Preparation And Properties Of TiB₂ Conductive PTC Composite Functional Materials

Polymer based PTC composites, as one of the most popular functional polymer materials in the domestic and foreign research, has been widely used in automatic temperature control heating system, overcurrent protection element manufacture and other fields.It still need for this kind of PTC material to improve the instability of the PTC properties and reduce the NTC effect in high temperature and other defects, besides the excellent PTC properties. In this dissertation, the PE/TiB2-CB composites, which shows obvious PTC effect, was prepared by melt-blending process. The SEM morphology was observed and the resistivity-temperature properties, thermal cycling electrical stability, thermal graphy were tested. The effect of tissue components and preparation process on the composites’ PTC properties and the PTC mechanism of the composite were investigated.The results show that there are obvious interface layer between TiB2-CB particles and the polyethylene matrix due to no chemical bond between them.In this experiment, the 7:3 ratio of HDPE, LDPE was choosed as matrix, with 24%TiB2 and 6%CB as conductive filler, followed by mixting in many times, grinding and drying treatment. The composites with crystallinity of 48.4%, room temperature resistivity of 0.65Ω·m, PTC intensity of 5.1 can be obtained by hot-pressed in 6MPa at 160℃,and then cooled in the air. The primary and secondary relationship of the process parameters on sample’s PTC characteristics is:thermoforming Cooling method>hot temperature>hot pressure. The PTC properties of composites increase at first and then decrease as the hot pressing pressure and temperature increasing. An appropriate heat pressing temperature and cooling method can increase the crystallinity of composites. The higher crystallinity corresponds to the more obvious PTC effect. The peak resistivity and PTC transition temperature (Ts) of the composites can be improved after annealed treatment at 120℃.In addition, the logp-T curve becomes steeper, and Infrared spectroscopy show the crystal structure of the system has not been destroyed. Compared with the poor resistance temperature stability of non crosslinked samples, the composites after cross-linked in water can get more stable repeated thermal cycling property and lower NTC effect in high temperature.The melting temperature range and the melting peak temperature of composite only depend on the characteristics of HDPE, LDPE matrix. The conductive fillers in polyethylene matrix is mainly distributed in the interface of crystalline and amorphous region and amorphous region, only a small amount in the stagnant zone crystalline matrix for thermal process and other factors. The PTC effect of composites comes mainly from the thermal expansion and the melting of polymer crystalline regions which lead to the destruction of conductive network. And the contribution of the matrix crystallization zone melting to the PTC properties of composite is higher than that of the thermal expansion of crystalline region.