Research On The Gradient Formation Mechanism Of PVB Based Electrically Conductive Composites By Olution Casting

Conductive polymeric composites (CPCs) are the dominant electrically conductivepolymeric materials in application because of the large-scale variation in the electricalconductivity, the favorable processability and low costs. Currently, electrically conductivefillers are uniform distribution through polymer matrix for almost all CPCs. Thepercolation volume of such materials is much high in a binary composite system withrandom packing in three-dimension, which result in a dramatic decrease on themechanical performance of the materials. In order to solve the contradiction betweenelectrically conductive performance and mechanical performance, the concept offunctionally gradient material (FGM) was introduced to prepare electrically conductivepolymeric composites, combining the advantages of electric conduction and gradientstructure tactfully. In this dissertation, metal filled gradient electrically conductivepolymeric composites were developed by use of solution casting technique on theprinciple of Stokes’ law. Also，the properties of these gradient materials were investigated.PVB solutions with different concentrations were prepared. The motion of fillerparticle through the solution and the gradient formation mechanism of the material wereinvestigated. Experimental facts show that the sedimentation velocity of filler particle isaffected by several factors, such as solution concentration, the speed of volatile solvent,particle size, density difference between particle and solution, and so on. The motion offiller particle through the solution can be analysed by Stokes’ law, and the relationshipbetween measured velocityν_m and theoretical velocityν_t was describedas:νm=A+Bν_t；By theoretical inference, the critical viscosity η_c formula is expressedFor PVB/Cu/ethanol system， the criticalconcentration C_c is about0.27.In this research, PVB/Cu and PVB/Ni series composites were prepared by use ofsolution casting. The effect of filler content and particle shape on the structure andperformance of the materials were investigated. Scanning Electron Microscope (SEM)and Energy Dispersive Spectrum (EDS) analysis show that the gradient distribution offiller particles in matrix formed along their thickness-direction. The conductive testingresults show that the order of magnitude of surface resistivity were kept at10~16Ω on resinrich side, while that declined to10~6Ω on filler rich side, and the percolation thresholdoccurred between3.0vol.%and4.5vol.%filler content at metal particles rich side. By contrast, The percolation volume of homogeneous conductive composites is about16vol.%. This result proved that the gradient distribution of fillers reduced the percolationthreshold, and therefore saved filler costs. Mechanical test demonstrated that the tensilestrength reduced insignificantly as the content of filler increases, which is very differentfrom homogeneous composites. That is to say, the gradient distribution of fillerguaranteed such material’s mechanical performance. Furthermore, these materials showgood shielding effectiveness (30-51dB).All experimental facts demonstrated that gradient polymeric composites can beprepared successfully using this technique. With electrically conductive fillers gradientlydistributed in polymer matrix, the contradiction between electrically conductiveperformance and mechanical performance is effectively resolved.