Reactive Molecular Dynamics Simulation Of Phosphorus Flame Retardant/Epoxy Resin Pyrolysis Products And Flame Retardant Mechanism

The development and use of flame retardant epoxy resin materials provide safety guarantee for the wide application of epoxy resin in military and civilian life.It is of importance to study the pyrolysis/combustion products and microscopic reaction mechanism for the flame retardant mechanism and practical application of flame retardant resins.During the experimental pyrolysis/combustion process,the products with secondary reaction or below the detection limit may result in the incomplete statistics of radicals and final products.The reactive molecular dynamics(Reax FF)method combining molecular dynamics with a chemical reaction force field can describe the process of bond breaking and forming,which provides a solution for studying complex flame retardant mechanisms at the molecular level.Based on this,the pyrolysis process of phosphorus-containing flame retardant epoxy resin was studied by Reax FF method.The laws of different temperature affecting the distribution of resin pyrolysis and combustion products were discussed,and the microscopic decomposition mechanism of epoxy resin was explained from the molecular level.The main contents are as follows.(1)Epoxy resin samples were prepared and their thermogravimetric(TG)curves were analyzed.Construction of rational cross-linked epoxy resin(EP)and epoxy/Hexa-(5-methyl-2-pyridyloxy)-cyclotriphosphazene(EP/HMPOP)models.The pyrolysis process of EP and EP/HMPOP at 500-3500 K was studied by Reax FF simulation method,including the initial reaction path,main products,pyrolysis,and flame retardancy mechanism.The C₄₀₊mass fraction fitting curves of EP at different temperatures were in good agreement with the TG data.The pyrolysis of EP and EP/HMPOP would release small molecular gases such as H₂O,CO,CH₂O,and C₂H₄ with activation energies of 141.72 k J/mol and 116.4k J/mol,respectively.The pyrolysis reaction of EP/HMPOP occurs more easily at a lower temperature,which was consistent with the reported results.The decomposition of EP was initiated by the cleavage of bonds containing nitrogen or oxygen atoms,and HMPOP molecules did not participate in the initial decomposition process of EP.HMPOP would crack to generate P·and N·to capture free radicals(such as H·and oxygen-containing fragments)of EP system,increase carbon residue and inhibit further decomposition of EP.HMPOP reduces the amount of C₀-C₄ fragments and harmful gases such as CH₂O.(2)The effects of stoichiometry,fuel-lean and fuel-rich conditions,and different temperatures on the combustion characteristics of EP and EP/HMPOP were studied.The pathway of O₂ involved in resin decomposition and its influence on the distribution of small molecular products were analyzed.The effect of O₂ concentration on the reaction rate was weaker than that of temperature.Under the stoichiometric and lean fuel conditions,more than90%of the mass in the system was occupied by small molecular gases,and the amount of C₄₀₊was very small.Combustion products for fuel-rich conditions were increased by insufficient combustion.Oxygen did not participate in the initial cleavage reaction of epoxy resin for the first time.However,it would react with fragments of epoxy resin cleavage in subsequent reactions,and directly promote the generation of C₁ and C₂ products.The participation of O₂ also led to a decrease in the activation energy of two systems,making the combustion reaction more likely to occur than pyrolysis reaction.Under lean fuel conditions,the effect of the flame retardant(HMPOP)was not obvious,and only had certain effect on the conversion of C₁ products.H₂O would react with HO₂·and O·to decrease its amount.(3)The pyrolysis and flame retardant performances of phosphate ester(pentaerythritol octahydrogen tetraphosphate,PEPA),inorganic(ammonium polyphosphate,APP),and phosphaphenanthrene(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide,DOPO)flame retardants on epoxy resins were also studied by the Reax FF method.Under the same phosphorus content,different types of phosphorus flame retardants showed different flame retardant effects.The decomposition of APP,PEPA,and DOPO was initiated by the cleavage of N-O,P-O,and P-H bonds respectively.Adding phosphorus flame retardants could effectively reduced the release of harmful gases,such as CH₂O and CO.Phosphorus fragments tended to generate PO₃,PO₄,or complex P-O-P-O structures during the pyrolysis process.These structures were connected with carbon fragments to form carbon cluster structures.Small amount of NH₂·,NH₃,NO,N₂ products were found in EP/APP system,which would dilute the flammable gas molecules during the pyrolysis reaction to achieve the flame retardant effect.