Lignin as a sustainable filler of polyvinyl chloride composites: Effects of ash content and loading levels on thermomechanical and combustion properties

Lignin is the second most abundant biopolymer and has generally been discarded as waste, but its availability, renewability, and unique properties have garnered popularity for its use in material applications. Its hydrophobic nature makes it a natural contender as a filler for polyvinyl chloride (PVC), a versatile polymer that is typically modified with a variety of toxic additives that can leach during thermal decomposition, whether through recycling or combustion. In this work, the applicability of Kraft lignin as a filler to PVC is explored, utilizing different grades of Kraft lignin from various purification methods and varying the loading of filler within PVC. The grade of lignin was a major aspect of the resulting mechanical properties due to the ash content, which decreased the elongation by up to 73 % in certain cases, but increased Young's modulus overall. Increasing concentrations of lignin amplified certain properties seen, with an optimal value at a loading of 18 wt%. Here, we see the most retained heat capacity (-5.14 %) and an increase to Young's modulus (+8.76 %). This loading slightly reduced the limiting oxygen index (LOI) (-9.60 %), but improved combustion indices by 50 to 80 %, through its charring behaviour. The industrially purified lignin maintained the largest elongation (-38.7 %) with similar combustion improvements, but lowered the heat capacity more (-21.3 %) compared to the laboratory purified lignin. Kinetic analysis showed a slight increase to the activation energy of dehydrochlorination and a change to the primary mechanism, shifting from nucleating to diffusion controlled.