Nanostructured lignin carriers for efficient flame retardant delivery in natural rubber composites

Abstract

Natural rubber (NR) is a renewable elastomer with broad industrial relevance but intrinsically poor flame resistance, a limitation that is further exacerbated in foamed structures. Conventional flame-retardant strategies typically require high filler loadings that compromise mechanical performance and processability. In this work, Kraft lignin-based nanocontainers (LNCs) were engineered as multifunctional carriers to deliver ammonium polyphosphate (APP) within an NR matrix, enabling simultaneous enhancement of flame retardancy and mechanical properties at low additive contents. LNCs were synthesized via interfacial crosslinking and stably incorporated into NR latex using surfactant-assisted dispersion, yielding nanocomposites with preserved particle sizes of approximately 300 nm and minimal aggregation after coagulation and drying. At a loading of 10 wt% LNC, the resulting NR composites exhibited a 35% improvement in comprehensive combustion indices, a 43% reduction in peak heat release rate, and a 57% decrease in linear burn rate relative to neat NR, while achieving a UL-94 HB rating where the control failed. Concurrently, mechanical performance was significantly improved, with a 127% increase in toughness alongside gains in strength, modulus, and elongation at break. Notably, foamed NR/LNC composites demonstrated further enhancement in flame resistance, exhibiting higher limiting oxygen index values and nearly half the linear burn rate of their solid counterparts, indicating a synergistic interaction between the intumescent nanocontainers and the porous foam architecture. Overall, lignin nanocontainer-mediated delivery of flame retardants provides an effective, bio-based strategy to balance fire safety and physicomechanical performance in natural rubber systems, outperforming conventional bulk additive approaches.