The Gas-phase Flame-retardant Mechanisms Of Polyethylene/Triphenyl Phosphate Revealed By Molecular Dynamics Simulation

Polyethylene(PE)has become a widely used material in civil and military industries because of its excellent physical and mechanical properties,however,the flammability of PE limits its application.In order to reduce the fire risk of PE materials,flame retardants are added to inhibit the pyrolysis and combustion processes.Phosphate ester,a typical flame retardant for PE,can generate phosphorus-containing free radicals,which can actively capture the H and HO radicals emitted by the burning PE so that it can weaken or terminate combustion chain branching reactions,therefore retarding the combustion.However,the combustion process of flame-retardant composites is complex and rapid,and it is difficult to detect and analyze the dynamic evolution of the transient intermediates and reaction mechanism with the existing experimental techniques,which ultimately result in the study of the flame-retardant mechanism not being readily available.To cope with the above problems,this paper studies the gasphase flame retardant mechanism of PE/TPP(triphenyl phosphate)using reactive force field molecular dynamics simulation(ReaxFF MD).Firstly,the effect of heating rate and temperature on PE pyrolysis was studied using ReaxFF MD simulations.Based on the simulation results of PE non-isothermal pyrolysis,the PE pyrolysis process was divided into four stages:structural adjustment and activation,initial pyrolysis,deep degradation and post pyrolysis.It was found that increasing the heating rate caused a shift towards higher temperatures in all stages of PE pyrolysis,while low heating rates favored the production of more gaseous products.Moreover,when the temperature was too high and the residence time was too long,the carbon chain of the product increased in isothermal pyrolysis.The temperature range of 2500-2750 K was found to promote PE pyrolysis to produce more of the combustible gases C2H4 and C3H6,under both isothermal and non-isothermal conditions.The PE pyrolysis process is predominated by free radical chain mechanism,with βfragmentation being the main reaction to produce C2H4.The results of ReaxFF MD simulations at different temperatures elucidate the pyrolysis process and the chemical reaction mechanism of TPP.The results show that the P-O bond of TPP is less stable than the C-O bond,and is more likely to undergo pyrolysis to produce phenoxy radical(C6H5O)and OPO2(CH5)2.The intermediate product OPO2(C6H5)2 is readily subject to C6H5(benzene radical)and OPO2C6H5.The main phosphorus-containing products from the pyrolysis of TPP are PO2 and PO3 radicals.The pathways of PO2 generation are TPP+OPO2(C6H5)2→O2POC6H5→PO2 and TPP→OPO2(C6H5)2→OPOC6H5→PO2,while the pathways of PO3 generation are TPP→OPO2(C6H5)2→O2POC6H5→PO3.The hydrocarbon products from TPP pyrolysis are mainly H2 and C2H2.The gas-phase flame retardant mechanism of PE/TPP was revealed by conducting ReaxFF MD simulations on various C2H4/TPP combustion systems under different temperature conditions.The effect of TPP concentration on the combustion of ethylene(C2H4)and the impact of temperature on the flame-retardant effect of TPP are analyzed.The gas-phase flame retardant mechanism of TPP is related to phosphorus-containing oxides.PO2 can actively capture H,HO,O and HO2 radicals,while HOPO2 mainly captures H and HO radicals.Meanwhile,PO3 and HOPO can only capture H radicals and HO radicals,respectively.After analyzing the number of reactions related to phosphorus-containing compounds,the importance of different phosphorus-containing compounds can be ranked as follows:PO2>HOPO2>HOPO>PO3.HO+PO2+M→HOPO2+M,HOPO2+H→PO2+H2O,HO+PO2+M→HOPO2+M and HOPO2+H→PO2+H2O are the flame-retardant mechanisms of organophosphorusbased flame retardants in previous studies.In this study,not only the above results were found,but also PO2+O→PO3,PO2+HO2→HOPO+O2,PO3+H→HOPO2 and HOPO2+HO→H2O+PO3 were proposed.The addition of TPP to C2H4 combustion systems has been found to effectively reduce the number of reactive radicals,with the most significant effect being observed on the HO radical.In C2H4/TPP combustion systems where the molar fraction of TPP is in the range of 0-10%,the most effective reduction in the number of HO,H and O radicals is achieved when the molar fraction of TPP reaches 10%.The effect of temperature on the flame retardancy of TPP was analyzed for a system with a 10%molar fraction of TPP.When the temperature is≥3250 K,TPP was effective in reducing the number of HO radicals.The inhibition of O radicals by the addition of TPP only occurred at relatively high temperatures(≥3750 K).