| With the demand of human beings for high quality production and life,higher requirements are put forward for the material industry.Polymer materials are widely used in microelectronics industry because of their low price,easy processing and good comprehensive performance.Polyvinyl alcohol(PVA)is a kind of water-soluble biodegradable polymer material with a large amount of-OH in its molecular structure,which has great advantages in gas barrier,antistatic and other aspects,and has good application value in the microelectronics industry.However,the poor heat dissipation and fire saferty performance has affected the reliability and service life of devices.Hexagonal boron nitride(h-BN)with two-dimensional lamellar structure is considered to be an excellent filler for the preparation of high-performance thermal-conductivity and flame retardant polymer composites due to its good thermal stability at high temperature,high thermal conductivity,chemical stability and lamellar barrier property.Therefore,it is of great significance to improve the mechanical performance,thermal conductivity and flame retardancy of PVA with the use of h-BN.At present,there are the following problems in the preparation and performance research of high-performance thermal-conductivity and flame retardant polymer composites with h-BN as functional filler:(1)Preparation efficiency of h-BN nanosheet.For base research,chemical vapor deposition(CVD)and other methods are often used to prepare h-BN nanosheet.However,for large-scale applications in polymer materials,these methods affect the preparation efficiency of h-BN nanosheet.(2)Mechanical enhancement efficiency.h-BN nanosheet is easy to agglomerate in the polymer matrix,and it is difficult to form a good dispersion system,which seriously affects the mechanical enhancement effect of two-dimensional nanosheet on polymers.(3)Thermal conductivity.The poor interface compatibility between h-BN nanosheets and polymer matrix is likely to increase the thermal resistance effect between the interfaces,thus reducing the thermal conductivity of the h-BN sheet in the material.(4)Flame retardant efficiency.The lamellar barrier effect of h-BN nanosheets gives the polymer a certain flame retardant performance at a low addition level,but its flame retardant efficiency is greatly reduced by the effects of agglomeration and interface incompatibility.In this paper,the efficient preparation method of h-BN nanosheets was first explored from the point of view of the design and preparation of high performance thermal conductivity and flame retardant PVA composites.Then,the biomimetic polydopamine(PDA)was used to organically treat the surface of the h-BN sheet and construct an interface compatible coating to enhance the mechanical enhancement effect of the h-BN sheet on PVA.On the basis of organic modification,transition metal oxides was grown in situ on the surface of h-BN nanosheet by using the active site provided by the organic layer to prepare the core-shell type multifunctional hybrid sheet material,and its comprehensive influence on the mechanical properties,thermal conductivity and flame retardancy of PVA composites was studied.In order to further improve the catalytic performance of h-BN nanosheets in the combustion process of PVA composites,h-BN hybrid materials containing different metal salt components were constructed by hydrothermal synthesis method and the effects of different metal elements on the flame retardant properties,carbon layer formation characteristics and pyrolysis product release behavior of PVA are studied.In addition,through the organic flame retardant modification of the h-BN nanosheet surface,the dispersion characteristics of the h-BN nanosheets in the matrix and its interfacial compatibility with the matrix were also improved,reducing the interface thermal resistance and enhancing the flame retardancy of h-BN.The main contents of this paper were as follows:Part one:h-BN nanosheets were prepared by high temperature heat treatment and ultrasonic stripping in aqueous phase.The reason was that oxygen entered into the lattice of h-BN and destroyed the original dense structure of particles at high temperature,effectively weakening the van der Waals forces between adjacent sheet structures.When the water molecules enter the loose particles in the dense structure,h-BN can be quickly exfoliated into two-dimensional sheets.In order to verify the mechanical enhancement effect of h-BN sheets on PVA materials,PDA was used to strip the organic modification of h-BN nanosheet to improve the interface compatibility between inorganic lamellar particles and PVA.The construction of the PDA organic layer on the h-BN surface promotes the dispersion of the