The Research About PTC Composites Base On HDPE/CB

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. This kind of PTC composite has been widely used in many fields. In this article, some factors influencing on the room temperature resistivity and intensity of the PTC composite are considered such as the proportion of composite based on HDPE/CB, mold pressing conditions and assistant additive et al. Finally, composite samples are obtained with 8 PTC intensity and stable performance. The characteristic of composite are depended on the performance of components. It is investigated that all kind of components in the sample influence the resistivity based on HDPE/CB composites at the room temperature. The least room temperature electrical resistance of composites can be obtained with 43~48wt% carbon black,8 wt% zinc stearate, 1 wt% cross-linking agent DCP, 5～17wt% anti-flammability agent aluminium hydroxide and 1.5～3wt% anti-oxidant 246. Mould technics is one of the most important aspects that influence the resistivity and PTC intensity. The influence of the mould technics on the performance of HDPE/CB based composite has been considered by studied the mold pressure, mold pressing temperature, mold pressing time and the time of composites cooling to room temperature. According to the crossing analysis method, we got some very important conclusions as follow: those factors influencing the room resistivity are obvious in mold pressing temperature, mold pressing time and cooling time, but influence of mold pressing pressure is low. On the other hand, those factors influencing on the PTC effect of the composite are obvious in mold pressing temperature, mold pressing time and mold pressing pressure, but influence on PTC intensity of the composite is low in the cooling time. To obtain HDPE/CB composites with low room temperature resistivity and high PTC intensity, a optimum mold pressing technics is designed that mold pressing pressure is 8 MPa, mold pressing temperature is 155℃, mold pressing time is 15 minutes, and cooling time is 40 minutes. The sample followed as the optimal techincs that the PTC intensity is 8.41, and the NTC intensity is nearly zero in the sample, the HDPE/CB composites resistivity is still good and the PTC intensity of composites is stable after several heat-cool circles. By adding appropriate amount of EVA to HDPE/CB composite, it can change characteristic temperature of the composite from 131℃to 122℃.When the amount of EVA is less than 27.6wt%, HDPE-EVA/CB composite has PTC intensity larger than 7 and NTC intensity smaller than 0.5 in intensity. After several heat-cool circles, the HDPE/CB composites resistivity performance is still excellent and stable. The PTC intensity of composite will become low gradually and the NTC intensity is reverse with the graphite added in HDPE/CB-GP (graphite) composite. To ensure the useful value of the composite, namely, HDPE /CB-GP composite have a PTC intensity larger than 6, and NTC intensity smaller than 1.5, the amount of graphite should be lower than 17.2wt%. When HDPE/CNT composite contains 5wt% carbon nano-tubes, the resistivity becomes low obviously. Though ㏒R～T curve of the HDPE/CNT composite takes on PTC characteristic peak, the value of PTC intensity is too small to reach the request. In HDPE/CB-CNT composite system, it is no obvious influence on room temperature resistivity and PTC intensity of composite when CNT content is low than 1wt%. The intensity becomes low evidently and the room temperature resistivity is no change obviously when CNT content of the composition is more than 1wt%. When CNT content of the composition is more than 11wt%, the electric performance is excellent, but the PTC intensity disappears nearly. The PTC intensity of HDPE/CB composite may be explained according to the mechanism: it results in a conduct electricity passage may form as carbon black grain get in touch each other. When the temperature rises to melting point of HDPE nearby, due to the expansion coefficient of the HDPE is larger than carbon black, carbon black electric chain will be broken with HDPE expanded. It causes resistivity of composite rise 103～108 times quickly and form strong PTC intensity.