Grafting polycaprolactone onto alkaline lignin for improved compatibility and processability

Industrial lignin has a lower compatibility and processability than other polymers during the preparation of composites. To valorize the industrial lignin from spent liquor of alkaline pulping mills, polycaprolactone was grafted onto alkaline lignin to improve its compatibility and processability in this investigation. The lignin-g-polycaprolactone copolymer was prepared by the ring-opening polymerization of the ε-caprolactone monomer under anhydrous conditions and was catalyzed by n-butyl titanate. The techniques of FTIR, 31P NMR and 1H NMR were employed to investigate the reaction mechanisms, including the reaction preference of hydroxyl groups and the average length of the PCL arms. Eventually, the composites of lignin-g-PCL and PCL were prepared by melt blending, and the results demonstrated that the compatibility and processability of lignin was significantly improved by the grafting modification, and that the fracture surface of composite, as well as the mechanical properties, was strongly affected by the average length of the PCL arms grafted on lignin. The best mechanical performance of the composite with tensile strength and bending strength of 1.22 ± 0.05 MPa and 2.23 ± 0.10 MPa, respectively, was achieved by the lignin-g-PCL sample with averaged PCL arm length about 19. This study paves the way for a wider utilization of lignin in composites and biomaterials.