Preparation Of Boron Nitride-based Composite Flame Retardants And Study Of Their Effects On Flame-retardant Performance Of Epoxy Resin

Boron nitride nanosheets（BNNS）as a kind of novel materials with two-dimensional（2D） layered structure are potential flame retardants,because they can act as physical barriers upon polymer burning as graphene does.However,similar to graphene,unfunctionalized BNNS not only have poor flame retardancy but also are liable to stack.To address this issue,we introduce several compounded components to prevent the stacking of BNNS and improve their flame-retardant performance.Firstly,a novel BF-BNNS nanocomposite flame retardant was prepared by surface functionalization of BNNS with flame retardant bismuth ferrite（BF）nanoparticle in order to inhibit the stacking of BNNS and improve their dispersion performance as well as flame-retardant performance in epoxy resin（EP）matrix.Secondly,a new type of superparamagnetic ZF-BNNS nanocomposite flame retardant was prepared by functionalizing BNNS with nano zinc ferrite（ZF）.The dispersion state of ZF-BNNS in EP matrix was manipulated by applying dynamically adjusted magnetic field,thereby maintaining the flame-retardant effect and barrier effect of BNNS and improving their flame-retardant performance via their ordered orientation in the EP-matrix composites.Finally,a three-dimensional ammonium polyphosphate/boron nitride/zinc ferrite（APP/BN/ZF）flame retardant aerogel skeleton was constructed,followed by the injection of EP into the aerogel skeleton to increase the dosage of the flame-retardant and enhance its dispersion in EP matrix,thereby affording a flame-retardant composite with significantly improve flame-retardancy.The main contents and results are as follows:1.Preparation and Characterization of BF-BNNS Nanocomposite Flame Retardant and Investigation of Its Effects on Flame-retardant Performance of EPBF-BNNS nanocomposite flame retardant was prepared by hydrothermal method.The structure of the as-prepared BF-BNNS nanocomposite flame retardant was characterized by transmission electron microscopy（TEM）and X-ray powder diffraction（XRD）;and its effects on the flame-retardant performance of EP were investigated.TEM and XRD characterization results reveal that the as-prepared BF-BNNS nanocomposite consists of binary phases BF and BNNS;and the BF nanoparticles on the surface of BNNS have a size of around 15 nm.Cone calorimetry results show that BF-BNNS nanocomposite exhibits good flame-retardant performance,and the addition of 3.0 wt%of BF-BNNS reduces the peak heat release rate（PHRR）,peak smoke release（PSPR）and peak CO production（PCOP）of EP by 34.7%,35.6%,and 50.0%,respectively.Dynamic thermomechanical analysis（DMA）results demonstrate that BF-BNNS can significantly increase the glass transition temperature and storage modulus of EP,showing good dynamic thermomechanical properties.With a view to the dispersion state of BF-BNNS in EP,the morphology and composition of char residue,and EP pyrolysis process,it is concluded that BF can improve the dispersion performance of BNNS in EP and reduce the amount of the released aromatic compounds and CO toxic gases among EP pyrolysis products.Besides,the BF-BNNS flame-retardant can form a continuous and dense char layer with high graphitic degree,thereby adding to the flame-retardant performances.2.Preparation and Characterization of ZF-BNNS Nanocomposite Flame Retardant as well as Its Magnetically Controlled Arrangement and Flame-retardant Performance in EP MatrixZF-BNNS nano-hybrid flame retardant was successfully prepared by a solvothermal method.The structure of the as-prepared ZF-BNNS nano-hybrid was characterized by XRD and TEM.The as-prepared ZF-BNNS nano-hybrid flame retardant was then added in EP matrix and manipulated to orderly distributed under dynamically adjusted magnetic field;and the effect of the magnetically manipulated orientation of ZF-BNNS on the flame-retardant performance of EP matric was investigated.Results indicate that the as-prepared ZF-BNNS nano-hybrid flame retardant exhibits superparamagnetic performance,and the ZF nanoparticles on the surface of BNNS have a size of around 50 nm.The SEM image of the fractured surface of ZF-BNNS/EP shows that ZF-BNNS nanofillers exhibit ordered orientation in EP matrix.Cone calorimetry test results show that the oriented ZF-BNNS nano-hybrid has good flame-retardant effect,due to the horizontal barrier effect of BNNS.With a view to the dispersion state of BF-BNNS in EP matrix,the morphology and composition of char residue,and EP pyrolysis process,it is concluded that the oriented ZF-BNNS is favorable for BNNS to efficiently exert barrier effect,thereby better blocking heat,pyrolysis products,and oxygen.In the meantime,ZF-BNNS can enhance the degree of graphitization and the density of the char layer,which also contributes to the excellent flame-retardant performance.3.Construction of 3D APP/BN/ZF Aerogel Flame Retardant and Study of Its Effects on Flame-retardant Performance of EPA three-dimensional APP/BN/ZF aerogel skeleton was prepared by a simple freeze-drying method.EP was then injected into the aerogel skeleton to afford EP flame-retardant composite.The structure of the APP/BN/ZF 3D aerogel skeleton was characterized by XRD,Fourier transform infrared spectroscopy（FT-IR）,and scanning electron microscopy（SEM）.The effects of3D APP/BN/ZF composite aerogel on the flame-retardant performance of EP were investigated.Findings demonstrate that,after freezing-drying treatment,APP is connected with BNNS via hydrogen bonding to form APP/BN/ZF composite aerogel possessing three-dimensional porous structure and low density.The porous structure of aerogel flame retardant is destructed after vacuum infusion of EP,forming a dense composite with 24.0 wt%of the flame-retardant.As compared with those of EP matrix,the limiting oxygen index（LOI）of the EP flame-retardant composite is increased from 21.2 to 28.7;and its peak heat release rate（PHRR）and total heat release（THR）are reduced by 86.2%and 86.5%,exhibiting enhanced flame-retardant effect.The analysis of the char residue and pyrolysis process of the EP composite demonstrates that the EP flame-retardant composite has a high graphitization degree and a high thermal stability of char layer.In the meantime,its released non-combustible products are significantly enhanced and its released combustible organic hydrocarbons and aromatic compounds are significantly reduced,which also accounts for its excellent flame-retardant performance.Moreover,polydimethylsiloxane（PDMS）and hydrophobic SiO₂ can be used as the starting materials to construct micro/nano structure on the surface of EP flame-retardant composite,thereby endowing it with superhydrophobicity.