Construction of Zinc Ion-reinforced Melamine Phosphate Flame Retardant for Synergistic Enhancement of Flame Retardancy and Energy Absorption in Ethylene–Vinyl Acetate Foams

Abstract

The foam materials used in cushioning and protective applications often face a trade-off between flame retardancy and energy absorption. Ethylene–vinyl acetate (EVA) foams are lightweight, flexible, and highly flammable, which restricts their broader engineering applications. In this study, a multifunctional additive, silane-modified zinc ion–reinforced melamine phosphate (ZnMP), was developed via in situ ion complexation and surface grafting to overcome the incompatibility between flame retardancy and mechanical performance in conventional intumescent systems. Compared to pristine melamine phosphate (MPP), ZnMP forms a coordinated structure through Zn²⁺ interactions with nitrogen- and phosphorus-containing groups. When incorporated into the EVA foams at 15 wt%, ZnMP acted as an efficient heterogeneous nucleating agent, generating a dense microporous structure. Meanwhile, the silane coupling agent enhanced the interfacial interaction between the ZnMP and the EVA matrix, enabling effective reinforcement. Consequently, the EVA/ZnMP foam exhibited significantly improved mechanical properties, including a 144.3% increase in storage modulus, 21.4% increase in energy adsorption, and 88.6% cyclic compression retention after 10 cycles (80% strain). In addition, Zn²⁺ promoted the formation of a compact char layer during combustion, leading to excellent flame-retardant performance with a limiting oxygen index (LOI) of 25.3%, UL-94 V-0 rating, and a 41.0% reduction in the peak heat release rate. This ion-coordination and interfacial modification strategy offers a promising route for developing lightweight EVA foams with balanced mechanical reinforcements and flame-retardant properties for cushioning and protective applications.