Reactive Molecular Dynamics Simulations For Flame Retarded Polyethylene With Melamine Phosphate And Pentaerythritol

Due to the flame retardant reaction temperature is too high, the reaction rate is too fast, the path is complex and changeable, so it is difficult to get the flame retardant mechanism by the means of experiment. In this paper, reactive molecular dynamics simulations using ReaxFF reactive force field are carried out under different temperature(1500,1750 and 2000 K), MP content(10%,20% and 30%) and MP/PER ratio(3:1, 2:1, 1:1, 1:2, 1:3) to compare the thermal decomposition of pure PE with flame retardant PE by melamine phosphate(MP) and with flame retardant PE by MP/pentaerythritol(PER) compound. The flame retardant mechanism was obtained, which can preliminary forecast the level of the flame-retardant properties theoretically. Molecular dynamics simulations can provide a new idea for the theoretical research of flame retardant, which can not only save the cost, shorten the cycle, and can be a theoretical guidance for design, research, and performance prediction of new flame retardant. There are mainly three aspects discussed in the paper:(1) Reactive molecular dynamics simulations were carried out on oxidation of PE at different temperature. Results show that with the increase of temperature, the pre equilibrium and induction time become shorten, potential energy reaches the peak faster. It can also accelerate the thermal oxidative degradation of PE, and consumption rate of PE an O2, and generation rate of CO2 and H2 O. The time to achieve the dynamic equilibration of the system become shorter. At lower temperature the form of final products are C 1-C10,C11-C20 and C21-C30, while C1-C10 is the main form existence at higher temperature. The main mechanism of oxidation of PE is complex free radical chain reaction, including chain initiation, growth, branching and termination. In addition, the pyrolysis of PE can also occure, such as β-scission, intra-molecular H-shift, branching and H-abstrction.(2) Reactive molecular dynamics simulations and experiments were carried out on oxidation of flame-retardant PE by MP at different temperature and MP addition. The results indicate that at low temperature 1500 K, large carbon clusters are formed as final products, while no carbon clusters formed at high temperature 2000 K. Simulation results show that with the increase of the MP content, the reaction rate slow down, and the drop of potential energy decrease, while more carbon clusters generate. At the same time, the more content of MP is, the better flame-retardant properties is. Experimental data indicate that with the increase of MP content, the limiting oxygen ind ex(LOI) increase, while the peak heat release rate(PHRR), total heat release(THR) and heat release capacity decrease It shows that the flame retardant property is better, which is consistent with the simulation results. Apart from the free radical chain reaction of oxidation of PE, the mechanism also include that pyrolysis and dehydration condensation and polymerization of MP, which can generate inorganpyrophosphate, metaphosphate, phosphite, partial phosphite and PO radical.(3) Reactive molecular dyna mics simulations and experiments were carried out on oxidation of flame-retardant PE by MP/PER at different temperature and proportion of MP/PER. Results show that large carbon clusters are formed as final products, but with the increase of temperature, the amount of carbon clusters is gradually less. When the ration of MP/PER is 2:1, the reaction rate slow down, and the drop of potential energy decline least, and most carbon clusters generate. Experimental results show that the LOI increase by the order of 2:1>3:1>1:1>1:2>1:3>PE, PHRR, THR and THR decrease by the order of the 2:1<3:1<1:1<1:2<1:3<PE, the flame retardancy increase by the order of 2:1>3:1>1:1>1:2>1:3>PE, which is consistent simulation results. In addition, the main mechanism of MP/PER compound flame retardant is own dehydration carbonization of PER, as well as dehydration and carbonization of MP and PER, which generate three-dimensional carbon layer, play the role of oxygen insulation, heat insulation.