Comparative study on the synergistic reinforcement of lignin between carbon black/lignin and silica/lignin hybrid filled natural rubber composites

When the content of carbon black (CB)/silica is constant, the combination of lignin and CB/silica offers obvious advantages over previously reported partial replacement of carbon black/silica by lignin. Considering the distinction between CB/lignin and silica/lignin interactions, the effects of lignin concentration on the vulcanization behavior, crosslink density, filler dispersion, and physical and mechanical properties of the NR composites were investigated. The results showed that the incorporated lignin extended the scorch time (ts1) and optimum vulcanization time (tc90) of the sulfur-cured NR/CB/lignin compounds. The addition of lignin was beneficial for reducing the networking of CB. Although the moduli at 100% and 300% elongation decreased, the NR/CB/lignin composites still exhibited good tensile strength and elongation at break, especially with the addition of lignin at less than 30 phr. For the silica-filled NR composites, the extent of vulcanization of the NR/silica/lignin compounds slightly increased, and the vulcanization rate showed little change. The introduction of lignin plays a significant role in the reinforcement effect of silica-filled NR composites. The strengthened interaction between silica/lignin and NR and the improved filler dispersion significantly contributed to improved reinforcement. However, severe filler networking was observed when the lignin concentration was too high (up to 50 phr), resulting in decreased tensile strength. It is worth mentioning that due to significant reinforcement and the little changes in the hardness of the silica-filled NR vulcanizates by the addition of lignin, the combination of lignin and silica becomes more practical and potential for the formulation optimization of industrial rubber products.