Study On The Electrical Properties And PTC Behavior Of Carbonaceous Fiber/Polymer Composites

In recent years, conductive polymer composites (CPCs) have been paid more and more attention and used in a wide variety of industrial applications such as self-regulating heater, current protection devices, microswitches sensors and electromagnetic interference shields. These CPCs are consisted of insulating polymer composites incorporated with conductive particles such as carbon black, carbon fibers, metal particles or conducting polymers such as polyaniline.One of the most attractive features of conductive polymer composites lies in their positive temperature coefficient (PTC) effect, which is characterized by a drastic rise in volume resistivity as temperature approaches the melting point of the matrix resin. The PTC effect was first reported by Frydman in 1948. Up to now, there is still not a comprehensive theory to describe the PTC phenomenon. The most common explanation for the PTC behavior is that when the conductive polymer is heated, conductive pathways are broken due to thermal expansion of the polymer, leading to an increase in resistivity. PTC materials have been used widely for many applications such as sensors, self-regulating heaters, over-temperature protectors, over-current protectors and switching materials. The negative temperature coefficient (NTC) effect following the PTC transition, which is induced by rearrangement of conductive fillers in low viscous polymer melts, is a main drawback for the application of PTC materials. Thus the NTC effect must be eliminated. Crosslinking polymer composite is the common method of eliminating the NTC effect. If the agglomerate of conductive filler is restricted, the NTC effect would be eliminated. According to this consideration, we try to increase the viscosity of composite by filler and matrix, replacing the crosslinking method, to eliminate the NTC effect.In recent years, carbon fiber filled conductive polymer composites have attracted the more attention of the researchers. Carbon fiber possesses a certain aspect ratio, and it is regarded as a rigid and conductive chain formed by many carbon black particles. Therefore, the resistivity of polymer composite filled with carbon fiber is lower than that of composite filled with carbon black at same filler loading. In the filled polymer composites, the carbon fibers are not easy to aggregate together and therefore the NTC effect can be decreased or eliminated. Especially, as a common sense, the short fibers with small aspect-ratio are easy to be separated by thermal expansion, indicating better PTC effect and fewer NTC effect than those of the composites with common carbon fibers.In this paper, the carbonaceous fibers, namely carbon fiber (CF) and graphite fiber (GF) were filled with polymer matrix as conductive filler. The effect factors of the electrical properties of these composites were investigated in detail, as followed: The effect of process conditions on the resistivity of polyvinylidene fluoride (PVDF) composites filled with GF and CF were studied; the PTC/NTC behaviors of CF/PVDF and GF/PVDF composites were studied; the electrical properties and PTC/NTC behaviors of GF/PVDF/UHMWPE ternary composites were studied; and the PTC behaviors of CF-filled amorphous EPDM rubber composites were explored.One of attentive problem associated with the production of conductive fibers filled polymer composite is to obtain reproducible conductivity. Processing conditions, especially those involving considerable shear of the polymer-fiber filler mixture, can have a greater effect on the aspect ratio, dispersion and distribution of filler, and on the conductive properties. Therefore, it is necessary to select a standard processing condition for the manufacture of such composites having reproducible conductive properties. In the second chapter of our thesis, the effects of processing conditions (mixing time, mixing temperature, rotor speed, vulcanization pressure and nip gap) on the conductivity of composites were discussed. The results indicated that the different breakage of GF and CF leaded to the change in the resistance of composites, which was influenced by processing conditions. The optimal conditions for GF and CF have been fumbled: the optimal mixing time for GF is 8 minutes and for CF is 10 minutes, the range of optimal rotor speed is 15-25rev./min, the optimal mixing temperature is 170-180°C, the optimal vulcanization pressure is 