Bioinspired delayed-exposure Co3O4@ZIF-8 nanoreactor for decoupled charring and CO oxidation in polypropylene composites

Abstract

Conventional intumescent flame retardants typically generate localized hypoxic environments within the char layer, leading to incomplete carbonization and substantial CO emission, particularly in intrinsically flammable polyolefins. Metal oxides, owing to their high catalytic activity and tunable redox states, have emerged as promising additives for smoke suppression and toxicity mitigation in polyolefin systems. However, excessive catalytic activity can concurrently accelerate oxidative decomposition of the protective char. Inspired by the protective architecture and stimuli-responsive release of natural seeds, we report a bioinspired interfacial chemical engineering strategy to construct a core–shell Co₃O₄@ZIF-8 catalyst. This confinement architecture enables spatiotemporal decoupling of char formation and CO oxidation. During early combustion, the catalyst facilitates the rapid formation of a compact char barrier serving as an efficient thermal shield. Upon reaching elevated temperatures, Co₃O₄ becomes exposed via the ZIF-8 carrier, efficiently catalyzing CO oxidation and markedly reducing toxic emissions. This strategy achieves a 91.7 % reduction in the peak heat release rate (PHRR), an 83.9 % decrease in total heat release (THR), and a 54.1 % reduction in CO emission, while delivering outstanding flame-retardant performance (LOI=32 %, UL-94 V-0 rating). Collectively, this work establishes a bioinspired core–shell confined catalyst for simultaneous fire safety and toxicity suppression in flame-retardant polypropylene.