sheets in matrix,which in turn increases the tensile strength and elongation at break of the PVA materials.In addition,h-BN@PDA can effectively improve the thermal stability of PVA,promote the formation of dense carbon layer and inhibit the release of pyrolysis products.Part two:Based on organic modification,the active sites provided by the organic layer were used to grow TiO2 to prepare a core-shell multifunctional hybrid nanosheets to further increase the contact area between the nanosheets and PVA.The h-BN hybrid structure with special interface characteristics can enhance the interface interaction between the lamellar and the matrix,thus reducing the thermal resistance of the interface,effectively forming a heat conduction network to facilitate the heat transfer,and enhance the thermal conductivity of the composite.At high temperature,the addition of h-BN@PDA@TiO2 increased the carbon content of PVA composites,and promoted the high temperature thermal stability of PVA composites.Meanwhile,the multi-functional hybrid nanosheets can effectively inhibit the heat release during the combustion of PVA composites through the barrier effect of nanosheets of nanosheets and catalytic formation of high-quality compact carbon protective layer.In addition,the release of toxic pyrolysis products was also inhibited in the study of PVA pyrolysis products.These phenomena confirm that the core-shell multifunctional hybrid nanosheets can significantly improve the fire safety performance of PVA composites.Part three:In order to further improve the catalytic performance of h-BN in the combustion process of PVA composites,an environmentally friendly h-BN@ZnFe2O4 flame retardant hybrids are synthesized by hydrothermal method.The effect of hybrid on the flame retardancy,the formation of carbon layers and the release behavior of pyrolysis products was studied.h-BN@ZnFe2O4 hybrid effectively inhibits the re-aggregation of h-BN nanosheets and promotes their dispersion in matrix.The study showed that the thermal degradation of PVA can be delayed with the physical barrier of the hybrid.As the content increased,the heat release of composites also decreased,which was mainly due to the interaction of h-BN nanosheets and carbon layers.At the same time,ZnFe2O4 can be thermally decomposed to Zn and Fe oxides,which further promoted the formation of compact carbon layers.Part four:In order to analyze the influence of metal compositions on the flame retardant and catalytic efficiency of h-BN,h-BN@ZnMoO4 hybrids were prepared by self-assembly method.Then,the mechanical properties,flame retardant properties and smoke suppression and toxicity reduction behavior of PVA composites were studyed.As the contents increased,the tensile strengths of composites also increased.Meanwhile,the realese of heat,smoke release and pyrolysis product release of the samples all showed a downward trend.This was mainly due to the interaction of h-BN and carbon layers,which hindered the escape of gaseous pyrolysis products and inhibited the transfer of heat and oxygen.At the same time,ZnMoO4 further promoted the formation of the dense carbon layers during the thermal decomposition of the sample.Part five:The h-BN nanosheets can also be used in cooperation with phosphorus-containing flame retardants through surface organic flame retardant modification to achieve the purpose of simultaneously improving the efficiency of mechanical enhancement,thermal conductivity and flame retardancy.The hexachlorotripolyphosphazene was bonded on the surface of h-BN nanosheets with its active P-Cl,then ethylenediamine was further grafted on the surface of h-BN nanosheets to prepare a multifunctional organic-inorganic hybrid fillers.The fractured surface morphologies showed that composites with organic flame-retardant modified h-BN nanosheets had a wavy crack shape on the cross-section,and the fracture morphology was relatively rough,which confirmed the improved interface interaction.The improved interface helped h-BN to form a thermally conductive network in PVA to facilitate heat transfer.The h-BN nanosheets and the modified layer containing phosphorus and nitrogen can synergistic effect.On the one hand,the two-dimensional h-BN can disperse in PVA matrix,acting as a physical barrier;On the other hand,the catalysis of thermal decomposition products was conducive to the formation of a dense carbon layer.During the polymer combustion process,the dense carbon layer can prevent the entry of external oxygen,slow down the volatilization and diffusion of combustible gases,delay the further degradation of the composite material,and improve the fire safety and thermal stability of PVA composites. |
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