Pyrolysis And Flame Retardant Mechanism Of Flame Retarded Polycarbonate Based On Reactive Force Field

Polycarbonate（PC）is widely used in electronic appliances,automotive,aerospace,daily necessities and other industries.PC is put forward higher requirements on its flame retardancy because of the frequent fire accidents related to electronic and electrical products in recent years.Silicon-containing flame retardant is an excellent and halogen-free flame retardant,which has good dispersibility and compatibility in polymer matrix.Because of the complexity in the reaction at high temperature,the further study the flame retardant mechanism of silicon-containing flame retardant on PC was studied.The pyrolysis process and combustion characteristics of organic-inorganic hybrid octaphenylsilsequioxane（OPS）flame retarded PC was analyzed at the molecular level by using reactive force field molecular dynamics（ReaxFF）.At the same time,inorganic silicon dioxide（SiO₂）and organicα,ω-dichloropolydimethylsiloxane（DPS）,the effects of different temperatures on the pyrolysis and combustion products of PC/SiO₂ and PC/DPS systems were explored.The decomposition mechanism of PC/silicon-containing flame retardant was explained from the molecular level.The main contents are as follows.（1）The molecular model of PC and PC/OPS was constructed,and their pyrolysis process at 2000-4000K was studied by using ReaxFF simulation method.The initial decomposition mechanism,products distribution and flame-retardant mechanism were analyzed.It was found that the decomposition of PC molecule chain was caused by the breaking of C-O bonds,and the decomposition of OPS molecule was caused by C-Si bond rupture.The rupture of some C-Si bonds made OPS change from cage structure to layer or network structure.These has been proved to inhibit the volatilization of heat and flammable volatile gas products,and then improved the flame retardancy of materials.From the potential energy and decomposition time of PC and PC/OPS composites,the addition of OPS slowed down the decomposition rate of PC molecular chain and improved the thermal stability of composites,which are consistent with the reported results.The content of CO₂,CO,C₆H₅OH and their derivatives produced by pyrolysis of PC/OPS composites was lower than that of pure PC in the later stage of reaction.This was because that the adsorption of oxygen-containing small molecules by unsaturated Si atoms produced by decomposition of OPS inhibited these harmful substances.In addition,OPS promoted the combination of some products into carbon clusters and thermally stable organic fragments.（2）The combustion characteristics of PC and PC/OPS under oxygen-lean,stoichiometric and oxygen-rich conditions and different temperatures were studied.Unsaturated Si atoms would adsorb oxygen in the reaction process to expand the Si-O structure,which was beneficial to the formation of thermal insulation and oxygen insulation protective layer.PC/OPS could maintain a certain smoke suppression performance at low temperature and low oxygen conditions.However,PC/OPS lost this effect at high temperature and high oxygen concentration,which was due to the full combustion of PC/OPS system at high temperature and high oxygen concentration.In addition,PC/OPS also produced more water.OPS improved the activation energy of PC combustion reaction,and inhibitd the combustion reaction process of PC materials.（3）Two flame retardant molecular models,PC/SiO₂ and PC/DPS,were established,and their pyrolysis and flame-retardant properties were studied by ReaxFF method.The addition of three flame retardants reduced the potential energy of the composites and made the composites in a relatively stable state.All three flame retardant systems had inhibitory effects on smoke and harmful gases,and the organic-inorganic hybrid PC/OPS system had the best comprehensive performance.Compared with pure PC,the activation energy of each system was increased,which indicated that silicon-containing flame retardant could inhibit the pyrolysis reaction of materials.Silicon-containing flame retardant could inhibit smoke and harmful gases,improve residual carbon and delay thermal decomposition.Among them,organic-inorganic hybrid OPS had the best effect.Therefore,ReaxFF method is effectively used to study the pyrolysis mechanism of PC/silicon-containing flame retardant composites,and provides a new method for theoretical prediction and analysis of flame retardancy.