10-20MPa, and the optimal nip gap is 3mm.In the third chapter of our thesis, the conductive properties and PTC/NTC behaviors for GF/PVDF and CF/PVDF composites were compared. It was found that the percolate curves shape of GF/PVDF composites were similar to that of CF/PVDF composites. The percolate threshold of the GF/PVDF composites (5wt%) was smaller than that of CF/PVDF composites (8wt%). We studied the resistivity-temperature behavior of the both composites, and found that the composite containing filler content around the percolation threshold exhibits the highest PTC intensity, the PTC intensity of GF/PVDF composite was larger one order than that of CF/PVDF composite, and the PTC transition temperature of GF/PVDF composite is lower than that of CF/PVDF composite. Moreover, it was found that CF can be more effective to eliminate the NTCeffect compared with GF. By DSC, SEM, OM and revolving rheometer analysis, the various breakage of fiber was the main factor which resulted in the differences of PTC intensity, PTC transition temperature and NTC effect. According to the different action of GF and CF, we prepared GF/CF/PVDF ternary composite. Addition of 2wt%CF into the composite with 7wt%GF can effectively eliminate the NTC phenomenon of the composite, and the composite showed a larger PTC intensity and a higher PTC transition temperature.In the fourth chapter of our thesis, in order to increase the viscosity of composite, ultra-high molecular weight polyethylene (UHMWPE) was selected as the second matrix to fill with the composite, 6wt% of GF loading was fixed, and the effect of UHMWPE content on the electrical and PTC/NTC behavior of GF/PVDF/UHMWPE ternary composites was investigated. It was found that with the increase of UHMWPE content, the resistivity of system decreases firstly and then increases. When the PVDF/UHMWPE weight ratio is 95/5, the composite showed a minimum of resistivity. With increasing the PVDF/UHMWPE weight ratio, the resistivity of composites increased gradually. By SEM observation, it was found that the morphology of UHMWPE can affect the dispersion of fibers. At low UHMWPE content, the UHMWPE particles uniformly dispersed in the PVDF matrix, few GF particles can penetrate these particles due to the very high viscosity of UHMWPE, and most of GF particles were forced to disperse in PVDF matrix. With increasing the UHMWPE content, the morphology of UHMWPE was transformed from particles to sheet structure, and many GF can penetrate easily the sheet formed by UHMWPE. Addition of UHMWPE to PVDF matrix results in the occurrence of volume-exclusion effect and volume-expansion effect. The resistivity of composites depended on the both effects. At low UHMWPE content, the volume-exclusion effect wasPTC/NTC behavior; PVDF/UHMWPE; Conductive EPDM rubber. preponderant, resulting in the decrease of resistivity. When the UHMWPE content reached a certain value, the volume-expansion effect was preponderant, and the resistivity increased gradually. Addition of UHMWPE can also affect PTC/NTC behavior of composites. The composite without UHMWPE showed a notable NTC effect, but for GF/PVDF/UHMWPE system, when the PVDF/UHMWPE weight ratio is lower than or equals to 95/5, the NTC effect can be completely eliminated. When PVDF/UHMWPE weight ratio reached 70/30, the system showed two jumps in resistivity at 140°C and 160°C, respectively, namely two-step PTC effect. This phenomenon was attributed to the melting of PVDF and UHMWPE crystallites, respectively.All of the PTC matrixes used were crystalline or semicrystalline polymer. However, amorphous rubbers were not considered to use as the PTC materials. Many researchers studied the resistivity-temperature behaviors of carbon black filled rubber composites, such as EPDM, NBR, SR, and found that these conductive rubbers showed an obvious NTC effect or a weak PTC effect. In the fifth chapter of our thesis, CF, which has a certain aspect ratio, was filled with the ethylene-propylene-diene (EPDM). We researched the resistivity-temperature behaviors of CF/EPDM and CB/EPDM composites with various filler content. It was found that CF/EPDM composite showed a PTC effect, while CB/EPDM composite showed a NTC effect. According to this phenomenon, we proposed that the resisitivity-temperature behavior of conductive EPDM rubber composites completely depended on the shape of carbonaceous fillers. I-U characteristics indicated that the CF/EPDM rubber composites can be used as heating materials.