Exploring lignin types in circular by design polyethylene blends for injection moulding applications: interactions and properties

Abstract

In the context of developing circular by design lignin-based linear low-density polyethylene (LLDPE) blends, the interactionsbetween lignin-lignin and lignin-LLDPE using three different types of lignin—hardwood and softwood kraft lignin, andhardwood sulphite lignin (designated as SUZ, IND, and VIX, respectively)—were studied. This work focuses on improvingboth mechanical properties and biodegradability to design products that minimize plastic waste and enhance environmentalperformance, in alignment with the principles of a circular economy. The unmodified lignins were thoroughly characterizedand reduced in size (38–75 μm) before melt blending with LLDPE in a twin-screw extruder in proportions up to 10%. Thepelletized 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 exhibitedgood dispersion (particles less than 200 nm) and combination of stiffness (0.7 GPa), tensile strength (11 MPa), elongation atbreak (47%) and thermal stability (455 °C). Additionally, biodegradation tests conducted through soil burial and Aspergillusniger solid medium revealed a weight loss ten times greater than that of neat LLDPE over 90 and 180 days, respectively, ata 10% lignosulfonate content. Tensile strength was poor for the SUZ-based blends. The LLDPE-IND blends exhibited com-parable tensile strength to LLDPE-VIX although biodegradation and thermal properties were lower. The Coulomb forces insulphite lignins, with monovalent counterions, promote lignin disaggregation and miscibility during the melt and thermalprocessing, enhancing interfacial interactions and the final properties of the LLDPE blends.