Kraft Lignin as a Sustainable Flame Retardant Additive for Polymer Composites

Abstract

Plastic waste is a widespread and continuing issue, including the leaching of toxic additives from microplastics. Lignin is the second most abundant biopolymer, and although previously considered as waste and used as fuel, lignin’s availability and unique properties have garnered popularity as a sustainable and functional additive in many material applications. In this thesis, the versatility of lignin is explored at varying levels of valorization. The first part of this work explored three differing purities of lignin and their effect on the properties of polyvinyl chloride (PVC) composites. These were tested alongside composites with differing concentrations of lignin and were all subject to various thermomechanical and flammability testing. Additionally, the decomposition kinetics of PVC and a lignin-PVC composite were explored. The industrially purified lignin outperformed the other purities mainly in mechanical properties, but the laboratory purified lignin retained the most heat capacity compared to the control. Additionally, at a concentration of 18 wt.% lignin, combustion indices were improved by 50 to 80%, but the elongation at break of these composites were reduced by 38.7%. The second part of this work focused on incorporating a hydrophilic FR (ammonium polyphosphate) into natural rubber (NR), a hydrophobic polymer, by utilizing nano-containers constructed via the crosslinking of lignin. The properties of the lignin nanocontainers (LNCs) were studied to best enhance the dispersion of this filler within the NR at varying concentrations. These composites were tested alongside bulk incorporated lignin & FR as well as foamed samples to best determine the value brought by the LNCs. At a concentration of just 10 wt.% LNC, various flammability parameters improved and outperformed the bulk incorporated sample. This work demonstrated the versatility of lignin to by adding value to polymeric materials without additional modification or being modified to improve its incorporation as a nanomaterial.