Preparation And Emi Shielding Performance Of Modified Chopped Carbon Fiber Foamed Composites

With the development of communication technology and precision electronic equipment,the requirements of electromagnetic shielding protection level are increasingly higher.The development of high-performance electromagnetic shielding materials has become one of the urgent needs in the field of electromagnetic information.At present,a single electromagnetic shielding material can’t meet the needs of modern high electromagnetic shielding protection level.Electromagnetic shielding materials are developing from the original single homogeneous to multi-dimensional composite structure.Composite electromagnetic shielding materials with high electromagnetic shielding performance,light weight,flexibility and high strength have attracted extensive attention.In this paper,the foamed coating material was prepared by physical foaming with chopped carbon fiber（CF）as conductive filler and waterborne polyurethane（WPU）as resin matrix.On this basis,the conductive carbon fiber（CF）was modified by polydopamine（PDA）and magnetic Fe₃O₄nanoparticles.Then the CF hybrid material was prepared.After blending and foaming with WPU,the conductive cloth based WPU/（CF/PDA/Fe₃O₄）foam coating composite with high electromagnetic shielding performance was prepared.In order to achieve high electromagnetic shielding performance and light elasticity,the short cut carbon fiber with high conductivity was selected as the conductive filler,and combined with waterborne polyurethane,the light flexible WPU/CF foamed coating material was prepared by physical foaming method.The test results show that when the length of carbon fiber is fixed（3 mm）,when the mass ratio of CF increases from 2%to 10%,the pore diameter on the surface of foamed coating material gradually increases,the uniformity of pore size distribution decreases,and the electromagnetic shielding performance gradually increases,but the increasing trend of shielding performance begins to slow down when the mass ratio of carbon fiber increases to 6%.When the mass ratio of carbon fiber exceeds 8%,the young’s modulus and tensile strength of the material decrease;When the mass ratio of carbon fiber is fixed（6%）,with the increase of CF length from 1 mm to 6 mm,the electromagnetic shielding performance of foamed coating increases first and then slows down,and the young’s modulus and tensile strength also increase;In general,the preferred parameter of carbon fiber of foaming coating material is 3 mm length to 6%mass ratio.The electromagnetic shielding performance of foaming coating material reaches 24.9 d B in the frequency of 30 MHz-1500MHz,and has certain flexibility and elasticity.In order to improve the impedance matching of carbon fiber and further improve its electromagnetic shielding performance,the hybrid material was prepared by combining conductive CF and magnetic Fe₃O₄.Firstly,the surface of carbon fiber was pretreated by dopamine polymerization catalyzed by Cu SO₄/H₂O₂,and then Fe₃O₄magnetic nanoparticles were grown on the surface of polydopamine modified carbon fiber by solvothermal method.Through SEM test,it is found that Cu SO₄/H₂O₂can efficiently catalyze the polymerization reaction,and a layer of polydopamine film is evenly coated on the surface of CF,which effectively increases the active groups on the surface of CF and is beneficial to improve the adhesion between CF and Fe₃O₄in the subsequent process;Fe₃O₄nanoparticles were uniformly loaded on the surface of CF after solvothermal treatment.The mass ratio of carbon fiber to iron source is an important factor to adjust the electromagnetic properties of hybrid materials.The electromagnetic shielding properties of hybrid materials increase first and then decrease with the increase of iron source ratio.When the mass ratio of carbon fiber to iron source is 1:5,CF/PDA/Fe₃O₄material has the highest electromagnetic shielding performance,up to 41.5 d B at 18 GHz,and the electromagnetic shielding efficiency reaches 99.99%,which is obtained through the synergistic influence of a large number of interface polarization and electro-magnetic loss.In order to endow the foamed coating material with good mechanical properties,the hybrid foamed coating material was coated on the conductive fabric with high mechanical properties,and the conductive cloth based WPU/（CF/PDA/Fe₃O₄）foamed coating composite was prepared.Compared with pure hybrid foam coating material,the surface pore size of conductive cloth based hybrid foamed coating composite decreases,the number of pores increases and the uniformity of pore size distribution increases;Young’s modulus and tensile strength were improved;Because the base cloth has good conductivity,it can act as the bottom electromagnetic wave reflection layer after compounding with the foamed coating material,which can further improve the reflection and absorption loss of electromagnetic wave,so as to enhance the overall electromagnetic shielding performance of the composite material.In the frequency of 30 MHz-1500 MHz,the average electromagnetic shielding performance of conductive cloth based hybrid foamed coating composite can reach 59.2 d B,which is 30.9 d B higher than that of hybrid foam coating and 109.18%higher than that of hybrid foaming coating.This subject combines the electric loss and magnetic loss electromagnetic shielding materials,and on this basis,combines the porous structure with the interlayer composite structure.The electromagnetic shielding performance was enhanced through the combination of electric-magnetic materials and multi-dimensional structure design.On the premise of high electromagnetic shielding performance,it has high mechanical strength and certain flexibility,which has a certain reference for the preparation and development of multi-dimensional electromagnetic shielding materials.