Study On Enhancing The Flame Retardancy And Thermal Stability Of Bisphenol A Epoxy Thermosets

Epoxy resin is a thermosetting polymer（EP for short）with a three-dimensional network structure.Because of its excellent dielectric properties,mechanical properties,dimensional stability and corrosion resistance,it is used as electrical insulation materials,corrosion-resistant coatings,glass fiber reinforced plastics/composites,etc.and is used in national defense,military industry and national economy,department.However,since the epoxy resin structure contains many covalent bonds with poor thermal stability,a large number of small flammable gas molecules will be generated during thermal decomposition,making it a flammable fire hazard.Adding flame retardants to the epoxy resin matrix is a common way to achieve flame retardancy.However,most of the additive flame retardants impart flame retardant properties to the epoxy resin while inevitably lowering the initial decomposition temperature of the substrate,thus limiting the use of epoxy resin materials in areas requiring high heat resistance.Both diethyl aluminum hypophosphite（Al Pi）and melamine polyphosphate（MPP）have good flame retardancy,but when they are used alone,they will inevitably lower the initial decomposition temperature of the material.At the same time,the single-component.The addition amount is often large and the dispersibility is not good.Therefore,how to improve the efficiency of Al Pi and MPP flame-retardant epoxy resins and avoid the sacrifice of thermal stability of flame-retardant materials has important research value and significance.The main work of this paper and the main research results obtained are as follows:（1）Use Al Pi with gas phase and condensed phase flame retardant effect as flame retardant,and polyimide powder（PI）with high heat resistance and low thermal conductivity as synergist to prepare flame retardant epoxy resin composite material.Thermogravimetric analysis（TGA）is used to characterize the thermal stability of materials;limiting oxygen index（LOI）,vertical combustion（UL-94）,cone calorimetry（Cone）are used to test the flame retardant properties of materials;tensile test materials are used mechanical properties and use scanning electron microscope（SEM）to observe its cross-sectional morphology.The LOI test results show that the addition of 2 wt%Al Pi can increase the oxygen index of epoxy resin to33.5%,achieve vertical combustion to UL-94 V-0 level,and significantly reduce heat release rate,total heat release and total smoke production.However,the initial decomposition temperature(T_5%)of epoxy resin decreased by 12°C.Interestingly,adding only 0.1%of PI can not only further increase the LOI to 35.5%,but also increase the initial decomposition temperature of EP/2%Al Pi to 352°C,which is close to the initial decomposition temperature of Neat EP（354°C）.This may be because PI’s high heat resistance and low thermal conductivity make it hinder the heat transfer rate in the substrate,thereby delaying the decomposition of the substrate.（2）Using the high heat resistance of basalt fiber and the catalytic effect of its metal oxide components,chopped basalt fiber will enhance the flame retardancy and thermal stability of Al Pi flame-retardant epoxy resin.Use TGA to analyze its thermal stability,use LOI,UL-94and cone calorimetry to evaluate its flame retardancy,use SEM to evaluate the compatibility and dispersion of flame retardants and basalt fibers in the substrate,and test the material of the tensile properties.After compounding 0.5 wt%BF in EP/2%Al Pi,the initial decomposition temperature(T_5%)increased from 343°C to 356°C,which was 2°C higher than the initial decomposition temperature of Neat EP.In addition,adding 0.5 wt%of BF can increase the LOI of EP/2%Al Pi from 33.5%to 35.5%,and still pass UL-94 V-0.Although the addition of BF leads to the"wick"effect,which makes HRR increase instead of decreasing,BF can play a blocking role in the small-scale flame combustion test and in the initial stage of ignition to achieve thermal stability and LOI improvement.（3）Using our laboratory’s self-made trimethylol melamine（MOH-S）as the carbon source and gas source,and MPP as the acid source and gas source,the two are combined to construct an intumescent flame-retardant epoxy resin system.Its flame retardant performance is evaluated by testing its limiting oxygen index and vertical combustion grade,and its thermal stability is analyzed by thermogravimetry.The addition of 18%MPP just increased the LOI value to 25.5%and reached UL-94 V-1.The total addition amount of MPP and MOH-S was controlled to 18 wt%,and then preparation of EP/18%（MPP/MOH-S）by different mass ratio of MPP to MOH-S（4:1,3:1,and 2:1）.When the mass ratio of MPP to MOH-S is 4:1,the LOI value was significantly increased to 29.5%,and the highest vertical combustion level UL-94 V-0 can be achieved.Under this mass ratio,the minimum MPP content required for EP to obtain UL-94 V-0 is reduced to 14.4 wt%.The addition of MOH-S significantly improved the carbon layer expansion and flame retardant efficiency of EP/18%MPP.