Synergistic P-N charring agents to enhance flame retardancy of ethylene-vinyl acetate (EVA): Insights from experimental and molecular dynamic simulations

Previous charring agents had nonignorable defects, such as a low charring rate and poor compatibility with acid and gas sources in intumescent flame retardants (IFRs), leading to poor flame retardant efficiency. Thus, we designed a novel P-N macromolecule charring agent named poly(piperazinyl phosphamide) (PA) and confirmed the chemical structure of PA. A small amount of 7.5 wt% PA in EVA/ammonium polyphosphate (APP)/PA improved the limit oxygen index (LOI) to 35.7 ± 0.58% from 19.0 ± 0.33% and reached the level of UL-94 V–0. This implies that PA was an efficient charring agent in IFRs. Moreover, the peak heat release rate (PHRR) and total heat released (THR) dramatically declined by 90.73% and 97.70%, respectively. This suggests that the addition of PA further hinders the diffusion of combustible substances, forming a dense and sturdy char layer in the condensed phase. In detail, the decomposition of PA produces many aromatic structures, such as C = N, C = C, P = O, and P–N. Simultaneously, APP can defer the cleavage of C–N bonds for PA and promote the formation process of the benzene ring, which is clearly conducive to charring. To estimate the flame retardant micromechanism of EVA/APP/PA at high temperatures, this work utilized molecular dynamics simulation based on reaction force field (ReaxFF-MD) to research at the atomic scale. The potential energy, decomposition time, main gas products, and free radical products of the materials were simulated, which demonstrated that EVA/APP/PA has better flame retardancy than equal amounts of EVA/APP owing to the addition of PA. Suppressing the generation of combustible gases (CH4, C2H4, C6H6) and facilitating the binding of molecular products into char residues to form organic molecular structures with good stability is consistent with the experimental results. Proving that the PA was the key to boosting the flame retardancy of EVA by hindering the spread of combustible gases. A novel way for analysing and predicting the flame-retardant mechanism of EVA at the atomic level was proposed.