Functionalization The Epoxy Resin Through Core-shell Flame Retardants And Interficial Regulation

Epoxy resin,as one of the most versatile thermosetting polymers,has already been employed in numerous fields,such as coatings,microelectronic packaging,adhesive and aerospace.Similar to most of polymeric materials,high flammability of epoxy resin leads to fire risk and significantly limits its application in some areas.Thus,improving the non-flammability of epoxy resin has attracted increasing attention.Integration of hybrid elements such as nitrogen,phosphorus,boron,silicon and transition metals into the polymers,and regulation of the material structure,are two basic principles to achieve flame retardancy.Addition of flame retardants,intrinsic flame-retardant polymers and preparation of polymer nanocomposites are the main methods to change the element composition of polymer materials.Herein,We develop a simple and controllable coating strategy to decorate a variety of inorganic nanoparticles with boronate polymer,thus forming core-shell structure flame retardant.By introducing these into epoxy resin(EP),we have found that the thermal stability,flame retardant and mechanical performances of the as formed nanocomposites greatly rely on the shell thickness.Also,our design effectively overcome the disadvantage of some inorganic particles with single function or have obvious defects in flame retardant application,improve the compatibility between inorganic nanoparticles and polymer matrix and regulate the corresponding interface.The main achievements of this research are displayed as follow:(1)Building blocks for boronate polymer shell including six-armed catechol monomer(denote as HAC)and two-armed boronic monomer(denote as DMB)were synthesized,h-BN was exfoliated and in-situ edge-hydroxylated.Based on the formation of B-N dative bond and the hydrogen bond interaction between HAC and edge-hydroxylated h-BN nanosheets,we prepared h-BN-OH@CPBPs with controllable shell thicknesses by regulating the momomers amonts,which were respectively denoted as h-BN-OH@CPBP 15,h-BN-OH@CPBP40 and h-BN-OH@CPBP75.We confirmed the core-shell structure by scanning electron microscope(SEM),transmission electron microscope(TEM)and atomic force microscope(AFM).The structure and composition of h-BN-OH@CPBPs were characterized by Attenuated total reflectance Fourier transform infrared(ATR-FTIR)spectra,X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).The thermogravimetric analysis(TGA)results showed the h-BN-OH@CPBPs had high thermal stability.The corresponding weight percentages of boronate polymer were very close to theoretical ratio,indicating a high coating efficiency of this approach.We then introduced h-BN-OH@CPBPs with different shell thicknesses into the EP to form nanocomposites.SEM and TEM images of EP/h-BN-OH@CPBP40 indicated that h-BN-OH@CPBP40 could keep its core-shell structure and exhibited uniform distribution in EP matrix.We consider that decorating h-BN-OH with a reactive and soft CPBP shell,can evidently improve the compatibility and binding affinity between particle flame retandant and EP.(2)The influence of the shell thickness and loading amount of h-BN-OH@CPBPs on flame retardancy of the nanocomposites were evaluated.h-BN-OH@CPBPs with thicker shell is benefit to enhance the LOI value and shorten the burning time but not conducive to reduce the heat release rate,indicating that the synergistic flame-retardant effect between the core and shell is dependent on the boronate shell thicknesses.The nanocomposites show the highest limiting oxygen index(LOI)of 28.1%and reach V-0 rating in the vertical burning test at a loading amount of 2.5 wt.%h-BN-OH@CPBPs.Also the heat and smoke releases of nanocomposites during burning are significantly reduced.The possible flame-retardant mechanism was proposed:when EP/h-BN-OH@CPBPs nanocomposites are ignited,phosphazene-containing boronate polymer shell can synergistically catalyze char formation,participate in the carbonization reaction and release a large quantity of CO2 to dilute the combustible gases.Simultaneously,undecomposed h-BN-OH aggregates in the char residues to form a barrier,which can prevent the inner matrix from fire hazard.(3)The mechanical performances were investigated by differential scanning calorimetry(DSC),dynamic thermomechanical analysis(DMA)and three point bending tests.The results showed that introduction of h-BN-OH@CPBP40 could enhance the glass transition temperature(Tg)and mechanical properties.The Tg value of nanocomposites was improved as the shell thickness of h-BN-OH@CPBPs increased,and dynamic mechanical properties of nanocomposites also varied due to the thickness of the shell.By fixing the loading amount of h-BN-OH@CPBPs at 2.5 wt.%,we investigated the effect of CPBP shell thickness on the mechanical performance of the nanocomposites.In the case of 75 nm CPBP shell thickness,the flexural modulus of the nanocomposite(EP-2.5-1)was enhanced slightly.But 75 nm of thickness CPBP shell had no positive impact on the flexural strength of it.On the contrary,h-BN-OH@CPBP with 15 nm shell thickness could boost the flexural strength of nanocomposite(EP-2.5-2)by 33.58%comparing to the pure EP.However,its flexural modulus decreased apparently.Notably,h-BN-OH@CPBP40 of shell thickness could simultaneously improve flexural strengths and modulus of the nanocomposites.Probably,this was due to the interfacial interaction change induced by different shell thicknesses of h-BN-OH@CPBPs.Therefore,the performance of nanocomposites can be optimized by interfacial regulation.(4)Based on the idea of multiple mechanisms synergistic effect,core-shell structure Cu2O@BP nanocubes flame retardant was designed to combine the smoke suppression and high flame retardancy,and was characterized by SEM,TEM,FTIR,XRD,XPS and TGA.The Cu2O@BP was introduced into the epoxy matrix with the amount of 5 wt.%to form nanocomposite,and the thermal stability,flame retardation and mechanical properties of the nanocomposite were investigated.Comparing with the pure epoxy resin and EP/Cu2O nanocomposite,the introduction of Cu2O@BP greatly improved the carbonization of epoxy resin matrix during combustion,and maintained the excellent smoke suppression property of Cu2O,which significantly improved the flame retardant performance of nanocomposites.The reason for the flame retardancy improvement of nanocomposites is related to the synergistic effect between cuprous oxide and boronate polymer.The three-point bending test shows that the introduction of Cu2O couldn’t damage the mechanical properties of the matrix.on the contrary,it could also be boosted,which may be caused by the enhancement of interface compatibility between the epoxy resin matrix and cuprous oxide by boronate polymer.(5)The phosphazene-containing and DOPO-based boronate was coating to the surface of magnesium hydroxide,thus forming Mg(OH)2@BP-Ce core-shell structure flame retardant.The morphology,structure and thermal stability of the flame retardant were characterized.Then,introducing Mg(OH)2@BP-Ce and Mg(OH)2 with the amount of 5 wt.%to epoxy matrix to forming nanocomposites,respectively.By analyzing the heat release rate during combustion,we find that carbonization has an important effect on the reduction of heat release rate.Boronate with catalytic carbonization effect can make up for the lack of carbonization ability of magnesium hydroxide and improve its interfacial compatibility with epoxy resin.