Study On Graphite (Carbon) Fiber/Carbon Black/Polymer PTC Conductive Composites

Resistivity of PTC conductive composites based on polymer increases with ambient temperature rising, when the temperature rises to vicinity of the polymer melting point, the resistance rapidly increases (3-8) orders of magnitude instantaneous, that is called PTC effect. PTC composites have been used widely in electromagnetic shielding materials, self-regulating heating, over-current protection, thermal-overload protection and other fields utilizing the large resistivity jump nique properties of composites. Pepole have devoted their intrest to the study and develop, due to the advantage of easy molding process, light weight and the resistivity sudden changes more sensitively. But after PTC transition temperature, the resitivty decreases quikly because of the the rearrangement of conductive fillers, named NTC effect. The resistiviy decreases after the formation of conductive path. That is a fatal defect to the composites, and with poor electrical reproducibility after undergoing repeat heat cycles, limited their application prospects. Thereby improving the stability of PTC conductive composite is imperative, especially eliminating the NTC effect.CB as conductive filler prepared PTC materials have been extensively researched, with high resistivity, large filler content, and made them difficulty in molding process, exhibiting less stability. In this paper ,we choosed GF as conductive filler with high aspect ratio structure and high conductivity ,based on the preliminary work. In 2008, the first user was a doctor in our research group, based on his work and carried out the unfinished part.In the past years, people always tried to eliminate NTC phenomenon by mesns of radiation crosslinking. This paper design idear started with the influencing factor of the fillers distribution in the composites, adopted non-cross-linking meatures to explor the essence of improving NTC effect, and enhanced the stability of composites. We prepared four systems including eleven kinds composites. One-filler of single base systerms: (HG-1P)CB/PVDF, (ACET)CB/PVDF, GF/PVDF, GF/EVA, (ACET)CB/EVA;Two-fillers of single base systerm: GF/CB/EVA; One-filler of two bases systerms: GF/HDPE/PVDF, GF/EVA/PVDF; Filler-modified systerms: Oxidize-GF/EVA, Oxidize-GF/EVA/PVDF, Oxidize-GF/PVDF.The first part: prepared (HG-1P)CB/PVDF, (ACET)CB/PVDF, GF/PVDF composites through traditional melt blending process. CB include (HG-1P)CB with high structure and large surface area and the other (ACET)CB reverses to (HG-1P)CB. Compared the PTC/NTC behavior of three kinds of composites, we believed that different shape and structure of CB and GF had important effects on PTC properties, especially (HG-1P)CB with high structure and large surface area formed conductive path easily undergoing heat process, which resulted in unstable properties, so not suitable for PTC materials. While the (ACET)CB/PVDF composite required large filler contents to meet certain conductivity. GF/PVDF composite showed lower percolation threshold and room temperature resistivity, high PTC intensity advantages, but the composite also had NTC effect, not esaily dispersed because of its bundle structure relativel to the granular structure CB.DSC tests showed that GF/PVDF andCB/PVDF composites, exhibited large volume expansion when the temperature approached the polymer melting point, and destroyed the conductive path, turning up large resistivity jump, so the melting of polymer crystalies was the reason for the PTC effect emergence. After heat treatment at 140℃10h on GF/PVDF and CB/PVDF composites, the PTC effect of GF/PVDF composite had improved significantly: the perfect crystallization, improved the crystallinity, decreased the room temperature resistivity, the resistivity had little change with temperature before the PTC switch temperature, and the PTC switch temperature increased, but the NTC effect still existed.The second part: Because of the special structure of PVDF, there is a strong interaction like hydrogen bonds among the molecular chains, resulting in the chains arranged closely. The C-F interval arrangement on the chains, forming certain shielding room beacause of the C-F mutual exclusion, while inhibits the invading of the conductive filler. So we selected EVA as polymer, the chain with ester groups, prepared GF/CB/EVA composites. The long aspect ratio of GF and spherical CB had the positive cooperativity in conductivity, and solved the large filler contents problem, while GF dispersion improved because of the CB dispersed evenly. Usually the fibrous filler can increase the viscosity and improve the NTC effect. But we found that the hydrogen bonds between the CB with -OH,-COOH and EVA matrix was important contributions to improve the NTC effect, because of inhibiting conductive filler agglomeration at high temperature.The third part: Although the special structure of PVDF have some shielding effect on dispersion of conductive fillers, we selected PVDF as a second polymer to prepare two polymer blends composites, utilized its good chemical stability, and easy processing properties. as known to us preparing blends composites was an effect mean to reduce the filler contents and eliminate NTC effect, people usually increased the viscosity of systerm to eliminate NTC effect through selecting high viscosity UHWMPE as the second group, while the viscosity was too high to process. So we replaced it by PVDF a stable property polymer as second group, and prepared GF/HDPE/PVDF composites and GF/EVA/PVDF composites. Studied the influence on PTC/NTC behavior with PVDF addition and fixed GF content 6%,With PVDF addition increased, the room-temperature resistivity of GF/HDPE/PVDF composites showed a sharp decline. HDPE/PVDF weight ratio was 1/1, room-temperature resistivity reduced by nearly 5 orders of magnitude compared with GF/HDPE composite, which turned the GF/HDPE insulator into a PTC material with lower room temperature resistivity, good PTC effect, and weaken NTC effect. When the HDPE/PVDF was 1/2, there was a clear double-PTC effect with forming the two continuous phase structure, delaying and declining the NTC effect, at the same time the resitivity turned up large jump near the PVDF melting temperature, PTC intensity had greatly improved relative to the GF/PVDF composite.Double-PTC effect emerged in the GF/EVA/PVDF composite at a certain percentage, which delayed and improved NTC effect. While the room temperature resistivity didn't show obvious change with PVDF addition. Contrasted the viscosity of the polymer: EVA>PVDF>HDPE,and the GF filled the polymer had the same relations corresponding to the polymer by rotating rheometer at a certain shear rate. GF firstly localized in HDPE with lower viscosity, then dispersed at the interface between HDPE and PVDF. GF dispersed in the PVDF with its addition and at the interface between EVA and PVDF, at PVDF melting temperature the PTC intensity first increased and then decreasesed with the PVDF addition. GF dispersed unevenly in their matrix due to the special structure of PVDF. Therefore, the polymer viscosity and polymer structure were important factors for GF dispersing.The last part: We found that the conductive filler characters, surface groups and the polymer structure, viscosity were important factors affecting the PTC performance based on former experimental results. The chemistry function between CB surface polar functional groups and EVA ester functional group was the key reason to remove the NTC effect. Therefore, we selected GF as conductive filler with superior performance, and by chemical modification means to increase their surface active groups, used EVA as matrix with special functional groups, tried to enhance the chemical reaction between GF and EVA matrix, in order to eliminate NTC. We confirmed that GF had large number of -OH and -COOH groups on the surface after nitric acid oxidation, the surface became rough, and grooves became deeper through FITR test and SEM characterization. Contrasted Oxidize-GF/EVA, Oxidize-GF/EVA/PVDF and Oxidize-GF/PVDF tests, found that between the Oxidize-GF and EVA not only had physical adsorption but also exited hydrogen bond. The chemical interaction between GF and EVA matrix was the basic reason for eliminating the NTC effect and improving the composites stability.