Preparation,Structure And Performance Control Of Thermally Conductive And Flame-Retardant Silicone Rubber Composites

With the arrival of the 5G era,electronic components with high performance e.g.,miniaturization,lightweight,integration and intelligence are developing rapidly.However,the accumulate heat generated from components during the serving would greatly affect the stability,reliability and service life of electronic products.In addition,when the heat accumulates to a certain degree,this would induce a serious fire tissue.Therefore,developing high thermal conductivity and flame retardant material to address the above problems has obtained considerable interest during the past several years.Silicone rubber has been widely used as structural materials in aerospace,automotive machinery,electronic devices and other fields due to its excellent properties e.g.,good thermal stability,electric insulation,high and low temperature resistance,outstanding mechanical properties and easy processing.Unfortunately,silicone rubber still shows low thermal conductivity(?0.2 W/m K)and poor flame retardancy,which cannot meet the critical need of high-power,high-density and highly integrated electronic components for heat dissipation and flame retardancy.Traditional method is to improve the thermal conductivity and flame retardancy of silicone rubber via incorpating different functional fillers.Especially,high content of thermally conductive filler and flame retardants are generally needed,while this casues the increase in the viscosity of the material prepolymer and the processing difficulty.There is few papers to disclose the effect of different size and morphology of fillers on various physical proeprties of the silicone rubber.In addition,constructing the 3D thermally conductive network of nanofiller in the silicone rubber can produce high thermal conductivity at a relatively low content.However,the poor compatibility between the filler and matrix could lead to the decreased mechanical properties,and most of them involve in the use of the harmful solvent,which is not applicable for large-size product.Therefore,we conducted the thermally conductive silicone rubber composites to understand the related composition-structure-property relationship,and the the main researches were mainly introduced as following:(1)Based on liquid vulcanized silicone rubber(LSR),three thermally conductive fillers with different morphologies are selected,i.e.spherical alumina(Al₂O₃)?granular aluminum nitride(Al N)?lamellar boron nitride(BN).The thermally conductive silicone rubber composite material was prepared by a simple mechanical blending method,and the effects of the morphology and content of the thermally conductive fillers on its thermal conductivity,flame retardancy and other properties were investigated.The results revealed that at the same filling amount,due to its large specific surface area,the lamellar BN can more easily contact each other in the silicone rubber matrix than the corresponding Al₂O₃Al N,which is attributed to the formation of interconnected network for heat conduction path.When the filler loading is 120 Phr,the thermal conductivity of BN/LSR-120 composite material is 1.04 W/m K,which is about four times the thermal conductivity of pure silicone rubber(0.25 W/m K),and the thermal conductivity increases as much as 316%.Compared with the Al N/LSR-120(0.56 W/m K)and Al₂O₃/LSR-120(0.47 W/m K)composite materials,the thermal conductivity of BN/LSR-120 are enhanced by 0.48 W/m K and 0.57 W/m K,respectively.In addition,owing to the compact dense BN network in the silicone rubber matrix,the BN/LSR-120 composites also exhibit better excellent flame retardancy and thermal stability than other two composite systems at the same content.(2)In order to improve the heat conduction path and resolve the problems of high viscosity and difficult processing caused by high filling conent of BN,we used granular Al N and lamellar BN to prepare hybrid silicone rubber composites.At a fixed filling content of 120 Phr,a series of multi-component thermally conductive silicone rubber composite materials were prepared by simply adjusting the content ratio of BN and Al N.The effect of the filler ratio on the thermal conductivity of silicone rubber composite materials was explored to optimize the processing and performance of the composites.It was found that the combined use of hybrid fillers with different morphologies not only reduces the viscosity,but also further enhances its thermal conductivity.When the content ratio of BN to Al N is 1:1,the prepared B1A1 composite material can obtain the highest synergistic enhancement,and its thermal conductivity can reach 1.16 W/m K,which is five times higher than that of B0A1(0.56 W/m K)and B1A0(1.04 W/m K)prepared by filling single Al N and BN.In addition,the vertical combustion test and thermal stability test results also demonstrated that the excellent flame retardancy and good thermal stability were obtained in the B1A1 composites.(3)In order to reduce the interface thermal resistance between the filler and the matrix,a vinyl triethoxy silane coupling agent(VTES)containing carbon-carbon double bonds is used to functionalize the BN and thus improve the wettability of the BN surface and enhance its affinity with the matrix,thus leading to the improved thermal conductivity and increased mechanical properties of the composite.Comparision of various properties of the silicone rubber composites prepared before and after modification confirmed that the VTES modification not only promoted the dispersion of BN in the silicone rubber matrix,but also enhance the interfacial interaction between BN and silicone rubber matrix.As a result,the mechanical property and thermal conductivity were improved without altering its flame resistance.When the filler loading is 120 Phr,the thermal conductivity of the V-BN/LSR-120 composite prepared by the functionalized boron nitride(V-BN)is 1.23 W/m K,which is?392%increase compared with that of pure silicone rubber,and the value is much higher than the thermal conductivity of the BN/LSR composite.Moreover,the surface treatment also promoted the stress transferring from the matrix to the filler,thus producing a higher mehcnaical strength.