Catalytic effect of metal ions on flame-retardant cellulose paper with montmorillonite through layer-by-layer self-assembly

A simple, environmentally friendly and efficient montmorillonite (MMT)-metal ion (Me: Mg²⁺, Ca²⁺, Fe³⁺ and Zn²⁺)-carboxymethyl cellulose (CMC) organic–inorganic hybrid film is successfully applied to flame retardant cellulose paper through layer-by-layer self-assembly (LbL) technology. Using SEM, TGA, XPS, air permeability test and horizontal flammability test to characterize its surface morphology, thermal stability, air permeability and flame retardancy. The results show that MMT-Me-CMC system can endow cellulose paper with high flame retardancy properties, and different metal ions have different contributions to the flame retardancy. MMT with multi-layer structure and high specific surface area is beneficial to reduce the air permeability of paper, thereby effectively reducing the flammability of flammable pyrolysis gas and the transfer of heat. The XPS analysis shows that the Zn²⁺ flame retardant system promotes the formation of graphitized char in the condensed phase. Meanwhile, the possible flame-retardant mechanism of the system is clarified. Zn²⁺ replaces Al³⁺ in MMT, thus increasing the active acidic catalytic sites on MMT. This promotes the carbonization of degradation products and forms graphitized clay-rich char. This work provides a simple and efficient LbL preparation strategy and provides a reference for the catalytic role of metal ions in the flame retardancy of cellulose paper.