Research On Microcellular Injection Molding Polypropylene Matrix Conductive Composite And Its Electromagnetic Shielding Properties

With the development of modern communication technology and precision electronic equipment,the problem of electromagnetic radiation pollution has become increasingly prominent.Conductive polymer composites（CPCs）reinforced with carbon nanofillers have become a research hotspot in the field of electromagnetic shielding due to their light weight,flexible conductivity control and wide absorption band.However,due to the strong van der Waals force between carbon nanofillers,they are easy to aggregate during the processing,which limits their enhancement effect.It is still a big challenge to improve the dispersion of carbon nanofillers and build a uniform conductive network.Introducing in-situ fiber forming process into microcellular foaming injection molding system can greatly improve the microcellular foaming performance of polymer by using heterogeneous nucleation of fibrils.The improved cell structure can enhance the volume exclusion effect and promote the dispersion of conductive fillers.In this paper,polypropylene（PP）was used as matrix,multi walled carbon nanotubes（CNTs）as carbon nano filler,and polytetrafluoroethylene（PTFE）as in-situ fiber reinforced phase to prepare microporous electromagnetic shielding composites with good mechanical properties and high absorption.The main contents of this paper are as follows:1.PP/CNT composites with different CNT content were prepared by melt blending and injection molding.The crystallization behavior,rheological properties and electrical properties of PP/CNT composites with different ratios were analyzed.CNT enhanced the young’s modulus of PP/CNT composites from 212.06 MPa to 332.14 MPa.The appearance of storage modulus plateau in low frequency region and the formation of loss factor peak indicate that CNT overlap each other to form a physical network structure.In addition,the addition of CNT can significantly improve the conductivity of PP from1.05×10^-14S/cm to 3.75×10^-6S/cm,and the electromagnetic shielding performance from 0.6d B to 12.4d B.2.A series of PP/CNT composite foams with adjustable pore structure were prepared by supercritical N₂microcellular foaming injection.The effect of CNT content on the foaming behavior,electrical conductivity and electromagnetic shielding properties of PP microporous foam was investigated.The results show that CNT improves the microcellular foaming properties of PP matrix.The introduction of cell structure promotes the rearrangement and connection of CNTs to form conductive network,which increases the reflection times and path of electromagnetic wave between cell wall and filler.The electromagnetic shielding performance of composite microporous material with 1wt%CNT increases from 8.6d B of solid material to 12.5d B.3.In order to further improve the dispersion of CNT,PP/CNT/PTFE ternary composite microporous materials were prepared by in-situ fiber-reinforced technology and microcellular foaming injection molding process.The effects of PTFE content on the crystallization behavior,rheological properties,microcellular injection foaming behavior and electromagnetic shielding properties of the composites were revealed.PTFE formed a network structure of high aspect ratio fiber and physical entanglement in PP.The self-lubricating effect of PTFE greatly improves the elongation at break of the matrix.The heterogeneous nucleation effect of PTFE fibrils with high aspect ratio was obvious.The cell density increased from 7.52×10⁶cells/cm³to 6.76×10⁷cells/cm³,and the cell diameter decreased from 38.79μm to 26.43μm.Compared with PP/CNT microporous samples,the electromagnetic shielding performance of PP/CNT/PTFE microporous samples with the same CNT content increased from 12.6d B to 24.8d B,showing good electromagnetic shielding performance.