Exploring lignin types in circular by design polyethylene blends for injection moulding applications

Abstract

In the context of developing circular by design lignin-based linear low-density polyethylene (LLDPE) blends, the interactions between lignin-lignin and lignin-LLDPE using three different types of lignin—hardwood and softwood kraft lignin, and hardwood sulphite lignin (designated as SUZ, IND, and VIX, respectively)—were studied. This work focuses on improving both mechanical properties and biodegradability to design products that minimize plastic waste and enhance environmental performance, in alignment with the principles of a circular economy. The unmodified lignins were thoroughly characterized and reduced in size (38–75 μm) before melt blending with LLDPE in a twin-screw extruder in proportions up to 10%. The pelletized blends were further processed into dumbbell-shaped specimens by injection moulding. Thermal, morphological, mechanical and biodegradation tests were performed on the specimens. The LLDPE blend containing 10% VIX exhibited good dispersion (particles less than 200 nm) and combination of stiffness (0.7 GPa), tensile strength (11 MPa), elongation at break (47%) and thermal stability (455 °C). Additionally, biodegradation tests conducted through soil burial and Aspergillus niger solid medium revealed a weight loss ten times greater than that of neat LLDPE over 90 and 180 days, respectively, at a 10% lignosulfonate content. Tensile strength was poor for the SUZ-based blends. The LLDPE-IND blends exhibited comparable tensile strength to LLDPE-VIX although biodegradation and thermal properties were lower. The Coulomb forces in sulphite lignins, with monovalent counterions, promote lignin disaggregation and miscibility during the melt and thermal processing, enhancing interfacial interactions and the final properties of the LLDPE blends.