Enhancing flame retardancy and smoke suppression of melamine urea-formaldehyde resin with phosphorus-based metal complexes: A comparative study of Mg²⁺, Ca²⁺, and Al³⁺

Melamine urea-formaldehyde resin (MUF) is widely used as an adhesive in wood processing, where it acts like the skeletal structure of the human body, binding various wood components to form engineered wood products. Like wood, MUF is flammable, and its organic components exacerbate smoke and toxic gas release during combustion, a challenge addressed by composite flame retardants combining organic retardants with metal ions for improved performance. In this study, a phosphorus-based ligand (PAP) was used to complex with Mg²⁺, Ca²⁺, and Al³⁺, forming PAP@Mg, PAP@Ca, and PAP@Al complex flame retardants that, when applied to MUF, enhance its flame retardancy, smoke suppression, while maintaining coating performance and bonding strength. Cone calorimeter tests revealed that PAP@Mg and PAP@Al reduced the total heat release rate (THR) of MUF by 32.50 % and 20.43 %, respectively, while decreasing total smoke release (TSR) by 55.56 % and 67.61 %, along with suppressing CO and CO<inf>2</inf> emissions. Under non-flaming pyrolysis conditions, all three flame retardants lowered MUF's specific optical smoke density (Ds<inf>4min</inf>) by more than 63.2 %. Notably, PAP@Mg showed the best performance in reducing toxic gases (HCN, CO) and combustible gases (CH<inf>4</inf>, C<inf>2</inf>H<inf>6</inf>), demonstrating superior smoke suppression and toxicity reduction. This study evaluates the impact of different metal types on the flame-retardant and smoke-suppressing properties of composite flame retardants, offering new insights for enhancing the combustion safety of MUF-based wood materials and expanding their potential applications.