A study on enhancing the electrical properties of polymer/conductive filler composite

Abstract

Recently, IT devices such as e-books, cellular phones, flat-screen TVs, and digital cameras have become thinner and integrated, and automobiles have been equipped with various electronic devices. These trends have led to issues such as electromagnetic interference and exposure of human beings to electromagnetic waves. Metallic materials have superior electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE). However, they also have some disadvantages, such as their high weight and limited design options due to high material processing costs and a difficulty for complicated molding. While polymers have some advantages such as lightness and ease of processing, they usually have no electrical properties. Therefore it is very important to impart electrical conductivity and EMI shielding property to polymer/filler composites for dramatically widening the range of polymer applications. As a method of impart the electrically conductivity to polymer, one approach is to have the polymer itself be given an electrical conductivity by providing rigid and conjugated polymer backbone. The current inherently conductive polymers are typically difficult to process owing to their poor mechanical properties, high viscosity, and high melting point. Another approach for imparting electrical conductivity to insulating polymers is to add electrically conductive filler. One method for compositing electrically conductive filler with insulating polymer is to use metallic fillers with superior electrical conductivity, such as magnesium, aluminum, copper, nickel, silver, and stainless steel. However, except magnesium and aluminum, all these metallic fillers have much higher densities than polymer and their use increases the weight of parts. Magnesium has a low corrosion resistance and need to be treated for corrosion protection. Also for aluminum has a dense oxide layer on the surface, while it has the role of preventing corrosion and oxidation of aluminum, it has a problem of lowering the effectiveness of filler when the aluminum particles are small enough to be composited with polymer. The most commonly used industrial method is to add carbon fillers such as graphite, carbon black, and carbon fiber (CF), to insulating polymers. Graphite and carbon black have the advantages of low cost, and high electrical conductivity

however, they requires high contents to produce conductivity, causing increase in viscosity, poor degassing behavior, and degradation of processability of the composites. CF has the advantages of high electrical conductivity, high rigidity, and high reinforcing effectiveness in fibrous shape. Similar to graphite and carbon black, they have the disadvantage that a high content of CF increases the viscosity of the composite. This lowers processability and degrades the exterior quality of the product because of CF protruding from the product surface. Carbon nano-fillers such as carbon nanotube (CNT), carbon nanofiber, graphite nanoplatelet, and graphene, are promising alternatives, which can have large effects even when present in small amounts as they have nano-scale dimensions and large specific surface areas. However, they have some limitations such as high cost, poor dispersibility in a polymer matrix due to van der Waals and Coulomb attractions, and limited adding amount due to high specific surface area. Consequently, carbon nano-fillers are significantly effective to reduce the percolation thresholds of the polymer nanocomposites, however, the conductivity values are typically less than 1 S/cm. For imparting electrical properties to insulating polymer with maintaining the properties of the matrix as much as possible, it is very important to enhance the efficiency of conductive fillers. Therefore, It is needed to understanding the mechanism of producing the electrical properties of the conducting composites and investigate the effects of various factors on the electrical properties. Furthermore, it is also important to understand the synergistic effect and its mechanism of two kinds of fillers. First of all, for understanding the mechanism of producing the electrical conductivity of electrically conducting polymer composites, the cause of differences in the electromagnetic interference shielding efficiency (EMI SE) of polyamide (PA)/carbon fiber (CF) and polycarbonate (PC)/CF composites at same CF content was explored. As the CF content increases, EMI SE of PA/CF continuously increases, however, that of PC/CF becomes saturated from a CF content 25 wt%. Theoretically electrical conductivity and magnetic permeability effect on EMI SE, their properties were measured. It was found that the magnetic permeability of PA/CF is similar to that of PC/CF but the electrical conductivity of PA/CF is higher than that of PC/CF at CF content over 25wt%. To identify the cause of differences in the electrical conductivity in PA/CF and PC/CF, the length and orientation structure of CF within these composites was observed. It was determined that compared to PC/CF, PA/CF has longer fiber length and more domains where the fiber orientation is random, resulting in higher electrical conductivity. Secondly, carbon fibers (CF) and two types of carbon nanotubes (CNTs) were investigated using polyamide 6,6 (PA) as a matrix for identifying their synergistic mechanism. One type of CNT was sized with polyurethane (PU-CNT), and the other was not sized (N-CNT). When PA was compounded with each CNT separately, it was found that PU-CNT, which showed better compatibility with PA, formed looser and larger agglomerates, resulting in better electrical conductivity as compared to N-CNT. To observe the synergistic effect of CF and CNT, PA/CF/CNT composites were examined. At a low CF content, PU-CNT produced significantly synergistic effect while N-CNT did not. However, at a high CF content, both PU-CNT and N-CNT showed synergistic effects due to the improved dispersibility of CNTs. Furthermore, the effect of PU-CNT is more 10 times of that of CF alone, while the effect of N-CNT is just three times of that of CF alone, indicating that the size of CNT agglomerates is also important for synergistic effect. From these results, it is concluded that the two mechanisms producing the synergistic effect of CF and CNT are the formation of a co-supporting network of CNT/CF and the dispersion of CNT by CF. Lastly, a new method of polymer/low-melting-point metal alloy (LMA)/elongated light metal composite fabrication is proposed to solve problems of polymer/metal composites. The first step is mixing light metal powders with LMA at a temperature above the melting point of the LMA. The second step is cold extrusion of the LMA/light metal powders to fabricate LMA/elongated light metal. Thus, the metal filler with the density of ~4.5 g/cm3 was obtained. The last step is compounding a polymer with the LMA/elongated light metal at the processing temperature of the polymer above the melting points of the LMA. The effects of the length and the cross-sectional shape of the elongated light metal on the morphology of the LMA/elongated light metal in the polymer matrix were studied, as were electrical conductivities and mechanical properties of the composites. As the length and/or the cross-sectional aspect ratio of the elongated light metal was increased, the domains of LMA/elongated light metal formed more networks so that the electrical conductivity increased, and specific surface area of LMA/elongated light metal increased so that notched Izod impact strength was improved. Thus, the polymer/LMA/elongated light metal composites were fabricated without degrading processability even at 60 vol% metal filler loading and the electrical conductivities over 103 S/cm were achieved. In this study, the effects of various factors on the electrical properties of the electrical conductive composites were investigated and the synergistic mechanisms of two kinds of fillers were identified. On the basis of these results, a new filler system was proposed, and the electrical conductive polymer composite can be fabricated with enhancement of the electrical and mechanical properties without degrading the processability of the composites.