| Electronic equipment and communication technology bring convenience to society,but also bring electromagnetic radiation pollution problems.Therefore,polymer based on electromagnetic shielding materials with light texture,machinability,corrosion resistance,low cost and other excellent characteristics have come into the attention of researchers.Segregated structure and uniform structure are two typical structures of polymer based on electromagnetic shielding materials.Composite materials with these two structures have broad application prospects in communication transmission,aerospace,electronic equipment,especially in the field of electromagnetic shielding protection,due to their advantages such as preparation,processability,flexible structural design,and performance adjustment.In this research,starting from the conductive network structure design of electromagnetic shielding materials,we prepared electromagnetic shielding composites based on uniform structure and segregated structure respectively by regulating the dispersion of conductive fillers in the polymer matrix.Through the structure morphology regulation and performance test,we revealed the internal laws between structure and conductive performance,electromagnetic shielding performance,mechanical performance,thermal stability and flame retardant performance.In addition,we used multilayer graphene(MLG)as conductive filler to design a novel semi-melt processing method,which can stably build a perfect segregated structure in the low melt viscosity polymer multi-block polyesterimide(BPEI),so that it has an efficient and stable conductive path,providing a new idea for building a conductive network of low viscosity system segregated structure.The research contents of this paper are as follows:(1)BPEI/MLG composites with uniform structure were prepared by melt blending and molding.Firstly,we adjusted the micro morphology of uniform structure by processing parameters such as MLG content,processing temperature,blending time and blending rate,so as to explore the law of conductive network morphology changing with parameters.The results showed that the increase of MLG content can reduce the distance between filler particles and form a better conductive network,but it also increased the contact opportunities and lead to agglomeration;High processing temperature can provide low polymer melt viscosity and fluid power,which was conducive to filler dispersion;The proper blending rate and time promoted the dispersion of MLG,but too large blending rate and too long blending time made MLG tend to agglomerate.Secondly,based on the micro morphology,the relationship between the structure and the conductivity,electromagnetic shielding and mechanical properties of the composite were studied.It was found that the composite with more perfect uniform structure often has better conductivity and electromagnetic shielding properties: the percolation value of the system was 1.05 vol%,and the conductivity is 52.9 S/m.When the material thickness was 2.0 mm and the MLG content was 5.0wt%,the electromagnetic shielding performance of the composite material reached 33.4 d B.In addition,we revealed that the uniform structure’s electromagnetic shielding mechanism dominated by reflection.For mechanical properties,a small amount of MLG loading was beneficial to the mechanical properties,which was due to the inherent strength of MLG,the mechanical coupling between MLG and polymer,and less agglomeration.However,excessive MLG addition aggravated the agglomeration and weaken the mechanical properties and the composite under 5.0 wt% MLG loading still showed a tensile strength of 41.5 MPa and a tensile modulus of 1.8 GPa.(2)BPEI/MLG electromagnetic shielding composite with segregated structure was prepared by a simple mechanical mixing method.Here,we use the single variable principle to explore the influence of molding temperature and MLG loading on the morphology of segregated structure.The research showed that the gradually increase of MLG contendt can broaden the conductive path of the material and build a perfect segregated structure,while the high molding temperature will make MLG separate from the BPEI interface,either agglomerate in a large scale or disperse into the BPEI matrix,which will destroy the segregated structure.Moreover,we prepared the electromagnetic shielding composite with segregated structure had the most perfect conductive path without obvious defects at the optimized temperature of 270 °C and MLG content of 5.0 wt%.Secondly,we studied the relationship between the perfection of the segregated structure and the conductivity,electromagnetic shielding performance,mechanical performance,thermal stability,and flame retardancy.The research showed that the more perfect the segregated structure is,the higher the effective utilization rate of the filler is,and the more efficient the conductive path is.Therefore,the better the conductivity and electromagnetic shielding performance of the composite material have:an extremely low percolation value of 0.39 vol% and an ultra-high conductivity of 313.5 S/m,the material has an excellent electromagnetic shielding efficiency of 62.2 d B at a thickness of 2.3 mm.In addition,we also revealed that the system is an electromagnetic shielding mechanism dominated by reflection,and its conductor mechanism follows the conductive path theory.For mechanical properties,the construction of the segregated structure is unfavorable to the tensile strength and tensile modulus of the material,and this trend will gradually increase with the increase of MLG content.However,under the loading of 5.0 wt% MLG,the composite still has a tensile strength of34.9 MPa and a tensile modulus of 1.5 GPa,which is due to the good interfacial adhesion between MLG and BPEI.Finally,the rigid aromatic skeleton and ester bond of BPEI bring excellent intrinsic thermal stability and flame retardancy,while the addition of MLG will introduce more carbon residue layers,further expanding this advantage,making the initial decomposition temperature of the composite as high as 465 °C,LOI value of 32.7%,flame retardant grade of V-0 and no droplet drop. |
PDF Full Text Request