-PBS/BSG bio-composite (after four and five extrusion stages) exhibit greater rigidity, thermal stability, and reduced ductility compared to pure PBS, limiting its recyclability. Thermomechanical degradation induces the formation of radicals and branching in the PBS macromolecular chain, which becomes less hydrophilic (by 53%, almost reaching the 90° threshold) and rougher (by 38%) after five processing cycles. The presence of BSG in the five-times recycled bio-composite (r5-PBS/BSG) increases both its hydrophilicity (by 15%) and surface roughness (by approximately 17%) and limits the formation of carboxyl groups induced by thermomechanical degradation.