Combustion Behaviors And Char Structure Evolution Of Polycarbosilane/magnesium Hydroxide/polyethylene And Flame Retardant Mechanism

The flame retardants and flame retardant materials with high efficiency,fire safety and environment friendly characteristics have been the hot research topic in the field of flame retardant materials.Magnesium hydroxide(MH),as one of the important non-toxic halogen-free flame retardants,has been widely used in wire and cable industry.Due to the structure and the flame retardant mechanism of MH,its limited flame retardant efficiency gives rise to the high loading level in flame retardant materials,which leads to the deteriorated properties of flame retardant materials.The halogen-free flame retardant technology of magnesium hydroxide with high-efficiency still faces a big challenge in the view of scientific and technical points.In this dissertation,a ceramizable flame retardant and its flame retardant materials based on MH were designed and prepared,taking the flame retardant mechanism in the condensed phase into account,The ceramic behavior of ceramizable MH and its effect on flame retardancy,combustion and thermal degradation of composites were studied.The mechanism of ceramizable flame retardancy was revealed by studying the composition and structure of residual char in condensed phase.Polycarbosilane(PCS)was selected as ceramizable additive to prepare halogen-free,non-toxic,high efficiency flame retardant,which greatly reduced the loading level of the flame retardant and obtained self-supporting ceramic char layer.The process of thermal degradation and the structure evolution of residual char were studied by means of SEM,TG,TG-FTIR,GC-MS,DSC,XRD,XPS and other modern means of analysis and characterization.The flame retardant mechanism was proposed,which would be a suggested solution to meet the requirements of flame retardant materials with halogen-free,non-toxic,high-efficient characteristics in the field of wire and cable,and provide a theoretical reference for the development of high effective inorganic flame retardants.PCS modified MH was prepared by surface modification technology.The interaction between PCS molecule and MH surface was proved to be combined by intermolecular forces—hydrogen bond,which made it possible for the ceramization of MH.Magnesium silicate formed during the thermal process.As a result,the ceramic intermediates from PCS and magnesium oxide from MH were bonded together by Si-O-Mg chemical bond,and the ceramic char layer formed during the combustion of PCS/MH/PE.A significant synergistic flame retardant effect was revealed between MH and PCS by the research on flame retardancy and combustion behavior,and the oxygen index(OI)of the composites was significantly improved by PCS.As a result,the OI of 35 % can reach with 26 wt.% MH and 4 wt.% PCS,which exceeds the bromine flame retardant system and the traditional intumescent flame retardant system at the same loading.According to combustion behaviors,the peak heat release rate(pHRR)was reduced by 36 % and the total heat release(THR)by 11 % at the PCS/MH ratio of 1/9 and the 50 wt.% loading of PCS/MH.The fire safety of PCS/MH/PE was significantly improved.The relationship between char structure and thermal stability,flame retardant property and combustion behaviors of PCS/MH/PE was established.PCS improved the thermal stability of MH and shorten the temperature gap between MH and PE,which matched the thermal degradation of PE and stabilized the PE macromolecular chains.Accordingly,the combustion of PCS/MH/PE was suppressed due to an effective char layer formed in short time.The flame retardant mechanism was proposed on the basis of ceramic char layer.Because no obvious catalytic charring effects in the condensed phase and no obvious changes in the gas phase were revealed,the ceramic char played a key role in the flame retardant mechanism due to the ceramic function of PCS.The compact char on the surface and the porous char in the bulk improved the heat and mass insulation effect of magnesium oxide connected by magnesium silicate.In addition,the ceramic reaction between the PCS and MH enhanced the strength of the residual char and maintained the porous char structure.