Study On Electrical Heating Property Of Polyethylene-Based Composites

Conductive polymer composites (CPCs) are one of the most important and interesting areas in polymer composite research. CPCs is a kind of electrical resistor which easily and quantitatively converts electrical energy to thermal energy via Ohmic joule heating. Research on the electrical heating properties is important for broadening conductive composite material applications. In this paper, composites with high density polyethylene (HDPE) as a matrix, carbon fiber (CF), carbon nanotube (CNT) as filler were prepared by melt blending. The research dealt with that y-ray irradiation treatment to increase thermal stability of composite, and effect of hybrid filler on improvement of electrical conductivity of composite, and carbon paper composite for promoting filler dispersion. The main contents and results are as follows:Electrical and dielectric and electrical heating properties of the composite films filled with individual CF are investigated systematically by melt mixing. The threshold value of HDPE/CF composites is 10 wt%(5.26 vol%) CF content. The resistivity of HDPE/CF composite shows obvious temperature dependence. The PTC intensity of the composites with 10 wt% CF content is up to 1010 ? cm. Surface heating temperature (Ts) of the composites is strongly dependent on the filler content and applied voltage. The characteristic growth time constant is 35-120s when the applied voltage is 2-9V. The AC resistivity of the composites shows a strong dependence on frequency before the threshold value and partly dependence on frequency after the threshold value. The dielectric permittivity and loss of the HDPE/CF composites varies with filler content, frequency and temperature. Composite film containing 10 wt% CF was exposed to the 60Co y-ray irradiation source at different doses in order to obtain an electrical heating material with good thermal and mechanical properties. The Ts values of the irradiated composite films are dependent on the applied voltage and the irradiation dose. In addition, the thermal expansion of the irradiated composite films decreased as the irradiation dose increased, and the E' of the composites increased as the irradiation dose increased. Using a rechargeable battery as a voltage source to evaluate the self-heating property of the irradiated composite films, surface temperature reached 54.2? and lasted for 6 h for the sample irradiated at 250 KGy.Electrical and dielectric and electrical heating properties of the composite films filled with individual CNT by melt mixing were also investigated systematically. The threshold value of HDPE/CNT composites is 2 wt%(1.12 vol%) which is far lower than that of the HDPE/CF composite system. However, the resistivity of HDPE/CNT composites doesn't show obvious temperature dependence. Surface heating temperature (Ts) of the composites is strongly dependent on the filler content and applied voltage. Compared with HDPE/CF composites, the surface temperature of HDPE/CNT composite is slightly lower at the same filler content, indicating that not only the content of the conductive filler, but also morphology and dispersed state of the filler affect the electrical heating property. Moreover, the melting point, degree of crystallinity and thermal stability of HDPE/CNT composite are higher than HDPE/CF composites when the filler content is the same. The AC resistivity and the dielectric permittivity and loss of the HDPE/CNT showed a similar regularity with HDPE/CF composites.Based on the above-mentioned research, HDPE composite films filled with hybrid filler of CF and CNT are prepared by melt mixing in order to study the synergistic effect of hybrid filler. Electrical and self-heating properties of the composite films are investigated. Results show that:the electrical resistivity of composite films filled with hybrid fillers is lower than those with single filler; the composite films filled with hybrid fillers displayed more excellent self-heating performance than those with single filler when the total content of filler is the same, such as a higher surface temperature (Ts), a more rapid temperature response, and a better thermal stability. Therefore, the combination of CNTs and CFs is believed to have a synergetic effect on improvement of the electrical and self-heating properties of composite films. The synergy can be considered to be the result of the fibrous filler CFs acting as long distance charge transporters and the CNTs serving as an interconnection between the fibers by forming local conductive paths. Using a rechargeable battery as the applied voltage source to evaluate the self-heating property of the composite films filled with hybrid filler, a heating temperature of 57.6? is achieved, which can last for 6 h.Moreover, the morphology, electrical heating property and mechanical property of the carbon nanopaper (CNP) and HDPE/CNP composites are discussed. The CNP is prepared by suspension filtration and the HDPE/CNP composite is fabricated by hot press method. The CNP with flat, smooth and porous structure possess of a certain level of strength and flexibility. The tensile strength of the CNP is 1.91 MPa. The tensile strength and modulus of HDPE/CNP composite are 24.88 MPa 1953.17 MPa respectively, which are higher than those of HDPE and HDPE/CNT composite prepared by mixing blend. The surface temperature reaches 113.1? for HDPE/CNP composite with 8 wt% filler content at the applied voltage of 9V. For the HDPE/CNT composite, the surface heating temperature is only 34.9? at same filler content and applied voltage. This indicates that that the electrical heating property of HDPE/CNP sandwiched composites is much better than that of the mixing blend of HDPE and carbon nanotubes.