| PTC (Positive Temperature Coefficient) composite is the material which has the non-linear and positive temperature electric resistance coefficient. The value of the electric resistance of the material will increase 103-109 quantitatively within narrow temperature range when the temperature reaches characteristic temperature of PTC composite.As it is naturally abundant and low price, graphite is used as conductive filler material in present work, and high density polyethylene is uses as based material, succeed prepared polymer based conductive composite with distinct positive temperature coefficient. The behavior of crystallization and the distribution of graphite were also studied by means of SEM, DTA. And the experiment result is anatomized in this paper.The effects of the graphite content and the matrix crystallity on PTC effect were studied, and the mechanism of PTC effect was discussed. It is supposed that the PTC effect is relation to the crystalline behavior of matrix. The experiment show that the sensitivity temperature of HDPE/graphite material is between 90℃-135℃, the resistivity increase above 6 orders of magnitude. At first, positive temperature coefficient increasing along with temperature rising. The max positive temperature coefficient occurrence when temperature attaining 130℃. When temperature reached melting point, the negative temperature coefficient will appeared. This material temperature sensitive range is different as the graphite content change. On the whole, along with the graphite content increasing, the begin point of the temperature–sensitive range raise also. The rule is consistent with this material Tg. The graphite weight reached 30% could obtain obviously positive temperature coefficient. The effect of graphite size on PTC effect was first studied systematically. The experimental results indicate that the PTC effect is related to the particle diameter of graphite. So a balance between the PTC intensity and the room temperature resistivity is achieved by using 50μm graphite in the composite.The mechanism property drastically descends owing to addition of the graphite. So it is necessary to add the additives in the composite which can improve the mechanism property, maintain the PTC intensity. The comprehensive performance of graphite, Dioctyl-Phthalate (DOP) and talc powder filled high density polyethylene (HDPE) composites was studied in this paper. The orthogonal design experiment was taken to optimize the content of the filler. The composite as prepared under the optimum condition has an excellent comprehensive performance.The effect of dioctyl phthalate(DOP), dibutyl phthalate(DBP) and dioctyl sebacate (DOS) on the comprehensive performance of the composite were studied. EPDM was employed to improve the comprehensive performance of HDPE/ graphite composite. For HDPE/graphite system, due to EPDM's low crystallization, the crystallization of this composite was also descended. But the percolation threshold of the graphite changed a little and this also confirmed with the model that we depicted. The conductive passways improved or the widen speed slowed down; the conductive passways can be opened in wide range of the graphite content, made the PTC curves widen.Anneal is a very important measure for repeat and stability of the material preparation. After anneal, the low temperature resistivity will rise commonly. Consider conductive additive interphase property is very important for graphite powder. Particular, conductive additives were cleanout through organic solvent. This measure could make the resistivity reduce 1 order of magnitude.The effect of graphite size, additives, materials blends and processing technique on the comprehensive performance of the composite were detailed studied in this paper. The study result reveals that HDPE/graphite system has good property of self-limiting temperature. It has a good appliance prospect.
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