Hydrophobic modification of lignin for rubber composites

Lignin being the second most abundant natural biopolymer after cellulose constitutes between 18 wt% and 35 wt% of wood. Due to its abundance, sustainability, renewability, and the ability to undergo numerous modifications through chemical reactions, there is a significant interest in lignin valorization for material applications. However, its hydrophilicity, lack of melt processability, and poor dispersibility have hindered its wide scale application. Structural modifications have been reported to counter its detrimental properties effectively. Thus, in the present work, a novel silylation reaction was employed to successfully modify kraft lignin and enhance its hydrophobicity. The modification was verified by using nuclear magnetic resonance and infrared spectroscopies. The change in the wettability and dispersibility in various solvent systems indicated the increase in hydrophobicity due to the modifications. This enabled it to better disperse in natural rubber composites. Lignin being the hard moiety imparted mechanical strength, while, NR being the soft moiety, provided it with elastomeric characteristics in the composites. The incorporation of 5 wt% modified lignin in the hydrophobic NR matrix led to a 44.4% increase in the tensile strength. With higher loadings of filler in composites, an increase in elastic moduli and intensity of Payne effect were recorded. The hypothesized dispersibility improvement of the modified lignin in NR compared to its unmodified counterparts was also evident from the morphological analysis.