A Study On Preparation And Bonding Of Electromagnetic Shielding Plywood Laminated With Conductive Film

Electromagnetic shielding plywood is a branch of electromagnetic shielding wood-basedcomposites. It has the characteristics of environmental soundness, stable shielding performance,and easy convertibility of functions. It will be a new generation of indoor green functionaldecorative material in preventing electromagnetic signal leakage and external electromagneticradiation interference. The studies on its manufacturing, structure, performance andapplications will help to widen the research field of wood-based composites, and it will alsoprovide scientific and theoretical basis to sustainable management and development ofplantation.First of all, the process of "spreading-scattering-molding-pressing" was adopted toprepare copper fiber/urea-formaldehyde conductive film in this thesis, then its structuralfeatures, conductivity, piezoresistivity and electromagnetic shielding effectiveness were studied.Based on above study, corresponding model of conductivity and shielding mechanism theorywere established. Secondly, based on summarizing and integrating the multilayer and interlayerelectromagnetic shielding theories, laminated shielding structure model was proposed in thisthesis, which "laminating" and "compositing" the conductive film and the larch veneer toproduce a new type electromagnetic shielding plywood; then coaxial tests, electromagneticshielding effectiveness and bonding properties were studied. Meanwhile, through analysis onthe bonding method in the design of electromagnetic compatibility structure and consideringthe characteristics of electromagnetic shielding plywood, the bonding interface was designedinto 3 types: inclined interface, opposite interface and ladder interface. The interfaces are fixedby conductive glue, non- conductive glue and copper foil. Coaxial test method, shielding roomwindow method and spherical antenna method were adopted for the first time in studying thebonding technology of electromagnetic shielding plywood in the electromagnetic compatibilitydesign. Finally, according to the shear strength standards of plywood, a test was designed forstudying the relation between functional invalidation and intense invalidation. The results andinnovative points are as follows:1 The conductive film belongs to "percolation" system and has piezoresistivecharacteristics. With the increasing of copper fiber filling volume, the conductive structure ofthe conductive film will gradually develop into "three dimensional electricity conductivenetwork" from "two-dimensional electricity network", and its shielding effectiveness willbecome stronger. The relationship between the volume resistivity of film and the content of copper fiber volume is the following:ρ=ρ_cΦ^-4.9245, and the relationship of the averageelectromagnetic shielding effectiveness with the filling rate and the thickness of film is as thefollowing: SE=-45.7987+110.0766t+16.2136lnw-22.9793tlnw.2 By sealing the edge with conductive dope, the contact resistance between the coaxialtest sample and the internal and external conductor will be reduced and the tested value will benear to the real shielding effectiveness. The laminated shielding structure may improve theshielding effectiveness of material, the shielding effectiveness of a 200 g/m² double-layerconductive film laminated plywood is in the range of 39.3 to 61.75 dB, with the average valueat 49.65 dB. Under the precondition of equal total filling volume, with the increase of fillingvolume, the shielding effectiveness of double-layer conductive film laminated plywood willstart to become stronger than that of monolayer conductive film laminated plywood. The morethe filling volume is, the difference between the effectiveness of the foregoing two types ofplywood will be larger. This thesis thus is the first one to call this filling volume "double layershielding reversion point". In this experiment, the reversion point of the copper fiberfilling volume is 50 g/m², which may have strong directive significance on real production.3 SE of electromagnetic shielding plywood by copper foil connection (window methodtest) fluctuates between 56.39 and 75.15 dB, its average value is 65.86 dB and the shieldingeffect is good; for the cabinet produced in copper foil connection method, the maximum valueof whole electromagnetic shielding (spherical dipole sub-antenna method) is 45.09 dB, theminimum value is 22.98 dB; within the frequency range of 30～240 MHz, the cabinet can reachClass B requirement for universal military electronic equipment cabinet and may be applied tosituations which demand highly electromagnetic compatibility. Through comparison, copperfoil connection is regarded as the best bonding method.4 After soaking 3 hours, the shear strength of electromagnetic shielding plywood cansatisfy related national standards; after soaking 6 hours or 9 hours, the shear strength decreaseddrastically, lower than the national standards. However, after soaking 9 hours, the average SEof plywood is above 35dB, the decreased average value is between 2.54 and 9.25 dB, whichshows that under the circumstance of strength loss of plywood, the decrease of SE is notobvious, and thus it will still keep certain shielding effect.